# Fiber Optic Chemical Nanosensors Based on  Engineered Single-Walled Carbon Nanotubes: A Review

Authors: M. Consales; A. Cutolo; M. Penza; P. Aversa; M. Giordano; A. Cusano
Journal: Journal of Sensors
 (2008)

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## Abstract

In this contribution, a review of the development of high-performance optochemical nanosensors based on the integration of carbon nanotubes with the optical fiber technology is presented. The paper first provide an overview of the amazing features of carbon nanotubes and their exploitation as highly adsorbent nanoscale materials for gas sensing applications. Successively, the attention is focused on the operating principle, fabrication, and characterization of fiber optic chemosensors in the Fabry-Perot type reflectometric configuration, realized by means of the deposition of a thin layer of single-walled carbon nanotubes (SWCNTs) upon the distal end of standard silica optical fibers. This is followed by an extensive review of the excellent sensing capabilities of the realized SWCNTs-based chemical nanosensors against volatile organic compounds and other pollutants in different environments (air and water) and operating conditions (room temperature and cryogenic temperatures). The experimental results reported here reveal that ppm and sub-ppm chemical detection limits, low response times, as well as fast and complete recovery of the sensor responses have been obtained in most of the investigated cases. This evidences the great potentialities of the proposed photonic nanosensors based on SWCNTs to be successfully employed for practical environmental monitoring applications both in liquid and vapor phase as well as for space. Furthermore, the use of novel SWCNTs-based composites as sensitive fiber coatings is proposed to enhance the sensing performance and to improve the adhesion of carbon nanotubes to the fiber surface. Finally, new advanced sensing configurations based on the use of hollow-core optical fibers coated and partially filled by carbon nanotubes are also presented.


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## Body

## 1. Introduction

Environmental monitoring is required
to protect the public and the environment from toxic contaminants and pathogens
that can be released into a variety of media including air, soil,
and water.
Air pollutants include sulfur dioxide, carbon monoxide, nitrogen dioxide, and
volatile organic compounds (VOCs), which originate from sources such as vehicle
emissions, power plants, refineries, and industrial and laboratory processes.
Soil and water contaminants can be classified as microbiological, radioactive,
inorganic, synthetic organic, and VOCs. Pesticides and herbicides are applied directly to plants and soils,
and incidental releases of other
contaminants can originate from spills, leaking pipes, underground storage
tanks, waste dumps, and waste repositories. Some of these contaminants can
persist for many years and migrate through large regions of soil until they
reach water resources, where they may present an ecological or human-health
threat. The current monitoring methods are mainly based on offsite laboratory
analyses and are costly and time-consuming. In addition, limitations in
sampling and analytical techniques occur [1, 2]. For this reason, a need exists
for accurate, inexpensive, continuous, and long-term monitoring of
environmental contaminants using sensors that can be operated on site.Chemical sensors can be in most cases
schematically described as composed of a sensitive part which, interacting with
the surrounding environment, collects and concentrates molecules at or within
the surface undergoing physical changes, and of an opportune transducer which converts these changes of the sensing part into
interpretable and quantifiable terms.
The heart of the chemical sensor is the sensitive element
which is the interface between transducer and external  environment so that the
nature, the selectivity, and sensitivity of the sensor depend upon these
interactive materials. Good materials to use as sensing part should optimize
specific interactions with a target analyte or narrow class of analytes that should provide a
fast and reversible diffusion of the penetrants and small recovery times, and also they should maintain
the physical state as well as their
geometry over several cycles of use,  in order to avoid hysteretic effects, and thus to ensure
the reproducibility [3]. Candidate materials for chemical sensors include
polymers, organic monolayers, ceramics, metals semiconductors, nanostructured
and porous materials (nanomaterials, molecular sieves, sol-gels, aerogels),
biomolecules, and combination thereof. Of course, adequate and detailed
physical, chemical, and structural characterizations of the exploited sensing
materials are essential for a deep understanding of their properties and for a
rational design of the whole chemical sensor and of its performances.The natural step following the
selective recognition of an analyte from the sensitive layer is the signal
transduction, and thus the choice of an opportune technique to read the
physical or chemical changes occurring at the sensing part. Transducing
approaches can include mechanical (acoustic wave, micromechanical),
electrochemical, optical, thermal, and electronic types. Each has strengths and
weaknesses relative to the particular application. Each transduction principle
can be implemented in a variety of configurations, and fabricated by multiple
approaches, resulting in many different sensing platforms.A number of chemical sensors have
been developed for environmental monitoring applications [4], a classification
of which can be carried out upon their principal physics and operating
mechanisms. The most exploited transduction principles in chemical sensing are
the mass change and the resistivity/conductivity change of the sensitive
element occurring on exposure to and consequent sorption of the molecules of
target environmental analytes. The first physical parameter is in many cases
measured by the shift in the resonant frequency of an oscillating piezoelectric
crystal. Depending on the kind of vibrational wave propagated in the crystal, a
mass sensor can be classified as quartz crystal microbalance (QCM) or surface
acoustic wave (SAW) sensors [5–8]. They typically use a thin polymeric film as
sensitive layer [9], however SAW and QCM-based chemical sensors using other
sensitive coatings have been proposed [10, 11]. Instead,
resistivity/conductivity changes are typically detected by conductometric
measurement carried out on sensitive materials (mainly semiconducting metal
oxides and conjugated polymers) deposited between two electrodes. These sensor
technologies carry the name of metal oxide semiconductors (MOS) and conductive
organic polymer (COP) sensors [12]. During the last two decades, however, a
remarkable interest has been also focused on optical transduction principles
for the measurement of chemical and biological quantities [13]. Since the
development of the first optical sensors for the measurement of CO2 and O2 concentrations
[14], a large variety of devices based upon optical methods have been used in
chemical sensors and biosensors including ellipsometry, spectroscopy
(luminescence, phosphorescence, fluorescence, Raman), interferometry (white
light interferometry, modal interferometry in optical waveguide structures),
spectroscopy of guided modes in optical waveguide structures (grating coupler,
resonant mirror), and surface plasmon resonance [15–25]. In these sensors, a
desired quantity is determined by measuring the refractive index, absorbance,
and fluorescence properties of analyte molecules or of a chemooptical transducing
medium.Optical fiber sensors are also very
attractive in chemical sensing applications due to some unique characteristics derived by the use of
optical fibers, one of the most outstanding characteristics of which is their
ability to transmit light over large distances with low losses allowing a
sensor head to be remotely located from the instrumentation. This feature is
particularly useful for sensing in harsh environments where hazardous chemicals
may be present or extremes in temperature occur. In addition to the small size,
light weight and high flexibility of fibers allow access to areas that would be
otherwise difficult to reach.Provided the optical power density is
within certain limits, fiber optic chemical sensors are much safer in explosive
environments compared with sensors involving electrical signals, where a spark
may trigger a gas explosion. Optical signals are immune to electrical or
magnetic interference from, for example, power lines and electrical machinery.
Furthermore, optical fibers have the capability of carrying a huge amount of
information, much greater than that carried by electrical wires. Fiber optic
sensing is very versatile, since the intensity, wavelength, phase, and
polarization of light can all be exploited as measurement parameters, and
several wavelengths launched in the same fiber in either direction form
independent signals. This gives the possibility of monitoring several chemicals
with the same fiber sensor or even simultaneously monitoring unwanted
environment parameters variations which could drastically affect the chemical
concentration measurements, such as the temperature or disturbance of the
fiber. Multiplexing of fiber optic systems is also relatively easy, allowing
expensive source or analysis instrumentation to be shared among a number of
sites.In this contribution, a review on the
integration of carbon nanotubes as advanced nanostructured sensitive coatings
with the optical fiber technology for the development of high-performance optochemical
nanosensors to exploit for a wide range of environmental monitoring
applications is carried out. In particular, the excellent sensing capabilities
of the realized photonic chemosensors demonstrated against VOCs and other
pollutants in different environments (air and water) and operating conditions
(room temperature and cryogenic temperatures) will be reviewed. As it will be
seen in the following sections, ppm and sub-ppm chemical detection limits have
been obtained in most of the investigated cases.


## 2. Carbon Nanotubes as Advanced Materials for Chemical Sensing

The search for new advanced materials
is an important area of contemporary research in numerous disciplines of
science. Great attention has been paid in recent years to nanostructured
materials of different chemical composition, produced as nanoparticles,
nanowires, or nanotubes. Similarly, there has already been great interest in
their preparation, properties, and applications in the literature. As a matter
of fact, with the development of nanoscience and nanotechnology, a large number
of literatures on one-dimensional nanostructured materials, including tubes,
rods, belts, and wires in this area, have been published every year [26]. These
materials have their unique structures which are dominated by a wire-like structure
whose diameter varies over a broad range from several tens of nanometers to a
micrometer. In particular, carbon-based nanostructures exhibit unique
properties and morphological flexibility, which renders them inherently
multifunctional and compatible with organic and inorganic systems.Carbon nanotubes (CNTs), discovered
by Iijima in 1991 [27], are at the forefront of the novel nanoscale
investigations and nanostructure effects due to their unique electronic,
chemical, structural, optical, mechanical, and thermal properties depending on
their specific hollow nanostructure with surface-arranged carbon atoms
organized in rolled one-dimensional seamless tubes [28, 29]. They are
considered as a new form of pure carbon, and can essentially be thought of as
layers of graphite rolled-up into a tube to form a cylinder with diameter of
few nanometers and length ranging from 1 to 100 microns. To date, CNTs are
building blocks considered as the most promising functional nanomaterial for
future miniaturized gas nanosensors due to their hollow nanostructure and high
specific surface area which provide attractive characteristics for gas sensing
applications. In fact, due to their unique morphology, CNTs possess the
excellent capability to reversibly adsorb molecules of environmental pollutants
undergoing a modulation of their electrical, geometrical, and optical
properties, such as conductivity, refractive index, and thickness. CNTs can be
distinguished in SWCNTs or multiwalled carbon nanotubes (MWCNTs) depending on whether
only one layer or many layers of graphite are concentrically rolled up
together, and can behave either as metallic or semiconducting, depending upon
their diameter and chirality (the way the hexagons are arranged along the
tubule axis) [30]. SWCNTs are a very important variety of carbon nanotubes because they
exhibit important electrical and sensing properties that are not shared by the MWCNTs
variants. A further distinction is usually made between closed-end or
opened-end CNTs depending upon whether the ends of the tubes are capped or
uncapped [31].


### 2.1.
Chemical Sensors Based on Carbon Nanotubes: State of the Art

The special geometry of carbon
nanotubes and their characteristic of being all surface reacting materials
offer great potential applications as chemical sensor devices with excellent sensitivities
and fast responses [32]. Most of the sensors based on CNTs are field effect
transistors (FET), since much interest has been focused in the past on the
study of the changes in their electrical properties as a consequence of the
interaction with gaseous and VOC molecules: many studies have shown that
although carbon nanotubes are robust and inert structures, their electrical
properties are extremely sensitive to the effects of charge transfer and
chemical doping by various molecules. The electronic structures of target
molecules near the semiconducting nanotubes cause measurable changes to the
nanotubes electrical conductivity. Nanosensors based on changes in electrical
conductance are highly sensitive, but they are also limited by factors such as
their inability to identify analytes with low adsorption energies, poor
diffusion kinetics, and poor charge transfer with CNTs [33].Kong et al. [34] were probably the
first to show that CNTs can be used in chemical sensors since exposing SWCNTs
to electron withdrawing (e.g., NO2) or donating (e.g., NH3)
gaseous molecules dramatically increases or decreases the electrical resistance
of SWCNTs in the transistor scheme. In addition, CNTs-based sensors
demonstrated a fast response and a higher sensitivity than, for example,
solid-state sensors at room temperature.More or less in the same period,
it
was demonstrated that the electrical conductance of SWCNTs could be modified in
presence ofO2 [35]. The effect of the adsorption of
several gas compounds in SWCNTs was also described [36], as well as those of water
vapor on the electrical resistance of an SWCNT [37]. Shortly afterwards,
Fujiwara et al. [38] studied the N2 and O2 adsorption
properties of SWCNT bundles and their structures. All these studies opened the
door to the development of chemical sensors based on CNTs.Sensing devices based not only on the
changes in the electrical properties of CNTs but also on other principles were
proposed. For example, bundles of SWCNTs [39] have measured the thermoelectric
qualitative response to a variety of gases (He, N2, H2, O2,
and NH3). Sumanasekera et al. [40] created a thermoelectric chemical
sensor to measure the easily detectable and reversible thermoelectric power
changes of SWCNTs when they are in contact with He, N2, and H2.
Chopra et al. [41] developed a circular disk resonator coated with SWCNTs using
a conductive epoxy, which selectively detects the qualitative presence of
several gases (NH3, CO, Ar, N2, and O2) due to
changes in the dielectric constant and shifts in the resonant frequency.Wei et al. [42] demonstrated a gas
sensor depositing CNT bundles onto a piezoelectric quartz crystal. This sensor
detected CO, NO2,  H2, and N2 by detecting
changes in oscillation frequency and was more effective at higher temperatures
(200°C). Penza et al. [43] developed SAW and QCM sensors coated with SWCNTs and MWCNTs and used
them to detect VOCs such as ethanol, ethylacetate, and toluene by measuring the
downshift in the resonance frequency of the acoustic transducers.Carbon nanotubes can be easily
functionalized with molecules enabling the specific interaction with target chemicals,
thus improving the typically low selectivity of CNTs-based devices. In this
way, different types of sensors based on molecular recognition interactions can
be developed, allowing the development of nanosensors that are highly selective
and sensitive. Chen et al. [44] used a noncovalent functionalized FET based on
SWCNTs  for selectively recognizing target proteins in solution. Azamian  et al.
[45] immobilized glucose oxidase on SWCNTs and enhanced the catalytic signal by
more than one order of magnitude compared to that of an activated macrocarbon
electrode. Zhao et al. [46] worked with horseradish peroxidase and Sotiropoulou
et al. [47] worked with enzymes. Barone et al. [48] developed a device for
β-D-glucose sensing in solution phase. They also showed two distinct mechanisms
of signal transduction: fluorescence and charge transfer.Recently, further interest has also been
devoted to the possibility of changing the optical and/or geometrical
properties of SWCNTs upon adsorption of target analyte molecules, enabling to
exploit such materials for the development of optochemical nanosensors for a
wide range of environmental monitoring applications, from chemical detection in gaseous
or liquid phase [6, 49–53] to hydrogen detection at cryogenic temperatures suitable
for aerospace applications [51]. In particular, the possibility of exploiting
such materials in conjunction with the optical fiber technology could enable
the development of optoelectronic noses and tongues capable of air and water
quality monitoring, characterized by ppm and sub-ppm resolutions, good recovery
features, and fast responses, as it will be seen in Section 4.


## 2.1.
Chemical Sensors Based on Carbon Nanotubes: State of the Art

The special geometry of carbon
nanotubes and their characteristic of being all surface reacting materials
offer great potential applications as chemical sensor devices with excellent sensitivities
and fast responses [32]. Most of the sensors based on CNTs are field effect
transistors (FET), since much interest has been focused in the past on the
study of the changes in their electrical properties as a consequence of the
interaction with gaseous and VOC molecules: many studies have shown that
although carbon nanotubes are robust and inert structures, their electrical
properties are extremely sensitive to the effects of charge transfer and
chemical doping by various molecules. The electronic structures of target
molecules near the semiconducting nanotubes cause measurable changes to the
nanotubes electrical conductivity. Nanosensors based on changes in electrical
conductance are highly sensitive, but they are also limited by factors such as
their inability to identify analytes with low adsorption energies, poor
diffusion kinetics, and poor charge transfer with CNTs [33].Kong et al. [34] were probably the
first to show that CNTs can be used in chemical sensors since exposing SWCNTs
to electron withdrawing (e.g., NO2) or donating (e.g., NH3)
gaseous molecules dramatically increases or decreases the electrical resistance
of SWCNTs in the transistor scheme. In addition, CNTs-based sensors
demonstrated a fast response and a higher sensitivity than, for example,
solid-state sensors at room temperature.More or less in the same period,
it
was demonstrated that the electrical conductance of SWCNTs could be modified in
presence ofO2 [35]. The effect of the adsorption of
several gas compounds in SWCNTs was also described [36], as well as those of water
vapor on the electrical resistance of an SWCNT [37]. Shortly afterwards,
Fujiwara et al. [38] studied the N2 and O2 adsorption
properties of SWCNT bundles and their structures. All these studies opened the
door to the development of chemical sensors based on CNTs.Sensing devices based not only on the
changes in the electrical properties of CNTs but also on other principles were
proposed. For example, bundles of SWCNTs [39] have measured the thermoelectric
qualitative response to a variety of gases (He, N2, H2, O2,
and NH3). Sumanasekera et al. [40] created a thermoelectric chemical
sensor to measure the easily detectable and reversible thermoelectric power
changes of SWCNTs when they are in contact with He, N2, and H2.
Chopra et al. [41] developed a circular disk resonator coated with SWCNTs using
a conductive epoxy, which selectively detects the qualitative presence of
several gases (NH3, CO, Ar, N2, and O2) due to
changes in the dielectric constant and shifts in the resonant frequency.Wei et al. [42] demonstrated a gas
sensor depositing CNT bundles onto a piezoelectric quartz crystal. This sensor
detected CO, NO2,  H2, and N2 by detecting
changes in oscillation frequency and was more effective at higher temperatures
(200°C). Penza et al. [43] developed SAW and QCM sensors coated with SWCNTs and MWCNTs and used
them to detect VOCs such as ethanol, ethylacetate, and toluene by measuring the
downshift in the resonance frequency of the acoustic transducers.Carbon nanotubes can be easily
functionalized with molecules enabling the specific interaction with target chemicals,
thus improving the typically low selectivity of CNTs-based devices. In this
way, different types of sensors based on molecular recognition interactions can
be developed, allowing the development of nanosensors that are highly selective
and sensitive. Chen et al. [44] used a noncovalent functionalized FET based on
SWCNTs  for selectively recognizing target proteins in solution. Azamian  et al.
[45] immobilized glucose oxidase on SWCNTs and enhanced the catalytic signal by
more than one order of magnitude compared to that of an activated macrocarbon
electrode. Zhao et al. [46] worked with horseradish peroxidase and Sotiropoulou
et al. [47] worked with enzymes. Barone et al. [48] developed a device for
β-D-glucose sensing in solution phase. They also showed two distinct mechanisms
of signal transduction: fluorescence and charge transfer.Recently, further interest has also been
devoted to the possibility of changing the optical and/or geometrical
properties of SWCNTs upon adsorption of target analyte molecules, enabling to
exploit such materials for the development of optochemical nanosensors for a
wide range of environmental monitoring applications, from chemical detection in gaseous
or liquid phase [6, 49–53] to hydrogen detection at cryogenic temperatures suitable
for aerospace applications [51]. In particular, the possibility of exploiting
such materials in conjunction with the optical fiber technology could enable
the development of optoelectronic noses and tongues capable of air and water
quality monitoring, characterized by ppm and sub-ppm resolutions, good recovery
features, and fast responses, as it will be seen in Section 4.


## 3. Optochemical Nanosensors in Reflectometric Configuration: Principles, Fabrication, and Characterization

This section is focused on the
description and operating principle of the optoelectronic sensing configuration
adopted for the realization of the carbon nanotubes-based optochemical nanosensors
(i.e., the reflectometric configuration), and the optoelectronic scheme for
their robust, continuous, and real-time interrogation. The deposition technique
exploited for the SWCNTs integration with the optical fiber technology, the
molecular engineering Langmuir Blodgett (LB) method, is also presented and
described in detail together with an extensive characterization of the structural
and morphological properties of the fabricated chemosensitive coatings.


### 3.1. Sensing Configuration and Operating
Principle

The reflectometric configuration is
essentially based on a low finesse and extrinsic Fabry-Perot (FP)
interferometer and, as schematically shown in Figure1, uses a thin sensitive
film deposited at the distal end of a properly cut and prepared silica optical
fiber (SOF) to produce an FP cavity. The thin film acts as an optical cavity
where the first mirror is given by the fiber/sensitive layer interface whereas
the second one is given by the sensitive layer/external medium interface.SEM images of (a) a properly cut bare fiber and (b) of a fiber tip
coated by a thin sensitive layer; (c) schematic view of the Fabry-Perot-based
configuration.
(a)(b)(c)First described in 1899 by Fabry and
Perot [54], the interferometer known by their names makes use of multiple
reflections between two closely spaced surfaces. In fact, the light is
partially reflected each time it reaches the second surface, resulting in
multiple offset beams which can interfere with each other (see Figure 2).Figure 2
Schematic representation of the
multiple reflected beams in a Fabry-Perot cavity.The amount of light reflected at the
first interface can be calculated as the sum of the multiple reflected beams
and is strongly influenced even by very small changes of the distance between
the two surfaces (the sensitive layer thickness) or its optical properties (the
sensitive layer refractive index) [55]. This explains the massive use of such
configuration in fiber optic-based sensing in the two past decades, especially
for the detection and measurements of various physical, chemical, and
biomedical parameters [56, 57]. All these characteristics, combined with the
possibility of integrating a number of sensitive materials with the optical
fibers by means of very simple, low-cost, and versatile deposition techniques,
make it one of the most attractive and useful optoelectronic configurations especially
suitable for practical applications.The principle of operation of an optoelectronic sensor
in reflectometric configuration relies thus on the fact that a modulation of
the intensity of light reflected at the fiber-sensing overlay interface occurs
due to small changes of the layer thickness(dfilm) and its complex
refractive index (n˜film). As a matter of fact, the fiber-film
reflectance can be expressed as [58]
(1)R=|r12+r23⋅e−i⋅k˜film1+r12⋅r23⋅e−i⋅k˜film|2with(2)r12=nf−n˜filmnf+n˜film,r23=n˜film−nextn˜film+next,k˜film=2π⋅(2⋅n˜film⋅dfilm)λ=4π⋅n⋅dfilmλ−i4π⋅k⋅dfilmλ=βfilm−iα⋅dfilm,where n˜film=n−i⋅k is the square root of the complex dielectric
constant (ε˜film=εr−i⋅εi) of the sensitive fiber coating, α=4π⋅k/λ is the overlay absorption coefficient, nf and next are the optical fiber and external medium refractive index,
and λ is the optical wavelength. Thus, the reflectance changes due to the chemical
interaction between the sensing overlay and the target analyte can be expressed
as follows:
(3)ΔR=(δRδn)⋅Δn+(δRδk)⋅Δk+(δRδdfilm)⋅Δdfilm=Sn⋅Δn+Sk⋅Δk+Sd⋅Δdfilm,
where Sn, Sk, and Sd are the sensitivities against the variations of the real and imaginary parts of
the complex refractive index and against the overlay thickness, respectively.
It is
noteworthy that these three sensitivity factors Sn,
Sk, and Sd strongly depend upon the geometrical and
electro-optical properties of the sensitive nanocoatings and upon the
environmental condition (e.g., vapor or liquid phase), and for this reason they
have to be properly considered case by case.


### 3.2. Reflectometric Sensor Interrogation
Scheme

So far, a variety of schemes have
been proposed for the interrogation of a fiber optic sensor based on the FP cavity,
the most used ones relying on spectrum-modulating approach, and single
wavelength reflectometry [59]. Here the attention has been focused on this last
technique, which is simple to implement and requires just few widespread
commercial and low-cost optoelectronic components while preserving excellent
performances. In addition, it enables the fabrication of cost-effective,
reliable, robust, and portable equipments, which are factors of crucial
importance for in situ and long-term monitoring applications and for the
desired technology transfer to the market. The typical interrogation scheme
enabling the single wavelength reflectance monitoring of an FP cavity realized
on the distal end of an optical fiber is shown in Figure 3(a).(a) Schematic illustration of the typical interrogation scheme adopted
for the single wavelength reflectance monitoring of an optical cavity realized
upon the fiber tip. (b) Image of the compact and portable equipment exploited
for the interrogation of the FP-type optical fiber sensors.
(a)(b)The sensitive interface is lighted by a pigtailed superluminescent
light emitting diode (SLED) operating in one of the two main telecommunication
wavelengths (1310 nm and 1550 nm). A source controller is used to keep the SLED
at a constant temperature and feed it with an input current, which allows the
source to emit an optical power of few mW. This power is split by a2×2
in-fiber directional coupler, providing the necessary connections between light
source, sensing interface, and two InGaAs photodiodes. The first photodiode is
for the measurement of the reflected signal:
(4)Vsignal=k1⋅Z1⋅Psource⋅R,
where Psourse is the power emitted by the source, k1
is a constant which takes account of the power losses due to the
coupling factor of the 2×2 directional coupler and the photodiode responsivity,
Z1 represents a gain factor introduced by the first photoreceiver
device. The
second arm of the 2×2 coupler is used to provide a  valid intensity monitoring
channel for the compensation of the sensor response. As a matter of fact, the
second photodiode provides a voltage signal directly related to the optical
power levels within the optical chain according to
(5)Vsource=k2⋅Z2⋅Psource,
where k2(k2≠k1) takes into
account the losses coming from the connection between source, 2×2 coupler, and
photodiode and Z2(Z2≠Z1) is a gain factor
introduced by the second photoreceiver device. The intensity compensation is
obtained by considering the ratio between the voltage signals at the two
photoreceivers according to
(6)I=VsignalVsource=k1⋅Psource⋅Z1⋅Rk2⋅Z2⋅Psource=α⋅R,
where α=k1⋅Z1/k2⋅Z2
takes into account all the setup parameters. As
evident,  the normalized response of the sensor is only sensitive to the change in
the reflectance induced by chemical sorption within the sensing overlay and not
to the fluctuations of the optical power levels along the whole measurement
chain. In the followings, the relative change of the sensor output ΔI/I0 has been considered
(where I0 is the output
signal in the reference or initial condition), which, in turn, corresponds to
the relative reflectance change occurring at the fiber-sensitive layer
interface (ΔR/R0).Synchronous detection is typically
implemented to enhance the system performances, by amplitude modulating the
light source at 500 Hz and retrieving the photodetector voltages by using a
dual channel lock-in amplifier. In addition, a Time Division Multiplexing (TDM)
approach is typically exploited to perform the quasisimultaneous interrogation
of up to eight optical probes by means of a multichannel fiber optic switch.
Finally, the optical data can be stored in a notebook controlling the sensing
process in LabView software by a NI-DAQ card. The overall setup was properly
engineered in order to provide robust, compact and portable equipment (see Figure3(b)).


### 3.3. Optochemical Nanosensor Fabrication

The realization of homogeneous thin
films of SWCNTs with a controllable thickness is an important basis for the
future development of their scientific understanding and technological
applications. Proper manipulation techniques are required for applying thin films
of carbon nanotubes on substrates that do not allow direct grow methods.
Although various proposals exist for their incorporation into devices, in
single tube or thin film architectures [60], here the LB technique has been
chosen as a way to transfer nanometer-scale layers of SWCNTs upon either bare
optical fibers or cadmium arachidate (CdA) buffer-linker material, previously
deposited (by the same technique) upon the fiber end in order to improve the
carbon nanotubes adhesion on the sensors surface. The CdA has been chosen as
buffer material due to its peculiar amphiphilic molecular structure suitable
for LB deposition process [61, 62].The LB-technique is one of the most
promising techniques for preparing such thin films as it enables the precise
control of the monolayer thickness, homogeneous deposition of the monolayer
over large areas, and the possibility of making multilayer structures with
varying layer composition [63]. An additional advantage of the LB technique is
that monolayers can be transferred on almost any kind of solid substrate.
However, these advantages have to be traded with the low speed of the
deposition procedure as well as the limited number of materials suitable for
this technique. As schematically represented in Figure 4, the molecules of the
films to be deposited are firstly dispersed onto the surface of a subphase,
typically oriented with the hydrophobic part upwards and with the hydrophilic
one immersed in water. Subsequently, a reduction of the surface area occupied
by each molecule is performed by means of moving barriers in order to produce a
solid phase of a given surface pressure in which the molecules are densely
packed forming a highly ordered array [64]. From this phase, the molecules can
be transferred to a properly cleaned and prepared solid substrate by its
dipping through the condensed Langmuir layer.Figure 4
Schematic representation of the
Langmuir-Blodgett deposition procedure.As the solid phase is reached only at
high surface pressures, a continuous reduction of the moving barriers is
performed when the molecules are transferred from the subphase to the substrate
in order to keep the surface pressure constant, ensuring that the solid phase
is maintained. Repeated dipping of the same substrate is also possible,
resulting in the deposition of a thin film with one monolayer at a time. The precise control
and uniformity of film thickness in the submicron range allowed by the LB
technique is essential especially for the optical interference principle used
in the reflectometric configuration to convert refractive index changes into
optical intensity changes.For SWCNT film deposition, a solution
(0.2 mg/mL) of SWCNT pristine material in chloroform has been spread onto a
subphase constituted by deionized water (18 MΩ) with 10−4 M of CdCl2. The subphase
pH and the temperature were 6.0 and 23°C, respectively. The monolayer has been
compressed with a barrier rate of 15 mm/min up to a surface pressure of 45 mN/m. The single layer has been deposited with a dipping rate of 3 mm/min and
the transfer ratio of the monolayers from the subphase to the surface of the
optical fiber sensor substrate, either bare or already coated by 20 monolayers
of CdA [50], was in the range of 0.5 to 0.7. After a proper drying of 12 hours
overnight, the sensing multilayers deposited were ready for the vapor testing.
The SWCNT pristine material has been purchased from Carbon Nanotechnologies
Inc. (Houston, Tex,
USA), and contains primarily
SWCNTs with a small amount (approx. 1%) of amorphous carbon and 4–6% of catalyst Fe
particles. The nanotube diameters are roughly Gaussian distributed and centered
at about 2 nm. The raw material of commercial nanotubes has been used
as-bought, without any purification treatment. The samples were prepared by
sonicating SWCNT suspension in chloroform for 1 hour at room temperature prior
to the deposition. For the fabrication of the carbon nanotubes-based probes,
before the LB deposition procedure the standard silica optical fibers have been
previously accurately polished from the acrylic protection and cleaved with a
precision cleaver in order to obtain a smooth and plane surface. Then, they
have been washed in chloroform and dried with gaseous nitrogen to be ready for
the CdA or SWCNT deposition. The number of CdA and/or SWCNT monolayers
deposited onto the sensor surface have been controlled by simply choosing how
many times the substrate is dipped inside or lifted from the solution
containing the monolayer to transfer.


### 3.4. Sensor Characterization: Structural and
Morphological Features of SWCNT Overlays

The rational design of a chemical
sensor and of its performances is something which is possible only if the
sensitive material properties and the way they are affected by different
deposition parameters or ambient conditions are well known and understood. To
this aim an extensive characterization of the as-bought SWCNT powders as well
as of the deposited LB SWCNT films has been carried out in order to investigate
their structural and morphological features. Such characterization involved
X-ray diffraction (XRD) and Raman spectroscopy analyses, high-resolution
transmission electron microscopy (HRTEM), and scanning electron microscopy (SEM)
observations. In particular, Figure5 reports the typical XRD spectrum of
as-bought SWCNT powder (1 mg) material. The pattern exhibits a well-defined
graphite-like (002) diffraction peak at 2θ = 26.5°, and a broad band centered at lower 2θ = 22° from amorphous carbon or nonnanotube
carbon material.Figure 5
X-ray diffraction spectrum obtained from HiPco SWCNT powder (1 mg).Further XRD analyses performed on CdA
multilayers deposited on glass substrates evidenced a CdA monolayer spacing of
about 2.8 nm for cadmium arachidate. The same measurements, performed with an
SWCNT multilayer, revealed a carbon nanotubes monolayer spacing of about 2.0 nm
[50].The HRTEM images of an SWCNT powder,
reported in Figure6 at (a) low and (b) high magnifications, confirm the
nanometric dimension of the carbon tubes and reveal the presence between them
of some Fe metal particles, a typical catalyst used in the HiPco production
process of carbon nanotubes. Generally, the SWCNT powders contain not only
carbon tube nanostructures but also carbonaceous particles such as amorphous
carbon, fullerenes, and transition metals introduced as a catalyst during the
synthesis. [65, 66].(a), (b) High resolution TEM images of SWCNT powder at (a) low and (b)
high magnifications, (c) SEM images of CdA-buffered SWCNT LB films deposited
upon an SOF sensor substrate.
(a)(b)(c)These impurities sometimes hinder the
fine characteristics of the carbon nanotubes and limit the best performance of
the SWCNTs-based functional devices. For this reason, effective purifications
for the removal of the by-products (amorphous carbon, fullerenes, catalyst
metal particles) of SWCNT synthesis are needed in order to fully exploit their
outstanding properties. Finally, Figure6(c) reports the typical SEM images of
CdA-buffered LB SWCNT films deposited upon an SOF tip. It demonstrates the
success of the integration of carbon nanotubes with the optical fiber and
reveals their attitude to adhere one to each other forming bundles or ropes
with a spaghetti-like arrangement. In the investigated cases, such CNT mats are
characterized by mean diameter and length which have been estimated to be in
the range 5–40 nm and 1–15 μm, respectively.Also Raman spectroscopy analyses have
been conducted to characterize the fabricated LB films based on SWCNTs already
deposited upon the optical fiber tip. To this aim, a Raman microscope
functioning in backscattering configuration employing an HeNe laser (633 nm)
and 50x and 100x objective lenses was used. The results are shown in Figure7,
where the typical Raman spectrum of SWCNTs has been reported.Figure 7
Typical Raman spectrum of an LB SWCNT film directly deposited upon the
optical fiber tip.They confirm the single-walled and
semiconducting nature of the CNTs [67] as well as their integration with the
optical fiber transducers. The characteristic multipeak feature “G-band” at
about 1580 cm−1, corresponding to carbon atom vibration tangentially
with respect to the nanotube walls, together with the less remarkable
disorder-induced “D-band” peak typically in the range 1300–1400 cm−1,
representing the degree of defects or dangling bonds
[67], can be
easily revealed. The large ratio of G to D peaks here observed gives us an
indication of an ordered structure of the deposited SWCNT overlay, whereas the
observation of the two most intense G peaks (labeled G+ and G−) confirms the
absence of MWCNTs.


## 3.1. Sensing Configuration and Operating
Principle

The reflectometric configuration is
essentially based on a low finesse and extrinsic Fabry-Perot (FP)
interferometer and, as schematically shown in Figure1, uses a thin sensitive
film deposited at the distal end of a properly cut and prepared silica optical
fiber (SOF) to produce an FP cavity. The thin film acts as an optical cavity
where the first mirror is given by the fiber/sensitive layer interface whereas
the second one is given by the sensitive layer/external medium interface.SEM images of (a) a properly cut bare fiber and (b) of a fiber tip
coated by a thin sensitive layer; (c) schematic view of the Fabry-Perot-based
configuration.
(a)(b)(c)First described in 1899 by Fabry and
Perot [54], the interferometer known by their names makes use of multiple
reflections between two closely spaced surfaces. In fact, the light is
partially reflected each time it reaches the second surface, resulting in
multiple offset beams which can interfere with each other (see Figure 2).Figure 2
Schematic representation of the
multiple reflected beams in a Fabry-Perot cavity.The amount of light reflected at the
first interface can be calculated as the sum of the multiple reflected beams
and is strongly influenced even by very small changes of the distance between
the two surfaces (the sensitive layer thickness) or its optical properties (the
sensitive layer refractive index) [55]. This explains the massive use of such
configuration in fiber optic-based sensing in the two past decades, especially
for the detection and measurements of various physical, chemical, and
biomedical parameters [56, 57]. All these characteristics, combined with the
possibility of integrating a number of sensitive materials with the optical
fibers by means of very simple, low-cost, and versatile deposition techniques,
make it one of the most attractive and useful optoelectronic configurations especially
suitable for practical applications.The principle of operation of an optoelectronic sensor
in reflectometric configuration relies thus on the fact that a modulation of
the intensity of light reflected at the fiber-sensing overlay interface occurs
due to small changes of the layer thickness(dfilm) and its complex
refractive index (n˜film). As a matter of fact, the fiber-film
reflectance can be expressed as [58]
(1)R=|r12+r23⋅e−i⋅k˜film1+r12⋅r23⋅e−i⋅k˜film|2with(2)r12=nf−n˜filmnf+n˜film,r23=n˜film−nextn˜film+next,k˜film=2π⋅(2⋅n˜film⋅dfilm)λ=4π⋅n⋅dfilmλ−i4π⋅k⋅dfilmλ=βfilm−iα⋅dfilm,where n˜film=n−i⋅k is the square root of the complex dielectric
constant (ε˜film=εr−i⋅εi) of the sensitive fiber coating, α=4π⋅k/λ is the overlay absorption coefficient, nf and next are the optical fiber and external medium refractive index,
and λ is the optical wavelength. Thus, the reflectance changes due to the chemical
interaction between the sensing overlay and the target analyte can be expressed
as follows:
(3)ΔR=(δRδn)⋅Δn+(δRδk)⋅Δk+(δRδdfilm)⋅Δdfilm=Sn⋅Δn+Sk⋅Δk+Sd⋅Δdfilm,
where Sn, Sk, and Sd are the sensitivities against the variations of the real and imaginary parts of
the complex refractive index and against the overlay thickness, respectively.
It is
noteworthy that these three sensitivity factors Sn,
Sk, and Sd strongly depend upon the geometrical and
electro-optical properties of the sensitive nanocoatings and upon the
environmental condition (e.g., vapor or liquid phase), and for this reason they
have to be properly considered case by case.


## 3.2. Reflectometric Sensor Interrogation
Scheme

So far, a variety of schemes have
been proposed for the interrogation of a fiber optic sensor based on the FP cavity,
the most used ones relying on spectrum-modulating approach, and single
wavelength reflectometry [59]. Here the attention has been focused on this last
technique, which is simple to implement and requires just few widespread
commercial and low-cost optoelectronic components while preserving excellent
performances. In addition, it enables the fabrication of cost-effective,
reliable, robust, and portable equipments, which are factors of crucial
importance for in situ and long-term monitoring applications and for the
desired technology transfer to the market. The typical interrogation scheme
enabling the single wavelength reflectance monitoring of an FP cavity realized
on the distal end of an optical fiber is shown in Figure 3(a).(a) Schematic illustration of the typical interrogation scheme adopted
for the single wavelength reflectance monitoring of an optical cavity realized
upon the fiber tip. (b) Image of the compact and portable equipment exploited
for the interrogation of the FP-type optical fiber sensors.
(a)(b)The sensitive interface is lighted by a pigtailed superluminescent
light emitting diode (SLED) operating in one of the two main telecommunication
wavelengths (1310 nm and 1550 nm). A source controller is used to keep the SLED
at a constant temperature and feed it with an input current, which allows the
source to emit an optical power of few mW. This power is split by a2×2
in-fiber directional coupler, providing the necessary connections between light
source, sensing interface, and two InGaAs photodiodes. The first photodiode is
for the measurement of the reflected signal:
(4)Vsignal=k1⋅Z1⋅Psource⋅R,
where Psourse is the power emitted by the source, k1
is a constant which takes account of the power losses due to the
coupling factor of the 2×2 directional coupler and the photodiode responsivity,
Z1 represents a gain factor introduced by the first photoreceiver
device. The
second arm of the 2×2 coupler is used to provide a  valid intensity monitoring
channel for the compensation of the sensor response. As a matter of fact, the
second photodiode provides a voltage signal directly related to the optical
power levels within the optical chain according to
(5)Vsource=k2⋅Z2⋅Psource,
where k2(k2≠k1) takes into
account the losses coming from the connection between source, 2×2 coupler, and
photodiode and Z2(Z2≠Z1) is a gain factor
introduced by the second photoreceiver device. The intensity compensation is
obtained by considering the ratio between the voltage signals at the two
photoreceivers according to
(6)I=VsignalVsource=k1⋅Psource⋅Z1⋅Rk2⋅Z2⋅Psource=α⋅R,
where α=k1⋅Z1/k2⋅Z2
takes into account all the setup parameters. As
evident,  the normalized response of the sensor is only sensitive to the change in
the reflectance induced by chemical sorption within the sensing overlay and not
to the fluctuations of the optical power levels along the whole measurement
chain. In the followings, the relative change of the sensor output ΔI/I0 has been considered
(where I0 is the output
signal in the reference or initial condition), which, in turn, corresponds to
the relative reflectance change occurring at the fiber-sensitive layer
interface (ΔR/R0).Synchronous detection is typically
implemented to enhance the system performances, by amplitude modulating the
light source at 500 Hz and retrieving the photodetector voltages by using a
dual channel lock-in amplifier. In addition, a Time Division Multiplexing (TDM)
approach is typically exploited to perform the quasisimultaneous interrogation
of up to eight optical probes by means of a multichannel fiber optic switch.
Finally, the optical data can be stored in a notebook controlling the sensing
process in LabView software by a NI-DAQ card. The overall setup was properly
engineered in order to provide robust, compact and portable equipment (see Figure3(b)).


## 3.3. Optochemical Nanosensor Fabrication

The realization of homogeneous thin
films of SWCNTs with a controllable thickness is an important basis for the
future development of their scientific understanding and technological
applications. Proper manipulation techniques are required for applying thin films
of carbon nanotubes on substrates that do not allow direct grow methods.
Although various proposals exist for their incorporation into devices, in
single tube or thin film architectures [60], here the LB technique has been
chosen as a way to transfer nanometer-scale layers of SWCNTs upon either bare
optical fibers or cadmium arachidate (CdA) buffer-linker material, previously
deposited (by the same technique) upon the fiber end in order to improve the
carbon nanotubes adhesion on the sensors surface. The CdA has been chosen as
buffer material due to its peculiar amphiphilic molecular structure suitable
for LB deposition process [61, 62].The LB-technique is one of the most
promising techniques for preparing such thin films as it enables the precise
control of the monolayer thickness, homogeneous deposition of the monolayer
over large areas, and the possibility of making multilayer structures with
varying layer composition [63]. An additional advantage of the LB technique is
that monolayers can be transferred on almost any kind of solid substrate.
However, these advantages have to be traded with the low speed of the
deposition procedure as well as the limited number of materials suitable for
this technique. As schematically represented in Figure 4, the molecules of the
films to be deposited are firstly dispersed onto the surface of a subphase,
typically oriented with the hydrophobic part upwards and with the hydrophilic
one immersed in water. Subsequently, a reduction of the surface area occupied
by each molecule is performed by means of moving barriers in order to produce a
solid phase of a given surface pressure in which the molecules are densely
packed forming a highly ordered array [64]. From this phase, the molecules can
be transferred to a properly cleaned and prepared solid substrate by its
dipping through the condensed Langmuir layer.Figure 4
Schematic representation of the
Langmuir-Blodgett deposition procedure.As the solid phase is reached only at
high surface pressures, a continuous reduction of the moving barriers is
performed when the molecules are transferred from the subphase to the substrate
in order to keep the surface pressure constant, ensuring that the solid phase
is maintained. Repeated dipping of the same substrate is also possible,
resulting in the deposition of a thin film with one monolayer at a time. The precise control
and uniformity of film thickness in the submicron range allowed by the LB
technique is essential especially for the optical interference principle used
in the reflectometric configuration to convert refractive index changes into
optical intensity changes.For SWCNT film deposition, a solution
(0.2 mg/mL) of SWCNT pristine material in chloroform has been spread onto a
subphase constituted by deionized water (18 MΩ) with 10−4 M of CdCl2. The subphase
pH and the temperature were 6.0 and 23°C, respectively. The monolayer has been
compressed with a barrier rate of 15 mm/min up to a surface pressure of 45 mN/m. The single layer has been deposited with a dipping rate of 3 mm/min and
the transfer ratio of the monolayers from the subphase to the surface of the
optical fiber sensor substrate, either bare or already coated by 20 monolayers
of CdA [50], was in the range of 0.5 to 0.7. After a proper drying of 12 hours
overnight, the sensing multilayers deposited were ready for the vapor testing.
The SWCNT pristine material has been purchased from Carbon Nanotechnologies
Inc. (Houston, Tex,
USA), and contains primarily
SWCNTs with a small amount (approx. 1%) of amorphous carbon and 4–6% of catalyst Fe
particles. The nanotube diameters are roughly Gaussian distributed and centered
at about 2 nm. The raw material of commercial nanotubes has been used
as-bought, without any purification treatment. The samples were prepared by
sonicating SWCNT suspension in chloroform for 1 hour at room temperature prior
to the deposition. For the fabrication of the carbon nanotubes-based probes,
before the LB deposition procedure the standard silica optical fibers have been
previously accurately polished from the acrylic protection and cleaved with a
precision cleaver in order to obtain a smooth and plane surface. Then, they
have been washed in chloroform and dried with gaseous nitrogen to be ready for
the CdA or SWCNT deposition. The number of CdA and/or SWCNT monolayers
deposited onto the sensor surface have been controlled by simply choosing how
many times the substrate is dipped inside or lifted from the solution
containing the monolayer to transfer.


## 3.4. Sensor Characterization: Structural and
Morphological Features of SWCNT Overlays

The rational design of a chemical
sensor and of its performances is something which is possible only if the
sensitive material properties and the way they are affected by different
deposition parameters or ambient conditions are well known and understood. To
this aim an extensive characterization of the as-bought SWCNT powders as well
as of the deposited LB SWCNT films has been carried out in order to investigate
their structural and morphological features. Such characterization involved
X-ray diffraction (XRD) and Raman spectroscopy analyses, high-resolution
transmission electron microscopy (HRTEM), and scanning electron microscopy (SEM)
observations. In particular, Figure5 reports the typical XRD spectrum of
as-bought SWCNT powder (1 mg) material. The pattern exhibits a well-defined
graphite-like (002) diffraction peak at 2θ = 26.5°, and a broad band centered at lower 2θ = 22° from amorphous carbon or nonnanotube
carbon material.Figure 5
X-ray diffraction spectrum obtained from HiPco SWCNT powder (1 mg).Further XRD analyses performed on CdA
multilayers deposited on glass substrates evidenced a CdA monolayer spacing of
about 2.8 nm for cadmium arachidate. The same measurements, performed with an
SWCNT multilayer, revealed a carbon nanotubes monolayer spacing of about 2.0 nm
[50].The HRTEM images of an SWCNT powder,
reported in Figure6 at (a) low and (b) high magnifications, confirm the
nanometric dimension of the carbon tubes and reveal the presence between them
of some Fe metal particles, a typical catalyst used in the HiPco production
process of carbon nanotubes. Generally, the SWCNT powders contain not only
carbon tube nanostructures but also carbonaceous particles such as amorphous
carbon, fullerenes, and transition metals introduced as a catalyst during the
synthesis. [65, 66].(a), (b) High resolution TEM images of SWCNT powder at (a) low and (b)
high magnifications, (c) SEM images of CdA-buffered SWCNT LB films deposited
upon an SOF sensor substrate.
(a)(b)(c)These impurities sometimes hinder the
fine characteristics of the carbon nanotubes and limit the best performance of
the SWCNTs-based functional devices. For this reason, effective purifications
for the removal of the by-products (amorphous carbon, fullerenes, catalyst
metal particles) of SWCNT synthesis are needed in order to fully exploit their
outstanding properties. Finally, Figure6(c) reports the typical SEM images of
CdA-buffered LB SWCNT films deposited upon an SOF tip. It demonstrates the
success of the integration of carbon nanotubes with the optical fiber and
reveals their attitude to adhere one to each other forming bundles or ropes
with a spaghetti-like arrangement. In the investigated cases, such CNT mats are
characterized by mean diameter and length which have been estimated to be in
the range 5–40 nm and 1–15 μm, respectively.Also Raman spectroscopy analyses have
been conducted to characterize the fabricated LB films based on SWCNTs already
deposited upon the optical fiber tip. To this aim, a Raman microscope
functioning in backscattering configuration employing an HeNe laser (633 nm)
and 50x and 100x objective lenses was used. The results are shown in Figure7,
where the typical Raman spectrum of SWCNTs has been reported.Figure 7
Typical Raman spectrum of an LB SWCNT film directly deposited upon the
optical fiber tip.They confirm the single-walled and
semiconducting nature of the CNTs [67] as well as their integration with the
optical fiber transducers. The characteristic multipeak feature “G-band” at
about 1580 cm−1, corresponding to carbon atom vibration tangentially
with respect to the nanotube walls, together with the less remarkable
disorder-induced “D-band” peak typically in the range 1300–1400 cm−1,
representing the degree of defects or dangling bonds
[67], can be
easily revealed. The large ratio of G to D peaks here observed gives us an
indication of an ordered structure of the deposited SWCNT overlay, whereas the
observation of the two most intense G peaks (labeled G+ and G−) confirms the
absence of MWCNTs.


## 4. Environmental Monitoring Applications: Experimental Results

This
section provides a
review of the results obtained during the last few years of research focused on
the development of SWCNTs-coated optochemical nanosensors for numerous environmental
monitoring applications. In particular, in the following sections VOC detection
in air, chemical trace monitoring in water, andH2 detection at
cryogenic temperature suitable for aerospace applications will be presented. The
results here reported reveal the potentiality of the SWCNTs-coated chemosensors
to be employed as very promising optochemical devices for applications related
to the environmental monitoring either in air or in liquid phase, at room
temperature and likewise at a temperature as low as 113 K.


### 4.1. Room Temperature Detection of VOCs in Air

The investigation and
characterization of the VOC detection performance of SWCNTs-coated optochemical
nanosensors have been carried out by means of an experimental setup ad hoc
designed and realized, a schematic representation of which is reported in Figure8. In particular, the optical fiber probes have been located in a properly designed
test chamber realized in stainless steel. The volume of the test chamber was
1200 mL, the total flow rate per exposure has been kept constant at 1000 mL/min
and the vapors have been generated by the bubbling method. The gas flow rate
has been controlled by a mass flow-meter driven by a controller-unit
communicating with a PC via standard RS-485 serial bus. The controller unit was
able to drive up to eight different gas-channels, and the gas flow rate in each
gas channel was regulated by a dedicated mass flow meter with a full scale of
the mass flow ranging from 10 to 1000 mL/min. Numerous tests have been
performed by using nitrogen or dry air as reference and carrier gas to
transport the generated vapors inside the test ambient: dry air has been chosen
because of the higher stability demonstrated by the optical fiber sensor
signals. All the experimental measurements have been conducted at room
temperature.(a) Schematic representation and
(b) photogram of the experimental setup used for the vapor testing.
(a)(b)The capability of
SWCNT overlays of undergoing changes in their complex refractive index and
thickness as consequence of the adsorption of target analyte molecules has been
demonstrated for the first time in 2004 [49]. In that case, LB films consisting
of SWCNT bundles were transferred upon the optical fiber tip by using a buffer
LB multilayer of CdA predeposited on the sensor surface in order to promote the
adhesion of the CNTs. The fabricated probes were exploited for the detection of
several VOCs, such as isopropanol,  methanol, ethanol, toluene, and xylene.
However, in 2005, multilayers of SWCNTs with different thicknesses were
successfully deposited directly upon the optical fiber surface by a
modification of the LB process [68], resulting in an improvement of the sensing
performance of the unbuffered configuration with respect to the buffered case,
especially in terms of sensor sensitivity. As an example, Figure 9 shows the
highest relative reflectance changes (ΔR/R0) exhibited by a fiber
optic chemosensor coated with a CNTs film composed of four SWCNT monolayers
(namely, SOF-4) when exposed to 30-minute decreasing concentration pulses of xylene
vapors with respect to the counterpart optoelectronic sensor arranged in the
CdA buffered configuration (2 monolayers of SWCNTs deposited onto 20 monolayers
of CdA). In both cases, significant reflectance changes occurred upon target
analyte exposure as a consequence of the variation in the SWCNT overlay
refractive index promoted by the toluene molecules adsorption inside the
sensitive material. Both sensors exhibited the capability of detecting traces
of the chemical under investigation at few tens of ppm levels combined with a
fast response, a complete reversibility of the sensor responses (which grants
the reuse of the sensor after a given measurement), and a marked dependence of
the response times upon the analyte concentration.Figure 9
ΔR/R0 occurred on xylene
vapor exposure to CdA-buffered and unbuffered SWCNTs-based optochemical
nanosensors, at room temperature.However, the sensor SOF-4
provided response changes more than one order of magnitude higher than the ones
provided by the counterpart probe in the CdA-buffered configuration. In light
of these results, strong interest was devoted to the investigation of the
sensing capabilities of SWCNTs-based optochemical sensors arranged in the
unbuffered configuration against several VOCs at room temperature.Figure10(a) reports the results of
the toluene vapor testing carried out by exposing at room temperature the
transducers SOF-4 to four pulses of toluene with concentration in the range 54–93 ppm. The results
obtained confirmed the behavior of the fiber optic nanosensor exhibited during the
xylene testing: as a matter of fact, also in this case an optical reflectance
increase on exposure was observed as a consequence of the analyte molecule
adsorption. A less pronounced dependence of the response time on the toluene
concentration was noticed, revealing that different adsorption dynamics occur
depending on the VOC under investigation, as it will be examined more in detail
in Section 4.1.2. Also, a slight drift in the signal baseline can be
recognized, due to little thermal changes in the not perfectly thermostated
test chamber. After 5 hours of vapors testing, the total ΔR/R0 due
to a temperature drift of 2.4°C was approximately 2.8⋅10−3.
Furthermore, in order to investigate the reliability of the proposed
transducers, a repeatability test has been carried out for the same
optoelectronic nanosensor.(a) Time responses of the sensor
SOF-4, exposed to four decreasing concentration pulses of toluene vapors, at
room temperature; (b) repeatability test, carried out by exposing the same sensor to 30-minute pulses of xylene vapors with a concentration as low as 21 ppm.
(a)(b)The results are shown
in Figure10(b), where the reflectance changes that occurred as a consequence
of two exposures at 21 ppm of xylene, at room temperature, have been reported.
The exposure time has again been kept constant at 30 minutes. As evident, the optochemical
probe is demonstrated to be highly repeatable and reliable also at very low
analyte concentrations. The results of the repeatability test have to be
considered at constant temperature, because no thermal variation occurred
during it. However, it is evident that the monitoring of thermal drifts and the
compensation of their effects on the sensor response are strictly required in
order to not reduce the system performance. The temperature monitoring could be
implemented by means of appropriate Fiber Bragg Grating temperature sensors,
which could be separately inserted in the test ambient or even integrated with
the optical fiber probe [69]. Similarly, the SWCNTs-based optoelectronic chemosensors
demonstrated a relevant sensitivity also to humidity changes within the test
environment [68]. As a matter of fact, controlled humidity test performed using
an SOF sensor coated by two monolayers of SWCNTs onto 20 monolayers of CdA
revealed a sensitivity as high as −5.4⋅10−4 for 1% humidity change, thus requiring a proper calibration and compensation of
the sensors response, especially when high accuracy is necessary.Figure11 reports the
calibration curves of the nanosensor SOF-4 obtained against toluene and xylene
vapors at room temperature, which revealed an almost linear behavior in the
investigated concentration ranges. In addition, the sensor sensitivities against
both analytes, calculated as the relative reflectance change upon concentration
unit (Sanalyte=ΔR/R0/C) [70], evidenced a higher response to
xylene (Sxylene=1.1⋅10−3 ppm−1) than to toluene (Stoluene=4.7⋅10−4 ppm−1). As a matter of fact, the SOF sensitivity to xylene is more
than two times higher than the one to toluene, as evidenced by the ratio
between them (Sxylenesof/Stoluenesof=2.4).Figure 11
Calibration curves of the sensor
SOF-4, exposed at room temperature to toluene and xylene vapors.By considering the minimum detectable
values achievable with the exploited optical interrogation device (ΔR/R0min=1.34⋅10−4),
resolutions of
approximately 290 ppb and 120 ppb have been estimated for toluene and xylene,
respectively. These excellent resolutions are more than three orders of
magnitude higher than those obtained by means of a fluorosiloxane polymer-based
surface plasmon resonance (SPR) optical fiber sensor (900 ppm and 190 ppm, resp.,
for toluene and xylene) [71], and more than two orders of magnitude higher than
that provided by a multimodal optical fiber sensor sensitized by phenyl-modified
porous silica (100 ppm and 20 ppm, resp.) [72].


#### 4.1.1. Effects of the Number of SWCNT Monolayers and CdA Buffer on the
Sensor Sensitivities

In the present section, the influence
of the number of SWCNTs monolayers deposited upon the optical fiber end-face on
the sensor performance will be discussed. The differences between the
performance of the buffered and unbuffered configurations are also better
discussed. To achieve this aim, four optochemical sensors coated by a different
number of SWCNTs, directly deposited onto the bare substrates and also buffered
by 20 LB monolayers of CdA, have been simultaneously exposed to toluene and xylene
vapors.Figure12(a) reports the relative
reflectance changes versus the toluene concentration for all the tested nanosensors,
at room temperature. It can be seen that an increase of the sensitivity has
been obtained by passing from two (0.9⋅10−4 ppm−1) to four monolayers of SWCNTs (4.7⋅10−4 ppm−1) directly deposited onto bare SOFs (see Figure 12(b)). On the
contrary, the optical chemosensors coated by a higher number of carbon
nanotubes monolayers (twelve), and thus by a thicker SWCNT film, exhibited a
negative sensitivity to toluene (−0.7⋅10−4 ppm−1). This means that for this sensor the fiber-film interface
reflectance decreases on exposure. This is due to the fact that the film
reflectance, and thus also the sensor sensitivity, is strongly dependent upon
the thickness and refractive index of the CNT ovelay, in accordance with (1).
As a matter of fact, depending upon the geometric features of the film
deposited onto the optical fiber end, a sensor can exhibit either a positive or
a negative sensitivity to a target analyte.Comparison between the (a) calibration curves and (b) sensitivities
obtained by exposing four fiber optic chemosensors coated by a different number
of SWCNT monolayers to toluene vapors, at room temperature.
(a)(b)For
the same reason, the CdA buffer multilayer, whose optical properties are quite
similar to those of the standard optical fibers, and thus not optimized for the
particular configuration exploited, dramatically decreases the sensitivity of
optical fiber sensors (−0.4⋅10−4 ppm−1). It is
noteworthy, however, that by choosing a buffer-linker material whose optical
and geometrical features (such as the multilayer thickness and refractive
index) are well optimized for the specific configuration, one could be able to strongly
enhance the SOF sensor performance. From Figure 13, it can be seen that the
four fiber optic chemosensors exhibited the same behaviors also in case of
xylene vapor testing. Hence, it demonstrates that the inversion of the
optoelectronic sensor sensitivities is not due to the particular analyte
tested, but to the optical configuration exploited. This feature could be very
useful for pattern recognition analysis in case of multitransducer approaches,
where complementary sensors are exploited to improve the analyte
discrimination.Comparison between the (a)
calibration curves and (b) sensitivities obtained by exposing four fiber optic chemosensors
coated by a different number of SWCNT monolayers to xylene vapors, at room
temperature.
(a)(b)


#### 4.1.2. Response and Recovery Time Analysis

The experimental results so far
reported evidenced a dependence of the sensor response times upon the analyte
concentration. For this reason, a detailed analysis of the adsorption dynamic
features of the proposed chemosensors towards both the tested hydrocarbons is
required. To this aim, in this section an investigation of the response and
recovery times of the sensor SOF-4, exposed to decreasing concentration pulses
of toluene and xylene vapors, at room temperature, is reported. The exposure
time was 30 minutes for each pulse. The response (recovery) time has been
calculated as the time needed for the output signal to pass from 10% to 90%
(from 90% to 10%) of the total signal variation occurring on vapor exposure
(recovery). The results obtained in the case of toluene are shown in Figure14(a),
and reveal a response time that increases with the analyte concentration. The
minimum response time, obtained for 54 toluene ppm, was approximately 7 minutes
while the maximum one, obtained for 93 ppm, was approximately 11 minutes. In
addition, a quite constant recovery time as low as approximately 5 minutes has
been estimated for the same transducer.Response and recovery times
obtained by exposing the sensor SOF-4 to (a) toluene and (b) xylene vapors, at
room temperature.
(a)(b)Similar results have been observed in
case of xylene exposures (see Figure14(b)), with the SOF response times
increasing from approximately 4 minutes (3 ppm) to approximately 18 minutes (39 ppm). In addition, the recovery times of both sensors are quite constant as a
function of the xylene concentration, with their values estimated to be of approximately
6 minutes. By comparing the response and recovery times towards the two tested
VOCs, it turned out that the dynamic responses of the sensor SOF-4 are slower
in the case of xylene vapors than in the case of toluene ones. This effect
could be attributed to a different adsorption kinetics of the two target
analyte molecules inside the CNT sensitive nanocoatings. It is noteworthy,
however, that the response times of the proposed fiber optic chemosensors are
relatively good taking into account the volume of the test chamber and the
total flow rate per exposure.It is also important noting that these
times could be improved by depositing a lower amount of carbon nanotubes upon
the sensor substrates, for example by decreasing the number of SWCNT
monolayers. However, this could reduce the sensor sensitivities. As a matter of
fact, Figure15 reports a bar plot showing the mean response and recovery times
of the sensors SOF-2 and SOF-4, obtained for a 17 ppm xylene vapors exposure,
at room temperature. It can be clearly observed that, even if a decrease of the
sensor mean response time (from 9 to 6 minutes) as well as of the mean recovery
time (from 5 to 2 minutes) can be achieved by reducing the SWCNT layer
thickness, this results in a strong diminution of the sensor sensitivities. In
fact, as observed in Section 4.1.1, a reduction from 1.1⋅10−3 ppm−1 to 3⋅10−4 ppm−1 has been observed by passing from the sensor SOF-4 to SOF-2.
This means that the choice of the geometric features of the sensitive overlay
has to be made by considering the tradeoff that exists between sensor
sensitivity and response (and recovery) time.Figure 15
Comparison between mean response
and recovery times of the fiber optic chemosensors SOF-2 and SOF-4, obtained
for a 17 xylene ppm exposure, at room temperature.


### 4.2. Chemical Trace Detection in Water

In this section, the attention has
been focused on the feasibility of exploiting the excellent sensing properties
of CNTs for the development of high-performance optoelectronic sensors capable
of chemical trace detection in aqueous environment, at room temperature.A schematic view of the experimental
setup exploited for the chemical trace detection in water is reported in Figure16. The SWCNTs-based optochemical transducers in reflectometric configuration
have been inserted in a Pyrex beaker containing pure water. The presence within
the test ambient of the analyte under investigation has been promoted by its
injection inside the beaker. The injected volume has been chosen, each time, in
order to obtain the desired analyte concentration. The polluted water has been
continuously stirred to ensure the maximum dispersion of the analyte. In
addition, after each analyte exposure, the capabilities of the SOF sensors to
recover the initial steady state level have been investigated by restoring the
initial condition of pure water. To this aim, pure water has been continuously
injected in the test chamber, while the contaminated water, previously present
in it, contemporarily stilled out.Figure 16
Schematic view of the
experimental setup exploited for the chemical trace detection in water.Before performing the toluene
detection measurements, the stability within an aqueous environment of the
SWCNT sensitive layers, deposited upon the fiber optic end-face, has been
investigated. To the aim, a chemooptic sensor coated by twelve monolayers of
SWCNTs, whose sensing performance against toluene in air was already tested [6],
has been successively inserted in a beaker containing pure water and also left
for several days inside it. Figure 17 reports the ΔR/R0 of the
optical probe occurred in correspondence of its insertion in water. It can be
seen that, as it was inserted in water, a strong and fast reflectance decrease
occurred as a consequence of the change in the external medium refractive
index. This effect is followed by a slow variation of the reflectance itself
(see the inset in Figure 17) until the equilibrium value is reached, which can
be attributed to the interaction between the sensitive layer and the water
molecules. In addition, once reached the equilibrium, the reflectance signal is
stable, demonstrating that no degradations of the SWCNT sensitive layer
occurred.Figure 17
ΔR/R0 of the SWCNTs-based sensor occurred as a consequence of its insertion in water.Once the carbon nanotubes stability has
been verified, the same SOF sensor has also been tested for the room
temperature detection of toluene in water. The analyte concentration range was 20–80 ppm
(calculated asμL/L). The results obtained are shown in Figure 18,
where the ΔR/R0 that have occurred upon the injections of increasing
concentrations of toluene have been reported. It can be observed that in
correspondence of each toluene injection the interaction between analyte
molecules and sensitive overlay results in a significant decrease of the
reflectance. In addition, the CNTs-based SOF sensor provides complete
recoveries of the steady-state value of uncontaminated water, which
demonstrates its excellent desorption features. The slight drift observed in
the sensor baseline can be attributed to thermal changes since the test chamber
is not perfectly thermostated. However, this aspect, as already evidenced in
the previous sections, can be easily addressed by using simple procedures to
compensate the effects of temperature variations on the sensor response.Figure 18
Time responses of the SOF sensor
coated by 12 monolayers of SWCNTs, exposed to increasing concentrations of
toluene, at room temperature.An estimation of the influence of noise on the sensor response
has also been provided by calculating the noise effect on the sensor outputI in the steady state levels
corresponding to the absence of analyte within the chamber. By considering the
steady-state level in the worst case (maximum scattering in the sensor level)
over a time interval of 15 minutes, a standard deviation σN from the mean value of the sensor output I0 (I0∼0.9424)
of approximately 6⋅10−4 can
be appreciated. By these results, a minimum detectable change of the
sensor output ΔI/I0min (and thus a ΔR/R0min) of 6⋅10−4 has been estimated.In Figure19(a), the sensing
performance of the SOF sensor coated by twelve monolayers of SWCNTs against
toluene in air and aqueous
environments has been compared.(a) Comparison between the
calibration curves of the SOF sensor coated by 12 monolayers of SWCNTs against
toluene obtained in air and aqueous environments; (b) response and recovery
times of the SOF sensor, exposed to toluene with concentration in the range 20–80 ppm.
(a)(b)Here, the reflectance changes that occurred
in the two cases have been plotted versus the toluene concentration. As can be
observed, higher linearity has been obtained in case of detection in water
environment. Furthermore, a comparison of the sensor sensitivity in the two different
environments in the case of toluene detection can be carried out in the
concentration range 40–80 ppm since in
this range a linear behavior of the sensor calibration curve has been observed
also for the case of detection in air. The results revealed a sensitivity in
air higher (approx.1.5⋅10−4 ppm−1) than that obtained in the case of water environment (approx.
1.2⋅10−4 ppm−1). The differences in the two characteristics can be attributed
to the dependence of the reflectance upon the surrounding refractive index
combined to different adsorption characteristics occurring in liquid
environment, especially if the kinetic is diffusion limited. Taking into
account the sensitivity of the sensor against toluene and considering the ΔR/R0min (6⋅10−4)
obtainable with the exploited instrumentation, a resolution of 5 ppm has been
estimated.Finally, an analysis of the response
(t10–90) and recovery (t10–90) times of the optical
probes has been carried out. The results obtained for the detection in water
are shown in Figure 19(b) and reveal characteristic times slightly dependent upon
the analyte concentration, with the mean response time (approx. 20 minutes)
higher than the mean recovery one (approx. 4 minutes). In Figure 20, a
comparison between the mean characteristic times obtained with the SOF sensors
coated by 12 monolayers of SWCNTs in the two different environments is
reported. It can be seen that, also in this case, a difference has been
observed with the response and recovery times obtained against toluene in air,
which is approximately 8 minutes and approximately 28 minutes, respectively.Figure 20
Comparison between the response and
recovery times of the proposed SOF sensors in case of toluene detection in
water and air.


### 4.3. Gaseous Hydrogen Detection at Cryogenic
Temperatures

The amazing characteristic of SWCNTs
of being highly adsorbent materials has been successfully exploited also for
the detection of gaseous hydrogen (H2) at cryogenic temperatures. As
a matter of fact, the experimental results provided in this section will reveal
the excellent potentialities of the proposed SWCNTs-based optochemical sensors
in this interesting environmental and industrial application.CryogenicH2 is often
utilized as the primary fuel source by rocket engine because of the benefits
deriving from hydrogen combustion properties. Unfortunately, gaseous hydrogen
has a very low flammability limit and low ignition energy, so that it becomes a
very dangerous and volatile substance to work with during the rocket engine
development stage. Hydrogen leakage into an oxidizing environment can easily
ignite a catastrophic explosion. For this reason, the development of accurate
and reliable devices for the detection of hydrogen leakage at cryogenic temperatures
is of crucial importance for human safety. In 1981, Lundström introduced the
first H2 sensor based on thin films of Palladium (Pd) deposited on
the top of the channel of an MOSFET device [73, 74]. Since then, this issue has
been widely studied by the researchers and a number of hydrogen sensors have
been proposed [75, 76]. However, among the big amount of sensing devices, fiber
optic detectors have strongly attracted the attention of the scientific
community in this field, especially because of their high sensitivity and lack
of sparking possibilities, guaranteed by the removal of all electrical power
from the test site. To date, several configurations of fiber-optic hydrogen
sensors have been proposed, based on different transducing principles and materials.
[77, 78]. The most exploited material in this field is Pd since it exhibits
strong thickness and refractive index changes on H2 molecule
adsorption at room temperature. However, at cryogenic temperatures Pd looses
its excellent sensing properties [79], thus the main effort in this field is
devoted to the research of new sensitive materials that are able to provide
high sensitivity and good desorption features as well as fast responses at very
low temperatures. Recent reports have demonstrated a high reversible adsorption
of molecular hydrogen in carbon nanotubes and graphitic nanofibers [80, 81]. In
light of this argument, SWCNTs (of both closed-end and opened-end types) have
been exploited as sensitive nanostructured coatings of reflectometric SOF sensors
for the detection of gaseous H2 at a temperature as low as 113 K [51].
In the following, the experimental results obtained by using three
reflectometric sensors, coated respectively by two and six monolayers of
closed-end SWCNTs (namely, 2_CeSWCNTs and 6_CeSWCNTs, resp.) and two monolayers
of opened-end SWCNTs (2_OeSWCNTs), are reported.In order to test theirH2 sensing performance, the sensors were located in a cylindrical chamber,
properly designed and realized in order to minimize the possible thermal
variations. The chamber was, in turn, inserted within a tank containing
cryogenic nitrogen (see Figure 21) with the aim of reaching the very low
temperature of 113 K.(a) Schematic view and (b),
(c) images of the experimental setup exploited for theH2 detection
at cryogenic temperatures.
(a)(b)(c)The temperature has been monitored by
a copper/copper-nickel thermocouple located close to the optical sensor head.
Nitrogen gas has been continuously injected in the cylindrical chamber in order
to reduce the humidity content inside the test ambient before the strong
temperature decreasing due to the insertion of the chamber in the cryogenic
tank. To expose the SOF sensors to gaseous hydrogen with concentrations as low
as 4% and 1%, nitrogen has also been mixed with gaseous argon containing
hydrogen.The results obtained are shown in Figure22, where the ΔR/R0 of the sensors (a) 2_CeSWCNTs, (b) 6_CeSWCNTs,
and (c) 2_OeSWCNTs, exposed to decreasing concentration pulses of hydrogen at a
temperature of 113 K, have been reported. As evident, all the optical probes
provided significant reflectance changes as a consequence of the gaseous H2 adsorption within the sensitive nanomaterials. In particular, good sensitivity
and complete recovery of the nitrogen signal after both hydrogen concentration
pulses have been observed for the two CeSWCNTs-based sensors, while good
sensitivity but no recovery of the steady-state value has been observed for the
OeSWCNTs-based one.Transient
responses of the three fiber optic probes (a) 2_CeWCNTs, (b) 6_CeWCNTs, and (c)
2_OeWCNT to decreasing concentration pulses of gaseous hydrogen (<5%), at
113 K.
(a)(b)(c)In addition, the bar plot shown in Figure23, in which the amplitude of the relative reflectance changes (|ΔR/R0|) obtained
for the tested SWCNTs-based probes has been reported, clearly reveals that for
the probe 2_CeSWCNTs, the ΔR/R0 obtained for 4% and 1% of H2 are almost similar (the ratio is approx. 1.1). In this case, a saturation of
the fiber overlay sorption capability could explain the weak mismatch between
the sensor responses.Figure 23
ΔR/R0 occurred to the three
tested SOF sensors, on H2 exposure.Differently, for the case of the
sensor 6_CeSWCNTs, a decrease of theH2 concentration of four times
leads to a relative reflectance diminution of approximately three times. In
this case, however, due to the increase of nanotubes monolayers, higher
saturation threshold is expected.Furthermore, a study of the response
and recovery times (t10–90 and t90–10, resp.) has been
carried out. The results obtained have been reported in Figure 24 and reveal
that the probes 2_CeSWCNTs and 6_CeSWCNTs are characterized by the same
response times, (approx. 4 minutes), but have slightly different recovery times
(approx. 9 and 11 minutes, resp.). This evidences that the number of carbon
nanotube monolayers influences the sensor recovery features. In addition, the
probe 2_OeSWCNTs showed a response time (approx. 5 minutes) higher than that
obtained with the CeSWCNTs-based sensors. This could be attributed to the fact
that, while in the case of the CeSWCNTs, the H2 molecules adsorption
takes place only on the surface of the single carbon tubes and in the
interstitial sites between the tubes, in the case of the counterpart OeSWCNTs,
the hydrogen molecule adsorption takes place also inside the carbon tubes,
requiring, for this reason, more time for the establishment of the equilibrium
condition with the external environment.Figure 24
Mean
response and recovery times for the three tested SOF sensors.


### 4.4. Discussion

Here, a brief discussion concerning some
of the typical drawbacks arising from the use of carbon nanotubes as sensitive
elements of reflectometric optical fiber sensors is carried out, which
evidences some currently unsolved matters, but, at the same time, opens the way
to further investigations and research activities.First of all, the attitude of SWCNTs to
randomly distribute in tangled networks of nanotubular chains and densely
aggregate mats causes a not very high repeatability of the distribution of
carbon nanotubes upon the sensor surface as well as their optical properties.
These drawbacks could be overcome by means of an alignment of the carbon
nanotubes onto the sensor substrate, which should provide a better and almost
predictable SWCNT distribution upon them.The adhesion of the carbon nanotubes
upon the fiber substrate also requires further investigations and improvements,
especially when the SWCNTs-based sensors are exploited for in water chemical
detection applications, for which particularly hard operating conditions could
even promote a detachment of the sensitive overlay from the optical fiber tip. A
first attempt to improve the adhesion of SWCNT overlays to the distal end of
the optical fibers is reported in the next section and relies on their
embedding in CdA host-matrix in order to obtain nanocomposites with tailored
amount of nanotube filler contents.Finally, another issue to address is
related to the poor selectivity of the proposed SWCNTs-based sensors towards a
given chemical specie. This means that in case of a multicomponent gas mixture
with interfering analytes, the information provided by the sensors could be
ambiguous and no straightforward information about the test environment could
be achieved. This is a very common aspect in chemical sensing applications as
demonstrated by the strong effort which is currently devoted by the researchers
on how to improve the sensor discrimination abilities among different analytes.
Basically, there are two possible approaches in the attempt to increase the
chemical sensor selectivity, and both have to be followed in synergy. The first
one is a direct approach which relies on the sensitive layer functionalization
in order to have a higher affinity of the material towards specific chemical
species [82]. The second one is based on the use of a hybrid system composed of
multiple transducers coated by the same material (or, in the most general case,
by different materials) in the form of an array. Here, sensors with a poor
selective response, when considered collectively, provide unique patterns
typical for each analyte. The generated response patterns are interpreted by
pattern-recognition algorithm for the selective detection [83]. This approach
has already been investigated and exploited by the authors [50], and is
currently under investigation.


## 4.1. Room Temperature Detection of VOCs in Air

The investigation and
characterization of the VOC detection performance of SWCNTs-coated optochemical
nanosensors have been carried out by means of an experimental setup ad hoc
designed and realized, a schematic representation of which is reported in Figure8. In particular, the optical fiber probes have been located in a properly designed
test chamber realized in stainless steel. The volume of the test chamber was
1200 mL, the total flow rate per exposure has been kept constant at 1000 mL/min
and the vapors have been generated by the bubbling method. The gas flow rate
has been controlled by a mass flow-meter driven by a controller-unit
communicating with a PC via standard RS-485 serial bus. The controller unit was
able to drive up to eight different gas-channels, and the gas flow rate in each
gas channel was regulated by a dedicated mass flow meter with a full scale of
the mass flow ranging from 10 to 1000 mL/min. Numerous tests have been
performed by using nitrogen or dry air as reference and carrier gas to
transport the generated vapors inside the test ambient: dry air has been chosen
because of the higher stability demonstrated by the optical fiber sensor
signals. All the experimental measurements have been conducted at room
temperature.(a) Schematic representation and
(b) photogram of the experimental setup used for the vapor testing.
(a)(b)The capability of
SWCNT overlays of undergoing changes in their complex refractive index and
thickness as consequence of the adsorption of target analyte molecules has been
demonstrated for the first time in 2004 [49]. In that case, LB films consisting
of SWCNT bundles were transferred upon the optical fiber tip by using a buffer
LB multilayer of CdA predeposited on the sensor surface in order to promote the
adhesion of the CNTs. The fabricated probes were exploited for the detection of
several VOCs, such as isopropanol,  methanol, ethanol, toluene, and xylene.
However, in 2005, multilayers of SWCNTs with different thicknesses were
successfully deposited directly upon the optical fiber surface by a
modification of the LB process [68], resulting in an improvement of the sensing
performance of the unbuffered configuration with respect to the buffered case,
especially in terms of sensor sensitivity. As an example, Figure 9 shows the
highest relative reflectance changes (ΔR/R0) exhibited by a fiber
optic chemosensor coated with a CNTs film composed of four SWCNT monolayers
(namely, SOF-4) when exposed to 30-minute decreasing concentration pulses of xylene
vapors with respect to the counterpart optoelectronic sensor arranged in the
CdA buffered configuration (2 monolayers of SWCNTs deposited onto 20 monolayers
of CdA). In both cases, significant reflectance changes occurred upon target
analyte exposure as a consequence of the variation in the SWCNT overlay
refractive index promoted by the toluene molecules adsorption inside the
sensitive material. Both sensors exhibited the capability of detecting traces
of the chemical under investigation at few tens of ppm levels combined with a
fast response, a complete reversibility of the sensor responses (which grants
the reuse of the sensor after a given measurement), and a marked dependence of
the response times upon the analyte concentration.Figure 9
ΔR/R0 occurred on xylene
vapor exposure to CdA-buffered and unbuffered SWCNTs-based optochemical
nanosensors, at room temperature.However, the sensor SOF-4
provided response changes more than one order of magnitude higher than the ones
provided by the counterpart probe in the CdA-buffered configuration. In light
of these results, strong interest was devoted to the investigation of the
sensing capabilities of SWCNTs-based optochemical sensors arranged in the
unbuffered configuration against several VOCs at room temperature.Figure10(a) reports the results of
the toluene vapor testing carried out by exposing at room temperature the
transducers SOF-4 to four pulses of toluene with concentration in the range 54–93 ppm. The results
obtained confirmed the behavior of the fiber optic nanosensor exhibited during the
xylene testing: as a matter of fact, also in this case an optical reflectance
increase on exposure was observed as a consequence of the analyte molecule
adsorption. A less pronounced dependence of the response time on the toluene
concentration was noticed, revealing that different adsorption dynamics occur
depending on the VOC under investigation, as it will be examined more in detail
in Section 4.1.2. Also, a slight drift in the signal baseline can be
recognized, due to little thermal changes in the not perfectly thermostated
test chamber. After 5 hours of vapors testing, the total ΔR/R0 due
to a temperature drift of 2.4°C was approximately 2.8⋅10−3.
Furthermore, in order to investigate the reliability of the proposed
transducers, a repeatability test has been carried out for the same
optoelectronic nanosensor.(a) Time responses of the sensor
SOF-4, exposed to four decreasing concentration pulses of toluene vapors, at
room temperature; (b) repeatability test, carried out by exposing the same sensor to 30-minute pulses of xylene vapors with a concentration as low as 21 ppm.
(a)(b)The results are shown
in Figure10(b), where the reflectance changes that occurred as a consequence
of two exposures at 21 ppm of xylene, at room temperature, have been reported.
The exposure time has again been kept constant at 30 minutes. As evident, the optochemical
probe is demonstrated to be highly repeatable and reliable also at very low
analyte concentrations. The results of the repeatability test have to be
considered at constant temperature, because no thermal variation occurred
during it. However, it is evident that the monitoring of thermal drifts and the
compensation of their effects on the sensor response are strictly required in
order to not reduce the system performance. The temperature monitoring could be
implemented by means of appropriate Fiber Bragg Grating temperature sensors,
which could be separately inserted in the test ambient or even integrated with
the optical fiber probe [69]. Similarly, the SWCNTs-based optoelectronic chemosensors
demonstrated a relevant sensitivity also to humidity changes within the test
environment [68]. As a matter of fact, controlled humidity test performed using
an SOF sensor coated by two monolayers of SWCNTs onto 20 monolayers of CdA
revealed a sensitivity as high as −5.4⋅10−4 for 1% humidity change, thus requiring a proper calibration and compensation of
the sensors response, especially when high accuracy is necessary.Figure11 reports the
calibration curves of the nanosensor SOF-4 obtained against toluene and xylene
vapors at room temperature, which revealed an almost linear behavior in the
investigated concentration ranges. In addition, the sensor sensitivities against
both analytes, calculated as the relative reflectance change upon concentration
unit (Sanalyte=ΔR/R0/C) [70], evidenced a higher response to
xylene (Sxylene=1.1⋅10−3 ppm−1) than to toluene (Stoluene=4.7⋅10−4 ppm−1). As a matter of fact, the SOF sensitivity to xylene is more
than two times higher than the one to toluene, as evidenced by the ratio
between them (Sxylenesof/Stoluenesof=2.4).Figure 11
Calibration curves of the sensor
SOF-4, exposed at room temperature to toluene and xylene vapors.By considering the minimum detectable
values achievable with the exploited optical interrogation device (ΔR/R0min=1.34⋅10−4),
resolutions of
approximately 290 ppb and 120 ppb have been estimated for toluene and xylene,
respectively. These excellent resolutions are more than three orders of
magnitude higher than those obtained by means of a fluorosiloxane polymer-based
surface plasmon resonance (SPR) optical fiber sensor (900 ppm and 190 ppm, resp.,
for toluene and xylene) [71], and more than two orders of magnitude higher than
that provided by a multimodal optical fiber sensor sensitized by phenyl-modified
porous silica (100 ppm and 20 ppm, resp.) [72].


### 4.1.1. Effects of the Number of SWCNT Monolayers and CdA Buffer on the
Sensor Sensitivities

In the present section, the influence
of the number of SWCNTs monolayers deposited upon the optical fiber end-face on
the sensor performance will be discussed. The differences between the
performance of the buffered and unbuffered configurations are also better
discussed. To achieve this aim, four optochemical sensors coated by a different
number of SWCNTs, directly deposited onto the bare substrates and also buffered
by 20 LB monolayers of CdA, have been simultaneously exposed to toluene and xylene
vapors.Figure12(a) reports the relative
reflectance changes versus the toluene concentration for all the tested nanosensors,
at room temperature. It can be seen that an increase of the sensitivity has
been obtained by passing from two (0.9⋅10−4 ppm−1) to four monolayers of SWCNTs (4.7⋅10−4 ppm−1) directly deposited onto bare SOFs (see Figure 12(b)). On the
contrary, the optical chemosensors coated by a higher number of carbon
nanotubes monolayers (twelve), and thus by a thicker SWCNT film, exhibited a
negative sensitivity to toluene (−0.7⋅10−4 ppm−1). This means that for this sensor the fiber-film interface
reflectance decreases on exposure. This is due to the fact that the film
reflectance, and thus also the sensor sensitivity, is strongly dependent upon
the thickness and refractive index of the CNT ovelay, in accordance with (1).
As a matter of fact, depending upon the geometric features of the film
deposited onto the optical fiber end, a sensor can exhibit either a positive or
a negative sensitivity to a target analyte.Comparison between the (a) calibration curves and (b) sensitivities
obtained by exposing four fiber optic chemosensors coated by a different number
of SWCNT monolayers to toluene vapors, at room temperature.
(a)(b)For
the same reason, the CdA buffer multilayer, whose optical properties are quite
similar to those of the standard optical fibers, and thus not optimized for the
particular configuration exploited, dramatically decreases the sensitivity of
optical fiber sensors (−0.4⋅10−4 ppm−1). It is
noteworthy, however, that by choosing a buffer-linker material whose optical
and geometrical features (such as the multilayer thickness and refractive
index) are well optimized for the specific configuration, one could be able to strongly
enhance the SOF sensor performance. From Figure 13, it can be seen that the
four fiber optic chemosensors exhibited the same behaviors also in case of
xylene vapor testing. Hence, it demonstrates that the inversion of the
optoelectronic sensor sensitivities is not due to the particular analyte
tested, but to the optical configuration exploited. This feature could be very
useful for pattern recognition analysis in case of multitransducer approaches,
where complementary sensors are exploited to improve the analyte
discrimination.Comparison between the (a)
calibration curves and (b) sensitivities obtained by exposing four fiber optic chemosensors
coated by a different number of SWCNT monolayers to xylene vapors, at room
temperature.
(a)(b)


### 4.1.2. Response and Recovery Time Analysis

The experimental results so far
reported evidenced a dependence of the sensor response times upon the analyte
concentration. For this reason, a detailed analysis of the adsorption dynamic
features of the proposed chemosensors towards both the tested hydrocarbons is
required. To this aim, in this section an investigation of the response and
recovery times of the sensor SOF-4, exposed to decreasing concentration pulses
of toluene and xylene vapors, at room temperature, is reported. The exposure
time was 30 minutes for each pulse. The response (recovery) time has been
calculated as the time needed for the output signal to pass from 10% to 90%
(from 90% to 10%) of the total signal variation occurring on vapor exposure
(recovery). The results obtained in the case of toluene are shown in Figure14(a),
and reveal a response time that increases with the analyte concentration. The
minimum response time, obtained for 54 toluene ppm, was approximately 7 minutes
while the maximum one, obtained for 93 ppm, was approximately 11 minutes. In
addition, a quite constant recovery time as low as approximately 5 minutes has
been estimated for the same transducer.Response and recovery times
obtained by exposing the sensor SOF-4 to (a) toluene and (b) xylene vapors, at
room temperature.
(a)(b)Similar results have been observed in
case of xylene exposures (see Figure14(b)), with the SOF response times
increasing from approximately 4 minutes (3 ppm) to approximately 18 minutes (39 ppm). In addition, the recovery times of both sensors are quite constant as a
function of the xylene concentration, with their values estimated to be of approximately
6 minutes. By comparing the response and recovery times towards the two tested
VOCs, it turned out that the dynamic responses of the sensor SOF-4 are slower
in the case of xylene vapors than in the case of toluene ones. This effect
could be attributed to a different adsorption kinetics of the two target
analyte molecules inside the CNT sensitive nanocoatings. It is noteworthy,
however, that the response times of the proposed fiber optic chemosensors are
relatively good taking into account the volume of the test chamber and the
total flow rate per exposure.It is also important noting that these
times could be improved by depositing a lower amount of carbon nanotubes upon
the sensor substrates, for example by decreasing the number of SWCNT
monolayers. However, this could reduce the sensor sensitivities. As a matter of
fact, Figure15 reports a bar plot showing the mean response and recovery times
of the sensors SOF-2 and SOF-4, obtained for a 17 ppm xylene vapors exposure,
at room temperature. It can be clearly observed that, even if a decrease of the
sensor mean response time (from 9 to 6 minutes) as well as of the mean recovery
time (from 5 to 2 minutes) can be achieved by reducing the SWCNT layer
thickness, this results in a strong diminution of the sensor sensitivities. In
fact, as observed in Section 4.1.1, a reduction from 1.1⋅10−3 ppm−1 to 3⋅10−4 ppm−1 has been observed by passing from the sensor SOF-4 to SOF-2.
This means that the choice of the geometric features of the sensitive overlay
has to be made by considering the tradeoff that exists between sensor
sensitivity and response (and recovery) time.Figure 15
Comparison between mean response
and recovery times of the fiber optic chemosensors SOF-2 and SOF-4, obtained
for a 17 xylene ppm exposure, at room temperature.


## 4.1.1. Effects of the Number of SWCNT Monolayers and CdA Buffer on the
Sensor Sensitivities

In the present section, the influence
of the number of SWCNTs monolayers deposited upon the optical fiber end-face on
the sensor performance will be discussed. The differences between the
performance of the buffered and unbuffered configurations are also better
discussed. To achieve this aim, four optochemical sensors coated by a different
number of SWCNTs, directly deposited onto the bare substrates and also buffered
by 20 LB monolayers of CdA, have been simultaneously exposed to toluene and xylene
vapors.Figure12(a) reports the relative
reflectance changes versus the toluene concentration for all the tested nanosensors,
at room temperature. It can be seen that an increase of the sensitivity has
been obtained by passing from two (0.9⋅10−4 ppm−1) to four monolayers of SWCNTs (4.7⋅10−4 ppm−1) directly deposited onto bare SOFs (see Figure 12(b)). On the
contrary, the optical chemosensors coated by a higher number of carbon
nanotubes monolayers (twelve), and thus by a thicker SWCNT film, exhibited a
negative sensitivity to toluene (−0.7⋅10−4 ppm−1). This means that for this sensor the fiber-film interface
reflectance decreases on exposure. This is due to the fact that the film
reflectance, and thus also the sensor sensitivity, is strongly dependent upon
the thickness and refractive index of the CNT ovelay, in accordance with (1).
As a matter of fact, depending upon the geometric features of the film
deposited onto the optical fiber end, a sensor can exhibit either a positive or
a negative sensitivity to a target analyte.Comparison between the (a) calibration curves and (b) sensitivities
obtained by exposing four fiber optic chemosensors coated by a different number
of SWCNT monolayers to toluene vapors, at room temperature.
(a)(b)For
the same reason, the CdA buffer multilayer, whose optical properties are quite
similar to those of the standard optical fibers, and thus not optimized for the
particular configuration exploited, dramatically decreases the sensitivity of
optical fiber sensors (−0.4⋅10−4 ppm−1). It is
noteworthy, however, that by choosing a buffer-linker material whose optical
and geometrical features (such as the multilayer thickness and refractive
index) are well optimized for the specific configuration, one could be able to strongly
enhance the SOF sensor performance. From Figure 13, it can be seen that the
four fiber optic chemosensors exhibited the same behaviors also in case of
xylene vapor testing. Hence, it demonstrates that the inversion of the
optoelectronic sensor sensitivities is not due to the particular analyte
tested, but to the optical configuration exploited. This feature could be very
useful for pattern recognition analysis in case of multitransducer approaches,
where complementary sensors are exploited to improve the analyte
discrimination.Comparison between the (a)
calibration curves and (b) sensitivities obtained by exposing four fiber optic chemosensors
coated by a different number of SWCNT monolayers to xylene vapors, at room
temperature.
(a)(b)


## 4.1.2. Response and Recovery Time Analysis

The experimental results so far
reported evidenced a dependence of the sensor response times upon the analyte
concentration. For this reason, a detailed analysis of the adsorption dynamic
features of the proposed chemosensors towards both the tested hydrocarbons is
required. To this aim, in this section an investigation of the response and
recovery times of the sensor SOF-4, exposed to decreasing concentration pulses
of toluene and xylene vapors, at room temperature, is reported. The exposure
time was 30 minutes for each pulse. The response (recovery) time has been
calculated as the time needed for the output signal to pass from 10% to 90%
(from 90% to 10%) of the total signal variation occurring on vapor exposure
(recovery). The results obtained in the case of toluene are shown in Figure14(a),
and reveal a response time that increases with the analyte concentration. The
minimum response time, obtained for 54 toluene ppm, was approximately 7 minutes
while the maximum one, obtained for 93 ppm, was approximately 11 minutes. In
addition, a quite constant recovery time as low as approximately 5 minutes has
been estimated for the same transducer.Response and recovery times
obtained by exposing the sensor SOF-4 to (a) toluene and (b) xylene vapors, at
room temperature.
(a)(b)Similar results have been observed in
case of xylene exposures (see Figure14(b)), with the SOF response times
increasing from approximately 4 minutes (3 ppm) to approximately 18 minutes (39 ppm). In addition, the recovery times of both sensors are quite constant as a
function of the xylene concentration, with their values estimated to be of approximately
6 minutes. By comparing the response and recovery times towards the two tested
VOCs, it turned out that the dynamic responses of the sensor SOF-4 are slower
in the case of xylene vapors than in the case of toluene ones. This effect
could be attributed to a different adsorption kinetics of the two target
analyte molecules inside the CNT sensitive nanocoatings. It is noteworthy,
however, that the response times of the proposed fiber optic chemosensors are
relatively good taking into account the volume of the test chamber and the
total flow rate per exposure.It is also important noting that these
times could be improved by depositing a lower amount of carbon nanotubes upon
the sensor substrates, for example by decreasing the number of SWCNT
monolayers. However, this could reduce the sensor sensitivities. As a matter of
fact, Figure15 reports a bar plot showing the mean response and recovery times
of the sensors SOF-2 and SOF-4, obtained for a 17 ppm xylene vapors exposure,
at room temperature. It can be clearly observed that, even if a decrease of the
sensor mean response time (from 9 to 6 minutes) as well as of the mean recovery
time (from 5 to 2 minutes) can be achieved by reducing the SWCNT layer
thickness, this results in a strong diminution of the sensor sensitivities. In
fact, as observed in Section 4.1.1, a reduction from 1.1⋅10−3 ppm−1 to 3⋅10−4 ppm−1 has been observed by passing from the sensor SOF-4 to SOF-2.
This means that the choice of the geometric features of the sensitive overlay
has to be made by considering the tradeoff that exists between sensor
sensitivity and response (and recovery) time.Figure 15
Comparison between mean response
and recovery times of the fiber optic chemosensors SOF-2 and SOF-4, obtained
for a 17 xylene ppm exposure, at room temperature.


## 4.2. Chemical Trace Detection in Water

In this section, the attention has
been focused on the feasibility of exploiting the excellent sensing properties
of CNTs for the development of high-performance optoelectronic sensors capable
of chemical trace detection in aqueous environment, at room temperature.A schematic view of the experimental
setup exploited for the chemical trace detection in water is reported in Figure16. The SWCNTs-based optochemical transducers in reflectometric configuration
have been inserted in a Pyrex beaker containing pure water. The presence within
the test ambient of the analyte under investigation has been promoted by its
injection inside the beaker. The injected volume has been chosen, each time, in
order to obtain the desired analyte concentration. The polluted water has been
continuously stirred to ensure the maximum dispersion of the analyte. In
addition, after each analyte exposure, the capabilities of the SOF sensors to
recover the initial steady state level have been investigated by restoring the
initial condition of pure water. To this aim, pure water has been continuously
injected in the test chamber, while the contaminated water, previously present
in it, contemporarily stilled out.Figure 16
Schematic view of the
experimental setup exploited for the chemical trace detection in water.Before performing the toluene
detection measurements, the stability within an aqueous environment of the
SWCNT sensitive layers, deposited upon the fiber optic end-face, has been
investigated. To the aim, a chemooptic sensor coated by twelve monolayers of
SWCNTs, whose sensing performance against toluene in air was already tested [6],
has been successively inserted in a beaker containing pure water and also left
for several days inside it. Figure 17 reports the ΔR/R0 of the
optical probe occurred in correspondence of its insertion in water. It can be
seen that, as it was inserted in water, a strong and fast reflectance decrease
occurred as a consequence of the change in the external medium refractive
index. This effect is followed by a slow variation of the reflectance itself
(see the inset in Figure 17) until the equilibrium value is reached, which can
be attributed to the interaction between the sensitive layer and the water
molecules. In addition, once reached the equilibrium, the reflectance signal is
stable, demonstrating that no degradations of the SWCNT sensitive layer
occurred.Figure 17
ΔR/R0 of the SWCNTs-based sensor occurred as a consequence of its insertion in water.Once the carbon nanotubes stability has
been verified, the same SOF sensor has also been tested for the room
temperature detection of toluene in water. The analyte concentration range was 20–80 ppm
(calculated asμL/L). The results obtained are shown in Figure 18,
where the ΔR/R0 that have occurred upon the injections of increasing
concentrations of toluene have been reported. It can be observed that in
correspondence of each toluene injection the interaction between analyte
molecules and sensitive overlay results in a significant decrease of the
reflectance. In addition, the CNTs-based SOF sensor provides complete
recoveries of the steady-state value of uncontaminated water, which
demonstrates its excellent desorption features. The slight drift observed in
the sensor baseline can be attributed to thermal changes since the test chamber
is not perfectly thermostated. However, this aspect, as already evidenced in
the previous sections, can be easily addressed by using simple procedures to
compensate the effects of temperature variations on the sensor response.Figure 18
Time responses of the SOF sensor
coated by 12 monolayers of SWCNTs, exposed to increasing concentrations of
toluene, at room temperature.An estimation of the influence of noise on the sensor response
has also been provided by calculating the noise effect on the sensor outputI in the steady state levels
corresponding to the absence of analyte within the chamber. By considering the
steady-state level in the worst case (maximum scattering in the sensor level)
over a time interval of 15 minutes, a standard deviation σN from the mean value of the sensor output I0 (I0∼0.9424)
of approximately 6⋅10−4 can
be appreciated. By these results, a minimum detectable change of the
sensor output ΔI/I0min (and thus a ΔR/R0min) of 6⋅10−4 has been estimated.In Figure19(a), the sensing
performance of the SOF sensor coated by twelve monolayers of SWCNTs against
toluene in air and aqueous
environments has been compared.(a) Comparison between the
calibration curves of the SOF sensor coated by 12 monolayers of SWCNTs against
toluene obtained in air and aqueous environments; (b) response and recovery
times of the SOF sensor, exposed to toluene with concentration in the range 20–80 ppm.
(a)(b)Here, the reflectance changes that occurred
in the two cases have been plotted versus the toluene concentration. As can be
observed, higher linearity has been obtained in case of detection in water
environment. Furthermore, a comparison of the sensor sensitivity in the two different
environments in the case of toluene detection can be carried out in the
concentration range 40–80 ppm since in
this range a linear behavior of the sensor calibration curve has been observed
also for the case of detection in air. The results revealed a sensitivity in
air higher (approx.1.5⋅10−4 ppm−1) than that obtained in the case of water environment (approx.
1.2⋅10−4 ppm−1). The differences in the two characteristics can be attributed
to the dependence of the reflectance upon the surrounding refractive index
combined to different adsorption characteristics occurring in liquid
environment, especially if the kinetic is diffusion limited. Taking into
account the sensitivity of the sensor against toluene and considering the ΔR/R0min (6⋅10−4)
obtainable with the exploited instrumentation, a resolution of 5 ppm has been
estimated.Finally, an analysis of the response
(t10–90) and recovery (t10–90) times of the optical
probes has been carried out. The results obtained for the detection in water
are shown in Figure 19(b) and reveal characteristic times slightly dependent upon
the analyte concentration, with the mean response time (approx. 20 minutes)
higher than the mean recovery one (approx. 4 minutes). In Figure 20, a
comparison between the mean characteristic times obtained with the SOF sensors
coated by 12 monolayers of SWCNTs in the two different environments is
reported. It can be seen that, also in this case, a difference has been
observed with the response and recovery times obtained against toluene in air,
which is approximately 8 minutes and approximately 28 minutes, respectively.Figure 20
Comparison between the response and
recovery times of the proposed SOF sensors in case of toluene detection in
water and air.


## 4.3. Gaseous Hydrogen Detection at Cryogenic
Temperatures

The amazing characteristic of SWCNTs
of being highly adsorbent materials has been successfully exploited also for
the detection of gaseous hydrogen (H2) at cryogenic temperatures. As
a matter of fact, the experimental results provided in this section will reveal
the excellent potentialities of the proposed SWCNTs-based optochemical sensors
in this interesting environmental and industrial application.CryogenicH2 is often
utilized as the primary fuel source by rocket engine because of the benefits
deriving from hydrogen combustion properties. Unfortunately, gaseous hydrogen
has a very low flammability limit and low ignition energy, so that it becomes a
very dangerous and volatile substance to work with during the rocket engine
development stage. Hydrogen leakage into an oxidizing environment can easily
ignite a catastrophic explosion. For this reason, the development of accurate
and reliable devices for the detection of hydrogen leakage at cryogenic temperatures
is of crucial importance for human safety. In 1981, Lundström introduced the
first H2 sensor based on thin films of Palladium (Pd) deposited on
the top of the channel of an MOSFET device [73, 74]. Since then, this issue has
been widely studied by the researchers and a number of hydrogen sensors have
been proposed [75, 76]. However, among the big amount of sensing devices, fiber
optic detectors have strongly attracted the attention of the scientific
community in this field, especially because of their high sensitivity and lack
of sparking possibilities, guaranteed by the removal of all electrical power
from the test site. To date, several configurations of fiber-optic hydrogen
sensors have been proposed, based on different transducing principles and materials.
[77, 78]. The most exploited material in this field is Pd since it exhibits
strong thickness and refractive index changes on H2 molecule
adsorption at room temperature. However, at cryogenic temperatures Pd looses
its excellent sensing properties [79], thus the main effort in this field is
devoted to the research of new sensitive materials that are able to provide
high sensitivity and good desorption features as well as fast responses at very
low temperatures. Recent reports have demonstrated a high reversible adsorption
of molecular hydrogen in carbon nanotubes and graphitic nanofibers [80, 81]. In
light of this argument, SWCNTs (of both closed-end and opened-end types) have
been exploited as sensitive nanostructured coatings of reflectometric SOF sensors
for the detection of gaseous H2 at a temperature as low as 113 K [51].
In the following, the experimental results obtained by using three
reflectometric sensors, coated respectively by two and six monolayers of
closed-end SWCNTs (namely, 2_CeSWCNTs and 6_CeSWCNTs, resp.) and two monolayers
of opened-end SWCNTs (2_OeSWCNTs), are reported.In order to test theirH2 sensing performance, the sensors were located in a cylindrical chamber,
properly designed and realized in order to minimize the possible thermal
variations. The chamber was, in turn, inserted within a tank containing
cryogenic nitrogen (see Figure 21) with the aim of reaching the very low
temperature of 113 K.(a) Schematic view and (b),
(c) images of the experimental setup exploited for theH2 detection
at cryogenic temperatures.
(a)(b)(c)The temperature has been monitored by
a copper/copper-nickel thermocouple located close to the optical sensor head.
Nitrogen gas has been continuously injected in the cylindrical chamber in order
to reduce the humidity content inside the test ambient before the strong
temperature decreasing due to the insertion of the chamber in the cryogenic
tank. To expose the SOF sensors to gaseous hydrogen with concentrations as low
as 4% and 1%, nitrogen has also been mixed with gaseous argon containing
hydrogen.The results obtained are shown in Figure22, where the ΔR/R0 of the sensors (a) 2_CeSWCNTs, (b) 6_CeSWCNTs,
and (c) 2_OeSWCNTs, exposed to decreasing concentration pulses of hydrogen at a
temperature of 113 K, have been reported. As evident, all the optical probes
provided significant reflectance changes as a consequence of the gaseous H2 adsorption within the sensitive nanomaterials. In particular, good sensitivity
and complete recovery of the nitrogen signal after both hydrogen concentration
pulses have been observed for the two CeSWCNTs-based sensors, while good
sensitivity but no recovery of the steady-state value has been observed for the
OeSWCNTs-based one.Transient
responses of the three fiber optic probes (a) 2_CeWCNTs, (b) 6_CeWCNTs, and (c)
2_OeWCNT to decreasing concentration pulses of gaseous hydrogen (<5%), at
113 K.
(a)(b)(c)In addition, the bar plot shown in Figure23, in which the amplitude of the relative reflectance changes (|ΔR/R0|) obtained
for the tested SWCNTs-based probes has been reported, clearly reveals that for
the probe 2_CeSWCNTs, the ΔR/R0 obtained for 4% and 1% of H2 are almost similar (the ratio is approx. 1.1). In this case, a saturation of
the fiber overlay sorption capability could explain the weak mismatch between
the sensor responses.Figure 23
ΔR/R0 occurred to the three
tested SOF sensors, on H2 exposure.Differently, for the case of the
sensor 6_CeSWCNTs, a decrease of theH2 concentration of four times
leads to a relative reflectance diminution of approximately three times. In
this case, however, due to the increase of nanotubes monolayers, higher
saturation threshold is expected.Furthermore, a study of the response
and recovery times (t10–90 and t90–10, resp.) has been
carried out. The results obtained have been reported in Figure 24 and reveal
that the probes 2_CeSWCNTs and 6_CeSWCNTs are characterized by the same
response times, (approx. 4 minutes), but have slightly different recovery times
(approx. 9 and 11 minutes, resp.). This evidences that the number of carbon
nanotube monolayers influences the sensor recovery features. In addition, the
probe 2_OeSWCNTs showed a response time (approx. 5 minutes) higher than that
obtained with the CeSWCNTs-based sensors. This could be attributed to the fact
that, while in the case of the CeSWCNTs, the H2 molecules adsorption
takes place only on the surface of the single carbon tubes and in the
interstitial sites between the tubes, in the case of the counterpart OeSWCNTs,
the hydrogen molecule adsorption takes place also inside the carbon tubes,
requiring, for this reason, more time for the establishment of the equilibrium
condition with the external environment.Figure 24
Mean
response and recovery times for the three tested SOF sensors.


## 4.4. Discussion

Here, a brief discussion concerning some
of the typical drawbacks arising from the use of carbon nanotubes as sensitive
elements of reflectometric optical fiber sensors is carried out, which
evidences some currently unsolved matters, but, at the same time, opens the way
to further investigations and research activities.First of all, the attitude of SWCNTs to
randomly distribute in tangled networks of nanotubular chains and densely
aggregate mats causes a not very high repeatability of the distribution of
carbon nanotubes upon the sensor surface as well as their optical properties.
These drawbacks could be overcome by means of an alignment of the carbon
nanotubes onto the sensor substrate, which should provide a better and almost
predictable SWCNT distribution upon them.The adhesion of the carbon nanotubes
upon the fiber substrate also requires further investigations and improvements,
especially when the SWCNTs-based sensors are exploited for in water chemical
detection applications, for which particularly hard operating conditions could
even promote a detachment of the sensitive overlay from the optical fiber tip. A
first attempt to improve the adhesion of SWCNT overlays to the distal end of
the optical fibers is reported in the next section and relies on their
embedding in CdA host-matrix in order to obtain nanocomposites with tailored
amount of nanotube filler contents.Finally, another issue to address is
related to the poor selectivity of the proposed SWCNTs-based sensors towards a
given chemical specie. This means that in case of a multicomponent gas mixture
with interfering analytes, the information provided by the sensors could be
ambiguous and no straightforward information about the test environment could
be achieved. This is a very common aspect in chemical sensing applications as
demonstrated by the strong effort which is currently devoted by the researchers
on how to improve the sensor discrimination abilities among different analytes.
Basically, there are two possible approaches in the attempt to increase the
chemical sensor selectivity, and both have to be followed in synergy. The first
one is a direct approach which relies on the sensitive layer functionalization
in order to have a higher affinity of the material towards specific chemical
species [82]. The second one is based on the use of a hybrid system composed of
multiple transducers coated by the same material (or, in the most general case,
by different materials) in the form of an array. Here, sensors with a poor
selective response, when considered collectively, provide unique patterns
typical for each analyte. The generated response patterns are interpreted by
pattern-recognition algorithm for the selective detection [83]. This approach
has already been investigated and exploited by the authors [50], and is
currently under investigation.


## 5. Swcnts-based Nanocomposites as Advanced Nanocoatings

As already discussed in Section4.4,
the adhesion of the carbon nanotubes to the fiber substrate and the poor repeatability
of the deposition process (especially concerning the distribution of the tubes
upon the fiber tip as well as the optical properties of the fabricated layer)
represent two important issues to be improved for an extensive use of the
proposed SWCNTs-based optochemical sensors for practical applications and for
their effective transfer to the market. The alignment of carbon nanotubes upon
the sensors substrate and the embedding of controlled quantity of them inside a
host-matrix of a foreign material for the synthesis of nanocomposites with
tailored amount of nanotube filler contents have been identified as possible
ways to overcome these drawbacks. In fact, the CdA was used in the past as
buffer-linker material to promote the adhesion of SWCNT thin films to the fiber
end-face. Here the attention has been focused on the latter solution. In
particular, the sensing performances of fiber optic chemosensors based on
nanocomposite overlays of SWCNTs embedded in a CdA matrix have been
investigated against several chemical pollutants both in air and water
environments, at room temperature.


### 5.1. Fabrication and Characterization of CdA/SWCNTs-Based Fiber Optic Chemosensors

The Langmuir-Blodgett deposition
process has been exploited again as a way to transfer thin films of
SWCNTs-based nanocomposites upon the distal end of properly prepared
single-mode optical fibers. The CdA has been chosen as host-matrix material to
incorporate the SWCNTs in the nanocomposite due to its peculiar amphiphilic
molecular structure suitable for LB deposition process and because of the
know-how already experienced by the authors in the integration of such material
and the optical fiber transducers [49, 50]. Two separate solutions of arachidic
acid in chloroform and SWCNTs in chloroform have been mixed in order to prepare
a final solution of chloroform with arachidic acid (0.25 mg/mL) and SWCNTs
(0.19 mg/mL). The concentrations and the volumes of the initial solutions were
chosen to obtain a weight percentage of the filler component (SWCNTs) with
respect to the matrix-component (CdA) of approximately 75 wt.%. However,
different concentrations of arachidic acid and SWCNTs in the final solution
could also be exploited for the preparation of composites with different weight
percentages. The mixed solution was then accurately dispersed and stirred in an
ultrasonic bath for 1 hour. Subsequentially, 160 μL of the mixed solution were spread onto a
subphase constituted by acetate buffer with CdCl210−4 M. The pH and the temperature of the subphase
were kept constant at 6.0 and 20°C, respectively. The monolayer of the
nanocomposite was compressed with a barrier rate of 15 mm/min up to a surface
pressure of 27 mN/m. The single composite monolayer was deposited upon the
fiber surface with a dipping rate of 14 mm/min. The optical fibers used for the
deposition have been previously accurately polished from the acrylic protection
and cleaved with a precision cleaver. Then, they have been washed in chloroform
to remove any
residual coating. Repeated dipping of the fiber substrates through the
condensed Langmuir layer has been performed, resulting in the deposition of
multilayered CdA/SWCNT films one monolayer at a time.Figure25(a) shows the typical SEM
photogram of a multilayer constituted by twenty monolayers of CdA/SWCNT nanocomposite
with a filler weight percentage of 75 wt.% (deposited upon a glass substrate). The
typical bundle disposition of the carbon nanotubes together with their good
cadmium arachidate-assisted adhesion to the surface can be easily observed. A
detailed morphological and structural characterization of the carbon
nanotubes-based composites deposited by the LB method has been carried out by
Penza et al. [84]. Instead, here, the results of Raman spectroscopy analysis
are reported, conducted with the aim of characterizing the fabricated nanolayers
of CdA/SWCNTs (already deposited upon the optical fiber tip) and comparing them with the
standard SWCNTs one. To achieve this purpose, a Raman microscope functioning in
backscattering configuration employing an HeNe laser (633 nm) and 50x and 100x
objective lenses was used. The results are shown in Figure 25(b), where the typical
Raman spectra of SWCNTs and carbon nanotubes-based composite layers, already
deposited upon the fiber end-face, have been reported.(a) SEM image of a CDA/SWCNT overlay with a filler content percentage of
75 wt.%, deposited upon a glass substrate; (b) typical Raman spectra of SWCNT
and CdA/SWCNT LB overlays directly deposited upon the optical fiber end-face.
(a)(b)No significant differences between
the two recorded Raman spectra can be observed, thus evidencing that no
degradation of the CNT occurred due to their inclusion within the CdA matrix.


### 5.2. Room Temperature Toluene and Xylene Vapor
Detection

The room temperature sensing
performance of the fabricated probes have been investigated both in air and
water environments and have been compared with the ones obtained with the same
transducers coated by unbuffered SWCNT overlays. Here the attention is focused
on the toluene and xylene vapors detection by using a sample coated by 20
monolayers of nanotubes-based composite with an SWCNT filler weight percentage
of 75 wt.%. The results obtained from the experimental measurements are shown
in Figure26. Here the relative reflectance changes occurred as a consequence
of the exposure to (a) toluene and (b) xylene vapors have been reported. The
analyte adsorption within the SWCNTs-based nanocomposite overlay promoted
significant changes of the optical length of the FP sensing cavity (defined as
the product between its thickness and refractive index), resulting in a
decrease of the fiber-film reflectance. The results also reveal the capability
of the optoelectronic transducer to detect very low concentrations of the
tested pollutants at ppm levels, as well as its quite good attitude to recover
the initial baseline signal upon the complete analyte molecules desorption.Time response of the optochemical sensor coated by 20 monolayers of
CdA/SWCNT nanocomposite (75 wt.%), exposed to different concentrations of (a)
toluene and (b) xylene vapors, at room temperature.
(a)(b)Furthermore, an almost linear
behavior in the sensor calibration curves (reported in Figure27) has been
found towards the two organic vapors in the investigated ranges (0–83 ppm and
0–44 ppm for toluene and xylene, resp.), and a higher sensitivity against
xylene vapors (1.2⋅10−3 ppm−1) was obtained with respect to toluene ones (5⋅10−4 ppm−1).Figure 27
Comparison between the sensor characteristic curves obtained for toluene
and xylene vapors.Also, an analysis of the mean
response (t10–90) and recovery (t90–10) times (see Figure 28) revealed that the optochemical probe coated by twenty monolayers of
CdA/SWCNT nanocomposite is characterized by a faster dynamic in case of toluene
exposure (32 and 39 minutes, resp.) than in the xylene one (36 and 41 minutes, resp.).
It is noteworthy that the relatively high response times can be attributed to
the high number of composite monolayers (and thus to the overlay thickness used
for the sensor fabrication). Also, the content of SWCNTs can influence the
diffusion times as well as the sensor sensitivity. These aspects, however, are
still under investigation.Figure 28
Mean response and recovery times obtained exposing the optochemical
sensor coated by 20 monolayers of CdA/SWCNT nanocomposite (75 wt.%) to toluene
and xylene vapors, at room temperature.The sensing performance of the
fabricated probe can be compared with the results obtained with the probe
coated by four monolayers of SWCNTs directly deposited upon the fiber end
(SOF-4), which demonstrated toluene and xylene sensitivities of approximately4.7⋅10−4 ppm−1 and 1.1⋅10−3 ppm−1 and mean response times of 9 and 11 minutes, respectively (see
Section 4.1) [6]. The results obtained reveal that slightly higher
sensitivities have been obtained by using CdA/SWCNT composites, combined with
longer mean response times, probably due to the different number of monolayers
and overlay composition in the two cases.


### 5.3. Chemical Trace Detection in Aqueous
Environments

Once their excellent VOC adsorption
capabilities in air at room temperature have been verified, the sensing characteristics of
the CdA/SWCNT nanocomposite have been investigated also for the aromatic
hydrocarbon detection in aqueous environment. To this aim, the same sensor used
in air environment was tested for the detection of low concentrations of
toluene and xylene in water by using the experimental setup already reported
and explained in detail in Section4.2. Figure 29 reports the transient
responses of the fiber optic probe as a consequence of several toluene
injections with concentrations ranging from 20 to 100 ppm (μL/L). Like in the air case, a significant reflectance
decrease occurred on analyte adsorption within the sensing nanolayer. In
addition, a good repeatability has been observed in the sensor response when
exposed to two successive 100 ppm xylene injections.Figure 29
Time responses of the optochemical sensor coated by 20 monolayers of CdA/SWCNT
nanocomposite (75 wt.%), exposed to different concentrations of toluene in
water at room temperature.Furthermore, Figure30 demonstrates
that also in the case of detection in aqueous ambient a linear dependence
exists between the fiber-film reflectance change and the concentration of the
two organic analytes, with a higher affinity towards xylene (the sensor
sensitivity is 1.0⋅10−3 ppm−1) than toluene (4⋅10−4 ppm−1). These sensitivities are slightly lower than those obtained
for the chemical detection in air (see the previous section).Figure 30
Comparison
between the sensor characteristic curves obtained in correspondence of toluene
and xylene injections in water environment.However, strong differences have been
observed between the dynamics
of the sensor responses in the two environments, with the mean response and
recovery times against toluene (11 and 7 minutes, resp.) and xylene (14 and 7
minutes) in water (see Figure31) considerably lower than the ones evidenced by
the CdA/SWCNTs-based sensor in air (this effect could be due to the difference
of the two test chambers and of the diffusion kinetic in the two environments). Finally, in
aqueous ambient the proposed sensor exhibited enhanced performances with
respect to unbuffered SWCNT films especially in terms of sensor sensitivity,
for which enhancement of four times has been observed [52].Figure 31
Mean response and recovery times obtained exposing the optochemical
sensor coated by 20 monolayers of CdA/SWCNT nanocomposite (75 wt.%) to toluene
and xylene in water, at room temperature.


## 5.1. Fabrication and Characterization of CdA/SWCNTs-Based Fiber Optic Chemosensors

The Langmuir-Blodgett deposition
process has been exploited again as a way to transfer thin films of
SWCNTs-based nanocomposites upon the distal end of properly prepared
single-mode optical fibers. The CdA has been chosen as host-matrix material to
incorporate the SWCNTs in the nanocomposite due to its peculiar amphiphilic
molecular structure suitable for LB deposition process and because of the
know-how already experienced by the authors in the integration of such material
and the optical fiber transducers [49, 50]. Two separate solutions of arachidic
acid in chloroform and SWCNTs in chloroform have been mixed in order to prepare
a final solution of chloroform with arachidic acid (0.25 mg/mL) and SWCNTs
(0.19 mg/mL). The concentrations and the volumes of the initial solutions were
chosen to obtain a weight percentage of the filler component (SWCNTs) with
respect to the matrix-component (CdA) of approximately 75 wt.%. However,
different concentrations of arachidic acid and SWCNTs in the final solution
could also be exploited for the preparation of composites with different weight
percentages. The mixed solution was then accurately dispersed and stirred in an
ultrasonic bath for 1 hour. Subsequentially, 160 μL of the mixed solution were spread onto a
subphase constituted by acetate buffer with CdCl210−4 M. The pH and the temperature of the subphase
were kept constant at 6.0 and 20°C, respectively. The monolayer of the
nanocomposite was compressed with a barrier rate of 15 mm/min up to a surface
pressure of 27 mN/m. The single composite monolayer was deposited upon the
fiber surface with a dipping rate of 14 mm/min. The optical fibers used for the
deposition have been previously accurately polished from the acrylic protection
and cleaved with a precision cleaver. Then, they have been washed in chloroform
to remove any
residual coating. Repeated dipping of the fiber substrates through the
condensed Langmuir layer has been performed, resulting in the deposition of
multilayered CdA/SWCNT films one monolayer at a time.Figure25(a) shows the typical SEM
photogram of a multilayer constituted by twenty monolayers of CdA/SWCNT nanocomposite
with a filler weight percentage of 75 wt.% (deposited upon a glass substrate). The
typical bundle disposition of the carbon nanotubes together with their good
cadmium arachidate-assisted adhesion to the surface can be easily observed. A
detailed morphological and structural characterization of the carbon
nanotubes-based composites deposited by the LB method has been carried out by
Penza et al. [84]. Instead, here, the results of Raman spectroscopy analysis
are reported, conducted with the aim of characterizing the fabricated nanolayers
of CdA/SWCNTs (already deposited upon the optical fiber tip) and comparing them with the
standard SWCNTs one. To achieve this purpose, a Raman microscope functioning in
backscattering configuration employing an HeNe laser (633 nm) and 50x and 100x
objective lenses was used. The results are shown in Figure 25(b), where the typical
Raman spectra of SWCNTs and carbon nanotubes-based composite layers, already
deposited upon the fiber end-face, have been reported.(a) SEM image of a CDA/SWCNT overlay with a filler content percentage of
75 wt.%, deposited upon a glass substrate; (b) typical Raman spectra of SWCNT
and CdA/SWCNT LB overlays directly deposited upon the optical fiber end-face.
(a)(b)No significant differences between
the two recorded Raman spectra can be observed, thus evidencing that no
degradation of the CNT occurred due to their inclusion within the CdA matrix.


## 5.2. Room Temperature Toluene and Xylene Vapor
Detection

The room temperature sensing
performance of the fabricated probes have been investigated both in air and
water environments and have been compared with the ones obtained with the same
transducers coated by unbuffered SWCNT overlays. Here the attention is focused
on the toluene and xylene vapors detection by using a sample coated by 20
monolayers of nanotubes-based composite with an SWCNT filler weight percentage
of 75 wt.%. The results obtained from the experimental measurements are shown
in Figure26. Here the relative reflectance changes occurred as a consequence
of the exposure to (a) toluene and (b) xylene vapors have been reported. The
analyte adsorption within the SWCNTs-based nanocomposite overlay promoted
significant changes of the optical length of the FP sensing cavity (defined as
the product between its thickness and refractive index), resulting in a
decrease of the fiber-film reflectance. The results also reveal the capability
of the optoelectronic transducer to detect very low concentrations of the
tested pollutants at ppm levels, as well as its quite good attitude to recover
the initial baseline signal upon the complete analyte molecules desorption.Time response of the optochemical sensor coated by 20 monolayers of
CdA/SWCNT nanocomposite (75 wt.%), exposed to different concentrations of (a)
toluene and (b) xylene vapors, at room temperature.
(a)(b)Furthermore, an almost linear
behavior in the sensor calibration curves (reported in Figure27) has been
found towards the two organic vapors in the investigated ranges (0–83 ppm and
0–44 ppm for toluene and xylene, resp.), and a higher sensitivity against
xylene vapors (1.2⋅10−3 ppm−1) was obtained with respect to toluene ones (5⋅10−4 ppm−1).Figure 27
Comparison between the sensor characteristic curves obtained for toluene
and xylene vapors.Also, an analysis of the mean
response (t10–90) and recovery (t90–10) times (see Figure 28) revealed that the optochemical probe coated by twenty monolayers of
CdA/SWCNT nanocomposite is characterized by a faster dynamic in case of toluene
exposure (32 and 39 minutes, resp.) than in the xylene one (36 and 41 minutes, resp.).
It is noteworthy that the relatively high response times can be attributed to
the high number of composite monolayers (and thus to the overlay thickness used
for the sensor fabrication). Also, the content of SWCNTs can influence the
diffusion times as well as the sensor sensitivity. These aspects, however, are
still under investigation.Figure 28
Mean response and recovery times obtained exposing the optochemical
sensor coated by 20 monolayers of CdA/SWCNT nanocomposite (75 wt.%) to toluene
and xylene vapors, at room temperature.The sensing performance of the
fabricated probe can be compared with the results obtained with the probe
coated by four monolayers of SWCNTs directly deposited upon the fiber end
(SOF-4), which demonstrated toluene and xylene sensitivities of approximately4.7⋅10−4 ppm−1 and 1.1⋅10−3 ppm−1 and mean response times of 9 and 11 minutes, respectively (see
Section 4.1) [6]. The results obtained reveal that slightly higher
sensitivities have been obtained by using CdA/SWCNT composites, combined with
longer mean response times, probably due to the different number of monolayers
and overlay composition in the two cases.


## 5.3. Chemical Trace Detection in Aqueous
Environments

Once their excellent VOC adsorption
capabilities in air at room temperature have been verified, the sensing characteristics of
the CdA/SWCNT nanocomposite have been investigated also for the aromatic
hydrocarbon detection in aqueous environment. To this aim, the same sensor used
in air environment was tested for the detection of low concentrations of
toluene and xylene in water by using the experimental setup already reported
and explained in detail in Section4.2. Figure 29 reports the transient
responses of the fiber optic probe as a consequence of several toluene
injections with concentrations ranging from 20 to 100 ppm (μL/L). Like in the air case, a significant reflectance
decrease occurred on analyte adsorption within the sensing nanolayer. In
addition, a good repeatability has been observed in the sensor response when
exposed to two successive 100 ppm xylene injections.Figure 29
Time responses of the optochemical sensor coated by 20 monolayers of CdA/SWCNT
nanocomposite (75 wt.%), exposed to different concentrations of toluene in
water at room temperature.Furthermore, Figure30 demonstrates
that also in the case of detection in aqueous ambient a linear dependence
exists between the fiber-film reflectance change and the concentration of the
two organic analytes, with a higher affinity towards xylene (the sensor
sensitivity is 1.0⋅10−3 ppm−1) than toluene (4⋅10−4 ppm−1). These sensitivities are slightly lower than those obtained
for the chemical detection in air (see the previous section).Figure 30
Comparison
between the sensor characteristic curves obtained in correspondence of toluene
and xylene injections in water environment.However, strong differences have been
observed between the dynamics
of the sensor responses in the two environments, with the mean response and
recovery times against toluene (11 and 7 minutes, resp.) and xylene (14 and 7
minutes) in water (see Figure31) considerably lower than the ones evidenced by
the CdA/SWCNTs-based sensor in air (this effect could be due to the difference
of the two test chambers and of the diffusion kinetic in the two environments). Finally, in
aqueous ambient the proposed sensor exhibited enhanced performances with
respect to unbuffered SWCNT films especially in terms of sensor sensitivity,
for which enhancement of four times has been observed [52].Figure 31
Mean response and recovery times obtained exposing the optochemical
sensor coated by 20 monolayers of CdA/SWCNT nanocomposite (75 wt.%) to toluene
and xylene in water, at room temperature.


## 6. Hollow-Core Fiber Optic Sensors: Towards Advanced Sensing Configurations

In this last section, a novel sensing
scheme is presented, based on the integration of SWCNTs with special fibers,
the hollow-core optical fibers (HOFs). HOFs are composed of bidimensional
arrays of air holes of micrometric dimensions running along the fiber length in
which the light is confined within the air core by means of the photonic
bandgap (PBG) constituted by the microstructured cladding. This special feature
of the HOFs allows one to develop new functionalized devices by filling the fiber
holes with proper materials. As a matter of fact, the integration between HOFs
and specific materials enables the final device to own advanced properties in
terms of tunability [85–87] and sensitivity [53]. In particular, it can lead to
the development of novel optoelectronic chemosensors that exploit the changes
of the HOF
guiding
properties due to the presence of the sensitive material within the holey
structure [53]. The experimental data here reported will demonstrate the
success of the SWCNT partial filling within the HOF
holes and the capability of the novel
sensing devices thus formed to detect VOC traces.


### 6.1. HOF
Sensor Fabrication and Characterization

With reference to Figure32, the HOFs-based
sensing probes are composed of a piece of HOF,
spliced at one end with an SOF and covered and partially filled with SWCNTs at
the other termination. The SWCNT deposition, performed before the HOF-SOF
splicing, has been carried out at atmospheric pressure by means of the
molecular engineered LB technique (see Section 3.3).(a) Schematic view of the
transverse section of an HOF
and its fundamental mode field distribution; (b) schematic view of HOF-based
sensor probe.
(a)(b)Similarly to any reflectometric
configuration, also in this case the sensing principle relies basically on the
dependence of the sensing probe reflectance on the optical properties of the
sensitive materials. Nevertheless, in this case, the key factor in the
determination of the sensing performance is the dependence of the sensing probe
reflectance on the guiding properties of the portion of HOF  filled with SWCNTs.The injection of the LB suspension
inside the HOF holes and thus the adhesion of carbon nanotubes onto the
air-hole sides were guaranteed by the dipping of a piece of HOF, having one
termination not connected (atmospheric pressure), through the CNT suspension,
perpendicularly to it for a depth of approximately 1 mm [88]. For the HOF-SOF
splicing, the open end of each HOF
piece has been spliced to the input end of an SOF by using an electrical arc discharge
system (Fujikura FSM-50S). The fiber ends, both cleaved, have been aligned and
pressed against each other using the splicer precision motors. A proper
splicing procedure, involving a series of ten arcs with short duration (100 ms)
and high power (80 bit) [53], has been developed to avoid the collapse of the air
holes. The process was previously tested and the losses were estimated to be
approximately 1dB by comparing the power transmitted in butt coupling
configuration with that after the optical fiber fusion. In addition, the
bonding achieved between the two fibers, weak with respect to the bending, has
been enforced by using a heating protection sleeve. Several sensors have been
realized with this technique by varying the fabrication features such as the HOF
length and the number
of SWCNT monolayers. However, here an extensive analysis of two samples is reported,
the former one (namely SENSOR 1) constituted of a 13.3 cm long HOF coated by
ten monolayer of SWCNTs; the latter (namely SENSOR 2) constituted of a 4 cm
long HOF coated by twenty SWCNTs monolayers.In order to investigate the typical morphological
characteristics of the fabricated HOF
sensors, several samples have been analyzed by means of SEM imaging. The
retrieved images showed quite uniform CNTs-based overlays covering the fiber
holes. The typical attitude of SWCNTs to randomly aggregate mats was revealed
also in this case. As an example, Figure33 reports the SEM image of an HOF
before and after the
deposition of sixteen monolayers of SWCNTs.SEM image of an HOF
(a) before and (b) after the deposition of sixteen monolayers of SWCNTs.
(a)(b)To better evidence the SWCNT
distribution, only a
detail of the HOF
cladding has been shown, revealing an ultrathin weblike overlay covering the
holes of the microstructured fiber. This also demonstrates the success of the
carbon nanotubes integration with the HOFs.A reflectance characterization of the
realized probes have also been carried out within the HOF bandwidth by using a
tunable laser source and an optical spectrum analyzer in synchronous mode allowing
1 pm wavelength resolution in the spectral range 1520–1620 nm. The reflectance
spectra obtained for the two probes under investigation (reported in Figure34)
exhibit several interference fringes and also their envelope offers a quite
periodic behavior. In particular, with regard to SENSOR 1, two different
harmonic contents (fast and slow) can be clearly distinguished, suggesting the
presence of a double cavity interferometer. From the wavelength separation of
the fringes, the optical path length weighted by the medium effective
refractive index of each interferometer can be retrieved according to the
following equation [89]:
(7)2⋅neq⋅d=λ1⋅λ2(λ2−λ1),where d is the length of the
interferometer, neq is the medium effective refractive index of the
propagating mode, λ1 and λ2 are the wavelengths
corresponding to two adjacent maxima.Reflectance of (a) SENSOR 1 and (b) SENSOR 2. The arrows indicate the
wavelengths corresponding to two adjacent envelope relative maxima.
(a)(b)By applying (7) at the reflectance
spectrum of SENSOR 1, optical path lengths weighted by the medium effective
refractive index of approximately 26.6 cm and 40 μm have been estimated. The
former one coincides approximately with the double length of the HOF piece (considering the effective refractive index of
the HOF
not
filled by SWCNTs equal to 1).  The latter one represents the extension of the
region, on micrometric scale, constituted by the deposited SWCNTs, which are
either within the HOF holes or outside of the HOF
end face. The two
cavities, hence, can be attributed to the piece of HOF
and to the SWCNT region. In turn, from
the analysis of the reflectance of SENSOR 2, although  the fast oscillating component is again
clearly observable, the envelope of the reflectance seems to be periodic mostly
in the wavelength range 1545–1575 nm while it
is less recognizable a
periodic behavior for the envelope in the remaining spectral range.
Nevertheless, optical path lengths weighted by the medium effective refractive
index of approximately 8 cm and 460 μm have been estimated. The fast oscillating behavior is still
attributed to the piece of HOF,
which is approximately 4 cm long, whereas the SWCNTs seem to cover a region of hundreds of microns
consistently with the larger amount of SWCNTs deposited in this latter case
(twenty monolayers). Also, on the basis of previous works dealing with the
SWCNT deposition onto the end-face of standard fibers, it results that ten and
twenty monolayers of SWCNTs are not able to produce an external cavity able to
produce interference fringes in the investigated wavelength range. Hence, the
results obtained from the spectral characterization of both sensors reveal that
the carbon nanotubes penetrated at least into the HOF central hole and their distribution along the HOF
axis, which however cannot be assumed
spatially uniform, extends for tens and hundred of microns, respectively. It is
noteworthy that (7) is rigorously valid under the hypothesis of transparent
media which in turn is not truly verified for carbon nanotubes-based materials.
In addition, the estimation of the equivalent cavity length by means of the
interference fringes together with the trivial measurement of the length of the
HOF piece before the deposition is not able to
discriminate the fraction of nanotubes penetrated inside the holes from the
fraction of nanotubes which composes an external overlay at HOF
termination [53]. Nevertheless, an
insight of the double interferometer behavior of the sensing probe is obtained
and an approximated estimation of the extension of the SWCNTs region can be
retrieved.The far field emerging from the
fabricated sensors has also been collected by means of an infrared vidicon
camera (Hamamatsu C2741-03) while a narrowband laser source at 1550 nm lighted the
sensing probes. The end face of the samples has been positioned in the nearby
of the receiving lens. In Figure35, the far field transmittances of SENSOR 1 and SENSOR 2 have been reported, respectively. In the case of SENSOR 1, it presents the typical
quasi-Gaussian shape of the field associated to the HOF
fundamental mode, which has been reported
for comparison in Figure 35(c). At the bottom of the Gaussian waveform, little
peaks of circular shape are observable, corresponding to the cladding modes
also observed in the far field plot of Figure 35(c). The presence of the SWCNTs
hence is not able to strongly modify the field distribution of the propagating
mode while it confers the double interferometric behavior evidenced before. In
addition, a slightly higher light content can be observed in correspondence of
the HOF
cladding in the coated case. This effect demonstrates a slightly worse
confinement of the filled HOF
consistent with refractive index contrast reduction induced by a partial
filling of the cladding holes.Distribution of the far field transmittances of (a)SENSOR 1, (b) SENSOR
2, and (c) a bare HOF.
(a)(b)(c)With regards toSENSOR 2, instead, a strong modification of the field distribution
occurs and the amount of power transmitted is strongly reduced. The emerging
field presents a circular crown shape and the core mode is not more visible. In
other words, the larger amount of SWCNTs penetrated within the holes are able
to suppress the fundamental core mode, thus leading to a strong diminution of
the confinement power. This circumstance is attributed to the modification of
the PBG occurring as a consequence of the cladding and core hole filling which make
the functionalized HOF
not more able to guide meaningfully the light. The PBG modification affects
particularly SENSOR 2 which was
fabricated with a higher number of monolayers with respect to SENSOR 1. As a consequence, an overall
deeper infiltration of the carbon nanotubes and also their higher density
within the HOF
holes occurred. Nevertheless, on the basis of the aforementioned reflectance
characterization, it results that the light going through the sensing probe 2
experiences a double interferometer as far as the reflectance spectrum shows an
envelope almost periodic. In light of the absence of the fundamental mode in
the transmitted far field and in combination with the reflectance
characterization, it can be deduced that the propagating mode going through the
HOF
filled with
SWCNTs is not just reflected at the HOF-SWCNTs interface but it experiences
along the SWCNT region a distributed mirror with losses before being suppressed. Consequently, the
extension of the SWCNT region within the HOF
could be underestimated. The penetration depth of the SWCNTs within the holes
and their spatial distribution are currently under investigation as well as the
effects of the deposition parameters on the resulting functionalized HOF
and it will be the
object of further work. In summary, the far field characterizations in
combination with the spectral characterizations reveal the success of the SWCNT
deposition onto the distal end of the HOF
[53] and demonstrate the partial filling of the nanotubes within the
microstructured holey-core optical fibers. Also, the obtained results show how
by a proper control of the filling procedure, especially in terms of spatial
distribution, it is possible to modify the PBG and thus the propagation
properties of HOFs for specific applications.


### 6.2. Modeling

In light of the reflectance and far
field characterizations results reported so far, the sensing probe reflectance
ofSENSOR 1 can be modeled as that of
a Fabry Perot-type double interferometer while the behavior of SENSOR 2 cannot be trivially assumed
like the one of a simple double interferometer. In case of SENSOR 1,  the reflectance can be written according to the following
equation:
(8)R=|(r12+r23⋅(1−A12)⋅ei⋅βHOF+r34⋅(1−A12)⋅(1−A23)⋅ei⋅(βHOF+βSWCNTs)+r12⋅r23⋅r34⋅ei⋅βSWCNTs)÷(1+r12⋅r23⋅ei⋅βHOF+r12⋅r34⋅(1−A23)⋅ei⋅(βHOF+βSWCNTs)+r23⋅r34⋅ei⋅βSWCNTs)|2
with
(9)r12=nSOF−nHOFnSOF+nHOF,r23=nHOF−nSWCNTsnHOF+nSWCNTs,r34=nSWCNTs−nextnSWCNTs+next,βHOF=(4⋅πλ)⋅nHOF⋅dHOF,βSWCNTs=(4⋅πλ)⋅nSWCNTs⋅dSWCNTs.
The symbols r, n, d, and A represent,
respectively, the reflection coefficient, the complex effective refractive
index, the cavity length, and the absorbance (due to the coupling and splicing
losses), while the
subscript indices indicate the interface they refer to and the
subscript labels indicate the referred cavity.
In particular, the subscript 12 refers to the SOF-HOF
interface, 23 to the HOF-SWCNTs interface, and 34 to the SWCNTs-external medium
interface, while the subscript
labels SOF, HOF, and SWCNTs have an obvious meaning. The
reflectance R is ruled by several factors involved in the propagation of the
lightwave through the HOF and the HOF
filled with SWCNTs. In
particular, any change in the SWCNT refractive index affects the reflectance R
through the reflection coefficients r23 and r34, the
absorbance A23, and the propagation constant βSWCNTs according to (8) and (9).It is noteworthy that the sensor
sensitivity to the  changes in the refractive index of the sensitive material
used to functionalize the HOFs, which in turn are induced by its interaction
with the chemicals under investigation, cannot be thought as derived by a
simple Fabry-Perot effect in light of the PBG modifications. For instance, at
the interface between the HOF and the HOF
filled with SWCNTs,
coupling losses due to the modal overlap between modes propagating in the
sections with different guiding properties affect the reflectanceR. This means
that as the complex refractive index of the SWCNTs is modified by chemical
sorption, consequent changes are expected in the coupling coefficients as well
as in the reflectance R. On the other hand, also the propagation losses within
the HOF
region
filled with SWCNTs depend upon the SWCNT refractive index and hence contribute
to the reflectance variations.In summary, the changes in the
complex effective refractive index of SWCNTs within the filled region of HOF
determine, in
combination with the penetration depth of the carbon nanotubes, the sensing
probe reflectance changes by affecting the propagation features of the sensing
probe itself. This means that the key factor in the determination of the
performance of this class of sensors relies in the possibility to exploit the
aforementioned losses variability in order to get enhanced performance with
respect to traditional SWCNTs-based SOF sensors.


### 6.3. Experimental Results

In order to investigate the sensing
capability of the fabricated HOF
sensors, they have been placed in the test chamber schematically described in Section5.1 and exposed to different concentrations of VOC vapors. At the same time,
the sensors under test were interrogated with the same interrogation system
exploited for the SOF sensors (described in detail in Section 3.2), allowing
the continuous monitoring of the probe reflectance. It is noteworthy that since
the light source used offers a bandwidth much wider than the separation of the
faster interference fringes, the sensing performance exploited by each sensor
basically depends on the slow features of the reflectance spectrum and thus
mainly on the SWCNT cavity. During the measurements, the temperature within the
chamber has been monitored by using a commercial thermocouple. The sensor
sensitivity to temperature variations was previously characterized and used in
order to properly compensate the sensor output. In particular, a comparison
among the performances of SENSORS 1 and 2 is carried out by considering their
ΔR/R0 due to toluene exposure (see Figure 36(a)). To this aim, both
sensors have been exposed to three toluene pulses of 45 minutes with increasing
concentrations in the ppm range.(a) Time responses of SENSORS 1 and 2 to three toluene vapor pulses, with
concentrations in the range 84–164 ppm, at room
temperature, (b)ΔR/R0 of SENSOR 1 corresponding to the steady
states reached on exposure to different concentration pulses of toluene vapors.
(a)(b)On exposure, the reflectance of
SENSOR 1 decreased linearly with the toluene concentration as confirmed by
further exposure measurements resumed in Figure36(b). Differently, SENSOR 2
reflectance increased upon exposure presenting a significantly lower and of
opposite sign sensitivity. Moreover, SENSOR 2 turned out to be not able to
reach a steady state within 45 minutes especially for high concentration
exposures; this also limits the maximum reflectance change observed during the
exposure tests.The higher response times and the
lower sensitivity of SENSOR 2 can be explained on the basis of the higher
extension of the SWCNT region (higher diffusion times and higher losses in the
filled region). In fact, the target VOC molecules able to interact with the
SWCNTs lead to changes in the SWCNT agglomerate complex refractive index and
the induced variations are detected as fast as short is the SWCNT region. In
addition, as demonstrated by the far field characterization, the CNT
penetration depth in SENSOR 2 makes the functionalized HOF
not more able to guide the light and thus
it reduces the capability of the light to interact with the sensitive material.
Further investigation is required to assess the sensitivity dependence on the
SWCNT distribution within the HOF
structure and also to identify characterization features able to predict the
correct functioning and the performance of the final device.


## 6.1. HOF
Sensor Fabrication and Characterization

With reference to Figure32, the HOFs-based
sensing probes are composed of a piece of HOF,
spliced at one end with an SOF and covered and partially filled with SWCNTs at
the other termination. The SWCNT deposition, performed before the HOF-SOF
splicing, has been carried out at atmospheric pressure by means of the
molecular engineered LB technique (see Section 3.3).(a) Schematic view of the
transverse section of an HOF
and its fundamental mode field distribution; (b) schematic view of HOF-based
sensor probe.
(a)(b)Similarly to any reflectometric
configuration, also in this case the sensing principle relies basically on the
dependence of the sensing probe reflectance on the optical properties of the
sensitive materials. Nevertheless, in this case, the key factor in the
determination of the sensing performance is the dependence of the sensing probe
reflectance on the guiding properties of the portion of HOF  filled with SWCNTs.The injection of the LB suspension
inside the HOF holes and thus the adhesion of carbon nanotubes onto the
air-hole sides were guaranteed by the dipping of a piece of HOF, having one
termination not connected (atmospheric pressure), through the CNT suspension,
perpendicularly to it for a depth of approximately 1 mm [88]. For the HOF-SOF
splicing, the open end of each HOF
piece has been spliced to the input end of an SOF by using an electrical arc discharge
system (Fujikura FSM-50S). The fiber ends, both cleaved, have been aligned and
pressed against each other using the splicer precision motors. A proper
splicing procedure, involving a series of ten arcs with short duration (100 ms)
and high power (80 bit) [53], has been developed to avoid the collapse of the air
holes. The process was previously tested and the losses were estimated to be
approximately 1dB by comparing the power transmitted in butt coupling
configuration with that after the optical fiber fusion. In addition, the
bonding achieved between the two fibers, weak with respect to the bending, has
been enforced by using a heating protection sleeve. Several sensors have been
realized with this technique by varying the fabrication features such as the HOF
length and the number
of SWCNT monolayers. However, here an extensive analysis of two samples is reported,
the former one (namely SENSOR 1) constituted of a 13.3 cm long HOF coated by
ten monolayer of SWCNTs; the latter (namely SENSOR 2) constituted of a 4 cm
long HOF coated by twenty SWCNTs monolayers.In order to investigate the typical morphological
characteristics of the fabricated HOF
sensors, several samples have been analyzed by means of SEM imaging. The
retrieved images showed quite uniform CNTs-based overlays covering the fiber
holes. The typical attitude of SWCNTs to randomly aggregate mats was revealed
also in this case. As an example, Figure33 reports the SEM image of an HOF
before and after the
deposition of sixteen monolayers of SWCNTs.SEM image of an HOF
(a) before and (b) after the deposition of sixteen monolayers of SWCNTs.
(a)(b)To better evidence the SWCNT
distribution, only a
detail of the HOF
cladding has been shown, revealing an ultrathin weblike overlay covering the
holes of the microstructured fiber. This also demonstrates the success of the
carbon nanotubes integration with the HOFs.A reflectance characterization of the
realized probes have also been carried out within the HOF bandwidth by using a
tunable laser source and an optical spectrum analyzer in synchronous mode allowing
1 pm wavelength resolution in the spectral range 1520–1620 nm. The reflectance
spectra obtained for the two probes under investigation (reported in Figure34)
exhibit several interference fringes and also their envelope offers a quite
periodic behavior. In particular, with regard to SENSOR 1, two different
harmonic contents (fast and slow) can be clearly distinguished, suggesting the
presence of a double cavity interferometer. From the wavelength separation of
the fringes, the optical path length weighted by the medium effective
refractive index of each interferometer can be retrieved according to the
following equation [89]:
(7)2⋅neq⋅d=λ1⋅λ2(λ2−λ1),where d is the length of the
interferometer, neq is the medium effective refractive index of the
propagating mode, λ1 and λ2 are the wavelengths
corresponding to two adjacent maxima.Reflectance of (a) SENSOR 1 and (b) SENSOR 2. The arrows indicate the
wavelengths corresponding to two adjacent envelope relative maxima.
(a)(b)By applying (7) at the reflectance
spectrum of SENSOR 1, optical path lengths weighted by the medium effective
refractive index of approximately 26.6 cm and 40 μm have been estimated. The
former one coincides approximately with the double length of the HOF piece (considering the effective refractive index of
the HOF
not
filled by SWCNTs equal to 1).  The latter one represents the extension of the
region, on micrometric scale, constituted by the deposited SWCNTs, which are
either within the HOF holes or outside of the HOF
end face. The two
cavities, hence, can be attributed to the piece of HOF
and to the SWCNT region. In turn, from
the analysis of the reflectance of SENSOR 2, although  the fast oscillating component is again
clearly observable, the envelope of the reflectance seems to be periodic mostly
in the wavelength range 1545–1575 nm while it
is less recognizable a
periodic behavior for the envelope in the remaining spectral range.
Nevertheless, optical path lengths weighted by the medium effective refractive
index of approximately 8 cm and 460 μm have been estimated. The fast oscillating behavior is still
attributed to the piece of HOF,
which is approximately 4 cm long, whereas the SWCNTs seem to cover a region of hundreds of microns
consistently with the larger amount of SWCNTs deposited in this latter case
(twenty monolayers). Also, on the basis of previous works dealing with the
SWCNT deposition onto the end-face of standard fibers, it results that ten and
twenty monolayers of SWCNTs are not able to produce an external cavity able to
produce interference fringes in the investigated wavelength range. Hence, the
results obtained from the spectral characterization of both sensors reveal that
the carbon nanotubes penetrated at least into the HOF central hole and their distribution along the HOF
axis, which however cannot be assumed
spatially uniform, extends for tens and hundred of microns, respectively. It is
noteworthy that (7) is rigorously valid under the hypothesis of transparent
media which in turn is not truly verified for carbon nanotubes-based materials.
In addition, the estimation of the equivalent cavity length by means of the
interference fringes together with the trivial measurement of the length of the
HOF piece before the deposition is not able to
discriminate the fraction of nanotubes penetrated inside the holes from the
fraction of nanotubes which composes an external overlay at HOF
termination [53]. Nevertheless, an
insight of the double interferometer behavior of the sensing probe is obtained
and an approximated estimation of the extension of the SWCNTs region can be
retrieved.The far field emerging from the
fabricated sensors has also been collected by means of an infrared vidicon
camera (Hamamatsu C2741-03) while a narrowband laser source at 1550 nm lighted the
sensing probes. The end face of the samples has been positioned in the nearby
of the receiving lens. In Figure35, the far field transmittances of SENSOR 1 and SENSOR 2 have been reported, respectively. In the case of SENSOR 1, it presents the typical
quasi-Gaussian shape of the field associated to the HOF
fundamental mode, which has been reported
for comparison in Figure 35(c). At the bottom of the Gaussian waveform, little
peaks of circular shape are observable, corresponding to the cladding modes
also observed in the far field plot of Figure 35(c). The presence of the SWCNTs
hence is not able to strongly modify the field distribution of the propagating
mode while it confers the double interferometric behavior evidenced before. In
addition, a slightly higher light content can be observed in correspondence of
the HOF
cladding in the coated case. This effect demonstrates a slightly worse
confinement of the filled HOF
consistent with refractive index contrast reduction induced by a partial
filling of the cladding holes.Distribution of the far field transmittances of (a)SENSOR 1, (b) SENSOR
2, and (c) a bare HOF.
(a)(b)(c)With regards toSENSOR 2, instead, a strong modification of the field distribution
occurs and the amount of power transmitted is strongly reduced. The emerging
field presents a circular crown shape and the core mode is not more visible. In
other words, the larger amount of SWCNTs penetrated within the holes are able
to suppress the fundamental core mode, thus leading to a strong diminution of
the confinement power. This circumstance is attributed to the modification of
the PBG occurring as a consequence of the cladding and core hole filling which make
the functionalized HOF
not more able to guide meaningfully the light. The PBG modification affects
particularly SENSOR 2 which was
fabricated with a higher number of monolayers with respect to SENSOR 1. As a consequence, an overall
deeper infiltration of the carbon nanotubes and also their higher density
within the HOF
holes occurred. Nevertheless, on the basis of the aforementioned reflectance
characterization, it results that the light going through the sensing probe 2
experiences a double interferometer as far as the reflectance spectrum shows an
envelope almost periodic. In light of the absence of the fundamental mode in
the transmitted far field and in combination with the reflectance
characterization, it can be deduced that the propagating mode going through the
HOF
filled with
SWCNTs is not just reflected at the HOF-SWCNTs interface but it experiences
along the SWCNT region a distributed mirror with losses before being suppressed. Consequently, the
extension of the SWCNT region within the HOF
could be underestimated. The penetration depth of the SWCNTs within the holes
and their spatial distribution are currently under investigation as well as the
effects of the deposition parameters on the resulting functionalized HOF
and it will be the
object of further work. In summary, the far field characterizations in
combination with the spectral characterizations reveal the success of the SWCNT
deposition onto the distal end of the HOF
[53] and demonstrate the partial filling of the nanotubes within the
microstructured holey-core optical fibers. Also, the obtained results show how
by a proper control of the filling procedure, especially in terms of spatial
distribution, it is possible to modify the PBG and thus the propagation
properties of HOFs for specific applications.


## 6.2. Modeling

In light of the reflectance and far
field characterizations results reported so far, the sensing probe reflectance
ofSENSOR 1 can be modeled as that of
a Fabry Perot-type double interferometer while the behavior of SENSOR 2 cannot be trivially assumed
like the one of a simple double interferometer. In case of SENSOR 1,  the reflectance can be written according to the following
equation:
(8)R=|(r12+r23⋅(1−A12)⋅ei⋅βHOF+r34⋅(1−A12)⋅(1−A23)⋅ei⋅(βHOF+βSWCNTs)+r12⋅r23⋅r34⋅ei⋅βSWCNTs)÷(1+r12⋅r23⋅ei⋅βHOF+r12⋅r34⋅(1−A23)⋅ei⋅(βHOF+βSWCNTs)+r23⋅r34⋅ei⋅βSWCNTs)|2
with
(9)r12=nSOF−nHOFnSOF+nHOF,r23=nHOF−nSWCNTsnHOF+nSWCNTs,r34=nSWCNTs−nextnSWCNTs+next,βHOF=(4⋅πλ)⋅nHOF⋅dHOF,βSWCNTs=(4⋅πλ)⋅nSWCNTs⋅dSWCNTs.
The symbols r, n, d, and A represent,
respectively, the reflection coefficient, the complex effective refractive
index, the cavity length, and the absorbance (due to the coupling and splicing
losses), while the
subscript indices indicate the interface they refer to and the
subscript labels indicate the referred cavity.
In particular, the subscript 12 refers to the SOF-HOF
interface, 23 to the HOF-SWCNTs interface, and 34 to the SWCNTs-external medium
interface, while the subscript
labels SOF, HOF, and SWCNTs have an obvious meaning. The
reflectance R is ruled by several factors involved in the propagation of the
lightwave through the HOF and the HOF
filled with SWCNTs. In
particular, any change in the SWCNT refractive index affects the reflectance R
through the reflection coefficients r23 and r34, the
absorbance A23, and the propagation constant βSWCNTs according to (8) and (9).It is noteworthy that the sensor
sensitivity to the  changes in the refractive index of the sensitive material
used to functionalize the HOFs, which in turn are induced by its interaction
with the chemicals under investigation, cannot be thought as derived by a
simple Fabry-Perot effect in light of the PBG modifications. For instance, at
the interface between the HOF and the HOF
filled with SWCNTs,
coupling losses due to the modal overlap between modes propagating in the
sections with different guiding properties affect the reflectanceR. This means
that as the complex refractive index of the SWCNTs is modified by chemical
sorption, consequent changes are expected in the coupling coefficients as well
as in the reflectance R. On the other hand, also the propagation losses within
the HOF
region
filled with SWCNTs depend upon the SWCNT refractive index and hence contribute
to the reflectance variations.In summary, the changes in the
complex effective refractive index of SWCNTs within the filled region of HOF
determine, in
combination with the penetration depth of the carbon nanotubes, the sensing
probe reflectance changes by affecting the propagation features of the sensing
probe itself. This means that the key factor in the determination of the
performance of this class of sensors relies in the possibility to exploit the
aforementioned losses variability in order to get enhanced performance with
respect to traditional SWCNTs-based SOF sensors.


## 6.3. Experimental Results

In order to investigate the sensing
capability of the fabricated HOF
sensors, they have been placed in the test chamber schematically described in Section5.1 and exposed to different concentrations of VOC vapors. At the same time,
the sensors under test were interrogated with the same interrogation system
exploited for the SOF sensors (described in detail in Section 3.2), allowing
the continuous monitoring of the probe reflectance. It is noteworthy that since
the light source used offers a bandwidth much wider than the separation of the
faster interference fringes, the sensing performance exploited by each sensor
basically depends on the slow features of the reflectance spectrum and thus
mainly on the SWCNT cavity. During the measurements, the temperature within the
chamber has been monitored by using a commercial thermocouple. The sensor
sensitivity to temperature variations was previously characterized and used in
order to properly compensate the sensor output. In particular, a comparison
among the performances of SENSORS 1 and 2 is carried out by considering their
ΔR/R0 due to toluene exposure (see Figure 36(a)). To this aim, both
sensors have been exposed to three toluene pulses of 45 minutes with increasing
concentrations in the ppm range.(a) Time responses of SENSORS 1 and 2 to three toluene vapor pulses, with
concentrations in the range 84–164 ppm, at room
temperature, (b)ΔR/R0 of SENSOR 1 corresponding to the steady
states reached on exposure to different concentration pulses of toluene vapors.
(a)(b)On exposure, the reflectance of
SENSOR 1 decreased linearly with the toluene concentration as confirmed by
further exposure measurements resumed in Figure36(b). Differently, SENSOR 2
reflectance increased upon exposure presenting a significantly lower and of
opposite sign sensitivity. Moreover, SENSOR 2 turned out to be not able to
reach a steady state within 45 minutes especially for high concentration
exposures; this also limits the maximum reflectance change observed during the
exposure tests.The higher response times and the
lower sensitivity of SENSOR 2 can be explained on the basis of the higher
extension of the SWCNT region (higher diffusion times and higher losses in the
filled region). In fact, the target VOC molecules able to interact with the
SWCNTs lead to changes in the SWCNT agglomerate complex refractive index and
the induced variations are detected as fast as short is the SWCNT region. In
addition, as demonstrated by the far field characterization, the CNT
penetration depth in SENSOR 2 makes the functionalized HOF
not more able to guide the light and thus
it reduces the capability of the light to interact with the sensitive material.
Further investigation is required to assess the sensitivity dependence on the
SWCNT distribution within the HOF
structure and also to identify characterization features able to predict the
correct functioning and the performance of the final device.


## 7. Conclusion and Future Prospects

In conclusion, in this paper the
combination of advanced nanostructured coatings based on single-walled carbon
nanotubes with the optical fiber technology for the development of high-performance
optochemical nanosensors has been reviewed. The sensitive material integration
has been performed by the customization of the well-known molecular engineered
Langmuir-Blodgett deposition technique that has been here used to transfer
ultrathin layers based on SWCNTs upon the distal end of silica optical fibers.
The adopted optical configuration is based on an extrinsic low-finesse
Fabry-Perot interferometer, with its principle of operation relying, therefore,
on the measurement of the changes in the amount of power reflected at the
fiber-film interface occurring as a consequence of the changes in the optical (complex
refractive index) and geometrical properties (thickness) of the sensitive elements.
Such modifications are, in turn, caused by the interaction of the sensing
layers with the target analyte molecules present in the environment.Since the heart of a chemical sensor
is the sensitive layer, from which the main performances of the sensing device are derived, a strong
effort has been devoted to the investigation of the carbon nanotube overlays
features. For this reason, extensive structural and morphological
characterizations either of the as-bought SWCNT powders or of the deposited LB SWCNT
films have been carried out by means of X-ray diffraction and Raman spectroscopy
analyses, high-resolution transmission electron microscopy, and scanning
electron microscopy observations. They confirmed the nanostructured dimensions
of the exploited class of sensitive material as well as its successful transfer upon the optical
fiber surface. The realized chemical sensors have been tested against VOCs and
other pollutants in different conditions ranging from gaseous to aqueous
environments, up to harsh environments at cryogenic temperatures suitable for
space application. In most of the investigated cases, the fiber optic chemosensors
coated by SWCNTs demonstrated their strong potentiality as well as the ability
of detecting environmental pollutants around or well below the ppm threshold. The
sensing performance of the proposed optochemical probes can be easily tailored
by a proper choice of the number of SWCNT monolayers. This choice, however, has
to be made by taking account of the tradeoff between sensor sensitivity and
response time. Furthermore, the use of CdA/SWCNTs-based nanocomposites as novel
sensitive nanocoatings has also been presented, revealing their strong
potentiality to be successfully employed for chemical sensing both in liquid
and gas phases. In addition, new advanced sensing configurations have been
proposed, based on the use of hollow-core optical fibers coated and partially
filled by carbon nanotubes.


### 7.1. Future Prospects

As already discussed in Section4.4,
one of the major concerns with chemical sensors is how to improve their
discrimination ability among different analytes. In fact, we are still far away
from having synthetic and tunable materials able to mimic the tremendous
molecular recognition capability of the biological receptors such as enzymes.
However, there are two possible approaches in the attempt to increase the
chemical sensor selectivity, and both should be followed in synergy. The first
one is a direct approach which relies on the sensitive layer functionalization in
order to have a higher affinity of the sensitive elements towards specific
chemical species [82]. The second one is based on the use of a hybrid system
composed of multiple transducers coated by different sensitive materials in the
form of an array. Here, sensors with a poor selective response, when considered
collectively, provide unique patterns typical for each analyte. The generated
response patterns are interpreted by pattern-recognition algorithm for the
selective detection [83]. On this line of argument, the future work will be
mainly devoted to the second solution. In particular, the use of chemical
sensor arrays, based either on a single class of materials (SWCNTs) deposited
on transducers with different operating principles, or on various types of
materials (SWCNTs, nanoporous polymers, and metal oxides) [90–92] combined with
a single type of transducers,  or even on a multimaterial and multitransducers
approach, will be investigated. At the same time, particular attention will be
focused on the definition and application of suitable pattern recognition
algorithms to interpret the results of the multielement arrays.Further interest will also be devoted
to the resolution of some unsolved matters, first of all to the improvement of
the repeatability of the CNT overlay deposition process by means of their
alignment upon the optical fiber surface. From this approach, an almost
predictable distribution of the carbon tubes is expected, which should lead to
an improved repeatability of the integration process. In addition, the optical
anisotropy characteristics of such aligned carbon nanotubes overlays, first
discovered at the optical communication wavelength in 2005 by Yoo et al. [93],
could also be investigated and exploited for chemical sensing.Future works will also deal with a
deeper investigation of the dependence of the sensing performances of the
SWCNTs composites-based transducers on the carbon nanotube filler content
within the CdA host-matrix. In particular, since the content of SWCNTs can influence
the sensitivity and dynamic characteristics of the composites-based sensors and
since the sensitive area (i.e., the one corresponding to the core region of the
fiber, which has a diameter of approx. 9μm) is shorter than the one upon which
the overlay is effectively deposited (the whole diameter of the fiber is of
approx.125 μm), the carbon nanotubes dispersion within the host material
represent a challenging issue for which much attention should be paid.Finally, appropriate integration
procedures for the selective filling of the HOF
holes and to control the SWCNT spatial
distribution inside them are under investigation. This could lead to ad hoc
modifications of the photonic bandgap offered by the functionalized hollow-core
optical fibers, which could
be very useful for the development of high-performance sensing
configurations as well as new photonic devices.


## 7.1. Future Prospects

As already discussed in Section4.4,
one of the major concerns with chemical sensors is how to improve their
discrimination ability among different analytes. In fact, we are still far away
from having synthetic and tunable materials able to mimic the tremendous
molecular recognition capability of the biological receptors such as enzymes.
However, there are two possible approaches in the attempt to increase the
chemical sensor selectivity, and both should be followed in synergy. The first
one is a direct approach which relies on the sensitive layer functionalization in
order to have a higher affinity of the sensitive elements towards specific
chemical species [82]. The second one is based on the use of a hybrid system
composed of multiple transducers coated by different sensitive materials in the
form of an array. Here, sensors with a poor selective response, when considered
collectively, provide unique patterns typical for each analyte. The generated
response patterns are interpreted by pattern-recognition algorithm for the
selective detection [83]. On this line of argument, the future work will be
mainly devoted to the second solution. In particular, the use of chemical
sensor arrays, based either on a single class of materials (SWCNTs) deposited
on transducers with different operating principles, or on various types of
materials (SWCNTs, nanoporous polymers, and metal oxides) [90–92] combined with
a single type of transducers,  or even on a multimaterial and multitransducers
approach, will be investigated. At the same time, particular attention will be
focused on the definition and application of suitable pattern recognition
algorithms to interpret the results of the multielement arrays.Further interest will also be devoted
to the resolution of some unsolved matters, first of all to the improvement of
the repeatability of the CNT overlay deposition process by means of their
alignment upon the optical fiber surface. From this approach, an almost
predictable distribution of the carbon tubes is expected, which should lead to
an improved repeatability of the integration process. In addition, the optical
anisotropy characteristics of such aligned carbon nanotubes overlays, first
discovered at the optical communication wavelength in 2005 by Yoo et al. [93],
could also be investigated and exploited for chemical sensing.Future works will also deal with a
deeper investigation of the dependence of the sensing performances of the
SWCNTs composites-based transducers on the carbon nanotube filler content
within the CdA host-matrix. In particular, since the content of SWCNTs can influence
the sensitivity and dynamic characteristics of the composites-based sensors and
since the sensitive area (i.e., the one corresponding to the core region of the
fiber, which has a diameter of approx. 9μm) is shorter than the one upon which
the overlay is effectively deposited (the whole diameter of the fiber is of
approx.125 μm), the carbon nanotubes dispersion within the host material
represent a challenging issue for which much attention should be paid.Finally, appropriate integration
procedures for the selective filling of the HOF
holes and to control the SWCNT spatial
distribution inside them are under investigation. This could lead to ad hoc
modifications of the photonic bandgap offered by the functionalized hollow-core
optical fibers, which could
be very useful for the development of high-performance sensing
configurations as well as new photonic devices.



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*Source: 936074-2008-09-23.xml*