Sensors and Materials, Vol. 24, No. 6 (2012) 275–302
MYU Tokyo
S & M 0885

Micro- and Nanosensors
for Medical and Biological Measurement
Peter Rolfe*,1,2
Oxford BioHorizons Ltd., 12 Park View Rd., Berkhamsted, Hertfordshire, HP4 3EY, UK
2Harbin Institute of Technology, Harbin, China

1(Received October 11, 2011; accepted December 8, 2011)
Key words: biomedical measurement, micro/nanosensors, MEMS/NEMS, lab-on-a-chip,
photonics, invasive and noninvasive sensors, biocompatibility, biomedical metrology

Micro- and nanosensors have evolved rapidly in the last few decades and they
have expanding roles within biology and medicine, where measurement scienceand technology is of key importance. The targets for measurement include a huge
number of simple and complex molecules, physical quantities such as pressure, force,
displacement and flow, and electrical and magnetic phenomena arising from the heart,brain, muscles and nerves. Routine clinical care of patients currently benefits from theuse of macro- and microscale sensors based on electrical, electrochemical, acoustic,
piezoelectric and optical principles. Disposable electrodes for recording biopotentials,
such as the electrocardiogram and electroencephalogram, are common, whereas invasive
electrochemical and optical fibre sensors for pressure, blood gases and pH are useful inintensive care. Microscale immobilised enzyme glucose sensors are largely confined tothe analysis of small blood samples, their invasive use still facing technical challenges.
Sensors constructed to the nanoscale using quantum dots and carbon nanotubes are
now rapidly emerging, being aimed at more complex biomolecules such as DNA.
Nanoparticles in general and surface-enhanced Raman spectroscopy also play importantroles in these developments. The impact of micro- and nanosensors on the fundamental
understanding of major biomedical challenges and on clinical diagnosis and care arehighlighted here.

1.

Introduction

Sensors and instrumentation for medical and biological measurement evolved steadilyin the first half of the 20th century, establishing certain diagnostic devices, such as theCorresponding author: e-mail: PeterRolfe@aol.com

*

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electrocardiograph, the electronic sphygmomanometer for blood pressure measurement,
and physiological monitors for anaesthesia and intensive care, as the classical symbolsof biomedical instrumentation within healthcare.(1) Although complete measurement
systems comprising of sensors and the associated electronics and displays are, of course,
essential, the very significant challenges of sensor design and construction have, overmany decades, attracted particular attention.(2) In recent years, therefore, there has been
the emergence of increasingly sophisticated and complex sensing devices and these arebeing tested or utilised in a growing number of biomedical situations for a diverse rangeof target measurands.
The application areas for medical and biological sensors have been evolving as thenature and priorities of basic research and routine clinical care have themselves moved
forward. At one end of the spectrum is the fundamental biological research that isneeded, for example, to understand the molecular phenomena that are so important incontrolling the behaviour of cells, tissues and organs, and this is now heavily dependent
upon the use of sensors. Then, there are needs for sensors in physiologically based
clinical research, acute clinical care, and emerging fields such as tissue engineering,
regenerative medicine, nanomedicine, and drug discovery. At the other end of thespectrum are the growing needs to develop measurement systems to be used for selfdiagnosis and self-care by individuals in their home and during normal daily life. In thelatter case, developments in wireless sensor networks are now aimed at achieving socalled ubiquitous healthcare,(3) and sensors constitute key components of such systems.
The variety of target measurands of interest and importance is now extremely diverse.
Physical variables, such as pressure, force, flow, and displacement, are important for bothbasic research as well as for routine clinical care in fields such as biomechanics, orthopaedics,
cardiorespiratory medicine, and intensive care. The intrinsic electrochemical nature of
biological cells can be interrogated through measurement of electrical fields, charge,
and magnetic flux. Such measurements are important for cardiac, musculo-skeletal,
and cerebral monitoring. Then, at the molecular level, sensors are needed for gases,
ions, proteins, DNA, bacteria, and viruses, as well as many other chemical compoundsof importance for diagnosis of diseases such as diabetes, kidney and liver failure, andcancer.
The practical sensor devices developed to meet these diverse needs have been basedon a number of important sensing principles. The earliest medical sensors for pressure
or stress/strain utilising electrical principles embodied in resistive bridge strain gauges
served the field well,(4) and these still have applications.(5) Electrochemical sensors,
especially those based on amperometry(6) and potentiometry,(7,8) have been and continue
to be key contributors in the vast range of chemical measurements currently undertaken.
The measurement of bioelectrical phenomena also depends on the use of electrodes,(9)commonly used in the electrocardiograph (ECG), electroencephalograph (EEG) andelectromyograph (EMG). Of major importance at present are sensors based on opticalprinciples, including absorbance, reflectance, fluorescence, and interferometry. The useof optical fibres to achieve such measurements was an important early development,(10)whereas planar fabrication techniques have also provided many options for complex
optical sensor configurations.(11)Sensors and Materials, Vol. 24, No. 6 (2012)277

It is evident that current sensor advances owe much to key technological innovations
that have had a particular impact on device fabrication, improving reliability andcomplexity at the microscale as well as opening the door to nanoscale devices. For
example, semiconductor chip fabrication based on lithographic techniques continues
to achieve ever-decreasing feature dimensions, now in the range of 10–20 nm, throughefforts aimed at developing laser sources with adequate power at wavelengths in theextreme ultraviolet band around 13 nm. Important contributions have also been madeby advances in such fields as micro- and nano-electromechanical systems (MEMS/
NEMS),(12) microfluidics,(13) lab-on-a-chip(14) and photonics-based micro- and nanodevices
and systems using optical waveguides and plasmonics.(15) The progressive evolution of
sensor fabrication technologies has moved the medical sensor field quite distinctly fromthe macroscale, through the microdomain and now into the nanoworld.(16) This enables
the complexities of cellular and intracellular processes to be examined in detail at thefundamental level. In fact, there have also been significant benefits to research in sensor
science and technology from the inspiration that derives from natural biological sensory
systems,(17) for example, from the senses of sight, hearing, taste, smell and touch.(18) This
has led to important activities in so-called biomimicry and biomimetics.(19)In this review, the evolution of micro- and nanosensors used for biological andmedical measurement is explored by considering sensing techniques, theoreticalissues and practical challenges as well as applications. It is recognised that the use ofa dimensional prefix, such as macro-, micro-, or nano-, can be interpreted in different
ways, leading to some misinterpretations. For example, some use the prefix to refer tothe dimensions of the complete packaged sensor device, whereas others use it to indicate
the size of the sensing element alone. A complete sensor device may be anything from100 µm to 5 mm in diameter, but the sensing element could be just 20 nm in diameter.
A comparison between the dimensions of the measurement site and of the sensor istherefore presented here. Also, the different interpretations of the dimensional prefix are
highlighted in an effort to clarify the variants of sensor terminology currently being usedby researchers, users, and manufacturers.

2.

Sensor Design Constraints

2.1

Measurands

The recognised biomedical measurement application areas that exist have createddemands for sensors in terms of both the required measurands and the constraints on
sensor performance and physical configuration, including size and form. Table 1 showsa broad classification of the measurands and the biomedical systems and measurementsites appropriate for biomedical measurements.
The measurement of physical variables includes pressure, force, flow, displacement,
and acceleration. Temperature measurement is also a basic requirement, especiallyfor clinical monitoring. Among the chemical species of importance, oxygen, O2, andcarbon dioxide, CO2, are vital in clinical care as well as in basic research. Measurementof nitric oxide, NO, has been of major interest since its roles in fundamental cellular
processes became known.(20) Sensors are needed for the measurement of a number of278

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Table 1
Broad classification of measurands and measurement sites.
Type
PhysicalMeasurand
Pressure, force, flow, displacement,
acceleration, temperature
ChemicalO2, CO2, NO, pH, Ca++, K+, Na+,
Cl−, C6H12O6 glucose, (NH2)2CO urea,(CH3)2CO acetone, bacteria, DNA,virus, biomarkers, drugs, antigen,antibody
Electrical, magnetic ECG, EMG, EEG, MCG, MEG, EOG

Systems and sites
Cardiac, respiratory, skeletal, gastrointestinal, central nervous system
Intravascular, renal, hepatic, tissue,
noninvasive, breath, urine, saliva, invitro
Cardiac, cerebral, muscles, ocularions, particularly H+, Ca++, K+, Na+, and Cl−. These ions have profound influences on the
behaviour of cells, which in turn control many physiological processes. Measurementsof a variety of other more complex chemical compounds, such as glucose, which isof major importance in diabetes, urea and acetone,(21) of relevance to kidney function,bring further significant challenges for sensor design and construction. Electricalsignals including the so-called biopotentials, such as the ECG, EMG, and EEG, remain
important for clinical diagnosis but are now increasingly being reassessed alongsidemagnetic measurements. Magnetocardiography, MCG, and magnetoencephalography,
MEG, might offer advantages over the ECG and EEG.

2.2

Measurement site

Designing practical sensors that can be fabricated cost-effectively for reliable use, ineither basic research or in routine clinical care, must take account of the intended site forthe sensor to be located. This is important in order that the sensor geometry and overall
dimensions can be designed to meet all practical and safety constraints. In addition tosensor shape, form and dimensions, it is also vital that the materials from which thedevices are fabricated are chosen to ensure that any materials-related risks are minimised.
In this context, the investigation of the biocompatibility of sensors is vital,(22,23) and thematerials from which the devices are fabricated are key factors in this.
There are three broad categories of measurement site that are helpful when
considering these matters of sensor configuration and materials:a) Invasive measurement. This involves the placement of the sensing device within
living tissues, in either animal or human subjects. Devices may be inserted into
an artery or vein, or be placed within the gastrointestinal tract or respiratory tract.
Invasive sensors may also be inserted into cells or tissues being cultured in thelaboratory.
b) Noninvasive measurement.(24) In this case, the sensor is positioned on or near thesurface of the living tissue. For clinical monitoring, this typically involves fixationof the sensing device to the skin surface. In cell culture bioreactors, it may involve
sensors positioned either outside the culture chamber, or at the surface of a single
cultured cell.

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Sensors and Materials, Vol. 24, No. 6 (2012)c) Ex vivo and in vitro measurement. This involves the sensing device being situatedwithin a sample chamber or other apparatus where it analyses the sample, which
could be taken from a patient or experimental animal or it could be taken from abioreactor.
With invasive and noninvasive measurement techniques, the sensor is required tocapture data directly from the intact subject, patient, or organism. For this reason, bothtypes of measurement are also referred to as being in vivo, literally “within the intact
living organism”. On the other hand, both ex vivo and in vitro mean “outside the intact
living organism”, but the latter specifically means “in glass”. Thus, ex vivo and in vitro
measurements particularly relate to the analysis of samples removed from the intact
organism and such samples include blood, urine, saliva, breath, tissue or cells.

2.3

Sensor device dimensions

The physical size of the sensor devices used for the very wide range of work inbiology and medicine is an important factor in terms of design and fabrication. Clearly,
the size of the overall sensor device, its shape or form, must be appropriate for themeasurement site and the sample volume in which the target measurands are located.
It is relevant to consider the dimensions of biological structures and humananatomical features for the scale of sensing devices to be put into context; this is shownin Table 2.
It can be seen that there are nanoscale items, structures, and features, not only
as atoms and individual molecules but also as subcellular organelles, such as thecytoskeleton. DNA strands have subnanometre width dimensions but can be very long,as much as 3 m but are coiled into a ball in the nucleus. Whole cells, having a varietyof shapes, vary from red blood cells that are discoid in shape and 2×8 µm2 in size, toTable 2This gives an approximate range of the sizes of elements, structures, features, tissues and organs.
The size range extends from less than 1 nm for atoms to several tens of centimetres for humanorgans.
Structure/Feature
Atoms and molecules
Cytoskeleton (dia)Size
0.2–10 nm
5–25 nm

DNA (length)
Organelles
Cell nucleus
Mitochondria
Cells
Red blood cells
Microvasculature (dia)Arteries, veins
Kidney, heart, brain

0.34 nm × Nbp
1–5 µm5 µm
0.1×2 µm
5–100 µm
2×8 µm
5–10 µm
1–200 mm
10–20 cmComment
Actin filaments (AF), intermediate filaments (IF),microtubules
Nbp number of base pairs; 3 m total length
Intracellular elements
The cell’s ‘power house’
Cell shapes are diverse
Discoid in shape
Capillaries, venules

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Sensors and Materials, Vol. 24, No. 6 (2012)skeletal muscle cells that can be 100 µm in diameter and 3 cm long. Blood vessels areimportant in the context of sensor placement within the body since they can provide
convenient access to blood, which is a key fluid for biochemical analysis as well as for
direct blood pressure and flow measurement.
Measurements made ex vivo/in vitro frequently involve the removal of a sample fromthe subject or organism and placement within an analytical chamber. The nature of thesample and the available volume may impose limits on both the sensor and chamber inwhich the measurement is made. Examples of such samples are breath, blood, urine,
faeces, saliva, and biopsy tissues, including both hard tissues such as bone and the morecommon soft tissues. Blood and other fluid sample volumes may be as large as 1–5 mLfrom human adults but as low as 50 µL from premature babies and laboratory animals.
In the widely used microarrays(25) for protein and DNA analysis, a typical sample volume
is 10 µL.
In situations where it is important to reduce the sample volume needed, or tominimise the influence that the mere presence of the sensor has on the biologicalenvironment in which it is situated, there have always been efforts to reduce the overall
sensor device size. This is where advances in fabrication methods and technologies, suchas MEMS and NEMS, play crucial roles and where recent developments have moved
the state-of-the-art in terms of sensing phenomena firmly from the microdomain into thenanoworld.
The terminology of sensing devices based on dimensional criteria can be open tointerpretation, and may, therefore, only be of limited value, unless there is agreement
among users as to the specific meaning of dimensional prefixes. As has already been
mentioned above, complete sensor devices usually require appropriate packaging and, inmany cases, this significantly increases the overall dimensions. The notable exceptions
to this are dye molecules and nanoparticles that can be delivered to invasive sites, where
they sense the target measurand, yet may be interrogated, or read-out, remotely throughexternally applied EM or magnetic fields. In the case of these two examples, dyes andnanoparticles, there is no packaging required for the sensor and so it is valid to use theterm nanosensor for the active element even if very large instrumentation is needed tocomplete the measurement system. Some nanoparticles, such as nanowires, can be usedas the basis of sensor devices in which they are combined with other elements, suchas transistors, in order to fabricate a complete device. In this case, the sensor is still ananosensor but the complete device, for example, a virus sensor, is a large-scale deviceof several mm2. In this case, there is no real merit in using a dimensional prefix todescribe the complete device, but it is likely to be branded a nanosensor simply because
it incorporates such a sensor.
Further terminological issues arise in the field of electrodes used for electrical andelectrochemical measurements. Some electrodes are used for intracellular measurement
and they therefore need to have an active tip with a diameter of 1–10 µm. This sensing
part is clearly a microsensor. However, the shank of the electrode is considerably larger,
in the region of 1 mm, to achieve the desired robustness for practical use. Thus, thedevice consists of a microscale sensing tip and a macroscale support. In this field, thecomplete devices are universally referred to as microelectrodes despite the fact that the281

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overall device is macroscale. A similar situation arises with sensors used for direct intraarterial pressure measurement, where, for example, a 50-µm-diameter fibre optic sensor
is mounted on the tip of a catheter 3–7 mm in diameter. Strictly speaking, this overall
device is not a microsensor, although it does contain one.

3.

Key Sensing Principles and Techniques

Within the vast and growing fields of micro- and nanosensors, there are several key
underlying sensing principles upon which practical sensors are based. Two major classesof sensing principles are, firstly, electrical and electrochemical sensing and, secondly,
photonic or optical sensing. Devices based on these two classes of sensing have playedsuch important roles in biomedical measurement as to warrant special description of themost important underlying principles.

3.1

Electrical and electrochemical sensing

Some of the most widely used biomedical sensors are based on electrical andelectrochemical principles. These are convenient in that they usually combine thesensing element with the transducing function to produce a current or a potential inproportion to the target measurands.

3.1.1 Ag/AgCl reference electrodeThe simplest examples of electrochemical sensors are seen in the electrodes used forbiopotential recording, as in ECG, EMG, EOG, and EEG. These electrodes are usuallymacroscale, merely being required to collect small electrical potentials (0.1–1.0 mV)
and feed them to a high-gain high-input-impedance amplifier. They can, nevertheless,be of microscale.(26) They mostly utilise the all-important silver/silver chloride (Ag/
AgCl) electrode, which, in the presence of a high concentration of chloride ions, Cl−, canestablish a very stable half-cell potential. For this reason, the Ag/AgCl electrode is used
as a reference electrode in many other applications, including in a variety of microscale
devices.
Ag/AgCl electrodes are used with an electrolyte to supply the very necessary Cl− ions,and typically, this is KCl. Then, metal Ag produces Ag+ ions in the electrolyte solution
and these combine reversibly with the Cl− ions provided by the electrolyte. A balanceis achieved by electrons in the Ag metal flowing through a connecting wire. The two
chemical relationships are as follows.
Ag++Cl−Ag

AgCl

Ag++e−(1a)(1b)

In addition to its use for collecting biopotentials, the Ag/AgCl electrode is widelyused in chemical sensors as a stable reference to allow a current or potential to bemeasured. The Nernst equation is used frequently in connection with electrode or
biological cell potentials, and it can be written as282

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E = E0−

RT
lnQ,
nF

(2)

where E is the cell potential under nonstandard conditions, E0 is the standard cellpotential, R is the universal gas constant, T is the absolute temperature, n is thevalency, F is Faraday’s constant, and Q is the reaction quotient, which is the ratio of theconcentration of products to the concentration of reactants. Substituting for T, n, and F,
and converting to log10 gives the familiar form:
E = E0−0.059
log10Q .
n(3)

Under some conditions, the reaction quotient may equal unity and the cell potential
will then simply be equal to the standard cell potential of +0.223 V.

3.1.2 Amperometry

Of particular interest among electrochemical approaches are amperometric andpotentiometric sensors for gases, ions, and metabolites.
The simplest amperometric sensor, which is amenable to microscale fabrication,
comprises of a noble metal working electrode (Au, Ag, Pt) held at an appropriate
potential with respect to a Ag/AgCl reference for reduction or oxidation of the target
measurands. A current is then produced in proportion to the concentration of themeasurands. There has been special interest in the measurement of O2 and H2O2 withsuch sensors.
The practical use of amperometric microsensors in biological fluids, such as blood,requires the use of permeable polymer membranes to surround the electrochemical cellso that diffusion of the target molecule to the working electrode can be controlled. With
this arrangement, it is possible to achieve a useful relationship between current andconcentration derived using Faraday’s law and Fick’s law:
Id = n

FADs
cs ,
a

(4)

where Id is the diffusion current, n is valency, F is Faraday’s constant, A is the area of theworking electrode, Ds is the diffusion coefficient of the measurand in the membrane, a isthe membrane thickness, and cs is the concentration of the measurand.

3.1.3 Potentiometry

The second major electrochemical method with which to create sensors is
potentiometry. This method is particularly useful for the measurement of ions andit operates by generating a potential difference across an ion-selective membrane inproportion to ion activity. Ion-selective membranes have been mostly based on glasses,
but polymer materials containing appropriate ionophores have been increasingly used.
For the commonly used pH electrode, the relationship between the hydrogen ion activity,

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Sensors and Materials, Vol. 24, No. 6 (2012)[H+], and the potential difference appearing across the glass membrane, E, is given by
E = E0+2.3

RTlog10[H+] .
F(5)We define pH as
pH = −log[H+].

(6)

At room temperature, 2.3RT/F = 0.0592 V, thus, by substitution, we then have the linear
relationship between E and pH as
E = E0−0.0592pH.

(7)

pH sensors based on ion-selective membranes are therefore assessed in part by theslope of their E vs pH response, which, to be in accord with an ideal Nernstian response,should be −0.0592 V.

3.2

Photonic sensing principles

The many types of interaction between light, more broadly electromagnetic (EM)energy, and matter have provided extensive and diverse opportunities for the creation ofsensing techniques and devices. In biomedical measurement, there is particular interestin those parts of the EM spectrum in the ultraviolet (UV), visible (Vis) and near infrared(NIR). This corresponds to wavelengths ranging from UV 220–400 nm (frequencies of
1,350–740 THz), Vis 380–760 nm (frequencies of 790–400 THz), and NIR 760–2,500
nm (frequencies of 400–120 THz). The photonic sensing methods in these EM bands
might reasonably be considered to be inherently nanoscale and, in fact, reference tonanophotonics and nanooptics as specialist entities supports this idea. However, despite
this, the fact remains that the complete optical devices or assemblies may be considerablylarger.

3.2.1 Absorption and scattering

Much is known about the atomic and molecular interactions of EM waves or photons,
and some of the most useful phenomena for sensing are absorption and scattering. As
EM energy passes through matter, its intensity may diminish due to either absorptive or
scattering process or a combination of both depending on the properties of the matter andthe wavelength of the EM energy. In the case of purely absorptive matter, the LambertBeer law can be used to relate the incident intensity, Io, the concentration of the absorber, [C],the transmitted intensity, I, the path length of light through the matter, L, and the molar
extinction coefficient of the absorber, ε, as
I = Ioe−ε[C]L,from which the transmittance, T, and absorbance, A, can be derived:

(8)

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T = log (I/Io) = −ε[C]L,(9)

A = log (Io/I) = ε[C]L.

(10)

The simplest use of these relationships for absorption measurement is in sensors
where a dye is arranged to sense a chemical or physical event or process, such as
oxygenation. For a more comprehensive spectrophotometric measurement, thewavelength of the interrogating EM energy is varied. Thus, for three absorbers and using
three wavelengths (see Fig. 1):Aλ1 = (ε1,λ1[C]1 + ε2,λ1[C]2 + ε3,λ1[C]3)L,(11a)

Aλ2 = (ε1,λ2[C]1 + ε2,λ2[C]2 + ε3,λ2[C]3)L,(11b)

Aλ3 = (ε1,λ3 [C]1 + ε2,λ3[C]2 + ε3,λ3[C]3)L.

(11c)

These can be solved to allow calculation of C1, C2, and C3.
The processes of elastic and inelastic scattering are also used extensively inbiomedical sensing. One category of inelastic scattering is photoluminescence,
which entails the absorption and reradiation of photons. One important type ofphotoluminescence is fluorescence in which the energy of the reemitted photons,
although it occurs rapidly, within around 10 ns, is less than that of the absorbed photons.
This lower energy means that the wavelength of the emitted energy is increased ascompared with that of the incident energy. This is given in the equation proposed by
Planck:E = hc/λ = hv.

ε3,λ1

(12)

ε

ε2,λ1
ε1,λ1

λ2

λ1

λ3

λ

Fig. 1. This shows absorption spectra of three chemical species: dashed line, 1; dotted line, 2;
solid line, 3. Interrogation with three wavelengths allows the three chemical concentrations to be
calculated. Shown are the extinction coefficients of the three species at λ1 (© Peter Rolfe).

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Here, E is the energy, h is Planck’s constant, c is the speed of light, λ is the
wavelength, and ν is frequency. Fluorescence is utilised in sensors in which afluorophore sensitive to a target molecule is incorporated in some way with the sensing
device. Typically, the fluorescence is quenched by the target molecule, Q, thereby
creating an inverse relationship between fluorescence intensity and target concentration (seeFig. 2). This process is described using the Stern-Volmer Relationships:(Io/I−1) = (τo/τ−1) = Ksv[Q] = Kqτo[Q].

(13)Here, Io and I are the luminescence intensities in the absence and presence of thequenching analyte, and τo and τ are the lifetimes of the excited state in the absence andpresence of the quenching analyte, respectively. Ksv is the overall quenching constantand Kq is the bimolecular quenching constant. Another type of photoluminescence isphosphorescence in which the internal energy transfer involves intersystem crossing,which is a slow process compared with that taking place in fluorescence.
Another important type of inelastic scattering is that named after Raman. There are
two forms: with Stokes Raman, the scattered photon has less energy than the absorbed
photon; with anti-Stokes Raman, the emitted photon has more energy than the absorbed
photon. Whereas fluorescence requires the incoming wave to have a specific frequency,
the Raman effect can occur for any incoming frequency.

3.2.2 Plasmonics

Of growing importance in sensor development are approaches based on plasmonics.
This field is concerned with the study and use of plasmons, which are quanta of plasma

Excited stateδE = hc/ λ 1

Fluorescence

λ2Ground state
QuenchingFig. 2. This shows how fluorescence may be quenched, for example, by oxygen.

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oscillations and are considered as quasiparticles. Surface plasmons, which exist very
close to surfaces, are particularly important since they interact with light, producing
surface polaritons. These EM waves produced when light arrives at a dielectric-metalinterface propagate parallel to the interface. The surface polaritons can be influenced bysurface events such as particle attachment or change in refractive index, and this is usedas the basis of sensing.
The excitation of surface plasmons by light, termed surface plasmon resonance (SPR),
can be achieved by passing the light through a prism and creating an evanescent wave
(EW) at the interface between the prism surface and air or a thin metal film (Fig. 3). The
EW is created when the angle of incidence of the light beam at the interface is greaterthan the critical angle, producing total internal reflection (TIR). For different angles of
the incident wave, the intensity of the reflected wave is at a minimum when resonanceoccurs. Changes of refractive index produced by surface events, such as antigenantibody binding, change the resonance angle and can therefore be quantified.
In order to calculate the depth of the ‘field of view’ of this kind of sensor, one
examines the intensity of the EW, EEW, as it diminishes exponentially in a direction
normal to the interface:
EEW = Eoe−z/d .

(14)

2

This then allows the depth of penetration, d2, to be derived:z

Au

EEW = Eoe−z / d 2
d2

(b)

Au

θ i >θ c© Peter Rolfe

(a)

(c)

Fig. 3. (a) This shows the creation of an evanescent wave, EEW, at the interface between a prismand an external medium. Incident light approaches the interface at an angle, θi, which is greaterthan the critical angle θc. (b) This shows the addition of a thin Au film in the Otto configuration and (c)the Kretchmann configuration.

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d2 = λ/2�n1[sin2θi−(n1/n2)2],(15)

where n1 and n2 are the refractive indices in the prism and external medium, respectively,and θi is the angle of incidence and θc is the critical angle. Deposition of a thin metallic
film, of Au or Ag, on or near the prism surface allows the EW to produce the surface
polaritons. As shown in Fig. 3(b), in the Otto configuration, the metal film is positioned
near the surface, whereas in the Kretchmann configuration (c), it is deposited directlyon the surface. A further important variant of these plasmonic principles is to use
nanoparticles with thin-film Au surfaces to produce localised surface plasmon resonance(LSPR). Moreover, it is useful to employ metallic films or particles to enhance Raman
scatter, as in surface-enhanced Raman scatter (SERS).

4.

In Vivo Applications

4.1

Invasive measurement

The insertion of measurement devices into a patient can be justified in several clinicalsituations. For example, patients who are under intensive care are likely already to haveinvasive procedures and the use of a sensor placed within tissues or the intravascularspace could produce valuable information. In experimental animals, invasive devices
may be justified in terms of the quality of the data that they could yield.

4.1.1 General considerations

The invasive approach to measurement involves the insertion of a device into theorganism under investigation, be that a healthcare patient or a laboratory biologicalpreparation, including an animal. This would appear to provide the most direct means
with which to perform the measurement and may therefore be considered to constitutethe preferred method when one considers matters such as accuracy and precision.
Metrological considerations are always important in biomedical measurement andthese can become particularly challenging when an invasive approach is used. Sensors
and their associated instrumentation need to be calibrated periodically, and there needs
to be a quality control protocol in place to ensure that the derived data, especially whenused for critical diagnosis or therapeutic monitoring, are of appropriate precision andaccuracy.

4.1.2 Tissue and intravascular sensors

There are important clinical and basic research applications for invasive sensors
that are designed to be inserted into the vascular system, or embedded in tissues, orinserted into single cells. The sensors of relevance in these areas can be either micro- or
nanoscale, as will be seen from the dimensional data in Table 2. However, it is importantto recognise that in many cases, the dimensions of the sensing element are often
considerably smaller than the overall size of the device that is fabricated. Thus, a nanoscale sensor may well need to be much larger to be usable in practice.

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The established medical applications of invasive sensors are in the clinical care of
critically ill patients who may be under intensive care, or coronary care or undergoing
major surgery. The patients may be a fetus or newborn baby, children, or adults. The
care of these patients involves the continuous monitoring of important physiologicalvariables, such as blood gases, ions, metabolites, blood pressure, and blood flow. The
aims are, firstly, to enable changes in the patient’s condition to be detected as rapidly
as possible and, secondly, to visualise and evaluate the effects of therapies so that thesecan be speedily optimised. Continuous real-time monitoring of many key variables canbe achieved by placing sensors directly inside the vascular space. Figure 4 shows amicrosensor at the tip of a flexible polymer catheter suitable for insertion into an arteryor a vein where it can detect one or more target measurands.
The range of physical and chemical measurands for which intravascular sensors
have been developed is extensive. Measurement of arterial and venous blood pressure
is vital in critical care, and micropressure sensors based on semiconductor strain gauges
are used routinely for this task. Such pressure measurement in these patients can be
performed by inserting a flexible polymer catheter into an artery or vein in the wrist.
In newborn babies, the cut umbilical cord provides access to both the arterial andvenous spaces so that catheters can be inserted conveniently. Pressure measurement
catheters are filled with saline and then an external pressure sensor can be attached to thecatheter. Alternatively, a micropressure sensor can be inserted directly into the artery,as shown in Fig. 4. In this case, the sensing element is a semiconductor strain gauge
but interferometric fibre optic sensors based on a Fabry-Perot cavity are also possible.(27)Figure 5 shows such a device, in which the displacement of a flexible diaphragmis measured as a result of changes in the interference pattern between reflected andreference beams. Direct measurement of pressure inside the head (intracranial) or chest(intrapleural) may be required and these sensors are also suitable for this purpose.
The topic within the clinical microsensors field of most intense research effort has
been that of chemical or molecular sensors.(28) Devices have been developed for bloodgases (O2, CO2), ions (H+, K+, Na+, Ca++, Cl−), molecules linked to metabolic processes(glucose, lactate, urea, creatinine), drugs, hormones, and microorganisms involved insensing
element

cathetersupportsignalblood
1.0 mmblood
sampling© Peter Rolfe

Fig. 4. This shows the general format of an intravascular sensor.

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diaphragm
compliance, C

cavityLsdielectric
mirrorpathlength
difference, 2Ls© Peter Rolfe

Fig. 5. This shows an optical pressure sensor based on a Fabry-Perot Cavity. The path length
difference is measured by interferometry.

infection, such as bacteria and viruses. These sensors can be fabricated with microwires (Au,
Ag and Pt), of 1 to 20 µm diameter, in amperometric and potentiometric electrochemical
devices. Figure 6(a) shows a microsensor based on amperometry for insertion into tissue
for measurement of PO2. This working electrode is held at −600 mV with respect to areference electrode positioned adjacent to it.
Amperometric and potentiometric microsensors are also available for more complex
molecules such as metabolites, drugs, and hormones. A glucose microsensor is shownin Fig. 6(b). Here, an enzyme is used to achieve the selective sensing function and so
this is a true biosensor — a device with which sensing is achieved through biologicallyinspired means. This device has glucose oxidase enzyme immobilised within cellulose
acetate, held between an outer glucose-permeable membrane of polyetherurethane andan inner layer of cellulose acetate. The enzyme catalyses the reaction between thesubstrate, glucose, and O2, thereby producing gluconic acid and H2O2. The reaction canbe followed by measuring changes in pH, or PO2 or the production of H2O2. The latterapproach is shown in Fig. 6(b), with the oxidation of H2O2 being carried out at a Au
anode.(29)The measurement of ions (e.g., pH) has been achieved with microscale glass
electrodes, and these have been suitable for both tissue insertion as well as forincorporation into flexible catheters for intravascular use. In addition to the use of glass
membranes for ion sensing, chemically sensitive field-effect transistors, CHEMFETs,
can be used, not only for the measurement of ions but many other target molecules. For
example, enzymes can once again be used to catalyse specific reactions that are then290

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Pt black

2.0 µm

Ag/AgCl cathodePt anode

membranes
800 µmglass insulation
(b)

(a)

Ag/AgCl internal reference
10 µm

polymer + ionophore© Peter Rolfe 2011(c)Fig. 6. Electrochemical sensors for invasive cell and tissue measurements. (a) A microneedle
electrode for intracellular use. (b) An enzyme electrode using a triple-membrane structure
contained within a hypodermic needle. (c) A coated wire ion-selective electrode using a polymer
membrane containing an ionophore (see text).

followed by an ion-selective FET. A further alternative to glass ion-selective membranes
is to use polymer membranes containing an ionophore for the target ion [see Fig 6(c)].
For example, the antibiotic valinomycin can be used as an ionophore for the K+ ion.
The advantage of the polymer ion-selective membrane is that they allow much simpler
fabrication by dip-coating processes. There are many ionophores now available andothers being developed to achieve greater selectivity and stability through the use offluorous phases.(30)
Among the various types of invasive sensor, those designed for intravascularimplantation are of major importance. Figure 7 shows an amperometric microsensor
fabricated within a flexible tube that is suitable for the intravascular monitoring of PO2.
A Pt working electrode and a Ag/AgCl reference electrode together with a buffered KClelectrolyte are contained within an oxygen-permeable polymer chamber. As before,
a current proportional to PO2 is produced by the reduction of the O2 diffusing into thechamber. This general approach has also been used for the fabrication of sensors tomeasure PCO2, H+, K+, glucose, urea, and O2 saturation. In the latter case, the sensor
utilises spectrophotometric principles, based on the oxygen-dependant optical absorption
spectra of oxy- and deoxyhaemoglobin.(31)
Optical sensing is a very important approach for clinical monitoring, and glass or
polymer optical fibres are widely used as the basis of microsensor construction. These
fibres are combined with chromophores or fluorophores to allow a wide range of291

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Supporting
catheter

Reference:chlorided silver
wire

Diffusion
membrane
0.8 mmElectrolyte chamber:

Working electrode:

Buffered KCl

Insulated Pt wire cut
cleanly© Peter Rolfe

Fig. 7. An amperometric electrochemical sensor for invasive use.

species to be sensed, including gases and ions.(32) Figure 8 shows a fibre optic sensor
for O2 measurement. A fluorophore is adsorbed to microbeads within an O2-permeable
envelope. Oxygen diffuses into the envelope and it quenches the fluorescence, which isdetected by means of two optical fibres.
Despite the apparent advantages of invasive methods, the direct placement of anengineered device into a human subject or an animal preparation carries with it certain
disadvantages and design challenges. In clinical situations, there are potential hazards
to be considered, including infection, and this is also a significant concern in animal
preparations. Furthermore, if devices are used within the subject or preparation forextended periods of time, such as a few days or longer, then undesirable interactions
between the device and the biological environment can take place. Such interactions
can produce immunological reactions from the host, as well as adversely affect thedevice performance. Interactions take place between the invasive sensor surfaces andthe surrounding biological fluids; for example, proteins will be adsorbed to the sensor
surface, cells can then adhere and this can lead to a coating of the sensor or encapsulation(see Fig. 9). Improving the biocompatibility of invasive sensors is still necessary and it
is being approached with the use of materials that are derived from natural biomaterials,
such as the cell membrane.(33,34)

4.2

Noninvasive measurement

The key feature of noninvasive measurement is that the whole process should
be nondisturbing to the subject and to the target measurands. In the case of clinicalmonitoring, this means that not only is the patient as free as possible from any
encumbrance of attached devices but also any potential hazards that might be associatedwith the measurement devices are minimised. When this is considered within thecontext of basic biological research, for example, in the study of cells that are cultured

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300 µmanalyte [Q]permeable
membranefluorophore

fibre

λex

λ em

Fig. 8 (left). A photonic sensor based on fluorescence quenching. A fluorophore is adsorbed tomicrobeads. Two optical fibres carry exciting and emission beams (© Peter Rolfe).
Fig. 9 (right). This shows an SEM image of the surface of a membrane-covered intra-arterial
sensor after removal from a patient. Proteins are seen adsorbed to the membrane surface and there
are also cells attached (© Peter Rolfe).

in the laboratory, noninvasive measurement devices seek to avoid disruption of cellular
integrity and behaviour.

4.2.1 Skin surface chemical sensing

An important sensor configuration for noninvasive measurement by attaching sensors
to the patient on the surface of the body is shown in Fig. 10. The skin surface sensor
is depicted, and the generalised structure of the skin, with three of its layers, is shown.
Devices based on this approach are many and varied. Electroanalytical sensors can beused for the estimation of the arterial blood gases, O2 and CO2. Gases diffuse throughskin and if the cutaneous blood flow is maximal, achieved by heat-induced vasodilatation
at around 42°C, then the skin surface PO2 and PCO2 are approximately equal to arterialvalues.(35–37) With these electrically heated devices, O2 is measured with membranecovered Pt-cathode amperometric microsensors, whereas CO2 is derived from the pH of
an electrolyte solution measured with a potentiometric pH sensor.
The indirect estimation of blood gas levels using skin surface devices requires
validation, and this is done by comparison with data from the in vitro analysis ofwithdrawn blood samples.(38) One limitation is in subjects who are in shock and whose
peripheral circulation may be compromised.

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Sensors and Materials, Vol. 24, No. 6 (2012)s.c.

skin surface
devices.b.
dermis© Peter Rolfe

Fig. 10. Noninvasive measurement of tissue chemical constituents can be achieved with skinsurface devices based on electrochemistry or photonics.

4.2.2 Expired gas analysis

For assessing breathing in some patients, it is useful to measure respired gas flow,
volume and composition using micro-anemometers, pneumotachographs, and gas
analysers. The analysis of expired gases has, until quite recently, only focussed on
the measurement of O2 and CO2 for clinical purposes and, to a lesser extent for special
respiratory studies, on N2 and helium. However, the detection of very low concentrationsof expired molecules is of growing interest as a possible noninvasive way to detect
important disease processes within the body. Early work in this area was carried out
with large laboratory instruments, including mass spectrometers, but there are now
possibilities to consider the use of microscale and indeed nanoscale sensor arrays. Suchapproaches are bioinspired, in that they attempt to mimic biological olfactory principles,
embodied in the artificial nose and the e-nose.(39) Fundamental cellular processes could
be studied by this approach, and also, diseases such as kidney failure and cancer may
possibly be detected early.

4.2.3 Photonic tissue interrogation

Near-infrared spectroscopy (NIRS) has been of interest for several decades as apossible means with which to measure blood oxygenation in deep tissues such as thebrain and skeletal muscle.(40,41) Biological tissues are relatively transparent to EM
radiation in the NIR part of the spectrum, that is, for λ = 750–2,500 nm. At the sametime, there are useful absorption bands in this region for biological compounds that areimportant for both basic biological investigations and clinical diagnosis. For example,
oxy- and deoxyhaemoglobin have well-known absorption spectra in the NIR, thereby
allowing blood oxygenation and blood volume determinations in the brain and, insome circumstances, in other organs. Moreover, the intracellular respiratory enzyme

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cytochrome aa3 also has a characteristic absorption band at around 830 nm, offering thepossibility for intracellular oxidative processes to be studied.
In order to perform in vivo NIRS measurements, it is not necessary to use complex
sensors. Instrumentation utilising laser diodes and semiconductor detectors are used,
but the probes attached to the subject are quite simple, usually comprising of fibre optic
bundles. The related technique of pulse oximetry, however, does use sensors with which
to illuminate tissue and detect the diffusely backscattered or transmitted light.(42) The
absorption characteristics of oxy- and deoxyhaemoglobin are then used to calculate
oxygen saturation. An important related technique is pulse glucometry for blood glucose
estimation utilising the optical pulsatile signal from a finger.(43)5.

Ex Vivo/In Vitro Measurement

In addition to the importance of microsensors for continuous real-time clinicalmonitoring, there is growing activity in the development of multiple sensors, both micro
and nano, for multianalyte processing of biological samples, such as blood, urine, saliva,
breath, tissues, and cells. One important area of work aims to create the so-called ‘labon-a-chip’,(14) in many forms, whereby analyses previously carried out with large-scalelaboratory instruments may be performed with simple handheld instruments. Oncesamples are removed from a patient, or from a laboratory preparation, a sample chamberis then used for the analysis and this can be a microfluidic or nanofluidic assembly
incorporating sensing features. The rapid growth in the development of MEMS and,more recently, NEMS techniques has facilitated the creation of compact analyticalplatforms with large-scale sensing capabilities.
Analysis of blood samples can be achieved very near the patient by means of a sensor
chip having an appropriate sample input (see Fig. 11). Once infused or aspirated into thedevice, the sample fluid enters a channel that is configured so as to bring it into contact
with a number of sensors, either sequentially or simultaneously. The simplest device canmicrofluidic
channelschip
sensors

sampleinlet 60 µm

substrateconnector© Peter Rolfe

Fig. 11. Blood samples are taken from a microneedle and aspirated into a microfluidic chamber.
Chip sensors for a variety of biological compounds then perform the analysis.

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Sensors and Materials, Vol. 24, No. 6 (2012)use thick-film sensors, for example, for the measurement of blood gases (PO2, PCO2)
and pH. Semiconductor sensors (ISFET, ChemFET, ImmunoFET) are now used in thistype of device, offering a wide range of possible analytes including gases, ions, proteins,
drugs, and hormones. Optical sensors based on semiconductor chips can also be
included, offering the enormous power of spectrophotometric analysis. Surface plasmon
resonance (SPR) is another powerful optical technique that has been utilised in sensors,for example, for receptor- or antigen-antibody-based sensors.
The application area where there has been the most significant impact of sensor arraysis that of genetic research, which has aimed to establish gene-related predisposition
to disease.(25) The DNA microarray (Fig. 12), or gene chip, consists of an array of
specific segments of single-stranded DNA attached to, typically, a glass slide, and whenthe matching complementary segment of DNA combines with this (hybridisation), a
fluorophore emits radiation. The pattern of fluorescent microdots is then analysed as animage. Microarrays are now being used to study the relationships between genes and the
behaviour of cells, for example, to discover methods for the early detection of cancers,
heart and cerebrovascular diseases, and in tissue engineering.
Copying nature, or biomimetics, provides a rich source of inspiration in sensor arraydevelopment.(17,19) Biological cells forming the basis of sensory systems for vision,
hearing, olfaction (the e-nose(39)), touch, and taste have been emulated in solid-state

singlestrand
DNA

(a)

(b)

Fig. 12. This shows the principle of the DNA microarray. In (a), a single strand of DNA is
attached to glass. When the complementary target is recognised, hybridisation occurs withfluorescence emission. In (b), an array of ssDNAs that have been hybridised displays the patternof recognised targets. This is analysed by computer image processing (© Peter Rolfe).

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Sensors and Materials, Vol. 24, No. 6 (2012)devices, producing biomimetic chips. These sensor arrays often combine processing in
the form of neural networks to achieve very high selectivity, and they are beginning tohave an impact on the new approach to organ repair and replacement within the field of
cellular engineering.(17)6.

Cell Research and Engineering

6.1

Towards nanoparticles

Basic cell research continues to rely on advances in measurement technologies,
including sensors and microscopy. The ability to culture cells in specialised bioreactors
opens up numerous opportunities for micro- and nanosensor designs. Considering
single cells, there are important constraints on sensor design firstly relating to scale andsecondly to the issue of cell disruption (see § 2.3 above). Although microneedle sensors,
based on electrochemistry or photonics, have, over many years, been used for valuable
work in single-cell experiments, their limitations in terms of disruption of cellular
integrity and disturbance of normal behaviour are often questioned.
As with clinical measurement, described above in § 4, measurement of tissue
PO2 and pH has been of major interest for basic physiological systems research and,indeed, in single-cell studies. Microcirculation studies have been based on the use ofO2 microelectrodes, of 1–10 µm diameter, as well as with the use of oxygen-quenchedphosphorescent dyes.(44) In fact, amperometric microsensors have been a key part of thisfield of research since the middle of the last century and devices have been designed for
intracellular as well as skin surface measurement.(45) The microscale Pt cathode, with orwithout a covering gas diffusion membrane, has been, once again, of significant value.
More recent fabrication technologies have led to thin-film and thick-film devices witharrays of cathodes for spatial measurements.
Improvements of microneedle sensors continue to be reported. An ultramicro
Ag/AgCl reference electrode having a glass capillary tip outer diameter of 1.0 µm andinner diameter of 0.5±0.2 µm has been developed and its assessment for intracellularmeasurement reported.(26) A salt bridge was used to make contact between an internal
300-µm-diameter chlorided silver wire rather than fabricating a Ag microtip.
Nanosensors are many and varied and are already having an impact on cell and tissue
research. These sensors are based, among other things, on optical, electric, magnetic,
acoustic, and immunological principles.(46) Nanoparticles, NPs, such as nanoshells,
nanowires, carbon nanotubes (CNT), and quantum dots (QDs), can be used to createsuch sensors. Gold nanoparticles (GNs) can sense their dielectric environment and thisphenomenon can be harnessed in sensor design. The GNs can be functionalised with amolecular recognition element and thereby act as biosensors. Nanoshells, constructedwith a spherical dielectric (e.g., silica) core surrounded by a gold or silver coating ofa few nanometre thickness, can be interrogated using an EM wave at an appropriate
wavelength, from UV to IR, to create a plasmon resonance. This significantly increases
the light intensity, which is of benefit in the application of Raman spectroscopy, andthese plasmon resonance effects are utilised in surface-enhanced Raman spectroscopy,
SERS, and localised surface plasmon resonance, LSPR.

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Nanoparticle sensors have been developed for the measurement of a wide range ofimportant measurands including O2, a number of ions, free radicals, and electric andmagnetic fields. To measure O2, the NP can utilise the well-proven approach basedon fluorescence quenching, by incorporating the standard ruthenium-based indicatordye, [Ru(dpp)3]2+, together with a reference dye into the appropriate NP, e.g., silica.
However, it has been shown that the precise choice of dye and the method of fabrication
are important.(47,48) NP sensors for ions, such as H+, K+, and Ca++, can use hydrophilic
particles such as polyacrylamide, once again loaded with an appropriate dye. A
different approach has been described for pH measurement in which a silica shell isused to encapsulate a rhodamine-β-iso-thiocyanate H+-sensitive core.(49) In this case, awide operating range of pH 3–10 was achieved. This is important as has recently beenreported, and the use of two fluorophores was described to achieve an intracellular pH
range of 3.2–7.0.(50)
Nanowire sensors can utilise the measurement of conductivity change that takes placewhen a macromolecule binds to the nanowire surface. Figure 13 shows how a nanowire
can be used to create antigen-antibody nanosensors, in this case, for the detection of a
microorganism. This approach has been reported for the design of a virus nanosensor.(51)
Semiconductor QDs can be used effectively to act as fluorescent labels, having
advantages over conventional dyes. They are more stable than conventional dyes, havea higher fluorescent yield, and have good resistance to photobleaching. Typically, theQD is a CdSe core surrounded by a ZnS shell, with an overall diameter of 2–10 nm (seeFig. 14). The anticipated elevation of an electron stimulated by a UV photon is given as

antibody∆σ© Peter Rolfe

micro-organism
σ - conductivity

Fig. 13. This illustrates the use of a nanowire to create an antigen-antibody nanosensor for thedetection of microorganisms. As antibodies recognise their target, the conductivity of the nanowirechanges.

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CdSe/ZnS
quantum dot
confined-excitonradius, rc-e-+
Bohr-excitonradius, rB-e

Fig. 14. This shows a quantum dot, QD, having a physical radius less than the Bohr-exciton radius
for the CdSe/ZnS material. The resulting confined-exciton radius leads to emission wavelengths
that are tunable by choosing the QD size, ranging from 2–10 nm (© Peter Rolfe).

the Bohr-exciton radius, rB-e, with an associated emission wavelength of λB-e produced
on return to ground state. However, the QD radius, rc-e, is chosen to be less than rB-e,
which leads to quantum confinement. Consequently, the confined emission wavelength,
λc-e, can be determined by choice of the QD radius. In cells and tissues, QDs can beused to localise or track mesenchymal stem cells when transplanted into tissues or whencocultured with other cell types.(52) QDs are also now used in an increasing range ofsensors,(53) for example, based on fluorescence resonance energy transfer (FRET) for
DNA analysis.(54) Carbon nanotubes are also offering exciting possibilities for sensor
fabrication, due to their unusual electrical and physical properties. A glucose biosensor
has been reported, in which an array of CNTs have been attached to a platinum substrate,
glucose oxidase has been immobilised to the CNTs, and direct electron transfer from the
enzyme to the Pt electrode has been achieved.(55)

6.2

Cells in bioreactors

The fields of cellular engineering, tissue engineering, and tissue regeneration are
evolving rapidly. The culture of cells and tissues allows experiments to be controlled,
and the use of specialised bioreactors has become important for this. The evolutionof bioreactors has also been accompanied by the development of comprehensive
measurement techniques that allow improved observation as well as the control of thecell and tissue culture processes.(56) Figure 15 shows a schematic of a bioreactor inwhich cells are stirred during the culture process. Three options for monitoring theprocesses are indicated: invasive, noninvasive, and shunt. This is very similar to clinicalmonitoring and, in fact, very similar sensors can be used in both applications. Invasive
bioreactor monitoring is carried out with catheter-tip probes for O2, CO2, pH, andglucose, whereas noninvasive monitoring can utilise NIR spectroscopy and fluorescencebased reporter films attached to the internal glass wall or window. Nanoparticle sensors
are also suitable for use in bioreactors to enable the study of complex cellular processes.
The shunt measurement approach involves the transfer of bioreactor contents,
including cells if necessary, through a measurement chamber for analysis. This is a very

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invasive
sensing

sampling‘shunt’noninvasive
sensing

Fig. 15. This depicts a typical rotational bioreactor. Three approaches are possible formeasurement: invasive sensing, noninvasive sensing, and sampling shunt. A variety of sensing
techniques is possible including electroanalysis and a wide range of photonic methods, such as IR
spectroscopy, fluorescence, and optical coherence tomography (© Peter Rolfe).

versatile approach, allowing complex measurements to be made on a periodic basis.
The measurement chamber may use individual sensors or a sensor array, based on thefull range of measurement principles. Once again, NIR spectroscopy has much to offer,
for example, to carry out protein analysis of the withdrawn samples.(57) Vibrational
spectra of proteins are also very important for analysis in the NIR region. The repeating
backbone of proteins is responsible for the particular modes: NH stretch at 3,300 cm−1,
CO stretch at 1,600 to 1,700 cm−1, and NH deformation at 1,500 to 1,550 cm−1.
An example of a specialised flow-through bioreactor is shown in Fig. 16. This
assembly allows the monolayer culture of endothelial cells, with flow of nutrientsand metabolic products. Individual sensors and sensor arrays can be included in thisbioreactor depending on the experimental objectives. For example, metabolic processes
can be followed using electrochemical and photonic sensors.

7.

Discussion and Conclusions

There is enormous diversity in the range of micro- and nanosensors that haveemerged over the last half-century or are now in the course of development. The focus
on the scale of sensors becomes somewhat blurred when one fully takes into account the
totality of a sensing device, but there is no doubt that the move into the nanoscale is ofsubstantial importance and not just a technological oddity. Whilst the roles of macro- andmicrosensors continue to be vital in clinical care, for example to measure blood pressure,the ECG, and blood gases, the potential of the emerging families of nanosensors to makeenormous contributions to basic research is evident. Nanoparticles (shells, spheres,rods, wires, tubes, and dots) are already being used in biological systems, including cells

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sensors 1

Culturefluid flowFisensors 2

sensors 4

cells ortissuesensors 3

100 µmFig. 16. This flow-through bioreactor allows basic studies on cell behaviour, from metabolic
processes to cell signalling. The four sensor assemblies can be chosen according to the applicationand include electrochemical and photonic devices (© Peter Rolfe 2005).

and tissues, and also the lungs and gastrointestinal tract. The potential risks of NPs arerecognised and are the subject of debate and research. For example, CdS quantum dots
can liberate toxic cadmium if exposed to appropriate levels of UV. Clearly, therefore,
every application must be examined thoroughly to evaluate risks.
Nanoscience could also contribute to ongoing developments of microsensors, forexample, to help solve the problems associated with the biocompatibility of invasive
sensors. It is known that interactions between implanted devices and biological fluids
depend on surface properties and these could be tailored with nanomaterial strategies.
The micro/nanoworld offers exciting new prospects to achieve sensing of complex
molecules. Research must continue so that the potential for these technologies to move
us closer to achieving the early detection of serious illness through personal healthscreening in the home as well as in the hospital ultimately becomes a reality.

Acknowledgements
The author is grateful to students and staff who have worked with him at HIT, Harbin,
China, the University of Genova, Italy, and Kanazawa University, Japan.

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