# Vapor Sensing Using Conjugated Molecule-Linked   Au Nanoparticles in a Silica Matrix

Authors: Shawn M. Dirk; Stephen W. Howell; B. Katherine Price; Hongyou Fan; Cody Washburn; David R. Wheeler; James M. Tour; Joshua Whiting; R. Joseph Simonson
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

Cross-linked assemblies of nanoparticles are of great value as chemiresistor-type sensors.  Herein, we report a simple method to fabricate a chemiresistor-type sensor that minimizes the swelling transduction mechanism while optimizing the change in dielectric response. Sensors prepared with this methodology showed enhanced chemoselectivity for phosphonates which are useful surrogates for chemical weapons. Chemoselective sensors were fabricated using an aqueous solution of gold nanoparticles that were then cross-linked in the presence of the silica precursor, tetraethyl orthosilicate with theα-,
ω-dithiolate (which is derived from the in situ deprotection of
1,4-di(Phenylethynyl-4′,4″-diacetylthio)-benzene (1)
with wet triethylamine). The cross-linked nanoparticles and silica matrix were drop coated onto interdigitated electrodes having 8 μm spacing. Samples were exposed to a series of analytes including dimethyl methylphosphonate (DMMP), octane, and toluene. A limit of detection was obtained for each analyte. Sensors assembled in this fashion were more sensitive to dimethyl methylphosphonate than to octane  by a factor of 1000.


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## Body

## 1. Introduction

The ability to
effectively detect harmful chemical agents, including chemical weapons, is a
huge concern in today’s world. Numerous
types of chemicals can be used as chemical warfare agents, each with widely
different structures, making detection of them increasingly difficult. One
approach to chemical detection involves the use of sensors comprised of
conductive elements including carbon particles [1], carbon nanotubes [2–4], and metal
nanoparticles [5–14], each
embedded in a nonconductive matrix applied to a set of interdigitated
electrodes (IDEs) to provide sensors known as chemiresistors. In the above cases, the nonconductive matrix
imparts chemoselectivity. Typically in metal nanoparticle-based chemiresistors,
this matrix is the coating on metal nanoparticles.Chemiresistors
may be probed by evaluating the current passing though the sensor as a bias is
applied to one of the IDEs. Several publications have described the signal
transduction mechanism as an activated tunneling model that contains two terms
as shown in the following equation
[10, 15, 16]: (1)σ=σ0e(−δβ)e(−Ea/RT).In this expression, σ is
the electronic conductivity of the film, δ is the interparticle distance, β is the electronic coupling
coefficient (effectively a measure of the density of states (DOS) available
between conducting particles), Ea is the activation energy for electron
transfer between adjacent conducting particles, R is the gas constant, and T is
the absolute temperature. The first exponential factor takes into account the
effect of nanoparticle spacing and the DOS overlaps between nanoparticles, and
the second exponential term is related to the permittivity of the film.In
most cases, the analyte affects both factors of the activated tunneling model serving
both as a solvent, swelling the nanoparticle/matrix, as well as changing the
effective dielectric constant of the sensing film. In the case of low
dielectric constant analytes, the first exponential factor dominates, and
conductivity decreases. In the case of analytes with high dielectric constants,
the second factor dominates, and conductivity increases in the presence of the
analyte.In this article, we report the assembly of amine-capped
nanoparticles in the presence of both thiolate cross-linker and silica
precursor. This nanoparticle cross-linker silica assembly was then deposited
onto interdigitated electrodes and exposed to a series of analyte vapors in
varying concentrations. Selectivity is provided by both the cross-linking
ligand and the silica matrix surrounding the nanoparticles. The silica
precursor was chosen to minimize the contribution of the first exponential
factor of the activated tunneling mechanism by effectively fixing the value ofδ, that is,
by creating a relatively rigid matrix which exhibits little swelling. This
allows the activated tunneling change upon exposure to be dominated by the
second factor in order to increase the sensitivity of the sensor to high
dielectric constant (k) compounds like dimethyl methylphosphonate DMMP (k=22.3) [17].


## 2. Experimental Details

### 2.1. Materials and Methods

All reactions were carried out under a dry nitrogen atmosphere.1H NMR spectra were obtained on a 400 MHz Bruker (Fremont, Calif, USA) Avance-400.
Proton chemical shifts (δ) are reported relative to tetramethylsilane
(TMS). UV-vis measurements were performed on a Perkin-Elmer (Waltham, Mass, USA) Lambda 25.
Centrifugation was performed using a Fisher (Suwanee, Calif, USA)  Scientific Marathon 8 K centrifuge.
Hydrogen tetrachloroaurate (p.a.), tetraoctylammonium bromide (98%), cetyl trimethyl
ammonium bromide (99+%), 1,4-diiodobenzene (98%), trimethylsilylacetylene
(98%), dodecylamine (98%), dichlorodimethylsilane (99%), tetraethyl
orthosilicate (98%), and sulfuric acid (reagent grade) were purchased and used
as received from Acros. (Geel, Belgium) Sodium borohydride (99%), potassium
carbonate (99%), tris(dibenzylideneacetone)dipalladium(0), copper(I) iodide
(99.999%), triphenylphosphine (99%), tetrahydrofuran (anhydrous, 99.9%,
inhibitor free), 1,2-dichloroethane (99%),
and hydrogen peroxide (30% solution in water, ACS Reagent) were purchased and used as
received from Aldrich.
(Milwaukee, Wis, USA) Toluene (certified ACS), methylene chloride (HPLC grade), chloroform (HPLC
grade), carbon disulfide (reagent grade), dimethylacetamide (certified), and
hexanes (HPLC grade) were purchased from Fisher (Suwanee, Calif, USA) and used as received.
Triethylamine (TEA, reagent grade) was purchased from Fisher (Suwanee, Calif, USA) and vacuum was
transferred over calcium hydride prior to use in coupling reactions.
4-Iodobenzenesulphonyl chloride (95%) was purchased and used as received from Lancaster (Windham, NH, USA).


### 2.2. Preparation of Au Nanoparticles

#### 2.2.1. Thiol-Capped Au Nanoparticles

The
preparation of the capped gold nanoparticles followed the Brust method [18].
The synthesis of gold nanoparticles involved the transfer of hydrogen
tetrachloroaurate (0.35 g, 0.87 mmol) dissolved in water (25 mL) to a solution
of tetraoctylammonium bromide (2.19 g, 4.01 mmol) in toluene (80 mL). After the gold salt transferred into the
organic phase, the aqueous phase was discarded. Octanethiol (26μL, 0.15 mmol)
was added, and sodium borohydride (0.38 g, 10.0 mmol) in water (25 mL) was
added too. The
solution turned dark red almost immediately, and after 15 minutes the organic
phase was separated and passed through a 0.45μm Teflon filter. The organic
solvent was almost completely removed by rotary evaporation, and the resulting
solid material was dissolved in a minimal amount of toluene (~3 mL) followed by
precipitation with ethanol. The ethanolic suspension was subjected to
centrifugation (2000 rpm for 10 minutes) to collect the nanoparticles after
decanting the supernate. The precipitation procedure was repeated, and the
nanoparticles were stored at −20°C.


#### 2.2.2. Amine-Capped Au Nanoparticles

The preparation
followed a modified Brust method [18]. The dodecylamine replaced the thiol
component. Tetrachloroaurate (112 mg) was dissolved with water (25 mL) and
poured into a 500 mL round bottomed flask. Tetraoctylammonium bromide (0.37 g,
0.68 mmol) was dissolved into toluene (25 mL) and added to the round bottom
flask with stirring. Once the aqueous layer was clear, a solution of
dodecylamine (574 mg in toluene, 25 mL) capping agent was added. A fresh
solution of sodium borohydride (165 mg in water, 25 mL) was added, and the
color immediately changed to very dark red.
The contents were stirred for 1 hour. The layers were separated, and the
aqueous layer was discarded. The organic solvent was almost completely removed
by rotary evaporation. The solid material was dissolved in a minimal amount of
toluene (~2 mL) and then precipitated with ethanol. The suspension was
subjected to centrifugation (2000 rpm for 10 minutes) to collect the
nanoparticles, and the decanted liquid was discarded. The precipitation procedure was repeated, and
the nanoparticles were stored at −20°C.


#### 2.2.3. Transfer of Au Nanoparticles into Water [19]

Au nanoparticles (100μg) dissolved
in chloroform (3 mL) were added to a 100 mL Erlenmeyer flask.  A solution of cetyl trimethyl ammonium bromide (20μg) dissolved in water (10 mL) was also added to the Erlenmeyer flask. The two-phase
mixture was heated until all the chloroform was removed, and the nanoparticles
were transferred to the aqueous phase. The nanoparticles were characterized
using TEM after the transfer was complete. The nanoparticles had an average
size range of 4–6 nm (radius for
several batches).


### 2.3. Preparation of Oligophenylene Ethynylene

#### 2.3.1. General Pd/Cu Coupling Reaction Procedures [20]

All solids,
including the aryl halide, alkyne, copper iodide, triphenylphosphine, and
palladium catalyst, were
added to an oven dried sealed glass tube. The atmosphere was removed via
vacuum and replaced with dry argon. THF, remaining liquids, and triethylamine
were added during stirring. The reaction was then heated if required.
Upon cooling, the reaction mixture was filtered via gravity filtration to
remove the solids and then diluted with methylene chloride. The reaction
mixture was extracted with aqueous ammonium chloride (×3). The organic layer was dried with magnesium sulfate and then filtered,
and the solvent was removed via rotary evaporation.


#### 2.3.2. General Procedure for the Deprotection of Trimethylsilyl-Protected Alkynes [21]

The protected alkyne, potassium
carbonate (5 equivalents per protected alkyne), methanol, and methylene
chloride were added to a round bottom flask equipped with a stir bar.
The reaction was stirred at room temperature, and upon completion the reaction
mixture was diluted with methylene chloride and washed with brine (3×). The organic layer was dried over anhydrous
magnesium sulfate, and the solvent was removed via rotary evaporation.


#### 2.3.3. Synthesis of 1,4-Bis(Trimethylsilylethynyl)Benzene [22]

Diiodobenzene (3.30 g, 10 mmol),
tris(dibenzylideneacetone)dipalladium(0) (0.28 g, 0.30 mmol), copper(I) iodide
(0.23 g, 1.20 mmol), triphenylphosphine (0.63 g, 2.40 mmol), trimethylsilyl
acetylene (3.2 mL, 22.0 mmol), triethylamine (5.6 mL), and tetrahydrofuran (20 mL) were coupled according to the general coupling procedure above. The crude product was carried directly onto
the next synthetic step.


#### 2.3.4. Synthesis of 1,4(Diethynyl)Benzene [22]

1,4-Bis(trimethylsilylethynyl)benzene was deprotected
with potassium carbonate (8.3 g, 60.0 mmol), methanol (50 mL), and
dichloromethane (50 mL) according to the general deprotection procedure above
to yield 1.5 g (100%) of a crystalline pale white solid,1H NMR (400 MHz, CDCl3) 7.42 (s, 4H), 3.15 (s, 2H).


#### 2.3.5. Synthesis of S-Acetyl-4-Iodothiophenol [23]

4-iodobenzenesulphonyl
chloride (6.1 g, 20.0 mmol) was added to a 250 mL round bottom flask,
and the atmosphere was removed via vacuum and replaced with argon. Zinc powder (4.6 g, 70.0 mmol)
was added to a 500 mL three-neck round bottom flask equipped with a stir
bar and an addition funnel, and the atmosphere was removed via vacuum and
replaced with argon. Dimethylacetamide
(5.6 mL, 60.0 mmol) and 1,2-dichloroethane (100 mL) were added to the
250 mL round bottom flask, and the contents were transferred via cannula into the
addition funnel. 1,2-dichloroethane
(100 mL) and dimethyldichlorosilane were added to the 500 mL three-neck
flask (8.5 mL, 70.0 mmol). The contents
of the addition funnel were added drop-wise over a period of 30 minutes to the
500 mL flask. After the complete
addition, the reaction was heated to 75°C for 2 hours. The reaction mixture was
cooled to room temperature, and potassium carbonate (1.52 g, 11.0 mmol) and
acetyl chloride (5.7 mL, 80.0 mmol) were added to the reaction flask; the
contents were stirred overnight. The
crude reaction was worked up with brine and extracted with dichloromethane. The
organic phase was dried over magnesium sulfate and then filtered, and the solvents were removed via rotary
evaporation to yield 5.4 g (97%) of a crystalline white solid, 1H
NMR (400 MHz, CDCl3) 7.72 (dt, J  =  8.4, 2 Hz, 2H), 7.11 (dt, J  =  8.4 Hz, 2.4 Hz, 2H) 2.41 (s, 3H).Synthesis of 1,4-Di(phenylethynyl-4′,4′′-Diacetylthio)-Benzene (1) [24]
S-Acetyl-4-iodothiophenol
(2.78 g, 10.0 mmol), 1,4(diethynyl)benzene (0.63 g, 5.0 mmol),
tris(dibenzylideneacetone)dipalladium(0) (0.14 g, 0.15 mmol), copper iodide
(0.11 g, 0.60 mmol), triphenylphosphine (0.31 g, 1.20 mmol), triethylamine (5.6 mL), and tetrahydrofuran (20 mL) were coupled according to the general coupling
procedure above. The crude product was purified via column chromatography (1:1
hexanes:dichloromethane) to yield 1.21 g (82%) of a pale brown solid,1H
NMR (400 MHz, CDCl3) 7.61 (m, 8H) 7.46 (d, J  =  8.4 Hz, 4H) 2.45 (s, 6H).


### 2.4. Cross-Linking the Au Nanoparticles

An aqueous solution of Au nanoparticles (0.5 mL) and1 (100 μL, 1 mmol in THF) were
combined. Triethylamine (5 μL) was then
added, and a color change was evident within 5 minutes. Tetraethyl orthosilicate (TEOS, 10 μL) was
then added and agitated. UV-vis measurements were
taken within 5 minutes after the TEA and TEOS additions to confirm
cross-linking.


### 2.5. Cross-Linked Nanoparticle Film Deposition

Interdigitated
electrode devices were cleaned with acetone (30 seconds bath and rinse) and
rinsed with DI water. The device was then submerged in Piranha (1:1 30%
hydrogen peroxide and conc. sulfuric acid) for 2 minutes and rinsed with
copious amounts of DI water.Caution: piranha is a very strong
oxidizer and reacts violently with organics. The surface was dried
with CO2. Prior to Au nanoparticle/silica deposition, the
interdigitated electrode containing substrate was electrically characterized as
open using a digital multimeter. The films were produced by flooding the
surface with the cross-linked Au nanoparticle/silica solution and allowing the
solvent to evaporate. The films that were produced in this manner had
reproducible initial resistances. The thickness was measured with atomic force
microscopy (AFM).


### 2.6. Substrate Exposure to Analytes

Dimethyl methylphosphonate,
toluene, and octane were evaluated as analytes.
The analytes are dissolved in 1 mL CS2 and injected into a
Hewlett Packard 5850 split injection gas chromatograph equipped with 1 meter
100μm ID capillary column coated with a polydimethylsiloxane stationary phase
at ambient temperature. The carrier gas was hydrogen, and the split was 0.33.
The flow rate through the column was 28 sccm. The injection port was heated to
250°C. Gas flow exited the capillary column and entered a custom-made test
fixture described below. Gas left the
test fixture and returned to the HP FID detector heated to 250°C. The injection
volume was 1μL,
and repeated injections were performed with an autoinjection tower for
reproducibility.After the Au nanoparticle/silica assembly, the
functionalized substrate was placed in a custom-made silco-coated stainless steel test fixture where
the assembled substrate sits in a recessed slot. The lid of the test fixture
contained pogo pins that were
connected to a Keithley 6487 current preamp voltage source. The lid included a
milled channel to allow gas flow over the 8μm interdigitated electrodes. The
lid was sealed with an external O-ring. The voltage source and current
measurement instruments were controlled by a custom LabVIEW program. Prior to
analyte exposure, an I(V) trace was performed by cycling the voltage from 0 mV
to 100 mV to −100 mV to 0 mV in order to determine the resistance of the
working sensor device. During an analyte exposure, 100 mV was typically applied
to the substrate, and the current was measured.


## 2.1. Materials and Methods

All reactions were carried out under a dry nitrogen atmosphere.1H NMR spectra were obtained on a 400 MHz Bruker (Fremont, Calif, USA) Avance-400.
Proton chemical shifts (δ) are reported relative to tetramethylsilane
(TMS). UV-vis measurements were performed on a Perkin-Elmer (Waltham, Mass, USA) Lambda 25.
Centrifugation was performed using a Fisher (Suwanee, Calif, USA)  Scientific Marathon 8 K centrifuge.
Hydrogen tetrachloroaurate (p.a.), tetraoctylammonium bromide (98%), cetyl trimethyl
ammonium bromide (99+%), 1,4-diiodobenzene (98%), trimethylsilylacetylene
(98%), dodecylamine (98%), dichlorodimethylsilane (99%), tetraethyl
orthosilicate (98%), and sulfuric acid (reagent grade) were purchased and used
as received from Acros. (Geel, Belgium) Sodium borohydride (99%), potassium
carbonate (99%), tris(dibenzylideneacetone)dipalladium(0), copper(I) iodide
(99.999%), triphenylphosphine (99%), tetrahydrofuran (anhydrous, 99.9%,
inhibitor free), 1,2-dichloroethane (99%),
and hydrogen peroxide (30% solution in water, ACS Reagent) were purchased and used as
received from Aldrich.
(Milwaukee, Wis, USA) Toluene (certified ACS), methylene chloride (HPLC grade), chloroform (HPLC
grade), carbon disulfide (reagent grade), dimethylacetamide (certified), and
hexanes (HPLC grade) were purchased from Fisher (Suwanee, Calif, USA) and used as received.
Triethylamine (TEA, reagent grade) was purchased from Fisher (Suwanee, Calif, USA) and vacuum was
transferred over calcium hydride prior to use in coupling reactions.
4-Iodobenzenesulphonyl chloride (95%) was purchased and used as received from Lancaster (Windham, NH, USA).


## 2.2. Preparation of Au Nanoparticles

### 2.2.1. Thiol-Capped Au Nanoparticles

The
preparation of the capped gold nanoparticles followed the Brust method [18].
The synthesis of gold nanoparticles involved the transfer of hydrogen
tetrachloroaurate (0.35 g, 0.87 mmol) dissolved in water (25 mL) to a solution
of tetraoctylammonium bromide (2.19 g, 4.01 mmol) in toluene (80 mL). After the gold salt transferred into the
organic phase, the aqueous phase was discarded. Octanethiol (26μL, 0.15 mmol)
was added, and sodium borohydride (0.38 g, 10.0 mmol) in water (25 mL) was
added too. The
solution turned dark red almost immediately, and after 15 minutes the organic
phase was separated and passed through a 0.45μm Teflon filter. The organic
solvent was almost completely removed by rotary evaporation, and the resulting
solid material was dissolved in a minimal amount of toluene (~3 mL) followed by
precipitation with ethanol. The ethanolic suspension was subjected to
centrifugation (2000 rpm for 10 minutes) to collect the nanoparticles after
decanting the supernate. The precipitation procedure was repeated, and the
nanoparticles were stored at −20°C.


### 2.2.2. Amine-Capped Au Nanoparticles

The preparation
followed a modified Brust method [18]. The dodecylamine replaced the thiol
component. Tetrachloroaurate (112 mg) was dissolved with water (25 mL) and
poured into a 500 mL round bottomed flask. Tetraoctylammonium bromide (0.37 g,
0.68 mmol) was dissolved into toluene (25 mL) and added to the round bottom
flask with stirring. Once the aqueous layer was clear, a solution of
dodecylamine (574 mg in toluene, 25 mL) capping agent was added. A fresh
solution of sodium borohydride (165 mg in water, 25 mL) was added, and the
color immediately changed to very dark red.
The contents were stirred for 1 hour. The layers were separated, and the
aqueous layer was discarded. The organic solvent was almost completely removed
by rotary evaporation. The solid material was dissolved in a minimal amount of
toluene (~2 mL) and then precipitated with ethanol. The suspension was
subjected to centrifugation (2000 rpm for 10 minutes) to collect the
nanoparticles, and the decanted liquid was discarded. The precipitation procedure was repeated, and
the nanoparticles were stored at −20°C.


### 2.2.3. Transfer of Au Nanoparticles into Water [19]

Au nanoparticles (100μg) dissolved
in chloroform (3 mL) were added to a 100 mL Erlenmeyer flask.  A solution of cetyl trimethyl ammonium bromide (20μg) dissolved in water (10 mL) was also added to the Erlenmeyer flask. The two-phase
mixture was heated until all the chloroform was removed, and the nanoparticles
were transferred to the aqueous phase. The nanoparticles were characterized
using TEM after the transfer was complete. The nanoparticles had an average
size range of 4–6 nm (radius for
several batches).


## 2.2.1. Thiol-Capped Au Nanoparticles

The
preparation of the capped gold nanoparticles followed the Brust method [18].
The synthesis of gold nanoparticles involved the transfer of hydrogen
tetrachloroaurate (0.35 g, 0.87 mmol) dissolved in water (25 mL) to a solution
of tetraoctylammonium bromide (2.19 g, 4.01 mmol) in toluene (80 mL). After the gold salt transferred into the
organic phase, the aqueous phase was discarded. Octanethiol (26μL, 0.15 mmol)
was added, and sodium borohydride (0.38 g, 10.0 mmol) in water (25 mL) was
added too. The
solution turned dark red almost immediately, and after 15 minutes the organic
phase was separated and passed through a 0.45μm Teflon filter. The organic
solvent was almost completely removed by rotary evaporation, and the resulting
solid material was dissolved in a minimal amount of toluene (~3 mL) followed by
precipitation with ethanol. The ethanolic suspension was subjected to
centrifugation (2000 rpm for 10 minutes) to collect the nanoparticles after
decanting the supernate. The precipitation procedure was repeated, and the
nanoparticles were stored at −20°C.


## 2.2.2. Amine-Capped Au Nanoparticles

The preparation
followed a modified Brust method [18]. The dodecylamine replaced the thiol
component. Tetrachloroaurate (112 mg) was dissolved with water (25 mL) and
poured into a 500 mL round bottomed flask. Tetraoctylammonium bromide (0.37 g,
0.68 mmol) was dissolved into toluene (25 mL) and added to the round bottom
flask with stirring. Once the aqueous layer was clear, a solution of
dodecylamine (574 mg in toluene, 25 mL) capping agent was added. A fresh
solution of sodium borohydride (165 mg in water, 25 mL) was added, and the
color immediately changed to very dark red.
The contents were stirred for 1 hour. The layers were separated, and the
aqueous layer was discarded. The organic solvent was almost completely removed
by rotary evaporation. The solid material was dissolved in a minimal amount of
toluene (~2 mL) and then precipitated with ethanol. The suspension was
subjected to centrifugation (2000 rpm for 10 minutes) to collect the
nanoparticles, and the decanted liquid was discarded. The precipitation procedure was repeated, and
the nanoparticles were stored at −20°C.


## 2.2.3. Transfer of Au Nanoparticles into Water [19]

Au nanoparticles (100μg) dissolved
in chloroform (3 mL) were added to a 100 mL Erlenmeyer flask.  A solution of cetyl trimethyl ammonium bromide (20μg) dissolved in water (10 mL) was also added to the Erlenmeyer flask. The two-phase
mixture was heated until all the chloroform was removed, and the nanoparticles
were transferred to the aqueous phase. The nanoparticles were characterized
using TEM after the transfer was complete. The nanoparticles had an average
size range of 4–6 nm (radius for
several batches).


## 2.3. Preparation of Oligophenylene Ethynylene

### 2.3.1. General Pd/Cu Coupling Reaction Procedures [20]

All solids,
including the aryl halide, alkyne, copper iodide, triphenylphosphine, and
palladium catalyst, were
added to an oven dried sealed glass tube. The atmosphere was removed via
vacuum and replaced with dry argon. THF, remaining liquids, and triethylamine
were added during stirring. The reaction was then heated if required.
Upon cooling, the reaction mixture was filtered via gravity filtration to
remove the solids and then diluted with methylene chloride. The reaction
mixture was extracted with aqueous ammonium chloride (×3). The organic layer was dried with magnesium sulfate and then filtered,
and the solvent was removed via rotary evaporation.


### 2.3.2. General Procedure for the Deprotection of Trimethylsilyl-Protected Alkynes [21]

The protected alkyne, potassium
carbonate (5 equivalents per protected alkyne), methanol, and methylene
chloride were added to a round bottom flask equipped with a stir bar.
The reaction was stirred at room temperature, and upon completion the reaction
mixture was diluted with methylene chloride and washed with brine (3×). The organic layer was dried over anhydrous
magnesium sulfate, and the solvent was removed via rotary evaporation.


### 2.3.3. Synthesis of 1,4-Bis(Trimethylsilylethynyl)Benzene [22]

Diiodobenzene (3.30 g, 10 mmol),
tris(dibenzylideneacetone)dipalladium(0) (0.28 g, 0.30 mmol), copper(I) iodide
(0.23 g, 1.20 mmol), triphenylphosphine (0.63 g, 2.40 mmol), trimethylsilyl
acetylene (3.2 mL, 22.0 mmol), triethylamine (5.6 mL), and tetrahydrofuran (20 mL) were coupled according to the general coupling procedure above. The crude product was carried directly onto
the next synthetic step.


### 2.3.4. Synthesis of 1,4(Diethynyl)Benzene [22]

1,4-Bis(trimethylsilylethynyl)benzene was deprotected
with potassium carbonate (8.3 g, 60.0 mmol), methanol (50 mL), and
dichloromethane (50 mL) according to the general deprotection procedure above
to yield 1.5 g (100%) of a crystalline pale white solid,1H NMR (400 MHz, CDCl3) 7.42 (s, 4H), 3.15 (s, 2H).


### 2.3.5. Synthesis of S-Acetyl-4-Iodothiophenol [23]

4-iodobenzenesulphonyl
chloride (6.1 g, 20.0 mmol) was added to a 250 mL round bottom flask,
and the atmosphere was removed via vacuum and replaced with argon. Zinc powder (4.6 g, 70.0 mmol)
was added to a 500 mL three-neck round bottom flask equipped with a stir
bar and an addition funnel, and the atmosphere was removed via vacuum and
replaced with argon. Dimethylacetamide
(5.6 mL, 60.0 mmol) and 1,2-dichloroethane (100 mL) were added to the
250 mL round bottom flask, and the contents were transferred via cannula into the
addition funnel. 1,2-dichloroethane
(100 mL) and dimethyldichlorosilane were added to the 500 mL three-neck
flask (8.5 mL, 70.0 mmol). The contents
of the addition funnel were added drop-wise over a period of 30 minutes to the
500 mL flask. After the complete
addition, the reaction was heated to 75°C for 2 hours. The reaction mixture was
cooled to room temperature, and potassium carbonate (1.52 g, 11.0 mmol) and
acetyl chloride (5.7 mL, 80.0 mmol) were added to the reaction flask; the
contents were stirred overnight. The
crude reaction was worked up with brine and extracted with dichloromethane. The
organic phase was dried over magnesium sulfate and then filtered, and the solvents were removed via rotary
evaporation to yield 5.4 g (97%) of a crystalline white solid, 1H
NMR (400 MHz, CDCl3) 7.72 (dt, J  =  8.4, 2 Hz, 2H), 7.11 (dt, J  =  8.4 Hz, 2.4 Hz, 2H) 2.41 (s, 3H).Synthesis of 1,4-Di(phenylethynyl-4′,4′′-Diacetylthio)-Benzene (1) [24]
S-Acetyl-4-iodothiophenol
(2.78 g, 10.0 mmol), 1,4(diethynyl)benzene (0.63 g, 5.0 mmol),
tris(dibenzylideneacetone)dipalladium(0) (0.14 g, 0.15 mmol), copper iodide
(0.11 g, 0.60 mmol), triphenylphosphine (0.31 g, 1.20 mmol), triethylamine (5.6 mL), and tetrahydrofuran (20 mL) were coupled according to the general coupling
procedure above. The crude product was purified via column chromatography (1:1
hexanes:dichloromethane) to yield 1.21 g (82%) of a pale brown solid,1H
NMR (400 MHz, CDCl3) 7.61 (m, 8H) 7.46 (d, J  =  8.4 Hz, 4H) 2.45 (s, 6H).


## 2.3.1. General Pd/Cu Coupling Reaction Procedures [20]

All solids,
including the aryl halide, alkyne, copper iodide, triphenylphosphine, and
palladium catalyst, were
added to an oven dried sealed glass tube. The atmosphere was removed via
vacuum and replaced with dry argon. THF, remaining liquids, and triethylamine
were added during stirring. The reaction was then heated if required.
Upon cooling, the reaction mixture was filtered via gravity filtration to
remove the solids and then diluted with methylene chloride. The reaction
mixture was extracted with aqueous ammonium chloride (×3). The organic layer was dried with magnesium sulfate and then filtered,
and the solvent was removed via rotary evaporation.


## 2.3.2. General Procedure for the Deprotection of Trimethylsilyl-Protected Alkynes [21]

The protected alkyne, potassium
carbonate (5 equivalents per protected alkyne), methanol, and methylene
chloride were added to a round bottom flask equipped with a stir bar.
The reaction was stirred at room temperature, and upon completion the reaction
mixture was diluted with methylene chloride and washed with brine (3×). The organic layer was dried over anhydrous
magnesium sulfate, and the solvent was removed via rotary evaporation.


## 2.3.3. Synthesis of 1,4-Bis(Trimethylsilylethynyl)Benzene [22]

Diiodobenzene (3.30 g, 10 mmol),
tris(dibenzylideneacetone)dipalladium(0) (0.28 g, 0.30 mmol), copper(I) iodide
(0.23 g, 1.20 mmol), triphenylphosphine (0.63 g, 2.40 mmol), trimethylsilyl
acetylene (3.2 mL, 22.0 mmol), triethylamine (5.6 mL), and tetrahydrofuran (20 mL) were coupled according to the general coupling procedure above. The crude product was carried directly onto
the next synthetic step.


## 2.3.4. Synthesis of 1,4(Diethynyl)Benzene [22]

1,4-Bis(trimethylsilylethynyl)benzene was deprotected
with potassium carbonate (8.3 g, 60.0 mmol), methanol (50 mL), and
dichloromethane (50 mL) according to the general deprotection procedure above
to yield 1.5 g (100%) of a crystalline pale white solid,1H NMR (400 MHz, CDCl3) 7.42 (s, 4H), 3.15 (s, 2H).


## 2.3.5. Synthesis of S-Acetyl-4-Iodothiophenol [23]

4-iodobenzenesulphonyl
chloride (6.1 g, 20.0 mmol) was added to a 250 mL round bottom flask,
and the atmosphere was removed via vacuum and replaced with argon. Zinc powder (4.6 g, 70.0 mmol)
was added to a 500 mL three-neck round bottom flask equipped with a stir
bar and an addition funnel, and the atmosphere was removed via vacuum and
replaced with argon. Dimethylacetamide
(5.6 mL, 60.0 mmol) and 1,2-dichloroethane (100 mL) were added to the
250 mL round bottom flask, and the contents were transferred via cannula into the
addition funnel. 1,2-dichloroethane
(100 mL) and dimethyldichlorosilane were added to the 500 mL three-neck
flask (8.5 mL, 70.0 mmol). The contents
of the addition funnel were added drop-wise over a period of 30 minutes to the
500 mL flask. After the complete
addition, the reaction was heated to 75°C for 2 hours. The reaction mixture was
cooled to room temperature, and potassium carbonate (1.52 g, 11.0 mmol) and
acetyl chloride (5.7 mL, 80.0 mmol) were added to the reaction flask; the
contents were stirred overnight. The
crude reaction was worked up with brine and extracted with dichloromethane. The
organic phase was dried over magnesium sulfate and then filtered, and the solvents were removed via rotary
evaporation to yield 5.4 g (97%) of a crystalline white solid, 1H
NMR (400 MHz, CDCl3) 7.72 (dt, J  =  8.4, 2 Hz, 2H), 7.11 (dt, J  =  8.4 Hz, 2.4 Hz, 2H) 2.41 (s, 3H).Synthesis of 1,4-Di(phenylethynyl-4′,4′′-Diacetylthio)-Benzene (1) [24]
S-Acetyl-4-iodothiophenol
(2.78 g, 10.0 mmol), 1,4(diethynyl)benzene (0.63 g, 5.0 mmol),
tris(dibenzylideneacetone)dipalladium(0) (0.14 g, 0.15 mmol), copper iodide
(0.11 g, 0.60 mmol), triphenylphosphine (0.31 g, 1.20 mmol), triethylamine (5.6 mL), and tetrahydrofuran (20 mL) were coupled according to the general coupling
procedure above. The crude product was purified via column chromatography (1:1
hexanes:dichloromethane) to yield 1.21 g (82%) of a pale brown solid,1H
NMR (400 MHz, CDCl3) 7.61 (m, 8H) 7.46 (d, J  =  8.4 Hz, 4H) 2.45 (s, 6H).


## 2.4. Cross-Linking the Au Nanoparticles

An aqueous solution of Au nanoparticles (0.5 mL) and1 (100 μL, 1 mmol in THF) were
combined. Triethylamine (5 μL) was then
added, and a color change was evident within 5 minutes. Tetraethyl orthosilicate (TEOS, 10 μL) was
then added and agitated. UV-vis measurements were
taken within 5 minutes after the TEA and TEOS additions to confirm
cross-linking.


## 2.5. Cross-Linked Nanoparticle Film Deposition

Interdigitated
electrode devices were cleaned with acetone (30 seconds bath and rinse) and
rinsed with DI water. The device was then submerged in Piranha (1:1 30%
hydrogen peroxide and conc. sulfuric acid) for 2 minutes and rinsed with
copious amounts of DI water.Caution: piranha is a very strong
oxidizer and reacts violently with organics. The surface was dried
with CO2. Prior to Au nanoparticle/silica deposition, the
interdigitated electrode containing substrate was electrically characterized as
open using a digital multimeter. The films were produced by flooding the
surface with the cross-linked Au nanoparticle/silica solution and allowing the
solvent to evaporate. The films that were produced in this manner had
reproducible initial resistances. The thickness was measured with atomic force
microscopy (AFM).


## 2.6. Substrate Exposure to Analytes

Dimethyl methylphosphonate,
toluene, and octane were evaluated as analytes.
The analytes are dissolved in 1 mL CS2 and injected into a
Hewlett Packard 5850 split injection gas chromatograph equipped with 1 meter
100μm ID capillary column coated with a polydimethylsiloxane stationary phase
at ambient temperature. The carrier gas was hydrogen, and the split was 0.33.
The flow rate through the column was 28 sccm. The injection port was heated to
250°C. Gas flow exited the capillary column and entered a custom-made test
fixture described below. Gas left the
test fixture and returned to the HP FID detector heated to 250°C. The injection
volume was 1μL,
and repeated injections were performed with an autoinjection tower for
reproducibility.After the Au nanoparticle/silica assembly, the
functionalized substrate was placed in a custom-made silco-coated stainless steel test fixture where
the assembled substrate sits in a recessed slot. The lid of the test fixture
contained pogo pins that were
connected to a Keithley 6487 current preamp voltage source. The lid included a
milled channel to allow gas flow over the 8μm interdigitated electrodes. The
lid was sealed with an external O-ring. The voltage source and current
measurement instruments were controlled by a custom LabVIEW program. Prior to
analyte exposure, an I(V) trace was performed by cycling the voltage from 0 mV
to 100 mV to −100 mV to 0 mV in order to determine the resistance of the
working sensor device. During an analyte exposure, 100 mV was typically applied
to the substrate, and the current was measured.


## 3. Results and Discussion

### 3.1. Preparation of the Nanoparticle/Silica Film

Initially
Brust-type [18] nanoparticles capped with alkyl thiols were transferred into
water by the method of Fan and used to create a nanoparticle film. The strategy
employed was to mix the nanoparticles, a silica precursor (tetraethyl orthosilicate),
and a cross-linking molecule 1 together in the presence of a reagent to
facilitate both the hydrolysis of the tetraethyl orthosilicate and the
deprotection of the thioacetate groups as shown in Scheme 1.Scheme 1
Shows the assembly process used to form phenylene ethynylene crosslinked Au nanoparticles in a silica matrix.Two
common methods used to deprotect thiol acetate groups include NH4OH
in THF [25] and H2SO4 in CH2Cl2/CH3OH
[26]. In our case, we were limited to water as the solvent, and attempts to
deprotect the thiol acetate groups with these protocols in the presence of the
silica precursor tetraethyl orthosilicate and the amine-capped Au nanoparticles
proved unsuccessful. Attempted ammonium hydroxide which facilitated
deprotection of the thioacetate group resulted in a very fast TEOS condensation
rate, and a silica gel resulted within seconds of the base addition. This
gellation precluded the desired organic cross-linking of the metal
nanoparticles. In the case of the H2SO4 facilitated
deprotection, the rate of condensation of the hydrolyzed tetraethyl
orthosilicate was slower than the base-catalyzed case; however, the
deprotection of the thioacetate in water was still slower, so that adequate
cross-linking or the nanoparticles were again precluded (as determined by NMR).Sharporenko
et al. successfully demonstrated that a thiol acetate group could be
deprotected with wet triethylamine in
situ to form a self-assembled monolayer [27]. The use of this deprotection method resulted
in a useable ratio of the TEOS condensation rate to thiol acetate deprotection
rate in the work described here. A color change was apparent within minutes,
confirming cross-linking of the Au nanoparticles (NPs) due to the expected
change in plasmon resonance. If the
solution was allowed to react (cross-link) for several more minutes (~10 minutes),
the nanoparticles precipitated from solution.
This color change was confirmed and measured with the use of UV-visible (UV-vis) absorption
spectroscopy of the nanoparticles in aqueous solution. Absorption spectra of
the NPs were acquired throughout the cross-linking process to observe changes
in the plasmon resonance. A shift correlates to a change in resonance
attributed to the cross-linking of the NPs. These shifts are observed in the
spectra displayed in Figure 1.UV-vis spectra of (a) alkyl amine capped NPs only and crosslinked, and (b) alkanethiol-capped NPs only and the1-crosslinked Au-NP. The shifts are 45 and 34 nm, respectively.
(a)(b)We
found that dodecylamine-capped Au nanoparticles were much easier to cross-link
(and gave a larger red shift in UV-vis) than the corresponding thiol-capped Au
nanoparticles, presumably due to faster ligand exchange kinetics.
Dodecylamine-capped Au nanoparticles were prepared according to the method of
Brust [18] except that dodecylamine was used in place of the alkanethiol
capping agent as shown in Scheme 2.Scheme 2
Preparation of alkyl amine gold nanoparticles by a modification of the Brust method [18].Cross-linked Au/silica films of dodecylamine-capped
nanoparticles were prepared by drop-casting onto Piranha-cleaned 8μm Au
interdigitated electrodes after a period of 5 minutes from the addition of
triethylamine. The solvent was allowed to evaporate
and air dry for a
period of no less than 3 hours. The drop cast films were characterized using
AFM to measure the thickness of the assembled film. The typical film thickness was determined to
be 25 nm.


### 3.2. The Use of the Dodecylamine-Capped Nanoparticle/Silica Films as Sensor Elements

The coated
substrate was mounted into a test fixture that was placed inside a GC equipped
with a flame ionization detector (FID) in line with the column (see Scheme3). The device was exposed to various
concentrations of DMMP, toluene, and octane dissolved in carbon disulfide.Scheme 3
The layout of the test structure used to determine analyte selectivity and sensitivity.Figure2 shows a typical plot of the normalized (The plot was normalized by dividing the change in current upon exposure to analyte by the initial current.) change in current, as a 1000-ppm-DMMP
(904 ppm maximum solution concentration (v/v), not taking the GC split into
account) sample is injected onto the column. The first peak observed is the CS2 solvent peak followed by a second slightly broader DMMP peak. The blue trace is
the normalized change in signal from an interdigitated electrode that was
functionalized with Au nanoparticles and the silica precursor TEOS. The red
trace is the normalized change in signal from an interdigitated electrode that
was functionalized with Au nanoparticles cross-linked with 1
and the silica precursor TEOS. The 1-crosslinked
nanoparticle-functionalized device was more sensitive to the same concentration
of DMMP, as reflected by the eight-fold increase in detector response.Figure 2
1000 ppm DMMP exposure (1000 ppm max) to TEOS  +  NP (blue line) and TEOS  +  PE  +  NP films (red line). The inset shows an expansion of the DMMP peak.Figure3 shows the FID response to the same 1000 ppm DMMP injection both with and
without the custom-made test fixture in line with the FID. The peak width of the CS2 is not
dramatically widened with the test fixture in line; however, the DMMP peak
appears never to reach
the FID detector. This could be evidence
of the gettering ability of the assembled film. Based on this evidence, part of
the film selectivity to DMMP could arise from the silica matrix surrounding the
Au NPs.Figure 3
The response of the FID detector to a 1000 ppm DMMP in
CS2 injection with (▪), and without a TEOS  +  NP  +  PE         (◆) film before the FID detector. The inset shows an expansion of the DMMP peak.The
normalized signal response to octane, toluene, and DMMP at various
concentrations is shown in Figure4 and was used to determine the sensitivity
of the electrode sensor [13]. Sensitivity is usually defined as the slope
of the output signal for a given set of concentrations; as such sensitivity of
the assembled sensor was determined for octane (8.0 e-4% change/ppm),
toluene (1.4 e-2% change/ppm), and DMMP (8.0 e-1%
change/ppm) between the range of 100 ppm and 50 000 ppm.Figure 4
Comparison of the response of films consisting of TEOS  +  PE  +  NP when exposed to octane (◆), toluene (▴), and DMMP (▪). The response of the film is presented as a log/log plot for clarity.The
limits of detection (LoD) for octane (190 ppm in CS2), toluene (80 ppm in CS2), and DMMP (60 ppm in CS2) were determined
from a plot of the log normalized signal change versus log concentration. Based
on this plot, a simple extrapolation was used in combination with an arbitrary
signal to noise ratio of five (corresponding to 10% noise) to determine the
LoD. In practice, these LoD values were not realized in our experiments likely
due to a nonoptimized test cell configuration that leads to band broadening and
sensitivity loss. No effort was made to reduce the dead volume of the sensor
cell.The
nanoparticles used in the sensing matrix had an average radius size of 4–6 nm, and no
attempt was made to synthesize nanoparticles of different sizes or of different
metals. Presumably, the electron transport properties of the sensing film could
be tailored with modifications of both the metal as well as the size of the
nanoparticles and will be the subject of future work.


### 3.3. Role of the Au Nanoparticle to Silica Ratio

Several experiments were performed where the Au nanoparticle to TEOS ratio
was modified, and the IDEs were evaluated upon exposure to DMMP, as shown in Figure5. Experiments with large Au
nanoparticle:TEOS ratios show results consistent with a conduction mechanism
dominated by the first exponential factor of the activated tunneling model.
Accordingly, the conductivity was decreased in the presence of both the DMMP (k=22.3) and the
solvent CS2 (k=2.6) [28]. Experiments with small Au
nanoparticle:TEOS ratios demonstrated results consistent with a conduction
mechanism dominated by the second exponential factor of the activated tunneling
model, and showed an increase in conductivity for DMMP. The sample prepared
with 0.31 mg Au/10 mL TEOS exhibited the most sensitivity to DMMP, whereas the
smallest Au nanoparticle:TEOS ratio (0.16 mg Au/10 mL TEOS) was not as
sensitive. In the latter case, the sensitivity to DMMP is attributed to slow
diffusion of the analyte into the sensing film during this nonequilibrium
measurement, due to the highly cross-linked nature of the sensing matrix.Response of gold nanoparticle/10 mL TEOS coated IDEs when exposed to 10 000 ppm DMMP in CS2. The amount of Au nanoparticles was decreased while keeping the TEOS amount fixed. (a) contained 1.25 mg Au NP:10 uL TEOS, (b) contained 0.63 mg Au NP:10 uL TEOS, (c) contained 0.31 mg Au NP:10 uL TEOS, (d) contained 0.16 mg Au NP:10 uL TEOS.
(a)(b)(c)(d)


## 3.1. Preparation of the Nanoparticle/Silica Film

Initially
Brust-type [18] nanoparticles capped with alkyl thiols were transferred into
water by the method of Fan and used to create a nanoparticle film. The strategy
employed was to mix the nanoparticles, a silica precursor (tetraethyl orthosilicate),
and a cross-linking molecule 1 together in the presence of a reagent to
facilitate both the hydrolysis of the tetraethyl orthosilicate and the
deprotection of the thioacetate groups as shown in Scheme 1.Scheme 1
Shows the assembly process used to form phenylene ethynylene crosslinked Au nanoparticles in a silica matrix.Two
common methods used to deprotect thiol acetate groups include NH4OH
in THF [25] and H2SO4 in CH2Cl2/CH3OH
[26]. In our case, we were limited to water as the solvent, and attempts to
deprotect the thiol acetate groups with these protocols in the presence of the
silica precursor tetraethyl orthosilicate and the amine-capped Au nanoparticles
proved unsuccessful. Attempted ammonium hydroxide which facilitated
deprotection of the thioacetate group resulted in a very fast TEOS condensation
rate, and a silica gel resulted within seconds of the base addition. This
gellation precluded the desired organic cross-linking of the metal
nanoparticles. In the case of the H2SO4 facilitated
deprotection, the rate of condensation of the hydrolyzed tetraethyl
orthosilicate was slower than the base-catalyzed case; however, the
deprotection of the thioacetate in water was still slower, so that adequate
cross-linking or the nanoparticles were again precluded (as determined by NMR).Sharporenko
et al. successfully demonstrated that a thiol acetate group could be
deprotected with wet triethylamine in
situ to form a self-assembled monolayer [27]. The use of this deprotection method resulted
in a useable ratio of the TEOS condensation rate to thiol acetate deprotection
rate in the work described here. A color change was apparent within minutes,
confirming cross-linking of the Au nanoparticles (NPs) due to the expected
change in plasmon resonance. If the
solution was allowed to react (cross-link) for several more minutes (~10 minutes),
the nanoparticles precipitated from solution.
This color change was confirmed and measured with the use of UV-visible (UV-vis) absorption
spectroscopy of the nanoparticles in aqueous solution. Absorption spectra of
the NPs were acquired throughout the cross-linking process to observe changes
in the plasmon resonance. A shift correlates to a change in resonance
attributed to the cross-linking of the NPs. These shifts are observed in the
spectra displayed in Figure 1.UV-vis spectra of (a) alkyl amine capped NPs only and crosslinked, and (b) alkanethiol-capped NPs only and the1-crosslinked Au-NP. The shifts are 45 and 34 nm, respectively.
(a)(b)We
found that dodecylamine-capped Au nanoparticles were much easier to cross-link
(and gave a larger red shift in UV-vis) than the corresponding thiol-capped Au
nanoparticles, presumably due to faster ligand exchange kinetics.
Dodecylamine-capped Au nanoparticles were prepared according to the method of
Brust [18] except that dodecylamine was used in place of the alkanethiol
capping agent as shown in Scheme 2.Scheme 2
Preparation of alkyl amine gold nanoparticles by a modification of the Brust method [18].Cross-linked Au/silica films of dodecylamine-capped
nanoparticles were prepared by drop-casting onto Piranha-cleaned 8μm Au
interdigitated electrodes after a period of 5 minutes from the addition of
triethylamine. The solvent was allowed to evaporate
and air dry for a
period of no less than 3 hours. The drop cast films were characterized using
AFM to measure the thickness of the assembled film. The typical film thickness was determined to
be 25 nm.


## 3.2. The Use of the Dodecylamine-Capped Nanoparticle/Silica Films as Sensor Elements

The coated
substrate was mounted into a test fixture that was placed inside a GC equipped
with a flame ionization detector (FID) in line with the column (see Scheme3). The device was exposed to various
concentrations of DMMP, toluene, and octane dissolved in carbon disulfide.Scheme 3
The layout of the test structure used to determine analyte selectivity and sensitivity.Figure2 shows a typical plot of the normalized (The plot was normalized by dividing the change in current upon exposure to analyte by the initial current.) change in current, as a 1000-ppm-DMMP
(904 ppm maximum solution concentration (v/v), not taking the GC split into
account) sample is injected onto the column. The first peak observed is the CS2 solvent peak followed by a second slightly broader DMMP peak. The blue trace is
the normalized change in signal from an interdigitated electrode that was
functionalized with Au nanoparticles and the silica precursor TEOS. The red
trace is the normalized change in signal from an interdigitated electrode that
was functionalized with Au nanoparticles cross-linked with 1
and the silica precursor TEOS. The 1-crosslinked
nanoparticle-functionalized device was more sensitive to the same concentration
of DMMP, as reflected by the eight-fold increase in detector response.Figure 2
1000 ppm DMMP exposure (1000 ppm max) to TEOS  +  NP (blue line) and TEOS  +  PE  +  NP films (red line). The inset shows an expansion of the DMMP peak.Figure3 shows the FID response to the same 1000 ppm DMMP injection both with and
without the custom-made test fixture in line with the FID. The peak width of the CS2 is not
dramatically widened with the test fixture in line; however, the DMMP peak
appears never to reach
the FID detector. This could be evidence
of the gettering ability of the assembled film. Based on this evidence, part of
the film selectivity to DMMP could arise from the silica matrix surrounding the
Au NPs.Figure 3
The response of the FID detector to a 1000 ppm DMMP in
CS2 injection with (▪), and without a TEOS  +  NP  +  PE         (◆) film before the FID detector. The inset shows an expansion of the DMMP peak.The
normalized signal response to octane, toluene, and DMMP at various
concentrations is shown in Figure4 and was used to determine the sensitivity
of the electrode sensor [13]. Sensitivity is usually defined as the slope
of the output signal for a given set of concentrations; as such sensitivity of
the assembled sensor was determined for octane (8.0 e-4% change/ppm),
toluene (1.4 e-2% change/ppm), and DMMP (8.0 e-1%
change/ppm) between the range of 100 ppm and 50 000 ppm.Figure 4
Comparison of the response of films consisting of TEOS  +  PE  +  NP when exposed to octane (◆), toluene (▴), and DMMP (▪). The response of the film is presented as a log/log plot for clarity.The
limits of detection (LoD) for octane (190 ppm in CS2), toluene (80 ppm in CS2), and DMMP (60 ppm in CS2) were determined
from a plot of the log normalized signal change versus log concentration. Based
on this plot, a simple extrapolation was used in combination with an arbitrary
signal to noise ratio of five (corresponding to 10% noise) to determine the
LoD. In practice, these LoD values were not realized in our experiments likely
due to a nonoptimized test cell configuration that leads to band broadening and
sensitivity loss. No effort was made to reduce the dead volume of the sensor
cell.The
nanoparticles used in the sensing matrix had an average radius size of 4–6 nm, and no
attempt was made to synthesize nanoparticles of different sizes or of different
metals. Presumably, the electron transport properties of the sensing film could
be tailored with modifications of both the metal as well as the size of the
nanoparticles and will be the subject of future work.


## 3.3. Role of the Au Nanoparticle to Silica Ratio

Several experiments were performed where the Au nanoparticle to TEOS ratio
was modified, and the IDEs were evaluated upon exposure to DMMP, as shown in Figure5. Experiments with large Au
nanoparticle:TEOS ratios show results consistent with a conduction mechanism
dominated by the first exponential factor of the activated tunneling model.
Accordingly, the conductivity was decreased in the presence of both the DMMP (k=22.3) and the
solvent CS2 (k=2.6) [28]. Experiments with small Au
nanoparticle:TEOS ratios demonstrated results consistent with a conduction
mechanism dominated by the second exponential factor of the activated tunneling
model, and showed an increase in conductivity for DMMP. The sample prepared
with 0.31 mg Au/10 mL TEOS exhibited the most sensitivity to DMMP, whereas the
smallest Au nanoparticle:TEOS ratio (0.16 mg Au/10 mL TEOS) was not as
sensitive. In the latter case, the sensitivity to DMMP is attributed to slow
diffusion of the analyte into the sensing film during this nonequilibrium
measurement, due to the highly cross-linked nature of the sensing matrix.Response of gold nanoparticle/10 mL TEOS coated IDEs when exposed to 10 000 ppm DMMP in CS2. The amount of Au nanoparticles was decreased while keeping the TEOS amount fixed. (a) contained 1.25 mg Au NP:10 uL TEOS, (b) contained 0.63 mg Au NP:10 uL TEOS, (c) contained 0.31 mg Au NP:10 uL TEOS, (d) contained 0.16 mg Au NP:10 uL TEOS.
(a)(b)(c)(d)


## 4. Conclusion

Interdigitated
electrodes were functionalized by a flood coating technique, using dodecylamine-capped
Au nanoparticles cross-linked with a phenylene ethynylene oligomer in a silica
matrix. Control of the nanoparticle:TEOS
ratio enables control over which factor dominates in the activated tunneling
model. Sensors prepared with 0.31 mg Au
nanoparticles:10μL TEOS exhibited the greatest increase in conductance upon
exposure to DMMP, whereas sensors prepared with 1.25 mg Au nanoparticles:10μL
TEOS showed decreases in conductivity upon exposure to DMMP. Devices
functionalized with 0.31 mg Au nanoparticles:10μL TEOS were 1000 times more
sensitive to DMMP than to octane.



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*Source: 481270-2008-12-30.xml*