Chemical functionalization of electrodes for detection of gaseous nerve agents with carbon nanotube field-effect transistors

Article abstract of DOI:10.1039/c1cc11517k

An innovative sensor for the detection of nerve agents in the gas phase based on a carbon nanotube field-effect transistor was developed. A high sensitivity to organophosphorus gases was obtained by modifying gold electrodes with specific tailor-made self

Full text of DOI:10.1039/c1cc11517k

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COMMUNICATION
Chemical functionalization of electrodes for detection of gaseous nerve
agents with carbon nanotube field-effect transistorsw
Michael Delalande,a Simon Clavaguera,a Momar Toure,a Alexandre Carella,*a
´
Stephane Lenfant, Dominique Deresmes, Dominique Vuillaume and Jean-Pierre Simonato*
b
b
b
a
Received 16th March 2011, Accepted 7th April 2011
DOI: 10.1039/c1cc11517k
An innovative sensor for the detection of nerve agents in the
gas phase based on a carbon nanotube field-effect transistor
was developed. A high sensitivity to organophosphorus gases was
obtained by modifying gold electrodes with specific tailor-made
self-assembled monolayers.
The sulfur-containing reactive moiety towards OP agents, 3,
was prepared in 2 steps from the scaffold of 1,5,7-trimethyl-
3-azabicyclo[3.3.1]nonane-7-methanol (1).15
In this communication, we report the fabrication of
CNTFET sensors modified at the electrode with organic
species very sensitive to OPs, and their characterization with
the Sarin simulant, namely diphenylchlorophosphate (DPCP).
In order to graft the reactive species onto gold electrodes,
we first prepared the thioacetate precursor 2 by Sonogashira
coupling of 1 with 4-iodo-phenylthioacetate (Scheme 1). However,
molecule 2 is not stable. Even stored in the solid state and at
low temperature, this compound undergoes intermolecular
transesterification which generates by-products.
The advent of international terrorism requires us to prevent
potential attacks. In particular toxic gases are amongst the
simplest materials to use and can lead to tragic consequences.1
Organophosphorus compounds (OPs) are extremely neurotoxic
compounds. They have already been used, like for instance
Sarin during the attack of Tokyo’s subway in 1995. Several
new sensing concepts have recently been reported for the
detection of these chemical warfare agents including colori-
metric and fluorimetric spectroscopies,2–4 chemiresistors,5,6
enzymatic assays,7 gas chromatography/mass spectrometry,8
microcantilevers,9 ion-mobility spectrometry10 and others.
However, there is still a need for highly sensitive and low-cost
sensors. Since the pioneering work of Dai11 and Snow,12
carbon nanotube field-effect transistor (CNTFET) based sensors
have been shown to be good candidates for the detection
of gas traces.13,14 Their effectiveness is mainly ascribed to an
extreme sensitivity to electrostatic changes at the surface of the
CNTs and/or an alteration of the Schottky barrier at the
CNT/metal interface. A recent work described the use of OP
sensitive molecules anchored on silicon nanowires in which
the reaction of a primary alcohol with an OP initiates an intra-
molecular cyclization to generate a quaternary ammonium
salt. This leads to a detectable charge formation in the vicinity
of a semiconductor.15
Therefore, we synthesized the corresponding disulfide 3 in
high yield from 2 by treatment with sodium hydroxide. The
compound 3 is stable for months and allows efficient formation
of SAMs on gold. Its structure was unambiguously supported
by 1H and 13C NMR, IR spectroscopy and mass spectrometry.
SAMs of tolane disulfide derivative 3 on a gold-coated
silicon wafer were obtained by immersing the gold surface
overnight in a freshly prepared 0.7 mM solution of acetone at
room temperature.
The water contact angle of the sample (86.6 Æ 3.71) is
consistent with the literature data.19 Furthermore, the grafting
of molecules on gold was also confirmed by XPS experiment
(see ESIw). Exposure to vapours of DPCP induces a significant
downshift of the contact angle (72.8 Æ 4.41), in agreement with
a higher hydrophilic surface obtained after reaction.
It is now well established that self-assembled monolayers
(SAMs) can drastically modify the work function of gold
electrodes.16–18 We thus chose to create OP sensitive electrodes
by synthesizing specific molecules, and grafting them onto
electrodes of highly sensitive CNTFET transducers.
a CEA Grenoble/LITEN/DTNM, 17 rue des Martyrs,
38054 Grenoble Cedex 9, France.
E-mail: jean-pierre.simonato@cea.fr
b
´
IEMN/CNRS, BP 60069, Avenue Poincare,
59652 Villeneuve d’Ascq Cedex, France
w Electronic supplementary information (ESI) available: Synthesis of
molecule 3; preparation of monolayer and characterization by contact
angle and XPS; device fabrication; carbon nanotube dispersion and
spreading; KPFM measurements. See DOI: 10.1039/c1cc11517k
Scheme 1 Synthesis of 3 from Kemp’s triacid.
c
6048 Chem. Commun., 2011, 47, 6048–6050
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Fig. 1 (a) Schematic structure of the CNTFET with gold electrodes
functionalized by 3 (b) SEM image of the SWCNT network.
Devices with bottom gate/bottom contact geometry (Fig. 1a)
were fabricated on a heavily doped (100) silicon wafer
(n++, 2–5 mO cm) covered by a thermally grown silicon dioxide
layer (100 nm). The source and drain electrodes of the device
were patterned by optical photolithography, followed by
Ti (5 nm) and Au (30 nm) evaporation and lift-off. The length
and width of the CNTFET channels are respectively 10 mm and
1000 mm.
Fig. 2 Normalized response to the CNTFET sensors upon exposure
to DPCP vapours with bare Au electrodes (squares; VDS = À1 V,
VGS = 10 V) and Au electrodes functionalized with 3 (circles;
VDS = À1 V, VGS = 4 V).
A spray-coating technique was then used to spread SWCNT
(CoMoCat SG65) networks on the device from a dispersion of
SWCNTs in N-methylpyrrolidone prepared by sonication and
centrifugation (Fig. 1b). Functionalizaton of the CNTFET
gold electrodes was performed as described above. Electric
measurements were performed on a home-made probe station
with an Agilent 4155C semiconductor parameter analyzer. Devices
were exposed to DPCP vapours below 1 ppm concentration in
air. Experiments were initially carried out using dry air in
order to avoid potential hydrolysis of DPCP, however experi-
ments realized with relative humidity ranging from 0 to 90%
showed similar detection behaviour. The sensors with and
without sensitive SAM were electrically characterized before
and after exposure to DPCP to determine the best value of
Fig. 3 (a) Transfer curves of CNTFET before (grey triangles) and
after (black squares) functionalization of electrodes with 3, and after
exposure to less than 1 ppm of DPCP (white circles). (b) Work
function variation of gold electrodes functionalized with 3 before
and after exposure to DPCP vapours (KPFM measurements).
V
GS (gate-source voltage) to apply in direct monitoring of OP
traces. The transfer characteristics (IDS vs. VGS) were measured
by applying VDS = À1 V as drain-source voltage while sweeping
the gate voltage VGS from +15 to À20 V.
the current. To check variations in the gold electrode work
functions, we carried out Kelvin probe force microscopy
(KPFM) measurements on control samples (see details in ESIw).
The KPFM results shown in Fig. 3b confirm this hypothesis.
After the functionalization with 3, the WF is reduced by ca.
360 meV (reference is the bare Au electrode). After reaction
with DPCP, the WF increases by about 110 meV, leading to a
WF difference of ca. 250 mV compared to bare Au. Since the
WF did not completely return to its initial value (bare Au),
these WF variations cannot totally account for the fact that
the drain current after reaction with DPCP returns to, or even
overcomes, the initial value. It is likely that other mechanism(s)
need to be taken into account. For instance, a gas-induced
doping of the CNTs caused by an electron charge transfer
from CNT to the ammonium–an electron acceptor–could
also explain the shift of the transfer curves towards positive
voltage observed in devices with bare and functionalized
electrodes.11
Fig. 2 shows on a logarithmic scale the responses of CNTFET
devices as a function of time at optimized VGS, with bare and
functionalized gold electrodes.
Pristine CNTFET sensors exhibit sensitivity to DPCP vapours
with an increase of the current by a factor B13 and a time
constant t of 198 s, as determined by fitting the experimental
curve with an exponential function derived from mass action
model.20 Interestingly, after gold electrode functionalization
with 3, the response of CNTFET to DPCP vapours was highly
improved. Current increases much faster (t B10 s) and the
I
on/Ioff ratio reaches a factor of B100.
Fig. 3a shows typical transfer curves of CNTFET before
(triangles) and after (squares) functionalization of the electrodes,
and after exposure to less than 1 ppm of DPCP in air (circles).
After the gold electrode functionalization, a significant decrease
is observed for the ‘‘ON’’ current suggesting an increase in the
Schottky barrier for holes,21 for instance as due to a decrease
of the gold electrode work function (WF). The quaternary
ammonium species generated after DPCP exposure could alter
the dipole layer of the monolayer and thus return the work
function of gold electrodes towards its initial value (i.e. reduce
the Schottky barrier for holes), resulting in an enhancement of
In summary, we report an innovative sensor based on
CNTFET using electrodes functionalized with a SAM of OP
sensitive molecules. The main sensing mechanism is ascribed
to the modification of the Shottky barrier at the gold-SWCNT
interface when OP compounds chemically react with the receptors,
and thus alter the work function of the gold electrodes.
c
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This journal is The Royal Society of Chemistry 2011