Sensing of alkylating agents using organic field-effect transistors

Article abstract of DOI:10.1002/adfm.200901598

Alkylating agents are simple and reactive molecules that are commonly used in many and diverse fields, such as organic synthesis, medicine and agriculture. Some highly reactive alkylating species are also being used as blister chemical warfare agents. The detection and identification of alkylating agent is not a trivial issue because of their high reactivity and simple structure. Here, a novel polythiophene derivative that is capable of reacting with alkylating agents is reported, along with its application in direct electrical sensing of alkylators using an organic field-effect transistor, OFET, device. Upon reacting with alkylators, the OFET containing the new polythiophene analogue as its channel becomes conductive, and the gate effect is lost; this is in marked contrast to the response of the OFET to innocent vapors, such as alcohols and acetone. By following the drain-source current under gate bias, one can easily follow the processes of absorption of the analyte to the polythiophene channel and their subsequent reaction.

Full text of DOI:10.1002/adfm.200901598

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Sensing of Alkylating Agents Using Organic Field-Effect
Transistors
By Yair Gannot, Carmit Hertzog-Ronen, Nir Tessler,* and Yoav Eichen*
for a specific mass or the detection of the
presence of a specific atom in the alkylating
Alkylating agents are simple and reactive molecules that are commonly used
in many and diverse fields, such as organic synthesis, medicine, and
agriculture. Some highly reactive alkylating species are also being used as
blister chemical warfare agents. The detection and identification of alkylating
agent is not a trivial issue because of their high reactivity and simple
structure. Here, a novel polythiophene derivative that is capable of reacting
with alkylating agents is reported, along with its application in direct electrical
sensing of alkylators using an organic field-effect transistor, OFET, device.
Upon reacting with alkylators, the OFET containing the new polythiophene
analogue as its channel becomes conductive, and the gate effect is lost; this is
in marked contrast to the response of the OFET to ‘‘innocent’’ vapors, such as
alcohols and acetone. By following the drain–source current under gate bias,
one can easily follow the processes of absorption of the analyte to the
polythiophene channel and their subsequent reaction.
[
8]
agent (e.g., S in mustard gas).
In recent years, we, as well as other
[9–11]
groups,
have reported on the photo-
induced electron transfer (PET)-based
detection of alkylating agents based on
the quaternization of a Lewis base nucleo-
phile, usually a tertiary amine group, which
serves also as a quencher to the lumino-
[10,11]
phore.
The application of a lumines-
centsensorarraytechniquewasshowntobe
very useful in identifying the nature of the
[
10]
electrophile. Nevertheless, direct electri-
cal detection of such agents was never
demonstrated. Direct electrical detection of
chemical substances may be achieved in a
[12]
variety of methods, of which field-effect
modulation of the conductivity of an
[
13]
organic layer seems most promising, due to the potential of
signal amplification and gain of information on the captured
[
14]
substance from different types of gate manipulations.
The
1
. Introduction
ability of building arrays of field-effect transistors, each bearing a
different active material, offers promising routes to the detection
and identification of diverse substances in solution as well as in the
Alkylating agents, such as benzyl bromide, dichloromethane, and
dimethyl sulfate are simple molecules commonly used in organic
synthesis as reagents and solvents. Other alkylating agents are
[15]
gas phase.
Herein, we report on the preparation and characterization of an
organic field-effect transistor (OFET) having a channel layer made
of a novel co-polymer, 1, synthesized from 3-hexyl-thiophene, 2,
[
1]
[2]
used as soil sterilizers, anticancer drugs, and unfortunately
[
3]
also as chemical warfare agents. Alkylating agents readily react
with many nucleophiles in the human body, rendering them toxic
0
0
00
0
and 4-[2,2 ;5 ,2 ]terthiophen-3 -yl-pyridine, 3. The new OFET
responds to even minute concentrations of alkylating agents and is
capable of recognizing them among harmless substances, such as
ethanol, isopropanol, and water.
[
4,5]
and/or mutagenic.
The combination of their extensive use,
high toxicity, and easy manufacturability renders some of these
agents perfectterror warfare agents. There istherefore aclear need
for simple, sensitive, and informative tools for their detection and
identification in solutions and especially in the gas phase.
Two approaches have been pursued in developing effective
chemical sensing means for detecting alkylators; one is based on
following the change of optical properties of a nucleophile upon
2
. Results and Discussion
[
6,7]
Co-polymer 1 was prepared according to Scheme 1. Compound 4,
-(4-pyridyl)-thiophene, was prepared in 75% yield using
reacting with alkylators,
while the other is focused on the quest
3
palladium-catalyzed cross coupling between thiophene-3-boronic
acid, 5, and4-bromopyridine, 6. Compound4wasdibrominatedin
the 2 and 5 positions using N-bromosuccinimide (NBS) in acetic
acid, affording 2,5-dibromo-3-(4-pyridyl)-thiophene, 7, in 60%
yield. Palladium-catalyzed cross coupling of 7 with two equivalents
of 2-(tri-n-butylstannyl) thiphene, 8, afforded 3 in 80% yield. Co-
polymer 1 was obtained as a bright red solid, with number- and
[
*] Prof. Y. Eichen, Y. Gannot, C. Hertzog-Ronen
Schulich Faculty of Chemistry, Technion - Israel Institute of Technology
Technion City, 32000, Haifa (Israel)
E-mail: chryoav@tx.technion.ac.il
Prof. N. Tessler
Nanoelectronics Center, Department of Electrical Engineering
Technion - Israel Institute of Technology
Technion City, 32000, Haifa (Israel)
E-mail: nir@ee.technion.ac.il
weight-averaged molecular weights of
M
n
¼ 37 000 and
M ¼ 97 000 (by gel permeation chromatography, GPC), in 17%
w
DOI: 10.1002/adfm.200901598
yield by FeCl -catalyzed co-polymerization of 3 and 2 from a 1:10
3
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of a film of 1 before and after (1-R) exposure to
methyl iodide, 9, a common alkylating agent.
As can be seen from the figure, most of the
effect of the alkylation process on the optical
properties of the polymer is expressed in the
photoluminescence spectrum, where the red
shoulder is blue-shifted from 1.67 to 1.70 eV,
and in the increase of the intensity of the blue
shoulder at 1.80 eV. There seem also to be a
slight change in the absorption tail to shorter
wavelengths, but it is too small to be
quantified in this type of measurement.
OFETs were fabricated by spin-coating a
0 nm thick film of 1 atop the FET structure
5
using a previously described procedure.
[
16]
A
schematic cross-section of the OFETis shown
in Figure 2a, while Figure 2b shows the top
view of the interdigitated source–drain
electrodes.
Figure 3 depicts the OFET’s electrical
characteristics measured for a FET having 1
as the channel. Figure 3a shows the OFET’s
output characteristics, i.e., the drain–source
currents as a function of drain–source voltage
Scheme 1. Preparation of co-polymer 1. i) Ba(OH)
2
ꢂ 8H
2
3 4
O þ Pd(Ph P) ; ii) NBS in acetic acid; iii)
3 4 3 2 2
Pd(Ph P) ; iv) FeCl in chloroform; v) H N–NH .
for several gate–source voltages (V
g
¼ 0, –10,
and –20 V). The black lines and the inset show
the device response prior to exposure to 9,
while the gray overlapping lines show the
response afterwards. As can be clearly seen in
the figure, prior to exposure to the alkylating
agent (methyl iodide), the device exhibits
standard FET output characteristics with a
slight indication of doping, probably due to
[
17]
Scheme 2. Generalized reaction scheme of 1 with alkylating agents.
atmospheric oxygen.
The post-exposure
curvesshowconsiderablyhighercurrentsand
a resistor-like response with no effect due to
the gate bias. This is a clear indication for high-level p-type doping
associated with the exposure to 9. Figure 3b shows the OFET’s
transfer characteristics, i.e., the drain–source current as a function
of gate–source voltage, for fixed drain–source voltage
(
V
ds ¼ ꢁ10 V). The black line shows the device response prior to
exposure, while the gray horizontal line shows the response after
exposure to 9. Consistent with the results presented in Figure 3a,
the transfer characteristics of the FET prior to exposure show
current modulation due to the gate–source voltage. The post-
exposure gray horizontal line shows almost no dependence on the
gate voltage. At zero gate bias, V ¼ 0, the exposure of the FET to 9
g
enhances the current by almost two orders of magnitude, while at
¼ ꢁ20 V the factor is only 2–3. Assuming that the mobility is
V
g
Figure 1. The optical density, OD, and photoluminescence, PL, spectra of
a film of co-polymer 1 (2
Dashed lines: The spectra of the film before exposure; black lines: the
charge density independent and not affected by the doping, one
can use the above results to estimate the doping density according
n
3
m
; n ¼ 0.95 ꢀ 0.005, m ¼ 0.05 ꢀ 0.005) at 80 K.
ꢁ2
toEquation1,whereCox isthegatecapacitance(inunitsofFcm ),
Vg isthegate–sourcebias(inunitsofV), qistheelementarycharge,
and r2D is the dopant surface density.
spectra of the same film after exposure to 9.
solution in chloroform. NMR characterization reveals that 1 is
composed of 2
n
3
m
, where n ¼ 0.95 ꢀ 0.005 and m ¼ 0.05 ꢀ 0.005.
1
1
r2D ¼ 2 ꢂ COX Vg ¼ 2 ꢂ
ꢂ 35 ꢃ 10ꢁ ꢂ 20
9
Co-polymer 1 reacts with alkylating agents according to
Scheme 2 both in solutions and as a film when reacted with
their vapors. Figure 1 depicts the absorption and emission spectra
1:6 ꢃ 10ꢁ
19
q
1
2
ꢁ2
¼ 8:75 ꢃ 10 cm
(1)
1
06
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Figure 3. a) Output and b) transfer characteristics measured on a FET
having 1 as the channel. The black lines and the inset show the device
response prior to exposure to 9, while the gray (overlapping in a) lines show
the response after exposure to 9.
Figure 2. a) A schematic cross-section of the OFET structure. b) Top view
of the interdigitated contacts structure (channel length is 2 mm).
Assuming the doping is uniformly distributed across the
1
8
ꢁ3
ꢄ50 nm film, the doping density is r3D ¼ 1.75 ꢃ 10 cm . The
detection scheme of 1-based OFETs involves the formation of a
covalent bond between the analyte (the alkylating agent) and
polymer in the channel. As this process is not reversible, all
available reaction sites are bound to react. The number of reaction
sites (pyridine rings, see Scheme 2) represents also the upper
detection limit of the device. The total weight of the polymer
1
0
ꢄ9 ꢃ 10 analyte molecules will induce complete saturation of all
active sites of the polymer.
A similar effect of exposure to alkylating agents on the output
and transfer characteristics could be observed when FETs, bearing
1 as the channel, were exposed to chloromethoxy ethane, 10,
1-chloro-2-ethylsulfanyl ethane, 11, and benzyl bromide, 12 (see
Supporting Information, SI).
ꢁ9
embraced by the source and drain electrodes, M, is 10
g
assumingadensityof1 gcm ), andisobtainedusingEquation2,
ꢁ
3
(
where W is the width of the device, L is the length, and h is the
thickness of the polymer film.
In contrast, exposure of the OFET to vapors of non-reactive
materials thatareincapable offormingcovalentbondswith 1, such
as methanol, 13, results with the deterioration of the output and
transfer characteristics, as can be seen in Figure 4. The same
behavior could be observed upon exposure of OFETs containing 1
as the channel layer to isopropanol, 14 and acetone, 15 (see SI).
A similar behavior of deterioration of the field-effect character-
istics could also be observed in OFETs lacking the ability to
M ¼ W ꢃ L ꢃ h ꢃ r ¼ 1 ꢂ 2 ꢃ 10ꢁ4 ꢂ 5 ꢃ 10ꢁ6 ꢂ 1 ¼ 10ꢁ9
g
(2)
Co-polymer 1 was found to consist of 5% monomer 3, i.e.,
ꢃ 10ꢁ11g or ꢄ1.5 ꢃ 10ꢁ13 mol. This means that reacting with
chemically react with alkylating agents, such as FETs having poly-
[18]
5
(3-hexylthiophene) (P3HT), 16, as the active channel.
For
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ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 4. a) Output and b) transfer characteristics of FET having 1 as the
channel layer. The black lines show the device response at ambient
conditions, while the gray lines show the response after exposure to 13.
Figure 5. a) Output and b) transfer characteristics of FET having P3HT, 16,
as the channel. The black lines show the device response at ambient
conditions, while the gray lines show the response after exposure to 11.
example, Figure5 depicts the output and transfer characteristics of
a FET having 16 as the channel before and after exposure to 11.
The effect of alkylation on FETs having 1 as the channel can be
rationalized by following the effect of alkylation on the charge
distribution in the polymer. Co-polymer 1 is composed of an
electron-rich polythiophene skeleton with pendant pyridine
groups.Uponquaternization, thepyridinegroupsaretransformed
into electron-deficient pyridinium groups. These electron-poor
groups attract charge density from the polythiophene skeleton,
thus partially doping it (Scheme 3).
substances with FETs containing 16. After a while, the source–
drain current starts increasing with time indicating that 9 starts to
react with the polymer.
3. Conclusions
A novel polythiophene derivative that is capable of reacting with
alkylating agents was prepared and characterized. The new
polymer was used for direct electrical detection of alkylators using
an OFET device. Upon reacting with alkylators, the OFET
containing the new polythiophene derivative as the channel layer
becomes conductive, and the gate effect is lost; this is in marked
contrast to the response of the OFETto ‘‘innocent’’ vapors, such as
alcohols and acetone.
Assumingalkylationinducesdoping(Scheme3), thenumberof
charge carriers that are formed in the film located between the
source and drain electrodes is only ꢄ2% of the total number of
pyridine moieties available in the same volume of polymer. By
Figure 6 depicts the drain–source current as a function of the
exposure time to vapors of 9 at a constant gate–source voltage of
ꢁ
10 V. Upon introducing the methyl iodide vapors to the
atmosphere (marked with an arrow), the drain–source current
drops,mostprobablyindicatingtheprocessofabsorptionofvapors
by the polymer and consequent changes to its morphology and
packing. The drop in the drain–source current is expected in view
of the similarity between this process and what is found for
nonreactive substances withFETs containing1andforanyofthese
1
08
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render it conductive. Major research effort is
currently directed towards OFETsensing using
[
19]
stable organic semiconductors,
towards multispot ‘‘artificial nose’’ sensing
devices.
as well as
[
20]
4. Experimental
Materials: Highly regioregular 16 was purchased
from Rieke Metals, Inc (4002-EE). See Scheme 4 for
chemical structures of 9–16.
4
-Thiophen-3-yl-pyridine, 4: 4-Bromopyridine hydro-
chloride (1.23 g, 6.32 mmol), Pd(PPh (0.35 g,
.12 mmol), Ba(OH) O (4.98 g, 9.49 mmol),
3 4
)
3
2
ꢂ 8H
2
and thiophene-2-boronic acid (0.89 g, 6.97 mmol)
were dissolved in a mixture of in 82 mL dimethyl ether
2
(DME) and 22 mL of H O under an inert atmosphere,
and the mixture was stirred overnight at 85 8C. After
cooling to room temperature, the reaction mixture
was extracted with three portions of dichloromethane.
Scheme 3. Proposed doping mechanism.
The combined organic layers were washed with
water and brine and dried over sodium sulfate, and the solvents were
evaporated. The crude product was purified using column chromatography
(
(
silica, 1:99 v/v methanol:dichloromethane) affording 6 as a white solid
1.7 g, 75%).
1
Melting point (mp): 138 8C; H NMR (300 MHz, CDCl
3
, tetramethylsi-
lane—TMS, 25 8C, d): 7.43(d, 2H), 7.45 (d,2H), 7.64 (dd,1H), 8.69 (d, 2H);
13
3
C NMR (125 MHz, CDCl , 25 8C, d): 120.73, 123.05, 125.63, 127.06,
1
39.41, 142.54, 150.34; Chemical ionization mass spectrometry, CIMS
þ
þ
(
m/z): 162 [MþH ], 161 [M ].
4-(2,5-Dibromo-thiophen-3-yl)-pyridine, 7: A mixture of 4 (1.6 g, 10 mmol)
and NBS, (4.4 g, 24 mmol) in 150 mL acetic acid was stirred overnight at
room temperature. The mixture was then poured on ice, made slightly
basic, and extracted with three portions of dichloromethane. The combined
organic layers were dried over sodium sulfate, filtered, and concentrated.
The crude product was purified using column chromatography (silica,
7
5:25 v/v hexane:ethyl acetate) affording 7 as a white solid (1.9 g, 60%).
1
mp: 263.4 8C; H NMR (300 MHz, CDCl
3
, TMS, 25 8C, d): 7.03(d, 1H),
13
7
.33 (d, 1H), 7.46 (d, 2H), 8.63 (d, 2H); C NMR (125 MHz, CDCl , 25 8C,
3
d): 123.00, 126.78, 128.28, 130.84, 138.12, 142.61, 149.69; Matrix-assisted
laser desorption/ionization time-of-flight MS, MALDI-TOF MS (m/z): 319
Figure 6. Time response of an FET having 1 as the channel. Methyl iodide,
þ
9, is introduced at T ¼ 10 min. The processes of initial absorption of the
[M ].
0
0
4-[2,2(;5(,2 ]Terthiophen-3(-yl-pyridine, 3: Compounds 7, (0.23 g,
.7 mmol) and 8, (0.57 g, 1.5 mmol) and Pd(PPh (0.07 g, 0.07 mmol)
analyte and subsequent reaction with the polymer are clearly observed.
0
3 4
)
were dissolved in 20 mL of dry toluene under an inert atmosphere. The
mixture was refluxed for 48 h, then cooled to room temperature, and
concentrated under reduced pressure. The crude product was purified
using column chromatography (silica, 20:80 v/v ethyl acetate:hexane)
affording 3 as a yellowish powder (0.18 g, 80%).
1
mp: 91.3 8C; H NMR (300 MHz, CDCl
3
, TMS, 25 8C, d): 2.87(s, 4H),
.98–7.00(dd, 2H), 7.05–7.06(dd, 1H), 7.17(s, 1H), 7.22–7.23(dd, 1H),
6
7
(
13
.27–7.28(dd, 2H), 7.30–7.31(dd, 2H), 8.47–8.51(dd, 2H); C NMR
, 25 8C, d): 125.6, 126.1, 126.9, 127.6, 128.5, 129.2, 129.4,
125 MHz, CDCl
3
1
29.8, 136.2, 138.0, 138.3, 138.6, 145.5, 151.8; MALDI-TOF (m/z): 325.0
þ
[
M ].
Scheme 4. Chemical structures of 9–16.
00
Poly-(4-[2,2(;5(,2 ]terthiophene-3(-yl-pyridine-co-3-hexylthiophene), 2
n m
3 ,
1: FeCl (2.35 g, 13.2 mmol) was added in four portions over 12 h to a
3
following the drain–source current under gate bias, one can easily
follow the processes of absorption of the analyte to the
polythiophene channel and its subsequent reaction. The detection
limit of the present device is calculated to be at least in the range of
solution of 3 (0.22 g, 0.66 mmol) and 2 (0.56 g, 3.3 mmol) in 36 mL of
chloroform; then 100 mL of ethanol were added. The precipitate was
collected, dissolved in chloroform, and washed with a 5% aqueous
hydrazine solution (a de-doping process). The organic phase was dried
over sodium sulfate then concentrated at low temperature under reduced
pressure. The polymer was separated from its shorter oligomers by
disolution in a minimum amount of tetrahydrofuran and reprecipitation
with methanol. The polymer was further purified using chromatography
1
0
11
1
0 ꢁ10 bound molecules.
Like many other polythiophenes, 1 is sensitive to oxidizing
agents, such as halogens. These, and other substances, dope it and
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(
silica, 50:50 v/v THF:hexane) affording 1 as a bright red solid, M
and M
¼ 97 000 (GPC), in 17% yield.
H NMR: (300 MHz, CDCl , TMS, 25 8C, d): 0.83(br), 1.25–1.28(br),
.51–1.62(br), 2.49(br), 2.73(s, 1H), 6.91(br), 8.57 (br). NMR character-
n
¼ 37 000
[
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m ¼ 0.05 ꢀ 0.005.
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Device Fabrication and Electrical Measurements: OFETs were fabricated
using the procedure described previously [16]. The bottom-contact FET
substrate was prepared using standard clean-room procedures and
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consisted of highly doped Si (gate), SiO (insulator), and patterned gold
1
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in the experiments was 2 mm, and the channel width, W, was 10 000 mm.
The co-polymer 1 was spin-coated directly onto the FET substrate, which
was then subjected to thermal annealing (110 8C in vacuum for 3 h).
Subsequently the FETs were kept under ambient atmosphere for at least a
day before measuring their electrical characteristics and their response to
exposure to the various chemicals. This procedure established a baseline
that includes the effects of ambient atmosphere on the FETs performance
and allowed better resolution of the effects originating from the exposure of
the devices to the different alkylating agents. FET characteristics were
measured on a probe station using a semiconductor parameter analyzer
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(Agilent 4155B). After measuring the baseline properties, the FET was
placed in a closed vessel pumped down to 0.1 atm, and then a drop of the
material to be detected was inserted into the vessel and allowed to fully
vaporize such that the final concentration was roughly 1000 ppm. The
devices were kept in the vessel for a day to ensure that the FET
characteristic reached steady state before performing the post-exposure
measurements.
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