Discrimination of alkyl and aromatic amine vapors using TTF-TCNQ based chemiresistive sensors

Article abstract of DOI:10.1039/c6cc08237h

We report a chemiresistive sensor approach based on a TTF-TCNQ charge transfer material, which can real-time detect and distinguish the vapors of alkyl amine and aromatic amine species under ambient conditions, based on the dramatic difference in the kine

Full text of DOI:10.1039/c6cc08237h

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DOI: 10.1039/C6CC08237H
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Discrimination of Alkyl and Aromatic Amine Vapors Using TTF-
TCNQ Based Chemiresistive Sensors†
Received 00th January 20xx,
Accepted 00th January 20xx
Chen Wang, Na Wu, Daniel L. Jacobs, Miao Xu, Xiaomei Yang, and Ling Zang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report a chemiresistive sensor approach based on the TTF- sensors for oxidizing or electron withdrawing gases22-23, taking
TCNQ charge transfer material, which can real-time detect and advantages of the high density of carriers. However, the
distinguish the vapors of alkyl amine and aromatic amine species sensing of reducing gases (e.g., amines) with the same TTF-
under ambient conditions, based on dramatic difference on the TCNQ material has been barely reported. In this work, we
kinetics of the electric current recovery processes after the demonstrate a new approach of using TTF-TCNQ as a
exposure of the two amine species.
chemiresistive material to detect and discriminate alkyl and
aromatic amines according to their kinetic difference in the
response signals. In our further control experiment, by using
pure TCNQ material (in morphology of microfibers), we
confirmed that the observed sensing discrimination was
unique in the TTF-TCNQ system, which was probably
originated from the different charge transfer interaction of
alkyl and aromatic amines at the interface of TTF-TCNQ.
Volatile amines are important biomarkers1 and common
pollutants2-4. The discrimination between alkyl and aromatic
amine vapors during the detection impacts on quick diagnosis
of diseases5-7
,
food preservation8-11
,
and environment
protection12-13. Compared to the traditional spectroscopic
approaches, such as ion mobility spectroscopy, mass
spectroscopy and Raman spectroscopy, chemical sensors
usually feature the good portability and low cost14. But due to
the similar electron donating nature of alkyl and aromatic
amines, the design of chemical sensors to discriminate them
meets challenges. For example, the sensors based on
photoinduced charge transfer (PCT) may fail on the
discrimination between the two amines15, while the chemical
reaction sensors usually require large dose of amines in the
reactions16-17. The sensor array approach may be universal for
gas detection. But to clearly discriminate such similar vapors,
the number of array channels would be unavoidably raised,
which increases the work and cost on sensor array
preparation, prior analyte training, and complicated data
Figure 1. (a) The molecular structures of TTF and TCNQ. (b) Photos of acetonitrile
solution of TTF (left), TCNQ (middle), and as prepared TTF-TCNQ microfibers (right). (c)
An optical microscopy image of TTF-TCNQ microfibers (scale bar = 50 µm).
The TTF-TCNQ microfibers were fabricated via a surfactant-
free solution-based method (Figure 1b, detailed in SI), since
the large surface-to-mass ratio of fibril structures is favorable
for the chemiresistive sensing24-25 (Figure 1c). The X-ray
diffraction (XRD) measurement of these microfibers confirmed
the clean cocrystalline structure by comparing with the
powder simulated XRD spectra from the previously obtained
single crystal structures of TTF, TCNQ and TTF-TCNQ (Figure
S1). Then the TTF-TCNQ microfibers were transferred onto
interdigitated electrodes (IDEs) patterned on glass to make the
chemiresistor sensor chip with uniform surface dispersion of
the microfiber materials (Figure S2). Interestingly, when the
electric field along the TTF-TCNQ material exceeded 5×104
V·m-1, the conductivity irreversibly decreased to a low state
likely due to the thermally caused disorder of the crystal
interpreting18-19
.
Therefore, to improve the detection
efficiency, a specific sensor material that can distinguish
between alkyl and aromatic amines in real-time detection is
desired for practical applications.
Tetrathiafulvalene
(TTF)
and
7,7,8,8-
tetracyanoquinodimethane (TCNQ) (Figure 1a) form 1:1 charge
transfer complex, TTF-TCNQ, which constitutes metallic
conductor with high charge mobility and density at room
temperature20-21. TTF-TCNQ materials have been studied as
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(Figure S2a). This phenomenon was observed for the sensor amines and the sensor material should be much stronger than
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chips with finger gap of 10 and 100 µm. To avoid this nonlinear that between aromatic amines andDOthI:e10.1s0a3m9/eC6CmCa0t8e2r3i7aHl.
I-V region, the TTF-TCNQ chemiresistors were tested under Considering the high electron deficiency of TCNQ (a strong
low voltage bias (0.1 V). For the sensing tests, the vapors of electron acceptor) and the strong basicity (or nucleophilicity)
seven alkyl amines (butylamine, hexylamine, dodecylamine, of amines, a donor-acceptor type interaction would be
trimethylamine,
benzylamine,
cyclohexylamine,
and expected between amines and TCNQ, either in the format of
dibutylamine) and five aromatic amines (aniline, p-toluidine, o- neutral complex or further ionic charge separation pair
toluidine, N,N-dimethylaniline and 4-fluoroaniline) were amine+–TCNQ– (with the latter being much stronger bound and
selected and diluted to the concentration range of 90 to 120 becoming harder to be dissociated). As to be further discussed
ppm for comparative investigations (with the exception for below, the irreversible response of alkyl amines was likely due
dodecylamine, for which the saturated vapor pressure at room to their stronger basicity, which affords the formation of
temperature, 18 ppm, was used without further dilution).
charge pair through steady state charge separation.
Upon the exposure to the amine vapors, the electrical Nonetheless, bindings with both types of amines to TCNQ led
current measured over TTF-TCNQ showed similar and instant to fierce competition to the TTF-TCNQ complex, and thus a
decreases, but with the removal of the amine vapors, the decrease in conductivity (or current) as observed in Figure 2.
recovery of signal showed remarkable differences between the
Table 1. Comparison of recovery times (RT) of the TTF-TCNQ chemiresistor when
exposed to different amine vapors (from data in Figure 2).
alkyl and aromatic amines (Figure 2). To make quantitative
comparison between the signal recovery of different amines,
we introduced a term named “recovery time” (RT), which was
defined as the period of time from the end of amine exposure
to the time when 1/e×100% (36.8%) of the current change
remains. It was found that after the removal of aromatic
amines the decrease in current was quickly and almost fully
recovered, with RTs less than 3 s (Table 1). So the
chemiresistive sensing response to aromatic amines can be
considered as
a reversible process, implying that the
interaction between aromatic amines and the sensor material
is relatively weak, likely in the order of van der Waals force,
which usually dominates the intermolecular interaction in
reversible chemical sensors26.
Indeed, there is the possibility that the observed reversible
response of aromatic amines was due to the steric hindrance
caused by the aromatic rings, which often weakens the
intermolecular interactions. To exclude this possibility, we
selected an alkyl amine that is substituted with a benzene
group, namely benzylamine. It was interesting to observe that
this amine gave similar irreversible response as other alkyl
amines shown in Figure 2a, implying that the strong donor-
acceptor interaction (dominated by the amine moiety)
sufficiently surpasses the steric effect. Nonetheless, the steric
hindrance effect of side groups was indeed observed among
the aromatic amines themselves as shown in Figure 2b,
wherein the sensing response magnitude of N,N-
dimethylaniline was about 60% lower and the RT was 40%
shorter than the unsubstituted aniline tested under the same
concentrations. Such significant difference in response can
reasonably attributed to the bulkier dimethyl groups.
Moreover, to exclude the possibility that observed different
responses may be caused by the variation between devices
(e.g., fabrication fluctuations), we used a single TTF-TCNQ
chemiresistor sensor to test both aniline and hexylamine
(representing aromatic and alkyl amine, respectively) by
exposing to each amine three times consequently as shown in
Figure S4. The results demonstrated good consistency
between the three times of exposure to the same amine, and
more importantly the dramatic difference in signal recovery
(reversible vs. irreversible) still remains between the aromatic
and alkyl amines, the same as observed in Figure 2. It is thus
Figure 2. Change of electrical current of TTF-TCNQ chemiresistors upon exposure to the
vapor of (a) alkyl amines (from top to bottom): butylamine (110 ppm), hexylamine (120
ppm), dodecylamine (18 ppm), triethylamine (100 ppm), benzylamine (95 ppm),
cyclohexylamine (90 ppm), and dibutylamine (100 ppm), and (b) aromatic amines (from
top to bottom): aniline (100 ppm), o-toluidine (100 ppm), p-toluidine (100 ppm), N,N-
dimethylaniline (100 ppm), and 4-fluoroaniline (100 ppm). Each amine was tested for
four consequent cycles. Bias voltage was 0.1 V.
On the contrast, all the alkyl amines led to almost
irreversible current decreases, as observed during the time
frame of testing, 120 s, regardless the molecular weights
(butylamine or dodecylamine) or the steric conformations
(primary, secondary or tertiary) of the amines (Table 1). To this
regard, the RTs for alkyl amines are simply marked as > 120 s,
as shown in Table 1. Clearly, the interaction between alkyl
2 | J. Name., 2012, 00, 1-3
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confirmed that aromatic and alkyl amines can be distinguished likely that the observed irreversible response of alVkieywl aArmticilne Oesnlianet
from the reversible vs. irreversible signal recovery as measured TTF-TCNQ was due to the local stabilDizOaIt:i1o0n.10o3f9/Cth6eCCc0h8a23r7gHe
over the TTF-TCNQ chemiresistor.
separation pair of TCNQ––amine+, for which the strong
To study the different sensing responses between alkyl and electrostatic attraction makes it hard to dissociate. On the
aromatic amines, we re-performed the experiments under the other hand, the aromatic amines would be in favor of forming
same setups, but using pure TCNQ material as the neutral donor-acceptor complex with TCNQ at the same
chemiresistive sensor, which was fabricated by drop-casting a interface, thus making it reversible for desorption. To support
TCNQ solution onto the same IDEs. Self-assembly of TCNQ such polarity effect on interfacial binding, we carried out
molecules led to the formation of microfibers, whose comparative UV-vis spectral measurements for aniline and
dimension sizes were similar to the microfibers of TTF-TCNQ hexylamine (representing aromatic and alkyl amine,
(Figure S5). As a strong electron acceptor, TCNQ functions as a respectively) in mixture with TCNQ in three different organic
typical n-type semiconductor with electron as the major solvents, acetonitrile, chloroform, and toluene, representing
charge carrier21. Binding with electron donors like amines polar, medium polar and non-polar mediums, respectively. As
would increase the electrical conductivity of TCNQ through shown in Figure S4, in the polar solvent, like acetonitrile, both
charge transfer interaction. Indeed, as shown in Figure 3, both aniline and hexylamine formed the anionic radical of TCNQ–, as
alkyl and aromatic amine vapors caused significant increase in confirmed by the characteristic absorption bands in the visible
electrical current of TCNQ, in sharp contrast to the decrease in region 650-900 nm33. The higher concentration of TCNQ–
current observed for TTF-TCNQ (Figure 2). Another difference produced with hexylamine was mainly due to its stronger
from the case of TTF-TCNQ was that all the amines basicity (or electron donating power). With the decreasing the
demonstrated quick reversible responses (Figure 3), with the polarity from acetonitrile to chloroform, significant amount of
RTs comparable between alkyl and aromatic amines (Table 2). TCNQ– could still be detected with hexylamine, whereas aniline
This observation indicates that the binding of the two types of generated only the neutral donor-acceptor complex
amines with TCNQ was similar in strength, likely in the format (characteristic of the broad absorption in the visible range)34.
of donor-acceptor complex, which was reversible for With the further decreasing the solvent polarity to toluene,
dissociation. However, for TTF-TCNQ the surface binding of neither of the two amines efficiently produced the charged
alkyl amines was much stronger, leading to irreversible sensing species, indicating the lack of stabilization of the charge
response (Figure 2), while the aromatic amines remain about separation. The results of Figure S6 revealed clearly that the
the same reversible response with RTs in about the same order charge separation species of TCNQ with alkyl amines formed
as for pure TCNQ (Table 1 and 2). Such dramatic difference much more easily than that with aromatic amines, so in a
observed for alkyl amines between TTF-TCNQ and pure TCNQ certain polarity range (for example, in chloroform), the charge
can be attributed to the different polarity at the surface of the transfer complex between TCNQ and alkyl amines could
two materials.
develop into ions, while that with aromatic amines still stayed
in neutral state. Such different dependence on polarity may
suggest that alkyl amines bound to TTF-TCNQ in the format of
TCNQ––amine+ pair at the interface, while the aromatic amines
took the format of neutral donor-acceptor complex. The
stronger electrostatic interaction of TCNQ––amine+ explains
the irreversible sensing response observed for the alkyl
amines, and the relatively weaker complexation between
aromatic amine and TCNQ is consistent with the observed
reversible response in both TCNQ and TTF-TCNQ
chemiresistors. The strong charge transfer interaction
between alkyl amine and TCNQ was also evidenced by infrared
spectroscopy (IR) characterization of the TTF-TCNQ cocrystal35
mixed with dodecylamine (Figure S7). Moreover, due to the
competitive D-A interaction with TCNQ between TTF and
adsorbed amine, the desorption of amines from the surface of
pure TCNQ would be slower than that from the surface of TTF-
Figure 3. Change of electrical current of TCNQ chemiresistors upon exposure to the
vapor of (a) alkyl amines (from top to bottom): butylamine (110 ppm), hexylamine (120
ppm), dodecylamine (18 ppm), triethylamine (100 ppm), benzylamine (95 ppm),
cyclohexylamine (90 ppm), and dibutylamine (100 ppm), and (b) aromatic amines (from
top to bottom): aniline (100 ppm), o-toluidine (100 ppm), p-toluidine (100 ppm), N,N-
dimethylaniline (100 ppm), and 4-fluoroaniline (100 ppm). Each amine was tested for TCNQ cocrystal. This is consistent with the signal recovery
four consequent cycles. Bias voltage was 10 V.
data shown in Table 1 and 2, where the recovery times of
aromatic amines measured on the TTF-TCNQ material are
As a typical charge transfer (metallic) material, the degree
of charge transfer was reported over 0.5 and its interface was
believed to be more polar than the pure, neutral organic
materials such as TCNQ27-29. In general, polar medium (solvent)
is conducive to stabilizing the geminate pair of charge
separation species such as TCNQ––amine+ in this study30-32. It is
overall faster than those measured on the pure TCNQ material
under the same experimental conditions.
It was also interesting to note that the recovery time of
alkyl amines on TCNQ is strongly dependent on the molecular
weight, i.e., the larger the molecule the longer the recovery
takes. For example, among the three primary amine analogues
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4
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tested, the RT was determined 0.6 s for butylamine, 2.7 s for
hexylamine, and 17 s for dodecylamine (Table 2), showing
clear trend of increase with molecular weight. Similar
molecular weight dependence of signal recovery kinetics was
also observed for other chemiresistive sensors26. The slow
recovery of large analytes is mostly due to the more difficult
desorption process. For the similar reason, larger molecules
are easier to condense on surface, resulting in more efficient
increase in sensor signal. For example, under the similar vapor
concentrations, the signal amplitude generated by butylamine
was about 50% smaller than hexylamine. Such molecular
weight effect may be incorporated into the sensor systems to
enhance the differential sensing by comparing the recovery
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