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Sensors and Actuators B 190 (2014) 529– 534

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l h om epage: www.elsev ier .com/ locat e/snb

ighly sensitive detection of trinitrotoluene in water byhemiresistive sensor based on noncovalently amino functionalizedingle-walled carbon nanotube

iangming Wei ∗, Dejiong Lu, Jian Wang, Hao Wei, Jiang Zhao, Huijuan Geng, Yafei Zhang ∗

ey Laboratory for Thin Film and Microfabrication of the Ministry of Education, Institute of Micro/Nano Science and Technology,hanghai Jiao Tong University, Shanghai 200240, PR China

r t i c l e i n f o

rticle history:eceived 5 July 2013eceived in revised form 25 August 2013ccepted 4 September 2013vailable online 13 September 2013

a b s t r a c t

We developed a chemiresistive sensor based on 1-pyrenemethylamine (PMA) functionalizedsingle-walled carbon nanotube (SWCNT) networks for highly sensitive and rapid detection of 2,4,6-trinitrotoluene in water. In this sensor, the SWCNT network was deposited between interdigitatedelectrodes, and the functional PMA molecule was noncovalently attached to the sidewall of the SWCNTsvia �–� interaction. The amino substituent of PMA could selectively interact with 2,4,6-trinitrotoluene

eywords:arbon nanotubehemiresistorsqueous electronic Sensorunctionalizationrinitrotoluene

to form negative charged complexes on the SWCNT surface. These charged complexes can act as effec-tive molecular gate and strongly change the electrical conductance of the SWCNTs, enabling the sensorseasily to detect 2,4,6-trinitrotoluene at 10 parts-per-trillion (ppt) concentration in aqueous solutions,with the response time of less than 1 min, without the need of pre-concentration of the analytes. Thefunctionalized sensors also show excellent selectivity toward 2,4,6-trinitrotoluene over those interferingorganic molecules.

. Introduction

The development of miniaturized smart sensors to detect tracend ultra-trace explosive molecules is crucial for homeland securitys well as environment and human health [1]. 2,4,6-TrinitrotolueneTNT) is one of the best-known explosives, which is not only aecurity threat, but also has been recognized as important envi-onmental pollutions due to water and soil contamination [2]. TNTs a toxicant as well as a carcinogen for human being [3,4]. It can tar-et the liver, spleen and the circulatory system, leading to severelyamage to human health. The maximum allowable concentra-ion in drinking water set by the U.S. Environmental Protectiongency is 2 parts-per-billion (ppb) [2,5]. So far, many laboratory

echniques, such as ion mobility spectrometry (IMS), mass spec-rometry (MS), high-performance liquid chromatography (HPLC)nd surface enhanced Raman spectroscopy (SERS) have been devel-ped to detect TNT and its metabolites. But these methods requirexpensive and bulk instruments or tedious sample preparation pro-esses. There is a strong demand to develop miniaturized smart

ensors for fast and real-time detection of ultra-trace TNT.

The miniaturized smart sensors based on one-dimensional1D) nanostructures have been received much attention because

∗ Corresponding authors. Tel.: +86 21 34204322 116; fax: +86 21 34205665.E-mail addresses: lmwei@sjtu.edu.cn (L. Wei), yfzhang@sjtu.edu.cn (Y. Zhang).

925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2013.09.017

© 2013 Elsevier B.V. All rights reserved.

of their fast and high sensitivity toward molecular adsorption.Some types of one-dimension (1D) nanomaterials, such as siliconnanowires [6], metal oxide nanowires [7] and carbon nanotubes[8–11] have been used to fabricate TNT sensors. Among thesematerials, single-walled carbon nanotubes (SWCNTs) receive spe-cial attention. SWCNTs are nearly ideal quantum wires with everyatom on the surface [12], which are expected to exhibit excellentsensitivity toward surrounding chemical species [13]. SWCNTsalso possess high aspect ratio, good environmental stability, andexcellent mechanical and electronic properties. These featuresmake SWCNTs ideal sensing materials for compact, low cost,low power and potable sensor devices [13–15]. Multiple types ofSWCNT devices, such as SWCNT field-effect-transistors (FETs) [16],SWCNT chemiresistors [17] and chemicapacitors [18] have beendeveloped for analyte detection. Among them, the chemiresistorswhich are based on the simple change in resistance in response tothe binding of analytes are very attractive because of their simplestructure and the ease of high precise measurement [17,19]. How-ever, the pristine SWCNTs always demonstrate weak response andlow selectivity toward specific molecules [20]. So, there is a need tofunctionalize SWCNTs to improve both the sensitivity and the selec-tivity of SWCNT sensors [20–23]. Recently, SWCNT-FET sensors

functionalized by biomimetic molecules, such as polydiacetylene[2] and peptide [9] have been reported to detect TNT in water anda significant improvement in the sensitivity and selectivity wasobtained. When a submonolayer of unbundled SWCNT network

530 L. Wei et al. / Sensors and Actuators B 190 (2014) 529– 534

Fig. 1. (A) The illustrative schematic of the SWCNT chemiresistive sensors. The straight black and yellow lines indicating SWCNTs and metal electrodes, respectively; (B)F age ofd

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E-SEM image of the electrode structure of SWCNT chemiresistors; (C) FE-SEM imeposited at different position on the electrode patterning wafer (E).

as used to fabricate SWCNT-FET sensors, a rapid response to TNTolution (less than 2 s) was also observed [8]. However, the currentWCNT sensors for practical detection of TNT molecules stillannot satisfy the requirement in terms of sensitivity, selectivity,imple structure and easy operation. Herein, we demonstrateighly sensitive (down to ppt level) and rapid detection of TNTresponse time < 30 s when exposure to 10 ppt TNT) in watersing SWCNT chemiresistive sensors, which was noncovalentlyunctionalized by 1-pyrenemethylamine (PMA). The functionalMA molecules strongly bond onto the surface of SWCNTs via �–�nteraction. The amino substituent of PMA could selectively inter-ct with TNT to form negative charged complexes on the SWCNTurface (Scheme 1). These charged complexes can act as effectiveolecular gate and strongly change the electrical conductance of

WCNTs, enabling the sensors easily to detect trinitrotoluene atarts-per-trillion (ppt) level. Our functionalized SWCNT chemire-istive sensor also exhibited excellent selectivity to TNT overhose interfering molecules, such as 2,6-dinitrotoluene (DNT) and,4-dinitrotoluene (2,4-DNT).

. Experimental details

.1. SWCNT network deposition and sensor fabrication

To fabricate our gas sensors, the Si/SiO2 wafer substrate wasrst ultrasonically rinsed with toluene, acetone, ethanol, deionizedDI) water and Piranha solution (98% H2SO4:30% H2O2 = 3:1(v/v)) tolean the wafer. This clean wafer was immersed in aminopropyltri-thoxysilane (APS) aqueous solution (1.5 mM) for 2 h, subsequentlyashed by DI water and kept in a vacuum evaporator at 120 ◦C

or 1 h to form the amino-terminated monolayer on the surface ofhe Si/SiO2 substrate. The purified carboxyl-functionalized SWCNTsCarbon Solutions Inc) were ultrasonically dispersed in DI water for

h. The pretreated Si/SiO2 substrate modified with an APS mono-ayer was immersed in the SWCNT suspension, followed by rinsing

ith ethanol and DI water and drying with the aid of nitrogenow.

SWCNTs bridging the conducting channels; (D) FE-SEM images showing SWCNTs

The SWCNT sensors were fabricated using standard microfab-rication procedures as our previous report [19]. The interdigitatedelectrode fingers were made by sputtering 10 nm Cr and 180 nm Auonto the patterned photoresist mold. We introduced lift-off processto remove the photoresist. Finally the electrodes were ultrason-ically rinsed in acetone and ethanol repeatedly, washed with DIwater thoroughly and then dried by nitrogen flow before they wereused.

The functionalization of the SWCNT sensors was performedby immersing the SWCNT sensor chip in 1-pyrenemethylaminehydrochloride (PMA·HCl) solution (0.036% in water) for 24 h, andthen rinsing the sensor chip by DI water and ethanol to removeexcessive PMA·HCl. The resulting PMA·HCl functionalized sensorchip was further immersed in triethylamine for 24 h to transformPMA·HCl to PMA. Last, the functionalized sensor chip was washedthoroughly by ethanol and dried at 60 ◦C.

2.2. Sensor testing system

The electrical signal of the sensor was monitored by using asemiconductor parameter analyzer (Agilent 4156C). First, a dropof DI water (10 �L, containing 0.05% DMSO) was dropped onto thesensor chip such that entire channel of electrodes was covered. Sen-sor measurements were performed by measuring current versustime at a constant voltage (0.5 V). After a stable baseline electri-cal signal was obtained, a drop of TNT solution (2 �L, containing0.05% DMSO) was added to the droplet of water (10 �L). A solutionwith higher concentration of TNT was added to the existing dropletafter the current stabilized. This was repeated with multiple con-centrations. All sensing measurements were carried out at roomtemperature (25 ◦C).

2.3. Characterization

The morphologies of the SWCNTs and the sensors were observedby field emission scanning electron microscopy (FE-SEM, Carl Zeiss

L. Wei et al. / Sensors and Actuators B 190 (2014) 529– 534 531

Scheme 1. Possible TNT sensing mechanism of the 1-pyrenemethylamine (PMA)functionalized SWCNT sensors. PMA was first strongly bounded onto the surface ofSWCNTs via �–� interaction. The amino groups of PMA can electively bind TNTand form negative charge complexes on the surface of SWCNTs. These chargedcomplexes can act as effective molecular gate and strongly change the electricalc

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In this study, PMA was used to functionalize the SWCNTs. PMAcan strongly bond onto the surface of SWCNTs via �–� interaction[25]. PMA also provides an electron-rich amino group which can

onductance of SWCNTs.

ltra 55). X-ray photoelectron spectra (XPS) were recorded on ahimadzu/Kratos AXIS Ultra XPS spectrometer.

. Results and discussion

The uniform and high-density SWCNT networks on the Si/SiO2afer were fabricated with a solution-based self-assemble method

24]. Briefly, the Si/SiO2 substrate was first functionalized with APSo form an amino-terminated monolayer on its surface. The debun-led carboxyl-SWCNTs were easily absorbed and deposited on themino functionalized Si/SiO2 substrate via electrostatic interac-ion. Following deposition of SWCNTs, the metal electrodes wereatterned by standard photolithography. The schematic and FE-EM images of the SWCNT chemiresistor structures were shownn Fig. 1 panels (A) and (B), respectively. The interdigitated elec-rodes possess a channel length of 600 �m and a channel width of0 �m. The FE-SEM images show that the high density and uniformWCNT networks bridged the interdigitated electrodes and acted

s conducting channels (Fig. 1C). We also took the FE-SEM imagest nine different locations on the wafer to determine the depositionniformity of SWCNT networks (Fig. 1D and E). It can be seen thatuch uniform and high-density nanotube networks were achieved

Fig. 2. (A) Photographs of TNT solution after PMA was added. (B and C) were thepure TNT and pure PMA solution, respectively.

throughout the 3 in. wafer, suggesting our method is feasible tomanufacture device on a large scale.

Fig. 3. (A) XPS survey spectra of SWCNT network before (a) and after (b) function-alized by PMA. (B) High resolution XPS N1s spectra of SWCNT network before (a)and after (b) functionalized by PMA.

532 L. Wei et al. / Sensors and Actuators B 190 (2014) 529– 534

Fig. 4. (A) Response of the PMA functionalized SWCNT sensor (the sensor was first covered by 10 �L DI water reservoir) toward 2 �L DI water. (B) The normalized currentr differc and afe

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elative to initial baseline (I/I0) of the PMA functionalized sensor upon exposure tooncentration. The �I is calculated from (B), indicating the current change before

xposure to 1 ppb and 1 ppm of TNT solution.

electively bind electron-deficient TNT, leading to formation of neg-tive charged complexes on the surface of SWCNTs [6]. Two kinds oftrong interactions may account for the formation of such negativeharged complexes: (1) The charge transfer from the electron-richmino groups to electron-deficient aromatic rings of TNT leads tohe formation of Meisenheimer complexes [26–31]; (2) The TNT

olecule which is known a Brønsted–Lowry acid can be deproto-ated at methyl groups by basic amine to formation of TNT anion29–31]. This negative charge on TNT anion is resonantly stabi-ized by three electron-withdrawing nitro groups, and distributedhroughout the molecule. The formation of TNT anion is evidencedy the fact that the TNT solution changes from colorless to deep redpon PMA was added (Fig. 2). These reversible charged complexesre close to the SWCNT surface and can act as effective molecularating elements to strongly change the electrical conductance ofhe SWCNTs.

Fig. 3 shows the XPS spectra of the SWCNT sensors before andfter functionalization by PMA. As expected, the SWCNT networkxhibits a C1s peak (∼284.5 eV) and an O1s peak (∼532 eV) aris-ng from physically adsorbed oxygen and oxygen groups on theefect sizes. From the XPS spectrum, negligible N elements can bebserved (atom ratio: N/C < 1%) in the SWCNT sensor chips beforeunctionalization. These negligible N elements were attributed tohe APS molecules which were introduced onto the SiO2/Si sub-trate to assist SWCNT deposition, as mentioned above. However,

pronounced increase in the N content (N/C = 4.1%) was observedfter the SWCNT network was functionalized by PMA. This result

onfirms the functionalization of PMA on the surface of SWCNTetworks.

The conductance between two electrodes was measuredo investigate the sensor response. To obtain a stable baseline

ent concentrations of TNT; (C) Plot of average relative response (�I/I0) versus TNTter a given TNT solution is added; (D) The I/I0 of the pristine SWCNT sensor upon

electrical signal, a drop of DI water (10 �L, containing 0.05% DMSO)acting as a water resource was first dropped onto the sensor chip tocover the entire channel of electrodes. After a stable baseline elec-trical signal was obtained, a drop of TNT solution (2 �L) was addedto the water reservoir (10 �L). A drop with higher concentration ofTNT was added to the existing droplet after the current stabilized.Fig. 4A shows that no change in current was observed when purewater drop (without TNT) was added into the water reservoir.But the functional sensor exhibits fast response and ultrasensitiveto the presence of TNT. For instance, it gave 10.1% conductancechange upon exposure to 10 ppt of TNT with a response time of lessthan 30 s, without the need for pre-concentration step (Fig. 4B).With the increasing TNT concentration, the conductance changeincreased (Fig. 4C). At higher concentration, the response tendsto saturate, which might be ascribed to the saturated adsorp-tion of TNT on the surface of SWCNTs and hence leading to thesaturation response. Importantly, the control “unfunctionalized”SWCNT sensors gave no observable signal upon exposure tohigh concentration of TNT (1 ppb) (Fig. 4D). This result confirmsthat the functional molecule PMA plays a key role in enhancingthe sensitivity of our sensors. It should be mentioned that theAPS molecules which were introduced onto the wafer to assistdeposition of SWCNTs can also provide amino groups. Our resultshows that these APS did not significantly enhance the sensitivityof our sensors (without PMA, the sensor can detect ∼1 ppm TNT,see Fig. 4D), which might due to their low population (N/C < 1%),and the fact that the APS molecules were buried by SWCNTs, which

may block TNT to interact with APS. We also observed that our PMAfunctionalized SWCNT sensor can be regenerated after the testedsensor was immerged in plenty of DI water/DMSO and then gentlywashed by DI water/DMSO (see Fig. S1 in supporting information).

L. Wei et al. / Sensors and Actuato

Fig. 5. The �I/I of the PMA functionalized SWCNT sensor upon exposure to 100 ppbos

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single-walled carbon nanotube chemiresistive sensors for ultrasensitive andhighly selective organophosphor vapor detection, Nanotechnology 22 (2011)

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f TNT, 2,6-DNT, 2,4-DNT, 1,3-DNB and 1-NB. Before exposure to one analyte, theensor was thoroughly washed by DI water to remove the adsorbed impurity.

Supplementary material related to this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.snb.2013.09.017.

Several important factors might account for the high sensitiv-ty of our functional sensor toward TNT: (1) The �–� interactionetween PMA and SWCNTs is strong enough to firmly bind PMAn the surface of SWCNTs [25]; (2) the PMA molecules are shortrganic molecules (<1 nm in length). Therefore, the formed chargedomplexes are expected to be close to the surface of the SWCNTensing elements; Since the electrostatic potential that arises fromharges on the analyte molecule decays exponentially toward zeroith distance [6,32], the short distance between SWCNT and the

harges is essential for optimal sensing; (3) in our sensing system,he sensing experiment was performed in DI water (salt-free). Inhis case, the Debye length is expected to be about 1 �m [6]. Thisarge screening length makes the SWCNT conductance very sen-itive to the formation of charged complex pairs on the SWCNTurface; (4) the negative charged complex TNT molecule is reso-antly stabilized by neighboring nitro groups, resulting in stableomplexes and enhanced sensitivities.

The sensitivity and selectivity of the sensors to some struc-urally related nitroaromatic chemical derivatives including,6-dinitrotoluene (2,6-DNT), 2,4-dinitrotoluene (2,4-DNT), 1,3-initrobenzene (1,3-DNB) and 1-nitrobenzene (1-NB) were also

nvestigated (Fig. 5). The results clearly show that our functional-zed SWCNT sensors show higher sensitivity toward TNT than thosenterfering molecules. For example, the conductance change of theensors upon exposure to 100 ppb of TNT was 37%, whereas thathange was only 1.2% upon exposure to 100 ppb of 1-NB. Fig. 5 alsohows that the nitrosubstituted aromatic molecules with strongerlectron-withdrawing nature, such as TNT and 2,6-DNT, induce aarger conductivity change than those nitrosubstituted aromatic

olecules with less electron-withdrawing nature (such as 1-NB),uggesting the former molecules have a higher ability to createharge-transfer complexes with the functional amino groups. Espe-ially, all these interfering molecules gave no observable signals atoncentrations lower than 1 ppb, thus making our functional sensorighly suitable for the ultra-trace detection of TNT.

. Conclusions

In conclusion, we have successfully developed a PMA func-ionalized SWCNT chemiresistive sensor for highly sensitive andapid detection of TNT in water. The selective interaction of TNTith PMA on the surface of SWCNTs caused significant change in

[

rs B 190 (2014) 529– 534 533

the conductance of the SWCNT sensors, which made the sensorseasily detect TNT at ppt concentration level with the responsetime of less than 1 min, without the need of pre-concentration ofthe analytes. Moreover, our functionalized sensors show excellentselectivity toward TNT over those related compounds with nitrogroups. The simple structure of the chemiresistors combining withthe easy and efficient functionalization approach makes our sensorideal candidate for fabrication of large sensor arrays. We hope oursensors with a number of amine derivatives, in the near future,could offer a universal platform for the real-time, supersensitiveand selective detection of various types of explosives.

Acknowledgment

We thank for the financial support from the National NaturalScience Foundation of China (No. 51272155, 61376003), the Foun-dation for SMC Excellent Young Teacher in Shanghai Jiao TongUniversity.

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Biographies

Liangming Wei is currently an associate professor at Shanghai Jiaotong University,China. His research interests include preparation of carbon-based nanomaterialsand their use for in sensors and energy storage.

Dejiong Lu is an MS candidate at Prof. Zhang’s group in Shanghai Jiaotong University.Her research focus now includes nanomaterials and their application for sensor andsolar cells.

Jian Wang is a PhD candidate at Prof. Yafei Zhang’s group in Shanghai Jiao TongUniversity. His research focus now includes the preparation of nanowires, nanotubesand their use for gas sensors.

Hao Wei is currently an associate professor at Shanghai Jiaotong University, China.His research interests include preparation of nanomaterilas and their use for inenergy storage.

Jiang Zhao is a PhD candidate at Prof. Yafei Zhang’s group in Research Institute ofMicro/Nano Science and Technology, Shanghai Jiao Tong University. His researchfocus now includes the preparation of CNT and CNT field emission display.

Huijuan Geng is a PhD candidate at Prof. Zhang’s group in Shanghai Jiaotong Uni-

sensor and solar cells.

Yafei Zhang is currently a professor at Shanghai Jiaotong University, China. Hisresearch interests include synthesis of nanomaterials and their applications in nan-odevice.