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Single-walled carbon nanotube-based chemo-capacitive sensors with molecular receptors for selective detection of chemical warfare agents Sun Gu Song, Seonggyun Ha, Hye Jin Cho, Minhe Lee, Dawoon Jung, Jae-Hee Han, and Changsik Song ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01713 • Publication Date (Web): 24 Dec 2018 Downloaded from http://pubs.acs.org on December 26, 2018
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ACS Applied Nano Materials
Single-walled Carbon Nanotube-based Chemo-capacitive Sensors with Molecular Receptors for Selective Detection of Chemical Warfare Agents Sun Gu Song†, Seonggyun Ha†, Hye Jin Cho†, Minhe Lee†, Dawoon Jung‡, Jae-Hee Han‡,* and Changsik Song†,* †
Department of Chemistry, Sungkyunkwan University, Suwon, Gyeonggi 16419, Korea.
‡
Department of Energy/IT, Gachon University, Seongnam, 13120, Korea.
KEYWORDS: chemical warfare agent, chemo-capacitive sensor, single-walled carbon nanotube, dielectrophoresis, sensor array, molecular receptor. ABSTRACT: Single-walled carbon nanotubes (SWCNTs) were fabricated using AC dielectrophoresis into chemocapacitive sensors and molecular receptors were applied for the selective detection of several chemical warfare agents (CWAs). The selective responses toward nerve simulants (G and V), choking, and blister agents as well as a pesticide were investigated with specific receptor molecules that were either covalently functionalized or non-covalently coated onto the surface of the SWCNTs. The SWCNT-based chemo-capacitive sensors showed reproducibility and sensitivity to 200 ppb for several target molecules. The fabricated sensor arrays were assessed for the selective detection of 6 different CWAs and the principal component analysis demonstrated their specificity. VX, a real nerve agent, was tested on the fabricated SWCNT-based chemo-capacitive sensor coated with a thiourea-functionalized siloxane polymer and the successful detection of VX at 100 ppb, confirmed that our SWCNT-based sensors are suitable for practical applications.
Introduction During the past few decades, various sensor technologies for detecting chemical warfare agents (CWAs) have been developed using infrared and Raman spectroscopy for remote monitoring2,1, ion mobility spectroscopy3, flame photometry4, photoionization5, electrochemical detection6, and surface acoustic wave (SAW) detection7. To date, most commercially available detectors are large and require long warm-up times, high power consumption, and considerable time for operator training and maintenance8, hampering their use in quick-and-ubiquitous detection. Chemo-resistive9,10,11,12,13 or -capacitive sensors have the potential to solve these problems since they can be operated at ambient temperatures, feature simple operation and response, consume a minimal amount of power, and can be fabricated into small sizes with low-cost and reproducibility14. However, most chemo-resistive sensors, which detect changes in electrical resistance in response to the chemical environment, exhibit limited selectivity because of their sensitivity to humidity, rendering the management of the interfering signals quite difficult.14-15 However, chemo-capacitive sensor signals originating from the changes of capacitance in dielectric polymer films16 can be easily applied with less interference from moisture signals as compared to those in electrically conductive films. Chemo-capacitive sensors have been applied in the detection of volatile organic compounds (VOCs), toxic industrial chemicals17, and humidity18 by vapor sorption to polymers, resulting in dielectric and permittivity changes.
For the detection of CWAs, various sensing materials have been studied in the chemo-capacitive or chemo-resistive sensors.19 Among the materials tested, carbon nano-materials are promising candidates for sensor fabrications because of their excellent physical properties and high electrical conductance.20 As one-dimensional nanowires of highly environmentalsensitive electronic properties, single-walled or multi-walled carbon nanotubes (SWCNTs or MWCNTs) have been utilized in chemical sensors for pH, bio-molecules (DNA and proteins), VOCs, and CWAs or their simulants.21. The electrical transduction using CNTs can be modulated by pinning carriers or perturbing doping levels in the nanotubes, by at the CNT-CNT interfaces and/or CNT-metal electrodes (Schottky barrier), all of which can give rise to sensory signals (especially resistive outputs).22 One of the most important and critical issues in CNT-based sensors is to exhibit selective responses to target analytes. Functionalization of receptor molecules to CNTs has been utilized to achieve the selectivity. One approach to attach “selective” receptor molecules is to non-covalently functionalized CNT surfaces with small molecules, polymers (wrapping), or even metal nanoparticles.23 On the other hand, covalent functionalization via diazonium chemistry or other bond formations are also an effective way to introduce receptor molecules.21,24. Chemo-capacitive sensors incorporating CNTs generally produce better signal-to-noise responses than chemo-resistive sensors because the conductance of CNTs is more easily controlled by the gate voltage.25 In addition, the baseline capacitance of CNT networks may affect sensing performance since
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the number of interconnections correlates with the surface area of the CNT networks and may influence the detection of CWAs.26 For these reasons, dielectrophoresis (DEP) is an ideal method as it can efficiently regulate the amount of SWCNTs between electrodes, resulting in controlled morphology and reproducible baseline capacitance.27 In DEP, SWCNT molecules are assembled between electrodes under electric fields, and the SWNT amounts are controlled by the applied voltage, frequency and time.28 DEP has been already utilized to align SWCNTs or MWCNTs between electrodes and those devices have already been applied as various sensors for pH, biomolecules, volatile organic compounds (VOCs), and even CWAs simulants. Although there have been developed various SWCNT-based chemosensors, they mostly utilized resistive measurements (i.e., conductance or resistance changes of charge carriers). Chemo-resistive sensors certainly have advantages such as simple operation and low cost, but they are known to exhibit limited selectivity because of their highly sensitive response to humidity, rendering the management of the interfering signals quite difficult. On the other hand, chemo-capacitive sensor signals originating from the changes of capacitance in a dielectric layer can be easily applied with less interference from moisture compared to those in electrically conductive films. It seems that DEP-aligned SWCNTs have been rarely applied as “chemo-capacitive” sensors. Furthermore, with SWCNT-DEP chemo-capacitive sensors, selective discrimination of various CWAs or simulants with molecular receptors has not been achieved. Herein, we fabricated SWCNT chemo-capacitive sensors and arrays using DEP and various receptor molecules were used for selective CWA detection. Unlike previous sensors which were developed only for organophosphorus agents such as dimethylmethylphosphonate (DMMP), we attempted to detect various CWAs (simulants) in a single sensor platform. Most importantly, we successfully observed sensing signals from real CWAs such as VX and CG. These SWCNT-DEP sensors are suitable for practical applications. Firstly, the DEP process we utilized is appropriate for the large scale production of delicate sensing devices by the fast but controlled deposition of SWCNTs, enabling stable baseline capacitances with an automated DEP apparatus for the large scale fabrications. Secondly, only small amount of SWCNTs has been used for the sensor fabrication, which would make our sensors competitive (or reasonable) in prices. Thirdly, mobile applications should be facile through Bluetooth connections to smart phones since the electronics of our SWCNT-DEP sensors were relatively simple and easily connectable. Lastly, from the point of view of the receptor molecules, the successful measurements of real VX, not its simulant, strongly indicate that our approach to selectively discriminate CWAs (or simulants) with molecular receptors should be applicable.
Experimental section Materials. CWA simulants (Dimethylmethylphosphonate, 2chloroethyl phenyl sulfide, malathion and parathion) were purchased from Sigma-Aldrich and used without further purification. CG gas was purchased from RIGAS with N2 in 10 ppm. SWCNTs (6,5 chirality,
