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The Direct and Selective Electrochemical Vapour Trace Detection of Organic Peroxide Explosives via Surface Decoration Vadim Krivitsky, Boris Filanovsky, Vladimir Nadakka, and Fernando Patolsky Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00257 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Analytical Chemistry

The Direct and Selective Electrochemical Vapour Trace Detection of Organic Peroxide Explosives via Surface Decoration Vadim Krivitsky1#, Boris Filanovsky1#, Vladimir Naddaka1 and Fernando Patolsky1,2,3*

1School

of Chemistry, the Raymond and Beverly Sackler Faculty of Exact Sciences Tel-Aviv University, Tel Aviv 69978, Israel

2

The Center for Nanoscience and Nanotechnology, Tel Aviv University

3 Department

of Materials Science and Engineering, The Iby and Aladar Fleischman Faculty of Engineering, Tel-Aviv University, Tel Aviv 69978, Israel

Email: [email protected]

# These authors contributed equally to this work

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Abstract

The ability to detect traces of highly energetic explosive materials sensitively, selectively, accurately, and rapidly could be of enormous benefit to civilian national security, military applications, and environmental monitoring. Unfortunately, the detection of explosives still poses a largely unmet arduous analytical problem, making their detection an issue of burning immediacy, and a massive current challenge in terms of research and development. Although numerous explosive detection approaches have been developed, these methods are usually time-consuming, require bulky equipment, tedious sample preparation, a trained operator, cannot be miniaturized and lack the ability to perform automated real-time high-throughput analysis, strongly handicapping their mass deployment. Here, we present the first demonstration of the 'direct' electrochemical approach for the sensitive, selective and rapid vapor trace detection of TATP and HMTD, under ambient conditions, unaffected by the presence of oxygen and hydrogen peroxide species, down to concentrations lower than 10ppb. The method is based on the use of Ag-nanoparticles-decorated (AgNPs) carbon microfibers air-collecting electrodes (CFE), which allow for the selective direct detection of the organic peroxide explosives, through opening multiple redox routes, not existent in the undecorated carbon electrodes. Finally, we demonstrate the direct and rapid detection of TATP and HMTD explosive species from real-world air samples.


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Introduction The ability to detect traces of highly energetic explosive materials sensitively, selectively, accurately, and rapidly could be of enormous benefit to civilian national security1, military applications2, and environmental monitoring3. Unfortunately, the detection of explosives still poses a largely unmet arduous analytical problem, making their detection an issue of burning immediacy, and a massive current challenge in terms of research and development.4,5 This is mainly due to the requirement to sensitively and selectively detect traces of explosive species in any given encountered environment, a problem further exacerbated by the inherently low vapor pressures displayed by most of these compounds. For instance, the powerful explosives RDX (1,3,5-trinitroperhydro-1,3,5-triazine) and HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) exhibit extremely low vapor pressures of a few parts-per-trillion (ppt) and part-per-quadrillion (ppq), respectively.6 Although numerous explosive detection approaches have been developed, these methods are usually time-consuming, require bulky equipment, tedious sample preparation, a trained operator, cannot be miniaturized and lack the ability to perform automated real-time highthroughput analysis, strongly handicapping their mass deployment.7 Triacetone triperoxide (TATP) and hexamethylene triperoxide diamine (HMTD) are the major peroxide-based explosives, very popular among terrorists, as they can be readily synthesized from commercially available chemicals.8 Therefore, there is an urgent need for the rapid and inexpensive detection of these "home-made" explosives, especially at checkpoints of mass-transit facilities, and other government and public facilities. Although explosives that contain peroxide groups have been known about in the scientific community since the late 19th century, they have become publically known only recently.9-13 While a large number of techniques have been reported for the detection of nitro-based explosives such as TNT, there is significant difficulty in using conventional photometric devices for the detection of peroxide-based explosives. This is partially attributable to the lack of nitro or other chromophoric functional groups present in the structure of TATP and HMTD molecules14. There are a number of direct detection methods that do not require chemical decomposition, or derivatization, of these compounds, like MS, LC/MS, GC-MS, ion mobility spectrometry and HPLC15-19. However, these methods suffer from relatively high costs, extensive operator skill, and limited field portability.



On the other hand, several approaches were proposed for the 'indirect' detection of peroxide-based explosives, mainly based on their chemical decomposition to hydrogen peroxide, H2O2, under acidic conditions or UV radiation, and its subsequent detection by spectroscopic or electrochemical methods.8,20-24 Electrochemical detection of the peroxide-based explosives is one of the most prospective methods, because of its inherent potential high sensitivity, selectivity, simplicity, and low cost.25,26 In spite of these inherent potential advantages, the 'direct' electrochemical detection of di-alkyl peroxides has shown to be challenging, due to their very negative reduction potential, rendering their electrochemistry inaccessible in aqueous solutions. Alternatively, H2O2 exhibit reduction potentials well within the potential window of water, and thus have been the target for the development of numerous detection methods. Recently, an 'indirect' electrochemical TATP sensor has been demonstrated by Wang and co-workers, using a Prussian-blue, "artificial peroxidase" modified electrode to detect H2O2 formed from the acid 22 or UV 23 decomposition of TATP, achieving detection limits of ~0.1-50M. Cheng and co-workers 27-29 proposed a method based on acid-treatment of TATP, followed by detection of the released H2O2 (and/or hydroperoxides) by the electrocatalytic reduction of a FeII/IIIethylenediaminetetraacetate complex at a glassy carbon electrode, yielding a detection limit of 0.89 µM for TATP. Furthermore, an indirect detection approach for TATP was proposed,30 based on the rapid redox reaction of peroxides (HMTD, benzoyl peroxide, t-butyl peroxide, TATP and H2O2) with the bromide ion, leading to the formation of bromine, yielding a detection limit of 8.5 μM, 16.3 μM and 14.9 μM for TATP, HMTD, and H2O2, respectively. Also, electrogenerated chemiluminescence (ECL) detection method for HMTD, using AgNO3 as an ECL enhancing agent in acetonitrile at a platinum electrode, was proposed by Parajuli and Miao,31 yielding a detection limit of 50 μM. Additional numerous methods for the indirect electrochemical, or electrogenerated chemiluminescence, detection of peroxide-based explosives has been reported32, based on the chemical decomposition of the peroxide-based molecule, followed by the electrochemical detection of the resulting hydrogen peroxide species. Recently, a few colorimetric and fluorimetric detection schemes were proposed only for the detection of TATP, showing sensitivity or detection times limitations33,34. Clearly, all of these reported methods are strongly limited by several handicapping factors: (1) The detection is 'indirect', requiring prior chemical decomposition of the organic peroxide species into hydrogen peroxide, by acid or UV treatment (2) These approaches, due to their 'indirect' nature, are time consuming (3) The lowest limit of detection exhibited by most of these methods, >1M, 4 ACS Paragon Plus Environment

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may be sufficient for the detection of the highly volatile TATP molecules, but will hardly serve the detection of the lowly volatile HMTD explosive. (4) The selective detection of these explosives through the final sensing of hydrogen peroxide, a highly abundant chemical, may be easily screened by the presence of hydrogen peroxide in a large number of commercial products of daily use, i.e. cosmetics and toothpaste, thus, leading to an unacceptably large number of false-positive results under real-world conditions. (5) Furthermore, detection by many of these approaches can be readily obscured by the presence of oxygen in the detection samples, thus requiring the performance of tedious degassing steps prior detection. In all cases, the presence of peroxide-based explosives cannot be selectively discriminated from potential contamination of the tested samples with hydrogen peroxide, or other peroxide-based non-harmful chemicals. Thus, the development of 'direct' approaches for the selective and sensitive electrochemical detection of TATP and HMTD is highly required. A method for the direct electrochemical reduction of several dialkyl-peroxides in aprotic organic solvents (such as N,N-dimethylformamide, dimethyl sulphoxide, and acetonitrile), in the presence of aromatic electron transfer catalysts, i.e. such as anthracene, was proposed.35,36 Besides the use of organic solvents, this method requires the removal of dissolved oxygen before detection. 27,37,38 The electrochemical reduction peak of dissolved oxygen species, or/and contaminant H2O2, strongly interfere with the detection of the alkyl peroxides. Under these conditions, oxygen and H2O2 species, usually present at concentrations considerably higher than the detected explosives traces, are practically reduced at the same potentials as peroxide-based explosives. These handicapping issues render the unselective direct reduction of dialkyl-peroxides in organic solvents impractical for the development of real-world detectors for TATP and HMTD. Here, we present the first demonstration of the 'direct' electrochemical approach for the sensitive, selective and rapid vapor trace detection of TATP and HMTD, under ambient conditions, unaffected by the presence of oxygen and hydrogen peroxide species, down to concentrations lower than 10ppb. The method is based on the use of Ag-nanoparticles-decorated (AgNPs) microcarbon fibers air-collecting electrodes (CF), which allow for the selective direct detection of the organic peroxide explosives, through opening multiple redox routes, not existent in the undecorated carbon electrodes. Finally, we demonstrate the direct and rapid detection of TATP and HMTD explosive species from real-world air samples.



Materials and Methods Chemicals.

Hexamethylenetetramine

(99+%),

tetra-n-butylammoniumtetrafluoroborate

(TBABF4, 99+%, electrochemical grade), hydrogen peroxide (30%), anhydrous citric acid (99.5+%), sulfuric acid (95-98%), silver benzoate (99%), acetone (99.8+%), acetonitrile (99.8%), N, N-dimethylformamide (99.8%), Nafion® 117 solution (~5% in a mixture of lower aliphatic alcohols and water), ethanol (99.5+%), and 1,2-dichloroethane (99.8%) were purchased from Sigma-Aldrich, Israel, and used without further purification. Microfibers carbon (FC) electrodes (180µm thickness, type SpectraCarb 2050A-1050) was purchased from Engineered Fiber Technology (USA). DIW was obtained from Millipore Mill-Q water 18 MΩ·cm. Synthesis of TATP and HMTD. TATP and HMTD are very dangerous materials that may explode under impact, friction, static electricity, and temperature changes. Always handle these materials with extreme caution!!! The synthesis of TATP and HMTD must be carried out by qualified personnel, under the use of appropriate safety precautions, and synthesized in small quantities (0.1 g). TATP was synthesized by reaction of acetone with 30% H2O2 in the presence of sulfuric acid at -20 oC. For long-term storage, TATP was kept in a dichloromethane solution. HMTD was obtained by reaction of hexamethylenetetramine with 30% H2O2 in the presence of anhydrous citric acid, at ice cooling. For long-term storage, HMTD is always kept wet. (See Supplementary Materials sEction for details) Results and Discussion The properties of porous microcarbon fibers electrodes make them great candidates to serve as working electrodes for the detection of explosives materials. They practically allow the vapor collection and pre-concentration of traces of tested explosives species, and the subsequent direct electrochemical detection. Due to their extremely large active surface, they allow the effective 'trapping' of molecular species through their physicochemical adsorption on the carbon fibers surface. Furthermore, carbon fibers electrodes are highly conductive and mechanically stable, thus serving as excellent potential candidates as detection electrodes, for both the filtering and collection of air samples and the final electrochemical detection of the chemical species under test. 6 ACS Paragon Plus Environment

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However, dissolved oxygen, as well as H2O2 traces, can both interfere with peroxide-based explosives detection. Oxygen may be reduced at the same potentials as peroxide-based explosives, resulting in complete masking of their signal. Regularly, 10-15 minutes of deaeration is required in order to remove dissolved oxygen to an acceptable level for cathodic detection, which is too long to be applied in field conditions.39,40 Traces of H2O2, which may be found in field conditions, might overlap with the reduction peak of peroxide-based explosives. Moreover, peroxide-based explosives detection methods that are based on the detection of H2O2, may lead to "false positive" detection.27,37,38 In this context, we first tested the electrochemical behavior of the explosive TATP on 'unmodified' CF electrodes at high pH conditions (in a solution containing deionized water, 50% acetonitrile and a 0.1 M quaternary ammonium salt electrolyte TBABF4 at pH 12), aiming at separating between the peaks of H2O2, oxygen and the peroxide-based explosive analytes, Figure 1a. The high pH sensing conditions improve both the stability of the peroxide-based molecular species and widen the electrochemical working window down to almost -2V. Two peaks can be discerned from the scanning voltammetry curve at 0.75V and 1.37V, corresponding to the electrochemical reduction of oxygen and TATP respectively. Clearly, using our CF electrodes at these experimental conditions allows the full separation of the dissolved oxygen electroreduction signal from the TATP signal. A clear linear concentration-dependent behavior for TATP is observed as expected, with the lowest detection limit of ca.200ppb. Unfortunately, the co-addition of H2O2 to the TATP tested solution, at a concentration of 40 ppm, does not lead to a separated reduction peak for the smaller peroxide, and the reduction potentials of both, H2O2 and TATP fully overlap at around 1.37 V (Figure 1a, pink curve). Thus, these results limit the use of 'unmodified' CF electrodes as potential sensors for the detection of TATP, due to their incapability to discriminate between the organic-peroxide explosive species and H2O2. Figure 1 Silver is a well-known catalyst for the decomposition reaction of H2O241, therefore, it was suggested that modifying the FC electrodes with stable silver nanoparticles, may facilitate the separation of the signals of H2O2 and peroxide-based explosives. For this purpose, we developed a procedure dedicated to the decoration of the FC electrodes with silver nanoparticles of controlled density, by the co-deposition of a silver benzoate in a Nafion thin film from a mixture 7 ACS Paragon Plus Environment


of silver benzoate, Nafion and acetonitrile. After the deposition of the silver benzoate-Nafion film, a single electrochemical preconditioning reduction step was performed by scanning the potential from 0V to -1.6V (staying at -1.6V for a period of 10 seconds). This step is expected to lead to the reduction of the silver ions in the silver benzoate salt embedded in the Nafion film, leading to the formation of silver clusters of controlled density and dimensions, Supplementary Figure S1. As expected, the above-mentioned silver benzoate modification procedure on the surface of the FC electrodes, resulted in complete separation of the signals of TATP and H2O2, Figures 1b and d. Due to the organometallic properties of silver benzoate, the silver efficiently absorbed to the microcarbon-fibers, and converted to nanoparticles as a result of the applied negative voltage prior to the analyte detection, while the Nafion served as coating exchange membrane,42 that prevented the washing of silver benzoate and silver nanoparticles from the surface of the working electrode. Figure 1d demonstrates the two separate peaks obtained for H2O2 (at ~ -0.45 V) and for TATP (at ca.-1.05V). Moreover, the silver benzoate modification described enabled a linearly-correlated concentration-dependent detection of TATP (R=0.995), with the lowest detection limit in a solution of ca. 200ppb in solution, Figures 1b and c. Notably, both electroreduction signals, for H2O2 and TATP, display a considerable shift towards less negative potential values after the proposed silver benzoate modification procedure, with a shift of ca. 900mV and 400mV for H2O2 and TATP respectively, Figures 1a and b. This observed shift plausibly stems from the modification of the carbon electrode with Nafion for the case of TATP electroreduction, and the precense of AgNPs for the case of the H2O2 electrocatalytic reduction. This demonstrates the catalytic properties of silver nanoparticles on the differential electroreduction of these peroxidebased chemical species. This differential electroreduction catalysis effect allows the complete separation and discrimination of the alkyl-peroxide TATP from the hydrogen peroxide signals, thus leading to the important capability to simultaneously detect and discriminate between these species. Notably, the use of bulk ionic silver species or other silver salts, eg. silver nitrate and silver perchlorate, did not lead to the positive observed detection outcome as described before through the silver benzoate modification. Additionally, we tested the silver-decorated CF electrodes in the detection of the explosive HMTD, Figure 2. Clearly, a concentration-dependent detection of HMTD was achieved as well, with the lowest limit of detection in a solution of ca. 250ppb, Figure 2a, and b. Also, the co-


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addition of 40ppm H2O2 did not affect the direct detection of HMTD, leading to the formation of two electroreduction separate discernable peaks at ca. -0.4V and -0.9V for H2O2 and HMTD respectively, Figure 2c. Figure 2 In order to pour light into the nature of the silver-decorated CF electrodes, the quality of the silver benzoate modification was evaluated by SEM analysis. As stated before, following chemical modification of the electrode, a single linear sweep voltammetry scan from 0 to -1.6 V was conducted. SEM-SE and SEM-BSE images demonstrated the formation of silver nanoparticles of 50-100nm in diameter, Figure 3b, which were distributed in low density around the carbon fibers surface. The same electrode was used again for 100 cycles of the same electrochemical procedure, then analyzed by SEM. SEM-SE and SEM-BSE images showed the formation of a larger density of silver nanoparticles of similar dimensions around the carbon-fibers electrode’s surface, Figure 3c. These experiments clearly demonstrate the density-controlled decoration of silver nanoparticles by the proposed procedure. Clearly, SEM analyses showed that a high density of silver nanoparticles decorated the surface of the FC electrodes after 100 cycles of linear sweep voltammetry scans (Figure 3C), while a much lower density of silver nanoparticles covered the surface of the FC following only one cycle of linear sweep voltammetry (Figure 3b). To validate the reduction of silver benzoate into silver nanoparticle by the electroreduction procedure, X-ray photoelectron spectroscopy (XPS) analysis was applied for the unmodified FC electrodes (Figure 3a), and following 1 and 100 cycles of linear sweep voltammetry scan cycles, Figure 3. The analysis showed a decrease in the concentrations of organometallic ionic (Organo-Ag) and oxide silver (oxide-Ag) atoms on the surface of the modified FC electrodes, in parallel to the increase in the concentration of metallic silver (Metalic Ag). Importantly, the presence of fluorine (F) and sulfur (S) atoms, indicates the electrode’s surface was successfully modified with Nafion. Figure 3 In order to further optimize the sensitivity performance of the AgNP-decorated FC electrodes, we performed a series of sensing experiments, this time using modified FC electrodes with higher silver nanoparticles density, after 10 scanning voltammetry scans. Indeed, this higher density of 9 ACS Paragon Plus Environment


decorated Ag nanoparticles led to a considerably improved sensitivity, the lowest limit of detection in a solution of 50ppb for TATP and 80ppb for HMTD, Figure 4a and b. For both explosives, an improvement in the H2O2 and explosives electroreduction peaks separation was also observed, with a peak separation of ca. 1V and 1.1V for TATP and HMTD respectively. This is due to an observed significant shift of the electroreduction signal of both explosives, the H2O2 remaining at a stable cathodic potential of ca.-0.4V, Supplementary Figure S2. Furthermore, the peaks of TATP and HMTD linearly depended on the concentrations of these explosives (R=0.997, a fact of importance in analytical applications. Figure 4 Additionally, further increasing of the silver nanoparticles density leads to a dramatic broadening of the electroreduction peaks of both explosives, and a pronounced decrease in the H2O2-toexplosives peaks separation, without any added sensitivity improvement, Supplementary Figures S3 and S4. For these reasons, an Ag nanoparticles density of 2.3×108 AgNPs/cm2 was found to be the optimal value for achieving the best sensitivity, as well as the best analytical discriminative capabilities. Importantly, the AgNPs-decorated FC electrodes are not affected by the presence of large amounts of potential analytical interferents, such as acetone, tobacco, organic solvents (ethanol, acetonitrile, etc.), perfumes, sugar, alcoholic beverages, and more. No signals are observed for none of these potential interferents at the described conditions. Notably, real-world vapor detection of TATP and HMTD was performed in our lab by the use of a home-built air-sampling device, Figure 5a (I and II). The device consists of a 4lt/min air sampling pump, a collection port suited for the assembly of our modified FC electrode and the required detection electronics board. Vapor sampling experiments were performed and showed the detection capabilities of our approach. Vapor samples collection of only 5 seconds were sufficient for the effective detection of TATP vapor traces (TATP displays a vapor pressure 0.015mmHg at 20oC), Figure 5b (III). Importantly, our electrodes are readily reusable after each sampling and sensing cycle, Figures 5a (III) and 5b (III). Also, a simulated real-world experiment on hidden explosives was performed (hidden inside a double-walled plastic bag and under the jacket, Figure 5b (I and II), demonstrating the capability of our proposed set-up to rapidly, selectively and 10 ACS Paragon Plus Environment

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sensitively detect the presence of a hidden small amount of TATP (10mg TATP), Figure 5b. Clearly, or FC modified electrodes are capable of collecting the molecular vapor traces of explosives through physicochemical adsorption and pre-concentrating these molecules for the subsequent electrochemical detection. Figure 5 Conclusions The decoration of FC electrodes with silver nanoparticles of controlled density was proven as an efficient method for the direct, sensitive and selective detection of peroxide-based explosives, in the presence of H2O2 and dissolved oxygen. The catalytic capabilities of the modification enabled to separate the signals of H2O2 and O2 from that of the peroxide-based explosive analyte, in a single cycle of linear sweep voltammetry measurement, without time-consuming preprocessing steps. Due to the distinct electroreduction peaks shape and their potential values, the analysis could distinguish between different peroxide-based explosives, adding multiplex sensing capabilities to a single working electrode. Simple linear sweep voltammetry measurements demonstrated high sensitivities and allowed detection of TATP and HMTD down to concentrations of ca. 100 ppb.43,44 Furthermore, increased sensitivity could be achieved by differential pulse voltammetry, Supplementary Figure S5, without extending the duration of the measurement.39 Finally, this approach demonstrated its capability to rapidly and sensitively detect TATP and HMTD directly from air samples after short collection time periods of only 3 seconds. Our electrodes display remarkable stability with no signs of analytical performance degradation after continuous use for several weeks and hundreds of sensing cycles. This approach represents the first demonstration on the rapid, selective and sensitive detection of peroxide-based explosives, TATP and HMTD, by simple straightforward and cost-effective electrochemical means, and not affected analytically by the presence of high concentrations of potential chemical interferents. Acknowledgments This work was in part financially supported by the Legacy Fund, Israel Science Foundation (ISF).



ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Figures S1-S5 Synthesis of TATP and HMTD. Detection of HMTD and TATP using the highest density of AgNPs, Detection of TATP by scanning voltammetry vs differential pulse voltammetry, Electrochemical studies parameters, Chemical modification, Silver nanoparticles density control, Scanning electron microscope characterization, X-ray photoelectron spectroscopy characterization X-ray photoelectron spectroscopy characterization, Error Analysis.


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Figure Captions Figure 1. (a) Unmodified FC electrode inability to separate between TATP and H2O2 electroreduction peaks. Electrochemical sensing of increasing concentrations of TATP in the absence (black curve) and presence of H2O2 (marked by red, blue, turquoise, and pink lines), in a background solution containing a mixture of NaOH in deionized water (pH=12) and acetonitrile at 7: 3 volume ratio, and 0.1 M TBABF4 (marked by black line). (inset) Molecular structure of the explosive TATP. (b) TATP detection by AgNPs-decorated CF electrodes. Concentrationdependent electrochemical sensing of TATP (marked by colored lines), in a background solution containing a mixture of NaOH in deionized water (pH=12) and acetonitrile at 7: 3 volume ratio, and 0.1 M TBABF4 (marked by the black line). The signal of TATP (marked by the green arrow) was used for extraction of the calibration curve. (c) The calibration curve extracted from the peaks of TATP and calculated according to the equation: ∆I = background current - current at the peak of TATP’s signal (marked by the green arrow in (b)). (d) Detection of TATP in the absence and presence of 500 ppm H2O2. 80 ppm TATP were analyzed alone (marked by red line), or in the presence of 500 ppm H2O2 (marked by the black line). (inset) The concentration-dependent detection of H2O2. The surface area of the microcarbon-fibers electrode was 0.35 cm2. The scan rate of linear sweep voltammetry was 0.1 V/second. Figure 2. HMTD detection by silver NP-decorated CF electrodes. (a) Electrochemical sensing of increasing concentrations of HMTD (marked by colored lines), in a background solution containing a mixture of NaOH in deionized water (pH=12) and acetonitrile at 7:3 volume ratio, and 0.1 M TBABF4 (marked by the black line). The signal of HMTD (marked by The green arrow) was used for extraction of the calibration curve. (inset) Molecular structure of the explosive HMTD. (b) The calibration curve extracted from the peaks of HMTD and calculated according to the equation: ∆I = background current - current at the peak of HMTD’s signal (marked by the green arrow in (a)). (c) Detection of HMTD in the presence of H2O2. 180 ppm HMTD were analyzed alone (marked by the black line), or in the presence of 40 ppm H2O2 (marked by red line). Please note that the signal peak of hydrogen peroxide appears at a potential of -0.4V and not at potentials of –0.9V where HMTD peak appears. The surface area of the microfibers carbon electrode was 0.35 cm2. The scan rate of linear sweep voltammetry registration was 0.1 V/second.



Figure 3. Surface analysis of silver benzoate-modified microfibers carbon electrodes. The surface of the FC electrode was imaged by a scanning electron microscope (SEM) using secondary electrons (SE) and backscattered electrons (BSE). The atomic concentrations of carbon (C), oxygen (O), silver (Ag), fluorine (F), and sulfur (S), were determined using X-ray photoelectron spectroscopy (XPS). The background solution contained a mixture of NaOH in deionized water (pH=12) and acetonitrile at 7: 3 volume ratio and 0.1 M TBABF4. The surface area of the FC electrode was 0.35 cm2. The scan rate of linear sweep voltammetry was 0.1 V/second. (a) 'Unmodified' FC electrode. (b) Silver benzoate-modified FC electrode following one linear sweep voltammetry scan, from 0 to -1.6 V. (c) Silver benzoate-modified FC electrode following 100 linear sweep voltammetry scans from 0 to -1.6 V. Figure 4. Detection of TATP using a higher density of AgNPs-decorated FC electrodes. (a) Detection of increasing concentrations of TATP (marked by colored lines) was conducted in background solution containing a mixture of NaOH in deionized water (pH=12) and acetonitrile at 7:3 volume ratio, and 0.1 M TBABF4 (marked by the black line). The stability of the electrode has been indicated by the reproducibility of the peak’s voltage of TATP (marked by green arrow), that was further used for the extraction of the calibration curve. (inset) Calibration curve based on the signals of TATP, and calculated according to the equation: ∆I = background current - current at the peak of TATP’s signal (marked by the green arrow in (a)). (b) Detection of HMTD using a higher density of AgNPs-decorated FC electrodes. Detection of increasing concentrations of HMTD (marked by colored lines) was conducted in background solution containing a mixture of NaOH in deionized water (pH=12) and acetonitrile at 7:3 volume ratio, and 0.1 M TBABF4 (marked by the black line). The stability of the electrode has been indicated by the reproducibility of the peak’s voltage of HMTD (marked by green arrow), that was further used for the extraction of the calibration curve. (inset) Calibration curve based on the signals of HMTD, and calculated according to the equation: ∆I = background current - current at the peak of HMTD’s signal (marked by the green arrow in (a)). Figure 5. Real-world detection of TATP samples. (a) Photographs of the detection set-up components, the CF electrode package, and the air-sampling pump (I and II). Linear sweep voltammetry curves (obtained at a scan rate of 0.1 V/second) of an Ag-NPs decorated FC 16 ACS Paragon Plus Environment

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electrode, in the presence of an electrolyte solution containing a mixture of NaOH in deionized water (pH=12) and acetonitrile at 7: 3 volume ratio and 0.1 M TBABF4, after sampling clean air (black line) and a TATP-containing air sample for 5 seconds (red line), at a distance of 20 cm from a paper-embedded 1mg TATP sample (III). (b) Real-world detection experiment of a concealed TATP-sample (I and II). Linear sweep voltammetry curves after sampling clean air (black line), a concealed-TATP air sample for 10 seconds (red line), at a distance of 20 cm from the TATP source as shown in the photographs, and the electrochemical signal obtained from the same electrode after two electroreduction scan cycles in the absence of TATP (III). Blue curves depict the electroreduction signal obtained after one cycle of detection, demonstating the self cleaning capabilities of the electrodes.



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Direct and Selective Electrochemical Vapor Trace Detection of

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