Electrochemical Determination of TNT, DNT, RDX and HMX with Gold

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Electrochemical Determination of TNT, DNT, RDX and HMX with Gold Nanoparticles/Poly(Carbazole-Aniline) Film– Modified Glassy Carbon Sensor Electrodes Imprinted for Molecular Recognition of Nitroaromatics and Nitramines #ener Sa#lam, Ay#em Üzer, Erol Erça#, and Re#at Apak Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00715 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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Analytical Chemistry

Electrochemical Determination of TNT, DNT, RDX and HMX with Gold Nanoparticles/Poly(Carbazole-Aniline) Film–Modified Glassy Carbon Sensor Electrodes Imprinted for Molecular Recognition of Nitroaromatics and Nitramines Şener Sağlam1, Ayşem Üzer1, Erol Erçağ2, Reşat Apak1,*,† 1

Istanbul University, Faculty of Engineering, Chem. Dept., 34320, Istanbul, Turkey Aytar Cad., Fecri Ebcioglu Sok., No. 6/8, Levent, 34340 Istanbul, Turkey *Corresponding author. Tel.: +90 212 473 7028; fax: +90 212 4737180. E-mail: [email protected]

2

ABSTRACT: Since nitroaromatic and nitramine type energetic materials, mostly arising from military activities, are persistent pollutants in soil and groundwater, on-site sensing of these hazardous chemicals has gained importance. A novel electrochemical sensor was designed for detecting nitroaromatic and nitramine type energetic materials, relying on gold nanoparticles (Aunano) – modified glassy carbon (GC) electrode coated with nitro-energetic memory poly(carbazole-aniline) copolymer (Cz-co-ANI) film (e.g., TNT memory–GC/P(Cz-co-ANI)-Aunano modified electrode). Current was recorded against concentration to build the calibration curves that were found to be linear within the range of 100-1000 µg L-1 for 2,4,6-trinitrotoluene (TNT) and 2,4- dinitrotoluene (DNT); 50-1000 µg L-1 for 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The corresponding limits of detection were 25 µg L-1 for TNT, 30 µg L-1 for DNT, and 10 µg L-1 for both RDX and HMX, using nitro-energetic memory–GC/P(Cz-co-ANI)-Aunano electrodes. These electrodes were used separately, and specific determinations were made in various mixtures of nitro-energetic materials. The developed method could be efficiently used in electroanalyzing nitroaromatics and nitramines in military explosives, i.e., comp B, octol and comp A5. The sensor electrodes were specific for the tested nitro-energetic compounds, and did not respond to paracetamol-caffeine based analgesic drug, acetylsalicylic acid (aspirin), sweetener and sugar that can be used as camouflage materials in passenger belongings. The developed method was statistically validated against the standard LC-MS reference method in contaminated clay soil samples containing TNT and RDX explosives. KEYWORDS: Nitro-energetic materials, Gold nanoparticles, Electrochemical sensors, Electropolymerization, Molecular imprinted polymers.

Nitroaromatic and nitramine type energetic materials enter the environment mostly by military actions1, and are persistent pollutants of soil and water at sites where energetic materials are stored, produced or consumed2. Nitroaromatic energetic materials are especially toxic and may cause health problems such as anemia, skin irritation, cataract and abnormal liver function in humans and animals3. Thus, there is considerable interest in the simple and low-cost determination of nitroexplosives in envorimental samples4. Different analytical techniques have been reported for detection of trace explosives such as gas and liquid chromatography coupled with mass spectrometry1,5-7, high performance liquid chromatography (HPLC)8,9, 10,11 12 spectrophotometry , reflectance spectroscopy and Raman spectroscopy13; on the other hand, many of the sophisticated techniques are relatively expensive, time-consuming and require qualified personnel. As an alternative to these methods, electrochemical methods are preferable because of their high sensitivity and selectivity, fast response, low cost, easy operation and portability14. Different coating materials have been used in preparing the working electrode for the determination of nitroaromatic and nitramine type energetic

materials. Nitro-explosives basically owe their electroactivity to the easily reducible nitro groups found in their structures15. Different working electrodes, such as glassy carbon electodes (GCE)16, carbon fiber electrodes2 and screen-printed carbon electrodes17, were used for the electrochemical detection of nitroaromatic and nitramine explosives. In addition, the working electrodes can be used as sensor electrodes when they are modified with different polymeric materials15 and nanoparticles18. Nanoparticle-based sensors have additional advantages such as high surface area, improved surface activity, and unique electrical/optical properties19. Molecular imprinting/memory techniques are used to prepare sensing materials with cavities that recognize a particular molecule in terms of shape, size and functional group. The potential analyte (for which a reasonable selectivity is aimed) is added as a template molecule during polymer synthesis, and this template is extracted from the matrix after polymer synthesis. The greatest advantages of molecular imprinted polymers (MIPs) are high selectivity and affinity to the target molecule20,21. Efforts for the determination of energetic materials with molecular imprinted polymers have gained importance in recent years. Shi et al.

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developed a graphene(GN)-polyaniline(PANI) nanocomposite that recognizes TNT on the electrode surface, and picric acid was used as a template analogue for the energetic material; the molecular imprinted electrode was prepared via synthesis taking more than 12 hours, and TNT was determined with the use of differential pulse voltammetry (DPV) and cyclic voltammetry (CV)22. Alizadeh et al. reported that MIP and non-imprinted polymer (NIP) were synthesized for selective TNT determination (with comparative evaluation) and used this material to prepare a carbon paste (CP) electrode with a nmol L-1-level detection limit for TNT; this sensor electrode was used for the detection of TNT in various soil and water samples4. Trammel et al. prepared a sensor electrode using imprinted diethylbenzene-bridged periodic mesoporous organosilicas (synthesized in about 5 days) for electrochemical determination of TNT, for which LOD was 13 µg L-1 23. Nie et al. proposed a DNT memory–electrochemical sensor by imprinting DNT on the surface of functionalized multiwalled carbon nanotubes/polyethyleneimine (fun-MWCNTs/PEI). Using this sensor, selective determination of DNT in the nmol L-1 range was achieved24. In the present study, nitro-energetic memory– electrochemical sensors in conjunction with the square wave voltammetry (SWV) technique were developed for highly sensitive and selective determination of nitroaromatic and nitramine type energetic materials. These sensor electrodes were prepared in two steps. In the first step, GC working electrodes were coated with the target energetic material (template) and carbazole-aniline monomers using electrochemical polymerization. TNT, DNT, RDX and HMX were used as template molecules. After this step, the surface of the energetic material memory–copolymer electrode was functionalized with gold nanoparticles. The designed electrodes were efficiently applied to the determinations of binary and multi-component mixtures of nitro-energetics in military explosives and energetic material–contaminated clay soil samples. A separate electrode was used for each explosive with special recognition of the target analyte. An important advantage of the proposed method was that it did not respond electroactive compounds that can be used as possible camouflage materials in similar appearance to explosives (carried as passenger belongings), such as paracetamol, caffeine, acetylsalicylic acid, aspartame and D-glucose; validation of the proposed sensing method was statistically performed against the reference LC-MS procedure. EXPERIMENTAL SECTION Safety Note. Nitroaromatics and nitramines are highly energetic explosives and bear an explosion risk during handling and loading when they are exposed to shock, friction, impact, fire, or electrostatic charge. This may result in detonation, deflagration, and violent burning reactions. Ingestion or inhalation of the toxic particles or vapors of these explosives must be avoided as well. For storage and laboratory studies, stock solutions prepared with appropriate solvents such as acetonitrile and acetone should be used. Chemicals, Solutions and Instruments. The explosive materials TNT (2,4,6-trinitrotoluene), DNT (2,4dinitrotoluene), RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazosin), Comp B composite explosive (containing 60 % RDX, 39 %TNT, and 1 % wax), octol composite explosive (containing 70% HMX and 30% TNT), and Comp A5 composite explosive

(containing 99% RDX and 1% wax), were procured from Makine Kimya Endustrisi Kurumu (MKEK: Machinery&Chemistry Industries Institution) under the guidance of Milli Savunma Bakanligi, Teknik Hizmetler Daire Baskanligi (Ministry of National Defense, Office of Technical Services) of Turkey. While preparing and diluting the solutions of selected explosives, HPLC grade extra pure acetonitrile (E. Merck) was used. The electrodes were cleaned using an alumina slurry from Baikowski International Corp. (0.05 mm, Baikalox 0.05CR). The supporting electrolytes for maintaining conductivity (used for analysis and electropolymerization, respectively) were tetrabutylammonium bromide (TBABr) and tetrabutylammonium perchlorate (TBAP) (Sigma-Aldrich). Isopropanol (Sigma-Aldrich), acetone and ethanol (both technical grade) were used for cleansing the electrodes. The electrode coating materials, Carbazole (Cz) and aniline (ANI), together with the other chemicals, were purchased from E.Merck (Darmstadt, Germany). The standard stock solutions of individual explosives as well as of military-purpose mixed explosives (Comp B, octol and comp A5) at 500 mg L-1 concentrations were prepared in extra pure acetonitrile (ACN). Gold (III) chloride solution (99.99% trace metals basis, 30% by wt. in dilute HCl) was utilized for depositing gold nanoparticles to GC electrode coated with energetic material memory–Cz-ANI copolymer, and the 0.04% (w/v) working solution was prepared from this stock solution. Voltammetric experiments were performed with a Gamry Instruments model Reference 600 potentiostat/galvanostat/ZRA interfaced to a PC computer and controlled Gamry Framework software. Glassy carbon (BASi stationary voltammetry electrodes; ø 1.6 mm, area 0.02cm2) was used as the working electrode. A platinum (Pt) electrode and an Ag/AgCl electrode were used as the auxiliary and reference electrodes, respectively. Contaminated clay soil samples were analyzed with a Shimadzu 8040 LC-MS liquid chromatograph-mass spectrometer. The FEI Model Quanta 450 FEG instrument was used for SEM measurements of the surface of the copolymer coated working electrode. Voltammetric Method Optimization. The parameters of electrode type, scan rate, solvent selection, and supporting electrolyte concentration were separately investigated. Characteristic reduction potentials for energetic materials were observed in voltammetric measurements made with a glassy carbon electrode chosen as the working electrode. A scan rate of 50 mV s-1 was selected, TBABr was adopted as the supporting electrolyte (with an optimal concentration of 0.04 mol L-1), and ACN was used as solvent in the work. Glassy carbon electrode pre-treatment. GC electrode polishing was performed with the use of an alumina suspension in the mode of circular movements of a few minutes duration, then washing with distilled water, and sonication in distilled water for 5 min. The electrode was resonicated for another 5 min in a mixture of isopropanolacetonitrile (1:1, v/v). It was understood that this procedure was sufficient for cleaning the electrode and that no peak was observed in the blank(baseline) solution during SWV scan. Electrochemical Polymerization of Energetic Material Memory–Cz-co-ANI Copolymer Film. The monomer mixture (2×10-2 mol L-1 Cz - 1×10-2 mol L-1 ANI - 100 mg L-1


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Analytical Chemistry energetic material) solution was prepared in ACN. When developing the analyte memory–modified working electrode, the energetic material was added at a concentration of 100 mg L-1 to monomer mixture solution (see: Supporting information for optimizing the {energetic material+carbazole+aniline} mixture ratio). Five milliliters of this solution were taken into the working cell, and 0.1 mol L-1 TBAP was added for electrochemical polymerization. Cyclic voltammetry was performed for polymerization within the potential range 0.0 V – 1.4 V, with 20 mV s-1 scan rate for 5 cycles. Eventually, for removing any unbound material from the surface, the modified GCE was rinsed with ACN. Au Nanoparticles Electrodeposition on the CopolymerCoated Electrode. The energetic material memory–GC/P(Czco-ANI) electrode was coated with Au nanoparticles with the use of cyclic voltammetry using 0.04 % (w/v) HAuCl4 (2.5 ml) + 0.1 mol L-1 H2SO4 (2.5 ml) solutions through electrochemical deposition. Coating was made within the (-0.4 V to 0.4 V) range at 50 mVs-1 scan rate. The quantity of deposited gold particles on the energetic material memory– P(Cz-co-ANI)/GCE surface was estimated by checking cycle numbers, at an optimal value of 40 cycles. The golden-colored electrode manufactured in this manner was named as energetic material memory–GC/P(Cz-co-ANI)-Aunano. (Impedance measurements and SEM image of the modified electrode are given in Supporting Information). Electrochemical Studies with Nitroaromatic and Nitramine Materials. The working solutions of 100-1000 µg L-1 TNT and DNT, 50-1000 µg L-1 RDX and HMX were prepared from the corresponding stock solutions, and 5 ml of solution was transferred to the measurement cell, to which 0.04 mol L-1 TBABr was added as supporting electrolyte. The modified electrode was cleaned in ACN for 5 min each time before measurement. SWV was carried out in a potential range between 0.2 V and -1.6 V, and peak potentials for each energetic compound were established. TNT memory– GC/P(Cz-co-ANI)-Aunano modified electrode for TNT determination, DNT memory–GC/P(Cz-co-ANI)-Aunano modified electrode for DNT determination, RDX memory– GC/P(Cz-co-ANI)-Aunano modified electrode for RDX determination, and HMX memory–GC/P(Cz-co-ANI)-Aunano modified electrode for HMX determination were used as sensor electrodes. Analysis of Synthetic and Real Energetic Material Mixtures. Binary and multi-component synthetic mixtures of TNT, DNT, RDX and HMX at different ratios were prepared and analyzed with the developed SWV method. Real energetic mixture samples of Comp B, Octol and Comp A5 were prepared in ACN at 1 mg L-1 and each of the components in the mixtures were determined with the proposed SWV method. Assay in The Presence of Electroactive Camouflage Materials. Paracetamol-caffeine based analgesic drugs, acetylsalicylic acid, aspartame-based sweetener and sugars were studied as possible interferents because of their similar color and appearance. One mg L-1 of TNT, DNT, RDX and HMX were determined in presence of 1000-fold (60 fold for caffeine) concentrations of electroactive compounds. The potential interferent compounds were tested with the developed SWV method. Validation of SWV Method Against LC-MS Using Contaminated Clay Soil Samples. One milliliter of 10 mg L-1

TNT and 1 mL of 15 mg L-1 RDX were mixed into 2 g clay soil for the preparation of contaminated clay soil sample. Two successive portions of 10 mL, followed by 5 mL of ACN were added to the clay soil sample, and kept in an ultrasonic bath for 5 min each time. The contents were transferred to centrifuge tubes, and centrifuged for 5 min at 5000 rpm, filtered with GF-PET and taken to a 25 mL flask with dilution to the mark25 (final concentrations were 400 µg L-1 TNT and 600 µg L-1 RDX). Subsequently, 5 mL of this solution was transferred into the working cell and each of the components were determined with the developed SWV method. For method validation against LC-MS method26, the sample was diluted 2-fold with ACN. LC-MS conditions: injection volume of sample was 10 µL and isocratic elution was used. Two different ammonium acetate solutions, each at 5 mmol L-1 concentration, in water and in pure methanol, were used as mobile phases A and B, respectively, at a flow rate of 0.3 mL min-1. LC-MS analysis was carried out using the negative ion mode electrospray ionization method and the ionization voltage was 3.5 kV. The product ion and procursor ion were 225.8 m/z and 196.1 m/z for TNT (collision energy: 15.0 V), and were 281.1 m/z and 46.15 m/z for RDX (collision energy: 10.0 V). Method validation against LC-MS determination of TNT and RDX were made by means of the statistical Student: t- and F-tests. RESULTS AND DISCUSSION Fabrication of Energetic Material Memory-Cz-ANI Copolymer Film on GCE. Electrochemical polymerization of carbazole-aniline monomer mixture (containing the energetic material) was carried out by cyclic voltammetry using a potential range from - 1.2 V to 1.4 V, scanning speed of 20 mV s-1, and 5 cycles. The CV of TNT memory–GC/P(Cz-coANI) electrode is seen in Fig. S-1, where an oxidation peak at 0.95 V and a reduction peak at 0.75 V for ANI, an oxidation peak at 1.30 V for Cz and a reduction peak at - 1.03 V for TNT were obtained. These peaks belong to the cation radical oxidation of the monomers. The quantity of polymer deposited on the electrode increased with increasing number of cycles and decreased with the amount of residual monomers in solution. When the energetic material memory–modified electrodes were individually developed with DNT, RDX and HMX, the reduction peaks were obtained at -1.0 V, -0.92 V and -0.97 V, respectively. Poly(Cz)-coated GCE showed electroactivity toward energetic materials, but the surface of the electrode exhibited physical instability and the coating could be spilled from the surface relatively easily after a number of measurements. Conversely, when PANI was used alone on the electrode, it would not show any signal for energetic materials. On the other hand, the copolymer (P[Cz-co-ANI]) was both physically stable (i.e., cracks and spills could not be observed on the surface of the electrode after several measurements) and electroactive for energetic materials. The copolymer-coated electrodes were tested for stability in a supporting electrolyte (without monomer) within a potential interval from -1.2 V to 1.4 V, scan speed of 20 mV s-1, and 5 cycles. The currents recorded at the end of the first and fifth cycles did not differ considerably. After this procedure, the remaining dimers or oligomers were entirely polymerized on the coated electrode surface. Additionally, the amount of surface-bound TNT was found to stabilize on the electrode



surface (as observed from the reduction peak current of TNT), although the number of cycles varied. Electrodeposition of Au-Nanoparticles on Energetic Material Memory–Cz-ANI Copolymer Coated GCE. The purpose of modifying the surface of modified working electrode with gold nanoparticles was to enable more sensitive determinations of energetic materials by protecting the copolymer layer and increasing the conductivity of the electrode surface. The reduction peak appearing at about 0.26 V in the first cycle (Fig. 1) could be attributed to the reduction of Au(III) to Au-nanoparticles on the surface of the energetic material memory–GC/P(Cz-co-ANI) electrode. With increasing the number of cycles, free Au3+ in the medium close by the electrode was reduced to Au0 and Aunano deposited on the electrode surface, leading to decrease of current. The stabilization of current occurred after 40 cycles, confirming that the maximal amount of Au nanoparticles accumulated on the surface of the electrode. Either recoating or special cleaning of the electrode (except cleaning in ACN for 5 min) was not required before each run, because the once-prepared energetic material memory–GC/P(Cz-co-ANI)-Aunano electrode could be used over and over during the measurements of the day.

Figure 1. Cyclic voltammograms of Au-nanoparticles on energetic material memory–GC/P(Cz-co-ANI) electrode.

AuNPs has been widely used to fabricate nanocomposites because of their unique chemical, physical and optical properties. AuNPs have many superior properties such as tunable morphology, chemical/mechanical stability, good electrical and surface properties, strong adsorption ability, small particle size and high surface reactivity27,28. The synergistic effects of the good elect rical conductivity of AuNPs were used to improve current response, electronic transmission, effective surface area and sensitivity of the sensor27,29. The imparted mechanical stability to the nanocomposite may be ascribed to the filling of vacancies (holes) in the composite structure by the adhesive gold nanoparticles30. CV Characterization of the Modified Electrodes. For the purpose of electrode characterization, CV was applied in monomer-free medium to bare GCE, TNT memory‒GC/P(Czco-ANI) electrode and TNT memory‒GC/P(Cz-co-ANI)Aunano electrode with a scan speed of 50 mV s-1 in the potential range between -1.2 V and 1.4 V.

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Figure 2. CV characterization of bare GCE, TNT memory‒ GC/P(Cz-co-ANI) and TNT memory‒GC/P(Cz-co-ANI)-Aunano electrodes.

As is apparent from Figure 2, reduction peaks could be obtained for both TNT memory‒GC/P(Cz-co-ANI) and TNT memory‒GC/P(Cz-co-ANI)-Aunano (an oxidation peak was obtained at about 1.0 V for polyaniline), showing that both electrodes had electroactivity and that Aunano-coating of the TNT memory‒GC/P(Cz-co-ANI) electrode caused an increase in this activity. Reduction peaks emerged at -0.97 V for the TNT memory‒GC/P(Cz-co-ANI) electrode, and at -1.06 V for TNT memory‒GC/P(Cz-co-ANI)-Aunano (reduction peak shifted to slightly lower potential). The bare GC electrode did not yield a noticeable peak under identical conditions. Determination of Nitroaromatic and Nitramine Type Energetic Materials with Modified Sensor Electrode. Nitroaromatic and nitramine type energetic materials were all electroactive and determined easily with voltammetric methods. The reduction reactions of these materials occurred in the negative potential range. The reduction potential of TNT and DNT commonly appeared at - 1.0 V, of RDX at - 1.03 V and of HMX at - 1.05 V. Square wave voltammograms and structures of all four explosives can be observed in Fig. 3. Calibration of TNT using TNT-memory–GC/P(Cz-co-ANI)Aunano and DNT using DNT-memory–GC/P(Cz-co-ANI)Aunano at -1.0 V potential, RDX using RDX-memory– GC/P(Cz-co-ANI)-Aunano at -1.03 V and HMX using HMXmemory–GC/P(Cz-co-ANI)-Aunano at -1.05 V yielded a linear correlation of current against concentration (Table 1). The smallest detectable amounts of explosives were 125 ng for TNT, 150 ng for DNT, and 50 ng for RDX and HMX. Table 1. Linear range, LOD and LOQ values for the tested nitroaromatic and nitramine energetic materials. TNT

DNT

RDX

HMX

Linear rangea

100-1000

100-1000

50-1000

50-1000

LODa

25

30

10

10

LOQa

83

100

33

33 a

In µg L units. The electroanalytical determinations of the nitroexposives were made with the aid of the calibration lines (A= mC+n), and the analytical performance figures (i.e., limits of detection -1


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Analytical Chemistry (LOD)=3σbl/m and quantification (LOQ)=10σbl/m, where σbl denoted the standard deviation of a blank and m the slope of the calibration curve) were collectively tabulated in Table 1 for TNT, DNT, RDX and HMX. The analytical sensitivity of the determination (m) varied between {(5.1-7.9)±(0.50.8)}×103 µA•L•µg-1, with a linear correlation coefficient (r) of 0.996-0.997 (Table S-1). Aside from the surface plasmon resonance (SPR) detection (measured as reflectance difference versus concentration) of RDX by molecularly imprinted composites, such as Kemp’s acid (1,3,5-trimethylcyclohexane1,3,5-tricarboxylic acid)–imprinted bisaniline-crosslinked Aunanoparticles composite31, useful information regarding the electrochemical determination of RDX and HMX with molecularly imprinted electrodes is almost absent in the literature. Therefore, it is remarkable that in this work, electrochemical sensing of RDX and HMX was accomplished with the proposed electrode at comparable (or better) sensitivity to that of TNT and DNT (Table 1), because intramolecular charge-transfer (between amine-N and nitro-N atoms) of RDX and HMX largely hinders intermolecular charge-transfer interactions with poly-amine/aniline functionalities. Determination of TNT was also performed with GC/P(Cz-co-ANI)-Aunano electrode and linear calibration range was found to lie within the concn. range of 10-50 mg L1 . When the analytical performance of the TNT-memory– GC/P(Cz-co-ANI)-Aunano electrode was compared with that of non-imprinted GC/P(Cz-co-ANI)-Aunano electrode, it was

found that TNT could be determined 100 times more sensitively (see Fig. S-4 in Supporting Information for details). Analytical Findings for Synthetic and Real Mixtures of Energetic Materials. Soils contaminated by the testing, manufacture, and disposal of explosives and ammunitions may contain mixtures of TNT, RDX and HMX, the most common found of the military-grade explosives32. Binary and multicomponent synthetic mixtures of TNT, DNT, RDX and HMX were prepared and determined with the proposed SWV procedure using energetic material-memory–GC/P(Cz-coANI)-Aunano electrodes. One mg L-1 of TNT was quantified in the presence of 5-fold concentration of DNT, RDX, HMX and (DNT+RDX+HMX) synthetic mixtures, and recovery of TNT was between 98.77-105.86 %. Similar to TNT, binary and multi-component synthetic mixtures were prepared for DNT, RDX and HMX. Recoveries of DNT, RDX and HMX were 97.31-101.13 %, 95.04-97.98 %, and 99.78-105.14 %, respectively (results were given in Tables S-2, S-3, S-4 and S5) (for five repetitive measurements; N=5). Real samples of energetic substances mixtures of Comp B, Octol and Comp A5 (at 1 mg L-1 each) were analyzed with the proposed SWV method. Using the proposed procedure, the recovery of TNT and RDX from Comp B, TNT and HMX from Octol, and finally RDX from Comp A5 samples were in the range of 98.3-102.1 % and 97.1-99.2 %, 98.5-99.8 % and 98.8-101.3 %, and finally 98.5-99.5 %, respectively(N=5).

Figure 3. Square wave voltammograms and structures of TNT (a), DNT (b), RDX (c) and HMX (d) recorded with energetic material memory–GC/P(Cz-co-ANI)-Aunano electrodes.

Interference of Electroactive Camouflage Materials to the Electrochemical Determination of Energetic Materials. Analgesic drug, acetylsalicylic acid, sweetener and sugar may be used as camouflage materials for TNT, DNT, RDX and

HMX, owing to their appearance and color similarities. Paracetamol-caffeine, acetylsalicylic acid, aspartame and Dglucose did not interfere with the determination of TNT, DNT, RDX and HMX (at 1 mg L-1 each) at 1000-fold (60-fold for



caffeine) concentrations levels, yielding energetic materials recoveries between 96.25-106.37 % (Table S-6 and Figure S5). Analytical Results for Contaminated Clay Soil Samples. TNT and RDX were analyzed each from 50 to 500 µg L-1 working solutions, and the mean values of three repetitive injections in LC-MS were used for calculations. The peak area versus concentration equations used for the calibration of TNT and RDX were given in Supporting Information. The contaminated clay soil sample was either analyzed in the original extract (by SWV) or in diluted form (analyzed by LC-MS); quantification of TNT with the developed voltammetric method and with LC-MS in the 1:2 (v/v) ACNdiluted solution yielded 396.1 ± 7.02 µg L-1 and 397.6 ± 8.56 µg L-1 concentration levels, respectively, in the main solution (N=5 repetitive runs for each method). RDX was also measured in the contaminated clay soil sample to give 593.9 ± 11.16 µg L-1 and 597.4 ± 7.06 µg L-1 concentrations with SWV and LC-MS, respectively. Statistics was used to compare the data found by the developed and reference LC-MS methods to essentially show no significant differences between accuracy and precision of results (Table S-7). CONCLUSIONS It was shown herein that molecularly imprinted nitroenergetic memory–electrochemical sensors may provide a rapid, low-cost, easily-prepared, mechanically/chemically stable, and greatly sensitive alternative for the analysis of complex samples of field residues of explosive substances. We named these sensor electrodes specific for TNT, DNT, RDX and HMX as energetic material memory–GC/P(Cz-co-ANI)Aunano electrodes. The molecularly imprinted polymer used in electrode preparation involved a copolymer coating synthesized from carbazole and aniline, both being electrondonor Lewis bases capable of forming charge-transfer complexes (σ- and π-complexes) with nitro-aromatic explosives having electron-attracting –NO2 groups. Polymerization and crosslinking of functional monomers in the presence of nitro-explosive templates led to the formation of complementary cavities in the matrix that were able to geometrically and chemically recognize the target analytes. The integration of Au-nanoparticles to the electrode surface enhanced the electrical conductivity and catalytic ability (fast response) of the manufactured electrodes. Although Aunano was frequently used for electrode synthesis, carbazole as a strong electron-donor was seldom used, which remarkably enhanced the target analyte sensitivity of the P(Cz-co-ANI) polymer coated electrode, also by increasing the diffusion and deeper contact of the tested explosives with the more easily accessible cavities of the 3-D polymeric matrix-Au nanoparticles network through π-π interactions33,34,35. The analytical sensitivity was further increased by the electron withdrawal of nitro-aromatics and σ-/π- donor abilities of amine-aniline functionalities in the copolymer structure36. In spite of the absence of efficient electrochemical determinations of RDX and HMX with molecularly imprinted electrodes in the literature, our work has remarkably revealed that these nitramines could be effectively determined with the proposed sensor electrodes at comparable or better sensitivities than nitro-aromatic explosives. This modified GC electrode showed a sensitivity enhancement in nitro-explosives determination by decreasing the LOD levels almost 100-fold (compared to that

of non-imprinted GC/P(Cz-co-ANI)-Aunano electrode). The developed energetic material memory–electrode was shown to have additional advantages of stability, reproducibility and repeated use without damage (not requiring pre-coating for at least 15 days). As opposed to the molecularly imprinted sensor electrodes suitable for a single nitro-aromatic explosive35, or to the tediously-prepared (some taking ½–6 days) similar nitroexplosive memory–electrodes reported in the literature4,18,22-24, we were able to prepare our GC/P(Cz-co-ANI)-Aunano sensor electrodes, each specific for a given nitro-explosive, within 1 hour by electropolymerization in a carbazole-aniline monomer mixture incorporating the target analyte. Additionally, the developed methodology was succesfully adopted to the analysis of synthetic mixtures of energetic compounds and military explosives such as comp B, octol and comp A5 that can be found in the environment as explosive residues, and it was also highly selective for the explosive analytes in preference to possible interferents (analgesic drug, acetylsalicylic acid, sweetener and sugar) encountered in passenger belongings for comouflage purposes. The developed voltammetric procedure was validated through statistics against LC-MS on contaminated clay soil samples. The developed voltammetric method is assumed to be useful for determining the shelf-life of military ammunition containing nitroaromatic and nitramine energetic materials, and also for field detection of explosive residues or for measuring the treatment levels of remediated land after military use.

ASSOCIATED CONTENT Supporting Information

Additional information as noted in text. The Supporting Information is available free of charge on the ACS Publications website. Supporting information contains; optimization of electropolymerization for energetic material‒carbazole-aniline mixtures, fabrication of energetic material memory-Cz-ANI copolymer film on GCE, impedance measurements of the modified electrodes, SEM image of the developed sensor electrode, electrochemical determination of TNT with GC/P(Czco-ANI)-Aunano electrode, slope, intercept, correlation coefficient of TNT, DNT, RDX and HMX determination, analytical results of synthetic mixtures of energetic materials, interference studies for electroactive compounds, calibration equations for TNT and RDX with using LC-MS, and statistical comparison of the developed method with reference (LC-MS) method (PDF).

AUTHOR INFORMATION Corresponding Author

* Fax: +90 212 4737180. E-mail: [email protected]

ORCID ID: 0000-0003-1739-5814 Present Addresses † Turkish

Academy of Sciences (TUBA), Piyade st. No:27, 06690 Çankaya, Ankara, Turkey

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to final version of manuscript. Notes

The authors declare no competing financial interest.


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Analytical Chemistry

ACKNOWLEDGMENT The authors acknowledge the kind donation of the tested explosive samples by the Ministry of National Defence, Office of Technical Services, and the Machinery & Chemistry Industries Institution (MKEK). The authors extend their gratitude to Istanbul University Research Fund (BAP) for the support given to the thesis project-23319. The authors wish to sincerely thank Professor Esma Sezer and Professor Belkıs Ustamehmetoğlu for their valuable discussions.

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Electrochemical Determination of TNT, DNT, RDX and HMX with Gold

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