Thioglycolic Acid-Modified Gold

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Diaminocyclohexane–Functionalized/Thioglycolic acid–Modified Gold Nanoparticles–based Colorimetric Sensing of TNT and Tetryl Ne#e Ular, Ay#em Üzer, Selen Durmazel, Erol Erça#, and Re#at Apak ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00709 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 15, 2018

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Diaminocyclohexane–Functionalized/Thioglycolic acid– Modified Gold Nanoparticles–based Colorimetric Sensing of TNT and Tetryl Neşe Ular†, Ayşem Üzer†, Selen Durmazel†, Erol Erçağ‡, and Reşat Apak*†§ †Analytical

Chemistry Division, Chemistry Department, Faculty of Engineering, Istanbul University-Cerrahpaşa, 34320 Avcilar, Istanbul, Turkey ‡Aytar Caddesi, Fecri Ebcioğlu Sokak, No. 6/8, Levent, Istanbul, 34340, Turkey KEYWORDS: Trinitrotoluene (TNT), tetryl, energetic materials, nitroaromatic explosives, gold nanoparticles (AuNPs), colorimetric sensor, spectrophotometric determination, charge-transfer

ABSTRACT: Detection of explosive residues in soil and post-blast debris is an important issue in sensor design for environmental and criminological purposes. An easy-to-use and low-cost gold nanoparticles (AuNPs)-based colorimetric sensor was developed for the determination of nitroaromatic explosives, i.e., trinitrotoluene (TNT) and tetryl, capable of analyte detection at picomolar (pM) levels. The sensor nanoparticles were synthesized by functionalizing the negatively-charged thioglycolic acid (TGA)-modified AuNPs with positively-charged (±)-trans-1,2-diaminocyclohexane (DACH) at a carefully calculated pH. The working principle of the sensor is charge-transfer (CT) interaction between the electron-rich free amino (–NH2) group of DACH and the electrondeficient –NO2 groups of TNT/tetryl, added to possible nanoparticle agglomerization via electrostatic interaction of TNTMeisenheimer anions with more than one cationic DACH-modified AuNPs. The limit of detection (LOD) and limit of quantification (LOQ) of the sensor were 1.76 pM and 5.87 pM for TNT, and 1.74 pM and 5.80 pM for tetryl, respectively. TNT, tetryl and tetrytol, extracted from nitroaromatic explosive‒contaminated soil sample, were determined with the proposed sensor, yielding good recoveries. The sensor could be selectively applied to various mixtures of TNT with common energetic materials such as RDX, HMX, PETN. Additionally, common soil ions (Cl-, NO3-, SO42-, K+, Mg2+, Ca2+, Cu2+, Fe2+, Fe3+ and Al3+) as well as detergents, sugar, sweeteners, acetylsalicylic acid (aspirin), caffeine and paracetamol-based painkiller drugs, which may be used as camouflage materials for explosives, either had no adverse effects or removable interferences on the detection method. The developed method was statistically validated against a GC–MS literature method.

Nitroaromatic energetic materials, also known as nitroaromatic compounds (NACs) consisting of a benzene ring substituted with one or more nitro-groups, are prevalently used for industrial purposes as precursors of dyes, polymers and pesticides, and for military activities or terrorist attacks as explosives. Therefore, the detection of NACs has become increasingly attractive in recent years due to homeland security, environmental and humanitarian implications.1-5 As one of the most significant nitroaromatic energetic materials, 2,4,6-trinitrotoluene (TNT) has a key role in agriculture, industry and mining, besides its role in military and terrorist activities for the manufacture of landmines and bombs. TNT has also caused significant biological and environmental damage through its toxic and mutagenic effects.6-9 The other important nitroaromatic compound, 2,4,6trinitrophenyl-N-methylnitramine (tetryl), is used in military equipments as propellant and detonator. Like TNT, tetryl is also responsible for environmental pollution in soil and groundwater, and may also cause biological damage.10 Due to environmental, criminological and security needs, development of selective and sensitive methods for the determination TNT and tetryl is an analytical challenge for building a safe society and a green environment. Up to now, numerous analytical methods with different working principles

have been devised for TNT and/or tetryl detection in environmental samples (e.g., soil and groundwater) using conventional techniques such as colorimetry,11-13 14-17 18-20 fluorometry, electrochemistry, surface-enhanced Raman spectroscopy6-8,21 as well as separation/detection techniques such as chromatography,22-24 ion mobility spectrometry25 and mass spectrometry.26 In addition, paperbased sensors, which have recently gained considerable attention for on-site detection27, have also been used for the identification of nitroaromatic compounds.28,29 Noble metal nanoparticles which can be prepared by different approaches (physical, chemical and green synthesis),30 especially gold (Au) and silver (Ag), can exhibit unique and tunable optical properties depending on their physical properties such as shape and size, owing to their surface plasmon resonance (SPR) behavior.31,32 Although silver has a higher plasmon quality than gold, Au is more often used than Ag in SPR studies because of its higher stability against oxidation.33,34 In order to improve opto-electronic properties and analytical selectivity, the surfaces of AuNPs were functionalized with amino (–NH2) and thiol (–SH)containing compounds (usually aminoacids) that show affinity toward AuNPs, such as cysteine,35 cysteamine,36,37 4aminothiophenol38 and 4-mercaptobenzoic acid.39 Although

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different modified AuNPs have different working mechanisms for detecting energetic materials (e.g., TNT, RDX, PETN), the mostly utilized common principle of these AuNPs–based methods is to rely on certain interactions, especially chargetransfer complexation between a π-electron-deficient acceptor (e.g., TNT and tetryl) and electron-rich σ- or π-donor (e.g., amines) leading to aggregation of AuNPs a result of Meisenheimer-type40 complex formation8,41 and subsequent electrostatic attractions between particles, such as aggregations between thiol-bound AuNPs and Meisenheimer complex-bound AuNPs.42 These type of interactions were exploited by Dasary et al.42 in TNT sensing with cysteine– modified AuNPs, where electrostatic attractions were further strengthened via hydrogen bonding interactions between cysteine carboxyls and amines of neighboring particles. Unfortunately, the aggregation of AuNPs may arise from the functionalization agent, cysteine itself, without analyte (TNT) depending on concentration. Utilizing these unique properties of gold nanoparticles and CT complexation-based donor-acceptor interactions, a selective molecular spectrophotometric sensor based on diaminocyclohexane–functionalized/thioglycolic acid– modified gold nanoparticles (DACH/TGA@AuNPs) was developed for the determination of nitro-energetic materials (TNT and tetryl) at picomolar (pM) levels in this study. The principle of the sensor is based on the charge-transfer (CT) interaction between one of the amino (–NH2) groups of DACH and –NO2 groups of TNT and/or tetryl via TGA-modified AuNPs. When the sensor interacted with TNT and/or tetryl, the SPR absorption band intensity of TGA-modified AuNPs decreased, and in the meantime, the band intensity arising from specific CT-interaction and subsequent nanoparticle agglomerization increased. Therefore, spectroscopic evaluation for this ratiometric sensor was made by taking the ratio of the absorption value at 650 nm to that at 520 nm (named as the ‘corrected absorbance’, varying linearly with analyte concentration). While the absorption value at 650 nm represented the new formed peak between 600-700 nm as a result of aggregation of modified AuNPs, the absorption value at 520 nm (i.e., intrinsic SPR wavelength of gold nanoparticles) belonged to the AuNP-TGA solution. EXPERIMENTAL SECTION Materials and Chemicals. Ethanol (EtOH) and acetonitrile (AcCN) were purchased from Sigma-Aldrich (St. Louis, Missouri, USA). Tetrachloroauric(III) acid (HAuCl4) was obtained from Aldrich (St. Louis, Missouri, USA). Trizma base (tris) and hydrochloric acid (HCl) were purchased from Sigma (St. Louis, Missouri, USA). Trisodium citrate dihydrate, thioglycolic acid (TGA), (±)-trans-1,2diaminocyclohexane (DACH), 2,4,6-trinitrophenol (picric acid, PA), sodium hydroxide (NaOH) and ammonium nitrate (AN, NH4NO3) were obtained from Merck (Darmstadt, Germany). The explosive materials TNT (pure), tetryl (pure), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX, which contains 85% active compound), 1,3,5,7-tetranitro-1,3,5,7tetraazacyclooctane (HMX, pure), pentaerythritol tetranitrate (PETN, pure), 2,4-dinitrotoluene (DNT, pure), Composite B (60% RDX + 39% TNT + 1% wax) and Octol (70% HMX + 30% TNT) were kindly supplied by the Mechanical and Chemical Industry Corporation (Makine Kimya Endustrisi Kurumu, MKEK) of Turkey. 1,3,5-Triamino-2,4,6trinitrobenzene (TATB) explosive was provided by

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AccuStandard. The clayey slime sample (containing 65.6% sand, 16.0% powder and 18.4% clay), used as matrix material for TNT-, tetryl- and tetrytol-sensing in the soil, was obtained from Istanbul University, Faculty of Forestry. Instruments. The spectra and absorption measurements were recorded in matched Hellma Suprasil quartz cuvettes using a Shimadzu UV-1800 UV–Vis spectrophotometer, and the thickness of the optical cuvettes was 10 mm. TEM measurements were performed by using a QImaging Retinga 4000R. Mettler Toledo Seven Compact pH-meter, IKA CMAG heater with magnetic stirring, Hettich-Universal 320 centrifuge, Wisd WiseCube incubator and Wisd WiseBath water bath were used for synthesis and modification of AuNPs. The developed method was validated against a literature GC–MS method43 utilizing a Thermo Scientific Trace gas chromatograph coupled with a DSQII mass spectrometer containing electron impact ionization and a quadrupole analyzer. GC was equipped with a Thermo TRHT5 column (15 m × 0.25 μm, ID 0.1 µm, 5% phenyl polycarborane siloxane). Preparation of Solutions. The preparation of all solutions used throughout the study was described in the ‘Supporting Information’. Synthesis, Modification and Characterization of AuNPs. AuNPs were synthesized according to a modification of Turkevich method.41 After the pH of the 40 mL 0.01% HAuCl4 solution (in dilute HCl) was adjusted to pH = 4 with 0.1 M NaOH, the final volume of the solution was diluted to 50 mL. The solution was taken up in a 250 mL reaction flask and stirred at 250 rpm for 15 min in a reflux condenser; then 1.75 mL of 1% (w/v) citrate solution was added, and the final solution was heated for 7 min under stirring. At the end of this time, the color change was observed from pale yellow to winered, and the flask was removed from hot plate and cooled to room temperature under stirring. The absorption spectrum of the synthesized AuNPs solution as well as the absorbances at 520 and 650 nm were recorded using a UV-Vis spectrophotometer. The synthesized AuNPs solution was allocated into centrifuge tubes in 5 mL portions, centrifuged at 10000 rpm for 20 min, and then the supernatants were disposed. An aliquot of 200 µL of 0.2 mM TGA solution was introduced to each centrifugate, and the solutions were shaken at 100 rpm in an incubator for 18 h at room temperature. At the end of this time, TGA-modified AuNPs solutions were dispersed by adding 2 mL of 1×10-5 M DACH and 0.2 mL of tris buffer (pH 8.3), and the solutions were kept at 65 oC during 3.5 h for DACH functionalization. After this period, all these solutions were combined and used for TNT and/or tetryl analysis. Recommended Procedure for TNT and Tetryl Quantification with the Sensor. A volume of 0.5 mL of DACH/TGA@AuNPs solution was introduced to a test tube, and then 1.5 mL each of EtOH:AcCN (4:1, v/v) solvent (blank) or sample (TNT and/or tetryl) solution at 10-3 – 10 µg L-1 initial concentration range was added to the tube. After 3 min of reaction time (Figure S-1), the absorption changes of each solution were measured against water at both 650 nm and 520 nm. The scheme for the proposed method can be summarized as follows: for blank solution, add 0.5 mL DACH/TGA@AuNPs + 1.5 mL EtOH:AcCN (4:1, v/v); measure at both 650 nm and

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520 nm against water (Vtotal = 2 mL). For sample solution, add 0.5 mL DACH/TGA@AuNPs + unknown TNT and/or tetryl solution (stand for 3 min); measure at both 650 nm and 520 nm against water (Vtotal = 2 mL) Determination of TNT in Complex Materials. The developed sensor was applied to 0.1 µg L-1 of TNT (initial concentration) in the presence of common energetic materials, i.e., tetryl, RDX, HMX and PETN, common soil ions, namely Cl-, NO3-, SO42-, K+, Mg2+,Ca2+, Cu2+, Fe2+, Fe3+ and Al3+, and possible camouflage materials exemplified by sugar {D-(+)glucose as representative compound}, sweetener, acetylsalicylic acid (aspirin) and paracetamol-caffeine based analgesic, separately, and then the recovery values of TNT were calculated (for details, see ‘Supporting Information’). Applying of the proposed sensor for real soil samples. To remove the impurities in the untreated clay loam sample (obtained from Istanbul University-Faculty of Forestry) for TNT, tetryl and tetrytol (having a proportion of ingredients of 70% tetryl‒30% TNT, w/w) determination in soil sample, 40 g soil sample was weighed and treated with 50 mL 0.1 M HCl for 15 minutes. Afterwards, the soil sample was washed with ultrapure water until the medium pH was stabilized and the soil sample was dried in a vacuum oven at 50 oC. A 2.0 gsample of dried soil was contaminated with 2.5 mL each of 10 mg L-1 TNT, tetryl and tetrytol in EtOH:AcCN (4:1, v/v). The contaminated soil samples were homogenized utilizing ultrasonic bath and then dried at 50 oC. Each of TNT-, tetryland tetrytol-contaminated soil samples was extracted with EtOH:AcCN (4:1, v/v) to give a final volume of 25 mL, and filtered (CHROMAFIL, PET-20/25). Analysis of each soil sample was carried out by applying the sensor procedure as described above after the extracted explosive samples were diluted 104-fold with EtOH:AcCN (4:1, v/v). Method Validation for the Developed Sensor against GC-MS using TNT and Tetryl Samples. For method validation, the reference GC–MS method was used as described in the literature.43 Application of GC–MS determination to the analytes (TNT and tetryl) and instrumental conditions were described in ‘Supporting Information’. Statistical Analysis. Descriptive statistical analyses were performed using Excel software (Microsoft Office 2013) for calculating the mean and the standard error of the mean. Results were expressed as the mean ± standard deviation (SD). Method validation against GC–MS determination43 of TNT and tetryl was made by means of Student t- and F-tests. RESULTS AND DISCUSSION Optimization of the Proposed Sensor. DACH was chosen as diamino compound in this study. pKa (the negative logarithm of the acid dissociation constant) values of DACH are pKa1 6.47 and pKa2 9.94. When pH is lower than pKa1, both –NH2 functional groups are protonated.44 In addition, only a single amino group is protonated at the arithmetic average of the pKa1 and pKa2 values, i.e., isoelectric pH. Using this information, it was aimed that when an amino group of DACH was protonated (–NH3+) and electrostatically interacted with the negatively-charged TGA-modified AuNPs to bind to the surface of the nanoparticles, the other non-protonated amino group would enter a charge-transfer interaction (by virtue of its free amine-N lone electron-pair) with the electron-attracting nitro-groups of TNT to form a donor-acceptor complex. For

this reason, the working pH was chosen as the arithmetic mean of the pKa1 and pKa2 values of DACH to be ≈ 8.3. Different buffers such as phosphate and tris prepared at this pH were tried and optimal results were reached with tris buffer at pH 8.3. After determining the appropriate pH, selection of the optimized temperature and incubation time was made in the modification of TGA@AuNPs with DACH. The best conditions for the modification of the TGA@AuNPs with DACH were determined to be 3.5 h incubation time in a 65 °C water bath at 150 rpm. Synthesis, Modification, Characterization and Reproducibility of AuNPs. AuNPs were synthesized according to a modification of Turkevich method,41 using citrate as reductant and capping agent. Intra-assay and interassay measurements showed that the characteristic SPR absorption peak of the AuNPs was observed at 520 nm (Figure S-2). Initially, based on other similar articles for the surface modification of AuNPs, it was thought that a diamine compound (DACH) could be covalently bound to the AuNPs surface via nitrogen groups. However, since no consistent results were obtained (possibly as a result of loose binding to nanoparticle surface), a bridging thiol compound between gold nanoparticles and diaminocyclohexane was considered. For this purpose, TGA was covalently bound to AuNPs through its –S atom which shows a strongly affinity to gold. The DACH functionalization was then carried out by electrostatic interaction via the free carboxylate group of TGA: (Au-SCH2-COO-…+H3N-DACH). The TEM images of AuNPs, synthesized with citrate and treated with TGA and DACH, in the presence of different concentrations (low and high) of TNT are collectively shown in Figure 1, demonstrating the aggregation ability of functionalized nanoparticles with increasing concentrations of TNT. The functionalized nanoparticles of 18 nm average diameter were agglomerated to the average sizes of 22 and 26 nm upon reaction with 1 and 100 µg/L TNT, respectively. Distinct aggregations of DACH-AuNPs in patches were apparent accompanying an increase in TNT concentration.

Figure 1. TEM images of (a) DACH/TGA@AuNPs, DACH/TGA@AuNPs in the presence of TNT at (b) low: 1 µg L-1 and (c) high: 100 µg L-1 concentration.

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The repeatability and reproducibility of the DACH/TGA@AuNPs were examined by comparing the UV/Visible spectra of three separate syntheses made on the same day (Figure S-3) and on different days (Figure S-4), respectively. There were no significant differences for both cases (Figures S-3 & S-4). The Working Principle of the Sensor. The general theory of charge-transfer (CT) complexation between an electrondeficient aromatic nitro-compound (Lewis acid) and an aminetype electron-donor (Lewis base) was established by Mulliken,45 and the resulting donor-acceptor CT-complexes exhibited high molar absorptivities enabling low levels of analyte (such as Ar-NO2) detection.46 Although a trinitrophenol (such as picric acid) may donate a proton to a cyclic amine to form an ammonium salt of picrate, the amine may partially donate its nitrogen electron (n-orbital) to the electron-withdrawing nitro-oxygen (π*-orbital) to form a highly colored CT-complex47 in Ar-NO2 compounds devoid of phenolic groups. Especially hydroxide, acetonate or methoxide (derived from deprotonated solvents) binding to the aromatic ring of an Ar-NO2 compound (such as TNT or tetryl) formed Meisenheimer-type CT-complexes40 stabilized by resonance. Modern theory states that among the molecular energy levels of a CT-complex, the electronic charge flow accompanying visible light absorption is from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) level, making the CT-complex visible and rendering naked-eye detection of analytes possible at trace concentrations. In our case, TGA easily formed a selfassembled monolayer onto AuNPs, followed by DACH binding from the (+)-charged ammonium-N to the carboxylate (R-COO-) end of TGA-AuNPs. This sensor nanoparticle could easily form a Meisenheimer-type CT-complex with TNT and/or tetryl through the free nitrogen atom (of DACH), explaining the intense color observed (Figure 2) and the subsequent pM (i.e., picomol L-1) sensitivities of the developed colorimetric sensor for TNT and tetryl (Scheme 1). The Meisenheimer anion derived from TNT may establish multiple electrostatic bonds with the cationic DACHfunctionalized TGA-AuNPs to bring about agglomerization of nanoparticles and red-shifts in SPR band maxima. The pMsensitivity reported in our work could not be achieved in solution without incorporating functionalized nanoparticles, and it should be added that the reagent blank of this study has been precisely defined, i.e., the absorbances were not recorded against water or solvent (as in other studies, making the reported sensitivities doubtful). The color forming the basis of measurements rapidly formed in 3 min, quite less than that of other similar methods reporting the formation of Meisenheimer complexes for TNT detection.

Figure 2. Color changes of the test tubes containing (1) synthesized AuNP, (2) AuNP + 2×10-4 M TGA (TGA modification), (3) TGA-modified AuNP + 1×10-5 M DACH (DACH modification), and (4) the sensor (DACH/TGA@AuNP) + 10 µg L-1 TNT.

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Scheme 1. Plausible mechanism of the developed sensor for TNT and tetryl determination (multiple interparticle interactions are not shown). Analytical Performance of the Proposed Sensor Applied to TNT and Tetryl Samples. When the analytical procedure for TNT and tetryl was applied at different concentrations, the solutions were observed on a color scale scrolling from red to blue, so a new peak emerged in the absorption spectrum within the range of 600-800 nm. This new peak derived from agglomerization/aggregation of nanoparticles, and since there were more than one possibility of interparticle attraction, the sizes of aggregated nanoparticles possibly varied in a wide range, thereby not allowing the selection of a single fixed peak wavelength for all measurements. For this reason, the absorbance at 650 nm was taken as the basis for monitoring the change in surface plasmon resonance. The unmodified citrate-reduced suspension of AuNPs presented an absorption peak at 520 nm which can be attributed to the surface plasmon resonance of the citratestabilized gold nanoparticles. TGA- and DACH-modification of the AuNPs suspension did not lead to a remarkable change in the spectra. A large change in the spectrum of the DACH/TGA@AuNPs was recorded after the addition of different concentrations of TNT and/or tetryl samples: a new and strong absorbance peak appeared at 650 nm, which can be ascribed to the absorbance of the TNT-/tetryl-induced aggregation of the DACH/TGA@AuNPs through the donoracceptor interaction between TNT/tetryl and DACH at the TGA modified-AuNPs surface. All absorbance measurements, including the reference solution, were made against ultrapure water. As the absorbance of the emerging peak at 650 nm increased, the absorbance of the original SPR peak at 520 nm decreased. Thus, to make a ratiometric quantitative evaluation, the absorbance ratio of the reference solution was subtracted from that of the sample solution to obtain a corrected absorbance, C. A., as shown in eq 1:

C.A. =

A650nm

A520nm

A650nm s

A520nm

b

(1)

C. A.: Corrected Absorbance, A: Measured Absorbance s: sample, b: blank

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With this calculation method, the absorbance value of the TNT/tetryl sample was independent of the amount of nanoparticles, and calibration equation was obtained with corrected absorbance (A′650/520) values (according to eq 1) obtained for TNT and tetryl. The ratio of A650 to A520 varied linearly with the logarithm of the concentration of TNT and tetryl. The developed sensor was applied to the determination of TNT, yielding the calibration line equation: A′650/520 = 7.21 × 10-2 Log CTNT + 2.1 × 10-1 (r = 0.9990), where A′650/520 was the corrected A650/A520 ratio (according to eq 1), and CTNT was the TNT concentration (in nM) in final solution. The developed sensor was also applied to the determination of tetryl, yielding the calibration line equation: A′650/520 = 2.86 × 10-2 Log CTetryl + 1.03 × 10-1 (r = 0.9988), where A′650/520 was the corrected A650/A520 ratio (according to eq 1), and Ctetryl was the tetryl concentration (in nM) in final solution. Visible spectra for aggregated DACH/TGA@AuNPs for varying TNT and tetryl concentrations, together with the color changes of the test tubes (inset figure), were shown in Figure 3 and Figure S-5, respectively.

Figure 3. Visible spectra of aggregated DACH/TGA@AuNPs obtained by applying the developed sensor to (1) the blank (4:1 EtOH:AcCN, v/v) and TNT samples at (2) 3.3×10-3 nM (3) 3.3×10-2 nM (4) 3.3×10-1 nM (5) 3.3 nM (6) 33 nM final concentration, the calibration curve of the relationship between A650nm/A520nm and the concentration of TNT and their color changes (inset figure). The analytical performance parameters of the developed sensor for TNT and tetryl assay were summarized in Table 1. Table 1. Analytical performance parameters of the developed sensor for TNT and tetryl. Analyte TNT

Linear rangea 3.3×10-3 – 33

Tetryl

2.6×10-3

– 26

LODb 1.76 1.74

CVc (%) IntraInterassay assay 0.51 0.79 0.64

0.83

in nM units at final concentration. blimit of detection, in pM units (LOD = 3σbl/antilogm, σbl denoting the standard deviation of a blank, and m showing the slope of the calibration line). ccoefficients of variation, as percentage (N = 5). a

Selectivity and Sensitivity of the Sensor. It was investigated whether other explosive substances (picric acid, PETN, HMX, AN, TATB, DNT and RDX) could also form a CT complex with DACH, as TNT and tetryl did in the absence of sensor. Without the proposed sensor, it was observed that TNT and tetryl (50 mg L-1) in solution phase could form an intense color difference with DACH (1.0×10-2 M) due to CT complexation, naturally at high concentrations of the analytes, as shown in Figure S-6. As opposed to the findings of Lin et al.48 who observed a small shoulder band with micromolar concentrations of DNT at around 600 nm, our method could be safely applied to DNT without any color formation due to the nonexistence of CT interaction. The consecutive pKa values of 1,2-ethanediamine used by Lin et al.48 are 10.71 and 7.56, while those of trans-DACH are 9.94 and 6.47, indicating the stronger basicity of the aliphatic diamine. This may explain the selectivity of DACH toward TNT, as less electronwithdrawing nitro-compounds could not produce Meisenheimer complexes from DACH. The same principle is true for nitrobenzene (NB) and nitrotoluene (NT). The lack of two nitro groups in the benzene ring of NB and NT may not enable partial negative charge distribution throughout the benzene ring. Owing to the missing electron-withdrawing nitro groups compared to that of TNT, NB and NT may not form strong donor-acceptor interaction with DACH at the TGAmodified AuNPs/solution interface, and therefore the aggregation phenomenon subject to this study may not be observed. Likewise, picric acid did not enter a donor-acceptor (DA) interaction with DACH due to its own intramolecular CT, and consequently did not produce any interference to the proposed method. Due to the mesomeric effect of the hydroxyl group of PA, the electron-rich hydroxyl group weakens the donor-acceptor interaction between PA and an electron-rich amine compound by forming an intramolecular charge transfer complex with electron-deficient nitro groups of PA. In fact, Jamil et al.8 directed other criticisms to ethylenediamine- and other amine-capped AuNPs sensor methods for TNT detection in relation to their unspecificity (i.e., bearing the risk of false-positives) and intolerance to aggregation by inert electrolytes. We may add two other criticisms to the existing ones, as easy removal of the surfacebound amine (because it is not supported by an underlayer of a self assembled thiol onto AuNPs) and incomplete description of the reagent blank in such sensing procedures (i.e., raising doubts about the pM sensitivities reported). In the light of our experience with the PVC-dicyclohexylamine colorimetric sensor for TNT detection developed in our laboratories,11 we can presume that the relative positioning of HOMO-LUMO levels in the Meisenheimer complex formed between DACHTGA-AuNPs and TNT enables selective visual detection of TNT without complex formation of the free nitrogen atom of cyclic amine with other Ar-NO2 compounds except tetryl, and this selectivity was also observed by Jamil et al.8 with their gold-cysteamine sensor for TNT, not forming CT complexes with DNT or picrate. In terms of the high sensitivity achieved in this work, our sensor over-satisfies the need for the rapid and precise screening of TNT in real water samples where the interim daily allowable concentration of TNT in wastewater is 0.04 mg/L8 and the EPA decontamination limit of 50 mg/kg for soil composting treatment.49 Only CT complexation between functionalized NPs and TNT could not be responsible for the

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extreme sensitivity provided by our ratiometric nanosensor (as these interactions would also prevail in solution phase devoid of sensor NPs), and the observed pM level detection limits were obviously brought about by interparticle attractions Evaluation of TNT Recovery in Complex Matrices. The possible interferences of other explosives (tetryl, RDX, HMX and PETN) used along with TNT were investigated. For this purpose, the developed sensor was applied to the mixture solutions of these explosives with TNT. The recoveries obtained for TNT in TNT-tetryl mixtures were in the range of 107–119%. Although the developed sensor could measure tetrytol as a double-base military explosive comprised of TNT and tetryl, these two nitro-compounds yielding CT complexes absorbing at the same wavelength could not be differentiated. Furthermore, the absorbances of TNT and tetryl at the proportions encountered in tetrytol were not additive, possibly resulting from different extents of nanoparticle aggregation with the CT complexes of these two nitro-compounds, because strict adherence to Beer’s law usually necessitates the production of a single chromophore in the color reaction of interest. The TNT recovery (%) values in the presence of other explosive substances (RDX, HMX and PETN) were summarized in Table S-1. The hexyl nitramine functional groups (–N-NO2) in RDX and HMX did not enable CT complexation with DACH because of the simultaneous presence of electron-withdrawing nitro groups and electrondonating amine groups in the same molecule. Thus, nitramines did not interfere with the proposed sensing method. Interference effects of common ions (Cl-, NO3-, SO42-, K+, Mg2+, Ca2+, Cu2+, Fe2+, Fe3+, Al3+) found in soil were investigated and TNT recoveries from these solutions were made in the range of 96–106%, excluding those of Mg2+, Ca2+, Cu2+, Fe2+, and Fe3+. To overcome the interference of Mg2+ cations to TNT determination, the limited solubility of salts containing these ions in EtOH/AcCN was exploited. Sodium and potassium salts of chloride, nitrate, and sulfate did not show interference at up to 5×104 mass ratios of the cation or anion to 0.1 µg L-1 TNT tested. The interferences of Ca2+, Cu2+, Fe2+ and Fe3+ cations were eliminated by masking these cations with EDTA. Thus, TNT recovery values were brought to the range of 100–106%. The proposed sensor was also separately applied to the mixtures of sugar {(D-(+)-glucose as representative compound}, sweetener, paracetamol-caffeine–based painkiller drug, acetylsalicylic acid and household detergent with TNT at 1-fold of TNT concentration. These camouflage materials carried by passengers as personal belongings in hand-held luggages had no significant responses (i.e., no false positives) as shown in the bar diagram (Figure 4).

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(possibly of a TNT Meisenheimer anion with several DACHfunctionalized cationic NPs) resulting in nanoparticle aggregation and red shifts in SPR bands. sensor in the presence of 0.1 µg L-1 TNT at 1-fold of TNT concentration. Recovery of TNT, tetryl and tetrytol from soil samples. The clayey soil sample was artificially contaminated with TNT, tetryl and tetrytol, and extracted with solvent. The sensor was applied to the soil extract and the recoveries of TNT, tetryl and tetrytol from this soil were 97%, 97% and 96%, respectively. Comparison of the performance of the developed sensor with those of other analytical sensors. The analytical performance parameters of the proposed sensor were compared with those of other nano-spectroscopic, electrochemical and paper-based sensors for TNT and/or tetryl determination (Table S-2). Validation of the proposed method against GC–MS. TNT and tetryl working solutions in acetonitrile at 0.5 – 8 mg L−1 and 5 – 25 mg L-1 concentrations, respectively, were analyzed with the GC–MS method existing in literature,43 and the mean values of three repetitive injections were used for calculations. The calibration equations between peak area and concentration were: Peak Area = 3.22 × 104 CTNT – 6.84 × 103 (r = 0.9993) for TNT, where CTNT was the TNT concentration. Peak Area = 2.30 × 103 Ctetryl – 8.36 × 103 (r = 0.9993) for tetryl, where Ctetryl was the tetryl concentration. Statistical comparison between the results of the proposed and reference GC–MS procedures applied to 0.01 mg L-1 TNT and tetryl samples in acetonitrile was made on N=5 repetitive analyses, essentially showing no significant difference between the results. Thus, the proposed method was validated against the GC–MS method; the t- and F-tests were used for comparing the population means and variances, respectively. The confidence levels used in validation of findings were 95% for both of t- and F-tests for TNT, and 95 % for t- and 99% for F-tests for tetryl. Statistical parameters of the developed method and the reference GC–MS method were summarized in Table 2.

Figure 4. Responses of sugar, sweetener, paracetamolcaffeine, acetylsalicylic acid and household detergent to the

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Table 2. Statistical comparison of the proposed sensor with GC – MS for TNT and tetryl determination. Analyte

TNT

Mean conc.

SD (σ)

9.7

4.7×10-4

10

2.1×10-4

Proposed Sensor (Spectrophotometric)

9.6

6.7×10-4

GC–MS

9.8

1.7×10-4

Method Proposed Sensor (Spectrophotometric) GC–MS

Sa,b

ta,b

ttableb

Fb

Ftableb

-

-

-

-

-

3.7×10-4

0.861

2.306

5.1

6.39

-

-

-

-

-

4.9×10-4

0.645

2.306

15.51

15.98

Tetryl

a S2 = ((n – 1)s 2 + (n – 1)s 2)/(n + n – 2) and t = (ā – ā )/(S(1/n + 1/n )1/2), where S is the pooled standard deviation, s and s 1 1 2 2 1 2 1 2 1 2 1 2 are the standard deviations of the two populations with sample sizes of n1 and n2, and sample means of ā1 and ā2 respectively (t has (n1 + n2 – 2) degrees of freedom); here, n1 = n2 = 5. b Statistical comparison made on paired data produced with proposed and reference methods; the results given only on the row of the reference method.

When the developed sensor and GC–MS sensing results are compared, it is seen that the limit of detection of the proposed colorimetric sensor (5.0×10-4 pg µL-1) is quite lower than that of the chromatographic method (0.18 ng µL-1) due to interparticle plasmon coupling of the aggregated nanoparticles. Although the LOD value of the GC–MS method43 is in ng µL-1, the calibration range is in the range of 1 – 10 µg µL-1. It can also be said that the developed method is more sensitive compared to a recent chromatography-based study (LOD: 0.1 pg µL-1 for TNT) developed by Marder et al.50 The developed method is advantageous in terms of sensitivity against the reference43 and the recent50 chromatographic method due to its wide range of calibration (3.3×10-3 – 33 nM for TNT). CONCLUSIONS A selective, sensitive, low-cost and easy-to-use colorimetric sensor was developed for TNT, tetryl and tetrytol, used as military explosives. The sensor was based on charge-transfer complexation between one of the –NH2 groups of DACH and –NO2 groups of NACs via TGA-modified AuNPs and subsequent aggregation of DACH/TGA@AuNPs interacting with TNT, and the observed sensitivity against a reagent blank was at picomolar levels, potentially enabling precise detection of minute amounts of TNT and/or tetryl in post-blast residues. The sensor was tested in real samples and validated against a reference GC‒MS method. The sensor also offered solutions for eliminating the interference effects of some common soil ions and of certain passenger belongings used as camouflage materials for energetic materials. Due to the excellent optoelectronic properties of nanomaterials, the sensor may be a part of the emerging explosive detection market in field measurements in regard to convertibility to a commercial kit format for homeland security.

ASSOCIATED CONTENT Supporting Information. Supporting information contains; (i) Preparation of solutions, (ii) Determination of TNT in complex materials.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]

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

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

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors wish to express their gratitude to the Ministry of National Defence, Office of Technical Services, and to the Mechanical & Chemical Industry Corporation (MKEK) for the donation of nitro- and composite explosive samples. The authors also extend their thanks to Istanbul University Research Fund (BAP Unit) for the support given to the M.Sc. Thesis Project56518.

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