Determination of hydrogen peroxide and triacetone triperoxide (TATP) with a silver nanoparticles—based turn-on colorimetric sensor

Sensors and Actuators B 247 (2017) 98–107

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Determination of hydrogen peroxide and triacetone triperoxide (TATP) with a silver nanoparticles—based turn-on colorimetric sensor Ays¸em Üzer, Selen Durmazel, Erol Erc¸a˘g 1 , Res¸at Apak ∗ Istanbul University, Faculty of Engineering, Chem. Dept., 34320, Istanbul, Turkey

a r t i c l e

i n f o

Article history: Received 7 December 2016 Received in revised form 27 February 2017 Accepted 2 March 2017 Available online 6 March 2017 Keywords: Hydrogen peroxide Triacetone triperoxide (TATP) Silver nanoparticles Colorimetry Explosive Sensor

a b s t r a c t Hydrogen peroxide (H2 O2 ) is a synthetic precursor and degradation product of peroxide-based explosives, such as TATP. We developed a colorimetric sensor that is selective, sensitive, cost-efficient and easy-to-use in conventional laboratories for the naked eye detection of hydrogen peroxide and for indirect determination of peroxide-based explosives (which are devoid of chromogenic/fluorogenic functional groups). We were able to partly oxidize zero-valent silver nanoparticles (Ag0 NPs) by H2 O2 to Ag+ under special conditions, and thus devise an indirect method for trace H2 O2 quantification by measuring the absorbance of the blue-colored diimine of TMB (3,3 ,5,5 - tetramethylbenzidine) at 655 nm, arising from TMB oxidation with Ag+ . TATP was acid-hydrolyzed to H2 O2 by an acidic cation-exchanger (Amberlyst-15) for potential field use. The limit of detection (LOD) of the sensor was 20 nM for H2 O2 and 0.31 mg L−1 for TATP. Common soil ions did not interfere, and TATP was analyzed in synthetic mixtures of other energetic materials. The responses of detergents, sweeteners, acetylsalicylic acid (aspirin) and paracetamol-based painkiller drugs, used as camouflage material in passenger belongings, were also examined. The developed method was statistically validated against the standard analytical methods of titanium(IV) oxysulfate (TiOSO4 ) for H2 O2 and GC–MS for TATP using t- and F- tests. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Hydrogen peroxide is a dual-function redox compound (i.e. that may act as both an oxidant and reductant) finding wide use in nutrition, pharmaceutical, textile and clinical industries. H2 O2 is a potent bleaching and disinfecting agent in water treatment, and may play key roles in atmospheric phenomena and biochemical processes [1–5]. Because it is a product of biochemical reactions catalyzed by oxidase enzymes (such as catalase, glucose oxidase, horseradish peroxidase, etc.), H2 O2 can be associated with a great many diseases such as diabetes, cancer, and aging [6,7]. Since peroxo-oxygen is unstable that may undergo disproportionation, H2 O2 is also used as a precursor for synthesis of homemade explosives (TATP, HMTD, etc.) classified under peroxide-based energetic materials [8–13]. In the late 1970 s and early 1980 s, peroxidebased explosives (TATP and HMTD) confiscated by the Israeli police were commonly used in terrorist attacks for their simple synthesis, accessibility of raw chemicals used for synthesis, and high explosive potential [14–16]. The determination of peroxide explosives has

∗ Corresponding author. E-mail address: [email protected] (R. Apak). 1 The researcher was an academic staff member of Istanbul University at the initiation of the work. http://dx.doi.org/10.1016/j.snb.2017.03.012 0925-4005/© 2017 Elsevier B.V. All rights reserved.

gained importance in recent years due to some historical terrorist actions such as 2005 London Subway bombing, 2009 failed terrorist attempt in a Northwest Airways flight, and Paris suicide bombing in 2015 [14]. Peroxide explosives lack spectroscopically active functional groups such as nitro substituents and aromatic rings, and therefore may elude common spectral detection techniques employed at check points of mass transport. TATP easily sublimes at room temperature, and does not leave post-blast residues unlike common nitro-explosives [17,18]. TATP may yield similar colored products as H2 O2 with the use of chromogenic reagents (such as titanyl oxalate) on microfluidic paper-based analytical devices, though at a much weaker color intensity [19]. Colorimetric determination of TATP is greatly facilitated when it is hydrolyzed to H2 O2 . Thus, hydrogen peroxide, which is a synthetic precursor and degradation product of TATP and HMTD, is accepted as a noteworthy compound for determination of peroxide-based explosives [20–23]. Considering all these facts, trace H2 O2 determination has become essential for both industrial and academic purposes. Relatively older assay methods such as titrimetry, spectrophotometry, fluorometry [24,25] for H2 O2 and colorimetry [26], spectroscopy [27–30], fluorometry [31,32], and chromatography [33,34] for TATP have started to be replaced by recently improved nanoparticlebased hydrogen peroxide sensors. Accordingly, nanosensors were designed for H2 O2 detection using metals like copper (Cu) [35], gold

A. Üzer et al. / Sensors and Actuators B 247 (2017) 98–107

(Au) [36], palladium (Pd) [37], silver (Ag) [38], or metal oxides like CuO [39], Fe3 O4 [40,41]. Silver nanoparticles are used as bactericide for their strong antimicrobial effects. One of the probable mechanisms leading to this effect (preventing microbial growth) is the generation of superoxide anion and hydroperoxyl radical, hydroxyl radical, hydrogen peroxide and singlet oxygen via Ag0 NPs and Ag+ [42]. Thermodynamically, hydrogen peroxide (E0 H2O2/H2O = 1.776 V) can act as an oxidant toward colloidal silver (E0 Ag + /Ag = 0.799 V) under optimal kinetic conditions. This provides an opportunity for Ag0 NPs−based determination of hydrogen peroxide. There are a number of spectrophotometric and LSPR−based methods in literature for the determination of H2 O2 using Ag0 NPs [43–47], however, their sensitivity and selectivity are quite low. Most of the existing AgNPsbased sensors for hydrogen peroxide are turn-off sensors, based on color fading at the LSPR wavelength of silver nanoparticles, however they suffer from problems associated with accuracy and precision. Therefore, the developed AgNPs method is considered essential for filling this gap. This work reports the development of a novel turn-on spectrophotometric sensor for H2 O2, which is easy-to-apply, stable, selective and of low cost, as required by infield/on-site determinations. 3,3 ,5,5 -Tetramethylbenzidine (TMB) is a chromogenic reagent commonly used for horseradish peroxidase−based determination of H2 O2 ; the reagent also has the ability to be selectively oxidized by Ag+ to a colored diimine product [48,49]. Combining these facts, we were able to oxidize zero-valent silver nanoparticles (Ag0 NPs) by H2 O2 to Ag+ under special conditions, and thus devise an indirect method for trace H2 O2 quantification by measuring the absorbance of TMB-diimine at 655 nm, arising from TMB oxidation with Ag+ . This colorimetric sensor proved to be more sensitive and selective than analogic turn-off nanoparticle sensors. The selected analytical wavelength is far from the effects of common interferents adversely affecting the intrinsic LSPR wavelength of Ag0 NPs around 400 nm. The explosive compound TATP can be easily converted by acidic hydrolysis to H2 O2 using the strongly acidic cation exchanger resin, Amberlyst-15, and subsequently determined with the proposed sensing method. This mild acid treatment also aids the discrimination of organic peroxides from inorganic ones. The interferences of some soil ions were investigated, as well as the possible ‘falsepositive’ effects of common passenger belongings and pills, such as aspartame−based sweeteners, acetylsalicylic acid, paracetamolcaffeine based analgesic drugs and household detergents which may serve as camouflage material to TATP having a similar color and appearance. The responses of nitro-explosive group chemicals (represented by TNT, RDX, PETN and NH4 NO3 ) to the sensor were also studied. 2. Materials and methods 2.1. Chemicals All reagents were analytical reagent grade unless otherwise stated. 3,3 ,5,5 -Tetramethylbenzidine (TMB), hydrogen perox® ® ide (%30, Suprapur ), silver nitrate (AgNO3 ) and Amberlyst-15 (hydrogen form, dry) were purchased from Merck Millipore and Sigma-Aldrich. All the other reagents were obtained from Merck, Sigma-Aldrich and Fluka. TNT, RDX and PETN samples were provided by the Mechanical and Chemical Industry Corporation (MKEK) of Turkey during previous projects. 2.2. Instrumentation The spectra and absorption measurements were recorded in matched Hellma Suprasil black quartz cuvettes using a Shimadzu

99

UV-1800 UV–vis spectrophotometer. The optical thickness of the cuvettes used in solution phase measurements was 10 mm. For validation of the proposed assay in the determination of TATP, a GC–MS instrument utilizing a Thermo Scientific Trace gas chromatograph coupled with a DSQII mass spectrometer containing electron impact ionization and quadrupole analyzer was used. GC was equipped with a Thermo 5MS column (30m × 320 ␮m × 0.25 ␮m). 2.3. Preparation of solutions The working solutions of hydrogen peroxide at 5 × 10−6 –8 × 10−5 M were daily prepared from the corresponding stock solutions of 1.0 × 10−3 M in ultrapure water. TMB stock solution was prepared daily at 1 × 10−2 M in ethanol. The acetate buffer solution at pH 4.0 (containing 2 M CH3 COOH and 2 M CH3 COONa) was prepared in ultrapure water. The stock solutions of NH3 and H2 SO4 were prepared at 4 M and 2 M, respectively, in water for neutralization and pH adjustments. Hydrogen peroxide at 4 × 10−5 M was assayed in the presence of 1- and 5-fold concentrations of common ions, namely SO4 2− , NO3 − , Mg2+ , Ca2+ , K+ , Cu2+ , Fe3+ , Fe2+ , Pb2+ , and Al3+ . All mixtures were prepared in ultrapure water. The working solutions of TATP at 10–250 mg L−1 were daily prepared from the corresponding stock solutions of 1000 mg L−1 in pure acetone (diluent was acetone – water; 1:10, v/v) for colorimetric sensing. For GC–MS analysis of TATP, acetonitrile was the preferred solvent and the working solutions of TATP at 1–10 mg L−1 were daily prepared from the corresponding stock solution at 500 mg L−1 in acetonitrile. Binary mixtures of common energetic materials, especially RDX, TNT, PETN and NH4 NO3 , were prepared at 1000 mg L−1 concentration (i.e., 10-fold of the analyte) in acetone-water (1:10, v/v) and TATP was added at a final concentration of 100 mg L−1 . Binary mixtures of household detergent, sweetener, acetylsalicylic acid and paracetamol were prepared at 1000 mg L−1 concentration (i.e., 10-fold of the analyte) in acetone-water media (1:10, v/v) and TATP was added at a final concentration of 100 mg L−1 . 2.4. Synthesis and characterization of AgNPs Silver nanoparticles were synthesized by using a modification of Lee-Meisel method developed by Wan et al. [50], based on NaBH4 reduction and citrate-capping of 4 nm average sized AgNPs. In this procedure, a mixture of a 20 mL of 1% (w/v) trisodium citrate solution and 75 mL of ultrapure water was heated to 70 ◦ C during 15 min under vigorous stirring. At the end of this time, a 1.7 mL volume of 1% (w/v) AgNO3 solution and then 2 mL of 0.1% (w/v) freshly prepared NaBH4 were quickly added to the mixture. The final reaction mixture was kept at 70 ◦ C under vigorous stirring for 1 h and cooled to room temperature. Characterization of the synthesized AgNPs was performed with the aid of a UV/Vis spectrophotometer after being diluted with ultrapure water (1:20, v/v). 2.5. Synthesis of TATP For synthesis of a small amount of TATP (not exceeding 100 mg), the literature method was applied with special safety precautions [51] (details given in Supplementary Material). 2.6. Recommended procedure for H2 O2 quantification The H2 O2 assay consisted of three parts, namely the catalytic degradation of H2 O2 over AgNPs, formation of blue color owing to oxidized TMB by Ag+ ions, and spectrophotometric determination of the oxidized TMB (i.e. diimine product) at 655 nm.

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A volume of 0.5 mL of concentrated AgNPs suspension, 0.5 mL ultrapure water and a 2 mL of H2 O2 solution were added to a test tube. The mixture was kept at room temperature for 30 min to allow Ag+ ions (presumably formed from the catalytic degradation of H2 O2 ) to be released from the surface of AgNPs. Finally, 0.5 mL of 2 M pH 4.0 acetate buffer solution was introduced to the test tube, followed by the quick addition of 0.5 mL of 1 × 10−2 M TMB solution. Upon release of Ag+ ions from the nanoparticles, TMB was oxidized to the blue colored diimine. After 5 min, the absorbance of the final solution at 655 nm was read against a reagent blank. The scheme for the method can be summarized as: add 0.5 mL AgNPs solution + 0.5 mL ultrapure water + (x) mL unknown H2 O2 solution in the ␮M range + (2.0-x) mL H2 O; (allow to stand for 30 min for H2 O2 degradation and/or Ag+ formation) + 0.5 mL of 2 M pH 4.0 buffer solution + 0.5 mL of 1 × 10−2 M TMB solution; measure A655nm against a reagent blank after 5 min of TMB addition. 2.7. Determination of H2 O2 in the presence of common soil ions Recovery values were calculated by preparing a H2 O2 solution with an initial concentration of 4 × 10−5 M, adding solutions of SO4 2− , NO3 − , Mg2+ , Ca2+ , K+ , Cu2+ , Fe3+ , Fe2+ , Pb2+ , and Al3+ (at 1- to 5-fold molar ratio to H2 O2 ), and then applying the proposed sensor. In order to eliminate the interference effects of these cations, Na2 EDTA as masking agent at a concentration of 1 × 10−2 M was used at different volume ratios (metal-EDTA ratio: 1:2.5 (v/v) for Ca2+ , Mg2+ , and Pb2+ ; 1:5 (v/v) for Fe2+ , Fe3+ , Al3+ , and Cu2+ ) as required for each cation, followed by the application of the proposed method

2.8. Real sample assay for H2 O2 For real sample assay, both the proposed sensor and reference TiOSO4 procedure [52] (described in ‘Supplementary Material’) were applied to a commercial 9% hydrogen peroxide solution which was originally used for hair color bleaching. The results of these applications were given in Table S2 and Table S3, respectively, for the proposed sensor and TiOSO4 reference method (Supplementary Material).

2.9. Hydrolysis of TATP to H2 O2 using a strongly acidic cation exchanger resin Hydrolysis of TATP was performed by batch method using Amberlyst-15 resin. One gram of Amberlyst-15 resin was weighed to a 50 mL-flask. The resin was swelled up in ethanol-water (1:1, v/v) medium for 40 min. After the solvent mixture was decanted, the resin was washed with 5% (v/v) H2 SO4 solution, and then 25 mL of TATP solution in acetone-water (1:10, v/v) was introduced to the flask. The flask was placed in an incubator at room temperature and agitated for 15 min at 100 rpm with the aim of TATP hydrolysis to H2 O2 . This step was repeated two times under the same conditions. The hydrolyzed solution was separated from the residual resin particles by filtration through quantitative filter paper. For stepwise neutralization, first 750 ␮L of 4 M NH3 and then 750 ␮L of 1 M NH3 were added (in this order) to the hydrolyzed solution to bring the final pH to about 7. The final volume was diluted to 100 mL with ultrapure water.

2.10. Application of the AgNPs-based sensor to TATP samples The recommended method was applied to 2 mL of the hydrolyzed TATP solution (see. 2.9) as described (see. 2.6).

2.11. Determination of TATP in Complex Materials In combination with 100 mg L−1 TATP, certain energetic materials (TNT, RDX, PETN, and NH4 NO3 ) and potential camouflage agents (acetylsalicylic acid, paracetamol, sweetener and household detergent) were separately applied to each mixture solution (10-fold of TATP, w/w), and percentage recovery values were recorded. 2.12. Statistical analysis Descriptive statistical analyses were performed using Excel software (Microsoft Office 2013) for calculating the means and the standard error of the mean. Results were expressed as the mean ± standard deviation (SD). Method validation against GC–MS determination of TATP was made by means of student (t-) and F-tests. (GC–MS determination of TATP was described in ‘Supplementary Material’) 3. Results and discussion 3.1. Detection principle of the sensor The detection principle of developed sensor is the partial oxidation of Ag0 NPs by H2 O2 to Ag(I) ions in neutral medium (Eq. (1)). Ag0 (NP) + 2H2 O2 Ag+ + O2 •− + 2H2 O

(1)

This equation represents the overall reaction, in which AgNPs presumably form a strongly oxidizing intermediate complex with H2 O2 (possibly Ag-O(H)-OH), which further reacts with another H2 O2 molecule to form Ag+ and superoxide [53]. The produced Ag+ was detected using a chromogenic reagent TMB acting as a selective and sensitive indicator for silver ions [48]. The absorbance arising from the color change (from yellow to blue) was measured by a visible spectrophotometer at 655 nm (Scheme 1). • The formation of superoxide anion radicals (O2 − ) during the catalytic decomposition of H2 O2 with alumina-supported metallic silver was first suggested by Ono and coworkers via ESR detection of superoxide radicals [54]. Since AgNPs are known to catalyze the decomposition of H2 O2 , O2 •− was hypothesized to act both as an oxidant for AgNPs and a reductant for Ag+ ions [55]. Aside from the fact that the reaction expressed by Eq. (1) is reversible through reformation of AgNPs [55], a complex interplay among AgNPs, Ag+ , O2 •− and H2 O2 seems to exist [53], preventing quantitative reactions to occur. Therefore, Ag+ formation (detected by TMB) from AgNPs−catalyzed degradation of H2 O2 has to be carefully optimized in order to develop a reliable colorimetric assay for trace hydrogen peroxide. 3.2. Reaction parameters for AgNPs-catalyzed H2 O2 degradation and TMB oxidation The preparation, characterization and stability of AgNPs used in hydrogen peroxide sensing were expressed in detail (Supplementary Material). The nanoparticles prepared by NaBH4 reduction and citrate capping (Table S1) exhibited maximum absorbance at the specific LSPR absorption wavelength (Fig. S1), were stable for 30 days (Fig. S2) and did not change from lot to lot of different syntheses (Fig.s S3 and S4). The average size and charge of the synthesized AgNPs were very important for their catalytic ability to degrade H2 O2 and partially produce Ag+ used in indirect quantification of hydrogen peroxide. The AgNPs-based sensor operated in two stages, namely the catalytic degradation of H2 O2 with AgNPs, and colorimetric detection of the formed Ag+ with TMB. Therefore, optimal pH, time (Fig. S5 & S6) and temperature for these two stages

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Scheme 1. Schematic presentation of the developed colorimetric sensor for H2 O2 determination (via a two-step reaction).

were determined (Supplementary Material). The optimal amounts of AgNPs (Fig. S7) and TMB (Fig. S8) used in the determination were specified. The reactivity of AgNPs toward H2 O2 was shown to decrease by decreasing pH, while at high pH, reformation of AgNPs by a back reaction (from Ag+ and O2 •− ) was essentially complete [42]. Therefore, the optimal pH setting (to a neutral pH of 7) for the first stage (involving catalytic degradation of H2 O2 to produce Ag+ ) was made to assure maximal H2 O2 degradation and minimal AgNPs formation (Supplementary Material). The optimal pH for the second stage (involving TMB oxidation by Ag+ ) was set to pH 4 in accordance with the recommendation of Liu and coworkers [48], and the initial concentration of acetic acid/acetate buffer was set to 2.0 M in order to bring about maximal aggregation of AgNPs so as to reveal the blue color of the TMB oxidation product (diimine) in a non-turbid solution (Supplementary Material). Within the concentration interval of trace hydrogen peroxide being tested, the time periods of 30 min and 5 min were determined to be optimal for the first and second stages of the reaction, respectively, at room temperature. Longer times and elevated temperatures were avoided to minimize back reactions (Supplementary Material). Ag+ formation from AgNPs should be optimized with respect to time, as it may only form from (AgNPs + H2 O2 ) reaction through an intermediate, and once formed, these products may reform AgNPs by a reverse reaction [42]. Thus, the overall Eq. (1) concerning Ag+ formation consists of two parts, Eq.s (2a) & (2b): AgNP + H2 O2 → intermediate +

intermediate + H2 O2 → Ag + O2 •− + H2 O

(2a) (2b)

Likewise, scavenging of superoxide radicals and reformation of AgNPs are associated with Eqs. (3a) & (3b): AgNP + O2 •− → AgNP∗− + O2

(3a)

AgNP∗− + Ag+ → AgNP + AgNP

(3b)

On the other hand, the actual chromophore for colorimetric determination, TMB-diimine, forms relatively rapidly from Ag+ oxidation of TMB, as seen from the time-dependent rise of 655 nm-absorbance of (TMB-Ag+ ) solution at room temperature [48]. It is possible that this color formation was further catalyzed by the presence of AgNPs [49]. Investigating the reactions represented by Eqs (2a), (2b) and (3a), (3b) between silver(I) ions and AgNPs, it is apparent that the formation of the indirect chromophore (Ag+ ) is kinetically delayed (because of the intermediate), and its consumption is inevitable due to the reverse reaction ending up with AgNPs reformation. Thus, a compromise should be found to maximize Ag+ formation and minimize its consumption; this critical time period was experimentally set in this work as 30 min. The involvement of superoxide radicals in AgNPs−catalyzed degradation of H2 O2 with

respect to Eq. (1) was also confirmed (Supplementary Material; Fig. S9). 3.3. Analytical figures of merit of the sensor applied to H2 O2 samples The proposed sensor was applied to H2 O2 solutions. The parameters of analytical performance for H2 O2 determination are shown in Table 1. The visible spectra of TMB-diimine product (obtained from oxidation of TMB with released Ag+ ions, produced from AgNPscatalyzed degradation of H2 O2 ), using varying concentrations of H2 O2 in the ␮M range, were shown in Fig. 1. 3.4. Recovery of H2 O2 in the presence of common soil ions The sensor was applied to common ions abundantly found in soil and groundwater (SO4 2− , NO3 − , Mg2+ , Ca2+ , K+ , Cu2+ , Fe3+ , Fe2+ , Pb2+ ve Al3+ ) at 1- and 5-fold concentrations of H2 O2 and the analyte recoveries are shown in Table 2. High concentrations of common ions should be avoided in noble metal nanoparticles–based assays because of the charge neutralization effect of salts causing nanoparticle aggregation. Fe2+ , Fe3+ and Pb2+ at 1-fold concentration, and Ca2+ , Mg2+ , Cu2+ ve Al3+ at 5-fold concentration of H2 O2 gave reduced recoveries for the analyte, possibly via affecting its catalytic degradation over AgNPs. All these interferences were eliminated by adding Na2 EDTA as masking agent at the volume ratios indicated in section 2.7 to obtain analyte recoveries ranging between 95 and 105% (Table 2). 3.5. TiOSO4 determination of H2 O2 for method validation TiOSO4 method [52] was applied to H2 O2 solutions prepared in ultrapure water within the concentration range of 2.5 × 10−4 –2.0 × 10−3 mol L−1 , and the mean values of three repetitive measurements were used for calculations. The calibration equation between absorbance and concentration was: A408nm = 7.11 × 102 CH2O2 –1.42 × 10−2 (r = 0.9999) where CH2O2 was the H2 O2 concentration (in mol L−1 ) in final solution. Comparing the molar absorptivities, this equation shows that the proposed method was nearly 40-times more sensitive than the reference titanyl oxysulfate colorimetric method. The proposed spectrophotometric method was validated against the TiOSO4 assay, and the results of statistical analysis were tabulated in Table 3, using n = 5 repetitive determinations of a standard H2 O2 solution at 4 × 10−5 mol L−1 . The t- and F-tests were used for comparing the population means and variances, respectively, and the confidence levels used in validation of findings were 95% and 99%, respectively, for t- and F-tests (Table 3). Basically, there were

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Table 1 Analytical performance of the proposed method for H2 O2 . Linear rangea

Slopeb -intercept values of the calibration lines

2.5 × 10−6 –4 × 10−5 (r = 0.9999)

2.81 × 104

a b c d e f

– 2.3 × 10−3

⑀c

LODd

2.81 × 104

2 × 10−8

CVse % (n = 5)

RSDf %

Intra-assay

Inter-assay

0.31

0.90

0.90−3.95

in mol L−1 units at final concentration. in mol L−1 units. molar absorptivity, in L mol−1 cm−1 units. limit of detection, in mol L−1 units (LOD = 3 ␴bl /m, ␴bl denoting the standard deviation of a blank and m showing the slope of the calibration line). coefficients of variation. relative standard deviation (N = 5).

Fig. 1. Visible spectra of TMB-diimine cation (obtained from oxidation of TMB with released Ag+ ions, produced from AgNPs-catalyzed degradation of H2 O2 ) at (1) 2.5 × 10−6 mol L−1 , (2) 5 × 10−6 mol L−1 , (3) 1 × 10−5 mol L−1 , (4) 2 × 10−5 mol L−1 , (5) 3 × 10−5 mol L−1 , (6) 4 × 10−5 mol L−1 final concentration of hydrogen peroxide.

no substantial differences between the precision and accuracy of results. 3.6. Comparison of the performance of the sensor with other H2 O2 sensors Endo et al. synthesized polyvinylpyrrolidone-coated silver nanoparticles as a LSPR-based sensor for hydrogen peroxide, but this turn-off sensor depending on disintegration of nanoparticles upon H2 O2 degradation basically lacked a linear calibration curve; even the logarithmic curve covering absorbance drops against several orders-of magnitude H2 O2 concentrations was quite nonlinear, e.g., 1 pM, 1 nM and 1 ␮M H2 O2 solutions gave rise to absorbance reductions of 14.3, 15.4 and 17.8%, respectively, rendering quantitative evaluation impossible [45]. With the same principle (i.e. degradation of AgNPs to yield a transparent solution), Filippo et al. developed a hydrogen peroxide sensor utilizing polyvinyl alcoholcoated silver nanoparticles; the duration of analysis (80 min), the absence of analytical figures of merit, and the non-linear calibration curve were the primary disadvantages of this method [44]. Another similar study from Vasileva et al. depended on starch-stabilized silver nanoparticles for quantifying H2 O2 over a wide concentration range, but unfortunately had no linear calibration curve even

for Log[H2 O2 ] [43]. Recently, Chen et al. [56] used AgNPs synthesized on the surface of graphene quantum dots (GQDs) for building a turn-off sensor for hydrogen peroxide, but although this sensor was reported to yield linear responses between 0.5 and 100 ␮M H2 O2 , the absorbance values of 14 concentrations (in the range 100–0 ␮M) were very close (0.0–0.6). The common feature of all these LSPR sensors was the lack of a quantitative evaluation at low H2 O2 concentrations due to the inherent weakness of these turn-off sensors, as the catalyst (AgNPs) degraded the analyte (H2 O2 ) which in turn degraded the catalyst partly by non-chemical (mechanical) means, and yet the analyte quantitation had to be made by measuring the LSPR absorption of catalyst. Although turn-off AgNPs sensors (i.e. based on absorbance decrease) were claimed not to require chromogenic reagents (because of the exploitation of the intrinsic LSPR band of AgNPs), it was either not possible to generate a linear calibration curve or the obtained absorbance values were very low. Among a limited number of turn-on sensing methods, peroxidase-like activity of certain nanostructures were exploited to catalyze the reaction between H2 O2 and a special dye (e.g., TMB, ABTS). For this purpose, Wang and Wei [47] developed a method for H2 O2 determination via Fe3 O4 magnetic nanoparticles catalysis, where hydrogen peroxide−derived ROS oxidized ABTS to

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Fig. 2. Visible spectra of TMB-diimine cation (obtained from oxidation of TMB with released Ag+ ions, produced from AgNPs-catalyzed degradation of H2 O2 derived from the hydrolysis of TATP) at A. The color images of the test tubes containing (b) blank, (1) 1.25 mg L−1 , (2) 3.125 mg L−1 , (3) 6.25 mg L−1 , (4) 12.5 mg L−1 , (5), 18.75 mg L−1 , (6) 25.0 mg L−1 , (7) 31.25 mg L−1 TATP (end product) is shown in the inset figure (B).

Table 2 Recovery of 2 × 10−5 M (final conc.) H2 O2 from common soil ions mixtures. InterferingIon

Interfering Ion Conc. (␮M, final conc.)

(Interferent/Analyte) mole ratio

InitialRecovery (%)

Ca2+

20 100

1 5

103 32

95

Mg2+

20 100

1 5

105 70

100

K+

20 100

1 5

100 95

Fe3+

20 100

1 5

85 –

101 98.7

Fe2+

20 100

1 5

80 –

95 95.5

Cu2+

20 100

1 5

102 45

103

Al3+

20 100

1 5

103 21

105

20 100

1 5

75 –

101 96.6

NO3 −

20 100

1 5

100 102

SO4 2−

20 100

1 5

100 105

Pb

2+

the colored cation radical. In this method, the absorbance values used for constructing the calibration curve were again considerably low; deviations were apparent from Beer’s law and linearity was not ideal. A turn-on peroxide sensor was developed by Jv et al.

Recovery (%) (after EDTA masking)

[57] using positively-charged gold nanoparticles (AuNPs) in the presence of TMB. The measured absorbances for the tested concentration range (2 ␮M–1.6 mM) were approximately in between 0.16 and 0.30. The exact reaction mechanism (claiming peroxidase-

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Table 3 Statistical comparison of the proposed method with TiOSO4 reference method for determination of H2 O2 . Method Proposed sensor TiOSO4

Mean Conc.(mg L−1 ) −5

3.97 × 10 4.06 × 10−5

Sa , b

SD (␴) −5

2.06 × 10 4.08 × 10−5

−7

3.23 × 10

ta

ttable b

Fb

Ftable b

2.11

2.30

3.93

6.39

a S2 = ((n1 − 1)s1 2 + (n2 − 1)s2 2 )/(n1 + n2 − 2) and t = (a¯ 1 − a¯ 2 )/(S(1/n1 + 1/n2 )1/2 ), where S is the pooled standard deviation, s1 and s2 are the standard deviations of the two populations with sample sizes of n1 and n2 , and sample means of a¯ 1 and a¯ 2 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.

mimic of AuNPs) was unclear; citrate-coated silver nanoparticles were also tried without success. The analytical performance characteristics of the proposed sensor were compared with those of other nano-spectroscopic sensors for H2 O2 determination (Table S4). Thus, the novel colorimetric sensor of this study constitutes a rare example of AgNPs-based turn-on sensors for hydrogen peroxide estimation, and is superior to color-fading sensors in regard to both precision of determination and colored product (diimine) formation occurring at a wavelength (655 nm) far from the LSPR wavelength of AgNPs around 400 nm, the latter of which is open to interference effects from near-UV absorbing constituents of complex matrices. The developed sensor is more sensitive than a very limited number of turn-on sensors. In this work, in order to achieve higher sensitivity, we exploited a turn-on colorimetric sensing mechanism in which H2 O2 oxidized AgNPs to Ag+ , which in turn oxidized 3,3 ,5,5 tetramethylbenzidine. In other words, the indirect chromophore is the silver ions (Ag+ ) emerging as a result of partial oxidation of AgNPs under optimized conditions. However, this is an indirect determination which does not utilize either AgNPs or Ag+ ions by itself, because Ag+ oxidizes a reporter dye (TMB) to a bluecolored diimine having maximal absorptivity at 655 nm, enabling the detection of H2 O2 at a LOD as low as 20 nM. In this case, hydrogen peroxide owes its high molar absorptivity to its indirect determination with an intensely colored dye, TMB-diimine. He et al. [42] hypothesized that AgNPs and hydrogen peroxide may form Ag+ ions through the reactions represented by Eq.s (2a) & (2b). On the other hand, Liu et al. [48] reported that the detection limit for silver(I) ions in the TMB colorimetric estimation was about 50 nM at a signal-to-noise ratio of 3. The stoichiometry of Eqs. (2a) and (2b) and the high molar absorptivity of the proposed method (Table 1) together explain the low LOD we achieved for H2 O2 . While the AgNPs-based sensor was being developed, minimal reaction time, practical usage and sensor stability were considered. For this purpose, optimal conditions were determined such as AgNPs synthesis, pH of medium, effects of buffer concentration, and reaction time (Supplementary Material). The developed sensor gave reproducible linear responses over a reasonable H2 O2 concentration range at micromolar level. Borohydride-reduced and citrate-capped AgNPs to be used in the developed method were synthesized, as described in literature by Wan et al. [50]. The characteristic peak of these AgNPs showed maximal absorption wavelength at 391 nm, which did not change during intraand inter-assay measurements. The sensor was stable for almost 30 days at +4 ◦ C (Supplementary Material).

advantages in terms of its strongly acidic character through sulphonic acid groups, as the hydrolysis procedure does not require concentrated acid solution and neutralization can be achieved using a low amount of the weak base NH3 , added to the fact that the resin can be successively reused after regeneration. The agitation procedure carried out in a resin suspension of TATP solution was consecutively repeated twice for increasing the hydrolysis yield of TATP to H2 O2 . Optimization studies of the hydrolysis procedure were given in Supplementary Material. The analytical performance parameters for TATP determination are shown in Table 4. Visible spectra of TMB-diimine cation (obtained from oxidation of TMB with released Ag+ ions, produced from AgNPs-catalyzed degradation of H2 O2 derived from the hydrolysis of TATP) for varying TATP concentrations, together with the color changes of the test tubes, are shown in Fig. 2. 3.8. Results of TATP determination in commonly used explosives and camouflage materials The percentage recoveries of TATP at 100 mg L−1 initial concentration from mixtures containing nitro-explosives (TNT, RDX and PETN) and ammonium nitrate (the main component of ANFO) at 10-fold amounts were very close to 100%. The recoveries were 105%, 105%, 101% and 99%, respectively, for mixtures containing TNT, RDX, PETN and NH4 NO3 . Household detergent, sweetener, acetylsalicylic acid and paracetamol-based painkiller drugs can be used as camouflage because of the color and appearance similarities to TATP. For this reason, they were studied as a source of (false positive) interference. When a detergent is dissolved in water, it yields a mixture of hydrogen peroxide (which eventually decomposes to water and oxygen) and sodium carbonate [59]. The resulting hydrogen peroxide is a potential interference for the proposed assay. For eliminating this effect, we exploited the solubility differences between the analyte and detergent, and applied selective acetone extraction for dissolving TATP. After extraction of TATP from detergent, sweetener, acetylsalicylic acid and paracetamol mixtures, the proposed assay was applied. Recoveries of TATP from detergent, sweetener, acetylsalicylic acid and paracetamol added samples were 99%, 97%, 98% and 95%, respectively. Additionally, the proposed method was directly applied to unspiked detergent, sweetener, acetylsalicylic acid and paracetamol-caffeine. The bar diagram (Fig. 3) shows that these camouflage materials carried by passengers as personal belongings in hand-held luggages had no significant responses (i.e. no false positives).

3.7. Application of the sensor to TATP determination

3.9. GC–MS determination of TATP for method validation

The mass spectrum of the synthesized TATP was checked in GC–MS. The characteristic ion generated by the analyte having an (m/z) value of 43 was observed at SIM mode, which was compatible with the reference values from both GC–MS library and literature [58]. The use of an Amberlyst-15 resin in the recommended hydrolysis procedure for conversion of TATP to H2 O2 provided certain

TATP working solutions in acetonitrile at 1–10 mg L−1 concentrations were analyzed with GC–MS, and the mean values of three repetitive injections were used for calculations. The calibration equations between peak area and concentration were: PeakArea = 1.16 × 105 CTATP + 8.01 × 104 (r = 0.9994)

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105

Table 4 Analytical performance of the proposed method for TATP. Linear rangea

Slopeb -intercept values of the calibration lines

⑀c

LODd

CVse % (n = 5) Intra-assay

Inter-assay

1.25–31.25 (r = 0.9997)

2.81 × 10−2 −

6.9 × 103

0.31

1.6

1.8

a b c d e f

4.21 × 10−2

RSDf %

0.72−5.39

in mg L−1 units at final concentration. in mg−1 L units. molar absorptivity, in L mol−1 cm−1 units. limit of detection, in mgL−1 units (LOD = 3 ␴bl /m, ␴bl denoting the standard deviation of a blank and m showing the slope of the calibration line). coefficients of variation. relative standard deviation (N = 5).

Table 5 Statistical comparison of the proposed method with GC–MS for TATP determination. Method

Mean Conc.(mg L−1 )

SD (␴)

Sa , b

ta

ttable b

Fb

Ftable b

Proposed sensor GC–MS

98.84 98.48

0.37 0.09

0.27

2.10

2.30

14.25

6.39

a S2 = ((n1 − 1)s1 2 + (n2 − 1)s2 2 )/(n1 + n2 − 2) and t = (a¯ 1 − a¯ 2 )/(S(1/n1 + 1/n2 )1/2 ), where S is the pooled standard deviation, s1 and s2 are the standard deviations of the two populations with sample sizes of n1 and n2 , and sample means of a¯ 1 and a¯ 2 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.

Fig. 3. Responses of (1) 100 mg L−1 TATP, (2) 1000 mg L−1 acetylsalicylic acid, (3) 1000 mg L−1 paracetamol, (4) 1000 mg L−1 sweetener, (5) 1000 mg L−1 household detergent to the sensor.

ducibility (CVH2O2 0.3% intra-day and 0.9% inter-day; CVTATP 1.6% intra-day and 1.8% inter-day) and linear response (linear correlation coefficient: r = 0.9999 for H2 O2 and 0.9997 for TATP) within a reasonable range of micromolar concentrations of hydrogen peroxide (LODH2O2 = 20 nM). This sensor has a better chance for being used in conventional laboratories using nanotechnology because it is nonlaborious, cost-effective and stable (i.e. maintaining its spectral characteristics over one month when prepared under optimal conditions). The explosive TATP can be easily determined in the field because it is readily hydrolyzed with a strongly acidic cation exchange resin to H2 O2 (minimizing solvents and pretreatments) with an LOD of 0.31 mg L−1 for TATP. Compared to sensitive but expensive and laborious LC– and GC– MS/MS methods, this practical method enabling naked eye detection of the reaction product may better fit the needs of crime scene investigations, easening the burden of criminological laboratories for fast screening and decision making in the analysis of large numbers of samples. Acknowledgements

Statistical comparison between the results of the proposed and reference GC–MS procedures applied to 5 mg L−1 TATP in acetonitrile was made on 5 repetitive analyses, essentially showing no significant difference between the results. Thus, the proposed procedure was validated against GC–MS method; the t- and F-tests were used for comparing the population means and variances, respectively [58]. The confidence levels used in validation of findings were 95% and 99%, respectively, for t- and F-tests (Table 5). 4. Conclusions This work reports the development of a colorimetric H2 O2 and TATP sensor using silver nanoparticles. The analyte (H2 O2 ) oxidized AgNPs to Ag+ , which in turn selectively oxidized the chromogenic reagent (TMB) to a blue-colored diimine. The sensor operating at 655 nm (far from the interferences of near-UV absorbing constituents) was optimized for quantitating H2 O2 and TATP, and analytical figures of merit determined. Although a great many turn-off sensors for hydrogen peroxide exist in literature utilizing the principle of AgNPs degradation to a colorless product, the developed sensor is a turn-on sensor having distinct advantages of analytical sensitivity (ε = 2.81 × 104 M−1 cm−1 for H2 O2 ), repro-

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Biographies Ays¸em Üzer received the Ph.D. degree in 2009 and she is currently an Associate Professor of Faculty of Engineering Chemistry Department at Istanbul University in Turkey. Her academic studies and expertise are mainly focused on field analysis of trace explosives and other environmentally hazardous substances, including spectroscopic and electroanalytical studies and development of nano-sensors. Selen Durmazel received the M.Sc. degree in 2016 in Istanbul University, Chemistry Department and she is currently Ph.D. student in Analytical Chemistry Division. Her research interests are synthesis of nanoparticles and development of chemical sensors. g received his Ph.D. in Istanbul University in 1995. His research interErol Erc¸a˘ ests are environmetal chemistry, heavy metal removal by solvent extraction and ion exchange; sorption of heavy metals and pesticides from water onto unconventional sorbents, development of atomic and molecular spectroscopic methods for trace elements in biological tissue and blood samples, antioxidant vitamins, and polynitro-explosives.

107

Res¸at Apak received his Ph.D. in Istanbul University in 1982 and he is currently a Professor of Faculty of Engineering Chemistry Department at Istanbul University in Turkey. He is head of the Analytical Chemistry Division in Chemistry Department since 1990. He is principal member of Turkish Academy of Sciences (TUBA) since 2012. His research interests focus on development of analytical methods for the (molecular and atomic) spectrophotometric determination of trace pollutants (e.g., residual pesticides and polynitro-explosives in soil) and biologically important compounds such as antioxidant vitamins, flavonoids and other polyphenols; devising analytical methods for the total antioxidant capacity assay of foodstuffs and human plasma, individual determination of antioxidant vitamins, flavonoids, and plasma antioxidants.