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Sensors and Actuators B 247 (2017) 98–107

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo u r nal homep age: www.elsev ier .com/ locate /snb

etermination of hydrogen peroxide and triacetone triperoxideTATP) with a silver nanoparticles—based turn-on colorimetric sensor

ys em Üzer, Selen Durmazel, Erol Erc ag1, Res at Apak ∗

stanbul University, Faculty of Engineering, Chem. Dept., 34320, Istanbul, Turkey

r t i c l e i n f o

rticle history:eceived 7 December 2016eceived in revised form 27 February 2017ccepted 2 March 2017vailable online 6 March 2017

eywords:ydrogen peroxideriacetone triperoxide (TATP)ilver nanoparticles

a b s t r a c t

Hydrogen peroxide (H2O2) 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-usein conventional laboratories for the naked eye detection of hydrogen peroxide and for indirect determina-tion of peroxide-based explosives (which are devoid of chromogenic/fluorogenic functional groups). Wewere able to partly oxidize zero-valent silver nanoparticles (Ag0NPs) by H2O2 to Ag+ under special condi-tions, and thus devise an indirect method for trace H2O2 quantification by measuring the absorbance ofthe blue-colored diimine of TMB (3,3′,5,5′- tetramethylbenzidine) at 655 nm, arising from TMB oxidationwith Ag+. TATP was acid-hydrolyzed to H2O2 by an acidic cation-exchanger (Amberlyst-15) for potentialfield use. The limit of detection (LOD) of the sensor was 20 nM for H2O2 and 0.31 mg L−1 for TATP. Com-

olorimetryxplosiveensor

mon 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 painkillerdrugs, used as camouflage material in passenger belongings, were also examined. The developed methodwas statistically validated against the standard analytical methods of titanium(IV) oxysulfate (TiOSO4)

TATP

for H2O2 and GC–MS for

. Introduction

Hydrogen peroxide is a dual-function redox compound (i.e. thatay act as both an oxidant and reductant) finding wide use in

utrition, pharmaceutical, textile and clinical industries. H2O2 is potent bleaching and disinfecting agent in water treatment, anday play key roles in atmospheric phenomena and biochemical

rocesses [1–5]. Because it is a product of biochemical reactionsatalyzed by oxidase enzymes (such as catalase, glucose oxidase,orseradish peroxidase, etc.), H2O2 can be associated with a greatany diseases such as diabetes, cancer, and aging [6,7]. Since

eroxo-oxygen is unstable that may undergo disproportionation,2O2 is also used as a precursor for synthesis of homemade explo-

ives (TATP, HMTD, etc.) classified under peroxide-based energeticaterials [8–13]. In the late 1970′s and early 1980′s, peroxide-

ased explosives (TATP and HMTD) confiscated by the Israeli police

ere commonly used in terrorist attacks for their simple synthesis,

ccessibility of raw chemicals used for synthesis, and high explosiveotential [14–16]. The determination of peroxide explosives has

∗ Corresponding author.E-mail address: rapak@istanbul.edu.tr (R. Apak).

1 The researcher was an academic staff member of Istanbul University at thenitiation of the work.

ttp://dx.doi.org/10.1016/j.snb.2017.03.012925-4005/© 2017 Elsevier B.V. All rights reserved.

using t- and F- tests.© 2017 Elsevier B.V. All rights reserved.

gained importance in recent years due to some historical terroristactions such as 2005 London Subway bombing, 2009 failed terroristattempt in a Northwest Airways flight, and Paris suicide bomb-ing in 2015 [14]. Peroxide explosives lack spectroscopically activefunctional groups such as nitro substituents and aromatic rings,and therefore may elude common spectral detection techniquesemployed at check points of mass transport. TATP easily sublimesat room temperature, and does not leave post-blast residues unlikecommon nitro-explosives [17,18]. TATP may yield similar coloredproducts as H2O2 with the use of chromogenic reagents (suchas titanyl oxalate) on microfluidic paper-based analytical devices,though at a much weaker color intensity [19]. Colorimetric deter-mination of TATP is greatly facilitated when it is hydrolyzed toH2O2. Thus, hydrogen peroxide, which is a synthetic precursor anddegradation product of TATP and HMTD, is accepted as a note-worthy compound for determination of peroxide-based explosives[20–23]. Considering all these facts, trace H2O2 determination hasbecome essential for both industrial and academic purposes. Rela-tively older assay methods such as titrimetry, spectrophotometry,fluorometry [24,25] for H2O2 and colorimetry [26], spectroscopy[27–30], fluorometry [31,32], and chromatography [33,34] for TATP

have started to be replaced by recently improved nanoparticle-based hydrogen peroxide sensors. Accordingly, nanosensors weredesigned for H2O2 detection using metals like copper (Cu) [35], gold

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Au) [36], palladium (Pd) [37], silver (Ag) [38], or metal oxides likeuO [39], Fe3O4 [40,41].

Silver nanoparticles are used as bactericide for their strongntimicrobial effects. One of the probable mechanisms leading tohis effect (preventing microbial growth) is the generation of super-xide anion and hydroperoxyl radical, hydroxyl radical, hydrogeneroxide and singlet oxygen via Ag0NPs and Ag+ [42]. Thermody-amically, hydrogen peroxide (E0

H2O2/H2O = 1.776 V) can act as anxidant toward colloidal silver (E0

Ag + /Ag = 0.799 V) under optimalinetic conditions. This provides an opportunity for Ag0NPs−basedetermination of hydrogen peroxide. There are a number of spec-rophotometric and LSPR−based methods in literature for theetermination of H2O2 using Ag0NPs [43–47], however, theirensitivity and selectivity are quite low. Most of the existing AgNPs-ased sensors for hydrogen peroxide are turn-off sensors, basedn color fading at the LSPR wavelength of silver nanoparticles,owever they suffer from problems associated with accuracy andrecision. Therefore, the developed AgNPs method is consideredssential for filling this gap. This work reports the developmentf a novel turn-on spectrophotometric sensor for H2O2, which isasy-to-apply, stable, selective and of low cost, as required by in-eld/on-site determinations.

3,3′,5,5′-Tetramethylbenzidine (TMB) is a chromogenic reagentommonly used for horseradish peroxidase−based determinationf H2O2; the reagent also has the ability to be selectively oxidizedy Ag+ to a colored diimine product [48,49]. Combining these facts,e were able to oxidize zero-valent silver nanoparticles (Ag0NPs)

y H2O2 to Ag+ under special conditions, and thus devise an indirectethod for trace H2O2 quantification by measuring the absorbance

f TMB-diimine at 655 nm, arising from TMB oxidation with Ag+.his colorimetric sensor proved to be more sensitive and selectivehan analogic turn-off nanoparticle sensors. The selected analyticalavelength is far from the effects of common interferents adversely

ffecting the intrinsic LSPR wavelength of Ag0NPs around 400 nm.he explosive compound TATP can be easily converted by acidicydrolysis to H2O2 using the strongly acidic cation exchanger resin,mberlyst-15, and subsequently determined with the proposedensing method. This mild acid treatment also aids the discrimi-ation of organic peroxides from inorganic ones. The interferencesf some soil ions were investigated, as well as the possible ‘false-ositive’ effects of common passenger belongings and pills, suchs aspartame−based sweeteners, acetylsalicylic acid, paracetamol-affeine based analgesic drugs and household detergents whichay serve as camouflage material to TATP having a similar color

nd appearance. The responses of nitro-explosive group chemicalsrepresented by TNT, RDX, PETN and NH4NO3) to the sensor werelso studied.

. Materials and methods

.1. Chemicals

All reagents were analytical reagent grade unless otherwisetated. 3,3′,5,5′-Tetramethylbenzidine (TMB), hydrogen perox-de (%30, Suprapur

®), silver nitrate (AgNO3) and Amberlyst-15

®

hydrogen form, dry) were purchased from Merck Millipore andigma-Aldrich. All the other reagents were obtained from Merck,igma-Aldrich and Fluka. TNT, RDX and PETN samples wererovided by the Mechanical and Chemical Industry CorporationMKEK) of Turkey during previous projects.

.2. Instrumentation

The spectra and absorption measurements were recorded inatched Hellma Suprasil black quartz cuvettes using a Shimadzu

tors B 247 (2017) 98–107 99

UV-1800 UV–vis spectrophotometer. The optical thickness of thecuvettes used in solution phase measurements was 10 mm. For vali-dation of the proposed assay in the determination of TATP, a GC–MSinstrument utilizing a Thermo Scientific Trace gas chromato-graph coupled with a DSQII mass spectrometer containing electronimpact ionization and quadrupole analyzer was used. GC wasequipped with a Thermo 5MS column (30m × 320 �m × 0.25 �m).

2.3. Preparation of solutions

The working solutions of hydrogen peroxide at5 × 10−6–8 × 10−5 M were daily prepared from the correspondingstock solutions of 1.0 × 10−3 M in ultrapure water. TMB stocksolution was prepared daily at 1 × 10−2 M in ethanol. The acetatebuffer solution at pH 4.0 (containing 2 M CH3COOH and 2 MCH3COONa) was prepared in ultrapure water. The stock solutionsof NH3 and H2SO4 were prepared at 4 M and 2 M, respectively,in water for neutralization and pH adjustments. Hydrogen per-oxide at 4 × 10−5 M was assayed in the presence of 1- and 5-foldconcentrations of common ions, namely SO4

2−, NO3−, Mg2+, Ca2+,

K+, Cu2+, Fe3+, Fe2+, Pb2+, and Al3+. All mixtures were prepared inultrapure water. The working solutions of TATP at 10–250 mg L−1

were daily prepared from the corresponding stock solutions of1000 mg L−1 in pure acetone (diluent was acetone – water; 1:10,v/v) for colorimetric sensing. For GC–MS analysis of TATP, acetoni-trile was the preferred solvent and the working solutions of TATPat 1–10 mg L−1 were daily prepared from the corresponding stocksolution at 500 mg L−1 in acetonitrile. Binary mixtures of commonenergetic materials, especially RDX, TNT, PETN and NH4NO3,were prepared at 1000 mg L−1 concentration (i.e., 10-fold of theanalyte) in acetone-water (1:10, v/v) and TATP was added at afinal concentration of 100 mg L−1. Binary mixtures of householddetergent, sweetener, acetylsalicylic acid and paracetamol wereprepared 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 finalconcentration of 100 mg L−1.

2.4. Synthesis and characterization of AgNPs

Silver nanoparticles were synthesized by using a modification ofLee-Meisel method developed by Wan et al. [50], based on NaBH4reduction and citrate-capping of 4 nm average sized AgNPs. In thisprocedure, a mixture of a 20 mL of 1% (w/v) trisodium citrate solu-tion and 75 mL of ultrapure water was heated to 70 ◦C during 15 minunder vigorous stirring. At the end of this time, a 1.7 mL volume of1% (w/v) AgNO3 solution and then 2 mL of 0.1% (w/v) freshly pre-pared NaBH4 were quickly added to the mixture. The final reactionmixture was kept at 70 ◦C under vigorous stirring for 1 h and cooledto room temperature.

Characterization of the synthesized AgNPs was performed withthe aid of a UV/Vis spectrophotometer after being diluted withultrapure 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 H2O2 quantification

The H2O2 assay consisted of three parts, namely the catalyticdegradation of H2O2 over AgNPs, formation of blue color owing tooxidized TMB by Ag+ ions, and spectrophotometric determinationof 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 mLltrapure water and a 2 mL of H2O2 solution were added to a testube. The mixture was kept at room temperature for 30 min tollow Ag+ ions (presumably formed from the catalytic degradationf H2O2) to be released from the surface of AgNPs. Finally, 0.5 mLf 2 M pH 4.0 acetate buffer solution was introduced to the testube, followed by the quick addition of 0.5 mL of 1 × 10−2 M TMBolution. Upon release of Ag+ ions from the nanoparticles, TMB wasxidized to the blue colored diimine. After 5 min, the absorbancef the final solution at 655 nm was read against a reagent blank.

The scheme for the method can be summarized as: add 0.5 mLgNPs solution + 0.5 mL ultrapure water + (x) mL unknown H2O2olution in the �M range + (2.0-x) mL H2O; (allow to stand for0 min for H2O2 degradation and/or Ag+ formation) + 0.5 mL of 2 MH 4.0 buffer solution + 0.5 mL of 1 × 10−2 M TMB solution; measure655nm against a reagent blank after 5 min of TMB addition.

.7. Determination of H2O2 in the presence of common soil ions

Recovery values were calculated by preparing a H2O2 solutionith an initial concentration of 4 × 10−5 M, adding solutions of

O42−, NO3

−, Mg2+, Ca2+, K+, Cu2+, Fe3+, Fe2+, Pb2+, and Al3+ (at 1- to-fold molar ratio to H2O2), and then applying the proposed sen-or. In order to eliminate the interference effects of these cations,a2EDTA as masking agent at a concentration of 1 × 10−2 M wassed at different volume ratios (metal-EDTA ratio: 1:2.5 (v/v) fora2+, Mg2+, and Pb2+; 1:5 (v/v) for Fe2+, Fe3+, Al3+, and Cu2+) asequired for each cation, followed by the application of the pro-osed method

.8. Real sample assay for H2O2

For real sample assay, both the proposed sensor and referenceiOSO4 procedure [52] (described in ‘Supplementary Material’)ere applied to a commercial 9% hydrogen peroxide solution whichas originally used for hair color bleaching. The results of these

pplications were given in Table S2 and Table S3, respectively, forhe proposed sensor and TiOSO4 reference method (Supplementary

aterial).

.9. Hydrolysis of TATP to H2O2 using a strongly acidic cationxchanger resin

Hydrolysis of TATP was performed by batch method usingmberlyst-15 resin. One gram of Amberlyst-15 resin was weighed

o a 50 mL-flask. The resin was swelled up in ethanol-water (1:1,/v) medium for 40 min. After the solvent mixture was decanted,he resin was washed with 5% (v/v) H2SO4 solution, and then 25 mLf TATP solution in acetone-water (1:10, v/v) was introduced to theask. The flask was placed in an incubator at room temperature andgitated for 15 min at 100 rpm with the aim of TATP hydrolysis to2O2. This step was repeated two times under the same conditions.he hydrolyzed solution was separated from the residual resin par-icles by filtration through quantitative filter paper. For stepwiseeutralization, first 750 �L of 4 M NH3 and then 750 �L of 1 M NH3ere added (in this order) to the hydrolyzed solution to bring thenal pH to about 7. The final volume was diluted to 100 mL withltrapure water.

.10. Application of the AgNPs-based sensor to TATP samples

The recommended method was applied to 2 mL of theydrolyzed TATP solution (see. 2.9) as described (see. 2.6).

tors B 247 (2017) 98–107

2.11. Determination of TATP in Complex Materials

In combination with 100 mg L−1 TATP, certain energetic materi-als (TNT, RDX, PETN, and NH4NO3) and potential camouflage agents(acetylsalicylic acid, paracetamol, sweetener and household deter-gent) were separately applied to each mixture solution (10-fold ofTATP, w/w), and percentage recovery values were recorded.

2.12. Statistical analysis

Descriptive statistical analyses were performed using Excelsoftware (Microsoft Office 2013) for calculating the means andthe standard error of the mean. Results were expressed as themean ± standard deviation (SD). Method validation against GC–MSdetermination of TATP was made by means of student (t-) andF-tests. (GC–MS determination of TATP was described in ‘Supple-mentary Material’)

3. Results and discussion

3.1. Detection principle of the sensor

The detection principle of developed sensor is the partial oxi-dation of Ag0NPs by H2O2 to Ag(I) ions in neutral medium (Eq.(1)).

Ag0(NP) + 2H2O2Ag+ + O2•− + 2H2O (1)

This equation represents the overall reaction, in which AgNPspresumably form a strongly oxidizing intermediate complex withH2O2 (possibly Ag-O(H)-OH), which further reacts with anotherH2O2 molecule to form Ag+ and superoxide [53]. The produced Ag+

was detected using a chromogenic reagent TMB acting as a selectiveand sensitive indicator for silver ions [48]. The absorbance arisingfrom the color change (from yellow to blue) was measured by avisible spectrophotometer at 655 nm (Scheme 1).

The formation of superoxide anion radicals (O2•−) during the

catalytic decomposition of H2O2 with alumina-supported metallicsilver was first suggested by Ono and coworkers via ESR detectionof superoxide radicals [54]. Since AgNPs are known to catalyze thedecomposition of H2O2, O2

•− was hypothesized to act both as anoxidant for AgNPs and a reductant for Ag+ ions [55]. Aside fromthe fact that the reaction expressed by Eq. (1) is reversible throughreformation of AgNPs [55], a complex interplay among AgNPs, Ag+,O2

•− and H2O2 seems to exist [53], preventing quantitative reac-tions to occur. Therefore, Ag+ formation (detected by TMB) fromAgNPs−catalyzed degradation of H2O2 has to be carefully opti-mized in order to develop a reliable colorimetric assay for tracehydrogen peroxide.

3.2. Reaction parameters for AgNPs-catalyzed H2O2 degradationand TMB oxidation

The preparation, characterization and stability of AgNPs usedin hydrogen peroxide sensing were expressed in detail (Supple-mentary Material). The nanoparticles prepared by NaBH4 reductionand citrate capping (Table S1) exhibited maximum absorbance atthe specific LSPR absorption wavelength (Fig. S1), were stable for30 days (Fig. S2) and did not change from lot to lot of differentsyntheses (Fig.s S3 and S4). The average size and charge of the syn-thesized AgNPs were very important for their catalytic ability todegrade H2O2 and partially produce Ag+ used in indirect quantifi-

cation of hydrogen peroxide. The AgNPs-based sensor operated intwo stages, namely the catalytic degradation of H2O2 with AgNPs,and colorimetric detection of the formed Ag+ with TMB. Therefore,optimal pH, time (Fig. S5 & S6) and temperature for these two stages

A. Üzer et al. / Sensors and Actuators B 247 (2017) 98–107 101

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Scheme 1. Schematic presentation of the developed colori

ere determined (Supplementary Material). The optimal amountsf AgNPs (Fig. S7) and TMB (Fig. S8) used in the determination werepecified.

The reactivity of AgNPs toward H2O2 was shown to decrease byecreasing pH, while at high pH, reformation of AgNPs by a backeaction (from Ag+ and O2

•−) was essentially complete [42]. There-ore, the optimal pH setting (to a neutral pH of 7) for the first stageinvolving catalytic degradation of H2O2 to produce Ag+) was madeo assure maximal H2O2 degradation and minimal AgNPs forma-ion (Supplementary Material). The optimal pH for the second stageinvolving TMB oxidation by Ag+) was set to pH 4 in accordance withhe recommendation of Liu and coworkers [48], and the initial con-entration of acetic acid/acetate buffer was set to 2.0 M in order toring about maximal aggregation of AgNPs so as to reveal the blueolor of the TMB oxidation product (diimine) in a non-turbid solu-ion (Supplementary Material). Within the concentration intervalf trace hydrogen peroxide being tested, the time periods of 30 minnd 5 min were determined to be optimal for the first and secondtages of the reaction, respectively, at room temperature. Longerimes and elevated temperatures were avoided to minimize backeactions (Supplementary Material).

Ag+ formation from AgNPs should be optimized with respect toime, as it may only form from (AgNPs + H2O2) reaction through anntermediate, and once formed, these products may reform AgNPsy a reverse reaction [42]. Thus, the overall Eq. (1) concerning Ag+

ormation consists of two parts, Eq.s (2a) & (2b):

gNP + H2O2 → intermediate (2a)

ntermediate + H2O2 → Ag+ + O2•− + H2O (2b)

Likewise, scavenging of superoxide radicals and reformation ofgNPs are associated with Eqs. (3a) & (3b):

gNP + O2•−→ AgNP∗− + O2 (3a)

gNP∗− + Ag+ → AgNP + AgNP (3b)

On the other hand, the actual chromophore for colorimetricetermination, TMB-diimine, forms relatively rapidly from Ag+

xidation of TMB, as seen from the time-dependent rise of 655m-absorbance of (TMB-Ag+) solution at room temperature [48].

t is possible that this color formation was further catalyzed by theresence of AgNPs [49]. Investigating the reactions represented byqs (2a), (2b) and (3a), (3b) between silver(I) ions and AgNPs, it ispparent that the formation of the indirect chromophore (Ag+) isinetically delayed (because of the intermediate), and its consump-ion is inevitable due to the reverse reaction ending up with AgNPs

eformation. Thus, a compromise should be found to maximize Ag+

ormation and minimize its consumption; this critical time periodas experimentally set in this work as 30 min. The involvement of

uperoxide radicals in AgNPs−catalyzed degradation of H2O2 with

c sensor for H2O2 determination (via a two-step reaction).

respect to Eq. (1) was also confirmed (Supplementary Material; Fig.S9).

3.3. Analytical figures of merit of the sensor applied to H2O2samples

The proposed sensor was applied to H2O2 solutions. The param-eters of analytical performance for H2O2 determination are shownin Table 1.

The visible spectra of TMB-diimine product (obtained fromoxidation of TMB with released Ag+ ions, produced from AgNPs-catalyzed degradation of H2O2), using varying concentrations ofH2O2 in the �M range, were shown in Fig. 1.

3.4. Recovery of H2O2 in the presence of common soil ions

The sensor was applied to common ions abundantly found insoil and groundwater (SO4

2−, NO3−, Mg2+, Ca2+, K+, Cu2+, Fe3+, Fe2+,

Pb2+ ve Al3+) at 1- and 5-fold concentrations of H2O2 and the analyterecoveries are shown in Table 2. High concentrations of commonions should be avoided in noble metal nanoparticles–based assaysbecause of the charge neutralization effect of salts causing nanopar-ticle aggregation. Fe2+, Fe3+ and Pb2+ at 1-fold concentration, andCa2+, Mg2+, Cu2+ ve Al3+ at 5-fold concentration of H2O2 gavereduced recoveries for the analyte, possibly via affecting its catalyticdegradation over AgNPs. All these interferences were eliminated byadding Na2EDTA as masking agent at the volume ratios indicatedin section 2.7 to obtain analyte recoveries ranging between 95 and105% (Table 2).

3.5. TiOSO4 determination of H2O2 for method validation

TiOSO4 method [52] was applied to H2O2 solutions pre-pared in ultrapure water within the concentration range of2.5 × 10−4–2.0 × 10−3 mol L−1, and the mean values of three repet-itive measurements were used for calculations. The calibrationequation between absorbance and concentration was:

A408nm = 7.11 × 102CH2O2–1.42 × 10−2(r = 0.9999)

where CH2O2 was the H2O2 concentration (in mol L−1) in final solu-tion. Comparing the molar absorptivities, this equation showsthat the proposed method was nearly 40-times more sensitivethan the reference titanyl oxysulfate colorimetric method. Theproposed spectrophotometric method was validated against theTiOSO4 assay, and the results of statistical analysis were tabu-lated in Table 3, using n = 5 repetitive determinations of a standard

H2O2 solution at 4 × 10−5 mol L−1. The t- and F-tests were used forcomparing the population means and variances, respectively, andthe confidence levels used in validation of findings were 95% and99%, respectively, for t- and F-tests (Table 3). Basically, there were

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

Table 1Analytical performance of the proposed method for H2O2.

Linear rangea Slopeb-intercept values of the calibration lines �c LODd CVse % (n = 5) RSDf %

Intra-assay Inter-assay

2.5 × 10−6–4 × 10−5

(r = 0.9999)2.81 × 104 – 2.3 × 10−3 2.81 × 104 2 × 10−8 0.31 0.90 0.90−3.95

a in mol L−1 units at final concentration.b in mol L−1 units.c molar absorptivity, in L mol−1 cm−1 units.d 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).e coefficients of variation.f relative standard deviation (N = 5).

F with

2 ) 3 × 1

nr

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ntueeertddcacsst

ig. 1. Visible spectra of TMB-diimine cation (obtained from oxidation of TMB

.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

o substantial differences between the precision and accuracy ofesults.

.6. Comparison of the performance of the sensor with other2O2 sensors

Endo et al. synthesized polyvinylpyrrolidone-coated silveranoparticles as a LSPR-based sensor for hydrogen peroxide, buthis turn-off sensor depending on disintegration of nanoparticlespon H2O2 degradation basically lacked a linear calibration curve;ven the logarithmic curve covering absorbance drops against sev-ral orders-of magnitude H2O2 concentrations was quite nonlinear,.g., 1 pM, 1 nM and 1 �M H2O2 solutions gave rise to absorbanceeductions of 14.3, 15.4 and 17.8%, respectively, rendering quan-itative evaluation impossible [45]. With the same principle (i.e.egradation of AgNPs to yield a transparent solution), Filippo et al.eveloped a hydrogen peroxide sensor utilizing polyvinyl alcohol-oated silver nanoparticles; the duration of analysis (80 min), thebsence of analytical figures of merit, and the non-linear calibration

urve were the primary disadvantages of this method [44]. Anotherimilar study from Vasileva et al. depended on starch-stabilizedilver nanoparticles for quantifying H2O2 over a wide concentra-ion range, but unfortunately had no linear calibration curve even

released Ag+ ions, produced from AgNPs-catalyzed degradation of H2O2) at (1)0−5 mol L−1, (6) 4 × 10−5 mol L−1 final concentration of hydrogen peroxide.

for Log[H2O2] [43]. Recently, Chen et al. [56] used AgNPs synthe-sized on the surface of graphene quantum dots (GQDs) for buildinga turn-off sensor for hydrogen peroxide, but although this sensorwas reported to yield linear responses between 0.5 and 100 �MH2O2, the absorbance values of 14 concentrations (in the range100–0 �M) were very close (0.0–0.6). The common feature of allthese LSPR sensors was the lack of a quantitative evaluation at lowH2O2 concentrations due to the inherent weakness of these turn-offsensors, as the catalyst (AgNPs) degraded the analyte (H2O2) whichin turn degraded the catalyst partly by non-chemical (mechanical)means, and yet the analyte quantitation had to be made by mea-suring the LSPR absorption of catalyst. Although turn-off AgNPssensors (i.e. based on absorbance decrease) were claimed not torequire chromogenic reagents (because of the exploitation of theintrinsic LSPR band of AgNPs), it was either not possible to generatea linear calibration curve or the obtained absorbance values werevery low.

Among a limited number of turn-on sensing methods,peroxidase-like activity of certain nanostructures were exploited

to catalyze the reaction between H2O2 and a special dye (e.g.,TMB, ABTS). For this purpose, Wang and Wei [47] developed amethod for H2O2 determination via Fe3O4 magnetic nanoparticlescatalysis, where hydrogen peroxide−derived ROS oxidized ABTS to

A. Üzer et al. / Sensors and Actuators B 247 (2017) 98–107 103

Fig. 2. Visible spectra of TMB-diimine cation (obtained from oxidation of TMB with released Ag+ ions, produced from AgNPs-catalyzed degradation of H2O2 derived fromthe 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 2Recovery of 2 × 10−5 M (final conc.) H2O2 from common soil ions mixtures.

InterferingIon InterferingIon Conc.(�M, final conc.)

(Interferent/Analyte)mole ratio

InitialRecovery (%) Recovery (%)(after EDTAmasking)

Ca2+ 20 1 103100 5 32 95

Mg2+ 20 1 105100 5 70 100

K+ 20 1 100100 5 95

Fe3+ 20 1 85 101100 5 – 98.7

Fe2+ 20 1 80 95100 5 – 95.5

Cu2+ 20 1 102100 5 45 103

Al3+ 20 1 103100 5 21 105

Pb2+ 20 1 75 101100 5 – 96.6

NO3− 20 1 100

100 5 102

tuaw

SO42− 20 1

100 5

he colored cation radical. In this method, the absorbance values

sed for constructing the calibration curve were again consider-bly low; deviations were apparent from Beer’s law and linearityas not ideal. A turn-on peroxide sensor was developed by Jv et al.

100105

[57] using positively-charged gold nanoparticles (AuNPs) in the

presence of TMB. The measured absorbances for the tested con-centration range (2 �M–1.6 mM) were approximately in between0.16 and 0.30. The exact reaction mechanism (claiming peroxidase-

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

Table 3Statistical comparison of the proposed method with TiOSO4 reference method for determination of H2O2.

Method Mean Conc.(mg L−1) SD (�) Sa,b ta ttableb Fb Ftable

b

Proposed sensor 3.97 × 10−5 2.06 × 10−5 3.23 × 10−7 2.11 2.30 3.93 6.39TiOSO4 4.06 × 10−5 4.08 × 10−5

here Sp ively (

nce m

mwaoS

rotfwipl

eHtiorbcthd[iovaat

reatscawTta3

3

G(w[

y

a S2 = ((n1 − 1)s12 + (n2 − 1)s2

2)/(n1 + n2 − 2) and t = (a1 − a2)/(S(1/n1 + 1/n2)1/2), wopulations with sample sizes of n1 and n2, and sample means of a1 and a2 respectb Statistical comparison made on paired data produced with proposed and refere

imic of AuNPs) was unclear; citrate-coated silver nanoparticlesere also tried without success. The analytical performance char-

cteristics of the proposed sensor were compared with those ofther nano-spectroscopic sensors for H2O2 determination (Table4).

Thus, the novel colorimetric sensor of this study constitutes aare example of AgNPs-based turn-on sensors for hydrogen per-xide estimation, and is superior to color-fading sensors in regardo both precision of determination and colored product (diimine)ormation occurring at a wavelength (655 nm) far from the LSPRavelength of AgNPs around 400 nm, the latter of which is open to

nterference effects from near-UV absorbing constituents of com-lex matrices. The developed sensor is more sensitive than a very

imited number of turn-on sensors.In this work, in order to achieve higher sensitivity, we

xploited a turn-on colorimetric sensing mechanism in which2O2 oxidized AgNPs to Ag+, which in turn oxidized 3,3′,5,5′-

etramethylbenzidine. In other words, the indirect chromophores the silver ions (Ag+) emerging as a result of partial oxidationf AgNPs under optimized conditions. However, this is an indi-ect determination which does not utilize either AgNPs or Ag+ ionsy itself, because Ag+ oxidizes a reporter dye (TMB) to a blue-olored diimine having maximal absorptivity at 655 nm, enablinghe detection of H2O2 at a LOD as low as 20 nM. In this case,ydrogen peroxide owes its high molar absorptivity to its indirectetermination with an intensely colored dye, TMB-diimine. He et al.42] hypothesized that AgNPs and hydrogen peroxide may form Ag+

ons through the reactions represented by Eq.s (2a) & (2b). On thether hand, Liu et al. [48] reported that the detection limit for sil-er(I) ions in the TMB colorimetric estimation was about 50 nM at

signal-to-noise ratio of 3. The stoichiometry of Eqs. (2a) and (2b)nd the high molar absorptivity of the proposed method (Table 1)ogether explain the low LOD we achieved for H2O2.

While the AgNPs-based sensor was being developed, minimaleaction time, practical usage and sensor stability were consid-red. For this purpose, optimal conditions were determined suchs AgNPs synthesis, pH of medium, effects of buffer concentra-ion, and reaction time (Supplementary Material). The developedensor gave reproducible linear responses over a reasonable H2O2oncentration range at micromolar level. Borohydride-reducednd citrate-capped AgNPs to be used in the developed methodere synthesized, as described in literature by Wan et al. [50].

he characteristic peak of these AgNPs showed maximal absorp-ion wavelength at 391 nm, which did not change during intra-nd inter-assay measurements. The sensor was stable for almost0 days at +4 ◦C (Supplementary Material).

.7. Application of the sensor to TATP determination

The mass spectrum of the synthesized TATP was checked inC–MS. The characteristic ion generated by the analyte having an

m/z) value of 43 was observed at SIM mode, which was compatible

ith the reference values from both GC–MS library and literature

58].The use of an Amberlyst-15 resin in the recommended hydrol-

sis procedure for conversion of TATP to H2O2 provided certain

is the pooled standard deviation, s1 and s2 are the standard deviations of the twot has (n1 + n2–2) degrees of freedom); here, n1 = n2 = 5.ethods; the results given only on the row of the reference method.

advantages in terms of its strongly acidic character through sul-phonic acid groups, as the hydrolysis procedure does not requireconcentrated acid solution and neutralization can be achievedusing a low amount of the weak base NH3, added to the fact that theresin can be successively reused after regeneration. The agitationprocedure carried out in a resin suspension of TATP solution wasconsecutively repeated twice for increasing the hydrolysis yield ofTATP to H2O2. Optimization studies of the hydrolysis procedurewere given in Supplementary Material.

The analytical performance parameters for TATP determinationare shown in Table 4.

Visible spectra of TMB-diimine cation (obtained from oxidationof TMB with released Ag+ ions, produced from AgNPs-catalyzeddegradation of H2O2 derived from the hydrolysis of TATP) for vary-ing TATP concentrations, together with the color changes of the testtubes, are shown in Fig. 2.

3.8. Results of TATP determination in commonly used explosivesand camouflage materials

The percentage recoveries of TATP at 100 mg L−1 initial con-centration from mixtures containing nitro-explosives (TNT, RDXand PETN) and ammonium nitrate (the main component of ANFO)at 10-fold amounts were very close to 100%. The recoveries were105%, 105%, 101% and 99%, respectively, for mixtures containingTNT, RDX, PETN and NH4NO3.

Household detergent, sweetener, acetylsalicylic acid andparacetamol-based painkiller drugs can be used as camouflagebecause of the color and appearance similarities to TATP. For thisreason, they were studied as a source of (false positive) interference.When a detergent is dissolved in water, it yields a mixture of hydro-gen peroxide (which eventually decomposes to water and oxygen)and sodium carbonate [59]. The resulting hydrogen peroxide is apotential interference for the proposed assay. For eliminating thiseffect, we exploited the solubility differences between the ana-lyte and detergent, and applied selective acetone extraction fordissolving TATP. After extraction of TATP from detergent, sweet-ener, acetylsalicylic acid and paracetamol mixtures, the proposedassay 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 wasdirectly applied to unspiked detergent, sweetener, acetylsalicylicacid and paracetamol-caffeine. The bar diagram (Fig. 3) showsthat these camouflage materials carried by passengers as personalbelongings in hand-held luggages had no significant responses (i.e.no false positives).

3.9. GC–MS determination of TATP for method validation

TATP working solutions in acetonitrile at 1–10 mg L−1 concen-trations were analyzed with GC–MS, and the mean values of threerepetitive injections were used for calculations. The calibration

equations between peak area and concentration were:

PeakArea = 1.16 × 105CTATP + 8.01 × 104(r = 0.9994)

A. Üzer et al. / Sensors and Actuators B 247 (2017) 98–107 105

Table 4Analytical performance of the proposed method for TATP.

Linear rangea Slopeb-intercept values of the calibration lines �c LODd CVse % (n = 5) RSDf %

Intra-assay Inter-assay

1.25–31.25(r = 0.9997)

2.81 × 10−2 − 4.21 × 10−2 6.9 × 103 0.31 1.6 1.8 0.72−5.39

a in mg L−1 units at final concentration.b in mg−1 L units.c molar absorptivity, in L mol−1 cm−1 units.d 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).e coefficients of variation.f relative standard deviation (N = 5).

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

Method Mean Conc.(mg L−1) SD (�) Sa,b ta ttableb Fb Ftable

b

Proposed sensor 98.84 0.37 0.27 2.10 2.30 14.25 6.39GC–MS 98.48 0.09

a S2 = ((n1 − 1)s12 + (n2 − 1)s2

2)/(n1 + n2 − 2) and t = (a1 − a2)/(S(1/n1 + 1/n2)1/2), where Spopulations with sample sizes of n1 and n2, and sample means of a1 and a2 respectively (

b Statistical comparison made on paired data produced with proposed and reference m

F −1 −1

1d

rtscwri

4

TArasatido

References

ig. 3. Responses of (1) 100 mg L TATP, (2) 1000 mg L acetylsalicylic acid, (3)000 mg L−1 paracetamol, (4) 1000 mg L−1 sweetener, (5) 1000 mg L−1 householdetergent to the sensor.

Statistical comparison between the results of the proposed andeference GC–MS procedures applied to 5 mg L−1 TATP in acetoni-rile was made on 5 repetitive analyses, essentially showing noignificant difference between the results. Thus, the proposed pro-edure was validated against GC–MS method; the t- and F-testsere used for comparing the population means and variances,

espectively [58]. The confidence levels used in validation of find-ngs were 95% and 99%, respectively, for t- and F-tests (Table 5).

. Conclusions

This work reports the development of a colorimetric H2O2 andATP sensor using silver nanoparticles. The analyte (H2O2) oxidizedgNPs to Ag+, which in turn selectively oxidized the chromogeniceagent (TMB) to a blue-colored diimine. The sensor operatingt 655 nm (far from the interferences of near-UV absorbing con-tituents) was optimized for quantitating H2O2 and TATP, andnalytical figures of merit determined. Although a great many

urn-off sensors for hydrogen peroxide exist in literature utiliz-ng the principle of AgNPs degradation to a colorless product, theeveloped sensor is a turn-on sensor having distinct advantagesf analytical sensitivity (ε = 2.81 × 104 M−1cm−1 for H2O2), repro-

is the pooled standard deviation, s1 and s2 are the standard deviations of the twot has (n1 + n2 − 2) degrees of freedom); here, n1 = n2 = 5.ethods; the results given only on the row of the reference method.

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 correla-tion coefficient: r = 0.9999 for H2O2 and 0.9997 for TATP) within areasonable range of micromolar concentrations of hydrogen per-oxide (LODH2O2 = 20 nM). This sensor has a better chance for beingused in conventional laboratories using nanotechnology because itis nonlaborious, cost-effective and stable (i.e. maintaining its spec-tral characteristics over one month when prepared under optimalconditions). The explosive TATP can be easily determined in thefield because it is readily hydrolyzed with a strongly acidic cationexchange resin to H2O2 (minimizing solvents and pretreatments)with an LOD of 0.31 mg L−1 for TATP. Compared to sensitive butexpensive and laborious LC– and GC– MS/MS methods, this prac-tical method enabling naked eye detection of the reaction productmay better fit the needs of crime scene investigations, easeningthe burden of criminological laboratories for fast screening anddecision making in the analysis of large numbers of samples.

Acknowledgements

The authors wish to express their gratitude to the Ministry ofNational Defence, Office of Technical Services, and to the Mechan-ical & Chemical Industry Corporation (MKEK) for the donation ofnitro- and composite explosive samples. The authors extend theirthanks to Istanbul University Research Fund (BAP Unit) for the sup-port given to M. Sc. Thesis Project-58243. One of the authors (Dr.Ays em Uzer) extends her thanks to Istanbul University ResearchFund for the support given to BEK Project-2016-20178, whichenabled her to present a part of this work in the “XIII. Conference onOptical Chemical Sensors and Biosensors 2016 International Meet-ing” in Graz Austria.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.snb.2017.03.012.

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Actua

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(e.g., residual pesticides and polynitro-explosives in soil) and biologically impor-tant compounds such as antioxidant vitamins, flavonoids and other polyphenols;devising analytical methods for the total antioxidant capacity assay of foodstuffsand human plasma, individual determination of antioxidant vitamins, flavonoids,and plasma antioxidants.

A. Üzer et al. / Sensors and

iographies

ys em Üzer received the Ph.D. degree in 2009 and she is currently an Associaterofessor of Faculty of Engineering Chemistry Department at Istanbul Universityn Turkey. Her academic studies and expertise are mainly focused on field analy-is of trace explosives and other environmentally hazardous substances, includingpectroscopic and electroanalytical studies and development of nano-sensors.

elen Durmazel received the M.Sc. degree in 2016 in Istanbul University, Chemistryepartment and she is currently Ph.D. student in Analytical Chemistry Division.er research interests are synthesis of nanoparticles and development of chemical

ensors.

rol Ercag received his Ph.D. in Istanbul University in 1995. His research inter-

sts are environmetal chemistry, heavy metal removal by solvent extraction andon exchange; sorption of heavy metals and pesticides from water onto unconven-ional sorbents, development of atomic and molecular spectroscopic methods forrace elements in biological tissue and blood samples, antioxidant vitamins, andolynitro-explosives.

tors B 247 (2017) 98–107 107

Res at Apak received his Ph.D. in Istanbul University in 1982 and he is currentlya Professor of Faculty of Engineering Chemistry Department at Istanbul Universityin Turkey. He is head of the Analytical Chemistry Division in Chemistry Depart-ment since 1990. He is principal member of Turkish Academy of Sciences (TUBA)since 2012. His research interests focus on development of analytical methods forthe (molecular and atomic) spectrophotometric determination of trace pollutants