Direct Blue Light-Induced Autocatalytic Oxidation of

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Direct Blue Light-induced Autocatalytic Oxidation of o-Phenylenediamine for Highly Sensitive Visual Detection of Triaminotrinitrobenzene Jinhu Wang, Hua Li, Yanhua Cai, Dunju Wang, Liang Bian, Faqin Dong, Haili Yu, and Yi He Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00759 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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Analytical Chemistry

Direct Blue Light-induced Autocatalytic Oxidation of o-Phenylenediamine for Highly Sensitive Visual Detection of Triaminotrinitrobenzene Jinhu Wanga, Hua Lic, Yanhua Caib, Dunju Wanga, Liang Biand, Faqin Dongd, Haili Yua, Yi He* a

a State Key Laboratory of Environment-friendly Energy Materials, Sichuan Co-Innovation Center for New Energetic Materials, Southwest University of Science and Technology, Mianyang 621010, P. R. China. b Chongqing Key Laboratory of Environmental Materials and Remediation Technology, Chongqing University of Arts and Sciences, Yongchuan 402160, P. R. China. c Materials Characterization & Preparation Center, Southern University of Science and Technology, Shenzhen 518055, China. d Key Laboratory of Solid Waste Treatment and Resource Recycle, Ministry of Education, State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang 621010, Sichuan, China. *Corresponding

author:

Prof.

Dr.

Yi

He,

Tel:

+86-816-6089889, Email: [email protected].

ACS Paragon Plus Environment

+86-816-6089885,

Fax:

Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT：o-Phenylenediamine (OPD)-based chromogenic reactions are worthy tools for development of visual colorimetric assays. The chromogenic reactions are usually triggered by various oxidants, which is not easily tunable and incompatible with some analytes. Herein, we report that direct blue light irradiation can induce the autocatalytic oxidation of OPD to generate 2,3-diaminophenazine (oxidized-state OPD, oxOPD). The autocatalytic photooxidation reaction mechanism of OPD is mainly ascribed to the resonant energy transfer between ectronically excited oxOPD and dissolved oxygen to form singlet state oxygen with a high oxidation capacity, which accelerates the oxidation of OPD. We demonstrate that under neutral and alkaline environment, the photo-induced autocatalytic oxidation of OPD is able to be further enhanced by triaminotrinitrobenzene (TATB) explosive because of its inhibition effect on the aggregation caused quenching phenomenon of oxOPD. On this basis, a straightforward visual colorimetric assay for TATB with a tunable dynamic range is developed. This assay has capable of detecting TATB explosive concentrations as low as 2.7 nM. Notably, the obvious color change after addition of TATB enables a naked-eye readout with the lowest detectable TATB concentrations of 60 nM.

KEYWORDS:

Triaminotrinitrobenzene

explosive,

o-phenylenediamine, autocatalysis, phooxidation


colorimetric

detection,

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INTRODUCTION Facile detection of trace explosives is very crucial due to the need for home safety and the increasing threat from the terrorism. To date, a large number of studies have reported on the detection of several explosives, including trinitrotoluene, picric acid, nitroglycerine, hexahydro-1,3,5-trinitro-1,3,5-triazine, triacetone triperoxide, and pentaerythritol tetranitrate1-14. However, the detection of relatively new explosive such as triaminotrinitrobenzene (TATB) that is a nuclear weapon explosive is quite limited. There are only a few reports about the use of surface-enhanced Raman scattering (SERS) for detection of trace TATB explosive15. Apparently, the SERS-based detection methods require the sophisticated equipment that is complex, expensive and time-consuming, and sample treatment procedure. In addition, the SERS technique can be easily affected by operation conditions, and it is difficult to realize the quantitative determination of TATB with a good accuracy. These drawbacks inevitably render the SERS assays challenging for rapid, low-cost and in-field detection of TATB explosive. Hence there is an urgent need to develop new detection strategy that does not involve the complicated instrument and tedious sample pretreatment for fast and reliable detection of TATB. Visual colorimetric assays gain increasing attention in recent years thanks to their cost-effectiveness, portability, and simplicity16-19. The analysis results can be conveniently visualized according to the anlayte-induced color change. These outstanding features make them very desirable for the detection of various analytes20-26. The key component in colorimetric assays is how to efficiently transform the analyte concentration into visual color change that is directly observed with naked eye. Accordingly, great efforts have been made to develop chromogenic systems for colorimetric methods. In particular, o-phenylenediamine (OPD) as an important signal reporter has been used in colorimetric assays because of its unique physicochemical property, in which OPD is oxidized by heavy metal ions or hydrogen peroxide, generating yellow 2,3-diaminophenazine which is called oxidized OPD (oxOPD)27-29. Based on the changes of color and absorption spectra, heavy metal ions



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(Ag+, Cu2+, Hg2+) and pyrophosphatase have been determined by OPD-involved sensing system27,29. Nevertheless, the use of OPD-based chromogenic system for visual colorimetric detection of TATB explosive is not explored. In this work, we find that the blue light is able to induce the autocatalytic oxidation of OPD without the need of external oxidants. Interestingly, the introduction of TATB is capable of further improving the pho-oxidation of OPD, and the enhancement mechanism is clarified as well. Inspired by this experimental phenomenon, a straightforward visual colorimetric assay for TATB with a tunable dynamic range is innovatively constructed. This facile and simple visual colorimetric assay achieves highly sensitive and selective detection of TATB. Further testing using real water samples affirms the potential application of the present colorimetric assay for practical explosive determination.

EXPERIMENTAL SECTION Chemicals and instrument. OPD, ascorbic acid, thiourea, ethanol, and methanol were purchased from ChengDu KeLong Chemical Co., Ltd. Sodium hydroxide, hydrochloric acid, sodium carbonate, and 1, 3-diphenylisobenzofuran were purchased from Sinopharm Chemical Reagent Co., Ltd. TATB, 2, 4, 6-trinitrotoluene, cyclotetramethylene-tetranitramine,

cyclotrimethylenetrinitramine,

2,

4,

6-trinitrophenol, hexanitrohexaazaisowurtzitane, and nitroguanidin explosives are provided by China Academy of Engineering Physics. All the chemicals are analytical-reagent grade and used without further purification. Ultraviolet-visible (UV-vis) absorption spectra of all the samples are performed on a UV-1800 UV-vis spectrophotometer (Shimadzu, Japan). Mass spectra are carried out on a Q-Exactive mass spectrometer. Procedure for autocatalytic oxidation of o-phenylenediamine. 5 mM OPD was firstly prepared in deionized (DI) water. After that, 3 mL OPD solution (5 mM) was transferred into a glass vial (5 mL), followed by irradiation using 12 W blue light-emitting diode (LED) as shown in Figure S1. The UV-vis absorption spectra and photographs at different irradiation time were collected. To investigate the


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experimental conditions such as OPD concentration, pH, LED color, the indicator and scavengers of reactive oxygen species, the similar operation procedures were conducted. General protocol for visual colorimetric detection of triaminotrinitrobenzene. The standard solutions of TATB with different concentrations from 0.01 μM to 25 μM were prepared by serial dilution of TATB stock solution. Then, the TATB standard solution and OPD solution (5 mM) were injected into a 5 mL glass vial, and the final volume of detection mixture is maintained at 3 mL. After blue light irradiation (12 W) for 5 min, the UV-vis absorption spectra and corresponding photographs were recorded. For examining the feasibility of this visual colorimetric assay for real water determination, the river water and lake water were collected from Fujiang River (Mianyang, China) and Jiuzou Lake (Mianyang, China) on 5 November 2018. Recovery experiments were carried out by using 50-fold diluted river and lake water samples spiked with four concentrations of TATB (0.5, 2, 6, and 10 μM). The spiked samples were analyzed by this colorimetric assay.

RESULTS AND DISCUSSION Blue light-induced autocatalytic oxidation of OPD. The photo-oxidation of OPD is studied by UV-vis absorption spectroscopy. As depicted in Figure 1a, when OPD solution (5 mM) is irradiated by blue LED for 12 min, the colorless OPD aqueous solution gradually turns to yellow in color and displays a strong UV-vis absorption peak at 415 nm, which is the characteristic absorption peak of oxOPD. The successful generation of oxOPD is also certified by mass spectra as shown in Figure S2. On the contrary, the OPD solution placed in the dark environment still remains colorless and shows a weak UV-vis absorption in the visible region (Figure 1a). Meanwhile, in order to further demonstrate the presence of the direct photo-oxidation of OPD, we regulate the oxidation reaction of OPD through switching on and off the blue light. As shown in Figure 1b, the OPD solution, as anticipated, is able to respond to the blue light stimulus and exhibits an increase in absorbance at 415 nm.



Figure 1. Direct blue light-triggered autocatalytic oxidation of OPD. (a) UV-vis absorption spectra and the corresponding optical image of 5 mM OPD aqueous solution with and without blue irradiation for 12 min (pH 5.5). (b) Photo-oxidation of 5 mM OPD (absorbance at 415 nm) upon alternating irradiation with blue light (pH 5.5). (c) UV-vis absorption spectra of 4 mM OPD solution at different irradiation time (pH 5.5). The time gaps between the measurements are 20 s. (d) The irradiation time dependence of the absorbance of 4 mM OPD solution at 415 nm. However, when the blue light is removed, the oxidation reaction of OPD is stalled (Figure 1b). We cycle the blue light on and off, yielding a gradual absorbance increase during the on cycles. These results unambiguously confirm the photo-oxidation of OPD. Also, this oxidation reaction is time-dependent (Figure 1c), and the absorbance of oxOPD at 415 nm and irradiation time perfectly follows an exponential curve with a correlation coefficient of 0.9991 (Figure 1d). The exponential dependence is typical of autocatalytic reactions30, proving that the blue light-induced oxidation of OPD is an autocatalytic reaction. Simultaneously, the autocatalytic oxidation of OPD is associated with the experimental conductions such


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as pH and OPD concentration as displayed in Figure S3 and Figure S4. The optimized pH is 5.5 for pho-oxidation of OPD. The result can be clarified as follows. OPD can bind proton (H+) to form OPDH22+ in a strong acidic medium, which can not be oxidized because it is not easy to donate electrons during the oxidation of OPD. In a neutral and alkaline environment, the protonation of OPD will not occur, resulting in a bad water solubility because of its hydrophobic benzene ring structure. The balance between protonation and water solubility is responsible for the pH-dependent oxidation property of OPD, and a weak acidic environment (pH 5.5) is applicable to oxidize OPD. Autocatalytic oxidation mechanism. Subsequently, the autocatalytic oxidation mechanism of OPD solution is investigated as well. We firstly study the effect of the wavelengths of LED lamp on the oxidation of OPD solution. The OPD oxidation reaction is triggered by using LED lamps with different colors such as blue (λmax=445 nm), green (λmax=535 nm), and red (λmax=650 nm) as shown in Figure 2a. Only blue LED lamp is capable of initiating the oxidation of OPD, this experimental phenomenon is consistent with the UV-vis absorption spectra of oxOPD (Figure 1a). This result suggests that the blue light-induced autocatalytic oxidation of OPD results from photo-activation of oxOPD. Additionally, to examine the role of dissolved oxygen in the photo-oxidation of OPD process, nitrogen gas (N2) is bubbled into the reaction solution for 30 min before blue light irradiation. As illustrated in Figure 2b, there is almost no absorption change at N2-saturated OPD solution, implying that the dissolved oxygen plays a key role in the blue light-induced autocatalytic oxidation of OPD. As it is well known, some fluorescent molecules can be utilized as the photosensitizer for generation of reactive oxygen species (ROS) in the presence of dissolved oxygen31,32. Accordingly, we can reasonably infer that the photo-oxidation of OPD is derived from the ROS that is produced by oxOPD under blue light excitation. To confirm our hypothesis, the influence of ascorbic acid (AA) as an efficient ROS scavenger is first studied (Figure S5)33, 34, and the result affirms that it significantly inhibits the autocatalytic oxidation of OPD even at a low concentration.



These results testify the generation of ROS during the photo-oxidation of OPD process.

Figure 2. Reaction mechanism of blue light-induced oxidation of OPD. (a) UV-vis absorption spectra of 5 mM OPD solution under irradiation with different colors of LED light for 8 min (pH 7). (b) UV-vis absorption spectra of photo-oxidation of 5 mM OPD at air-saturated and N2-saturated solutions (pH 7). (c) The changes in UV-vis absorption spectra of 1, 3-diphenylisobenzofuran (10 μM) upon mixing with 5 mM OPD under blue light illumination (time interval, 20 s). (d) Energy level diagram of oxOPD and the corresponding singlet oxygen production during the light irradiation. (e) Schematic illustration of autocatalytic oxidation of OPD induced by blue light.


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To identify the ROS species, 1,3-diphenylisobenzofuran (DPBF), a singlet state oxygen (1O2) indicator, is introduced into the OPD oxidation reaction35. The oxidation concentration of initial OPD is estimated to be 2.2 μM through the Beer-Lambert law using the reported molar absorption coefficient of oxOPD (1.67×104 M-1 cm-1)36. As indicated in Figure 2c, the absorption band of DPBF at 410 nm obviously decreases upon blue light irradiation. It is also noted that a shoulder at about 440 nm is discovered, which is from the absorption spectra overlapping of DPBF and oxOPD. Apart from DPBF, the effect of other 1O2 scavenger such as sodium azide (NaN3) is also evaluated (Figure S6)37. After addition of NaN3, the photo-oxidation reaction is greatly restrained. In contrast, as for the OPD oxidation reaction, the UV-vis absorption spectra do not change when methanol that is an effective scavenger for superoxide radical is added to the reaction system (Figure S7)38. These results indicate that 1O2 is main ROS and cause the photo-oxidation of OPD, which is in good agreement with the experimental results from the green laser-caused oxidaiton of OPD39,40. Howbeit, the oxOPD has a weak absorption at the laser wavelength (532 nm) as depicated in Figure 2a and 2b. At the same time, the laser has a strong energy and significant thermal effect. Oppositely, the LED is one of cold light sources, and it does not produce thermal and has a low energy. These advantages make the blue LED suitable for fabrication of visual colorimetric assays. For more in-depth understanding of the production of 1O2, the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and energy levels of oxOPD are calculated according to the density functional theory method using Gaussian 98 (Figure S8 and Figure 2d). The energy difference between the lowest excited singlet state (S1) and higher excited triplet state (T2) is calculated to be about 0.5 eV. Consequently, the S1→T2 intersystem crossing (ISC) can occur. Electrons in T2 convert iso-energetically to the lowest excited triplet state (T1) that is called internal conversion (IC), which decays back to S0 state via phosphorescence. Because the phosphorescence is spin forbidden and has a long lifetime (microseconds to seconds), and it is in a position to transfer its energy to normal oxygen to form highly reactive 1O2.



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Based on these experimental and theoretical results, a possible reaction pathway of pho-oxidation of OPD is summarized in Figure 2e. Firstly, OPD is partially oxidized by dissolved oxygen to yield oxOPD. Then, the resulting oxOPD is excited by blue light to create ectronically excited oxOPD (oxOPD*), accompanying the production of highly reactive 1O2 which accelerates the oxidation of OPD. With the oxOPD concentration increasing, the rate of pho-oxidation reaction of OPD becomes much faster. As a result, an exponential curve between the absorbance of oxOPD and irradiation time is obtained (Figure 1d), and an autocatalytic pho-oxidation reaction emerges. TATB explosive enhanced autocatalytic oxidation of OPD. Significantly, the autocatalytic photo-oxidation reaction of OPD can be further expedited when TATB explosive is added. As illustrated in Figure 3a, the UV-vis absorption of pho-oxidation reaction of OPD at pH 7.0 is weak after blue light illumination for short time (5 min). Nevertheless, the absorbance of this reaction system at 415 nm can be enhanced to 7.8 times in the presence of 6 μM TATB explosive. Similarly, the fluorescence

of

oxOPD

is

also

enhanced

by

TATB

explosive,

and

a

hypsochromic shift on fluorescent peak can be observed as shown in Figure 3b. As the oxOPD is more hydrophobic than OPD, the oxOPD easily aggregates in solution by π–π stacking interactions. To authenticate the aggregation caused quenching (ACQ) phenomenon of oxOPD, we dissolve the oxOPD in acetone and measure its fluorescence (Figure 3c). It can be seen that the fluorescence intensity of oxOPD in acetone is much higher than that of oxOPD in aqueous solution. Besides, the TATB can not evidently improve the photo-oxidation reaction of OPD under the acidic conditions because it is more prone to be protonated (Figure S9). These results indicate that the ACQ effect of oxOPD is a main factor for dominating its photochemical property of oxOPD. On the other hand, TATB explosive molecules associate together via intermolecular hydrogen bonding self-assembly to form two-dimensional sheet arrangement in aqueous solution (Figure 3d)41. The planar molecular configuration and groups that are suitable for the formation of hydrogen bonds of TATB assembly


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can interact with oxOPD via non-covalent interactions such as π-π stacking, hydrogen bonds, and hydrophobic effect, which avoids the aggregation of oxOPD and refrains the ACQ effect. This result leads to the occurrence of fluorescence enhancement (Figure 3b) and the improvement of photo-oxidation property (Figure 3a). In the meantime, there is another possibility that TATB assembly formation may occur on OPD, which may suppress the photochemical production of oxOPD. However, the concentration of OPD in our experiment is kept at 5 mM, which is much higher than that of TATB (0.01-10 μM). Thus, the suppression effect of TATB can be ignored.

Figure 3. TATB explosive-enhanced blue light-induced oxidation of OPD. (a) UV-vis absorption spectra and the corresponding photographs of photo-oxidation of OPD in the absence and presence of 6 μM TATB explosive after blue light irradiation for 5 min. (b) Photoluminescence spectra of oxOPD solution without and with 6 μM TATB explosive. (c) Effect of acetone (94%, v/v) on the photoluminescence spectra of oxOPD solution. (d) Enhancement mechanism of the photo-oxidation of OPD induced by TATB explosive.



Visual colorimetric detection of TATB with tunable dynamic range. The enhancement effect of TATB on the autocatalytic photo-oxidation reaction of OPD is appropriate to fabricate a visual colorimetric assay. In order to estimate the analytical performance, a series of TATB concentrations are determined. Figure 4a describes that the UV-vis absorption spectra of the photo-oxidation reaction of OPD before and after exposure of TATB with different concentrations at pH 7.0. Figure 4b shows that the absorbance at 415 nm (A415

nm)

gradually increases with increasing TATB

concentration, and a good linear relationship is obtained between A415 nm and TATB concentration in the range of 0.1-10 μM. The linear regression equation can be expressed as A415

nm

= 0.07 + 0.056 CTATB (μM), with a correlation coefficient

R2=0.996. The limit of detection (LOD) for TATB is found to be 30 nM according to 3σ/slope (σ is the standard deviation of blank solutions). More importantly, the linear analytic range of our visual colorimetric assay for TATB is tunable via changing the pH of reaction solution. As shown in Figure 4c and Figure 4d, when the pH is altered from 7 to 11, and the UV-vis absorption peak of oxOPD redshifts to 430 nm because the protonation of oxOPD does not occur at alkaline medium, resulting in the electron-donative effect of two free amino groups in oxOPD. The linearity range for TATB is 0.01-1 μM at pH 11, and the linear regression equation is A430 nm = 0.17+ 0.31 CTATB (μM), R2=0.99. The corresponding LOD is found to be 2.7 nM. The tunable dynamic range property of this visual colorimetric assay can match TATB concentrations in different environments. Furthermore, the distinct color change of the detection system is observed as shown in Figure 4e, and the lowest detectable TATB concentrations with naked eye is as low as 60 nM. Likewise, the analytical performance of this assay at pH 5.5 is investigated as shown in Figure S10. While a good linear relationship between TATB concentration and absorbance value can not be obtained.


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Figure 4. Visual detection of TATB explosive. (a) UV-vis absorption spectra of detection system in the presence of TATB explosive with different concentrations at pH 7. (b) Calibration curve for absorbance of oxOPD located at 415 nm versus TATB concentration at pH 7. (c) UV-vis absorption curves of detection system in response to varying concentrations (0-1 μM) of TATB. (d) Calibration curve of TATB explosive at pH 11. (e) Photographs of the detection system after introduction of different concentrations of TATB. (f) The absorbance at 415 nm of detection system containing equal molar concentrations of different types of explosives (1 μM). Reaction conditions: 5 mM OPD and blue light irradiation for 5 min. Error bars represent the standard deviations of three measurements. The selectivity of this visual colorimetric assay is next examined. Except for TATB, other explosives (their chemical structures are given in Figure S11), including



2,4-dinitrotoluene

(DNT),

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2,4,6-trinitrotoluene

(TNT),

cyclotetramethylene-tetranitramine (HMX), cyclotrimethylenetrinitramine (RDX), 2,4,6-trinitrophenol

(TNP),

hexanitrohexaazaisowurtzitane

(CL-20),

and

nitroguanidin (NG), are tested using the present colorimetric assay. As can be seen in Figure 4f, a strong colorimetric response is obtained after addition of TATB explosive, whereas no apparent absorbance is found in the presence of other explosives. It reveals that the present visual colorimetric assay has a good selectivity for the detection of TATB explosive. The good selectivity is ascribed to the fact that the strong electron withdrawing property of these explosives from nitro groups, which can quench the fluorescence of oxOPD via photoinduced electron transfer42 (Figure S12), inhibiting the formation of 1O2 under blue light illumination. Contrarily, TATB explosives contain both three amino and nitro groups, and the intramolecular photoinduced electron transfer occurs, which does not cause the fluorescence quenching but enhances the fluorescent intensity (Figure S12). Encouraged by the good sensitivity and selectivity of this colorimetric assay, real water samples such as river water and lake water are determined. The collected water samples are filtered, diluted and spiked with different concentrations of TATB. Soon afterwards, recovery tests are carried out based on the linear regression equations in diluted river water and lake water (Figure S13 and Figure S14), which are summarized in Table 1. The analytical recoveries span from 94% to 101.5% and 96% to 98.6% in river water and lake water, respectively. Correspondingly, the relative standard deviations (n=3) for all samples are within 6%. These experimental data demonstrate that this colorimetric assay has an acceptable accuracy and the feasibility for the determination of TATB explosive in natural water samples.


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Table 1 Quantitative results of detection of spiked TATB in 50-fold diluted natural water samples. Samples

Spiked (μM)

Measured (μM)

Recovery (%)

RSD (%)

0.5

0.47

94

2.5

2

1.83

91.5

5.5

6

6.09

101.5

4.7

10

9.66

96.6

4.5

0.5

0.48

96

1.5

2

1.93

96.5

4.7

6

5.78

96.3

4.1

10

9.86

98.6

2.6

River water

Lake water

CONCLUSIONS In summary, we have demonstrated a visual colorimetric assay for TATB based on its enhancement effect on the blue light-induced autocatalytic oxidation of OPD. The enhanced photo-oxidation of OPD by TATB is attributed to the non-covalent interactions between OPD and TATB assembly, which inhibits the ACQ effect of oxOPD. The obvious color change after addition of TATB for naked-eye detection is successfully realized at a concentration of 60 nM. Further by virtue of UV-vis spectroscopy, TATB can be selectively determined with a tunable dynamic range, and the LOD is down to 2.7 nM at pH 11. What is more, the present visual colorimetric assay is applied to determine TATB in real water samples. Overall, compared with the reported assay for TATB, the fabricated colorimetric method does not require



expensive instrument and external oxidants, and it has exceptional merits such as naked-eye readout, good sensitivity, outstanding quantitatively capability as well as satisfactory anti-interference property. We envision that this assay can be employed for practical public safety analysis.

ASSOCIATED CONTENT Supporting Information Figure S1-S14. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS The support of this research by the Longshan Scholars Programme of Southwest University of Science and Technology (Grant No. 17LZX449 and 18LZX204), China Academy of Engineering Physics Foundation (Grant No. 18zh005603), and China Aerodynamics Research and Development Center Foundation (Grant No. JPD20170142) are gratefully acknowledged.

REFERENCES (1) Hu, Z.; Deibert, B. J.; Li, J. Chem. Soc. Rev. 2014, 43, 5815-5840. (2) Wan, W.-M.; Tian, D.; Jing, Y.-N.; Zhang, X.-Y.; Wu, W.; Ren, H.; Bao, H.-L. Angew. Chem. Int. Edit. 2018, 57, 15510-15516. (3) Wang, B.; Lv, X.-L.; Feng, D.; Xie, L.-H.; Zhang, J.; Li, M.; Xie, Y.; Li, J.-R.;


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