TNP - ACS Publications - American Chemical Society

Dec 17, 2014 - Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. •S Supporting ...
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A Label-Free Fluorescence Sensing Approach for Selective and Sensitive Detection of 2,4,6-Trinitrophenol (TNP) in Aqueous Solution Using Graphitic Carbon Nitride Nanosheets Mingcong Rong,† Liping Lin,† Xinhong Song,† Tingting Zhao,§ Yunxin Zhong,∥ Jiawei Yan,∥ Yiru Wang,† and Xi Chen*,†,‡ †

Department of Chemistry and the MOE Key Laboratory of Spectrochemical Analysis & Instrumentation, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China ‡ State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, 361005, China § Xiamen Huaxia Vocational College, Xiamen, 361005, China ∥ Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China S Supporting Information *

ABSTRACT: An effective and facile fluorescence sensing approach for the determination of 2,4,6-trinitrophenol (TNP) using the chemically oxidized and liquid exfoliated graphitic carbon nitride (g-C3N4) nanosheets was developed. The strong inner filter effect and molecular interactions (electrostatic, π−π, and hydrogen bonding interactions) between TNP and the gC3N4 nanosheets led to the fluorescence quenching of the g-C3N4 nanosheets with efficient selectivity and sensitivity. Under optimal conditions, the limit of detection for TNP was found to be 8.2 nM. The proposed approach has potential application for visual detection of TNP in natural water samples for public safety and security.

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for their detection. Since TNT and TNP possess extremely strong electron affinity,17 their discrimination from a mixture using fluorescence detection with photo-induced electron transfer is very difficult. TNP is an environmentally deleterious substance and has far more violent explosive power than the better-known TNT.18 TNP contaminates groundwater and soil, and it damages the skin, eyes, and respiratory systems of human beings and wildlife,16,19 because of its higher water solubility. Therefore, there is an urgent need to explore new fluorescent materials or reagents that are highly selective and sensitive for the determination of TNP. Graphitic carbon nitride (g-C3N4), which is a typical semiconductor, is known for its applications in sensing,20−25 drug delivery and bioimaging,26−28 the oxygen reduction reaction,29−31 peroxidase-like catalysis of hydrogen peroxide,32−34 and photoelectrocatalysis.35−38 In contrast with bulk materials, g-C3N4 nanosheets of atomic-scale thickness and nanoscale could greatly promote their photoresponse and electroresponse, because of their high specific surface area, and this is confirmed by both theoretical calculation and corresponding application studies.26,39−41 In order to gain a better performance, great efforts have been made to prepare an

itroaromatic explosives are widely used in the preparation of matches and fireworks, forensic investigations, and the aviation and military industries, and they can accumulate in natural waters over the long-term during their testing, usage, storage, and dumping. Because of their contamination of the environment, the risk to human and wildlife health, and the growing public security concerns, selective and sensitive detection of such compounds has attracted increasing attention.1,2 Nitroaromatic explosives such as 2,4,6-trinitrophenol (picric acid, TNP), 2,4,6-trinitromethylbenzene (TNT), and 2,4-dinitrotoluene (DNT) are common components of industrial explosives and are found worldwide in landmines, making them a major focus in explosive detection.1,2 Currently, many methods are used to detect nitroaromatic explosives, including infrared and Raman spectroscopy,3 mass spectrometry,4,5 solid-phase microextraction,6,7 X-ray imaging techniques,8 surface-enhanced Raman spectroscopy,9,10 ion mobility spectroscopy,11 and thermal neutron analysis.12 Unfortunately, most of these methods suffer limitations, such as a complicated synthesis process or labeling procedure, which is timeconsuming, expensive, and involves the complex operation of sophisticated instruments, all of which impede their widespread application. In recent years, fluorescence detection methods have gained increased attention, because of their high sensitivity and simplicity.13−16 Although extremely high sensitivity toward nitroaromatic explosives has been demonstrated, there still remains the great challenge to find a fast and selective method © 2014 American Chemical Society

Received: October 25, 2014 Accepted: December 17, 2014 Published: December 17, 2014 1288

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Analytical Chemistry atomic-scale-thickness g-C3N4 nanosheet, such as using liquidphase exfoliation,26,39 chemical oxidation,42,43 and hydrothermal methods.27,44 g-C3N4 nanosheets obtained using these methods have been widely used in the fluorescence detection of a variety of ions and biological molecules.44−47 However, until now, there has not been enough attention to the determination of nitroaromatic explosives using g-C 3 N4 nanosheets.48 In the present work, with the chemically oxidized and liquid exfoliated g-C3N4 nanosheets, a label-free fluorescence sensing approach was developed for the highly selective and sensitive determination of TNP in an aqueous solution. Because of the strong inner filter effect (IFE) and molecular interactions (electrostatic, π−π, and hydrogen bonding interactions) between TNP and the g-C3N4 nanosheets, the fluorescence of the g-C3N4 nanosheets gradually decreased with the addition of TNP. The proposed approach also showed its applicability to natural water samples spiked with TNP.

was prepared by heating 10.0 g of melamine in a muffle furnace for 3 h to 550 °C and maintaining it for another 4 h in air. In the next process, 1.0 g of the finely ground bulk g-C3N4 powder was added into 100 mL of 5 M HNO3 and given ultrasonic treatment for 4 h before being refluxed at 120 °C for 24 h. After natural cooling to room temperature, the refluxed g-C3N4 was then centrifuged at 10 000 rpm for 30 min and then washed three times with ultrapure water to neutral. Finally, the g-C3N4 nanosheets stock solution (the concentration of g-C3N4 is 250 μg/mL) was obtained by collecting the last remaining supernatant and then placing this under ultrasonic reaction for 16 h. Fluorescence Sensing of TNP and Other Nitroaromatic Explosives. Fifty microliters (50 μL) of the stock solution of g-C3N4 nanosheets was mixed with a 10 mM TrisHCl buffer (pH 8.0) containing different concentrations (0 to 300 μM) of TNP or other nitroaromatic explosives. The final volume of the solution was kept at 1.0 mL. After 5 min of reaction at room temperature, the fluorescence spectra of the solutions were collected in the wavelength range from 400 nm to 590 nm (excitation wavelength: 310 nm). Selectivity Measurements of TNP. In the investigation of the selectivity of TNP in the developed method, several nitroaromatic explosives and metal ions were selected as coexisting substances. The concentration of each nitroaromatic explosive, as well as Hg2+ and Cu2+, was 5 μM, and the concentration of other metal ions was selected as 20 μM. The same detection conditions were selected as mentioned above. Determination of TNP in Lake Water and Seawater Samples. The lake water samples were obtained from Xiamen University campus, and the seawater sample was collected from the sea around Xiamen University. The samples were centrifuged at 12 000 rpm for 20 min twice and then the supernatant filtered through the 0.22 μm water phase membrane. Fifty microliters (50 μL) of the as-synthesized stock solution of g-C3N4 nanosheet was injected into 10 mM Tris-HCl buffer (pH 8.0) solution containing a 0.5 mL water sample and different concentrations of TNP. The final volume of the solution was 1 mL. The fluorescence spectra of the solutions were collected after reaction for 5 min at room temperature. Preparation of the Test Papers. First, a piece of water phase microporous membrane filter paper (50 mm × 0.22 μm) was placed into a Buchner funnel (50 mm with 61 pores). Second, 20 mL of the stock solution of g-C3N4 nanosheets (5 mg) was repeatedly suction filtering through the filter paper. Then, the filter paper was naturally dried to keep it flat. After that, we cut off the filter paper to keep the middle sheet, which contained nine holes. After dripping 1.5 μL of TNP solution with nine different concentrations into the nine filtration pores, the test paper was placed at room temperature to evaporate the solvent thoroughly. Time-Resolved Decay Measurements. To test the reaction mechanism between g-C3N4 nanosheets and TNP, time-resolved decay measurements of 50 μL of the stock solution of g-C3N4 nanosheets at different concentrations (0, 0.05, 0.10, 0.5, 1, 5, and 50 μM) of TNP were detected in 10 mM Tris-HCl buffer (pH 8.0). The final volume of the solution was kept as 1.0 mL. The excitation light was 297 nm (the excitation light source closest to 310 nm), and the emission wavelength was 438 nm.

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EXPERIMENTAL SECTION Materials. Melamine was purchased from the Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China); 4-nitrophenol (NP), nitrobenzene (NB), methylbenzene (MB), phenol (PHE), and metal nitrates or chlorides of analytical grade were obtained from the Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China); 4-nitrotoluene (99%, NT) and TNP (0.1 mg/mL in acetonitrile) were obtained from J&K Scientific (Beijing, China); 2,4-dinitrophenol (DNP) was purchased from AccuStandard (USA); DNT and TNT (1 mg/mL in methyl alcohol) were purchased from Aladin, Ltd. (Shanghai, China); and Tris-HCl was obtained from Sigma− Aldrich, Inc. (USA). All chemicals were of analytical grade and used as received without further purification. Ultrapure water with a resistivity of 18.2 MΩ cm−1 was obtained from a Millipore purification system (Millipore, USA) and used for the experiments. Water phase microporous membrane filters (50 mm × 0.22 μm) were purchased from Xiamen Green Reagent Glass Instrument Co., Ltd. (Xiamen, China). Instrumentation. Ultraviolet−visible light (UV-vis) absorption spectra and fluorescence spectra were obtained with a spectrophotometer (Model UV2550, Shimadzu, Japan) and an spectrophotometer (Model F-4500, Hitachi, Japan); Fourier transform infrared (FT-IR) spectra were obtained using a Nicolet 330 spectrophotometer (Thermo Electron Corp., USA); the fluorescence lifetime was obtained using a FluoroMax-4 spectrofluorometer (Horiba Jobin-Yvon, France); transmission electron microscopy (TEM) and atomic force microscopy (AFM) images were collected using a Model JEM1400 microscopy system (JEOL, Japan) at an acceleration voltage of 120 kV and a 5500 SPM system (Agilent, America) with silicon probes; dynamic light scattering (DLS) and ζpotential results were obtained with a Nano-ZS (Malvern Instruments, U.K.); X-ray powder diffraction (XRD) results were obtained with a Rigaku Model Ultima IV XRD diffractometer (Rigaku, Japan) equipped with graphite monochromatized high-intensity Cu Kα radiation (λ = 1.5417 Å); and X-ray photoelectron spectroscopy (XPS) measurements were obtained using a PHI Quantum 2000 XPS system (Physical Electronics, USA) with Al Kα radiation (ℏυ = 1486.60 eV). Preparation of the g-C3N4 Nanosheets. These g-C3N4 nanosheets were prepared based on previous reports with minor modification.42,47 In a typical synthesis, first, bulk g-C3N4 1289

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Figure 1. (A) SEM image of the bulk g-C3N4. (B) TEM image of the g-C3N4 nanosheets. (C) AFM image of the g-C3N4 nanosheets (inset shows the height of the g-C3N4 nanosheets). (D) XRD patterns of the bulk g-C3N4 and the g-C3N4 nanosheets. (E) FT-IR spectrum of the g-C3N4 nanosheets. (F) XPS spectrum of the g-C3N4 nanosheets. (G) C 1s spectrum of the g-C3N4 nanosheets. (H) N 1s spectrum of the g-C3N4 nanosheets. (I) O 1s spectrum of the g-C3N4 nanosheets.

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RESULTS AND DISCUSSION Materials Characterization. As mentioned above, g-C3N4 nanosheets of atomic-scale thickness and nanoscale can greatly promote photoresponse and electroresponse, compared with bulk g-C3N4 materials. The bulk g-C3N4 was characterized using scanning electron microscopy (SEM), as shown in Figure 1A; the solid rocky-like agglomerates consist of irregular folded flakes approximately several micrometers in size. The g-C3N4 nanosheets were characterized by means of TEM, AFM, XRD, FT-IR, and XPS. Typical TEM and AFM images of g-C3N4

nanosheets showed a mean diameter of 90 nm and a thickness ranging from 2.18 nm to 3.83 nm (see Figures 1B and 1C), indicating that the as-prepared g-C3N4 nanosheets comprised less than 12 C−N layers. Dynamic light scattering (DLS) revealed a mean hydrated radius of 101 nm of the as-prepared g-C3N 4 nanosheets (see Figure S1 in the Supporting Information). XRD patterns of the g-C3N4 nanosheets are shown in Figure 1D; the strong XRD peak centered at 27.4° (d = 0.325 nm) corresponds to the typical graphitic interlayer stacking (002) peak of g-C3N4. The small peak at 13.1° (d = 1290

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Scheme 1. Schematic Illustration for the Sensitive and Selective Fluorescence Detection of TNP Based on the g-C3N4 Nanosheets

Figure 2. (A) UV-vis absorption spectra of the g-C3N4 nanosheets and TNP, and the theoretical and experimental spectra of the sum of the g-C3N4 nanosheets and TNP. (B) The emission spectra of the g-C3N4 nanosheets under different excitation wavelengths ranging from 300 nm to 370 nm. (C) Fluorescence responses of the g-C3N4 nanosheets in different pH solutions (10 mM Tris-HCl buffer). (D) Fluorescence excitation and emission spectra of the g-C3N4 nanosheets in the presence and absence of 5 μM TNP (inset photographs represent the g-C3N4 nanosheets (a) in daylight, (b) under 365 nm UV light, (c) in the presence of 100 μM TNP under daylight, and (d) in the presence of 100 μM TNP under 365 nm UV light).

O−H stretching.26 As shown in Figures 1F−I, the g-C3N4 nanosheets contained three major chemical elements (C, N, O) with a C/N/O ratio of 0.515/0.199/0.286, the relatively high oxygen content may have been caused by some oxygencontaining groups generated during the chemical oxidation and liquid exfoliation, which was similar to the mechanism reported in hydrothermal-treated fluorescence g-C3N4.44 The C 1s spectrum showed three peaks, centered at 284.8, 286.2, and 288.5 eV, attributed to graphitic carbon, sp3-bonded carbon with oxygen (C−OH), and sp2-bonded carbon (N−CN). The N 1s spectrum showed four peaks, centered at 398.5, 399.6, 400.6, and 404.3 eV, corresponding to sp2 hybridized aromatic nitrogen bonded to carbon atoms (CN−C),

0.676 nm) corresponded to the periodic in-plain structural packing feature within the sheets.46,49 After the chemical oxidization and liquid exfoliation, the interlayer stacking distance slightly decreased from 0.325 nm to 0.322 nm (27.7° of the g-C3N4 nanosheets), which indicated that the bulk g-C3N4 was successfully exfoliated into nanoscale layered structures. FT-IR and XPS measurements were further performed to explore the chemical composition of the gC3N4 nanosheets. The peak at 811 cm−1 (Figure 1E) was attributed to the vibration of the triazine ring. The peaks around 1000 and 1800 cm−1 represented the stretching modes of CN heterocycles (C−N(−C)−C and C−NH−C). The peaks between 3000 and 3600 cm−1 corresponded to N−H and 1291

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Figure 3. (A) Fluorescence responses of the g-C3N4 nanosheets in the presence of different concentrations of TNP (0, 0.05, 0.1, 0.15, 0.25, 0.3, 0.4, 0.5, 0.8, 1, 2.5, 5, 7.5, 10, 25, 50, 75, 100, 150, 200, 250, and 300 μM) in 10 mM Tris-HCl buffer (pH 8.0). The inset figure presents the relationship between F0/F and the concentration of TNP. (B) Fluorescence responses of the g-C3N4 nanosheets in the presence of different concentrations of TNP within the linear range. The inset figure presents the two linear ranges of ((F0/F) − 1) and the concentration of TNP. (C) Fluorescence quenching efficiencies ((F0/F) − 1) obtained from different nitroaromatic explosives. (D) Fluorescence responses of the g-C3N4 nanosheets in the presence of 5 μM different nitroaromatic explosives, F0 and F represent the fluorescence intensities at 438 nm in the absence and presence of TNP and other nitroaromatic explosives.

the similar impurities could also be found in the hydrothermal method, and its influence on the fluorescence peak of 438 nm was negligible.44 The most intense peak of 438 nm was obtained with an excitation wavelength at 310 nm (Figure 2B). In order to obtain the optimal conditions for further analytical applications, the effects of pH and the amount of stock solution of the g-C3N4 nanosheets on the fluorescence intensity were investigated. As shown in Figure 2C, the fluorescence intensity of the g-C3N4 nanosheets gradually decreased from pH 1 to pH 7, but remained almost constant from pH 8 to pH 13. The results may have been caused by the terminal amino groups on the g-C3N4 nanosheets, resulting in them being more stable under acid conditions, which could also be revealed by the positive ζ-potential of the g-C3N4 nanosheets solutions (see Figure S2 in the Supporting Information). Based on the results in Figure S3 in the Supporting Information, 50 μL of the stock solution was selected as the optimal amount and was applied in the experiments that followed. The stability of the g-C3N4 nanosheets in the presence of various concentrations of NaCl was also investigated. The fluorescence intensity change of the g-C3N4 nanosheets could almost be neglected, even in 1 M NaCl solution (see Figure S4 in the Supporting Information), revealing the possibility of their application in high ionic strength environments. Considering the practical testing conditions, in the experiments that followed, we chose 50 μL of the g-C3N4 nanosheets stock solution in 10 mM Tris-HCl buffer (pH 8.0) with a overall

tertiary nitrogen bonded to carbon atoms (N−(C)3) and C− NH bond, quaternary N bonded to three C in the aromatic cycles and charging effects or positive charge localization in heterocycles.46 The O 1s spectrum showed two peaks, centered at 531.8 and 532.8 eV, ascribed to N−CO and C−OH. In our experiments, we found that the fluorescence of the gC3N4 nanosheets could be efficiently turned off in the presence of TNP, because of the strong IFE and molecular interactions (electrostatic, π−π, and hydrogen bonding interactions) between them, and because there was a wide overlap between the absorption spectrum of TNP and the fluorescence excitation and emission spectra of the g-C3N4 nanosheets. The high acidity, benzene ring, and hydroxyl of TNP make it possible to interact with the basic sites terminal amino of the triazine ring on the g-C3N4 nanosheets through electrostatic, π−π, and hydrogen bonding interactions. The possible mechanism for this phenomena is illustrated in Scheme 1. Spectroscopic Properties of the g-C3N4 Nanosheets. In order to reveal the spectroscopic properties of the g-C3N4 nanosheets, the UV−vis absorption and fluorescence emission spectra of the g-C3N4 nanosheets were studied. They presented a strong absorption peak at 310 nm and a shoulder peak at 365 nm (Figure 2A). Under excitation wavelengths from 300 nm to 370 nm, there was one main fluorescence emission peak at a wavelength of 438 nm and a small shoulder peak of 500 nm, which could be caused by the impurity produced in the synthetic process. The same fluorescence peak at ∼500 nm for 1292

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Table 1. Summary of the Stern−Volmer Equations at 438 nm for the Fluorescence Quenching of the g-C3N4 Nanosheets by TNP and Other Nitroaromatic Explosivesa

a

nitroaromatic explosive

number of samples, n

KSV (× 104 M−1)

intercept (μM)

TNP TNP TNT DNT NT NB PHE MB

8 7 7 9 9 6 N N

24.8 5.95 0.300 0.350 0.590 0.09 N N

0.995 1.09 0.846 1.01 1.00 0.952 N N

linear range coefficient 0−0.5 0.5−10 50−400 0−250 0−300 10−200

μM μM μM μM μM μM N N

correlation 0.995 0.996 0.993 0.986 1.00 0.995 N N

selectivity factor, SFb 1 0.0121 0.0141 0.0238 0.00363 N N

N = not present. bSF = the ratio between the KSV value of TNP and that of the other six quenchers.

(Figure 3D) indicated that the influence of other nitroaromatics was negligible. In addition, the influence of some coexisting metal ions was detected. The results, which are shown in Figure S6 in the Supporting Information, indicated that there were negligible influences for the selected coexisting metal ions, showing the excellent selectivity of the proposed method in the determination of TNP. Based on the above study, the possible mechanisms of the gC3N4 nanosheets and TNP are listed as follows: (1) fluorescence resonance energy transfer (FRET) or IFE between the g-C3N4 nanosheets and TNP; (2) the formation of a donor−acceptor charge-transfer complex between π-electronrich donors (the g-C3N4 nanosheets) and TNP; and (3) molecular interactions (such as electrostatic, π−π, and hydrogen bonding interactions) between the g-C3N4 nanosheets and TNP. Both in the FRET and IFE process, there should generally be a good spectral overlap between the absorption spectrum of a quencher and the emission spectrum of a fluorescent agent. As displayed in Figure 4A, TNP presented a wide absorption spectrum from 300 nm to 480 nm, with a strong absorption peak at 353 nm. Meanwhile, under the excitation of 310 nm, the g-C3N4 nanosheets exhibited a strong fluorescence emission peak at 438 nm. In order to ascertain the main reason for the selectivity, we explored the fluorescence lifetime of the g-C3N4 nanosheets with various concentrations of TNP. As shown in Figure 4B, the lifetime of the g-C3N4 nanosheets in the absence and presence of different concentration of TNP remained constant under an excitation of 297 nm, and the fluorescence decay was fitted using a three-exponential decay function to yield a lifetime of 6.08 ns. These results excluded the possibility of the FRET process between the g-C3N4 nanosheets and TNP. Therefore, the IEF could be considered as one major process in the fluorescence quenching. Furthermore, the selectivity toward TNP could also be explained by the IEF mechanism. As shown in Figure S7 in the Supporting Information, there was little overlap between the UV-vis absorption spectra of the other six nitroaromatic explosives and the fluorescence excitation and emission spectra of the g-C3N4 nanosheets, which resulted in poor IFE efficiency. In addition, as shown in Figure 2A, the experimental UV−vis absorption spectrum of TNP upon the addition of the g-C3N4 nanosheets was very similar to the theoretical one. There is only a slight difference in the intensity. This phenomenon indicated that the interaction between TNP and the g-C3N4 nanosheets was weak, and no Meisenheimer complex between them could be formed.14 This conclusion could also be verified from the results indicated as shown in Figure S8 in the Supporting Information. Obviously, no new absorption peak but increased absorption intensity could be

volume of 1.0 mL. As revealed in Figure 2D, the addition of TNP into the g-C3N4 nanosheets led to fluorescence quenching of both their excitation and emission spectra. The kinetic characteristic of the reaction system was investigated. As shown in Figure S5 in the Supporting Information, with the addition of 100 μM TNP into the g-C3N4 nanosheets solution, a rapid equilibration of the reaction could be achieved within 5 min. Using quinine sulfate as a fluorescence reference, the relative quantum yield of the as prepared g-C3N4 nanosheets at 310 nm was estimated to be 5.5% (see Table S1 in the Supporting Information). Fluorescence Determination of the g-C3N4 Nanosheets toward TNP. In order to evaluate the selectivity, the fluorescence performance of the g-C3N4 nanosheets in the presence of nitroaromatic explosives was studied. In the experiments, different concentrations of TNP and another six nitroaromatic explosives (TNT, DNT, NT, NB, MB, and PHE; the detailed information is summarized in Table S2 in the Supporting Information) were selected. Figure 3A shows that the fluorescence intensity at 438 nm gradually decreased with the addition of TNP, even at a low TNP concentration of 50 nM, which revealed that the g-C3N4 nanosheets were much more sensitive to the other six nitroaromatic explosives. The quenching results could be quantitatively treated with the Stern−Volmer equation,

F0 = 1 + KSV[PA] F where F0 and F represent the fluorescence intensity of the gC3N4 nanosheets in the absence and presence of TNP; [PA] is the concentration of TNP; and KSV is the Stern−Volmer constant. Two distinct linear ranges were shown between the fluorescence intensity and the concentration of TNP ranging from 0 to 0.5 μM and 0.5 to 10 μM, with a limit of detection (LOD) of 8.2 nM at a signal-to-noise ratio of 3 (Figure 3B). The value was comparable with (or superior to) the LODs obtained from other fluorescence methods.14,48,50,51 Tritration of all the nitroaromatic explosives studied in the fluorescence quenching experiments was undertaken, and the parameters for each quencher related to the Stern−Volmer plots model are listed in Table 1. Except PHE and MB, the other nitroaromatic explosives were fitted close or related to the Stern−Volmer model. The ratio of the KSV of TNP to the nitroaromatic explosives, which is defined as the selectivity factor (SF) in Table 1, suggested the superior selectivity for TNP, compared to the others. To further investigate the selectivity, the fluorescence characteristics of the g-C3N4 nanosheets mixed with other nitroaromatics in the absence and presence of TNP (5 μM, within the linear range) were explored. The results 1293

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investigated. The quenching efficiency decreased in the order TNP > DNP > NP, which is in accordance with their order of acidity (see Figure S9B in the Supporting Information). As shown in Figure S9A in the Supporting Information, the overlap between the absorption spectra of DNP or NP and the fluorescence excitation and emission spectra of the g-C3N4 nanosheets is comparable or even superior to those of TNP, resulting in a comparable or better IFE effect. The molecular interactions can also be well-verified by a red shift in the emission peak upon the addition of high concentrations of TNP into the g-C3N4 nanosheets (Figure 3A).16,50,52,53 Thus, compared to other nitroaromatic explosives, the g-C3N4 nanosheets exhibited a much higher fluorescence quenching response toward TNP, because of a favorable IFE and molecular interactions, including electrostatic, hydrogen bonding, and π−π interactions. Determination of TNP in Water Samples and Test Papers. The applicability of the proposed approach was tested in the determination of TNP in natural water samples. A recovery study was carried out on the samples spiked with standard TNP solutions (0.75, 1.5, and 4.5 μM) to evaluate the method that we developed. The obtained recoveries of the samples varied from 91.2% to 124%, suggesting the applicability of the proposed method to natural water samples (see Table 2). Table 2. Determination of TNP Spiked in Water Samples (n = 3)a added TNP concentration (μM) 0.75 1.5 4.5 0.75 1.5 4.5 0.75 1.5 4.5

Figure 4. (A) UV−vis absorption spectrum of TNP and the fluorescence excitation and emission spectra of the g-C3N4 nanosheets. (B) Time-resolved decay of the g-C3N4 nanosheets with different concentrations of TNP in 10 mM Tris-HCl buffer (pH 8.0). The inset table presents the lifetime value. (C) Photographs of the filter paper (a) without and (b) with the g-C3N4 nanosheets; photographs of the filter paper with the g-C3N4 nanosheets having 1.5 μL of TNP solution added at different concentrations (from left to right: 0, 0.01, 0.05, 0.5, 5, 50, 100, 500, and 5000 μM) (c) in the daylight and (d) under 365 nm UV light.

concentration found (μM) Water Sample 1 0.909 1.76 4.95 Water Sample 2 0.803 1.60 5.56 Water Sample 3 0.684 1.45 4.22

recovery (%)b

RSD (%)

121 117 110

1.62 0.688 1.38

107 106 124

0.356 0.554 1.27

91.2 96.6 93.9

2.35 1.37 0.352

a

Samples 1 and 2 are lake water; sample 3 is seawater. bRecovery (%) = 100 × (concentration found/concentration added).

The relative standard deviation (RSD) of each sample was below 2%, revealing the high reproducibility and precision of this approach. We also made a comparison of our method to those of others (see Table S3 in the Supporting Information), although the LOD of the proposed one is not the lowest, the high sensitivity and selectivity and the simplicity made it a novel and important approach in the determination of TNP. A fluorescent filter paper was prepared by repeatedly suction filtering 20 mL of the stock solution of g-C3N4 nanosheets through the filter paper. The large lamellar structure of the gC3N4 nanosheets allowed them to be stacked together in the filtration pores. The blue fluorescence of the g-C3N4 nanosheets was immediately quenched in varying degrees by different concentrations of TNP, and it can be clearly seen under a portable 365 nm UV lamp (Figure 4C). It can be indicated in Figure 4 that the detection of such test paper was 0.5 μM (the fourth pore, equals to 0.75 pmol). The g-C3N4 nanosheets based paper sensor combined with fingerprint

observed with the increasing concentration of TNP, which excluded the possibility of a charge-transfer mechanism between TNP and the g-C3N4 nanosheets. Moreover, the fluorescence quenching could be caused by some molecular interactions such as electrostatic and hydrogen bonding interactions between the hydroxyl (−OH) on the benzene ring of electron-deficient TNP and the free basic sites (−NH2, −NH−, and −OH on the surface) of the electron-rich g-C3N4 nanosheets, and π−π interaction between their benzene and triazine rings. To confirm the electrostatic interaction (an acid− base interaction) in the fluorescence detection process, the fluorescence characteristics of some nitroaromatic explosives containing one hydroxyl group, such as DNP and NP, were 1294

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Analytical Chemistry lifting techniques has potential for application in the fields of public security and safety for the visual detection of TNP; however, for now, it still has some limits, and further optimization is needed for practical applications.

(13) Nagarkar, S. S.; Joarder, B.; Chaudhari, A. K.; Mukherjee, S.; Ghosh, S. K. Angew. Chem., Int. Ed. 2013, 52, 2881−2885. (14) Wang, Y.; Ni, Y. Anal. Chem. 2014, 86, 7463−7470. (15) Ma, Y.; Li, H.; Peng, S.; Wang, L. Anal. Chem. 2012, 84, 8415− 8421. (16) Dinda, D.; Gupta, A.; Shaw, B. K.; Sadhu, S.; Saha, S. K. ACS Appl. Mater. Interfaces 2014, 6, 10722−10728. (17) Xu, B.; Wu, X.; Li, H.; Tong, H.; Wang, L. Macromolecules 2011, 44, 5089−5092. (18) Dong, M.; Wang, Y. W.; Zhang, A. J.; Peng, Y. Chem.Asian J. 2013, 8, 1321−1330. (19) Kaur, S.; Bhalla, V.; Vij, V.; Kumar, M. J. Mater. Chem. C 2014, 2, 3936−3941. (20) Lee, E. Z.; Jun, Y. S.; Hong, W. H.; Thomas, A.; Jin, M. M. Angew. Chem., Int. Ed. 2010, 49, 9706−9710. (21) Xu, H.; Yan, J.; She, X.; Xu, L.; Xia, J.; Xu, Y.; Song, Y.; Huang, L.; Li, H. Nanoscale 2014, 6, 1406−1415. (22) Tang, Y.; Su, Y.; Yang, N.; Zhang, L.; Lv, Y. Anal. Chem. 2014, 86, 4528−4535. (23) Cheng, C.; Huang, Y.; Tian, X.; Zheng, B.; Li, Y.; Yuan, H.; Xiao, D.; Xie, S.; Choi, M. M. Anal. Chem. 2012, 84, 4754−4759. (24) Cheng, C.; Huang, Y.; Wang, J.; Zheng, B.; Yuan, H.; Xiao, D. Anal. Chem. 2013, 85, 2601−2605. (25) Cheng, N.; Jiang, P.; Liu, Q.; Tian, J.; Asiri, A. M.; Sun, X. Analyst 2014, 139, 5065−5068. (26) Zhang, X.; Xie, X.; Wang, H.; Zhang, J.; Pan, B.; Xie, Y. J. Am. Chem. Soc. 2013, 135, 18−21. (27) Zhang, X.; Wang, H.; Zhang, Q.; Xie, J.; Tian, Y.; Wang, J.; Xie, Y. Adv. Mater. 2014, 26, 4438−4443. (28) Lin, L.-S.; Cong, Z.-X.; Li, J.; Ke, K.-M.; Guo, S.-S.; Yang, H.-H.; Chen, G.-N. J. Mater. Chem. B 2014, 2, 1031−1037. (29) Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Angew. Chem., Int. Ed. 2014, 53, 7281−7285. (30) Huang, H.; Yang, S.; Vajtai, R.; Wang, X.; Ajayan, P. M. Adv. Mater. 2014, 26, 5160−5165. (31) Tian, J.; Liu, Q.; Asiri, A. M.; Alamry, K. A.; Sun, X. ChemSusChem 2014, 7, 2125−2130. (32) Tian, J.; Liu, Q.; Ge, C.; Xing, Z.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. Nanoscale 2013, 5, 8921−8924. (33) Lin, T.; Zhong, L.; Wang, J.; Guo, L.; Wu, H.; Guo, Q.; Fu, F.; Chen, G. Biosens. Bioelectron. 2014, 59, 89−93. (34) Tian, J.; Liu, Q.; Asiri, A. M.; Qusti, A. H.; Al-Youbi, A. O.; Sun, X. Nanoscale 2013, 5, 11604−11609. (35) Li, Y.; Zhang, H.; Liu, P.; Wang, D.; Zhao, H. Small 2013, 9, 3336−3344. (36) Zhang, J.; Zhang, M.; Yang, C.; Wang, X. Adv. Mater. 2014, 26, 4121−4126. (37) Hou, Y.; Zuo, F.; Dagg, A. P.; Liu, J.; Feng, P. Adv. Mater. 2014, 26, 5043−5049. (38) Cheng, N.; Tian, J.; Liu, Q.; Ge, C.; Qusti, A. H.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. ACS Appl. Mater. Interfaces 2013, 5, 6815− 6819. (39) Yang, S.; Gong, Y.; Zhang, J.; Zhan, L.; Ma, L.; Fang, Z.; Vajtai, R.; Wang, X.; Ajayan, P. M. Adv. Mater. 2013, 25, 2452−2456. (40) Niu, P.; Zhang, L.; Liu, G.; Cheng, H.-M. Adv. Funct. Mater. 2012, 22, 4763−4770. (41) Ma, W.; Han, D.; Zhou, M.; Sun, H.; Wang, L.; Dong, X.; Niu, L. Chem. Sci. 2014, 5, 3946−3970. (42) Chen, L.; Huang, D.; Ren, S.; Dong, T.; Chi, Y.; Chen, G. Nanoscale 2013, 5, 225−230. (43) Xu, J.; Zhang, L.; Shi, R.; Zhu, Y. J. Mater. Chem. A 2013, 1, 14766−14772. (44) Zhang, S.; Li, J.; Zeng, M.; Xu, J.; Wang, X.; Hu, W. Nanoscale 2014, 6, 4157−4162. (45) Tian, J.; Liu, Q.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. Anal. Chem. 2013, 85, 5595−5599. (46) Tang, Y.; Song, H.; Su, Y.; Lv, Y. Anal. Chem. 2013, 85, 11876− 11884.

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CONCLUSIONS In this study, using the chemically oxidized and liquid exfoliated g-C3N4 nanosheets, an effective and facile fluorescence approach was developed for the selective and sensitive determination of TNP in aqueous solution. The proposed method was revealed to be independent of the coexistence of other nitroaromatic explosives or metal ions. An LOD of 8.2 nM could be obtained for the determination of TNP. Based on the results from the time-resolved decay measurements, the high sensitivity and selectivity of the method were induced mainly by the IFE process, as well as the interactions between TNP and the g-C3N4 nanosheets. The method was applied in the determination of trace amounts of TNP in water samples, and this demonstrated the application potential of fluorescent g-C3N4 nanosheets.

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 592 2184530. Fax: +86 592 2184530. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was financially supported by the National Nature Scientific Foundation of China (Nos. 21175112, 21375112) and the Program of Science and Technology of Xiamen for University Innovation (No. 3502Z20143025), which are gratefully acknowledged. Furthermore, we would like to extend our thanks to Professor John Hodgkiss of The University of Hong Kong for his assistance with English.

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REFERENCES

(1) Salinas, Y.; Martinez-Manez, R.; Marcos, M. D.; Sancenon, F.; Costero, A. M.; Parra, M.; Gil, S. Chem. Soc. Rev. 2012, 41, 1261− 1296. (2) Germain, M. E.; Knapp, M. J. Chem. Soc. Rev. 2009, 38, 2543− 2555. (3) Lopez-Lopez, M.; Garcia-Ruiz, C. TrACTrends Anal. Chem. 2014, 54, 36−44. (4) Barron, L.; Gilchrist, E. Anal. Chim. Acta 2014, 806, 27−54. (5) Badjagbo, K.; Sauve, S. Crit. Rev. Anal. Chem. 2012, 42, 257−271. (6) Berg, M.; Bolotin, J.; Hofstetter, T. B. Anal. Chem. 2007, 79, 2386−2393. (7) Guerra-Diaz, P.; Gura, S.; Almirall, J. R. Anal. Chem. 2010, 82, 2826−2835. (8) Wells, K.; Bradley, D. A. Appl. Radiat. Isot. 2012, 70, 1729−1746. (9) Dasary, S. S.; Singh, A. K.; Senapati, D.; Yu, H.; Ray, P. C. J. Am. Chem. Soc. 2009, 131, 13806−13812. (10) Sylvia, J. M.; Janni, J. A.; Klein, J. D.; Spencer, K. M. Anal. Chem. 2000, 72, 5834−5840. (11) Eiceman, G. A.; Stone, J. A. Anal. Chem. 2004, 76, 390 A−397 A. (12) Vourvopoulos, G.; Womble, P. C. Talanta 2001, 54, 459−468. 1295

DOI: 10.1021/ac5039913 Anal. Chem. 2015, 87, 1288−1296

Article

Analytical Chemistry (47) Zhang, X. L.; Zheng, C.; Guo, S. S.; Li, J.; Yang, H. H.; Chen, G. Anal. Chem. 2014, 86, 3426−3434. (48) Chen, H.-Y.; Ruan, L.-W.; Jiang, X.; Qiu, L.-G. Analyst 2014, DOI: 10.1039/c4an01693a. (49) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. Nat. Mater. 2009, 8, 76−80. (50) Nagarkar, S. S.; Desai, A. V.; Ghosh, S. K. Chem. Commun. 2014, 50, 8915−8918. (51) Sk, M. P.; Chattopadhyay, A. RSC Adv. 2014, 4, 31994−31999. (52) Bragança, A. M.; Araújo, M.; Bonifácio, V. D. B. Part. Part. Syst. Char. 2014, DOI: 10.1002/ppsc.201400120. (53) Ding, A.; Yang, L.; Zhang, Y.; Zhang, G.; Kong, L.; Zhang, X.; Tian, Y.; Tao, X.; Yang, J. Chem.Eur. J. 2014, 20, 12215−12222.

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DOI: 10.1021/ac5039913 Anal. Chem. 2015, 87, 1288−1296