Highly sensitive and selective paper sensor based on carbon quantum dots for visual detection of TNT residues in groundwater

Accepted Manuscript Title: Highly sensitive and selective paper sensor based on carbon quantum dots for visual detection of TNT residues in groundwater Author: Xike Tian Hui Peng Yong Li Chao Yang Zhaoxin Zhou Yanxin Wang PII: DOI: Reference:

S0925-4005(16)32051-2 http://dx.doi.org/doi:10.1016/j.snb.2016.12.079 SNB 21453

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

28-9-2016 14-12-2016 15-12-2016

Please cite this article as: Xike Tian, Hui Peng, Yong Li, Chao Yang, Zhaoxin Zhou, Yanxin Wang, Highly sensitive and selective paper sensor based on carbon quantum dots for visual detection of TNT residues in groundwater, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.12.079 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highly sensitive and selective paper sensor based on carbon quantum dots for visual detection of TNT residues in groundwater Xike Tian, Hui Peng, Yong Li∗, Chao Yang, Zhaoxin Zhou, Yanxin Wang Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China *Corresponding author. Tel.: +86 027 67883739. Email address: [email protected]

  Graphical abstract  NH 4

CH3 NH2

CDs

O 2N

NH 2 NH2

+

PET

CH 4

O 2N

NO2

NO2

CDs NO 2

  CDs were modified by ethylenediamine, and it can be used as a highly sensitive and selective TNT  sensor in water solution at pH 7.0. The nanosensor can detect TNT with a low detection limit of  0.213 µmol/L, and the interfering ions can not influence its selectivity.      NO2

Highlights •Carbon quantum dots were prepared via a simple hydrothermal method. •The nanosensor exhibited excellent sensitivity and selectivity towards TNT. •The detection limit of TNT is as low as 0.213 µmol/L. •The portable paper sensor was developed for TNT detection in actual water samples. Abstract Nowadays, selective, sensitive and visual detection of 2,4,6-trinitrotoluene (TNT) explosives residues in groundwater system has been a great challenge for human security. Herein, a visual fluorescence paper sensor for accurate and on-site detection of TNT in groundwater system by linking the recognition molecule onto the surface of the carbon quantum dots (CDs) was introduced. The fluorescence of CDs based

4 

nanosensor can be selectively and efficiently quenched by TNT through a photo-induced electron transfer effect between primary amino groups and TNT explosives. The fluorescence quantum yield of this kind of nanosensor is up to 32.25％ and the detection limit reaches as low as 0.213 µmol/L. The interference molecules, such as the nitrobenzene derivatives have no influence on the sensitive detection of TNT. Furthermore, the paper sensor based on the CDs probe has been prepared, which can efficiently detect TNT residues in groundwater system. This developed nanosensor possesses the great capability of on-site and visual detection of TNT residues in groundwater without any further processing step. Keywords: TNT detection, Carbon quantum dots, Photo-induced electron transfer, Detection limit

1 Introduction As a very effective and relatively safe explosive, 2,4,6-trinitrotoluene (TNT) is provided with high explosive power, high chemical stability and low sensitivity to impact, and it has played an important role in the military and some industrial fields. [1]. Thus contaminated groundwater is usually found at the areas where the TNT is manufactured, packaged, transported and used, and the environment is being forced to confront the problems of the water pollution [2-4]. Water is the integral composition to the human and the contaminated water will do great harm to human. The existence of TNT will exert great damages to the circulatory system, liver, spleen and immune system. Moreover, the mutagenicity and carcinogenicity of TNT and its biological degradation products also have great damages to aquatic and land creatures. Therefore, it is of great importance to develop highly selective, sensitive detection methods for real-time and on-site TNT monitoring in groundwater to protect human health [5-9]. A great number of traditional detection methods, such as high performance liquid chromatography (HPLC) [10, 11], liquid chromatography-mass spectrometry (LC-MS) [12], gas chromatography-mass spectrometer (GC-MS) [13, 14], surface-enhanced Raman scattering (SERS) [15, 16], surface plasmon resonance (SPR) [17] and so on, have been investigated and applied for TNT residues detection due to their high 5 

sensitivity and quick response. However, their sophisticated vapor sampling, instrument calibration and preconcentration procedures have greatly hampered the wide applications [18, 19]. Fluorescence method is considered as one of the most promising approaches for TNT sensing because of its inherent sensitivity, high selectivity, and easy operation. Organic fluorescence dyes which are considered as a most common material with high sensitivity and easy operation, has been used for a long time. But it causes more environment pollution owing its high toxicity, bad biodegradability and excessive heavy metals content [20-22]. Quantum dots (QDs) have been the center of attention for fluorescence detection in the past few years. Different from organic fluorescent dyes, QDs, which show a brand new class of fluorescent nanoprobes, are superior to organic fluorescent dyes due to their narrow and symmetric emission, tunable emission color, unique optical and electronic properties, benign environment and high quantum yield. However, they still possess a series of limitations, such as weak stability, tedious preparation procedures, high toxicity, which greatly hindered their wide applications.[23-25]. Carbon quantum dots (CDs) are a new class of carbon nanomaterial which have been acknowledged as discrete, spheroid particles with sizes below 10 nm [26]. This kind of materials is able to overcome the restrictions of QDs, which possess outstanding properties including chemical inertness, excellent photo stability, favorable biocompatibility, low toxicity, good water solubility, cheap cost and simple synthesis [27]. Inspired by these advantages, there is a great interest in the synthesis of kinds of CDs  and it has been widely used in analysis and detection [28-32].  Yan et al linked CDs with TeCd quantum dots by glutathione (GSH) to form a single excited two-emission ratio probes for detecting NO2 [33].  Xiao et al used L-cysteine as precursors to synthesize CDs for the detection of Cr6+ [34].  Dong et al used polyethyleneimine to functionalize CDs for Cu2+ detection [35].  Herein, CDs were prepared via a simple and facile hydrothermal method using citric acids as the precursor, and also the as-synthesized CDs were functionalized by many ammine groups to obtain the nanosensor. When TNT residues were added into the nanosensor, the fluorescence was quenched greatly, showing that the as-prepared 6 

nanosensor can detect TNT residues in water. In addition, the interfering molecules have no any influences on the selective detection of TNT. The sensing mechanism was investigated and found that the electron-deficient TNT and electron-rich primary amine on the surface of CDs to form stable Meisenheimer complexes would induced photoinduced electron transfer (PET) effect, thus caused the fluorescence quenching. Furthermore, the portable paper sensor based on the obtained CDs nanosensor was developed to detect TNT residues visually and sensitively. The present work promises an effective method to develop sensitive, selective and portable nanosensor for the visual detection of trace TNT in groundwater. And we can detect trace TNT in the actual water samples with the naked eyes. In addition, this kind of design mechanism can be utilized for a variety of applications in sensing a series of pollutants in groundwater.

2 Experimental sections 2.1 Materials and instrumentation Citric acid, ethylenediamine, hydrogen peroxide, TNT standard solution (in methanol), alcohol, ethylene glycol, isopropyl alcohol, acetone, methylene chloride, p-nitrophenol, p-nitrotoluene, 2-nitroaniline, 2-nitrobenzoic acid, nitrobenzene, p-nitrobenzyl

alcohol,

3-chloronitrobenzene,

p-nitroacetophenone,

p-nitrophenylhydrazine, 2-nitrobenzenemethanol, phosphate buffer and polyvinyl alcohol were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China) and used without any further purifications.  Ultra-pure water (18.25 MΩ cm−1) was used throughout all the experiments. Powder X-ray diffraction (XRD) data were obtained on a Rigaku D/MAXRC X-ray diffractometer with a Cu Ka radiation source (45.0 kv, 50.0 mA). A Perkin-Elmer LS-55 fluorescence spectrometer was used to measure the fluorescence of the sample solution with the condition of a 1 cm quartz cuvette at room temperature and excitation and emission slit widths of 10 nm. Transmission electron microscopy (TEM) was performed with a JEOL 2000EX (JEOL, Japan) operated at 200 kV. The Fourier Transform infrared spectroscopy (FT-IR) spectra were collected 7 

with an EQUNOX 55 instrument using the diffuse reflectance scanning disc technique from 4000 to 400 cm−1 at room temperature.  Particle size distribution and Zeta potential were recorded by a Zetasizer Nano ZS90.  Ultraviolet–visible (UV–Vis) spectra were measured on a Perkin Elmer Lambda 35 spectrophotometer. A PXSJ-226 digital pH meter (Shanghai, China) was used to measure pH values of the aqueous solutions. 2.2 Synthesis of CDs based fluorescence nanosensor CDs were obtained via a hydrothermal method. In detail, 5 g of citric acid was dissolved in a mixed solution including 0.5 mL of ethylenediamine and 50mL ultra-pure water to form a clear solution. After stirring for 15 min, the mixture solution was heated for 7 h under 180 ℃ in polytetrafluoroethylene autoclave. Then, the mixture solution was homogenized and kept at room temperature; large dots in the mixture were removed by centrifuging at 10000 rpm. The precipitation was discarded, and the solution was filtered three times by the 0.22 μm membrane filter, and then transfer the solution into dialysis bag (MW=500), the dialysis of solution lasted over 48 h to obtain the CDs solution. Excessive ethylenediamine was added into the obtained CDs solution in the presence of hydrogen peroxide. After treated with ultrasound for 1h, the mixture solution was oscillated for 24 h. The pH of this mixture solution was adjusted at 7 with 1M HCl to remove redundant ethylenediamine. The mixture solution was centrifugalized at 10000 rpm to remove impurities. Subsequently the dialysis of the solution was carried out for 48 h. Finally, brown powder (CDs@NH2) was obtained by freeze drying for 72 h, 0.5mg of the brown powder was dissolved in 10 mL ultra-pure water, and then the mixture was diluted for 10 times. The obtained 5 mg/L CDs solution was stored at 4 ℃ for further use. 2.3 Fluorescence quantum yield measurement of CDs based fluorescent nanosensor To measure the fluorescence quantum yield of CDs, rhodamine6G was set as a reference. The rhodamine 6G was diluted in five concentration gradients, then the UV-vis absorption spectra fluorescence emission spectrum was measured, and the same series solution was put in place in 1cm cuvette for, the CDs was measured under 8 

the same treatment. Finally the ratio was calculated by UV-vis absorption and the integral of fluorescence emission spectrum, The quantum yield of the CQDs was measured by following the equation[36, 37], where Φ is the quantum yield, η is the refractive index, and GradCDs and GradST represent the ratio of CDs and rhodamine6G mentioned above. Φ

Φ

Grad Grad

η η

2.4 TNT detection in aqueous media Briefly, 30 μL of 5 mg/L CDs@NH2 solution was diluted in 1mL cuvette solution for fluorescence measurement;  the pH of the mixture was adjusted to 7 by phosphate buffer. Then 30 μL of 1×10-6 mol/L TNT was added in each time and TNT quench the fluorescence in an overt way. The relative fluorescence intensity was measured under a single wavelength excitation of 337 nm, and the dual emission peaks were recorded at 431 nm. The fluorescent nanosensor with common interference factors were performed the same procedure and interference-free environment. 2.5 Preparation of paper-based sensor. In order to fabricate the CDs@NH2-based paper sensor for visual detection of TNT, 1 g of poly (vinyl alcohol) (PVA-1788) was first added in 20 mL of water in a flask. The mixture solution was then heated on the stove until the PVA was dissolved completely and the clear PVA solution was kept at room temperature for further use. Then, 1 mL of CDs fluorescence sensor solution was added into the clear PVA solution (5 mL) and stirred vigorously to form a clear solution. The mixture solution was then dripped onto a common filter paper (D = 0.45 µM) and dried at room temperature for 30 mins. After drying, the filter paper that contains CDs@NH2 was cut into strips for the further detection [18, 38-43]. 2.6 TNT detection of actual water samples The effect of on-site visual measurement of TNT using the paper-based sensor was carefully investigated. In short, the groundwater sample was obtained under the Jianghan Plain. Firstly, the sample was filtered by 0.45 μm microporous membrane. 9 

Then 20 μL CDs@NH2 was added in the solution, the mixture solution was bathed in 25 ℃ water for 2 h. Finally, the mixture solution was diluted by ultra-pure water for fluorescence responses detection. A series of TNT solution (1×10-9, 5×10-9, 1×10-8, 5 ×10-8, 1×10-7, 5×10-7 mol/L) was dropped onto the prepared paper sensor, then the paper-based sensor strips were immersed in the solutions for 10 s and taken out air-dried under ambient condition. The fluorescence color responses of the paper-based sensor were observed under a UV lamp with excitation wavelength of 365 nm.

3. Results and discussion The obtained CDs with low toxicity and high fluorescence quantum yield of 32.25% were prepared via a facile hydrothermal approach with citric acids as the precursor. Then primary amino groups were modified onto the surface of CDs through a passivation process. The prepared fluorescence nanosensor (CDs@NH2) has showed a blue single-emission peak at 431 nm under the excitation wavelength of 337 nm. In the presence of TNT, the electron-deficient TNT and electron-rich primary amino groups on the surface of CDs would form the stable Meisenheimer complexes. And this kind of stable Meisenheimer complexes could be the acceptor to the CDs. The  highest occupied molecular orbital (HOMO) of acceptor’s lone pair of electrons is between the HOMO and the lowest unoccupied molecular orbital (LUMO) of the fluorophore (CDs). After excited, the lone pair electron was transferred to the HOMO of the fluorophore, and thus the radioactive transitions of the fluorophore have been prevented, resulting in PET effect, and then caused the fluorescence quenching. The preparation route and sensing mechanism of the visual fluorescence sensor are shown in Fig. 1.

3.1 Characterization of CDs@NH2 nanosensor XRD pattern of CDs@NH2 (Fig. 2a) showed that a peak appears in the range of 20 to 25o. The peak can be assigned to the (002) plane of graphite, indicating that the prepared nanosensor is amorphous. The functionalization and surface functional 10 

groups of CDs and CDs@NH2 were characterized by FT-IR (Fig. 2b). In the spectrum of CDs, the peak at 3431.33 is the O-H stretching vibration peak, and the peak at 1698.06 can be assigned to the C=O stretching vibration. Compared with CDs, the characteristic peaks at 3188.74 and 3139.50 for the spectrum of CDs@NH2 can be assigned to the N-H functional groups, indicating that primary amino groups have been decorated on the CDs successfully. The size and morphology of CDs@NH2 were characterized by TEM, as shown in Fig. 2c. TEM image shows that the CDs@NH2 is of with narrow size (below 10 nm) and a spherical morphology. CDs@NH2 solution was diluted to suitable concentration to be characterized by Zeta potential (Fig.2d). The average size of CDs@NH2 ranges from 1 to 9 nm, the majority of particles in the 4 nm, which was consistent with the TEM images. The ζ potential of CDs@NH2 in aqueous media was also determined as +2.38 mV, which indicates that there are many positive charges on the CDs owing to the existence of primary amino groups. Due to these positive charges, electrostatic repulsion appeared between each CDs nanoparticles, which led to the great dispersibility in solution and it can be further used for detection in aqueous media.

3.2 Optimal TNT detection conditions for CDs@NH2 nanosensor In order to investigate the optical properties of CDs, under same excitation wavelength (337 nm) and room temperature, fluorescence study of CDs and CDs@NH2 was carried out in Fig.3. And seen, the CDs exhibit bright blue under a UV lamp (365 nm). Even both of CDs and CDs@NH2 possess the strong fluorescence emission intensity, the modification of CDs with ethylenediamine has resulted in a fluorescence intensity increase significantly. It may be due to that the surface passivation effect induced by the primary amino groups decorated on the CDs results in the increase of the fluorescence intensity. The class of the fluorescence mechanism arises from surface-related defective sites – generally any sites which have imperfect sp2 domains will lead to surface energy traps. Both sp2 and sp3 hybridized carbons and other functionalized cause surface defects. Owing to functionalization or surface passivation, the surface defects become more stable to expedite more effective 11 

radioactive recombination of surface-confined electron sand holes, which achieving brighter fluorescence emissions [44-47]. Typically, the fluorescent CDs nanosensor can exhibit different luminous characteristics under different conditions. Here we took pH and solvent into account, at first, we prepared different pH (from 4 to 10) buffer solution, then different pH buffer solution was added in each 1cm cuvette, 10μl of the CDs@NH2 solutions was added next for fluorescence measurements. And the fluorescence measurements were performed under same ambient conditions, the entire process was carried out in the absence of other interference substances. The effect of pH (from 4 to 10) on the fluorescence intensity of CDs@ NH2 was studied (Fig. 4a and 4b). With an increasing pH values from 4 to 7, the fluorescence intensity increases drastically. While when pH value is above 7, the fluorescence intensity doesn’t increase continuously, showing that pH values have no important role any more. All these mean that the prepared CDs@NH2 nanosensor has strong and stable fluorescence intensity in the neutral or alkaline environment. So the phosphate buffer solution with pH at 7 was chosen for the TNT detection in groundwater. Subsequently, the influence of solvent on fluorescent CDs nanosensor was investigated: different solvents including water, ethylene glycol (EG), isopropyl alcohol (IA), acetone and methylene chloride (MC) were added into 1cm cuvette individually, then 10μl of the CDs@NH2 solutions was added in different solvent for fluorescence

measurements.

All

mixture

solutions

were

measured

under

above-mentioned conditions. Fig. 4c.d shows the fluorescence intensity of fluorescent nanosensor in different solvents including water, ethylene glycol, isopropyl alcohol, acetone and methylene chloride. We can find that CDs@ NH2 possess strong fluorescence intensity in water while the nanosensor emitted the lowest fluorescence in methylene chloride. So water is chosen as the TNT detection solvent.

3.3 Sensitive and selective detection of TNT residues in water To evaluate the sensing ability of the CDs@NH2 nanosensor, the fluorescence response of the fluorescent nanosensor toward different concentrations of TNT was 12 

carried out under the optimal conditions. After 30 μL of 5 mg/L CDs@NH2 solution was diluted in 1mL cuvette, 30 μL of 1×10-6 mol/L TNT was added in each time and the relative fluorescence intensity was measured under a single wavelength excitation of 337 nm. As a result, the quenching of CDs@ NH2 fluorescence by TNT was rapid and sensitive, even at a very low concentration. As shown in Fig. 5a, the fluorescence intensity decreased greatly with the addition of TNT, and when the concentration of TNT reached to 1×10-7 mol/L in the cuvette, we can find that the fluorescence of the nanosensor almost disappeared. The fluorescence intensity decrease of nanosensor was proportional to the TNT concentration and showed a good linearity in Fig. 5b. According to Lineweaver-Burk K

formula:

C

, the binding constant of CDs and

TNT can be calculated as 3.9×106 L·mol-1, indicating that TNT molecules bind with CDs greatly. The fluorescence intensity decrease of nanosensor was proportional to the TNT concentration and showed its linearity can be shown in Fig. 5 (b), indicating the detection range reaches from 0 to 1 μM. Furthermore, the detection limit of the prepared CDs nanosensor towards TNT residues in groundwater also can be determined as low as 0.213 µmol/L under the optimal experimental conditions. Compared with other reported TNT nanosensors, we can find the detection limit of the prepared CDs is quite superior to the fluorescence quantum dots [48], silica nanoparticles [31] and so on. In order to investigate the selectivity of the nanosensor to TNT, TNT and various nitroaromatic derivates, such as p-nitrophenol (PNP), p-nitrotoluene (PNT), 2-nitroaniline (2NN), 2-nitrobenzoic acid (2NA), nitrobenzene (NB), p-nitrobenzyl alcohol

(PNA),

3-chloronitrobenzene

(3CN),

p-nitroacetophenone

(PNAT),

p-nitrophenylhydrazine (PNPH), 2-nitrobenzenemethanol (2NBM) were diluted into cuvette individually with the phosphate buffer (pH=7), fluorescence intensity was measured in the presence of 30 μL of 5 mg/L CDs@NH2 under the same experimental conditions. As seen from Fig.5c, when TNT alone was added into the nanosensor solution, the fluorescence at 431 nm quenched greatly. While when other interfering 13 

molecules were added, we can easily found that the fluorescence has no or little decrease, showing that our prepared nanosensor possesses the efficient ability for selective detection of TNT in groundwater. Furthermore, interfering experiments were conducted by adding 0.05 μM TNT to the nanosensor solution containing 0.1 μM other nitroaromatic. The fluorescence measurements were performed under the same ambient conditions (Fig. 5d As shown, with the addition of TNT, the fluorescence intensity of nanosensor decreased greatly and could not be interfered under the presence of other nitroaromatics even when the concentration of the interference substances were greatly higher than that of TNT. So we can conclude that CDs@NH2 has a good selectivity for TNT detection in groundwater.

3.4 Paper sensor based on CDs@NH2 for TNT detection in actual groundwater As mentioned, instant on-site visual detection of the trace TNT in aqueous solution is very crucial for security needs [54]. For this purpose, a paper-based sensor has been developed. The layer thickness of the paper-based sensor was measured as 0.53mm. And also, the layer thickness of four different parts of the portable paper-based sensor was determined as 0.53mm, 0.53mm, 0.54mm, and 0.53mm, indicating that the paper-based sensor possess a good uniformity. Furthermore, the portable paper-based sensor has also been reproduced. Experimental results indicated the newly produced paper sensors also can detect TNT in groundwater, and the similar fluorescence responses can be observed, which confirmed that paper-based sensor had good reproducibility. In addition, the cellulose paper was very stable in the dark, and no any degradation of fluorescence color and sensitivity were observed after the storage for at least several weeks, and the entire paper sensor exhibited a bright blue fluorescence emission color under the irradiation of UV light (365nm). In order to verify the instant on-site visual detection of TNT, we took groundwater sample under the Jianghan Plain for further practical detection. As shown in Fig. 6a, with increasing TNT concentrations (1×10-9, 5×10-9, 1×10-8, 5×10-8, 1×10-7, 5×10-7 mol/L), the 14 

fluorescence emission intensity decreased gradually. When the paper sensor was used to detect trace TNT, we can find the fluorescence became darker and darker under a 365 nm UV lamp, which is consistent with the fluorescence emission spectra. These results indicate that the portable paper nanosensor is highly desirable for the rapid and convenient visual detection of TNT in aqueous solution.

4 Conclusions In summary, a kind of new nanosensor for trace TNT detection in groundwater was synthesized via a simple and facile hydrothermal method, and the nanosensors were systematically characterized by the physicochemical methods. The as-prepared nanosensor can efficiently detect TNT in groundwater with the detection limit as low as 0.213 µmol/L under the optimal experimental conditions. Experimetnal results indicated that the primary amino groups decorated on the CDs would react with the TNT molucles to form the stable Meisenheimer complex due to the electrostatic effect, which then induced the fluorescence quenching due to the PET mechanism. The prepared CDs@NH2 nanosensor possesses the high selectivity toward other nitoarmatic comounds. Furthermore, a paper-based sensor for the visual detection of TNT has been successfully developed by immobilization of the CDs@NH2 on cellulose paper. The present work promises an effective means to develop sensitive, selective and portable nanosensor for visual detection of trace TNT in groundwater, and this kind of design mechanism can be utilized for a variety of applications in sensing a series of pollutants in groundwater.

Acknowledgments This work was supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. 41521001) and the National Natural Science Foundation of China (No. 51371162) and the “Fundamental Research Funds for the Central Universities”.

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Xike Tian is a Professor at the Faculty of Material Science and Chemical Engineering,  China University of Geosciences (Wuhan). He got his Ph.D. degree from Institute of  Solid  State  Physics,  Chinese  Academy  of  Sciences.  His  current  research  interests  include  environmental  nanomaterials,  optical  detection  technology  and  prevention  treatment research of hazardous substances in the environment.    Hui Peng is currently pursuing his master degree at Faculty of Material Science and  Chemistry, China University of Geosciences (Wuhan). His research interests focused  on  the  design  and  preparation  of  fluorescent  probes  for  chemical  and  biological  sensing.    Yong Li is a Lecturer at Faculty of Materials Science and Chemistry, China University  of Geosciences (Wuhan). He obtained his Ph.D. degree from Lanzhou University. And  his  current  research  focused  on  the  optical  detection  technology  on  environmental  pollutants.    Chao  Yang  is  an  Associate  professor  at  Faculty  of  Materials  Science  and  Chemistry,  China  University  of  Geosciences  (Wuhan).  His  research  focused  on  separation  and  detection technologies of the environmental pollutants.    Zhaoxin Zhou is an Associate professor at Faculty of Materials Science and Chemistry,  China University of Geosciences (Wuhan). His research focused on optical detection  technology on pollutant in the environment and oilfield sewage disposal.    Yanxin  Wang  is  a  Professor  of  Environmental  Hydrogeology  and  the  President  of  China  University  of  Geosciences  at  Wuhan.  His  research  work  has  been  focused  on hydrogeochemistry  and  groundwater  contamination.  Prof.  Wang  has  made  great  efforts  in  studying  mechanisms  of  groundwater  contamination  and  cost‐effective  technologies of site remediation, to provide theoretical and technological support for  safe supply of drinking water.  20 

  Fig. 1 Schematic illustration of the fabrication of the CDs@NH2 nanosensor and its sensing mechanism of TNT    

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  Fig.2 (a) XRD pattern of CDs; (b) FT-IR spectra of CDs and CDs@NH2; (c) TEM image of CDs@NH2 (d) the size distribution of CDs@NH2 characterized by Zeta potential    

22 

  Fig.3 Fluorescence spectra of CDs and CDs@NH2 with excitation at 337 nm

23 

  Fig. 4 (a, b) Influence of the pH on the fluorescence intensity of fluorescent nanosensor; (c, d) Influence of different solvents on the fluorescence of fluorescent nanosensor    

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  Fig.5 (a) Fluorescence spectral change of the fluorescent nanosensor upon addition of different amounts of TNT (increasing 30 μL of 1×10-6 mol/L TNT was diluted in 1 cm cuvette solution for each time); (b) Linear fitting curve of fluorescence intensity with respect to the TNT concentrations (The resultant linear detection ranges from 0 to 1 μM), the inset image shows the corresponding fluorescence color change under UV lamp (365 nm); (c) Fluorescence response of the nanosensor to different nitroaromatic and TNT (0.1 μM for all matter, excitation at 337 nm); (d) The interference studies of the nanosensor toward TNT. The black bars represent the fluorescence response of the nanosensor to interference substances (p-nitrophenol (PNP), p-nitrotoluene (PNT), 2-nitroaniline (2NN), 2-nitrobenzoic acid (2NA), nitrobenzene (NB), p-nitrobenzyl alcohol (PNA), 3-chloronitrobenzene (3CN), p-nitroacetophenone (PNAT), p-nitrophenylhydrazine (PNPH), 2-nitrobenzenemethanol (2NBM),0.05 μM for TNT, 0.1 μM for the interference substances.) The red bars represent the change of emission that occurred following the subsequent addition of 0.1 μM of TNT to the above solutions.     

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  Fig. 6 (a) Fluorescence intensity responses of practical water samples for paper sensor (b)  The fluorescence images of the paper nanosensor upon the addition of different concentrations of TNT (1×10-9, 5×10-9, 1×10-8, 5×10-8, 1×10-7, 5×10-7 mol/L). All the images were taken under a 365 nm UV lamp.      

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Table 1 Comparison of the detection limit of as-prepared CDs based nanosensor with other reported TNT nanosensors Method

Detection limit

References

Fluorescence graphene quantum

2.2µmol/L

[48]

Mesoporous silica nanoparticles

0.598 µmol/L

[31]

Electrochemical diamond sensors

0.11 µmol/L

[49]

Amine-Capped ZnS-Mn2+

2.81µmol/L

[50]

Molecularly Imprinted Polymers

3µmol/L

[32]

SPR immunosensor

1µmol/L

[51]

Upconversion luminescence

0.426 µmol/L

[30]

1µmol/L

[52]

Capillary array electrophoresis chip

0.44 µmol/L

[53]

This work

0.213 µmol/L

this work

dots

Nanocrystals

nanosensor A smartphone-based biosensor

   

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