A convenient and universal platform for sensing environmental nitro-aromatic explosives based on amphiphilic carbon dots

Environmental Research 177 (2019) 108621

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A convenient and universal platform for sensing environmental nitroaromatic explosives based on amphiphilic carbon dots

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Jingwen Wanga, Yushan Yanga, Guoying Suna, Min Zhenga,∗, Zhigang Xieb,∗∗ a School of Chemical Engineering, School of Chemistry and Life Science, Advanced Institute of Materials Science, Changchun University of Technology, 2055 Yanan Street, Changchun, Jilin, 130012, PR China b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin, 130022, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Amphiphilic carbon dots Sensor 2,4,6-Trinitrophenol Environmental detection Indicator paper

2,4,6-trinitrophenol (TNP) is environmentally deleterious substance that has been of pressing societal concern. Therefore, developing a convenient and reliable platforms for its fast and efficient detection is of paramount importance from security point of view. Herein, amphiphilic fluorescent carbon dots (CDs) were prepared by a simple solvothermal method. CDs exhibit high selectivity and sensitivity on TNP in the polar and apolar solvent and even natural water samples. Moreover, the simple and portable indicator paper can be prepared conveniently and used for sensing TNP visually with high sensitivity and fast response. Research findings obtained from this study would assist in the development of portable devices for the on-site and real-time detection of environmental hazards.

1. Introduction As a nitroaromatic compound, 2,4,6-Trinitrophenol (TNP) is identified as an explosive and toxic pollutant. TNP is hazardous to human safety and health, which can bring out severe effects on the respiratory system, central nervous system and cardiovascular system etc (Lichtenstein et al., 2014; Chen et al., 2016; Huynh et al., 2016; Devi and Ahmaruzzaman, 2017). Thus, developing a reliable and rapid strategy for detecting TNP with high sensitivity has become a burning issue nowadays. Although diverse technological methods such as mass spectrometry (Pamme et al., 2002), electrochemistry (Wang and Fan, 2009) and ion mobility spectrometry (Pavlačka et al., 2016) have been used for the detection of TNP, the drawbacks of expensive instrumentation, complicated pretreatment procedures and time-consuming analysis restrict their incorporation into a portable system for real time and on-site analysis. Consequently, there is an ongoing effort to develop materials and techniques for detecting TNP in a convenient and low cost manner. Recently, many fluorescent sensors such as metal organic frameworks (Joarder et al., 2015; Bagheri et al., 2016; Xing et al., 2017; Pan et al., 2017; Ye et al., 2015), metal nanoclusters (Zhang et al., 2016a; Enkin et al., 2014) and quantum dots (Ye et al., 2016; Fan et al., 2017; Wang and Ni, 2014) on explosives have been widely investigated due to their easy manipulation and high sensitivity.

∗

However, the shortcomings of the current fluorescent materials including toxicity, low TNP selectivity and poor stability are still the hurdles for their potential applications. As one kind of new carbon-based zero-dimensional nanomaterial, carbon dots (CDs) possess a substantial amount of advantages of a cheap and wide source, easy preparation, low toxicity and excellent biocompatibility (Zhu et al., 2017a; Zhi et al., 2018; Jiang et al., 2015; Zhao et al., 2018; Ma et al., 2017). These remarkable properties make them promising candidates for the applications in sensing (Dong et al., 2012; Qu et al., 2013; Saini et al., 2017; Gong et al., 2019; Zheng et al., 2013, 2016a; Babaee et al., 2019), bioimaging (Qu et al., 2015; Yang et al., 2009, 2017; Zhu et al., 2013; Karakoçak et al., 2018; Zhang et al., 2016b; Zheng et al., 2016b; Wang et al., 2015), diagnosis (Zheng et al., 2015; Li et al., 2017; He et al., 2018) and nanomedicine (Peng et al., 2017; Li et al., 2018a, 2018b; Chaudhary et al., 2017; Zheng et al., 2014; Zhu et al., 2017b). Considering the green preparation route and enchanting fluorescence of CDs, a type of fluorescence assay was developed for the detection of TNP. Although there are some reports on CDs-based sensors for TNP in water or organic solvents (Siddique et al., 2018; Xu et al., 2018; Ren et al., 2018; Ju et al., 2018), as far as we know, few amphiphilic CDs were reported and applied for practical TNP detection. Herein, amphiphilic fluorescent CDs were synthesized via a facile

Corresponding author. Corresponding author. E-mail addresses: [email protected] (M. Zheng), [email protected] (Z. Xie).

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https://doi.org/10.1016/j.envres.2019.108621 Received 11 July 2019; Received in revised form 31 July 2019; Accepted 1 August 2019 Available online 01 August 2019 0013-9351/ © 2019 Elsevier Inc. All rights reserved.

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Scheme 1. Schematic diagram for synthesizing amphiphilic fluorescent CDs and their application in TNP detection.

used were analytical reagent grade without further purification.

solvothermal method from P-Phenylenediamine and L-Histidine (Scheme 1). The as prepared CDs are well dispersible in a variety of polar and nonpolar solvents such as water, formamide, acetonitrile, N,N-dimethylformamide (DMF), methanol, ethanol, acetone, tetrahydrofuran (THF) and ethyl acetate (EA). Moreover, CDs exhibit similar optical behaviors and highly selective detection toward TNP over other nitro-explosives in the different mediums with the detection limit of 1.09 μM (water), 0.32 μM (formamide), 0.33 μM (acetonitrile), 0.34 μM (DMF), 0.53 μM (methanol), 0.31 μM (ethanol), 0.23 μM (acetone), 0.28 μM (THF) and 0.43 μM (EA). The applications of CDs for sensing TNP in real water samples were also explored and the results indicate that CDs still possess high sensitivity and selectivity toward TNP with the detection limit of 1.31 μM (tap water) and 0.99 μM (lake water). The most important is that an efficient indicator paper for visual detection of TNP was prepared conveniently by casting CDs solution on filter paper. This kind of solid sensor can sense TNP visually with high sensitivity and selectivity. Our work opens up a new avenue for on-site and real-time detection of environmental hazards without any sophisticated processes or devices.

2.2. Characterizations The morphology of the nanoparticles was measured using transmission electron microscopy (TEM) performed on a JEOL JEM-1011 electron microscope operating at an acceleration voltage of 100 kV. The crystalline structure was recorded by using an X-ray diffractometer (XRD) (Bruker AXS D8 Focus), using Cu Ka radiation. Raman spectrum was conducted on Horiba JY LabRAM HR Evolution Raman spectrometer. Fourier Transform Infrared (FT-IR) spectrum was recorded on a Bruker Vertex 70 spectro-meter. X-ray photoelectron spectra (XPS) were carried out using a Thermo Scientific ESCALAB 250 Multitechnique Surface Analysis. UV–Vis absorption spectra were recorded on Shimadzu UV-2450 spectrophotometer. Fluorescence emission spectra were recorded on a PerkinElmer LS-55 phosphorescence spectrophotometer. The absolute fluorescence quantum yield was measured on a Hamamatsu Photonic Multi-Channel Analyzer C10027. 2.3. Preparation of CDs

2. Experimental In a typical synthesis, 0.108 g of P-Phenylenediamine and 0.155 g of L-Histidine were dissolved in 10 mL of ethanol, then the mixture was sealed in a poly(tetrafluoroethylene)-lined autoclave and heated at 160 °C for 5 h. After cooled to room temperature, the crude product was filtered through a microporous membrane. And subjected to a dialysis via dialysis bag (MWCO=3000) for 24 h. Finally, the obtained solution was freeze-dried under vacuum conditions, and the purified CDs were obtained.

2.1. Materials P-Phenylenediamine (97%), L-Histidine (99%), p-nitrochlorobenzene (NC, 98%), p-nitrophenol (NP, 98%) and formamide (AR, 99%) were purchased from Aladdin Ltd (Shanghai, China). The nitroaromatics that include nitrobenzene (NB, 99%), 1,3-dinitrobenzene (DNB, 98%) and 1-chlor-2,4-dinitrobenzene (CDNB, 99%) were purchased from Energy Chemical Technology Corporation (Shanghai, China). 2,4,6-trinitrophenol (TNP, 99%) was purchased from Xilong Chemical Co., Ltd (China). Ethanol (AR, ≥99.7%), acetonitrile (AR, ≥99.5%), N,N-dimethylformamide (AR, ≥99.5%), methanol (AR, ≥99.5%), tetrahydrofuran (AR, ≥99.5%) and ethyl acetate (AR, ≥99.5%) were purchased from Tian Jin Fuyu Fine Chemical Co.,Ltd. Acetone (AR, ≥99.5%) was purchased from Beijing Chemical Works. Ultrapure water (18.2 MΩ/cm) was obtained from a Milli-Q ultrapure system (Shanghai, China). The South lake water was obtained from the South Lake. Tap water and South lake water were filtered using filter paper to remove the suspended substance. All chemicals

2.4. Procedures for detecting nitroaromatic compounds 1 mL of CDs solution (20 μg/mL) was mixed with 1 mL of different concentrations (0, 2, 4, 10, 20, 40, 60, 80, 100, 120, 140 μM) of nitroaromatic compounds to obtain the mixture of CDs (10 μg/mL) and nitroaromatics (0, 1, 2, 5, 10, 20, 30, 40, 50, 60, 70 μM). After thorough shaking, the mixture was incubated at room temperature for 1 min. The fluorescence quenching behaviors of CDs were monitored by a fluorescence spectrometer under the excitation of 360 nm. The detection limit and Ksv can be calculated from the Stern-Volmer curves. 2

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Fig. 1. (a) Emission spectra of CDs in different solutions. (b) TEM image of CDs, insert: Diameter distribution of CDs. (c) XRD pattern of CDs. Raman (d) and FT-IR (e) spectra of CDs. (f) XPS survey of CDs. High-resolution C1s (g), N1s (h), and O1s (i) XPS spectra, respectively.

3. Results and discussion

investigated using XPS analysis. The XPS survey spectrum (Fig. 1f) of CDs are mainly composed of three peaks at 284, 398, and 531 eV, which are attributed to C1s, N1s and O1s, respectively (Huang et al., 2018). The high-resolution XPS spectra of C1s (Fig. 1g) can be well deconvolved into three peaks, which correspond to C−C/C=C (284.6 eV), C−N (285.4 eV) and C=O (287.6 eV), respectively (Qiao et al., 2018). The N1s peak (Fig. 1h) can be deconvoluted into two peaks of N−H (398.9 eV) and C=N−C (400.1 eV) (Wang et al., 2018). The O1s spectrum (Fig. 1i) displays one fitted peak of C=O (532.0 eV) (Zhang et al., 2018; Yang et al., 2012). The XPS analyses agreed well with the FT-IR measurements of CDs. The photoluminescence (PL) and UV–vis absorption spectra of CDs in ethanol are shown in Fig. 2. The two peaks at 246 and 300 nm (Fig. 2a) in the UV–Vis absorption spectrum of CDs correspond to π-π* and n-π* transitions (Lim et al., 2015; Wang et al., 2017). Besides, we also notice there are two bands at 262 and 365 nm in the excitation spectrum of CDs, indicating that more than one kind of excitation energy is trapped on the surface of CDs (Yang et al., 2011; Sun et al., 2016). In addition, the emission band of CDs is centered at 446 nm under the excitation of 360 nm, which implying the blue fluorescence of CDs. Moreover, the results of fluorescence spectra of CDs (Fig. 2b) indicate that CDs show nearly excitation-independent PL behavior when the excitation wavelength is from 350 nm to 400 nm with the maximum excitation and emission wavelength at 360 and 446 nm, respectively. The excitation-independent PL behavior is due to the low number of surface defects and the uniform nature of CDs (Zhi et al., 2014). Besides, the fluorescence quantum yield is important properties for fluorescent materials. The absolute fluorescence quantum yield of CDs was measured to be 5.80%. In addition, the photostability of CDs was studied (Fig. 2c). There is no significant decrease in the fluorescence intensity under 6 h of light irradiation, showing the excellent

3.1. Synthesis and characterization of CDs CDs were successfully synthesized with P-Phenylenediamine and LHistidine as raw materials, which are well dispersible in a variety of polar and nonpolar solvents. Under 360 nm of excitation (Fig. 1a), CDs show different emission wavelengths in different solvents such as water (488 nm), formamide (451 nm), acetonitrile (428 nm), DMF (429 nm), methanol (457 nm), ethanol (446 nm), acetone (430 nm), THF (421 nm) and EA (427 nm). The morphologies and size distributions of CDs were characterized by transmission electron microscopy. It can be clearly seen from Fig. 1b that CDs are monodispersed near-spherical nanoparticles with average diameters of 23.3 ± 2.7 nm. To better understand the crystallinity of CDs, XRD characterization were performed. As shown in Fig. 1c, there is a characteristic peak at 2θ = 24.1°, which is similar to the graphite lattice spacing (002) (Mao et al., 2014; Park et al., 2014; Rodríguez-Padrón et al., 2018; Chen et al., 2014). The Raman spectrum of CDs (Fig. 1d) displays two broad peaks at around 1372 and 1588 cm−1, which are corresponded to the D band (sp3-hybridized) and G band (sp2-hybridized), respectively. The relative intensity of the disordered D band and crystalline G band (ID/IG) for the CDs is around 0.89, indicating that they have a similar structure to graphite. Besides, the chemical bonds and functional groups on CDs are further investigated using FT-IR spectroscopy (Fig. 1e). The broad bands spanning from 3720 to 3100 cm−1 correspond to the stretching vibrations of O−H and N−H. The low-intensity peaks from 3000 to 2858 cm−1 are allocated to the C−H stretching vibrations. Four peaks at approximately 1610, 1518, 1400 and 1263 cm−1 are ascribed to the stretching vibrations of C=O, C=N, C=C and C−N vibration, respectively. Moreover, the surface elements and groups of CDs were also 3

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Fig. 2. (a) UV–vis absorption (black solid line), excitation (red solid line) and emission (blue solid line) spectra of CDs in ethanol. (b) Fluorescence spectra of CDs under different wavelengths of excitation. (c) Photostability of CDs under green light (16 mW) irradiation. Fluorescence spectra of CDs in the presence of different concentrations of TNP (d), NC (e), NP (f), NB (g), DNB (h) and CDNB (i). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

sensitivity study, Stern-Volmer plots of the CDs probe with different concentrations of nitro-based explosives in the range of 0–70 μM were investigated. The results in Fig. 3c showed that the I0/I-1 of CDs was increased obviously after the addition of TNP, however almost negligible change was observed after addition of other nitro-based explosives, suggesting that CDs exhibit dominant selectivity on TNP over other explosives. Furthermore, the quenching consequences could be quantitatively analyzed with the Stern-Volmer (S-V) equation：

photostability of CDs. 3.2. CDs as fluorescent sensors of nitroaromatic compounds Although the definite luminescence mechanism of CDs has always been debated, the charming optical properties constantly encourage scholars to explore their valuable applications. In this study, the assynthesized CDs (10 μg/mL) were used for highly selective nitroaromatics sensing. To compare the selectivity of CDs toward explosive nitrocompounds, the emission spectra of CDs were recorded upon the addition of TNP (Fig. 2d), NC (Fig. 2e), NP (Fig. 2f), NB (Fig. 2g), DNB (Fig. 2h) and CDNB (Fig. 2i). As shown in Fig. 2d–i, with the addition of nitroaromatic compounds in the range of 0–70 μM to CDs ethanol solution, the fluorescence intensities show varying degrees of decrease. Especially, the fluorescence of CDs was found significantly quenched with increasing TNP concentrations, indicating that CDs are very sensitive to TNP and can act as a nanoprobe for TNP detection. The fluorescence intensities of CDs versus different concentrations of nitro-explosives were plotted and shown in Fig. 3a. With the increase of concentration from 0 to 70 μM, the fluorescence intensity of CDs decreased by different values: TNP (574.37) > NP (121.48) > NC (38.71) > CDNB (32.69) > NB (26.15) > DNB (22.14). To further investigate the selectivity of fluorescent CDs toward TNP, fluorescence quenching titrations were carried out using other nitroaromatic compounds such as NC, NP, NB, DNB and CDNB. As can be seen from Fig. 3b, compared with other nitro-explosives, the fluorescence quenching efficiency of CDs upon the addition of TNP is extraordinarily high (~96.15%). These results again demonstrate that CDs are much more sensitive to TNP than the other nitroaromatic compounds. For the

I0/ I = K SV [A] + 1 where I0 is the original fluorescence intensity without analyte, I is the fluorescence intensity with analyte, KSV is the quenching constant of [M]−1, and [A] is the molar concentration of analyte. The PL intensities of CDs were plotted versus the concentration of TNP which shows an exponential S-V curve. From the inset of Fig. 3d, a linear curve at lower concentrations (0–20 μM) could be noticed. But the S-V curve deviates from linear and increases exponentially at higher concentrations. This nonlinearity of the S-V curve of CDs was perhaps owing to a combination of static and dynamic quenching. This kind of nonlinear curve also implies an effective fluorescence energy transfer mechanism present in the quenching process. An exponential quenching equation was applied to fit the nonlinear S-V curve:

I0/I = a exp (κ [A]) + b where a, b and κ are constants. The result indicates that the curve can be fitted to I0/I-1=0.1722exp(0.0711[TNP])+0.0465 very well, and the correlation coefficient (R2) reaches 0.9986 as displayed in Table S3. The quenching constant value for TNP was calculated to be 12243 M−1, 4

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Fig. 3. (a) PL intensity changes of CDs upon the addition of various concentrations (0–70 μM) of nitroaromatics. (b) Fluorescence quenching efficiency obtained for CDs upon the addition of 70 μM different nitroaromatics (with their corresponding chemical structures) in ethanol. (c) Stern–Volmer plots of the CDs probe with different nitro-based explosives at various concentrations in ethanol. (d) Fitting curve of the nonlinear Stern-Volmer plot for CDs against TNP by exponential quenching equation in ethanol, inset shows the fitting curve of the linearity of the Stern-Volmer plot for TNP of 0–20 μM. (e) Absorption spectra of various nitroaromatic compounds and the emission spectrum of CDs in ethanol. (f) The photos of CDs ethanol solution with addition of various nitroaromatic compounds under 365 nm UV lamp. (g) Selectivity and anti-interference property of the CDs probe to TNP in ethanol. Fluorescence intensity of CDs probe towards TNP (70 μM), NC (70 μM), NP (70 μM), NB (70 μM), DNB (70 μM) and CDNB (70 μM). Black bar: probe plus inspected species; Red bar: probe in the presence of inspected species and TNP. (h) Fluorescence quenching efficiency obtained for CDs upon the addition of 70 μM TNP in different solution: (1) ultrapure water, (2) tap water, (3) South Lake water, (4) formamide, (5) acetonitrile, (6) N,N-dimethylformamide, (7) methanol, (8) ethanol, (9) acetone, (10) tetrahydrofuran, (11) ethyl acetate. (Data are presented as the average from three independent measurements.). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

using the product of constants a and κ (Liu et al., 2010; Wei et al., 2015). Besides, the quenching constant values for various nitroaromatic compounds are displayed in Table S1, and the quenching constant of TNP is remarkably larger than other nitro-explosives, demonstrating the predominant selectivity of TNP over other nitro-compounds. The non-linearity of the S-V plot for TNP suggests that an energy transfer mechanism is perhaps present in the quenching process. The possibility of resonance energy transfer depends on the degree of overlap between the fluorescence emission spectrum of the donor (the fluorophore) and the absorption spectrum of the acceptor (the analyte) (Sun et al., 2015; Sadhanala and Nanda, 2015; Liang et al., 2016; Zhuo et al., 2015). Absorption spectra of various nitroaromatic compounds and the emission spectrum of CDs are described in Fig. 3e, the emission spectrum of CDs largely overlaps with the absorption spectrum of TNP, while there is almost none overlap for other nitroaromatics, confirming the high sensitivity and selectivity of CDs towards TNP. The Standard deviation (σ) of the blank measurements (Table S2) was obtained by fluorescence responses (20-times of consecutive scanning on the Fluorescence Spectrophotometer). Therefore, the detection limit was calculated to be 0.31 μM (by using 3σ/κ, σ as the standard deviation and κ as the slope). Specificity is another critical indicator to a probe. Fig. 3f shows the visual change of CDs ethanol solution with addition of various nitroaromatic compounds under 365 nm UV lamp. The solution containing TNP could be easily distinguished from others by naked eye

detection. As seen in Fig. 3g, the responses of the CDs probe (black bar) are highly selective toward TNP compared with other interfering agents. Furthermore, the fluorescence of all the above samples was discovered to be greatly reduced upon the subsequent addition of TNP (Fig. 3f). The PL emissions of all the above samples were found to be obviously reduced upon the subsequent addition of TNP (Fig. 3g, red bar). These findings clearly demonstrate excellent selectivity and antiinterference capacity of the as-fabricated CDs probe towards TNP. For the practical utility, we explored CDs to detect TNP in different solutions. As shown in Fig. 3h, CDs exhibit different quenching efficiency for TNP in different solvents: ultrapure water (61.87%), tap water (57.36%), South Lake water (64.68%), formamide (78.66%), acetonitrile (87.22%), N,N-dimethylformamide (84.71%), methanol (68.63%), ethanol (96.15%), acetone (95.67%), tetrahydrofuran (93.40%), ethyl acetate (88.32%). Obvious decrease of the fluorescence is observed in different solutions with the addition of 0–70 μM of TNP, which is very helpful for practical applications (Figs. S1–10). The result shows that the Stern-Volmer plot can be fitted to exponential quenching equation perfectly in different solutions, and all of the correlation coefficient (R2) reaches > 0.99 as displayed in Table S3. The quenching constant value for TNP was calculated to be > 104 M−1. The detection limit of TNP by CDs is less than 0.53 μM in all organic solvents. Moreover, the selectivity of this probing platform was evaluated in the real water systems, as shown in Table S3, CDs probe for TNP have similar detection limit in ultrapure water (1.09 μM), tap water (1.31 μM)

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Table 1 Comparison of current and reported CDs-based sensors for TNP detection. Material

Detection media

LOD (μM)

Species detected

Ref.

Terbium-doped Carbon dots Metal–Organic Framework Metal−Organic Framework Metal−Organic Framework Metal−Organic Framework Metal−Organic Framework Ag nanoclusters Carbon dots Carbon dots Quantum dots Carbon dots Carbon dots Carbon dots Carbon dots Carbon dots Amphiphilic carbon dots

Water Water Water DMF N,N-dimethylacetamide Water Water Water Water Water Water Water Ethanol–water Ethanol Water Lake water, tap water, water, formamide, acetonitrile, DMF, methanol, ethanol, acetone, THF and EA

0.2 0.0129 0.56 0.0001 1.40 0.051 0.095 0.2 0.0191 0.127 2.0 0.36 0.23–1.31

TNP TNP TNP TNP TNP TNP TNP TNP TNP TNP TNP TNP TNP TNP TNP TNP

Chen et al. (2016) Joarder et al. (2015) Bagheri et al. (2016) Xing et al. (2017) Pan et al. (2017) Ye et al. (2015) Zhang et al. (2016a) Ye et al. (2016) Fan et al. (2017) Wang and Ni (2014) Zheng et al. (2014) Li et al. (2018b) Zhu et al. (2017b) Siddique et al. (2018) Zhang et al. (2018) Present Work

and lake water (0.99 μM). Compared to other CDs-based sensors (Table 1), the amphiphilic CDs can probe TNP in a variety of mediums with low detection limit and provide a more convenient and universal strategy for detecting TNP in environmental surveillance with high applicability and effectiveness. In order to demonstrate the rapid and easy manipulation portability of CDs for TNP detection, the indicator papers for the visual detection were prepared by immersing filter papers into various concentrations (0–2000 μg/mL) of CDs solution and drying them at room temperature. As displayed in Fig. 4a, the indicator paper immobilized with 200 μg/ mL of CDs displays the strongest fluorescence under UV light, so it was chosen as the indicator paper for TNP detection. The indicator papers immobilized with 200 μg/mL of CDs were subjected to different concentrations of TNP, as shown in Fig. 4b, the fluorescence intensity of indicator papers is weaken gradually with increasing the concentration of TNP from 0 to 2000 μM, indicating the fast and naked-eye sensitive detection for TNP (Fig. 4b). In addition, the selectivity of this indicator paper on TNP over other nitroaromatic explosives was carried out (Fig. 4c). The fluorescence of the test paper treated with TNP is completely quenched, by contrast, there is no significant fluorescence change for the groups treated with other explosives. These results indicate that this portable nanosensor can successfully distinguish TNT from other explosives with great selectivity. To further investigate the high selectivity of CDs-based indicator paper for TNP, competition experiments of TNP over other nitroaromatic explosives were conducted. The results (Fig. 4d) show that this CDs-based indicator paper can also

successfully probe TNP in the mixtures. Consequently, CDs have enormous potentials for TNP detection in environmental monitoring. These results demonstrated the excellent advantages of this kind of CDsbased indicator paper including fast response, high sensitivity and practicability. Therefore, CDs can behave a promising candidate for real-time and on-site sensing TNP in the laboratory and environment.

4. Conclusions In summary, amphiphilic CDs were prepared via solvothermal method from P-Phenylenediamine and L-Histidine. CDs exhibit high sensitivity and selectivity on TNP in apolar and polar solvents and even real water samples with fast response (1 min). Moreover, the CDs-based indicator paper can be conveniently prepared and used for distinguishing TNP in 1 min by the naked eyes. This pioneering work will open a new avenue toward the development of CDs for the convenient and precise detection of environmentally harmful nitroaromatic explosives.

Acknowledgements The financial support from the National Natural Science Foundation of China (No. 51873023) and Talent Development Fund of Jilin Province.

Fig. 4. (a) The photos of the indicator papers immobilized with different concentrations (0–2000 μg/mL) of CDs under 365 nm UV illumination. (b) Photographs of CDs-based indicator papers (immobilized with 200 μg/mL of CDs) with various concentrations (0–2000 μM) of TNP. (c) Photographs of CDs-based indicator papers treated with nitroaromatic explosives (2000 μM). (d) Photographs of CDs-based indicator papers treated by the mixtures of TNP (2000 μM) with other nitroaromatic explosives (2000 μM). 6

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Appendix A. Supplementary data

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