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Simple and Sensitive Fluorescent and Electrochemical Trinitrotoluene Sensors Based on Aqueous Carbon Dots Lingling Zhang, Yujie Han, Jinbo Zhu, Yanling Zhai, and Shaojun Dong Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac5043686 • Publication Date (Web): 20 Jan 2015 Downloaded from http://pubs.acs.org on January 25, 2015

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Simple and Sensitive Fluorescent and Electrochemical Trinitrotoluene Sensors Based on Aqueous Carbon Dots Lingling Zhang, Yujie Han, Jinbo Zhu, Yanling Zhai, Shaojun Dong* State Key Laboratory of Electroanalytical chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, China University of Chinese Academy of Sciences, Beijing, 100049, China E-mail: [email protected]. Tel: +86-431-85262101; Fax: +86-431-85689711

ABSTRACT: Aqueous N-rich carbon dots (CDs), prepared by microwave-assisted pyrolysis method, are applied as a dual sensing platforms for both the fluorescent and electrochemical detection of 2, 4, 6-trinitrotoluene (TNT). The fluorescent sensing platform is established on the strong TNT-amino interaction which can quench the photoluminescence of amino functionalized CDs through charge transfer. The resultant linear detection ranges from 10 nM to 1.5 µM with a fast response time of 30 s. Glassy carbon electrode modified with CDs exhibits fine capability for TNT reduction with the linear range from 5 nM to 30 µM, better than that obtained by fluorescent method. Moreover, the minimum distinguishable response concentration with respect to these two methods is down to the nanomolar level with a high specificity and sensitivity.

As a kind of environmental contaminants, explosive nitroaromatic compound 2, 4, 6-trinitrotoluene (TNT) has drawn the social concern. In World Wars, huge amounts of TNT were released into the soil, lakes and rivers. Nowadays, TNT continues to be a major ingredient of ordnance in military and terrorism, causing environment detriment and threatening public security.1-3 Moreover, TNT is toxic to creatures. It is not only the growth inhibiter to fungi, bacteria and plants but also a mutagenic carcinogen to the human beings.3 Exposure to TNT may cause pancytopenia, a disorder of the bloodforming tissues in humans and other mammals.4, 5 Thus, developing practicable analytical platforms to monitor ultratrace levels of TNT is urgent and necessary. Carbon dots (CDs), as an emerging type of carbon nanomaterials, have attracted dramatic interest in recent years for their extraordinary optical, electronic and biochemical properties,6 such as green synthesis,7-11 excellent solubility,7, 10 easy functionalization, stable photoluminescence (PL), high electrochemical activity.12, 13 low toxicity8 and good biocompability.14 As a green substitute for the toxic transition metal quantum dots, CDs have found their more promising applications in a broad range of areas such as electrocatalysis,12, 13 bioimaging,14, 15 sensing,16-18 electrochemiluminescence,19, 20 drug/gene delivery,21 and other optoelecteonic field.11, 22, 23 Among the variety of reported approaches for CDs synthesis, micro-assisted pyrolysis of proper biomass is a rapid and convenient route for mass production.10 In this work, we successfully synthesized N-rich CDs by one-step microwave porolysis of citric acid (CA) with the existence of passivation reagent, ethylenediamine (EDA). Thanks to the introduction of the primary amine group, the asprepared CDs possess excellent water-solubility, ground-

ing for their extensive applications in analysis and bioanalysis fields. It is well known that strong acid-base pairing interaction happens between the electron-rich amino groups and electron-deficient nitroaromatic rings.24-26 TNT, a typical electron−deficient nitro compound, can selectively interact with primary amines to form TNT-amine complexes through the charge transfer from the donor (amine group) to the acceptor (nitroaromatic group). This specific interaction makes it feasible to design the surface chemistry of nanomaterials for the selective and sensitive detection of TNT.26-28 In this context, fluorescent detection platform has been developed on the basis of the interaction between TNT and the amine group on CDs, which quenches the photoluminescence of CDs. Additionally, attributed to the electrochemical activity of CDs, an electrochemical TNT sensor has also been constructed based on the concentration effect of amine group on CDs towards TNT. Thus, for the first time, both the fluorescent and electrochemical detection of TNT have been realized with the use of water-soluble CDs.

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Figure 1. (A) TEM image (inset: high resolution TEM image) and (B) XPS spectrum of the as-synthesized CDs; (C) the PL spectra of the as-synthesized CDs: excitation spectrum (a) was obtained at the emission wavelength of 455 nm and the emission spectra were obtained with the excitation wavelength of 280 nm (b), 300 nm (c), 320 nm (d), 390 nm (e), 340 nm (f), 380 nm (g), 350 nm (h), 370 nm (i) and 360 nm (j); (D) EIS obtained at the bare GC electrode (a) and the CDs/GC electrode (b).

The synthesis of CDs is according to the literature10 with a little modification (in supporting information). Figure 1A displays the transmission electron microscope (TEM) image of the as-prepared CDs. It is found that they are mono-disperse nanoparticles with the diameter of ca. 3 nm. The high resolution TEM image (inset of Figure 1A) demonstrates that CDs are highly crystalline and the lattice fringes with a spacing of 0.21 nm assigned to (100) facet of graphitic carbon, suggesting a nanocrystalline sp2 carbon structure.29 The component of CDs is characterized by X-ray photoelectron spectroscopy (XPS) as shown in Figure 1B. The result shows that CDs contain C, O, and N elements and the ratio of them are 59.92%, 27.05%, 13.03%, respectively. In addition, Fourier transformed infrared spectroscopy (FTIR) is used to characterize the formation of surface functional groups, seen in Figure S1 in supporting information. The broad bands around 3300 cm-1 indicated the vibrations of O-H and NH and the peaks at 2922 cm-1 and 2854 cm-1 are attributed to –CH2 vibrations. The peaks at 1664 cm-1, 1560 cm-1, 1380 cm-1 and 1051 cm-1 were assigned to the amide I (C=O), amide II (N–H), C-N and C-O vibrations respectively. The PL spectra of the synthesized CDs indicate that upon the optimal excitation of 36o nm, a strong emission peak at 455 nm is observed. Electrochemical impedance spectroscopy (EIS) is used to assess the charge-transfer resistance (Rct) at the electrode surface, which is represented by the semicircle diameter of EIS. As illustrated in Figure 1D, CDs modified glassy carbon electrode (noted as CDs/GC electrode) exhibits much lower Rct than bare GC electrode does, suggesting the improved electron flux and accelerated electron shuttle between the electrolyte and the electrode substrate.

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Figure 2. (A) The emission spectrum of CDs (a) and absorption spectra of EDA (b), TNT (c) and EDA-TNT mixture (d) with their corresponding photos in the inset; (B) The PL kinetics of CDs with the corresponding photos before (a) and after (b) 0.5 µM TNT addition; (C) The PL intensity responding to different concentrations of TNT and the corresponding calibration plotting (a-j: blank, 1 nM, 10 nM, 50 nM, 150 nM, 250 nM, 500 nM, 1 µM, 1.5 µM and 2.5 µM); (D) The PL changes responding to different interferences of 5 µM as well as 0.5 µM TNT.

To evaluate the feasibility of quenching-based TNT detection, an assay to illustrate TNT-amine interaction was first carried out. Figure 2A shows the absorption spectra of EDA, TNT and their mixture, respectively. Compared to TNT and EDA, a new absorption peak with wide range from 460 nm to 520 nm appears in the mixture and the formation of chromophoric complex is a rapid process, verifying the strong interaction between the electrondeficient aromatic ring of TNT and amino groups. The emission peak at 455 nm of CDs overlaps well with the absorption spectrum of TNT-amine complex, leading to charge transfer from fluorescent CDs to the aromatic rings of TNT and consequently quenching the fluorescence emission of CDs strongly. In order to achieve sensitive TNT detection, some factors should be optimized. The concentration of CDs is selected as 25 µg mL-1 and ultrapure water is chosen for the quenching assays rather than buffer solutions (Figure S2 and S3). Afterwards, quenching kinetics test for 0.5 µM TNT is implemented (Figure 2B). Apparently, the reaction achieves a plateau in ca. 30 s and such a fast sensing process favors for the rapid detection in real-time analysis system. Figure 2C displays the TNT concentration-dependent fluorescence change of CDs. It is found that the fluorescence gradually decreases with the continuous addition of TNT due to the continuous formation of TNT−amine complex. The variation of fluorescence is linear over the concentration of TNT from 10 nM to 1.5 µM (R2=0.997, inset of Figure 2C) and the detection limit is estimated to be 1 nM. The selectivity of this sensing platform was assessed by testing its tolerance to high concentration of TNT analogues, such as nitrobenzene (NB), 2-nitrotoluene (2-NT), 4nitrotoluene (4-NT), 2,4-dinitrotoluene (2,4-DNT) and 2,6-dinitrotoluene (2,6-DNT). As shown in Figure 2D, the quenching ratio for these analogues of 5 µM is much lower than that for 0.5 µM TNT, suggesting neglectable interference in the real sample analysis.

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Figure 3. (A) DPVs obtained at the bare GC electrode (a, b) and CDs/GC electrode (c, d) in 0.1 M pH 7.0 N2-saturated PBS (containing 0.2 M KCl) with (b, d) and without (a, c) 5 µM TNT; (B) DPVs responding to different concentrations of TNT at CDs/GC electrode with the magnified low concentration region in the inset (a-t: blank, 1 nM, 5 nM, 10 nM, 20 nM, 30 nM, 80 nM, 130 nM, 230 nM, 330 nM, 830 nM, 1.33 µM, 2.33 µM, 3.33 µM, 4.33 µM, 9.33 µM, 14.33 µM, 19.33 µM, 29.33 µM,and 39.33 µM); (C) The plotting linear relationship between peak currents at -0.37 V and TNT concentrations; (D) Chronoamperometric response obtained at CDs/GC electrode to successive injection of 5 µM NB (a), 2NT (b), 4-NT (c), 2, 4-DNT (d), 2, 6-DNT (e) and TNT at a constant potential of -0.37 V.

Electrochemical methods have found their broad applications in small molecule sensing due to their advantages of reasonable sensitivity, rapid response, low cost, simple procedure and ease of miniaturization. A variety of carbonaceous materials have exhibited satisfied electrochemical activity and regarded as the ideal electrode mateirials, such as pyrolytic graphite, carbon nanotubes, carbon nanofibres, reduced graphene oxide, etc. CDs, a newcomer to the carbonaceous materials family, also possessed favorable merits in electrochemical applications, which have not drawn enough attention to date. Herein, CDs/GC electrodes are prepared for electrochemical reduction of TNT. Amino functionalization enables CDs on electrode surface to enrich enough TNT molecules through the strong charge transfer interaction between TNT and CDs, leading to enhanced detection of TNT. Figure 3A compares the differential pulse voltammograms (DPVs) responding to 5 µM TNT at bare GC electrode and CDs/GC electrode. Three well-defined reduction waves are assigned to the stepwise reduction of three different nitro groups of TNT to the corresponding hydroxylamine and amine. The reduction current of the first wave at -0.37 V recorded at CDs/GC electrode is ca. 4-times higher than that at bare GC electrode. It is assumed that, except for the strong amine-TNT interaction, small quantum dots size and high degree of graphitization also contribute to the excellent catalytic activity of CDs. DPV responses to different concentrations of TNT are recorded in Figure 3B. In low concentrations, only the first wave can be observed. Within the increase of TNT addition, the other two peaks appear sequentially and the peak currents of the three waves increase in proportion with TNT concentration. As shown in Figure 3C,

plotting concentration dependent peak currents obtained at -0.37 V yields a good linear relationship ranging from 5 nM to 30 µM (R2 = 0.998, N=3), and according to the minimum distinguished curve in the inset Figure 3B, the limit of detection is 1 nM. The electrochemical selectivity was evaluated by comparing the DPV responses of a series of potential interferences at the CDs/GC electrode (Figure S5). It is found that the peak number is equal to the amount of nitro groups on the nitroaromatics. NB, 2NT and 4-NT give a reduced peak located at -0.60 V, 0.67 V and -0.62 V, respectively, and 2, 4-DNT and 2, 6DNT exhibited two reduction peaks. It is worthy to note that even though those reduction peaks overlap with each other seriously, the peak at -0.37 V assigned to TNT is independent and far away from other peaks. Such a significant difference guarantees the high specificity. Chronoamperometric curve obtained at -0.37 V (Figure 3D) depicts that the addition of those interferences wound not impact the detection of TNT, confirming its excellent capability to distinguish TNT from others. In addition, the stability was tested by comparing the electrocatalytic behaviors of the as-prepared CDs/GC electrode before and after one week (Figure S6), it is found that its activity can maintain ca. 96%, suggesting the satisfied stability. Table 1 shows the previous reports with respect to fluorescent and electrochemical detection of TNT based on different kinds of nanomaterials. In comparison with these results, the present electrochemical system possesses wider detection range, even though the comparable limit of detection. To estimate the practicality in real sample analysis, ultrapure water is replaced by tap water to prepare the buffer solution in which extra 50 nM target analyte TNT was added. The recovery in five parallel measurements ranges from 98.3% to 103.1%, suggesting the good practicality and reproducibility in real sample determination.

In summary, N-rich CDs have been synthesized successfully through a microwave-assisted pyrolysis method and applied in both the fluorescent and electrochemical TNT determination. The fluorescent sensing platform is established on the strong TNT-amino interaction which can quench the PL of CDs through charge transfer. The resultant linear detection ranges from 10 nM to 1.5 µM with a fast response time of 30 s. GC electrode modified with CDs exhibits fine capability for TNT reduction with the linear range from 5 nM to 30 µM, better than that obtained by fluorescent method. Moreover, the mini-

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mum distinguishable response concentration with respect to these two methods is down to nanomolar level with a high specificity, which is not only attributed to the high intrinsic activity and large surface area of CDs but also beneficial from the accumulation effect of amino functionalization. In comparison to fluorescent determination, electrochemical method does not require expensive instruments, thus is more practical and powerful in the field of chemical analysis.

ASSOCIATED CONTENT Supporting Information Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Tel: +86-431-85262101; Fax: +86-431-85689711

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (No. 21375123) and the 973 Project (No. 2011CB911002).

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Simple and sensitive fluorescent and electrochemical trinitrotoluene sensors based on aqueous carbon dots.

Aqueous N-rich carbon dots (CDs), prepared by the microwave-assisted pyrolysis method, are applied as a dual sensing platform for both the fluorescent...
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