Planar Is Better: Monodisperse Three Layered MoS2 Quantum Dots as

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Planar Is Better: Monodisperse Three Layered MoS2 Quantum Dots as Fluorescent Reporters for 2, 4, 6-Trinitrotoluene Sensing in Environmental Water and Luggage Cases Hui Zhu, Hui Zhang, and Yunsheng Xia Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04893 • Publication Date (Web): 11 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018

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Analytical Chemistry

Planar Is Better: Monodisperse Three Layered MoS2 Quantum Dots as Fluorescent Reporters for 2, 4, 6-Trinitrotoluene Sensing in Environmental Water and Luggage Cases Hui Zhu, Hui Zhang, and Yunsheng Xia* Key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China.

ABSTRACT: In this study, we present a simple but effective fluorescent system for highly sensitive and versatile sensing of 2, 4, 6-trinitrotoluene (TNT) using few layered planner MoS2 quantum dots (QDs) as reporters. Excitation independent emitting MoS2 QDs were first fabricated by the proposed ultrasound-hydrothermal based top-down method assisted by carbon-free hydroxylamine hydrochloride. The obtained pristine MoS2 QDs were then modified with cysteine for introducing amino groups as TNT binding sites. The as-prepared MoS2 QDs possess a planar structure, which can more adequately interact with flat aromatic TNT molecules due to π-π attraction and decreased steric effects, as compared with traditional spherical/quasi-spherical QDs. As a result, they exhibit extremely high sensitivity for TNT sensing (1 nM and 2 ng for solution and substrate assay, respectively). The common ions containing in environmental water samples do not interfere with the sensing. Furthermore, the QDs decorated test paper shows an instantaneous (within 1 min) responses to trace amounts of deposited TNT, and the fluorescence quenching can even be well visualized by naked eyes. Due to favorable analytical performances, the proposed MoS2 QDs based TNT sensing system has potential applications in both environmental water monitoring and security screening. 1

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KEYWORDS: Layered MoS2 quantum dots; 2, 4, 6-Trinitrotoluene (TNT); Fluorescent sensing; Environmental water monitoring; Security screening

INTRODUCTION 2, 4, 6-Trinitrotoluene (TNT), due to its high-powered strength, has been widely employed in mining, military industries, and even terrorist explosive attacks.1 Furthermore, as one of deleterious substances, it is registered in the U.S. Environmental Protection Agency’s list of priority pollutants for environmental remediation.2 So, the development of sensing platforms for TNT analytical detection has always attracted considerable research efforts.3-5 For different detection goals, the sensing systems ought to possess corresponding features. For example, for security screening, ultra-trace TNT residual deposited on various surface (luggage cases, packages, envelopes, fingerprints) is expected to be rapidly detected. So, the sensing systems should be competent for solids/substrates detection, and several characteristics, such as sensitive, on-site, real-time, portable, as well as easily signal readout are needed. While in environmental monitoring, the sensing processes are often conducted in solution. Considering that the concentrations of containing TNT molecules are often rather low, and the related samples (from soil, various environmental water samples, etc.) are usually complex, high sensitivity and selectivity are becoming critical. Among various methods/techniques, fluorescent sensing has been extensively studied due to its favorable properties and versatile applications.6-10 Towards various fluorophores (organic small molecules, polymers and inorganic NPs), inorganic nanocrystals, or named as quantum dots (QDs) have been well employed as fluorescent reporters for TNT sensing because of their excellent optical properties and sophisticated surface modification.11-19 Despite these 2


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achievements, there are a few issues and even problems still be concerned. First of all, conventional QDs are spherical/quasi-spherical in morphology, and their large curvature is adverse to sufficiently interacting with flat aromatic TNT molecules. As a result, the detection sensitivity is limited. Then, to enhance the sensitivity, we presented several AuNP-QDs hybrid assembly based platforms,20,21 and the signal readout results from replacement reaction induced disassembly by TNT analytes. However, these systems can only work in solution and their applications are accordingly restricted. Third, most previously used QDs (CdS, CdSe, and CdTe based materials) contain Cd ions, whose high toxicity hampers their applications. Therefore, it is urgent to explore eco-friendly QDs based sensing systems, in which TNT analytes can be highly sensitive and versatilely assayed for different types of samples. Such system would greatly enrich the kit of nano-assay, and well promote the applications of nanomaterials in the fields of analytical chemistry. MoS2, as one of layered materials, has attracted much interest because of its electronic, optical, and catalytic properties.22-24 Different from large-area MoS2 monolayers, MoS2 QDs made by few-layer MoS2 with reduced size and well-defined shape, which possesses enhanced fluorescent, catalytic, photocatalytic, and electrochemical efficiencies due to quantum confinement effects.25-30 Up to now, fluorescent MoS2 QDs have been employed for sensing and bio-imaging applications.31-34 We envision that MoS2 QDs, especially fabricated by top-down method, are one of appropriate fluorescent reporters for TNT sensing: In addition to Cd-free, their flat shape is more favorably interact with aromatic ring based TNT molecules, as compared with spherical ones. Such enhanced interaction is promising for higher sensitivity. In this study, we have developed a facile but effective sensing system for highly sensitive assay of 3



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TNT explosive in both environmental water samples and luggage cases, using few layered MoS2 QDs as fluorescent reporters. To this end, we first present an ultrasound-hydrothermal based top-down method for the fabrication of monodisperse three-layer MoS2 QDs from bulk MoS2 layer with the assistance of hydroxylamine hydrochloride. The obtained products are 5.0 ± 0.4 nm in diameter with 2.0 ± 0.3 nm thickness. Notably, the as-prepared MoS2 QDs exhibit excitation independent emission behaviors (emission peak ~ 398 nm), which is different from most previous excitation dependent MoS2 QDs. To better interact with TNT molecules, the pristine QDs were further modified with cysteine molecules for introducing amino groups as TNT binding sites. The resulting cysteine modified MoS2 (cysteine@MoS2) QDs can well preserve their morphology, only the emission efficiency enhanced from 3.2 to 5.6%. The QDs’ emission can be selectively and strongly quenched by TNT molecules based on electron transfer effects. The fluorescence responses of the MoS2 QDs to TNT analytes are more dramatic as compared with TNT analogues (2,4-dinitrotoluene (DNT), nitrobenzene (NB), trinitrophenol (TNP)) and common ions. The detect limit is as low as 1 nM, which is substantially more sensitive than that of Cd-based QDs systems. The fluorescent quenching is independent on the QDs’ assembly/aggregation, and the quenching is instant (within 1 min) for substrate based interactions. So, in addition to bulk solution, it is competent to detect trace of TNT contamination (2 ng) on substrates using the corresponding test paper. Due to favorable analytical performances, the proposed MoS2 QDs based system has been successfully used for assaying trace of TNT with different states, including in natural water samples and deposited on luggage surface. Scheme 1.

4


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EXPERIMENTAL SECTION Materials. Hydroxylamine hydrochloride, MoS2 powder, humic acid and L-cysteine were purchased from Aladdin. TNT, DNT, NB, TNP and nitromethane were supplied by Sigma-Aldrich. NiSO4·6H2O, CuSO4·5H2O, Zn(NO3)2·6H2O, CdCl2·2.5H2O, AgNO3, KCl, NaCl, FeCl3, CaCl2, Li2CO3, FeSO4·7H2O, CoCl2·6H2O, Pb(NO3)2, Cr3NO3, Al2(SO4)3·18H2O, MnSO4·H2O, MgSO4, NaHCO3 and BaCl2·2H2O were acquired from Shanghai Chemical Reagent Co. All solutions were prepared with deionized water (18.25 MΩ·cm). Apparatus. Fluorescence spectra were recorded by a Hitachi F-4600 fluorescence spectrophotometer with excitation wavelength being 380 nm. A Hitachi U-2910 spectrometer was used to record the UV−visible spectra. Fluorescence lifetime was recorded on a Fluorolog-3 spectrofluorometer (Horiba JobinYvon). Fourier transform infrared (FT-IR) spectra were measured from a KBr window on a PerkinElmer PE-983 FT-IR spectrophotometer. Transmission electron microscopy (TEM) photographs were taken with a HT-7700 Hitachi microscope at an accelerating voltage of 100 kV. High-resolution TEM (HRTEM) characterizations were carried out by Tecnai G2 20 ST (FEI) under the accelerating voltage of 200 kV. The solutions were analyzed for particle sizes and ζ-potential values using dynamic light scattering (DLS, Zetasizer Nano ZS series, Malvern Instruments) with 633 nm laser wavelength. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an Axis Ultra DLD spectrometer. The thicknesses of MoS2 QDs were measured by atomic force microscopy (AFM) using a Bruker Dimension Icon AFM. Fabrication of the Pristine MoS2 QDs. The as-prepared MoS2 QDs were fabricated through an ultrasound-hydrothermal method using hydroxylamine hydrochloride as stripping agents. In brief, 0.06 g of MoS2 powder and 0.2 g of hydroxylamine hydrochloride were dispersed in 9 mL of 5



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water. Then, ultrasonic process was conducted for 6 h along ice bath for keeping its temperature. Last, the mixture was transferred into a 20 mL Teflon-lined stainless steel autoclave follow by added 9 mL water. The resulting solution was further reacted at 200 °C for 12 h. Purification of the Pristine MoS2 QDs. First, the mixture was centrifugated at 8000 rpm for 10 min to remove bulky MoS2. Second, the supernatant was further filtered twice through a 0.22 µm nylon filter. Finally, the obtained product was dialyzed for 24 h through a 1000 kD filter. Modification of the MoS2 QDs. The purified MoS2 QDs (5 mL) were transferred into a 20 mL Teflon-lined stainless steel autoclave. Then, 0.05 g cysteine and 9 mL water were added, and the mixture was further reacted at 100 °C for 12 h. The resulting cysteine@MoS2 QDs were stored at 4 °C for the research of their microstructure and optical properties. Procedures for TNT Sensing. 100 µL of PBS (0.01 M, pH = 7.0) buffer solution and 40 µL of purified cysteine@MoS2 QDs (24 ng mL-1) were placed in a series of 5 mL centrifuge tubes. Then, different concentrations of TNT solution were added. The resultant solution was diluted to 1.7 mL and mixed thoroughly. After incubation for 20 min, their absorption spectra were recorded at ambient conditions. Procedures for TNT Sensing in Environmental Water Samples. Tap water (from the lab), and pond water (from Jinghu Lake, Wuhu), were measured after three times filtration using 0.22 µm filters. For the sensing, PBS (0.01 M, 100 µL) buffer solution (pH = 7.0) and 40 µL of purified cysteine@MoS2 QDs (24 ng mL-1) were placed in a series of 5 mL centrifuge tubes. Then, 50 µL of pure or TNT spiked water samples were first added. The resultant solution was diluted to 1.7 mL and mixed thoroughly. After incubation for 20 min, their fluorescence spectra were measured. Visual Detection of TNT Residues on Luggage Surface. First, a piece of filter paper (D = 25 6


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mm) was immerged into the cysteine@MoS2 QDs (1.28 ng mL-1). After 10 min, the filter paper was removed from the solution and kept still in dark for drying. Then, 4 µL of dilute TNT solution with different concentrations (0, 0.5, 2.5, 12.5, 50 µg mL-1) were dropped on a luggage, respectively. The amounts of TNT deposited on the substrates were estimated based on their concentration and volume. Last, the paper was pressed on the contaminated spots. The fluorescence quench responses of the indicating paper were observed under a UV lamp (8 W, λmax = 365 nm).

RESULTS AND DISCUSSION In terms of top-down method for MoS2 QDs fabrication, organic small molecules (ethanol,33,35 N,N-dimethylformamide,34 N-methyl-2-pyrrolidone,36) and an elevated temperature are often adopted. At high temperature, these organic molecules probably polymerize, carbonize and form carbon nanodots. Obviously, such byproducts would strongly disturb the optical property study and even applications of MoS2 QDs because carbon nanodots possess similar size and fluorescence

emission.

To

avoid

this

potential

problem,

we

herein

present

an

ultrasound-hydrothermal based approach for top-down fabrication of mondisperse MoS2 QDs from bulk MoS2 layer using hydroxylamine hydrochloride as assistant agents. Because no any organic molecules are employed, the formation of emitting carbon nanodots can be completely excluded. Figure 1A shows a typical large scale TEM image of the obtained MoS2 QDs. The diameter of the products is 5.0 ± 0.4 nm with only 8% size distribution. The highly paralleled and ordered lattice fringe with 0.27 nm d-space is observed in the HRTEM image (Figure 1B), which corresponds to 7



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the (100) faces of MoS2 material. Based on AFM measurement (Figure 1C), the height of the particles is about 2.0 ± 0.3 nm, indicating that the products are three-layered MoS2 QDs.37 Obviously, with the assistance of hydroxylamine hydrochloride, the combination of ultrasound and hydrothermal processes is effective for top-down fabrication of monodisperse MoS2 QDs. Herein, the roles of hydroxylamine hydrochloride might be similar to that of H2SO4 molecules.38 However, at present, their exact stripping effects have been not clear at molecular level. As described in Figure 1E (black curve), the UV-Vis spectrum has a distinct absorption peak at 220 nm, which is assigned to the excitonic features of MoS2 QDs. By using a 300 nm excitation, the MoS2 QDs exhibit a symmetric emission profile, and the peak locate at 398 nm (red curve in Figure 1E). Correspondingly, the MoS2 QDs containing solution shows a bright blue fluorescence under a 365 nm UV lamp. Their emission quantum yield is 3.2% using quinine sulphate as reference. Figure 1F is a series of emission spectra excited by different excitonic wavelength. As excitonic wavelength < 280 nm, a weak peak appears at shorter wavelength (~ 300 nm), which shows a gradual bathochromic shift with the increase of the excitonic wavelength. Obviously, such signals come from the scattering effect of the particle containing solution. It should be noted that the emission peak of the MoS2 QDs is at around 400 nm, which is almost independent on the excitation wavelength. Such property is obviously different from most previously reported excitation-dependent

emitting

MoS2

QDs.

Herein,

the

MoS2

QDs

possess

an

excitation-independent emission property, which is probably attributed to two reasons. First, the MoS2 QDs possess uniform size. According to quantum confinement effect, emission wavelength would be shifted to longer wavelength with the increase of particle size. As shown in Figures 1A and 1B, the size distribution of the as-prepared particles is only 8%, which is in favor of a fixed 8


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emission wavelength. Second, the products are pure MoS2 QDs. As described in Experimental Section, no any organic molecules were employed in the present fabrication system. As a result, the obtained products do not contain the impurity of carbon nanodots. It is well known that one of distinct properties of carbon nanodots is excitation-dependent emission wavelength. Conceivably, if some carbon nanodots by-products formed (from the carbonization of the assistant organic molecules) during the fabrication of MoS2 QDs, the study of MoS2 QDs’ emission property would be substantially confused. In the present work, our study is focused on the sensing performances of planar MoS2 QDs for TNT analytes, and the emission property of MoS2 QDs (excitation-dependent or excitation-independent emitting wavelength) will be systematically investigated in next work. Figure 1.

The as-prepared MoS2 QDs have a layered morphology, which should more facilitate to interact with planar structured molecules due to decreased steric effect, π-π interactions, etc. To test this hypothesis, we investigated their interactions with TNT molecules. As shown in Figure S2A in Supporting Information, even for the pristine MoS2 QDs, their fluorescence can be observably quenched as the concentration of the added TNT is as low as 50 nM, which is almost compared with some modified (possess binding sites for TNT analytes) QDs based fluorescence systems. Such fluorescent quenching effects might be attributed to the π-π binding effects between the layered MoS2 QDs and planar TNT molecules, as indicated by a noticeable DLS size increase (Figure S2B in Supporting Information). These results indicate that the interactions of MoS2 QDs and TNT molecules are rather dramatic. 9



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To further enhance the sensitivity, we then modified the MoS2 QDs by cysteine, because NH3 groups can strongly interact with TNT molecules and form Meisenheimer complexes by electron transfer effects. FT-IR technique was first used for detecting the modification processes. As shown in Figure S3 in Supporting Information, a series of signals from various groups within cysteine molecules17 (such as N-H stretching vibrations (3190 cm−1), C-H stretching vibrations (2969 cm−1), C=O asymmetric stretching vibrations (1604 cm-1) and C-N stretching vibrations (1350 cm-1)) are distinctly observed in the modified products; at the same time, S-H symmetric stretching band (2550 cm-1) disappears. After modification, the ζ-potential values of the QDs containing solutions change from +0.8 to -15 mV (Figure S4 in Supporting Information). These results preliminarily indicated that cysteine molecules bound onto the MoS2 QDs’s surface by S-Mo bond.17 To further demonstrate cysteine modification, XPS characterization was conducted. As described in Figures 2A and 2B, after cysteine modification, a new S-C bond at 161.5 eV (green curve in Figure 2B) is observed. Furthermore, AFM technique was then employed for studying the modification. As shown in Figure 2C, the modified products are spherical and well dispersed on Si substrate; Furthermore, the line scan shown in the insets of Figure 2C indicates that the particles’ height is 2.5 ± 0.5 nm, which is some larger than that of the pristine ones (2.5 vs. 2.0 nm). Obviously, both mean detection (FT-IR, ζ-potential and XPS) and individual particle measurement (AFM) clearly demonstrated that the MoS2 QDs are successfully modified by cysteine molecules. As shown in TEM and HRTEM images (Figure 2E and its inset), the MoS2 QDs can well keep their shape and structure after cysteine modification. After modification, the DLS size slightly increase from 4.8 to 6.5 nm (Figure 2F), which on the one hand, further demonstrates the successful cysteine modification; and on the other hand, indicates that the MoS2 QDs are well dispersed and do not 10


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aggregate each other. Compared with the pristine MoS2 QDs, the modified products possess an enhanced fluorescence emission (The QY increases from 3.2 to 5.6%) and a slightly longer emission peak (from 398 to 413 nm). Accompanied by steady fluorescence changes, the emission life time perceptibly decreased from 4.8 to 4.2 ns (Figures 1H and 2H). Qualitatively, the evolutions of the above fluorescence parameters can be understood as: the cysteine modification processes decrease the surface traps/defects of the pristine MoS2 QDs, which causes the QY enhancement, emission bathochromic-shift, as well as lifetime decrease.39,40 Similar with the pristine MoS2 QDs, the modified products also possess excitation-independent emitting behaviors (Figure S5 in Supporting Information). For convenience, the cysteine modified MoS2 are named as cysteine@MoS2 QDs in the following sections. Figure 2.

We then studied the interactions of the cysteine@MoS2 QDs with TNT molecules. It is known that the benzene ring of TNT molecule is highly electron deficient due to the synergistic electron-withdrawing effect of its 2,4,6-trinitro substituent groups. As a result, it can react with electron-rich primary amine and form Meisenheimer complex (Figure 3A). Because of this reaction, primary amine groups often act as binding sites for TNT sensing. As shown in the TEM image (Figure S8 in Supporting Information), the cysteine@MoS2 QDs do not aggregate in the presence of TNT, which is further demonstrated by in-situ DLS measurements (After reaction with TNT analytes, the DLS size of the modified MoS2 QDs only slightly increases from 6.5 to 7.7 nm, bottom of Figure 2F vs. Figure 3B). AFM technique was further employed to study their interactions. As shown in Figure 3C, the MoS2 QDs well disperse on the substrate, which is in 11



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well agreement with TEM and DLS observations. At the same time, the QDs’ height increases from 2.5 to 3.0 nm, indicating the binding of the QDs and TNT molecules. As compared with the pristine MoS2 QDs, the modified ones can be more dramatically quenched by TNT molecules. As described in Figure 3E, the cysteine@MoS2 QDs exhibit an observable fluorescence decrease even the concentration of the added TNT is as low as 2 nM. Along with steady state fluorescence quenching, the emission lifetime also well quenched from 4.2 to 1.5 ns (Figure 2H vs. Figure 3F), indicating a strong electron transfer from the QDs to TNT analytes.16 There is a good linear relationship (IF = 499.92 - 73.15 lgCTNT, R = 0.992.) between the fluorescence intensities and TNT concentrations in the range of 2 to 800 nM (Figure 3G). The detection limit can low to 1 nM, which is substantially more sensitive than that of electron transfer quenching systems, using traditional Cd-based QDs as reporters.11-14,41-51 Because both the MoS2 QDs and TNT molecules have planner shape, which is in favor of their sufficient contact and causes more effectively electron transfer fluorescence quenching. In contrast, previous Cd-based QDs often possess a spherical/quasi-spherical shape, and their large curvature is probably adverse to their sufficient interact with TNT molecules. Compared with TNT, other three analogues (DNT, NB, and TNP, Figure 3H and Figures S9A-S9C in Supporting Information) and non-aromatic nitro-containing molecule (nitromethane, Figure S9D in Supporting Information) only exhibit very slight quenching effects at same conditions, probably due to their weaker electron deficiency and/or decreased interactions with the MoS2 QDs. Figure 3.

In the following, the sensing application potentials of the modified MoS2 QDs were investigated. First, we assessed TNT molecules monitoring in environmental water samples. The corresponding 12


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interference experiments were studied. As shown in Figures 4A and 4B, some familiar transition/heavy metal ions and the common eight ions in natural water do not interfere with TNT sensing as their concentrations are dozens of times higher than that of the analytes. Although Pb2+ and Hg2+ with tens of µM concentration level exhibit a little fluorescence quenching for the modified MoS2 QDs, which would do not cause virtual disturbance due to their concentrations are often rather low at common conditions. In some cases, environmental water might contain humic acid, the interference test was accordingly conducted. As shown in Figure S10A in Supporting Information, humic acid can cause observable fluorescent quenching as their concentrations reach several to tens of µg mL-1 levels. Fortunately, the average molecular weight of humic acid is much larger than that of TNT molecule (4700-3040052,53 vs. 227). So, the containing humic acid in water samples can be removed by ultrafiltration processes, which effectively eliminates their interference (Figure S10B in Supporting Information). Then, the sensing of TNT in several water samples (pond and tap water samples) was further investigated, using the cysteine@MoS2 QDs as reporters. As shown in Figures 4C and 4D, almost no fluorescent responses are observed until the samples are spiked with some TNT. Furthermore, the fluorescence quenching degrees are in agreement with the analyte concentrations, even the added TNT is as low as 10 nM (relative errors < 13%). These results demonstrate that this method has potential in environmental applications for TNT sensing. Figure 4. Figure 5.

It is documented that TNT explosives tend to contaminate and remain adhering to the package surface even by extremely cautious handling. Thus, instant on-site visual detection of trace TNT 13



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deposited on various packages is very crucial for security-screening needs. It is known that deposited TNT residues can be captured by the formation of Meisenheimer complexes and lifted from the surface to a TNT-indicating paper, which provides a facile way for the corresponding detection. Because the present sensing principle is based on (TNT-MoS2 QDs) binding instead of analytes modulated particle aggregation/assembly states, it is probably competent for security check applications by substrate-based detection. For this purpose, we first prepared the cysteine@MoS2 QDs containing test paper. As shown in Figure S11, after the immersion in MoS2 QDs’ solution for 10 min, the filter paper can emit the characteristic blue fluorescence. We then employed the cysteine@MoS2 QDs-treated paper for the detection of TNT deposited on the surface of luggage cases. For security-screening, test speed is very critical. As shown in Figure 5B, trace of TNT can cause a substantially fluorescence quenching within 1 min, such quick response is significant for security check applications. Conceivably, for test paper based assay, the interactions of TNT and the MoS2 QDs is instant and do not need a diffusion process, which probably causes the rapid responses. As shown in Figure 5C, even 2 ng of TNT residue can cause an observable fluorescence quenching of the test paper, which is several times better than Cd-based QDs detection systems.12 Then, the quenching spots become darker and darker with the gradual increase of the amounts of deposited analyte targets. Such ultrahigh sensitivity and quality dependent quenching efficiency for deposited TNT residues are very appropriate to security-screening applications. Finally, it is found that the test papers possess a few differences in color hues as they are illuminated by different angled UV light (vertical and parallel for Figures 5B and 5C, respectively.). The exact reason is not clear at present. However, the different angled 14


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UV illumination does not impact the observation for the TNT quenched spots (Figure 5B vs. Figure 5C).

CONCLUSIONS In summary, we present an ultrasound-hydrothermal system for fabricating planar, monodisperse, and excitation independent emitting MoS2 QDs, which are then acted as fluorescent reporters for TNT sensing. Compared with conventional QDs based TNT sensing systems, the present one is not only eco-friendly, but possess higher sensitivity because of more adequate interaction of layered MoS2 QDs and the analyte targets. Due to favorable analytical performances, the proposed platform shows application potentials in environmental water sample monitoring and security screening.

AUTHOR INFORMATION Corresponding Author *Fax: +86-553-3869303. Phone: +86-553-3869303. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Nos. 21775004 and 21422501) and Foundation for Innovation Team of Bioanalytical Chemistry.

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Scheme 1. Schematic Presentations of Fabrication Processes (A), Modification (B) of the MoS2 QDs, and the Resultant MoS2 QDs for TNT Sensing (C).

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Figure 1. Characterizations of the pristine MoS2 QDs. TEM (A) and HRTEM (B) and images of the MoS2 QDs. (C) AFM image (the inset is the corresponding height profile analyses) of the MoS2 QDs. (D) Histograms of the size and height distribution results, which were obtained from 100 particles, respectively. (E) UV-vis absorption and fluorescence spectra of the MoS2 QDs. (F) Emission spectra of the MoS2 QDs excited by different wavelengths. (G) Plots of emission peaks vs. excitation wavelength. (H) Fluorescence lifetime of the MoS2 QDs.

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Figure 2. Characterizations of the cysteine@MoS2 QDs. XPS spectra of the pristine (A) and the cysteine modified (B) MoS2 QDs. (C) AFM image (the inset is the corresponding height profile analyses) of the cysteine@MoS2 QDs. (D) Histograms of the size and height distribution results, which were obtained from 100 particles, respectively. (E) TEM and HRTEM (inset) images. (F) DLS sizes of the pristine (up) and the cysteine modified (down) MoS2 QDs. (G) Fluorescence emission spectra of the pristine and the cysteine modified MoS2 QDs. (H) Fluorescence lifetime of the cysteine@MoS2 QDs.

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Figure 3. TNT sensing using the cysteine@MoS2 QDs as fluorescent reporters. (A) UV-vis absorption spectra of TNT (0.5 mM) in the absence and presence of cysteine (1.2 mg). (B) DLS size of the cysteine@MoS2 QDs after reaction with TNT. (C) AFM image of the cysteine@MoS2 QDs (the inset is the corresponding height profile analyses) (D) Histograms of size and height distribution results after reaction with TNT molecules, which were obtained from 100 particles, respectively. (E) Evolution of fluorescence spectra of the cysteine@MoS2 QDs (0.6 ng mL-1) with increasing TNT concentrations. (F) Fluorescence lifetime of the cysteine@MoS2 QDs in the presence of TNT molecules (100 µM). (G) Plots of the cysteine@MoS2 QDs fluorescence intensities versus TNT concentrations. (H) Fluorescence responses of the cysteine@MoS2 QDs to TNT and its three analogues (DNT, NB and TNP).

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Figure 4. (A) Fluorescence responses of the cysteine@MoS2 QDs (0.6 ng mL-1) to common transition/heavy metal ions. The concentration of Hg2+ is 25 µM and other ion concentrations are 100 µM. (B) Fluorescence responses of the cysteine@MoS2 QDs to eight common ions (All the ion concentrations are 100 µM.) in environmental water samples. Sensing of TNT in pond (C) and tap (D) water samples by the cysteine@MoS2 QDs. F0 and F are the fluorescence intensities of the cysteine@MoS2 QDs in the absence and presence of the corresponding substances, respectively. 24


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Figure 5. (A) The processes for performing visual detection of TNT residues on luggage surfaces using the cysteine@MoS2 QDs decorated filter paper. The lifted TNT residue is visualized under UV illumination. (B) Trace of TNT (100 ng) can cause a substantially fluorescence quenching by the reaction time for 0 min, 1 min, 5 min, 15 min, and 30 min, respectively, and (C) a luggage with amounts of 0, 2, 10, 50 and 200 ng, respectively. All the images were taken under the illumination of a 365 nm UV lamp.

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for TOC only:

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Planar Is Better: Monodisperse Three Layered MoS2 Quantum Dots as

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