Article Cite This: Anal. Chem. 2018, 90, 3942−3949
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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* Key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China S Supporting Information *
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 planar MoS2 quantum dots (QDs) as reporters. Excitation-independent emitting MoS2 QDs were first fabricated by using the proposed ultrasonic−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, 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 QDsdecorated test paper shows an instantaneous (within 1 min) response to trace amounts of deposited TNT, and the fluorescence quenching can even be well-visualized by the naked eye. Because of favorable analytical performances, the proposed MoS2 QDsbased TNT sensing system has potential applications in both environmental water monitoring and security screening.
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nanocrystals (or quantum dots (QDs)) have been wellemployed as fluorescent reporters for TNT sensing, because of their excellent optical properties and sophisticated surface modification.11−19 Despite these achievements, there are a few issues and even problems still drawing concern. 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 gold nanoparticle−QDs (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 a system would greatly enrich the nanoassay kit, and well promote the applications of nanomaterials in the field of analytical chemistry.
ecause of its high-powered strength, 2,4,6-trinitrotoluene (TNT) has been widely employed in mining, military industries, and even terrorist explosive attacks.1 Furthermore, as a deleterious substance, it is registered in the U.S. Environmental Protection Agency’s list of priority pollutants for environmental remediation.2 Therefore, the development of sensing platforms for TNT analytical detection has always attracted considerable research efforts.3−5 For different detection goals, the sensing systems should possess corresponding features. For example, for security screening, ultratrace TNT residual deposited on various surfaces (luggage cases, packages, envelopes, fingerprints) is expected to be rapidly detected. Therefore, 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 TNT molecules contained in substances 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 parameters. Among various methods/techniques, fluorescent sensing has been extensively studied, because of its favorable properties and versatile applications.6−10 With regard to various fluorophores (organic small molecules, polymers, and inorganic nanoparticles (NPs)), inorganic © 2018 American Chemical Society
Received: November 26, 2017 Accepted: February 11, 2018 Published: February 11, 2018 3942
DOI: 10.1021/acs.analchem.7b04893 Anal. Chem. 2018, 90, 3942−3949
Article
Analytical Chemistry
addition to bulk solution, it is competent to detect traces of TNT contamination (2 ng) on substrates using the corresponding test paper. Because of favorable analytical performances, the proposed MoS2 QDs-based system has been successfully used for assaying traces of TNT with different states, including its presence in natural water samples and deposited on luggage surfaces.
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 consist of 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 Until now, fluorescent MoS2 QDs have been employed for sensing and bioimaging applications.31−34 We envision that MoS2 QDs, especially fabricated using a top-down method, are one of appropriate fluorescent reporters for TNT sensing. In addition to being Cdfree, their flat shape interacts more favorably with aromaticring-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 the highly sensitive assay of TNT explosives in both environmental water samples and luggage cases, using fewlayered MoS2 QDs as fluorescent reporters. To this end, we first present an ultrasonic−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 and 2.0 ± 0.3 nm thick. Notably, the as-prepared MoS2 QDs exhibit excitation-independent emission behaviors (emission peak at ∼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 is enhanced from 3.2% to 5.6%. The QDs’ emission can be selectively and strongly quenched by TNT molecules, based on electron transfer effects. (See Scheme 1.)
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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 Model F-4600 fluorescence spectrophotometer with the excitation wavelength being 380 nm. A Hitachi Model U2910 spectrometer was used to record the ultraviolet−visible light (UV-vis 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 Model PE-983 FT-IR spectrophotometer. Transmission electron microscopy (TEM) photographs were taken using a Model HT-7700 microscope (Hitachi) 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. Xray photoelectron spectroscopy (XPS) measurements were carried out on an Axis Ultra DLD spectrometer. The thicknesses of the 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 ultrasonic−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 water. Then, the ultrasonic process was conducted for 6 h in an ice bath, to maintain its temperature. Last, the mixture was transferred into a 20 mL Teflon-lined stainless steel autoclave, followed by the addition of 9 mL of 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 kDa 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 of cysteine and 9 mL of 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. A buffer solution volume of 100 μL of PBS (0.01 M, 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, different concentrations of TNT
Scheme 1. (A) Schematic Presentations of Fabrication Processes; Modification of (B) the MoS2 QDs and (C) the Resultant MoS2 QDs for TNT Sensing
The fluorescence responses of the MoS2 QDs to TNT analytes are more dramatic, compared with TNT analogues (2,4dinitrotoluene (DNT), nitrobenzene (NB), trinitrophenol (TNP)) and common ions. The detection limit is as low as 1 nM, which is substantially more sensitive than that of Cd-based QDs systems. The fluorescent quenching is independent of the QDs’ assembly/aggregation, and the quenching is instant (within 1 min) for substrate-based interactions. Therefore, in 3943
DOI: 10.1021/acs.analchem.7b04893 Anal. Chem. 2018, 90, 3942−3949
Article
Analytical Chemistry
Figure 1. Characterizations of the pristine MoS2 QDs. (A) TEM and (B) HRTEM 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 versus excitation wavelength. (H) Fluorescence lifetime of the MoS2 QDs.
assistant agents. Because no 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 the (100) faces of MoS2 material. Based on AFM measurement (Figure 1C), the height of the particles is ∼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 ultrasonic and hydrothermal processes is effective for topdown 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 not been clear at the 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 is located at 398 nm (see the 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 sulfate as a reference. Figure 1F is a series of emission spectra excited by different excitonic wavelengths. For excitonic wavelengths of
