Talanta 144 (2015) 1036–1043

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The use of tungsten disulfide dots as highly selective, fluorescent probes for analysis of nitrofurazone Xinrong Guo a, Yong Wang a, Fangying Wu a, Yongnian Ni a,b,n, Serge Kokot c a

Department of Chemistry, Nanchang University, Nanchang 330031, China State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang 330047, China c School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Brisbane 4001, Australia b

art ic l e i nf o

a b s t r a c t

Article history: Received 2 May 2015 Received in revised form 15 July 2015 Accepted 19 July 2015 Available online 19 July 2015

Tungsten disulfide (WS2) is a two-dimensional transition metal dichalcogenide, which is of particular interest because it has highly anisotropic bonding, which leads to strongly anisotropic electrical and mechanical properties. Thus, in this work, a simple hydrothermal process was developed to produce photoluminescence from WS2 dots. This was achieved in the presence of sodium tungstate and reduced L-glutathione; the emitted fluorescence produced a quantum yield as high as 0.066. The WS2 dots and the associated fluorescence were investigated with the use of transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared and UV–vis spectroscopies. The WS2 dots were used as a fluorescent probe to analyze nitrofurazone (NFZ). The associated fluorescence resonance energy transfer (FRET) mechanism was also investigated and the emitted fluorescence was found to be linear in the range of 0.17–166 μmol L  1 with a detection limit of 0.055 μ mol L  1. The proposed method was successfully applied for analysis of NFZ in nasal drops and water samples. & 2015 Elsevier B.V. All rights reserved.

Keywords: Tungsten disulfide dots Fluorescence analysis Nitrofurazone

1. Introduction Graphene is a two-dimensional (2D) crystalline substance, which has unique electronic, thermal, optical, mechanical and chemical properties, and it has been widely used since its discovery in 2004 [1–3]. These unique properties depend on the atomic-layer thickness of graphene and its 2D morphology. Such properties have stimulated research for other kinds of 2D nanomaterial. These materials are generally inorganic substances similar to graphite; particularly, they are substances, which are layered materials of ultrathin nanosheets of transition metal dichalcogenides (TMDCs) with strong in-plane bonding and weak out-of-plane interactions. Such structures facilitate exfoliation into two-dimensional layers of single unit cell thickness, which has been widely used in a many applications including solid lubricants, catalysts, batteries, various nanoelectronic and energy storage devices [4–7]. To date, a number of methods have been developed to prepare TMDC nano-sheets, -tubes and -particles. These methods include liquid exfoliation [8,9], hydrothermal synthesis [10,11], pyrolysis [12], chemical vapor deposition [13] and n Corresponding author at: Department of Chemistry, Nanchang University, Nanchang 330031, China. Fax: þ 86 791 3969500. E-mail addresses: [email protected] (Y. Ni), [email protected] (S. Kokot).

http://dx.doi.org/10.1016/j.talanta.2015.07.055 0039-9140/& 2015 Elsevier B.V. All rights reserved.

electrochemical Li-intercalation [14]. TMDCs have a wide variety of mechanical and electronic applications with semiconductors of MoS2 [15,16], MoSe2 [17,18], WS2 [19,20] and WSe2 [21,22]. In particular, the WS2 material consists of 2D covalently bonded S– W–S layers separated by a van der Waals gap. Also, weak van der Waals forces hold the adjacent sulfur sheets together with an S– W–S layer sequence [5,23]. Recently, WS2 has attracted particular attention because of its highly anisotropic bonding influences the strongly anisotropic electrical and mechanical properties, which are useful in catalytic reactions [19], as solid state lubricants [24], solar cells [25] and field-effective transistors [26]. Also it has been noted that the mono-layered WS2 sheets, which are characterized by strong luminescence, change from indirect to direct band gap semi-conductors; this occurs when WS2 is converted from multi- to monolayer sheets [9,26]. Such unique physical and chemical properties of the mono-layered sheets are observed because the WS2 can be significantly affected by the edge structure and atomic defects associated with small nanostructures [27,28]. Elsewhere [26], bulk WS2 was exfoliated to form monolayers of WS2 quantum dots (QDs) with side lengths in the range of 8–15 nm; such WS2 QDs fluoresce strongly and this suggests the presence of direct band gaps. However, while the above information is readily available, comparatively little is known about the optical properties of WS2 materials, especially those concerned with photoluminescence

X. Guo et al. / Talanta 144 (2015) 1036–1043

(PL). Nitrofurazone (NFZ, 5-nitro-2-furaldehyde semicarbazone) is a nitrofuran, which is the first therapeutic 5-nitrofuran drug [29]. NFZ is an effective fungicide, amoebicide and skin sensitizer, which resists various allergies and infections related to animals and humans [30]. It can also be used for the treatment of enteritis, and boils caused by coli or salmonella bacteria. Therefore, NFZ is still used as a broad-spectrum, antimicrobial drug [31,32]. However, NFZ is often found in animal food and thus, it can find its way into muscles, liver and milk of animals as residual. It has serious effects on human health [33]. In addition, this drug is potentially carcinogenic and mutagenic, and can promote mammary tumor growth in mammals [31]. Consequently, rapid and sensitive analytical methods for NFZ are required, particularly in the fields of bioassay and chemistry. In this context many methods have been developed: spectrophotometry [34], chromatography [35], high performance liquid chromatography (HPLC) with fluorescence detection (HPLC-FLD) [36], voltammetry [30], FI-CL method [37] and fluorescence [38]. In this work, a completely new approach was proposed. Thus, the initial aim was to synthesize novel photo-luminescent WS2 dots, which could emit strong fluorescence with a high quantum yield. This was to be achieved with the use of a hydrothermal process involving sodium tungstate and reduced L-glutathione. Given that the latter step was successful, a novel photoluminescence sensor containing the WS2 dots was to be constructed and used as a fluorescent probe for the quantitative analysis of NFZ in water and in other samples.

2. Experimental

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fluorescence decay time of the WS2 dots was recorded on an F900 Fluorimeter (Edinburgh Analytical Instruments, Livingston, UK). A Thermo Nicolet FTIR fluorescent 380 spectrometer (Thermo Electron Scientific Instruments, Madison, USA) was used to measure the Fourier transform infrared spectra. 2.3. Synthesis of WS2 dots The WS2 dots were synthesized by a simple hydrothermal process similar to the synthesis of MoS2 in our laboratory [7,11]. Sodium tungstate (Na2WO4  2H2O; 0.0625 g) was dissolved in 12.5 mL water, and then 0.1 M HCl was added to adjust the pH to 6.5. This mixture and 1.268 g reduced GSH were transferred to a beaker containing 50 mL water. After a 5 min ultra-sonication, the mixture was transferred to a 100 mL Teflon-lined stainless steel autoclave, which was sealed tightly, and heated at 220 °C for 24 h. After cooling in air, the supernatant WS2 dots were collected after being centrifuged for 15 min at 12,000 rpm. The WS2 dots were then stored in a refrigerator at 4 °C. 2.4. Analysis of NFZ with the use of the WS2 dots WS2 dots (1.93 ng mL  1, 200 μL) and 2.5 mL, pH 7.0 B-R buffer (0.04 mol L  1) were sequentially added into a quartz cuvette, and the solution was mixed thoroughly. An NFZ (50 μL) solution was added to the above mixture, and the whole solution was again mixed thoroughly at room temperature (25 70.5 °C). In total, there were 23 such solution samples, each at a different concentration in the range of 0.17–166 μmol L  1. A quenched fluorescence spectrum of each solution was collected in the 340– 550 nm range at the excitation wavelength of 320 nm. A calibration plot for NFZ was obtained by this procedure.

2.1. Chemicals and materials Sodium tungstate (Na2WO4  2H2O, Hongqi Chemical Co., Jiangsu, China), reduced L-glutathione (GSH, Aladdin Chemistry Co., Shanghai, China), and nitrofurazone (NFZ, Sigma-Aldrich Co., Shanghai, China) were purchased from local chemical shop and their aqueous solutions with suitable concentration were prepared as needed. A series of Britton-Robinson (B-R) buffers with different pHs, was prepared by mixing 0.04 mol L  1 mixed acid made of 85% H3PO4, CH3COOH and H3BO3 and 0.2 mol L  1 NaOH in different proportions. In general, the chemicals used were Analytical Grade reagents, and all glassware was cleaned sequentially with chromic acid solution and aqua regia; then the glassware was thoroughly rinsed with double distilled water at least three times. Freshly double distilled water was used in all experiments. 2.2. Apparatus UV–vis absorption spectra were recorded on an Agilent 8453 Spectrophotometer (Agilent Technologies, Santa Clara, CA, USA). The fluorescence measurements were made on an LS-55 Luminescence spectrometer (Perkin Elmer Co., MA, USA). X-ray diffraction (XRD) spectra were collected with the use of a Bede D1 high-resolution X-ray diffractometer system (Bede Co., UK). Transmission electron microscopy (TEM) images, and selected area electron diffraction (SAED) measurements were obtained with a JEM-2010 instrument (JEOL Co., Japan), and the operational settings of the associated point, linear resolutions and the accelerating voltage were set to 0.23 nm, 0.14 nm and 200 kV, respectively. X-ray photoelectron spectroscopy (XPS) measurements were made with the use of an ESCALAB-MKII Spectrometer (VG Scientific Ltd., UK) using Al-Kα radiation. Fluorescence of WS2 dots was observed by exposing its solution to a ZF-2 UV lamp (Qilinbeier Instrumental Co., Haimen, China) set to 365 nm. The

3. Results and discussion 3.1. Characterization of the prepared WS2 dots The WS2 dots, which were synthesized by the hydrothermal method, were characterized by several techniques, including XRD, TEM, XPS and FT-IR. The XRD spectra of WS2 dots (Fig. 1A) clearly showed three major diffraction peaks, i.e. the reflections at 14.1°, 33°, and 39.8°, which corresponded to the (002), (100), and (103) planes. The relatively weak intensity of these peaks indicated that the as-prepared WS2 dots contained only a few crystals and displayed a hexagonal phase structure [39]. The reflection at 22.3° was assigned to the GSH compound (Fig. S1, Supplementary data) [40]. A typical TEM image of the WS2 dots (Fig. 1B) and the particle size distribution histogram (top inset) showed that the surface structure was not particularly uniform and was covered with tiny particles, which varied in size between 6 and 14 nm (average value: 10.35 nm). These observations are consistent with those from previous studies [26]. Additionally, the selected area electron diffraction (SAED) pattern (bottom inset) of the WS2 dots further indicated the presence of a polycrystalline crystal structure. The XPS technique was performed to provide more information on the chemical states of W and S in the WS2 dots; the spectra (Fig. 1C) displayed the elemental peaks of W and S, together with Na, N, C and O from the reaction compounds involved in the hydrothermal synthesis. High-resolution XPS spectra for W 4f and S 2p were also collected (see Fig. 1D and E). As shown in Fig. 1D, the W 4f spectrum consisted of three peaks, and two of these at about 33.5 and 34.1 eV were characteristic of peaks representing the W 4f7/2 and W 4f5/2 binding energy lines, respectively, i.e. for the W (IV) oxidation state in the WS2 dots [5,26,39]. The peak at

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Fig. 1. (A) XRD pattern of a sample of WS2 dots; (B) TEM image of the WS2 dots. Insets: (1) particle size distribution histogram of WS2 dots, and (2) the SAED pattern; (C) XPS survey spectrum of WS2 dots; (D) high-resolution peak-fitting XPS spectra of the W 4f region from WS2 dots; (E) high-resolution peak-fitting XPS spectra of the S 2p region from WS2 dots; (F) FT-IR spectrum of WS2 dots.

approximately 35.7 eV could be attributed to the W 4f7/2 binding energies, which originated from the W(VI) state in the tungsten oxide [26,41]. Also, there were two peaks of the S 2p spectrum (Fig. 1E), which arose from the 161.7 and 163.0 eV. These peaks were attributed to the S 2p3/2 and S 2p1/2 binding energy lines of the WS2 dots, i.e. for the S(II) oxidation state [7,26,42]. Also, the W: S molar ratio was 1:2.3, which confirmed the formation of WS2 dots. The FT-IR spectrum of the WS2 dots was presented together with the one from the GSH (Fig. 1F). For this compound, the peaks at 3127 and 3024 cm  1 were assigned to the N–H (NH3 þ ) stretching band (νN–H), and that at 2524 cm  1 to the S–H stretching band (νS–H) [43]. Also, the peaks at 1712 and 1598 cm  1 were assigned to the C ¼O stretching band of the carboxylic group (νC ¼ O), and a strong N–H deformation peak from the amide bond (δN–H) was present at 1537 cm  1 [44]. It was noted that the characteristic peak of the S–H stretching band disappeared when WS2 dots were formed in the presence of GSH, and the peaks at νN–H and δN–H shifted to 3120 and 1600 cm  1, respectively. Also, the peak for νC ¼ O became relatively broad. These results suggested that GSH coordinated with W(IV) ions to form the WS2 dots. Thus, the binding involved the sulfhydryl, amino and the carboxylic groups [44]. 3.2. Optical properties of WS2 dots A two-dimensional contour map (Fig. 2A) was obtained from typical excitation-emission matrices (EEMs) collected from the WS2 dots spectra – λex ¼320 nm and λem ¼ 405 nm. Excitationemission fluorescence spectra (Fig. 2B) from WS2 dots samples indicated that in the 260–420 nm range the emission increased up to 320 nm and then decreased quite rapidly; additionally, the fluorescence emission peak showed a corresponding shift from 400 to 460 nm. UV–vis absorption and fluorescence spectra from the WS2 dots

(Fig. 2C) showed an absorption peak at 297 nm [26], and a strong emission fluorescence peak at about 405 nm. The associated excitation wavelength was 320 nm as descried above. Also, the WS2 dots solution emitted strong blue fluorescence on exposure to UV light at 365 nm (inset, Fig. 2C). The quantum yield (QY) of WS2 dots was calculated to be 0.066 on excitation at 324 nm (Table 1); quinine sulfate was used as the fluorescent standard (QY of the quinine sulfate¼0.546) [7]. This QY value was larger than that from the carbon quantum dots (CQDs) – 0.019 [45], MoS2 QDs (0.026) [7] and the exfoliation of WS2 QDs (0.004) during the associated synthesis in solution [26]. Additionally, WS2 dots not only had the strong fluorescence, but also were very stable; even after being stored for 3 months in a refrigerator at 4 °C, the samples still exhibited a strong fluorescence. 3.3. WS2 dots as a fluoresce probe and the mechanism of the analysis of NFZ During the synthesis of WS2 dots with the use of the hydrothermal procedure, GSH was the sulfur donor, which released the H2S and also reduced the Na2WO4. In addition, this compound participated in the formation of the WS2 dots (see Scheme 1). The dots, when submitted to UV irradiation, emitted strongest blue fluorescence at the excitation wavelength of 320 nm. After adding the NFZ, the fluorescence intensity at 405 nm was significantly reduced, and no blue fluorescence peak was evident. It appeared that the absorption band of the NFZ was effectively overlapped by the emission band of WS2 dots (Fig. 3A). These observations suggested that significant fluorescence quenching occurred after the addition of the NFZ, and this indicated that a fluorescence resonance energy transfer (FRET) had occurred between the WS2 dots and the NFZ (Fig. 3A) [46]. The UV–vis spectra of WS2 dots (Fig. 3B) showed that in the presence of NFZ, the theoretical absorption spectrum of WS2 dots was very similar to the experimental one, and the intensities of

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these two absorption bands were very similar. The results implied that there was only a weak interaction between the WS2 dots and NFZ; apparently, no Meisenheimer complex was formed [7]. Therefore, the possibility of a charge-transfer mechanism between WS2 dots and NFZ can be excluded. To obtain further understanding of the fluorescence quenching mechanism, the decay time measurements of WS2 dots in the presence of NFZ were investigated (decay profiles-Fig. 3C). In the absence of NFZ, the WS2 dots fluorescence had a lifetime of 4.78 ns, which decreased to 3.79 ns after the addition of NFZ. This result revealed that the quenching mechanism of WS2 dots in the presence of NFZ was the outcome of dynamic quenching [47]. 3.4. Optimization of experimental conditions To improve the performance of the sensor based on the WS2 dots for the detection of NFZ, two experimental conditions were optimized-the reaction time and the pH of the medium. It was observed that after the addition of NFZ to the reaction mixture, the fluorescence intensity of WS2 dots was quenched quite rapidly and the quenching efficiency reached 40% as compared to that observed just in the presence of the WS2 dots (Fig. S2(A), see Supplementary material). In the former case, i.e. in the presence of the NFZ, fluorescence remained constant for at least 10 min. This indicated that the quenching process was stable and rapid. The effect of pH on the fluorescence intensity of the WS2 dots was also investigated. The fluorescence intensity of the WS2 dots in the presence or absence of NFZ increased slowly with the increase of pH from 2.0 to 4.0. Thereafter the fluorescence intensity remained almost constant in the pH range of 4.0–9.0. This suggested that WS2 dots were less affected in mildly acidic or basic medium, (Fig. S2(B), Supplementary material). Thus, when the different values of the fluorescence intensity of the WS2 dots samples with or without the NFZ were compared, pH 7.0 was chosen as the optimal value [38]. 3.5. Analytical performance of the sensor containing WS2 dots

Fig. 2. (A) Two-dimensional contour map obtained from typical excitation-emission matrices (EEMs) of WS2 dots. The arrow represented the position of the fluorescence peak; (B) fluorescence emission spectra of WS2 dots at different excitation wavelengths (spectral range: 260–420 nm); (C) UV–vis and fluorescence spectra of WS2 dots. Inset: Photo of a fluorescing WS2 dots aqueous solution on irradiation at 365 nm. Table 1 Quantum yield of WS2 dots. Sample

Quinine sulfate WS2 dots

3.6. Application of the WS2 dot sensor for the detection of NFZ in various samples

Excitation (nm) Absorbance Integrated em. intensity 324 324

Under the optimized experimental conditions, NFZ solutions of different concentrations were added to the B-R buffer (pH 7.0) and the WS2 dots solution. Fluorescence intensity of each mixture was recorded at 405 nm, and it was found that it decreased with increasing concentration of NFZ; a gradual spectral red-shift was observed. This was ascribed to the FRET effect (Fig. 4A), which indicated that the F0/F (quenching ratio) versus NFZ concentration was almost linear in the range of 0.17–166.0 μM range, and the regression equation was: F0/F¼ 0.0413cNFZ (μM) þ 1.072. The correlation coefficient was 0.9996 (n ¼3) and the limit of detection (LOD) was 0.055 μM (S/N¼3). Compared with some traditional methods for the same analysis (Table 2), the novel method described in this work produced somewhat better results with LOD and its linear range; also, it was quite rapid and simple to perform. When compared with some other fluorescence sensor methods [38,48], the behavior of WS2 dots as a fluorescent probe showed a much better detection limit. Also, it should be noted that WS2 dots are environmentally friendly and harmless in the presence of biological cells or organisms [26].

QY (%)

0.0106

4.31  105

54.6

0.0108

4

6.60

5.21  10

The photoluminescence sensor based on the WS2 dots was applied for the analysis of NFZ in nasal drops, which were obtained from a Chinese medicine shop. Under the optimized experimental conditions, a series of nasal drops of different concentration were added to 2.95 mL of a solution containing the WS2

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Scheme 1. The proposed fluorescent sensing process for the analysis of NFZ; the process was based on the quenching effect observed in the presence of the WS2 dots.

dots (1.54 ng L  1), and adjusted to pH 7.0 with the B-R buffer (0.04 mol L  1). It was found that the fluorescence intensity at 405 nm decreased with increasing concentration of the NFZ analyte in the nasal drop samples (Fig. 4B), and a plot of the quenching ratio F0/F versus concentration of the above mentioned analyte in the nasal drops was linear in the range of 0.33– 26.56 μM. The regression equation was

F0/F = 0.0409cNFZ (μM) + 1.084. The correlation coefficient was 0.998 (n¼ 3). Compared with the standard plot described above, the slopes of the standard and measured regression lines were quite similar, and the concentration of NFZ in the nasal drop sample was found to be 0.20 70.05 mg mL  1. The sensor based on the WS2 dots was also applied for the analysis of spiked a water samples. Standard addition solutions of NFZ at different concentrations were added to the lake water (Table 3). The %Recovery results of these spiked water samples were calculated according to the standard curve, which ranged from 98.2% to 99.4%. Also, the relative standard deviation (RSD) ranges were less than 5%. These results indicated the novel method described above was a practical approach for the analysis of NFZ in water samples. 3.7. Application of the WS2 dots sensor for the detection of NFZ – an interference study Residues of the potentially harmful NFZ have been detected in the muscle, liver and milk samples of animals [33]; there were many common sources of this compound including animal feed, water and soil, among others. While the sensor based on the WS2 dots may be used satisfactorily for the detection of NFZ, there could be some interfering substances presented in the analytical samples. In general, such potential interferences included metal and non-metal ions as well as common organic compounds. Thus, in this work a range of such interfering agents were tested-metal ions: Ca2 þ , Pb2 þ , Li þ , Cd2 þ , K þ , Mg2 þ , Ba2 þ , Mn2 þ , Zn2 þ , Al3 þ and NH4 þ (each 1  10  4 M), Fe3 þ and Ce4 þ (8.1  10  6 M), Ni2 þ (3.3  10  4 M), Co2 þ (6.67  10  5 M), Fe2 þ (1.62  10  5 M); amino acids: proline (Pro), aspartic acid (Asp), arginine (Arg), leucine (Leu), valine (Val), threonine (Thr), glutamine (Gin), histidine (His), glycine (Gly), Asparagine (Asn), serine (Ser), methionine (Met), isoleucine (Ile), glutamate (Glu), phenylalanine (Phe), alanine (Ala)

and tyrosine (Tyr) (each 3.3  10  4 M). For each amino acid at the above specified concentration, the relative error was less than 5% in all cases, Fig. S3 (Supplementary material). This diagram showed the effect of each interfering substance as a normalized spectral intensity, (F0–F)/F0. It was evident that most of the interference effects with the majority of metal ions were small, even at 16 fold concentration level. Most of the amino acids also showed only a small interference effect at the 40 fold concentration level. The interferences of the metal ions were summarized in Table 4. It was shown that the proposed method could be used to determine NFZ in natural samples for the interferences of metals ions (with low level concentrations) were small. It should be noted that Fe2 þ , Co2 þ , Fe3 þ and Ce4 þ , even at low concentration levels, had a small interfering effect, which may be attributed to the formation of the complexes between the WS2 dots and the interfering substances. A significant decrease in selectivity of the WS2 dots sensor for the analysis of NFZ was noted in the above four cases. In general, the proposed method, which uses the WS2 dots as a fluorescent probe, is simple, inexpensive, highly sensitive and selective for the detection of NFZ.

4. Conclusions A simple hydrothermal process was used to develop a method of analysis for the important compound, Nitrofurazone, (NFZ), which is used as a treatment against fungicides, amoebicides and other skin ailments. NFZ is also, effective against various allergies and infections related to animals, humans and livestock. The method involves the emission of blue fluorescence from WS2 dots with the use of sodium tungstate and reduced L-glutathione; the associated fluorescence quantum yield was found to be as high as 0.066. Importantly, this fluorescence was quenched by the NFZ analyte. The new WS2 dots fluorescence method had a linear range of 0.17–0.166 μmol L  1, and the detection limit was 0.055 μ mol L  1. In addition, the proposed method successfully analyzed NFZ in nasal drops and lake water samples.

Acknowledgments The authors gratefully acknowledge the financial support of this study by the National Natural Science Foundation of China,

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Fig. 4. (A) Fluorescence emission spectra of WS2 dots (1.93 ng mL  1) collected at different concentrations of the NFZ solution in pH 7 B-R buffer (0.04 mol L  1). NFZ concentrations were: 0.0, 0.17, 0.33, 0.66, 0.10, 1.33, 1.66, 3.32, 4.98, 6.64, 8.30, 9.96, 11.62, 13.28, 14.96, 16.60, 19.92, 23.24, 26.56, 29.88, 33.20, 49.80, 66.40, 99.60 and 166 μmol L  1. Inset: a calibration plot – (F0/F) versus concentration of NFZ (F0 and F-fluorescence intensities without or with NFZ, respectively); (B) Fluorescence emission spectra of WS2 dots (1.93 ng mL  1) in the presence of the NFZ nasal drops at different concentrations (0, 0.33, 0.66, 0.10, 1.33, …, 26.56 μmol L  1). Inset: calibration plot – (F0/F) versus concentration of NFZ (F0 and F-fluorescence intensities recorded from the nasal spray samples with or without NFZ). Table 2 Analysis of NFZ – a performance comparison of the new WS2 dots method with other available ones. Fig. 3. (A) Spectral overlap between the emission spectrum of WS2 dots (1.93 ng mL  1) and the absorption spectrum of NFZ (16.7 μmol L  1). Inset: photos of an aqueous solution containing WS2 dots being irradiated at 365 nm in (a) the absence, and (b) the presence of NFZ; (B) UV–vis spectra of the WS2 dots and the NFZ solution; note-theoretical and experimental profiles were the sum of the WS2 dots and NFZ spectra; (C) Time-resolved fluorescence decay of WS2 dots, which was recorded in the presence or absence of NFZ (10 μmol L  1).

China (NSFC–21365014 and NSFC–21305061), the Natural Science Foundation of Jiangxi Province, China (20132BAB213011 and 20132BAB203011), the Education Department Science Foundation of Jiangxi Province, China (GJJ13026), and the State Key Laboratory of Food Science and Technology of Nanchang University (SKLF– ZZA201302).

Methodsa

Dynamic range (μ mol L  1)

LOD (μ mol L  1)

Recovery (%) Refs.

DPV FI-CL Fluorescence Fluorescence HPLC-DAD-MS HPLC-DAD WS2 dots

2.5–37.5 0.5–50.0 1.0–1000 6.0–800 0.25–0.51 2.5–13.0 0.17–166

0.80 0.10 0.80 4.50 0.25 – 0.05

100.0 100.0 99.9 99.4 499.0 499.9 100.0

[30] [37] [38] [48] [49] [50] This work

a DPV ¼differential pulse voltammetry, FI-CL¼ flow injection chemiluminescence.

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Table 3 Recoveries of NFZ (μM) from spiked water samples (n¼ 3). [11] Sample

Detected Added (m mol L  1)

Longtenga NDb ND ND

3.32 8.30 16.6

Found (m mol L  1)

Recovery (%) RSD (%)

3.29 8.15 16.5

99.0 98.2 99.4

[12] 1.50 1.20 3.60

a Water samples were collected from the Longteng lake located in the Nanchang University campus. b Not detected.

Table 4 Effect of coexisting metal ions on analysis of NFZ (8.3  10  6 mol L  1). Ions

Concentration (mol L

[13]

[14]

[15]

[16]

1

) [17]

None Ca2 þ Pb2 þ Li þ Cd2 þ Kþ Mg2 þ Ba2 þ Mn2 þ Zn2 þ Al3 þ NH4 þ Fe3 þ Ce4 þ Ni2 þ Co2 þ Fe2 þ a

6

NFZ of 8.3  10 M (1.00)a 1.0  10  4 (0.97) 1.0  10  4 (0.99) 1.0  10  4 (0.97) 1.0  10  4 (0.99) 1.0  10  4 (0.97) 1.0  10  4 (0.98) 1.0  10  4 (0.97) 1.0  10  4 (0.96) 1.0  10  4 (0.98) 1.0  10  4 (1.02) 1.0  10  4 (0.97) 8.1  10  6 (0.96) 8.1  10  6 (1.03) 3.3  10  4 (0.96) 6.67  10  5 (1.01) 1.62  10  5 (0.98)

3

1.0  10 (0.63) 1.0  10  3 (0.67) 1.0  10  3 (0.74) 1.0  10  3 (0.75) 1.0  10  3 (0.74) 1.0  10  3 (0.86) 1.0  10  3 (0.71) 1.0  10  3 (0.84) 1.0  10  3 (0.85) 1.0  10  3 (1.22) 1.0  10  3 (0.82) 6.67  10  5 (0.73) 6.67  10  5 (1.13) 9.9  10  4 (0.93) 1.0  10  4 (1.12) 4.8  10  5 (0.90)

[18]

[19]

[20]

[21]

[22]

[23]

The values in the brackets are calculated as relative peak height (F/F0).

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.talanta.2015.07. 055.

[24] [25]

[26]

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The use of tungsten disulfide dots as highly selective, fluorescent probes for analysis of nitrofurazone.

Tungsten disulfide (WS2) is a two-dimensional transition metal dichalcogenide, which is of particular interest because it has highly anisotropic bondi...
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