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ZnS nanoparticles for high-sensitive fluorescent detection of pyridine compounds
Journal of Alloys and Compounds 559 (2013) 39–44
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ZnS nanoparticles for high-sensitive fluorescent detection of pyridine compounds Zhe Li, Jiantao Ma, Yanqing Zong, Yi Men ⇑ Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, PR China
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Article history: Received 28 August 2012 Received in revised form 13 January 2013 Accepted 16 January 2013 Available online 29 January 2013 Keywords: ZnS Nanoparticles Fluorescent sensor Pyridine
a b s t r a c t Water-soluble ZnS nanoparticles (NPs) capped with alpha-thioglycerol (TGO) have been synthesized through a chemical precipitation method. The nanoparticles were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), UV–Vis absorption spectroscopy, fluorescence spectroscopy, and fluorescence decay spectroscopy. Results showed that the TGO-capped ZnS NPs exhibited the cubic zinc blende structure, and the average size was found to be 2.94 nm. Compared with the bulk ZnS, the band-gap energy of the nanoparticles (4.40 eV) rose significantly due to the strong quantum confinement. The TGO-capped ZnS NPs showed a characteristic blue luminescence corresponding to two emission peaks at 419 nm and 460 nm associated with the defect states of the nanoparticles. Such functionalized nanoparticles can be used as fluorescent sensor for the determination of pyridine compounds because they quenched the fluorescence of the nanoparticles effectively. The detection limit was 6.76 105 M for pyridine. The quenching mechanism was studied in detail, and the results demonstrated the existence of dynamic quenching processes. The proposed sensing method is not only sensitive, simple, fast and low cost, but also meaningful for practical applications. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, nanostructure materials are not only at the very frontier of fundamental materials research, but they have also entered into people’s daily life step by step [1–3]. With the deteriorating trend of environmental pollution and the rising awareness of the public vulnerability to the chemical and biological threats, the detection techniques with both high sensitivity and reliability are demanded urgently. The application of nanostructure materials to the design of chemical sensors has attracted considerable attention due to their tremendous specific surface, high activity, excellent anti-photobleaching and tunable emission [4–6]. As a developing semiconductor material star, ZnS nanoparticles (NPs) are low toxicity materials with a wide band gap [7] of 3.72 eV (cubic zinc blende structure) and 3.78 eV (hexagonal wurtzite structure), exhibiting remarkable optical and electrical properties [8–11], which suggest that they may be particularly well-suited for the manufacture of novel sensor. At present, ZnS NPs can be used to detect the DNA molecular [12,13], protein [14], organic molecule [15–17], pH value [18,19], metal ion [20–23], etc. Pyridine and its derivatives are important chemical raw materials, which are mainly used as solvent and intermediate in the production of agricultural chemicals, dyestuffs, additives, drugs, and others. However, this foul-smelling and toxic substance could in⇑ Corresponding author. Tel./fax: +86 010 58805186. E-mail address: [email protected] (Y. Men). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.01.076
trude into the human living environment through the discharge of various industrial effluents and exhaust gases. Besides, pyridine and its derivatives are frequently found in the cigarette smoke [24], and also generated during the putrefaction processes of certain foodstuffs [25]. If inhaled, ingested or absorbed through skin, these chemicals can cause many potentially harmful effects on human body, which include nausea, vomiting, headaches, coughing, asthmatic breathing, laryngitis and even cancers [26]. Consequently, the detection of pyridine has become a heightened need for the health of human and the development of environment-friendly society. Various approaches have been established for the detection of pyridine, such as barbituric acid spectrophotometry [27], high-performance liquid chromatography [28,29], gas chromatography [30–32], liquid chromatography–mass spectrometry (LC–MS) [24], gas chromatography–mass spectrometry (GC–MS) [33,34], optical fiber sensing method [35], but the method based on the fluorescent of nanomaterials has not been reported so far. Herein, we report a very simple method for the synthesis of TGO-capped ZnS NPs with a narrow size distribution and good optical properties. Compared with other traditional methods, the preparation is a one-step approach using non-toxic and low cost raw materials, and the obtained nanoparticles are water-soluble, which could expand their applying areas particularly in chemical and biological fields. When the pyridine is added to TGO-capped ZnS NPs aqueous solution, it could attach to the surface of ZnS NPs, and result in a significant fluorescence quenching. Based on this phenomenon, a novel fluorescent sensor for the determination
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of pyridine compounds is proposed, and the performance of this ZnS NPs sensor exhibits high sensitivity, simple, fast and low detection limit. The possible mechanism of fluorescence quenching is also discussed. 2. Experimental The TGO-capped ZnS NPs were synthesized by chemical precipitation method as follows: 1.00 g Zn (CH3COO)22H2O (Tianjin Bodi Chemical Holding Co., Ltd.) and 1.20 g alpha-thioglycerol (TGO, Shanghai TCI Development Co., Ltd.) were dissolved in 285 mL secondary distilled water (homemade). The TGO acts as a capping agent to prevent the agglomeration of particles and stabilize the ZnS NPs. The pH value of the mixed solution was adjusted to 11.20 by 2 M NaOH solution. After about 30 min of bubbling nitrogen to drive off the oxygen from the reaction system, 10 ml of 0.214 M Na2S9H2O (Wako Pure Chemical Industries, Ltd.) solution was slowly injected to the above solution to yield a stoichiometric ratio of 0.47 for [S2]/[Zn2+]. The mixture was heated to reflux for 18 h. After cooling down to the room temperature naturally, the colorless and transparent liquid was concentrated to 20 ml through the rotary evaporator, and 100 ml ethanol was then added to generate white precipitations, which was followed by centrifuging and washing with ethanol several times. Finally, the products were dried in vacuum for 24 h. During the fluorescence measurement, the concentration of TGO-capped ZnS NPs aqueous solution was stabilized at 0.20 mg/ml, and 6.67 102 M Na2HPO4– KH2PO4 buffer solution (pH = 8.86) was chosen for the system to keep the pH value constant. A known concentration of pyridine solution was added into ZnS NPs solution and mixed entirely to monitor the change of fluorescence emission intensity. High-resolution transmission electron microscope (HRTEM) images were observed on a JEOL JEM-2010 electron microscope operated at 200 kV, and NanoMeasurer 1.2 software was used to evaluate the size of each nanoparticle. The wide-angle X-ray diffraction (XRD) measurement was determined by a PAN-alytical X’Pert PRO MPD X-ray diffractometer using a Cu Ka radiation source. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet-380 FT-IR spectrometer. UV–Vis absorption measurement of TGO-capped ZnS NPs was carried out with a Purkinje General TU-1901 UV–Vis spectrometer. Fluorescence spectra were achieved by a Varian Cary Eclipse fluorescence spectrophotometer at the room temperature. Fluorescence decay behaviors were measured by an Edinburgh FLS920 transient/steady-state luminescence spectrometer using a nanosecond flash lamp as excitation sources.
Scheme 1. Possible surface structure of TGO-capped ZnS NPs.
3. Results and discussion FT-IR spectra of TGO and TGO-capped ZnS NPs are shown in Fig. 1. A broad absorption band in the 3000–3696 cm1region is associated with the stretching vibrations of hydroxyl groups in bound water and TGO. The peaks at 2930 and 2879 cm1 are assigned to the stretching vibrations of C–H groups of TGO on the surface of ZnS NPs. The deformation vibrations of the methine and methylene groups in TGO located at 1633 and 1417 cm1 are also found. The characteristic vibrations at 1064 and 1035 cm1 belong to the C–O group in TGO. Moreover, the absorption peak of the S–H vibration in TGO at about 2554 cm1 cannot be observed, indicating that the sulfhydryl groups of the TGO have successfully attached to the surface of ZnS NPs [36], which leads the ZnS NPs to be water-soluble. Possible surface structure of TGOcapped ZnS NPs is shown in Scheme 1. X-ray diffraction studies were carried out to establish the ZnS crystallite structure and obtain an approximate average size of the ZnS NPs, as shown in Fig. 2a. The three peaks appearing at about 28.6°, 47.8°, and 56.6° respectively correspond to the (1 1 1), (2 2 0), and (3 1 1) planes of zinc blende crystal structure, which match well with the standard card (JCPDS 05-0566) denoted in Fig. 2b. As expected, the XRD peaks of ZnS NPs are prominently broadened compared to the corresponding bulk form, which is the characteristic of nanosized particles. The average size of the crystallite (D) was estimated from the line broadening of the XRD peaks by using the Debeye–Scherrer equation [37],
D¼
0:9k b cos h
Fig. 1. FT-IR spectra of TGO and TGO-capped ZnS NPs.
diffraction peaks expressed in radians, and h is the angle of diffraction. After the contribution from Ka2 radiation was removed by fitting a cubic spline via the MDI Jade 5.0 software, the calculated crystallite size is around 2.88 nm, and a comparison of this size with the size determined from HRTEM will be discussed later in the article. Fig. 3a shows a high-resolution transmission electron microscopy (HRTEM) image of TGO-capped ZnS NPs, revealing that the
ð1Þ
where k is the wavelength of X-ray radiation (for Cu Ka1 radiation, k = 1.5406 Å), b is the full-width at half-maximum (FWHM) of the
Fig. 2. (a) The wide-angle XRD pattern for TGO-capped ZnS NPs. (b) Standard XRD pattern of bulk ZnS with the zinc blende structure (JCPDS 05-0566).
Z. Li et al. / Journal of Alloys and Compounds 559 (2013) 39–44
sample is composed of an abundance of quasi-spherical particles. Another HRTEM image with a large number of ZnS NPs is presented in Fig. S1. Diameter distribution of TGO-capped ZnS NPs is summarized in Fig. 3b, which was obtained by measuring the diameter of 233 nanoparticles on several HRTEM images. The well-dispersed NPs have a narrow size distribution with a relative standard deviation of 6.7%. The average diameter is determined to be 2.94 nm, which is very close to the size calculated by XRD. Fig. 3c exhibits the enlarged HRTEM image of a single ZnS nanoparticle, the lattice fringes of nanocrystals in NPs have a spacing of nearly 0.31 nm, corresponding to the (1 1 1) plane of the cubic ZnS phase. Selective area electron diffraction (SAED) pattern of TGO-capped ZnS NPs is shown in Fig. 3d, which demonstrates a set of concentric rings in-
41
stead of sharp spots, as a result of the random orientation of NPs. The three diffraction rings can be indexed as (1 1 1), (2 2 0), and (3 1 1) reflections, confirming the zinc blend structure in good agreement with the XRD results. The UV–Vis absorption spectrum of TGO-capped ZnS NPs is presented in Fig. 4a, the main optical absorption of TGO-capped ZnS NPs is located in the spectral region from 233 nm to 330 nm, and could reflect the band-gap energy of TGO-capped ZnS NPs. Since ZnS is a direct band-gap semiconductor, the relationship between the absorption coefficient (a) and the direct band-gap energy (Eg) is given as follows [38,39].
ðahmÞ2 ¼ Aðhm EgÞ
ð2Þ
where hm is the incident photon energy, and A is the edge-width parameter. The band-gap energy (Eg) of ZnS NPs can be determined by extrapolation of the straight line portion of the (ahm)2 versus hm plot on hm axis at a = 0. As shown in Fig. 4b, the value of Eg for TGOcapped ZnS NPs is calculated to be 4.40 eV, which is higher than the bulk ZnS (3.72 eV). This obvious blue-shift is due to the strong influence of quantum confinement of TGO-capped ZnS NPs. Fig. 5 shows the fluorescence emission spectra of TGO-capped ZnS NPs (aq). When the excitation wavelength is held at 298 nm, the maximum emission wavelength of TGO-capped ZnS NPs (aq) occurs at 431 nm, exhibiting a characteristic blue luminescence. Moreover, the emission spectrum is very broad and asymmetric, and should be superposition of multiple emission components.
Fig. 3. (a) HRTEM image and (b) diameter distribution histogram of TGO-capped ZnS NPs. (c) Enlarged single ZnS nanoparticles showing the lattice fringes. (d) Selective area electron diffraction (SAED) pattern of TGO-capped ZnS NPs.
Fig. 4. (a) UV–Vis absorption spectrum of TGO-capped ZnS NPs (aq). (b) Plot of (ahm)2 versus photon energy hm for the TGO-capped ZnS NPs.
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To identify the exact peak positions of the spectra, a Gaussian curve fitting is performed, and gives two Gaussian curves. As shown in Fig. 5, the Gaussian curve fits well to the experimental curve (R2 = 0.9977). Therefore, the emission spectra of ZnS NPs mainly consist of two emission peaks at 419 nm and 460 nm. It is known that the luminescence properties of NPs are greatly influenced by many factors such as synthesis conditions, particle size, crystal shape, doped situation, and defect states [7,40]. During the synthesis of TGO-capped ZnS NPs, the stoichiometric ratio of [S2] to [Zn2+] was kept at 0.47, and it could form a large number of sulfur vacancies [41,42], which are doubly ionized donor centers. Possibly, sulfur vacancies can act as common electron traps and lie just below the conduction band. Furthermore, due to the high surface to volume ratio in nanometer regime, zinc vacancies are also formed at surface of ZnS NPs, which can act as hole traps above the valence band. When the ZnS NPs are excited, the photogenerated carriers (electrons and holes) recombine with each other via different routes such as direct band to band recombination and defect-related recombination, which could result in various fluorescence emissions. In our case, the emission peak at 419 nm can be assigned to the recombination of electrons at the sulfur vacancy donor level (Vs) with holes at the valence band (VB) [41–44], and the 460 nm emissions can be ascribed to the transfer of electrons from Vs to zinc vacancy acceptor level (Vzn) [45–47]. Fig. 6 displays the effect of pyridine on the fluorescence emission of TGO-capped ZnS NPs (aq). It is observed that the fluorescence intensity reduces gradually with the increasing pyridine concentration, indicating that the fluorescence of TGO-capped ZnS NPs could be quenched significantly by pyridine. Under the illumination of a UV light (254 nm), the fluorescence quenching can even be recognized with the naked eyes (Fig. 6). When pyridine was added into ZnS NPs (aq), the TGO attached to the surfaces of ZnS NPs can be replaced by pyridine effectively. Meanwhile, the defects or dangling bonds on the surface of ZnS NPs also provide the possibility of conjoining with pyridine directly. In both cases, the r donation can be formed between N atom of pyridine and Zn atom of ZnS, which generates pyridine cations through a charge-transfer from N atom to Zn atom. Once ZnS NPs were excited, pyridine cations can act as electron acceptors, and seize the photoexcited electrons at the defect state immediately [48]. After the electron transfer from ZnS NPs to pyridine, the added pyr-
Fig. 5. Fluorescence emission spectra (hollow red circles) of TGO-capped ZnS NPs (aq) at an excitation wavelength of 298 nm. (The slit widths of excitation and emission were both 5 nm.) The individual components by Gaussian fitting are shown in the dashed black lines, and the sum of individual fitting line is shown in the solid black line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
idine possibly conducts a process of non-radiative electron–hole recombination, which brings that the emissions at 419 and 460 nm are both quenched. To further understand and verify the proposed quenching mechanism, the fluorescence decay behaviors of TGO-capped ZnS NPs in the absence and presence of pyridine were measured (Fig. 7). The decay curves exhibit multiexponential decay patterns, which can be fitted well by the two-order equation. The results (Table 1) illustrate clearly that the fluorescence lifetime of TGOcapped ZnS NPs was decreased with the addition of quencher. Therefore, dynamic quenching of fluorescence may take place, and the decline of fluorescence intensity with the increasing pyridine concentration reflects the ultrafast electron transfer from NPs to pyridine. The schematic diagram of the pyridine quenching the fluorescence is shown in Scheme 2. The fluorescence quenching data are generally analyzed by the Stern–Volmer equation, I0/I = 1 + KSV[Q], where I and I0 are the fluorescence intensities of the nanoparticles in the presence and absence of quencher (pyridine), respectively, KSV is the Stern– Volmer quenching constant and [Q] is the concentration of pyridine. As is shown by the Stern–Volmer plot in Fig. 8, at the quencher concentration range of 1.16 105–1.40 102 M, the linear relation between I0/I and the quencher concentration is very fine (R2 = 0.9967), and the fitted linear regression equation is I0/ I = 1.022 + 197.3[Q]. When the signal-to-noise ratio equals 3, the detection limit was 6.76 105 M. A further study on the TGO-capped ZnS NPs fluorescence sensor is also made to detect 2,20 -bipyridine. The fluorescence intensity of ZnS NPs solution showed a gradual decrease with the addition of 2,20 -bipyridine (Fig. S2), which was similar to the phenomena of pyridine’s addition. The fluorescence quenching data are also approached by a Stern–Volmer equation (Fig. S3), and the detection limit is calculated to be 1.78 106 M, which is lower than that of pyridine. In the process, the two nitrogen atoms of 2,20 -bipyridine can coordinate with Zn atom of ZnS NPs and form a stable five-membered ring structure. Meanwhile, 2,20 -bipyridine has a stronger ability to seize photoexcited electrons from the defect state than that of pyridine, and could quench the fluorescence of
Fig. 6. Fluorescence emission spectra of TGO-capped ZnS NPs (aq) after adding pyridine with different concentrations at an excitation wavelength of 298 nm. (The slit widths of excitation and emission were both 5 nm. The concentrations of pyridine were (a) 0.00 M; (b) 1.16 105 M; (c) 2.89 105 M; (d) 1.73 104 M; (e) 4.61 104 M; (f) 6.33 104 M; (g) 1.04 103 M; (h) 1.87 103 M; (i) 2.69 103 M; (j) 3.91 103 M; (k) 5.14 103 M; (l) 6.36 103 M; (m) 8.8 103 M; (n) 1.41 102 M.) The inset shows the fluorescence image of TGOcapped ZnS NPs (aq) and its with pyridine, which were taken under the illumination of a UV light (254 nm).
Z. Li et al. / Journal of Alloys and Compounds 559 (2013) 39–44
Fig. 7. Fluorescence decay behaviors of TGO-capped ZnS NPs at room temperature in the absence and presence of pyridine. Samples were excited at 298 nm, and the decay data was collected at the maximum of the fluorescence. The concentration of pyridine used for this experiment was 1.70 103 M.
Table 1 Fit results of the fluorescence decay measurements before and after quenching of TGO-capped ZnS NPs.
TGO-capped ZnS NPs TGO-capped ZnS NPs + pyridine
s1 (ns)
s2 (ns)
v2
82.44 71.62
406.55 323.54
1.146 1.032
43
Fig. 8. The Stern–Volmer plot of the pyridine concentration dependence of the fluorescence intensity of TGO-capped ZnS NPs.
influence of strong quantum confinement. The fluorescence properties of TGO-capped ZnS NPs were investigated in depth, the peaks located at 419 nm and 460 nm were assigned to the transfer of electrons from Vs to VB, and Vs to VZn, respectively. When pyridine was added into TGO-capped ZnS NPs (aq), the fluorescence intensity could be decreased significantly, which is caused by dynamic quenching of fluorescence. Based on this phenomenon, the water-soluble ZnS nanoparticles can act as a novel fluorescent sensor for pyridine compounds, which has the characteristics of sensitivity, simplicity, swiftness and low detection limit, showing a potential application value and the market prospect for detecting the pyridine compounds. Acknowledgments This work was supported by the Key Technologies R&D Program for the 12th Five-Year Plan (2012BAJ24B04-3) of the Ministry of Science and Technology of the Peoples Republic of China and by the Fundamental Research Funds for the Central Universities, China. Appendix A. Supplementary material
Scheme 2. Schematic energy level diagram showing possible emission process in TGO-capped ZnS NPs, and the emissions quenched by pyridine. (VB: valence band, CB: conduction band, Vs: sulfur vacancy donor level, VZn: zinc vacancy acceptor level, s: hole, d: electron.)
the ZnS NPs more effectively. The above results illustrated that the TGO-capped ZnS NPs are good sensors to pyridine and its derivatives, especially to 2,20 -bipyridine. Therefore, the TGO-capped ZnS NPs fluorescence sensor can be also used in many fields such as pharmaceutical, pesticide and environmental protection for the rapid and simple determination of pyridine compounds. 4. Conclusions In summary, water-soluble ZnS NPs were successfully fabricated through a simple chemical precipitation method. Formation of TGO-capped ZnS NPs was confirmed by FT-IR measurement. TGO-capped ZnS NPs exhibited the cubic zinc blende structure with an average diameter of 2.94 nm. The band-gap energy of TGO-capped ZnS NPs was estimated to be 4.40 eV, showing the
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ZnS nanoparticles for high-sensitive fluorescent detection of pyridine compounds Zhe Li, Jiantao Ma, Yanqing Zong, Yi Men ⇑ Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, PR China
a r t i c l e
i n f o
Article history: Received 28 August 2012 Received in revised form 13 January 2013 Accepted 16 January 2013 Available online 29 January 2013 Keywords: ZnS Nanoparticles Fluorescent sensor Pyridine
a b s t r a c t Water-soluble ZnS nanoparticles (NPs) capped with alpha-thioglycerol (TGO) have been synthesized through a chemical precipitation method. The nanoparticles were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), UV–Vis absorption spectroscopy, fluorescence spectroscopy, and fluorescence decay spectroscopy. Results showed that the TGO-capped ZnS NPs exhibited the cubic zinc blende structure, and the average size was found to be 2.94 nm. Compared with the bulk ZnS, the band-gap energy of the nanoparticles (4.40 eV) rose significantly due to the strong quantum confinement. The TGO-capped ZnS NPs showed a characteristic blue luminescence corresponding to two emission peaks at 419 nm and 460 nm associated with the defect states of the nanoparticles. Such functionalized nanoparticles can be used as fluorescent sensor for the determination of pyridine compounds because they quenched the fluorescence of the nanoparticles effectively. The detection limit was 6.76 105 M for pyridine. The quenching mechanism was studied in detail, and the results demonstrated the existence of dynamic quenching processes. The proposed sensing method is not only sensitive, simple, fast and low cost, but also meaningful for practical applications. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, nanostructure materials are not only at the very frontier of fundamental materials research, but they have also entered into people’s daily life step by step [1–3]. With the deteriorating trend of environmental pollution and the rising awareness of the public vulnerability to the chemical and biological threats, the detection techniques with both high sensitivity and reliability are demanded urgently. The application of nanostructure materials to the design of chemical sensors has attracted considerable attention due to their tremendous specific surface, high activity, excellent anti-photobleaching and tunable emission [4–6]. As a developing semiconductor material star, ZnS nanoparticles (NPs) are low toxicity materials with a wide band gap [7] of 3.72 eV (cubic zinc blende structure) and 3.78 eV (hexagonal wurtzite structure), exhibiting remarkable optical and electrical properties [8–11], which suggest that they may be particularly well-suited for the manufacture of novel sensor. At present, ZnS NPs can be used to detect the DNA molecular [12,13], protein [14], organic molecule [15–17], pH value [18,19], metal ion [20–23], etc. Pyridine and its derivatives are important chemical raw materials, which are mainly used as solvent and intermediate in the production of agricultural chemicals, dyestuffs, additives, drugs, and others. However, this foul-smelling and toxic substance could in⇑ Corresponding author. Tel./fax: +86 010 58805186. E-mail address: [email protected] (Y. Men). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.01.076
trude into the human living environment through the discharge of various industrial effluents and exhaust gases. Besides, pyridine and its derivatives are frequently found in the cigarette smoke [24], and also generated during the putrefaction processes of certain foodstuffs [25]. If inhaled, ingested or absorbed through skin, these chemicals can cause many potentially harmful effects on human body, which include nausea, vomiting, headaches, coughing, asthmatic breathing, laryngitis and even cancers [26]. Consequently, the detection of pyridine has become a heightened need for the health of human and the development of environment-friendly society. Various approaches have been established for the detection of pyridine, such as barbituric acid spectrophotometry [27], high-performance liquid chromatography [28,29], gas chromatography [30–32], liquid chromatography–mass spectrometry (LC–MS) [24], gas chromatography–mass spectrometry (GC–MS) [33,34], optical fiber sensing method [35], but the method based on the fluorescent of nanomaterials has not been reported so far. Herein, we report a very simple method for the synthesis of TGO-capped ZnS NPs with a narrow size distribution and good optical properties. Compared with other traditional methods, the preparation is a one-step approach using non-toxic and low cost raw materials, and the obtained nanoparticles are water-soluble, which could expand their applying areas particularly in chemical and biological fields. When the pyridine is added to TGO-capped ZnS NPs aqueous solution, it could attach to the surface of ZnS NPs, and result in a significant fluorescence quenching. Based on this phenomenon, a novel fluorescent sensor for the determination
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of pyridine compounds is proposed, and the performance of this ZnS NPs sensor exhibits high sensitivity, simple, fast and low detection limit. The possible mechanism of fluorescence quenching is also discussed. 2. Experimental The TGO-capped ZnS NPs were synthesized by chemical precipitation method as follows: 1.00 g Zn (CH3COO)22H2O (Tianjin Bodi Chemical Holding Co., Ltd.) and 1.20 g alpha-thioglycerol (TGO, Shanghai TCI Development Co., Ltd.) were dissolved in 285 mL secondary distilled water (homemade). The TGO acts as a capping agent to prevent the agglomeration of particles and stabilize the ZnS NPs. The pH value of the mixed solution was adjusted to 11.20 by 2 M NaOH solution. After about 30 min of bubbling nitrogen to drive off the oxygen from the reaction system, 10 ml of 0.214 M Na2S9H2O (Wako Pure Chemical Industries, Ltd.) solution was slowly injected to the above solution to yield a stoichiometric ratio of 0.47 for [S2]/[Zn2+]. The mixture was heated to reflux for 18 h. After cooling down to the room temperature naturally, the colorless and transparent liquid was concentrated to 20 ml through the rotary evaporator, and 100 ml ethanol was then added to generate white precipitations, which was followed by centrifuging and washing with ethanol several times. Finally, the products were dried in vacuum for 24 h. During the fluorescence measurement, the concentration of TGO-capped ZnS NPs aqueous solution was stabilized at 0.20 mg/ml, and 6.67 102 M Na2HPO4– KH2PO4 buffer solution (pH = 8.86) was chosen for the system to keep the pH value constant. A known concentration of pyridine solution was added into ZnS NPs solution and mixed entirely to monitor the change of fluorescence emission intensity. High-resolution transmission electron microscope (HRTEM) images were observed on a JEOL JEM-2010 electron microscope operated at 200 kV, and NanoMeasurer 1.2 software was used to evaluate the size of each nanoparticle. The wide-angle X-ray diffraction (XRD) measurement was determined by a PAN-alytical X’Pert PRO MPD X-ray diffractometer using a Cu Ka radiation source. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet-380 FT-IR spectrometer. UV–Vis absorption measurement of TGO-capped ZnS NPs was carried out with a Purkinje General TU-1901 UV–Vis spectrometer. Fluorescence spectra were achieved by a Varian Cary Eclipse fluorescence spectrophotometer at the room temperature. Fluorescence decay behaviors were measured by an Edinburgh FLS920 transient/steady-state luminescence spectrometer using a nanosecond flash lamp as excitation sources.
Scheme 1. Possible surface structure of TGO-capped ZnS NPs.
3. Results and discussion FT-IR spectra of TGO and TGO-capped ZnS NPs are shown in Fig. 1. A broad absorption band in the 3000–3696 cm1region is associated with the stretching vibrations of hydroxyl groups in bound water and TGO. The peaks at 2930 and 2879 cm1 are assigned to the stretching vibrations of C–H groups of TGO on the surface of ZnS NPs. The deformation vibrations of the methine and methylene groups in TGO located at 1633 and 1417 cm1 are also found. The characteristic vibrations at 1064 and 1035 cm1 belong to the C–O group in TGO. Moreover, the absorption peak of the S–H vibration in TGO at about 2554 cm1 cannot be observed, indicating that the sulfhydryl groups of the TGO have successfully attached to the surface of ZnS NPs [36], which leads the ZnS NPs to be water-soluble. Possible surface structure of TGOcapped ZnS NPs is shown in Scheme 1. X-ray diffraction studies were carried out to establish the ZnS crystallite structure and obtain an approximate average size of the ZnS NPs, as shown in Fig. 2a. The three peaks appearing at about 28.6°, 47.8°, and 56.6° respectively correspond to the (1 1 1), (2 2 0), and (3 1 1) planes of zinc blende crystal structure, which match well with the standard card (JCPDS 05-0566) denoted in Fig. 2b. As expected, the XRD peaks of ZnS NPs are prominently broadened compared to the corresponding bulk form, which is the characteristic of nanosized particles. The average size of the crystallite (D) was estimated from the line broadening of the XRD peaks by using the Debeye–Scherrer equation [37],
D¼
0:9k b cos h
Fig. 1. FT-IR spectra of TGO and TGO-capped ZnS NPs.
diffraction peaks expressed in radians, and h is the angle of diffraction. After the contribution from Ka2 radiation was removed by fitting a cubic spline via the MDI Jade 5.0 software, the calculated crystallite size is around 2.88 nm, and a comparison of this size with the size determined from HRTEM will be discussed later in the article. Fig. 3a shows a high-resolution transmission electron microscopy (HRTEM) image of TGO-capped ZnS NPs, revealing that the
ð1Þ
where k is the wavelength of X-ray radiation (for Cu Ka1 radiation, k = 1.5406 Å), b is the full-width at half-maximum (FWHM) of the
Fig. 2. (a) The wide-angle XRD pattern for TGO-capped ZnS NPs. (b) Standard XRD pattern of bulk ZnS with the zinc blende structure (JCPDS 05-0566).
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sample is composed of an abundance of quasi-spherical particles. Another HRTEM image with a large number of ZnS NPs is presented in Fig. S1. Diameter distribution of TGO-capped ZnS NPs is summarized in Fig. 3b, which was obtained by measuring the diameter of 233 nanoparticles on several HRTEM images. The well-dispersed NPs have a narrow size distribution with a relative standard deviation of 6.7%. The average diameter is determined to be 2.94 nm, which is very close to the size calculated by XRD. Fig. 3c exhibits the enlarged HRTEM image of a single ZnS nanoparticle, the lattice fringes of nanocrystals in NPs have a spacing of nearly 0.31 nm, corresponding to the (1 1 1) plane of the cubic ZnS phase. Selective area electron diffraction (SAED) pattern of TGO-capped ZnS NPs is shown in Fig. 3d, which demonstrates a set of concentric rings in-
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stead of sharp spots, as a result of the random orientation of NPs. The three diffraction rings can be indexed as (1 1 1), (2 2 0), and (3 1 1) reflections, confirming the zinc blend structure in good agreement with the XRD results. The UV–Vis absorption spectrum of TGO-capped ZnS NPs is presented in Fig. 4a, the main optical absorption of TGO-capped ZnS NPs is located in the spectral region from 233 nm to 330 nm, and could reflect the band-gap energy of TGO-capped ZnS NPs. Since ZnS is a direct band-gap semiconductor, the relationship between the absorption coefficient (a) and the direct band-gap energy (Eg) is given as follows [38,39].
ðahmÞ2 ¼ Aðhm EgÞ
ð2Þ
where hm is the incident photon energy, and A is the edge-width parameter. The band-gap energy (Eg) of ZnS NPs can be determined by extrapolation of the straight line portion of the (ahm)2 versus hm plot on hm axis at a = 0. As shown in Fig. 4b, the value of Eg for TGOcapped ZnS NPs is calculated to be 4.40 eV, which is higher than the bulk ZnS (3.72 eV). This obvious blue-shift is due to the strong influence of quantum confinement of TGO-capped ZnS NPs. Fig. 5 shows the fluorescence emission spectra of TGO-capped ZnS NPs (aq). When the excitation wavelength is held at 298 nm, the maximum emission wavelength of TGO-capped ZnS NPs (aq) occurs at 431 nm, exhibiting a characteristic blue luminescence. Moreover, the emission spectrum is very broad and asymmetric, and should be superposition of multiple emission components.
Fig. 3. (a) HRTEM image and (b) diameter distribution histogram of TGO-capped ZnS NPs. (c) Enlarged single ZnS nanoparticles showing the lattice fringes. (d) Selective area electron diffraction (SAED) pattern of TGO-capped ZnS NPs.
Fig. 4. (a) UV–Vis absorption spectrum of TGO-capped ZnS NPs (aq). (b) Plot of (ahm)2 versus photon energy hm for the TGO-capped ZnS NPs.
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To identify the exact peak positions of the spectra, a Gaussian curve fitting is performed, and gives two Gaussian curves. As shown in Fig. 5, the Gaussian curve fits well to the experimental curve (R2 = 0.9977). Therefore, the emission spectra of ZnS NPs mainly consist of two emission peaks at 419 nm and 460 nm. It is known that the luminescence properties of NPs are greatly influenced by many factors such as synthesis conditions, particle size, crystal shape, doped situation, and defect states [7,40]. During the synthesis of TGO-capped ZnS NPs, the stoichiometric ratio of [S2] to [Zn2+] was kept at 0.47, and it could form a large number of sulfur vacancies [41,42], which are doubly ionized donor centers. Possibly, sulfur vacancies can act as common electron traps and lie just below the conduction band. Furthermore, due to the high surface to volume ratio in nanometer regime, zinc vacancies are also formed at surface of ZnS NPs, which can act as hole traps above the valence band. When the ZnS NPs are excited, the photogenerated carriers (electrons and holes) recombine with each other via different routes such as direct band to band recombination and defect-related recombination, which could result in various fluorescence emissions. In our case, the emission peak at 419 nm can be assigned to the recombination of electrons at the sulfur vacancy donor level (Vs) with holes at the valence band (VB) [41–44], and the 460 nm emissions can be ascribed to the transfer of electrons from Vs to zinc vacancy acceptor level (Vzn) [45–47]. Fig. 6 displays the effect of pyridine on the fluorescence emission of TGO-capped ZnS NPs (aq). It is observed that the fluorescence intensity reduces gradually with the increasing pyridine concentration, indicating that the fluorescence of TGO-capped ZnS NPs could be quenched significantly by pyridine. Under the illumination of a UV light (254 nm), the fluorescence quenching can even be recognized with the naked eyes (Fig. 6). When pyridine was added into ZnS NPs (aq), the TGO attached to the surfaces of ZnS NPs can be replaced by pyridine effectively. Meanwhile, the defects or dangling bonds on the surface of ZnS NPs also provide the possibility of conjoining with pyridine directly. In both cases, the r donation can be formed between N atom of pyridine and Zn atom of ZnS, which generates pyridine cations through a charge-transfer from N atom to Zn atom. Once ZnS NPs were excited, pyridine cations can act as electron acceptors, and seize the photoexcited electrons at the defect state immediately [48]. After the electron transfer from ZnS NPs to pyridine, the added pyr-
Fig. 5. Fluorescence emission spectra (hollow red circles) of TGO-capped ZnS NPs (aq) at an excitation wavelength of 298 nm. (The slit widths of excitation and emission were both 5 nm.) The individual components by Gaussian fitting are shown in the dashed black lines, and the sum of individual fitting line is shown in the solid black line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
idine possibly conducts a process of non-radiative electron–hole recombination, which brings that the emissions at 419 and 460 nm are both quenched. To further understand and verify the proposed quenching mechanism, the fluorescence decay behaviors of TGO-capped ZnS NPs in the absence and presence of pyridine were measured (Fig. 7). The decay curves exhibit multiexponential decay patterns, which can be fitted well by the two-order equation. The results (Table 1) illustrate clearly that the fluorescence lifetime of TGOcapped ZnS NPs was decreased with the addition of quencher. Therefore, dynamic quenching of fluorescence may take place, and the decline of fluorescence intensity with the increasing pyridine concentration reflects the ultrafast electron transfer from NPs to pyridine. The schematic diagram of the pyridine quenching the fluorescence is shown in Scheme 2. The fluorescence quenching data are generally analyzed by the Stern–Volmer equation, I0/I = 1 + KSV[Q], where I and I0 are the fluorescence intensities of the nanoparticles in the presence and absence of quencher (pyridine), respectively, KSV is the Stern– Volmer quenching constant and [Q] is the concentration of pyridine. As is shown by the Stern–Volmer plot in Fig. 8, at the quencher concentration range of 1.16 105–1.40 102 M, the linear relation between I0/I and the quencher concentration is very fine (R2 = 0.9967), and the fitted linear regression equation is I0/ I = 1.022 + 197.3[Q]. When the signal-to-noise ratio equals 3, the detection limit was 6.76 105 M. A further study on the TGO-capped ZnS NPs fluorescence sensor is also made to detect 2,20 -bipyridine. The fluorescence intensity of ZnS NPs solution showed a gradual decrease with the addition of 2,20 -bipyridine (Fig. S2), which was similar to the phenomena of pyridine’s addition. The fluorescence quenching data are also approached by a Stern–Volmer equation (Fig. S3), and the detection limit is calculated to be 1.78 106 M, which is lower than that of pyridine. In the process, the two nitrogen atoms of 2,20 -bipyridine can coordinate with Zn atom of ZnS NPs and form a stable five-membered ring structure. Meanwhile, 2,20 -bipyridine has a stronger ability to seize photoexcited electrons from the defect state than that of pyridine, and could quench the fluorescence of
Fig. 6. Fluorescence emission spectra of TGO-capped ZnS NPs (aq) after adding pyridine with different concentrations at an excitation wavelength of 298 nm. (The slit widths of excitation and emission were both 5 nm. The concentrations of pyridine were (a) 0.00 M; (b) 1.16 105 M; (c) 2.89 105 M; (d) 1.73 104 M; (e) 4.61 104 M; (f) 6.33 104 M; (g) 1.04 103 M; (h) 1.87 103 M; (i) 2.69 103 M; (j) 3.91 103 M; (k) 5.14 103 M; (l) 6.36 103 M; (m) 8.8 103 M; (n) 1.41 102 M.) The inset shows the fluorescence image of TGOcapped ZnS NPs (aq) and its with pyridine, which were taken under the illumination of a UV light (254 nm).
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Fig. 7. Fluorescence decay behaviors of TGO-capped ZnS NPs at room temperature in the absence and presence of pyridine. Samples were excited at 298 nm, and the decay data was collected at the maximum of the fluorescence. The concentration of pyridine used for this experiment was 1.70 103 M.
Table 1 Fit results of the fluorescence decay measurements before and after quenching of TGO-capped ZnS NPs.
TGO-capped ZnS NPs TGO-capped ZnS NPs + pyridine
s1 (ns)
s2 (ns)
v2
82.44 71.62
406.55 323.54
1.146 1.032
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Fig. 8. The Stern–Volmer plot of the pyridine concentration dependence of the fluorescence intensity of TGO-capped ZnS NPs.
influence of strong quantum confinement. The fluorescence properties of TGO-capped ZnS NPs were investigated in depth, the peaks located at 419 nm and 460 nm were assigned to the transfer of electrons from Vs to VB, and Vs to VZn, respectively. When pyridine was added into TGO-capped ZnS NPs (aq), the fluorescence intensity could be decreased significantly, which is caused by dynamic quenching of fluorescence. Based on this phenomenon, the water-soluble ZnS nanoparticles can act as a novel fluorescent sensor for pyridine compounds, which has the characteristics of sensitivity, simplicity, swiftness and low detection limit, showing a potential application value and the market prospect for detecting the pyridine compounds. Acknowledgments This work was supported by the Key Technologies R&D Program for the 12th Five-Year Plan (2012BAJ24B04-3) of the Ministry of Science and Technology of the Peoples Republic of China and by the Fundamental Research Funds for the Central Universities, China. Appendix A. Supplementary material
Scheme 2. Schematic energy level diagram showing possible emission process in TGO-capped ZnS NPs, and the emissions quenched by pyridine. (VB: valence band, CB: conduction band, Vs: sulfur vacancy donor level, VZn: zinc vacancy acceptor level, s: hole, d: electron.)
the ZnS NPs more effectively. The above results illustrated that the TGO-capped ZnS NPs are good sensors to pyridine and its derivatives, especially to 2,20 -bipyridine. Therefore, the TGO-capped ZnS NPs fluorescence sensor can be also used in many fields such as pharmaceutical, pesticide and environmental protection for the rapid and simple determination of pyridine compounds. 4. Conclusions In summary, water-soluble ZnS NPs were successfully fabricated through a simple chemical precipitation method. Formation of TGO-capped ZnS NPs was confirmed by FT-IR measurement. TGO-capped ZnS NPs exhibited the cubic zinc blende structure with an average diameter of 2.94 nm. The band-gap energy of TGO-capped ZnS NPs was estimated to be 4.40 eV, showing the
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