2. MATERIALS AND METHODS
2.1 Materials
All chemicals used were of analytical grade or of the highest purity available. All solutions were prepared with double-distilled, deionised Milli-Q water (18 MΩ cm). Zinc acetate dihydrate Zn (CH3COO) 2. 2H2O, sodium sulfide (Na2S) and polyvinylpyrrolidone (PVP) were purchased from Sigma-Aldrich with high purity. Working standard solutions were prepared daily in deionised water. How I- prepared and standardization
2.2 Synthesis of PVP capped ZnS nanoparticles (PVP-ZnSNps)
The PVP-ZnSNps were prepared by a soft chemical method as reported by G.De and co workers with some modification [24]. A 5mg of PVP was dissolved in 50 ml of water and stirred for 20 minutes. Then 5 ml of 0.1 M Zn (CH3COO) 2. 2H2O solution was added with constant stirring. The pH of the solution was adjusted to 8.0 by 0.1 N NaOH. Further, 10 ml of freshly prepared 0.05 M aqueous solution of Na2S was added drop wise to get a transparent colorless aqueous dispersion of PVP-ZnSNps. This dispersion was stirred for 20 min and then refluxed for 8 h. Here we maintain 1:1 molar ratio of Zn: S. The dispersed nano particles were collected from aqueous solution with the addition of a known amount of acetone and by centrifugation at 8000 rpm. Immediate flocculation of nanoparticles occurred. To remove unreacted sulfide and excess PVP, the particles were washed thrice with acetone and water. The purified PVP-ZnSNps were dried under vacuum.
2.3 Physicochemical Characterization of PVP-ZnSNps
Nanoparticle characterization is essential to establish a control on the size of the nanoparticles during synthesis and for understanding the morphology as well as their applicability. Herein, the optical properties of the nanoparticles were measured by UV-Vis absorption and fluorescence spectroscopy through Jasco V-570 UV-Vis. Spectrometer and Fluorolog Horiba Jobin Yvon spectro fluorimeter respectively at room temperature (25+ 2°C). Interactions between the ligand and the nanoparticles were evaluated by FT-IR spectra; which was recorded on Bruker Tensor-27 FT-IR spectrometer. Transmission electron micrograph (TEM) was recorded by JEOL, JEM-2100(200 kV) to observe the morphology of nanoparticles. Particle size was measured by DLS measurements using a Metrohm Microtrac-NanotracTM 10.5.2. instrument.
2.4 Detection of Iodide (I-)
The procedure followed to investigate the anion recognition ability of PVP-ZnSNps is as follows. Stock solutions (1Ã-10-2 M) of various anions were prepared and diluted when required. A series of solutions were prepared in 5 ml volumetric flasks each containing 0.5 ml of various anions (1Ã-10-5 M), PVP-ZnSNps (1ml, 1Ã-10-5 M) and phosphate buffer solution (2 ml, pH 7.4). The final volume of the resulting mixture was made up to 5 ml by the addition of deionized water. The fluorescence spectra were obtained using a fluorescence spectrophotometer operated at an excitation wavelength of 485 nm.
加拿大代写 The List Of Abbreviations
2.5 Analysis of real samples
Seawater (collected from the Gulf of Khambhat, Gujarat) and river water samples (collected from Sabarmati River Ahmedabad) and local tap water was filtered through 0.22 µm membrane filter paper and used for analysis without any further purification process.
Samples of edible salt (0.3g) were dissolved in 5 ml deionized water. Prior to analysis these sample solutions were treated for 10 minutes with 5.0 mM ascorbic acid to reduce IO3− to I−. The resulting solution was filtered through 0.22 µm membrane filter paper and used for analysis.
For the urine samples; 2 ml of acetonitrile and 6 ml of de-ionized water was added alongwith 2 ml urine in centrifuge tubes. The tubes were vortex mixed for 1 min. and centrifuged at 1500 rpm for 15 minutes. The supernatant of these solutions were taken and filtered through a 0.22 µm membrane filter paper prior to use.
3. RESULTS AND DISCUSSION
PVP capped ZnSNps have been synthesized to develop sensor for anions. The prepared nanoparticles were characterized by FT-IR, TEM, DLS, UV-Vis.spectrophotometry and fluorescence spectroscopy. The PVP-ZnSNps were then investigated for its interaction with anions like F-, Cl-, Br-, I-, NO2-, NO3-, S-2, ClO4-, CN-, IO3- etc.. It was observed that the PVP-ZnSNps is very selective for iodide ions and hence method was developed for the detection and estimation of I- by spectrophotometry and fluorescence measurements.
3.1 Physicochemical Chemical Characterization and optical properties of
PVP-ZnSNps
Fig. FT-IR spectra of ZnS Nps and PVP-ZnSNps
The FT-IR study of PVP-ZnSNps was carried out to understand the interaction between PVP and ZnS nanoparticles. As shown in Fig.1, PVP shows its characteristic absorption peaks which are a total match with reported literature data. The peaks observed in the range of 2681-2953 cm-1, 1452- 1509 cm-1 and 1363 cm-1 are attributed to C-H bonding. The strong absorption peaks around 1291 cm-1 and 1682 cm-1 are due to C=O bonding. In the case of PVP-ZnSNps no new peaks are seen nor do any of the existing peaks vanish. However, the peaks in the range of 2681-2953 cm-1, 1452- 1509 cm-1 and 1291 cm-1 are slightly broadened and the intensity of the peak at 2681-2953 cm-1 has decreased. This may be due to the coordination between the nitrogen atom of PVP and Zn+2 ions of ZnS nanoparticles. The spectra clearly indicate that PVP acts as capping agent on ZnSNps and does not interact chemically with the nanoparticles.
Fig. Absorption spectra of ZnSNps, PVP-ZnSNps and PVP-ZnSNps + I-
Fig.2 shows the absorption spectra of uncapped ZnSNps and PVP-ZnSNps. For the PVP capped ZnS Nps, the absorption peak appeared at around 288 nm. The capping of ZnS Nps with PVP did not cause shifting of absorption peak position (λmax); indicating no new chemical bond formation between ZnSNps and PVP. This clearly supports our conclusions from the FT-IR spectra. Additionally, the absorption behavior confirms that the size of the ZnSNps remains stable after capping and increasing intensity of the peak indicates more crystalline nature of nanoparticles after capping.
The change in the optical properties of ZnSNps on capping with PVP was studied using its fluorescence (FL) emission spectra as presented in Fig 3. The uncapped nanoparticles showed a very broad emission peak at 487 nm; capping with PVP causes a small shift towards shorter wavelength (blue shift, 2 nm: 487ïƒ 485 nm) and a fourfold increase in the intensity of the peak.
Fig. (a) FL spectra of PVP capped and uncapped ZnSNps. (b) Effect of PVP concentration on the FL intensity of ZnSNps
It is well known that the photooxidation of semiconductor nanoparticles could result in the photobleaching due to its surface defects [37]. Therefore, we may conclude that PVP capping causes the inhibition of photooxidation of the ZnSNps by decreasing the surface defects. Furthermore, when the effect of PVP concentration on the luminescence properties of ZnSNps was studied (Fig.3(b), it was observed that with increasing amount of PVP, the FL intensity of ZnSNps dramatically increases. This can be explained based on the fact that with increasing concentration of PVP the hanging bonds and surface defects on the nanoparticles are reduced; hence they become more stable, consequently increasing the FL intensity. But for concentrations >0.5 M, the FL intensity was found to have stabilized, indicating that the surface of ZnSNps was saturated with PVP. Therefore, a 0.5 M solution of PVP was selected to prepare PVP-ZnSNps.
Moreover, the line width of the PVP-ZnSNps FL spectrum is relatively narrow (with the full width at half-maximum of 35 nm), indicating that the capped PVP-ZnSNps have a narrow size distribution.
3.2 Particle Size and Surface Morphology of PVP-ZnSNps The particle size and surface morphology of Ncs and DNcs were studied by DLS and SEM respectively. The size of Ncs and DNcs were found to be around 110 ± 10 nm and 130 ± 10 nm respectively by DLS (Fig.5 a,b). The
Fig. 4 TEM images of (a) ZnSNps (b) PVP-ZnSNps (C) HRTEM image of PVP-ZnSNps
Transmission electron microscopy (TEM) was used to study the morphology of ZnSNps and PVP-ZnSNps. The TEM images of ZnSNps and PVP-ZnSNps are shown in Fig.4a,b. TEM images of uncapped ZnSNps shows neither good quality images nor any visible lattice fringes in the HRTEM. This clearly indicated the poor crystalline nature of uncapped ZnSNps. The TEM images of PVP-ZnSNps show that the capped particles are monodispersed and uniform. The HRTEM image of PVP-ZnSNps (Fig.4c) shows the noticeable lattice fringes, which indicated that the synthesized PVP-ZnSNps are crystalline in nature.
The diameter of ZnSNps increases after PVP capping. The average sizes of ZnSNps and PVP-ZnSNps as measured using DLS were found to be 1.7 nm and 29 nm respectively (Fig.5a,b). The increase in particle size by DLS measurements and TEM images confirms the surface coating provided by PVP and the increase in crystalline nature of ZnSNps due to this coating.
Fig.5 DLS histogram of (a) ZnSNps (b) PVP-ZnSNps
加拿大代写 The List Of Abbreviations
