3.3 Effect of pH
The optical properties of semiconductor nanoparticles are highly dependent on their surrounding environmental conditions such as pH. Hence the effect of pH on the fluorescence spectra of PVP-ZnSNps was studied by subjecting it to a pH range of 4.0 to 13.0. The results obtained from the study show that the FL intensity of PVP-ZnSNps was almost stable in the pH range 5.0-10.0 (Fig.6).
Fig. 6 Effect of pH values on the FL intensity of PVP-ZnSNps
At low pH the FL intensity of PVP-ZnSNps decreased significantly. It is clear that at lower pH, ZnSNps dissolve and produce surface defects; thus the FL intensity of PVP-ZnSNps decreases. In contrast, at a higher pH above 10.0, the electrostatic repulsion between ZnSNps and the negative charge of the solution prevent agglomeration of NPs, which results in the considerable increase of the FL intensity of PVP-ZnSNps. In general, pH of biological samples is around 7.0. Hence, pH 7.4 was finally selected for further analysis.
3.4 Stability PVP-ZnSNps
Fig.7 Stability of ZnSNps and PVP-ZnSNps
The storage stability at room temperature of uncapped ZnSNps and PVP-ZnSNps was evaluated in aqueous solution at room temperature (Fig.7). It was found that the FL intensity of PVP-ZnSNps increased gradually up to 7 days and then remained stable for 30 days with no change in intensity or wavelength of emission. On the other hand, the intensity of ZnSNps increased up to 5 days, then suddenly dropped, and the FL peak of ZnSNps shifted to a longer wavelength. This clearly indicated an increase in the size of nanoparticles implying that the particles had started aggregating.
These results were supported by DLS histograms of capped and uncapped. The DLS studies evidently show an increment in the sizes of uncapped nanoparticles. Hence we may conclude that PVP conserves the optical properties of ZnSNps for a long period of time by reducing the surface defects and inhibiting photo oxidation.
3.5 Selectivity Anion Sensing
Fig.8 (a) Effect of relevant anions (1Ã-10-5 M) (b) different iodide salt on the FL intensity of PVP-ZnSNps
The sensing properties of PVP-ZnSNps toward various anions such as F-, Cl-, Br-, I-, NO2-, NO3-, S-2, ClO4-, CN-, IO3- was studied. These anions are very common and have significant influences on the environment and human physiology, especially the halogen elements due to their similar chemical properties [33]. The response in the FL intensity of PVP-ZnSNps was observed upon the addition of above all anions (0.5 ml, 1Ã-10-5 M).
Interestingly, with the addition of the iodide salt; the colorless solutions of the PVP-ZnSNps changes into yellow rapidly indicating a red shift in the absorptions spectra. The FL intensity of PVP-ZnSNps was quenched by almost 95% by iodide solution (Fig.8a) and a red shift (3 nm: 485ïƒ 488 nm) was also observed in the emission peak. The shift (red shift, 155 nm: 288ïƒ 443 nm) in the absorption peak was more significant compared to the emission peak (Fig.2). The remaining anions have a slight or almost no effect on the FL intensity of PVP-ZnSNps. It reveals that PVP-ZnSNps are highly selective towards iodide ion amongst all the tested anions. Additionally, we also inspected the influence of different cations on the iodide sensing capacity of PVP-ZnSNps by taking a solution (0.5ml, 1Ã-10-5 M) of different iodide salts like NaI, KI, LiI, NH4I, AgI and CuI (Fig.8b) and observing the change in the FL intensity of PVP-ZnSNps. The results obtained confirmed that PVP-ZnSNps is selectively sensitive towards iodide ions and is independent of the nature of salt taken.
3.6 PVP-ZnSNps interaction with I-
Fig.9 Effect of iodide solution on FL intensity of ZnSNps and PVP-ZnSNps
The effect of iodide ion solution (3Ã-10-8, 50µl) on FL spectra of PVP-ZnSNps and ZnSNps (Fig.9) reveal that the FL intensity of PVP-ZnSNps were more quenched by iodide ions than the bare ZnSNps. Furthermore, in UV-visible spectroscopy with the addition of iodide ion a new absorption peak is formed at 488nm; it suggests a complex formation between PVP-ZnSNps and I- (Fig.1). In addition by comparing the FT-IR spectra of PVP-ZnSNps before and after addition of I- some significant variation was observed; the peak intensity of C=O (1682cm-1) found in PVP-ZnSNps reduces after I- addition and it becomes broad with increasing concentration of iodide. This result clearly indicated that the iodide interact with C=O of PVP unit. It was also observed that interaction peaks of PVP and Zn+2 ions (2681-2953 cm-1, 1452- 1509 cm-1 and 1291 cm-1 ) was also affected by iodide addition; they became slightly broad and less intense after the addition of I- ions. This observation suggests that ZnSNps also take a part in the interaction between PVP-ZnSNps and I-. From all these results it is very clearly indicates that the PVP and ZnSNps both interact with iodide ions and moreover PVP capping facilitates the interaction between nanoparticles and the iodide ion; hence is beneficial for the iodide detection (Fig.10).
Fig.10 Proposed interaction of I- with PVP-ZnSNps
3.7 Iodide detection
To evaluate the sensitivity of the PVP-ZnSNps towards iodide ions, the intensity of absorption and FL spectra was measured after addition of various concentration of I- ions (1Ã-10-4, 1Ã-10-5, 1Ã-10-6, 1Ã-10-7, 5Ã-10-8, 3Ã-10-8, 2Ã-10-8,1Ã-10-8, 8Ã-10-9, 6Ã-10-9, 4Ã-10-9, 2Ã-10-9,1Ã-10-9 M). The results show (Fig.11a) that upon the addition of iodide a new absorption peak is formed and λmax is shift almost 157 nm. The absorption intensity gradually decreases with lower concentration of iodide solution and the intensity of the yellow color also decreases with the concentration.
加拿大代写 The List Of Abbreviations
Fig.11 Effect of iodide concentration on (a) absorption intensity (b) FL intensity of PVP-ZnS Nps
Meanwhile, the FL intensity of PVP-ZnSNps was quenched with the increasing concentration of iodide (Fig 11b). Quenching started with the addition of 1Ã-10-9 M (0.5 ml) iodide solution. FL intensity was found to be quenched to the maximum (almost 100%) on the addition of 1Ã-10-4 (0.5 ml) iodide solutions. Our observations suggest that fluorescent detection is more sensitive than absorption detection. After the addition of 2Ã-10-8 M solution of iodide the response of absorption intensity remain almost constant in contrast the FL intensity noticeably respond up to the addition of 1Ã-10-9 M iodide solution.
In general, fluorescence quenching can happen through various mechanisms, like ground-state complexation, charge-transfer phenomena, electronic energy transfer, fluorescence resonance energy transfer (FRET), heavy atom effect, magnetic perturbations, etc [38-39]. In this case electron charge transfer via a heavy- atom effect is proposed which is coherently similar with the reports of Valiyaveettil and Jang [40-41]. The "heavy-atom" enhances spin-orbit coupling by the interaction between excited molecular entity and an atom with high atomic number [42]. In our case, the "heavy atom" effect is involved in quenching the fluorescence intensity of PVP-ZnSNps by heavy atom I-. Furthermore, the semiconductor nanoparticles oxidized the iodide ion into iodine [37]. In this case, the excitation of ZnSNps in PVP-ZnSNps generates electron-hole pair. The added iodide ions are oxidized by these holes and form I2. The rapid color change of PVP-ZnSNps solution after addition of iodide ion also gives visible confirmation of I2 formation.
ZnS + hvex ZnS (e- / h+)
ZnS (e- / h+) + I- ZnS(e-) + (2I- . h+) I2
Fig.8a shows that the FL intensity PVP- ZnSNps is not affected by the addition of any other anion except iodide. It reveals the selectivity of this sensor towards iodide. In particular, halide anions are selective toward sensors according to their basicity, affinity, size and shape. [33, 49]. The selectivity towards iodide of this sensor can be due to the high atomic mass of iodide ion. Atomic weight of other halides are much insubstantial than iodide (126.90), so they are incapable of causing a spin-orbit coupling (heavy atom effect) resulting in the oxidation of the anion.
In addition, there are two types of quenching, one is static quenching through the formation of a complex and the other is the dynamic quenching due to the random collisions between the emitter and the quencher. For any quenching mechanism, the electron/energy transfer is involved from the emitter to the quencher and each can be quantitatively described by the Stern-Volmer studies [42]. The rate of quenching can be determined by using the slope of the Stern-Volmer plot.
I0/I = 1+ KSV [S]
Here [S] is the concentration of iodide, I is the FL intensity of PVP-ZnSNps at any given iodide concentration. I0 is FL intensity of PVP-ZnSNps.
Fig. 12 The Stern-Volmer plot in the linear range (1Ã-10-9 M- 1Ã-10-7 M)
data plotted as I0/I vs. [I-]
The Stern-Volmer plot reveals that the plot is linear up to the concentration of 1Ã-10-7 M of iodide and then starts curving upwards (Fig.12). This clearly indicates that the quenching process occurs by a static mechanism till the concentration of 1Ã-10-7 M of iodide and at higher concentrations quenching occurs via a dynamic mechanism. The unique UV-visible absorption spectrum with the addition of iodide in PVP-ZnSNps also supports the static quenching mechanism process [43].
Furthermore, below 1Ã-10-7 M, the obtained experimental data of the Stern-Volmer plot can be satisfactorily fitted by a linear regression calibration equation (Fig.12). A good linear relationship (r>0.99) could be used to determine iodide. The limit of detection calculated by 3σ IUPAC criteria was 3.4 nM.
加拿大代写 The List Of Abbreviations
4. CONCLUSION
In conclusion, we have presented a novel dual-channel "colorimetry and fluorometry" recognition of iodide ions (I-) with high sensitivity and selectivity in aqueous real samples. Iodide quenched the fluorescence response of PVP-ZnS Nps in a concentration dependent manner pointing out the feasibility of the method for the quantitative measurement of I- with lower detection limit of 5.4 nM. Furthermore the color of Nps solution turned from colorless to pale yellow due to the complex formation and followed by the oxidation of the iodide ions. This "turn off" optical sensor was found to be highly sensitive and free of interference from all tested anions. In so far as we know, this is the first report describing highly specific recognition of an anion by ZnSNps. The experimental results reported here open up an inventive approach of quick and dependable identification of iodide. Due to its great practical potential for the dual channel detection of iodide in real samples this cost-effective sensing system could be developed in to a strip based kit system for on spot analysis.
