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We have developed a simple, rapid, and cost-effective fluorescent turn off nanosensor, by polyvinylpyrrolidone capped ZnS nanoparticles for the selective and sensitive detection of iodide ions in high briny solution, edible salt and other real samples. PVP capped ZnS nanoparticles has been prepared by simple green chemical synthesis using Zn-acetate di-hydrate, sodium sulphide and polyvinylpyrrolidone (PVP) at room temperature in aqueous solution. PVP modified ZnS nanoparticles were approximately 29 nm in diameter with intense, narrow fluorescence properties and long term stability than uncapped ZnSNps. The PVP-ZnSNps provide a highly sensitive method for the determination of iodide via significant FL intensity quenching. The optimum pH range for iodide determination is 5 to 10. Under optimal conditions, the relative FL intensity decreases linearly with increasing iodide concentration in the range 2Ã- 10-9 to 1Ã-10-7 M with a detection limit of 3.4 Ã- 10-9 M. No other ions excluding iodide can induce a considerable FL quenching. The quenching mechanism can be demonstrated by the "heavy atom" effect for the quenching of PVP-ZnSNps FL intensity; this mechanism induced the oxidation of quencher ion (iodide ion). This method is simple and relatively free from interference of closely associated ions and is successfully applied to the determination of iodide in real samples.
The design and construction of efficient anion sensors is an emerging research area because they play a major role in biological systems and play a crucial role in industrial and environmental processes . Some micro and macro molecules have been described as selective anion-responsive receptors for sensing of sulphides , fluoride [3-4] nitrates , and cyanide . However, most of these molecules involve complex synthetic procedures; have poor water solubility or weak photo-stability which makes the process of anion sensing rather complicated [7-8].
Most of these complexities can be surpassed by using metal nanoparticles as sensing agents. Their synthetic procedures are simple as well as straightforward and they have fascinating semiconducting, piezoelectric, and pyroelectric properties making them smart and sensitive sensors [9-10]. Among these semiconductor metal nanoparticles have gained considerable attention due to their size dependent physical and optical properties. Additionally, these nanoparticles have high surface area and show fast electron communication features [11-12]. The detection and determination of anions by these nanosensors is usually done by spectroscopic or electrochemical techniques [13-15]. In the various spectroscopic methods for detection, fluorescence seems to be the most promising due to its high sensitivity, quick response and facile quantification of signal.
Of the various semiconductor nanoparticles, zinc sulphide (ZnS) nanoparticles are the least toxic and are characterized with a wide band gap at room temperature, high index of refraction and high transmittance in the visible range . Due to these properties ZnS nanoparticles have been widely used as an important phosphor for photoluminescence, electroluminescence and cathodoluminescence [17-19]. They can be easily synthesized in an aqueous medium and are less expensive than any other semiconductor nanoparticles with good photocatalytic properties. Hence they have been extensively used as nanosensors for a range of ions and biomolecules [20-21]. Furthermore, the key properties such as optical stability and aqueous solubility of these nanoparticles can be tuned by the surface modification with organic ligands [22-24]. The presence of such ligands on the surface generates new traps, facilitating an enhancement in the selectivity and efficiency of optical reactions occurring on the surface of nanoparticles. For example, sodium thioglycolate capped ZnS nanoparticles have been used as a sensor for human serum albumin (protine) , L-carnitine modified ZnS nanoparticles functions as a selective optical probe of mercury ion  and thiolactic acid capped ZnS nanoparticles have been used as sensors for silver ions .
One of the most frequently used organic capping agents for metal nanoparticles is poly (N-vinyl-2-pyrrolidone) (PVP). This water-soluble polymer covers nanoparticle surface via physical and chemical bonding; hence restricts particle-particle contact and prevents agglomeration of nanoparticles. It has been extensively used as a protecting agent against agglomeration of metal colloids in the well-known polyol process [25-26]. Recently PVP has been reported as a capping agent with ZnS nanoparticles for detection of cholesterol .
Iodine, as we know, is a crucial trace element on earth and mainly exists in seawater; elemental iodine is used as a chemical reagent in some organic chemical synthesis, in medicine etc; it also plays essential roles in neurological activity and thyroid gland function [28-29]. In fact, the world health organization (WHO) has stated that the biggest cause for mental retardation on a global scale is due to iodine deficiency. Altered levels of iodide stimulate various diseases, such as cretinism, congenital abnormalities and goiter . The daily intake of iodine for human nutrition should be controlled above certain limits; hence the need to determine iodine in the body, foodstuffs and drinks is very important. In previous literature, several methods has been reported for the detection and determination of iodide in trace levels; such as gas chromatography in combination with mass spectrometry, ion chromatography, chemiluminescence, potentiometry, colourimetry ,optical and redox method etc [1,31-35]. Many of these methods are frequently used although these approaches are time consuming, laborious, expensive and less sensitive. Thus, it is crucial to develop simple, sensitive and rapid analytical sensor for the estimation of iodide in different samples in trace level.
In the present work we sought to combine the interesting optical properties of ZnS nanoparticles with the complexing ability of PVP for the detection of iodide ions. Here, we have synthesized PVP capped ZnS nanoparticles and used them for the detection of iodide ions. PVP was specifically chosen as the capping agent because of its well known complexing ability with electron acceptors such as iodide . It was expected that the complexed iodide on the surface will interact more efficiently with the ZnS nanoparticles. The improved interaction may cause sufficient changes in the optical properties of the nanoparticles facilititating detection.
2. MATERIALS AND METHODS
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 MÎ© 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 . 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.
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 Âµm 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 IO3âˆ’ to Iâˆ’. The resulting solution was filtered through 0.22 Âµm 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 Âµm 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
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 (Î»max); 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 . 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 Â± 10 nm and 130 Â± 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
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 . 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, 50Âµl) 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 Î»max 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.
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 . 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 . 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 . 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 .
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 3Ïƒ IUPAC criteria was 3.4 nM.
3.8 Comparison of the present method with the previously reported methods
Limit of detection
0-10 Ã-10âˆ’6 M
1 -1000 Ã-10âˆ’9 M
50 Ã-10âˆ’9 M
3.94Ã-10âˆ’6 - 5.51Ã-10âˆ’5 M
1.0Ã-10âˆ’6 - 1.0Ã-10âˆ’4 M
8.0 Ã-10âˆ’7 M
Mercuric(II)-(p-(di methyl amino) benzylidene) thio semicarbazide Complax
0 - 4.00Ã-10 âˆ’6 M
Fluorescein-5-iso thio cyanate-modified Au NPs
10.0-600.0 Ã-10âˆ’9 M
10 Ã-10âˆ’9 M
Triethanolamine-capped CdSe quantum dots
0 - 3.5Ã-10âˆ’5 M
Anthracene - 5,10,15,20-tetraphenylporphyrin (TPP) Complex
1.0 Ã- 10-6 - 2.5 Ã- 10-4 M
2Ã- 10-9 to 1Ã-10-7 M
3.4 Ã-10âˆ’9 M
Table 1 : Comparision table of proposed nanosensor with previously reported I- sensor
Table 1 shows the comparision of the previously reported method with the sensor reported in the present study. It can be seen that the present method is more sensitive than the other reported methods and also very selective.
Fig.13 The fluorescence quenching of PVP-ZnSNps in the presence of I- and the mixtures with equal amount of other anions
Furthermore, iodide sensing was carried at various concentration of iodide in the presence of equivalent amount of other anions (Fig.13) to understand the interference of these anions on the proposed method. The results showed that the fluorescence quenching rates with and without other anions is almost same. This confirms that the sensor is equally sensitive in presence of other ions also or in other words supplementary anions do not interfere on the sensitivity of PVP-ZnSNps nanosensor.
3.9 Real Samples analysis
In order to confirm the applicability of PVP-ZnSNps nanosensor for analyzing iodide in real samples, they were applied to detect and determine iodide in sea water, river water, tap water, urine sample and edible salt sample. The real samples were analyzed by standard addition method. A known amount of standard iodide solution was spiked in an unknown real sample and the possibility of applying the present optical sensor for analysis of samples was tested by determining the recovery of known amounts of iodide ions added to the samples.
Amount of added iodide
Amount of founded iodide
Recovery (%) (n=3)
0.01 Ã- 10-7
0.08 Ã- 10-7
0.5 Ã- 10-7
1.617 Ã- 10-7
1.621 Ã- 10-7
1.674 Ã- 10-7
2.112 Ã- 10-7
0.01 Ã- 10-7
0.08 Ã- 10-7
0.5 Ã- 10-7
1.420 Ã- 10-7
1.417 Ã- 10-7
1.497 Ã- 10-7
1.94 Ã- 10-7
0.01 Ã- 10-7
0.08 Ã- 10-7
0.5 Ã- 10-7
1.182 Ã- 10-7
1.195 Ã- 10-7
1.259 Ã- 10-7
0.01 Ã- 10-7
0.08 Ã- 10-7
0.5 Ã- 10-7
0.982 Ã- 10-7
0.977 Ã- 10-7
1.052 Ã- 10-7
1.464 Ã- 10-7
0.01 Ã- 10-7
0.08 Ã- 10-7
0.5 Ã- 10-7
15.121 Ã- 10-7
14.912 Ã- 10-7
15.019 Ã- 10-7
15.496 Ã- 10-7
Table 2 : Real sample analysis
The results (Table 2) show good agreement between added and detected concentration of the iodide in real samples. There was no interference observed by the accompanying compounds, indicating the specificity of the probe. The results obtained were excellent with recovery (between 98 .6% to 101.5%) in real samples. This confirms that the proposed sensor can be effectively used to determine the content of iodide in real samples.
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.