Corresponding author: Guttena Veerabhadram ( gvbhadram@osmania.ac.in ) © 2018 Dasari Ayodhya, Guttena Veerabhadram.
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Citation:
Ayodhya D, Veerabhadram G (2018) Synthesis and characterization of N, O-donor Schiff base capped ZnS NPs as a sensor for fluorescence selective detection of Fe 3+, Cr 2+ and Cd 2+ ions. Modern Electronic Materials 4(4): 151-162. https://doi.org/10.3897/j.moem.4.4.35062
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We report the simple synthesis of zinc sulfide nanoparticles (ZnS NPs) by a co-precipitation method using Schiff base, (2-[(4-methoxy-phenylimino)-methyl]-4-nitro phenol) as a capping agent. Here, Schiff base is also used as N, O-donor ligand to control the morphology of NPs and fluorescence interactions. The formation of ZnS NPs and their optical, structural, thermal properties and morphologies were studied by means of UV–vis DRS, fluorescence, FTIR, XRD, SEM, TEM, zeta potential and TGA. The optical properties and quantum confinement effect of the products were confirmed by means of spectroscopic measurements. XRD and TEM image shows that the synthesized ZnS NPs have cubic structures with a diameter of about less than 10 nm. The prepared ZnS NPs exhibited as a selective probe detection of Fe3+, Cr2+ and Cd2+ ions by fluorometrically and the emission band which disappears in the presence of increasing concentrations of Fe3+, Cr2+ and Cd2+ ions. Based on the fluorescence quenching of the NPs in the presence of metal ion of interest, the feasibility of their determinations was examined according to the Stern-Volmer equation. Our work suggested that Schiff base capped ZnS NPs could be a potential selective sensor in the detection of heavy metal ions.
ZnS NPs, schiff base, fluorescence sensor, selective probe, metal ion detection
Nanostructure-based semiconductor materials are of great importance for several technological applications due to their optical and thermal properties. The design and fabrication of metal sulfide nanoparticles with tunable properties for advanced applications have drawn a great deal of attention in the field of nanotechnology due to the quantum size and surface effects. These unique properties make them suitable for many applications. Wide direct band gap semiconductor materials like ZnS have gained special notice due to their size-dependent properties and widespread technological applications. The semiconductor-based NPs and composites such as ZnS, CdS, CdSe, CdS/Fe3O4, CdS/TiO2, CdIn2S4, Ag2S, Ag2S/SiO2, Ag2S/TiO2, Bi2S3, NiS, CoS, CuS, HgS and PbS with varied band gap have been studied due to their wide use in optoelectronics, electronics, light-emitting diodes, electroluminescence, flat panel displays, infrared windows, sensors, lasers, bio-devices and catalytic applications [
In recent years, many methods have been developed to prepare semiconductor NPs using physical and chemical techniques. Physical methods such as liquid microwave irradiation [
Development of novel fluorescent sensors has attracted significant interest for selective and sensitive detection of metal ions in environmental and biological samples [
In the aspect of environmental pollution includes various heavy ions such as cations (Pb2+, Cd2+, Hg2+, Fe3+, Cr2+ and Cu2+ etc.) and anions (X−, IO3−, ClO4−, NO2−, S2−, CN− and N3− etc.) in different media was harmful to the environment as well as human health. Therefore, the detection of heavy ions is owing to their biological importance or environmental harm, have attracted more and more attention. There are many kinds of qualitative and quantitative methods for detection of various cations and anions in different media, using various techniques such as titrimetry, voltammetry, flow-injection analysis, inductively coupled plasma atomic emission spectroscopy, electrochemical methods, chromatography, chemiluminescence, colorimetry and fluorescence spectrometry [
Schiff bases, derived from the condensation of primary amines and aldehydes or ketones are characterized by the anil-linkage –HC=N–, possess structural similarities with natural biological substances. They have a wide variety of applications in biological, inorganic, clinical and analytical fields [
We report in this paper, the synthesis of Schiff base capped ZnS NPs using a simple co-precipitation method. The prepared ZnS NPs were characterized by several physicochemical techniques such as UV–vis DRS, PL, FTIR, XRD, SEM, TEM, and TGA. The prepared ZnS NPs exhibited simple cubic structure with the average size of the NPs is about 10 nm. The synthesized ZnS NPs were used as fluorescent sensors for detection of metal ions such as Mn2+, Ba2+, Al3+, Cd2+, Cr2+, Cu2+, Fe3+, Ni2+, Co2+ and Ag+. But Fe3+, Cr2+ and Cd2+ metal ions caused a significant fluorescence quenching of ZnS NPs. The prepared ZnS NPs as selective probes for the detection of several heavy metal ions, especially Fe3+, Cr2+ and Cd2+ in μM range of concentrations. Furthermore, the proposed NPs as sensors were employed for the determination of metal ions with satisfactory results.
Zinc acetate dihydrate, sodium sulfide, cadmium acetate, copper acetate, nickel acetate, chromium chloride and cobalt chloride chemicals were purchased from Sigma-Aldrich (97%, Bangalore Bonded Warehouse, India); manganese chloride, barium chloride and aluminium chloride were obtained from SD Fine-Chem Limited (Worli Road, Mumbai, India); ferric nitrate and silver nitrate were purchased from Finar Limited (Ellisbridge, Ahmedabad, India). Double distilled water was used throughout this experiment. All the reagents were analytical grade and used without further purification.
ZnS NPs were synthesized by co-precipitation method by adding an equal amount of Zn2+ solution and S2– as precipitating anion formed by decomposition of sodium sulfide nonahydrate. To prepare Schiff base capped ZnS NPs, 25 ml of 0.25 M zinc acetate and an equal quantity of 0.25 M sodium sulfide were dissolved separately in double distilled water. The solutions were stirred for 30 min using magnetic stirrer. The prepared 2-[(4-methoxy-phenylimino)-methyl]-4-nitrophenol was used as a capping agent to prevent agglomeration of ZnS NPs. In a separate beaker, 5×10−3 M of Schiff base was dissolved in 10 ml of methanol and was stirred. To the stirred solution of zinc acetate, a solution of Schiff base was poured drop by drop. After 30 min, the solution of sodium sulfide was poured drop by drop similarly. A very fine precipitate appeared soon after the addition of sodium sulfide. After formation of a white colored precipitate, the resulting ZnS precipitate was collected, filtered, washed with double distilled water and absolute ethanol several times to remove the unreacted chemicals, and finally dried in a furnace at 80 °C for 5 h. The probable reaction mechanism of formation of Schiff base capped ZnS NPs by the co-precipitation method is shown as follows in Fig.
X-ray diffraction (XRD) pattern was recorded on X’Pert PHILIPS, 30 kV, 40 mA using nickel-filtered CuKα radiations (λ = 1.5406 Å). The UV–vis diffuse reflectance spectra (UV–vis DRS) were recorded using a Shimadzu 3600 spectrophotometer in the spectral range of 200–800 nm. Transmission electron microscopy (TEM) was performed on TECNAI G2 and the microscope was operated at 200 kV. Samples were prepared by dispersing the powder in water. Imaging was recorded by depositing few drops of suspension on a carbon coated 400 mesh Cu grid. The solvent was left to evaporate before imaging. Scanning electron microscopy (SEM) images of fabricated ZnS NPs were obtained using ZEISS EVO18 electron microscope. Fourier transforms infrared (FTIR) spectra on KBr pellet were measured on a Shimadzu spectrophotometer in the range of 4000–400 cm−1. The photoluminescence (PL) spectrum was measured with an RF–5301PC spectrofluorometer (Shimadzu, Japan). The thermal behavior and degradation of the ZnS NPs were investigated by thermogravimetric analysis (TGA) in the temperature range of 40 to 800 °C at a heating rate of 10 °C min−1, under the nitrogen atmosphere. Zeta potential measurements were determined with the Zetasizer Nano ZS (Malvern Instruments, UK).
Fluorescence spectroscopy studies were carried out in order to evaluate the ability of the receptors to operate as cation sensors. As most of the tested metal ions are highly toxic and have adverse effects on human health, all experiments involving heavy metal ions and other toxic chemicals were performed with protective gloves. The waste solutions containing heavy metal ions were collectively reclaimed to avoid polluting the environment. The nitrate, acetates and chloride salts of the metal (Mn2+, Ba2+, Al3+, Cd2+, Cr2+, Cu2+, Fe3+, Ni2+, Co2+ and Ag+) were dissolved in double distilled water to prepare 1×10–3 M stock solutions. A solution of Schiff base capped ZnS NPs was prepared in double distilled water. Titration experiments were carried out in 1 cm quartz cuvette at room temperature. 2mL of ZnS NPs solution was placed in the quartz cell and the fluorescence spectrum was recorded. It was then titrated by successive additions in small portions (10 mL) of the solution of corresponding metal salt (1×10–6 M) and fluorescence intensity changes were recorded at room temperature. Various concentrations (10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500 μM) of Fe3+, Cr2+ and Cd2+ (0.5 ml) were added to the ZnS solution (2.5 ml) to test the sensitivity limits of Schiff base capped ZnS NPs. The mixtures were measured within seconds after adding metal ions by a fluorescence spectrophotometer (at λem = 430 nm, λex = 340 nm).
The optical methods take advantage of observing the essential characteristics of the nanomaterials without significantly modifying or permanently damaging them due to their non-contact and non-invasive nature. Nanoparticles are ideal for ultrasensitive and multiple applications in optical sensing, so there is a need to explore the optical properties of these nanoparticles [
One of the most important optical properties of the ZnS NPs is their PL emission, which depends on the size, shape, and surface energetic states, which are further influenced by surface passivation [
FTIR spectrum of the Schiff base capped ZnS NPs is shown in Fig.
XRD analysis was carried out to determine the crystalline nature of the Schiff base capped ZnS NPs and shown in Fig.
(1)
where k = 0.95, D is the particle size, λ is the wavelength of CuKα radiation and β is the corrected full width at half maximum of the diffraction peak. From the XRD pattern, the observed size of the Schiff base capped ZnS NPs is 14 nm at the high intense peak of (111) plane. The lattice parameters are calculated by the formula:
1/d2 = 1/a2(h2 + k2 + l2),
where a is lattice parameter, dhkl is the interplanar separation corresponding to Miller indices (h, k and l). The estimated value of the lattice constant (a) is 5.26 Å.
The morphology and size of the ZnS NPs were analyzed by SEM, energy-dispersive X-ray spectroscopy (EDX), TEM and selected area electron diffraction (SAED) as shown in Fig.
Zeta potential measurements have been carried out to check the stability of dispersed nanoparticles. Nanoparticles with less than 20 nm diameter have high mobility in solution due to the Brownian motion which highly affects the stability of particles. These electrostatic repulsions between particles depend on the pH. To attain higher zeta potential, nanoparticles should be away from the isoelectric point, therefore the zeta potential values are recorded for optimized pH. The negative zeta potential of ZnS suspensions in aqueous solution changes to positive and negative in the presence of cationic and anionic surfactants, respectively. These transformations in zeta potential sign can be crucial in defining the adsorption modes of surfactant aggregates at the NP surface in an aqueous medium. The adsorbed cationic and anionic surfactant aggregates have been thought to generate positively and negatively charged slipping plane respectively, by forming nearly bi-layer structure over the nanoparticle surface. The value of zeta potential for Schiff base capped ZnS NPs at pH 9 is −26.7 mV. These results are on par with zeta potential values of aqueous ZnS suspensions reported by Mehta et al. [
To determine the thermal stability and crystalline conditions of the synthesized Schiff base capped ZnS NPs, thermogravimetry analysis (TGA) was investigated as shown in Fig.
The fluorescence detection of various aqueous metal ions was performed at room temperature. For an effective sensor, the high selectivity for the target analyte over potentially competitive species is required. Competition experiments were carried out by recording the changes of the fluorescence intensity before and after adding the metal ions into the probe solution. To test the selectivity of Schiff base capped ZnS NPs for the metal ions; we investigated the fluorometric response in the presence of various metal ions at the concentration of 1 µM. The addition of 10 mL of 1×10-6 M of an aqueous solution of Mn2+, Ba2+, Al3+, Cd2+, Cr2+, Cu2+, Fe3+, Ni2+, Co2+ and Ag+ ions to the ZnS NPs, did not produce significant fluorescence intensity changes. Nevertheless, upon the addition of Fe3+, Cr2+ and Cd2+ ions to the solution containing ZnS NPs and other metal ions, immediate decrease in fluorescence emission was observed. The fluorescence intensity of probe towards the surveyed metal ions is displayed in Fig.
Fluorescence quenching of the ZnS NPs is also strongly affected by the concentration of some metal ions. This phenomenon arises from binding metal ions of interest to the surface of NPs as an acceptor and changing the surface state of NPs, which can be described clearly by the well-known Stern-Volmer equation:
(2)
Where, F0 and F are the fluorescence intensity in the absence and presence of the quencher (for example, Fe3+ ions), KSV is the Stern-Volmer quenching constant, and Q is the concentration of the quencher. The linear relationship (R2 = 0.98) of the Stern-Volmer plot of F0/F versus the metal ion concentration suggests that only one type of quencher is available and affects the fluorophore. In other words, the phenomenon of PL quenching can be used for the determination of quencher if a linear regression is found between the concentration of metal ion of interest and PL quenching just according to Stern-Volmer equation. In this case, the slope of the linear relationship is proportional to KSV if the intercept is on level with 1. Evidently, obtaining higher KSV values implies that the measurement is carried out with greater sensitivity. With regards to this criterion, PL quenching of Schiff base capped ZnS NPs was investigated in the presence of several heavy metal ions including Fe3+, Cr2+ and Cd2+ with different concentrations.
To understand the interaction between metal ions (Fe3+, Cr2+and Cd2+) and ZnS NPs, the response characteristics of nanoparticles to metal ions were systematically studied by fluorescence spectroscopy. As shown in Fig.
The fluorescence response of Schiff base capped ZnS NPs; in the presence of carious concentrations of (a) Fe3+, (b) Cr2+ and (c) Cd2+ ions; (d) bar diagram of the effect of Fe3+, Cr2+ and Cd2+ concentrations (10 to 100 μM) in the ZnS fluorescence intensity; (e) the relationship between relative concentrations (10 to 500 μM) of Fe3+, Cr2+ and Cd2+ with ZnS fluorescence intensity; (f) the relation between fluorescence response (F0/F) of ZnS NPs and the concentration of Fe3+, Cr2+ and Cd2+ from 10 to 100 μM.
The bar diagram of the different concentrations (10–100 µM) of these metal ions (Fe3+, Cr2+ and Cd2+) shows that the fluorescence intensity of ZnS NPs as a concentration dependent and it was shown in Fig.
Comparison of the previously reported methods with this work in the selective detection of Fe3+, Cr2+ and Cd2+ ions.
S. No. | Samples | Metal ion | Concentration | Detection limit | Mode of detection | Ref. |
1 | Carbon NCs | Fe3+ | 0.01–100 μM | 0.001 μM | Fluorometric | [53] |
2 | B,N,S-co-doped CDs | Fe3+ | 0.3–546 µM | 90 nM | Colorimetric,Fluorometric | [54] |
3 | Au NPs | Fe3+ | 0–180 µM | 11.3 nM | Colorimetric | [55] |
4 | Eu3+-Gd2O3NPs | Fe3+ | 0–10 ppm | 1.48 ppm | Fluorometric | [56] |
5 | Schiff base-ZnS NPs | Fe3+ | 10–500 µM | 10.24 µM | Fluorometric | Present study |
6 | Au NPs | Cr6+ | 10–200 nM | 0.9 nM | Electrochemical | [57] |
7 | CdS NPs | Cr3+ | 0.016–0.260 M | 16×10−9 M | Fluorometric | [58] |
8 | Rhodamine based sensor | Cr2+ | 0.07–3.5 mM | 64 μM | Fluorometric | [59] |
9 | Schiff base-ZnS NPs | Cr2+ | 10–500 µM | 31.48 µM | Fluorometric | Present study |
10 | Fe3O4NPs | Cd2+ | 1–15×10–8 M | 1.5×10-8 M | Fluorometric | [60] |
11 | MoS2 NSs | Cd2+ | 0–11.5 µM | 7.2×10-8 M | Optical | [61] |
12 | Ag2S QDs | Cd2+ | 0–100 µM | 546 nM | Fluorometric | [62] |
13 | Schiff base-ZnS NPs | Cd2+ | 10–500 µM | 64.56 µM | Fluorometric | Present study |
In summary, we report a simple method to fabricate and stabilize ZnS NPs using Schiff base by co-precipitation method. HR-TEM analysis showed that the synthesized NPs were less than 10 nm in size. The cubic phase of synthesized Schiff base capped ZnS NPs was observed from the XRD. The optical properties of the ZnS NPs were investigated by UV–vis DRS and PL spectroscopy. The obtained ZnS NPs showed a band gap of 3.98 eV, which is in agreement with published literature for cubic zinc blende structure. The interactions among ZnS NPs and metal ions were investigated using fluorescence studies. Under optimal conditions, the developed sensor was successfully employed to determine Fe3+, Cr2+ and Cd2+ ions in real samples and proved to be selective and as well as sensitive. The ZnS NPs exhibited good fluorescence quenching selectivity to Fe3+, Cr2+ and Cd2+ ions. Concentration experiments showed that there existed two parts of a linear relationship between fluorescence intensity and concentration of Fe3+, Cr2+ and Cd2+ ions in the range of 10–500 μM. The limit of detection (LOD) was estimated to be 10.24 μM, 31.48 μM and 64.56 μM for Fe3+, Cr2+ and Cd2+ ions, respectively. The output of this study clearly suggests that synthesized Schiff base capped ZnS NPs can be used as a promising nanomaterial for efficient fluorescent material for quenching and sensing applications.
The authors would like to acknowledge the Head, Department of Chemistry, Osmania University for providing the necessary facilities. One of the authors, D. Ayodhya wishes to thank the UGC, New Delhi which supported this work. The authors would like to thank DST-FIST, New Delhi, India for providing necessary analytical facilities in the department.