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Design and development a novel uranyl sensor based on FePt/ZnIn2S4 core-shell semiconductor nanostructures
⁎Corresponding authors. zeynali.ph@gmail.com (Hossein Zeynali), mhmf1359@yahoo.com (Mohammadhassan Motaghedifard)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
In this study, the FePt core, ZnIn2S4 shell and FePt-ZnIn2S4 core-shell nanostructures were successfully synthesized using solvothermal process. Temperature dependent hysteresis behavior of as-synthesized nanoparticles (NPs) FePt and FePt/ZnIn2S4 showed super paramagnetic response at 300 K and ferromagnetic properties with a coercive field (Hc) of 2830 Oe and 970 Oe at 2 K, respectively. Also, the blocking temperature (TB) estimating by the peak in ZFC curves was about 26 K for FePt and about 46 K for FePt/ZnIn2S4 NPs. After the identification process of nanostructure, using electrochemical methods, the behavior of FePt-ZnIn2S4 core-shell@PGE was studied in 0.1 M phosphate buffer solution (PBS) containing 5.0 mM [Fe(CN)6]3−/4−. The EIS complex plane plots showed a drastic change in the charge transfer resistance of the probe redox reaction as a function of UO22+ concentration. This behavior was used for construction of the calibration curve, and a linear range from 0.5 to 10.0 μM UO22+ with a detection limit of 71.7 nM.
Keywords
Core-shell nanostructures
Solvothermal process
Magnetization behavior
Uranyl glue
Electrochemical study
1 Introduction
In the last decade, there has been a growing high interest in one-dimensional nanostructures such as nanofibers, (Cheng et al., 2014), nanobelts (Xing et al., 2011), nanotubes (Chi et al., 2014), and nanorods (Carvajal et al., 2014; Khayatian et al., 2014), because of their unique properties and potential applications in many fields (Cheng et al., 2014; Xing et al., 2011; Chi et al., 2014; Carvajal et al., 2014; Khayatian et al., 2014; Wang et al., 2013; Hieu et al., 2014). Metal nanomaterials exhibit many unique electronic, optical, catalytic properties that are different from those of the corresponding bulk metal materials (Dotzauer et al., 2006). Because of advantages such as high catalytic activities, efficient electron transfer rate, large specific surface area and good chemical stability of the metal nano- materials (Yan et al., 2001; Balamurugan et al., 2011), in recent years, many researchers have developed electrochemical sensors based on nanoparticles. Recently, hybrid nanomaterials having core-shell structures based on shells such as ZnIn2S4 have started gaining attention of researchers in sensor technologies (Shim et al., 2013).
The only member of the AB2X4 family of semiconductors with a layered structure, ZnIn2S4 has been extensively studied because of its important potential uses in charge storage, electrochemical recording, thermoelectricity, and photocatalysis especially (Lei et al., 2003; Romeo et al., 1973; Seo et al., 1999). Various physical and chemical methods have been developed to prepare hybrid metal based nanoparticles of ZnIn2S4 such as nanosheet, nanotubes, nanoribbons, nanowires, and flower-like microspheres for solar water splitting (Peng et al., 2011; Chen et al., 2015; Zhou et al., 2013). On the other hand, many structures of ZnIn2S4 such as Cu, Co metal-doped ZnIn2S4 photocatalysts (Shen et al., 2008; Yuan et al., 2013), Multi-Walled Carbon Nanotubes/ZnIn2S4 (Chai et al., 2012), CdS/ZnIn2S4 (Yu et al., 2012), CuInS2/ZnIn2S4 (Bozhko et al., 2013), In2S3/ZnIn2S4 (Mei et al., 2013), and Reduce Graphene Oxide/ZnIn2S4 (Zhou et al., 2013) have been synthesized which were been improved the photo catalytic activity of them in compare to ZnIn2S4.
In other side, the attachment of nanostructured ZnIn2S4 material with structurally well-defined morphology onto a conductive substrate has been difficult to achieve and remains as a challenge. This disadvantage imposes barriers in the construction of device because stronger binding of electrode films onto conductive substrate is a necessary prerequisite for the fabrication of devices, especially for solar cells. Therefore, in present work, in order to improve the catalytic activity and well-defined distribution of ZnIn2S4 nanoparticles on the surface of electrode, we report the synthesis of core-shell FePt-ZnIn2S4 composite by a novel solvothermal method. This preparation method can effectively make ZnIn2S4 nanoparticles distribute on the surface of FePt super paramagnetic/conductive nanostructures.
Uranium is a ubiquitous metal of special concern because of its high toxicity and radioactivity (Li and Zhang, 2012; Li et al., 2009). In the past half-century, uranium has been widely used in many fields such as nuclear power station, nuclear weapons, industrial and medical fields. Recently, its usage is increasing with the development of nuclear energy. Therefore, human beings have a high chance of being exposed to uranium, which can cause serious harms to human health (Fang et al., 2010). Therefore, the study of uranium has already become a hot research topic in many fields such as environmental science, biological science, medical science and toxicology.
The methods including inductively coupled plasma atomic emission spectrometry (ICPAES) (Fujino et al., 2000), inductively coupled plasma-mass spectrometry (ICP-MS) (Ejnik et al., 2000), ion chromatography (IC) (Grudpan et al., 1995) and capillary zone electrophoresis (CZE) (Lu et al., 1998), graphite furnace atomic absorption spectrometry (GFAAS) (Khan and Yasmin, 2003), fluorimetry (Hong et al., 1989), spectrophotometry (Khan et al., 2001), neutron activation analysis (Byrne and Benedik, 1988) and voltammetry (Davis and Gray, 1964) have been used for determination of uranium concentrations. However, these methods require expensive equipments and higher running cost and their availability is limited.
Among different strategies explored to reach this goal, electro analytical methods are simpler in means of equipment and expenditure and yet are potent tools for analysis of uranium in watery environments. Electro analysis has a major advantage over the aforementioned techniques, as it allows speciation of uranium in solution by direct measurement, and as such may generally be regarded as a nondestructive detection method.
According to our knowledge, no report has been published on determination of UO22+ using core-shell nanostructures based on electrochemical sensors. So, the purpose of this study was; (i) firstly, solvothermal synthesis of FePt-ZnIn2S4 core-shell nanostructures and its characterization; and in continuation, (ii) application of FePt-ZnIn2S4 core-shell@PGE as a new electrode for UO22+ monitoring in phosphate buffer solution.
2 Experimental
2.1 Materials and physical measurements
Platinum (II) acetyl acetonate (97%), tris (acetyl acetonato) iron (III) (99.9%), Indium(III) Nitrate Pentahydrate, In(NO3)3·5H2O (99.9%), anhydrous ethyl alcohol and hexane were purchased from Merck and 1,2-hexadecanediol (90%), oleic acid (99%) and oleyl amine (70%), Thioacetamide (TAA, >99%) and Zinc acetyl acetonate hydrate powder, Zn(acac)2 (99.9%), from Sigma-Aldrich. All the chemicals were used without further purification. X-ray diffraction (XRD) patterns were recorded by a Philips-X’PertPro, X-ray diffractometer using Ni-filtered Cu Kα radiation at scan range of 10 < 2θ < 80. The energy dispersive X-ray spectroscopy (EDXS) analysis was obtained on a Philips EM208 microscope. Transmission electron microscope (TEM) images were obtained on a Philips EM208S transmission electron microscope with an accelerating voltage of 100 kV.
The electrochemical studies were carried out using an Autolab potentiostat-galvanostat PGSTAT 35 (Ecochemie Utrecht, Netherlands), equipped with the GPES 4.9,006 software and FRA 4.9 software. The experiments were carried out in a three-electrode glass cell with Ag/AgCl (3.0 M KCl) as a reference electrode, a platinum wire as counter electrode and a pencil graphite electrode (PGE) as working electrode. The EIS measurements were done in a 5.0 mV ac amplitude potential superimposed on the formal potential of the redox probe (E°′ = +0.250 V) in a wide frequency range from 10 kHz to 100 mHz. All experiments were carried out at an ambient temperature of 25 ± 0.1 °C.
Magnetic measurements were done in a cryogenic free vibrating sample magnetometer operating using a closed-cycle He cryostat. The PPMS DynaCool (Quantum Design) in VSM option were used with a sensitivity of 0.016 mT, operating magnetic field up to 9 T and temperature range of 1.8–300 K. Sample holders were cylindrical rods made of polypropylene with a diameter of about 3 mm. The sample occupies a height of about 2 mm inside the rod.
2.2 Synthesis of FePt core nanoparticle
Nuclear magnetic FePt nanoparticles as a core were synthesized using solvothermal method under standard airless process in accordance with the following command: initial precursors containing iron (III) acetylacetonate (0.50 mmol), platinum (II) acetylacetonate (0.25 mmol), 1,2-hexadecanediol (2.5 mmol), oleic acid (5.0 mmol), oleyl amine (5.0 mmol) were mixed in 10 mL benzyl ether and magnetically stirred for 20 min under a flow of N2 atmosphere. Then the mixture was heated to boiling benzyl ether at a rate of approximately 5 °C/min and kept at this temperature for 15 min. After completion of the growth of nanoparticles, the heat source was separated from the reaction vessel allowing the mixture to cool at room temperature. The resulting black nanoparticles were isolated by adding 40 mL of ethanol and centrifugation. Then, the ethanol impurities were separated and black precipitation was dispersed in hexane in the presence of oleic acid and oleyl amine. In order to completely remove the extra impurities, the centrifugation was repeated twice. Finally, the FePt core nanoparticles were dispersed with the stabilizer in hexane.
2.3 Synthesis of ZnIn2S4 shell nanostructures
For synthesis of ZnIn2S4 nanostructures as a shell, stoichiometric quantities of thioacetic acid (TAA) (150 mg), In (NO3)3·5H2O (300 mg), Zn (acac)2 (130 mg), 1, 2 hexadecane diol (365 mg), oleic acid (1200 µL), oleyl amine (1225 µL) were mixed in 10 mL benzyl ether and magnetically stirred under a flow of N2 atmosphere for 20 min at room temperature. Then, the resulting mixing was heated from 100 °C to boiling point of benzyl ether (300 °C) at a rate of approximately 5 °C/min. After the reaction, the heat source was removed from the reaction vessel allowing the mixture to cool at room temperature. Then, the resulting nanostructures were filtrated and separated by adding 40 ml of ethanol and centrifugation in 8000 rpm for 10 min. Finally, the nanostructures were dried in a vacuum oven at a temperature of 70 °C for two hours.
2.4 Synthesis of FePt-ZnIn2S4 core-shell nanostructures
The process for the synthesis of FePt-ZnIn2S4 core-shell nanostructures was done as following: based on stoichiometry, a controlled amount of TAA (150 mg), In (NO3)3·5H2O (300 mg) and Zn (acac)2 (130 mg) were dissolved in a mixture of 10 mL benzyl ether, 1,2 hexadecane diol (365 mg), oleic acid (1200 µL), oleyl amine (1225 µL). The result was mixed in and magnetically stirred under a flow of N2 atmosphere for 20 min at room temperature. Then, the synthesized FePt core nanoparticles dissolved in 10 mL hexane were added to the reaction vessel to be mixed with other ingredients and magnetically stirred under a flow of N2 atmosphere for 20 min at room temperature. For hexane evaporation, the reaction vessel was heated from room temperature to 100 °C for 20 min. Then, the mixture was heated from 100 °C to boiling point of benzyl ether (300 °C) at a rate of approximately 5 °C/min. After the reaction, the heat source was removed from the mixture allowing it to cool at room temperature. The core-shell nanostructures were filtrated and separated by adding 40 mL of ethanol and centrifugation in 8000 rpm for 10 min. At final, the obtained nanostructures dissolved in a mixture of oleic acid and oleyl amine in the presence of Hexane (10 mL) were centrifuged to separate impurities from synthesized nanoparticles.
2.5 Fabrication of FePt-ZnIn2S4@PGE
Firstly, the pencil graphite electrode (PGE) was polished on fine emery paper and then polished by slurry of alumina powder containing 0.05 μm grains of Al2O3. After this, the surface of electrode was rinsed with distilled water and dried at 25 °C. Then, 5 μL of the fine dispersed solution of the FePt-ZnIn2S4 nanoparticles in dimethylformamide (DMF) was dropped onto the surface of cleaned PGE and was dried in a hot air flow at 50 °C. After modification, the FePt-ZnIn2S4@PGE was washed with double distilled water, dried at room temperature and applied to electrochemical experiments. Scheme 1 shows summary of synthesis process and modifying the electrode using core shell nanostructures.
3 Results and discussions
3.1 Characterization of FePt, ZnIn2S4 and FePt-ZnIn2S4 nanostructures
Fig. 1a shows a TEM image of as-synthesized FePt nanoparticles. According to Fig. 1b, average diameter of FePt core particles (〈d〉), based on log-normal fitting, is about 4.1 nm and standard deviation (σ) of about 0.36 nm, indicate narrow size distribution (σ/〈d〉) of 0.09. Inset photography in Fig. 1b, shows HRTEM an image of a FePt particle which is highly spherical and mono dispersed.
Fig. 2(a–d) shows TEM and HRTEM images with different magnification of as-synthesized FePt/ZnIn2S4 core/shell nanoparticles which have also spherical shapes and are well isolated. HRTEM image of FePt/ZnIn2S4 (Fig. 2c and d) indicates that FePt cores are individually surrounded by ZnIn2S4 crystals.
Energy Dispersive Spectroscopy (EDS) analysis were employed to investigate the chemical composition of as-synthesized FePt and ZnIn2S4 nanostructures. The EDS profile of Fig. 2(e), reveals that equal atomic composition of FePt nanostructures, which is clear from the elemental peaks. The Peaks of element In, Zn and S are detected in the EDS profile (Fig. 2f). Atomic ratio of elements Zn:In:S is very close to 1:2:4, which is confirming the purity of the nanostructures.
Fig. 3A (a–c) shows the XRD patterns of FePt, ZnIn2S4 and FePt/ZnIn2S4 nanoparticles, respectively. As clearly can be seen, the pattern of FePt/ZnIn2S4 structure shows existence of two set peaks: one set is a chemically disordered fcc FePt phase and the other is hexagonal structured ZnIn2S4. The XRD peak intensities of Fig. 3A.c have been considerably decreased compared to Fig. 3A.a and A.b. Also the diffraction peaks of Fig. 3A.c are broader than the peaks of Fig. 3A.a and A.b. The reduction in peak intensities and peak broadness of the core/shell nanoparticles may be ascribed to the decrease of inter planer separation of crystal lattice. These results definitively showed that well defined FePt/ZnIn2S4 core/shell nanoparticles were successfully synthesized.
Fig. 3B.a-b shows the UV–Vis absorption spectra of the as-synthesized ZnIn2S4 and FePt/ZnIn2S4 core/shell nanoparticles. Both samples present strong similar absorption profiles with absorption edge at about 490 nm. From the absorption edge values, the corresponding band gap energy (Eg) for the ZnIn2S4 and FePt/ZnIn2S4 samples is 2.53 eV. The very slight shift of absorption edge for the FePt/ZnIn2S4 sample may be due to the large crystallite size and different particle morphology.
Temperature dependent hysteresis behavior of as-synthesized FePt and FePt/ZnIn2S4 showed super paramagnetic response at 300 K and ferromagnetic properties with a coercive field (Hc) of 2830 Oe and 970 Oe at 2 K, respectively. The coercivity values of FePt NPs drops sharply as temperature raised from 2 K to 4 K, 15 K and 20 K corresponding to 2830 Oe, 2130 Oe, 400 Oe and 160 Oe respectively, indicating that the size of the nanoparticles are thermally unstable. This is consistent with the low magneto crystalline anisotropy of fcc structure of the nanoparticles (Fig. 4A and C). The saturation magnetization of FePt is 1.65 emu/g at 300 K, which is much lower than the value of 7.73 emu/g at 2 K, being this is typical behavior of fcc FePt nanoparticles.
Temperature dependent magnetization in zero field cooled (ZFC) and field cooled (FC) of FePt and FePt/ZnIn2S4 NPs is shown in Fig. 4B and D. The blocking temperature (TB) estimated by the peak in ZFC curve is about 26 K for FePt and about 46 K for FePt/ZnIn2S4 NPs. It indicates that ZnIn2S4 thin layer covers individual FePt NPs, which is consistent with the TEM and XRD results. Therefore inter particle separation increase in ZnIn2S4 coated FePt NPs, the reducing magnetic dipole-dipole interacting, this leads to a significant shift in TB to higher temperature. Furthermore both ZFC-FC overlap a wide temperature range above TB, meaning that the particles are non-interacting. The saturation magnetization of FePt/ZnIn2S4 NPs at 2 K decreased to 2.248 emu/g compared to FePt (7.73 emu/g), could be attributed to the presence of non magnetic ZnIn2S4 compound in the core/shell structure. The magnetic properties of as-synthesized FePt and FePt/ZnIn2S4 are summarized in Table 1 as extracted from Fig. 4(A–D).
| Sample | fcc-FePt | fcc Fet/ZnIn2S4 |
|---|---|---|
| Tb | 26 K | 46 K |
| Hc @ 2 K (Oe) | 2830 | 970 |
| Hc @ 300 K (Oe) | 0 | 0 |
| MS @ 2 K (emu/g) | 7.73 | 2.24 |
| MS @ 300 K (emu/g) | 1.65 | 0.11 |
3.2 Electrochemical application of FePt-ZnIn2S4 core-shell NSs
Cyclic voltammetry (CV) method was used to evaluate electrochemical behavior of different electrodes. So, CVs for 5.0 mM Fe(CN)63−/4− in 0.1 M phosphat buffer solution (PBS) on the bare PGE, FePt nanoparticles@PGE, FePt-ZnIn2S4 core-shell@PGE and UO22+@FePt-ZnIn2S4 core-shell@PGE were drawn (Fig. 5). The quasi reversible one-electron redox behavior of ferricyanide ions was observed on the bare PGE with a peak separation (ΔEp) of 0.086 V at the scan rate of 50 mV s−1. After the modification of the electrode using FePt nanoparticles, the peak currents for Fe(CN)63−/4− increased about 1.37 times than for bare PGE, while the ΔEp decreased (0.078 V). By immobilization of FePt-ZnIn2S4 core-shell nanostructures onto PGE, the peak current and peak potential unexpectedly changed for redox activity of Fe(CN)63−/4− at the surface of core-shell electrode. In this case, ΔEp was found to be 0.177 V with reduced peak current about 4.2 times than bare PGE. The sharp decreasing in current can be due to the inhibitory effects of core-shell coating at the surface of electrode against diffusion of the probe ions to the surface of electrode. After the accumulation of uranyl ions on the surface of electrode, (UO22+@FePt-ZnIn2S4 core-shell@PGE), the redox responses of Fe(CN)63−/4− was changed. The reason for small increase in faradic current observed for UO22+@FePt-ZnIn2S4 core-shell@PGE in the presence of [Fe(CN)6]3−/4− is a resultant of electrostatic attraction which exists between positively charged surface and negatively charged probe, and blocking effect of UO22+@FePt-ZnIn2S4 core-shell with dominant effect of electrostatic attraction. These results are summarized in Table 2.
| Electrode | Ea (V) | Ia (µA) | Ec (V) | Ic (µA) |
|---|---|---|---|---|
| Bare-PGE | 0.237 | 8.85 | 0.150 | −9.13 |
| FePt NPs/PGE | 0.228 | 12.11 | 0.150 | −12.03 |
| FePt-ZnIn2S4 @ PGE | 0.282 | 2.11 | 0.105 | −2.21 |
| UO22+@FePt-ZnIn2S4 @ PGE | 0.253 | 5.424 | 0.134 | −5.60 |
In an attempt to clarify the differences among the electrochemical performance of the bare PGE, FePt nanoparticles@PGE, FePt-ZnIn2S4 core-shell@PGE and UO22+@FePt-ZnIn2S4 core-shell@PGE, electrochemical impedance spectroscopy (EIS) was used as a procedure to study electrodes surface. EIS is a suitable technique for investigation of the electrode surface dependent charge transfer process (interfacial properties, i.e., resistance and capacitance) (Bonanni et al., 2012; Bagheri et al., 2013). The experiments are done in Fe(CN)63−/4− redox probe by focus on the variations of the charge transfer resistance (Rct) between solution and electrode surface. By immobilization of nanostructures in each step, the value of Rct for modified electrode was changed. To construe data obtained from Nyquist plots, an equivalent circuit model was used to fit the results. In this circuit, Rs, Q and Rct represent solution resistance, a constant phase element model parameter which is sometimes shown as Y0 or T and named as double layer parameter and electron transfer resistance, respectively. W is a finite-length Warburg short-circuit term coupled to Rct, which accounts for the Nernstian diffusion. In our work, the analysis of Nyquist plot showed a large decrease in Rct at FePt nanoparticle@PGE compared to bare PGE in 0.1 M PBS solution (Fig. 6), indicating a significant increase in the rate of charge transfer. However, after fabrication of PGE with FePt-ZnIn2S4 core-shell, the semicircle increased distinctively, indicating that FePt-ZnIn2S4 core-shell could inhibited the electron transfer between the electrochemical probe [Fe(CN)6]3−/4− and bare PGE (Fig. 6). With the accumulation of uranyl ions on the surface of the FePt-ZnIn2S4 core-shell@PGE, these ions act as an intermediary exchange of electrons between the [Fe(CN)6]3−/4− and the surface of electrode. Therefore, by increasing the concentration of UO22+ onto core-shell nanostructures, gradually reduced charge transfers resistance. The results are summarized in Table 3.
| Electrode | Rs (Ω) | Rct (Ω) | Q (sn Ω−1) | W | |
|---|---|---|---|---|---|
| Y0 | n | ||||
| Bare-PGE | 267.76 | 2908.9 | 3.26 × 10 −5 | 0.824 | 2.03 × 10−4 |
| FePt NPs @ PGE | 244.89 | 564.2 | 9.39 × 10 −6 | 0.904 | 2.26 × 10 −4 |
| FePt-ZnIn2S4 @ PGE | 265.21 | 18723 | 6.47 × 10 −6 | 0.733 | 3.06 × 10 −4 |
| UO22+@FePt-ZnIn2S4 @ PGE | 253.46 | 5303.6 | 2.55 × 10 −5 | 0.710 | 1.85 × 10 −4 |
3.3 pH optimization for accumulation of UO22+
According to sensitivity of uranyl ions to the presence of phosphate ions in solution, the sodium acetate/acetic acid buffer solution was selected and optimized at pH = 4.0 for continue measurements which 0.025 M of sodium perchlorate (NaClO4) was added to this solution to create the perfect environment for transferring and accumulation of uranyl ions from the bulk solution to the surface of modified electrode. Then, Nyquist plots were obtained in 0.1 M PBS in presence 5.0 mM [Fe(CN)6]3−/4− probe from accumulation of 1.0 μM of UO22+ onto FePt-ZnIn2S4 core-shell@PGE at different pH values of 0.1 M sodium acetate/acetic acid buffer solution containing 0.025 M of sodium perchlorate. The Rct of UO22+@FePt-ZnIn2S4 core-shell@PGE was strongly depended upon pH of the pre-concentration solution and the best conditions was achieved around pH = 4.0.
3.4 Optimization of accumulation time
To optimize the accumulation time of UO22+ ions, the Nyquist plots of FePt-ZnIn2S4 core-shell@PGE against immersion time in UO22+ solution were draw in [Fe(CN)6]3−/4− probe solution at the formal potential (DC, 0.25 V) and a frequency range of 0.1 Hz–100 kHz with an ac amplitude of 5.0 mV. Then, the changing procedure for charge transfer resistance (Rct) of 5.0 mM [Fe(CN)6]3−/4− at the UO22+@FePt-ZnIn2S4 core-shell@PGE as a function of immersion time of the FePt-ZnIn2S4 core-shell@PGE electrode in UO22+ solution was investigated. According to results, a decreasing in Rct was observed when the immersion time increases from 0 to 25 min, which in times longer, the charge transfer resistance did not change. Therefore, 25 min was chosen as the optimum immersion/accumulation time for adsorption of UO22+ in this work.
3.5 Calibration curve and limit of detection for UO22+
The EIS measurements obtained under the optimized conditions at a constant DC potential of +0.25 V vs. Ag/AgCl reference electrode on FePt-ZnIn2S4 core-shell@PGE, after accumulation of UO22+ from solutions containing different concentrations of UO22+ are presented in Fig. 7. The charge transfer resistance of modified electrode was decreased by increasing the UO22+ concentrations in accumulation solution (Fig. 7A). With approximating experimental data using [R(Q[RW])] as CPE model (Fig. 7B), the values of Rct and other parameters were extracted. These behaviors allowed constructing of the calibration curve for determination of UO22+. Two linear calibration curve ΔRct/Ω = 1918.9 [UO22+] + 1560.9, R2 = 0.9935 from 0.5 to 6.25 μM UO22+ and ΔRct/Ω = 586.69 [UO22+] + 9536.1, R2 = 0.9989 from 6.25 to 10.0 μM UO22+ and a detection limit of 71.7 nM was found by using ΔRct as a function of UO22+, where ΔRct = Rct in UO2=0 − Rct in UO2=x. To compare among our paper and other papers, it is important that investigated several parameters simultaneously; novelty of sensor (No papers have yet been reported for application of core shell nanostructures in dedection of uranyl ions.), detection limit, linear rang of calibration curve, simplicity for preparation, selectivity and sensitivity of electrode. With comparison between these factors, it seems that this modified electrode has novelty with agreeable detection limit, selectivity than to other cations as interferences and agreeable dynamic range for uranyl against other reported papers. Table 4 shows summary of this comparison. Also, The relative standard deviations of ΔRct as 1.32% (n = 4) were found for 1.5 μM UO22+, respectively, which is very small implying the repeatability of the electrode response obtained by using EIS method.
| Sensor | Detection method | Limit of detection | Linear dynamic range | Ref |
|---|---|---|---|---|
| Uranyl ionic imprinted polymer | Potentiometric | 1.0 µM | 3.0–6000 1 µM | Abu-Dalo et al. (2016) |
| 6-O-palmitoyl-l-ascorbic acid (PAA)-modified graphite (GRA) electrode | Differential pulse voltammetry (DPV) | 1.8 µg L−1 | 2.7–67.5 µg L−1 | Dimovasilis and Prodromidis (2011) |
| Carboxylated graphene modified Glassy carbon electrode | DPV | 130 nM | 0.05–5. 0 µM | Ziółkowski et al. (2017) |
| Tetraphenylethene-based fluorescent sensor | Colorimetric | – | 1.0–20.0 µM | Wena et al. (2016) |
| POCl3/mercapto ethanol/Au | Square wave voltammetry | <1 µM | 1.0–10.0 µM | Becker et al. (2009) |
| PBED optical sensor | Spectrophotometery | 0.999 µM | 3.99–80.6 µM | Ghaedi et al. (2012) |
| FePt-ZnIn2S4 core-shell@PGE | EIS | 71.7 nM | 0.5–10.0 µM | This work |
3.6 Interference study and real sample analysis
To evaluate the ability of FePt-ZnIn2S4 core-shell@PGE in sensing of UO22+ ions, the effect of some cations and anions, such as Pb2+, K+, NH4+, Ni2+, Zn2+, Mn2+, Cu2+, Cr2O72−, CO32−, Cl− and SO42− was investigated.
In accordance to the ±5.0% relative error (Honarmand et al., 2016), the interferences ions did not show any negative impact on the measurement of ions uranyl. These results indicated that under optimized conditions, the selective and sensitive determination of UO22+ ions could be possible in spiked samples at the FePt-ZnIn2S4 core-shell@PGE. Therefore, the recovery tests were measured based on standard addition method by spiking different concentrations of UO22+ solution into watery samples. Recoveries were found to be 96.3 and 98.0% for 2.0 μM UO22+, respectively. The obtained results were summarized in Table. 5.
| Sample | Added (µM) | Found (µM) | Recovery (%) | Recovery standard deviation | Content of sample (µM) |
|---|---|---|---|---|---|
| Synthetic sample | |||||
| (I) | 1.00 | 2.96 | 98.0 | 0.03 | 2.0 |
| (II) | 4.50 | 9.70 | 96.3 | 0.14 | 5.4 |
| Mineral water | 2.00 | 1.95 | 97.0 | 0.07 | No or < LOD |
4 Conclusions
Solvothermal method was used for the synthesis of FePt core, ZnIn2S4 shell and FePt ZnIn2S4 core-shell nanostructures in the presence of 1,2-hexadecanediol, oleyl amine and amyl alchol as a reducing agents. According to chemical affinity of modified electrode to uranyl ion adsorption, the FePt ZnIn2S4 core-shell@PGE was fabricated and applied for measuring of uranyl cations. So, by accumulation of UO22+ from solutions containing different concentrations of UO22+, a detection limit of 71.7 nM of UO22+ was found by EIS method. The RSD in peak current (1.32%, n = 4) for modified electrode shown good repeatability of modified electrode in absorbance of UO22+.
Acknowledgment
This work was supported by the Islamic Azad University Kashan Branch. Access to TAIL-UC facility funded under QREN-Mais Centro Project ICT-2009-02-012-1890 is gratefully acknowledged.
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