Structural and morphological tuning of Cu-based metal oxide nanoparticles by a facile chemical method and highly electrochemical sensing of sulphite

A facile one-step chemical method is introduced for the successful synthesis of Cu2O, CuO and CuNa2(OH)4 crystal structures and their electrochemical properties were also investigated. X-ray diffraction studies revealed that these copper-based oxide nanoparticles display different crystal structures such as cubic (Cu2O), monoclinic (CuO) and orthorhombic [CuNa2(OH)4]. The microstructural information of nanoparticles was investigated by transmission electron microscopy. It shows attractive morphologies of different orientation such as rod like structure, nanobeads and well-aligned uniform nanorod for Cu2O, CuO and CuNa2(OH)4, respectively. Electrochemical sensing of sulphite (SO32−) on these three copper-based oxide modified electrodes was investigated. Among the three different crystal structures, CuO shows promising electrocatalytic activity towards oxidation of sulphite. A linear variation in peak current was obtained for SO32− oxidation from 0.2 to 15 mM under the optimum experimental condition. The sensitivity and detection limit were in the order of 48.5 µA cm−2 mM−1 and 1.8 µM, respectively. Finally, practical utility of CuO modified electrode was demonstrated for the estimation of sulphite in commercial wine samples.

A facile one-step chemical method is introduced for the successful synthesis of Cu 2 O, CuO and CuNa 2 (OH) 4 4 , respectively. Electrochemical sensing of sulphite (SO 3 2− ) on these three copper-based oxide modified electrodes was investigated. Among the three different crystal structures, CuO shows promising electrocatalytic activity towards oxidation of sulphite. A linear variation in peak current was obtained for SO 3 2− oxidation from 0.2 to 15 mM under the optimum experimental condition. The sensitivity and detection limit were in the order of 48.5 µA cm −2 mM −1 and 1.8 µM, respectively. Finally, practical utility of CuO modified electrode was demonstrated for the estimation of sulphite in commercial wine samples.

crystal structures and their electrochemical properties were also investigated. X-ray diffraction studies revealed that these copper-based oxide nanoparticles display different crystal structures such as cubic (Cu 2 O), monoclinic (CuO) and orthorhombic [CuNa 2 (OH) 4 ]. The microstructural information of nanoparticles was investigated by transmission electron microscopy. It shows attractive morphologies of different orientation such as rod like structure, nanobeads and wellaligned uniform nanorod for Cu 2 O, CuO and CuNa 2 (OH)
Electrochemical sensor research is one of the important areas because of its application in numerous fields including drug, industrial, food, environmental and so on. Sulphite (SO 3 2− ) is used as a food additive and also as an inhibitor to prevent the microbial reactions 1 . Sulphite improves the appearance of foods and wines and maintains their quality as well. The US-FDA recommended levels of SO 3 2− in food are below 10 mg/kg and liquid items 10 mg/L -12 . Excess amount of SO 3 2− in food products leads to form number of symptoms which includes asthma in our human body and number of changes in the organoleptic properties of raw materials. In some cases, till now SO 3 2− was used in wine and some other food as an additive because other additives are not found for the replacement of SO 3 2− . On the other hand, SO 3 2− is a precursor to produce acid rain, which acidifies the water bodies, soil and harms trees, crops, monuments and buildings 3,4 . Therefore, estimation of SO 3 2− is significant in the trace analysis of food, surface water and drinking water and so on.
The Association of Analytical Chemists (AOAC) has recommended Monier-Williams method as a standard method for the detection of sulphite. Besides long analysis time, the conventional titrimetric method also suffers from poor precision. In the pursuit of suitable alternative, a number of methods have been established for the detection of SO 3 2− such as ion chromatography 5 , FIA/gas diffusion 6 , spectrophotometric detection 7-12 , flow injection analysis (FIA) 13 , colorimetric titration 14 , chemiluminescence 15,16 , and electrochemical estimation [17][18][19] . Among these methods, electrochemical technique is attractive due to high selectivity, sensitivity, wide concentration range, low-cost, simplicity and so on. The electrooxidation of SO 3 2− on conventional electrodes shows high over potential due to sluggish electron transfer 20,21 . Therefore, the electrochemical method based on modified electrodes is an attractive approach for the detection of SO 3 2− . Nanomaterials accomplish an essential part in the electrochemical sensing of trace amount food additives, pharmaceutical compounds, harmful pollutants and heavy metal ions as nanomaterials show different  [22][23][24] . Nowadays, sensing platform based on nanostructured metal oxides especially copper oxide, CuO nanocomposites with other transition metal oxides and carbon materials were explored for determination of different analytes. CuO is a P-type semiconductor with narrow bandgap (E g ) of 1.2 eV, which shows attractive properties such as high electrical conductivity, good stability, efficient electrode in photovoltaics, high mechanical strength, high catalytic activity and high temperature durability. Moreover, CuO is low cost and abundant non-toxic semiconductor [25][26][27][28][29] . The CuO based materials are widely used in gas sensor, photocatalyst and lithium ion electrode materials. Recently, a number of electroactive sensor platforms were extensively used to analyse pharmaceutical and biologically important compounds [30][31][32][33][34] . Numerous methods including sol-gel, hydrothermal, sonochemical, thermal evaporation, microwave irradiation and electrochemical approach have been reported for preparation of CuO, Cu 2 O and other coper oxide-based nanomaterials. Even though some of the above methods seem as simple, but it is hard to control crystal structures and morphology using a single method. Therefore, it is essential to design and development of a unique synthesis method for preparation of different crystal structures with different morphology by the single method in industrial-scale. In this direction, for the first time we have prepared different types of Cu based nanomaterials such as CuNa 2 (OH) 4

Synthesis of the copper-based metal oxides.
To synthesis various structure of copper-based oxide nanoparticles, a simple solution phase method was used. Typically, 0.2 M copper (I) thiocyanate (CuSCN) was dissolved in de-ionized water. Next, 1 g of PVP was added into the above solution and stirred until completely dissolved. Then, 0.25, 0.5, 1.0 M NaOH pellets were dropped into the above solution followed by addition of constant volume of 5 ml hydrazine hydrate. The resultant solution was stirred for 2 h at room temperature. Then the obtained precipitate was washed by repeated centrifugation at 4000 rpm for 20 min. Finally, the wet samples were dried at 120 °C for 6 h.

Materials and electrochemical characterizations.
Characterization of Cu based metal oxide nanoparticles were carried out by using following techniques. The morphological study and energy dispersive X-ray analysis (EDX) were performed using scanning electron microscopy (SEM) by SEM-JEOL JSM-6380LV. X-ray diffraction pattern (XRD) of the powder samples was obtained with PW3040/60 X'pert PRD X-ray powder diffractometer equipped with a scintillation counter using Cu Kα radiation (λ = 0.1540 nm). The transmission electron microscopy (TEM) characterization was carried out using JEM 2100 F with 200 kV acceleration voltages. ESCA+Omicron UK XPS system was used for X-ray photoelectron spectroscopy (XPS) analysis with an Mg-Kα source. The functional group analysis was performed with Thermo Nicolet 200. In the electrochemical studies we used three electrode system and AUTOLAB PGSTAT302N (NOVA) instrument. Working electrodes are CuO/GCE, Cu 2 O/GCE and CuNa 2 (OH) 4 /GCE, reference electrode is saturated calomel electrode and counter electrode is platinum wire. In order to remove the dissolved oxygen, the experimental solution was purged with high purity inert N 2 gas.

Results and discussion
Physical characterization of the samples. X-ray diffraction is the most widely recognized study to evaluate structural and quality of the samples. The size of powder samples can be estimated reliably from the broadening of diffraction peaks. Additionally, the crystallite size which depends on the width of diffraction peaks was approximately evaluated using the Debye-Scherrer's equation 35 . As can be seen in Fig. 1, all the diffraction peaks obtained from Cu 2 O sample are well matched with standard data (JCPDS No. 01-077-0199). The calculated crystallite size was about 22.2 nm. The diffraction peaks obtained for CuO nanoparticles are well indexed to the monoclinic structure of CuO (JCPDS No. 01-080-0076) with superior crystal quality. No additional peaks were obtained, which demonstrates the high quality of product. The above result clearly shows that 0.5 M NaOH is more favorable concentration for the preparation of high quality monoclinic CuO nanoparticles. The average crystallite size was found to be in the range of 21.6 nm. More interestingly, orthorhombic structure of copper sodium hydroxide (CuNa 2 (OH) 4 Fig. 2b,d,f, respectively. The SEM micrographs clearly showed that the particles were roughly agglomerated with homogeneous morphologies. The EDAX result showed only the elements present in the spectra for the corresponding samples and not for any other impurities or secondary products. The obtained EDX results confirmed that only Cu and O ions are present in the prepared nanoparticles (Fig. 2b,d) with the same ratio proportion as defined at the time of experiment. The atomic percentage of element of the corresponding samples is presented in table (inset of Fig. 2b,d,e).
To evaluate exact sizes and morphology of the nanoparticles, TEM measurement was performed. Figure 3a,b illustrates the morphology of highly crystalline Cu 2 O nanoparticles. The TEM images showed the presence of rod like morphology with an average diameter of 20-25 nm and length of ~ 500 nm. Uniform beads like morphology of CuO are depicted in Fig. 3d,e. The average size of the nano beads is about 25 nm. As shown in Fig. 3g,h, a bunch of transparent uniform nanorods structure obtained for CuNa 2 (OH) 4 when 1 M of NaOH was used. TEM results clearly revealed that variation of precursor concentration (NaOH) is not only modified the crystal structure but also tuned the morphology. To further investigate structural information, SAED patterns were recorded for all the three samples of Cu 2 O, CuO and CuNa 2 (OH) 4 , the obtained results are presented in Fig. 3c,f,i, respectively. The well-distinguished fringes in the SAED pattern confirm the highly crystalline nature of samples. Figure 4a shows survey spectrum of XPS analysis for CuO nanoparticles. The XPS result shows (Fig. 4b) existence of two binding energies, for Cu 2p of CuO sample, at 933.8 eV (Cu 2p 3/2 ) and 953.5 eV (Cu 2p 1/2 ) with a difference of 19.7 eV, which proves the formation of copper (II) oxide 35 . The presence of two satellite peaks at higher binding energies of 941.4 eV and 961.6 eV are typical of materials having d9 configuration in their ground state that obviously shows the presence of Cu 2+35,36 . As well-documented, the spectra of the O1s (Fig. 4c) core level for CuO can be deconvoluted into two components located at 530.10 eV and 530.96 eV 37 . These two parts are attributed to the different chemical state of oxygen, where the peak at lower binding energy ascribed to the oxygen (O 2− ) associating with Cu 2+ ion in the CuO structure. Figure 4d shows the characteristic binding energy of 283.6 eV corresponding to C 1 s.
FT-IR spectroscopy was used to identify the functional groups present in the materials. Figure 5 shows FT-IR spectra of Cu 2 O, CuO and CuNa 2 (OH) 4 nanostructures. Among the FT-IR spectra, CuNa 2 (OH) 4 nanoparticle shows a strong broad absorption band from 2500 to 3750 cm −1 corresponds to hydroxyl (OH) functional groups presented in the compound. In the range between 1700 and 1000 cm −1 several peaks were observed. The peak around 1611.2 cm −1 can be assigned to C=C. The strong peak appeared around 1350 cm −1 is attributed to the deformation vibration of C-H band while low intensity peaks appeared between 900 and 700 cm −1 also assigned to the aromatic bending vibration of C-H group. The strong absorption peaks observed in the range of 500-700 cm −1 are due to the vibrational modes of CuO and Cu 2 O nanostructures 38 .
Optical absorption behaviour is one of the most important fundamental properties in revealing the energy band gap and optoelectronic applications. Figure 6a,b shows UV-visible absorbance spectra of the as-prepared Cu 2 O, CuO and CuNa 2 (OH) 4 nanostructures recorded by ultrasonically dispersing in de-ionized water. The

Electrochemical studies. Comparison of electrocatalytic activity of Cu 2 O, CuNa 2 (OH) 4 and CuO modified
electrodes and pH effect. Figure 7a shows CV studies of copper-based metal oxides such as Cu 2 O, CuNa 2 (OH) 4 and CuO modified electrodes in 0.5 mM SO 3 2− at a scan rate of 5 mV s −1 . All the three catalysts, CuNa 2 (OH) 4 , Cu 2 O and CuO, show electrocatalytic response towards SO 3 2− oxidation at the potential of 640, 550 and 490 mV, respectively. The potential difference between CuNa 2 (OH) 4 -Cu 2 O, Cu 2 O-CuO and CuNa 2 (OH) 4 -CuO were 90 mV, 150 and 60 mV, respectively. The current value was found to be higher for CuO compared to CuNa 2 (OH) 4 and Cu 2 O. It shows that the CuO has high conductivity with attractive electrocatalytic ability towards oxidation of SO 3 2− . As shown in Fig. 7b, electrochemical stability studies were also carried out for all the three materials in the presence of 5 mM SO 3 2− using cyclic voltammetry by continuously recording 30 cycles. All the three modified electrodes show the electrochemical response for the oxidation of SO 3 2− . Unfortunately, the current response decreases continuously after few cycles in the case of Cu 2 O and CuNa 2 (OH) 4 . On the other hand, CuO shows stable response and the current decrease is negligible in comparison with other two modified electrodes. The above cyclic voltammetry studies clearly demonstrated that CuO possesses higher catalytic activity and better stability compared to other two electrocatalysts. Therefore, CuO modified electrode was chosen as the best catalyst and hence most favourable for further studies on electrochemical sensing of SO 3 2− . As shown in Fig. 7c, electrooxidation of SO 3 2− occurs on CuO/GCE at less positive potential with enormous current compared to the same on  We anticipated that the electrochemical oxidation of SO 3 2− would be depend on the solution pH. In order to examine the DPV response of SO 3 2− at CuO/GCE, measurement was carried out in 3 mM SO 3 2− with varying pH of 6.0, 6.5, 7.0, 7.5 and 8.0 as illustrated in Fig. S1a. A plot of current density versus pH is shown in Fig. S1b. It can be noticed that the oxidation peak current was higher in the case of pH 7. Further increase in pH leads to decrease in the oxidation current. This phenomenon clearly explains that proton is involved in the process of electrochemical oxidation of SO 3 2− . Hence, 0.1 M PBS (pH 7) was selected as supporting electrolyte throughout the experiments.
(1)   . CV result clearly exhibited that the oxidation peak current increases with raising the scan rate. While increasing the scan rate, the potential shifted towards higher potential value. Figure 8b shows a plot of SO 3 2− oxidation peak current density (Ipa) versus square root of the scan rate from 5 to 200 mV s −1 . Further, log (current density) vs log (scan rate) plot exhibits a slope value of ~ 0.5 (not shown here). The above results show that the overall reaction was controlled by diffusion 41 . . It clearly showed that CuO/ GCE has appreciable electrocatalytic activity for the sensing of SO 3 2− . From the Fig. 9b, the analytical parameter such as sensitivity, limit of detection (LOD) were found to be 48.5 µA cm −2 mM −1 and 2.23 µM (LOD = 3σ/S; σ is the standard deviation and S is the sensitivity) with the correlation coefficient (R 2 ) of 0.9953. The corresponding linear regression equation was y = 48.5C (SO 3 2− ) + (− 14.7). Similarly, the performance of CuO/GCE for the detection of SO 3 2− was explored by DPV technique also. As shown in Fig. 9c, SO 3 2− oxidation peak was observed at 0.47 V in DPV technique. The oxidation current increases with increasing SO 3 2− concentration from 0.005 to 15 mM. From the calibration curve shown in Fig. 9d, the linear range, LOD and sensitivity were found to be 0.005-15 mM, 1.42 µM and 29.93 µA cm −2 mM −1 (R 2 = 0.9906), respectively. The linear regression equation was y = 29.93C (SO 3 2− ) + (− 8.9). Table 1 displays the electrochemical sensor parameter of our proposed sensor along with previous reports based on metal, metal oxides and carbon materials. It can be noticed that the performance of CuO/GCE based sensor is comparable with previous works. . Using PBS solution, the wine samples were diluted and the required amount of the sample was injected into the supporting electrolyte without further pre-treatment. The CuO/GCE was used for sensing SO 3 2− in the wine samples. A characteristic catalytic peak of SO 3 2− was observed at 0.5 V. Cyclic voltammetric measurement was performed for the estimation of SO 3 2− in the wine sample as shown in Fig. S2a and the corresponding calibration curve is displayed in Fig. S2b. The sensor parameters of real samples obtained are summarized in Table 2. The proposed sensor shows recovery in the range of 99-100.9%, which indicates that the proposed CuO based electrochemical sensing platform can be used for the determination of SO 3 2− in wine samples.

Concentration effect.
Repeatability, stability, fabrication reproducibility and interference studies of CuO/GCE. In order to assess the fabrication reproducibility of proposed sensor, five modified electrodes were fabricated under identical fabrica-    Fig. 10a. The RSD was found to be 4.1%, which indicates good reproducibility of the sensors made in the same way. Typically, the response of a modified electrode to an analyte decreases after several measurements. In this context, we have evaluated the repeatability of CuO modified electrode in sulphite estimation by continuously monitoring the electrochemical response of 5 mM SO 3 2− for every 4 min interval for 10 measurements using DPV techniques as shown in Fig. 10b. The RSD for the obtained currents was 3.6%, which indicates extraordinary repeatability of the sensor in SO 3 2− determination. In order to examine the selectivity of CuO/ GCE for the detection of SO 3 2− , the influence of foreign species on the detection of 0.5 mM of SO 3 2− was investigated by DPV. The study was carried out under 200 fold higher concentration of interfering species including BaCl 2 , NaI, PO 4 2− , glucose, fructose, oxalic acid, tartaric acid, malic acid, citric acid, sodium thiosulphate, NaCl, NaBr, Na 2 HPO 4 , NaNO 3 , Cu(CH 3 COO) 2 , (NH 4 ) 2 CO 3 , CaCl 2 , MgCl 2 and Na 2 SO 4 which are not influencing the sensing of SO 3 2− as shown in Fig. 10c. Further, CV technique was used for the measurement of stability of the proposed sensor by continuously recording 50 cycles in 5 mM SO 3 2− , as displayed in Fig. 10d, the decrease in current response was negligible. The above studies reveal that the proposed senor demonstrates good repeatability, selectivity, fabrication reproducibility and stability for the sensing of SO 3 2− . The long-time stability of the CuO/GCE was examined by CV as illustrated in Fig. 11a for the detection of 5 mM SO 3 2− . After 15 days, the experiment was carried out using the same procedure and the electrode retained 94% current value, which indicates high storage stability of CuO based sensor in SO 3 2− estimation and the

Conclusions
The Cu 2 O, CuNa 2 (OH) 4 and CuO nanostructures have been effectively synthesized by single stage chemical method. The microscopic studies obviously revealed the phase purity, surface morphology, functional groups and elemental composition. Electrocatalytic activity was evaluated using CV and DPV techniques. The Cu 2 O, CuNa 2 (OH) 4 and CuO catalysts exhibited electrocatalytic activity, however, higher current was observed in the case of CuO/GCE for the detection of SO 3 2− . The limit of detection and sensitivity of CuO/GCE for the electrochemical estimation of SO 3 2− were found to be 1.42 µM and 29.93 µA cm −2 mM −1 , respectively. The low-cost and environmental friendly CuO modified electrode is an excellent platform for the electrooxidation of SO 3 2− as well as exhibited appreciable electrochemical durability in neutral medium. The present work proposes a new methodology for the sensing of SO 3 2− using CuO sensor. This proposed sensor was used for the quantitative detection of SO 3 2− in commercial wine samples thereby opening a new avenue for assessing food quality.