Fabrication of potato-like silver molybdate microstructures for photocatalytic degradation of chronic toxicity ciprofloxacin and highly selective electrochemical detection of H2O2

In the present work, potato-like silver molybdate (Ag2MoO4) microstructures were synthesized through a simple hydrothermal method. The microstructures of Ag2MoO4 were characterized by various analytical and spectroscopic techniques such as XRD, FTIR, Raman, SEM, EDX and XPS. Interestingly, the as-prepared Ag2MoO4 showed excellent photocatalytic and electrocatalytic activity for the degradation of ciprofloxacin (CIP) and electrochemical detection of hydrogen peroxide (H2O2), respectively. The ultraviolet-visible (UV-Vis) spectroscopy results revealed that the potato-like Ag2MoO4 microstructures could offer a high photocatalytic activity towards the degradation of CIP under UV-light illumination, leads to rapid degradation within 40 min with a degradation rate of above 98%. In addition, the cyclic voltammetry (CV) and amperometry studies were realized that the electrochemical performance of Ag2MoO4 modified electrode toward H2O2 detection. Our H2O2 sensor shows a wide linear range and lower detection limit of 0.04–240 μM and 0.03 μM, respectively. The Ag2MoO4 modified electrode exhibits a high selectivity towards the detection of H2O2 in the presence of different biological interferences. These results suggested that the development of potato-like Ag2MoO4 microstructure could be an efficient photocatalyst as well as electrocatalyst in the potential application of environmental, biomedical and pharmaceutical samples.


Results and Discussion
The crystalline structure and phase purity of the as-synthesized samples were determined by using XRD pattern as shown in Fig 642) and (731) reflection planes, respectively. Aforementioned planes are well related to the standard XRD report of cubic phase structured Ag 2 MoO 4 with the space group of Fd3m 51 . From the XRD pattern, it was clearly revealed that the as-synthesized product is β -Ag 2 MoO 4 36 . The appearance of sharp and high intense peaks demonstrated the higher crystalline nature of cubic Ag 2 MoO 4 phase. There is no any other peaks were appeared which related to the Ag 2 O or MoO 3 phase, indicates the as-synthesized Ag 2 MoO 4 was homogeneous solid.
FTIR and Raman spectroscopy is an important tool for analyzing the involvement of functional groups present in the as-synthesized Ag 2 MoO 4 . In the FTIR spectra (Fig. 2B), the absorption peaks at 3280 and 1650 cm −1 correspond to the O-H stretching and bending vibrations of the water molecules, respectively 52 . The peak at 645 cm −1 is confirmed the Ag-O stretching vibration of Ag 2 MoO 4. The sharp peak at 891 cm −1 attributed to the anti-symmetric Mo-O stretching in tetrahedral MoO 4 2− ion 53 . Raman spectra showed (Fig. 2C) the peaks at 896, 782, 382 and 305 cm −1 which are due to the ν 1 (A g ), ν 3 (E g ), ν 4 (B g ) and ν 2 (A g ) symmetric and asymmetric stretching vibration modes of Ag 2 MoO 4 , respectively. The vibration modes at 896 and 782 cm −1 could be ascribed to the symmetric stretching vibration of Mo-O bond of the MoO 4 unit and the asymmetric stretching vibrations of the molybdate ion, respectively. The peaks at 382 and 305 cm −1 were clearly indicated the ν 4 and ν 2 bending vibration modes of tetrahedral MoO 4 54 .
The surface morphology of the as-prepared microstructures was investigated by SEM. Figure 3(A-C) illustrated the general views of the different magnifications of the obtained Ag 2 MoO 4 microstructures. The images are clearly displayed the potato-like structure of Ag 2 MoO 4 which seems like bunches of potatoes with clean and fairly smooth surfaces, the average diameters of microstructures about 1-2 μ m. Energy dispersive x-ray spectra (EDX) were used to identify the elements present in the as-prepared Ag 2 MoO 4 microparticles, as depicted in Fig. 3D. The EDX spectra showed the peaks at approximately 0.5, 2.4 and 3 keV reveal the presence of O, Mo, and Ag elements in the material without any other significant impurities.   The bandgap energy of the Ag 2 MoO 4 is an important parameter for the selection of suitable kind of light source needed for the degradation purposes. The UV-Vis (Diffuse reflectance) absorption spectrum of Ag 2 MoO 4 microparticles is shown in Fig. 4A. The results shows that the relation between the normalized absorption of the photocatalyst and wavelength with a range of 200-800 nm. The most part of absorption spectra of Ag 2 MoO 4 falls in the UV region, a broad steep from 300 to 420 nm which corresponds to the band gap energy value from 3-3.34 eV. The band gap value was calculated using Tauc's equation and the graph plotted (α hν ) 2 against (hν) as can be seen in Fig. 4B. The calculated band gap energy value is 3.14 eV. On the other hand, the bandgap of Ag 2 MoO 4 is significantly altered compared to that of previous reports 36,43,44,50 . The oxygen vacancy created in the crystal lattice of the Ag 2 MoO 4 is leads to the distortion in the energy levels and influenced the bandgap which may be attributed to the effect of hydrothermal environment on the surface microstructures.
X-ray photoelectron spectroscopy (XPS) was used to evaluate the information about the chemical composition and chemical status of the as-synthesized Ag 2 MoO 4 , as shown in Fig. 5. The overall XPS spectrum in Fig. 5A shows the coexistence of elements Mo, C, Ag and O within the as-prepared Ag 2 MoO 4 microparticles and no other impurities were detected, which are in good agreement with EDX report. In addition, the presence of C peak at 284.9 eV is ascribed to the adventitious hydrocarbon from the XPS instrument and it is inherent. High resolution scanning XPS spectra clearly confirms the Ag 3d, Mo 3d, and O 1s level, which is fitted by using the Gaussian fitting method, as shown in Fig. 5(B-D). In Fig. 5B, the Ag 3d spectra displays the two peaks at 368.7 and 374.4 eV  attributed to the Ag 3d 5/2 and Ag 3d 3/2 electron binding energy in Ag 2 MoO 4 , respectively 55 . The peaks at 233.2 and 236.3 eV ascribed to the Mo 3d 5/2 and Mo 3d 3/2 binding energy of Mo 3d, respectively. The major binding energy peaks Mo 3d 5/2 and Mo 3d 3/2 are separated by 3.1 eV, which belongs to the Mo 6+ oxidation state as depicted in Fig. 5C 56 . The high intense peaks at around in the range of 530.5-533.6 eV revealed the presence of O 1s core level 57 in Ag 2 MoO 4 (Fig. 5D). Hence, the obtained XPS results clearly confirmed that the valence of Ag, Mo and O are + 1, + 6 and − 2, respectively, which is very good agreement with the phase structure of Ag 2 MoO 4 .
Electrochemical impedance spectroscopy (EIS) was used to investigate the changes of the electrode surface during the fabrication process. The nyquist curves of the EIS spectra was observed using bare GCE (a) and Ag 2 MoO 4 modified GCE (b) in 0.1 M KCl containing 5.0 mM K 3 Fe(CN) 6 /K 4 Fe(CN) 6 (Fig. 6). The diameter of  the semicircle indicates the electron transfer resistance (R ct ) of the electrode. This resistance controls the electron transfer kinetics of redox probe at the electrode interface. From the EIS results, the Ag 2 MoO 4 modified GCE (b) shows larger semicircle than bare GCE (a) reveals that the Ag 2 MoO 4 modified GCE can increase the electron transfer resistance on electrode surface, because it is hindered the electron transfer of K 3 Fe(CN) 6 /K 4 Fe(CN) 6 , thus confirming the successful modification of Ag 2 MoO 4 on the GCE surface.
Photocatalytic activity. The photocatalytic behavior of as-prepared Ag 2 MoO 4 microparticles was performed against the degradation of CIP under UV-light illumination, as illustrated in Fig. 7A. The absorbance spectrum shows the progressive degradation of CIP and the main absorption peak of CIP was observed at 276 nm and other small peaks were also completely diminished within 40 min. The degradation percentage of CIP solution was estimated from the relative intensity of absorbance in UV-visible spectra. The relative intensity of absorbance was decreased and reached almost zero within 40 min, reveals that the Ag 2 MoO 4 microparticles degraded the 98% of the CIP solution. Initially, the utmost decrement of CIP was observed which could be attributed to the competence of CIP with hydroxyl radicals generated by UV-light photoexcitation of Ag 2 MoO 4 microparticles. However, as the reaction proceeds, the formation of by-products from degradation might compete with the hydroxyl radicals and adsorption sites on the catalyst surface. Hence, the gradual degradation of CIP was observed.
Under similar degradation conditions, commercially available Ag 2 MoO 4 and commercial TiO 2 powder were also employed as a photocatalyst and their accurate comparison was carried out which depicted in Fig. 7B. The photocatalytic activity of commercial Ag 2 MoO 4 and TiO 2 on CIP degradation exhibited a poor efficiency and the corresponding degradation percentages are observed around 38% and 32%, respectively. Moreover, there is no significant degradation was observed either in the absence of light or in the absence of a catalyst. The results clearly confirmed the as-synthesized Ag 2 MoO 4 shown a superior photocatalytic efficacy over the commercial Ag 2 MoO 4 and TiO 2 powder for the degradation of CIP solution.
Catalyst dosage is an important parameter that can significantly influence the rate of photodegradation. Figure 7C shows the efficiency of CIP degradation (%) against the effect of catalyst loading by varying the catalyst amount from 30 to 80 mg/mL, wherein the concentration of CIP and intensity of the light were maintained as constant. It can be seen that the rate of photocatalytic degradation was maximum at 50 mg/mL, due to the generation of more number of electron-hole pairs. However, the addition of excess amount of photocatalyst can blocks the existing active sites and interfere with the diffusion of photons. As a result, the photocatalytic degradation of CIP was decreased when increase the concentration of photocatalyst over 50 mg/mL 58 .
Effect of initial concentration of CIP solution on the photodegradation was also investigated and the concentration varied from 10 to 30 mg/L under identical conditions, the results are shown in Fig. 7D. In the present case ~98% degradation was achieved at 20 mg/L whereas 83% and 77% degradation were observed at 25 and 30 mg/L concentration of CIP, respectively. This is due to the screening of light by the CIP solution and the less number of photons to reach the Ag 2 MoO 4 surface. Hence, the electron-hole pair generation is reduced greatly and consequently, the degradation efficiency decreased.
Generally, the reactive oxidative species (ROS) viz hydroxyl radical (·OH), superoxide radical anion (O 2 ·− ), hole (h + ) and electron (e − ) involved in the photocatalytic reaction 59 . The photocatalytic mechanism of the degradation of CIP is represented in the following equations The semiconductor photocatalyst generally undergoes excitation under light illumination with energy greater than the bandgap while the e − excited from the valence band (VB) to the conduction band (CB) leaves h + in the VB 60 . In the present study, Ag 2 MoO 4 was irradiated by UV light which produced the e − in the CB and h + in the VB, as illustrated in the Eq. 1. The e − in the CB were reacted with the oxygen molecule to form O 2 ·− and the O 2 ·− was reacted with the water molecule to form ·HO 2 , as shown in Eqs 2 and 4. Furthermore, h + in the VB adsorbed water molecules and reacted to form ·OH, as given in Eq. 2. The ROS formed in the photocatalytic reaction facilitated the degradation of the CIP by the stepwise photocatalytic reduction process (Eqs 5 and 6). The high photocatalytic activity as well as recycling ability of the catalyst is a vital issue for long-term use in practical applications. To evaluate the sustainability of Ag 2 MoO 4 , the recycling experiments were carried out for the degradation of CIP, as illustrated in Fig. 7E. The photocatalytic activity of the Ag 2 MoO 4 on CIP degradation did not show a significant loss and attains more than 90% of degradation rate even after the five successive cycles with every cycle lasting for 40 min, which indicating that the potato-like Ag 2 MoO 4 microstructure is a very effective and highly stable photocatalyst. Thus, the obtained results also indicated a good reusability of the catalyst. Moreover, the poor loss of catalytic activity during the recycle performance is due to the accumulation of impurities on the surfaces of the photocatalyst.  Fig. 8B. The reduction peak current of H 2 O 2 was linearly increased with increasing the H 2 O 2 concentration from 0 to 1 mM, which revealing electrocatalytic activity of Ag 2 MoO 4 modified GCE toward the H 2 O 2 . Furthermore, the low level detection, sensitivity and linear range of H 2 O 2 were clearly discussed in amperometric (i-t) section.  -j). When increasing the scan rate from 20 to 200 mV/s, the reduction peak current of H 2 O 2 was increased and the peak potential was shifted towards the more negative side. The peak current of H 2 O 2 reduction is directly proportional to the scan rate (Fig. 9B) (Correlation co-efficient R 2 = 0.998), indicating that the electrode process is surface controlled process 61 .   Figure S1. When the pH value increased from 3 to 5, the reduction peak current increased and then decreased gradually while increasing the pH more than 7. However, higher peak currents are observed at pH 7. Therefore, Ag 2 MoO 4 modified GCE has good electrocatalytic activity at pH 7 and the reduction of H 2 O 2 is pH dependence. Therefore, we chosen pH 7 is optimized pH and this pH was used further electrochemical measurements.

Amperometric determination of H 2 O 2 at Ag 2 MoO 4 modified GCE. The amperometric i-t technique
is one of the most important method to determine the electrocatalytic activity of modified electrodes in electrochemical sensor and biosensor applications. In the present work, we have utilized an amperometric method to estimate the performance of Ag 2 MoO 4 modified GCE toward H 2 O 2 detection. In this regards, Ag 2 MoO 4 modified rotating disc glassy carbon electrode (RDGCE) was performed in continuously stirred pH 7 solution by applying constant potential at − 0.5 V with rotation speed at 1200 rpm. Figure 10 Fig. 10), the obtained sensitivity and limit of detection (LOD) of the sensor is around 9.8 μ Aμ M −1 cm −2 and 0.03 μ M, respectively. The above results suggesting that the Ag 2 MoO 4 modified RDGC electrode has good electrocatalytic activity towards H 2 O 2 . The analytical parameters, such as LOD, linear range, and sensitivity, of H 2 O 2 sensor was compared with various modified electrodes are summarized in Table 1. Clearly, the Ag 2 MoO 4 modified RDGCE, reported here, exhibits good sensitivity and LOD over a wide linear range of H 2 O 2 concentration compared to other reports [62][63][64][65][66][67][68][69][70][71][72][73][74][75] .
Selectivity is very important study in the electrochemical sensor. In order to investigate selectivity, the proposed sensor was used to detect the H 2 O 2 in the presence of variety of biological interferences such as catechol (b), fructose (c), lactose (d), sucrose (e), glucose (f), hydroquinone (g), ascorbic acid (h), uric acid (i), dopamine (j) and epinephrine (k) with 50-fold excess concentration of each analytes, as shown in Fig. 11. The Ag 2 MoO 4 modified RDGCE exhibited well-defined response towards each 100 μ M H 2 O 2 (a). No remarkable responses were monitored for the 50-fold excess concentration of biological interferences as mentioned above. Hence, the Ag 2 MoO 4 modified RDGCE has excellent selectivity towards the determination of H 2 O 2. Furthermore, we have studied the stability of our proposed sensor by amperometric (i-t) techniques and the results obtained as it can be seen in Figure S2. The prepared sensor retains its 98.2% of its initial response of H 2 O 2 after prolongs runs up to 2600 s, which suggesting the good stability of the sensor.
In summary, a potato-like Ag 2 MoO 4 microparticles were successfully prepared through a simple hydrothermal method. Different analytical and spectroscopic methods were used to confirm the structural nature of Ag 2 MoO 4 microparticles. All the obtained results are strongly evidenced that the prepared compound shows like pristine Ag 2 MoO 4 without any other impurities. The as-prepared Ag 2 MoO 4 microparticles explored excellent photocatalytic activity towards the degradation of chronic toxicity CIP under UV-light illumination and 98% of the CIP was degraded within 40 min. On the other hand, the Ag 2 MoO 4 microparticles were used to fabricate the sensor electrode for the detection of H 2 O 2 in the potentially interfering biological substances. The Ag 2 MoO 4 modified GCE revealed a high electrocatalytic activity with wide linear range, good stability and low detection limit than the previous reports. Finally, the Ag 2 MoO 4 represents an interesting and promising candidate for photocatalytic

Experimental Section
Materials. Silver nitrate (AgNO 3 ), sodium molybdate dihydrate (Na 2 MoO 4 .2H 2 O), urea (CH 4 N 2 O), hydrogen peroxide (H 2 O 2 ) and all chemicals were purchased from Sigma-Aldrich and used further purification. Ciprofloxacin drug was purchased from Thiruvengadam Medicals, Virudhunagar, India. All other chemicals were of analytical grade and used without further purification. The phosphate buffer solution (PBS) was prepared using 0.05 M Na 2 HPO 4 and NaH 2 PO 4 and all the required solutions were prepared using deionized water (DI).
Synthesis of silver molybdate. In a typical synthesis, 0.5 M of Na 2 MoO 4 .2H 2 O and 0.1 M of AgNO 3 were dissolved in 60 mL DI water. Then, 0.3 g of urea (10 mL) was gradually added into the above mixture under vigorous stirring at room temperature for 30 min. The mixture was transferred into 100 mL Teflon-lined autoclave and maintained at 120 °C for 8 h in an oven. The autoclave was then cooled down to room temperature naturally, and the obtained yellow product was collected by centrifugation and washed thoroughly with DI water and ethanol for three times and dried overnight at 80 °C.  Characterization. The powder X-ray diffraction (XRD) analysis was carried out using PANalytical X'Pert PRO X-ray diffractometer measured with Cu-Kα radiation ( λ = 1.54178 Å) in the 2θ range of 10-100°. X-ray photoelectron spectroscopy (XPS) measurements were carried out using ULVAC-PHI 5000 VersaProb instrument. Scanning electron microscope (SEM) and Energy dispersive X-ray spectra (EDX) were probed using Hitachi S-3000 H and HORIBA EMAX X-ACT, respectively. Raman spectra were collected in an NT-MDT confocal Raman microscopic system with an exciting laser wavelength of 488 nm and the laser spot-size is around 0.5 μ m. Fourier transform infrared spectroscopy (FTIR) was recorded by Shimadzu FT-IR 3000 spectrometer in the diffuse reflectance mode at a resolution of 4 cm −1 , the sample was pressed into KBr disc with a weight ratio of sample to KBr of 1:100 in the range of 4000-400 cm −1 . UV-Visible diffused reflectance (DRS) spectrum of the sample was taken from Shimadzu UV-2600 spectrophotometer and BaSO 4 was used as a reflectance reference material. The absorption spectra in the photocatalytic degradation process were monitored by Shimadzu 2100 UV-Visible spectrometer. The electrochemical impedance spectroscopy (EIS) was performed by XPOT (ZAHNER elektrik instrument). Cyclic voltammetry (CV) and Amperometric (i-t) experiments were carried out using CHI 405a work station and PINE instrument, respectively. All the electrochemical studies were carried out in three-electrode cell system, glassy carbon electrode (GCE surface area = 0.071 cm 2 ) was used as a working electrode, platinum wire and standard Ag/AgCl electrodes were used as a counter and reference electrode, respectively.
Photocatalytic activity. The photocatalytic activity of the as-prepared Ag 2 MoO 4 was evaluated via degradation of CIP solution under UV light illumination (λ = 200~400 nm). In a typical activity, 50 mg/mL of catalyst was dispersed in 100 mL aqueous solutions of CIP (20 mg/L). Prior to illumination, the solution mixture was stirred magnetically for 30 min in the dark to establish the adsorption-desorption equilibrium between CIP and as-prepared Ag 2 MoO 4 photocatalyst. Then, the solutions were illuminated by UV light (λ max = 365 nm) to induce photocatalytic reaction. During the irradiation, 5 mL of the solution was withdrawn at 5 min time intervals and centrifuged to remove the catalyst. The obtained clear liquor was analyzed by UV-Vis spectrometer to determine the concentration changes of CIP.
Fabrication of silver molybdate modified GCE. Before surface modification, the GCE was mirror like polished with 0.05 μ m alumina slurry and rinsed with DI to remove the alumina particles on the GCE surface. After that the GCE was sonicated for 1 min containing ethanol and water (1:1 ratio). About 5 mg/mL of the as-prepared Ag 2 MoO 4 was re-dispersed in DI water and about 8 μ L (optimized concentration) of the dispersed Ag 2 MoO 4 was drop casted on the GCE surface. Then it was allowed to dry at room temperature, followed by the dried Ag 2 MoO 4 modified GCE was rinsed with DI water to remove the loosely attached catalyst on the GCE surface. The obtained Ag 2 MoO 4 modified GCE was used to further electrochemical experiments. Then, it was stored in room temperature when not in use.