In the next step the spectroscopic signature of our samples

In the next step, the spectroscopic signature of our samples was probed. The UV–Vis spectrophotometry studies for L/IL solution were carried out using Cecil, model CE-7200 apparatus at 268nm and 270nm as a function of analyte ascorbic acid and oxalic acid concentration (Fig. 6). A clear increase in absorbance with analyte concentration (1–20mM) was observed. From the magnitude of absorbance, we could find the maximum binding constant which was 0.50mM−1 for oxalic acid and 0.12mM−1 for ascorbic acid. The surface morphology of L/IL/ITO electrodes with different analytes were characterized using scanning electron microscope (SEM) is shown in Fig. 7. Fourier transform infrared spectroscopy (FTIR) of L/IL/ITO electrode with different analytes was done to identify the structural and conformational changes based on comparison of peak intensities shown in Fig. 8. FTIR spectra of laponite with [C2mim][Cl] exhibited broad band at around 3200–3460cm−1 which was assigned to –OH stretching vibration. The bands observed at 950cm−1 and 1150cm−1 corresponding to Si–O and Si–O–Si stretching vibrations. Also the band at 660cm−1 corresponded to Mg–O vibration. The peak at 1643cm−1 and 1426cm−1 corresponded to the imidazolium ring bend, and the CH2 bend in the spectra. The study of L/IL solutions with selective analytes is shown in Fig. 8 where we could observe that some peaks were shifted in case of different analytes. When compared with oxalic acid we observed an increase in intensity of dhpg bands and shifting of peak at 2112cm−1 to 1878cm−1. Also peaks at 757cm−1, 907cm−1, 1175cm−1 (imidazole ring deformation), and 1300cm−1 (H–C–C stretching) were observed in the spectrum. No change either in peak position and intensity was observed in case any of the analytes for 3200–3460cm−1. In case of ascorbic acid the intensity increase was less as compared to oxalic acid. A new small peak occurred at 1894 and 1719cm−1 and the peak at 2095cm−1 did not show shift as in case of oxalic acid, but the area of the band decreased. At lower wave number we observe the shifting of peaks at 773cm−1 (C–H out of plane bending of imidazole ring), 916cm−1 (C–H out of plane bending of imidazole ring), and 1100cm−1 (H–C–N bending). On comparing with cholesterol, a new peak at 2621cm−1 and 2939cm−1 appeared. In this case the intensity was maximum of all the analytes. The suppression of peak at 1342cm−1 was also noticed. Further, at low wave number we could observe the peak shifting. This shifting of FTIR peaks led us to conclude the structural changes in L/IL were due to interaction of them with the analytes. These changes in IR spectra could be due to the bond formation/interaction taking place between the NH⋯OH, CO⋯HO etc. The fabrication and sensing mechanism for L/IL/ITO for sensing oxalic acid is depicted in Fig. 9.
Conclusion The aim of our work was to construct a electrochemical biosensor using nanoclay laponite and ionic liquid, 1-ethyl-3-methyl imidazolium chloride [C2mim][Cl] film deposited on indium tin oxide (ITO) glass electrode. The biosensor fabricated through this was used for the determination of oxalic acid, ascorbic acid and cholesterol. We characterized the L/IL/ITO electrode using cyclic voltammetry (CV), scanning electron microscope (SEM), UV–Vis spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR). The biosensor showed maximum sensitivity for oxalic acid in the range of 1–20mM. Thus, fabricated L/IL/ITO biosensor was successfully employed for determination of oxalic acid. This platform offers the promise to develop strip-based analyte sensors that do not require stringent storage conditions. Comparison of different oxalic acid sensors which were reported in literature is shown in Table 2.
Conflict of interest
Acknowledgments N.J. acknowledges University Grants Commission, Government of India for a Research Fellowship. This work was supported by a grant from Department of Science and Technology (DST), Government of India. K.R. acknowledges receipt of DST-Inspire Faculty Award. Authors are thankful to Advanced Research Instrumentation Facility of the University for allowing us access to FTIR facility.