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Herein, we report the synthesis and functionalization of gadolinium oxide nanoparticles (Gd2O3 NPs) to fabricate a highly efficient immunosensor for the detection of Vibrio cholera toxin (CT). Gd2O3 NPs were produced in a straightforward manner utilizing the microwave irradiation technique using a domestic microwave oven. X-ray diffraction, transmission electron microscopy, and spectroscopic techniques were used to characterize the structural and physical aspects of Gd2O3 NPs. The Gd2O3 NPs were then functionalized with 3-(Aminopropyl) triethoxysilane (APTES) and electrophoretically deposited onto an ITO-coated glass substrate. The anti-CT monoclonal antibodies were covalently attached to the APTES-Gd2O3/ITO electrode via EDC-NHS chemistry, followed by bovine serum albumin (BSA). For CT detection, electrochemical response experiments using BSA/anti-CT/APTES-Gd2O3/ITO immunoelectrodes were carried out (5–700 ng mL−1). The immunoelectrode demonstrated an outstanding electrochemical reaction against CT, with a sensitivity of 8.37 mA ng−1 mL cm−2 and a detection limit of 1.48 ng mL−1.

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A report on the synthesis of carbon-quantum-dot-embedded iron oxide nanoparticles (CQD@Fe3O4NPs) and their improved electrochemical studies is presented. Fe3O4NPs and CQD@Fe3O4NPs were synthesized by the wet-chemical co-precipitation method. X-ray diffraction measurements exhibited pure cubic phase with Fd3m space group in Fe3O4NPs and CQD@Fe3O4NPs. Fourier-transform infrared spectroscopy measurements confirmed the functionalization of Fe3O4NPs with CQDs. Dynamic light scattering measurements revealed a hydrodynamic radius of 520 nm and 319 nm for Fe3O4NPs and CQD@Fe3O4NPs, respectively. Moreover, zeta potential measurements showed positively charged Fe3O4NPs and negatively charged CQD@Fe3O4NPs. High-resolution transmission electron microscopy measurements showed nearly spherical structure with an average size of around 7 nm for Fe3O4 in both samples, whereas CQDs were nearly 2 nm in size in CQD@Fe3O4NPs. A biocompatibility study showed that CQD@Fe3O4NPs were more biocompatible than the bare Fe3O4NPs. CQD@Fe3O4NPs were then dispersed in chitosan (CHIT) solution, and drop-casted onto an indium tin oxide (ITO) glass substrate for further study. Atomic force microscopy results showed improved surface roughness of the CQD@Fe3O4−CHIT/ITO electrode, providing a better biosensing platform. The electrochemical response studies of CQD@Fe3O4−CHIT/ITO also showed enhanced electrochemical signal compared to Fe3O4−CHIT/ITO electrodes. Thus, a CQD@Fe3O4−CHIT/ITO electrode was used for the detection of vitamin D2 (10–100 ng ml−1) using a differential pulse voltammetry technique. The sensitivity and limit of detection were obtained as 0.069 µA ng−1 ml cm−2 and 2.46 ng ml−1, respectively.