Lipase has been gaining attention as the recognition element in electrochemical biosensors. Lipase immobilization is important to maintain its stability while providing excellent conductivity. In this study, a lipase electrochemical biosensor immobilized on a copper-centred metal-organic framework integrated with reduced graphene oxide (lipase/rGO/Cu-MOF) was synthesized by a facile method at room temperature. Response surface methodology (RSM) via central composite design (CCD) was used to optimize the synthesis parameters, which are rGO weight, ultrasonication time, and lipase concentration, to maximize the current response for the detection of p-nitrophenyl acetate (p-NPA). The results of the analysis of variance (ANOVA) showed that all three parameters were significant, while the interaction between the ultrasonication time and lipase concentration was the only significant interaction with a p-value of less than 0.05. The optimized electrode with parameters of 1 mg of rGO, 30 min ultrasonication time, and 30 mg mL-1 lipase exhibited the highest current response of 116.93 μA using cyclic voltammetry (CV) and had a residual standard error (RSE) of less than 2% in validation, indicating that the model is suitable to be used. It was characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and Fourier transform infrared spectroscopy (FTIR), where the integration of the composite was observed. Immobilization using ultrasonication altered the lipase's secondary structure, but reduced its unorderly coils. The electrochemical and thermal analysis showed that the combination of Cu-MOF with rGO enhanced the electrochemical conductivity and thermostability.
Discovering the distinctive photophysical properties of semiconductor nanoparticles (NPs) has made these a popular subject in recent advances in nanotechnology-related analytical methods. Semiconductor NPs are well-known materials that have been widely used in photovoltaic devices such as optical sensors and bioimaging, and dye-sensitized solar cells (DSSCs), as well as for light-emitting diodes (LEDs). The use of a narrow-bandgap semiconductor such as CdS NPs in the photoelectrochemical (PEC) detection of chemicals and biological molecules plays a key role as a photosensitizer and promotes some specific advantages in light-harvesting media. Their size-controlled optical and electrical properties make NPs fascinating and promising materials for a variety of nanoscale photovoltaic devices. Moreover, charge injection from the narrow bandgap to the adjacent material leads to efficient charge separation and prolongs the electron lifetime by the elimination of the charge carrier recombination probability. In this regard, a single photon enables the production of multiple photogenerated charge carriers in CdS NPs, which subsequently boosts the effectiveness of the photovoltaic devices. In particular, the present review article highlights the recent emerging PEC detection methods based on CdS NPs, specifically related to the direct and indirect interactions of NPs with target analytes. The current opportunities and challenges in achieving real-world applications of CdS-based PEC sensing are also presented.