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Here, a novel biosensing platform for the detection of SARS-CoV-2 usable both at voltammetric and impedimetric mode is reported. The platform was constructed on a multi-walled carbon nanotubes (MWCNTs) screen-printed electrode (SPE) functionalized by methylene blue (MB), antibodies against SARS-CoV-2 spike protein (SP), a bioactive layer of chitosan (CS) and protein A (PrA). The voltammetric sensor showed superior performances both in phosphate buffer solution (PBS) and spiked-saliva samples, with LOD values of 5.0±0.1 and 30±2.1 ng/mL, compared to 20±1.8 and 50±2.5 ng/mL for the impedimetric sensor. Moreover, the voltammetric immunosensor was tested in real saliva, showing promising results.

In this paper we report on a facile, cost effective and environmental friendly green synthesis method of gold and silver nanoparticles (NPs) by using quercetin as reducing agent. The obtained NPs were characterized by transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), dynamic light scattering (DLS) and UV-Vis spectroscopy and parameters such as pH, ionic strength and temperature, effectively affecting shape and size of NPs, have been carefully studied and optimized. The obtained results showed that the synthesized NPs were circular in shape with an average diameter of 5 and 8 nm for the AuNPs and the AgNPs, respectively. The “green” NPs, showing increased electroactive areas (AEA) and electronic transfer rate constants (k0), were successively used to fabricate a novel third generation lactose biosensor based on cellobiose dehydrogenase from Trametes villosa (TvCDH). The TvCDH/AuNPs based lactose biosensor revealed the best results showing very efficient DET and a detection limit for lactose of 3.5 mM, a large linear range from 10 to 300 mM, a high sensitivity (5.4 μA mM−1 cm−2) and long-term stability.

The method based on the confidence limits of a regression straight line is applied to the linear portion of the calibration curve of a biosensor in order to determine the limit of detection. A practical application gives a detection limit of an ethanol biosensor greater than that calculated by using the method of the signal-to-noise-ratio, S/N=3, but the value is more reliable because it accounts for the variability in the response at analyte additions. A simple approximation of the method is also presented.

The kinetics of the electron transfer between NADH and ferrocenecarboxylic acid (FMCA) catalyzed by diaphorase from Clostridium kluveri (EC 1.8.1.4.) was studied by voltammetry. A commercially available digital simulation program permitted one to determine the rate constants of the catalytic system by a fitting procedure. A rate constant of 3×104 M−1 s−1was found for the reaction between diaphorase and mediator while a value of 8 M−1 s−1was found for the parallel competitive reaction between NADH and mediator. In addition a comparison of the present method to other data treatment procedures reported in the literature and a brief evaluation of their limits of applicability were carried out.