Contribution of rotational diffusivity towards the transport of antigens in heterogeneous immunosensors

Higher capture efficiency in heterogeneous immunosensors is desirable for the detection of cancer biomarkers at low concentrations. The process of the capture of these antigens is transport limited since the rates of antigen/antibody reactions are faster. In the case of non-flow systems, diffusive transport has contributions from both translational and rotational phenomena. Since the contribution of the rotational diffusivity is comparatively less explored in the literature, we have studied the same for three antigens – bovine serum albumin (BSA), prostate specific antigen (PSA) and C-reactive proteins (CRP). We quantified the rotational diffusivities using the time resolved fluorescence anisotropy method, and further quantified the contribution of the rotational diffusivities to the overall diffusivity of the antigens, and also studied the effect of the process parameters – temperature and pH of the solution. With an increase in temperature, the rotational diffusivity increased showing Arrhenius dependence while with the variation of pH, it showed a non-monotonic behavior having maxima closer to the isoelectric point of the corresponding antigens. This interesting behavior of the pH values could be attributed to lesser electro-viscous effects when the antigen molecule is neutral around its isoelectric point. The optimization of the pH and temperature for the immunosensors could be utilized to design efficient immunosensors.