Refractive index change caused by biomolecular adsorption and structural transformations of adsorbed molecules in ultrasensitive plasmonic biosensors.

Adsorption-based plasmonic refractometric biosensors have a great potential for applications in environmental protection, medicine and industry, as they are highly sensitive detectors of a broad range of biological analytes. Their response is determined by the temporal change of the effective refractive index (RI) of the sensing area caused by biomolecules adsorption. Many biomolecules undergo a structural transformation upon a contact with the sensing surface, causing a change in their affinity towards adsorption sites. Different forms of molecules also have different RI, which affects the sensing area effective RI. Adsorption leads to depletion of biomolecules from the sample, affecting the instantaneous rate of change of the number of adsorbed molecules, and thus also the effective RI change. Therefore, it is necessary to investigate the influence of these phenomena on the sensor response. In this work, we derive the first mathematical model that describes the change in the effective RI of the sensing area by taking into account reversible adsorption, structural transformation of adsorbed biomolecules, and sample depletion. We then use that model to analyze influences of these processes on the RI change by studying an exemplary protein adsorption. The results have shown the significant influence of these processes on the sensing area effective RI, and thus on the sensor time response, particularly when extremely low biomolecule concentrations are to be detected. The use of the presented model prevents erroneous interpretation of measurement results in ultrasensitive plasmonic biosensors, which may occur if using models that do not take into account structural transformation and analyte depletion. [ABSTRACT FROM AUTHOR]