Thickness dependence of nanocrystalline tin oxide thin films in capacitive biosensor characterization

Nanocrystalline SnO2 thin films of varying thicknesses (250-50 nm) have been prepared on p-type Si by Langmuir Blodgett technique. Nanocrystalline SnO2/polyaniline (Si/SnO2/PANI) thin film heterostructure has been studied for label free, real time electrochemical detection for model immunoglobulin interactions. Sensor responses were studied using impedance methods, wherein transient capacitance gave fast response for antigen-antibody interaction in picomolar concentration range. From the study of sensor structures with varying film thicknesses, it was observed that SnO2 film with less than 200 nm thickness were effected with insulating (dielectric) properties whereas for thickness higher than 200 nm, it was n-type semiconducting characteristics. This is due to effects of charge on the surface adsorbed species as revealed by capacitance voltage characteristics, suggesting these characteristics as indicators of ionic transport effects in nanostructured metal oxide films in electrochemical cell configuration. The current study clearly states the possibility of fabricating cast effective and stable capacitive sensors with SnO2 films with thickness less than 100 nm. The study helps in optimizing the sensing method for capacitive/conductive biosensors based on nanostructured semiconductor thin films.