To evaluate the sensitivity of FET biosensor with the present technique of signal amplification, we assess several concentrations(400 pM, 400 fM, 400 aM) of prion protein samples, which added to the sensing surface and then followed with the chemical modification. Fig. 3b shows the Vg for prion detection at different concentrations before and after chemical modification. When the prion protein adsorbed in the FET sensing surface is without treated by any chemical modification, the FET responses was very small around −6 mV at the concentration of 400 pM and can be negligible at smaller concentration (400 fM and 400 aM).