The enhancement of FET responses is suggested mainly due to the increase of net positive charge of prion protein adsorbed on the surface, which caused by the presence of additional amino groups and removal some carboxyl groups. Theoretically, each single conversion of carboxyl group into amine-ester species in the protein residues means the increase of 1 unit charge for the net positive charge of prion. Then, the further modification by protein conjugation with ethylenediamine can yield extra positive charge from additional amino groups.