A schematic illustration for the chemical modification of prion protein is shown in Fig. 1(a). The carboxyl group ( COOH),which is negatively charged species in physiological buffer solution,was converted into the neutral species and then further modified to be positively charged species. We used EDC/NHS coupling reaction to block the carboxyl group to be neutral species, followed with conjugation reaction using ethylenediamine to create more cationic groups (amino groups).