Amino-functionalized silica nanoparticles (SiO2–NH2) were
prepared using the modified Stöber method [28–30]. Briefly,
4 ml of tetraethoxysilane (TEOS) was added to a mixture of
3.3 ml ammonium hydroxide and 47 ml of ethanol with stirring,
the reaction was continued 24 h; the resulting silica colloidal
dispersion was functionalized with APTES by quickly added
0.3 ml of APTES with a vigorously stirring. The mixture was
stirred overnight. The nanoparticle was purified by centrifugation
and redispersion in ethanol. Above procedure was repeated
three times. Meanwhile, bare silica nanoparticles (SiO2) obtained
under the same condition without addition of APTES