The procedure was done following the steps outlined by Jang et al. (44). Briefly, for the functionalization of silica particles, 0.23 g of silica pellet (as prepared above) was redispersed in 40 ml of fresh ethanol in a sonication bath. Under stirring with a Teflon-coated magnetic bar, 4 ml of ammonia solution (0.727 M in ethanol) was added. APTES solution (170 μl; 0.011 M in ethanol) was then slowly added dropwise to the silica dispersion. After 15 hours, the resultant solution was washed with ethanol five times by centrifugation to remove the unreacted APTES molecules.