Silica nanoparticles were synthesized according to the Stöber method [24]. Briefly, 1.5 mL of TEOS was added dropwise to a solution containing 50 mL of ethanol, 1.0 mL of deionized water, and 3.0 mL of ammonium hydroxide (28%) under magnetic stirring. After the mixture was stirred at 40 ◦C for 4 h, an additional 1.0 mL of TEOS was added and the reaction was allowed to continue for another 12 h with stirring. The obtained silica nanoparticles were purified by centrifugation (10,000 rpm, 10 min) and re-dispersed in water three times.