Ester-silica nanoparticles (NPs) were prepared using a facile two-surfactant synthesis route in neutral medium involving silica (tetraethyl orthosilicate, TEOS) and organosilica (3-(trimethoxysilyl)propyl 5-(trimethoxysilyl)pentanoate, PMe) precursors. The ester hydrolysis generates carboxylic acid groups, which confer a strongly negative zeta potential to the NPs at neutral pH. (337)