However a very limited member of articles describes
the modification of carboxyl group to silica nanoparticle
surface. Carboxyl groups on silica surface were obtained by
trichlorocyanoethylsilane (TCES) [19], polymer [25] and succinic
anhydride [26,27] through different methods, especially
succinic anhydride converts amino groups into carboxyl groups
easily without higher temperature or additional catalyst [28,29].