Monodispersed raw silica nanoparticles (RSNPs) with the particle size of 40 nm were successfully fabricated by condensation reaction of tetraethylorthosilicate in methanol with high concentration ammonia (1.2 M). The RSNPs were treated with the coupling agent 3-aminopropyltrimethoxysilane (APTMS) for grafting amine groups on the surface to obtain the amino-functionalized silica nanoparticles (ASNPs). The chemical structure and surface morphology of RSNPs and ASNPs were characterized by Fourier-transform infrared spectra, solid-state NMR spectra and scanning electron microscopy. In addition, a method to quantify the grafted amine groups on the surface of ASNPs was developed by using the ninhydrin assay. The ninhydrin analysis showed that 60 mol % of the APTMS molecules were immobilized on the surface, that is, 4.4 amine groups per nm2 of surface area were bonded on nonporous ASNPs. The weight loss of particles obtained from thermogravimetry analysis indicated the amount of grafted amine groups and was used as a reference to compare with the value determined from ninhydrin method.
