Quantification of the extent of functionalization on silica nanoparticle surface is crucial in a variety of applications. This work aims to evaluate particle properties and extent of functionalization on silica nanoparticles synthesized under acidic and basic conditions via co-condensation of tetraethyl orthosilicate with 3-aminopropyltrimethoxysilane in the sol-gel process. The silica nanoparticles properties were analyzed via field emission scanning electron microscope, transmission electron microscopy, dynamic light scattering, and Brunauer-Emmett-Teller methods; whereas, the extent of functionalization was evaluated using Fourier transform infrared spectroscopy, X-ray photoelectron spectrometer, and thermogravimetric analyzer. Results showed that the functionalized silica nanoparticles have a lower tendency to agglomerate and highly monodispersed as compared to unfunctionalized. The surface areas of acid and base-catalyzed nanoparticles were obtained as 618.8 and 514.7 m2/g, respectively. FTIR spectra, XPS scans, and TGA curves confirmed the presence of alkyl and amine functional groups on the functionalized surfaces. The extent of functionalization (N/Si) was obtained as 0.296 and 0.196, and the percentages of functional groups attached on the surface were found to be 6.80 and 5.92 for acid and base-catalyzed nanoparticles, respectively. The overall results suggest that acidic catalysis is a better approach for the synthesis of surface-modified or organofunctionalized silica nanoparticles. © 2019 Elsevier B.V.