Surface functionalization has numerous applications worldwide. Silicon oxide has been a research material of choice. However, tin dioxide (SnO₂) films are employed in many applications especially in gas sensors, and little studied in regard to functionalization. Thus, they were chosen to be functionalized via 3-aminopropyltriethoxysilane (APTES). Different synthesis parameters were tested such as APTES grafting by vapor or liquid phases deposition. In liquid, many parameters were investigated: water presence, reaction times, and APTES concentration. The presence and reactivity of grafted amine-terminated film on SnO₂ were carried out by Alexa Fluor® molecules. In addition, APTES grafting was characterized using attenuated total reflectance Fourier transform infrared spectroscopy and X-ray photoelectron spectrometry techniques. These characterizations showed how synthesis parameters affect the amount and thickness of APTES films. Optimal liquid silanization parameters were determined in order to obtain a saturated SnO₂ surface with APTES molecules. Importantly, the addition of 5 vol% H₂O to the APTES solution provided denser surface coverage, by hydrolyzing the ethoxy groups to silanol. An almost 50% improvement over anhydrous liquid and vapor methods was obtained.