Owing to its low toxicity toward living organisms and specific optical properties, we promote the use of zinc oxide (ZnO) as an alternative to existing semiconductor-based materials for developing new bioimaging techniques. ZnO nanoparticles (NPs) were prepared using the laser ablation technique in oxygen reactive atmosphere at room temperature by ablating a commercial high-purity ZnO target. The surface functionalization of ZnO NPs was successfully achieved using a dry chemical reactor with ammonia/argon mixture plasma. The roles of various plasma ions in the surface interaction with ZnO NPs were investigated to understand the mechanism of functionalization by quadrupole mass spectrometry. (C) 2014 The Japan Society of Applied Physics