This work reports the synthesis of US nanoparticles (NPs) capped with ammonia and triethylamine by using a precipitation method The obtained particles were characterized by using X-ray diffraction (XRD). transmission electron microscope (TEM) and spectroscopic techniques The particle size for NPs capped with ammonia (6.2 nm) and triethylamine (2.4 nm) was calculated from XRD patterns using Scherrer formula For ammonia capping, the optical properties are typical of localized surface states However, the confinement effects play a significant role in the optical properties of triethylamine capping The emission of US NPs is quenched by Flufenamic acid (FLF) drug. At pH 7. the quenching efficiency of triethylamine capped CdS NPs is greater than that of ammonia capping The interaction between FLF and ammonia capped US NPs proceeds via electrostatic mode, whereas for methylamine capping the interaction might be due to electron transfer process In acidic media, the fluorescence quenching of ammonia capped CdS in the presence of FLF was followed by a fluorescence enhancement and blue shift of fluorescence emission This might be a result of the dissociation of the CdS-ammonia nanosystem, due to protonation of the surface In such case, removing the capping agent. leads to faster aggregation of the particles. (c) 2009 Elsevier B.V. All rights reserved