Polyvinyl-pyrrolidone capped spherical cadmium sulphide quantum dots (CdS-PVP QDs), 2-6 nm in size, were developed as a selective turn-on fluorescence nanosensor for monohydrogen phosphate ion (HPO) in aqueous medium. Fluorescence intensity of CdS-PVP QDs significantly increased with addition of HPO ions, whereas the other common inorganic ions had very little effect on the fluorescence intensity. The proposed sensor may be efficiently used for the detection of HPO ions at a low level of concentration up to 213 nM in real urine sample. Cell imaging study indicates that the CdS-PVP QDs are cell permeable and can detect the intracellular distribution of HPO ions under fluorescence microscope. The CdS-PVP QDs showed considerable activity against Staphylococcus aureus also.