Abstract: CdS semiconductor nano-particles were synthesized in aqueous solution by using mercaptoethylic acid as the stabilizer as well as surface-modification reagent.The average size of CdS nano-particles estimated from TEM graph is ca.10 nm.The CdS nano-particles exhibited strong ultraviolet absorption at 292 nm and fluorescence emission at 524.8 nm.When surface-modification of CdS nano-particles were replaced with pepsin,ultraviolet absorption peak kept same,fluorescence emission intensity decreased as well as fluorescence peak shifted to 462.4nm.Temperature and pH value influenced surface-modification of CdS nano-particles.Under optimal conditions,a concentration of 2～20 mg/L of pepsin could be determined on the basis of fluorescence intensity decrease ratio of CdS nano-particles,with a detection limit 3σ of 0.13 mg/L (n=10).The proposed method was applied to the detection of the concentration of pepsin in human gastric juice.

Key words: CdS nano-particles, pepsin, surface-modification, fluorescence determination