Shape-controlled synthesis of protein-conjugated CdS nanocrystals (NCs) and study on the binding of Cd2+/CdS to trypsin

Protein-conjugated CdS nanocrystals (NCs) with different morphology have been synthesized under biomimetic condition using trypsin as capping agent in aqueous medium. The reaction parameters including concentration of trypsin, pH value, reaction time, and temperature have a major influence on the morphology and optical property of CdS NCs. XRD, selected area electron diffraction (SAED), TEM, HRTEM, and EDS characterizations were used to investigate the structure, composition, morphology, and size of as-prepared products. The binding reaction between Cd2+/CdS and trypsin was investigated systematically through various spectroscopic methods. UV-vis, FT-IR, photoluminescence (PL) spectra, and conductivity analysis of Cd2+-trypsin suggest that Cd2+ ions could coordinate with the functional groups of trypsin and induce the formation of unfolding and loosening structure in protein molecules, and the change of protein conformation was also verified by circular dichroism (CD) spectra. This interaction increased local supersaturation of Cd2+ ions around the groups of trypsin and reduced the nucleation activation energy of CdS nuclei, which favored heterogeneous nucleation in trypsin matrix and resulted in the formation of inorganic-organic hybrid materials. The functional integrity of the enzyme conjugated to CdS NCs was studied by monitoring the enzymatic activity of CdS-trypsin conjugates. The fluorescence of CdS NCs is dependent strongly on trypsin which passivates the surface of NCs.