Synthesis and characterizations of ultra-small ZnS and Zn (1− x) Fe xS quantum dots in aqueous media and spectroscopic study of their interactions with bovine serum albumin

Abstract

This work reports a new experimental methodology for the synthesis of ultra small zinc sulfide and iron doped zinc sulfide quantum dots in aqueous media. The nanoparticles were obtained using a simple procedure based on the precipitation of ZnS in aqueous solution in the presence of 2-mercaptoethanol as a capping agent, at room temperature. The effect of Fe 3+ ion concentration as dopant on the optical properties of ZnS was studied. The size of quantum dots was determined to be about 1 nm, using scanning tunneling microscopy. The synthesized nanoparticles were characterized by X-ray diffraction, UV–Vis absorption and photoluminescence emission spectroscopies. The presence and amount of iron impurity in the structure of Zn (1− x) Fe x S nanocrystals were confirmed by atomic absorption spectrometry. A blue shift in band-gap of ZnS was observed upon increasing incorporation of Fe 3+ ion in the iron doped zinc sulfide quantum dots. The photoluminescence investigations showed that, in the case of iron doped ZnS nanoparticles, the emission band of pure ZnS nanoparticles at 427 nm shifts to 442 nm with appearance of a new sharp emission band around 532 nm. The X-ray diffraction analysis indicated that the iron doped nanoparticles are crystalline, with cubic zinc blend structure, having particle diameters of 1.7 ± 022 nm. Finally, the interaction of the synthesized nanoparticles with bovine serum albumin was investigated at pH 7.2. The UV–Vis absorption and fluorescence spectroscopic methods were applied to compare the optical properties of pure and iron doped ZnS quantum dots upon interaction with BSA. It was proved that, in both cases, the fluorescence quenching of BSA by the quantum dots is mainly a result of the formation of QDs–BSA complex in solution. In the steady-state fluorescence studies, the interaction parameters including binding constants ( K a ), number of binding sites ( n), quenching constants ( K ′ S V ), and bimolecular quenching rate constants ( k q ) were determined at three different temperatures and the results were then used to evaluate the corresponding thermodynamic parameters Δ H, Δ S and Δ G.