Cadmium-free aqueous synthesis of ZnSe and ZnSe@ZnS core–shell quantum dots and their differential bioanalyte sensing potential

Abstract

Herein we report a facile and cadmium-free approach to prepare water-soluble fluorescent ZnSe@ZnS core–shell quantum dots (QDs), using thioglycolic acid (TGA) ligand as a stabilizer and thiourea as a sulfur source. The optical properties and morphology of the obtained core–shell QDs were characterized by UV–vis and fluorescence spectroscopy, transmission electron microscopy (TEM), energy-dispersive x-ray analysis (EDX), x-ray diffraction (XRD), electrophoresis and dynamic light scattering (DLS) techniques. TEM analysis, and electrophoresis data showed that ZnSe core had an average size of 3.60 ± 0.12 nm and zeta potential of −38 mV; and for ZnSe@ZnS QDs, the mean size was 4.80 ± 0.20 nm and zeta potential was −45 mV. Compared to the core ZnSe QDs, the quantum yield of these core–shell structures was higher (13% versus 32%). These were interacted with five common bioanalytes such as, ascorbic acid, citric acid, oxalic acid, glucose and cholesterol which revealed fluorescence quenching due to concentration dependent binding of analytes to the core only, and core–shell QDs. The binding pattern followed the sequence: cholesterol < glucose < ascorbic acid < oxalic acid < citric acid for ZnSe, and cholesterol < glucose < oxalic acid < ascorbic acid < citric acid for core–shell QDs. Thus, enhanced binding was noticed for the analyte citric acid which may facilitate development of a fluorescence-based sensor based on the ZnSe core-only quantum dot platform. Further, the hydrophilic core–shell structure may find use in cell imaging applications.