ZnS@CdS core/shell quantum dots as photokilling agents for pathogenic bacteria

Recently, research into various kinds of hybrid nanocrystals, such as core/shell quantum dot (QD) nanocomposites, has become an attractive topic in the field of antibacteria and anti-mold technology [1]. These hybrid systems exhibit new properties and functionalities due to the strong interaction between the two different functional components. Several groups have attempted to fabricate the reverse type-I core/shell QDs, where a material with narrower band gap was overgrown onto the core with a wider band gap, and a significant red-shift of the band gap has been observed. In these nanostructures, shell acts as electron and hole sinks and hinder the recombination of photoinduced electrons and holes in the core. Subsequently, the photoinduced electrons can be captured by the adsorbed O2 and the holes can be trapped by the surface hydroxyl, both resulting in the formation of hydroxyl radical species. It was known that hydroxyl radical species are harmful to microbes due to their high oxidative capacity.Herein, the ZnS@CdS core/shell QDs, which is conventionally considered as a reverse type-I structure, were prepared and then structural, morphological (shape and size) and optical properties of these nanostructures were characterized by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM) and UV-Vis and fluorescence spectroscopy techniques. Detailed investigations of the antibacterial photocatalytic activity of QDs were also carried out using disc diffusion and broth tube dilution methods against E. coli and S. saprophyticus as model strains of gram-negative and gram-positive bacteria. Compared to ZnS and CdS, the as-formed ZnS@CdS nanohybrid was found to give highly improved bactericidal activity and recyclability toward both gram-positive and -negative cells. Moreover, the minimum inhibitory concentration values of ZnS@CdS nanocomposites were 400.0 μg ml-1 for E. coli and 300.0 μg ml-1 for S. saprophyticus, respectively. On the basis of our results, we can conclude that the ZnS@CdS core/shell nanocomposite is a promising candidate for the photocatalytic destruction of bacterial cells.