Red luminescent manganese-doped zinc sulphide nanocrystals and their antibacterial study.

Water soluble, uniform-sized ZnS:Mn²⁺ nanocrystals (NCs) have been prepared using a simple co-precipitation method with a methanol and water binary mixture as a reaction medium. The structure of the prepared ZnS:Mn²⁺ NCs is cubic with a mean size distribution of 3-5 nm. Photoluminescence (PL) studies showed emission at ∼612 nm, which is 22 nm red shifted as compared with the reported literature. This red shift could be attributed to the observed distortion in the imaged lattice plane. The capping effect of pepsin, citric acid and biotin on the optical properties of ZnS:Mn²⁺ NCs has been examined and the maximum enhancement in PL Intensity was found in the case of biotin. The synthesised ZnS:Mn²⁺ NCs were characterized by X-ray Diffraction (XRD), transmission electron microscopy (TEM), X-ray photoemission spectroscopy (XPS) for investigation of their structural properties. Because of the high PL intensity, biotin capped ZnS:Mn²⁺ NCs were further investigated for their anti-bacterial activity against gram negative and gram positive bacteria. These NCs show broad spectrum antibacterial activity against both types of bacteria having an MIC value of 100 ng ml^-1 for B. subtilis.