Biocompatible and highly luminescent manganese doped zinc sulfide (ZnS:Mn) nanocrystals of average particle size 10 nm have been synthesized by capping with a novel amino acid ligand, L-citrulline. Though there are many reports on the bioimaging applications of nanostructured semiconductors, the present study focused on the detection of a special type of metal accumulating bacteria, Lysinibacillus fusiformis. This bacterium has significant applications in the disposal of metal components from industrial effluents. In this context, the detection of this bacterium is quite important and the present work demonstrates a novel technique for this bacterial detection. The synthesized nanocrystals were attached to Lysinibacillus fusiformis and characteristics of the bioconjugated system were studied. The blue shift observed in the ultraviolet-visible absorption and photoluminescence spectra of the bioconjugated system, confirms conjugation of the Lysinibacillus fusiformis with L-citrulline-capped ZnS:Mn. When the bioconjugated system (capped ZnS:Mn + bacteria) was observed using a fluorescent microscope under excitation wavelengths 365.4 nm (ultraviolet), 435.8 nm (blue) and 546.1 nm (green), fluorescence emissions were obtained in yellow, green and red regions respectively. The study of relative growth of Lysinibacillus fusiformis in the presence of L-citrulline-capped ZnS:Mn proves biocompatible property of these nanocrystals and their tunable color properties under different excitation wavelengths make them ideal for biolabeling applications.

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