Excitation induced tunable emission in biocompatible chitosan capped ZnS nanophosphors

Colloidal semiconductor nanomaterials exhibit color modulation which can be useful for various optoelectronic and biolabelling applications. Previous studies on CdSe and CdTe have shown tunable color modulation by varying the size of nanomaterials but toxicity of cadmium has created doubt for its end applications as biosensor. The recent work on ZnS: Mn2+ doped semiconductor has shown some viability for biolabelling but for tunable behavior particles of different doping concentration needs to be synthesized. In the present work all the above problems were considered and viable solution has been given to use ZnS: Mn²⁺ doped semiconductor using chitosan as capping agent. Here we report deliberate color modulation for chitosan capped ZnS: Mn²⁺ nanoparticles (NPs) synthesized in single step instead of many samples of different size. The tunable behavior is achieved by varying the excitation wavelength in same sample. Shifting of emission peak from dopant related emission at 590 nm (d states of Mn²⁺) to 481 nm defect related emission (Sulfur vacancy to Zn²⁺) and 421 nm (sulfur vacancy to valence band) has been observed. The work illustrates coupling between Frenkel exciton in organic polymers with Wannier exciton in inorganic semiconductors which has been observed for the present nanophosphors. Synthesized chitosan capped ZnS: Mn²⁺ NPs have been characterized by x-ray diffraction, high-resolution transmission electron microscopy, energy dispersive x-ray, and Fourier transform infrared for structural studies. UV visible absorption spectroscopy and Photoluminescence studies were done to observe optical behavior. Due to nontoxic nature of chitosan capped ZnS: Mn²⁺ with tunable color properties in single sample, these materials can be used as biolabels. Biocompatibility of chitosan functionalized ZnS:Mn quantum dots has been also verified with bacteria (staphylococcus aureus).