In this study, green-ZnS nanoparticles (g-ZnS NPs) were synthesized via chemical precipitation using aqueous solutions of ZnCl2 and Na2S at the presence of green tea solution. The ternary hybrid nanocomposite of green-ZnS@ Graphen oxide@Chitosan (g-ZnS@GO@CS) was also prepared through in situ chemical deposition of g-ZnS nanoparticles on GO@CS composite. The obtained nanostructures were characterized by FTIR, SEM, XRD, EDX, X-map, photoluminescence (PL), TGA, and UV-DRS analyses. Based on the results, the g-ZnS@GO@CS composite possessed cauliflower morphology in which ZnS NPs were homogeneously distributed on the composite matrix. Crystallographic investigations revealed the fcc phase of ZnS NPs. Moreover, the crystallite sizes of ZnS, g-ZnS, and g-ZnS@GO@CS samples were estimated as 28.8, 17.9, and 14.8 nm, respectively. Using UV–vis spectroscopy, the bandgap energy of g-ZnS@GO@CS composite, pure ZnS, and g-ZnS were determined 3.02, 3.37, and 4.42 eV, respectively; suggesting a significant reduction in the bandgap of the composite sample due to the synergic effect of GO@CS. Among various g-ZnS@GO@CS composites with different GO@CS contents (4, 8, and 12 wt%) the sample containing 4 wt% GO@CS exhibited the highest photoluminescence intensity. The g-ZnS@GO@CS nanocomposite displayed higher antibacterial activity (growth inhibition zone of 27.6 mm) on Gram-positive Staphylococcus aureus bacteria compared to g-ZnS NPs (growth inhibition zone of 23.5 mm). No significant bioactivity was, however, observed against Gram-negative bacteria (Escherichia coli). According to the results, g-ZnS@GO@CS composite with reduced bandgap, enhanced PL intensity, and smaller ZnS NPs can offer better antibacterial activity as compared to pristine g-ZnS NPs
