In this work, we report a one-step green synthesis of gold nanoparticles (AuNPs) by microwave irradiation using nontoxic and biodegradable polysaccharide chitosan as a reducing and stabilizing agent. The interaction between gold nanoparticles with the amine group of chitosan was confirmed by Fourier transform infrared spectroscopy analysis, and the stability of the nanoparticle is ascertained by zeta potential measurements. Transmission electron microscopy photograph and dynamic light scattering measurements confirmed the average size of gold nanoparticles as 25 nm. The ability of the synthesised gold nanoparticles as a catalyst for the reduction of eosin dye in the presence of NaBH 4 was monitored by means of spectrofluorometry and spectrophotometry. It is found that the NaBH 4 -induced reduction of eosin is enhanced in the presence of AuNPs even without a catalyst. Time-resolved fluorescence decay studies also confirmed the reduction of eosin in the presence of AuNPs.