The UV–Vis spectra were recorded every 2 min for over 2 h after Cu2+ ion was added. The results show that the higher the Cu2+ concentration the faster the appearance of the second peak at 640 nm, indicating the rapid formation of nano-assemblies. Furthermore, the spectrum in the range of 600–800 nm is evidence of the presence of prominent d–d bands, strongly suggested the coordination bonding of the l-Cysteine to the Cu2+ ions