When L-cysteine and AgNO3 solutions were mixed, Ag+ ions got bounded to the L-cysteine through S-Ag bond, resulting in a colorless liquid (Fig. 1b); upon microwave irradiation, the Ag+ ions were reduced to Ag and the reaction mixture turned into light yellow-green when the molar ratio of Ag+ and L-cysteine was 1.5:1, confirming the appearance of AgNPs