The amino acid asparagine (ASP) was used as a benign reducing and stabilizing agent for the production of monodisperse gold nanoparticles (AuNPs) using green chemistry principles. With an increasing concentration of ASP (0.5 to 10 mM), the absorbance intensity at 525 nm increased; however, no effects on the color, size, or shape of the AuNPs were observed. Transmission electron microscope (TEM) images showed that the AuNPs were either hexagonal or spherical in shape and had an average size of approximately 10 ± 5 nm. Facile colorimetric assays of the AuNPs were applied to detect a variety of heavy metal ion species in water. In this study, the selective detection of arsenic ions (As (III) ions) by quenching, aggregation, and/or red-shifting of the surface plasmon resonance (SPR) was successfully achieved. The AuNPs sensor was sustainable as a visual colorimetric detection system and spectral assay of hazardous As (III) ions in the reaction medium; thus, it will be useful for aqueous assessment without using any sophisticated or expensive instruments.