Colloidal suspensions of AgNPs mixed with cysteine aliquots were placed in glass cuvettes with an optical path length of 1 cm. The laser beam was focused through an optical lens permanently mounted in a cuvette holder. Four accumulations (60-second-long each) were averaged for every spectrum. The colloids were activated just before spectrum acquisition by adding one to four successive 40 µL aliquots of 0.5 M KNO3 solution for a satisfactory signal-to-noise ratio. Samples prepared at relatively lower temperatures (e.g. 25 °C or 40 °C) generally changed color from gray-brown to dark green upon addition of the second KNO3 aliquot, indicating AgNPs aggregation. The color change did not occur in samples prepared at temperatures higher than 40 °C which coincided with a worse signal-tonoise ratio in the SERS spectra. Similarly, aggregation was not observed upon addition of cysteine alone under the experimental conditions of this study .