Label-free Colorimetric Sensing of α-Amino Acids Based on Surface-enhanced Photochromic Phenomena of Molybdenum (VI) Oxide Nanoparticles

Due to the biological importance of alpha-amino acids and their derivatives, the development of novel colorimetric probes for these molecules has been an active research area in recent years. In this study, surface-enhanced photochromic phenomena by various a-amino acids (L-aspartic acid [Asp], L-leucine [Leu], and L-lysine [Lys]) adsorbed on molybdenum(VI) oxide (MoO3) nanoparticles in an aqueous system were investigated by means of UV-vis absorption spectrometry. No derivatization of a-amino acids is required, and eventually highly accurate determinations are obtained from the UV-induced MoO3 photochromic coloration. Under optimal conditions in this system, limit of quantitation (LOQ) values of 3.45 X 10(-6), 2.08 X 10(-6), and 1.53 X 10(-6) mol dm(-3) for Asp, Leu, and Lys, respectively, were achieved at pH 2. During UV irradiation, the detection process could be visually observed by naked eyes. A simple and unique approach to explore sensitive "label-free" colorimetric sensing toward a-amino acids is proposed.