Label-free colorimetric analysis strategies based on adsorption-responsive surface-enhanced photochromic phenomena of tungsten(VI) oxide nanoparticles for amino acids.

Herein, we present a colorimetric detection method based on the surface-enhanced photochromic phenomenon of tungsten (VI) oxide (WO) nanocolloid particles for α-amino acid (AA) molecules, including -aspartic acid (Asp), -glutamic acid (Glu), -histidine (His), -isoleucine (Ile), -leucine (Leu), -lysine (Lys), -phenylalanine (Phe), and -valine (Val). The UV-induced photochromic phenomena in the AA/WO binary aqueous systems were investigated using UV-Vis absorption spectrometry. The adsorption properties of the AA molecules on the surface of the WO nanocolloid particles have been identified using a combination of adsorption isotherm analysis and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. A good linear correlation between the concentration of the AAs adsorbed on the surface of the WO nanocolloid particles and the initial photochromic coloration rate in the corresponding UV-irradiated WO colloidal aqueous solution was obtained with over three orders of magnitude, indicating that the surface-enhanced photochromic phenomenon of the WO nanocolloid particle can be used to detect the AA molecules. In addition, based on the results of the UV-Vis absorption, ATR-FTIR, and adsorption isotherm analyses, we have experimentally demonstrated that the AA/WO binary aqueous system with inner-sphere adsorbed Ile, Leu, Lys, or Val molecules on the surface of the WO nanocolloid particles exhibits a more significant surface-enhanced photochromic phenomenon than the system with outer-sphere adsorbed Asp, Glu, His, or Phe molecules. The strong inner-sphere adsorption of the AA molecules successfully improved the limit of detection. This study provides valuable insights into a "label-free" colorimetric assay system based on the surface-enhanced photochromic phenomenon of the WO nanocolloid probe.