Cyclodextrin-Assisted Surface-Enhanced Photochromic Phenomena of Tungsten(VI) Oxide Nanoparticles for Label-Free Colorimetric Detection of Phenylalanine.

Herein are presented the results of experiments designed to evaluate the effectiveness of host-guest interactions in improving the sensitivity of colorimetric detection based on surface-enhanced photochromic phenomena of tungsten(VI) oxide (WO) nanocolloid particles. The UV-induced photochromic coloration of WO nanocolloid particles in the presence of aromatic α-amino acid (AA), l-phenylalanine (Phe) or l-2-phenylglycine (Phg), and heptakis(2,3,6-tri--methyl)-β-cyclodextrin (TMβCDx) in an aqueous system was investigated using UV-vis absorption spectrometry. The characteristics of the adsorption modes and configurations of AAs on the WO surface have also been identified by using a combination of adsorption isotherm analysis and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). A distinct linear relationship was observed between the concentration of AAs adsorbed on the WO nanocolloid particles and the initial photochromic coloration rate in the corresponding UV-irradiated colloidal WO in aqueous media, indicating that a simple and sensitive quantification of AAs can be achieved from UV-induced WO photochromic coloration without any complicated preprocessing. The proposed colorimetric assay in the Phe/TMβCDx/WO ternary aqueous system had a linear range of 1 × 10 to 1 × 10 mol dm for Phe detection, with a limit of detection of 8.3 × 10 mol dm. The combined results from UV-vis absorption, ATR-FTIR, and adsorption isotherm experiments conclusively indicated that the TMβCDx-complexed Phe molecules in the Phe/TMβCDx/WO ternary aqueous system are preferentially and strongly inner-sphere adsorbed on the WO surface, resulting in a more significant surface-enhanced photochromic phenomenon. The findings in this study provided intriguing insights into the design and development of the "label-free" colorimetric assay system based on the surface-enhanced photochromic phenomenon of the WO nanocolloid probe.