Smart app-based on-field colorimetric quantification of mercury via analyte-induced enhancement of the photocatalytic activity of TiO-Au nanospheres.

We have devised a unique strategy for highly sensitive, selective, and colorimetric detection of mercury based on analyte-induced enhancement of the photocatalytic activity of TiO-Au nanospheres (TiO-Au NSs) toward degradation of methylene blue (MB). Through electrostatic interactions, Au nanoparticles are attached to poly-(sodium 4-styreneulfonate)/poly(diallyldimethylammonium chloride) modified TiO nanoparticles, which then form an Au shell on each TiO core through reduction of Au with ascorbic acid. Notably, the deposition of Hg species (Hg/CHHg) onto TiO-Au NSs through strong Au-Hg aurophilic interactions speeds up catalytic degradation of MB. The first-order degradation rates of MB by TiO-Au NSs and TiO-Au-Hg NSs are 1.4 × 10 min and 2.1 × 10 min, respectively. Using a commercial absorption spectrometer, the TiO-Au NSs/MB approach provides linearity (R = 0.98) for Hg over a concentration range of 10.0 to 100.0 nM, with a limit of detection (LOD) of 1.5 nM. On the other hand, using a low-cost smartphone app that records the color changes (ΔRGB) of MB solution based on the red-blue-green (RGB) component values, the TiO-Au NSs/MB approach provides an LOD of 2.0 nM for Hg and 5.0 nM for CHHg, respectively. Furthermore, the smartphone app sensing system has been validated for the analyses of various samples, including tap water, lake water, soil, and Dorm II, showing its great potential for on-line analysis of environmental and biological samples. Graphical Abstract ᅟ.