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 TiO2–Au nanospheres (TiO2–Au NSs) toward degradation of methylene blue (MB). Through electrostatic interactions, Au nanoparticles are attached to poly-(sodium 4-styreneulfonate)/poly(diallyldimethylammonium chloride) modified TiO2 nanoparticles, which then form an Au shell on each TiO2 core through reduction of Au3+ with ascorbic acid. Notably, the deposition of Hg species (Hg2+/CH3Hg+) onto TiO2–Au NSs through strong Au–Hg aurophilic interactions speeds up catalytic degradation of MB. The first-order degradation rates of MB by TiO2–Au NSs and TiO2–Au–Hg NSs are 1.4 × 10−2 min−1 and 2.1 × 10−2 min−1, respectively. Using a commercial absorption spectrometer, the TiO2–Au NSs/MB approach provides linearity (R2 = 0.98) for Hg2+ 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 TiO2–Au NSs/MB approach provides an LOD of 2.0 nM for Hg2+ and 5.0 nM for CH3Hg+, 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.
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This work was funded by the Ministry of Science and Technology (MOST), Taiwan (grant no. 103-2923-M-002-002-MY3). P.R. and A.P.P. are grateful to MOST for a postdoctoral fellowship under the contract number MOST 104-2811-M-002-154 and MOST 104-2811-M-002-153, respectively. We would also like to thank Ms. S.-J. Ji and Ms. C.-Y. Chien of Precious Instrument Center (National Taiwan University) for their assistance in SEM and EDX analysis.
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The authors declare no conflict of interest.
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Ravindranath, R., Periasamy, A.P., Roy, P. et al. Smart app-based on-field colorimetric quantification of mercury via analyte-induced enhancement of the photocatalytic activity of TiO2–Au nanospheres. Anal Bioanal Chem 410, 4555–4564 (2018). https://doi.org/10.1007/s00216-018-1114-7
- Methylene blue
- Smartphone app
- TiO2–Au nanospheres