A novel chemical sensor for the colorimetric detection of toxic mercuric salts in water exhibits submicromolar sensitivity, according to the chemists in the U.K. who designed the system.
The sensor is based on a mesoporous nanocrystalline TiO2 film sensitized with a commercially available ruthenium dye. The film's color changes from red to orange when it is immersed in an aqueous solution of a Hg2+ salt.
"There is no previous report of TiO2 films being used for detecting metal salts," Palomares says. "Because of its excellent optical transparency, high surface area, and the ease of modifying the surface, TiO2 is an ideal material on which to build colorimetric sensors."
Using the dye-sensitized film, the researchers can detect Hg2+ concentrations as low as 20 µM with the naked eye. Spectrophotometric detection using the film enabled them to sense Hg2+ concentrations down to around 0.3 µM--that is, 0.5 ppm of Hg2+. The group also showed that the film's color change in the presence of Hg2+ was insensitive to the presence of other water-polluting metal cations such as Cu2+ and Cd2+ and various anionic species.
The sensor offers a number of advantages over existing chemical sensors, according to the team. For example, devices based on thin films of gold require complicated electronic circuits and operate at 150–300 °C, whereas the Imperial College sensor works at room temperature. Sensors using polymer composites exhibit relatively limited sensitivity, and those that employ luminescent organic materials usually function only in organic solvents.
"Similarly, limitations are encountered with biosensing of Hg2+, which requires the use of buffering solutions and long equilibration times before the reading can be carried out," the authors observe.
They attribute the color change of their sensor to the Hg2+ ion-induced transformation of the ruthenium complex's thiocyanate ligand. "We are currently investigating the detailed mechanism of the transformation and also expanding our approach to other metal-NCS complexes and other analytes," Palomares says.