Colour Catcher® sheet beyond the laundry: a low-cost support for realizing porphyrin-based mercury ion sensors

The detection of mercury ions in drinkable water is a challenging task, because this heavy metal should be detected in very low concentrations. While analytical laboratory instruments can allow reaching the required sensitivity, the need for continuous monitoring of water samples by chemical sensors is more difficult. In this work, we report the development of a simple colorimetric approach based on the color variation of a free base porphyrin upon interaction with Hg(II) ions. The support is made by Color Catcher (R) strip embedded by the tetra (4-carboxyphenyl)porphyrin; the anionic character of the porphyrin allows a uniform and stable deposition onto the positively charged solid support. An optical platform, composed of low cost electronic devices, such as LED and webcam, coupled to a PC has been developed to digitalize the images of the strip color changes upon the flux of water solution containing Hg(II) ions. Homemade software developed in Matlab, using the Hue parameter, was exploited to analyze the color changes, allowing reaching a LOD three times below the Hg(II) legal limit stabilized by WHO. The device also showed good selectivity towards other potential interferent metal ions, demonstrating the potentialities of this approach in the real field.