Colorimetric determination of perfluorocarboxylic acids using porphyrin hosts and mobile phone photographs

Perfluorinated carboxylic acids (PFCAs) are widespread, potentially harmful, and difficult to detect pollutants. Here we investigate the use of three colorimetric porphyrin host molecules with chain lengths derived from different PFCA precursors as the first visual-based sensors for a range of different sized perfluorocarboxylates. We found that modifications to the length of the fluorinated chains led to subtleties in binding preferences and the resultant colorimetric response (RGB). Host-guest interactions were investigated with UV-visible spectroscopy, and ImageJ software analysis was used to relate RGB information from digital photographs with binding and perceived colors. The CIE76 formula for color difference was used to formalise the visual estimation of PFCA concentrations with color charts produced from raw RGB information. Using the RGB information collected from a mobile phone photograph, the color responses were parameterized and calibrated using known concentrations of perfluorooctanoic acid so that total PFCA concentrations could be estimated with less than 20% error across a 10 ppb (parts per billion)-16 ppm (parts per million) range. Functionalized porphyrin molecules were found to change color in the presence of perfluorocarboxylic acids (PFCAs). The porphyrin molecules allow for simple visual- and color space-based detection of PFCAs across a range of concentrations.