Colourimetric PFAS detection

Per- and polyfluoroalkyl substances (PFAS) are a class of synthetic pollutants predominantly saturated with fluorine atoms. These molecules are used in the manufacturing of a vast range of commercial products due to useful physio-chemical properties such as hydrophobicity and chemical resistivity. It is these properties that have made PFAS ubiquitous in items such as water-resistant clothing, stain-resistant fabrics, fast-food packaging, aqueous firefighting foams (AFFF), and non-stick cookware, but also in our bodies and the environment. The emerging concern of toxicity coupled with prevalent bioaccumulation led to regulations restricting the use of common PFAS such as perfluorooctanoic acid (PFOA), perfluorohexanesulfonic acid (PFHxS), and perfluorooctanesulfonic acid (PFOS).
As a class of molecules, PFAS exhibit peculiar behaviours and unexpected properties that can complicate research and analysis. The available literature is dispersed across analytical protocols and remediation procedures, and is littered with non-descript terminology that makes working with PFAS challenging for a non-specialist chemist. A tutorial review that amalgamates and translates PFAS information into a format accessible to a non-specialist chemist is included. The purpose of this tutorial was to enable successful PFAS research in all areas of chemistry, and allow multidisciplinary audiences to assess the validity of results from published PFAS studies. Here it acts as a preface to the project objective.
The aim of this thesis was to investigate the use of supramolecular chemistry as a PFAS detection method. Molecular recognition and small molecule sensing are intermolecular events that can be used for sensing and detection. In this instance, a host molecule was designed to interact with the guest molecule PFOA. The host molecule was composed of a porphyrin base, anion binding groups, and fluorinated chains designed to interact with the PFAS guest molecule. The synthetic pathway to this receptor included isomeric purification of 5,10,15,20-tetrakis(2- aminophenyl)-21H,23H-porphine, allowing for the acyl substitution of perfluorooctanoyl chloride to yield a tetra-amido “picket fence” porphyrin. The porphyrin provided a strong chromophore and scaffold for the anion binding amides, to which the perfluorinated chains were available to interact with the fluorinated guest molecules. The receptor was screened with a range of interferent anions and PFAS guest molecules using UV-visible spectroscopic analysis. The host molecule was found to be highly responsive and selective to perfluorocarboxylic acids (PFCAs), and the binding constants for nine PFCAs were determined from UV-visible spectroscopic titrations. Application of this molecule for detection of PFOA from spiked soil samples was demonstrated with the visual detection of PFOA at 3 ppm with minimal sample preparation.
After demonstrating the use of α,α,α,α -5,10,15,20-tetra- (2-amidophenyl- pentadecafluoro-octanoyl) porphyrin for PFOA detection, additional host molecules were explored by installation of a range of different anion binding groups including squaramide, urea, thiourea, and triazole. The installation of these moieties required fine tuning of synthetic pathways to overcome isomeric conversion of the host scaffold, whilst accommodating the often challenging incorporation of fluorinated functionalities. The impact of these modifications was probed using UV-visible spectroscopy, NMR spectroscopy, and the determination of binding constants to assess the response to common PFAS. It was found that the amido derivatives were the most responsive and practical sensor molecules for PFAS, in terms of the intensity of the colourimetric response and synthetic accessibility. The application of the host molecules and the detection platform was also investigated, including the use of solid supports such as silica or paper for the detection of PFAS from water or vapour. Other emerging PFAS were also considered, including GenX (hexafluoropropylene oxide dimer acid), which was detected in vapour from a 1 ppm aqueous solution using paper doped with a tetra-amido host molecule.
The length of the perfluorinated chains were also modified, allowing for fine tuning of the colourimetric responses. A tetra-amido porphyrin with perfluorodecane-length chains was shown to provide similar colour responses to each of the tested PFCAs, making it a useful sensor for the total detection of PFCAs. The molecule was combined with known quantities of PFOA to create a colourimetric calibration that could be used for the visual estimation of total PFCA concentrations. This was first explored with visual perceptions and ∆E colour comparison values, which measure the how different or similar two colours are to each other, and further enhanced using parameterisation of RGB values extracted from mobile phone photographs, allowing detection of total PFCA concentrations from 10 ppb to 16 ppm.
PFCAs are a subset of common anionic PFAS, but contamination includes perfluorosulfonic acids, such as PFOS which represent an additional challenge to detect due to their aggregation properties and surfactant behaviours. Initial screening of the host molecules with PFOS did not provide a persistent colourimetric response, and it was considered that the host was not readily available to interact with the surfactant like PFOS in solution. To better target PFOS, the hydrophobic tetra-amido porphyrin with perfluorooctane-length chains was suspended in aqueous micellular solutions. A range of surfactants were screened, and an assembly method was developed that allowed for the host to be suspended in an aqueous micellar solution. These suspensions were combined with aqueous PFOS samples to elicit a colour change which was investigated using UV-visible spectroscopy, and assessed using visual perceptibility, ∆E, and mobile phone photograph RGB information.
The investigations presented here show the first examples of selective, colourimetric PFAS detection using porphyrin host molecules, and represent advancement towards a low cost, rapid visual PFAS sensor