Tailoring Thermoresponsive Poly(N-Isopropylacrylamide) Toward Sensing Perfluoroalkyl Acids

Widespread distribution of poly- and perfluoroalkyl substances (PFAS) in the environment combined with concerns for their potentially negative health effects has motivated regulators to establish strict standards for their surveillance. The United States Environmental Protection Agency issued a cumulative domestic threshold of 70 ppt for water supplies, and this bar is even lower in some local districts and other countries. Monitoring PFAS consequently requires sensitive analytical equipment to meet regulatory specifications, and liquid chromatography with tandem mass spectroscopy (LC/MS/MS) is the most common technique used to satisfy these requirements. Though extremely sensitive, the instrument is often burdened by pretreatment regimens, sedentation, and user proficiency barriers that encumber or limit its effectiveness. As an alternative, polymeric strategies for detecting PFAS are promising candidates for funneling recognition, transduction, and receptor elements into a single sensing platform to overcome some of the hurdles affecting LC/MS/MS. Toward this end, poly(N-isopropylacrylamide) (PNIPAM), an extensively studied thermoresponsive polymer, is a hydrogel with tailorable swelling properties dependent upon its polymeric composition and surrounding media. This polymer holds a lower critical solution temperature (LCST) around 32 °C that marks its transition from a relatively hydrophilic, swollen state to a hydrophobic, collapsed state once heated, and prior research indicates that surfactants such as sodium dodecyl sulfate can heavily influence the temperature at which this transition occurs and the ultimate swelling ratio for crosslinked hydrogels. Two particularly concerning fluorosurfactants, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), were hypothesized to act similarly to their non-fluorinated analogs by augmenting the swelling of PNIPAM in a dose-dependent manner. The effect of these fluoropollutants on PNIPAM was therefore studied to identify 1) if PFOS and PFOA would have an appreciable effect on the swelling behavior of varying PNIPAM morphologies, 2) if the swelling response could be enhanced by adding functional comonomers into the PNIPAM backbone, and 3) if the swelling behavior could be outfitted with Förster resonance energy transfer (FRET)-compatible dyes to signal the contaminants’ concentration. As such, crosslinked PNIPAM hydrogels were functionalized with fluorinated comonomers to induce fluorine-fluorine attraction amongst the polymers and their analytes to strengthen their recognition capability and microgels were equipped with FRET-capable dyes to achieve a fluorescent transduction motif indicative of the contaminants’ presence. Results indicated that PFOS augments the swelling of PNIPAM hydrogels significantly while PFOA causes microgels to collapse at temperatures below their innate LCST. FRET primarily replicated swelling observations as expected for the distance-mediated fluorescent phenomenon. Though the fluoropollutants generated appreciable swelling perturbations at concentrations within the micromolar range, additional functionalization is necessary to exploit the molecular-level interactions between PNIPAM and target fluorosurfactants to yield detection limits within the range needed for environmental applications