Assessing the perfluoroalkyl acid-induced swelling of Förster resonance energy transfer-capable poly(N-isopropylacrylamide) microgels

Abstract

As a method to combat the extensive contamination of poly- and perfluoroalkyl substances (PFAS) in water supplies, poly(N-isopropylacrylamide) (PNIPAM) microgels copolymerized with 2,2,2-trifluoroethylacrylate (TFEA) represent a potential sensing tool for recognizing PFAS at dilute aqueous concentrations. The microgels exhibit exceptional temperature responsiveness, transitioning from a swollen z-average diameter of 890.8 ± 19.8 nm to a collapsed diameter of 246.4 ± 10.3 nm below and above their lower critical solution temperature, respectively, for non-fluorinated gels, offering broad size fluctuations that are susceptible to coadded contaminants. Monitoring size perturbations as a function of analyte concentration, the polymers were observed to deswell in the presence of perfluorooctanoic acid, octanoic acid, phenol, and sodium 1-octane sulfonate while tetraethylammonium perfluorooctane sulfonate (TPFOS) augmented swelling. Adding up to 40 mol% TFEA to the networks lowered the concentration at which the microgels’ normalized z-average diameter demonstrated a significant deviation from 0.25 mM to 0.1 mM for TPFOS, indicating fluorophilicity as a key contributor to the copolymers’ associative capacity. Implanting Förster resonance energy transfer-compatible dyes, cyanine 3 and cyanine 5, into non-fluorinated microgels largely reiterated results from light scattering, as expected for the size-dependent energy transfer mechanism. Including dyes did, however, reinforce the customizability of this system, leaving windows open for functionalization with other signal transduction motifs to lower the detection limits of the polymer further. The swelling changes for PNIPAM microgels stimulated by the acidic constituents of PFAS highlight the polymer as a candidate for detecting the substances following additional development.