Thermo- and light-regulated fluorescence resonance energy transfer processes within dually responsive microgels

Abstract

We report on the fabrication of thermo- and light-responsive P(NIPAM-DMNA-NBDAE-RhBEA) microgels consisting of N-isopropylacrylamide (NIPAM), photocleavable moieties, 5-(2′-(dimethylamino)ethoxy)-2-nitrobenzyl acrylate (DMNA), fluorescence resonance energy transfer (FRET) donors, 4-(2-acryloyloxyethylamino)-7-nitro-2,1,3-benzoxadiazole (NBDAE), and rhodamine B-based FRET acceptors (RhBEA) via the free radical emulsion polymerization technique. Thermo-induced collapse and swelling of responsive microgels above and below the lower critical solution temperatures (LCSTs), respectively, can finely tune the spatial proximity between NBDAE and RhBEA dyes, leading to the facile modulation of FRET efficiencies. Upon UV irradiation (pH 8.5), DMNA moieties within microgels undergo photolysis reactions and the formation of sodium carboxylate residues on microgel scaffolds results in elevated LCST. Thus, UV irradiation of microgel dispersions in the intermediate temperature range (between the LCST of original microgels and that of UV-irradiated microgels) can directly lead to the re-swelling of initially collapsed microgels. The incorporation of FRET pairs (NBDAE and RhBEA dyes) allow for the in situ monitoring of thermo- and UV irradiation-induced volume phase transitions (VPTs) of the reported dually responsive microgels. This work represents the first report of thermoresponsive microgels with VPTs tunable by photolabile 2-nitrobenzyl ester moieties.