Reversible switching of perylene emission colors by thermo-responsive volume changes in poly(N-isopropylacrylamide) nanoparticles

In this study, we prepared poly( N -isopropylacrylamide) (PNIPAM) nanoparticles (NPs) via soap-free emulsion radical polymerization of N -isopropylacrylamide (NIPAM) in water using N , Nʹ -methylenebis(acrylamide) (BIS) as a crosslinker and potassium persulfate (KPS) as an initiator. Perylene (Per), a hydrophobic fluorescent dye, was introduced into the obtained NPs by swelling the NPs in a Per-containing THF solution, followed by washing and redispersion in water. We found that the fluorescence of Per was reversibly tuned by temperature-induced changes in the local environment within the NPs. Below the lower critical solution temperature (LCST), the NPs swelled and became more hydrated, and excimer emission from Per was enhanced, probably because hydrophobic Per molecules were localized in limited hydrophobic regions within the hydrated PNIPAM network, promoting local Per–Per associations. Above the LCST, the NPs contracted due to dehydration, making the interior more hydrophobic; however, the excimer-to-monomer emission ratio decreased and monomer emission became dominant. These spectral changes, together with the temperature-dependent fluorescence excitation spectra monitored at 443 and 550 nm, support a temperature-induced change in the association state of particle-associated Per. The different excitation profiles are consistent with contributions from spectroscopically distinct Per populations to the monomer and long-wavelength emissions. However, the precise nanoscale spatial distribution of Per within the nanoparticles was not directly determined in the present study.