Manipulation of aggregation-induced emission of thermoresponsive fluorescent polymers having Au(I)–S groups for a fluorescent chemosensor

Abstract Aggregation-induced emission (AIE) is a new technique for enhancing fluorescence. To demonstrate manipulation of near-infrared AIE by thermal stimuli for analytical use, we synthesized a thermoresponsive fluorescent polymer containing Au(I)-S bonds as near infrared fluorophores. We also examined the polymer for use as a fluorescent chemosensor based on the manipulation of AIE. The thermoresponsive fluorescent polymer was prepared from poly(N-isopropylacrylamide-co-methacrylic acid) by introducing thiol groups which were further modified to form fluorescent Au(I)-S bonds. The thermoresponsive fluorescent polymer aggregated and then fluoresced in the near infrared region due to AIE when heated above the phase transition temperature of 32 °C. The change in fluorescence intensity of the polymer was reversible by changing the solution temperature. Because the residual carboxyl groups make the thermoresponsive fluorescent polymer negatively ionizable, neutralization by the addition of positively charged analytes facilitates its aggregation to enhance fluorescence above the phase transition temperature. When hemoglobin, which has an isoelectric point of 6.8, was added to a pH 6.0 polymer solution at 40 °C, the polymer aggregated to enhance its fluorescence as a result of neutralization. A linear relationship between the hemoglobin concentration and the fluorescence intensity of the polymer was obtained for 0.5 to 2.0 g/L hemoglobin with a detection limit (3 s) of 0.014 g/L.