BODIPY-Based Chain Transfer Agent: Reversibly Thermoswitchable Luminescent Gold Nanoparticle Stabilized by BODIPY-Terminated Water-Soluble Polymer

Abstract

Well-defined poly(N-isopropylacrylamide)s (PNIPAMs) with boron dipyrromethene (BODIPY) in the terminal end were prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization of NIPAM with BODIPY-chain transfer agent (CTA), which was synthesized by the reaction of pyrrole and BODIPY derivative having benzyl chloride in the meso position. The controlled character of the polymerization of NIPAM was confirmed by the formation of the narrow molecular weight distribution products and linear increase of molecular weight with the feed ratios of [NIPAM]/[CTA]. The RAFT-prepared PNIPAM exhibited strong green luminescence with emission maxima at 558 nm upon excitation with the absorption maxima at 526 nm and was used for the aqueous synthesis of stabilized gold nanoparticles (AuNPs). The in situ reduction of the RAFT-prepared polymers and HAuCl(4) resulted in the formation of stable modified AuNPs with size distributions of ∼10 nm in water solution. Temperature-dependent change in photoluminescence (PL) spectra (excited at 526 nm) of the RAFT-prepared polymers measured in water showed an increase in emission intensity with the rise at temperature (20 → 40 °C). On the contrary, AuNPs exhibited a decrease in emission intensity with increasing of temperature, resulting that shrinking of the distance between BODIPY units and Au core in the AuNP across border LCST gives rise to both quenching of emission of BODIPY unit by Förster resonance energy transfer (FRET) and dye quenching of aggregated BODIPY units. The emission/quenching of AuNP occurs reversibly and efficiently, regardless of the heating and cooling cycle.