Mechanistic investigation on fluorescence instability of AIE polymeric nanoparticles with a susceptible AIEgen prepared in miniemulsions

Abstract

Fluorescence stability is a key index for application of aggregation-induced emission (AIE) polymeric nanoparticles (NPs) as fluorescent probes. In this work, the temperature-dependent fluorescence stability of AIE polystyrene (PSt) NPs with a susceptible AIE luminogens (1-methyl-1,2,3,4,5-pentaphenylsilole, MPPS), which was named as PSt/MPPS NPs, was thoroughly investigated. The PSt/MPPS NPs in aqueous emulsions showed an abnormally fast fluorescence loss at the temperatures close to or above the glass transition temperature of the matrix polymer. The poor fluorescence stability of PSt/MPPS NPs at elevated temperatures could be ascribed to the continuous hydrolysis of MPPS. Firstly, the MPPS molecules at the surface of NPs undergo hydrolysis. At elevated temperatures, the incorporated MPPS inside the NPs can freely diffuse to the particle/water interface to undergo hydrolysis under the drive of MPPS concentration difference between the interior and surface of NPs, leading to an almost complete fluorescence quenching. Based on the fluorescence loss mechanism of the PSt/MPPS NPs, several effective strategies were designed to improve fluorescence stability of AIE polymer/MPPS NPs, for example through using matrix polymers with a relatively less hydrophobicity or a higher glass transition temperature to replace PSt, or fabrication of a less hydrophobic polymer shell on the PSt/MPPS NPs.