Aggregation-induced emission polymer nanoparticles with pH-responsive fluorescence

Aggregation-induced emission (AIE) polymer nanoparticles (PNPs) including spherical and cylindrical micelles and vesicles were prepared to have a pH-tunable fluorescence response by virtue of the amphiphilic block copolymer self-assembly technique. The poly(M1)-b-poly(M2-co-M3) block copolymers were prepared from ring-opening metathesis polymerization (ROMP) of norbornene-based monomers M1, M2, and M3, in which the hydrophilic poly(M1) had poly(ethylene glycol) side chains and the hydrophobic poly(M2-co-M3) possessed reactive pentafluorophenyl ester (in M2) and AIE-active tetraphenylethene (in M3) side groups. The spherical and cylindrical micelles and vesicles were then self-assembled from poly(M1)-b-poly(M2-co-M3) with different block ratios in selective solvents of THF/water. This produced self-assembly with hydrophobic microdomains aggregated by poly(M2-co-M3) blocks and dispersing shells formed by hydrophilic poly(M1) blocks. In the presence of reactive pentafluorophenyl ester groups, the hydrophobic microdomains could be crosslinked by reacting with the diamine crosslinkers to chemically fix the self-assembly morphology and produce the stable PNPs. Based on the same activated ester chemistry, stable PNPs with varied morphologies and pH-responsive AIE properties were prepared by further post-functionalizing the crosslinked hydrophobic microdomains. Using N,N-diethylethylenediamine as a post-functionalization agent to introduce the diethylamino groups, the resultant PNPs emitted weak fluorescence when the pH < 4 and strong fluorescence when the pH > 5 in water. Comparatively, using β-alanine as a post-functionalization agent to introduce the carboxylic groups, the resultant PNPs emitted weak fluorescence when the pH > 10 and strong fluorescence when the pH < 8 in water. In addition, the pH-dependant fluorescence “turn on/off” properties were obtained for the post-functionalized PNPs when reversibly varying the pH value of the aqueous solution between 2 and 11.