Ionic aggregation-induced emission dye with bulky counterions for preparation of bright near-infrared polymeric nanoparticles.

Dyes exhibiting aggregation-induced emission (AIE) are attractive building blocks for the preparation of bright fluorescent nanomaterials. AIEgens are especially efficient in pure dye aggregates, whereas they are much less explored as dopants in NPs built of hydrophobic polymers. Here, we describe an approach that combines cationic AIEgens with bulky hydrophobic counterions (fluorinated tetraphenylborates) that enables preparation of small and bright AIEgen-loaded polymeric NPs. To this end, we synthesised a cationic tetraphenylethylene (TPE) derivative and studied its salts with counterions of different sizes and hydrophobicities. In organic solvent/water mixtures, all these salts exhibited typical AIE behaviour, whereas only salts with bulky hydrophobic counterions exhibited strongly red-shifted emission in the near-infrared (NIR) region. Encapsulation of these salts into poly(methyl methacrylate-co-methacrylic acid) (PMMA-MA) NPs revealed that bulky counterions ensure (i) formation of small (∼50 nm) AIEgen-loaded polymeric NPs; (ii) good fluorescence quantum yield (up to 30%); and (iii) NIR emission reaching 700 nm. By contrast, AIEgens with small inorganic anions (perchlorate and hexafluorophosphate) blended with PMMA-MA produced large aggregates with emission in the far-red region. Single-particle microscopy revealed that our 50 nm AIEgen-loaded PMMA-MA NPs were 6-fold brighter than the NIR emitting quantum dots (QD705). These NPs feature low cytotoxicity and compatibility with live cell imaging, in contrast to large aggregates of AIEgens with small inorganic counterions that failed to internalize into the cells. The present work shows that combination of cationic AIEgens with bulky counterions opens new routes for the preparation of bright polymer-based nanomaterials.