Ionic Aggregation‐Induced Emission: Bulky Hydrophobic Counterions Light Up Dyes in Polymeric Nanoparticles

Abstract

Dye-loaded polymeric nanoparticles emerge as powerful bioimaging tools, but their assembly is challenged by aggregation-caused quenching (ACQ) of flat fluorophores. Aggregationinduced emission (AIE) proposes an effective solution against ACQ by exploiting propellershaped neutral fluorophores without polymeric blending. Fighting ACQ of ionic dyes in polymeric nanoparticles can be achieved by bulky hydrophobic counterions. Here, we aim to generate an AIE phenomenon in poorly emissive ionic dyes using bulky counterions. Three cationic hemicyanine dyes of styryl pyridinium family have been synthesized: two planar fluorophores featuring ACQ and one propeller-shaped AIE fluorophore. We found that in water bulky fluorinated tetraphenylborates can light up all three dyes, including planar non-AIE fluorophores. The described "ionic" AIE (iAIE) with bulky counterions enables preparation of 50-nm dye-loaded polymeric NPs showing 40 % quantum yield at 500 mM dye loading, whereas small anions favor poorly emissive large aggregates. Single-particle microscopy reveals that these NPs emit without blinking 50-fold brighter than quantum dots-605 (at 470-nm excitation). iAIE opens the route to assembling charged intrinsically non-AIE fluorophores into bright fluorescent (nano)materials.

Lanthanide‐based bulky counterions against aggregation‐caused quenching of dyes in fluorescent polymeric nanoparticles

“…[ 38 ] Owing to their capacity to control aggregation of cationic dyes, BHC found yet another application in the formulation of dye‐loaded polymeric nanoparticles, [ 21,30 ] ultra‐small fluorescent micelles, [ 39 ] and ultrabright dye‐counterion aggregates. [ 40–43 ]…”

“…[38] Owing to their capacity to control aggregation of cationic dyes, BHC found yet another application in the formulation of dye-loaded polymeric nanoparticles, [21,30] ultra-small fluorescent micelles, [39] and ultrabright dye-counterion aggregates. [40][41][42][43] Originally, the concept of bulky counterions for preventing ACQ in polymer NPs was developed based on perfluorinated tetraphenyl borate (F5-TPB). [21] Later studies showed that these counterions can be replaced by other bulky, hydrophobic, and especially fluorinated counterions based on aluminates [44] and boron hydrides.…”

Ultrabright Fluorescent Polymeric Nanofibers and Coatings Based on Ionic Dye Insulation with Bulky Counterions

“…Previously, we introduced hydrophobic counterions as an effective tool to minimize ACQ and improve the brightness of the dyes inside the polymeric nanoparticles. [42][43][44][45] Bulky counterions play a role of a spacer between the dye molecules inside the nanoparticles and reduce the dye aggregation, whereas the hydrophobicity of the counterions helps in better dye encapsulation. 42,43,46 Indeed, the quantum yield of the encapsulated dyes in polymeric NPs improved with the size and fluorination level of the counterions, 42,46 providing proper dye-dye spacing for preventing ACQ.…”

Counterion Pairing Effects on a Flavylium Heptamethine Dye†

“…Net-neutral zwitterionic fluorophores have also been shown to aggregate less severely than cationic derivatives (19)(20)(21). Finally, strategies to control aggregation based on the relationship between counterion and dye have been applied in nanomaterials for imaging (22)(23)(24) and in solid-state applications (25). Indolenine cyanine dyes have shown reduced aggregation in poly(lactic-co-glycolic acid) (PLGA) nanoparticles when associated with bulky hydrophobic counterions (26).…”