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Dye-loaded polymer nanoparticles, due to their high brightness and potential biodegradability, emerge as a powerful alternative to quantum dots in bioimaging applications. To minimize aggregation-caused quenching of the loaded dyes, we have recently proposed the use of cationic dyes with bulky hydrophobic counterions (also known as weakly coordinating anions), which serve as spacers preventing dye pi-stacking inside nanoparticles. However, so far this approach of counterion-enhanced emission inside polymer NPs has been limited to one fluorinated tetraphenylborate (tetrakis(pentafluorophenyl)borate, F5-TPB). Herein, we show that the counterion-enhanced emission approach is not limited to tetraphenylborates and can be extended to other types of anions, such as the aluminium-based anion, Al[OC(CF3)3]4− (F9-Al), which is much easier to scale up, compared to F5-TPB. It is found that F9-Al strongly improves the encapsulation efficiency of the octadecyl rhodamine B dye compared to the perchlorate counterion (97 ± 2 vs. 51 ± 2%), being slightly better than F5-TPB (92 ± 4%). Similarly to F5-TPB, F9-Al can effectively prevent aggregation-caused quenching of rhodamine inside NPs made of the biodegradable polymer, poly(lactide-co-glycolide) (PLGA), even at 50 mM dye loading. According to single-particle microscopy, the obtained NPs are 33-fold brighter than commercial quantum dots QD585 at 532 nm excitation and exhibit complete ON/OFF switching (blinking), as was originally observed for NPs based on F5-TPB. Importantly, NPs loaded with the rhodamine/F9-Al ion pair entered the cells by endocytosis, showing no signs of dye leaching, in contrast to rhodamine perchlorate, which exhibited severe leakage from NPs with characteristic accumulation inside mitochondria. Moreover, F9-Al surpassed the F5-TPB anion in stability of dye-loaded NPs against leaching, which can be attributed to the higher hydrophobicity of the former. Overall, this work shows that counterion-enhanced encapsulation and emission of cationic dyes inside polymer NPs is a general approach for the preparation of stable and highly fluorescent nanomaterials for bioimaging applications.

An aluminium-based fluorinated counterion for enhanced encapsulation and emission of dyes in biodegradable polymer nanoparticles