Water-miscible organic J-aggregate nanoparticles as efficient two-photon fluorescent nano-probes for bio-imaging

Abstract

Many two-photon absorption (TPA) organic dyes are water insoluble and suffer from strong fluorescence quenching in aqueous media due to the self-aggregation effect. This seriously limits their applications as two-photon fluorescence (TPF) probes in bio-imaging. By employing a reprecipitation method, we prepared ultrabright water-miscible organic nanoparticles (ONPs) of 1,4-dimethoxy-2,5-di[4′-(cyano)styryl]benzene (COPV). The single-crystal structure reveals that the cooperation between π–π stacking and hydrogen-bonding interactions drives COPV molecules into a brickwork arrangement of J-aggregates, in which the coherent excitation delocalization reaches 2–3 molecules. Due to the superradiance of J-aggregates, COPV ONPs are highly emissive in aqueous media with a quantum yield >0.4; meanwhile, their TPA cross-section is greatly enhanced, probably due to exciton–vibration coupling. As TPF probes, COPV J-aggregate ONPs are 3–4 orders of magnitude brighter than conventional fluorescent dyes and an order of magnitude brighter than quantum dots. Moreover, these ONPs exhibit no obvious cytotoxicity at concentrations as high as 100 μg mL⁻¹. Our results demonstrate that ultrabright J-aggregate ONPs of COPV provide a new strategy to construct efficient TPF nano-probes for bio-imaging.