A quinoline-malononitrile-based fluorescent probe with aggregation-induced emission effect for the in vivo monitoring of viscosity†

Abstract

As an essential microenvironmental parameter, viscosity controls the diffusion of molecular species in cells to some extent during processes such as signaling, enzyme catalysis and biomolecular interactions. However, abnormal viscosity can lead to metabolic disorders and disease generation. Therefore, designing viscosity fluorescent probes for detecting viscosity changes in organisms is of great application value. Herein, a viscosity fluorescent probe (QM-C2) with aggregation-induced emission (AIE) effect was synthesized using quinoline-malononitrile with AIE properties as the electron acceptor and phenylcarbazole as the electron donor. Since the probe QM-C2 had a D–π–A structure, the phenylcarbazole moiety rotated freely in comparison to the quinoline-malononitrile moiety when it was in low-viscosity media, leading to a rapid energy depletion through a non-radiative transition process, which resulted in a weaker fluorescence. In contrast, the rotation of the molecular rotor was inhibited in high-viscosity media and the energy depletion of the non-radiative pathway was reduced, which resulted in an enhanced fluorescence. In addition, it was observed that common cationic, anionic and reactive oxygen species in the environment and in living organisms do not significantly interfere with the probe QM-C2, and it works effectively under a wide range of pH (pH = 4–10). Notably, the probe QM-C2 successfully monitored the viscosity changes induced by lipopolysaccharide, monensin and nystatin in zebrafish and nude mice.