In this study, we describe a rational design strategy to develop a latent ratiometric fluorescent pH probe from self-assembled seminaphthorhodafluor (SNARF) derivatives in aqueous conditions. Analysis of the characteristics of SNARF derivatives with protected phenolic groups allowed classification of the threshold between the assembled and monomeric states according to the Hansch–Fujita hydrophobic parameters for a substituent inserted as a SNARF-OH phenolic-protecting group. The esterase-activated latent ratiometric fluorescent pH probe SNARF-OBn(OAc) was characterized in the application of tuneable small-molecule aggregation and disaggregation. Before esterase treatment, SNARF-OBn(OAc) formed self-assembled clusters, emitting no fluorescence. The significant fluorescence was observed after esterase treatment because the self-assembled clusters of SNARF-OBn(OAc) diffuse as SNARF-OH monomers, demonstrating the intracellular application of SNARF-OBn(OAc) as a latent ratiometric fluorescent pH probe for floating cells. Furthermore, to directly visualize the state of self-assembled SNARF derivatives for investigating the mechanism of their cellular uptake, SNARF-Dan was rationally designed and applied for cellular imaging. Through the cellular application of SNARF-Dan, it was suggested that the self-assembled SNARF derivatives were introduced into the cell via macropinocytosis.

We report the synthesis and properties of two new seminaphthorhodafluor (SNARF) derivatives, SNARF-F and SNARF-Cl. Both these derivatives exhibit typical red shifts of absorption and fluorescence, and higher cell permeability as compared to traditional SNARF, while the pH-dependent dual-emission characteristics are well retained. In particular, the lower pKa (7.38) of SNARF-F makes it more suitable than traditional SNARF derivatives for intracellular applications.