A Red Emissive Two-Photon Fluorescence Probe Based on Carbon Dots for Intracellular pH Detection

Intracellular pH is closely related with many biological processes, including cellular proliferation, apoptosis, endocytic processes, signal transduction, and enzymatic activity. The use of fluorescent probes has become an essential method for intracellular pH detection, but existing fluorescent probes have substantial limitations, such as requiring tedious synthetic preparation, suffering from an inappropriate response range and insufficiently long emission wavelength. In this work, a red emissive two-photon fluorescence probe based on carbon dots (pH-CDs) is fabricated using a facile one-pot hydrothermal method for the monitoring of intracellular pH. pH-CDs possess a variety of superior properties, including high selectivity, excellent photostability, and low cytotoxicity. Furthermore, they exhibit a pH-sensitive response in the range of 1.0-9.0 and a linear range of 3.5-6.5, which is desirable for tracking the pH value in living cells. It is demonstrated that the pH-dependent fluorescence signal is regulated via switching between aggregation and disaggregation of CDs. More importantly, pH-CDs can be successfully applied to sense and visualize pH fluctuation in cells, tissue, and zebrafish. These findings suggest that the as-prepared pH-CDs probe has significant potential for practical application in living systems.