Tumor-targeted ratiometric fluorescent gold nanoclusters for monitoring apoptosis and quantifying pH in vivo

Precise measurements of tumor pH is essential for early cancer diagnose, treatment planning, and the development of anti-cancer drugs. Herein, we present a pH-sensitive, ratiometric fluorescent biosensor with tumor-targeting capabilities for real-time in vivo tumor pH detection. The biosensor, composed of glutathione-protected gold nanoclusters (AuNCs) conjugated with fluorescein-isothiocyanate (FITC), provides a reliable ratiometric fluorescence signal response to pH. The cyclic arginine-glycine-aspartic acid (c(RGDyC)) peptide was integrated into the sensor (denoted as c(RGDyC)-FITC@AuNCs) to confer tumor-targeting specificity. Upon excitation at 488   nm, c(RGDyC)-FITC@AuNCs exhibited dual-channel emissions at 515   nm and 710   nm, the ratio of which showed a 17-fold change across a pH range of 5.5 to 8.0. In addition, c(RGDyC)-FITC@AuNCs demonstrated excellent biocompatibility, long-term photostability, and low toxicity, enabling the quantitative monitoring of the pH changes during tumor cell apoptosis in real-time. Moreover, using tumor-bearing mouse models with induced acid-alkaline variations, c(RGDyC)-FITC@AuNCs successfully visualized and dynamically tracked in situ tumor pH. This biosensor offers a sensitive and accurate tool for tumor diagnosis, with significant potential in biomedical research and clinical studies.