Gold nanocluster-based fluorescence biosensor for targeted imaging in cancer cells and ratiometric determination of intracellular pH

• A ratiometric fluorescence biosensor for pH determination, FA-FITC@AuNC, was developed for specific bioimaging and biosensing in cancer cells at the same time.
• The developed biosensor can monitor pH gradients in a broad pH range of 6.0–7.8 with good sensitivity, high cyclic accuracy, and excellent selectivity over various metal ions and biological species.
• The pH biosensor was also successfully used in targeted imaging for FR+ve Hela cells and FR−ve lung carcinoma cells A549.
• This work can be extended to construct future ratiometric fluorescent biosensors for targeted imaging, drug deliver, or determination of other biomolecules, such as metal ions, proteins, and other biological relevant species in live cells, tissues, and animals.

The dysregulated pH is working as a mark of cancer. It is a challenge for developing a biosensor for targeted imaging in cancer cells and monitoring of intracellular pH. Here, a ratiometric fluorescence biosensor for pH determination was developed with targeted imaging into folate acceptor (FR)-rich cancer cells at the same time. AuNCs protected by bovine serum albumin (BSA) worked as reference fluorophore and fluorescein-isothiocyanate (FITC) acted as the response signal for pH. For targeted imaging of cancer cells, the AuNCs were simultaneously conjugated with folic acid (FA). The developed ratiometric biosensor can monitor pH with a wide linear range from 6.0–7.8 with a pKa at 6.84. Under every different pH condition, the probe showed high selectivity over various metal ions and amino acids with its fluorescence ratio stayed almost constant (<5%). It also showed good cyclic accuracy when pH switched between 6.0 and 8.0, as well as low cytotoxicity. The AuNC-based inorganic–organic nanohybrid biosensor showed good cell-permeability, low cytotoxicity, and long-term photostability. Accordingly, the pH biosensor was employed to gain targeted imaging in FR+ve Hela cells with FR−ve lung carcinoma cells A549 as comparison, and achieved to monitor the pH changes in Hela cells.