2D ratiometric fluorescent pH sensor for tracking of cells proliferation and metabolism

Extracellular pH plays a vital role no matter in physiological or pathological studies. In this work, a hydrogel, CD@Nile-FITC@Gel (Gel sensor), entrapping the ratiometric fluorescent probe CD@Nile-FITC was developed. The Gel sensor was successfully used for real-time extracellular pH monitoring. In the case of CD@Nile-FITC, pH-sensitive fluorescent dye fluorescein isothiocyanate (FITC) was chosen as the response signal for H⁺ and Nile blue chloride (Nile) as the reference signal. The developed fluorescent probe exhibited high selectivity for pH over other metal ions and amino acids. Meanwhile, the carbon-dots-based inorganic-organic probe demonstrated excellent photostability against long-term light illumination. In order to study the extracellular pH change in processes of cell proliferation and metabolism, CD@Nile-FITC probe was entrapped in sodium alginate gel and consequently formed CD@Nile-FITC@Gel. The MTT assay showed low cytotoxicity of the Gel and the pH titration indicated that it could monitor the pH fluctuations linearly and rapidly within the pH range of 6.0-9.0, which is valuable for physiological pH determination. As expected, the real-time bioimaging of the probe was successfully achieved.