Two-Step Hydrothermal Preparation of Carbon Dots for Calcium Ion Detection

It is well known that the calcium ion is essential for maintaining life activities in living organisms, and it is of great significance to detect the intracellular calcium concentration. For the detection of calcium ions, we developed a new type of fluorescent carbon dots (CDs), whose surface was modified by ethylenebis­(oxyethylenenitrilo)­tetraacetic acid (EGTA) through a secondary hydrothermal method. This is a simple and convenient chemical preparation method because all reactions are carried out in the same autoclave, and the final product is directly the EGTA-modified CDs. The CDs exhibit bright blue fluorescence, and as the calcium concentration increases, the fluorescence intensity drops sharply. The fluorescence quenching correlates with the concentration of calcium ions and has a good linearity in the range of 15–300 μM with a detection limit of 0.38 μM. The experimental results confirmed that the detection of calcium ions by CDs is a static fluorescence quenching process. Also, cytotoxicity test and cellular imaging experiments have shown that the CDs are nontoxic and biocompatible.