Engineering Surface Oxygen Functional Groups in Boron-Doped Carbon Dots for Improving Detection of Magnesium Ions Based on Chelation Enhanced Photoluminescence

Magnesium ion (Mg2+) is one of the most significant cations in living systems with involvement in many biochemical reactions cellular processes. Sensitive and specific detection of Mg2+ is therefore essential for various applications. Here, we report on the synthesis of boron doped carbon dots (BCDs) having a high concentration of oxygen surface states. BCDs are prepared from salyciladhyde and naphthalene-1-boronic acid using a versatile solvothermal method. The as prepared BCDs showed greenish-white luminescence at optimum dilution under UV illumination with quantum yield (QY) of 5.5% and exhibited high selectivity and sensitivity towards Mg2+ ion. Notably, the QY is increased by a factor of 12 after addition of Mg2+ ions to 65.7% together with an emission switching; the increased intensity can be attributed to chelation-enhanced photoluminescence (CHEP). Radiative rates are found to be increased, supporting the CHEP observation. Furthermore, the limit of detection is 0.4 µM, 100 times better than the recently reported carbon dot based Mg2+ sensor, much below the maximum permissible limit of Mg2+ ions in drinking water (0.1–0.8 mM). BCDs are also explored for detection of magnesium ions in lab tap water, seawater, and drinking mineral water with good precision. Further, density functional theory (DFT) calculations using B3LYP/6-31G(d) method for the BCDs and their Mg+2 complexes support the experimental observations.