Boron-doped Carbon Dots with Surface Oxygen Functional Groups as a Highly Sensitive and Label-free Photoluminescence Probe for the Enhanced Detection of Mg²⁺ Ions

Magnesium ion (Mg²⁺) is one of the most significant cations in living systems with involvement in many biochemical reactions and cellular processes and hence, sensitive and specific detection of Mg²⁺ is therefore essential for various applications. Here, we report the solvothermal synthesis of boron-doped carbon dots (BC10) with more oxygen surface states by using salicylaldehyde and naphthalene-1-boronic acid. The as-prepared BC10 showed greenish-white luminescence under 365 nm UV illumination with quantum yield (QY) of 5.5 % at optimum dilution with dimethyl sulfur oxide (DMSO) solvent. The BC10 in DMSO (DS-BC10) have shown high selectivity and sensitivity towards Mg²⁺ ion through the increased PL intensity due to chelation-enhanced photoluminescence (CHEP). The enhanced PL intensity was further supported by the increased QY by a factor of 12 after the addition of Mg²⁺ ions to 65.7 %. Moreover, the limit of detection of DS-BC10 is calculated to be 0.4 μM, which is 100 times better than the recently reported carbon dot-based Mg²⁺ sensor. Following that, DS-BC10 is also explored for detection of magnesium ions in lab tap water, seawater, and drinking mineral water with good precision. In addition, density functional theory (DFT) calculations using B3LYP/6-31G (d) method for the DS-BC10 and their Mg²⁺ complexes support the experimental observations.