Nitrogen‐Doped Citric Acid‐Based Carbon Dots: Low Carbon Footprint Synthesis and Mercury Detection

Abstract This study demonstrates the high‐pressure microwave‐assisted synthesis of nitrogen‐doped citric acid‐based carbon dots (N‐CAdot) using ammonium hydrogen carbonate (AHC) as the simplest form of nitrogen source, ammonium ion. In aqueous solution, AHC dissociates to release ammonium ions, while hydrogen carbonate ions act as a buffer, maintaining a slightly basic environment. The reaction mechanism is discussed, and optimal synthesis conditions are identified: a CA/AHC molar ratio of 1:12, reaction time of 3 min, and temperature of 200 °C. The estimated total carbon footprint of the synthesis process is 1.8 gCO 2 per mL of carbon dot solution. The resulting N‐CAdots exhibit a high quantum yield (42.5%), stability up to 30 days, and a broad pH tolerance (4.5–12). Under the optimized measurement conditions, the N‐CAdot solution enables selective detection of Hg 2+ with LOD and LOQ of 0.07 and 0.22 ppm, respectively. Recovery tests in spiked mineral water yield 92.5%–107.1%, confirming the method's reliability for the application of N‐CAdots in real‐sample analysis.