Luminescent nitrogen-doped carbon dots (NCDs) with high fluorescence quantum yield (46.01%) are synthesized from ammonium citrate and betaine hydrochloride by a simple and rapid green route of a one-pot hydrothermal method. The as-prepared NCDs exhibit excellent selectivity and sensitive detection for H+ and Hg2+ (detection limit of 1.69 nM for Hg2+). Finally, the low toxicity and highly photoluminescent NCDs are utilized to detect H+ and Hg2+ in SMMC-7721 cells by a laser scanning confocal microscope.