The human body requires iodine to develop and maintain proper metabolic balance. Worldwide, iodine deficiency affects two billion people and is the leading preventable cause of intellectual disability. Small amounts of iodine are needed for good health. However, large doses can eventually cause iodide goitre, hypothyroidism or myxedema. Children are especially sensitive to the effects of iodine. Because humans can be exposed to iodide via several different food chains, the development of on-site, real-time and reliable sensors for iodide is of great interest to ensure early diagnosis and improve management. We propose here a simple and low cost, yet sensitive and selective fluorescent ‘turn-off-on’ assay for rapid determination of iodide based on a combined carbon nanodots (CDs) and Hg2+ system. The fluorescence of CDs that was quenched by Hg2+ was restored and ‘turned on’ in the presence of iodide, which triggered a competitive reaction among CDs, Hg2+ and iodide. The recovered fluorescence intensity varied linearly with the concentration of iodide in the range of 0.05–5 μmol L–1, with a limit of detection as low as 46 nmol L–1. This approach shows excellent selectivity for iodine over the other anions.
