Understanding the Citric Acid-Urea Co-Directed Microwave Assisted Synthesis and Ferric Ion Modulation of Fluorescent Nitrogen Doped Carbon Dots: A Turn On Assay for Ascorbic Acid

摘要

Herein, nitrogen doped carbon dots (NCDs) were synthesised from citric acid and urea via a previously reported microwave assisted route. The NCDs shows emission maximum at 500 nm on excitation at 400 nm. The fluorescence of NCDs decreases slightly with increase in basicity of solution up to pH 7.5 and then increases again after pH 8.5, along with a blue-shift in tested alkaline pH. This pH dependent blue-shift indicates the presence of both carboxyl?carboxylate and phenol?pheno- late prototropic equilibrium in NCDs. Due to the special interaction of these phenolates and carboxylates on NCDs surface with di- or tri- valent heavy transition metal ions; it isdemonstrated that ferric ion (Fe3+ ion) can quench the fluorescence of NCDs. This Fe3+ induced static quenching of NCDs is a collaborative effect of inner filter effect, aggregation and ferromagnetism. However, Ascorbic acid (AA) can recover the fluorescence of Fe3+ quenched NCD with detection limit as low as 96 μM. This detection strategy has good selectivity towards AA over other antioxidants, saccharides, proteins and neurotransmitters. Furthermore, (spiked) human serum and (spiked) human urine were analysed and found good recovery percentage.