Chitosan and κ-carrageenan-derived nitrogen and sulfur co-doped carbon dots “on-off-on” fluorescent probe for sequential detection of Fe3+ and ascorbic acid

This study develops a high sensitive and selective "on-off-on" fluorescent probe for sequential detection of iron ion (Fe3+) and ascorbic acid (AA) based on nitrogen and sulfur co-doped carbon dots (N, S-CDs), which were synthesized by using chitosan and κ-carrageenan as raw materials through one-step hydrothermal protocol. The synthesized N,S-CDs possess particularly high quantum yield (QY = 59.31%), excellent stability and excitation dependent behavior, showing great potential for practical applications. Furthermore, N,S-CDs provided high selectivity and strong anti-interference to Fe3+ due to its fluorescence quenching performance, revealing a wide linear concentration range from 1 to 100 μM for the detection of Fe3+ ion with an extremely low limit of detection of 57 nM, and presented reliable and accurate results in actual sample detection of Fe3+. The overall fluorescence quenching mechanism of N,S-CDs with Fe3+ was due to the formation of N,S-CDs/Fe3+ initiated to the aggregation and electron transfer of N,S-CDs, resulting in the static quenching of fluorescence. More interestingly, AA could reduce Fe3+ to Fe2+ and efficaciously recover the quenched fluorescence of N,S-CDs/Fe3+. N,S-CDs/Fe3+ as "turn-on" fluorescent probe was further applied for detecting AA in a linear range of 0.5-90 μM with a detection limit of 38 nM.