Chitosan and κ-carrageenan-derived nitrogen and sulfur co-doped carbon dots "on-off-on" fluorescent probe for sequential detection of Fe and ascorbic acid.

This study develops a high sensitive and selective "on-off-on" fluorescent probe for sequential detection of iron ion (Fe) 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 Fe due to its fluorescence quenching performance, revealing a wide linear concentration range from 1 to 100 μM for the detection of Fe ion with an extremely low limit of detection of 57 nM, and presented reliable and accurate results in actual sample detection of Fe. The overall fluorescence quenching mechanism of N,S-CDs with Fe was due to the formation of N,S-CDs/Fe initiated to the aggregation and electron transfer of N,S-CDs, resulting in the static quenching of fluorescence. More interestingly, AA could reduce Fe to Fe and efficaciously recover the quenched fluorescence of N,S-CDs/Fe. N,S-CDs/Fe 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.