Carbon dots doped with sulfur and nitrogen (S,N-CDs) were utilised to design a paper-stripe based fluorescent probe for the detection of bilirubin. The S,N-CDs were synthesized through a microwave assisted route by using citric acid as carbon source and L-cysteine as a source of nitrogen and sulfur. The S,N-CDs exhibit bright blue fluorescence emission with a peak at 452 nm. Fluorescence is quenched by Fe(III) but selectively restored by bilirubin. The quenched fluorescent probe exhibit significant selectivity and sensitivity for bilirubin in the 0.2 nM to 2 nM concentration range, with a 0.12 nM detection limit. The method was applied to the determination of bilirubin in spiked human serum and urine samples. The method was used to design a paper based test stripe as a point of care device for visual bilirubin detection. Graphical abstract Schematic representation of sulphur and nitrogen doped carbon dots whose fluorescence is quenched by Fe(III) and turned on by bilirubin. Photograph of the corresponding system under day light and UV shows the feasibility of the phenomenon. The applicability of the assay was further extended by impregnating the probe on a filter paper.

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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↔phenolate 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 is demonstrated 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.

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