DFT, real sample and smartphone studies of fluorescent probe, N-doped carbon quantum dots for sensitive and selective detection of bilirubin

•Highly sensitive and selective detection of BR via FRET using NCQD as the fluorescent probe.•Achieved accurate quantification of BR with Ksv ≈ 7.48 × 105 M⁻¹ and LOD of 4.86 μM, linear range: 1.35–14.20 μM.•DFT studies confirm energy transfer mechanism in NCQD/BR complex.•Practical BR detection demonstrated in human serum and smartphone analyses. In this investigation, nitrogen-doped carbon quantum dots (NCQDs) were successfully synthesized, showcasing robust fluorescence characteristics and photostability. These NCQDs demonstrated biocompatibility and were utilized as effective sensing agents for bilirubin (BR) detection. Their remarkable Stern-Volmer constant, attributed to exceptional molar absorptivity and photoluminescence quantum yield, rendered them highly proficient in BR sensing. Various analytical techniques such as X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were employed to characterize the synthesized NCQDs, providing comprehensive insights into their structural, elemental, chemical composition, particle size, morphology, and optical properties. For sensing purposes, UV–Vis, steady-state, and time-resolved fluorescence spectroscopic techniques were utilized. The average particle size of the NCQDs was determined to be approximately 4.5 ± 0.3 nm. Notably, the Stern-Volmer constant (Ksv) for BR sensing from these quantum dots surpassed previously reported values in the literature. A linear relationship between emission intensity ratio and BR concentration within a range of 1.35–14.20 μM was observed. Accurate quantification of BR was achieved with a high Stern-Volmer constant (Ksv) S11 of approximately 7.48 × 105 M-1 and a limit of detection (LOD) of 4.86 μM. Leveraging the NCQDs as fluorescent probes, a highly sensitive and selective BR detection method based on the Förster resonance energy transfer (FRET) mechanism was established, with emission occurring at 360 nm. Density functional theory (DFT) studies supported FRET as the primary mechanism for fluorescence quenching, indicating electron transfer from NCQDs to BR, as evidenced by the energies of the lowest unoccupied molecular orbitals (LUMOs) of both NCQDs and BR. Real sample analyses of BR levels in human serum yielded a recovery range of 94.8 % to 104.21 %, validating the practical applicability of the method. Furt