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This work reports the preparation of amino acid-functionalized CQDs from citric acid by facile hydrothermal synthesis. The prepared N-doped CQDs exhibited excellent optical, physical, and chemical properties, and the differences were observed among the six different amino acids used as nitrogen dopants (Leu, Trp, Lys, Arg, Ala, His). Compared to the blank sample (without the addition of amino acids), N-doped CQDs have shown significantly higher quantum yield, also demonstrating the potential in metal ion sensing. The highest quantum yield of 36.45%, with a peak excitation/emission of 340/406 nm, was achieved using citric acid and amino acid Leu (CQD@Leu), treated at temperature of 180 °C during 9 h. The prepared samples were investigated toward metal ion selectivity (Ca2+, Cu2+, Fe3+, K+, Hg2+, Mg2+, Al3+, Mn2+, and Na+), and the CQD@Leu showed a selective and sensitive response upon the addition of Fe3+ ions. Therefore, CQD@Leu was selected for further investigation in Fe3+ detection in the model system and real well water samples. A developed model was described by a logistic function with a good coefficient of determination of R2 = 0.9982, while the linear range was determined in the concentration range from 0.3 mol dm−3 to 30 mol dm−3, with a determined limit of detection of LOD = 1.77 ± 0.01 mol dm−3 and limit of quantification of LOQ = 5.89 ± 0.04 mol dm−3. Furthermore, the results of the in vitro cytotoxicity test (MTT) with normal and tumor cell lines (MRC-5, HeLa, NCI-H358, and CaCo-2) clearly demonstrate the excellent biocompatibility of CQD@Leu.

Carbon quantum dots (CQDs) have recently emerged as innovative theranostic nanomaterials, enabling fast and effective diagnosis and treatment. In this study, a facile hydrothermal approach for N-doped biomass-derived CQDs preparation from Citrus clementina peel and amino acids glycine (Gly) and arginine (Arg) has been presented. The gradual increase in the N-dopant (amino acids) nitrogen content increased the quantum yield of synthesized CQDs. The prepared CQDs exhibited good biocompatibility, stability in aqueous, and high ionic strength media, similar optical properties, while differences were observed regarding the structural and chemical diversity, and biological and antioxidant activity. The antiproliferative effect of CQD@Gly against pancreatic cancer cell lines (CFPAC-1) was observed. At the same time, CQD@Arg has demonstrated the highest quantum yield and antioxidant activity by DPPH scavenging radical method of 81.39 ± 0.39% and has been further used for the ion sensing and cellular imaging of cancer cells. The obtained results have demonstrated selective response toward Fe3+ detection, with linear response ranging from 7.0 µmol dm−3 to 50.0 µmol dm−3 with R2 = 0.9931 and limit of detection (LOD) of 4.57 ± 0.27 µmol dm−3. This research could be a good example of sustainable biomass waste utilization with potential for biomedical analysis and ion sensing applications.

The fluorescent carbon quantum dots (CQDs) represent an emerging subset of carbonaceous nanomaterials, recently becoming a powerful tool for biosensing, bioimaging, and drug and gene delivery. In general, carbon dots are defined as zero-dimensional (0D), spherical-like nanoparticles with