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Preparation of Amino Acid Functionalized Carbon Quantum Dots from Citric Acid Efficient Fluorescent Nanoprobe for Selective Detection of Fe3+ Ions in Model Systems and in Well Water Samples

Carbon quantum dots (CQDs) represent a new and efficient form of carbon photoluminescent nanomaterials that have attracted enormous attention among the scientific community in the last few decades due to their excellent chemical and optical properties. Their chemical stability, biocompatibility/low toxicity, water solubility and optical efficiency represent a huge potential for a wide range of applications, especially in biomedical research and nanotechnology. CQDs are defined as zero-dimensional carbon nanomaterials, with typical sizes less than 10 nm, however, some literature report the preparation of spherical CQDs particles with a size diameter of approximately 50 nm. It is known that CQDs consist of a carbon core with a high content of sp3- hybridized carbon atoms, and a slightly lower content of sp2-hybridized carbon atoms. One of the many advantages of CQDs is the ability to tune the photoluminescent characteristics by variations in the size of the synthesized particles, the incorporation of different heteroatoms into the structure, or by the addition of functional groups to the particle surface, all to facilitate interactions with biological materials and to increase quantum yield (QY). Due to these characteristics, CQDs find various applications in theranostics, but also in sensing of different chemical species, including metal ions and biomolecules. CQDs can be synthesized by two different synthetic methods ; top-down and bottom- up approaches. The main advantage of the “bottom– up” approach, regarding the selection of chemical precursors and applying optimal operating conditions, makes “bottom–up” method more favorable technique for obtaining different CQDs. The most commonly reported synthetic routes for CQDs preparation are thermal/combustion techniques due to their simplicity, better variability of using different carbon sources, surface functionalization, environmental friendliness, and lower costs of synthesis. Therefore, this study is an example of amino acid-functionalized CQDs preparation 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 five different amino acids used as nitrogen dopants (Leu, Trp, Arg, Ala, His). Compared to the blank sample (without the addition of amino acids), N-doped CQDs have shown significantly higher quantum yield, demonstrating also the potential in cellular imaging. The highest quantum yield of 35.75% with a peak excitation/emission of 340/401 nm was achieved using citric acid and amino acid Leu (CQD@Leu), treated at temperature of 180°C during 9 hours. The prepared samples were investigated toward metal ion selectivity (Ca2+, Cu2+, Fe3+, K+, Hg2+, Mg2+, Al3+, Mn2+, and Na+), and the CQD@Leu showed selective and sensitive response (decrease in the fluorescence intensity) upon the addition of Fe3+ ions. Therefore, CQD@Leu was selected for further investigation in Fe3+ detection in model system and in real well water samples. The developed model was described by an exponential function with a suitable 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. These findings could demonstrate the potential application of the prepared N-doped CQDs as a fluorescent nanoprobe in ion sensing, elucidating also the influence of the different amino acids on fluorescence efficiency. The presented study may represent a novel and useful approach for developing low-cost and sensitive nanoprobe for practical applications, including those in analytical chemistry, biomedicine and environmental monitoring.

carbon quantum dots ; sensing ; water analysis

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