Cu²⁺ coordinated L-proline-derived carbon-nanodots for specific non-enzymatic detection of tyrosine

Carbon-nanodots (CDs) exhibit versatile emissive and electrochemical properties, which have been widely utilized in applications such as biosensing, bioimaging, and light-emitting devices. Amino acids provide an attractive means for introducing nitrogen (N-doping) and surface functionalities with structural diversity into CDs. In this work, water-dispersible, emissive and electrochemically active L-proline derived CDs (CDs-Pro) were synthesized directly from L-proline through a mild thermal process. The supramolecular complex of the CDs-Pro with Cu²⁺ ions (CDs-Pro/Cu²⁺) was evaluated for luminescence and electrochemical non-enzymatic detection of tyrosine (Tyr). The CDs-Pro and CDs-Pro/Cu²⁺ nanostructures were systematically characterized using various analytical methods to confirm the emissive core structure as well as the outer-shell functionality. The optical investigations report an interesting pH-dependent photoluminescence quantum yield in the range of 1.2 to 2.2%. The surfaces of screen-printed carbon electrodes modified with the CDs-Pro/Cu²⁺ nanostructures showed specific non-enzymatic electrochemical recognition of Tyr over a linear range of 16-550 µM, with a limit of detection (LOD) of 61.8 µM. Meanwhile, the water dispersion of CDs-Pro/Cu²⁺ nanostructures exhibited specific non-enzymatic photoluminescence detection of Tyr over the range 5-350 µM, with an LOD value of approximately 0.41 µM. Moreover, assay specificity was investigated using molecular modelling methods. The formation free energy (ΔG_form) values obtained for all investigated aromatic amino acids confirm a higher stability for the Pro/Cu²⁺/Tyr^2- system (-661.86 kJ mol^-1) compared to Pro/Cu²⁺/Trp (-640.60 kJ mol^-1) and Pro/Cu²⁺/Phe (-637.98 kJ mol^-1). Finally, this article addresses current challenges in the field of carbon-based nanostructures and outlines future research directions, emphasizing the potential of carbon-nanodots for the development of non-enzymatic assays for amino acid recognition.