2-Hydroxy-5-ethylphenylphosphonic acid and its application in carbon dot modification: synthesis, protonation, spectral properties and specific recognition of copper(II) ions.

In this work, we optimized the synthesis of 2-hydroxy-5-ethylphenylphosphonic acid (HEPPA) and synthesized carbon dots (CDs) modified with HEPPA (CDsHEPPA) for the recognition of metal ions. HEPPA was synthesized by 1,3-phosphate-phosphonate rearrangement of diethylphenylphosphates with subsequent hydrolysis of trimethylsilyl esters. The functional groups of CDsHEPPA were characterized using protonation constants and 31P NMR and IR spectra, which were compared with the spectra of HEPPA. The stability constants of HEPPA complexes with metal ions in water were determined and compared with the fluorescence properties of CDsHEPPA in the presence of metal ions. The specific recognition of copper(II) ions by CDsHEPPA was achieved using fluorescence quenching. Biological properties such as cytotoxicity and accumulation in HeLa cells were studied for both HEPPA and CDsHEPPA. CDsHEPPA showed lower cytotoxicity compared to HEPPA and the Cu2+-HEPPA complex. CDsHEPPA penetrated into cells and then diffusely distributed in the cytoplasm and perinuclear space. Incubation with copper(II) ions led to a decrease in the fluorescence intensity of CDsHEPPA in HeLa cells.