Holmium(III)–Carbon Nanodots Hybrid for Fluorescent Quenching Effect in the Oxalate Detection (C₂O₄)^2– and Their Different Photoinduced Charge Transfer Processes

In this work, carbon nanodots (CNDs) were synthesized via a pyrolysis carbonization method using <i>Rosa damascena</i> petals. The synthesized CNDs exhibit optical absorption in the UV region, with a tail extending out into the visible range. When these CNDs interact with Ho³⁺ ions through charge transfer processes, they form an REⁿ⁺-CNDs hybrid (Rare Earth-CNDs hybrid), resulting in fluorescence quenching in an aqueous solution. This fluorescence can be recovered by allowing the Ho³⁺-CNDs hybrid to interact with the oxalate anions. An in silico study supports this mechanism through density functional theory (DFT) analysis, which involves a charge transfer process in discrete systems of functionalized Ho³⁺-C₆₀. The above plot provides an approximation of the behavior of CNDs and highlights the electrostatic interaction in the [Ho-(C₂O₄)₃]^3- complex. Based on this phenomenon, an environmentally friendly fluorescent turn-off/on sensor was developed to detect holmium ions Ho³⁺ and oxalate ions in an aqueous solution. The quantitative detection of Ho³⁺ ranges from 1.0 × 10^-5 to 1.0 × 10^-4 M, while oxalate concentrations vary from 4.5 × 10^-5 to 3.6 × 10^-4 M. The concentration of both species exhibited a linear relationship with the fluorescence intensity, characterized by a correlation coefficient of <i>R</i> ² = 0.9801 and a Stern-Volmer constant <i>K</i> _SV = (1.09 × 10⁴ ± 6 × 10²) M^-1. In addition, the Laplacian and electronic values obtained, within Bader's quantum theory of atoms in molecules, revealed ionic interactions between holmium-(III) and oxalate anions. Moreover, an analysis of the Mulliken and Hirshfeld charges confirmed a charge transfer process from CNDs to Ho³⁺.