Hydrothermal synthesis of intrinsic carbon dots with tunable emission for fluorescent inks and variegated bioimaging applications

The current study presents the eco-friendly synthesis, detailed characterization, and versatile applications of hydrophilic wood sorrel carbon dots (WSCDs) via a one-step hydrothermal-assisted carbonization method. This simple and scalable approach involves direct carbonization of wood sorrel as a natural precursor, yielding WSCDs with a uniform spherical morphology and narrow size distribution (2.5–6.0 nm, predominantly 4.5 nm), confirmed through high-resolution transmission electron microscopy. Elemental analysis (Energy-Dispersive X-ray and X-ray photoelectron spectroscopy (XPS)) reveals a homogeneous distribution of carbon, nitrogen, and oxygen, while X-ray diffraction analysis (dominant peak at 23°) and Raman spectroscopy confirm a high degree of graphitization with a d-spacing of 0.21 nm. Functional groups such as hydroxyl, carboxyl, and carbonyl, contributing to hydrophilicity and versatile reactivity, were identified via Fourier transform infrared spectroscopy and XPS. Optical characterization demonstrates strong ultraviolet absorption (peaks at 245, 278, and 317 nm) and tunable fluorescence emission with excitation-dependent behavior, exhibiting remarkable photostability over 180 days and under ultraviolet irradiation. Biocompatibility studies indicate that WSCDs are non-toxic, as validated by 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays in Clone 9 hepatocyte and HCT-116 cells, with effective cellular internalization observed through confocal fluorescence microscopy. The practical application of WSCDs as fluorescent inks shows excellent performance, retaining clarity and fluorescence over extended periods, making them ideal for security printing and artistic purposes. This study emphasizes the potential of WSCDs as multifunctional materials for ecological inks and advanced imaging, highlighting their sustainable synthesis and broad industrial applicability.