Green synthesis of carbon quantum dots from fresh Citrus limon juice for Fe3+ ion detection and fluorescent security ink applications

Abstract

Citrus limon juice is an easily available and economically viable green chemical. Herein, we synthesized luminescent carbon quantum dots (CQDs) using fresh Citrus limon juice via a simple one-step hydrothermal carbonization process. We explored CQDs as simple and selective fast spectrofluorometric optical probes for ferric (Fe³⁺) ion sensing in a contaminated aqueous medium based on a simple PL turn-off mechanism. Ultraviolet-visible spectroscopy (UV-Vis), Fourier transform infrared (FT-IR) spectroscopy, dynamic light scattering (DLS), scanning electron microscopy (SEM), elemental mapping, transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and excitation-dependent fluorescence emission spectroscopy were used to investigate the morphology and optical properties of the carbon quantum dots and their sensitivity/selectivity for heavy metal ions. We observed that CQDs were spherical with an average size of ∼9 nm, with a substantial FLQY value of 14.19% and excellent optical properties. The performance of the synthesized sensor was promising for Fe³⁺ detection with a detection limit of 0.24 μM in the linear range of 0.073–9.37 μm. This chemo-sensor synthesized using a biomass precursor will pave the way for the evolution of CQDs through an easy, straightforward, green, and cost-effective approach, which aligns fully with the UN's Sustainable Development Goal (SDG) 3 (good health and well-being) and SDG 12 (responsible consumption and production).