Pearl-like Fluorescent Alginate/γ-Chitosan Beads Embedded with TiO2 and Carbon Dots for Visible-Light Self-Renewing Adsorption

Abstract

Pearl-like hybrid beads composed of alginate (AG), γ-chitosan (γ-CS), titanium dioxide (TiO2), and carbon dots (CDs) were successfully fabricated via a core–shell approach. In this architecture, the TiO2-alginate core provides active adsorption sites, while the γ-CS/CD shell imparts fluorescence functionality. The morphology and internal structure were examined using high-resolution transmission electron microscopy (HRTEM), which revealed a homogeneous distribution of TiO2 nanoparticles and CDs within the polymer matrix. Elemental composition and spatial distribution were confirmed by energy-dispersive X-ray spectroscopy (EDS). Interactions among alginate, γ-chitosan, TiO2, and CDs were further elucidated using Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), confirming successful composite formation without alteration of the TiO2 crystalline phase. Nitrogen adsorption–desorption analysis indicated a low BET surface area (0.35 m2·g–1), consistent with the macroscopic bead structure. Adsorption studies demonstrated effective removal of methylene blue (MB), with performance influenced by solution pH and initial dye concentration. Equilibrium data were well described by Langmuir and Freundlich isotherm models. Notably, fluorescence quenching occurred during MB adsorption, followed by fluorescence recovery under light irradiation, suggesting a light-responsive and reversible adsorption–desorption process. Overall, the AG/γ-CS/TiO2–CDs hybrid beads integrate adsorption and fluorescence functionalities within a sustainable biopolymer-based system, highlighting their potential as self-indicating materials for dye removal and environmentally relevant water treatment applications.