Straightforward Synthesis of Superparamagnetic Fe3O4-Carbon Dot Nanohybrids for Environmental Remediation: A Model Study Using Congo Red

Abstract Carbon dots (CDs) have attracted increasing interest as carbon-based nanomaterials due to their tunable surface chemistry, colloidal stability and low toxicity. Among them, magnetic carbon dots (MCDs) constitute a promising multifunctional subclass. In this work, magnetite-based magnetic carbon dots (Fe3O4-MCDs) were synthesized through a straightforward route using only ammonium ferric citrate and ethylenediamine as precursors. TEM revealed quasi-spherical Fe3O4-MCD nanohybrids with an average particle size of 12 ± 3 nm. XPS depth profiling employing Ar+ ion sputtering showed that these nanoparticles possess a magnetite-rich interior and a functionalized carbon-rich outer region. Furthermore, these nanoparticles self-assembled into hierarchical architectures with interparticle voids, resulting in a porous network with a specific surface area of 107 m2 g–1 and a pore volume of 0.431 cm3 g–1. In addition, the negligible coercivity and remanence are consistent with superparamagnetic-like behavior at room temperature, with a saturation magnetization of ∼60 emu g–1, facilitating rapid magnetic separation and redispersion. The Fe3O4-MCDs were evaluated for the adsorption of Congo red as a model pollutant, achieving an adsorption capacity of 121.6 mg g–1. Nonlinear kinetic analysis showed that adsorption followed the mixed 1,2-order model, while the Sips isotherm best described the equilibrium data. Overall, the results demonstrate that the Fe3O4-MCDs combine straightforward synthesis, high saturation magnetization, superparamagnetic behavior, and pollutant adsorption performance. These properties arise from their carbon/magnetite nanoarchitecture and engineered interfacial chemistry, promoting interactions with pollutants and highlighting their potential for multifunctional contaminant removal.