Humic acids-modified mesoporous silica encapsulating magnetite: crystal and surface characteristics

Abstract

Mesoporous silica nanoparticles incorporating magnetite nanospheres show promise for controlled drug delivery due to their high surface area and magnetic properties. This study explores the impact of humic acids (HA) on the structure and surface characteristics of magnetite within this mesoporous matrix using X-ray diffraction, Mössbauer spectroscopy, N₂ adsorption–desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic and electrophoretic light scattering (DLS, ELS), and magnetometry. Following synthesis and characterization of the magnetite nanospheres in a tetraethoxysilane-3-aminopropyltriethoxysilane copolymer matrix (M/TA), we demonstrate that HA modification induces significant structural anisotropy. Notably, the formation of plate-like Fe₃O₄ is observed. These structures can partially transform into a layered iron (oxyhydr)oxide structure with a high specific surface area and enhanced adsorption depending on the HA content. By adjusting the HA content, we can tune the surface area and sorption parameters of the composites. This is particularly evident in the two-fold increase in ciprofloxacin sorption capacity observed for M/TAHA compared to M/TA. This phenomenon is attributed to the prooxidant activity of HA, which partially oxidizes the magnetite phase to the observed (oxyhydr)oxide with a corresponding morphological transformation from spherical to plate-like. This interpretation is supported by the behavior of the suspensions in cyclic voltammetry experiments and by comparing the observed prooxidant effect of HA with the well-known antioxidant effect of HA in the stoichiometric form of magnetite. These findings contribute to a deeper understanding of the factors governing various properties in hybrid iron oxide-redox polyelectrolyte systems, holding potential for both prooxidant applications and the development of novel drug delivery systems.