Mechanism-resolved adsorption and J-aggregation of organic dyes on mesoporous cobalt ferrite-based nanoparticles

Mesoporous magnetic nanoadsorbents are a distinct class of functional materials that enable targeted control of adsorption pathways for various organic species under an external magnetic field. In this work, we present a versatile platform of rod-shaped (~100 × 30 nm) cobalt ferrite-based nanoparticles (NPs) with engineered bimodal mesoporosity (3-10 nm and 10-50 nm) to investigate how the molecular properties of adsorptives interact with nanostructure to govern adsorption behavior. Porosity was tuned by varying the annealing rates (5, 10, and 20 °C·min^-1) of a template akaganeite matrix coated with a cobalt hydroxide shell, resulting in NPs with distinct microstructures and textural properties that dramatically affect. This performance was systematically studied using three representative organic dyes: eriochrome blue SE (EB), methyl orange (MO), and methylene blue (MB), which differ in molecular structure and charge. The results reveal strikingly different mechanisms of adsorption for each dye. Narrower mesopores (3-10 nm) are critical for efficient adsorption of MO and MB. Despite carrying the same negative charge as the nanoparticle surface, MO shows high adsorption capacity (up to 62 mg·g^-1) in NPs with dominant 3-5 nm pores, which is 2.5 times higher than that of MB. MO adsorption proceeds via cooperative multilayer chemisorption, following pseudo-second-order kinetics, while MB undergoes primarily monolayer physisorption consistent with pseudo-first-order behavior. The most striking behavior is observed for EB. Upon contact with the NPs, EB consistently forms elliptical J-aggregates. At low concentrations, the majority of EB remains in molecular form and is effectively adsorbed (~190 mg·g^-1). However, at higher concentrations, the J-aggregates dominate and are largely desorbed into the solution. This concentration-dependent transition from adsorption to supramolecular release reveals a previously unreported dynamic behavior, highlighting the role of NPs as both adsorbents and inducers of self-assembly. These findings reveal diverse and sometimes counterintuitive adsorption pathways, demonstrating how mesoporous nanostructures can direct not only pollutant capture but also supramolecular organization at the nanoscale.