Magnetic Cellulose Nanocrystal Composites Functionalized with Benzalkonium Chloride for Potential Pathogen Control

This study presents the design and synthesis of multifunctional magnetic cellulose nanocrystal (CNC) composites engineered for targeted adsorption and broad-spectrum antimicrobial applications. The nanocomposites were fabricated by anchoring mixed-valence Fe3O4 magnetic nanoparticles on eco-friendly CNC frameworks, followed by surface functionalization with benzalkonium chloride (BC), a potent quaternary ammonium biocide. Both sulfated (S) and TEMPO-oxidized CNC (T) variants were investigated across Fe3O4:CNC mass ratios of 1:2 and 1:4. Comprehensive structural, magnetic, and morphological characterization confirmed successful surface modification, superparamagnetic responsiveness, and uniform component dispersion. Thermogravimetric analysis confirmed precise compositional tailoring, capturing up to 87 wt.% inorganic magnetite core fractions, 11−34 wt.% organic CNC matrix, and up to 11 wt.% bound BC surfactant. Density functional theory (DFT) simulations complemented these insights by mapping favorable BC binding configurations and localized electronic-structure alterations. Equilibrium adsorption isotherms followed a dominant Langmuir monolayer pathway, with saturation capacities of 178−224 mg g−1 and Langmuir correlation coefficients (R2) of 0.97−0.995. Quantum-chemical binding maps, together with robust within-variant interfacial charge inversions (e.g., from −22.1 to +8.5 mV for the 1:2 S formulation and from −20.4 to +11.2 mV for the 1:2 T formulation after BC addition), point to a cooperative physisorption−chemisorption transport mechanism. In vitro antibacterial assays showed pronounced, concentration-dependent inhibition zones of 7.9−12.8 mm against both Gram-positive (Listeria monocytogenes) and Gram-negative (Escherichia coli) foodborne pathogens, with bactericidal performance primarily correlating with BC loading and, to a lesser extent, with the underlying CNC surface chemistry. Together with the high adsorption capacity and magnetic recoverability of the CNC-Fe3O4 carrier, these results position BC@CNC-Fe3O4 nanocomposites as a promising platform for applications such as smart disinfection and active food-contact materials.