Functionalized halloysite nanotube–alginate composites as clay based platforms for real-time adsorption and QCM detection of hydrophobic biomolecule

Hydrophobic biomolecules such as vitamin D₃ are difficult to detect in aqueous systems because of low solubility and weak affinity to hydrophilic surfaces. In this study, halloysite nanotube–alginate composites were functionalized and evaluated by Quartz Crystal Microbalance (QCM) for real-time monitoring of vitamin D₃ adsorption. Two routes were compared: ball milling of halloysite (MBHA), generating fragmented surfaces with more active sites, and HTAB-assisted surfactant functionalization (MHHA), promoting hydrophobic interactions. FTIR, XRD, SEM, and AFM confirmed successful modification and morphological changes. The zeta potential shifted from −19.33 ± 1.25 mV (pristine HNTs) to 0.095 ± 0.12 mV after 3 h ball milling, while HTAB increased it to +29.44 ± 0.50 mV. QCM showed markedly improved sensing performance: at 75 ng/mL, the unmodified crystal exhibited 600 Hz (4.162 ng), while MBHA and MHHA reached 1100 Hz (7.631 ng) and 965 Hz (6.695 ng), respectively; MBHA thus provided an 83% higher signal than the unmodified crystal. Langmuir analysis indicated higher capacity for HTAB-modified HNTs (q_max = 5935.263) than ball-milled HNTs (5318.108), whereas ball milling showed stronger affinity (K_L = 0.48 vs 0.23). At higher vitamin D₃ levels, Avrami and PSO fits supported nucleation–growth and chemisorption, whereas at lower levels Boyd and intraparticle diffusion fits reflected diffusion-controlled uptake. Desorption was biphasic, with faster release for MBHA at higher loadings and more sustained release for MHHA under dilute conditions. Moreover, functionalized halloysite–alginate composites offer dual-functionality for real-time biosensing and controlled delivery of poorly soluble biomolecules in aqueous environments.