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Adsorption and Recovery of Polyphenolic Flavonoids Using TiO2-Functionalized Mesoporous Silica Nanoparticles

Khan, M. Arif, Wallace, William T., Islam, Syed Z., Nagpure, Suraj, Strzalka, Joseph, Littleton, John M., Rankin, Stephen E., Knutson, Barbara L.
ACS applied materials & interfaces 2017 v.9 no.37 pp. 32114-32125
2,2-diphenyl-1-picrylhydrazyl, X-radiation, adsorbents, adsorption, antioxidant activity, citric acid, ethanol, ligands, nanoparticles, nitrogen, porous media, quercetin, rutin, silica, sorption isotherms, surface area, synthesis, titanium dioxide, transmission electron microscopy
Exploiting specific interactions with titania (TiO₂) has been proposed for the separation and recovery of a broad range of biomolecules and natural products, including therapeutic polyphenolic flavonoids which are susceptible to degradation, such as quercetin. Functionalizing mesoporous silica with TiO₂ has many potential advantages over bulk and mesoporous TiO₂ as an adsorbent for natural products, including robust synthetic approaches leading to high surface area, and stable separation platforms. Here, TiO₂-surface-functionalized mesoporous silica nanoparticles (MSNPs) are synthesized and characterized as a function of TiO₂ content (up to 636 mg TiO₂/g). The adsorption isotherms of two polyphenolic flavonoids, quercetin and rutin, were determined (0.05–10 mg/mL in ethanol), and a 100-fold increase in the adsorption capacity was observed relative to functionalized nonporous particles with similar TiO₂ surface coverage. An optimum extent of functionalization (approximately 440 mg TiO₂/g particles) is interpreted from characterization techniques including grazing incidence X-ray scattering (GIXS), high-resolution transmission electron microscopy (HRTEM), and nitrogen adsorption, which examined the interplay between the extent of TiO₂ functionalization and the accessibility of the porous structures. The recovery of flavonoids is demonstrated using ligand displacement in ethanolic citric acid solution (20% w/v), in which greater than 90% recovery can be achieved in a multistep extraction process. The radical scavenging activity (RSA) of the recovered and particle-bound quercetin as measured by a 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay demonstrates greater than 80% retention of antioxidant activity by both particle-bound and recovered quercetin. These mesoporous titanosilicate materials can serve as a synthetic platform to isolate, recover, and potentially deliver degradation-sensitive natural products to biological systems.