Solid-phase extraction of flavonoids: A comprehensive review and comparative benchmarking of sorbent classes

Flavonoids are polyphenol-type secondary metabolites that plants do not synthesize for primary growth and development, but rather produce mostly for defense or for pigmentation in flowers. These substances are compounds with strong antioxidant activity that are used in the manufacture of medicines, cosmetic creams, and food supplements. Due to their low concentration in samples and the presence of interfering substances in the sample matrix, their analysis is difficult. To solve this problem, sample clean-up and concentration are necessary, for which various methods, including the solid-phase extraction (SPE) method, are used. In this method, sorbents play a fundamental role and act as selective extraction phases for matrix clean-up and analyte enrichment. In this review article, six classes of SPE sorbents that have been used so far for the extraction of flavonoids-porous frameworks (MOFs/COFs), carbon-based nanomaterials (graphene, CNTs), molecularly imprinted polymers (MIPs), macroporous resins, functionalized silica and metal oxides, and bio-based polymer composites (such as chitosan)-have been reviewed. In this review, the structural features of the sorbent (such as surface area, pore size, and functional groups) and the mechanisms affecting adsorption, including π-π interactions, hydrogen bonds, and electrostatic forces, have been evaluated. Recovery efficiency, chemical stability, regenerability (reusability), selectivity, and environmental compatibility have been investigated. This review provides a perspective on the effects of sample matrix and enrichment parameters and helps researchers identify promising SPE platforms for modern, high-throughput analysis of flavonoids.