Abstract
Tannins are eco-friendly, bio-sourced, natural, and highly reactive polyphenols. In the past decades, the understanding of their versatile properties has grown substantially alongside a continuously broadening of the tannins’ application scope. In particular, recently, tannins have been increasingly investigated for their interaction with other species in order to obtain tannin-based hybrid systems that feature advanced and/or novel properties. Furthermore, in virtue of the tannins’ chemistry and their high reactivity, they either physicochemically or physically interact with a wide variety of different compounds, including metals and ceramics, as well as a number of organic species. Such hybrid or hybrid-like systems allow the preparation of various advanced nanomaterials, featuring improved performances compared to the current ones. Consequently, these diverse-shaped materials have potential use in wastewater treatment or catalysis, as well as in some novel fields such as UV-shielding, functional food packaging, and biomedicine. Since these kinds of tannin-based hybrids represent an emerging field, thus far no comprehensive overview concerning their potential as functional chemical building blocks is available. Hence, this review aims to provide a structured summary of the current state of research regarding tannin-based hybrids, detailed findings on the chemical mechanisms as well as their fields of application.
Highlights
The plant-based polyphenolic tannins are the fourth most abundant form of terrestrial biomass extracted compounds right after cellulose, hemicellulose, and lignin [1]
Complex tannins were discovered as an additional group of tannins back in 1985, which consist of an ellagitannin unit as well as of a flavanoidunit, resulting in sophisticated structures
The results demonstrated that the combination of electrostatic interaction and π-π interaction favors the performance of malachite green adsorption [27]
Summary
The plant-based polyphenolic tannins are the fourth most abundant form of terrestrial biomass extracted compounds right after cellulose, hemicellulose, and lignin [1]. Complex tannins were discovered as an additional group of tannins back in 1985, which consist of an ellagitannin unit as well as of a flavanoidunit, resulting in sophisticated structures This tannin class was labeled as “non-classified” tannins and later the term “complex tannins” was introduced. Due to their complex structure and relatively low abundance, this class of tannin finds limited adaptation in tannin based applications [10] Apart from these conventional tannins, there are non-vascular plant (e.g., algae) tannins, so-called phlorotannins, which are generated from the polymerization of phloroglucinol [3,15]. Condensed tannins are prone to feature a heterocyclic reactivity as they undergo hydrolysis (degradation) and autocondensation reactions (heterocyclic ring opening and subsequent nucleophilic addition to another tannin unit) in catalyzed conditions [3].
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