Modulating the Antioxidant Activity of Thin Layer-by-Layer Films with Polyphenols

Abstract

Plants and fruits contain many antioxidant molecules among them polyphenols [1]. Such natural antioxidants can find applications in food science [2] and in particular for food packaging when the molecules of interest are integrated in a film or a membrane. Many polyphenols, among them tannic acid (Figure 1) deposit spontaneously at solid liquid interfaces in a versatile manner. The deposition of TA and other polyphenols occurring spontaneously on glasses from wine or from tea has been exploited recently in materials science [3,4]. Polyphenols contain redox active groups which can also act as coordination centers for metal cations like Fe3+. This chemical modality allows the deposition from polyphenol-metal cation mixtures to yield conformal coatings [5] or to obtain films by alternating adsorption steps of the polyphenol and metal cations according to a Layer-by-Layer (LBL) deposition method [6]. The same deposition strategy can be used to deposit polyphenol containing films by alternating their adsorption with that of a polymer [7], a polyelectrolyte [8] or proteins [9]. In these cases, the interactions responsible for the cohesion of the films are due to hydrogen bonding or to electrostatic interactions (polyphenols containing also weakly acidic groups able to become anionic at high enough pH). The deposition of films made from proteins and polyphenols is due to the interplay of many kinds of interactions among them some specific interactions with amino acids like L-proline [10]. Films containing TA and other polyphenols display some antioxidant activity [11]. The advantage of the LBL deposition method of a material with respect to direct functionalization by contact with a polyphenol containing solution, leading mostly to a deposited monolayer, is an increased stability and durability of the coating. Herein, it will be shown, that LBL films made from a polycation and TA are characterized by an amount of TA which is proportional to the number of deposition steps. The antioxidant activity of those (PAH-TA)n films (where n denotes the number of deposition cycles of the polycation and the polyphenol) scales also proportionally with the number of the deposition steps, as in other studies [11]. This means that all the polyphenol molecules in the film are accessible to the used probe, 2,2-diphenyl-1-picrylhydrazyl (Figure 1). The antioxidant properties of the films can be suppressed by depositing two capping bilayers made from poly (allylamine hydrochloride) (PAH) and poly (sodium 4-styrene sulfonate) (PSS). This new finding will be discussed in terms of possible applications in food packaging.