Layer-by-layer assembly of food-grade alginate/chitosan nanolaminates: formation and physicochemical characterization
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The alternate deposition of oppositely charged materials (layer-by-layer technique) is an effective approach to functionalize materials. Biopolymer-based nanolaminates obtained by the layer-by-layer technique can also be used to change the surface properties of food products or food contact materials. However, the final properties of nanolaminates may be affected by the conditions of the adsorbing solutions. The objective of this study was to form and characterize the physicochemical properties of nanolaminates assembled from alginate and chitosan solutions. The effect of pH, ionic strength and polysaccharide concentration on the properties of the adsorbing solutions was also evaluated. The ζ-potential, viscosity and whiteness index of the solutions were assessed before the assembly. Alginate/chitosan nanolaminates were characterized in terms of UV-visible spectroscopy, surface ζ-potential, contact angle, DSC analysis and SEM. The absorbance increased as a function of the number of polysaccharide layers on the substrate, suggesting an increase in the mass adsorbed. The surface ζ-potential of nanolaminates changed depending on the last polysaccharide deposited. Alginate layers were negatively charged, whereas chitosan layers were positively charged. Contact angles obtained in alginate layers were ≈ 10°, being mostly hydrophilic. Chitosan layers showed higher contact angle values (80°), indicating a more hydrophobic behavior. Microscopic examinations revealed the presence densely packed structures that corresponded to alginate/chitosan nanolaminates, having an estimated thickness of 700 nm. The results obtained in this work lay the basis for the rational design of polysaccharide-based nanolaminates in the food sector.