Cross-linked Chitosan and Poly(allyl amine) Thin Films
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Cross-linked Chitosan and Poly(allyl amine) Thin Films Caroline L. Schauer, Paul E. Schoen, and Frances S. Ligler Naval Research Laboratory, Center for Bio/Molecular Science and Engineering Code 6910, 4555 Overlook Ave. SW Washington, DC 20375-5348, U.S.A ABSTRACT The alternation of chitin layers with various compounds allows nature to create novel materials with selective refractive properties. Various organisms employ layered materials for survival; for example, in butterflies such materials cause the apparent disappearance and reappearance during flight to evade predators. The selective light refraction is achieved through control of the thickness and index of refraction of the multiple layers. With the ultimate goal of reproducing these layers, thin single films of chitosan, the soluble form of chitin, and poly(allyl amine) have been prepared and their optical properties analyzed. Ellipsometry, reflectance IR, reflectance spectroscopy, SEM and software modeling were used to characterize these films. Possible applications for optical layered materials range from medical diagnostics to environmental monitoring. INTRODUCTION Nature alternates chitin layers in order to control the apparent color in a wide variety of animals [1, 2]. For example, the intense coloration of butterflies, snakes, hummingbirds and arthropods is due to reflective interference by stacked thin layers of film comprised of alternating materials with high and low indices of refraction [3]. With the ultimate intent of reproducing this controlled reflectance, we fabricated single layers of chitosan, the soluble form of chitin, and poly(allyl amine) hydrochloride (PAH) (Figure 1). Both of these polymers could be used to make high quality films with a reflectance determined by film thickness. Different cross-linkers were examined for the ability to control the preparation of the films with respect to thickness and structural color. Two different procedures for cross-linking chitosan were employed. One, where the chitosan solution is cross-linked using Resimene before film formation and another, where chitosan is cross-linked using hexamethylene 1,6di(aminocarboxysulfonate) (HDISA) (Figure 1) after film formation. The addition of a plasticizer and use of coatings were investigated for the resulting film quality and solvent resistance. Since chitosan is known to adsorb heavy metal ions in solution, an investigation of the colored chitosan films was undertaken to identify their possible use as a metal ion sensor [4]. The thickness and color change of the thin films were measured after dipping into different salt and metal ion solutions. EXPERIMENTAL DETAILS Chitosan of medium molecular weight (approximately 240 kDA) and greater than 70% deacetylation was supplied by Protan (Raymond, WA). High (approximately 600 kDA) and low (approximately 150 kDA) molecular weight chitosan, phosphate buffer solution, pH 7.4 (PBS), and gluteraldehyde (GTA) were purchased from Sigma (St. Louis, MO). Polyvinyl butyral resin
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(Butvar, B-98) with a molecula
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