Adsorption of Chitosan on PET Films Monitored by Quartz Crystal Microbalance Tea Indest,* ,† Janne Laine, ‡ Volker Ribitsch, § Leena-Sisko Johansson, ‡ Karin Stana-Kleinschek, † and Simona Strnad † Laboratory for Characterization and Processing of Polymers, Faculty of Mechanical Engineering, University of Maribor, ⊥ Smetanova 17, SI-2000 Maribor, Slovenia, Laboratory of Forest Products Chemistry, Helsinki University of Technology, Post Office Box 6300, FI-02015 HUT, Finland, and Institute of Chemistry, Rheology, and Colloid Science, Karl Franzens University, ⊥ Heinrichstrasse 28, A-8010 Graz, Austria Received March 31, 2008; Revised Manuscript Received May 13, 2008 The adsorption behavior of chitosan on poly(ethylene terephthalate) (PET) model film surface was studied using the quartz crystal microbalance (QCM) technique. QCM with a dissipation unit (QCM-D) represents a very sensitive technique for adsorption studies at the solid/liquid interface in situ, with capability of detecting a submonolayer of adsorbate on the quartz crystal surface. Chitosan as well as PET were chosen for this study due to their promising biocompatible properties and numerous possibilities to be used in biomedical applications. As a first step, PET foils were activated by alkaline hydrolysis in order to increase their hydrophilicity. Model thin films were prepared from PET foils by the spin coating technique. The chemical composition of the obtained model PET films was analyzed using X-ray photoelectron spectroscopy (XPS) and their morphology was characterized by atomic force microscopy (AFM). Furthermore, the adsorption behavior of chitosan on these activated PET films and the influence of adsorption parameters (pH, ionic strength and chitosan solution concentration) were investigated in detail. Additionally, the surface chemistry and morphology of the PET films and the chitosan coated PET films were analyzed with XPS and AFM. 1. Introduction Adsorption of polysaccharides at a solid-liquid interface is the subject of intense investigation for technological as well as for biomedical applications. Therefore it is necessary to utilize special methods which provide direct information on the solid-liquid interactions. The quartz crystal microbalance (QCM) is a promising technique, which gives information about adsorption/desorption processes and is useful for sorption studies at the solid/liquid interface. 1,2 It is based on the change of the oscillating frequency of the piezoelectric quartz crystal device upon mass loading, with the capability of detecting a submono- layer of adsorbate 3 on the surface. Real-time measurements of the frequency shifts and energy dissipation due to the changes in mass and viscoelastic properties give information about the adsorbed layer. The chemical structure of PET with very few available polar groups (carboxyl, hydroxyl) on the surface results in low surface free energy and poor wettability. Although there are several other methods to increase PET hydrophilicity like radiation with plasma 4,5 and corona treatment, alkaline hydrolysis 6,7 is a standard and simple chemical pretreatment for PET surface activation. For the preparation of model surfaces of nonhydro- lyzed (PET-N) and hydrolyzed PET (PET-H) as substrates for adsorption studies, the spin coating technique was used. Spin coating is a very simple and fast technique for obtaining model surfaces. As the most appropriate solvent, 1,1,2,2-tetrachloro- ethane was chosen, and the spin-coated films were formed on silica quartz crystals. The surface chemistry of spin-coated PET-N and PET-H films was investigated with X-ray photo- electron spectroscopy (XPS) while the quality and smoothness of the PET-H film was characterized with atomic force microscopy (AFM). Furthermore, the adsorption of chitosan was investigated on such prepared model PET-N and PET-H films. Recently chitosan has attracted some attention due to its good biocompatibility 8 and biodegradability. 9 Chitosan macromolecules consist of glucosamine and N-acetylglucosamine units linked by 1-4 glycosidic bonds and is prepared from chitin by N-acetylation with alkali. 8,10,11 It is an inexpensive, nontoxic biopolymer with numerous biological functions such as antimicrobial/antibacterial activity, 12 antifungal activity, 9 hemocompatibility, hypocholes- terolemic activity, 13 the ability to accelerate wound healing, the ability to suppress some leukemia processes, 14 and it also has metal binding capacity. 15,16 The purpose of this study was to investigate the adsorption equilibrium under different (pH, ionic strength, and concentra- tion) conditions. The target was to learn if the adsorption process is driven by electrostatic or hydrophobic/entropic interactions and to prove if the adsorbed layer structure depends on the pH and the ionic strength of the solution. Additionally, the surface chemistry and morphology of the spin-coated PET-H films and the adsorbed chitosan layers (PET-HC) were analyzed with XPS and AFM. 2. Experimental Section 2.1. Materials. PET Foil. The original foil used for experiments was a Mylar polyethylene terephthalate (PET) foil, with a thickness of 175 μm. PET-N and PET-H Film Pretreatment. PET (Mylar, 175 μm) foil was immersed in 98% ethanol and cleaned in an ultrasonic bath for 10 min, washed thoroughly with demineralised water and air-dried. * To whom correspondence should be addressed. Tel.: +386-2-250-9646. Fax: +386-2-220-7990. E-mail: tea.indest@uni-mb.si. † University of Maribor. ‡ Helsinki University of Technology. § Karl Franzens University. ⊥ Member of the European Polysaccharide Network of Excellence (EPNOE). Biomacromolecules 2008, 9, 2207–2214 2207 10.1021/bm800333p CCC: $40.75  2008 American Chemical Society Published on Web 06/28/2008