AVS 52nd International Symposium
    Biomaterial Interfaces Wednesday Sessions
       Session BI1-WeM

Paper BI1-WeM7
Model Dielectric Functions for Adsorbed Protein Layers

Wednesday, November 2, 2005, 10:20 am, Room 311

Session: Protein-Surface Interactions
Presenter: H. Arwin, Linköping University, Sweden
Authors: H. Arwin, Linköping University, Sweden
J.A. Woollam, University of Nebraska, Lincoln
D.W. Thompson, University of Nebraska, Lincoln
Correspondent: Click to Email
A detailed knowledge about protein-surface interactions is of crucial importance for development of biomaterials, bioanalytical tools and biosensors as well as for understanding the mechanisms in protein-cell interactions. Ellipsometry is extensively used in these areas due to its nm-resolution in layer thickness and capability for in situ studies at solid-liquid interfaces. The outputs from an ellipsometer study are typically quantification of adsorbed surface mass and/or dynamics of protein adsorption. Recently also infrared ellipsometry has become available and optical signatures like amide bands in surface-bound proteins can be quantified. However, for layers of nm thickness, the analysis is not straight forward and it can be hard to separate thickness and refractive index of the protein layer, especially with single wavelength ellipsometry data. Spectroscopic ellipsometry can in some cases resolve this but very few studies are reported. Another major advantage if spectroscopic data are available is the possibility to model, i.e. to parameterize the wavelength dispersion of the refractive index. This also leads to noise reduction. In this report a model dielectric function (MDF) concept for protein layers is proposed. Notice that the square root of the MDF equals the refractive index. The MDF is based on a Cauchy dispersion to which Lorentzian and/or Gaussian resonances are added to account for electronic and vibrational excitations in ultraviolet/visible and infrared spectral regions, respectively. The use of the proposed MDF is exemplified on fibrinogen adsorbed on gold. With variable angle spectroscopic ellipsometry, data were recorded in the spectral region 0.2 to 30 µm (300-5000 cm@super-1@) before and after protein adsorption. In the analysis the fibrinogen layer thickness is obtained as well as the refractive index (the square root of an MDF) and the amide bands I, II and A are resolved. A generalization to protein layers in general will be discussed.