The correlation between the activity of urease immobilized to anodized sheet aluminium and the anodizing conditions

Abstract

The fixation of proteins to water insoluble materials has become an important field in chemistry and related disciplines [1]. Preference is often given to inorganic carriers because of their resistance of bacterial degradation. Moreover, the investigation of the interaction between proteins and nonbiological surfaces is of great practical interest with respect to the increasing use of prosthetic materials in the body [2]. Recently, we could show with the enzyme urease as an example that anodized sheet aluminium is a suitable carrier material for enzyme immobilisation [3, 4]. Further, the anodizing process leads to an adsorbens with a homogeneous surface, which is required for the interpretation of adsorption experiments. In this paper the influence of the different anodizing parameters on the activity of urease adsorbed to anodized sheet aluminum is described. The results are discussed with regard to the surface structure that has been studied by scanning electron micrographs. The anodizing of aluminium was carried out in sulfuric acid as electrolyte. The rolled Al-sheets were placed between two cathodes of lustrous carbon. A dc power supply allowed anodizing at constant voltage and constant current. The investigated parameters were: the concentration of sulfuric acid (c a), the applied voltage U and current density (i), the anodizing temperature (T a), the anodizing time (t a), and the contact time (t ad) between anodized sheet aluminium and the enzyme solution. We found that optimum activities are obtained if the anodizing conditions are: c a = 26 wt%, t a = 50 min, T a = 308 K, and i = 85 mA/cm 2. The connection between the single anodizing parameters and the activity of the adsorbed enzyme is as follows: in the starting experiment with U = 18 Volt, t a = 30 min, t ad = 40 min, T a = 298 K and c a between 2 and 40 wt% the best results were achieved with c a = 26 wt%; the corresponding i-value was 50 mA/cm 2. Higher acid concentrations lower the current density at which optimum activities are obtained. The activity itself differs only negligibly. At t a-values less than 20 min the activity of the immobilized is low. For 30 min < t a > 50 min the activity is raised by a factor of 1.5 to 2, dependent on c a. If T a is 308 K instead of 298 K the activity is three times as high as in the starting experiment. A nearly tenfold activity results if it is raised up to 80 mA/cm 2, and the activity increases again by a factor of 2 if t ad is extended by 100 min. The importance of the contact time t ad for the acitivity of the adsorbed enzyme distinctly increases for i > 50 mA/cm 2 as a consequence of the increasing porosity of the anodized sheet aluminium. The complex influence of the anodizing parameters on the surface structure and its correlation to the activity of the adsorbed enzyme is conclusively interpretable by means of scanning electron micrographs.