Covalent immobilization of glutamate dehydrogenase (GDH) onto activated Si/SiO2 supports was analyzed by both atomic force microscopy (AFM) and an enzymatic assay. When the concentration of 3-aminopropyltriethoxysilane used in the first derivatization step of the silicon surface was decreased, the specific enzymatic activity also decreased, whereas the mean roughness increased. Thus, the activity of immobilized GDH is critically dependent on the conditions for surface derivatization, and is inversely correlated with surface roughness. (c) 2005 Elsevier Inc. All rights reserved.