The invention relates to polysaccharide derivatives comprising a) at least one hydrophobic and b) at least one nitrogen-containing substituent; in particular, the derivative is a cellulose ether which, as substituent a), has a trialkylsilyl and, as substituent b), an aminoalkyl group. Furthermore, it relates to a process for immobilizing biomolecules on a coated sheet-like carrier material in which the biomolecules are attached at or in the coating and the coated sheet-like carrier material comprises within or outside the coating at least one such polysaccharide derivative. Additionally, in the process for preparing the mixed cellulose ether a solution of trialkylsilylcellulose in an organic solvent is admixed with a reactive aminoalkane derivative which is insoluble or only sparingly soluble in this solvent, the reaction is carried out and the end product is isolated.