The immobilization of biomolecules or the design technology of sensing interface is one of the key factors for piezoelectric immunosensors. In this paper, two technologies named as the self- assembled monolayers (SAMs) and the opposite-charged polyelectrolyte adsorption are combined for a novel immobilization of protein molecules applied to an immunosensor for detecting complement C_3 in human serum. An opposite-charged polystyrenesulfonate (PSS) layer forms on the cystamine (Cys) self-assembled monolayers (SAMs) on the surface of the quartz crystal microbalance (QCM) which electrostatically adsorbs antibodies of complement C_3 (anti-C_3) onto the QCM for immunoreacting complement C_3. The surface morphologies of the QCM were investigated by scanning electron microscopy (SEM) after being modified with Cys SAMs and PSS layer and then immobilizing anti- C_3, respectively. The conditions of self-assembling Cys SAMs and PSS layer as well as immobilizing anti-C_3 are optimized in detail. Compared to the glutaraldehyde binding approach, the antibodies immobilized by the PSS adsorption procedure present higher antibody activity in terms of greater frequency response to immunoreaction. It is found that this immunosensing system provides advantages of improved sensitivity, selectivity and reusability. The immobilization method developed here might offer a promising possibility in firmly immobilizing biomolecules for biosensors.

Key words: CYSTAMINE, POLYELECTROLYTE, IMMUNOASSAY, SENSORS