A Piezoelectric Biosensor as a Direct Affinity Sensor

It is well - known that the resonant frequency of an oscillating piezoelectric crystal can be affected by a change in mass at the crystal surface. 1 This method can be used as sensor for protein adsorption studies and for direct immunosensing. Up to now these studies have been performed by measuring the frequency in dry state, i.e. with the "dip and dry" technique which is rather cumbersome and time consuming. Recently we obtained some results with piezoelectric crystals used directly in liquid solutions. We will discuss a real-time monitoring of (i) adsorption of the human immunoglobulin Ig G (h -Ig G); (ii) the affinity reaction between covalently immobilized antigen (the pesticide 2,4 -D) and specific monoclonal antibodies (Mab anti- 2,4-D) from two different clones (clone F6C 10 and clone E2G2); and, (iii) immunoreactions between immobilized antigen and antibodies performing a competitive assay. All experiments show how the reaction under study is linked to a mass increase which can be monitored continuously in real time. Direct antigen - antibody interaction can, thus, be studied without any kind of label. A microprocessor controlled piezoelectric detector as sensor was employed to monitor in real time protein adsorption and immunoreactions using piezoelectric quartz crystals (AT-cut) with basic resonant frequency of 10 MHz. The adsorbed protein was an immunoglobulin (h -Ig G); in the immunosensing a covalent immobilized molecule (the pesticide 2,4 -D) formed the receptor for the immobilized ligand sample (Mab anti- 2,4- D) in a competitive assay.