Potential of the thickness shear mode acoustic immunosensors for biological analysis

This paper describes the principles underlying the most widely used mass-detecting immunosensor, the so-called quartz crystal microbalance or thickness shear mode resonator. The transducer consists in a circular quartz plate which is piezoelectrically excited in the thickness shear mode by means of a pair of planar electrodes. One of the transducer electrode is functionalized with antibodies that are covalently immobilized. The authors stress a very important aspect which has been neglected so far and deserves special attention for biosensors applications: The necessity is not only to consider mass effects, but also boundary viscoelastic properties, with respect to changes of the shear acoustic impedance when antibody binding reactions take place at the surface of a thickness shear mode acoustic sensor. Two representative examples are given for cases when 1. (1) changes of boundary liquid viscosity dominate (selective binding of erythrocytes by IgM), or 2. (2) mass effects dominate (binding of IgG by anti-IgG antibodies).