Dielectric-based biosensors

A basic background is given of the concepts and measurements of dielectric phenomena, and two examples of dielectric-based biosensors are described. The way in which the dielectric properties of immobilized urease can be used to monitor the hydrolysis of urea is first outlined, while the second example describes how the phenomenon of dielectrophoresis can be used to monitor the concentration and viability of suspensions of cells or micro-organisms. Introduction The dielectric properties of a liquid or solid material located between two plane-parallel electrodes may be characterized by measuring the conductance G and electrical capacitance C of such an assembly as a function of the frequency of an applied voltage across the electrodes. If the electrodes are of area A and separated by a distance d, then these two electrical quantities are defined by the following two equations: G = A d d (Siemens) C= AEq,/d (Farads) The conductivity u is the proportionality factor between the induced electric current density and the applied electric field and is a measure of the ease with which delocalized charge carriers such as electrons, protons and ions can move through the material under the influence of the field. The factor E” is the dielectric permittivity of free space and has the value of 8.854 x Farad per metre, while E is the permittivity of the material relative to that of free space ( E is often referred to as the dielectric constant of the material, but this is not the preferred term used by physicists). The relative permittivity is proportional to the ratio of the induced charge to the electric field, and is a measure of the extent to which localized charge distributions within the material can be distorted or polarized under the influence of the field. For biological materials such charges are mainly associated with electrical double layers occurring at membrane surfaces or around solvated macromolecules, or with (polar) molecules which possess a permanent electric dipole moment. Abbreviation used HMRA, hexamethylene bisacetamide. Each type of polarizable charge distribution or polar entity will exhibit its own characteristic response to an imposed electric field and, without going into the underlying theoretical details (which for biological materials are given in detail elsewhere [ 1-31), this is described by the relative permittivity being written as a complex function, having real and imaginary parts, of the form: