Impedance spectroscopy of interfaces, membranes and ultrastructures

For the past century, impedance spectroscopy has provided a non-invasive means of characterizing the electrical properties of many systems. Even today, it often provides the only non-invasive method for detailed structural-functional studies of these systems. This is especially so of systems in which important processes occur at the molecular level, such as those processes associated with biological and synthetic membranes and interfaces that form between solutions and various solids (e.g. metals and colloid particles). The fundamental concepts of impedance spectroscopy are re-examined and a review is given of the role that impedance spectroscopy has played in the development of our understanding of cellular and synthetic membranes, cell biophysics and ionic systems in general. Special emphasis is given to the problems associated with solution-electrode interfaces, as well as unstirred layers, which can plague measurements on biological systems and have led to much confusion in the past, A description is given of a new computer-controlled, four-terminal digital impedance spectrometer, which provides resolutions in impedance magnitude and phase of 0.002% and 0.001 degrees respectively over a frequency range of 10(-2) to 10(5) Hz and for impedances ranging from 10 to 10(9) Omega. We also describe impedance dispersions in terms of transfer functions which, when plotted along the negative frequency axis, yield ''spectra'' with distinct sharp peaks that identify fundamental frequency constants of the system. This ''control engineering'' form of presentation of impedance spectra demystifies the impedance analyses of these systems. The spectra and changes in these which occur as a result of perturbations to the system can be readily assessed and interpreted.