An Acoustic Pulse Electro-Kinetic Sensor

Abstract

There are many circumstances under which it is important to study the biochemical or biological activity of a surface in contact with a fluid. Protein adsorption is of particular interest - the affinity of proteins for a surface will determine the suitability of the underlying material for use in anything from kitchen utensils to biomedical implants. A surface that sheds dirt easily may be effective in preventing food-poisoning, but one that sustains a coat of non-denatured proteins is vital for producing a successful and long-lasting vascular implant. Numerous techniques exist for studying protein-surface interactions, each having their own strengths and weaknesses. Established methods often rely on the prior modification of the protein with some kind of label (e.g. radioactive iodine, or a fluorescent marker). More recently, efforts have focussed on improving techniques which do not, such as Surface Plasmon Resonance. In this dissertation, a new real-time, label-free system is described, which has notable advantages over existing methods. In particular, it offers flexibility and experimental simplicity at a substantially lower cost than many of its counterparts. The method involves detecting tiny electrical signals generated at the solid-liquid boundary when ultrasound strikes at an oblique angle; the information so obtained is a function of the charge distribution and the visco-elastic properties of the fluid at the interface. Two distinct electro-kinetic effects are identified and characterized, referred to as Double-Layer Compression (DLC) and the Parallel Vibration Potential (PVP). A DLC signal is generated when the distance between fluid-borne ions and the solid surface to which they are attracted is modulated by the acoustic variations in pressure. The Parallel Vibration Potential is generated when the same ions are caused to slip over the surface, in an oscillatory fashion. Apparatus has been designed and constructed for characterizing these effects. One effect (DLC) is shown to be useful for studying the electrochemical nature of the solid surface itself. The other (PVP) yields information on the type and density of proteins adsorbed at the surface; both protein-surface and protein-protein interactions have been monitored in this way.