lmmunobiosensors based on ion-selective electrodes

Ion-selective electrode (ISE) methods generally require large volume samples of more than a few milliliters. Therefore, it is highly desirable to devise a simple system for the ISE method to analyze ultratrace amounts of substances in a few microliters of sample solution. This chapter describes a novel and simplified approach for a microliter-ISE system devised by using commercially available conventional ISE without any modification. An essential feature of the present system is the use of a homemade plate-shaped stlver/silver halide reference electrode. Containers, such as a beakers, are not needed; instead, a small thin-layer space between a plate reference electrode and the flat bottom of the ISE sensor is conveniently used for holding a few microliters of sample solution. A schematic representation of the principle for the method is shown in Fig 1 . With this arrangement the necessary sample volume can be reduced to the microliter level (1 ). The thin-layer potentiometric system is extremely useful in immunoassay measurements in which the high selectivity without interference from other proteins and use of a sample volume as small as possible are the most important factors. It has been known in biochemistry that the antigen-antibody-complement reaction triggers the formation of “channel-like” holes across the liposome membrane. The combination of this channel-forming phenomenon with ion-selective electrode provides a unique electrochemical immunoassay (2 –9 ).