A New Microcapillary Device for Controlling Surface Conductivity Changes at Solid–Liquid Interface

Abstract

A new microcapillary device based on an interdigitated electrode array is presented. Microcapillaries with metal electrodes at their bottom are formed as trenches in SiO 2 and are open at the top. In an electrolyte solution, the ac current applied to electrodes flows from one capillary to another and is significantly affected by changes in surface conductance at the SiO 2 /electrolyte interface. The effect of electrostatically assembled charged polyelectrolyte layers on the surface conductance is studied.Surface charges at a solid/liquid interface play an important role in many systems, but their effect becomes more important at the micro-or nanoscale.1,2 For example, fluid flow control and sample transport in lab-on-a-chip devices depend strongly on the surface charges on the channel walls.3 Another important field is the design of biomedical materials, as charge formation at a biopolymer/liquid interface is of the highest importance in their applications. 4 To study the charges at insulator/electrolyte solution interfaces, conventional electrokinetic methods are most commonly used, among which are streaming current or streaming potential methods and electroosmosis. Experiments are usually performed on macroscopic surfaces with a simple geometry, such as capillaries or microchannels.3,5 However, it must be noted that the interpretation of electrokinetic measurement results is not straightforward and a lack of adequate experimental methods that permit the direct analysis of electrical charging at solid/liquid interfaces is recognized by specialists in the field.When a tangential electric field is applied at a solid/solution interface, owing to a higher concentration of ions within the electrical double layer (EDL), the local electric current can be higher than the typical current in the bulk electrolyte solution. For a long time, it was considered that surface conductivity is associated only with the migration of ions accumulated within the diffuse part of the EDL. However, accumulated experimental evidence facilitated the revision of the EDL model for a solid/ liquid interface 6,7 and led to the assumption that under an electric potential gradient, the ions that can migrate under the electric field are located in a stagnant layer of the EDL. Thus, the overall surface conductivity is due to the migration of ions confined within a stagnant layer and due to ions accumulated in a mobile region of the diffuse part of the EDL. The current density in both surface layers is determined by the concentration and mobility of ions.We have previously reported a three-dimensional impedimetric transducer 8,9 that, when in contact with an electrolyte solution, is highly sensitive to the charges present on its surface. In this work, the possibility of this device monitoring changes in the surface conductivity at a solid/liquid interface is studied.The device was fabricated using conventional microelectronic fabrication techniques. A silicon wafer covered with a 2500-nm-thick thermally grown SiO 2 layer was used as the substrate....