The Development of an Integrated Microengineered Ion Sensor (Inion) for Cesium

Abstract

An Ion Selective Conductimetic Microsensor (ISCOM) for the detection of cesium ions was devleoped. The operating principle of an ISCOM is based on the concentration dependant change in the bulk conductivity of an ion selective membrane. Changes in conductivity are achieved by salt extraction of the target ion into the bulk of the membrane that is formed on top of a planar interdigitated electrode, thus allowing the monitoring of the membrane conductivity.

Characterisation of the planar interdigitated electrode was undertaken using numerous techniques. This that that there was considerable inter-diffusion of the elemental layers comprising the subsurface structure of the electrode and allowed the identification of chromium as a contaminant. This was reported back to the electrode manufacturers and subsequently removed. Thermodynamic analsysis indicated that the elements of the surface of the electrode would not present any problems (background signal etc) during the normal operation of the ISCOM device.

A method for the deposition of the ion selective membrane onto the electrode was developed. Initially the membrane was formed in a well on the electrode surface that was created by the lamination of additional layers onto the device structure. This method was abandoned due to the high failure rates of these devices caused by solution seepage under the laminated layer resulting in it peeling away from the electrode structure. The second method used was the forming of a membrane by direct drop coating i.e. application of a drop of the membrane on to the electrode. This work indicated that greater sensitivity could be obtained with thinner membranes. The final method to be used was the spin coating of the ion selective membrane onto the electrode surface. This work demonstrated that the greatest sensitivity, and response times of less than 15 seconds, were achieved when spin coating was conducted at speed of 1000 rpm or grater. In addition to this the response of the ISCOMs was found to linearly related to the square root of the target ion concentration.

An analysis of the eeffect on device sensitivity of the composition of the ion selective membrane in terms of the relative level of membrane components was undertaken usign the experimental design software MODDE 6.0. This showed that the optimal membrane composition was 10% ionophore, 20% polymer and 70% pasticiser. Such membrances have a limit of detection of 4.95 x 10 M cesium.

The selectivity of the cesium ISCOM was analysed using a semi-empirical method develoepd for this sensor from data generated using a method based on teh Fixed Inference Method (FIM) for the determination of ISE selectivity. It was found that the selectivity of the cesium ISCOM was comparable to that reported for ISEs. Particularly the interference by rubidium in the ISCOM system is the lowest of any sensor system reported to date with logks value of -2.22.

The results for preliminary investigations into the device lifetime are also presented.