Miniaturization electrochemical sensors

W. Vonau, U. Enseleit, F. Gerlach, and S. Herrmann, Conceptions, materials, processing of miniaturized electrochemical sensors with planar membranes. Electrochim. Acta 49, 3745—3750 (2004). 

Electrochemical sensors have been used as the basis or as an integral part of many chemical and biosensor developments. The introduction of microelectrode assembly added a new dimension to electrochemical sensors, and, consequently, to chemical and biosensor research. In recent years, the advancement of microelectronic fabrication technology has provided new impetus to the development of micro or miniature electrochemical sensors. 

Miniaturized electrochemical sensors Electrochemical sensors are widely used in analytical chemistry, biology, and medicine [31]. Advantages of mass production of these sensors are beneficial in medical applications. 

The lightweight, low-cost and low-power properties of miniature electrochemical sensors make the sensors very suitable for deployment on novel platforms such as unmanned aerial vehicle (UAV). Such platforms can be advantageous if deployment of in situ sensors is impractical due to a high risk of an imminent explosive volcanic eruption, or where the region of volcanic degassing caimot practically be accessed using a handheld instrument. Some recent volcanological applicaticms are outlined below. 

Carbon monoxide and carbon dioxide can be measured using the FTIR techniques (Fourier transform infrared techniques see the later section on the Fourier transform infrared analyzer). Electrochemical cells have also been used to measure CO, and miniaturized optical sensors are available for CO 2 monitoring. 

Microfabrication technology has made a considerable impact on the miniaturization of electrochemical sensors and systems. Such technology allows replacement of traditional bulky electrodes and beaker-type cells with mass-producible, easy-to-use sensor strips. These strips can be considered as disposable electrochemical cells onto which the sample droplet is placed. The development of microfabricated electrochemical systems has the potential to revolutionize the field of electroanaly-tical chemistry. 

Electrochemical sensors have several disadvantages with respect to optical sensors (i) they are based on electrodes and require a reference electrode (ii) the liquid-liquid junction is easily perturbed by external factors (iii) they are sensitive to electrical interferences (iv) miniaturization is not easy and their cost is relatively high. However, optical sensors also have some disadvantages (i) ambient light can interfere (ii) the range over which the concentration of an analyte can be accurately measured is often limited (iii) they have generally limited long-term stability. 

Finally, SECM offers a new application area for miniaturized electrochemical and biochemical sensors. They can be used in connection with a positioning system to solve, for instance, problems of cell biology, material science, and interfacial geochemistry. Since SECM instruments are now available from different commercial sources, a much broader application in the electrochemical sensor community is expected within the next years. 

Ion-selective field-effect transistors (ISFETs) are ion sensors that combine the electric properties of gate-insulator field-effect transistors and the electrochemical properties of ion-selective electrodes (ISEs). ISFETs have attracted much attention for clinical and biomedical fields because they could contain miniaturized multiple sensors and could be routinely used for continuous in vivo monitoring of biological fluid electrolytes (e.g., Na+, K+, Ca +, Cl", etc.) during surgical procedures or at the bedside of the patients in clinical cate unit (2). 

For the probing of these microenvironments, appropriately-sized electrochemical sensors and electrodes are necessary, often of nanometre linear dimension, unless they can be incorporated in the walls of the electrochemical cell. By etching, disc microelectrodes with radii of as small as 2 nm have been fabricated [15] nevertheless, problems of interelectrode reproducibility can be large at such miniature electrodes. 

Macur, R. A., Leblanc, O. H., and Grubb, W. T., Miniature multifunctional electrochemical sensor for simultaneous carbon dioxide-pH measurements. U.S. Patent 3,905,889 (1975). 

Miniaturization and utilization of biocompatible materials for construction have allowed the electrochemical sensors to be successfully used for in vivo measurements. Usually, they are arranged as sensor detection systems in an array.316 Because spectrometric methods cannot be reliably applied for in vivo measurements, their uncertainty cannot be compared with that obtained by in vivo measurements using electrochemical sensors. The main problems for in vivo measurements are the sterilization of sensors, dimension of sensors (usually cannot exceed a nanometer magnitude order), and their geometric configuration. The calibration of sensors for in vivo measurements is also a problem because high-quality standards are necessary. 

Electrochemical sensors can be classified according to their mode of operation, e.g. conductivity/capacitance sensors, potentiometric sensors, and voltammetric sensors. Amperometric sensors can be considered a specific type of voltammetric sensor. The general principles of electrochemical sensors have been extensively described in other chapters of this volume or elsewhere. This chapter will focus on the fabrication of electrochemical sensors of micro or miniature size. 

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