Biosensors

Microelectrochemical Devices

M Lambrechts, W Sansen

• Publisher: CRC Press
• Year: 2021
• eBook ISBN: 9781000447835
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

Biosensors are analytical devices that combine a biologically sensitive element with a physical or chemical transducer to selectively and quantitatively detect the presence of specific compounds. Balancing basics, principles, and case studies, Biosensors: Microelectrochemical Devices covers the theory and applications of one class of biosensor-microelectrochemical devices. The book clearly explains microelectronic techniques used to produce these cheap, fast reacting, and disposable sensors with the aid of helpful diagrams and tables. Researchers and postgraduates active in the field of chemical sensors, analytical chemistry, or microelectronics will find this an invaluable reference.

CHAPTER 1
AN INTRODUCTION TO MICROELECTROCHEMICAL SENSORS

Sensors have become so important and so interesting because some of them allow us to extend and complete our own sensing capabilities. Moreover, sensors have become vital for control systems and robotics in general. Indeed, sensors are as important for modern electronic control systems as our own five main sense organs are for ourselves. Without our hearing, touch, smell, taste and vision sensing capacities we are vulnerable and limited in our capabilities. Sensors can best be seen as electronic equivalents for our sense organs. It is well understood however that the development of sensors is a quantum leap behind the realization of microelectronic devices. This is especially true for the sense organs that detect chemicals. On the other hand, nowadays high-resolution CCD cameras can provide vision for computers and extremely sensitive microphones can provide auditory information. To replace the touch sense, very precise temperature, force and pressure sensors exist; finally, tactile sensing is still in its infancy.

Electronic equivalents for the chemical senses, namely the taste and smell sense, do not exist at this moment. Also in our own biological systems, such as the immunological system and the glucose metabolism, chemical concentrations are measured and regulated with high precision. In order to replace or monitor these biological systems, extremely sensitive and selective sensors are necessary. Commercial chemical sensors are bulky, need regular calibration and are not selective and sensitive enough. However, if biological material is combined with existing chemical sensors into a biosensor, a similar selectivity and sensitivity as in biological systems can be obtained. Extensive research on this topic started a decade ago and has now resulted in several commercial sensors.

The best known example of a biosensor is without doubt the glucose sensor. As there is an extensive need for cheap disposable glucose sensors, the development of microelectrochemical glucose sensors with microelectronic techniques is the main topic in this book. Needle-type sensors are not discussed: they are too difficult to make, their characteristics are not sufficiently reproducible and hence they are expensive. The technology of microelectronics, on the other hand, has shown to be able to provide highly reproducible devices at low cost. Mass-fabrication techniques have been in development for over two decades now, so sensor elements are discussed which mainly use these planar technologies.

In the following sections of this introduction, the basic principles and terminology related to this work are detailed. In section 1.6, the structure of this book is described.

1.1 SENSORS, BIOSENSORS, BIOPROBES, TRANSDUCERS AND ACTUATORS; A DISCUSSION OF BUZZWORDS

Classification of sensors and related devices is a very popular activity, as usually at least one paper at each sensor conference is devoted to this topic. Classifications have been made according to signal domains (Middelhoek and Noorlag 1981, Middelhoek and Audet 1987), electronic components (Bergveld 1986), transduction modes (Janata and Bezegh 1988), applications (Kobayashi 1985) and biological criteria (Aizawa 1983, Lewis 1985). Because every classification has its exceptions and problems, there is some confusion over terminology. The following clarification of sensor jargon gives an overall view and is of course also subject to discussion.

1.1.1 Transducers, sensors and actuators

According to etymology, the word transducer is derived from the Latin verb 'transducere-traducere' which means 'to transfer-to translate'. Therefore, a device that transfers or translates energy from one kind of system to another, in the same or another form, is termed a transducer. In the instrumentation environment, a transducer is used to indicate a device that transfers a signal from one energy form into a signal in another energy form. The energy forms can be electrical, mechanical, optical, thermal, magnetic or radiant (Middelhoek and Noorlag 1981). A device where the input and the output belong to the same signal domain can be identified as a modifier (e.g. a transistor).

A sensor can now be defined as a transducer that converts a signal of some specific form into an electrical signal. An actuator, on the other hand, can be defined as a transducer that converts an electrical signal into a signal of another form, usually a mechanical signal (see also figure 1.1).

Examples of sensors are pressure sensors, pH sensors and phototransistors. Examples of actuators are solenoids, piezoelectric devices and laser diodes. An electrode at which hydrogen or oxygen is generated by applying a potential is an example of a chemical actuator. A display is a special kind of transducer that converts an electrical signal into a readable form; a CRT display, LCD screen or LED array are typical display devices that can be found almost everywhere nowadays.

Figure 1.1 Classification of transducers according to the application.

Figure 1.2 Schematic of a universal control system consisting of sensors, an electronic modifier, actuators and a display.

A universal electronic control system can be represented as consisting of sensors to convert input parameters into an electrical signal; these electrical signals are then processed by an electronic modifier, e.g. a microprocessor (see figure 1.2). The output of the modifier is converted by actuators into suitable stimuli or shown on a display. In this work attention is focused on the input section, i.e. the sensors. Miniature versions of the universal control system shown in figure 1.2 can be used as an implantable replacement for vital functions. Such an Internal Human Conditioning System (IHCS) suited for 'in vivo' applications is for example described in Sansen (1982).

Figure 1.3 The difference between biosensors and bioprobes.

1.1.2 Chemical, physical, mechanical and optical sensors

Sensors can be divided further according to the signal domain of the input signal. A division can be made between chemical and physical sensors. Physical sensors include mechanical, magnetic, thermal and optical sensors. In this book, all attention is directed towards chemical sensors. The chemical sensors can be divided further according to the transduction mode (see section 1.2).

1.1.3 Biosensors and bioprobes

A special type of chemical sensor is the biosensor. A biosensor is defined as a sensor that makes use of biological or living material for its sensing function. A glucose sensor is the best known example of a biosensor. The detection of glucose is based on the enzyme glucose oxidase, a biological component usually extracted from Aspergillus Niger (another source of this enzyme can be Penicillium notatum). An immunosensor is a biosensor that is based on immunological components. A bioprobe is defined as a sensor that measures vital functions of living beings. A blood pressure sensor is a typical example of a bioprobe. More examples of biosensors and bioprobes can be found in figure 1.3.

Biosensors are a subdivision of chemical sensors. Biosensors that measure immunological parameters are called immunosensors. It is important to notice that a sensor can be a biosensor as well as a bioprobe. An AIDS sensor is for example an immunosensor that is based on biological material, so it is a biosensor. Since it monitors the immunological system—a vital function—it is also a bioprobe. The same can be said about glucose sensors; however, if an enzymatic glucose sensor is used for monitoring an enzymatic reaction in a fermentation tank, then the glucose sensor is still a biosensor but not a bioprobe. A good example of an immunosensor that is not a bioprobe is a meat quality sensor. A well known example of a chemical sensor that is not a biosensor is the classical pH glass electrode.

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Table of contents

• Cover
• Half Title
• Series Page
• Title Page
• Copyright Page
• Dedication
• Contents
• PREFACE
• ACKNOWLEDGEMENTS
• 1 AN INTRODUCTION TO MICROELECTROCHEMICAL SENSORS
• 2 BASIC ELECTROCHEMICAL PRINCIPLES
• 3 MEASURING TECHNIQUES FOR SENSOR EVALUATION
• 4 PLANAR TECHNOLOGIES FOR MICROELECTROCHEMICAL SENSORS
• 5 CASE STUDIES ON MICROELECTROCHEMICAL SENSORS
• 6 THICK-FILM VOLTAMMETRIC SENSORS
• 7 CONCLUDING REMARKS
• LIST OF REFERENCES
• APPENDIX
• INDEX