Microsystems for Bioelectronics

Sensors at the micro-scale

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CHAPTER OUTLINE

Introduction.......................................................................................................................................... 91

Sensor basics....................................................................................................................................... 92

Analog signal ....................................................................................................................................... 93

Fundamental sensitivity limit of sensors: Thermal noise .......................................................................... 96

What information can be obtained from cells? ...................................................................................... 100

Sensors of bioelectricity ................................................................................................ ..................... 102

Chemical and biochemical sensors ...................................................................................................... 105

Planar ISFET sensors ............................................................................................................105

One-dimensional nanostructures for biosensing .......................................................................107

Thermal biosensors............................................................................................................................. 109

Basic principles ....................................................................................................................109

FET-type thermal sensors.......................................................................................................111

Thermoelectric sensors ..........................................................................................................112

Remarks on the state of the art of nano-biocalorimetry .............................................................116

Concluding remarks ............................................................................................................................ 116

Glossary of biological terms ................................................................................................................ 117

List of symbols.................................................................................................................................... 118

References ......................................................................................................................................... 119

4.1 INTRODUCTION

The nanomorphic cell must have mechanisms that enable it to obta in information about the external

environment and, in particular, about the state of a target living cell. It is stipulated that the nano-

morphic cell must measure external parameters in a non-invasive way. Thus, an essential function of

the nanomorphic cell for bio-applications is to receive and transform signals from its environment and

biological signals from the living cell into an electrical form for subsequent processing and analysis to

provide a basis for further actions. For the purposes of this discussion, the sensors must be of a size

commensurate with that of the artificial cell.

In this chapter, the basic principles of sensor elements are discussed and scaling projections for

micron-sized sensors are investigated. The barrier model (as in the FET) is shown to be a generic

abstraction for many classes of sensors. The concept of the signal-to-noise ratio is important for

understanding sensor scaling while the barrier formulation is used to derive a model for thermal

Johnson-Nyquist noise. It is argued that in many cases, sensors can be regarded as either digital or

CHAPTER

Microsystems for Bioelectronics the Nanomorphic Cell

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