Research in the field of biosensors has enormously increased over the recent years. Since the development of the first biosensor by Clark in 1962, where an amperometric oxygen electrode was immobilised with an enzyme (glucose oxidase),1 many efforts have been invested to create functional hybrid systems. These functional hybrid systems often benefit from the coupling of the unique recognition and signal-amplification abilities of biological systems, that have been developed and optimised during millions of years of evolution, with an artificial man-made signal detection and amplification system. Thus, the combination of knowledge in bioand electrochemistry, solid-state and surface physics, bioengineering, integrated circuit silicon technology and data processing offers the possibility of a new generation of highly specific, sensitive, selective and reliable micro (bio-)chemical sensors and sensor arrays. Moreover, the rapid development of silicon technology has stimulated the fabrication of miniaturised analytical systems such as mTAS (micro total analysis system), ‘lab on chip’ sensors, electronic tongue devices and electronic noses.2–17 Among the variety of proposed concepts and different types of biosensors, the integration of biologically active materials together with an ISFET (ion-selective field-effect transistor) is one of the most attractive approaches. The ISFET was invented by Bergveld18 in 1970 and has been introduced as the first miniaturised silicon-based chemical sensor. In spite of distinct difficulties with regard to practical applications, the great interest in ISFET-based biosensors, so-called biologically modified field-effect transistors (BioFETs), has generated a great number of publications, a flow that shows no sign of diminishing. The reason therefore is that silicon-based fieldeffect devices are currently being the basic structural element in a new generation of micro biosensors; they provide a lot of potential advantages such as small size and weight, fast response, high reliability, low output impedance, the possibility of automatic packaging at wafer level, on-chip integration of biosensor arrays and a signal processing scheme with the future prospect of low-cost mass production of portable microanalysis systems; moreover, their possible field of applications reaches from medicine, biotechnology and environmental monitoring through food and drug industries to defence and security. This paper gives a review of recent and significant advances in the research and development of BioFETs. In planing this review, we have chosen to focus mainly upon developments occurring during the last six years (from 1995 to the end of 2001). A computer search of the Science Citation Index has found that more than 400 publications concerning ISFETs and BioFETs have appeared from January 1995 to December 2001, indicating the intensity of research activities devoted to this important task. This review is in general limited to journal articles and usually does not include patents, conference proceedings, reports or PhD theses. Some references to important works reported prior to 1995 have also been added to provide additional source material. The review is organised as follows: The principles of the ISFET and BioFET are described in section 2. Recent advances in the development of various types of BioFETs are reviewed in section 3. Here, some examples of current applications of BioFETs are presented, too. Concluding points and future prospects of BioFETs are discussed in section 4. Michael J. Schöning was born in Bruchsal, Germany, in 1962. He received his diploma in 1989 and Ph.D. in 1993, both in electrical engineering, from the Technical University (TH) Karlsruhe. In 1989 he joined the Institute of Radiochemistry at the Research Centre Karlsruhe, Germany. Since 1993 he has been with the Institute of Thin Films and Interfaces at the Research Centre Jülich, and since 1999 he has been a Professor of applied physics at the University of the Applied Sciences Aachen, Germany. His research interests include silicon-based chemical and biological sensors, thin film techniques, solid-state physics, semiconductor devices and microsystem technology.

/pdf/recent-advances-in-biologically-sensitive-field-effect-1s078k27t4.pdf

Recent advances in biologically sensitive field-effect transistors (BioFETs)

Considerable progress has been made in the development of practical glucose sensors for in vivo use. In some designs, the sensor can be replaced by the patient, who "injects" a new sensor into a fresh site using a hypodermic needle. An open-loop hypoglycaemia sensor providing an alarm signal could be available soon, and it would be very useful in the management of diabetes. The sensor for this application would need to function for a minimum period of about 12 hours. Further developments in the glucose sensor area are expected to lead to a comfortable implantable sensor/pump device, which would provide closed loop control for a prolonged period. An acceptable system would be a sensor replaced every 2 to 3 days, coupled with a microprocessor-controlled pump replaced after 2 to 3 years. Further engineering input is needed to design the monitoring and control unit containing the alarm and pump systems. The world market in clinical analysis is about US $6 billion, and covers about 10% of the total expenses for health care. It is in the field of continuous microsensors that future competition for the clinical diagnostic market will take place. Funding in vivo projects is less attractive to industry than the more proven and well-established area of in vitro diagnostics. However, the benefits of in vivo monitoring to the health of the community, and the financial rewards to manufacturers are ultimately great. It is clearly of interest for industry to invest in the development and commercialisation of this high added-value technology. >

Continuous analyte monitoring to aid clinical practice

In one aspect, described herein are field effect chemical sensor devices useful for chemical and/or biochemical sensing. Also provided herein are methods for single molecule detection. In another aspect, described herein are methods useful for amplification of target molecules by PCR.

DNA sequencing and amplification systems using nanoscale field effect sensor arrays

The feasibility of immunosensors based on capacitance measurements on semiconductor-immobilized antibody-electrolyte heterostructures is being investigated. Capacitance measurements on biosensors succeed only if the successive biomolecular layers grafted onto the heterostructures are sufficiently electrically insulating and retain their recognizing ability (Jaffrezic-Renault et al., Sensors and Actuators B 15–16 (1993) 458–462; Bergveld, Biosensors and Bioelectronics 6 (1991) 55–72; Schasfoort et al., Anal. Chim. Acta 238 (1990) 323–329). However, as with other groups, our system showed no reproducible capacitance decrease on addition of antigen. The biolayer was not sufficiently insulating or did not have a suitable dielectric character. Ions were moving through or around this layer causing `shorting' of the system. In an attempt to drive the system toward exclusion of ions from the biolayer, an overlay of non-conducting polymer, in the form of polyethylene glycol (PEG), was added. Our results indicate that PEG forms an insulating layer on a bare chip surface, causing the capacitance to drop. The response was self-consistent and reproducible. On surfaces where there was protein pre-immobilized or pre-adsorbed, the PEG could not form a continuous, integral layer and a smaller drop in capacitance was observed. There is a difference in the response of single layers of protein, where only antibody is immobilized, and triple protein layers, where antibody was immobilized, antigen added and a secondary antibody bound. These results show the possibility of developing a differential capacitive biosensor.

A differential capacitive biosensor using polyethylene glycol to overlay the biolayer

A method for impedimetric detection of biomolecules with field-effect transistor (FET) arrays is presented. For biomolecular detection, the corresponding molecules such as single-stranded DNA or bovine serum albumine (BSA) were covalently immobilized to the gate structure of 16-channel open-gate FET devices. A micro spotter system was used to site-selectively attach biomolecules to selected channels out of the array. The differential transistor transfer function (TTF) of the FETs with attached biomolecules compared to free FETs was recorded. With this impedimetric technique it was possible to reliably detect biomolecules, because the recordings were not disturbed by the typical long-term drift of the sensors like in potentiometric readout mode. For transistor gates, where DNA sequences of different length were attached, small differences in the TTF spectra were detected. When BSA was covalently immobilized to the FETs clear differences in the TTF spectra were detected, which were independent on buffer pH variations around the isoelectric point of the protein. Based on the results presented in this article it can be concluded, that the TTF method detects passive components of the biomolecules like resistance and capacitance rather than surface charge effects.

Impedimetric detection of covalently attached biomolecules on field-effect transistors

The electrical resistance of a membrane containing charged proteins is a function of both concentration and mobility of the electrolyte ions present in the membrane, and thus it depends on the Donnan effect. We were able to determine the resistance of such a membrane deposited on an ISFET, by means of a simple a.c. impedance measurement. As an example a membrane containing cross-linked lysozyme was used. We found the membrane resistance to be inversely proportional to the fixed charge density, in good agreement with the theoretical description based on the Donnan theory.

/pdf/detection-of-charged-proteins-by-means-of-impedance-596k3mzdf2.pdf

Detection of charged proteins by means of impedance measurements

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Detection of charged proteins by means of impedance measurements