Light addressable potentiometric sensors

A constant bias potential is applied across the sensor in order to form a depletion layer at the insulator-semiconductor interface. The depth and capacitance of the depletion layer changes with the surface potential, which is a function of the ion concentration in the electrolytic solution. The variation of the capacitance is read out when the semiconductor substrate is illuminated with a modulated light and the generated photocurrent is measured by means of an external circuit. 

FIGURE 4.13 Schematic representation of a light-addressable potentiometric sensor (LAPS). 

The working principle of LAPS resembles that of an ion-selective field effect transistor (ISFET). hi both cases the ion concentration affects the surface potential and therefore the properties of the depletion layer. Many of the technologies developed for ISFETs, such as forming of ion-selective layers on the insulator surface, have been applied to LAPS without significant modification. 

Electron-hole pairs are only produced in areas illuminated by the light and spatially resolved photocurrents can be achieved using a focused scanning beam. Therefore the main advantage of LAPS is that it can work as a chemical imaging sensor the light-addressability of the LAPS allows one to obtain a two-dimensional map of the distribution of the ion concentration. 

The first LAPS utilized silicone nitride (S3N4) as a pH-sensitive layer [68]. A light-addressable high resolution pH imaging sensor was applied to the detection of spatially resolved metabolic activity of Escherichia coli colonies on agar medium [69]. For a silicone substrate thickness of 20 xm the reported spatial resolution was about 10 pm. The observed pH distribution was in good agreement with the results of simulation based on a two-dimensional diffusion model. 

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Several other approaches for detecting nucleic acids are reported in the literature, based, for example, on the light-addressable potentiometric sensor (LAPS) (Kung et al. 1990) or on acoustic wave devices (Su et al. 1996). 

Dill K., Song J.H., Blomdahl J.A., Olson J.D., Rapid, sensitive and specific detection of whole cells and spores using the light-addressable potentiometric sensor, J. Biochem. Biophys. Meth. 1997 3 161-166. 

Uithoven K.A., Schmidt J.C., Ballman M.E., Rapid identification of biological warfare agents using an instrument employing a light addressable potentiometric sensor and a flow-through immunofiltration-enzyme assay system, Biosens. Bioelectron. 2000, 14 761-770. 

The concept of light addressable potentiometric sensors (LAPS) was introduced in 1988 [67], LAPS is a semiconductor-based sensor with either electrolyte-insulator-semiconductor (EIS) or metal-insulator-semiconductor (MIS) structure, respectively. Figure 4.13 illustrates a schematic representation of a typical LAPS with EIS structure. A semiconductor substrate (silicone) is covered with an insulator (Si02). A sensing ion-selective layer, for instance, pH-sensitive S3N4, is deposited on top of the insulator. The whole assembly is placed in contact with the sample solution. 

D.G. Hafeman, J.W. Parce, and H.M. McConnell, Light-addressable potentiometric sensor for biochemical systems. Science 240, 1182—1185 (1988). 

Y. Mourzina, Y. Ermolenko, T. Yoshinobu, Y. Vlasov, H. lwasaki, and MJ. Schoning, Anion-selective light-addressable potentiometric sensors (LAPS) for the determination of nitrate and sulphate ions. Sens Actuators, B 91, 32—38 (2003). 

T. Yoshinobu, M.J. Schoning, R. Otto, K. Furuichi, Y. Mourzina, Y. Ermolenko, and H. lwasaki, Portable light-addressable potentiometric sensor (LAPS) for multisensor applications. Sens. Actuators, B 95, 352-356 (2003). 

FIGURE 1.3 Schematic drawing of a light addressable potentiometric sensor (LAPS) for pathogen detection. (Adapted from reference Gehring et al., 1998). 

Cytosensor Microphysiometer technology has been used to detect perturbation in mammalian cells (Hafner, 2000). The system measures small changes in extracellular acidification using a light addressable potentiometric sensor. If the metabolism is interfered with, acid excretion will be affected which could be sensitively measured by LAPS. In principle, this system should be suitable for monitoring pathogen interaction with mammalian cells. 

Tu, S. 1., Uknalis, J., and Gehring, A. (1999). Detection of immunomagnetic bead captured Escherichia coli 0157 FI7 by light addressable potentiometric sensor. /. Rapid Methods Automat. Microbiol. 7, 69-79. 

Light-addressable potentiometric sensors (LAPS) recent trends and applications... 

A.B.M. Ismail, T. Yoshinobu, H. Iwasaki, H. Sugihara, T. Yukimasa, I. Hirata and H. Iwata, Investigation on light-addressable potentiometric sensor as a possible cell-semiconductor hybrid, Biosens. Bioelectron., 18(12)(2003) 1509-1514. 

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