New trends in ion-selective electrodes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 96

Reference electrode

A

Insulating layer

Sample solution

Si substrate

Modulated light

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

The working principle of LAPS resembles that of an ion-selective fi eld effect tran-
sistor (ISFET). In 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 signifi cant modifi cation.
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 fi rst LAPS utilized silicone nitride (S 3 N 4 ) 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

m.
The observed pH distribution was in good agreement with the results of simulation
based on a two-dimensional diffusion model.
Ionophore-based solvent polymeric membranes were used as sensing layers for the
development of LAPS selective for lithium [70], potassium and calcium ions [71]. Anion-
selective LAPS for the determination of nitrate and sulfate ions were described [72].
LAPS offers a number of advantages in view of multisensor system applications.
Each point on the sensing surface can be individually accessed, and therefore multi-
sensor functions can be easily accomplished by depositing various sensor materials on
different sites of the sensing surface. A number of LAPS-based ion-selective multisen-
sor systems have been reported recently [73, 71, 74].
An enzyme deposited on the LAPS surface allows one to observe the spatial distri-
bution of a specifi c substrate. In a urea-selective sensor urease was immobilized on a
pH-selective LAPS [75].

µ m the reported spatial resolution was about 10 µ