Biomedical Engineering Reference
In-Depth Information
electrochemical devices as alternatives to the oxygen electrode are now avai-
lable, such as the microfabricated oxygen electrode, the thick-film electrode,
the ion-sensitive field effect transistor, and the surface photovoltage device.
In following section, we will describe the recent development and application
of cell-based biosensors for BOD measurement.
4.4.2
The Surface PhotoVoltage (SPV) and Its Application
The surface photovoltage (SPV) device, or light-addressable potentiometric
sensor (LAPS), is a silicon-based transducer to measure the surface poten-
tial of the device, especially the pH of the solution near the surface [45-48].
One side of the device is covered with silicon dioxide and/or silicon nitride
layers as the sensor side. The other side is mainly bare silicon and a parti-
ally deposited metal layer for ohmic contact. Illumination from the rear of
the SPV device induces a photovoltage. When only the sensor side of the
SPV device is immersed in an analyte solution, and the potential of the so-
lution is biased against the bulk silicon of the device using ohmic contact,
the silicon device acts as a MIS (metal insulator semiconductor) structure.
In this case, the solution plays the role of the “M” of MIS due to its con-
ductive nature. Because the depletion layer at the surface of the device has a
certain capacitance depending on the surface potential, the modulated photo-
voltage induces a photocurrent that depends on the sum of the bias potential
and the surface potential of the device. Thus, frequently modulated illumi-
nation induces a modulated photovoltage and ac photocurrent. The surface
potential can be estimated from the typical
CV
characteristics of the MIS
structure (Fig. 4.23). Finally, the SPV device gives the same response as the
Ion-Sensitive Field Effect Transistor (ISFET).
The SPV device has advantages compared with other pH sensors. The
fabrication process is simpler than for an ISFET. The multiplexing of dif-
ferent light sources in different locations allows multisensing (called LAPS
in a strict sense) without additional process complexity. SPV devices have
been used in some research, such as enzyme-linked immunoassays [49,50], and
measurement of taste compounds [51], hydrogen [52], and the metabolism of
animal cells [53-57].
The metabolism of cells produces acidic substances such as carbonate
ion and organic acid. The microbial metabolism is affected by many factors
in a cell's environment. Changes in the biological, chemical, and physical
environment of a cell are reflected in the production of acidic compounds.
Thus, when microorganisms or cells are immobilized on the sensor side of the
SPV device, the system can sense the biological information in the analyte
solution.
Recently, many methods have been developed for the measurement of
BOD. The BOD sensor consists of microbial cells immobilized on an oxy-
gen electrode and measures the current decrease resulting from a decrease
in dissolved oxygen [59-67]. Some other methods have been also reported
