Biomedical Engineering Reference
In-Depth Information
active derivatives. A common problem encountered in such methods is the pres-
ence of endogenous ammonia in the test samples, which means that the measured
response corresponds to the total ammonia (i.e., the ammonia produced by the
enzymatic reactions and that initially present in the sample). Hence, those sens-
ing systems that monitor the ammonium ions or ammonia are of poor interest in
the analysis of real samples containing endogenous ammonia. The choice of other
types of electrochemical transducers is preferred in order to avoid these serious
drawbacks.
The detection limit is defined as the lowest and the highest analyte concentra-
tion that can be detected by the biosensor considering the signal to noise ratio.
For a Clark electrode, a lower detection limit is up to 0.5 mg/L at 25
C. Lowering
the detection limit is the goal of many manufacturing technologies. The dynamic
range is calculated as the measured or extrapolated response of the probe to a
zero analyte concentration plus two (or three) standard deviations. Sometimes it is
advantageous to have a sensor that can measure a large concentration range. For
example, sensors giving a logarithmic signal output could go to multiple orders of
magnitude. However, a sensor should give the same signal each time it is put into a
fresh analyte solution. A measure of reproducibility is the coefficient of variation,
the ratio of the standard deviation to the mean value.
The response time is the time necessary to reach a steady state value. The re-
sponse time depends on the nature of receptor element. Biological receptors are
more sluggish than simple chemical receptors. The response time also depends
upon the analyte and product transport rate through different membrane layers.
Hence, the thickness and permeability of different layers are essential parameters.
For a Clark electrode, 90% of the steady state response is obtained within 20 sec-
onds after adding the analyte into the cell. For single enzymes, 1-3-minute response
times are reported. The electrochemical biosensors have a fast response time and a
relatively high signal-to-noise ratio. If a sensor is reused, then another parameter
to consider is the recovery time (i.e., the time before a sensor can analyze the next
sample). It should be reasonable for high throughput screening.
The biosensor stability is designed by the lifetime, which is defined as the stor-
age or operational time necessary for the sensitivity, within the linear concentra-
tion range, to decrease by a factor of 10% or 50%. There are three aspects of
lifetime: (1) the active lifetime of the biosensor in use, (2) the lifetime of biosensor
in storage, and (3) the lifetime of the biocomponent in storage prior to being im-
mobilized. Hence, it is necessary to specify whether the lifetime is a storage (shelf)
or operational (use) lifetime and to specify the necessary storage and operational
conditions. In addition, the mode of assessment to determine the lifetime should be
specified. The active lifetime of a biosensor depends upon the type of biocompo-
nent used and is application-dependent. A sensor lifetime can vary from a few days
to a few months. Generally, pure enzymes have the lowest stability, while cell and
tissue preparations have longer lifetimes.
°
9.5.2 Immobilization Strategies
Immobilization is defined as the attachment of a biosensing element to a surface
resulting in reduction or loss of mobility. It is critical that the biocomponent be
