Biomedical Engineering Reference
In-Depth Information
selectivity and limit of detection of the sensor when used, for example, in blood,
serum, urine and tissue will clearly be influenced strongly by interfering
components of the biological sample. It appears to be the case that wide scale
use of biosensors in, for example, clinical biochemistry, has not occurred
primarily because of this issue.
The placement of a solid transducer-probe combination into a biological
sample will result in a signal originating from a composite response, X
n
,
according to the following matrix:
4
d
n
4
t
3
n
g
|
1
X
n
¼
S
n1
C
1
þ
S
n2
C
2
þ
:
S
nn
C
n
ð
1
:
1
Þ
where C and S values represent the concentration of analyte and interferants in
proximity to the device, and the response sensitivities, respectively.
An enormous number of components would be expected to be involved in
this equation in terms of biological samples. A sensitive response (e.g. volts or
amps) implies maximization of one S value and, for a selective signal, a
minimization of all other S values. To a first approximation, it is necessary to
trap the analyte on the device surface to allow the sensor response and, as
mentioned above, to repel or avoid such binding of all other elements.
Accordingly the sensor signal will be a composite of the chemistry of
the attachment process and the physical perturbation caused by the
probe-analyte complex. This leads to some interesting aspects concerning the
nature of the couple between physical chemistry and the transduction
process. In certain cases, as will be seen in later sections, the mere presence of
the analyte can influence the transducer and the resulting signal is often
referred to as a 'mass response'. However, a situation can be envisaged where
a structural shift, such as a probe conformational change, and regardless of
whether the response is related to final state effects or the change itself, is
required for detection to take place. The physical chemistry of transduction in
this case is reminiscent of agonist versus antagonist interactions and will be
familiar to the biochemist community. All these mechanisms will obviously
be an intrinsic component of the sensitivity parameter, S
nn
, outlined above.
In summary, there are a number of key desirable properties that a biosensor
should possess, although some features are of course more important for some
applications than others:
d
n
3
.
Selectivity or even specificity (see ref. 4 for an excellent definition of these
parameters) with respect to the response to the analyte, as described
above, is a given. Non-specific binding through adsorption to the device
surface by interferants will clearly have a strong influence on selectivity,
especially if such components are at a high concentration .in the sample
under analysis.
High sensitivity is required with a resulting low value of limit of detection.
High accuracy is required in terms of concentration measurement for the
analyte. Signaling must be conducted with high reproducibility—a
precision issue.
