Biomedical Engineering Reference
In-Depth Information
A number of applications of the technology described above have appeared
with biodetection in mind. One example is the detection of Bacillus subtilis spores
via a silver-based spherical array.
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(These spores were employed as a surrogate
for anthrax spores.) Using laser excitation and spheres of dimension 600 nm on
the substrate, calcium dipicolinate extracted from the spores was detected by
observation of Raman spectral bands. The work was extended to anthrax spores
and even the design of a portable Raman spectrometer. A second example is
the detection of glucose. This experiment required the use of a partition layer
(self-assembled monolayer) on the silver substrate. Glucose bands were detected
following exposure to a saline solution of the molecule. In an effort to demon-
strate the use of the technique for 'real' samples, the silver array was subjected to
a solution of bovine serum albumin (BSA) prior to exposure to a glucose
solution. Glucose could then be assayed through the use of a spectral difference
technique. No attempt was made at the direct detection of the molecule in serum
and, indeed, it was not clear what the advantage of the method is compared with
the myriad of other methods for the determination of glucose.
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1.4.3.6 Photoacoustic Spectroscopy
Finally, in this section on optical approaches to biodetection we make concise
mention of the technique of photoacoustic spectroscopy, which involves the
excitation of a sample with electromagnetic radiation with detection being
achieved through acoustic physics. This technique has a long and distinguished
history in that one of the earliest studies was conducted by Alexander Graham
Bell in the last few years of the 19
th
Century. In essence laser light is absorbed
by the sample under study which produces local heating and, in turn, a pressure
wave with attendant acoustic wave generation. One of most important appli-
cations of the technology over many years has been the detection of gases such
as CO
2
and NO at trace levels. Indeed a device has been described for the
detection of the former molecule generated by an enzyme, which bears
resemblance to an enzyme electrode for the same purpose.
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In this case urease
was employed to generate the gas, which was allowed to permeate a membrane
which separated the photoacoustic cell from a solution of urea, the target
analyte. It is not clear what advantage these methods would have for such
determinations of enzyme activity. A further example, although not strictly
biosensor technology, is work related to detection molecules associated with
terrorism.
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A quantum cascade laser has been employed with a micro-
electromechanical system (MEMS) designed cell to detect parts per billion
(ppb) concentrations of dimethyl methylphosphonate, a nerve gas. It remains
to be seen if this technology will become competitive as a method in the
biosensor armamentarium.
1.4.4 Brief Summary of the Adjunct Technology Approach
To conclude this concise introduction to biosensor technology, we summarize
below a number of methodologies where more than one technique is employed
