Nanosensors in
Biomedical Field
Biosensors are sensors for
detecting biological entities such as proteins, drugs, specific viruses, cancer
cells etc. In vivo detection of these happens in variety of ways naturally. For
an example, when a body is first exposed to an allergen, the body creates
antibodies that will recognize that allergen if it appear again in the body.
This triggers allergy response system the body to release histamine. Glucose detection
is important in biosensing. Type I diabetics has to monitor their blood sugar
levels continuously. Nanoscale structures may advance it in a big way.
DNA sensing is another important
area in which nanosensing can play a potential role. Using the ability of DNA
to bind to a complementary strand and not to bind to anything else presence of any microorganism with known DNA sequence. For
instance, to sense the structure with the sequence CGCTTC a complementary strand
GCGCAAG can be used.
A single strand of, say, six
bases can contain 4,096 different combinations. Consequently, if a particular
biological target such as botulism or strep or scarlet fever has a known DNA
sequence, it is possible to target a short section of that DNA sequence that
can be uniquely sensed, without any errors, by an appropriate single-strand
complementary structure.
This is called DNA finger printing.
This is called DNA finger printing.
Generalization of this method
leads to lab-on-a-chip concept. Microlaboratories capable of sensing viral and
bacterial diseases are possible with this technology. Finally, biochips could
be used to sense either particular DNA signatures or particular protein
signatures known to be defects that can result in disease.
One of the great challenges of DNA
sensing is to amplify the effects of hybridization so that they can be easily
measured. One way to provide this amplification is to change the optical
properties of gold or silver nanodots that are attached to the DNA. Chad Mirkin,
Robert Letsinger, and their groups at Northwestern pioneered the combination of
quantum optical effects and molecular recognition (complementary DNA binding).
Their scheme and some actual results are shown in the following figure.
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| DNA Viral Detection |
The upper schematic shows how the
nanodots in a colorimetric sensor are brought together upon binding to the DNA
target (in this case anthrax). The clustered dots have a different color than
the unclustered ones as is shown in the photograph below them.
By exposing the single strands
of DNA that are attached to the gold nanodots, the sensor recognizes the target strands of DNA, which causes the gold nanospheres
to come closer together and, as in those recurring stained glass windows, change
color.
In a protein biosensor a molecular
nanostructure containing a biological binding site is attached to the gold
nanoparticles. The binding site is designed to recognize a particular protein analyte. When that analyte appears in solution,
it binds to the recognition site, which changes the chemical and physical environment of the
gold dot, whose color is then slightly changed. This change can be measured.
In
the electronic nose, a random polymer, or mix of polymers, is spread between
electrodes. When the molecules to be smelled land on the polymer(s), the
conductivity properties in particular regions will change in a particular way
that is specific to any given analyte.