Biomedical Engineering Reference
In-Depth Information
Table 7.1
Typical Values of Advection-Diffusion and Reaction
Parameters for Short DNA Strands
Number of
Basis Pairs
D (m
2
/s)
K
on
(m
3
/mole/s)
C
0
(mole/m
3
)
0.85 10
-10
0.34 10
-5
32
110
0.80 10
-10
0.24 10
-5
14
60
0.75 10
-10
0.46 10
-5
15
125
0.70 10
-10
0.29 10
-5
29
75
The principle is shown in Figures 7.51 to 7.54. The first step is the same as before:
functionalization of a surface of capture with ligands. The second step is most of
the time the most difficult because it requires finding molecules “analog” to the
targets, with an affinity to the ligand smaller than the target. This is the tricky part
that biologists and chemists have to resolve. These analogs can be marked, usually
by fluorescent marker. Then they are then immobilized to the ligands by a usual
sandwich-type reaction. The higher the saturation rate, the better for the sensitivity
of the detection. When the targets arrive in the vicinity of the surface of capture,
they displace some of the analogs, due to their higher affinity with the ligands. It
can be said they compete with the analogs to bind the ligands, hence, the name
competition
.
The result of the displacement is a change in the fluorescence level. At the lo-
cation of the surface of capture, the level of fluorescence decreases because of a
decrease in the marked analog surface concentration. Further down the microchan-
nel, the displaced analogs carry their fluorescent markers and there is a fluorescence
increase at the microchannel outlet.
In conclusion, the targets are not marked, just the analogs are marked, which
is more convenient. However, the process requires finding the best analogs possible
and is longer to set up. In a sense, displacement reactions are the negative of sand-
wich reactions.
Displacement Reaction Using FRET
A very interesting type of displacement reaction uses the fluorescence resonance en-
ergy transfer (FRET) principle [32, 33]. When a fluorophore is excited by a light at
the right wavelength, it emits light at a slightly shifted wavelength (Figure 7.55).
Figure 7.48
Step 1: Functionalization of the solid surface with ligand antibodies.
