Biomedical Engineering Reference
In-Depth Information
Fig. 14 Electrochemical
detection for label-free
immunoassays. Reprinted by
permission from Macmillan
Publishers Ltd: Nature
Biotechnology [ 125 ],
copyright (2005)
were split into four separate channel and then directed onto a CCD camera. Three
of the channels contained antibodies immobilised at the surface and the remaining
channel provided the reference. Binding of the antigens interacted with the eva-
nescent wave at the surface causing a change in the refractive index and hence a
change in the interference pattern. Using this device, they were able to detect
herpes simplex virus in the femtomolar range. More recently, the interferometric
principle has been extended to detect binding events in free solution using a
method called back-scattering interferometry by Bornhop et al. [ 7 ]. Here, the walls
of a rectangular microfluidic channel were used provide the interference pattern
and the scattered light was directed to a CCD array. Using this label-free method
the researchers were able to measure binding events of a range of binding partners
including protein A with IgG and calmodulin with calcium ions. The device was
even able to measure the picomolar dissociation constant of interleukin-2 with its
antibody. For POC applications, this method would require a pre-filtering step as
the measurement can suffer from refractive index problems caused by a complex
sample.
Electrochemical detection has been widely used for detection in microfluidic
immunoassays. Here the analyte concentration can be monitored via the change in
conductivity measured by an electrode fabricated within the microchannel. Field
effect transistors made from silicon nanowires offer high sensitivity without the
need for labels. These function as semiconductors where the antibody binding
results in a change in the dielectric environment of the nanowire and hence a
change in the conductance. An early application demonstrated the detection of four
cancer markers in serum with femtomolar sensitivity [ 125 ] (Fig. 14 ). This
approach is known to suffer from high levels of background signal in whole blood
samples, but recently a label-free electrochemical detection has been demonstrated
under physiological conditions [ 101 ]. Stern et al. used a microfluidic purification
chip to isolate antigens from whole blood and achieve quantitative detection of
two cancer markers in less than 20 min using only 20 ll of whole blood.
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