Biomedical Engineering Reference
In-Depth Information
to 500 nm have also been used [
64
,
65
]. An antibody-based recognition element was
incorporated inside the nanopores for capturing the target proteins. The electroless
plating of gold was performed on nanopores, and antibodies were immobilized by
using thiol-gold linkage chemistry. Human Insulin was used as a target
molecule. Mass spectroscopy was used to analyze the selective capturing of
the insulin. A 20-fold increase in mass spectroscopy intensities were reported.
Brogan et al. reported on the immobilization of rabbit anti-calf alkaline phosphates
(RACAP) Immunoglobulin G (IgG) on a surface modified with gold and made
use of thiol-gold interaction [
64
]. They showed that immunosurfaces with higher
antigen binding ratios can be made with specifically immobilized fragments as
compared with random immobilized fragments.
Transducing ligand binding mechanisms into electrochemical signals using nano-
porous polycarbonate membranes have also been reported [
66
,
67
]. The nanopores
were first coated with gold and the diameter was reduced to 30 nm. To improve
wetting, the sample was immersed into 10 mMTris solution for 2 h. Solution enriched
with galactose/glucose receptor (GGR) protein was added on the porous membrane.
The chip was left for a few hours for the diffusion of protein inside the nanopore.
After washing, new solution with D-glucose was added to one compartment of
the conductive cell. Upon ligand binding, hinge-twist motion was observed which
reduced the nanopore diameter and subsequently decreased the ionic current.
They also reported a reduction in sensitivity with increasing nanopore diameter.
They called their biosensor a “reagent-less electrochemical biosensor”.
SSNs also have a number of applications to be used as a biosensor for detecting
relatively large targets of size 10-300 nm. Uram et al. used relatively larger
nanopores (diameter of 650 nm) for selectively detecting icosahedral chlorella
virus [
68
]. Fabrication of a larger nanopore is a less demanding process. Antibodies
were immobilized on the walls of nanopores that helped in selective detection
of viruses based on resistive pulse sensing. Increases in the amplitude of the
resistive pulse were reported on the selective binding of antibodies and virus
particles. They reported that a virus concentration of 5
10
7
virus particle/mL
could be successfully analyzed by their device.
5.6 Summary and Targets
It has been shown that nanopores can be created using standard photolithography
followed by drilling with FIB. They can then be shrunk down to smaller diameters
through the use of either TEM shrinking or plasma polymer film (PPF) deposition.
Smaller nanopores are better because of the decreased translocation time,
thus allowing for more accurate sensing. Selective sensing is achieved by the
application of DNA probes in the nanopores, discriminating down to single-base
mismatch in the target molecules.
A target to meet in the future is to increase the detection resolution for the
functionalized nanopores. As it stands, DNA translocates very fast through a
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