Biomedical Engineering Reference
In-Depth Information
Selvaraju et al. (2007) point out that rapid and highly selective reactions can be applied to
signal amplification in biosensors using semi-heterogeneous nanocatalysts ( Daniel and
Astruc, 2004; Astruc et al., 2005 ). Das et al. (2006) and Lin et al. (2006) have used catalytic
reactions mediated by a nanocatalyst to detect proteins and nucleic acids. Selvaraju et al.
(2007) report that an ultralow detection limit is achievable using their magnetic beads assay.
Their method exploits the ease of separation and immunoreactions of the magnetic beds,
followed by signal amplification by the gold nanoparticles.
The authors used 8.1
0.8 nm size nanoparticles. They also covalently conjugated IgG to the
magnetic bead-Tosyl. According to the manufacturers, 40 m g/ml per mg of magnetic beads is
optimal for the conjugation of IgG to the magnetic beads. They further indicate that an anti-
mouse IgG-gold nanoparticle conjugate was prepared by direct adsorption of the IgG on the
8.1 nm gold nanoparticles ( Roth, 1982; Hermanson et al., 1996 ; Katz and Willner, 2004 ).
They used 8.1
0.8 nm gold nanoparticles. They report that a concentration of 40 m g IgG
per I mg of magnetic beads was generally used to conjugate IgG to the magnetic beads. They
further report that the IgG-magnetic beads conjugate is stable for at least 3 months if stored at
4 C. IgG was also directly adsorbed on the 8.1 nm gold nanoparticles to prepare the anti-
mouse IgG-AuN conjugate ( Roth, 1982; Hermanson, 1996; Katz and Willner, 2004 ). The
authors noticed that there was no significant change in the activity of the IgG-AuN (gold
nanoparticles) for about thirty days when the conjugate was used in the immunoassay.
To prepare the Fc-D (ferrocenyl-tethered dendrimer) modified electrode, the Fc-D was
immobilized on an ITO, using covalent bonding between the dendrimer amines and the car-
boxylic acids of a phosphonate dendrimer assembled monolayer ( Kim et al., 2003; Kwon
et al., 2006a,b ). Selvaraju et al. (2007) point out that bare ITO electrodes are not good for
the p -aminophenol (AP) as electrooxidation occurs at high potentials, where the background
current for electrolytes is high. Fc-D may be used to shift the oxidation potential to a less
positive value. These authors also indicate that the target protein is captured by both an
IgG-magnetic bead conjugate and an IgG-AuN conjugate. This forms the immunosensing
complex. This immunosensing complex is attracted to the Fc-D modified ITO electrode by
an external magnet. Praharaj et al. (2004) and Das et al. (2006) have reported that the gold
nanoparticles of the immunosensing complex generates p- AP by the catalytic reduction of
p -nitrophenol. This is a very fast reaction. This, along with the redox cycling of AP, note
Selvaraju et al. (2008), promotes signal amplification.
They recommend that to obtain low detection limits, the NSB between the Ig-G-gold
nanoparticles (AuN) and the Ig-G-magnetic beads (MB) along with the background current
should be minimized. They also report that the higher density of the immunosensing
complexes near the Fc-D modified electrode leads to a higher concentration of the p -AP gen-
erated by the gold nanoparticles (Au-N). Subsequently, that increases the anodic current of
the AP.
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