Biomedical Engineering Reference
In-Depth Information
affinity bioassays. Different inorganic colloid (QDs) nanocrystals (zinc sulfide,
cadmium sulfide, and lead sulfide) were used as encoding tags to differentiate
the signals of three DNA targets in connection to stripping voltammetric mea-
surements of the heavy metal dissolution products.
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These products yield well-
defined and resolved stripping peaks at −1.12V (Zn), −0.68V (Cd), and −0.53V
(Pb) at the mercury-coated glassy carbon electrode (vs. Ag/AgCl reference). The
position and size of these peaks reflect the identity and level of the corresponding
DNA target. The protocol is illustrated for the simultaneous detection of three
DNA sequences related to the BCRA1 breast cancer gene in a single sample in
connection to magnetic beads bearing the corresponding ONT probes. An effec-
tive and inexpensive multitarget electrochemical immunoassay based on the use
of similar tags is also reported.
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This multiprotein electrochemical stripping
detection capability with femtomolar, fmol, detection limits is also combined
with the efficient magnetic separation that minimizes nonspecific binding effects.
4.7.2.4 Potentiometric Analysis
Potentiometric analysis involving NP-based detection of protein and other bio-
molecules is another technique used with various possible medical applications.
Potentiometric sensors based on polymer membrane electrodes provide electro-
chemical signal that are extremely useful for measurements in small volumes.
Electrodes for potentiometric detection of cadmium ions with a detection limit
of 6 nM utilizing a Na
+
-selective electrode as pseudoreference facilitated mea-
surements in 150-µL samples.
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This system was applied in a potentiometric
immunoassay of mouse IgG using CdSe QD labels on a secondary antibody in a
sandwich immunoassay. The CdSe QDs were easily dissolved/oxidized in min-
utes with hydrogen peroxide maintaining a near-neutral pH. This potentiometric
immunoassay exhibited a detection limit of <10 fmol in 150-µL sample wells. This
system has the potential for detection of medically relevant protein biomarkers.
Another method for the sensitive sandwich assay detection of proteins
through the use of NMs of gold that were silver enhanced was detected oxida-
tively with silver ion-selective microelectrode.
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In this system, potentiometry
was used for ultrasensitive NM-based detection of protein interactions. This
approach is anticipated for bioassays with attractive detection limits for highly
sensitive bioaffinity assays.
4.7.2.5 Electrocatalytic Methods
The group of Polsky et al.
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have developed a new approach for electrochemical
immunoassay sensing in which Pd NPs were loaded onto an anti-TNF-α (tumor
necrosis factor-α) antibody to create an electrocatalytic antibody. Gold particles are
first covalently linked to the antibody that then act as a seed for growth of a palla-
dium shell. The Pd NPs have proven to be sensitive to the oxygen reduction reaction,
thereby requiring no additional reagents except the naturally present oxygen in the
solution. Electrochemical techniques like linear sweep voltammetry showed excel-
lent catalytic activity for the Au/Pd-modified antibody toward oxygen reduction. A
