Biomedical Engineering Reference
In-Depth Information
on the TNT surface in phosphate-buffered saline (PBS) solution that could
favor enzyme adsorption from electrostatic interactions between the negatively
charged groups on the TNT surface and the positively charged surface lysine
and arginine residues of lactate oxidase. As a result of this, a large quantity of
enzyme can be loaded into the electrode due to the large specific surface area
of the individual TNTs and the unique porous 3D network. Furthermore, this
architecture provides a friendly microenvironment for the immobilization of
enzymes that retain their biocatalytic activity. In their studies, cyclic voltam-
metry (CV) and amperometry tests revealed that the enzyme immobilized on
the TNTs maintained their substrate-specific catalytic activity. This structure
formation directly resulted in the excellent performance of the biosensor giv-
ing a sensitivity of 0.24 µA/cm
2
/mM, a 90% response time of 5 s, and a linear
response in the range from 0.5 to 14 mM.
4.4 COVALENT BINDING
An alternative way to avoid the instability and inactivation of biomolecules is
to covalently bind these to spacer or linker molecules that are attached to the
NM surfaces.
6,7,11,12,41,70,71
Low-molecular weight bifunctional linkers, which
have the anchor groups for their attachment to NM surfaces and the functional
groups for their covalent coupling to the target biomolecules, have been exten-
sively used in the generation of covalent-tethered conjugates of biomolecules
with various NMs.
72
Anchor groups such as thiols, disulfides, or phosphine
ligands are often used for the binding of the bifunctional linkers to Au, Ag,
CdS, and CdSe NPs.
73
Various terminal functional groups such as amine, active
ester, and maleimide groups are commonly used to covalently couple biological
compounds by means of carbodiimide-mediated esterification and amidation
reactions or through reactions with thiol groups.
72
Superparamagnetic NMs as labels or as capture surfaces in biosensors has
become a very important tool in research and in medicine.
74-76
These magnetic
NMs are especially designed for concentration, separation, purification, and
identification of molecules and specific cells. Magnetic separation technology
involving NMs is a simple technique based on a two-step process:
(1)
tagging or labeling of the desired biological entity with magnetic material and
(2)
separating these tagged entities using a magnetic separation device.
The magnetic NMs can be used as the binding platform or linker between the
analyte and the label (
Figure 4.4
). The strong and specific antibody-antigen
interaction provides a highly accurate way to label cells.
77
These magnetic NMs
coated with immunospecific agents have been successfully used to monitor lung
cancer cells
78
and breast cancer cells.
69,74,75
Hassen et al.
79
reported a new approach based on DNA hybridization for
detecting hepatitis B virus (HBV) and HIV virus involving non-faradic electro-
chemical impedance spectroscopy using modified magnetic NMs for the solid
