Biomedical Engineering Reference
In-Depth Information
prepared by anodic oxidation of titanium as follows: Titanium foil (0.80 mm thick) was
ultrasonically cleaned in distilled water and in acetone for 20 min. Next, after rinsing thor-
oughly the clean titanium foil electrodes (3
10 mm, geometrical area ca. 0.6 cm 2 ) was
first etched in an 18% HCl solution at 85ÂșC for 10 min. The etched titanium foil was
washed again and then subjected to anodization in a 1:8 acetic acid-water solution
containing 1.0 vol% hydrofluoric acid at 20 V for 45 min. This resulted in TiO 2 nanotube
arrays on the Ti substrate. The nanotubes were then applied to adsorb peroxidase enzyme.
Immobilized thionine on the surface was found to be electrochemically reduced. This
study demonstrates a method for biosensor design using a material with high surface area
and low electrical conductivity.
7.1.3
Ferromagnetic Particles
Recently magneto-electronics has shown great potential for biosensor design (9). This
approach is based on the interaction of magnetically labeled biomolecules with a magnetic
field sensor. These are termed as magneto resistive sensors. Magnetic nanoparticles coated
with biochemical ligands are being applied in such sensors for numerous biological and
medical applications. In particular biomagnetic biosensors can provide simple design and
rapid analysis. One of the approaches applied for magnetic bead labeled detection is based
on the principle of giant magneto-resistance (GMR) (10,11). When an external magnetic
field is applied to layers of alternating magnetic and nonmagnetic thin films (e.g., Fe and
Cr), this leads to a decrease in resistance of the thin films. This effect known as GMR
occurs due to the scattering of the electrons in the layers, which is influenced by the ori-
entation of the electron spin with respect to the magnetization direction of the magnetic
layers. Application of the external applied field aligns the magnets of all the layers in one
direction, and thus the scattering for electrons of one of the spins is reduced leading to the
reduction in electrical resistance. This detection principle has advantages over established
methods, and the sensitivity is promising. Several challenges that need to be met include
the design of suitable magnetic nanoparticles and their functionalization. Sensitive GMR
magnetic field sensors can be combined with suitable detection protocols such as
antibody- or antigen-labeled magnetic immunosensors.
In a separate approach for biosensor design magnetic beads have been utilized for the
immobilization of biomolecules. Recently, a phenol biosensor was developed based on the
immobilization of tyrosinase on the surface of modified magnetic MgFe 2 O 4 nanoparticles
(12). The tyrosinase was first covalently immobilized to core-shell (MgFe 2 O 4 -SiO 2 ) mag-
netic nanoparticles, which were modified with amino group on its surface. The resulting
magnetic bionanoparticles were attached to the surface of carbon paste electrode with the
help of a permanent magnet. The immobilization matrix provided a good microenviron-
ment for the retention of the bioactivity of tyrosinase. Phenol was determined by the direct
reduction of biocatalytically generated quinone species at
150 mV versus SCE. The
resulting phenol biosensor had high tyrosinase loading of the immobilization matrix.
Chung et al. (13) have demonstrated another method utilizing a substrateo-free bio-
magnetic sensing, which depends on the magnetic susceptibility of ferromagnetic particles
suspended in a liquid. The binding of the particles to the intended analyte modifies the
magnetic relaxation of these particles. This scheme has several advantages: (i) it requires
only one binding event; (ii) there is an inherent check of integrity; and (iii) the signal
contains additional information about the size of the target analyte.
In another recent report, magnetite (Fe 3 O 4 ) nanoparticles (average size 12 nm) modified
with electroactive Prussian blue (PB) were synthesized (14). The magnetic properties of the
sample were investigated by low-field alternating current susceptibility and
superconducting quantum interference device measurement; the results indicated super
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