15 Smart Platforms for Biomedical Applications
359
15.1.1.5 Biosensing
The sensing of biological analytes is at the core of clinical diagnostics. Magnetic
nanoparticles typically used in the biosensing space are iron oxide based, either
in core shell structures or functionalized for stability in solution [86]. There are a
variety of sensing device architectures for magnetic particles [30, 33, 34, 87–91]
but the primary architecture of interest has been magnetoresistance-based devices
[92]. An example of such a system would be a giant magnetoresistance (GMR) or
spin valve sensor-based assay [32, 93–96]. These are extremely sensitive magnetic
field sensors, and here the detection of the analyte is determined by the presence
of the stray field of a magnetic nanoparticle near the surface of the sensor. The
localization of the nanoparticle on the sensor occurs by specific binding events
between biomolecules such as antibody–antigen binding or complementary nucleic
acid sequences [93, 97, 98], where the sensor and nanoparticle are both functionalized
with the appropriate biomolecule. Magnetic approaches may be the most relevant
for assays in biological samples that do not allow for standard optical detection
techniques such as fluorescence. Line-of-sight considerations are generally eliminated with magnetic signals and as such magnetic nanoparticle-based assays may
be relevant for sensing molecules directly in fluids such as blood [98]. References
[82, 88, 89] and [91] provide a broader review of sensing device architectures and
nanoparticle formulations in this space (Fig. 15.3).
15.2 Lithographically Defined Particles
Solution suspended lithographically defined magnetic micro- and nanodiscs are a
relatively recent development [36, 99–102]. They have emerged because of the
interest in transferring the knowledge gained in the fabrication of magnetic memory
and logic devices to the biotechnology field. Lithographically defined particles are
planar in shape and are generally in the range of 200 nm–2 μm in planar dimensions
and 10–200 nm in thickness.
These microdiscs are fabricated with standard lithographic techniques, where
most generally a magnetic thin film is grown via physical vapour deposition on
patterned photoresist that is spun cast on a silicon wafer. The photoresist is then
dissolved and the structures atop the resist are lifted off into solution. The control
of magnetic thin-film growth allows for magnetic properties that can be engineered
to extreme precision, with resolutions for individual layer thicknesses in the sub-nm
range.
The primary challenge with lithographic microdiscs is ensuring a zero remanent
magnetization state since they are far too large to be superparamagnetic in nature, and
their stray fields are easily sufficient to drive strong magnetic agglomeration. There
are two primary approaches used to create microdiscs that do not agglomerate.
359
15.1.1.5 Biosensing
The sensing of biological analytes is at the core of clinical diagnostics. Magnetic
nanoparticles typically used in the biosensing space are iron oxide based, either
in core shell structures or functionalized for stability in solution [86]. There are a
variety of sensing device architectures for magnetic particles [30, 33, 34, 87–91]
but the primary architecture of interest has been magnetoresistance-based devices
[92]. An example of such a system would be a giant magnetoresistance (GMR) or
spin valve sensor-based assay [32, 93–96]. These are extremely sensitive magnetic
field sensors, and here the detection of the analyte is determined by the presence
of the stray field of a magnetic nanoparticle near the surface of the sensor. The
localization of the nanoparticle on the sensor occurs by specific binding events
between biomolecules such as antibody–antigen binding or complementary nucleic
acid sequences [93, 97, 98], where the sensor and nanoparticle are both functionalized
with the appropriate biomolecule. Magnetic approaches may be the most relevant
for assays in biological samples that do not allow for standard optical detection
techniques such as fluorescence. Line-of-sight considerations are generally eliminated with magnetic signals and as such magnetic nanoparticle-based assays may
be relevant for sensing molecules directly in fluids such as blood [98]. References
[82, 88, 89] and [91] provide a broader review of sensing device architectures and
nanoparticle formulations in this space (Fig. 15.3).
15.2 Lithographically Defined Particles
Solution suspended lithographically defined magnetic micro- and nanodiscs are a
relatively recent development [36, 99–102]. They have emerged because of the
interest in transferring the knowledge gained in the fabrication of magnetic memory
and logic devices to the biotechnology field. Lithographically defined particles are
planar in shape and are generally in the range of 200 nm–2 μm in planar dimensions
and 10–200 nm in thickness.
These microdiscs are fabricated with standard lithographic techniques, where
most generally a magnetic thin film is grown via physical vapour deposition on
patterned photoresist that is spun cast on a silicon wafer. The photoresist is then
dissolved and the structures atop the resist are lifted off into solution. The control
of magnetic thin-film growth allows for magnetic properties that can be engineered
to extreme precision, with resolutions for individual layer thicknesses in the sub-nm
range.
The primary challenge with lithographic microdiscs is ensuring a zero remanent
magnetization state since they are far too large to be superparamagnetic in nature, and
their stray fields are easily sufficient to drive strong magnetic agglomeration. There
are two primary approaches used to create microdiscs that do not agglomerate.