Biomedical Engineering Reference
In-Depth Information
with organic nanostructures that are designed specifically for biofunctional-
ization, with the overall goal of attaching them within neural cells configur-
ations. The structure of semiconducting nanoparticles enables the generation of
excitons, which are very sensitive to the external electric field. This sensitivity
can turn these nanoparticles into reporters with externally modulated fluor-
escence intensity spectra. They may be combined with selective molecular
binding moieties to confer sensitivity to changes in local neurotransmitter
concentrations. Quantum dots can be used as local optical reporters for
neuroscience, and for visualizing dynamic molecular processes in neurons and
glia on a large time scale, starting from seconds to many minutes, and on the
small size scale of the synaptic cleft (20 nm) of neuron-neuron interactions or
intracellular processes. Due to their intrinsic voltage sensitivity, they could be
used directly as optical readouts of membrane potential. These reporters must
be embedded into neural membranes (thickness
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2 nm) and able to react to
local electric fields as well as local chemical environments. Functionalization
with specific proteins enables the quantum dots to track receptors and func-
tional responses in neurons (e.g. to glycine, nerve growth factor, glutamate,
etc.).
Recent work using tools from atomic physics has shown that optically
manipulated color centers in diamond provide exceptionally sensitive magnetic
and electric field probes at sub-100 nm distances.
3
Diamond is uniquely suited
for studies of biological systems because it is chemically inert, cytocompatible,
and ideal for coupling to biological molecules. As a word of caution, it is
becoming increasingly apparent that cellular damage may well be a distinct
possibility as a consequence of interaction with nanoparticles. Studies of such
effects are now included in the field known as nanotoxicology.
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4.2 Nanotubes and Nanowires
Nanomaterials that can provide nanoscale topographical features have become
popular materials because culture substrates with nanoscale features have
significantly different effects on neuronal adhesion and growth. Vertical
nanowires were shown to selectively promote neuronal adhesion and guide
neurite growth even without any cell-adhesive coating.
4,5
Micropatterned
islands of tangled carbon nanotubes also showed similar spontaneous adhesion
and growth effects.
6
Guided neuronal growth was reported on various nano-
topographical substrates made of nanomesh carbon nanotubes,
7
electrospun
nanofibers
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or patterned polyurethane acrylate.
9
One-dimensional structures such as nanotubes and nanowires may be used
for highly local electrical measurements, for the delivery of photons to specific
locations, and for the local release or collection of chemicals. These types of
nanodevices could be used alone or combined with conventional organic
chromophores, which have been shown to greatly enhance optical signals,
hence acting as 'antennae' for light.
10,11
Indeed, membrane-bound and
antibody-linked gold nanoparticles have been already used for site-specific
measurements of membrane potential. Traditional organic chromophores have
