Lipid Membrane Electrostatics
The AFM has sufficient sensitivity to measure screened
electrostatic forces between the tip and sample in electrolyte
solutions. Using this interaction, we invented "Fluid
Electric Force Microscopy", which provides simultaneous
image topography and charge density contrast of biological
systems (Johnson
2003). When applied to model biomembranes, we find
charge contrast between the liquid-ordered and liquid-disordered
phases even though all of the lipids are zwitterionic,
and therefore expected to be neutral. Through force
spectroscopy we have determined that there is a repulsive
force interaction, and therefore negative effective
charge density, over the zwitterionic lipid membranes.
We are currently developing techniques to make quantitative
electrostatic measurements with the AFM and applying
them to supported lipid membranes.
Biological Nanophotonics
Noble metal nanoparticles exhibit strong optical extinction
at visible and near infrared (NIR) wavelengths due to
a localized surface plasmon resonance (LSPR) of their
free electrons. These unique optical properties and
their general lack of toxicity suggest several diagnostic
and therapeutic biomedical applications (review: Liao,
Nehl, Hafner NanoMedicine). Successful biomedical and
biological applications require materials that are well
controlled with well understood optical properties.
We therefore divide our efforts into three areas:
Synthesis and Manipulation
Gold nanoparticles can be synthesized by simple reduction
of gold ions in solution, resulting in spherical particles.
However, the addition of surfactants can direct the
growth of nanoparticles into complex shapes. Cetyltrimethylammonium
Bromide (CTAB) has been shown to produce gold nanorods
and other shapes with tunable optical properties. While
the surfactant clearly directs the growth, the detailed
chemistry is not understood. We have studied the synthesis
of gold nanorods both microscopically on surfaces and
spectroscopically in solution. For the latter, we developed
a quantitative analysis of the nanorod extinction spectra,
which yields the nanorod length and diameter throughout
the growth reaction. This provides microscopic growth
rates that we are now analyzing with traditional kinetic
models to understand these complex reactions.
In addition to enabling the synthesis of novel gold
nanoparticles, CTAB is also required for their stability.
If the concentration falls below 10 mM, CTAB synthesized
nanoparticles will aggregate. For biological applications,
this is a problem since CTAB is cytotoxic and would
likely interfere with nanoparticle/protein conjugation.
We have therefore worked out procedures to displace
the CTAB with more biologically friendly polymers (PEG).
Once free of their soapy prison, gold nanorods are prone
to processing, forming dense films on glass.
Optical Properties
Biosensing
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