Hafner Nano-Bio Lab
 
 

CURRENT RESEARH PROJECTS

 

Nanometer-scale tools and materials offer new insights into the workings of biological systems and can enable new sensing paradigms due to their unique physical properties. Our lab works at this Nano-Bio interface.

 
 

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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  Physics MS61; Rice University; 6100 Main St.; Houston, TX 77005; Anderson Biolab rm 302; p. 713-348-3205; f. 713-348-4150; e. hafner@rice.edu