...all that is gold does not glitter, not all those who wander are lost..

SERS for Membrane Structural Biology
Surface enhanced Raman scattering (SERS) provides vibrational spectra of molecules near gold nanoparticles, enabling specific chemical detection with simple optical measurements. The past decade has produced thousands of reports on increasing SERS sensitivity for the detection of small molecules such as toxins and explosives. However, SERS also serves as a powerful analytical tool to study chemical interfaces. We used SERS to study the surfactants that surround gold nanorods in solution, and discovered a structural transition that greatly affects further processing of the nanorod surface chemistry for biomedical applications (Lee 2011). Key to this discovery was the rapid decay of the SERS enhancement factor with distance from the nanoparticle surface. This spatial dependence can be used to determine structure in the chemical interface without molecular probes or labels. We are now applying SERS to study biomembrane structure, an area in great need of new characterization tools. We have successfully displaced the surfactant layer on gold nanorods with phospholipids and have confirmed the natural bilayer structure (submitted).



Gold Nanobelts
Long gold wires with nanoparticle-sized cross sections exhibit tunable plasmon resonances like nanorods and other shapes. We described these tunable resonances based on dark field microscopectroscopy (Anderson 2011). We also described interesting structures such as tapered and branched nanobelts (Payne 2013), as well as how the synthesis depends on the complex behavior of wormlike micelles (Payne 2014). Gold nanobelts also serve as plasmonic waveguides that provide strong quantum confinement of light due to their small size and cross sectional geometry (Anderson 2013).




Membrane Electrostatics
The atomic force microscope is highly sensitive to forces and can operate with the tip in solution. We used the AFM to map lateral charge distributions on biological samples in dilute electrolyte, including lipid membranes and DNA (Johnson 2003). An unexpected repulsive interaction between the negatively charged probe tips and zwitterionic lipids was found, which we showed was due to the lipid membrane dipole potential (Yang 2007, Yang 2008). The dipole potenial is a large internal potential different at the membraen interfacial region, which likely influences biomembrane function but is not widely studied because it is difficult to measure.




Plasmonic Biosensors
Plasmon resonances in gold nanoparticles are sensitive to the refractive index of their immediate nanoscale environment. The binding of molecules t the nanoparticle surface can therefore be detecting by simple spectral extinction measurements of the nanoparticles. In this way plasmonic nanoparticles can be used as low-tech biosensors. We demonstrated a real-time immunoassay based on this phenomenon that yielded the proper binding kinetics (Mayer 2008), and studied the sensitivity dependence on nanoparticle shape (Lee 2009). We also carried out the immunoassay with single nanoparticles to reach single molecule detection (Mayer 2010).



Single Particle Plasmonics
Most early measurements of nanoparticle plasmon resonances were from ensembles of similar nanoparticles suspended in colloidal solution. This left it unclear how the plasmon linewidth was affected by size differences among the nanoparticles and fundamental lifetimes of electronic excitations. We set up a dark field microspectrometer to measured the linewidth of the scattering from single gold nanoshells from Naomi Halas's group to shed some light on the matter (Nehl 2004). We also described the plasmonics of complex particles like gold nanostars (Nehl 2006).