The plasmon resonance of metal nanoparticles shifts upon refractive index changes of the surrounding medium through the binding of analytes. The use of this principle allows one to build ultra-small plasmon sensors that can detect analytes (e.g., biomolecules) in volumes down to attoliters. We use simulations based on the boundary element method to determine the sensitivity of gold nanorods of various aspect ratios for plasmonic sensors and find values between 3 and 4 to be optimal. Experiments on single particles confirm these theoretical results. We are able to explain the optimum by showing a corresponding maximum for the quality factor of the plasmon resonance.

Silver coating gold nanorods reduces the ensemble plasmon line width by changing the relation connecting particle shape and plasmon resonance wavelength. This change, we term "plasmonic focusing", leads to less variation of resonance wavelengths for the same particle size distribution. We also find smaller single particle linewidth comparing resonances at the same wavelength but show that this does not contribute to the ensemble linewidth narrowing.

We create large gold domains (up to 15 nm) exclusively on one side of CdS or CdSe/CdS quantum rods by photoreduction of gold ions under anaerobic conditions. Electrons generated in the semiconductor by UV stimulation migrate to one tip where they reduce gold ions. Large gold domains eventually form; these support efficient plasmon oscillations with a light scattering cross section large enough to visualize single hybrid particles in a dark-field microscope during growth in real time.

We compare the single-particle plasmonic sensitivity of silver and gold nanorods with similar resonance wavelengths by monitoring the plasmon resonance shift upon changing the environment from water to 12.5% sucrose solution. We find that silver nanoparticles have 1.2 to 2 times higher sensitivity than gold, in good agreement with simulations based on the boundary-elements-method (BEM). To exclude the effect of particle volume on sensitivity, we test gold rods with increasing particle width at a given resonance wavelength. Using the Drude-model of optical properties of metals together with the quasi-static approximation (QSA) for localized surface plasmons, we show that the dominant contribution to higher sensitivity of silver is the lower background polarizability of the d-band electrons and provide a simple formula for the sensitivity. We improve the reversibility of the silver nanorod sensors upon repeated cycles of environmental changes by blocking the high energy parts of the illumination light.

We create rod-shaped single crystalline gold-copper (Au x Cu (1-x) ) nanoparticles and verify the presence of copper in the particles with various direct and indirect optical and electron microscopy techniques. The particles grow from small, preformed gold seeds in a growth solution containing both copper and gold ions in the presence of a surfactant and a mild reducing agent. The presence of copper in the growth solution has a pronounced effect on the spectral characteristic of the resulting nanocrystals and reduces the total volume of the resulting particles. In contrast to spherical copper particles, our rod-shaped nanocrystals show a strong plasmon resonance and the copper content varies the plasmon resonance frequency and the plasmonic linewidth. Optical single particle plasmon-line-width observations suggest reduced plasmon damping at specific copper contents corresponding to stoichiometric particle compositions.