Mark E. Greene

Detecting proteins with excitons and plasmons: Nanobiotechnology

Researchers at the University of Michigan, Ohio University, and Pusan National University in Korea have constructed molecular spring assemblies to optically detect proteins in solution [Lee et al., Nat. Mater. (2007) 6, 291]. The assemblies are composed of Au nanoparticles attached to CdTe nanowires via poly(ethylene glycol), or PEG, linkers conjugated with antibodies. The device relies on the interaction between excitons, bound electron and hole pairs in the semiconducting nanowires, and plasmons, quantized oscillations of the conduction electrons in the metallic nanoparticles. The researchers conjugated antistreptavidin to the PEG linkers and then exposed the nanostructures to solutions with varying concentrations of streptavidin. When the nanowires are excited optically, shifts of up to 10 nm in the emission spectra are observed. The researchers attribute this shift to changes in the distances between the nanoparticles and nanowires and have developed a theoretical framework to explain the sensor effect in terms of the exciton mobility. “For a strong effect, the exciton drift time inside a nanowire should be comparable with the exciton lifetime,” explains Alexander O. Govorov of Ohio University. Decreasing the distance between the nanoparticle and nanowire decreases the exciton lifetime, leading to a blue shift in the emission spectrum of the exciton. The choice of nanowire and nanoparticle material are critical parameters to device performance. Replacement of Au with Ag nanoparticles, for example, would change the resonance frequency of the plasmons, explains Nicholas A. Kotov of the University of Michigan. “The exciton frequency then has to be changed to match it, in order to observe the same effects.” Govorov adds that to observe the blue shift, the exciton and plasmon energies should be close. “For Au nanoparticles and CdTe nanowires, this resonance is achieved,” he says. “Ag nanoparticles have a higher energy plasmon, so nanowires with a higher energy exciton such as InGaN are required.” Mark E. Greene