Optimization of nanoparticles for label free detection of single molecules via plasmonic resonances

Metal nanoparticles can show pronounced scattering and absorption maxima at certain wavelengths. At this wavelengths the electromagnetic field is strongly enhanced in volumes much smaller than the diffraction limit. This resonances are called localized surface plasmons and can be used to detect local refractive index changes in their modal volume by spectral shifts of the resonance. Extremely small amounts of molecules can be detected in this way without fluorescence labelling. In this work we show that principally the detection of a single molecule is possible. To reach this goal, the detection process muss be understood and optimized. We analyse this process with the help of perturbation theory and statistics. Absolute detection limits and optimization strategies for the nanoparticles as well as the optical setup are discussed. The concept is verified and illustrated by an analysis of crescent shaped gold nanoparticles. For this model system our approach is compared with experimental results and rigorous calculations based on the finite difference time domain method.