Dark-field spectroscopy on single plasmonic gold nanorods : new methods

Abstract

Nanomaterials find widespread application particularly as sensors in biochemistry and biophysics taking advantage of their sharp spectral plasmon absorption and scattering peak and its sensitivity to refractive index changes in the near surrounding of the particle. Having a plasmon resonance in the visible wavelength regime gold nanorods are used as refractive index sensors in single particle dark-field micro-spectroscopy, a technique that enables to resolve single particle plasmon shifts below 1 nm, which already permits single protein detection. A variety of possibilities to use dark-field microscopy on plasmonic gold particles is already reported reaching from in vivo diagnostics to basic research on plasmons. The improvement of these methods or development of new ones is a large field, reaching from sensor design to setup development.

For this purpose, I introduced silica coated nanoparticles as sensors in dark-field microscopy, which show versatile possibilities, although the accessibility of the sensor surface is decreased by a rigid but porous shell. Taking advantage of their porosity silica coated gold nanorods are able to filter out analytes exceeding a certain threshold in size and let smaller ones enter for detection, a principle highly requested for sensing in multi-component media like blood serum. Further having a rigid shell, whose thickness can be quantified in electron microscopy, these particles can be used to experimentally determine particle specific parameters like the sensing distance on single particle level.

As an improvement of an existing method (our so called NanoSPR), a technique to determine simultaneously the binding constants of an analyte with several receptors, I show that NanoSPR can also be used to identify the adsorbed species. Many proteins build dimers or higher ordered oligomers, whether between each other or with other components, but for some of those the reactive species is not yet identified. With an oligomer forming protein (IM30) as an example it was possible to identify the protomers or lower ordered oligomers of this protein as active binding species and determine their thermodynamic binding affinity from the same experiment.

Tuning a setup to high time resolution spectroscopy allows for measuring diffusion of particles through the sensing volume of a nanorod. I proved the feasibility of this method with the help of a model system of diffusing nanoparticles, whose diffusion times showed from physics expected dependencies. The consistency of this method was also shown by varying the nanoparticle material, size and their surface chemistry. Investigating the free diffusion of particles close to the sensor surface is the first step to investigate interactions between the particle (e.g. a protein) and the surface. If interactions play a role they lower the diffusion time of the system in thermodynamic equilibrium, an effect that can be used to extract binding constants free from unintended influence of mass transport. Measuring also weak interactions between the proteins and a surface covering molecule happening at short timescales is an interesting method in material science when working on anti-biofouling agents.