Due to tunable resonance frequency and compatible size, gold nanoparticles can act as labels for biological cells, enhancing Raman signals for molecular imaging and in vivo study. Nanoscale imaging of the cell systems typically requires scanning electron microscopy, which can only resolve dried cells, thus limiting its usefulness for biological applications. Likewise, plasmon-based sensing with a nanotip using external illumination generates large background energy levels, resulting in low signal-to-noise ratios.

Professors Li Shi and John R Howell at UT-Austin, along with Dr Alex Heltzel of PCKA, have conceived of a system to introduce a sub-diffraction limit input signal to a nanotip waveguide, allowing for greatly enhanced plasmonic coupling and reduced background noise. The team combined Mie scattering theory and a rigorous Maxwell equation solver to design a microsphere coupler suitable to pre-scatter an optical pulse for introduction to a 100 nm zinc-oxide nanowire waveguide. Introducing this signal directly to the waveguide greatly boosts the signal-to-noise ratio at the tip/biomarker junction, and is only possible through the microsphere scattering mechanism above the opaque silicon cantilever. The 3D simulations were distributed across two dozen processors for several days, predicting imaging fidelity of only ~25 nm.

The group has collaborated on the sensor design with researchers at the UT M.D. Anderson Cancer Center, and hopes that the microsphere coupler design will lead to in situ study of cells in their natural liquid environment. Prof. Li Shi's nanofabrication group at UT-Austin is currently refining the prototype sensor based on the microsphere coupler design.

The researchers reported their work in the journal Nanotechnology.