Noninvasive measurement of the refractive index of cell organelles using surface plasmon resonance microscopy
The goal of in-vivo imaging techniques is to extract useful information from living cells and organisms in the most gentle and non-invasive way possible. As a result, there are benefits to label-free techniques that utilize reflected light to measure the refractive index (RI) of cells, as opposed to fluorescence or other luminescence-based contrast mechanisms. Several well-established label-free techniques include quantitative phase imaging (QPI), optical diffraction tomography (ODT), and total internal reflection microscopy (TIR). While all of these techniques are able to quantify the RI in living cells, each has distinct drawbacks, which has limited their overall utility. For example, QPI is not well-suited for visualizing the RI of complex structures, and TIR is unable to provide sub-micron resolution. Here, Hassani and Kreysing present a surface plasmon resonance microscopy (SPRM) method that is able to quantify the RI of complex cell organelles with ~200 nm resolution. The technique parallels TIR, but achieves increased resolution by exciting localized plasmons on high-index coverslips coated by a thin layer of metal. In addition, by increasing the density of scanning points, the current SPRM system is able to overcome the speed limitations of previous scanning systems. It is expected that the unique capabilities of this technology will provide valuable insights into a variety of biological questions in the future.