Near-field penetrating optical microscopy: A live cell nanoscale refractive index measurement technique for quantification of internal macromolecular density

Quantification of intracellular nanoscale macromolecular density distribution is a fundamental aspect to understanding cellular processes. We report a near-field penetrating optical microscopy (NPOM) technique to directly probe the internal nanoscale macromolecular density of biological cells through quantification of intracellular refractive index (RI). NPOM inserts a tapered optical fiber probe to successive depths into an illuminated sample. A 50nm diameter probe tip collects signal that exhibits a linear relationship with the sample RI at a spatial resolution of approximately 50nm for biologically relevant measurements, one order of magnitude finer than the Abbe diffraction limit. Live and fixed cell data illustrate the mechanical ability of a 50nm probe to penetrate biological samples.