Localized surface plasmon enhanced cellular imaging using random metallic structures

We have studied fluorescence cellular imaging with randomly distributed localized near-field induced by silver nanoislands. For the fabrication of nanoislands, a 10-nm silver thin film evaporated on a BK7 glass substrate with an adhesion layer of 2-nm thick chromium. Micrometer sized silver square pattern was defined using e-beam lithography and then the film was annealed at ∼ 200°C. Raw images were restored using electric field distribution produced on the surface of random nanoislands. Nanoislands were modeled from SEM images. 488-nm p-polarized light source was set to be incident at 60°. Simulation results show that localized electric fields were created among nanoislands and that their average size was found to be ∼135 nm. The feasibility was tested using conventional total internal reflection fluorescence microscopy while the angle of incidence was adjusted to maximize field enhancement. Mouse microphage cells were cultured on nanoislands, and actin filaments were selectively stained with FITC-conjugated phalloidin. Acquired images were deconvolved based on linear imaging theory, in which molecular distribution was sampled by randomly distributed localized near-field and blurred by point spread function of far-field optics. The optimum fluorophore distribution was probabilistically estimated by repetitively matching a raw image. The deconvolved images are estimated to have a resolution in the range of 100-150 nm largely determined by the size of localized near-fields. We also discuss and compare the results with images acquired with periodic nanoaperture arrays in various optical configurations to excite localized plasmonic fields and to produce superresolved molecular images.