Nonlinear Scattering of Near-Infrared Light for Imaging Plasmonic Nanoparticles in Deep Tissue

Abstract

Nonlinear optical microscopy can obtain three-dimensionally resolved images within a specimen by exploiting the nonlinear light–matter interaction between the excitation light and sample. However, the image contrast significantly degrades with increasing observation depth because the emitted signal attenuates during light propagation through layers of the tissue before being detected. To obtain high contrast images from deep tissue, we developed saturated excitation (SAX) microscopy using the nonlinearity of near-infrared (NIR) plasmonic scattering from gold nanoshells and gold nanorods. SAX microscopy selectively detects the nonlinear component from the scattering signal generated by nanoparticle probes located at the center of the focal spot. By using this technique, background signals generated at out-of-focal positions are effectively removed. In addition, emitted signals in the NIR from nanoparticle probes efficiently transmit through biological tissue and are ideally suited to image deep parts of the tissue. We experimentally confirmed that scattering intensities from a single gold nanoshell and gold nanorod exhibit nonlinear relations with the excitation intensity of CW laser light at 780 and 1064 nm, respectively. We also demonstrated improvements of image contrast and spatial resolution at the depth of 400 μm in a phantom of muscle tissue by selectively detecting the nonlinear scattering signal component from gold nanoshells.