Plasmon Resonant Silica-Coated Silver Nanoplates as Contrast Agents for Optical Coherence Tomography

Silica-coated silver nanostructures are identified as potential contrast agents for visible and near-infrared bio-imaging applications due to their high optical extinction caused by localized surface plasmon resonance (LSPR), improved chemical stability, and lower toxicity. We demonstrate the potential of plasmon resonant silica-coated silver nanoplates as a contrast agent for optical coherence tomography (OCT). It is shown that, triangular-shaped silica-coated silver nanoplates (SSNPs) with a side length of 170 +/- 5 nm, base silver thickness of 10 +/- 1 nm, and silica shell thickness of 40 +/- 2 nm, exhibit higher optical extinction at a 1300 nm wavelength range, thus making them an excellent contrast agent for OCT imaging. Optical extinction characterization using OCT is found to be reasonably consistent with ultraviolet (UV)-Vis-near infrared (NIR) spectroscopy and finite difference time-domain (FDTD)-based analysis. Ex vivo studies on pig adipose tissue demonstrate that LSPR-induced enhanced scattering in SSNPs contributes to the OCT signal, leading to images with better contrast. Moreover, average A-scan profiles acquired at different time delays show the downward propagation of SSNPs and the extension of signal enhancement at the deeper regions. Speckle variance OCT images show that SSNPs are efficiently distributed over the targeted tissue region, demonstrating their applicability in a large lesion area.