The wait for robust nanoparticles with high extinction cross sections in a 1300-nm window is over. The opportunity for new exciting biomedical applications in this wavelength range is now open—thanks to Cheng-Kuang Lee and his colleagues from the National Taiwan University and the Chang Gung University, who introduced to the field of biomedical optics a new and promising type of biocompatible nanoparticles: gold nanorings with localized surface plasmon response (LSPR) at 1240 nm.

Most gold nanoparticles currently being used, including nanospheres, nanoshells, nanorods, and nanocages, offer LSPR below 900 nm. The gold nanorings, by contrast, can be easily fabricated to operate in the 1300-nm wavelength range. This spectral range is very important for bio-optical applications owing to the weak tissue scattering and consequently relatively long penetration depth. This makes gold nanorings attractive candidates for contrast-enhancement agents in optical imaging [especially optical coherence tomography (OCT)], photothermal therapy, and drug delivery.

In this paper, authors describe in detail the preparation process of a water solution of gold nanorings with two different LSPR wavelengths (1030 and 1240 nm). Then, by implementation of OCT, the extinction cross sections at 1310 nm for these two solutions were estimated and compared with those obtained from transmission measurements and numerical simulations. Both solutions were then independently delivered into a mouse liver, which was then continuously imaged with OCT, showing the successful tracking of the diffusion of those nanoparticles with LSRP close to the OCT imaging wavelength.

OCT technology, despite the recent progress in acquisition speeds and sensitivity, still suffers from a lack of good contrast agents because of the requirement of their coherent scattering. Therefore, any type of nanoparticle that scatters light in the OCT spectral range is a good candidate to be used in this field. This is especially true for the 1300-nm OCT imaging systems that are nowadays most commonly used for biomedical applications. As an interesting example, Cheng-Kuang Lee et al. demonstrate in this manuscript a method for tracking the diffusion of the nanorings in tissue. Additionally, by calculating a speckle intensity variance on consecutive OCT B-scans, one can find the local transport speed of nanoparticles.

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