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In the near-infrared optical window (NIR, 650 to 1000 nm), biological tissue generates a low intrinsic photoacoustic (PA) signal. This window can be utilized to reduce the background noise, and thus enhance visualization of contrast agents. Exogenous contrast agents have been investigated and co-applied with photoacoustic imaging such as organic materials (indocyanine green, astaxanthin, Prussian blue) and inorganic materials (gold nanorods, gold nanostars). However, these available contrast agents are usually associated with low thermal stability or large size, leading to unreliable photoacoustic signal. Conventional gold nanoparticles have an absorption peak of 520 nm, which corresponds to the absorption spectrum of hemoglobin. This study investigated the synthesis of novel ultrapure, biocompatible, and photostable chain-like gold nanoparticles (CGNPs), which shift the peak absorbance of GNPs from visible window (520 nm) to NIR window, while keeping the GNPs at a smaller size. These GNPs were fabricated by a femtosecond laser and were combined together with two organic polymers. The surface was then modified with PEG and conjugated with RGD ligands. The capacity of CGNPs for photoacoustic microscopy (PAM) and OCT were examined on 4 white New Zealand rabbits using a choroidal neovascularization (CNV) model. CNV was created by laser-induced retinal vein occlusion. Then, all animals were administered GNPs at concentration of 5 mg/mL. PAM and OCT were obtained before and after the injection at various time points, including 2 h, 4 h, 8 h, 24 h, days 2, 3, 5, 7, 9. 11, and 14. In vivo PAM and OCT imaging demonstrated that CNV was observed after the injection of CGNPs. In comparison with the signal before the injection, CGNPs produces 18- fold greater photoacoustic contrast and exhibits a 176 % increase in OCT signal, given the reduced background signal in the NIR window. The newly fabricated CGNPs have the capacity to improve visualization in living animals, while minimizing signal from hemoglobin and other endogenous contrast agents.
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