Coupling gold nanospheres into nanochain constructs for high-contrast, longitudinal photoacoustic imaging

Introduction: Photoacoustic (PA) imaging combines the advantages of ultrasound (US) and optical imaging, and has demonstrated huge benefits as non-invasive, non-ionizing, and real-time imaging modality. Exogenous contrast agents exhibiting strong optical absorption in the near-infrared (NIR) optical window ranging from 650 nm to 1300 nm) can substantially enhance PA imaging contrast, allowing visualization of a specific region of interest in vivo. Thus far, gold nanoconstructs (GNCs) have attracted significant attention as PA contrast agents due to their large absorption cross-section, versatile surface functionality, and cytocompatibility. Specifically, GNCs with high aspect ratios, such as gold nanorods (GNRs), exhibit robust PA responses due to strong NIR light absorption. However, photostability of existing anisotropic GNCs is inversely proportional to their aspect ratios, limiting their utility for longitudinal PA imaging applications. In this study, we hypothesized that coupling gold nanospheres (GNSs) into chain nanoconstructs could exhibit strong PA responses through plasmon coupling-induced NIR light absorption and enhanced photostability. Guided by computational simulations of the photothermal characteristics of GNS chain (GSC) constructs, we fabricated GSCs by introducing a ligand-mediated assembly of GNSs and evaluated GSCs for in vivo PA imaging of cancer-bearing mice.

Materials and

Methods: Optical characteristics of GSCs were computed by a finite-difference time-domain (FDTD) simulation (Lumerical Inc.). GSCs were fabricated via a ligand-aided assembly of 40 nm-sized GNSs. This process entails polydopamine layers on GSCs (P-GSCs), improving structural stability. The structural, optical, and PA properties of P-GSCs were measured using transmission electron microscopy (TEM), UV-vis-NIR spectroscopy, and US-guided PA imaging. For in vivo studies, P-GSCs were functionalized with cyclic Arg-Gly-Asp (cRGD) peptides to target integrin αvβ3 that is highly expressed on the cancer cell surface. The cRGD-tagged P-GSCs (cRGD-GSCs) were intravenously injected into breast tumor bearing mice, followed by US/PA tumor imaging (24-hour post-injection). Saline injection was used as a control for our tumor imaging experiment.

Results, Conclusions, and Discussions: We synthesized P-GSCs by dopamine-mediated assembly of 40 nm-sized GNSs (Figs. 1a-c). P-GSCs exhibited robust NIR light absorption due to inter-particle plasmon coupling of GNSs in close proximity (Fig. 1d). P-GSCs exhibited 80% higher PA signal compared to P-GNRs with a similar construct dimension and identical optical density (Fig. 1e). Furthermore, P-GSCs exhibited an approximately 50% higher photostability compared to P-GNRs (Figs. 1f, g). The stronger PA response and superior photostability of P-GSCs showcase their potential in outperforming existing plasmonic contrast agents for high-contrast PA imaging over multiple imaging sessions. Finally, we validated in vivo PA imaging capabilities of cRGD-GSCs through targeted tumor imaging. After systemic administration of the GSCs into tumor bearing mice, PA signal and contrast at the disease site significantly increased compared to the saline control (Figs. 1h-j). Together, results confirmed that our GSCs are capable of longitudinal PA imaging with high contrast.

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