Hyperbranched gold nanoconstructs for photoacoustic cancer imaging in the near-infrared optical window

Introduction:: Photoacoustic (PA) imaging is a non-invasive and non-ionizing modality that leverages the conversion of light to thermoacoustic waves using high-intensity nanosecond laser pulses. Exogenous contrast agents with strong optical absorption in the near-infrared (NIR) optical window (650 nm to 1300 nm) can greatly enhance PA contrast, allowing visualization of specific target regions. Gold nanoconstructs (GNCs) with high aspect ratios, such as gold nanorods (GNRs) and nanostars (GNSTs), have been widely utilized as PA contrast agents due to their strong NIR light absorption based on their anisotropic plasmon excitation. However, photostability of existing anisotropic GNCs is inversely proportional to their aspect ratios, limiting their utility for PA imaging. In this study, we hypothesized that hyperbranched gold nanoconstructs (HGNCs), that are based on the assembly of photostable gold nanospheres (GNSs), could exhibit high PA responses with enhanced photostability. Guided by computational simulations of their optical absorption and electric-field (E-field) enhancements, we synthesized HGNCs by introducing a surface blocker-aided growth of GNSs on a larger GNS template and evaluated HGNCs for in vivo PA imaging of breast cancer-bearing mice.


Materials and Methods:: Optical properties of HGNCs were calculated by a finite-difference time-domain (FDTD) simulation (Lumerical Inc.). HGNCs were synthesized via a surface blocker-aided growth of GNSs. The structural, optical, and PA properties of HGNCs were measured using transmission electron microscopy (TEM), UV-vis-NIR spectroscopy, and ultrasound (US)-guided PA imaging (Vevo LAZR, 700-900 nm). For in vivo studies, HGNCs were conjugated with Arg-Gly-Asp (RGD) peptides to target integrin α_vβ₃ that is highly expressed on the surface of MDA-MB 231 breast cancer cells. The RGD-HGNCs were intravenously injected into breast tumor bearing mice. The tumor was imaged 24 hours after the RGD-HGNC injection. Saline injection was used as a control for the in vivo tumor imaging experiment.


Results, Conclusions, and Discussions:: Results and Discussion: FDTD simulation results showed that HGNCs exhibited strong NIR light absorption with minimal scattering through intra-branch and inter-branch plasmon coupling (Figs. 1A, 1B). HGNCs (~ 80 nm) with a 6 nm branch thickness and 45 nm length were synthesized via surface blocker-aided growth of 35 nm-sized GNSs (Figs. 1C, 1D). HGNCs exhibited strong optical absorption at NIR frequencies (Fig. 1E). Due to the strong NIR light absorption, HGNCs generated high PA signals in the NIR window (Fig. 1F). Moreover, HGNCs exhibited approximately 50% higher photostability than GNRs and GNSTs, showing the potential of the HGNC contrast agent for PA imaging (Fig. 1G). Finally, we validated in vivo PA imaging capabilities of RGD-HGNCs through tumor imaging. After systemic injection of the RGD-HGNCs into tumor bearing mice, PA signals and contrast in the tumor region increased, compared to the saline control group (Figs. 1H, 1I). Together, results confirmed that HGNCs are capable of PA imaging with high contrast and imaging stability.

Conclusion: We designed and synthesized HGNCs with strong NIR light absorption and enhanced photostability to achieve high-contrast PA imaging of cancer in vivo. Future studies will investigate the HGNC structure by modifying branch density, length, diameter, and composition to maximize PA signals, contrast, and photostability.


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