Multifunctional Au Nanostars For X-ray Computed Tomography And Photoacoustic Imaging

Multimodal imaging has recently attracted intense attention because it combines the advantages of diverse imaging methods including positron emission tomography/computed tomography(PET/CT), magnetic resonance imaging(MRI)/PET, and MR/optical imaging. CT is one of the most commonly used imaging techniques in the clinic because of its high spatial resolution, fast scan speed, ease of forming 3D visual image and cost effectiveness. However, the inherent low sensitivity leads to poor soft-tissue contrast. To improve accuracy, complementary imaging methods are urgently needed. Photoacoustic(PA) imaging is a novel noninvasive and nonionizing imaging modality that has recently attracted significant interest. PA imaging combines the advantages of optical and acoustic imaging and provides deep tissue penetration(up to 5 cm), excellent spatial resolution(<1 mm), and high sensitivity. Therefore, the combination of CT imaging with PA imaging is an ideal dual-modal imaging modality, which could integrate functional and molecular information with anatomical images. The development of CT/PA imaging contrast agents may be of great importance for the clinical application.Most of the current dual-modality CT/PA contrast agents are composite materials, including Pd@Au nanoplates, Au@PB nanoparticles, WS2 nanosheets and Ta Ox@PPy nanoparticles. However, these nanomaterials have a complicated and multistep synthesis and potential toxic side-effects. This can decrease reproducibility and limit clinical utility. Thus the synthesis of ideal CT/PA imaging contrast agents with a facile method is meaningful.PA imaging can provide information on a cellular level with high sensitivity. Thus the dual-modal CT/PA imaging may be useful in the diagnosis of liver tumor or the evaluation of liver disease. The kinetics of nanoparticles in the liver and their biodistribution in organs are very important. In current studies, the content of inorganic nanoparticles were measured by inductively coupled plasma mass spectrometry(ICP-MS). While the complex method is invasive and need to isolate the organs. PA imaging show great potential in tracing liver kinetics and exploring biodistribution of nanoparticles. Some researches imaged ex vivo organs with photoacoustic microscopy(PAM).Therefore, we synthesized BSA-capped Au nanoparticles as CT/PA dual-modal imaging contrast agents, and characterized the nanoparticles with various methods. We studied the CT and PA contrast enhancement, biotoxicity and biodistribution of the nanoparticles. In addition, we explored the liver metabolism and organ imaging ex vivo with photoacoustic computed tomography(PACT), and imaged tumor after injection of BSA-Au NSs.Herein, we describe for the first time the use of BSA-coated nanostars(BSA-Au NSs)for combined CT/PA imaging in vivo via a facile synthesis and surface coating strategy. The size and morphology of BSA-Au NSs were observed by using a field-emission transmission electron microscope(TEM). The particle size distributions of BSA-Au NSs were determined by dynamic light scattering(DLS). Ultraviolet-visible-near infrared absorption spectra(UV-vis-NIR) were recorded. Fourier transform infrared spectroscopy(FTIR) was performed to confirm the presence of BSA on the surface of BSA-Au NSs. The cytotoxicity of BSA-Au NSs was assessed by cytotoxicity assay. We compared the X-ray attenuation property of BSA-Au NSs with Omnipaque, and measured the photoacoustic signal intensity of BSA-Au NSs and methylene blue(MB). The in vivo CT images were acquired after injection of BSA-Au NSs and Omnipaque at different time points. We observed the PA signal variation of liver after injection of BSA-Au NSs and the MB was used as control. We also imaged the ex vivo organs after 1 h of injection. The Au contents in various organs were determined by ICP-MS. The pathological sections of liver, spleen and kidney were used to assess the biotoxicity. We built the experimental animal model of murine mammary carcinoma to explore the feasibility of BSA-Au NSs imaging tumor. The experimental results are as follows:1. The TEM images showed that the BSA-Au NSs had an irregular and star-like morphology. The average hydrodynamic diameter of the BSA-Au NSs was 85 nm with a relatively narrow size distribution in water using DLS. The FTIR spectra confirmed the existence of BSA on the surface of BSA-Au NSs. UV-vis-NIR absorption spectrum showed that the BSA-Au NSs had a surface plasmon resonance(SPR) peak at 770 nm in NIR. There was no obvious cytotoxicity even at the highest concentation of 1 m M.2. The attenuation intensity of BSA-Au NSs was 1.34 times higher than iohexol at the same concentrations of the active element. The BSA-Au NSs had nearly 5.8-fold higher PA signal intensity than that of methylene blue at the same mass concentrations.3. The CT data confirm obvious contrast enhancement in liver. The increased contrast lasted for more than 2 hours without significant change. Despite the high liver accumulation, the liver signal had returned to baseline 24 h after administration. Furthermore, the CT signals of the intestines increased gradually, indicating bile excretion of the nanoparticles. In control group, iohexol resulted in no apparent tissue contrast enhancement and was quickly excreted by the kidney.4. In PA imaging,more blood vessels with higher signal intensity were observed after administration. There was efficient contrast enhancement of the vessels for more than 2 h, indicating the long circulation time of BSA-Au NSs. The shape and outline of the liver were observed with extremely high signal intensity. At 2 h post-injection, the PA signal intensity decreased and almost vanished at 24 h post-injection, suggesting efficient elimination of the contrast agents. After injection of MB, the liver was slightly observed.5. The PA images of ex vivo organs shows that the PA signal intensity of the injected group was higher than that of the control group. Furthermore, the signal intensity in the reticuloendothelial system(RES)(liver, spleen, etc.) was much higher than the other organs.6. The result of ICP-MS shows that the nanoparticles mainly accumulated in the liver and spleen. At 24 h after injection, the Au concentrations of liver and other organs decreased to relatively low levels versus that at 2 h.7. There was no apparent injury to the cellular structures in pathological result.8. The photoacoustic images of tumor show that the nanoparticles accumulated in tumor gradually.In summary, we prepared BSA-AuNSs with a facile method, and characterized the nanoparticles with various methods. The results of cytotoxicity and histopathology indicate that BSA-Au NSs have good biocompatibility. The dynamic contrast-enhancement imaging and the result of biodistribution showed the biodistribution and metabolism of the nanoparticls in mice. Compared with conventional CT contrast agent and small molecular PA contrast agent MB, BSA-Au NSs shows great advantages in imaging. BSA-Au NSs have great potential to be used both in CT and PA imaging. In addition, BSA-Au NSs have the advantage of easy surface-modification, which lay a foundation for our next work. We observed and semi-quantified the liver kinetics of the nanoparticles and imaged ex vivo organs by PACT, which provide a new and noninvasive approach for tracing materials.