Gold Nanorods as Contrast Agent for Photoacoustic Imaging (PAI) of Breast Cancer

Gold Nanorods (AuNRs) can play a fundamental role in the enhancement of photoacoustic imaging (PAI). This is due to their high and tunable optical absorption cross-section. Normally, the most convenient and highly stable AuNRs are synthesized using Cetyltrimethylammonium bromide (CTAB) as a surfactant. In this work, surface modification with poly ethylene glycol (PEGylation) and Silica coating (SiO2-coating) were applied to help in reducing toxicity of CTAB capped AuNRs. Breast cancer cell line (MDA-MB-231) was used to assess the modified AuNRs as contrast agents. A much brighter PAI of cells loaded with SiO2-coated AuNRs was obtained. Such an enhanced PA image is due to the larger cellular uptake, as observed from bright field microscope images taken for the cells. On the other hand, the corresponding PA image for cells with PEGylated AuNRs is diminished. The increase in cell uptake, in case of SiO2-coated AuNRs, could result due to the adsorption of protein, contained in the cell culture media, on the negatively charged silica surface, which increases intercellular uptake by receptor-mediated endocytosis. Moreover, the increase of the brightness of PAI of SiO2-coated AuNRs may be due to the lowering of the interfacial thermal resistance between gold and the surrounding solvent. This allows the particle to release more heat to its environment which increases the generated acoustic signals from these particles.