Silica-Coated Gold Nanorods as Photoacoustic Signal Nanoamplifiers

Abstract

Photoacoustic signal generation by metal nanoparticles relies on the efficient conversion of light to heat, its transfer to the environment and the production of pressure transients. In this study we demonstrate that a dielectric shell has a strong influence on the amplitude of the generated photoacoustic signal, and that silica coated gold nanorods of the same optical density are capable of producing about 3-fold higher photoacoustic signals than nanorods without silica coating. Spectrophotometry measurements and finite difference time domain (FDTD) analysis of gold nanorods before and after silica coating showed only an insignificant change of the extinction and absorption cross-sections, hence indicating that the enhancement is not attributable to changes in absorption cross-section resulting from the silica coating. Several factors including the silica thickness, the gold/silica interface, and the surrounding solvent were varied to investigate their effect on the photoacoustic signal produced from silica-coated gold nanorods. The results suggest that the enhancement is caused by the reduction of the gold interfacial thermal resistance with the solvent due to the silica coating. The strong contrast enhancement in photoacoustic imaging, demonstrated using phantoms with silica-coated nanorods, shows that these hybrid particles acting as "photoacoustic nano-amplifiers" are high efficiency contrast agents for photoacoustic imaging or photoacoustic image-guided therapy. KeywordsPhotoacoustic imaging; silica coated-gold nanorods; photoacoustic nano-amplifiers; medical and biological imaging; contrast agents Photoacoustic imaging is a non-ionizing and noninvasive imaging modality that combines the advantages of both optical and acoustic imaging. [1][2][3][4] In photoacoustic imaging, the intensity modulated electromagnetic radiation, e.g. a beam of pulsed laser light, is directed at the imaging target. The light is absorbed and converted to an outgoing thermoacoustic wave that can be detected by an ultrasound transducer and used to reconstruct images.5 -8 Since light is only used for heating and not for imaging, and acoustic waves are less scattered in optically turbid materials such as tissue, photoacoustic imaging can reach far deeper into turbid materials than purely optical imaging techniques.8 -9 The contrast in photoacoustic * To whom correspondence should be addressed. Telephone: (512) 471-1733. Fax: (512) imaging depends on the optical-to-acoustic conversion (optoacoustic) efficiency, i.e., how many incident photons can be absorbed and converted to heat, and how fast the generated heat can diffuse out from the target during thermoelastic expansion and wave generation. When a uniformly absorbing target is irradiated by pulsed light, the amplitude of the generated photoacoustic signal is proportional to the optical absorption and the thermalacoustic properties of the absorbing medium. In contrast, in a heterogeneous medium such as a weakly absorbing solvent containing plasmonic nanoparticles, the amp...

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