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23 February 2010 Design and synthesis of new gold nanoparticles for enhanced photoacoustic response
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Gold nanoparticles have been used as contrast agent in photoacoustic imaging to increase the detection sensitivity. For example, gold nanorods (AuNRs) have been used in time-intensity based flow estimation and used as nanoprobes to target cancer cells for early diagnosis and effective treatment. In this study, we aimed at the design and synthesis of a new type of gold nanoparticles with enhanced photoacoustic response. The key hypothesis is to create a nanostructure that allows anisotropic heat release. Specifically, such a structure results in higher heat flux transmitting outwards from the ends of the particle and therefore a greater temperature gradient can be created. To achieve this, a layer of SiO2 was coated along the longer axis of the gold nanorods, leaving both ends uncovered. These new particles are labeled as AuNR@nu-SiO2 with non-uniform ("nu") coating of silica. Experiments were performed to demonstrate the enhanced photoacoustic response from AuNR@nu-SiO2. The optical illumination was delivered by a Ti: Sapphire laser pumped by an Nd:YAG laser. A home-made photoacoustic transducer with a center frequency of 20 MHz detected the resulted acoustic signal. First, new types of particles coated with and without SiO2 were compared to bare AuNR in order to show the ability of the new nanostructure to enhance photoacoustic response. Second, the shape stability of the new particles was evaluated by measuring the photoacoustic responses versus time after high power laser irradiation. Third, the effect of thickness of SiO2 of AuNR@nu-SiO2 ranges from 1 nm to 15 nm was also evaluated. Results show that the mean intensity in photoacoustic image increase about 5 dB for AuNR@nu-SiO2 compared to bare AuNR. Also, it reveals that the normalized intensity for AuNR drops to below 0.6 while it is mostly larger than 0.7 in the case of AuNR@nu-SiO2 under pulse laser irradiation. In other words, the new type of nanoparticles is less susceptible to shape transformation. Moreover, it is indicated that the photoaocustic response increases slightly with the thickness of SiO2 and approach to an maximum response at 9 nm thickness. In short, these new particles can be used to achieve the same level of photoacoustic response with a fewer amount of particles, which means that there is less toxicity.
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Chen-Wei Wei, Carolina Poe, Chi-Meng Chen, Yi-Hsien Lee, Churng-Ren Chris Wang, and Pai-Chi Li "Design and synthesis of new gold nanoparticles for enhanced photoacoustic response", Proc. SPIE 7564, Photons Plus Ultrasound: Imaging and Sensing 2010, 75641S (23 February 2010);

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