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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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