Thermo-Optical Properties of Nanoparticles and Nanoparticle Complexes Embedded in Ice: Characterization of Heat Generati
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0964-R03-18
Thermo-Optical Properties of Nanoparticles and Nanoparticle Complexes Embedded in Ice: Characterization of Heat Generation and Actuation of Larger-Scale Effects Hugh H. Richardson1, Zachary N. Hickman1, Alyssa C. Thomas1, Martin E. Kordesch2, and Alexander O. Govorov2 1 Chemistry and Biochemistry, Ohio University, Athens, OH, 45701 2 Physics and Astronomy, Ohio University, Athens, OH, 45701
ABSTRACT We have investigated the thermo-optical properties of gold nanoparticles (NPs) embedded in an ice matrix. While an intense laser beam will not melt ice alone, resonant laser light of relatively weak intensity is able to melt ice embedded with Au NPs due to strong absorption in the regime of plasmon resonance. By recording time resolved Raman signals, we observe the melting process and determine the threshold melting power Pmelting (T ) , where T is the background temperature. The local temperature inside and around the NP complex depends strongly on its geometry and leads to a large scattering for the measured Pmelting as a function of the reduced temperature for different complexes. We can also characterize NPs that have been immobilized on glass surfaces by single particle spectroscopy. Results show that plasmon emission of Au NPs scales with the number of particles in an agglomerate. INTRODUCTION The optical properties of both semiconductor [1, 2] and metal nanoparticles [3-5] (NPs) have been intensively studied. Much interest has also been paid to heat generation by NPs under optical illumination, a process that involves absorption of incident photons and heat transfer from a NP to the surrounding matrix. The heating effect for metal NPs is strong due to mobile electrons and becomes enhanced due to plasmon resonance. The temperature increase at the surface of the NPs is challenging to measure, but is the most important parameter for incorporation of heated NPs into nano-medicine[6, 7]. In reference [4], an attempt to directly measure the surface temperature of Au NPs was achieved by embedding them in ice and driving them optically. The NP-surface temperature could then be determined by observing the power threshold for the melting process. Another approach was taken in ref [8] where the authors assembled a complex made of semiconductor and metal NPs linked by a temperature sensitive polymer chain. Since semiconductor NP emission strongly depends in the distance to the metal nanocrystal, the local temperature into the NP complex can be determined by measuring emission intensity. Biomedical applications of heated NPs rely on a simple mechanism [6, 7]. Using selective bio-molecular linkers, the NPs attach to tumor cells and heat generated by the opticallystimulated NPs destroys the tumor cells. It is essential that Au NPs are suitable for simultaneous molecular imaging and photo-thermal cancer therapy [6, 7], making fundamental studies of photo-thermal effects pertinent.
THEORY Under optical excitation, crystalline NPs fabricated of various materials can efficiently release heat. The electric field of th
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