PROJECT SUMMARY:

The chemical effects of ultrasound do not come from a direct interaction with molecular species. Instead, sonochemistry arises from acoustic cavitation: the formation, growth, and implosive collapse of bubbles in a liquid. Cavitational collapse produces intense local heating ("5000 K), high pressures (~1000 atm), and enormous heating and cooling rates (>10¹⁰ K/sec). Acoustic cavitation provides a unique interaction of energy and matter. A three-dimensional visualization of the comparison between sonochemistry and other forms of chemistry is shown below.

The field of sonochemistry has undergone a renaissance during the past decade. As our understanding of the nature of the chemical effects of ultrasound has grown, so too has the impact of sonochemistry on the chemical community and on a wide range of the physical sciences. Sonochemistry also has substantial strategic research significance to our industrial economy. Ultrasound already has major industrial applications, and the U.S. controls the world market in the production of ultrasonic equipment. Commercial generators of high?intensity ultrasound for large scale liquid processing is available off the shelf at quite modest costs. Ultrasonic cleaning now dominates both general purpose industrial and microelectronics applications, due to increasing restrictions on the use of halocarbon and other organic solvents. In addition, high intensity ultrasound has a large industrial base for welding, cutting, emulsification, solvent degassing, powder dispersion, cell disruption, and atomization. Thus, the development of sonochemistry and its applications will advance a global market in which the U.S. has a significant advantage.

The previous grant period was enormously productive, resulting in 47 publications (published or in press). These papers fell relatively evenly into five categories: (1) Cavitation Phenomena and Sonoluminescence, (2) Inorganic Materials Chemistry, (3) Applications to Heterogeneous Catalysis, (4) Biomaterials, and (5) Popularizations.

During the current grant period, experimental measurements of the conditions created during acoustic cavitation were successful. The effective temperatures during cavitation were probed using sonoluminescence spectra. It proved possible for the first time to experimentally measure the temperatures during cavitation. In this fashion, systematic control of the metal atom emission temperatures was demonstrated from 5000 K down to 2000 K.

In other work, the sonochemical decomposition of volatile organometallic precursors was shown to produce nanostructured materials in various forms with high catalytic activities. This has proved extremely useful in the synthesis of a wide range of nanostructured inorganic materials, including high surface area transition metals, alloys, carbides, oxides, and sulfides, as well as colloids of nm clusters.

Another important application has been the sonochemical preparation of biomaterials, most notably protein microspheres. Using high intensity ultrasound and simple protein solutions, a remarkably easy method to make both air-filled microbubbles and liquid-filled microcapsules was developed. This sonochemical method of microencapsulation uses high intensity ultrasound both to induce emulsification and to create oxidative crosslinking of protein cysteines. These microspheres are stable for months, and being slightly smaller than erythrocytes, can be intravenously injected. These protein microspheres have a wide range of biomedical applications, including uses for in vivo thermography, in vivo oximetry, MRI contrast agents, and drug delivery.

The overall goal in this research is to develop a fundamental understanding of the nature and applications of ultrasound in chemical reactivity. The specific objectives in this proposed work fall into three areas: 1) mechanistic and spectroscopic probes of the cavitation event, 2) novel inorganic materials synthesis and the sonochemical preparation of heterogeneous catalysts, and 3) biomaterials synthesis. In order to gain further understanding of the conditions present during cavitation, sonoluminescence will continue to be developed as a spectroscopic probe of the conditions created during bubble collapse. The use of ultrasound for the synthesis of novel inorganic materials will continue to focus on nanostructured and amorphous metals, alloys, carbides, oxides, and sulfides and their applications as heterogeneous catalysts for a variety of industrially important reactions. Finally, the development of the sonochemical synthesis of biomedical microstructures, especially proteinaceous microspheres, will be expanded. Biomedical applications of sonochemically prepared proteinaceous microspheres will be further developed, including their use as MRI agents. The functional properties of proteinaceous microspheres made from proteins with enzymatic activity will be delineated