| Abstract
| - High-power ultrasound is commonly used at relatively low frequencies (e.g., 20 kHz) to disperse andmodify micrometer and nanosized particles in liquids. However, our a priori hypothesis was that relativelyhigh frequency ultrasound is capable of modifying solid particles in aqueous solutions, because cavitationbubbles have a smaller resonance size at high frequency and would be more likely to violently collapseon the interface of submicrometer-sized particles. A combination of low-frequency (22.5 kHz) followedby high-frequency (70, 354, 803, and 1024 kHz) ultrasound was used to disperse and miniaturize amicrocrystalline powder of lithium phthalocyanine (LiPc), an electron paramagnetic resonance oxygen-sensitive probe, in aqueous solution. In the absence of a stabilizing agent, high-frequency sonolysisproduced nanosized particles that tended to agglomerate into clusters that were larger in size than theoriginal particles. Furthermore, all of the particles sonicated exhibited some degree of sonochemicaldegradation, as evidenced by color changes of the sonicated solutions. The addition of sodium dodecylsulfate (SDS) prior to high-frequency sonolysis of LiPc suspensions had a profound effect on stabilizingindividual particles in solution, thereby creating relatively monodispersed, nanosized particles in water.These particles retained their EPR activity; however, unlike the micrometer-sized LiPc particles, thenanosized LiPc particles were almost insensitive to oxygen. High-frequency ultrasound creates interestingmodifications to the LiPc particles, resulting in extremely thin, rod-shaped nanotubes that are not observedfollowing high-power, low-frequency ultrasound exposure.
- The microcrystalline form of lithium phthalocyanine (LiPc) can be dispersed in water using 22.5 kHz ultrasound. LiPc particles are paramagnetic, and their electron paramagnetic resonance absorption line width is sensitive to oxygen. Oxygen sensitivity stems from channels through the particles that are large enough to accommodate oxygen molecules. Extremely thin nanotubes are formed following further treatment of the particles with 354 kHz ultrasound, as shown.
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