Cavitation analysis of ultrasound experiments with silica nanoparticles.

Due to the recent development in the nanotechnology field for biomedical applications, especially regard the treatment of disease, a new kind of Mesoporous Silica Nanoparticles (MSN) are developed with the co-condensation synthesis and different chemical functionalizations are exploited to investigate the phenomena of cavitation and the production of radicals, when combined with the use of intrinsically safe Ultrasound (US) energies. The cavitation is the process of generation and oscillation of bubbles, using, as source, the gases dissolved in the solution, or carried by external elements such as the nanoparticles, guided by the variation of pressure produced by the US. The transient cavitation appears when the oscillating bubbles increase to a size which is no more able to follow the cyclic pressure variations and is characterized by the collapse of the bubbles with emission of high energies and localized increment of temperatures, which are the cause of the radicals generation. Before the treatment, the correct synthetization of the MSNs was tested through Dynamic Light Scattering, zeta potential, Brunauer-Emmett-Teller nitrogen sorption method, transmission electron microscopy and infrared transmission spectroscopy. All the MSN are tested, in parallel with a sample composed of silica Nanocup, using a Lipozero US emitter, at low US pressure for prolonged exposition, and with a focused transducer at high US pressure and limited exposition. Lipozero is a commercial instrument with an unfocused transducer used for aesthetic and lipo-reduction treatments. The samples were prepared in plastic wells of 24-well Petri dishes for cellular culture and sonicated at 1 MHz of frequency. three instruments monitored the cavitation activities: an invasive needle hydrophone, positioned inside the sample well, which acquires the pressure variations produced inside the well; a non-invasive biomedical and commercial echographer, which monitors the bubbles generation and their evolution in time; a Bruker spectometer for Electron Paramagnetic Resonance measurement, able to detect the radical produced by evaluating the amount of spin adduct (DMPO-OH) produced by the combination of 5,5-DiMethyl-1-Pyrroline-N-Oxide (DMPO), used as spin trap, with OH∙ radicals. The homemade focused transducer and a non-invasive cavitometer instead were used to observe the effect produced during short US exposition at a high pressure, the effects are visualized in an oscilloscope and transformed in the frequency spectra to observe the transient cavitation. In the experimental condition explored it was not possible to observe a noticeable increment of the radical generation with the particle administration. Nevertheless, from the cavitometer acquisitions emerges their capability to trap packet of gases inside their shell, released in short period during the US treatment. This peculiar aspect deserves further study to better understand the releasing process and the possibility of transport gases also in different mediums.