Free Radical Generation from High-Frequency Electromechanical Dissociation of Pure Water

Abstract

We reveal a unique mechanism by which pure water can be dissociated to form free radicals without requiring catalysts, electrolytes, or electrode contact by means of high-frequency nanometer-amplitude electromechanical surface vibrations in the form of surface acoustic waves (SAWs) generated on a piezoelectric substrate. The physical undulations associated with these mechanical waves, in concert with the evanescent electric field arising from the piezoelectric coupling, constitute half-wavelength “nanoelectrochemical cells” in which liquid is trapped within the SAW potential minima with vertical dimensions defined by the wave amplitude (∼10 nm), thereby forming highly confined polarized regions with intense electric field strengths that enable the breakdown of water. The ions and free radicals that are generated rapidly electromigrate under the high field intensity in addition to being convectively transported away from the cells by the bulk liquid recirculation generated by the acoustic excitation, thereby overcoming mass transport limitations that lead to ion recombination.

High frequency acoustic cell stimulation promotes exosome generation regulated by a calcium-dependent mechanism

“…4b, c). Unlike low-frequency ultrasound (10-100 kHz up to 1 MHz) typically used in sonoporation, the considerably higher frequencies and lower powers (one to two orders of magnitude) associated with the SRBW excitation or its surface acoustic wave counterpart do not generate any appreciable cavitation 57 to induce pore formation in the cell plasma membrane, which is known to inflict considerable damage to the cell. Rather, it was postulated in ref.…”

Non-contact ultrasound oocyte denudation

“…84 Furthermore, it has been indicated in the literature that high SAW undulatory displacements (≅10 nm, 10 MHz and power ≳ 10 V rms ) can locally accumulate charges sufficient to create a half wavelength "nano-electrochemical" cell that may split water and create free radicals. 85 As such, it is important to understand and identify the sources of electrical fields that may affect the performance of the device. Two main sources of unwanted electrical fields in our experimental design can be either through parasitic electric signal coupling from wire bonds, or electrical fields associated with SAW.…”

Sonoporation: Past, Present, and Future

“…When acoustic pressures are sufficiently high (e.g., 0.3-1.0 MPa at ≈1 MHz), [89][90][91][92] bubble collapse can be induced by acoustic waves thus generating a strong 'jetting' flow (i.e., fluid streaming), [93,94] which can further rupture the membrane of an adjacent cell as illustrated in Figure 1a. In addition to pressure, the inertial cavitation performance is dependent on acoustic wave frequency, [90][91][92]95,96] pulse repetition rate, [89] pulse duration, [89] initial bubble radius, [91,92] bubble properties, [90,95] temperature, [97] fluid properties, [90,98] wave-type, [99,100] and device energy efficiency. [99,100] Apart from the 'jetting' effect, inertial cavitation is also associated with shock waves generated from the bubble collapse.…”

“…In addition to pressure, the inertial cavitation performance is dependent on acoustic wave frequency, [90][91][92]95,96] pulse repetition rate, [89] pulse duration, [89] initial bubble radius, [91,92] bubble properties, [90,95] temperature, [97] fluid properties, [90,98] wave-type, [99,100] and device energy efficiency. [99,100] Apart from the 'jetting' effect, inertial cavitation is also associated with shock waves generated from the bubble collapse. These shock waves propagate and generate intense fluid streaming, resulting in the perforation of a cell membrane, [72,94,[101][102][103] as shown in Figure 1b.…”

“…[132] As shown in Figure 3g, by tuning input phases and amplitudes from multiple interdigital transducers, the controlled standing waves translated a bubble along a complex path to the desired location. Given that their device employed high-frequency surface acoustic waves, which are typically difficult to induce cavitation, [99,100] Meng et al leveraged an intense pulse of 24 MHz surface acoustic waves to successfully induce inertial cavitation. After sonoporation, PI (red) successfully entered the transiently porous cell (see Figure 3g).…”