## 1 Introduction
For particular US operating conditions, the as-formed microbubbles can collapse and locally generate very high pressures and temperatures [18] which allow the formation of ROS. This process can be achieved only by operating above specific conditions (cavitation threshold). The introduction in the medium of specific compounds, i.e., sonosensitizers, can lower this cavitation threshold and favor the formation of ROS, as discussed in the following. The first SDT systems came out in 1989 by [PERSON] et al. [19] who used the already-known photosensitizer hematoporphyrin as sonsensitizer. Most of the sonsentizers are organic molecules such as porphyrin derivatives, phthalocyaninesamong others [19; 20; 21]. However, these molecules are characterized by a high aggregation tendency in physiologic medium, and a minor cancer tissue selectivity, which decreases the therapy effectiveness [21]. The research for more efficient alternatives is in progress, e.g., [PERSON] et al. found that crystalline TiO\({}_{2}\) nanoparticles (NPs) activated by ultrasounds (US) can also work as sonsosensitizer to reduce the neoplastic tissue [22]. The above-mentioned compounds chemically react or introduce a large amount of bubbles into a specific medium (by their oscillation and/or their violent collapse) and behave like a chemical nanoreactor, leading to ROS generation [23]. The recognized advantage of SDT over the PDT is its higher tissue penetration, which allows the treatment of tumors located deeply in the body. The use of US in SDT is even manifold [21]. Apart from the well-known US imaging capabilities, it is also generally recognized that the combination of ultrasounds and sonosensitizers also lead to sonochemical reactions additional to cavitation, like sonoluminescence and pyrolysis reactions. These might contribute to a more efficient generation of highly toxic species and, therefore, it might further improve the overall efficacy of SDT treatments [24; 25; 26]. Semiconductor crystalline TiO\({}_{2}\) is a very well-known nanomaterial due to its photo-reactivity, photo-stability, and effective bactericidal performance. Crystalline TiO\({}_{2}\) (approved by the American Food and Drug Administration) has been widely used as food additive, as photo-active material in pharmaceutical, dermatological products (like in sunscreens), but also in paints, wastewater/drinking water membranes, in photovoltaic and photocatalytic devices [27; 28; 29; 30; 31]. It has also been demonstrated that crystalline TiO\({}_{2}\) is biologically inert in animals and humans, presents a good biocompatibility, and no toxicity in vitro or in vivo [32; 33]. For this reason, nanosized titania have found interesting and promising application in cancer therapy and diagnosis, i.e., theranostics [34], and specifically for SDT [35; 36]. Recently amorphous-TiO\({}_{2}\) has attracted great interest in the academic community [37; 38; 39; 40; 41; 42; 43; 44; 45; 46], where a set of new a-TiO\({}_{2}\) applications have been proved: high performance photocatalysts [38], dye sensitizers [43; 46], solar battery electrodes [44; 46], sodium ion anodes for rechargeable batteries [45], capacitors thin films [38], resistive random access memories [43], self-cleaning agents and water purifiers (in dye-polluted aqueous systems) [47], dehumidifiers [48], visible light photocatalysts [44], and biomedical devices [49; 50]. Some of the authors have proved the bactericidal performance of a-TiO\({}_{2}\) NPs and explained theoretically the amorphous semiconductor performances [51; 52; 53]. Furthermore, a-TiO\({}_{2}\) are often the first phases to form in many wet chemical synthetic routes (other times the first nuclei to form are anatase that rapidly evolve to oxy-hydroxy amorphous phases [37]). Amorphous TiO\({}_{2}\) is easy to process into different forms, its reduced order state allows a wide range of possible formulation with high levels of dopants.