Sonoluminescence (SL) refers to the emission of transient light from microscopic gas bubbles in a liquid medium subjected to intense ultrasonic irradiation [1]. The phenomenon occurs when acoustic energy induces rapid oscillation and subsequent collapse of microbubbles, resulting in localized high temperatures and pressures that produce
In biomedical science, ultrasound is already widely used for diagnostic imaging as it is widely exploited in chemistry for the synthesis of nanomaterials and for the acceleration of kinetics of reactions like transesterification and fermentation [9]. The coupling of ultrasonics with luminescent bubble dynamics provides a platform for enhancing imaging contrast, tracking biological processes, and triggering therapeutic effects. This convergence has stimulated global race into the exploitation of the biomedical potential of SL [10]. Representative image of the phenomena of SL depicting the emission of light by the implosive collapse of the gas filled microbubble in the liquid medium along the path of ultrasonic waves is shown in Fig. 1.
The optical emission of light in the nanosecond time scales from the implosive collapse of the gas filled micro bubble along the path of progress of the ultrasonic waves – The phenomena of microbubble sonoluminescence (MBSL).
SL arises from acoustic cavitation, a process in which ultrasound generates oscillating bubbles within a liquid medium. These bubbles undergo cycles of expansion and contraction due to pressure variations in the acoustic field. At sufficiently high acoustic intensities, the bubbles collapse violently, producing localized extreme conditions characterized by high temperature, pressure, and rapid energy release. During collapse, the compression of gas within the bubble leads to excitation of atoms and molecules. As these excited states relax, photons are emitted, resulting in a flash of light. The emitted light spectrum can vary depending on gas composition, liquid medium, and acoustic parameters. The phenomenon can occur as single-bubble sonoluminescence or multi-bubble sonoluminescence, each with distinct dynamics and emission characteristics [11]. Key factors influencing SL include acoustic frequency, pressure amplitude, bubble (cavity) size, gas composition, and surrounding fluid properties. These parameters determine the efficiency of energy conversion and the intensity of emitted light. Understanding these mechanisms is critical for biomedical applications, where precise control over acoustic and optical effects is necessary.
SL originates from the process of acoustic cavitation, in which ultrasonic waves propagate through a liquid medium and induce the formation, oscillation, and eventual collapse of gas-filled microbubbles. The alternating high-pressure and low-pressure cycles generated by the acoustic field cause these microbubbles to undergo rapid expansion and compression. When the acoustic intensity surpasses a critical threshold, the bubbles become dynamically unstable and collapse violently. This implosion leads to the concentration of energy within an extremely small volume, producing localized conditions of elevated temperature and pressure.
SL has emerged as a promising mechanism for enhancing biomedical imaging due to its ability to generate localized optical emissions deep within biological tissues. Unlike conventional optical imaging, which is limited by light scattering and absorption, SL enables internal light generation through ultrasound-driven cavitation. Ultrasound frequencies used for biomedical SL typically range between 0.1 MHz and 5 MHz. Lower frequencies in the kilohertz range are particularly effective for inducing cavitation and generating intense luminescent emissions, whereas higher mega Hertz frequencies are preferred for diagnostic imaging and controlled bubble oscillation. Acoustic pressure amplitudes and pulse durations also influence luminescence intensity and spatial resolution [12]. Microbubble contrast agents play a critical role in SL-based imaging. These gas-filled bubbles, commonly used in ultrasound diagnostics, act as cavitation nuclei. When exposed to acoustic waves, their oscillate and collapse, producing luminescent signals that can be detected and correlated with tissue structures. These signals enhance imaging contrast, enabling visualization of vascular networks, tumor microenvironments, and inflammatory regions. SL has also been explored for molecular imaging. Functionalized microbubbles and nanobubbles can be engineered to bind specific cellular receptors or biomarkers. Upon acoustic activation, localized luminescence indicates the presence of targeted molecules, facilitating disease detection at early stages. This approach has potential applications in oncology, cardiovascular imaging, and infection monitoring [13]. The integration of SL with photoacoustic imaging techniques further improves imaging performance. Combined acoustic and optical signals provide complementary information regarding tissue structure, oxygenation, and metabolic activity. Such hybrid imaging systems are being investigated globally for high-resolution, non-invasive diagnostics [14].
SL plays a significant role in therapeutic interventions where ultrasound-induced cavitation and luminescent emission activate biochemical processes. One of the most prominent applications is sonodynamic therapy, which involves ultrasound-mediated activation of sensitizing agents to generate reactive oxygen species. These reactive species induce apoptosis and necrosis in diseased cells, particularly cancerous tissues. Typical therapeutic ultrasound frequencies range from 0.02 MHz to 1 MHz for cavitation-driven therapy, as lower frequencies facilitate bubble collapse and reactive species production. Higher frequencies in the mega Hertz range are employed for targeted and controlled therapeutic effects. SLcontributes by providing localized photon emission that enhances activation of sensitizing molecules within tissues. Drug delivery systems benefit from SL through ultrasound-triggered permeability changes in cell membranes. Acoustic cavitation temporarily disrupts biological barriers, allowing targeted delivery of drugs and genes. Luminescent signals generated during therapy provide real-time monitoring of drug release and therapeutic efficacy [15]. Antimicrobial and antiviral applications arise from the generation of reactive oxygen species and localized heating during cavitation. These effects disrupt microbial membranes, denature proteins, and inhibit viral replication. SL-mediated processes are being explored for sterilization of medical devices, treatment of biofilms, and infection control in clinical environments [16]. Diagnostic applications include monitoring tissue response to therapy and assessing vascular perfusion. Luminescent signals correlate with cavitation activity and provide feedback regarding treatment intensity and localization. This enables optimization of therapeutic protocols and improves patient safety. The mechanism and function of sonodynamic therapy operating on SL with potential to kill the multidrug resistant bacteria is shown in Fig. 2.
A schematic diagram of various mechanisms and applications of antibacterial sonodynamic therapy [Adapted with permission from 16].
Schematic illustration of sonoluminescence-assisted and ultrasound-based dual-mode imaging using hybrid nanocrystals (ZnO and Lipase coated ZnO)
SL has significant applications outside the biomedical domain, particularly in chemistry, environmental science, and materials engineering. The extreme conditions generated during cavitation, including high temperatures and pressures, resulting in the SL drive chemical reactions and physical transformations. In sonochemistry, SL is associated with the formation of reactive radicals that initiate photochemical reactions. These processes are utilized in pollutant degradation, organic synthesis, synthesis of exotic nanomaterials and catalysis. Acoustic frequencies between 20 kHz and 100 kHz are commonly employed for chemical applications, as their produce strong cavitation effects [18]. Environmental remediation applications include water purification and degradation of persistent organic pollutants. SL enhances oxidation reactions that break down contaminants into less harmful compounds. This approach has been explored for wastewater treatment and industrial effluent management which is more significant in these days of rapid globalization that has resulted in population explosion, urbanization and severe deterioration of air, water and soil quality. In material’s science, SL contributes to synthesis of nanoparticles and nanostructured materials. The high-energy environment during bubble collapse promotes nucleation and growth of nanomaterials with unique properties. Acoustic parameters influence particle size, morphology, and crystallinity. Industrial applications include surface cleaning, emulsification, coatings and process monitoring. SL signals can be used as indicators of cavitation intensity and system efficiency. These processes are applied in microfabrication, pharmaceutical production, and chemical engineering [19]. Emerging research is exploring energy-related applications, including hydrogen production and catalytic reactions driven by cavitation. The ability of SL to generate localized extreme conditions provides opportunities for innovative technological developments across multiple scientific disciplines.