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Article

Ultrasonic Activation of Au Nanoclusters/TiO2: Tuning Hydroxyl Radical Production Through Frequency and Nanocluster Size

by
Takaaki Tsurunishi
,
Yuzuki Furui
and
Hideya Kawasaki
*
Department of Chemistry and Materials Engineering, Kansai University, 3-3-35, Yamate-cho, Suita 564-8680, Osaka, Japan
*
Author to whom correspondence should be addressed.
Molecules 2025, 30(3), 541; https://doi.org/10.3390/molecules30030541
Submission received: 9 December 2024 / Revised: 20 January 2025 / Accepted: 23 January 2025 / Published: 24 January 2025
(This article belongs to the Special Issue Research on Heterogeneous Catalysis—2nd Edition)
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Abstract

:
This study explores the sonocatalytic activity of gold nanoclusters (Au NCs) combined with titanium dioxide (TiO2) nanoparticles, forming Au NCs/TiO2 composites. The hybrid material significantly enhances hydroxyl radical (•OH) generation under ultrasonic conditions, attributed to high-energy cavitation bubbles formed during ultrasonication. The effects of frequency (200, 430, and 950 kHz) and power were systematically evaluated on Au144/TiO2 composites, identifying 430 kHz as optimal for •OH production due to its efficient cavitation energy. Au144 NCs function as electron traps, reducing electron–hole recombination in ultrasonically activated TiO2, thereby improving charge separation and enhancing •OH generation. Size-dependent effects were also studied, showing an efficiency trend of Au144 > Au25 > plasmonic Au nanoparticles > bare TiO2. These findings highlight the importance of ultrasonication frequency and Au NC size in optimizing sonocatalytic performance in the Au NCs/TiO2 composites, providing valuable insights for designing advanced sonocatalysts with applications in chemical synthesis, environmental remediation, and biomedical fields.

1. Introduction

The hydroxyl radical (•OH) is an exceptionally potent oxidizing agent found in various environments, including natural waters, the atmosphere, and interstellar space. As one of the most important reactive oxygen species (ROS), •OH plays a pivotal role in numerous chemical processes, particularly in environmental chemistry and biochemistry. In environmental applications, advanced oxidation processes (AOPs) are widely employed in wastewater treatment to degrade pollutants effectively [1,2]. In the medical field, •OH radicals are utilized in innovative therapeutic approaches such as photodynamic therapy and chemodynamic therapy, which target cancer cells through oxidative damage within the tumor microenvironment [3,4]. Advancing methods for enhanced •OH radical generation is critical for expanding its potential applications in both environmental remediation and healthcare.
Photocatalytic materials absorb light with energy exceeding their band gap, generating excited electrons and holes. These charge carriers migrate to the surface of the material, where they act as reducing and oxidizing agents, facilitating redox reactions. For example, the reduction of oxygen or the oxidation of water results in the formation of reactive oxygen species (ROS), such as superoxide radicals and •OH. These ROS play a key role in degrading organic compounds. However, a significant portion of the excited electrons and holes recombine before contributing to the reaction, releasing energy as heat and light instead [5,6].
In addition to photocatalysis, sonocatalysis involves the activation of materials, known as sonocatalysts, by ultrasound waves in liquids through high-energy acoustic cavitation. The collapse of cavitation bubbles generates extreme heat and pressure, causing the pyrolytic cleavage of water molecules. This process thermally dissociates water into •OH and hydrogen atoms, often accompanied by sonoluminescence. The effectiveness of sonocatalysts arises from the inherent sonocatalytic activity of semiconductors [6,7]. In the medical field, sonocatalysts are pivotal in sonodynamic therapy, where ultrasound activates the sonocatalysts to produce ROS, selectively destroying tumor cells. Thus, sonocatalysts play a vital role in tackling environmental challenges and advancing medical therapies [8,9].
Sonocatalysts, primarily composed of metals or metal oxides, are widely utilized in environmental remediation and therapeutic applications to facilitate chemical reactions and degrade persistent contaminants. Among these materials, titanium dioxide (TiO2) is one of the most extensively studied in sonocatalysis [6,7,10,11]. Recent research has aimed to enhance the efficiency of TiO2-based sonocatalytic processes and expand their applications in both medical and environmental fields [12,13,14,15,16,17,18]. Incorporating TiO2 nanoparticles (NPs) with noble metals, such as gold (Au), has been shown to improve their sonocatalytic performance [19,20,21]. Noble metals not only extend the absorption spectrum through surface plasmon resonance but also serve as electron sinks, facilitating electron transfer and reducing electron–hole recombination.
Gold nanoclusters (Au NCs), tiny aggregates of Au atoms with diameters smaller than 2 nm, exhibit unique properties distinct from bulk gold and larger plasmonic Au NPs [22,23,24]. These clusters display unique electronic states due to quantum-size effects. Structurally, Au NCs consist of a gold atom core surrounded by a protective layer of organic ligands. Their electronic structures, characterized by specific energy levels such as the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), vary with cluster size and significantly influence their physicochemical properties. As innovative nanomaterials, Au NCs hold great potential in catalysis and luminescence, primarily due to their size-dependent properties and exceptional stability [25,26,27].
In our previous research, we explored the sonocatalytic potential of Au NCs [28,29]. Specifically, we synthesized Au144 NC/TiO2 composites by integrating Au144 NCs with TiO2 NPs. The composites exhibited significantly enhanced sonocatalytic activity compared to TiO2 alone under ultrasonication at 1 MHz [30]. The sonocatalytic activation of the Au144/TiO2 composite is primarily driven by the generation of high-energy cavitation bubbles during ultrasonication. The characteristics of these cavitation bubbles, which are influenced by the power and frequency of ultrasonication, play a crucial role in determining the sonocatalytic performance of the composite. Despite the promising results, the current literature provides limited insight into how ultrasonication frequency and power affect the sonocatalytic efficiency of Au NC/TiO2-based catalysts. To address this gap, our ongoing research aims to optimize the sonocatalytic performance of Au144/TiO2 composites by systematically refining ultrasonic irradiation parameters, including frequency, intensity, and catalyst concentration. By identifying the optimal conditions for ultrasonic catalysis, we aim to maximize the efficiency of the composites. Additionally, we investigated the size-dependent effects of Au NCs (including plasmonic Au NPs) on the performance of Au NC/TiO2-based catalysts. Specifically, we compared Au144 NCs, Au25 NCs, and plasmonic Au NPs. These findings have the potential to broaden the application of these advanced sonocatalytic materials in diverse industrial and environmental contexts.

2. Results and Discussion

2.1. Preparation of Au144/TiO2 Nanocomposite

The Au144/TiO2 nanocomposite was prepared by adsorbing Au144 NCs onto TiO2 NPs. The UV–Vis absorption spectrum of the Au144(pMBA)60 NCs exhibited a peak at 280 nm, corresponding to 4–mercaptobenzoic acid (pMBA), along with a broad peak at 520 nm, as shown in Figure 1a. These spectral features are consistent with previously reported characteristics of Au144(pMBA)60 [30,31]. TEM analysis confirmed that the Au144 NCs, comprising 3 wt.%, were successfully loaded onto the TiO2 NPs without signs of aggregation. The observed particle size was less than 2 nm (Figure 1b). The reflectance spectra of Au144 NCs (3.0 wt.%)/TiO2 were evaluated using the Kubelka–Munk (K–M) function, as shown in Figure 1c. This analysis demonstrated UV–vis absorption attributed to the loaded Au144 NCs, which was notably absent in pristine TiO2. Additionally, X-ray fluorescence (XRF) measurements conducted on the Au144 NC (3.0 wt.%)/TiO2 powder samples determined the gold content to be 2.6 ± 0.2 wt.% (Figure 1d).

2.2. Sonocatalysis of TiO2 vs. Au144/TiO2

When ultrasound was applied to a reaction solution containing the Au144/TiO2 sonocatalyst, two primary pathways for the generation of •OH radicals were identified: (i) thermal cleavage of water at localized hot spots, and (ii) water oxidation facilitated by the ultrasonically excited sonocatalysts [6,7,8,9]. To evaluate the sonocatalyst’s ability to produce •OH radicals, we measured the fluorescence intensity increase at 425 nm in a disodium terephthalate (NaTA) solution under ultrasonic conditions at 430 kHz and 5 W, both with and without the sonocatalyst. The presence of •OH radicals was detected using NaTA, which reacts with •OH radicals to form fluorescent 2-hydroxy disodium terephthalate (HTA) [30,32,33]. The observed increase in fluorescence intensity in the catalyst-containing reaction solution (ΔPLt) was compared to that in the absence of the catalyst (ΔPL0). The difference, ΔIntsono = ΔPLt − ΔPL0, represents the net contribution of the sonocatalyst to •OH radical generation. This parameter, ΔIntsono, is depicted in Figure 2a,b and serves as a quantitative indicator of sonocatalytic activity. The fluorescence intensity increase in the presence of Au144/TiO2 was higher than that observed for TiO2 alone. Our previous study demonstrated that Au144 NCs alone do not contribute to an increase in •OH radical production under sonication conditions [30]. These findings demonstrate that the deposition of Au144 NCs onto TiO2 NPs enhances the sonocatalytic activity, particularly in the generation of •OH radicals.
Highly energetic ultrasonic cavitation can potentially damage the Au144/TiO2 catalyst. However, our previous study has demonstrated that ultrasound treatment does not compromise the structural integrity of the catalyst, as confirmed by the diffuse reflectance spectra of the Au144/TiO2 catalyst before and after ultrasonic irradiation [30]. Furthermore, TEM images of the Au144 (3 wt.%)/TiO2 catalyst were acquired following ultrasonic irradiation for 6 min at 430 kHz and 5 W. While slight particle growth was observed (indicated by arrows in Figure S1), no significant aggregation of Au NCs was detected. The stability of the Au144/TiO2 catalyst during ultrasonic irradiation is attributed to the coordination of pMBA ligands on the TiO2 surface. Specifically, the pMBA ligands attach to TiO2 via their carboxyl groups, which deprotonate and bond to the TiO2 surface in a bidentate coordination mode [34,35]. This interaction likely plays a critical role in preserving the stability of the Au144/TiO2 catalyst under the current ultrasonic conditions.
Understanding the dependence of sonocatalytic activity on catalyst concentration is critical for optimizing reaction conditions. An appropriate catalyst concentration ensures maximum •OH radical generation while avoiding potential drawbacks such as light scattering or shielding effects that can occur at higher concentrations. To identify the optimal concentration for the Au144/TiO2 catalyst, we evaluated its performance at varying concentrations. We measured ΔPLt over a 6 min period of ultrasonic irradiation at a frequency of 430 kHz and an intensity of 5.0 W on reaction solutions containing different concentrations of Au144/TiO2 (0.04, 0.1, 0.4, and 1.0 mg/mL). Plots of ΔIntsono over the 6 min duration of irradiation are presented in Figure 2c. Across all reaction solutions, a notable increase in ΔIntsono was observed, indicating the oxidation of NaTA to HTA, accompanied by the generation of •OH radicals via Au144/TiO2. The ΔIntsono values followed the order 0.1 mg/mL > 0.4 mg/mL > 0.04 mg/mL > 1.0 mg/mL, suggesting that the catalytic activity is maximized at a concentration of 0.1 mg/mL. This behavior may be explained by the mechanism of sonocatalyst photoexcitation via sonoluminescence light absorption [32]. At optimal concentrations, the catalyst efficiently absorbs sonoluminescence light, resulting in enhanced •OH radical generation. However, at excessive concentrations, the catalyst may block the sonoluminescence light, limiting the excitation to catalyst particles near the sonoluminescence source. Consequently, the ability to generate •OH radicals decreases as the concentration increases beyond 0.4 mg/mL. Based on these findings, the optimal concentration of the Au144/TiO2 catalyst was determined to be 0.1 mg/mL, and this concentration was maintained for all subsequent experiments to ensure maximum sonocatalytic activity.

2.3. Ultrasound Frequency and Power Dependence

The efficiency of sonocatalytic reactions is highly dependent on ultrasonic frequency and power, as these parameters directly influence the generation and collapse of cavitation bubbles. The cavitation dynamics vary with frequency and power, affecting the energy distribution and localized conditions at cavitation hot spots [6,7,8,9]. Understanding the optimal frequency and power for the Au144/TiO2 sonocatalyst is crucial for maximizing •OH radical production and ensuring efficient sonocatalytic performance. Furthermore, investigating these parameters provides insight into the mechanisms of ultrasonic activation and the role of cavitation in sonocatalysis.
We investigated the influence of ultrasonic frequencies on ΔIntsono at 200 kHz, 430 kHz, and 950 kHz, as shown in Figure 3a. For all reaction solutions containing Au144/TiO2, the ΔIntsono values were consistently higher than those of pristine TiO2. The observed effect of ultrasonic frequency on enhanced sonocatalytic activity followed the order 430 kHz > 200 kHz > 950 kHz. This trend indicates that the generation rate of •OH radicals was maximized at 430 kHz, driven by the superior sonocatalytic performance of the Au144/TiO2 catalyst. The frequency dependence can be attributed to variations in cavitation and sonoluminescence efficiency at different frequencies.
To further evaluate the impact of ultrasonic power on sonocatalytic performance, we analyzed ΔIntsono at 430 kHz with power levels of 1.0 W, 3.0 W, and 5.0 W, as shown in Figure 3b. The power levels represent electrical power inputs. The results revealed a power-dependent increase in ΔIntsono, following the order 5.0 W > 3.0 W > 1.0 W. However, power levels exceeding 5 W did not significantly enhance ΔIntsono and posed risks of equipment damage, such as deterioration of the ultrasonic transmission gel. These observations suggest that 5.0 W represents an optimal power level, balancing effective •OH radical generation with equipment safety. At lower power levels, insufficient cavitation activity limits radical production, while excessively high power levels may lead to inefficiencies caused by energy dissipation or hardware constraints.

2.4. Mechanistic Insights into Enhanced Sonocatalytic Activity in Au144/TiO2

The enhanced sonocatalytic activity of Au144/TiO2 compared to TiO2 alone demonstrates the enhanced effects of Au144 NCs deposited on TiO2. Figure 4 illustrates the proposed mechanisms driving enhanced •OH radical generation in the Au144/TiO2 sonocatalyst system under ultrasonic irradiation. This model highlights two key processes, ultrasonic cavitation-induced excitation and charge separation, which together contribute to the system’s superior sonocatalytic performance. During ultrasonic irradiation, acoustic pressure cycles lead to the formation, growth, and collapse of cavitation bubbles, as depicted in Figure 4a. The collapse of these bubbles creates extreme localized conditions, such as transiently high temperatures and pressures at cavitation hot spots. These conditions induce thermal effects and UV sonoluminescence in the wavelength range of 200–400 nm [36], which promote the excitation of electrons in TiO2 from the valence band (VB) to the conduction band (CB).
Hydroxyl radicals (•OH) are generated through the thermal decomposition of water under the extremely high temperatures and pressures created during the adiabatic collapse of ultrasonic cavitation bubbles. This process forms localized hot spots characterized by transiently elevated temperatures and pressures, resulting in the homolytic cleavage of water molecules and subsequent formation of •OH radicals [37].
In this study, we monitored the generation of •OH radicals in water (without the use of catalysts) under ultrasonic irradiation at varying frequencies (200 kHz, 430 kHz, and 950 kHz) and power levels (1 W, 3 W, and 5 W) using the NaTA method, as shown in Figure 5. The increased absorbance intensity observed at 430 kHz suggests a higher occurrence of cavitation events, more intense bubble collapses, or both, which collectively enhance hydroxyl radical production at this frequency. Consequently, the higher generation of •OH radicals at 430 kHz reflects more favorable cavitation conditions, such as an increased number of bubbles or more violent bubble collapses, conducive to the efficient excitation of the Au144/TiO2 system. This enhanced excitation likely contributes to the maximum sonocatalytic activity observed at 430 kHz.
Previous studies have shown that sonoluminescence intensity peaks at intermediate frequencies, such as approximately 360 kHz, while it is diminished at both higher frequencies (e.g., around 1000 kHz) and lower frequencies (e.g., around 100 kHz) [38,39,40]. These results suggest that the efficient ultrasonic excitation of the Au144/TiO2 catalyst observed at 430 kHz is likely driven by cavitation-induced sonoluminescence.
The above results highlight the critical role of cavitation dynamics in activating the Au144/TiO2 catalyst and enhancing its sonocatalytic efficiency. Future studies will be needed on directly examining bubble dynamics using advanced techniques, such as high-speed imaging or sonoluminescence measurements, to establish a clearer link between bubble behavior and sonocatalytic activity across different frequencies [41]. These investigations are expected to provide deeper mechanistic insights into the optimal performance observed at 430 kHz.
As for the deposition of Au144 NCs on TiO2, the excited electrons in TiO2 are subsequently transferred to the Au144 NCs. This efficient electron migration suppresses electron–hole recombination, thereby prolonging the lifetime of charge carriers. The enhanced charge separation facilitates more effective water oxidation, increasing the production of •OH radicals, as depicted in Figure 4b.
To validate this hypothesis, transient photocurrent and sonocurrent measurements were conducted for Au144/TiO2 composites and bare TiO2. Transient photocurrent and sonocurrent measurements serve as direct experimental evidence for enhanced charge separation in photocatalytic and sonocatalytic systems. A higher photocurrent or sonocurrent indicates more efficient charge carrier generation and reduced recombination of electron–hole pairs. These measurements are thus widely used to evaluate the ability of a material to facilitate charge separation under irradiation or ultrasonic excitation [42]. As shown in Figure 6a, Au144/TiO2 exhibited a higher photocurrent density under light exposure compared to bare TiO2, indicating improved charge separation and transfer. Similarly, transient sonocurrent measurements at 430 kHz (Figure 6b) demonstrated enhanced electron generation in Au144/TiO2 compared to TiO2 alone. These findings provide strong evidence that Au144 NCs play a crucial role in facilitating charge separation and improving the overall sonocatalytic performance of the system.

2.5. Size Dependence in Sonocatalytic Activity in Au/TiO2

The size of AuNCs deposited on semiconductors, such as TiO2, is a critical factor influencing the catalytic performance of composite materials. Optimizing the size of AuNCs can significantly improve light absorption, charge separation, active site availability, and overall catalytic efficiency [27,43]. The transition from metallic to molecular behavior in Au nanoparticles significantly influences their catalytic properties. Previous studies reveal that this transition occurs between Au333 and Au144, corresponding to sizes of approximately 1.7–2.3 nm. Larger nanoparticles, such as AuB520 and AuB940, exhibit metallic characteristics, whereas Au144 and smaller particles demonstrate molecular-like behavior. This size-dependent transition affects the catalytic performance of Au25, Au38, Au144 (1.7 nm), and Au333 (2.3 nm), as observed in both CO oxidation and the electrocatalytic oxidation of alcohol. Among these, Au144 displayed the highest catalytic performance, emphasizing the critical role of nanoparticle size and electronic structure in optimizing catalytic efficiency [44].
This study examines how the size of Au NCs influences the sonocatalytic performance of Au/TiO2 catalysts. The catalytic efficiency of variously sized Au NCs, including plasmonic Au nanoparticles (NPs), was evaluated (Figure 7). The ΔIntsono values, which represent sonocatalytic efficiency, followed the order Au144 > Au25 > plasmonic Au NPs > bare TiO2. These findings highlight the superior performance of Au NCs compared to plasmonic Au NPs within the Au/TiO2 catalyst system. Ultra-small Au NCs (<2 nm) possess exceptionally high surface energy at the atomic level, significantly enhancing their catalytic activity. In contrast, larger Au NPs (~2.4 nm), with a relatively reduced reactive surface area, demonstrate diminished catalytic performance.
Among the studied catalysts, Au144 NCs exhibited the highest performance, surpassing Au25 NCs by achieving an optimal balance between electron migration and charge separation. Smaller clusters, such as Au25 NCs, are known to inject photoexcited electrons into the TiO2 conduction band upon light absorption [45]. To test this hypothesis, we conducted additional experiments to evaluate photocatalytic •OH radical generation under visible light irradiation, selectively exciting Au144 NCs without activating TiO2. The results revealed no detectable •OH radical generation in the Au144 NC/TiO2 system under visible light (Figure S2), whereas the Au25 NCs/TiO2 system did exhibit •OH radical production (Figure S3). This finding suggests that excited electrons from Au144 NCs make a negligible contribution to •OH radical generation in this system. This behavior is likely due to the efficient electron injection capability of Au25 NCs into TiO2. While smaller clusters like Au25 can efficiently inject electrons, they are prone to increased charge recombination due to bidirectional electron transfer, which can diminish the overall efficiency of photocatalytic and sonocatalytic reactions. In contrast, larger clusters like Au144 may strike a better balance between electron injection and charge recombination, resulting in enhanced overall performance for specific applications.
The transition from Au NCs to plasmonic Au NPs, as well as the size-dependent effects and electronic structure variations, remains only partially understood in the Au/TiO2 sonocatalyst system. Future research should focus on elucidating the interplay between size and electronic structure using advanced spectroscopy and computational modeling. These insights will deepen our understanding of Au-based sonocatalytic systems, emphasizing the critical role of Au NC size and electronic structure in enhancing sonocatalytic performance.

3. Materials and Methods

3.1. Reagents

Hydrogen tetrachloroaurate (III) tetrahydrate (HAuCl4·4H2O) and methanol were sourced from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan). Disodium terephthalate (NaTA) and 4–mercaptobenzoic acid (pMBA) were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Sodium borohydride (99%) (NaBH4) was procured from Sigma-Aldrich (St. Louis, MO, USA). The TiO2 utilized was Aeroxide®P25. Deionized water was prepared using a water distillation apparatus (Aquarius RFD250; ADVANTEC, Tokyo, Japan).

3.2. Instruments

UV–visible (UV–vis) absorption spectroscopy and steady-state fluorescence spectroscopy were performed using a JASCO V-670 spectrophotometer (JASCO Corp., Tokyo, Japan) and RF-6000 spectrofluorometer (Shimadzu Corp., Kyoto, Japan), respectively. Diffuse reflectance UV–vis spectra were acquired using an Ocean Optics DH-2000-BAL deuterium–tungsten–halogen light source and an Ocean Optics USB4000 compact fiber optic spectrometer (Ocean Optics, Dunedin, FL, USA). The Au loading on TiO2 NPs was quantified using an energy-dispersive X-ray fluorescence (XRF) spectrometer (JSX-1000S Element Eye; JEOL Ltd., Tokyo, Japan). The crystal structure of the TiO2 NP powder was analyzed by X-ray diffractometry (XRD, D2 Phaser, Bruker AXS GmbH, Karlsruhe, Germany), employing a Cu-Kα radiation source (λ = 1.5406 Å) over a 2θ range of 10–80°, at an accelerating voltage of 30 kV and a current of 10 mA. The size and morphology of Au144/TiO2 were characterized using transmission electron microscopy (TEM; JEOL JEM2100 microscope operating at 200 kV, JEOL Ltd., Tokyo, Japan).

3.3. Synthesis of Au144(pMBA)60

According to our previous study [30], Au144(pMBA)60 NCs (Au144 NCs) were synthesized at an ambient temperature by mixing HAuCl4 aqueous solution (25 mM, 3 mL), pMBA solution (75 mM, 3 mL), deionized water (6.5 mL), and methanol (12.5 mL), with methanol comprising 50% of the total volume of 25 mL. After stirring the solution overnight until it turned colorless, NaBH4 was added in cold deionized water (500 mM, 227 µL) and the mixture was stirred for an additional 2 h. After the addition of methanol, the NCs were isolated by centrifugation and the purification step was repeated. Finally, the NCs were dried under reduced pressure to obtain the purified product.

3.4. Synthesis of Au25(pMBA)18

The synthesis of Au25(pMBA)18 nanoclusters (Au25 NCs) was conducted at room temperature in air, following a previously reported method [30]. The successful synthesis of Au25(pMBA)18 was confirmed through UV–vis spectroscopy, which revealed two prominent absorption bands at 450 nm and 670 nm, along with a broad shoulder around 800 nm. These spectral features align well with previously reported characteristics of Au25 NCs [30].

3.5. Synthesis of Au Nanoparticles

Gold nanoparticles (Au NPs) with a size of 2.4 nm were synthesized under ambient conditions and at room temperature following the reported procedure [46]. Initially, 2.7 mL of the 28 mM HAuCl4 solution and 2.4 mL of the 95 mM pMBA solution were mixed. This was followed by the gradual addition of 20 mL of the 106 mM NaOH solution. The resulting reaction mixture was stirred continuously for 20 h to ensure complete interaction of the components. The final concentrations of the key reactants in the reaction mixture were as follows: HAuCl4, 3 mM; pMBA, 9 mM; and NaOH, 100 mM. To reduce the gold ions, 28.3 mg of NaBH4 was dissolved in 5 mL of cold deionized water, yielding a 150 mM NaBH4 solution. Subsequently, a 0.25 mL aliquot of this NaBH4 solution was added to the reaction mixture, maintaining a molar ratio of NaBH4:Au = 1:2. The reaction was stirred overnight to facilitate reduction. To complete the synthesis, 29.2 mg of NaCl was added to the reaction mixture to achieve a final NaCl concentration of 10 mM. Methanol (80% v/v) was then introduced and the mixture was centrifuged at 6000 rpm for 10 min. The resulting precipitate was collected, dried, and stored, yielding the final Au NPs. The successful synthesis of Au NPs was confirmed through UV–vis spectroscopy, which revealed a plasmonic absorption band at around 520 nm [37].

3.6. Preparation of Au/TiO2 Composites

Following the method described in our previous report [30], a Au144 NC-supported TiO2 composite was fabricated at ambient temperature. In a reaction tube, 8 mL of an aqueous TiO2 dispersion (1 mg/mL) was combined with 80 μL of an aqueous Au144 NC solution (3.0 mg/mL). The mixture was stirred at 800 rpm for 17 h to promote the adsorption of Au144 NCs onto the TiO2 particles, yielding a composite with a 3 wt.% loading of Au144 NCs on TiO2. After stirring, the mixture was centrifuged at 6000 rpm for 15 min to separate the supernatant, leaving the Au144 NC (3 wt.%)/TiO2 composite as a precipitate. Similarly, composites of Au25 NCs (3 wt.%) and Au NPs (3 wt.%) supported on TiO2 were prepared using the same procedure.

3.7. Evaluation of Sonocatalytic Activity

The sonocatalytic activity of Au/TiO2 nanocomposites was assessed by measuring the generation of •OH radicals from water under ultrasonic stimulation. The presence of •OH radicals was detected using disodium terephthalate (NaTA), which reacts with •OH radicals to form fluorescent 2-hydroxy disodium terephthalate (HTA) [30]. The production of •OH radicals was monitored by measuring the fluorescence intensity of HTA at 425 nm (excitation at 315 nm). To ensure consistent ultrasonic transmission and eliminate air gaps, an ultrasonic transmission gel was applied between the transducer and a plastic dish during the experiment. The experimental setup was maintained at a constant temperature of 18 ± 2 °C using a temperature-controlled water bath equipped with a precision thermostat. The reaction solution was subjected to ultrasonic irradiation at intervals of 0, 2, 4, and 6 min using a QUAVA Mini QR-003 ultrasonic device (Kaijo, Hamura, Japan) at frequencies of 200 kHz, 430 kHz, and 950 kHz. After each irradiation, the mixture was centrifuged at 14,000 rpm for 10 min to separate the catalyst. The fluorescence intensity of the supernatant was measured at 425 nm to quantify the production of •OH radicals.

3.8. Transient Photocurrent/Sonocurrent Measuremetns in TiO2 and Au144 (3 wt%)/TiO2

To determine whether supporting Au144 NCs on TiO2 improves charge separation, transient photocurrent and ultrasonic current responses were measured using a three-electrode cell connected to a potentiostat (BAS, ALS611 DE). The measurement parameters were as follows: mode—amperometry i-t curve; initial potential—−0.1 V; sample interval—0.1 s. A Pt mesh served as the counter electrode and an Ag/AgCl electrode (BAS, RE-1B) was used as the reference electrode. The working electrode was prepared by dispersing 5 mg of the catalyst in a mixture of 375 µL of water containing 20 wt.% Nafion (50 µL) and 125 µL of 2-propanol. A 40 µL aliquot of this dispersion was applied onto a 1 cm2 fluorine-doped tin oxide (FTO) glass conductive surface and dried under reduced pressure overnight.
All measurements were performed in a 0.2 M Na2SO4 aqueous solution. The transient photocurrent response was recorded under UV light (394 nm, 22.46 mW/cm2) using a UV Spotlight Source (L5662, Hamamatsu Photonics, Shizuoka, Japan). For the transient ultrasonic current response, a 200 kHz, 50 W ultrasound device (QUAVA mini QR-003, Kaijo Corporation, Hamura, Japan) was employed. To minimize temperature elevation in the electrolyte due to ultrasonic irradiation, the reaction vessel was placed on a cooling plate submerged in a water-filled container.

4. Conclusions

In this study, we investigated the sonocatalytic capabilities of Au144/TiO2 composites, focusing on the generation of •OH radicals under varying ultrasonic frequencies, power levels, and NC sizes. Our findings demonstrate that the Au144/TiO2 composite exhibits superior sonocatalytic performance, particularly at an ultrasonic frequency of 430 kHz and a power setting of 5.0 W. This heightened activity is attributed to the optimal cavitation dynamics achieved at 430 kHz, which maximizes localized thermal effects and sonoluminescence, facilitating efficient •OH radical generation.
The size dependence of the Au NCs was also found to play a critical role in determining the sonocatalytic performance in Au/TiO2 composites. The comparative analysis revealed a clear efficiency trend: Au144 > Au25 > plasmonic Au NPs > bare TiO2. Larger clusters like Au144 demonstrated superior activity due to their discrete electronic structures, which enable better energy alignment with the TiO2 conduction band, facilitating enhanced charge separation and prolonged carrier lifetimes. In contrast, plasmonic Au NPs were less efficient under ultrasonic activation.
This research highlights the practical potential of the Au144/TiO2 system for applications in environmental remediation and medical treatments. The insights gained into the interplay between ultrasonic frequency, cavitation dynamics, and nanocluster size offer a pathway for developing advanced sonocatalytic materials. Ultimately, this study contributes to the broader field of chemical physics by emphasizing the significance of NP-enhanced sonocatalysis in addressing global challenges, including pollution control and sustainable healthcare solutions.

Supplementary Materials

The following supplementary materials can be downloaded at https://www.mdpi.com/article/10.3390/molecules30030541/s1: Figure S1. TEM image of Au144 (3 wt.%)/TiO2 catalyst after ultrasonic irradiation for 6 min at 430 kHz and 5 W; Figure S2. Fluorescence spectra of a NaTA solution under xenon lamp (150 W, λ > 420 nm): TiO2 alone and Au144 NC (3 wt.%)/TiO2; Figure S3. Fluorescence spectra of a NaTA solution under xenon lamp (150 W, λ > 420 nm): TiO2 alone and Au25 NC (3 wt.%)/TiO2.

Author Contributions

Conceptualization: H.K. formulated the overarching research goals and aims, while T.T. framed the specific hypotheses to be tested; methodology: T.T. and Y.F. designed the experimental and analytical processes; investigation: T.T. conducted the primary experiments and data collection; writing—original draft preparation: T.T. drafted the manuscript; writing—review and editing: H.K. provided critical review, commentary, and revisions to the manuscript; visualization: T.T. created all diagrams and visual aids that helped to interpret data; supervision: H.K. oversaw the project, including coordination and strategic guidance; project administration: H.K. managed the project administration, including compliance with funding and ethical requirements. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by JSPS KAKENHI (grant no. JP 22H01915) and Kansai University Fund for Supporting formation of strategic Research Centers (university initiative type).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Gligorovski, S.; Strekowski, R.; Barbati, S.; Vione, D. Environmental Implications of Hydroxyl Radicals (•OH). Chem. Rev. 2015, 115, 13051–13092. [Google Scholar] [CrossRef]
  2. Mahbub, P.; Duke, M. Scalability of Advanced Oxidation Processes (AOPs) in Industrial Applications: A Review. J. Environ. Manag. 2023, 345, 118861. [Google Scholar] [CrossRef]
  3. Li, X.; Luo, R.; Liang, X.; Wu, Q.; Gong, C. Recent Advances in Enhancing Reactive Oxygen Species Based Chemodynamic Therapy. Chin. Chem. Lett. 2022, 33, 2213–2230. [Google Scholar] [CrossRef]
  4. Chen, D.; Xu, Q.; Wang, W.; Shao, J.; Huang, W.; Dong, X. Type I Photosensitizers Revitalizing Photodynamic Oncotherapy. Small 2021, 17, 2006742. [Google Scholar] [CrossRef] [PubMed]
  5. Khan, H.; Shah, M.U.H. Modification Strategies of TiO2-Based Photocatalysts for Enhanced Visible Light Activity and Energy Storage Ability: A Review. J. Environ. Chem. Eng. 2023, 11, 111532. [Google Scholar] [CrossRef]
  6. Abdurahman, M.H.; Abdullah, A.Z.; Shoparwe, N.F. A Comprehensive Review on Sonocatalytic, Photocatalytic, and Sonophotocatalytic Processes for the Degradation of Antibiotics in Water: Synergistic Mechanism and Degradation Pathway. Chem. Eng. J. 2021, 413, 127412. [Google Scholar] [CrossRef]
  7. Qiu, P.; Park, B.; Choi, J.; Thokchom, B.; Pandit, A.B.; Khim, J. A Review on Heterogeneous Sonocatalyst for Treatment of Organic Pollutants in Aqueous Phase Based on Catalytic Mechanism. Ultrason. Sonochem. 2018, 45, 29–49. [Google Scholar] [CrossRef]
  8. Wang, X.; Zhong, X.; Gong, F.; Chao, Y.; Cheng, L. Newly Developed Strategies for Improving Sonodynamic Therapy. Mater. Horiz. 2020, 7, 2028–2046. [Google Scholar] [CrossRef]
  9. He, Z.; Du, J.; Miao, Y.; Li, Y. Recent Developments of Inorganic Nanosensitizers for Sonodynamic Therapy. Adv. Healthc. Mater. 2023, 12, 2300234. [Google Scholar] [CrossRef]
  10. Yang, Y.; Wang, X.; Qian, H.; Cheng, L. Titanium-Based Sonosensitizers for Sonodynamic Cancer Therapy. Appl. Mater. Today 2021, 25, 101215. [Google Scholar] [CrossRef]
  11. Maleki, A.; Seyedhamzeh, M.; Yuan, M.; Agarwal, T.; Sharifi, I.; Mohammadi, A.; Kelicen-Uğur, P.; Hamidi, M.; Malaki, M.; Al Kheraif, A.A.; et al. Titanium-Based Nanoarchitectures for Sonodynamic Therapy-Involved Multimodal Treatments. Small 2023, 19, 22062. [Google Scholar] [CrossRef]
  12. Priya, M.H.; Madras, G. Kinetics of TiO2-Catalyzed Ultrasonic Degradation of Rhodamine Dyes. Ind. Eng. Chem. Res. 2006, 45, 913–921. [Google Scholar] [CrossRef]
  13. Khataee, A.; Sheydaei, M.; Hassani, A.; Taseidifar, M.; Karaca, S. Sonocatalytic Removal of an Organic Dye Using TiO2/Montmorillonite Nanocomposite. Ultrason. Sonochem. 2015, 22, 404–411. [Google Scholar] [CrossRef] [PubMed]
  14. Khataee, A.; Kayan, B.; Gholami, P.; Kalderis, D.; Akay, S. Sonocatalytic Degradation of an Anthraquinone Dye Using TiO2-Biochar Nanocomposite. Ultrason. Sonochem. 2017, 39, 120–128. [Google Scholar] [CrossRef] [PubMed]
  15. Choi, Y.; Lee, D.; Hong, S.; Khan, S.; Darya, B.; Lee, J.Y.; Chung, J.; Cho, S.H. Investigation of the Synergistic Effect of Sonolysis and Photocatalysis of Titanium Dioxide for Organic Dye Degradation. Catalysts 2020, 10, 500. [Google Scholar] [CrossRef]
  16. Xu, D.; Ma, H. Degradation of Rhodamine B in Water by Ultrasound-Assisted TiO2 Photocatalysis. J. Clean. Prod. 2021, 313, 127758. [Google Scholar] [CrossRef]
  17. Sheikhmohammadi, A.; Asgari, E.; Nourmoradi, H.; Fazli, M.M.; Yeganeh, M. Ultrasound-Assisted Decomposition of Metronidazole by Synthesized TiO2/Fe3O4 Nanocatalyst: Influencing Factors and Mechanisms. J. Environ. Chem. Eng. 2021, 9, 105844. [Google Scholar] [CrossRef]
  18. Jonnalagadda, U.S.; Su, X.; Kwan, J.J. Nanostructured TiO2 Cavitation Agents for Dual-Modal Sonophotocatalysis with Pulsed Ultrasound. Ultrason. Sonochem. 2021, 73, 105530. [Google Scholar] [CrossRef]
  19. Deepagan, V.G.; You, D.G.; Um, W.; Ko, H.; Kwon, S.; Choi, K.Y.; Yi, G.R.; Lee, J.Y.; Lee, D.S.; Kim, K.; et al. Long-Circulating Au-TiO2 Nanocomposite as a Sonosensitizer for ROS-Mediated Eradication of Cancer. Nano Lett. 2016, 16, 6257–6264. [Google Scholar] [CrossRef] [PubMed]
  20. Cao, Y.; Wu, T.; Dai, W.; Dong, H.; Zhang, X. TiO2 Nanosheets with the Au Nanocrystal-Decorated Edge for Mitochondria-Targeting Enhanced Sonodynamic Therapy. Chem. Mater. 2019, 31, 9105–9114. [Google Scholar] [CrossRef]
  21. Li, F.; Pan, Q.; Ling, Y.; Guo, J.; Huo, Y.; Xu, C.; Xiong, M.; Yuan, M.; Cheng, Z.; Liu, M.; et al. Gold–Titanium Dioxide Heterojunction for Enhanced Sonodynamic Mediated Biofilm Eradication and Peri-Implant Infection Treatment. Chem. Eng. J. 2021, 413, 127412. [Google Scholar] [CrossRef]
  22. Zhang, B.; Chen, J.; Cao, Y.; Chai, O.J.H.; Xie, J. Ligand Design in Ligand-Protected Gold Nanoclusters. Small 2021, 17, 2004381. [Google Scholar] [CrossRef]
  23. Yang, Z.; Yang, X.; Guo, Y.; Kawasaki, H. Review on Gold Nanoclusters for Cancer Phototherapy. ACS Appl. Bio Mater. 2023, 6, 4504–4517. [Google Scholar] [CrossRef]
  24. Albright, E.L.; Levchenko, T.I.; Kulkarni, V.K.; Sullivan, A.I.; DeJesus, J.F.; Malola, S.; Takano, S.; Nambo, M.; Stamplecoskie, K.; Häkkinen, H.; et al. Atomically Precise Gold Nanoclusters: Synthesis, Properties, and Applications. J. Am. Chem. Soc. 2024, 146, 5759–5780. [Google Scholar] [CrossRef] [PubMed]
  25. Kawawaki, T.; Negishi, Y.; Kawasaki, H. Photo/Electrocatalysis and Photosensitization Using Metal Nanoclusters for Green Energy and Medical Applications. Nanoscale Adv. 2020, 2, 17–36. [Google Scholar] [CrossRef]
  26. Hossain, S.; Hirayama, D.; Ikeda, A.; Ishimi, M.; Funaki, S.; Samanta, A.; Kawawaki, T.; Negishi, Y. Atomically Precise Thiolate-Protected Gold Nanoclusters: Current Status of Designability of the Structure and Physicochemical Properties. Aggregate 2023, 4, e255. [Google Scholar] [CrossRef]
  27. Li, S.; Du, X.; Liu, Z.; Li, Y.; Shao, Y.; Jin, R. Size Effects of Atomically Precise Gold Nanoclusters in Catalysis. Precis. Chem. 2023, 1, 14–28. [Google Scholar] [CrossRef]
  28. Kawamura, K.; Hikosou, D.; Inui, A.; Yamamoto, K.; Yagi, J.; Saita, S.; Kawasaki, H. Ultrasonic Activation of Water-Soluble Au25(SR)18 Nanoclusters for Singlet Oxygen Production. J. Phys. Chem. C 2019, 123, 26644–26652. [Google Scholar] [CrossRef]
  29. Yagi, J.; Ikeda, A.; Wang, L.-C.; Yeh, C.-S.; Kawasaki, H. Singlet Oxygen Generation Using Thiolated Gold Nanoclusters under Photo- and Ultrasonic Excitation: Size and Ligand Effect. J. Phys. Chem. C 2022, 126, 19693–19703. [Google Scholar] [CrossRef]
  30. Kawamura, K.; Ikeda, A.; Inui, A.; Yamamoto, K.; Kawasaki, H. TiO2-Supported Au144 Nanoclusters for Enhanced Sonocatalytic Performance. J. Chem. Phys. 2021, 155, 124702. [Google Scholar] [CrossRef] [PubMed]
  31. Yan, N.; Xia, N.; Liao, L.; Zhu, M.; Jin, F.; Jin, R.; Wu, Z. Unraveling the Long-Pursued Au144 Structure by X-Ray Crysta lography. Sci. Adv. 2018, 4, eaat7259. [Google Scholar] [CrossRef]
  32. Fang, X.; Mark, G.; von Sonntag, C. OH Radical Formation by Ultrasound in Aqueous Solutions. Part I: The Chemistry Underlying the Terephthalate Dosimeter. Ultrason. Sonochem. 1996, 3, 57–63. [Google Scholar] [CrossRef]
  33. Zhao, G.; Chen, T.; Tang, A.; Yang, H. Modulating Coal-Derived Carbon toward Electrocatalytic Generation of Hydroxyl Radi cals for Organic Contaminant Removal. J. Mater. Chem. A 2024, 12, 7227–7236. [Google Scholar] [CrossRef]
  34. Yang, L.; Jiang, X.; Ruan, W.; Zhao, B.; Xu, W.; Lombardi, J.R. Adsorption Study of 4-MBA on TiO2 Nanoparticles by Surface-Enhanced Raman Spectroscopy. J. Raman Spectrosc. 2009, 40, 2000–2005. [Google Scholar] [CrossRef]
  35. Compeán-González, C.L.; Thomas, A.G.; Syres, K.L.; Cole, J.; Li, Z. 4-Mercaptobenzoic Acid Adsorption on TiO2 Anatase (101) and TiO2 Rutile (110) Surfaces. Surfaces 2022, 5, 238–250. [Google Scholar] [CrossRef]
  36. Didenko, Y.T.; Pugach, S.P. Spectra of Water Sonoluminescence. J. Phys. Chem. 1994, 98, 9742–9749. [Google Scholar] [CrossRef]
  37. Yasui, K. Production of O Radicals from Cavitation Bubbles under Ultrasound. Molecules 2022, 27, 4788. [Google Scholar] [CrossRef] [PubMed]
  38. Beckett, M.A.; Hua, I. Impact of Ultrasonic Frequency on Aqueous Sonoluminescence and Sonochemistry. J. Phys. Chem. A 2001, 105, 3796–3802. [Google Scholar] [CrossRef]
  39. Zhou, M.; Mohd Yusof, N.S.; Ashokkumar, M. Correlation between Sonochemistry and Sonoluminescence at Various Frequencies. RSC Adv. 2013, 3, 9319–9324. [Google Scholar] [CrossRef]
  40. Ji, R.; Pflieger, R.; Virot, M.; Nikitenko, S.I. Multibubble Sonochemistry and Sonoluminescence at 100 kHz: The Missing Link between Low- and High-Frequency Ultrasound. J. Phys. Chem. B 2018, 122, 6989–6994. [Google Scholar] [CrossRef] [PubMed]
  41. Yasui, K. Multibubble Sonoluminescence from a Theoretical Perspective. Molecules 2021, 26, 4624. [Google Scholar] [CrossRef] [PubMed]
  42. Lei, H.; He, Q.; Wu, M.; Xu, Y.; Sun, P.; Dong, X. Piezoelectric Polarization Promoted Spatial Separation of Photoexcited Electrons and Holes in Two-Dimensional g-C3N4 Nanosheets for Efficient Elimination of Chlorophenols. J. Hazard. Mater. 2022, 421, 126696. [Google Scholar] [CrossRef] [PubMed]
  43. Liu, Y.; Wierzbicka, E.; Springer, A.; Pinna, N.; Wang, Y. Influence of the Electronic Properties of the Ligand on the Photoelectrochemical Behavior of Au25 Nanocluster-Sensitized TiO2 Photoanode. J. Phys. Chem. C 2022, 126, 1778–1784. [Google Scholar] [CrossRef]
  44. Zhou, M.; Zeng, C.; Chen, Y.; Jin, R. Evolution from the Plasmon to Exciton State in Ligand-Protected Atomically Precise Gold Nanoparticles. Nat. Commun. 2016, 7, 13240. [Google Scholar] [CrossRef] [PubMed]
  45. Khan, R.; Naveen, M.H.; Abbas, M.A.; Lee, J.; Kim, H.; Bang, J.H. Photoelectrochemistry of Au Nanocluster-Sensitized TiO2: Intricacy Arising from the Light-Induced Transformation of Nanoclusters into Nanoparticles. ACS Energy Lett. 2021, 6, 24–32. [Google Scholar] [CrossRef]
  46. Shaltiel, L.; Shemesh, A.; Raviv, U.; Barenholz, Y.; Levi-Kalisman, Y. Synthesis and Characterization of Thiolate-Protected Gold Nanoparticles of Controlled Diameter. J. Phys. Chem. C 2019, 123, 28486–28493. [Google Scholar] [CrossRef]
Molecules 30 00541 g001
Figure 1. (a) UV–Vis absorption spectrum of Au144 NCs. (b) TEM image of Au144 (3 wt.%)/TiO2. (c) Absorption spectrum of Au144 (3 wt.%)/TiO2. (d) XRF spectrum of Au144 NC (3 wt.%)/TiO2.
Figure 1. (a) UV–Vis absorption spectrum of Au144 NCs. (b) TEM image of Au144 (3 wt.%)/TiO2. (c) Absorption spectrum of Au144 (3 wt.%)/TiO2. (d) XRF spectrum of Au144 NC (3 wt.%)/TiO2.
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Figure 2. Fluorescence spectra of a NaTA solution during ultrasonic irradiation for 6 min at 430 kHz and 5 W, with (a) Au144 NC (3 wt.%)/TiO2 and (b) TiO2 alone. Catalyst concentration: 0.1 mg/mL. (c) Fluorescence intensity increase (ΔIntsono) at 425 nm after 6 min of ultrasonic irradiation with varying concentrations of Au144 NC (3 wt.%)/TiO2: 0.04 mg/mL, 0.1 mg/mL, 0.4 mg/mL, and 1.0 mg/mL.
Figure 2. Fluorescence spectra of a NaTA solution during ultrasonic irradiation for 6 min at 430 kHz and 5 W, with (a) Au144 NC (3 wt.%)/TiO2 and (b) TiO2 alone. Catalyst concentration: 0.1 mg/mL. (c) Fluorescence intensity increase (ΔIntsono) at 425 nm after 6 min of ultrasonic irradiation with varying concentrations of Au144 NC (3 wt.%)/TiO2: 0.04 mg/mL, 0.1 mg/mL, 0.4 mg/mL, and 1.0 mg/mL.
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Figure 3. (a) Fluorescence intensity increase (ΔIntsono) at 425 nm after ultrasonic irradiation at 3 W for 6 min with TiO2 alone and Au144 (3 wt.%)/TiO2 at different ultrasonic frequencies: 200 kHz, 430 kHz, and 950 KHz. Catalyst concentration: 0.1 mg/mL. (b) Fluorescence intensity increase (ΔIntsono) at 425 nm after ultrasonic irradiation at 430 kHz for 6 min with Au144 (3 wt.%)/TiO2 at varying ultrasonic power levels: 1 W, 3 W, and 5 W. Catalyst concentration: 0.1 mg/mL.
Figure 3. (a) Fluorescence intensity increase (ΔIntsono) at 425 nm after ultrasonic irradiation at 3 W for 6 min with TiO2 alone and Au144 (3 wt.%)/TiO2 at different ultrasonic frequencies: 200 kHz, 430 kHz, and 950 KHz. Catalyst concentration: 0.1 mg/mL. (b) Fluorescence intensity increase (ΔIntsono) at 425 nm after ultrasonic irradiation at 430 kHz for 6 min with Au144 (3 wt.%)/TiO2 at varying ultrasonic power levels: 1 W, 3 W, and 5 W. Catalyst concentration: 0.1 mg/mL.
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Figure 4. Conceptual diagram illustrating enhanced •OH radical generation in Au144/TiO2 sonocatalyst system. (a) Thermal and sonoluminescence generation at cavitation hot spots during ultrasonication. (b) Enhanced mechanism for •OH radical generation involving the migration of excited electrons from TiO2 to Au144 NCs, simultaneously inhibiting electron–hole recombination.
Figure 4. Conceptual diagram illustrating enhanced •OH radical generation in Au144/TiO2 sonocatalyst system. (a) Thermal and sonoluminescence generation at cavitation hot spots during ultrasonication. (b) Enhanced mechanism for •OH radical generation involving the migration of excited electrons from TiO2 to Au144 NCs, simultaneously inhibiting electron–hole recombination.
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Figure 5. (a) Fluorescence intensity increase (ΔPL) in NaTA aqueous solutions (no catalyst) at 425 nm after 6 min of ultrasonic irradiation at 3 W, varying the ultrasonic frequencies of 200 kH, 430 kHz, and 950 kHz. (b) Fluorescence intensity increase (ΔPL) in NaTA aqueous solutions (no catalyst) at 425 nm after 6 min of ultrasonic irradiation at 450 kHz, varying the ultrasonic power of 1 W, 3 W, and 5 W.
Figure 5. (a) Fluorescence intensity increase (ΔPL) in NaTA aqueous solutions (no catalyst) at 425 nm after 6 min of ultrasonic irradiation at 3 W, varying the ultrasonic frequencies of 200 kH, 430 kHz, and 950 kHz. (b) Fluorescence intensity increase (ΔPL) in NaTA aqueous solutions (no catalyst) at 425 nm after 6 min of ultrasonic irradiation at 450 kHz, varying the ultrasonic power of 1 W, 3 W, and 5 W.
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Figure 6. (a) Transient photocurrent profile and (b) transient sonocurrent profile comparison between TiO2 and Au144 (3 wt%)/TiO2.
Figure 6. (a) Transient photocurrent profile and (b) transient sonocurrent profile comparison between TiO2 and Au144 (3 wt%)/TiO2.
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Figure 7. Fluorescence intensity increase (ΔIntsono) at 425 nm after ultrasonic irradiation at 5 W for 6 min with TiO2 alone and Au (3 wt.%)/TiO2 at an ultrasonic frequency of 430 kHz. Catalyst concentration: 0.1 mg/mL.
Figure 7. Fluorescence intensity increase (ΔIntsono) at 425 nm after ultrasonic irradiation at 5 W for 6 min with TiO2 alone and Au (3 wt.%)/TiO2 at an ultrasonic frequency of 430 kHz. Catalyst concentration: 0.1 mg/mL.
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Tsurunishi, T.; Furui, Y.; Kawasaki, H. Ultrasonic Activation of Au Nanoclusters/TiO2: Tuning Hydroxyl Radical Production Through Frequency and Nanocluster Size. Molecules 2025, 30, 541. https://doi.org/10.3390/molecules30030541

AMA Style

Tsurunishi T, Furui Y, Kawasaki H. Ultrasonic Activation of Au Nanoclusters/TiO2: Tuning Hydroxyl Radical Production Through Frequency and Nanocluster Size. Molecules. 2025; 30(3):541. https://doi.org/10.3390/molecules30030541

Chicago/Turabian Style

Tsurunishi, Takaaki, Yuzuki Furui, and Hideya Kawasaki. 2025. "Ultrasonic Activation of Au Nanoclusters/TiO2: Tuning Hydroxyl Radical Production Through Frequency and Nanocluster Size" Molecules 30, no. 3: 541. https://doi.org/10.3390/molecules30030541

APA Style

Tsurunishi, T., Furui, Y., & Kawasaki, H. (2025). Ultrasonic Activation of Au Nanoclusters/TiO2: Tuning Hydroxyl Radical Production Through Frequency and Nanocluster Size. Molecules, 30(3), 541. https://doi.org/10.3390/molecules30030541

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