
==== Front
Ultrason Sonochem
Ultrason Sonochem
Ultrasonics Sonochemistry
1350-4177
1873-2828
Elsevier

S1350-4177(24)00284-0
10.1016/j.ultsonch.2024.107036
107036
Original Research Article
ZnO quantum dots decorated BaTiO3 for cancer sonodynamic therapy
Zhang Xiaojian
Zhang Zhiqin
Yuan Honglei
Sun Xianke sunxianke666@163.com
⁎
School of Physics and Telecommunication Engineering, Zhoukou Normal University, Zhoukou 466001, China
⁎ Corresponding author. sunxianke666@163.com
19 8 2024
11 2024
19 8 2024
110 10703613 5 2024
10 8 2024
18 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Sonodynamic therapy depending on ultrasound irradiation, which generates reactive species to kill cancer cells, has attracted considerable attention due to the deep tissue penetration depth. However, the insufficient separation of electron/hole pairs induces its limited therapeutic efficiency. Herein, we use oxygen vacancy and ZnO quantum dots decoration techniques to enhance electron/hole separation and reactive species production. In oxygen vacancy-engineered BaTiO3, the higher oxygen vacancy concentration leads to more efficient adsorption of activate O2 and thus results in production of more radicals. In BaTiO3/ZnO heterostructures, the built-in electric field further improves separation of electron/hole pairs. The separated electron/hole react with O2/H2O to produce reactive species of •OH/∙O2- and kill cancer cells upon ultrasound irradiation. The work provides a guidance for sonosensitizers to tumor therapy.

Keywords

BaTiO3/ZnO
Oxygen vacancy
Heterostructure
Sonodynamic therapy
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pmc1 Introduction

Sonodynamic therapy is a safe and noninvasive therapeutic technique for treating deep-seated tumors deriving from photodynamic therapy, which stimulates sonochemical reactions depending on the acoustic cavitation effects of ultrasound via sonosensitizers and thus leads to the production of reactive species [1], [2]. Comparing with light used in photodynamic therapy, ultrasound has advantages of efficient tissue penetrability and accurate targeting ability, which can surmount deficiencies of tissue penetration depth and treatment efficacy to deep-seated tumors in photodynamic therapy [3]. Sonosensitizers can be organic or inorganic materials. A majority of organic sonosensitizers are phototoxic to skin and induce higher sensitivity of skin, which limits applications of organic sonosensitizers in sonodynamic therapy [4]. Inorganic sonosensitizers, mostly semiconductor nanoparticles and composites of them, develop rapidly because of their advantages of good biosafety and stability [2]. And various inorganic sonosensitizers, piezoelectric semiconductor, i.e. BaTiO3 [5], ZnO [6], and CoSnO3 [7], show potentials in sonodynamic therapy due to their excellent responsiveness to mechanical force. However, in most of piezoelectric sonosensitizers, the insufficient charge separation efficiency impedes the production of reactive species [3]. For enhancing production efficiency of reactive species, some approaches, such as surface defect engineering [8], doping [9], and formation of heterojunction [10], have been developed.

The construction of heterojunction between semiconductors results in the coupled interface, which leads to a band alignment or a rectifying contact as soon as the Fermi levels equilibrate at the interface [11]. This extends lifetimes of electrons (e − ) and holes (h+), and thus restricts rapid recombination of them [12]. So far, some semiconductor-heterojunction has been reported for sonodynamic therapy, such as Cu2−xO−BaTiO3 [10], Co3O4@TiO2-x [13], Bi/BiVO4 [14], and CoWO4/FeWO4 [15]. The results show that the construction of heterojunction brings about higher redox activity for production of reactive species. It is known that BaTiO3 owns remarkable piezoelectric effect but limited electric conductivity, but ZnO owns excellent electron mobility and relatively low piezoelectric coefficient [16]. In addition, it has been found that ZnO quantum dots show evident quantum size effects [17]: On one hand, the quantum size effect results in more arrivals of charge carriers to the surface via larger ratio of surface to volume; On the other hand, the quantum size effect leads to the enhancement of forbidden band gap width and thus more rapid diffusion of charge carriers to surface, which brings about higher densities of excess charges and thus relative easy conquest to potential barrier of photoreductive chemical process. Herein, we report synthesis, physical/chemical characteristics, and sonodynamic therapy performance of piezoelectric ZnO quantum dots decorated BaTiO3 heterostructure. The results show that the ultrasound induced electric field results in band bending and thus brings about more efficient production of reactive species. A straightforward annealing process under H2 results in reduction of Ti4+ in BaTiO3 and formation of oxygen vacancies (OV), which leads to higher catalytic activity of BaTiO3. More importantly, the heterojunction gives higher efficient production of hydroxyl (•OH) and superoxide (∙O2-) radicals than those of pure BaTiO3 and ZnO.

2 Synthesis and characterization

The details of preparation and characterization are provided in Supporting Materials. In the article, the obtained ZnO quantum dots and BaTiO3 nanoparticles are labeled as ZnO and BaTiO3. The heat-treated BaTiO3 nanoparticles under 5 %H2/95 %N2 atmosphere are labeled at BaTiO3(H2). The BaTiO3(H2)/ZnO heterostructure with the addition of 0.2 mol, 0.15 mol, 0.1 mol and 0.09 mol BaTiO3(H2) nanoparticles were labeled as BaTiO3(H2)/ZnO1, BaTiO3(H2)/ZnO2, BaTiO3(H2)/ZnO3 and BaTiO3(H2)/ZnO4, respectively.

3 Results and discussion

Fig. 1 shows X-ray diffraction (XRD) patterns of the prepared ZnO, BaTiO3, BaTiO3(H2), and BaTiO3(H2)/ZnO heterostructures. The XRD patterns of ZnO quantum dots and BaTiO3 nanoparticles fit well with the XRD data of hexagonal ZnO (JCPDs Card No. 05–0664) and tetragonal BaTiO3 (JCPDs Card No. 05–0626), respectively. No other diffraction lines corresponding to impurities are observed. These demonstrate the single and pure phase of them. After heat-treatment under H2/N2 atmosphere, the XRD patterns of the obtained BaTiO3(H2) nanoparticles barely change, suggesting that the heat-treatment has no influence on phase of products. For BaTiO3(H2)/ZnO1 and BaTiO3(H2)/ZnO2, the corresponding diffraction lines of ZnO are not detected on account of relative low content. For BaTiO3(H2)/ZnO3 and BaTiO3(H2)/ZnO4, the diffraction lines of ZnO are clearly observed, demonstrating the successful production of BaTiO3(H2)/ZnO heterostructures.Fig. 1 XRD patterns of the fabricated samples.

The microstructures of products are exhibited by images of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. S1 gives a SEM image of BaTiO3(H2) nanoparticles (A) and a TEM image of ZnO quantum dots (B). For BaTiO3, agglomerated nanoparticles with random distribution of grain size are observed. For ZnO quantum dots, they also tend to agglomerate together. Fig. 2A gives a SEM image of BaTiO3(H2)/ZnO3 heterostructure. Comparing with the BaTiO3(H2) nanoparticles, the microstructure of BaTiO3(H2)/ZnO3 heterostructure barely changes. Fig. 2B gives a TEM image of BaTiO3(H2)/ZnO3 heterostructure, showing that lots of nanoparticles with sizes smaller than 10 nm load on the surface of nanoparticles with sizes larger than 40 nm. To confirm the nanoparticles with different sizes, a high-resolution transmission electron microscopy (HRTEM) image is shown by Fig. 2C. The lattice fringes with a spacing of 2.84 Å correspond to (1 1 0) plane of BaTiO3 [18], and the lattice fringes with a spacing of 2.61 Å correspond to (0 0 2) plane of ZnO [19]. Fig. S2 provides a SEM image and corresponding elemental distribution maps of BaTiO3(H2)/ZnO3 heterostructure. The distribution of Ba, Ti, O and Zn is uniform. The zeta potential and hydrodynamic diameter are crucial characteristics for nanoparticles since they influence particles’ stability. Fig. S3 provides zeta-potential of the synthesized particles. The zeta-potential of the BaTiO3(H2) is much less negative than BaTiO3 due to the transitions from Ti4+ to Ti3+ [20]. For BaTiO3(H2)/ZnO heterostructures, the zeta-potential changes following the increase of the ZnO content, indicating the two moieties bind to each other [21]. Fig. S4 provides average hydrodynamic diameters of ZnO quantum dots, BaTiO3 and BaTiO3(H2) nanopatticles, as well as BaTiO3(H2)/ZnO3 heterostructure in water stored for 1 and 7 days. The average hydrodynamic diameters change slightly, indicating the excellent stability of them.Fig. 2 SEM (A), TEM (B) and HRTEM of BaTiO3(H2)/ZnO3 heterostructure.

X-ray photoelectron spectroscopy (XPS) is conducted for analyzing chemical state of elements. Fig. S5 gives the full XPS spectra of ZnO, BaTiO3, BaTiO3(H2), and BaTiO3(H2)/ZnO3. Corresponding energy peaks of Zn, Ba, Ti and O are confirmed in the XPS spectra. The comparison results of high resolution XPS (HR-XPS) spectra for Ba 3d, Ti 2p, Zn 2p and O 1 s are provided by Fig. 3. As shown in Fig. 3A, two peaks corresponding to Ba 3d5/2 and Ba 3d3/2 are observed, indicating the existence of Ba2+ in the perovskite BaTiO3 [16]. Shoulders correspond to Ba 3d of non-perovskite BaCO3 exist in HR-XPS spectra, which is difficult to avoid due to atmospheric CO2 [22]. Fig. 3B provides the HR-XPS spectra of Ti 2p for BaTiO3, BaTiO3(H2) and BaTiO3(H2)/ZnO3. Two peaks corresponding to Ti 2p3/2 and Ti 2p1/2 with a binding energy difference of 5.6 eV are observed, indicating the presence of Ti4+ in them [23]. For BaTiO3(H2) and BaTiO3(H2)/ZnO3, shoulders appear in HR-XPS spectra of Ti 2p, which corresponds to Ti3+ 2p and illustrates reduction from Ti4+ to Ti3+ after heat-treatment under 5 %H2/95 %N2 atmosphere [20]. Fig. 3C shows the HR-XPS spectra of Zn 2p for ZnO quantum dots and BaTiO3(H2)/ZnO3. Two peaks corresponding to Zn 2p3/2 and Zn 2p1/2 with a binding energy difference of 23.1 eV are observed, indicating Zn exits with the + 2 oxidation state [23]. The spectra also show that energy peaks of Ba 3d, Ti 2p and Zn 2p for BaTiO3(H2)/ZnO3 shift to larger values comparing with those of pure BaTiO3 and ZnO, which demonstrates the formation of heterostructure between BaTiO3 and ZnO, and is induced by the existence of built-in electric field in the heterojunction [24]. Fig. 3d shows the HR-XPS spectra of O 1 s for ZnO, BaTiO3, BaTiO3(H2), and BaTiO3(H2)/ZnO3. The energy peaks of lattice oxygen (OL) for ZnO and BaTiO3/BaTiO3(H2) locate at 530.4 and 529.9 eV, respectively. And the energy peak of OL for BaTiO3(H2)/ZnO3 locates at 530.1 eV, which also demonstrates the formation of heterostructure between BaTiO3 and ZnO [24]. For BaTiO3(H2) and BaTiO3(H2)/ZnO3, a shoulder peaking at 531.2 eV appears, which corresponds to OV [25]. The formation of OV is induced by the reduction of Ti4+ to Ti3+ (2Ti4+ + OL→2Ti3+ + OV+1/2O2). The formation of Ti3+ and OV is also confirmed by the electron paramagnetic resonance (EPR) results. As shown in Fig. S6, the signals at g = 2.00 and 1.97 correspond to VO and Ti3+-VO-Ti3+, respectively [26], [27]. The BaTiO3(H2) shows much stronger signals than that of BaTiO3, indicating much higher concentrations of VO and Ti3+ defects.Fig. 3 HR-XPS of Ba 3d (A), Ti 2p (B), Zn 2p (C) and O 1 s for the fabricated samples.

To valuate cancer cell killing effects of the fabricated nanoparticles, the cellular experiments to kill mouse 4 T1 breast cancer cells were carried out under ultrasound irradiation. Fig. 4 provides the experimental results without and with the presence of the fabricated nanoparticles. The killing effect shows clear dependence on concentration and type of added nanoparticle. As more nanoparticles are added, higher killing effect is obtained, which is induced by more generation of reactive species. Meanwhile, the BaTiO3(H2)/ZnO3 nanoparticles show the highest killing efficiency to 4 T1 cell. The viable cell percentage is about 23.6 % when its concentration is 100 μ g/mL. Its killing efficiency is respectively 2.1 and 1.7 times that of ZnO and BaTiO3, indicating more efficient generation of reactive species with the presence of BaTiO3(H2)/ZnO3 nanoparticles under ultrasound irradiation. Since H2O2 is generally overproduced in cancer cells under in vivo conditions, H2O2 (50 μ M) was then added in cellular experiments to simulate tumor microenvironment. After the addition of H2O2, the killing efficiency to 4 T1 cell is further enhanced. The killing efficiency is further enhanced to be 88.5 % for BaTiO3(H2)/ZnO3 nanoparticles with a concentration of 100 μ g/mL. This result demonstrates that the synergy of sonodynamic therapy and chemodynamic therapy results in a higher cell killing effect to 4 T1 cell than that of single sonodynamic therapy.Fig. 4 Cellular experiment results of killing mouse 4 T1 breast cancer cells under ultrasound irradiation.

To make sure about the mechanism of different killing efficiencies to 4 T1 cell, the active species were confirmed by trapping experiments using isopropanol (IPA), AgNO3, disodium EDTA (EDTA-2Na), 1,4-benzoquinone (BQ), and furfuryl alcohol (FFA) as scavenger of •OH, e−, h+, ∙O2- and singlet oxygen (1O2) in piezo-catalysis methylene blue (MB). Fig. S7 provides piezo-catalysis efficiencies of MB with the presence of ZnO, BaTiO3, BaTiO3(H2), BaTiO3(H2)/ZnO3 and BaTiO3(H2)/ZnO3 + H2O2 before and after the addition of different scavengers. Firstly, the degradation efficiencies obey the order of BaTiO3(H2)/ZnO3 + H2O2 > BaTiO3(H2)/ZnO3 > BaTiO3(H2) > BaTiO3 > ZnO, which is according with the killing efficiency of 4 T1 cell and indicates the different amounts of reactive species with the presence of different samples. Secondly, the degradation efficiency decreases highly after the addition of IPA and BQ scavengers, indicating that •OH and ∙O2- are prominent species in piezo-catalysis upon ultrasound irradiation. The generation of •OH and ∙O2- is further investigated via EPR measurements. Fig. 5 provides the EPR signals of •OH and ∙O2- radicals depending on ultrasound time and sample. As shown in Fig. 5A and B, characteristic EPR signals of •OH and ∙O2- appear upon ultrasound irradiation with the presence of BaTiO3(H2)/ZnO3. The intensity increases gradually over time and reaches the highest intensity after about two minutes. Fig. 5C and D provide the EPR signals of •OH and ∙O2- radicals after two minutes’ ultrasound irradiation with the presence of different samples. Clearly, the weakest and strongest EPR signals were observed by ZnO and BaTiO3(H2)/ZnO3 + H2O2 samples under fixed conditions, respectively. This is according with the killing efficiency of 4 T1 cell, indicating that the amounts of generated species depend on sample and thus it results in different killing efficiencies. The intracellular reactive oxygen species in 4 T1 cells under ultrasound irradiation with the presence of ZnO, BaTiO3, BaTiO3(H2) and BaTiO3(H2)/ZnO3 were quantitatively analyzed by flow cytometry. The results (Fig. 6) are according with the EPR results, revealing that the BaTiO3(H2)/ZnO3 processes the highest level of intracellular reactive oxygen species.Fig. 5 EPR spectra of •OH and ∙O2- radicals depending on time (A and B) and nanoparticles (C and D).

Fig. 6 Intracellular reactive oxygen species in 4 T1 cells under ultrasound irradiation with the presence of ZnO, BaTiO3, BaTiO3(H2) and BaTiO3(H2)/ZnO3.

It is known that ultrasound irradiation results in sonochemical and piezoelectric effects, which results in production of photoinduced e−/h+ pairs and polarization induced electron/hole (⊖/⊕) pairs, as shown by reaction (1) [12]:(1) Piezosemiconductor+ultrasonicvibration→e-/h++⊖/⊕

The following reactions (2), (3) for free charges occur or not depending on band structure and thus result in the production of reactive species or not.(2) e-/⊖+O2→∙O2-

(3) h+/⊕+H2O→∙OH

For photoinduced e−/h+ pairs, reaction (2) occurs when the conduction band potential (ECB) is less negative than that of O2/∙O2- (−0.28 eV vs. NHE), and reaction (3) occurs when the valence band potential (EVB) is more positive than that of H2O/•OH (2.01 eV vs. NHE) [10]. The detail calculation of Eg, ECB and EVB based on UV–vis absorbance spectra (Fig. S8A) and Tauc’s plots (Fig. S8B) is provided in Supporting Materials. The approximated Eg values for ZnO, BaTiO3 and BaTiO3(H2) are about 3.27, 3.39 and 3.35 eV, respectively. There is a Eg decrease for BaTiO3 after a thermal treatment process under 5 %H2/95 %N2 atmosphere, which is induced by the introduction of OV [28], [29]. Two unpaired e − will be retained in the lattice sites of original lattice O following the introduction of one OV, which results in the formation of a new mid-gap state, named OV state, below the conduction band and thus leads to the decrease of Eg [28]. The calculated ECB/EVB values for ZnO, BaTiO3 and BaTiO3(H2) are −0.35/2.92, −1.07/2.32 and −1.04/2.29 eV, respectively. The corresponding band structures of ZnO and BaTiO3(H2) are provided by Fig. 7.Fig. 7 Band structures of ZnO and BaTiO3(H2), as well as possible mechanism for production of reactive species.

Based on experimental results and band structures, the possible mechanism of cell killing was speculated. As shown in Fig. 7, the polarization induced ⊖/⊕ pairs result in production of •OH and ∙O2- radicals as they establish an internal electrical field in samples. The established internal electrical field leads to the opposite movements of photoinduced e−/h+ pairs and thus results in the generation of surface potential to encourage adsorption of dissociated ions [30]. For BaTiO3(H2), the higher OV concentration leads to more efficient adsorption of activate O2 and thus results in production of more radicals [31], which brings about more efficient cell killing. For BaTiO3(H2)/ZnO heterostructure, a built-in electric field is generated in the interface of BaTiO3(H2) and ZnO due to the formation of bound surface charges resulting from the piezoelectric polarization [24], as shown in Fig. 7. The photoinduced e − and h+ move toward inverse direction driven by the built-in electric field, which further improves separation efficiency of the photoinduced e−/h+ pairs and thus leads to more efficient production of reactive radical. As a result, the BaTiO3(H2)/ZnO heterostructures show higher cell killing efficiency than that of pure ZnO and BaTiO3. In addition, it is found from the cellular experiments (Fig. 4) that the BaTiO3(H2)/ZnO4 heterostructure show lower killing efficiency to 4 T1 cell than that of BaTiO3(H2)/ZnO3. It is induced by that the merged ZnO on BaTiO3 caused part of BaTiO3(H2)/ZnO composite interface to be ineffective [24]. And after the introduction of H2O2, the further enhanced killing efficiency to 4 T1 cell results from the more efficient generation of •OH by the disintegration of H2O2 upon ultrasound irradiation [32]. It is according with the EPR results. As shown in Fig. 5C and D, the signals of •OH become stronger after the addition of H2O2, but the signals of ∙O2- barely change.

4 Conclusion

ZnO quantum dots, BaTiO3 nanoparticles with engineering-produced oxygen vacancy, and ZnO/BaTiO3 heterostructure for sonodynamic therapy are fabricated. Under ultrasonic irradiation, free charge carriers are produced due to sonochemical and piezoelectric effects. Following the separation of charge carriers and reactions, reactive species of •OH and ∙O2- are generated to kill cancer cells. Via the oxygen vacancy engineering and ZnO/BaTiO3 heterostructure, the annihilate efficiency to cancer cell is enhanced due to more efficient production of reactive species. The work fulfills the progress in cancer therapy via sonodynamic effect.

CRediT authorship contribution statement

Xiaojian Zhang: Writing – original draft, Investigation. Zhiqin Zhang: Project administration, Methodology. Honglei Yuan: Validation, Data curation. Xianke Sun: Writing – review & editing, Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A Supplementary data

The following are the Supplementary data to this article:Supplementary Data 1

Acknowledgements

This work was supported by the High-level Talents Research and 10.13039/100018477 Startup Foundation Projects for Doctors of 10.13039/100012901 Zhoukou Normal University (ZKNUC2019019 ), and the 10.13039/501100012165 Key Technologies R&D Program of Henan Province (212102210266 ).

Appendix A Supplementary data to this article can be found online at https://doi.org/10.1016/j.ultsonch.2024.107036.
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