Silica-Coated GoldâSilver Nanocages as Photothermal Antibacterial

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Biological and Medical Applications of Materials and Interfaces

Silica Coated Gold-Silver Nanocages as Photothermal Antibacterial Agents for Combined Anti-Infective Therapy Shuangmei Wu, Aihua Li, Xiaoyi Zhao, Cunli Zhang, Bingran Yu, Nana Zhao, and Fu-Jian Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01149 • Publication Date (Web): 18 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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ACS Applied Materials & Interfaces

Silica Coated Gold-Silver Nanocages as Photothermal Antibacterial Agents for Combined Anti-Infective Therapy Shuangmei Wu,a,b,† Aihua Li,c,† Xiaoyi Zhao,a,b Cunli Zhang,a,b Bingran Yu,a,b Nana Zhao,a,b,* Fu-Jian Xua,b,* aState

Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China bKey Lab of Biomedical Materials of Natural Macromolecules (Beijing University of Chemical Technology), Ministry of Education, Beijing Laboratory of Biomedical Materials, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China cCollege of Materials Science and Engineering, Institute for Graphene Applied Technology Innovation, Laboratory of Fiber Materials and Modern Textiles, Growing Base for State Key Laboratory, Collaborative Innovation Center for Marine Biomass Fibers Materials and Textiles of Shandong Province, Qingdao University, Qingdao 266071, China. †Both authors contributed equally to this work. * To whom correspondence should be addressed E-mail addresses: [email protected] (F. J. Xu); [email protected] (N. Zhao).

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ABSTRACT Due to the abuse of antibiotics and the threats of antibiotic-resistance, bacterial infection is still one of the most difficult issues to be resolved. Thus, it is of great significance to explore novel antibacterial agents. In this paper, we investigate a type of silica coated gold-silver nanocages (Au-Ag@SiO2 NCs) as antibacterial candidates. Their intrinsic characteristics of photothermal property and sustained release of Ag ions were fully exploited for NIR-induced combined anti-infective therapy. The broad-spectrum antibacterial property of the as-prepared Au-Ag@SiO2 NCs was confirmed in vitro against Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative bacteria Escherichia coli (E. coli). In addition, Au-Ag@SiO2 NCs exhibit effective treatment of S. aureus biofilm with the assistance of NIR irradiation. More importantly, we assessed the in vivo antibacterial efficacy of Au-Ag@SiO2 NCs against S. aureus, which demonstrated sustainably enhanced therapeutic effects on a rat model with wound infection.

KEYWORDS: silica, gold-silver nanocages, photothermal agents, silver ions, antiinfective therapy.


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INTRODUCTION Bacterial infections are serious threats to human health and the death resulting from infections has been controlled effectively since the emergence of antibiotics.1 However, in the past few decades, the effectiveness of traditional antibiotics was greatly weakened due to the excessive use of antibiotics and the development of drug-resistant bacterial strains.2,3 In addition, several bacterial infections, especially those associated with biofilm formation, generally exhibits strong tolerance and insensitivity to antibiotics.4 Therefore, the development of high-efficiency antibacterial agents and strategies is urgently needed. A variety of antimicrobial agents including Ag,5-7 Au,8,9 ZnO,10 MoS2,11 graphene-based materials,12,13 cationic polymers,14-16 and peptides17,18 have been explored to combat bacterial infections. Among

various

anti-infective

therapies,

light-activated

therapies

including

photothermal therapy (PTT) and photodynamic therapy (PDT) attract extensive attention owing to their non-invasive nature, remote controllability and less adverse effects.19,20 Furthermore, the antimicrobial mechanisms of phototherapies are different from conventional antibiotic therapy. PDT utilizes photosensitizers to produce reactive oxygen species (ROS) to damage microbial cells, while PTT takes advantage of localized photoinduced hyperthermia.21,22 In this case, phototherapies could effectively kill antibioticresistant bacteria. Generally speaking, the effectiveness of PDT not only depends on photosensitizers and oxygen, but also is limited by the short lifetime of ROS and their short range of action. In contrast, PTT employing effective near-infrared (NIR) photothermal agents is hardly restricted by the working distance or microenvironment, which could be more desirable.23,24 In this regard, NIR light-activated photothermal



agents are considered as promising candidates for anti-infection therapy. Gold-silver nanocages (Au-Ag NCs) have attracted much attention due to their distinct optical properties while the surface plasmon resonance (SPR) absorption could be tunable in the NIR region by manipulating the Au/Ag ratio.25,26 Carrying the virtues of facile surface functionalization, Au-Ag NCs with hollow interiors and porous walls have been widely utilized in imaging, drug delivery, and PTT.27-30 Recently, Au-Ag NCs have also been exploited as antimicrobial agents. For example, Au-Ag NCs were employed to construct antibacterial platform through the combination of photothermal killing and targeted antibiotic delivery.31,32 Moreover, it is well known that Ag nanoparticles exhibit broad-spectrum antibacterial properties.33 Enhanced antibacterial activity of bimetallic Au-Ag nanoparticles was obtained, attributable to the high surface free energy of Ag atoms and the improved dispersion stability to accelerate the release of Ag ions.34,35 The antibacterial effects and mechanism of Au-Ag NCs were also investigated, where the destruction of cell membrane, ROS production, and cell apoptosis were involved.36 Although Au-Ag NCs are demonstrated as promising antimicrobial agents, combined anti-infective therapy exploiting their intrinsic characteristics including photothermal property and sustained release of Ag ions remains investigation. Furthermore, the evaluation of in vivo antibacterial performance of Au-Ag NCs is also important. Herein, we investigated the broad-spectrum antibacterial activity of Au-Ag NCs against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). The highly active surface Ag atoms in Au-Ag NCs are hypothesized to be released as Ag ions from the bimetallic structure.34 In addition, the hollow structure and thin, porous wall of AuAg NCs would also facilitate the release of Ag ions.25 Hyperthermia from phothothermal


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effect and Ag ions released from the NCs were both utilized for combined anti-infective therapy. Furthermore, silica coating was applied on the surface of Au-Ag NCs to improve the biocompatibility and realize sustained release of Ag ions, which could affect the antibacterial properties of NCs. The photothermal property of the resultant Au-Ag@SiO2 NCs under NIR laser irradiation was further verified. In this work, the antibacterial activity of Au-Ag@SiO2 NCs was investigated in vitro against S. aureus and E. coli as model Gram-positive and Gram-negative bacteria, as well as S. aureus biofilm. More importantly, the in vivo antibacterial activity of Au-Ag@SiO2 NCs was demonstrated in rat model with S. aureus infection.



EXPERIMENTAL SECTION Preparation of SiO2 Coated Au-Ag NCs. Ag nanocubes and Au-Ag NCs were synthesized as previously reported by Xia’s group and the detailed processes were shown in the Supporting Information.37,38 SiO2 coated Au-Ag NCs were prepared by hydrolysis and condensation reaction of tetraethoxysilane (TEOS) under basic conditions. Briefly, the as-prepared Au-Ag NCs (1.2 mg) were dispersed into 18 mL of isopropyl alcohol. Then deionized water (3.964 mL) and ammonium hydroxide (0.536 mL, 30%) was sequentially added into the solution containing Au-Ag NCs. Subsequently, TEOS (5 μL) was added and the reaction was allowed to proceed for 10 h with continuous stirring at room temperature. The obtained Au-Ag@SiO2 NCs were purified by centrifugation and washing with ethanol. In Vitro Cytotoxicity. The cytotoxicity of Au-Ag@SiO2 NCs was assessed in L929 cell line by MTT assay and the detailed procedures are shown in the Supporting Information. Antibacterial Assay. To investigate the antibacterial activity of Au-Ag@SiO2 NCs, Gram-negative bacteria (E. coli) and Gram-positive bacteria (S. aureus) were employed. The bacteria were seeded on 96-well plates at a density of 2 × 105 colony-forming units (CFU)/mL, followed by the treatment with different concentrations of Au-Ag@SiO2 NCs (from 0 to 1024 g/mL). The mass concentration of Au-Ag@SiO2 NCs was calculated by dividing the mass of Au-Ag@SiO2 NCs by the volume of solution. The mixed solution was divided into two groups. One group was irradiated with 808 nm NIR laser (1 W/cm2, illumination area of 0.8 cm2) for 5 min and the other group without NIR irradiation was denoted as the control group. Both groups were co-cultured at 37 °C for different time (0.5 and 12 h) with gentle shaking. Detailed procedures could be found in the Supporting


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Information. To further assess the antibacterial activity of Au-Ag@SiO2 NCs, a S. aureus biofilm model was also employed, which was described in details in the Supporting Information. In Vivo Anti-Infective Therapy. For the local infection mouse model, 25 μL of S. aureus suspension (4 × 108 CFUs) was inoculated over the wound region. The Wistar rats were divided in the following four groups: control group without infection or irradiation; W/O group with infection and without irradiation; NIR+ group with infection and AuAg@SiO2 NCs under NIR irradiation; NIR- group with infection and Au-Ag@SiO2 NCs without NIR irradiation. For Au-Ag@SiO2 NCs treatment groups, 50 μL of AuAg@SiO2 NCs (2 mg/mL) were applied and covered with sterile nonwoven fabrics on day 1, 2, 4 and 6. The rats from the PTT group were carried out under an 808 NIR laser for 5 min (1.5 W/cm2) after Au-Ag@SiO2 NCs were applied. On day 1, 3 and 7, the entire wounds with adjacent normal skin were harvested as previously reported.16 20 μL of the diluted solution was poured onto LB agar plate and subjected to culture for 24 h at 37 °C. The grown colonies on the plate were then counted for analysis. Histological analysis was performed following the previously reported procedures.16 More details could be found in the Supporting Information.



RESULTS AND DISCUSSION As illustrated in Scheme 1, the preparation of SiO2 coated Au-Ag nanocages (AuAg@SiO2 NCs) were achieved by silica coating on the surface of Au-Ag NCs via basecatalyzed hydrolysis of tetraethoxysilane (TEOS). Au-Ag NCs were prepared through the galvanic replacement reaction between Ag nanocubes and HAuCl4.37,38 The successful preparation of Ag nanocubes, Au-Ag NCs and Au-Ag@SiO2 NCs was evidenced by transmission electron microscope (TEM), UV-vis-NIR spectra and dynamic light scattering (DLS) measurements. From the TEM image shown in Figure 1a, well-defined Ag nanocubes were monodispersed. After Ag nanocubes reacted with a certain amount of HAuCl4 solution, Au-Ag NCs with hollow interiors and porous walls were produced (Figure 1b). The transformation of Ag nanocubes into Au-Ag NCs was also accompanied by the red shift of longitudinal SPR peak from blue (435 nm) to the NIR region (770 nm) (Figure 1d), confirming the formation of Au-Ag NCs with hollow structure. The mesoporous layer of SiO2 with a thickness of 10-15 nm was coated on the surface of AuAg NCs following the previously reported method,39 which could be clearly distinguished from TEM image (Figure 1c). Accordingly, the hydrodynamic size increased from ∼100 nm for Au-Ag NCs to ∼155 nm for Au-Ag@SiO2 NCs (Figure 1e). It was observed that the longitudinal SPR peak of Au-Ag NC@SiO2 NCs shifted to 804 nm, which was caused by the change in the refractive index of the medium (Figure 1d).31,40 The favorable adsorption peak encouraged us to testify the photothermal effect of AuAg@SiO2 NCs employing 808 nm NIR irradiation at the powder density of 1 W/cm2. The temperature evolution of the Au-Ag@SiO2 NC suspensions with different concentrations was measured, which could provide clues for the hyperthermia used for further


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antibacterial study. As shown in Figure 1f, the temperature of Au-Ag@SiO2 NC suspensions gradually rose with the irradiation time, while the temperature of water hardly increased under the same condition. In addition, the change in temperature was also dependent on the concentration of Au-Ag@SiO2 NCs. For example, the temperature of 50 μg/mL suspension rose from 20.7 to 57.4 oC after 10 min irradiation, while the temperature of 400 μg/mL suspension increased to 60.1 oC. These results imply the great potential of photothermal killing performance of the as-prepared Au-Ag@SiO2 NCs. To verify the released Ag ions from Au-Ag@SiO2 NCs, inductively coupled plasma mass spectrometry (ICP-MS) was carried out in the aqueous solution of NCs (1024 μg/mL). The concentrations of Ag ions released after 0.5 h without and with NIR irradiation were quantified to be ~1.42 and ~1.57 mg/L, respectively, indicating the promotion of Ag ions release by photothermal effect. After the incubation time of 12 h, the released amounts of Ag ions in the absence and presence of NIR irradiation were measured to be ~3.27 and ~3.76 mg/L, respectively. These results confirmed the contribution of Ag ions to the antibacterial activity of Au-Ag@SiO2 NCs. We checked the structure of Au-Ag@SiO2 NCs after 5 min NIR irradiation and 12 h incubation. Seeing from TEM image, negligible change was observed in the structure after the release of Ag ions (Figure S1, Supporting Information). Moreover, the temperature change before and after the release of Ag ions was comparable, indicating the good photothermal stability of Au-Ag@SiO2 NCs. To examine the antibacterial activity of Au-Ag@SiO2 NCs, S. aureus and E. coli were selected as the model Gram positive and Gram negative bacteria. After S. aureus cells were incubated with Au-Ag@SiO2 NCs of different concentrations under NIR irradiation



(808 nm, 1 W/cm2), the culture solution after 0.5 or 12 h was detected using colony counting method, as displayed in Figure 2a (NIR+ groups). At the same time, groups without NIR irradiation were carried out accordingly as the control (NIR- groups) while the antibacterial activity of Au-Ag@SiO2 NCs against E. coli was also assessed (Figure S2, Supporting Information). When the incubation time was 0.5 h, the minimum bactericidal concentration (MBC) values of the NIR+ groups were 256 and 512 μg/mL for S. aureus and E. coli, respectively. Therefore, Au-Ag@SiO2 NCs under NIR irradiation showed better antimicrobial activity against Gram-positive S. aureus than Gram-positive E. coli. In contrast, the control groups for S. aureus and E. coli without NIR irradiation hardly showed antibacterial effect when the concentration of AuAg@SiO2 NCs reached up to 1024 μg/mL. These results revealed obvious photothermal antibacterial activity of Au-Ag@SiO2 NCs against both S. aureus and E. coli. Meanwhile, judging from the results of the NIR- group, it was speculated that the bacterial killing effect from the released Ag ions was negligible, probably due to the relatively short incubation time of 0.5 h. In contrast, when the incubation time was extended to 12 h, MBC of the Au-Ag@SiO2 NCs toward S. aureus decreased to 128 μg/mL for the NIR+ group and 256 μg/mL for the NIR- group. The MBC against E. coli was found to be 8 μg/mL with or without NIR irradiation, as presented in Figure S3, Supporting Information. Compared with the MBC values for incubation time of 0.5 h, the MBC values of 12 h significantly decreased toward both S. aureus and E. coli, probably owing to the sustained release of Ag ions from Au-Ag@SiO2 NCs. Moreover, the longterm antibacterial activity of as-prepared Au-Ag@SiO2 NCs was probably owing to the presence of silica coating. The mesoporous feature of silica layer ensures the sustained


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release of Ag ions for a longer period of time. The lower MBC toward E. coli compared with S. aureus suggests that Ag ions released from Au-Ag@SiO2 NCs had better antibacterial activity against E. coli, consistent with the previously reported results.36 To further reveal antibacterial mechanism, scanning electron microscopy (SEM) was carried out to observe the morphology of bacterial cells after interaction with AuAg@SiO2 NCs. As shown in Figure S4 (Supporting Information), S. aureus and E. coli. without any treatments remained intact membranes after 5 and 12 h incubation (control group). When S. aureus and E. coli. cells were treated with Au-Ag@SiO2 NCs (1024 μg/mL) for 0.5 h in the absence of NIR irradiation (NIR- group), hardly any change could be observed, indicating the cells were almost viable. When the incubation time was extended to 12 h, sunken and collapsed cells could be found for both S. aureus and E. coli. These results confirm the contribution of Ag ions to destroy the membrane with sustained release from Au-Ag@SiO2 NCs after 12 h. For NIR+ groups, there were obvious wrinkled membranes for both bacterial cells even at the incubation time of 0.5 h, demonstrating the photothermal antibacterial activity of Au-Ag@SiO2 NCs. After 12 h, remarkable structural changes were observed for the NIR+ groups, exhibiting the longterm antibacterial activity of Au-Ag@SiO2 NCs resulting from both photothermal effect and sustained release of Ag ions. These phenomena coincide well with the antibacterial results obtained by colony counting method. In addition, to evaluate the cytotoxicity of Au-Ag@SiO2 NCs, the relative cell viabilities of L929 cells were determined by methyl thiazolyl tetrazolium (MTT) assay. After the cells were cultured with Au-Ag@SiO2 NCs at different concentrations for 24 h, it was observed that low cytotoxicity was exhibited when the concentration was below 256 μg/mL (Figure S5, Supporting Information).



Recently, biofilms have become one of the major health threats due to the intrinsically resistance to antibiotic therapy.4,41 As biofilm has extracellular polymeric substances (EPS) that provides a protective layer for microbial growth, bacteria in biofilm could be insensitive to most antibiotics. Therefore, the antibacterial efficacy of Au-Ag@SiO2 NCs against S. aureus biofilm was further tested. Figure 2b shows the three-dimensional (3D) confocal laser scanning microscopy (CLSM) images of S. aureus biofilms with different treatments after 7 day incubation. A large number of live cells were observed in the untreated control group and the group with Au-Ag@SiO2 NCs in the absence of NIR irradiation. On the contrary, there were rarely viable bacteria after laser irradiation was applied when the biofilm was exposed to Au-Ag@SiO2 NCs, indicating the efficient photothermal effect to eliminate the bacteria within biofilm. Encouraged by the excellent in vitro antibacterial activity, we further investigated the in vivo therapeutic efficacy of Au-Ag@SiO2 NCs in controlling infection of wounds. A bacterial infection model in rats was established with the local infection of S. aureus. The infected rats were randomly divided into the following four groups: NIR+ group (with infection and Au-Ag@SiO2 NCs under NIR irradiation), NIR- group (with infection and Au-Ag@SiO2 NCs without NIR irradiation), W/O group (with infection and without irradiation) and control group (no infection or irradiation). Figure 3a exhibits the photos of wounds on day 1, 3 and 7, demonstrating the therapeutic effects of different treatments. All groups applying S. aureus showed a certain degree of infection on day 1. Compared with the W/O group, the NIR- group exhibited a lighter degree of pyosis. The antibacterial activity of the NIR- group was speculated to result from the sustained release of Ag ions from Au-Ag@SiO2 NCs. Meanwhile, the wounds in the NIR+ group were irradiated with NIR laser for 5 min on day 1, 2, 4, and 6. On day 3, the wound in the 12 ACS Paragon Plus Environment

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NIR+ group showed the smallest area among all groups and on day 7, the pyosis around the wounds disappeared. After the 7-day treatment, the NIR+ group presented the fastest healing rate compared with other groups, confirming that NIR-induced hyperthermia from Au-Ag@SiO2 NCs could enhance the bacterial killing efficacy. The temperature increase on wounds treated with Au-Ag@SiO2 NCs under NIR irradiation was recorded by an IR thermal camera, as shown in Figure 3b. The local temperature was observed to increase rapidly from 32 to 66 oC after irradiation (808 nm, 1.5 W/cm2) for 5 min, verifying the excellent photothermal property of the as-prepared Au-Ag@SiO2 NCs. To further quantitatively evaluate the anti-infective therapeutic performance of AuAg@SiO2 NCs with PTT and released Ag ions, the bacteria around the wounds on day 1, 3, and 7 were cultured and counted, respectively (Figures 3c,d). The NIR+ group presented the lowest number of bacterial colonies compared with the other infection groups. It is to be noted that on day 3 and 7, the bacteria count from the NIR- group was also significantly lowered in contrast to the W/O group while this value was still high on day 1. These results coincide well with the hypothesis that the Ag ions released from AuAg@SiO2 NCs sustainably for anti-infective therapy. Furthermore, the superior bactericidal activity of the NIR+ group revealed the enhanced therapeutic effectiveness through the combination of PTT, consistent with the results depicted in Figure 3a. The wounds of rats were collected during the treatment processes and evaluated by hematoxylin and eosin (H&E) staining. The Neutrophils that were usually generated by infection were stained in blue to distinguish the infected tissues. As exhibited in Figure 4, histological results of the wounds in the NIR- and W/O groups possess obvious blue areas from day 1 to day 7, suggesting the tissues were seriously infected. Nevertheless, there were relatively less blue area in the NIR- group than the W/O group, attributed to 13 ACS Paragon Plus Environment


the release of Ag ions from Au-Ag@SiO2 NCs. Notably, the staining result from the NIR+ group is similar to the non-infected control group. What’s more, many new vessels were observed in the NIR+ group. The above results demonstrated the remarkable bactericidal activity and great potential of the photothermal Au-Ag@SiO2 NCs in combined anti-infective therapy.


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CONCLUSIONS In summary, we developed Au-Ag@SiO2 NCs as effective photothermal agents for combined anti-infective therapy. Through the treatment with Gram-positive bacteria (S. aureus) and Gram-negative bacteria (E. coli), the as-prepared Au-Ag@SiO2 NCs demonstrated impressive broad-spectrum antibacterial properties due to the combination of NIR-mediated hyperthermia and sustained release of Ag ions. The satisfactory bacterial killing effect of Au-Ag@SiO2 NCs was also assayed with a S. aureus biofilm model. The in vivo anti-infective performances were successfully demonstrated on a rat model with wound infection. These findings render Au-Ag@SiO2 NCs promising candidates against bacterial infections. Additionally, Au-Ag@SiO2 NCs with hollow interiors and porous walls might be explored as carriers to deliver antibiotics, together with Ag ions and NIR-induced hyperthermia for synergistic anti-infective therapy.



ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details and additional data such as photographs of E. coli colonies, quantitative analysis of bacterial colonies and cell cytotoxicity of Au-Ag@SiO2 NCs.

ACKNOWLEDGEMENTS This work was supported by the National Key Research and Development Program of China (Grant Nos. 2016YFA0201501 and 2017YFA0106100), National Natural Science Foundation of China (Grant Nos. 51773013, 21875014, and 51733001), and the Fundamental Research Funds for the Central Universities (Project Nos. BHYC1705A and XK1802-2), and the Fundamental Research Funds for the Central Universities and Research projects on biomedical transformation of China-Japan Friendship Hospital (No. PYBZ1826).


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Scheme 1. Schematic illustration of the preparation Au-Ag@SiO2 NC and corresponding antibacterial applications through the combination of photothermal killing and the released silver ions.


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Figure 1. TEM images of Ag nanocubes (a), Au-Ag NCs (b), and Au-Ag@SiO2 NCs. The insets in Figure a-c illustrate the enlarged images and the scale bar is 50 nm. (d) UVvis-NIR spectra and (e) Hydrodynamic sizes of Ag nanocubes, Au-Ag NCs and AuAg@SiO2 NCs. (f) Temperature elevation of water and aqueous solutions of AuAg@SiO2 NCs under NIR irradiation at 808 nm (1 W/cm2).



Figure 2. (a) Photographs of S. aureus bacterial colonies treated by Au-Ag@SiO2 NCs with different concentrations in the presence or absence of NIR irradiation, followed by incubation for 0.5 or 12 h. Scale bar: 45 mm. (b) CLSM images of S. aureus biofilm treated with Au-Ag@SiO2 NCs upon NIR irradiation (NIR+), Au-Ag NCs@SiO2 (NIR-) and PBS solution (Control), respectively.


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Figure 3. (a) Photographs of wound tissues with different treatments on day 1, 3, and 7. Scale bar: 1 cm. (b) Thermal images of rat treated with Au-Ag@SiO2 NCs before and after NIR irradiation for 5 min (808 nm, 1.5 W/cm2). (c) Photographs and (d) quantitative analysis of bacterial colonies from tissues of different treatment groups (n = 3, *p < 0.05). Scale bar: 45 mm.



Figure 4. H&E-stained tissue slices from the wound areas of different groups on day 1, 3 and 7. Images with the scale bar of 100 μm show the enlarged tissue slices in the corresponding boxes.


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Silica-Coated GoldâSilver Nanocages as Photothermal Antibacterial

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Silica-Coated GoldâSilver Nanocages as Photothermal Antibacterial