Photonic Crystal-Enhanced Photodynamic Antibacterial Therapy

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE22 Apr 2022Photonic Crystal-Enhanced Photodynamic Antibacterial Therapy Yujie Gao†, Xiaodong Chen†, Miaomiao Li, Lianbin Zhang and Jintao Zhu Yujie Gao† Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074 †Y. Gao and X. Chen contributed equally to this work.Google Scholar More articles by this author , Xiaodong Chen† Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074 †Y. Gao and X. Chen contributed equally to this work.Google Scholar More articles by this author , Miaomiao Li Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074 Google Scholar More articles by this author , Lianbin Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074 Google Scholar More articles by this author and Jintao Zhu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202201795 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Photodynamic antibacterial therapy (PDAT) is a kind of rejuvenating strategy that combats bacterial infection due to its admirable characteristics of noninvasiveness and broad-spectrum antibacterial capability. However, the efficiency of PDAT can be greatly hindered by limited light irradiation. Herein, we propose an enhanced PDAT by employing photonic composite films (PCFs) via slow photon and multiple scattering effects. The PCFs are obtained by UV light-initiated polymerization of poly(ethylene glycol) phenyl ether acrylate with a self-assembled SiO2 colloidal particle array, followed by the deposition of photosensitizers (PSs). The PCFs can prompt the PSs with matched absorption, which are deposited on their surface, to sufficiently utilize the incident light and generate more reactive oxygen species based on the slow photon phenomenon of photonic crystals and multiple scattering effects of the SiO2 colloidal particles. This finding demonstrates the great potential and significance of PCFs in the field of PDAT, which may reduce the requirements of excitation equipment and avoid damage to normal tissues from exposure to huge light energy. Download figure Download PowerPoint Introduction Bacterial infection is one of the most worrying public health problems worldwide.1–7 Many serious infectious diseases, including tetanus,8,9 pulmonary tuberculosis,10–12 and bacteremia,13,14 are closely related to pathogenic bacteria. Several strategies have been developed to combat pathogenic bacteria, including the use of antibiotics and inorganic antibacterial agents, which, however, may suffer the drawbacks of microbial resistance and systemic toxicity.4,15–20 Recently, photodynamic therapy (PDT) has been recognized as an emerging technique for antibacterial purposes, that is, photodynamic antibacterial therapy (PDAT).21–23 During the process of PDAT, photosensitizers (PSs) are excited by the appropriate light, and then the excited PSs react with the surrounding oxygen molecules to generate reactive oxygen species (ROS), which cause oxidative damage to the surrounding biomolecules, including proteins, lipids, and nucleic acids, and finally lead to the death of bacteria.24–28 Especially, due to fact that ROS does not act on specific bacterial targets, PDAT does not develop any bacterial resistance and has ignorable systemic toxicity.3,21,29 These unique characteristics of PDAT make it suitable for combating local bacterial infections and sterilizing the surface of medical equipment, and so on.30–33 PSs, excitation light, and molecular oxygen are the three necessary components to achieve effective PDAT.21,34 However, biological tissues usually absorb and scatter most of the incident light. Accordingly, only limited light can be utilized by PSs, further restricting the effective application of PDAT.35,36 With regard to the insufficient excitation source, increasing the intensity of the light source is the most direct strategy to enhance PDAT, while it not only requires a higher standard of equipment but also leads to security risks due to huge light energy.37 To this end, some attempts have been made to enhance PDAT without increasing the requirement for light intensity. For example, Wang et al.35 utilized the bioluminescence of luminol to enhance PDAT to treat microbial infections through the process of bioluminescence resonance energy transfer. More recently, a PDAT system was also constructed by combining electrochemiluminescence (ECL) and PS.38 Although the process of ECL was largely controllable, the charging technology and complicated construction of the instrument still require careful consideration. Therefore, the development of a facile, convenient yet effective strategy to overcome the deficient excitation light source in PDAT remains highly desirable. Photonic crystals (PCs) are a kind of optical material with periodic nanostructures comprised of materials with different dielectric constants, which have a photonic bandgap (PBG) and exhibit unique light manipulation performance and slow photon phenomenon; that is, the photon at the edge of the PBG has a slow group velocity.39,40 Recently, some studies have shown that the slow photon phenomenon and multiple scattering effects of PCs can significantly enhance the photoluminescence of fluorophores and the photocatalytic performance of photocatalysts via promoting photon capture.40–42 We hypothesize that PCs should also be able to manipulate the light in PDAT, thereby increasing the absorption of excitation light by PSs, ultimately improving the efficiency of PDAT. In this study, by using photonic composite films (PCFs) with PS deposited on their surface, we demonstrate that PCs can effectively enhance PDAT via their slow photon phenomenon and multiple scattering effects. The PCFs were obtained by UV light-initiated crosslinking of poly(ethylene glycol) phenyl ether acrylate (PEGPEA) with the self-assembled SiO2 colloidal particle array, which exhibited satisfactory mechanical stability and provided a platform for PSs deposition. Subsequently, the PS of rose bengal (RB) was deposited on the surface of PCFs, and their capabilities for producing ROS and killing bacteria under white light irradiation were investigated. We show that PCFs can effectively improve the production of ROS and antibacterial capability compared with amorphous composite films (ACFs) and polymer films (PFs) due to the unique characteristics of PCs, that is, the slow photon phenomenon and multiple scattering effects. This study presents a breakthrough in enhanced PDAT via an innovative combination of PCs and PDT and provides insights into the design of high-performance PDAT systems. Experimental Methods Materials PEGPEA (MW 324 Da) and RB (purity > 95%) were obtained from Sigma-Aldrich (Saint Louis, MO, United States). 2-Hydroxy-2-methylpropiophenone (Darocur 1173, purity > 97%) and Chlorin e6 (Ce6, purity > 90%) were purchased from Beijing InnoChem Science & Technology Co., Ltd. (Beijing, China). Tetraethyl orthosilicate (TEOS, purity > 98%), aqueous ammonia (purity > 28%), alcohol, and N-hexane were purchased from Sinopharm (Shanghai, China). Singlet oxygen sensor green (SOSG) was purchased from Invitrogen (Shanghai, China). Escherichia coli (CCTCC AB 93154) and Staphylococcus aureus (CCTCC AB 91093) were obtained from the China Center for Type Culture Collection. Deionized (DI) water was obtained with a water purification system (NW30VF, Heal Force). All chemicals were used as received without further purification. Preparation of the PCFs and deposition of PSs on the surface of the films SiO2 colloidal particles were initially prepared by using a modified Stöber method with different sizes by adjusting the amount of TEOS and ammonia, and dispersing them in ethanol (100 mg/mL).43 To prepare PCFs, a precursor containing SiO2 colloidal particle suspension, PEGPEA, and Darocur 1173 at a mass ratio of 40:60:1 was prepared. The precursor was heated in an oven at 70 °C for 12 h to obtain the assembled precursor with brilliant structural color and then infused into a gap between two glass slides. UV light irradiation (365 nm, 2 W/cm2, MXL001A, DUVTek) was conducted on the precursor for 1 min to obtain the PCFs. For comparison, ACFs with the randomly arranged SiO2 colloidal particles and PFs without SiO2 colloidal particles were also prepared through a similar process. To deposit RB on the surface of the films, the three types of films (PCFs, ACFs, and PFs) were initially cut into small pieces (1 cm × 1 cm). Subsequently, for each piece, 8 μL of RB solution (in the mixed solvent of ethanol and N-hexane at the ratio of 1:3, v/v) with the concentration of 250 μg/mL was dripped on the surface, and the solvent was allowed to evaporate at room temperature in dark conditions. Similarly, the films deposited with Ce6 or Ag3PO4 were obtained through the above procedure. Therein, Ag3PO4 was prepared in advance according to a previously reported method.44 Detection of ROS ROS generation was determined by using the SOSG probe. In detail, the SOSG powder was initially dissolved in methanol to obtain a stock solution with a concentration of 5 mM, which was diluted to 2 μM with DI water immediately before use. Then, the films deposited with PSs were placed in a 24-well plate, followed by the addition of 500 μL of SOSG solution into each well. Subsequently, the solution was exposed to white light with a light intensity of 10 W/m2 for 3 min. Then, the fluorescence spectra of the solutions were recorded by using a fluorescence spectrometer (FP-6500, Jasco, Japan) with an excitation of 494 nm. Photodynamic inactivation of bacteria S. aureus and E. coli were chosen as the model bacteria, which were cultivated in nutrient broth (NB, pH 7.4) at 37 °C with shaking. The initial optical densities of the two bacterial suspensions were determined at 600 nm by using a UV–vis spectrophotometer (UV 1800, Shimadzu, Japan) and fixed at 0.2 (OD600 nm = 0.2). Then, the films deposited with RB were placed in a 24-well plate, followed by the addition of 500 μL of the bacterial suspensions into each well. Subsequently, the suspension of S. aureus was irradiated by a white light (10 W/m2) for 2, 5, and 10 min, respectively, and the suspension of E. coli was irradiated by a white light (600 W/m2) for 5, 10, and 20 min, respectively. After irradiation, the bacterial suspensions were collected and placed in the dark for another 20 min. The bacteria treated without RB and light acted as a control, and the viability of the bacteria treated solely with RB or light was also studied. Then, the bacterial suspensions were diluted 22,500 times with sterilized phosphate-buffered saline (PBS) solution and inoculated 50 μL of the diluted bacterial suspensions on NB agar plates. Finally, photographs were taken after the incubation of the plates at 37 °C for 24 h. The antibacterial ratio was calculated by using the following equation: Antibacterial ratio = ( C 0 − C ) / C 0 × 100 % , (1)where C0 and C represent the number of colony-forming units in the control group and experimental group, respectively. Characterizations Scanning electron microscopy (SEM, SU8010, HITACHI, Japan) was used to observe the morphology of the SiO2 colloidal particles and the PCFs. UV–vis reflection spectra were measured with a USB4000 fiber optic spectrometer (Ocean Optics, United States). Optical photographs were recorded by using a digital camera (IXUS 105, Canon, Japan). Cross-sectional confocal laser scanning microscopy (CLSM) investigation (FV1000, Olympus, Japan) was carried out to measure the penetration depth of RB. The tensile test was performed on an IBTC-300SH machine (CARE Measurement & Control Co. Ltd., Tianjin, China) with a loading rate of 6 mm/min at room temperature. Results and Discussion Preparation of the PCFs and deposition of RB PCs can slow the group velocity of the photons at the edge of PBG, which in turn intensifies the interaction between photons and matter, further improving the light-harvesting efficiency.40,45–48 In this study, we rationally designed and fabricated PCFs with PSs deposition for enhanced PDAT. Figure 1 shows the structural design of the PCFs with the PSs deposited on the surface. The PCFs were prepared through the UV light-initiated polymerization of PEGPEA in the presence of the ordered SiO2 colloidal particle array.49 Because of the photonic structures from the ordered SiO2 colloidal particles arrangement, the PCFs exhibited sharp PBG (i.e., reflection peaks) with brilliant structural colors. The PBG of the PCFs were easily adjusted by varying the size of SiO2 colloidal particles. As shown in Figure 2a and Supporting Information Figure S1, by using SiO2 colloidal particles with sizes of 156, 174, and 218 nm, respectively, three different PCFs with their PBGs at ∼400–445 nm, ∼500–590 nm, and ∼615–710 nm, exhibiting blue, green, and red appearance, were obtained (denoted as PCF-blue, PCF-green, and PCF-red, respectively). Taking the PCF-green as an example, SEM measurements were used to characterize its microstructure. As shown in Figures 2b–2d, the thickness of the obtained PCF was ∼104 μm, and a well-ordered and non-close hexagonal packed arrangement of the SiO2 colloidal particles were observed on the surface and cross-sectional views, which was essential for generating a sharp PBG.50 Meanwhile, we found that the PCFs had good mechanical properties with an elongation at the break of 137% and tensile strength of 407 KPa ( Supporting Information Figure S2). Moreover, the PCFs also exhibited good stability as evidenced by the unchanged reflection spectra after 6 months of storage, ensuring their application as a platform for PDAT ( Supporting Information Figure S3). Figure 1 | Schematic illustration showing the preparation of PCFs deposited with PSs and their capability in enhancing ROS generation and enhanced PDAT. Download figure Download PowerPoint The PCF-green has an obvious reflection in the PBG between ∼500 and 590 nm, implying that it can prevent the propagation of light in this band, and the light will be back-reflected to the surface of the film. Hence, PSs with matched absorption wavelengths deposited on the surface of the PCF-green will absorb the reflected light, further leading to enhanced ROS generation efficiency. To this end, RB with a high absorption between 480 and 580 nm was chosen as a model PS, which approximately overlapped with the reflection wavelengths of the as-obtained PCF-green (Figure 2e). CLSM images were collected to observe the distribution of RB on the film.50 As shown in Figures 2f–2h, we can find that the RB molecules are homogeneously deposited on the surface of the PCF-green, and its distribution depth was ∼17 μm. Figure 2 | Preparation and characterization of the PCFs. (a) Normalized reflection spectra recorded by a fiber optic spectrometer with a white light source and corresponding photographs of PCFs with different PBGs prepared from SiO2 colloidal particles of different sizes. Scale bar in (a) is 5 mm. (b) The cross-sectional, (c) magnified surface, and (d) magnified cross-sectional SEM images of PCF-green. (e) The reflection spectrum of PCF-green and the absorption spectrum of RB. (f–h) Cross-sectional CLSM images of PCF-green deposited with RB. The red fluorescence signal originated from RB excited with the incident laser of 543 nm. Download figure Download PowerPoint Enhanced generation of ROS Having obtained the RB-deposited PCFs, we then verified their capability in enhancing ROS generation during the photodynamic process. Considering that both the slow photon effect of PCs and multiple scattering effects of SiO2 colloidal particles in the polymer matrix might contribute to the enhanced light-harvesting efficiency of RB, we also prepared PFs ( Supporting Information Figure S4) and ACFs ( Supporting Information Figure S5) with the randomly arranged SiO2 colloidal particles in the polymeric matrix for comparison. Meanwhile, PCF with PBG that is out of the absorption range of RB was also employed. To this end, SOSG, which emits fluorescence between ∼525 and 536 nm after oxidation, was employed to measure the ROS levels.31,51 Figure 3a reveals that under the irradiation of white light, which is commonly used to excite RB, for 3 min, maximum fluorescent intensity was observed in the PCF-green group and minimum fluorescent intensity in the PF group, indicating that RB on the PCF-green can generate the most ROS, while ROS generated in the PF generated the least. To intuitively demonstrate the influence of films on the production of ROS, we used PF as the control and defined the ratio of fluorescent intensity in different groups and that in the control group as an enhancement factor. According to the calculation, the enhancement factors were 1.70 ± 0.03, 2.21 ± 0.07, and 2.73 ± 0.17 for the ACF, PCF-red, and PCF-green groups, respectively (Figure 3b). The highest enhancement factor occurred in the PCF-green group. Because the PBG of PCF-green almost overlapped with the absorption spectrum of RB, the reflected light was sufficiently absorbed by RB. The slow photon phenomenon of PCs plays an important role in ROS enhancement. At the edge of PBG, the group velocity of light propagation was reduced, thereby intensifying the interaction of photons and PS, further leading to more ROS being generated. In addition, the multiple scattering effects of SiO2 colloidal particles in the polymer matrix also improved the absorption of light by RB. In contrast, the ACF and PCF-red exhibited a relatively lower enhancement of ROS production, which may solely be the result from the multiple scattering effects of SiO2 colloidal particles in the films.50 Figure 3 | PCF-green enhanced ROS production from RB under white light irradiation by using SOSG as a probe. (a) The fluorescent emission spectra of SOSG in different groups. (b) The corresponding enhancement factors in different groups. (c) Reflection spectra of PCF-green with different thicknesses. (d) The fluorescence emission spectra of SOSG in the PCF-green group with different thicknesses. Download figure Download PowerPoint Moreover, the effect of the thickness of the PCF on ROS generation was also investigated. From the reflection spectra, we found that the reflectivity of PCF-green increased with the thickness while their PBGs remained unchanged (Figure 3c). Subsequently, SOSG was also employed to demonstrate the capability of the PCF-green with different thicknesses in enhancing ROS production. As revealed in Figure 3d, stronger fluorescence was observed in the thicker (e.g., 100 and 200 μm) PCF-green groups, indicating that more ROS was generated, while relatively weaker fluorescent intensity was observed in the PCF-green group with the thickness of 50 μm. Considering that the greater influence of multiple scattering effects may be brought from the thicker film, the PCF-green with an optimized thickness of 100 μm was used in the following studies. To verify the generality of the enhanced ROS production by the PCs, we prepared Ce6 deposited PCF-blue ( Supporting Information Figure S6) and Ag3PO4 nanoparticles deposited PCF-green ( Supporting Information Figures S7 and S8), whose PBGs were roughly matched with the corresponding PSs and photocatalysts, respectively. Similarly, the ROS generation was also measured by adopting the SOSG probe. As expected, maximum fluorescent intensity was observed in the two PCF groups, indicating that Ce6 and Ag3PO4 on the PCFs generated the most ROS ( Supporting Information Figures S9 and S10). The above results demonstrated that PCFs are universal in enhancing the production of ROS by PSs or photocatalysts, and the improvement can be attributed to the increment of light-harvesting efficiency by PSs based on the slow photon effect of PCs and multiple scattering effects of SiO2 colloidal particles. In vitro antibacterial study Having demonstrated that PCs can enhance the capability of PSs or photocatalysts to produce ROS, we reasonably speculate that PCs can enhance the antibacterial effect of PDAT. To this end, we took S. aureus and E. coli as examples to demonstrate the enhanced PDAT based on RB, which was deposited on the surface of three different films (i.e., PCF-green, ACF, and PF). As shown in Figures 4a and 4b, when S. aureus was treated solely with RB or light (10 W/m2), all three different groups did not show any obvious effects on bacteria. In contrast, when being treated with RB and upon light irradiation for 2 min, the antibacterial ratio of the PCF group was 77.9% ± 2.3%, which was increased by ∼15.3% and ∼38.7% over those of the ACF group (62.6% ± 3.1%) and the PF group (39.2% ± 2.2%). The above results indicate that PCF can improve the PDAT effect of RB, which resulted from the enhancement of the light-harvesting efficiency of RB and further increased the production of ROS. However, further extending the irradiation time to 10 min, the antibacterial ratio of the PF group reached 98.2% ± 0.5%, which was almost the same with the ACF group (99.6% ± 0.3%) and the PCF group (99.7% ± 0.2%). These results indicate that the PCF can assist the PSs to utilize limited excitation light, thereby effectively producing ROS and displaying a higher antibacterial ratio. Figure 4 | Antibacterial activities of RB on the surface of different films under white light irradiation. Photographs of bacterial colonies formed by (a) S. aureus and (c) E. coli after being incubated in Petri dishes for 24 h at 37 °C. The scale bars in the last image in (a) and (c) apply to the other images. The antibacterial ratio against (b) S. aureus and (d) E. coli. The error bars represent the standard deviation. Download figure Download PowerPoint Compared with S. aureus, the PDAT effect of RB to E. coli was relatively weak, which is probably due to the obvious structural difference of their cell envelope. In other words, Gram-negative bacteria (e.g., E. coli) have a bilayer membrane structure with a dense outer layer, which may reduce the killing effect of PDAT.21,52,53 Hence, relatively long periods and harsh conditions were adopted for irradiation. We found that sole treatments of RB or light (600 W/m2) did not display any obvious effects on the bacteria. When the irradiation time was 5 min, the antibacterial ratio of the PCF group was 68.8% ± 3.0%, which was increased by ∼13.9% and ∼27.1% than those of the ACF group (54.9% ± 0.7%) and PF group (41.7% ± 1.2%). When the irradiation time was extended to 20 min, the antibacterial ratio of the PF group, the ACF group, and the PCF group reached 91.1% ± 1.8%, 95.4% ± 3.2%, and 99.3% ± 0.6%, respectively (Figures 4c and 4d). Thus, we can conclude that PCs can improve the antibacterial effect of PDAT when the excitation light is deficient, which not only reduces the requirements of excitation equipment but also avoids damage to normal tissues caused by huge light energy, which is of great significance in the field of PDAT. Conclusion We have demonstrated that PCs can effectively enhance the PDAT by using PCFs. When the absorption of PSs or photocatalysts matched with the PBGs of the PCFs, the slow photon phenomenon of PCs and multiple scattering effects of SiO2 colloidal particles improved the capability of PSs or photocatalysts to harvest light by intensifying the interaction between photons and matter, thereby enhancing the ROS production 2.7 times. In addition, because ROS destroys bacteria by causing oxidative damage to the biomolecules, the antibacterial ratio increased by 38.7% under light irradiation, realizing effective PDAT. The excitation light is one vital component in PDAT. Increasing the intensity of light is a direct way to improve the PDAT, which, however, inevitably increases the probability of tissue damage by huge light energy at the same time. Therefore, improving the utilization efficiency of PSs or photocatalysts to excitation light can be a safe, energy-saving, and effective strategy to enhance the efficiency of PDAT. We, therefore, believe that this finding provides a convenient yet robust strategy for enhancing the efficiency of PDAT without increasing the burden on equipment and avoids damage to normal tissues caused by huge light energy. Supporting Information Supporting Information is available and includes Figures S1–S10 and the characterization of Ag3PO4 nanoparticles. Conflict of Interest There is no conflict of interest to report. Funding Information This work was supported by the National Natural Science Foundation of China (52022032). Acknowledgments The authors thank the HUST Analytical and Testing Center for their support in the use of their facilities.