Fluoride-activated photothermal system for promoting bacteria-infected wound healing

Although photothermal therapy (PTT) employing nanozymes has shown excellent antibacterial potential, excessive heating generally harms host cells and hinders recovery. Herein, we report an innovative technique for acquiring the programmed temperature by managing the catalytic activity of nanozymes. The photothermal system of CeO2 + F− + TMB can obtain precise photothermal temperature by adjusting the concentration of fluoride ions under near-infrared irradiation. At the optimized photothermal temperature, the photothermal system affords fine photothermal antibacterial treatment with high-efficiency antibacterial effects against Staphylococcus aureus and Escherichia coli in vitro. In vivo wound healing experiments confirm that the system can effectively promote fibroblast proliferation, angiogenesis and collagen deposition with remarkable wound healing efficiency. This strategy offers a novel design concept for creating a new generation of PTT and opens the way for the creation of alternative antibiotics. Supplementary Information The online version contains supplementary material available at 10.1186/s12951-023-02091-y.


Introduction
Nanozymes are artificial enzyme mimics with excellent enzyme-like capabilities and distinctive nanomaterial physicochemical characteristics [1].They offer substantial benefits over natural enzymes, including cheap cost, high stability, outstanding durability, tunable activity, simple storage, and excellent reusability [2].Following the first report on the peroxidase-like activity of ferromagnetic nanoparticles in 2007, numerous nanozymes based on metals, metal oxides, carbons, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and other composite nanomaterials have been devised and manufactured [3][4][5][6][7].Great efforts have also been undertaken to explore the potential of nanozymes in the domains of biosensing, catalysis, clinical medicine, and environmental protection [8][9][10].
Bacterial infection is a global concern to public health that not only causes several terrible illnesses but also worsens the state of cancer patients and patients with epidemic diseases such as COVID-19 [11,12].Antibiotics are now the most widely used and effective method to treat bacterial illness [13].However, with the overuse of antibiotics, bacterial resistance continues to emerge and spread at an alarming rate [14,15].Compared with typical antibiotic treatment, physical stimulation-based antibacterial techniques offer comprehensive and effective antibacterial effects against both common and multidrug-resistant bacteria and may reduce the risk of bacterial resistance [16][17][18].Among these physical stimulation options, PTT is an efficient physical sterilizing approach that kills bacteria by inducing protein (including enzyme) denaturation, cell membrane rupture, cell cavitation, and cell fluid evaporation at elevated temperatures [19,20].Notably, near-infrared (NIR, 700-1100 nm) laser-mediated PTT affords an unique feature of deep tissue penetration without considerable light-induced cytotoxicity, which is beneficial for establishing antibacterial platforms for various deep therapies [21][22][23].
In treating infectious illnesses, nanozyme-based NIR PTT has drawn much interest lately owing to its higher efficacy and fewer side responses [24][25][26].PTT has specific requirements for photothermal temperature.Excessive temperatures can cause damage to host cells, damaging normal tissues while prolonging wound healing time [27,28].When the temperature is too low, the photothermal effect is not apparent, and it is impossible to eradicate microorganisms effectively.For the healing of infected wounds, studies have shown that a suitable temperature (more than 50 °C) efficiently prevents the multiplication of germs [29,30].The current photothermal temperature of nanozymes primarily adjusts the temperature by varying the material concentration, the power of the laser irradiation lamp source, and the irradiation time, but it suffers from cumbersome steps and an inability to achieve programmed temperature.Consequently, it is of utmost importance to discover a new strategy to manage the photothermal activity of nanozymes to obtain optimized photothermal temperature for better therapeutic effects.
Cerium oxide (CeO 2 ), an important rare earth metal oxide, contains mixed valence states of CeO 3 + and CeO 4

+
, high mechanical strength, high conduction band energy, exhibiting powerful catalytic oxidation ability, and offers the benefit of biocompatibility with relatively low or no toxicity to mammalian cells and advantage of being readily biodegradable [31,32].CeO 2 nanoparticles (CeO 2 NPs) have gained popularity as a nanozyme due to their biocompatibility and multienzyme-like properties [33].For instance, CeO 2 NPs display oxidase-like properties without needing hazardous H 2 O 2 as a co-substrate [34].The oxidase activity of naked CeO 2 nanozyme is relatively weak, which limits its use in antibacterial applications to some extent.Recently reports found that fluoride ions (F − ) could significantly improve the catalytic activity of CeO 2 nanozyme by altering the surface charge of CeO 2 for tuning the substrate adsorption affinity and facilitating electron transfer between substrates, oxygen and CeO 2 [35][36][37].Meanwhile, significant advancements have been achieved in the field of photothermal sensing and photothermal treatment based on the principle of substantial photothermal effect generated by the transmutation of TMB into oxidized TMB (oxTMB) [38][39][40][41][42][43].Inspired by this, we create a nanozyme-based NIR photothermal system (CeO 2 + F − + TMB) that can be activated by fluoride ions.The photothermal system can adjust the concentration of oxTMB by adjusting the concentration of fluoride ions, thereby achieving the precise photothermal temperature.In the absence of fluoride ions, the oxidase activity of the bare CeO 2 nanozyme is feeble, and the conversion of TMB to oxTMB is minimal, resulting in a minor photothermal effect.When fluoride ions are introduced to the system, CeO 2 oxidase activity steadily increases, and the concentration of fluoride ions is proportional to the concentration of oxTMB produced by TMB oxidation with CeO 2 nanozyme.Consequently, the photothermal temperature of the CeO 2 + F − + TMB system is linearly related to the concentration of fluoride ions within a given range.Above this, the temperature of the photothermal system can be changed by altering the concentration of fluoride ions, which can be used for photothermal antibacterial and in vivo wound healing.By altering the concentration of fluoride ions to acquire optimized photothermal temperature, the NIR photothermal system of CeO 2 + F − + TMB can provide high-efficiency antibacterial effects in vitro against Staphylococcus aureus (S. aureus, 99.8%) and Escherichia coli (E.coli, 99.3%).Further results show that the photothermal system exhibits good bactericidal activity and biocompatibility in vivo wound infection model (Scheme 1).This work not only sheds new light on the PTT strategy for antibacterial therapy and wound healing but also opens a new avenue for the development of alternative antibiotics.

Results and discussion
We first performed structural characterization of synthesized CeO 2 nanoparticles (CeO 2 NPs).The shape of CeO 2 NPs was not uniform, and most of those observed in TEM images were granular (Additional file 1: Figure S1).CeO 2 nanostructures were highly crystalline, as evidenced by HR-TEM images showing a continuous lattice and sharp fringes.In addition, SAED patterns and continuous lattices confirmed the crystalline characteristics and fluorite structure of CeO 2 NPs.The O and Ce elements were uniformly distributed on the surface of CeO 2 NPs according to the HAADF-STEM analysis (Additional file 1: Figure S2).XPS spectrum showed the presence of Ce, O and C as the main components and the Ce showed the mixed oxidation state of Ce 3+ and Ce 4+ , which was very important for the simulated oxidase-like properties of CeO 2 NPs (Additional file 1: Figure S3).Meantime, the characteristic diffraction peaks of the XRD spectrum at 2θ indicated the remarkable crystalline purity of the CeO 2 NPs produced (Additional file 1: Figure S4).
We then sought to determine whether fluoride ions could activate the photothermal activity of CeO 2 + TMB (Fig. 1a).As depicted in Additional file 1: Figure S5, since CeO 2 and TMB had essentially no absorption in the NIR region, we observed a minor temperature increase of 5.7 °C and 7.4 °C following NIR irradiation (808 nm) for approximately 100 s.A similar temperature rise of 7.4 °C was recorded in the presence of CeO 2 + F − .Due to the low oxidase catalytic activity of the CeO 2 nanozyme, only a tiny amount of TMB was catalyzed to be oxidized to oxTMB, resulting in weak absorption in the NIR region.The temperature increase of CeO 2 + TMB under NIR irradiation was only 14.6 °C.As expected, the presence of F − significantly increased the activity of CeO 2 oxidase to oxidize TMB to oxTMB with strong absorption in the NIR region.Therefore, the CeO 2 + F − + TMB system showed a robust photothermal effect with a temperature increase of 55.3 °C under NIR irradiation (Fig. 1b).Due to the good photothermal stability of oxTMB, no obvious photobleaching was detected in the CeO 2 + F − + TMB system after five cycles of on/off NIR irradiation (Fig. 1c).Further, the photothermal conversion efficiency (η) of CeO 2 + TMB and CeO 2 + F − + TMB was compared.The η of CeO 2 + TMB and CeO 2 + F − + TMB was determined to be 5.65% and 15.15%, validating our speculation that treating naked CeO 2 NPs with fluoride ions facilitated the improvement of the photothermal performance of CeO 2 + TMB (Fig. 1d, e).Accordingly, the above findings indicated that F − could effectively stimulate the photothermal conversion efficiency of the CeO 2 + TMB.
To optimize the performance of the CeO 2 + F − + TMB system, the photothermal assay investigated the impact of several parameters, including the concentration of CeO 2 NPs, buffer solution, and light power intensity.The optimized conditions were determined as CeO 2 NPs concentrate of 30 μg/mL, pH of 4.0, and power intensity of 2.3 W/cm 2 (Additional file 1: Figure S6).Furthermore, we validated the role of each module in the CeO 2 + F − + TMB system.In response to the glutathione (GSH), the oxidase activity was wholly eliminated owing to the reduction of CeO 2 NPs (Additional file 1: Figure S7), and the CeO 2 + F − + TMB system recorded a notable change of photothermal effect with the temperature change value from 56.2 °C to 7.7 °C (Additional file 1: Figure S8).Similarly, with the addition of 8-hydroxyquinoline (8-HQ, blocking the oxidation of TMB) to the CeO 2 + F − + TMB system, the temperature change value from 56.2 °C to 7.8 °C was observed, verifying the photothermal transformation mediated by oxTMB.Al 3+ , a chelating agent for F − , prevents F − from adhering to the surface of CeO 2 NPs.As predicted, a temperature change value from 56.2 °C to 12.8 °C was seen by the CeO 2 + F − + TMB system with the addition of Al 3+ (Additional file 1: Figure S9), which further proved the F − could effectively activate the photothermal conversion efficiency of the CeO 2 + F − + TMB system.
Based on the above results, we further investigated whether the NIR photothermal system of CeO 2 + F − + TMB could achieve programmed temperature by adjusting the concentration of F − .As shown in Fig. 1f, adding F − at concentrations ranging from 1 μM to 3 mM elicited a rapid dynamic response from the temperature output.The temperature rose linearly with F − concentration from 1 μM to 600 μM.On the contrary, the temperature change values dropped linearly with Al 3+ concentration from 0.02 mM to 0.5 mM (Fig. 1g).The potential interferences of Cl − , Br , and CH 3 COO − were explored to assess further the specific identification capacity of the CeO 2 + F − + TMB system for F − .As demonstrated in Additional file 1: Figure S10, the addition of potential interferences had no appreciable effect on the temperature change relative to the F − , demonstrating the outstanding selectivity of the CeO 2 + F − + TMB system toward F − .Therefore, herein, we constructed a NIR photothermal system of CeO 2 + F − + TMB that could obtain programmed temperature by varying the concentration of F − .Encouraged by the above data, to this end, we evaluated the antibacterial activity of CeO 2 + F − + TMB system for PTT in vitro.S. aureus (gram-positive organism) and E. coli (gram-negative creature) were chosen as typical strains to serve as model bacteria.No statistically significant distinction in the number of colonies was observed between the NIR illumination and dark settings of the control groups for both S. aureus and E. coli (Fig. 2a and Additional file 1: Figure S11a), proving that NIR illumination alone had no impact on microorganisms.In dark conditions, for the CeO 2 and TMB group, the survival rates of S. aureus (Fig. 2b) were 84.6% and 99.1%, and the survival rates of E. coli (Additional file 1: Figure S11b) were close to the control group.Due to the poor photothermal efficiency of CeO 2 and TMB, the CeO 2 and TMB group exhibited negligible antibacterial activity when exposed to 808 nm NIR illumination for about 7 min, with survival rates of 83.9% and 95.9% for S. aureus, and 101.6% and 81.2% for E. coli, respectively.This occurrence proved that CeO 2 and TMB did not display apparent antibacterial activity under dark and NIR light settings regarding the number of S. aureus and E. coli colonies.In the CeO 2 + TMB group, no substantial antibacterial effect was observed for S. aureus and E. coli in dark conditions.Under NIR irradiation (2.3 W/ cm 2 , 7 min), the CeO 2 + TMB group displayed a poor antibacterial effect with a 60.8% survival rate of S. aureus and 63.0% survival rate of E. coli, attributing to a low photothermal effect of CeO 2 + TMB.The S. aureus and E. coli in the F − groups exhibited survival rates of 79.3% and 89.9% when exposed to 808 nm light, which indicated that the F − barely impacted the bacteria activity.In contrast, under identical conditions for NIR irradiation, the antibacterial rates of the CeO 2 + F − + TMB group substantially boosted to 99.8% (S. aureus) and 99.3% (E.coli) in comparison to the CeO 2 + TMB group, showing that the F − could significantly enhance the photothermal conversion efficiency of the CeO 2 + TMB for PTT.To find out the association between F − dosage and bacteria sterilization efficiency, the F − concentration was altered to 0, 10, 50, 200, and 400 μM.Notably, under NIR illumination, the antibacterial activity of the CeO 2 + TMB group against S. aureus increased gradually with increasing F − concentration (Fig. 2c).For following antibacterial and wound healing studies, the optimal concentration of F − for PTT was established to be 400 μM due to the ultra-high antibacterial effectiveness (99.7%) and optimal photothermal temperature to guarantee good safety (minimal invasion) and efficient bacterial breakdown (Fig. 2d).Similarly, the antibacterial effect of CeO 2 + TMB against E. coli gradually increased with the increase of F − concentration under NIR irradiation (Additional file 1: Figure S12a), and the optimal antibacterial concentration of F − was also 400 μM (Additional file 1: Figure S12b).The negligible OD600 values of S. aureus (Fig. 2e) and E. coli (Additional file 1: Figure S13) treated in the CeO 2 + F − + TMB group under NIR irradiation, similar to the agar plate results, further revealed the potent antibacterial activity of CeO 2 + TMB could be effectively boosted by F − .
To examine the mechanism of antimicrobial action, bacterial SEM testing and bacterial live-dead fluorescence staining under light and dark settings were performed.As shown in Fig. 2f and Additional file 1: Figure S14, for control groups under the NIR illumination and dark settings, S. aureus and E. coli exhibited typical spherical and rod morphologies with smooth surfaces and undamaged cell walls, respectively similar to those of native living bacteria.After treatment with CeO 2 + TMB + NIR, a few disturbances could be seen on the cell wall, indicating minimal damage to the integrity of cell walls.However, the treatment of CeO 2 + F − + TMB + NIR group induced varying degrees of damage to the morphology of the bacterial cells, resulting in compression as well as a roughening of the bacterial surface.The effectiveness of the antibacterial treatment was further evaluated using live/dead fluorescence assays.The living bacteria were stained with blue dye during these tests, while the dead bacteria were stained with red dye.As evidenced by the absence of red fluorescence in Fig. 2g and Additional file 1: Figure S15, most S. aureus and E. coli were alive under NIR illumination and dark conditions for the control groups.Only a few red luminous cells were found for bacteria treated with CeO 2 + TMB + NIR group, implying a partial sterilization effect.Notably, treatment with CeO 2 + F − + TMB + NIR group caused a significant increase in the number of dead cells, as seen by the strong red fluorescence signal, indicating a substantial improvement in antibacterial effectiveness.In short, the potent antibacterial activity of CeO 2 + TMB could be well activated by F − under NIR illumination.
After thoroughly confirming the outstanding antimicrobial efficiency of CeO 2 + F − + TMB + NIR in vitro, we further investigated the potential in disinfecting and treating animal wounds.It was vital to assess the biosafety of antibacterial wound healing agents.Before constructing an infectious model in vivo, the cytotoxicity and hemocompatibility of CeO 2 + F − + TMB were evaluated.We employed NIH3T3 fibroblasts derived from mouse embryos to assess the cytotoxicity.According to the results of the cell counting kit-8 (CCK-8) experiment, CeO 2 + F − + TMB displayed little cytotoxicity over the 24 h observation period (Additional file 1: Figure S16).To further assess its biocompatibility, the hemolytic performance of CeO 2 + F − + TMB was measured after it had been grown with mouse red blood cells (RBCs).Positive and negative control RBCs were distributed separately in deionized water and normal saline to evaluate the hemolysis rate.The supernatant (water) from the positive control group had a deep red color (Additional file 1: Figure S17), whereas the supernatant (normal saline) from the negative control group and all other groups was transparent.Less than five percent hemolysis was identified in the group treated with CeO 2 + F − + TMB, indicating excellent blood compatibility.Together, the CeO 2 + F − + TMB system with excellent biocompatibility was feasible for in vivo anti-infective treatment.
Inspired by the remarkable antibacterial efficacy and excellent biocompatibility of the CeO 2 + F − + TMB system, we undertook wound management experiments in vivo.To determine the disinfects abscesses of the CeO 2 + F − + TMB system, we first established a skin model by injecting 100 μL of S. aureus (1 × 10 8 CFU/ mL) into the epidermis of a Balb/c mouse.The phototherapy mechanism for wound treatment was shown in Fig. 3a.We proceed to employ the CeO 2 + F − + TMB system as a positive control and acetate buffer as a negative control for in vivo wound healing investigations, which divided the mice into four groups, including control, control + NIR, CeO 2 + F − + TMB, and CeO 2 + F − + TMB + NIR.The wound receiving NIR phototherapy was monitored using a thermal imaging camera (Fig. 3b).After administering CeO 2 + F − + TMB injections to mice, the infected area showed a significant photothermal response when exposed to NIR light (808 nm, 2.3 W/ cm 2 ).The temperature rose rapidly and remained stable at 53.6 °C after about 7 min (Fig. 3c).The photothermal temperature of 53.6 °C in the infected site guaranteed good safety (minimal invasion) and efficient bacterial breakdown.In contrast, there was just a minor temperature change in the control group following acetate buffer treatment under the same conditions.Photographs of wound healing were obtained every four days across different treatment groups (Fig. 3d).The calculation of wound contraction was depicted in Fig. 3e.In all tested groups, the size of the wound decreased over time.Typically, among the four groups, the CeO 2 + F − + TMB + NIR group displayed, on average, the quickest wound healing (Fig. 3f ).Meanwhile, the in vivo photothermal response and wound healing investigations of the CeO 2 + TMB + NIR group were conducted to further demonstrate the role of F − in CeO 2 + F − + TMB + NIR photothermal system.As shown in Additional file 1: Figure S18, the temperature slowly increased over time and remained stable at 39.5 °C after about 7 min.The wound size reduced the wound area to 72.2% after 12 days, which was far lower than the 99.1% reduction in wound area by treatemnt of the CeO 2 + F − + TMB + NIR group (Additional file 1: Figure S19).Therefore, introducing F − could significantly enhance the wound healing effect of the CeO 2 + TMB + NIR in vivo.For evaluating the sterilization efficiency, bacteria from wound tissue on 2 h and day 7, were cultivated overnight on LB agar plates (Additional file 1: Figure S20).On day 7, the bacteria in the CeO 2 + F − + TMB + NIR group reduced significantly compared to the other groups.Moreover, to further evaluate the effect of wound healing, we selected the common antibiotic penicillin G as a comparison.The results in Additional file 1: Figure S21 showed that the wound size of thepenicillin G treated group and CeO 2 + F − + TMB + NIR treated group reduced the wound area to 94.1% and 99.1% after 12 days, respectively.Judging from the above data, the CeO 2 + F − + TMB NIR photothermal system could substantially improve bacteria-infected wound healing.
Furthermore, alteration in skin tissue was evaluated to corroborate the detail of the aforementioned wound healing.The histological investigation was performed using the H&E (hematoxylin and eosin) staining and Masson's trichrome staining.At a certain period after treatment, mice were euthanized, and tissues were obtained from the control, control + NIR, CeO 2 + F − + TMB, and CeO 2 + F − + TMB + NIR groups.Results from H&E staining revealed that on day 8, on the wound treated with CeO 2 + F − + TMB + NIR group, the epidermal layer was intact, and there were fewer inflammatory cells (blood cells and neutrophils), whereas the epidermal layer was compromised, and there were more inflammatory cells emerged in the skin tissue of control, control + NIR, and CeO 2 + F − + TMB groups (Fig. 4a).In addition, abundant fibroblasts and blood vessels were seen in CeO 2 + F − + TMB + NIR group, indicating improved tissue regeneration and wound healing.Masson's trichrome staining revealed that on day 8, the wound treated with CeO 2 + F − + TMB + NIR group had well-established collagen fibers and a dermal layer.In contrast, the wounds in the other treatment groups included many collagen fibers that had not been repaired (Fig. 4b).Additionally, compared to other groups, scar width treated with the CeO 2 + F − + TMB + NIR group decreased rapidly (Additional file 1: Figure S22).Furthermore, more collagen fibers were observed in the CeO 2 + F − + TMB + NIR group than in other treatment groups (Fig. 4c).Numerous inflammatory cytokines are essential in the initiation and progression of inflammatory disease.As demonstrated in Fig. 4d, e, the levels of pro-inflammatory cytokines IL-1β and IL-6 in the CeO 2 + F − + TMB + NIR group were substantially lower than in the other treatment groups, while the levels of anti-inflammatory cytokines IL-10 and growth factors VEGF were significantly higher.The data on inflammatory cytokines revealed that the treatment of the CeO 2 + F − + TMB + NIR group could effectively decrease the expression of IL-1β and IL-6 while up-regulating the expression of IL-10 and VEGF, hence reducing the inflammatory response.In short, wounds treated with CeO 2 + F − + TMB + NIR demonstrated minimal inflammation, good hemostasis, a high collagen fiber content, thick granulation tissue and quick wound healing.Consequently, CeO 2 + F − + TMB + NIR could accelerate the phase transition from hemostasis to inflammation, proliferation, and remodeling, facilitating wound healing.
Meanwhile, the in vivo biosafety of our proposed CeO 2 + F − + TMB + NIR antimicrobial system was further investigated to demonstrate its potential for future bacterial suppression applications.Monitoring the mice's body weight throughout the experimental treatments indicated no significant weight loss in either the CeO 2 + F − + TMB + NIR group or other treatment groups, proving that these treatments were not systemically toxic to mice (Additional file 1: Figure S23).Besides, the primary organs of the treated mice were taken for H&E staining, including the heart, liver, spleen, lung, and kidney, and no abnormalities or lesions were seen on the biopsy photographs (Fig. 5a).Additionally, the routine blood tests and blood biochemical analyses of the mice in the CeO 2 + F − + TMB + NIR group and other treatment groups showed no significant differences in any of the parameters, proving that the kidney and liver functions were unharmed (Fig. 5b).Collectively, these findings repeatedly validated the good biological security of antimicrobial therapy utilizing the CeO 2 + F − + TMB NIR photothermal system, revealing the great potential in clinical translation.

Conclusion
In summary, we have built a unique NIR photothermal system of CeO 2 + F − + TMB that can acquire the programmed temperature by managing the photothermal activity of nanozymes.By adjusting the concentration of fluoride ions to achieve optimized photothermal temperature, the CeO 2 + F − + TMB NIR photothermal system not only could realize high-efficiency antibacterial effects against S. aureus and E. coli in vitro, but also could guarantee good safety of minimal invasion.The CeO 2 + F − + TMB NIR photothermal system could also substantially improve bacteria-infected wound healing via anti-inflammatory by reduction of pro-inflammatory factors IL-1β and IL-6, upregulation of anti-inflammatory IL-10 and VEGF growth factors, stimulation of collagen deposition, and promotion of granulation tissue and

Fig. 3 aFig. 3 (
Fig. 3 a Schematic representations of wound infection model and treatment strategy based on CeO 2 + F − + TMB therapeutic platform.b Infrared thermal pictures, and c the rise in temperature at the wound sites of mice at the designated periods under 808 nm NIR illumination (2.3 W cm 2 ) for control (acetate buffer (pH 4, 20 mM)).andCeO 2 + F − + TMB (CeO 2 : 30 µg/mL, F − : 400 µM, TMB: 1 mM) groups.d Pictures depicting the progression of wound contraction from day 0 to day 12 in response to various treatments.e Quantitative analysis of the wound size in each group after different treatments.f The dynamic process diagram of wound trace for each treatment within 12 days.Error bars represent the standard deviation from the mean (n = 3) (See figure on next page.)