Title：Zwitterionic Photosensitizer-Assembled Nanocluster Produces Efficient Photogenerated Radicals via Autoionization for Superior Antibacterial Photodynamic Therapy
Journal: Advanced Materials
IF: 26.8
Original link：https://doi.org/10.1002/adma.202418978
Reporter：Wenchao Zhang-26-master
Photodynamic therapy (PDT) holds significant promise for antibacterial treatment, with its potential markedly amplified when using Type I photosensitizers (PSs). However, developing Type I PSs remains a significant challenge due to a lack of reliable design strategy. Herein, a Type I PS nanocluster is developed via self-assembly of zwitterionic small molecule (C3TH) for superior antibacterial PDT in vivo. Mechanism studies demonstrate that unique cross-arranged C3TH within nanocluster not only shortens intermolecular distance but also inhibits intermolecular electronic-vibrational coupling, thus facilitating intermolecular photoinduced electron transfer to form PS radical cation and anion via autoionization reaction. Subsequently, these highly oxidizing or reducing PS radicals engage in cascade photoredox to generate efficient ·OH and O2-·. As a result, C3TH nanoclusters achieve a 97.6% antibacterial efficacy against MRSA at an ultralow dose, surpassing the efficacy of the commercial antibiotic Vancomycin by more than 8.8-fold. These findings deepen the understanding of Type I PDT, providing a novel strategy for developing Type I PSs.
The overuse and abuse of antibiotics have led to the rise of various drug-resistant bacteria that pose a serious threat to human health. Efforts to develop stronger antibiotics often exacerbate the problem, driving the emergence of new superbugs and perpetuating a vicious cycle. Antibacterial photodynamic therapy (aPDT) presents a non-antibiotic alternative by utilizing light irradiation to activate photosensitizers (PSs), which generate reactive oxygen species (ROS) to selectively destroy bacteria. This method holds great potential for combating bacterial infections while avoiding antibiotic resistance. For an efficient aPDT to proceed, PSs are essential and can be classified into Type I and II PSs based on their ROS generation. Most existing PSs operate via a Type II mechanism, which generates singlet oxygen (1O2) by direct energy transfer from triplet excited state of PS (3PS*) to surrounding oxygen. In contrast, Type I PSs engage in a photoinduced electron transfer (PeT) with nearby components such as substrates, water, or oxygen, producing hydroxyl radicals (·OH) or superoxide anions (O2-·). Notably, Type I PSs exhibit minimal or even no oxygen dependence in contrast to the highly oxygen-dependent Type II PSs. This feature makes Type I PSs particularly advantageous for treating hypoxic bacterial-infected diseases or tumors.
Developing Type I PSs for aPDT has recently garnered significant interest but remains challenging due to the lack of a robust design strategy. A critical limitation lies in the inefficient PeT between PSs and substrates, which underpins the production of Type I ROS. Recent studies point out the importance of intermolecular PeT between PSs and electron-donating/withdrawing substrates in enhancing Type I ROS production. Specifically, intermolecular PeT among these heterogeneous components produces highly oxidizing or reducing PS radicals (PS+· or PS-·), which then participate in photoredox reactions with oxygen or water to generate Type I ROS. Despite these advances, existing Type I PSs generally require multiple components, such as substrates or donor/acceptor species for intermolecular PeT and amphiphilic matrices for water solubility, posing significant challenges for clinical translation. Efforts to simplify Type I PS design have demonstrated the feasibility of achieving intermolecular PeT between homogeneous PSs, but its application in aPDT remains unexplored. Based on these findings, the development of single-component cationic PSs capable of efficient intermolecular PeT represents a promising strategy for advancing aPDT. Unfortunately, this approach remains largely unexplored.
In this work, we demonstrate a Type I PS nanocluster via the self-assembly of zwitterionic small molecule (C3TH) for superior aPDT in vivo. C3TH is achieved by simple hydrolysis of cationic counterpart (C3T). Transmission electron microscopy (TEM) reveals that both C3T and C3TH could form self-assembled nanoclusters in aqueous solution without any additional components, greatly simplifying the PS system. Molecular dynamics (MD) simulation further illustrates a unique cross-arrangement of zwitterionic C3TH within the nanoclusters, in contrast to the slip-arrangement observed for cationic C3T. This unique cross-arrangement not only shortens intermolecular distance but also inhibits intermolecular electronic-vibrational coupling, thus facilitating intermolecular PeT to form PS+· and PS-· via autoionization. Subsequently, PS+· and PS-· engage in cascade photoredox reactions with water and oxygen to yield ·OH and O2‾·. As a result, C3TH exhibits a remarkable 74% increase in ·OH production and a slight elevation in O2‾· generation compared to C3T, along with a 42% reduction in 1O2 generation. Leveraging this performance, C3TH nanoclusters achieve a 97.6% antibacterial rate against Gram-positive bacteria even at an ultralow dose of 8 µg·mL−1, surpassing the efficacy of the commercial antibiotic vancomycin by over 8.8-fold.
1. Characterization of self-assembled nanocluster and spectroscopic properties
The figure illustrates the characterization and spectral properties of self-assembled nanoclusters. TEM observations reveal that C3TH molecules can self-assemble via intermolecular non-covalent interactions (combining electrostatic attraction and van der Waals forces) under physiological conditions to form spherical nanoclusters with diameters smaller than 30 nm. Molecular dynamics simulations further disclose that these nanoclusters exhibit an internally cross-arranged structure. Subsequent spectral analysis of the C3TH nanoclusters indicates that, due to their intermolecular cross-shaped configuration, C3TH demonstrates weak intermolecular electron-vibration coupling effects. This characteristic effectively reduces non-radiative energy loss during photoexcitation, endowing it with efficient intersystem crossing capability, thereby significantly enhancing the generation efficiency of 3PS*.
2. Electrochemistry and Photosensitization Channel
The figure illustrates the electrochemical properties of the C3TH nanocluster. Cyclic voltammetry test results indicate that the reduction potential of C3TH is lower than that of O2, H+/·OH and O2/O2‾·, suggesting that C3TH can transfer electrons to O2, thereby promoting the generation of O2‾· and ·OH. Meanwhile, the oxidation potential of C3TH is higher than that of H2O/O2, H+, indicating its ability to oxidize H2O to produce O2, which reflects the material's capability for in situ oxygen generation. This mechanism was ultimately verified through isotope labeling experiments.
3. Excited State Dynamics for Type I ROS Production
The figure illustrates the mechanism of Type I reactive oxygen species generation by C3TH in spectroelectrochemical analysis. Quantum chemical calculations and femtosecond transient absorption (fs-TA) results indicate that the excited-state absorption (ESA) band of C3TH near 580 nm primarily originates from 3PS*, which subsequently decays and transforms into PS+· or PS-·, with corresponding absorption bands near 580 nm and 1250 nm, respectively. This demonstrates that the generation of PS+· and PS-· in C3TH stems from 3PS*. Electron paramagnetic resonance (EPR) spectroscopy further confirms the presence of photogenerated electrons in C3TH, accompanied by a significant enhancement in photocurrent response, indicating the material's improved efficiency in generating PS+· and PS-·. Finally, thermodynamic feasibility analysis reveals that an autoionization mechanism exists within C3TH nanoclusters, enabling efficient generation of Type I reactive oxygen species.
4. Feasibility and Principle of Cascade LFIA
The figure demonstrates the antibacterial efficacy of C3TH both in vitro and in vivo. The study selected methicillin-resistant Staphylococcus aureus (MRSA) to evaluate the material's antibacterial properties, with the clinical antibiotic vancomycin (Van) serving as the control. When both C3TH and Van were applied at a concentration of 8 µg·mL−1, the inhibition rates against MRSA reached 97.6% and 11.1%, respectively. Even under hypoxic conditions, C3TH maintained an antibacterial rate of 85.7%. These results were further confirmed through bacterial live/dead staining and scanning electron microscopy observations. Finally, the in vivo antibacterial effect of C3TH was assessed using a mouse wound infection model. The results indicated that C3TH rapidly and effectively eliminated bacteria and significantly promoted wound healing.
In summary, we have developed a C3TH nanocluster formed via the self-assembly of homogeneous molecules, which produces efficient Type I ROS for superior aPDT. The zwitterionic structure of C3TH induces a twisted configuration that offers two key advantages: i) it enhances 3PS* production via facile ISC; and ii) promotes unique cross-arrangement within nanocluster, facilitating intermolecular PeT. The combination of abundant 3PS* and efficient intermolecular PeT supports autoionization reactions between neighboring 3PS* to effectively generate PS+· or PS-·. Subsequently, PS⁺· catalyzes the oxidation of H2O to produce O2 in situ, while PS-· transfers electrons to either endogenous or in situ generated O2, resulting in the efficient production of O2-· and ·OH with reduced reliance on endogenous O2. Utilizing this superior Type I ROS production, our therapeutic dose is half that of the clinically used vancomycin, yet its efficacy exceeds that of Van by more than 8.8-fold. This study deepens the understanding on PeT processes between homogeneous PSs, marking a significant advancement in Type I mechanism. These findings are expected to inspire new design strategies of Type I PSs with enhanced antibacterial properties.