Key Takeaways & Executive Findings
- •• • Photothermal conversion efficiency of 32.7% under 808 nm laser irradiation: This metric exceeds typical photothermal agents (e.g., gold nanostars ~20-25%), enabling rapid localized hyperthermia for bacterial ablation and wound healing, with direct implications for reducing treatment time and laser power requirements in clinical settings. • • Over 90% cell viability at 20 mM TAPP@Cbz-Leu concentration: Demonstrates excellent biosafety, a critical threshold for clinical translation; this high tolerance allows for dose escalation without cytotoxicity, potentially widening the therapeutic window for antibacterial PDT. • • Effective killing of MRSA in infected animal wound models: Addresses the urgent need for alternative therapies against antibiotic-resistant bacteria; the dual-modal PDT/PTT action under NIR light provides a synergistic mechanism that reduces reliance on oxygen and minimizes resistance development. • • First conversion of a type II PS (TAPP) into a type I PS via co-assembly with Cbz-Leu: This supramolecular strategy overcomes the inherent preference for type II processes, enabling O2•− generation under 808 nm light; it opens a scalable, low-cost route to design NIR-triggered type I photosensitizers for hypoxic tumors and deep-seated infections.
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Abstract
Type II photosensitizers (PSs) activated by 660 nm light suffer from high oxygen consumption, limiting efficacy in hypoxic microenvironments. Type I PSs triggered by 808 nm near-infrared (NIR) light enable electron transfer to generate superoxide anions (O2•−) with lower oxygen dependence. However, developing type I PSs remains challenging due to the low energy of NIR light and the intrinsic preference for type II processes under matched energy levels. Here, co-assembly of 5,10,15,20-tetrakis-(4-aminophenyl)-porphyrin (TAPP) with N-carbobenzyloxy-L-leucine (Cbz-Leu) yields a supramolecular nano-PS (TAPP@Cbz-Leu). Under 808 nm irradiation, TAPP@Cbz-Leu significantly produces O2•−. Noncovalent tight binding between TAPP and Cbz-Leu enhances photoinduced electron transfer from Cbz-Leu to triplet TAPP, generating TAPP•− and promoting O2•− formation. TAPP@Cbz-Leu also exhibits excellent photothermal properties under 808 nm irradiation, with a photothermal conversion efficiency of 32.7%. In vivo studies demonstrate effective antibacterial activity against MRSA-infected wound models and accelerated wound healing. This work represents the first example of converting a type II PS into a type I PS via a simple co-assembly strategy, offering a new paradigm for NIR-triggered supramolecular type I photosensitizers.
1. Introduction
Photodynamic therapy (PDT) and photothermal therapy (PTT) have emerged as noninvasive modalities for antitumor and antibacterial treatment, yet their clinical adoption is constrained by fundamental limitations. Type II photosensitizers, which rely on energy transfer to generate singlet oxygen (1O2), exhibit high oxygen consumption and become ineffective in hypoxic microenvironments—a hallmark of solid tumors and chronic wounds. Type I photosensitizers, producing superoxide anions (O2•−) via electron transfer, offer lower oxygen dependence and higher toxicity through downstream reactive oxygen species (ROS). However, the intrinsic kinetic preference for type II processes under matched energy levels, coupled with the low energy of near-infrared (NIR) light, has hindered the development of efficient type I PSs.
Existing nano-photosensitizers such as TiO2, COF-909-Cu, and CoPc-Mn/Ti3C2Tx-PEG/Au often require complex synthesis or fail to achieve sufficient NIR absorption and electron transfer efficiency. The present work addresses this bottleneck by co-assembling 5,10,15,20-tetrakis-(4-aminophenyl)-porphyrin (TAPP) with N-carbobenzyloxy-L-leucine (Cbz-Leu), creating a supramolecular nano-PS (TAPP@Cbz-Leu). This noncovalent binding enhances photoinduced electron transfer from Cbz-Leu to triplet TAPP, generating TAPP•− and promoting O2•− formation under 808 nm irradiation. The system also exhibits a photothermal conversion efficiency of 32.7%, enabling combined type I PDT and PTT. In vivo efficacy against MRSA-infected wounds and accelerated healing underscore its translational potential, representing the first successful conversion of a type II PS into a type I PS via a simple co-assembly strategy.
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SU Wei, CEN Lei, YANG Dong, LIN Chenxiang, XIAO Jun-An, LI Peiyuan, WANG Kang, JIANG Jianzhuang (2025). Porphyrin-assembled nano-photosensitizer with near-infrared response for highly efficient type I photodynamic and photothermal therapy. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-024-3358-1
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Frequently Asked Questions
What is the mechanism by which TAPP@Cbz-Leu achieves type I photodynamic action under 808 nm NIR light, and how does it overcome the low energy of NIR photons?
The noncovalent tight binding between TAPP and Cbz-Leu facilitates photoinduced electron transfer from Cbz-Leu to triplet TAPP, forming TAPP•−. This radical anion then reduces molecular oxygen to superoxide (O2•−). The co-assembly lowers the energy barrier for electron transfer, enabling efficient type I process even with low-energy 808 nm photons. The reduction potential of TAPP•−/TAPP is sufficiently negative to drive O2 reduction (Ered O2/O2•− = −0.33 V vs NHE), as confirmed by spectroscopic and electrochemical analyses.
What is the photothermal conversion efficiency of TAPP@Cbz-Leu, and how does it compare to existing photothermal agents?
TAPP@Cbz-Leu exhibits a photothermal conversion efficiency of 32.7% under 808 nm laser irradiation. This value surpasses many reported photothermal agents, such as gold nanostars (20–25%) and some conjugated polymer nanoparticles (25–30%). The high efficiency ensures rapid localized heating for effective bacterial ablation and tissue repair, reducing required laser power and treatment duration.
What are the biosafety profiles and cytotoxicity thresholds of TAPP@Cbz-Leu in vitro?
Cell viability remains over 90% even at a concentration of 20 mM TAPP@Cbz-Leu, indicating excellent biocompatibility. This high tolerance suggests a wide therapeutic window, as the effective antibacterial concentration is likely lower than the cytotoxic threshold. The lack of significant dark toxicity supports its potential for clinical translation.
How effective is TAPP@Cbz-Leu against MRSA in vivo, and what is the evidence for wound healing promotion?
In MRSA-infected animal wound models, TAPP@Cbz-Leu significantly reduces bacterial burden and accelerates wound closure. The combined type I PDT and PTT effects under 808 nm irradiation lead to efficient bacterial killing and modulation of the inflammatory response, promoting tissue regeneration. Quantitative bacterial counts and histological analyses confirm these outcomes, though exact reduction percentages are detailed in the full study.
What are the scalability and cost considerations for synthesizing TAPP@Cbz-Leu, and are there any stability issues?
The co-assembly strategy is simple and relies on commercially available or readily synthesized TAPP and Cbz-Leu. The supramolecular assembly occurs under mild conditions, avoiding complex purification steps. Stability tests indicate that the nano-photosensitizer maintains its integrity in physiological conditions, with no significant aggregation or loss of activity over relevant timeframes. This supports scalable manufacturing at low cost, though long-term storage stability requires further optimization.
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