Jointly Depleting Glutathione Based on Self-Assembled Nanomicelles for Enhancing Photodynamic Therapy

Photodynamic therapy (PDT) is one common ROS-generating therapeutic method with high tumor selectivity and low side effects. But the GSH-upregulation often alleviates its therapeutic efficiency. Here, we proposed a new strategy of jointly depleting GSH to enhance the therapeutic effect of PDT by preparing a nanomicelle by self-assembly method from GSH-activated photosensitizer DMT, curcumin, and amphiphilic polymer TPGS.

Integration of TADF Photosensitizer as “Electron Pump” and BSA as “Electron Reservoir” for Boosting Type I Photodynamic Therapy

Type I photosensitization provides an effective solution to the problem of unsatisfactory photodynamic therapeutic (PDT) effects caused by the tumor hypoxia. The challenge in the development of Type I mode is to boost the photosensitizer's own electron transfer capacity. Herein, we found that the use of bovine serum albumin (BSA) to encapsulate a thermally activated delayed fluorescence (TADF) photosensitizer PS can significantly promote the Type I PDT process to generate a mass of superoxide anions (O₂^•-). This Type I photosensitization opened a new strategy by employing BSA as "electron reservoir" and TADF photosensitizer as "electron pump". We integrated these roles of BSA and PS in one system by preparing nanophotosensitizer PS@BSA. The Type I PDT performance was demonstrated with tumor cells under hypoxic conditions. Furthermore, PS@BSA took full advantage of the tumor-targeting role of BSA and achieved efficient PDT for tumor-bearing mice in the in vivo experiments. This work provides an effective route to improve the PDT efficiency of hypoxic tumors.

Naphthofluorescein-based organic nanoparticles with superior stability for near-infrared photothermal therapy

Photothermal agents (PTAs) based on organic small molecules with near-infrared (NIR) absorption (700-900 nm) have attracted increasing attention in cancer photothermal therapy (PTT). However, NIR organic PTAs often suffer from poor stability. Fluorescein and its derivatives have been widely used in biological imaging and sensing due to their minimal cytotoxicity. But fluorescein and its derivatives have not been used in PTT because most of them don't have NIR absorption. In this work, two NIR naphthofluorescein derivatives, namely NFOM-1 and NFOM-2, were synthesized. In contrast to NFOM-1, NFOM-2 possesses an intramolecular hydrogen bonding network, which extends the absorption to the NIR region and significantly improves the photostability. NFOM-2 was encapsulated into an amphiphilic polymer (DSPE-mPEG2000) to obtain NFOMNPs as PTAs. Compared to the organic molecule NFOM-2, the absorption of NFOMNPs is broadened and further red-shifted to fit an 808 nm light source. Moreover, NFOMNPs exhibit good photothermal conversion efficiency (PCE, 40.4%, 808 nm, 1.0 W cm^-2), remarkable photostability and physiological stability, and significant PTT efficacy in vitro and in vivo was achieved. In other words, this study provides an intramolecular hydrogen bond network strategy and a fluorescein-based molecular platform to construct ultra-stable PTAs for efficient NIR PTT.

Time-Resolved Luminescent Sensing and Imaging for Enzyme Catalytic Activity Based on Responsive Probes

Enzymes, as a kind of biomacromolecules, play an important role in many physiological processes and relate directly to various diseases. Developing an efficient detection method for enzyme activity is important to achieve early diagnosis of enzyme-relevant diseases and high throughput screening of potential enzyme-relevant drugs. Time-resolved luminescence assay provide a high accuracy and signal-to-noise ratios detection methods for enzyme activity, which has been widely used in high throughput screening of enzyme-relevant drugs and diagnosis of enzyme-relevant diseases. Inspired by these advantages, various responsive probes based on metal complexes and metal-free organic compounds have been developed for time-resolved bioimaging and biosensing of enzyme activity owing to their long luminescence lifetimes, high quantum yields and photostability. In this review, we comprehensively reviewed metal complex- and metal-free organic compound-based responsive probes applied to detect enzyme activity through time-resolved imaging, including their design strategies and sensing principles. Current challenges and future prospects in this rapidly growing field are also discussed.

Thermally Activated Delayed Fluorescence Material: An Emerging Class of Metal-Free Luminophores for Biomedical Applications

The development of simple, efficient, and biocompatible organic luminescent molecules is of great significance to the clinical transformation of biomaterials. In recent years, purely organic thermally activated delayed fluorescence (TADF) materials with an extremely small single-triplet energy gap (ΔEST ) have been considered as the most promising new-generation electroluminescence emitters, which is an enormous breakthrough in organic optoelectronics. By merits of the unique photophysical properties, high structure flexibility, and reduced health risks, such metal-free TADF luminophores have attracted tremendous attention in biomedical fields, including conventional fluorescence imaging, time-resolved imaging and sensing, and photodynamic therapy. However, there is currently no systematic summary of the TADF materials for biomedical applications, which is presented in this review. Besides a brief introduction of the major developments of TADF material, the typical TADF mechanisms and fundamental principles on design strategies of TADF molecules and nanomaterials are subsequently described. Importantly, a specific emphasis is placed on the discussion of TADF materials for various biomedical applications. Finally, the authors make a forecast of the remaining challenges and future developments. This review provides insightful perspectives and clear prospects towards the rapid development of TADF materials in biomedicine, which will be highly valuable to exploit new luminescent materials.

Highly Efficient Aggregation-Induced Red-Emissive Organic Thermally Activated Delayed Fluorescence Materials with Prolonged Fluorescence Lifetime for Time-Resolved Luminescence Bioimaging

Organic thermally activated delayed fluorescence (TADF) materials are emerging as potential candidates for time-resolved fluorescence imaging in biological systems. However, the development of purely organic TADF materials with bright aggregated-state emissions in the red/near-infrared (NIR) region remains challenging. Here, we report three donor-acceptor-type TADF molecules as promising candidates for time-resolved fluorescence imaging, which are engineered by direct connection of electron-donating moieties (phenoxazine or phenothiazine) and an electron-acceptor 1,8-naphthalimide (NI). Theoretically and experimentally, we elucidate that three TADF materials possessed remarkably small ΔE_ST to promote the occurrence of reverse intersystem crossing (RISC). Moreover, they all exhibit aggregation-induced red emissions and long delayed fluorescence lifetimes without the influence of molecular oxygen. More importantly, these long-lived and biocompatible TADF materials, especially the phenoxazine-substituted NI fluorophores, show great potential for high-contrast fluorescence lifetime imaging in living cells. This study provides further a molecular design strategy for purely organic TADF materials and expands the versatile biological application of long-lived fluorescence research in time-resolved luminescence imaging.

A turn-on TADF chemosensor for sulfite with a microsecond-scale luminescence lifetime

A reaction-based luminescence chemosensor was synthesized for sulfite detection based on a fluorescein derivative with thermally activated delayed fluorescence (TADF). The chemosensor exhibited a fluorescence turn-on effect on sulfite with good sensitivity and selectivity. Importantly, utilizing the long luminescence lifetime of the TADF compound, the chemosensor realized photoluminescence lifetime imaging for sulfite in living cells with the luminescence lifetime distribution mainly around 14 μs.

Enhancing Intersystem Crossing to Achieve Thermally Activated Delayed Fluorescence in a Water-Soluble Fluorescein Derivative with a Flexible Propenyl Group

It is a challenge to rationally design an organic molecule with thermally activated delayed fluorescence (TADF) due to the intrinsically spin-forbidden transition. Meanwhile, those reported TADF organic molecules have difficulty to be directly applied in the field of biological and medical imaging because they usually have no water solubility. Here, a water-soluble TADF organic molecule DCF-BXJ was developed by introducing a flexible propenyl group into the commercial traditional fluorophore DCF (2,7-dichlorofluorescein). The flexible group provides nonradiative rotational motion, which causes an efficient energy level cross between the S₁ state and the T₂ state of DCF-BXJ. Results of transient absorption spectra and theoretical calculations supported that nonradiative rotational motion of the flexible group can enhance intersystem crossing (ISC) and bring out TADF. This work provides a new mechanism explanation for TADF existing in organic molecules.

Dual-Mode Detection of Bacterial 16S Ribosomal RNA in Tissues

The specific detection of pathogens has long been recognized as a vital strategy for controlling bacterial infections. Herein, a novel hydrophilic aromatic-imide-based thermally activated delayed fluorescence (TADF) probe, AI-Cz-Neo, is designed and synthesized by the conjugation of a TADF emitter with a bacterial 16S ribosomal RNA-targeted moiety, neomycin. Biological data showed for the first time that AI-Cz-Neo could be successfully applied for the dual-mode detection of bacterial 16S rRNA using confocal fluorescence imaging and time-resolved fluorescence imaging (TRFI) in both cells and tissues. These findings greatly expand the application of TADF fluorophores in time-resolved biological imaging and provide a promising strategy for the precise and reliable diagnosis of bacterial infections based on the dual-mode imaging of bacterial 16S rRNA by fluorescence intensity and fluorescence lifetime.

A dual-targeted theranostic photosensitizer based on a TADF fluorescein derivative

Specific diagnosis and therapy of cancer is still a challenge in biomedical research. Photodynamic therapy (PDT) has emerged as a novel therapeutic modality for cancer treatment. However, the traditional PDT photosensitizers often exhibit low specific selectivity. In this study, we have reported a dual-targeted theranostic photosensitizer FL-RGD by covalently conjugating tumor marker cyclic arginine-glycine-aspartic acid tripeptide (RGD) and a fluorescein derivative FL which has a property of thermally activated delayed fluorescence (TADF) and a long triplet lifetime for efficient PDT. The FL-RGD can target tumor tissues and further locate lysosomes of tumor cells to concurrently achieve the cancers' specific diagnosis and efficient treatment. The mechanism of its highly efficient PDT was attributed to the damage of lysosome via ¹O₂. Besides, FL-RGD has the potential to be utilized in depth imaging and treatment by two-photon excitation. The actual diagnosis performance of FL-RGD was proved by fluorescence imaging of living cells and tumor bearing mice. The therapy performance was proved by MTT assays, fluorescence-activated cell sorting (FACS) analysis and PDT experiments on tumor bearing mice. The research obviously exhibited the potential of FL-RGD for tumor theranostics in vivo and in vitro.

Nitroreductase-Activatable Theranostic Molecules with High PDT Efficiency under Mild Hypoxia Based on a TADF Fluorescein Derivative

High specificity detection and site-specific therapy are still the main challenges for theranostic anticancer prodrugs. In this work, we reported two smart activatable theranostic molecules based on a thermally activated delayed fluorescence fluorescein derivative. Nitroreductase induced by a mild hypoxia microenvironment of a solid tumor was used to activate the fluorescence and photodynamic therapy (PDT) efficiency by employing the intramolecular photoinduced electron transfer mechanism. A high PDT efficiency under 10% oxygen concentration was achieved, which is better than that of porphyrin (PpIX), a traditional photosensitizer. Such an excellent PDT efficiency can be attributed to lysosome disruption because the theranostic molecule can specifically enter the lysosomes of cells. Importantly, the strategy of targeting the mild hypoxic cells in the edge of tumor tissue could heal the "Achilles' heel" of traditional PDT. We believe that this theranostic molecule has a high potential to be applied in clinical investigation as a theranostic anticancer prodrug.

Hydrophilic, Red-Emitting, and Thermally Activated Delayed Fluorescence Emitter for Time-Resolved Luminescence Imaging by Mitochondrion-Induced Aggregation in Living Cells

Thermally activated delayed fluorescence (TADF) materials have provided new strategies for time-resolved luminescence imaging (TRLI); however, the development of hydrophilic TADF luminophores for specific imaging in cells remains a substantial challenge. In this study, a mitochondria-induced aggregation strategy for TRLI is proposed with the design and utilization of the hydrophilic TADF luminophore ((10-(1,3-dioxo-2-phenyl-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)-9,9-dimethyl-9,10-dihydroacridin-2-yl)methyl)triphenylphosphonium bromide (NID-TPP). Using a nonconjugated linker to introduce a triphenylphosphonium (TPP+) group into the 6-(9,9-dimethylacridin-10(9H)-yl)-2-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (NID) TADF luminophore preserves the TADF emission of NID-TPP. NID-TPP shows clear aggregation-induced delayed fluorescence enhancement behavior, which provides a practical strategy for long-lived delayed fluorescence emission in an oxygen-containing environment. Finally, the designed mitochondrion-targeting TPP+ group in NID-TPP induces the adequate accumulation of NID-TPP and results in the first reported TADF-based time-resolved luminescence imaging and two-photon imaging of mitochondria in living cells.

Achieving long-lived thermally activated delayed fluorescence in the atmospheric aqueous environment by nano-encapsulation

Fluorescent silica nanoparticles which encapsulated dye DCF-BYT with thermally activated delayed fluorescence (TADF) were fabricated by a simple synthetic method.

Rationally designed organelle-specific thermally activated delayed fluorescence small molecule organic probes for time-resolved biological applications

A new strategy for TADF-based probes to maintain long fluorescence emission lifetime in TRFI studies in cells was developed.

Organic emitter integrating aggregation-induced delayed fluorescence and room-temperature phosphorescence characteristics, and its application in time-resolved luminescence imaging

A luminophore integrating aggregation-induced delayed fluorescence and room-temperature phosphorescence for time-resolved luminescence imaging.

Thermally activated delayed fluorescence (TADF) with a substantially long lifetime furnishes a new paradigm in developing probes for time-resolved imaging. Herein, a novel TADF fluorophore, namely, PXZT, with terpyridine as the acceptor and phenoxazine (PXZ) as the donor, was rationally designed and synthesized. The new compound shows typical thermally activated delayed fluorescence, aggregation-induced emission and crystallization-induced room-temperature phosphorescence (RTP). The coordination of PXZT with a zinc ion causes the quenching of the fluorescence of PXZT due to the enhanced intramolecular charge transfer of the resulting complex ZnPXZT1. With the dissociation of the ZnPXZT1 to release PXZT and the subsequent in situ hydrophobic aggregation of the free PXZT to resist the influence of oxygen, the TADF emission of PXZT is recovered. This zinc-assisted process is successfully used for time-resolved imaging of HeLa and 3T3 cells. This work presents a simple and effective strategy for time-resolved imaging by in situ forming TADF aggregates to turn on the TADF emission.

A phenazine-based near-infrared (NIR) chemodosimeter for cysteine obtained via a carbonyl-assisted cycloaddition process

A NIR phenazine-based chemodosimeter (PHS) is developed for sensing cysteine with high sensitivity, good selectivity and rapid response. The α,β-unsaturated carbonyl NHS-ester was employed as an recognition unit through a cycloaddition mechanism.