Multifunctional Organic Fluorescent Probe with Aggregation-Induced Emission Characteristics: Ultrafast Tumor Monitoring, Two-Photon Imaging, and Image-Guide Photodynamic Therapy

A high-performance organic fluorescent probe with aggregation-induced emission properties for long-term tumor monitoring

Near-infrared organic fluorescent probes have great need in biological sciences and medicine but most of them are still largely unable to meet demand. In this work, a delicate multipurpose organic fluorescent probe (DPPM-TPA) with aggregation-induced emission performances is designed and prepared by facile method to reflect fluorescence labeling, two-photon imaging, and long-term fluorescent tracking. Specifically, DPPM-TPA NPs was constructed from 4-(diphenylamino)phenylboronic acid and DPPM-Br by classical Suzuki coupling reaction and then coated with F127. Such nanoprobe possessed high stability in diverse medium under ambient temperatures, low cytotoxicity, and brilliant fluorescence performance. More importantly, DPPM-TPA NPs showed excellent two-photon imaging and extraordinary long-term fluorescence tracing capacity to malignant tumor, and it can last up to 9 days. These results indicated that DPPM-TPA NPs is expected to serve as a fluorescent probe for photodiagnostic and providing a new idea for the development of long-term fluorescent tracker.

ATP Inhibition for Starvation/Mild Photothermal/Photodynamic Synergy Therapy Using Polypeptide Nanoparticles Conjugating 2-Deoxy-D-Glucose and Dye under NIR Phototheranostic Strategy

Rapid propagation of tumor cells requires plenty of energy, which is adenosine triphosphate (ATP) dependent. ATP inhibition in tumors not only results in the starvation of tumor cells but also down-regulation of the level of heat shock proteins (HSPs), which usually increase during traditional photothermal therapy (PTT), especially when the temperature is up 50 °C. 2-deoxy-D-glucose (2DG) is an anti-glycolytic reagent and can be used as an efficient agent for ATP inhibition in tumors. Compared with typical PTT, low-temperature mild photothermal therapy (MPTT) is receiving more and more attention because it avoids the high temperatures causing damage to the normal tissue, and the increase of HSPs which decrease PTT. Here, multifunctional polypeptide nanoparticles pDG@Ahx conjugating both a NIR probe Ahx-BDP and 2DG into the side chain of the amphiphilic polypeptide have been prepared. In vitro and in vivo studies reveal that the as-prepared nanoparticles achieve a synergistic effect of starvation/MPTT/PDT (photodynamic therapy), and it provides a new strategy to NIR-I/II fluorescence imaging-guided starvation/MPTT/PDT synergy therapy for tumors.

Functionalized Fluorescent Organic Nanoparticles Based AIE Enabling Effectively Targeting Cancer Cell Imaging

We report a fluorescent dye TM by incorporating the tetraphenylethylene (TPE) and cholesterol components into perylene bisimides (PBI) derivative. Fluorescence emission spectrum shows that the dye has stable red emission and aggregation-induced emission (AIE) characteristics. The incorporation of cholesterol components triggers TM to show induced chirality through supramolecular self-assembly. The cRGD-functionalized nanoparticles were prepared by encapsulating fluorescent dyes with amphiphilic polymer matrix. The functionalized fluorescent organic nanoparticles exhibit excellent biocompatibility, large Stokes' shift and good photostability, which make them effective fluorescent probes for targeting cancer cells with high fluorescence contrast.

Near-infrared AIEgens with high singlet-oxygen yields for mitochondria-specific imaging and antitumor photodynamic therapy

AIE-active photosensitizers (PSs) are promising for antitumor therapy due to their advantages of aggregation-promoted photosensitizing properties and outstanding imaging ability. High singlet-oxygen (¹O₂) yield, near-infrared (NIR) emission, and organelle specificity are vital parameters to PSs for biomedical applications. Herein, three AIE-active PSs with D-π-A structures are rationally designed to realize efficient ¹O₂ generation, by reducing the electron-hole distribution overlap, enlarging the difference on the electron-cloud distribution at the HOMO and LUMO, and decreasing the ΔE_ST. The design principle has been expounded with the aid of time-dependent density functional theory (TD-DFT) calculations and the analysis of electron-hole distributions. The ¹O₂ quantum yields of AIE-PSs developed here can be up to 6.8 times that of the commercial photosensitizer Rose Bengal under white-light irradiation, thus among the ones with the highest ¹O₂ quantum yields reported so far. Moreover, the NIR AIE-PSs show mitochondria-targeting capability, low dark cytotoxicity but superb photo-cytotoxicity, and satisfactory biocompatibility. The in vivo experimental results demonstrate good antitumor efficacy for the mouse tumour model. Therefore, the present work will shed light on the development of more high-performance AIE-PSs with high PDT efficiency.

A Versatile Theranostic Nanoplatform with Aggregation-Induced Emission Properties: Fluorescence Monitoring, Cellular Organelle Targeting, and Image-Guided Photodynamic Therapy

Photosensitizers (PSs) play a key role in the photodynamic therapy (PDT) of tumors. However, commonly used PSs are prone to intrinsic fluorescence aggregation-caused quenching and photobleaching; this drawback severely limits the clinical application of PDT, necessitating new phototheranostic agents. Herein, a multifunctional theranostic nanoplatform (named TTCBTA NP) is designed and constructed to achieve fluorescence monitoring, lysosome-specific targeting, and image-guided PDT. TTCBTA with a twisted conformation and D-A structure is encapsulated in amphiphilic Pluronic F127 to form nanoparticles (NPs) in ultrapure water. The NPs exhibit biocompatibility, high stability, strong near-infrared emission, and desirable reactive oxygen species (ROSs) production capacity. The TTCBTA NPs also show high-efficiency photo-damage, negligible dark toxicity, excellent fluorescent tracing, and high accumulation in lysosome for tumor cells. Furthermore, TTCBTA NPs are used to obtain fluorescence images with good resolution of MCF-7 tumors in xenografted BALB/c nude mice. Crucially, TTCBTA NPs present a strong tumor ablation ability and image-guided PDT effect by generating abundant ROSs upon laser irradiation. These results demonstrate that the TTCBTA NP theranostic nanoplatform may enable highly efficient near-infrared fluorescence image-guided PDT.

Ir(III) Complexes with AIE Characteristics for Biological Applications

Both biological process detection and disease diagnosis on the basis of luminescence technology can provide comprehensive insights into the mechanisms of life and disease pathogenesis and also accurately guide therapeutics. As a family of prominent luminescent materials, Ir(III) complexes with aggregation-induced emission (AIE) tendency have been recently explored at a tremendous pace for biological applications, by virtue of their various distinct advantages, such as great stability in biological media, excellent fluorescence properties and distinctive photosensitizing features. Significant breakthroughs of AIE-active Ir(III) complexes have been achieved in the past few years and great progress has been witnessed in the construction of novel AIE-active Ir(III) complexes and their applications in organelle-specific targeting imaging, multiphoton imaging, biomarker-responsive bioimaging, as well as theranostics. This review systematically summarizes the basic concepts, seminal studies, recent trends and perspectives in this area.

Aggregation-induced emission (AIE)-Based nanocomposites for intracellular biological process monitoring and photodynamic therapy

As a non-invasive visualization technique, photoluminescence imaging (PLI) has found its huge value in many biological applications associated with intracellular process monitoring and early and accurate diagnosis of diseases. PLI can also be combined with therapeutics to build imaging-guided theragnostic platforms for achieving early and precise treatment of diseases. Photodynamic therapy (PDT) as a quintessential phototheranostics technology has gained great benefits from the combination with PLI. Recently, aggregation-induced emission (AIE)-active materials have emerged as one of the most promising bioimaging and phototheranostic agents. Most of AIEgens, however, need to be chemically engineered to form versatile nanocomposites with improved their photophysical property, photochemical activity, biocompatibility, etc. In this review, we focus on three categories of AIE-active nanocomposites and highlight their application progresses in the intracellular biological process monitoring and PLI-guided PDT. We hope this review can guide further development of AIE-active nanocomposites and promote their practical applications for monitoring intracellular biological processes and imaging-guided PDT.

The Variance of Photophysical Properties of Tetraphenylethene and Its Derivatives during Their Transitions from Dissolved States to Solid States

The study of aggregation-induced emission luminogens (AIEgens) shows promising perspectives explored in lighting, optical sensors, and biological therapies. Due to their unique feature of intense emissions in aggregated solid states, it smoothly circumvents the weaknesses in fluorescent dyes, which include aggregation-caused quenching of emission and poor photobleaching character. However, our present knowledge of the AIE phenomena still cannot comprehensively explain the mechanism behind the substantially enhanced emission in their aggregated solid states. Herein, to systematically study the mechanism, the typical AIEgens tetraphenylethene (TPE) was chosen, to elucidate its photophysical properties, the TPE in THF/H₂O binary solvents, TPE in THF solvents depending on concentration, and the following direct conversion from a dissolved state to a precipitated solid state were analyzed. Moreover, the TPE derivatives were also investigated to supply more evidence to better decipher the generally optical behaviors of TPE and its derivatives. For instance, the TPE derivative was homogeneously dispersed into tetraethyl orthosilicate to monitor the variance of photophysical properties during sol–gel processing. Consequently, TPE and its derivatives are hypothesized to abide by the anti-Kasha rule in dissolved states. In addition, the factors primarily influencing the nonlinear emission shifting of TPE and its derivatives are also discussed.

Molecular engineering to achieve AIE-active photosensitizers with NIR emission and rapid ROS generation efficiency

Near-infrared (NIR) photosensitizers with rapid reactive oxygen species (ROS) production ability are in great demand owing to their promising performance toward boosting photodynamic therapy (PDT) and deep-tissue imaging, but molecular design guidelines for efficient photosensitizers are rarely elucidated. Herein, three AIEgens named DBP, TBP, and TBP-SO3 are designed and synthesized by precise donor-acceptor (D-A) molecular engineering to deeply understand the structure-property-application relationships. All the compounds exhibit AIE characteristics with strong long-wavelength emission in the aggregated state and are capable of efficiently producing ROS under white light irradiation. By controlling the ability of the D-A units, TBP-SO3 realizes NIR emission and more rapid ROS generation ability due to the promoted intersystem crossing processes compared with those of DBP and TBP. In addition, NIR-emitting TBP-SO3 is capable of specific endoplasmic reticulum targeting and excellent PDT treatment ability of cancer cells and bacteria. This successful example of molecular engineering paves a valuable way for developing advanced PSs with AIE properties, efficient ROS generation ability, and intense emission for fluorescence imaging PDT.

Two-Photon Absorption: An Open Door to the NIR-II Biological Window?

The way in which photons travel through biological tissues and subsequently become scattered or absorbed is a key limitation for traditional optical medical imaging techniques using visible light. In contrast, near-infrared wavelengths, in particular those above 1000 nm, penetrate deeper in tissues and undergo less scattering and cause less photo-damage, which describes the so-called "second biological transparency window". Unfortunately, current dyes and imaging probes have severely limited absorption profiles at such long wavelengths, and molecular engineering of novel NIR-II dyes can be a tedious and unpredictable process, which limits access to this optical window and impedes further developments. Two-photon (2P) absorption not only provides convenient access to this window by doubling the absorption wavelength of dyes, but also increases the possible resolution. This review aims to provide an update on the available 2P instrumentation and 2P luminescent materials available for optical imaging in the NIR-II window.

Emerging Strategies in Enhancing Singlet Oxygen Generation of Nano-Photosensitizers Toward Advanced Phototherapy

The great promise of photodynamic therapy (PDT) has thrusted the rapid progress of developing highly effective photosensitizers (PS) in killing cancerous cells and bacteria. To mitigate the intrinsic limitations of the classical molecular photosensitizers, researchers have been looking into designing new generation of nanomaterial-based photosensitizers (nano-photosensitizers) with better photostability and higher singlet oxygen generation (SOG) efficiency, and ways of enhancing the performance of existing photosensitizers. In this paper, we review the recent development of nano-photosensitizers and nanoplasmonic strategies to enhance the SOG efficiency for better PDT performance. Firstly, we explain the mechanism of reactive oxygen species generation by classical photosensitizers, followed by a brief discussion on the commercially available photosensitizers and their limitations in PDT. We then introduce three types of new generation nano-photosensitizers that can effectively produce singlet oxygen molecules under visible light illumination, i.e., aggregation-induced emission nanodots, metal nanoclusters (< 2 nm), and carbon dots. Different design approaches to synthesize these nano-photosensitizers were also discussed. To further enhance the SOG rate of nano-photosensitizers, plasmonic strategies on using different types of metal nanoparticles in both colloidal and planar metal-PS systems are reviewed. The key parameters that determine the metal-enhanced SOG (ME-SOG) efficiency and their underlined enhancement mechanism are discussed. Lastly, we highlight the future prospects of these nanoengineering strategies, and discuss how the future development in nanobiotechnology and theoretical simulation could accelerate the design of new photosensitizers and ME-SOG systems for highly effective image-guided photodynamic therapy.

Development of label-free fluorescent biosensor for the detection of kanamycin based on aptamer capped metal-organic framework

The abuse of antibiotics has caused serious threat to human health, so it is of great significance to develop a simple and sensitive method for the detection of trace residues of antibiotics in the environment and food. Herein, a novel label-free fluorescent biosensing platform based on the fluorescence change of aptamers-capped zeolitic imidazolate framework-8 (ZIF-8) @ 2,2',2″,2‴-((ethene-1,1,2,2-tetrayltetrakis (benzene-4,1-diyl)) tetrakis (oxy)) tetraacetic acid (TPE) through ATP-assisted competitive coordination reaction was designed for such an end. ZIF-8@TPE/Aptamer (Apt) emits strong fluorescence at 425 nm in HEPES buffer due to the aggregation induced luminescence properties of TPE molecules in confined state. Once kanamycin was added, the conformation of aptamer capped on the surface of ZIF-8@TPE changes because of the specific recognition of kanamycin with aptamer, leading to the collapse of ZIF-8 and release of TPE, accompanied with a dramatic decrease of fluorescence intensity. Under the optimal conditions, a good correlation was obtained between the fluorescence intensity of ZIF-8@TPE/Apt and the concentration of kanamycin ranging from 10 to 10³ ng/mL with a detection limit of 7.3 ng/mL. The satisfactory analytical performance of the assay for kanamycin detection suggests good prospect for its application in food safety analysis.

The fast-growing field of photo-driven theranostics based on aggregation-induced emission

Photo-driven theranostics, also known as phototheranostics, relying on the diverse excited-state energy conversions of theranostic agents upon photoexcitation represents a significant branch of theranostics, which ingeniously integrate diagnostic imaging and therapeutic interventions into a single formulation. The combined merits of photoexcitation and theranostics endow photo-driven theranostics with numerous superior features. The applications of aggregation-induced emission luminogens (AIEgens), a particular category of fluorophores, in the field of photo-driven theranostics have been intensively studied by virtue of their versatile advantageous merits of favorable biocompatibility, tuneable photophysical properties, unique aggregation-enhanced theranostic (AET) features, ideal AET-favored on-site activation ability and ready construction of one-for-all multimodal theranostics. This review summarised the significant achievements of photo-driven theranostics based on AIEgens, which were detailedly elaborated and classified by their diverse theranostic modalities into three groups: fluorescence imaging-guided photodynamic therapy, photoacoustic imaging-guided photothermal therapy, and multi-modality theranostics. Particularly, the tremendous advantages and individual design strategies of AIEgens in pursuit of high-performance photosensitizing output, high photothermal conversion and multimodal function capability by adjusting the excited-state energy dissipation pathways are emphasized in each section. In addition to highlighting AIEgens as promising templates for modulating energy dissipation in the application of photo-driven theranostics, current challenges and opportunities in this field are also discussed.

Near-Infrared Thermally Activated Delayed Fluorescence Nanoparticle: A Metal-Free Photosensitizer for Two-Photon-Activated Photodynamic Therapy at the Cell and Small Animal Levels

Thermally activated delayed fluorescence (TADF) materials with extremely small singlet-triplet energy offsets have opened new horizons for the development of metal-free photosensitizers for photodynamic therapy (PDT) in recent years. However, the exploration of near-infrared (NIR) TADF emitters for efficient two-photon-excited (TPE) PDT is still a formidable challenge, thus it has not been reported yet. In this study, purely organic photosensitizers (PSs) based on the TADF nanoparticles (NIR-TADF NPs) are designed for efficient TPE-PDT, which show excellent singlet oxygen generation ability. Thanks to the intrinsic two-photon excitation and NIR emission characteristics, the NIR-TADF NPs demonstrate promising potential in both single-photon-excited (SPE) and TPE NIR imaging. More importantly, the anti-tumor efficiency and biosafety of TADF-based PSs at the small animal level are confirmed in A549 tumor xenograft models under TPE laser irradiance, which will facilitate the practical biomedical applications of TADF materials. This work not only provides a promising strategy to develop metal-free PSs, but also expands the applied scope of TADF-based nanotherapeutics and advances their possible clinical translation in cancer therapy.

Recent Advances in AIEgen-Based Photodynamic Therapy and Immunotherapy

Cancer, one of the leading causes of death, has seriously threatened public health. However, there is still a lack of effective treatments. Nowadays, photodynamic therapy (PDT), relying on photosensitizers to trigger the generation of reactive oxygen species (ROS) for killing cancer cells, has been emerging as a noninvasive anti-cancer strategy. To enhance the overall anti-cancer efficacy of PDT, various approaches including molecular design and combination with other therapeutic techniques have been proposed and implemented. Especially, photodynamic immunotherapy that can effectively evoke the body's immune response has attracted much attention. Recently, a class of photosensitizers with aggregation-induced emission (AIE) character have shown unique promises, taking advantage of their profound fluorescence and ROS-generating ability in the aggregation state. Despite the promising results demonstrated by several groups, the associated studies are few and the mechanism of such AIEgen-based photodynamic immunotherapy has not been fully understood. This review discusses the recent advances in the AIEgen-based enhanced PDT with a special focus on the AIE photosensitizers for photodynamic immunotherapy, aiming to inspire more opportunities for in-depth investigation of the working principles in this emerging anti-cancer approach.

Microalgae-Derived Health Supplements to Therapeutic Shifts: Redox-Based Study Opportunities with AIE-Based Technologies

Reactive oxygen species (ROS) are highly reactive molecules, serve the normal signaling in different cell types. Targeting ROS as the chemical signals, different stress based strategies have been developed to synthesis different anti-inflammatory molecules in microalgae. These molecules could be utilized as health supplements in human. To provoke the ROS-mediated defence systems, their connotation with the associated conditions must be well understood, therefore, proper tools for studying ROS in natural state are essential. The in vivo detection of ROS with phosphorescent probes offers promising opportunities to study these molecules in a non-invasive manner. Most of the common problems in the traditional fluorescent probes are lower photostability, excitation intensity, slow responsiveness, and the microenvironment that challenge their performance. Some ROS-specific aggregationinduced emission luminogens (AIEgens) with pronounced spatial and temporal resolution have recently demonstrated high selectivity, rapid responsiveness, and efficacies to resolve the aggregation-caused quenching issues. The nanocomposites of some AIE-photosensitizers can also improve the ROS-mediated photodynamic therapy. These AIEgens could be used to induce bioactive components in microalgae through altering the ROS signaling, therefore are more auspicious for biomedical research. This study reviews the prospects of AIEgen-based technologies to understand the ROS mediated bio-physiological processes in microalgae for better healthcare benefits.

Practicable Applications of Aggregation-Induced Emission with Biomedical Perspective

Considerable efforts have been made into developing aggregation-induced emission fluorogens (AIEgens)-containing nano-therapeutic systems due to the excellent properties of AIEgens. Compared to other fluorescent molecules, AIEgens have advantages including low background, high signal-to-noise ratio, good sensitivity, and resistance to photobleaching, in addition to being exempt from concentration quenching or aggregation-caused quenching effects. The present review outlines the major developments in the biomedical applications of AIEgens-containing systems. From a literature survey, the recent AIE works are reviewed and the reasons why AIEgens are chosen in various biomedical applications are highlighted. The research activities on AIEgens-containing systems are increasing rapidly, therefore, the present review is timely.

Highly Efficient Multifunctional Organic Photosensitizer with Aggregation-Induced Emission for In Vivo Bioimaging and Photodynamic Therapy

Photosensitizers play a critical role in photodynamic therapy (PDT). Multifunctional organic nanoparticles (NPs) that possess bright fluorescence in aggregates, high singlet oxygen (¹O₂) quantum yield, near-infrared (NIR) absorption and emission, large Stokes shift, two-photon bioimaging, specific organelle targeting, high PDT efficiency, as well as good biocompatibility and photostability are ideal candidate photosensitizers for image-guided PDT. Due to its enhanced fluorescence and high ¹O₂ generation efficiency in aggregate states, photosensitizers with aggregation-induced emission (AIE) characteristics have attracted increasing interest in PDT. In this study, a new AIE-active Schiff base 5-(((5-(7-(4-(diphenylamino)phenyl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)methylene)amino)-3-methylthiophene-2,4-dicarbonitrile (TBTDC) based on a D-A-π-A skeleton has been designed and synthesized, and it can be readily encapsulated by Pluronic F-127 to form uniform nanoparticles. TBTDC NPs exhibit bright NIR emission at 825 nm with a Stokes shift up to 300 nm, impressive two-photon bioimaging capability with tissue penetration deep into 300 μm, high ¹O₂ generation quantum yield (0.552), specific targeting to lysosome, as well as good biocompatibility and photostability. Furthermore, TBTDC NPs present remarkable cytotoxicity for tumor cells and suppression of tumor growth in nude mice through reactive oxygen species generation upon white light irradiation. These results reveal that TBTDC NPs have great potential to become excellent candidates for multifunctional organic photosensitizers for two-photon bioimaging and image-guided PDT and are promising in future clinical applications.

Dual-Emission Carbonized Polymer Dots for Ratiometric pH Sensing, pH-Dependent Generation of Singlet Oxygen, and Imaging-Guided Dynamics Monitoring of Photodynamic Therapy

The pH environment in cancer cells has been demonstrated to display vital influences on the therapeutic effect of photodynamic therapy (PDT). It is very interesting to develop pH-responsive probes for simultaneous pH sensing and dynamics monitoring of the effects of PDT, and therefore assessing the correlation between them. In this study, a multifunctional fluorescence probe, dual-emission carbonized polymer dot (CPD) in blue and red regions, which uses ethylene imine polymer (PEI) and 4,4',4″,4‴-(porphine-5, 10, 15, 20-tetrayl) tetrakis (benzoic acid) (TCPP) as precursors through a one-step hydrothermal amide reaction, has been designed for ratiometric pH sensing, generating pH-dependent ¹O₂ for PDT of cancer cells, and investigating the dynamics effects of PDT through pH-guided imaging. The prepared CPDs were successfully used for ratiometric pH response, pH-dependent generation of ¹O₂, and dynamics monitoring PDT in HeLa cells. This study may provide an alternative strategy to prepare CPD-based theranostic integrated nanoprobes for PDT through the rational design of precursors.