Ultrabright organic dots with aggregation-induced emission characteristics for cell tracking

Preparation of AIEgen-based near-infrared afterglow luminescence nanoprobes for tumor imaging and image-guided tumor resection

Fluorescence imaging represents a vital tool in modern biology, oncology and biomedical applications. Afterglow luminescence (AGL), which circumvents the light scattering and tissue autofluorescence interference associated with real-time excitation source, shows remarkably increased imaging sensitivity and depth. Here we present a protocol for the design and synthesis of AGL nanoprobes with an aggregation-induced emission (AIE) effect to simultaneously red shift and amplify the afterglow signal for tumor imaging and image-guided tumor resection. The nanoprobe (AGL AIE dot) is composed of an enol ether format of Schaap's agent and a near-infrared AIE fluorogen (AIEgen) (tetraphenylethylene-phenyl-dicyanomethylene-4H-chromene, TPE-Ph-DCM) to suppress the nonradiative dissipation pathway. Pre-irradiating AGL AIE dots with white light could generate singlet oxygen to convert Schaap's agent to its 1,2-dioxetane format, thus initializing the AGL process. With the aid of AIEgen, the AGL shows simultaneously red shifted emission maximum (from ~540 nm to ~625 nm) and enhanced intensity (by 3.2-fold), facilitating better signal-to-background ratio, imaging sensitivity and depth. Intriguingly, the activated AGL can last for over 10 days. Compared with conventional approaches, our method provides a new solution to concurrently red shift and amplify afterglow signals for better in vivo imaging outcomes. The preparation of AGL AIE dots takes ~2 days, the in vitro characterization takes ~10 days (less than 1 day if not involving afterglow kinetic profile study) and the tumor imaging and image-guided tumor resection takes ~7 days. These procedures can be easily reproduced and amended after standard laboratory training in chemical synthesis and animal handling.

Ultrabright AIEdots with tunable narrow emission for multiplexed fluorescence imaging

AIEgen doped fluorescent nanodots (AIEdots) have attracted lots of attention, due to their superior characteristics as fluorescent probes, such as excellent photostability, large Stokes shift, high brightness and tunable emission. Unfortunately, most of the currently available AIEdots exhibit broad emission bandwidth, which limits their applications in multiplexed fluorescence imaging and detection. In this work, the strategy of designing and fabricating narrow emissive AIEdots (NE-AIEdots) with tunable wavelengths was presented by constructing a light-harvesting system with high energy transfer efficiency. Efficient intra-particle energy transfer from highly doped AIEgens, serving as the light-harvesting antenna, to the lightly doped narrow emissive fluorophore, resulted in high brightness and narrow emission. The emission band of NE-AIEdots with the full-width-at-half-maximum varied from 18 to 36 nm was 3-6.3 times narrower than that of traditional AIEdots. The single-particle brightness of NE-AIEdots was over 5-times that of commercial quantum dots under the same excitation and collection conditions. Taking advantage of the superior performance of these NE-AIEdots, multiplexed fluorescence imaging of lymph nodes in living mice was realized, which supported the future applications of NE-AIEdots for in vivo multiplexed labeling and clinical surgery.

Fluorescent Organic Small Molecule Probes for Bioimaging and Detection Applications

The activity levels of key substances (metal ions, reactive oxygen species, reactive nitrogen, biological small molecules, etc.) in organisms are closely related to intracellular redox reactions, disease occurrence and treatment, as well as drug absorption and distribution. Fluorescence imaging technology provides a visual tool for medicine, showing great potential in the fields of molecular biology, cellular immunology and oncology. In recent years, organic fluorescent probes have attracted much attention in the bioanalytical field. Among various organic fluorescent probes, fluorescent organic small molecule probes (FOSMPs) have become a research hotspot due to their excellent physicochemical properties, such as good photostability, high spatial and temporal resolution, as well as excellent biocompatibility. FOSMPs have proved to be suitable for in vivo bioimaging and detection. On the basis of the introduction of several primary fluorescence mechanisms, the latest progress of FOSMPs in the applications of bioimaging and detection is comprehensively reviewed. Following this, the preparation and application of fluorescent organic nanoparticles (FONPs) that are designed with FOSMPs as fluorophores are overviewed. Additionally, the prospects of FOSMPs in bioimaging and detection are discussed.

An Activatable Near-Infrared Afterglow Theranostic Prodrug with Self-Sustainable Magnification Effect of Immunogenic Cell Death

Herein, we report an activatable near-infrared (NIR) afterglow theranostic prodrug that circumvents high background noise interference caused by external light excitation. The prodrug can release hydroxycamptothecin (HCPT) in response to the high intratumoral peroxynitrite level associated with immunogenic cell death (ICD), and synchronously activate afterglow signal to monitor the drug release process and cold-to-hot tumor transformation. The prodrug itself is an ICD inducer achieved by photodynamic therapy (PDT). PDT initiates ICD and recruits first-arrived neutrophils to secrete peroxynitrite to trigger HCPT release. Intriguingly, we demonstrate that HCPT can significantly amplify PDT-mediated ICD process. The prodrug thus shows a self-sustainable ICD magnification effect by establishing an "ICD-HCPT release-amplified ICD" cycling loop. In vivo studies demonstrate that the prodrug can eradicate existing tumors and prevent further tumor recurrence through antitumor immune response.

Synthesis of Uniform Polymer Encapsulated Organic Nanocrystals through Ouzo Nanocrystallization

Nanocrystals (NCs) are widely used in optoelectronics, photocatalysis, and bioimaging. As the surface area to volume ratio increases with a decrease in the size of NCs, strategies to control the size of NCs are highly valuable for many applications. Given the importance of photoluminescent dyes, especially those with aggregation-induced emission, the transformation from an amorphous to a crystalline state can yield a drastic enhancement in their optical properties, which is of significance for biomedical applications. Till now, there is no general method available for the synthesis of small NCs with accurate control over the size and uniformity. Herein, a simple and general approach of ouzo nanocrystallization is presented for the synthesis of small (<100 nm) and highly uniform (polydispersity index~0.1) NCs with good control over the size. The process of nanoprecipitation is used to synthesize uniform nanoparticles (NPs) with different size, which is followed by solvent addition to form swollen NPs. Further, the amorphous core of swollen NPs is converted into NCs within polymer shell under Ouzo zone, which restricts NCs to grow above certain size. To demonstrate the general applicability of ouzo nanocrystallization, two different classes of luminescent materials are used as examples to fabricate small and highly uniform NCs.

A H2O2-activatable nanoprobe for diagnosing interstitial cystitis and liver ischemia-reperfusion injury via multispectral optoacoustic tomography and NIR-II fluorescent imaging

Developing high-quality NIR-II fluorophores (emission in 1000-1700 nm) for in vivo imaging is of great significance. Benzothiadiazole-core fluorophores are an important class of NIR-II dyes, yet ongoing limitations such as aggregation-caused quenching in aqueous milieu and non-activatable response are still major obstacles for their biological applications. Here, we devise an activatable nanoprobe to address these limitations. A molecular probe named BTPE-NO₂ is synthesized by linking a benzothiadiazole core with two tetraphenylene groups serving as hydrophobic molecular rotors, followed by incorporating two nitrophenyloxoacetamide units at both ends of the core as recognition moieties and fluorescence quenchers. An FDA-approved amphiphilic polymer Pluronic F127 is then employed to encapsulate the molecular BTPE-NO₂ to render the nanoprobe BTPE-NO₂@F127. The pathological levels of H₂O₂ in the disease sites cleave the nitrophenyloxoacetamide groups and activate the probe, thereby generating strong fluorescent emission (950~1200 nm) and ultrasound signal for multi-mode imaging of inflammatory diseases. The nanoprobe can therefore function as a robust tool for detecting and imaging the disease sites with NIR-II fluorescent and multispectral optoacoustic tomography (MSOT) imaging. Moreover, the three-dimensional MSOT images can be obtained for visualizing and locating the disease foci.

Differential Collective Cell Migratory Behaviors Modulated by Phospholipid Nanocarriers

Phospholipid nanocarriers have been widely explored for theranostic and nanomedicine applications. These amphiphilic nanocarriers possess outstanding cargo encapsulation efficiency, high water dispersibility, and excellent biocompatibility, which render them promising for drug delivery and bioimaging applications. While the biological applications of phospholipid nanocarriers have been well documented, the fundamental aspects of the phospholipid-cell interactions beyond cytotoxicity have been less investigated. In particular, the effect of phospholipid nanocarriers on collective cell behaviors has not been elucidated. Herein, we evaluate the interactions of phospholipid nanocarriers possessing different functional groups and sizes with normal and cancerous immortalized breast epithelial cell sheets with varying metastatic potential. Specifically, we examine the impact of nanocarrier treatments on the collective migratory dynamics of these cell sheets. We observe that phospholipid nanocarriers induce differential collective cell migratory behaviors, where the migration speed of normal and cancerous breast epithelial cell sheets is retarded and accelerated, respectively. To a certain extent, the nanocarriers are able to alter the migration trajectory of the cancerous breast epithelial cells. Furthermore, phospholipid nanocarriers could modulate the stiffness of the nuclei, cytoplasm, and cell-cell junctions of the breast epithelial cell sheets, remodel their actin filament arrangement, and regulate the expressions of the actin-related proteins. We anticipate that this work will further shed light on nanomaterial-cell interactions and provide guidelines for rational and safer designs and applications of phospholipid nanocarriers for cancer theranostics and nanomedicine.

Long-Term Dynamic Imaging of Cellular Processes Using an AIE Lipid Order Probe in the Dual-Color Mode

Lipid order in the cytoplasm membrane of eukaryotic cells undergoes dynamic changes in almost all cellular processes. Dynamically monitoring these changes is of essential biological significance and remains challenging. This work provides the first aggregation-induced emission probe, TPNPDA-C15, with highly three-dimensional specificity to cell membranes for fluorescent imaging of lipid order of live cells. TPNPDA-C15 displays red fluorescence enhancement with the viscosity increase while emits yellow fluorescence when aggregates form. Imaging analyses of giant unilamellar vesicles and live cells under osmotic shock by the probe demonstrate its sensitive response to the degree of phospholipids packing on artificial and cell membranes. Taking advantage of its superior low photocytotoxicity and high photostability, TPNPDA-C15 is further applied for long-term dynamic imaging of entire live cell physiological processes including apoptosis, ferroptosis, and mitosis in the dual-color mode. With the analysis of fluorescence signal changes in the two fluorescence channels, TPNPDA-C15 serves as a robust fluorescent probe for the imaging study of cellular dynamics.

Aggregation-induced emission (AIE) nanoparticles labeled human embryonic stem cells (hESCs)-derived neurons for transplantation

Transplantation of differentiated neurons derived from either human embryonic stem cells (hESCs) or induced pluripotent stem cells (iPSCs) is an emerging therapeutic strategy for various neurodegenerative diseases. One important aspect of transplantation is the accessibility to track and control the activity of the stem cells-derived neurons post-transplantation. Recently, the characteristics of organic nanoparticles (NPs) with aggregation-induced emission (AIE) have emerged as efficient cell labeling reagents, where positive outcomes were observed in long-term cancer cell tracing in vivo. In the current study, we designed, synthesized, and analyzed the biocompatibility of AIE-NPs in cultured neurons such as in mouse neuronal progenitor cells (NPCs) and hESC-derived neurons. Our data demonstrated that AIE-NPs show high degree of penetration into cells and presented intracellular long-term retention in vitro without altering the neuronal proliferation, differentiation, and viability. Furthermore, we have tracked AIE-NPs labeled neuronal grafts in mouse brain striatum in various time points post-transplantation. We demonstrated prolonged cellular retention of AIE-NPs labeled neuronal grafts 1 month post-transplantation in mouse brain striatum. Lastly, we have shown activation of brain microglia in response to AIE-NPs labeled grafts. Together, these findings highlight the potential application of AIE-NPs in neuronal transplantation.

Nanoparticles of Organic Electronic Materials for Biomedical Applications

Organic electronic materials play important roles in modern electronic devices such as light-emitting diodes, solar cells, and transistors. Upon interaction with light, these optically active materials can undergo different photophysical and photochemical pathways, providing unique opportunities for optimization of light emission via radiative decay, heat generation via nonradiative decay, and singlet oxygen production or phosphorescence emission via intersystem crossing, all of which open alternative opportunities for their applications in sensing, imaging, and therapy. In this Perspective, we discuss all of the pathways that determine the optical properties of high-performance organic electronic materials, focusing on the optimization of each pathway for photogeneration and relaxation of electronic excited states. We also examine nanoparticle (NP) fabrication techniques tailored to macromolecules and small molecules to render them into NPs with optimized size and distribution for biomedical applications and endow organic electronic materials with water dispersibility and biocompatibility. Lastly, we illustrate the in vitro and in vivo applications of some representative organic electronic materials after optimization of each relaxation pathway.

Highly stable semiconducting polymer nanoparticles for multi-responsive chemo/photothermal combined cancer therapy

Rationale: Structural stability and size controllability are critical issues to semiconducting polymer nanoparticles (SPNs), which currently show great potential for theranostic applications.

Methods: Herein, multi-responsive semiconducting polymer semi-interpenetrating nanoparticles (PDPP3T@PNIPAMAA IPNs) with highly stable structure and uniform size have been successfully designed by semi-interpenetrating technique.

Results: It is proposed for the first time that PDPP3T@PNIPAMAA IPNs were prepared with “reinforced concrete” particle structure, which is even resistant to organic solvent such as ethanol and THF. By adjusting the polymerization time, the obtained PDPP3T@PNIPAMAA IPNs exhibit uniform and controllable particle size with extremely low polydispersity index (~0.037) at 1 h of reaction time. The presence of pH/light/GSH multi-responsive semi-interpenetrating network in PDPP3T@PNIPAMAA IPNs dramatically increase their drug loading efficiency (92.64%), which is significantly higher than previously reported comparable SPNs-based drug delivery systems. Additionally, PDPP3T@PNIPAMAA-DOX IPNs further provide improved therapeutic efficacy by the combination of chemotherapy and photothermal therapy with controllably regulated release of doxorubicin (DOX). In vitro and in vivo results indicate that PDPP3T@PNIPAMAA-DOX IPNs are able to release drugs at controlled rate by pH/light/GSH regulation and offer PAI-guided chemo/photothermal combined therapy with excellent therapeutic efficacy.

Conclusions: The semi-interpenetrating network method may be generally extended for the preparation of a wide range of organic polymer nanoparticles to achieve ultrahigh structural stability, precise particle size controllability and excellent drug loading capacity.

Mechanistic Understanding of the Biological Responses to Polymeric Nanoparticles

Polymeric nanoparticles play important roles in the delivery of a multitude of therapeutic and imaging contrast agents. Although these nanomaterials have shown tremendous potential in disease diagnosis and therapy, there have been many reports on the failure of these nanoparticles in realizing their intended objectives due to an individual or a combination of factors, which have collectively challenged the merit of nanomedicine for disease theranostics. Herein, we investigate the interactions of polymeric nanoparticles with biological entities from molecular to organism levels. Specifically, the protein corona formation, in vitro endothelial uptake, and in vivo circulation time of these nanoparticles are systematically probed. We identify the crucial role of nanocarrier lipophilicity, zeta-potential, and size in controlling the interactions between nanoparticles and biological systems and propose a two-step framework in formulating a single nanoparticle system to regulate multiple biological effects. This study provides insight into the rational design and optimization of the performance of polymeric nanoparticles to advance their theranostic and nanomedicine applications.

Amphiphilic Polymer-Mediated Aggregation-Induced Emission Nanoparticles for Highly Sensitive Organophosphorus Pesticide Biosensing

Biosensing applications require signal reporters to be sufficiently stable and biosafe as well as highly efficient. Aggregation-induced emission (AIE) nanoparticles have proven to be capable of cell-imaging and cancer therapy; however, realizing sensitive detection of biomolecules remains a great challenge because of their instability, biotoxicity, and lack of modifiable functional groups. Herein, we report a self-assembling strategy to fabricate AIE nanoparticles (PTDNPs) through the dispersion of amphiphilic polymers (PTDs) in phosphate-buffered saline. The PTDs were prepared through radical copolymerization of N-(1,2,2-triphenylvinyl)-4-acetylaniline and dimethyl diallyl ammonium chloride. We found that the particle size, morphology, functional groups, and fluorescence property of PTDNPs can be fine-tuned. Further, PTDNPs-0.10 were chosen as signal reporters to detect organophosphorus pesticides (OPs) with the aid of gold nanoparticles. Their sensing performance on OPs is superior to that using C-dot/quantum dot/rhodamine B as the signal reporter. This study not only provides new possibilities to fabricate novel AIE nanoparticles with exceptional properties, but also facilitates the AIE nanoparticle's application for target analyte biosensing.

A novel AIE-active dye for fluorescent nanoparticles by one-pot combination of Hantzsch reaction and RAFT polymerization: synthesis, molecular structure and application in cell imaging

In recent years, amphiphilic AIE-active fluorescent organic materials with aggregation-induced emission (AIE) properties have been extensively investigated due to their excellent properties. This study describes the synthesis of a novel AIE-active dye of tetraphenylethylene diphenylaldehyde (TPDA). As compared with the reported fluorescent dye TPB, the fluorescence intensity of TPDA is significantly enhanced with the distinct red shift of emission wavelength. Subsequently, the corresponding novel polymers PEG-TPD were obtained through the one-pot combination of Hantzsch reaction and RAFT polymerization. The structure of PEG-TPD1 by the two-step process was similar with that of PEG-TPD2 by the one-pot method at the same feeding ratio of TPDA and PEGMA. The molecular weights (M_n) of the polymers PEG-TPD1 and PEG-TPD2 were respectively 52 000 and 28 000 with narrow polydispersity index (PDI), and their molar fractions of TPDA were respectively about 9.5% and 14.3%, indicating that the degree of Hantzsch reaction in the one-pot process was more complete. Subsequently, the effect of feed ratio of TPDA and PEGMA on polymer structure was further studied. It can be seen that the M_n of the polymers gradually increases as the proportion of TPDA increases. In aqueous solution, these amphiphilic PEG-TPD polymers tended to self-assemble into corresponding fluorescent polymer nanoparticles (FPNs). The diameter of PEG-TPD2 FPNs ranged from 200 to 300 nm, and their fluorescence emission spectra have maximum emission peak at 509 nm. The PEG-TPD FPNs have significant advantages such as good fluorescence intensity, high water dispersibility, good biocompatibility and easy absorption by cells, which can be attractively used in the field of bioimaging.

This work reported the fabrication of a novel AIE-active dye and its amphiphilic PEG-TPD fluorescent polymers via one-pot combination of RAFT polymerization and Hantzsch reaction, which showed potential applications in bioimaging.

Single-Molecular Near-Infrared-II Theranostic Systems: Ultrastable Aggregation-Induced Emission Nanoparticles for Long-Term Tracing and Efficient Photothermal Therapy

Second near-infrared (NIR-II, 1000-1700 nm) fluorescence bioimaging has attracted tremendous scientific interest and already been used in many biomedical studies. However, reports on organic NIR-II fluorescent probes for in vivo photoinduced imaging and simultaneous therapy, as well as the long-term tracing of specific biological objects, are still very rare. Herein we designed a single-molecular and NIR-II-emissive theranostic system by encapsulating a kind of aggregation-induced emission luminogen (AIEgen, named BPN-BBTD) with amphiphilic polymer. The ultra-stable BPN-BBTD nanoparticles were employed for the NIR-II fluorescence imaging and photothermal therapy of bladder tumors in vivo. The 785 nm excitation triggered photothermal therapy could completely eradicate the subcutaneous tumor and inhibit the growth of orthotopic tumors. Furthermore, BPN-BBTD nanoparticles were capable of monitoring subcutaneous and orthotopic tumors for a long time (32 days). Single-molecular and NIR-II-emitted aggregation-induced emission nanoparticles hold potential for the diagnosis, precise treatment, and metastasis monitoring of tumors in the future.

Robust Reconstruction of Fluorescence Molecular Tomography Based on Sparsity Adaptive Correntropy Matching Pursuit Method for Stem Cell Distribution

Fluorescence molecular tomography (FMT), as a promising imaging modality in preclinical research, can obtain the three-dimensional (3-D) position information of the stem cell in mice. However, because of the ill-posed nature and sensitivity to noise of the inverse problem, it is a challenge to develop a robust reconstruction method, which can accurately locate the stem cells and define the distribution. In this paper, we proposed a sparsity adaptive correntropy matching pursuit (SACMP) method. SACMP method is independent on the noise distribution of measurements and it assigns small weights on severely corrupted entries of data and large weights on clean ones adaptively. These properties make it more suitable for in vivo experiment. To analyze the performance in terms of robustness and practicability of SACMP, we conducted numerical simulation and in vivo mice experiments. The results demonstrated that the SACMP method obtained the highest robustness and accuracy in locating stem cells and depicting stem cell distribution compared with stagewise orthogonal matching pursuit and sparsity adaptive subspace pursuit reconstruction methods. To the best of our knowledge, this is the first study that acquired such accurate and robust FMT distribution reconstruction for stem cell tracking in mice brain. This promotes the application of FMT in locating stem cell and distribution reconstruction in practical mice brain injury models.

Aggregation-Induced Emission (AIE) Dots: Emerging Theranostic Nanolights

Theranostic nanolights refer to luminescent nanoparticles possessing both imaging and therapeutic functions. Their shape, size, surface functions, and optical properties can be precisely manipulated through integrated efforts of chemistry, materials, and nanotechnology for customized applications. When localized photons are used to activate both imaging and therapeutic functions such as photodynamic or photothermal therapy, these theranostic nanolights increase treatment efficacy with minimized damage to surrounding healthy tissues, which represents a promising noninvasive nanomedicine as compared to conventional theranostic approaches. As one of the most promising theranostic nanolights, organic dots with aggregation-induced emission (AIE dots) are biocompatible nanoparticles with a dense core of AIE fluorogens (AIEgens) and protective shells, whose sizes are in the range of a few to tens of nanometers. Different from conventional fluorophores that suffer from aggregation-caused quenching (ACQ) due to π-π stacking interaction in the aggregate state, AIEgens emit strongly as nanoaggregates due to the restriction of intramolecular motions. Through precise molecular engineering, AIEgens could also be designed to show efficient photosensitizing or photothermal abilities in the aggregate state. Different from ACQ dyes, AIEgens allow high loading in nanoparticles without compromised performance, which makes them the ideal cores for theranostic nanolights to offer high brightness for imaging and strong photoactivities for theranostic applications. In this Account, we summarize the recent advance of AIE dots and highlight their great potential as theranostic nanolights in biomedical applications. Starting from the design of AIEgens, the fabrication of AIE dots and their bioimaging applications are discussed. The exceptional advantages of superbrightness, high resistance to photobleaching, lack of emission intermittency, and excellent biocompatibility have made them reliable cross platform contrast agents for different imaging techniques such as confocal microscopy, multiphoton fluorescence microscopy, super-resolution nanoscopy, and light-sheet ultramicroscopy, which have been successfully applied for cell tracking, vascular disease diagnosis, and image-guided surgery. The integration of therapeutic functions with customized AIEgens has further empowered AIE dots as an excellent theranostic platform for image-guided phototherapy. Of particular interest is AIE photosensitizer dots, which simultaneously show bright fluorescence and high photosensitization, yielding superior performance to commercial photosensitizer nanoparticles in image-guided therapy. Further development in multiphoton excited photodynamic therapy has offered precise treatment with up to 5 μm resolution at 200 μm depth, while chemiexcited photodynamic therapy has completely eliminated the limitation of penetration depth to realize power-free imaging and therapy. With this Account, we hope to stimulate more collaborative research interests from different fields of chemistry, materials, biology, and medicine to promote translational research of AIE dots as the theranostic nanolights.

Progress and Trends in AIE-Based Bioprobes: A Brief Overview

Luminescent bioprobes are powerful analytical means for biosensing and optical imaging. Luminogens featured with aggregation-induced emission (AIE) attributes have emerged as ideal building blocks for high-performance bioprobes. Bioprobes constructed with AIE luminogens have been identified to be a novel class of FL light-up probing tools. In contrast to conventional bioprobes based on the luminophores with aggregation-caused quenching (ACQ) effect, the AIE-based bioprobes enjoy diverse superiorities, such as lower background, higher signal-to-noise ratio and sensitivity, better accuracy, and more outstanding resistance to photobleaching. AIE-based bioprobes have been tailored for a vast variety of purposes ranging from biospecies sensing to bioimaging to theranostics (i.e., image-guided therapies). In this review, recent five years' advances in AIE-based bioprobes are briefly overviewed in a perspective distinct from other reviews, focusing on the most appealing trends and progresses in this flourishing research field. There are altogether 11 trends outlined, which have been classified into four aspects: the probe composition and form (bioconjugtes, nanoprobes), the output signal of probe (far-red/near-infrared luminescence, two/three-photon excited fluorescence, phosphorescence), the modality and functionality of probing system (dual-modality, dual/multifunctionality), the probing object and application outlet (specific organelles, cancer cells, bacteria, real samples). Typical examples of each trend are presented and specifically demonstrated. Some important prospects and challenges are pointed out as well in the hope of intriguing more interests from researchers working in diverse areas into this exciting research field.

Polymeric nanorods with aggregation-induced emission characteristics for enhanced cancer targeting and imaging

Polymeric nanorods loaded with AIEgens are synthesized via nano-precipitation under ultrasound sonication, where prolonged sonication time could induce a nanodot-to-nanorod transition. These AIE nanorods, but not the nanodots, could be selectively internalized into cancer cells, which show better tumor accumulation, higher tumor penetration and more efficient in vivo cancer cell uptake.

AIEgen-Based Fluorescent Nanomaterials: Fabrication and Biological Applications

In recent years, luminogens with the feature of aggregation-induced emission (AIEgen) have emerged as advanced luminescent materials for fluorescent nanomaterial preparation. AIEgen-based nanomaterials show enhanced fluorescence efficiency and superior photostability, which thusly offer unique advantages in biological applications. In this review, we will summarize the fabrication methods of AIEgen-based nanomaterials and their applications in in vitro/in vivo imaging, cell tracing, photodynamic therapy and drug delivery, focusing on the recent progress.

Synthesis of fluorescent dendrimers with aggregation-induced emission features through a one-pot multi-component reaction and their utilization for biological imaging

Hyperbranched polymers have attracted wide research attention owing to their unique topological structure, physicochemical properties and great potential for applications such asadditives, drug delivery, catalysts and nanotechnology. Among these, the polyamidoamine(PAMAM) dendrimers are some of the most important dendrimers. However, the synthesis and biomedical applications of fluorescent PAMAM dendrimers have received only limited attention. In this work, we present a rather effective and convenient approach for synthesis of fluorescent PAMAM dendrimers with aggregation-induced emission (AIE) properties through a one-pot catalyst-free Mannich reaction under rather mild experimental conditions (e.g., low reaction temperature, air atmosphere in the presence of water). The obtained AIE-active amphiphiles (PhE-PAD) could self-assemble into fluorescent organic nanoparticles (FONs). The obtained AIE-active FONs (PhE-PAD FONs) were fully characterized, and their successful construction was confirmed by ¹H NMR spectroscopy, FT-IR spectroscopy and transmission electron microscopy. Fluorescence and UV-Visible absorption spectroscopy results demonstrated that the final PhE-PAD FONs showed strong yellow fluorescence, desirable photostability and good water dispersity. The cell viability evaluation and confocal laser scanning microscope imaging results suggested that PhE-PAD FONs possessed low cytotoxicity and excellent biocompatibility. Taken together, these results demonstrate that we have developed a facile and efficient strategy for the fabrication of AIE-active FONs, which possess many desirable features for biomedical applications.

Direct Imaging of Superwetting Behavior on Solid-Liquid-Vapor Triphase Interfaces

A solid-liquid-vapor interface dominated by a three-phase contact line usually serves as an active area for interfacial reactions and provides a vital clue to surface behavior. Recently, direct imaging of the triphase interface of superwetting interfaces on the microscale/nanoscale has attracted broad scientific attention for both theoretical research and practical applications, and has gradually become an efficient and intuitive approach to explore the wetting behaviors of various multiphase interfaces. Here, recent progress on characterizing the solid-liquid-vapor triphase interface on the microscale/nanoscale with diverse types of imaging apparatus is summarized. Moreover, the accurate, visible, and quantitative information that can be obtained shows the real interfacial morphology of the wetting behaviors of multiphase interfaces. On the basis of fundamental research, technical innovations in imaging and complicated multiphase interfaces of the superwetting surface are also briefly presented.

Dual modal ultra-bright nanodots with aggregation-induced emission and gadolinium-chelation for vascular integrity and leakage detection

The study of blood brain barrier (BBB) functions is important for neurological disorder research. However, the lack of suitable tools and methods has hampered the progress of this field. Herein, we present a hybrid nanodot strategy, termed AIE-Gd dots, comprising of a fluorogen with aggregation-induced emission (AIE) characteristics as the core to provide bright and stable fluorescence for optical imaging, and gadolinium (Gd) for accurate quantification of vascular leakage via inductively-coupled plasma mass spectrometry (ICP-MS). In this report, we demonstrate that AIE-Gd dots enable direct visualization of brain vascular networks under resting condition, and that they form localized punctate aggregates and accumulate in the brain tissue during experimental cerebral malaria, indicative of hemorrhage and BBB malfunction. With its superior detection sensitivity and multimodality, we hereby propose that AIE-Gd dots can serve as a better alternative to Evans blue for visualization and quantification of changes in brain barrier functions.

Ultrasmall Conjugated Polymer Nanoparticles with High Specificity for Targeted Cancer Cell Imaging

Fluorescent and biocompatible organic nanoparticles have attracted great interest in cancer detection and imaging, but the nonspecific cellular uptake has limited the detection specificity and sensitivity. Herein, the authors report the ultrasmall conjugated polymer nanoparticles (CPNs) with bright far-red/near-infrared emission for targeted cancer imaging with high specificity. The sizes of the ultrasmall CPNs are around 6 nm (CPN6), while large CPNs show sizes around 30 nm (CPN30). Moreover, CPN6 exhibits largely improved fluorescence quantum yield (η) of 41% than CPN30 (25%). Benefiting from the ultrasmall size, bare CPN6 shows largely suppressed nonspecific cellular uptake as compared to CPN30, while cyclic arginine-glycine-aspartic acid (cRGD) functionalized CPN6 (cRGD-CPN6) possesses excellent selectivity toward αvβ3 integrin overexpressed MDA-MB-231 cells over other cells in cell mixtures. The faster body clearance of CPN6 over CPN30 indicates its greater potentials as a noninvasive nanoprobe for in vivo and practical applications.

A bioorthogonal nanosystem for imaging and in vivo tumor inhibition

Bioorthogonal bond-cleavage reactions have emerged as promising tools for manipulating biological processes, still the therapeutic effect of these reactions in vivo needs to be explored. Herein a bioorthogonal-activated prodrug has been developed for bioimaging and therapy, which is composed of a Pd-mediated cleavable propargyl, a coumarin fluorophore and a potent anticancer drug. In vitro investigations show that, the presence of a Pd complex induces the cleavage of propargyl and subsequently trigger the cascade of reactions, thereby activating the coumarin fluorophore for imaging and releasing the anticancer drug for therapy. Both the prodrug and Pd complex were then separately encapsulated into phospholipid liposomes to form a two-component bioorthogonal nanosystem. The lyposomal nanosystem can be readily internalized by HeLa cells and displays strong intracellular fluorescence under one- or two-photon excitation, indicating the release of the active drug in cells as a result of the Pd-mediated bioorthogonal bond-cleavage reaction. More importantly, the nanosystem shows considerable high activity and exerts efficient inhibition towards tumor growth in a mouse model. This work demonstrates that, if properly formulated, a bioorthogonal system can perform well in vivo. This strategy may offer a new approach for designing bioorthogonal prodrugs with imaging and therapeutic capability.

Long wavelength excitable near-infrared fluorescent nanoparticles with aggregation-induced emission characteristics for image-guided tumor resection

Near infrared (NIR) fluorescence imaging (700-900 nm) is a promising technology in preclinical and clinical tumor diagnosis and therapy. The availability of excellent NIR fluorescent contrast agents is still the main barrier to implementing this technology. Herein, we report the design and synthesis of two series of NIR fluorescent molecules with long wavelength excitation and aggregation-induced emission (AIE) characteristics by fine-tuning their molecular structures and substituents. Further self-assembly between an amphiphilic block co-polymer and the obtained AIE molecules leads to AIE nanoparticles (AIE NPs), which have absorption maxima at 635 nm and emission maxima between 800 and 815 nm with quantum yields of up to 4.8% in aggregated states. In vitro and in vivo toxicity results demonstrate that the synthesized AIE NPs are biocompatible. Finally, the synthesized AIE NPs have been successfully used for image-guided tumor resection with a high tumor-to-normal tissue signal ratio of 7.2.

Fluorescent probes for nanoscopy: four categories and multiple possibilities

Nanoscopy enables breaking down the light diffraction limit and reveals the nanostructures of objects being studied using light. In 2014, three scientists pioneered the development of nanoscopy and won the Nobel Prize in Chemistry. This recognized the achievement of the past twenty years in the field of nanoscopy. However, fluorescent probes used in the field of nanoscopy are still numbered. Here, we review the currently available four categories of probes and existing methods to improve the performance of probes.

Dynamic intracellular tracking nanoparticles via pH-evoked “off–on” fluorescence

Strong fluorescence induced by spiropyran isomerized into merocyanine in low pH was utilized as a probe for efficient dynamic intracellular tracking.

Polyyne bridged AIE luminogens with red emission: design, synthesis, properties and applications

Construction of a donor–acceptor (D–A) structure and extension of π-conjugation are the commonly used strategies to shift the emission of luminophores to the red region.

Multifunctional AIEgens for Future Theranostics

The combination of diagnosis and therapeutics into one theranostics system has attracted great interest in life science and biomedical fields. The current theranostic platform largely relies on the integration of multiple materials with different functionalities. The all-in-one approach has the risk of high complicity with reduced reproducibility. Smart design of simple molecules born with multifunctions should represent one of the future directions in theranostics. Fluorogens with aggregation-induced emission (AIEgens) are one type of such smart materials, which have attracted increasing attentions in recent years. In this concept, the key frontier developments of simple AIEgens with multifunctions for imaging and therapy are presented, which include fluorescence-photoacoustic imaging, fluorescence-magnetic resonance imaging, fluorescence image-guided photodynamic therapy, fluorescence image-guided chemotherapy and photoacoustic image-guided photothermal therapy. The smart molecular design to endow each AIEgen with strong capability to simultaneously offer two or more theranostic functions should attract more scientists into this exciting research direction.

Organic Nanoparticles with Aggregation-Induced Emission for Bone Marrow Stromal Cell Tracking in a Rat PTI Model

Stem-cell based therapy is an emerging therapeutic approach for ischemic stroke treatment. Bone marrow stromal cells (BMSCs) are in common use as a cell source for stem cell therapy and show promising therapeutic outcomes for stroke treatment. One challenge is to develop a reliable tracking strategy to monitor the fate of BMSCs and assess their therapeutic effects in order to improve the success rate of such treatment. Herein, TPEEP, a fluorogen with aggregation-induced emission characteristics and near-infrared emission are designed and synthesized and further fabricated into organic nanoparticles (NPs). The obtained NPs show high fluorescence quantum yield, low cytotoxicity with good physical and photostability, which display excellent tracking performance of BMSCs in vitro and in vivo. Using a rat photothrombotic ischemia model as an example, the NP-labeled BMSCs are able to migrate to the stroke lesion site to yield bright red fluorescence. Immunofluorescence staining shows that the NP labeling does not affect the normal function of BMSCs, proving their good biocompatibility in vivo. These merits make TPEEP NP a potential cell tracker to evaluate the fate of BMSCs in cell therapy.

Aggregation-Induced Emission: Lighting up Cells, Revealing Life!

Understanding metabolism and dynamic biological events in cells, as well as physiological functions and pathological changes in organisms, is the major goal of biological investigations. It will improve our capability to diagnose and treat diseases, and will enhance personalized medicine. Fluorescence imaging is a powerful tool that plays an essential role in acquiring the comprehensive knowledge necessary to help reach this goal. Fluorescent molecules are crucial factors for obtaining high quality images. In contrast to conventional fluorogens with aggregation-caused quenching (ACQ) effect, molecules that show aggregation-induced emission (AIE) effect open up new avenues for fluorescence imaging. So far, a large variety of AIE probes have been developed and applied to bioimaging because of their outstanding characteristics, such as high fluorescence efficiency, excellent photostability and high signal-to-noise ratio (SNR). In this review, recent advances in AIE-based probes for biomedical imaging of intracellular microenvironments, natural macromolecules, subcellular organelles, intracellular processes, living tissues, and diagnosis and therapeutic evaluation of diseases in vivo are summarized. It is hoped that this review generates great research enthusiasm for AIE-based bioimaging, in order to promote the development of promising AIE probes and guide us to a better understanding of the biological essence of life.

A Porphyrin-Based Conjugated Polymer for Highly Efficient In Vitro and In Vivo Photothermal Therapy

Conjugated polymers have been increasingly studied for photothermal therapy (PTT) because of their merits including large absorption coefficient, facile tuning of exciton energy dissipation through nonradiative decay, and good therapeutic efficacy. The high photothermal conversion efficiency (PCE) is the key to realize efficient PTT. Herein, a donor-acceptor (D-A) structured porphyrin-containing conjugated polymer (PorCP) is reported for efficient PTT in vitro and in vivo. The D-A structure introduces intramolecular charge transfer along the backbone, resulting in redshifted Q band, broadened absorption, and increased extinction coefficient as compared to the state-of-art porphyrin-based photothermal reagent. Through nanoencapsulation, the dense packing of a large number of PorCP molecules in a single nanoparticle (NP) leads to favorable nonradiative decay, good photostability, and high extinction coefficient of 4.23 × 10⁴ m^-1 cm^-1 at 800 nm based on porphyrin molar concentration and the highest PCE of 63.8% among conjugated polymer NPs. With the aid of coloaded fluorescent conjugated polymer, the cellular uptake and distribution of the PorCP in vitro can be clearly visualized, which also shows effective photothermal tumor ablation in vitro and in vivo. This research indicates a new design route of conjugated polymer-based photothermal therapeutic materials for potential personalized theranostic nanomedicine.

Biocompatible Red Fluorescent Organic Nanoparticles with Tunable Size and Aggregation-Induced Emission for Evaluation of Blood-Brain Barrier Damage

Detection of damage to the blood-brain barrier (BBB) is important for the diagnosis of brain diseases and therapeutic drug evaluation. The widely used probe, Evans blue, suffers from low specificity and high toxicity in vivo. It is shown that organic nanoparticles with tuneable size, good biocompatibility, and aggregation-induced emission characteristics offer high detection specificity to detect BBB damage via a photothrombotic ischemia rat model.

Multifunctional Nanoparticles Self-Assembled from Small Organic Building Blocks for Biomedicine

Supramolecular self-assembly shows significant potential to construct responsive materials. By tailoring the structural parameters of organic building blocks, nanosystems can be fabricated, whose performance in catalysis, energy storage and conversion, and biomedicine has been explored. Since small organic building blocks are structurally simple, easily modified, and reproducible, they are frequently employed in supramolecular self-assembly and materials science. The dynamic and adaptive nature of self-assembled nanoarchitectures affords an enhanced sensitivity to the changes in environmental conditions, favoring their applications in controllable drug release and bioimaging. Here, recent significant research advancements of small-organic-molecule self-assembled nanoarchitectures toward biomedical applications are highlighted. Functionalized assemblies, mainly including vesicles, nanoparticles, and micelles are categorized according to their topological morphologies and functions. These nanoarchitectures with different topologies possess distinguishing advantages in biological applications, well incarnating the structure-property relationship. By presenting some important discoveries, three domains of these nanoarchitectures in biomedical research are covered, including biosensors, bioimaging, and controlled release/therapy. The strategies regarding how to design and characterize organic assemblies to exhibit biomedical applications are also discussed. Up-to-date research developments in the field are provided and research challenges to be overcome in future studies are revealed.

Far Red/Near-Infrared AIE Dots for Image-Guided Photodynamic Cancer Cell Ablation

We report a facile encapsulation approach to realize bright far red/near-infrared (FR/NIR) fluorescence and efficient singlet oxygen ((1)O2) production of organic fluorogens with aggregation-induced emission (AIEgen) and intramolecular charge transfer (ICT) characteristics for image-guided photodynamic cancer cell ablation. The synthesized AIEgen BTPEAQ possesses donor-acceptor-donor structure, which shows bright fluorescence in solid state. Due to the strong ICT effect, BTPEAQ exhibits poor emission with almost no (1)O2 generation in aqueous solution. Encapsulation of BTPEAQ by DSPE-PEG block copolymer yields polymer-shelled dots, which show enhanced brightness with a fluorescence quantum yield of 3.9% and a (1)O2 quantum yield of 38%. Upon encapsulation by silica, the formed SiO2-shelled dots show much improved fluorescence quantum yield of 12.1% but with no obvious (1)O2 generation. This study clearly demonstrates the importance of encapsulation approach for organic fluorophores, which affects not only the brightness but also the (1)O2 production. After conjugating the polymer-shelled AIE dots with cRGD peptide, the obtained BTPEAQ-cRGD dots show excellent photoablation toward MDA-MB-231 cells with integrin overexpression while keeping control cells intact.

Targeted imaging of EGFR overexpressed cancer cells by brightly fluorescent nanoparticles conjugated with cetuximab

To improve the treatment efficiency and reduce side effects in cancer therapy, accurate diagnosis of cancer cell types at a molecular level is highly desirable. Fluorescent nanoparticles (NPs) are especially suitable for detecting molecular biomarkers of cancer with advantages of superior brightness, easy decoration and high resolution. However, the conventional organic fluorophores, conjugated polymers, and inorganic quantum dots suffer from the drawbacks of aggregation-caused quenching (ACQ), low photostability, and heavy metal toxicity, respectively, which severely restrict their applications in NPs-based fluorescence imaging. To overcome these limitations, herein, we have developed fluorescent nanoparticles based on a t-BuPITBT-TPE fluorophore derived from aggregation-induced emission (AIE)-active tetraphenylethene. Through encapsulating t-BuPITBT-TPE within biocompatible DSPE-PEG and further decorating with a monoclonal antibody cetuximab (C225), the obtained t-BuPITBT-TPE-C225 NPs can be used for targeted imaging of non-small cell lung cancer cells with an overexpressed epidermal growth factor receptor (EGFR). The specific targeting ability of t-BuPITBT-TPE-C225 NPs has been well verified by confocal microscopy and flow cytometry experiments. The t-BuPITBT-TPE-C225 NPs have shown significant advantages in terms of highly efficient red emission, good bio-compatibility, and excellent photostability. This work provides a promising method for precise diagnosis of cancer cells by antibody-functionalized fluorescent NPs with high brightness.

Long-Term Tracking of the Osteogenic Differentiation of Mouse BMSCs by Aggregation-Induced Emission Nanoparticles

Bone marrow-derived mesenchymal stem cells (BMSCs) have shown great potential for bone repair due to their strong proliferation ability and osteogenic capacity. To evaluate and improve the stem cell-based therapy, long-term tracking of stem cell differentiation into bone-forming osteoblasts is required. However, conventional fluorescent trackers such as fluorescent proteins, quantum dots, and fluorophores with aggregation-caused quenching (ACQ) characteristics have intrinsic limitations of possible interference with stem cell differentiation, heavy metal cytotoxicity, and self-quenching at a high labeling intensity. Herein, we developed aggregation-induced emission nanoparticles decorated with the Tat peptide (AIE-Tat NPs) for long-term tracking of the osteogenic differentiation of mouse BMSCs without interference of cell viability and differentiation ability. Compared with the ability of the commercial Qtracker 655 for tracking of only 6 passages of mouse BMSCs, AIE-Tat NPs have shown a much superior performance in long-term tracking for over 12 passages. Moreover, long-term tracking of the osteogenic differentiation process of mouse BMSCs was successfully conducted on the biocompatible hydroxyapatite scaffold, which is widely used in bone tissue engineering. Thus, AIE-Tat NPs have promising applications in tracking stem cell fate for bone repair.