Near-Infrared Dioxetane Luminophores with Direct Chemiluminescence Emission Mode

A General Route for Chemiluminescence of n-Type Au Nanocrystals

The photoluminescence, electroluminescence, and electrochemiluminescence from nanocrystals (NCs) have been extensively exploited for both fundamental and applied investigation over two decades, while the understanding of chemiluminescence (CL) from NCs is still far from clear by now. Herein, a general route for triggering CL from NC luminophore is proposed by extensively exploiting the charge transfer between n-type NCs and oxidants. Oxidants, such as K₂S₂O₈, H₂O₂, KMnO₄, and NaClO, can chemically inject the hole onto the valence band (VB) of methionine-capped n-type AuNCs (Met@AuNCs) and enable the occurrence of efficient radiative-charge-recombination between the chemically injected exogenous VB hole and the pre-existed endogenous conduction band (CB) electron, which eventually results in single-color and defect-involved CL with the maximum emission wavelength around 824 nm. The CL of Met@AuNCs/oxidant is qualified for ultrasensitive CL immunoassay in a similar procedure to the biotin-avidin and magnetic separation involved commercial CL immunoassay and exhibits acceptable performance for linearly determining carcinoembryonic antigen from 50 pg/mL to 100 ng/mL with a limit of detection of 10 pg/mL (S/N = 3). This strategy provides a general route to develop nanoparticulate CL luminophores and might eventually enable CL multiplexing assay via extensively exploiting the CL of different wavebands.

Activatable Persistent Luminescence from Porphyrin Derivatives and Supramolecular Probes with Imaging-Modality Transformable Characteristics for Improved Biological Applications

Persistent luminescence without excitation light and tissue autofluorescence interference holds great promise for biological applications, but is limited by available materials with long-wavelength emission and excellent clinical potential. Here, we report that porphyrin derivatives can emit near-infrared persistent luminescence over 60 min after cessation of excitation light or on interaction with peroxynitrite. A plausible mechanism of the successive oxidation of vinylene bonds was demonstrated. A supramolecular probe with a β-sheet structure was constructed to enhance the tumor targeting ability and the photoacoustic and persistent luminescence signals. Such probes featuring light-triggered function transformation from photoacoustic imaging to persistent luminescence imaging permit advanced image-guided cancer surgery. Furthermore, peroxynitrite-activated persistent luminescence of the supramolecular probe also enables rapid and precise screening of immunogenic cell death drugs.

Chemiluminescent spiroadamantane-1,2-dioxetanes: Recent advances in molecular imaging and biomarker detection

Triggered chemiluminescence emission of spiroadamantane-1,2-dioxetanes to detect bioanalytes has fueled the emerging popularity of chemiluminescence imaging in live animals and cells. Recently, a structural evolution of the dioxetane scaffolds towards near-infrared emitters has been observed, and efforts have been made for quantitative and semi-quantitative detection of a wide range of analytes. In this review, we summarize the current chemiluminescence imaging developments of spiroadamantane-1,2-dioxetanes. Specifically, we look at examples which depict whole animal or cellular chemiluminescence imaging of small molecules and enzymes, as well as those that portray their potential diagnostic and therapeutic abilities, with an emphasis on analyte quantification and experimental parameters.

Modular Access to Diverse Chemiluminescent Dioxetane-Luminophores through Convergent Synthesis

Adamantyl-dioxetane luminophores are an important class of chemiluminescent molecular probes for diagnostics and imaging. We have developed a new efficient synthetic route for preparation of adamantyl-enolether as precursors for dioxetane chemiluminescent luminophores. The synthesis is convergent, using an unusual Stille cross-coupling reaction employing a stannane-enolether, to directly afford adamantyl-enolether. In a following simple step, the dioxetane is obtained by oxidation of the enolether precursor with singlet-oxygen. The scope of this synthetic route is broad since a large number of haloaryl substrates are either commercially available or easily accessible. Such a late-stage derivatization strategy simplifies the rapid exploration of novel luminogenic molecular structures in a library format and simplifies the synthesis of known dioxetane luminophores. We expect that this new synthetic strategy will be particularly useful in the design and synthesis of yet unexplored dioxetane chemiluminescent luminophores.

Polymeric agents for activatable fluorescence, self-luminescence and photoacoustic imaging

Numerous polymeric agents have been widely applied in biology and medicine by virtue of the facile chemical modification, feasible nano-engineering approaches and fine-tuned pharmacokinetics. To endow polymeric imaging agents with ability to monitor and measure subtle molecular or cellular alterations at diseased sites, activatable polymeric probes that can elicit signal changes in response to biomolecular interactions or the analytes of interest have to be developed. Herein, this review aims to provide a systemic interpretation and summarization of the design methodology and imaging utility of recently emerged activatable polymeric probes. An introduction of activatable probes allowing for precise imaging and classification of polymeric imaging agents is reported first. Then, we give a detailed discussion of the contemporary design approaches toward activatable polymeric probes in diverse imaging modes for the detection of various stimuli and their imaging applications. Finally, current challenges and future advances are discussed and highlighted.

Fluorescent probes for the detection of disease-associated biomarkers

Fluorescent probes have emerged as indispensable chemical tools to the field of chemical biology and medicine. The ability to detect intracellular species and monitor physiological processes has not only advanced our knowledge in biology but has provided new approaches towards disease diagnosis. In this review, we detail the design criteria and strategies for some recently reported fluorescent probes that can detect a wide range of biologically important species in cells and in vivo. In doing so, we highlight the importance of each biological species and their role in biological systems and for disease progression. We then discuss the current problems and challenges of existing technologies and provide our perspective on the future directions of the research area. Overall, we hope this review will provide inspiration for researchers and prove as useful guide for the development of the next generation of fluorescent probes.

Near-Infrared Afterglow Luminescence of Chlorin Nanoparticles for Ultrasensitive In Vivo Imaging

Afterglow imaging holds great potential for ultrasensitive biomedical imaging. As it detects photons after the cessation of real-time light excitation, autofluorescence can therefore be effectively eliminated. However, afterglow imaging is still in its infant stage due to the lack of afterglow agents with satisfactory lifetime, biocompatibility, and high luminescence brightness, particularly afterglow in the near-infrared region for in vivo applications. To address these issues, this study for the first time reports chlorin nanoparticles (Ch-NPs) emitting afterglow luminescence peaking at 680 nm with a half-life of up to 1.5 h, which is almost 1 order of magnitude longer than those of other reported organic afterglow probes. In-depth experimental and theoretical studies revealed that the brightness of the afterglow luminescence is strongly correlated with the singlet oxygen (¹O₂) capacity and the oxidizability of the chlorins. Benefitting from the ultralong half-life and the minimized imaging background, small metastatic tumor foci of 3 mm³ were successfully resected under the guidance of the afterglow luminescence generated upon a single shot of activation prior to the injection, which was impossible for conventional near-infrared fluorescence imaging due to tissue autofluorescence.

Chemiluminescent 1,2-Dioxetane Iridium Complexes for Near-Infrared Oxygen Sensing

Chemiluminescent iridium-based sensors which demonstrate oxygen dependent responses have been developed. The molecular probes, named IrCL-1, IrCL-2 and IrCL-3 consist of oxygen-sensitive iridium complexes attached to a spiroadamantane 1,2 dioxetane and operate via energy transfer from the chemiexcited benzoate to the corresponding iridium(III) complex. Complexing the iridium(III) center with π-extended ligands results in emission in the biologically relevant, near-infrared (NIR) region. All probes demonstrate varying oxygen tolerance, with IrCL-1 being the most oxygen sensitive. These probes have been further utilized for in vitro ratiometric imaging of oxygen, as well as for intraperitoneal, intramuscular and intratumoral imaging in live mice. To our knowledge, these are the first iridium-based chemiluminescent probes that have been employed for in vitro ratiometric oxygen sensing, and for in vivo tumor imaging.

Chemiluminescent Probes Based on 1,2-dioxetane Structures For Bioimaging

Chemiluminescent probes based on 1,2-dioxetane scaffold are one of the most sensitive imaging modalities for detecting disease-related biomarkers and can obtain more accurate biological information in cells and in vivo . Due to the elimination of external light excitation, the background autofluorescence problem in fluorescence technology can be effectively avoided, providing ultra-high sensitivity and signal-to-noise ratio for various applications. In this minireview, we highlight a comprehensive but concise overview of activatable 1,2-dioetxane-based chemiluminescent probes by reporting significant advances in accurate detection and bioimaging. The design principles and applications for reactive species, enzymes, and other disease-related biomarkers are systematically discussed and summarized. The challenges and potential prospects of chemiluminescent probes are also discussed to further promote the development of new chemiluminescence methods for biological analysis and diagnosis.

Amplification of Activated Near-Infrared Afterglow Luminescence by Introducing Twisted Molecular Geometry for Understanding Neutrophil-Involved Diseases

Understanding the mechanism and progression of neutrophil-involved diseases (e.g., acute inflammation) is of great importance. However, current available analytical methods neither achieve the real-time monitoring nor provide dynamic information during the pathological processes. Herein, a peroxynitrite (ONOO^-) and environmental pH dual-responsive afterglow luminescent nanoprobe is designed and synthesized. In the presence of ONOO^- at physiological pH, the nanoprobes show activated near-infrared afterglow luminescence, whose intensity and lasting time can be highly enhanced by introducing the aggregation-induced emission (AIE) effect with a twisted molecular geometry into the system. In vivo studies using three diseased animal models demonstrate that the nanoprobes can sensitively reveal the development process of acute skin inflammation including infiltration of first arrived neutrophils and acidification initiating time, make a fast and accurate discrimination between allergy and inflammation, and rapidly screen the antitumor drugs capable of inducing immunogenic cell death. This work provides an alternative approach and advanced probes permitting precise disease monitoring in real time.

Highly Selective and Sensitive Chemiluminescent Probe for Leucine Aminopeptidase Detection in Vitro, in Vivo and in human Liver Cancer Tissue

Leucine aminopeptidase (LAP) is involved in tumor cell proliferation, invasion, and angiogenesis, and is a well-known tumor marker. In recent years, chemiluminescence has been widely used in the field of biological imaging, due to it resulting in a high sensitivity and excellent signal-to-noise ratio. Here, we report the design, synthesis, and evaluation of the first LAP-activated chemiluminescent probe for LAP detection and imaging. The probe initially had no chemiluminescence but produced an extremely strong chemiluminescence after the release of the dioxetane intermediate in the presence of LAP. The probe had high selectivity over other proteases and higher signal-to-noise ratios than commercial fluorophores. Real-time imaging results indicated that the chemiluminescence was remarkably enhanced at the mice tumor site after the probe was injected. Furthermore, the chemiluminescence of this probe in the cancerous tissues of patients was obviously improved compared to that of normal tissues. Taken together, this study has developed the first LAP-activable chemiluminescent probe, which could be potentially used in protein detection, disease diagnosis, and drug development.

The first chemiluminescent probe for the detection of LAP is described. It shows a highly selective, sensitive and rapid chemiluminescence response for the detection of LAP in vitro and in vivo, and enables the differentiation of liver cancer.

From imines to amides via NHC-mediated oxidation

An efficient construction of amides through NHC-mediated oxidation of imines is described. This work has the advantages of wide scope, fast assembly and high yield, and can avoid the use of coupling agents, such as HATU, DCC, etc.

Direct and Indirect Chemiluminescence: Reactions, Mechanisms and Challenges

Emission of light by matter can occur through a variety of mechanisms. When it results from an electronically excited state of a species produced by a chemical reaction, it is called chemiluminescence (CL). The phenomenon can take place both in natural and artificial chemical systems and it has been utilized in a variety of applications. In this review, we aim to revisit some of the latest CL applications based on direct and indirect production modes. The characteristics of the chemical reactions and the underpinning CL mechanisms are thoroughly discussed in view of studies from the very recent bibliography. Different methodologies aiming at higher CL efficiencies are summarized and presented in detail, including CL type and scaffolds used in each study. The CL role in the development of efficient therapeutic platforms is also discussed in relation to the Reactive Oxygen Species (ROS) and singlet oxygen (1O2) produced, as final products. Moreover, recent research results from our team are included regarding the behavior of commonly used photosensitizers upon chemical activation under CL conditions. The CL prospects in imaging, biomimetic organic and radical chemistry, and therapeutics are critically presented in respect to the persisting challenges and limitations of the existing strategies to date.

Recent Advances in Aggregation-Induced Emission Materials and Their Biomedical and Healthcare Applications

The emergence of the concept of aggregation-induced emission (AIE) has opened new opportunities in many research areas, such as biopsy analysis, biological processes monitoring, and elucidation of key physiological and pathological behaviors. As a new class of luminescent materials, AIE luminogens (AIEgens) possess many prominent advantages such as tunable molecular structures, high molar absorptivity, high brightness, large Stokes shift, excellent photostability, and good biocompatibility. The past two decades have witnessed a dramatic growth of research interest in AIE, and many AIE-based bioprobes with excellent performance have been widely explored in biomedical fields. This review summarizes some of the latest advancements of AIE molecular probes and AIE nanoparticles (NPs) with regards to biomedical and healthcare applications. According to the research areas, the review is divided into five sections, which are imaging and identification of cells and bacteria, photodynamic therapy, multimodal theranostics, deep tissue imaging, and fluorescence-guided surgery. The challenges and future opportunities of AIE materials in the advanced biomedical fields are briefly discussed. In perspective, the AIE-based bioprobes play vital roles in the exploration of advanced bioapplications for the ultimate goal of addressing more healthcare issues by integrating various cutting-edge modalities and techniques.

Injectable Nanocomposite Implants Reduce ROS Accumulation and Improve Heart Function after Infarction

In a myocardial infarction, blood supply to the left ventricle is abrogated due to blockage of one of the coronary arteries, leading to ischemia, which further triggers the generation of reactive oxygen species (ROS). These sequential processes eventually lead to the death of contractile cells and affect the integrity of blood vessels, resulting in the formation of scar tissue. A new heart therapy comprised of cardiac implants encapsulated within an injectable extracellular matrix-gold nanoparticle composite hydrogel is reported. The particles on the collagenous fibers within the hydrogel promote fast transfer of electrical signal between cardiac cells, leading to the functional assembly of the cardiac implants. The composite hydrogel is shown to absorb reactive oxygen species in vitro and in vivo in mice ischemia reperfusion model. The reduction in ROS levels preserve cardiac tissue morphology and blood vessel integrity, reduce the scar size and the inflammatory response, and significantly prevent the deterioration of heart function.

Molecular Probes for Autofluorescence-Free Optical Imaging

Optical imaging is an indispensable tool in clinical diagnostics and fundamental biomedical research. Autofluorescence-free optical imaging, which eliminates real-time optical excitation to minimize background noise, enables clear visualization of biological architecture and physiopathological events deep within living subjects. Molecular probes especially developed for autofluorescence-free optical imaging have been proven to remarkably improve the imaging sensitivity, penetration depth, target specificity, and multiplexing capability. In this Review, we focus on the advancements of autofluorescence-free molecular probes through the lens of particular molecular or photophysical mechanisms that produce long-lasting luminescence after the cessation of light excitation. The versatile design strategies of these molecular probes are discussed along with a broad range of biological applications. Finally, challenges and perspectives are discussed to further advance the next-generation autofluorescence-free molecular probes for in vivo imaging and in vitro biosensors.

Multiple rapid-responsive probes towards hypochlorite detection based on dioxetane luminophore derivatives

In recent years, various methods for detecting exogenous and endogenous hypochlorite have been studied, considering its essential role as a biomolecule. However, the existing technologies still pose obstacles such as their invasiveness, high costs, and complicated operation. In the current study, we developed a glow-type chemiluminescent probe, hypochlorite chemiluminescence probe (HCCL)-1, based on the scaffold of Schaap's 1,2-dioxetane luminophores. To better explore the physiological and pathological functions of hypochlorite, we modified the luminophore scaffold of HCCL-1 to develop several probes, including HCCL-2, HCCL-3, and HCCL-4, which amplify the response signal of hypochlorite. By comparing the luminescent intensities of the four probes using the IVIS® system, we determined that HCCL-2 with a limit of detection of 0.166 μM has enhanced sensitivity and selectivity for tracking hypochlorite both in vitro and in vivo.

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Strategies for the design towards glow-type hypochlorite chemiluminescent probes.

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Methods to modify the hypochlorite luminophore scaffold.

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Applications of probes for the detection and imaging of hypochlorite in vitro and in vivo.

In Situ Generated Novel 1H MRI Reporter for β-Galactosidase Activity Detection and Visualization in Living Tumor Cells

For wide applications of the lacZ gene in cellular/molecular biology, small animal investigations, and clinical assessments, the improvement of noninvasive imaging approaches to precisely assay gene expression has garnered much attention. In this study, we investigate a novel molecular platform in which alizarin 2-O-β-d-galactopyranoside AZ-1 acts as a lacZ gene/β-gal responsive 1H-MRI probe to induce significant 1H-MRI contrast changes in relaxation times T 1 and T 2 in situ as a concerted effect for the discovery of β-gal activity with the exposure of Fe3+. We also demonstrate the capability of this strategy for detecting β-gal activity with lacZ-transfected human MCF7 breast and PC3 prostate cancer cells by reaction-enhanced 1H-MRI T 1 and T 2 relaxation mapping.

Hydrazine Hydrate and Dissolved Oxygen-Triggered Near-Infrared Chemiluminescence from CuInS2@ZnS Nanocrystals for Bioassays

The overwhelming majority of commercially available chemiluminescence (CL) assays are conducted in the eye-visible region. Herein, a near-infrared (NIR) aqueous CL strategy was proposed with CuInS₂@ZnS nanocrystals (CIS@ZnS NCs) as emitters. Hydrazine hydrate (N₂H₄·H₂O) could inject electrons into the conduction band of the CIS@ZnS NCs and simultaneously transformed to the intermediate radical N₂H₃^•. N₂H₃^• reduced dissolved oxygen (O₂) to O₂^-•, while the O₂^-• could inject holes into the valence band of the CIS@ZnS NCs. The recombination of electrons and holes at Cu⁺ defects in CIS@ZnS NCs eventually yielded efficient NIR CL at around 824.1 nm, which is the longest waveband for NCs CL to the best of our knowledge. The NIR CL could be conveniently performed in the neutral aqueous medium (pH 7.0) with a quantum yield of 0.0155 Einstein/mol and was successfully employed for constructing a signal-off CL biosensor with ascorbic acid as the analyte as well as a signal-on CL biosensor for determining ascorbate oxidase, which indicates that this NIR CL system has a promising potential for bioassays in diverse ways.

Fluorescence umpolung enables light-up sensing of N-acetyltransferases and nerve agents

Intramolecular charge transfer (ICT) is a fundamental mechanism that enables the development of numerous fluorophores and probes for bioimaging and sensing. However, the electron-withdrawing targets (EWTs)-induced fluorescence quenching is a long-standing and unsolved issue in ICT fluorophores, and significantly limits the widespread applicability. Here we report a simple and generalizable structural-modification for completely overturning the intramolecular rotation driving energy, and thus fully reversing the ICT fluorophores' quenching mode into light-up mode. Specifically, the insertion of an indazole unit into ICT scaffold can fully amplify the intramolecular rotation in donor-indazole-π-acceptor fluorophores (fluorescence OFF), whereas efficiently suppressing the rotation in their EWT-substituted system (fluorescence ON). This molecular strategy is generalizable, yielding a palette of chromophores with fluorescence umpolung that spans visible and near-infrared range. This strategy expands the bio-analytical toolboxes and allows exploiting ICT fluorophores for light-up sensing of EWTs including N-acetyltransferases and nerve agents.

Wavelength-Tunable, Long Lifetime, and Biocompatible Luminescent Nanoparticles Based on a Vitamin E-Derived Material for Inflammation and Tumor Imaging

Luminescence imaging is one of the most effective noninvasive strategies for detection and stratification of inflammation and oxidative stress that are closely related to the pathogenesis of numerous acute and chronic diseases. Herein biocompatible nanoparticles based on a peroxalate ester derived from vitamin E (defined as OVE) are developed. In combination with different fluorophores, OVE can generate luminescence systems with emission wavelengths varying from blue to the near-infrared light in its native and nanoparticle forms, in the presence of hydrogen peroxide (H₂ O₂ ). The OVE-based nanoprobes exhibit high luminescence signals with extremely long lifetime, upon triggering by inflammatory conditions with abnormally elevated H₂ O₂ . Activated neutrophils and macrophages can be illuminated by this type of luminescent nanoprobes, with luminescence intensities positively correlated with inflammatory cell counts. In mouse models of peritonitis, alcoholic liver injury, drug-induced acute liver injury, and acute lung injury, the developed luminescence nanoprobes enable precision imaging of inflammation and disease progression. Moreover, tumors expressing a high level of H₂ O₂ can be shined. Importantly, the OVE-based nanoplatform shows excellent in vitro and in vivo biocompatibility.

Chemiluminescence Detection of Hydrogen Sulfide Release by β-Lactamase-Catalyzed β-Lactam Biodegradation: Unprecedented Pathway for Monitoring β-Lactam Antibiotic Bacterial Resistance

β-Lactamase positive bacteria represent a growing threat to human health because of their resistance to commonly used antibiotics. Therefore, development of new diagnostic methods for identification of β-lactamase positive bacteria is of high importance for monitoring the spread of antibiotic-resistant bacteria. Here, we report the discovery of a new biodegradation metabolite (H2S), generated through β-lactamase-catalyzed hydrolysis of β-lactam antibiotics. This discovery directed us to develop a distinct molecular technique for monitoring bacterial antibiotic resistance. The technique is based on a highly efficient chemiluminescence probe, designed for detection of the metabolite, hydrogen sulfide, that is released upon biodegradation of β-lactam by β-lactamases. Such an assay can directly indicate if antibiotic bacterial resistance exists for a certain examined β-lactam. The assay was successfully demonstrated for five different β-lactam antibiotics and eight β-lactam resistant bacterial strains. Importantly, in a functional bacterial assay, our chemiluminescence probe was able to clearly distinguish between a β-lactam resistant bacterial strain and a sensitive one. As far as we know, there is no previous documentation for such a biodegradation pathway of β-lactam antibiotics. Bearing in mind the data obtained in this study, we propose that hydrogen sulfide should be considered as an emerging β-lactam metabolite for detection of bacterial resistance.

Design and Challenges of Sonodynamic Therapy System for Cancer Theranostics: From Equipment to Sensitizers

As a novel noninvasive therapeutic modality combining low-intensity ultrasound and sonosensitizers, sonodynamic therapy (SDT) is promising for clinical translation due to its high tissue-penetrating capability to treat deeper lesions intractable by photodynamic therapy (PDT), which suffers from the major limitation of low tissue penetration depth of light. The effectiveness and feasibility of SDT are regarded to rely on not only the development of stable and flexible SDT apparatus, but also the screening of sonosensitizers with good specificity and safety. To give an outlook of the development of SDT equipment, the key technologies are discussed according to five aspects including ultrasonic dose settings, sonosensitizer screening, tumor positioning, temperature monitoring, and reactive oxygen species (ROS) detection. In addition, some state-of-the-art SDT multifunctional equipment integrating diagnosis and treatment for accurate SDT are introduced. Further, an overview of the development of sonosensitizers is provided from small molecular sensitizers to nano/microenhanced sensitizers. Several types of nanomaterial-augmented SDT are in discussion, including porphyrin-based nanomaterials, porphyrin-like nanomaterials, inorganic nanomaterials, and organic-inorganic hybrid nanomaterials with different strategies to improve SDT therapeutic efficacy. There is no doubt that the rapid development and clinical translation of sonodynamic therapy will be promoted by advanced equipment, smart nanomaterial-based sonosensitizer, and multidisciplinary collaboration.

A fluorescent probe for specific detection of β-galactosidase in living cells and tissues based on ESIPT mechanism

β-galactosidase is of great significance to living organisms, which is an important marker of primary ovarian cancer and cellular senescence. To detect the activity of β-galactosidase, a novel fluorescent probe ESIPT-GAL which based on excited state intramolecular proton transfer (ESIPT) mechanism for detecting β-galactosidase is developed in this work with low background fluorescence and high sensitivity (Φ_F = 0.0045-0.2409). The fluorescence intensity at 552 nm of this probe increased by ~ 55 times with β-galactosidase addition (0-4 U/mL), and its detection limit is very low (3.9 × 10^-5 U/mL). In addition, the spectral data (pseudo-first-order rate: 1.303 min^-1) and enzyme kinetic parameter (V_max = 69.5 μΜ•S^-1) both show that the probe can achieve rapid response to β-galactosidase. Moreover, the probe has good water solubility, which ensures that it has good biocompatibility and can be easily applied to detect β-galactosidase in living cells and tissues. Importantly, the probe ESIPT-GAL can monitor β-galactosidase in deep mouse tissue sections (90 μm).

Molecular Design of d-Luciferin-Based Bioluminescence and 1,2-Dioxetane-Based Chemiluminescence Substrates for Altered Output Wavelength and Detecting Various Molecules

Optical imaging including fluorescence and luminescence is the most popular method for the in vivo imaging in mice. Luminescence imaging is considered to be superior to fluorescence imaging due to the lack of both autofluorescence and the scattering of excitation light. To date, various luciferin analogs and bioluminescence probes have been developed for deep tissue and molecular imaging. Recently, chemiluminescence probes have been developed based on a 1,2-dioxetane scaffold. In this review, the accumulated findings of numerous studies and the design strategies of bioluminescence and chemiluminescence imaging reagents are summarized.

ROS-responsive probes for low-background optical imaging: a review

Optical imaging is a facile tool for visualizing biological processes and disease progression, but its image quality is largely limited by light-induced autofluorescence or background signals. To overcome this issue, low-background optical-imaging techniques including chemiluminescence imaging, afterglow imaging and photoacoustic imaging have been developed, based on their unique working mechanisms, which are: the detection of light emissions from chemical reactions, the cessation of light excitation before signal collection, and the detection of ultrasonic signals instead of light signals, respectively. Stimuli-responsive probes are highly desirable for improved imaging results since they can significantly reduce surrounding interference signals. Reactive oxygen species (ROS), which are closely implicated in a series of diseases such as cancer and inflammation, are frequently employed as initiators for responsive agents to selectively change the imaging signal. Thus, ROS-responsive agents incorporated into low-background imaging techniques can achieve a more promising imaging quality. In this review, recent advances in ROS-responsive probes for low-background optical-imaging techniques are summarized. Moreover, the approaches to improving the sensitivity of probes and tissue penetration depth are discussed in detail. In particular, we highlight the reaction mechanisms between the probes and ROS, revealing the potential for low-background optical imaging.

Photoacoustic-Based Miniature Device with Smartphone Readout for Point-of-Care Testing of Uric Acid

Real-time and rapid detection of various biomarkers in body fluids has important significance for early disease diagnosis, efficient monitoring of treatment, and evaluation of prognosis. However, traditional detection methods not only require bulky and costly instruments but also are not suitable for the analysis of heterogeneous samples (e.g., serum and urine), limiting their applications in point-of-care testing (POCT). Herein, an integrated photoacoustic (PA) device with a smartphone as the acoustic signal readout has been constructed, greatly reducing the volume and cost of the instrument, and providing a potential miniature platform for POCT of clinical samples. By exploiting the electron transfer product of 3,3',5,5'-tetramethylbenzidine (TMB) (i.e., TMB⁺⁺) as the PA probe and hemin-graphene oxide (H-GO) complex as the peroxidase, quantitative analysis of uric acid was successfully performed by using only 30 μL of a sample solution. Due to the favorable stability of artificial enzymes, reaction reagents could be effectively embedded in agar gel to make a portable "test strip". Therefore, operators just need to drop clinical samples on the "test strip" for PA analysis, which is user friendly without requiring complex sample preparation steps. In addition, since the acoustic change mainly comes from the PA effect, it has a lower background signal than UV-vis and fluorescence analysis, greatly improving the analytical performance. The simplicity, low cost, and broad adaptability make this miniature PA device attractive for on-site detection, particularly in resource-limited settings.

Self-Immolative Difluorophenyl Ester Linker for Affinity-Based Fluorescence Turn-on Protein Detection

Currently most fluorogenic probes are developed for the analysis of enzymes, where a bond breaking or rearrangement reaction is required to transform a nonfluorescent enzymatic substrate into a fluorescent product. However, this approach cannot be used for proteins that do not possess enzymatic activities. In this article, we show that fluorogenic probes with a self-immolative difluorophenyl ester linker can mimic the bond disassembly processes of fluorogenic enzyme substrates for the rapid analysis of nonenzymatic proteins. Although numerous self-immolative reagents have shown promising applications in sensors, drug delivery systems, and material chemistry, all of them are triggered by either enzymes or small reactive molecules. In our strategy, the probe binds to the protein via a specific protein-ligand interaction, inducing a chemical reaction between the self-immolative linker and an amino acid of the protein, thereby triggering a cascade reaction that leads to the activation and release of the fluorogenic reporter. In contrast, a phenyl ester linker without the difluoro substituent cannot be triggered to release the fluorogenic reporter. With this probe design, live-cell imaging of extracellular and intracellular endogenous tumor marker proteins can be achieved with high selectivity and sensitivity.

Room-Temperature Phosphorescence Resonance Energy Transfer for Construction of Near-Infrared Afterglow Imaging Agents

Afterglow imaging that detects photons after cessation of optical excitation avoids tissue autofluorescence and thus possesses higher sensitivity than traditional fluorescence imaging. Purely organic molecules with room-temperature phosphorescence (RTP) have emerged as a new library of benign afterglow agents. However, most RTP luminogens only emit visible light with shallow tissue penetration, constraining their in vivo applications. This study presents an organic RTP nanoprobe (mTPA-N) with emission in the NIR range for in vivo afterglow imaging. Such a probe is composed of RTP molecule (mTPA) as the phosphorescent generator and an NIR-fluorescent dye as the energy acceptor to enable room-temperature phosphorescence resonance energy transfer (RT-PRET), ultimately resulting in redshifted phosphorescent emission at 780 nm. Because of the elimination of background noise and redshifted afterglow luminescence in a biologically transparent window, mTPA-N permits imaging of lymph nodes in living mice with a high signal-to-noise ratio. This study thus opens up a universal approach to develop organic RTP luminogens into NIR afterglow imaging agents via construction of RT-PRET.

A Dendron-Based Fluorescence Turn-On Probe for Tumor Detection

Specifically amplifying the emission signals of optical probes in tumors is an effective way to improve the tumor-imaging sensitivity and contrast. In this paper, the first case of dendron-based fluorescence turn-on probes mediated by a Förster resonance energy transfer (FRET) mechanism is reported. Dendrons up to the fourth generation with a hydrophilic oligo(ethylene glycol) scaffold are synthesized by a solid-phase synthesis strategy, and show precise and defect-free chemical structures. To construct the fluorescence turn-on probe, one Cy5.5 molecule is conjugated to the focal of a G3 dendron through a robust linkage and eight Black Hole Quencher 3 (BHQ-3) molecules are conjugated to its periphery through a PEG chain bearing a reductively cleavable disulfide linkage. By in vitro and in vivo experiments, it is demonstrated that the fluorescence of the dendron-based probe can be activated effectively and rapidly in the reductive environments of tumor cells and tissues, and the probe thus exhibits amplified tumor signals and weak normal tissue signals. Compared with the reported nanoscale turn-on probes, the dendron-based probe has several significant advantages, such as well-defined chemical structure, precisely controllable fluorophore/quencher conjugation sites and ratio, desirable chemical stability, and reproducible pharmacokinetic and pharmacological profiles, and is very promising in tumor detection.

Near-Infrared AIE Dots with Chemiluminescence for Deep-Tissue Imaging

Near-infrared (NIR) chemiluminescence (CL) emission is highly favorable for deep-tissue imaging, but chemically conjugated NIR CL emitters with the aggregation-induced emission (AIE) property for biotechnology are seldom reported. Herein, an AIE-active NIR CL emitter, TBL, is synthesized by conjugating luminol unit with electron-accepting benzothiadiazole and an electron-donating triphenylamine, and subsequently TBL dots are prepared by using F127 as the surfactant. The CL emission of TBL dots can last continuously for over 60 min and can be employed for quantitative (in vitro) and qualitative (in vivo) detection of ¹ O₂ . Strikingly, the NIR CL emission can penetrate through tissues with a total thickness of over 3 cm, exhibiting significantly better performance than NIR fluorescence emission and blue CL emission. Moreover, the successful differentiation of tumor and normal tissues by TBL-based CL imaging in vivo also paves the way for CL-guided cancer diagnosis and surgery.

Ratiometric pH Imaging Using a 1,2-Dioxetane Chemiluminescence Resonance Energy Transfer Sensor in Live Animals

Regulation of physiological pH is integral for proper whole body and cellular function, and disruptions in pH homeostasis can be both a cause and effect of disease. In light of this, many methods have been developed to monitor pH in cells and animals. In this study, we report a chemiluminescence resonance energy transfer (CRET) probe Ratio-pHCL-1, composed of an acrylamide 1,2-dioxetane chemiluminescent scaffold with an appended pH-sensitive carbofluorescein fluorophore. The probe provides an accurate measurement of pH between 6.8 and 8.4, making it a viable tool for measuring pH in biological systems. Further, its ratiometric output is independent of confounding variables. Quantification of pH can be accomplished using both common luminescence spectroscopy and advanced optical imaging methods. Using an IVIS Spectrum, pH can be measured through tissue with Ratio-pHCL-1, which is shown in vitro and calibrated in sacrificed mouse models. Intraperitoneal injections of Ratio-pHCL-1 into live mice show high photon outputs and consistent increases in the flux ratio when measured at pH 6, 7, and 8.

Boronate-Based Probes for Biological Oxidants: A Novel Class of Molecular Tools for Redox Biology

Boronate-based molecular probes are emerging as one of the most effective tools for detection and quantitation of peroxynitrite and hydroperoxides. This review discusses the chemical reactivity of boronate compounds in the context of their use for detection of biological oxidants, and presents examples of the practical use of those probes in selected chemical, enzymatic, and biological systems. The particular reactivity of boronates toward nucleophilic oxidants makes them a distinct class of probes for redox biology studies. We focus on the recent progress in the design and application of boronate-based probes in redox studies and perspectives for further developments.

Long-Lasting and Intense Chemiluminescence of Luminol Triggered by Oxidized g-C3N4 Nanosheets

Most of the known chemiluminescence (CL) systems are flash-type, whereas a CL system with long-lasting and strong emission is very favorable for accurate CL quantitative analysis and imaging assays. In this work, we found that the oxidized g-C₃N₄ (g-CN_OX) could trigger luminol-H₂O₂ to produce a long-lasting and intense CL emission. The CL emission lasted for over 10 min and could be observed by the naked eye in a dark room. By means of a CL spectrum, X-ray photoelectron spectra, and electron spin resonance spectra, the possible mechanism of this CL reaction was proposed. This strong and long-duration CL emission was attributed to the high catalytic activity of g-CN_OX nanosheets and continuous generation of reactive oxygen species from H₂O₂ on g-CN_OX surface. Taking full advantage of the long-lasting CL property of this system, we proposed one "non-in-situ mixing" mode of CL measurement. Compared with the traditional "in-situ mixing" CL measurement mode, this measurement mode was convenient to operate and had good reproducibility. This work not only provides a long-lasting CL reaction but also deepens the understanding of the structure and properties of g-C₃N₄ material.

Turn-on chemiluminescence probes and dual-amplification of signal for detection of amyloid beta species in vivo

Turn-on fluorescence imaging is routinely studied; however, turn-on chemiluminescence has been rarely explored for in vivo imaging. Herein, we report the design and validation of chemiluminescence probe ADLumin-1 as a turn-on probe for amyloid beta (Aβ) species. Two-photon imaging indicates that ADLumin-1 can efficiently cross the blood-brain barrier and provides excellent contrast for Aβ plaques and cerebral amyloid angiopathy. In vivo brain imaging shows that the chemiluminescence signal of ADLumin-1 from 5-month-old transgenic 5xFAD mice is 1.80-fold higher than that from the age-matched wild-type mice. Moreover, we demonstrate that it is feasible to further dually-amplify signal via chemiluminescence resonance energy transfer (DAS-CRET) using two non-conjugated smart probes (ADLumin-1 and CRANAD-3) in solutions, brain homogenates, and in vivo whole brain imaging. Our results show that DAS-CRET can provide a 2.25-fold margin between 5-month-old 5xFAD mice and wild type mice. We believe that our strategy could be extended to other aggregating-prone proteins.

Activity-Based Sensing: A Synthetic Methods Approach for Selective Molecular Imaging and Beyond

Emerging from the origins of supramolecular chemistry and the development of selective chemical receptors that rely on lock-and-key binding, activity-based sensing (ABS)-which utilizes molecular reactivity rather than molecular recognition for analyte detection-has rapidly grown into a distinct field to investigate the production and regulation of chemical species that mediate biological signaling and stress pathways, particularly metal ions and small molecules. Chemical reactions exploit the diverse chemical reactivity of biological species to enable the development of selective and sensitive synthetic methods to decipher their contributions within complex living environments. The broad utility of this reaction-driven approach facilitates application to imaging platforms ranging from fluorescence, luminescence, photoacoustic, magnetic resonance, and positron emission tomography modalities. ABS methods are also being expanded to other fields, such as drug and materials discovery.