Near-Infrared Chemiluminescent Probe for Real-Time Monitoring Singlet Oxygen in Cells and Mice Model

Near-Infrared Chemiluminescent Theranostics

Molecular chemiluminescence probes with near-infrared (NIR) emission offer promising benefits in deciphering complex pathological processes in a living system, as NIR chemiluminescence minimizes autofluorescence, enhances deep-tissue penetration, and improves signal-to-noise ratio. Molecular engineering using single-luminophore design and dual-luminophore design with intramolecular energy transfer provides ways to develop conventional chemiluminophore scaffolds into NIR chemiluminescence probes with ideal chemiluminescence quantum yield and half-life. By virtue of the structural diversity, 1,2-dioxetane-based NIR chemiluminophores with biomarker activity have been developed. This review summarizes the molecular design strategies of NIR chemiluminescence theranostic probes (NCTPs), followed by introducing activatable NCTPs with their biomedical applications for disease theranostics. Lastly, future perspectives and potential challenges of NIR chemiluminescence imaging in preclinical research and clinical translational potential are discussed.

An Activatable Chemiluminescent Self-Reporting Sulfur Dioxide Donor for Inflammatory Response and Regulation of Gaseous Vasodilation

Sulfur dioxide (SO2), being a novel gaseous signaling molecule, exhibits significant potential for application in the field of cardiovascular diseases. SO2 donors serve as crucial tools for the transportation and regulation of SO2 in vivo, facilitating the investigation of physiological roles associated with this molecule. However, the current therapeutic SO2 donors lack the capability to monitor the real-time release of SO2, thereby hindering accurate assessment of their therapeutic efficacy and target localization. Herein, we present an activatable chemiluminescent self-reporting SO2 donor (CL-SO2D) that can be selectively activated by peroxynitrite (ONOO-) to release SO2 and enable real-time visualization of the extent of release through chemiluminescent imaging. In vitro and cellular experiments demonstrate that CL-SO2D exhibits high selectivity and signal-to-noise ratio toward ONOO- and effectively facilitates the SO2 release process. Finally, CL-SO2D successfully achieved the response to the mouse inflammatory model and relieved vasoconstriction in zebrafish by releasing SO2 stimulated by ONOO-. The findings suggest that CL-SO2D exhibits impressive attributes in the diagnosis and treatment of SO2-related diseases, opening the gateway for developing low-background and high-sensitivity self-reporting SO2 donors.

Unprecedented Photoinduced-Electron-Transfer Probe with a Turn-ON Chemiluminescence Mode-of-Action

PeT-based fluorescent probes were demonstrated to be powerful tools for detection and imaging, owing to their significant fluorescence enhancement in response to specific targets. While numerous examples of fluorescence-based PeT have been frequently reported, there is not even a single example of a PeT probe that operates via a chemiluminescence mode. Here we report the first PeT-based turn-on probe that acts via a chemiluminescent operation mode. We designed, synthesized, and evaluated a novel chemiluminescent probe, featuring a PeT-based turn-on mechanism. The probe consists of a phenoxy-1,2-dioxetane, linked to an azide unit that acts as a PeT quencher. Upon cycloaddition of a strained cycloalkyne with the azide, a triazole-dioxetane is formed, which undergoes relatively slow chemiexcitation, resulting in a measurement window with an exceptionally high signal-to-noise ratio (over 5000-fold). The PeT-dioxetane probe could effectively detect and image two model proteins labeled with strained cycloalkyne units (Myc-DBCO and Max-DBCO) through either NHS or maleimide conjugations. Comparative analysis shows that our PeT-based chemiluminescent probe significantly outperforms a commercially available fluorescent analog. We anticipate that the insights gained from this study will facilitate the development of additional chemiluminescent probes utilizing various PeT-quenching pathways.

Universal sulfatase-based chemiluminescence biosensing platform: Validation via AFP detection in clinical blood samples

Enzyme-catalyzed chemiluminescence has been widely used in the field of biomedicine, especially in the test kit for various biomarkers. However, the currently reported enzyme-catalyzed chemiluminescence systems suffered from the addition of oxidizing substances, short emission wavelength, and susceptibility to interference by autofluorescence. In this paper, a universal sulfatase-based chemiluminescence system with NIR was developed, in which the designed substrate QM-CF could be transformed into 1,2-dioxetane derivate in the presence of sulfatase and oxygen. This system exhibited long emission wavelengths and CL half-time, a high signal-noise ratio, and without other additives. Importantly, the sulfatase-based chemiluminescence enzyme-linked immunoassay platform was successfully constructed and could be generally applied to detect biomarkers. As a proof of concept, the sulfatase-labeled AFP antibody and substate QM-CF were conveniently suitable for commercial AFP test kits, leading to satisfactory detection results of AFP in clinical blood samples.

Phenoxy-1,2-dioxetane-based activatable chemiluminescent probes: tuning of photophysical properties for tracing enzymatic activities in living cells

The use of chemiluminophores for tracing enzymatic activities in live-cell imaging has gained significant attention, making them valuable tools for diagnostic applications. Among various chemiluminophores, the phenoxy-1,2-dioxetane scaffold exhibits significant structural versatility and its activation is governed by the chemically initiated electron exchange luminescence (CIEEL) mechanism. This mechanism can be initiated by enzymatic activity, changes in pH, or other chemical stimuli. The photophysical properties of phenoxy-1,2-dioxetanes can be fine-tuned through the incorporation of different substituents on the phenolic ring and by anchoring them with specific triggers. This review discusses the variations in physicochemical properties, including emission maxima, quantum yield, aqueous solubility, and pKa, as influenced by structural modifications, thereby establishing a comprehensive structure-activity relationship. Furthermore, it categorises the probes based on different enzyme classes, such as hydrolase-sensitive probes, oxidoreductase-responsive probes, and transferase-activatable phenoxy-1,2-dioxetanes, offering a promising platform technology for the early diagnosis of diseases and disorders. The summary section highlights key opportunities and limitations associated with applying phenoxy-1,2-dioxetanes in achieving precise and effective enzyme assays.

Light/X-ray/ultrasound activated delayed photon emission of organic molecular probes for optical imaging: mechanisms, design strategies, and biomedical applications

Conventional optical imaging, particularly fluorescence imaging, often encounters significant background noise due to tissue autofluorescence under real-time light excitation. To address this issue, a novel optical imaging strategy that captures optical signals after light excitation has been developed. This approach relies on molecular probes designed to store photoenergy and release it gradually as photons, resulting in delayed photon emission that minimizes background noise during signal acquisition. These molecular probes undergo various photophysical processes to facilitate delayed photon emission, including (1) charge separation and recombination, (2) generation, stabilization, and conversion of the triplet excitons, and (3) generation and decomposition of chemical traps. Another challenge in optical imaging is the limited tissue penetration depth of light, which severely restricts the efficiency of energy delivery, leading to a reduced penetration depth for delayed photon emission. In contrast, X-ray and ultrasound serve as deep-tissue energy sources that facilitate the conversion of high-energy photons or mechanical waves into the potential energy of excitons or the chemical energy of intermediates. This review highlights recent advancements in organic molecular probes designed for delayed photon emission using various energy sources. We discuss distinct mechanisms, and molecular design strategies, and offer insights into the future development of organic molecular probes for enhanced delayed photon emission.

Chemiexcitation Acceleration of 1,2-Dioxetanes by Spiro-Fused Six-Member Rings with Electron-Withdrawing Motifs

The chemiluminescent light-emission pathway of phenoxy-1,2-dioxetane luminophores attracts growing interest within the scientific community. Dioxetane probes undergoing rapid flash-type chemiexcitation exhibit higher detection sensitivity than those with a slow glow-type chemiexcitation rate. We discovered that dioxetanes fused to non-strained six-member rings, with hetero atoms or inductive electron-withdrawing groups, present both accelerated chemiexcitation rates and elevated chemical stability compared to dioxetanes fused to four-member strained rings. DFT computational simulations supported the chemiexcitation acceleration observed by spiro-fused six-member rings with inductive electron-withdrawing groups of dioxetanes. Specifically, a spiro-dioxetane with a six-member sulfone ring exhibited a chemiexcitation rate 293-fold faster than that of spiro-adamantyl-dioxetane. A turn-ON dioxetane probe for the detection of the enzyme β-galactosidase, containing the six-member sulfone unit, exhibited a S/N value of 108 in LB cell growth medium. This probe demonstrated a substantial increase in detection sensitivity towards E. coli bacterial cells expressing β-galactosidase, with an LOD value that is 44-fold more sensitive than that obtained by the adamantyl counterpart. The accelerated chemiexcitation and the elevated chemical stability presented by dioxetane containing a spiro-fused six-member ring with a sulfone inductive electron-withdrawing group, make it an ideal candidate for designing efficient turn-on chemiluminescent probes with exceptionally high detection sensitivity.

Boosting Chemiexcitation of Phenoxy-1,2-dioxetanes through 7-Norbornyl and Homocubanyl Spirofusion

The chemiluminescent light-emission pathway of phenoxy-1,2-dioxetane luminophores is increasingly attracting the scientific community's attention. Dioxetane probes that undergo rapid, flash-type chemiexcitation demonstrate higher detection sensitivity than those with a slower, glow-type chemiexcitation rate. This is primarily because the rapid flash-type produces a greater number of photons within a given time. Herein, we discovered that dioxetanes fused to 7-norbornyl and homocubanyl units present accelerated chemiexcitation rates supported by DFT computational simulations. Specifically, the 7-norbornyl and homocubanyl spirofused dioxetanes exhibited a chemiexcitation rate 14.2-fold and 230-fold faster than that of spiro-adamantyl dioxetane, respectively. A turn-ON dioxetane probe for the detection of the enzyme β-galactosidase, containing the 7-norbornyl spirofused unit, exhibited an S/N value of 415 at a low enzyme concentration. This probe demonstrated an increase in detection sensitivity toward β-galactosidase expressing bacteria E. coli with a limit-of-detection value that is 12.8-fold more sensitive than that obtained by the adamantyl counterpart. Interestingly, the computed activation free energies of the homocubanyl and 7-norbornyl units were correlated with their CCsC spiro-angle to corroborate the measured chemiexcitation rates.

A de novo zwitterionic strategy of ultra-stable chemiluminescent probes: highly selective sensing of singlet oxygen in FDA-approved phototherapy

Singlet oxygen (1O2), as a fundamental hallmark in photodynamic therapy (PDT), enables ground-breaking clinical treatment in ablating tumors and killing germs. However, accurate in vivo monitoring of 1O2 remains a significant challenge in probe design, with primary difficulties arising from inherent photo-induced side reactions with poor selectivity. Herein, we report a generalizable zwitterionic strategy for ultra-stable near-infrared (NIR) chemiluminescent probes that ensure a highly specific [2 + 2] cycloaddition between fragile electron-rich enolether units and 1O2 in both cellular and dynamic in vivo domains. Innovatively, zwitterionic chemiluminescence (CL) probes undergo a conversion into an inert ketone excited state with an extremely short lifetime through conical intersection (CI), thereby affording sufficient photostability and suppressing undesired photoreactions. Remarkably, compared with the well-known commercial 1O2 probe SOSG, the zwitterionic probe QMI exhibited an ultra-high signal-to-noise ratio (SNR, over 40-fold). Of particular significance is that the zwitterionic CL probes demonstrate excellent selectivity, high sensitivity, and outstanding photostability, thereby making a breakthrough in real-time tracking of the FDA-approved 5-ALA-mediated in vivo PDT process in living mice. This innovative zwitterionic strategy paves a new pathway for high-performance NIR chemiluminescent probes and high-fidelity feedback on 1O2 for future biological and medical applications.

Stimuli-responsive linkers and their application in molecular imaging

Molecular imaging is a non-invasive imaging method that is widely used for visualization and detection of biological events at cellular or molecular levels. Stimuli-responsive linkers that can be selectively cleaved by specific biomarkers at desired sites to release or activate imaging agents are appealing tools to improve the specificity, sensitivity, and efficacy of molecular imaging. This review summarizes the recent advances of stimuli-responsive linkers and their application in molecular imaging, highlighting the potential of these linkers in the design of activatable molecular imaging probes. It is hoped that this review could inspire more research interests in the development of responsive linkers and associated imaging applications.

Leveraging Long-Distance Singlet-Oxygen Transfer for Bienzyme-Locked Afterglow Imaging of Intratumoral Granule Enzymes

Dual-locked activatable optical probes, leveraging the orthogonal effects of two biomarkers, hold great promise for the specific imaging of biological processes. However, their design approaches are limited to a short-distance energy or charge transfer mechanism, while the signal readout relies on fluorescence, which inevitably suffers from tissue autofluorescence. Herein, we report a long-distance singlet oxygen transfer approach to develop a bienzyme-locked activatable afterglow probe (BAAP) that emits long-lasting self-luminescence without real-time light excitation for the dynamic imaging of an intratumoral granule enzyme. Composed of an immuno-biomarker-activatable singlet oxygen (1O2) donor and a cancer-biomarker-activatable 1O2 acceptor, BAAP is initially nonafterglow. Only in the presence of both immune and cancer biomarkers can 1O2 be generated by the activated donor and subsequently diffuse toward the activated acceptor, resulting in bright near-infrared afterglow with a high signal-to-background ratio and specificity toward an intratumoral granule enzyme. Thus, BAAP allows for real-time tracking of tumor-infiltrating cytotoxic T lymphocytes, enabling the evaluation of cancer immunotherapy and the differentiation of tumor from local inflammation with superb sensitivity and specificity, which are unachievable by single-locked probes. Thus, this study not only presents the first dual-locked afterglow probe but also proposes a new design way toward dual-locked probes via reactive oxygen species transfer processes.

A Tandem-Locked Chemiluminescent Probe for Imaging of Tumor-Associated Macrophage Polarization

Tumor-associated macrophages (TAMs) play a role in both pro- and anti-tumor functions; and the targeted polarization from M2 to M1 TAMs has become an effective therapy option. Although detection of M1 TAMs is imperative to assess cancer immunotherapeutic efficacy, existing optical probes suffer from shallow tissue penetration depth and poor specificity toward M1 TAMs. Herein, we report a tandem-locked NIR chemiluminescent (CL) probe for specific detection of M1 TAMs. Through a combined molecular engineering approach via both atomic alternation and introduction of electron-withdrawing groups, near-infrared (NIR) chemiluminophores are screened out to possess record-long emission (over 800 nm), record-high CL quantum yield (2.7 % einstein/mol), and prolonged half-life (7.7 h). Based on an ideal chemiluminophore, the tandem-locked probe (DPDGN) is developed to only activate CL signal in the presence of both tumour (γ-glutamyl transpeptidase) and M1 macrophage biomarkers (nitric oxide). Such a tandem-lock design ensures its high specificity towards M1 macrophages in the tumor microenvironment over those in normal tissues or peripheral blood. Thus, DPDGN permits noninvasive imaging and tracking of M1 TAM in the tumor of living mice during R837 treatment, showing a good correlation with ex vivo methods. This study not only reports a new molecular approach towards highly efficient chemiluminophores but also reveals the first tandem-locked CL probes for enhanced imaging specificity.

Dual-Functional AIE Fluorescent Probe for Visualization of Lipid Droplets and Photodynamic Therapy of Cancer

Abnormal lipid droplets (LDs) are known to be intimately bound with the occurrence and development of cancer, allowing LDs to be critical biomarkers for cancers. Aggregation-induced emission luminogens (AIEgens), with efficient reactive oxygen species (ROS) production performance, are prime photosensitizers (PSs) for photodynamic therapy (PDT) with imaging. Therefore, the development of dual-functional fluorescent probes with aggregation-induced emission (AIE) characteristics that enable both simultaneous LD monitoring and imaging-guided PDT is essential for concurrent cancer diagnosis and treatment. Herein, we reported the development of a novel LD-targeting fluorescent probe (TDTI) with AIE performance, which was expected to realize the integration of cancer diagnosis through LD visualization and cancer treatment via PDT. We demonstrated that TDTI, with typical AIE characteristics and excellent photostability, could target LDs with high specificity, which enables the dynamic tracking of LDs in living cells, specific imaging of LDs in zebrafish, and the differentiation of cancer cells from normal cells for cancer diagnosis. Meanwhile, TDTI exhibited fast ROS generation ability (achieving equilibrium within 60 s) under white light irradiation (10 mW/cm2). The cell apoptosis assay revealed that TDTI effectively induced growth inhibition and apoptosis of HeLa cells. Further, the results of PDT in vivo indicated that TDTI had a good antitumor effect on the tumor-bearing mice model. Collectively, these results highlight the potential utility of the dual-functional fluorescent probe TDTI in the integrated diagnosis and treatment of cancer.

Chemiluminescent Conjugated Polymer Nanoparticles for Deep-Tissue Inflammation Imaging and Photodynamic Therapy of Cancer

Deep-tissue optical imaging and photodynamic therapy (PDT) remain a big challenge for the diagnosis and treatment of cancer. Chemiluminescence (CL) has emerged as a promising tool for biological imaging and in vivo therapy. The development of covalent-binding chemiluminescence agents with high stability and high chemiluminescence resonance energy transfer (CRET) efficiency is urgent. Herein, we design and synthesize an unprecedented chemiluminescent conjugated polymer PFV-Luminol, which consists of conjugated polyfluorene vinylene (PFV) main chains and isoluminol-modified side chains. Notably, isoluminol groups with chemiluminescent ability are covalently linked to main chains by amide bonds, which dramatically narrow their distance, greatly improving the CRET efficiency. In the presence of pathologically high levels of various reactive oxygen species (ROS), especially singlet oxygen (1O2), PFV-Luminol emits strong fluorescence and produces more ROS. Furthermore, we construct the PFV-L@PEG-NPs and PFV-L@PEG-FA-NPs nanoparticles by self-assembly of PFV-Luminol and amphiphilic copolymer DSPE-PEG/DSPE-PEG-FA. The chemiluminescent PFV-L@PEG-NPs nanoparticles exhibit excellent capabilities for in vivo imaging in different inflammatory animal models with great tissue penetration and resolution. In addition, PFV-L@PEG-FA-NPs nanoparticles show both sensitive in vivo chemiluminescence imaging and efficient chemiluminescence-mediated PDT for antitumors. This study paves the way for the design of chemiluminescent probes and their applications in the diagnosis and therapy of diseases.

Near-Infrared Chemiluminescence Imaging of Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) has a high prevalence but is poorly managed for cancer patients due to the lack of reliable and sensitive diagnostic techniques. Molecular optical imaging can provide a noninvasive way for real-time monitoring of CIPN; However, this is not reported, likely due to the absence of optical probes capable of imaging deep into the spinal canal and possessing sufficient sensitivity for minimal dosage through local injection into the dorsal root ganglia. Herein, a near-infrared (NIR) chemiluminophore (MPBD) with a chemiluminescence quantum yield higher than other reported probes is synthesized and a NIR activatable chemiluminescent probe (CalCL) is developed for in vivo imaging of CIPN. CalCL is constructed by caging MPBD with calpain-cleavable peptide moiety while conjugating polyethylene glycol chain to endow water solubility. Due to the deep-tissue penetration of chemiluminescence and specific turn-on response of CalCL toward calpain (a hallmark of CIPN), it allows for sensitive detection of paclitaxel-mediated CIPN in living mice, which is unattainable by fluorescence imaging. This study thus not only develops a highly efficient chemiluminescent probe, but also presents the first optical imaging approach toward high-throughput screening of neurotoxic drugs.

Photosensitizer-singlet oxygen sensor conjugated silica nanoparticles for photodynamic therapy and bioimaging

Intracellular singlet oxygen (1O2) generation and detection help optimize the outcome of photodynamic therapy (PDT). Theranostics programmed for on-demand phototriggered 1O2 release and bioimaging have great potential to transform PDT. We demonstrate an ultrasensitive fluorescence turn-on sensor-sensitizer-RGD peptide-silica nanoarchitecture and its 1O2 generation-releasing-storing-sensing properties at the single-particle level or in living cells. The sensor and sensitizer in the nanoarchitecture are an aminomethyl anthracene (AMA)-coumarin dyad and a porphyrin or CdSe/ZnS quantum dots (QDs), respectively. The AMA in the dyad quantitatively quenches the fluorescence of coumarin by intramolecular electron transfer, the porphyrin or QD moiety generates 1O2, and the RGD peptide facilitates intracellular delivery. The small size, below 200 nm, as verified by scanning electron microscopy and differential light scattering measurements, of the architecture within the 1O2 diffusion length enables fast and efficient intracellular fluorescence switching by the tandem ultraviolet (UV)-visible or visible-near-infrared (NIR) photo-triggering. While the red emission and 1O2 generation by the porphyrin are continually turned on, the blue emission of coumarin is uncaged into 230-fold intensity enhancement by on-demand photo-triggering. The 1O2 production and release by the nanoarchitecture enable spectro-temporally controlled cell imaging and apoptotic cell death; the latter is verified from cytotoxic data under dark and phototriggering conditions. Furthermore, the bioimaging potential of the TCPP-based nanoarchitecture is examined in vivo in B6 mice.

Activatable Near-Infrared Versatile Fluorescent and Chemiluminescent Dyes Based on the Dicyanomethylene-4H-pyran Scaffold: From Design to Imaging and Theranostics

Activatable fluorescent and chemiluminescent dyes with near-infrared emission have indispensable roles in the fields of bioimaging, molecular prodrugs, and phototheranostic agents. As one of the most popular fluorophore scaffolds, the dicyanomethylene-4H-pyran scaffold has been applied to fabricate a large number of versatile activatable optical dyes for analytes detection and diseases diagnosis and treatment by virtue of its high photostability, large Stokes shift, considerable two-photon absorption cross-section, and structural modifiability. This review discusses the molecular design strategies, recognition mechanisms, and both in vitro and in vivo bio-applications (especially for diagnosis and therapy of tumors) of activatable dicyanomethylene-4H-pyran dyes. The final section describes the current shortcomings and future development prospects of this topic.

Time-gated luminescent probes for lysosomal singlet oxygen: Synthesis, characterizations and bioimaging applications

BACKGROUD

Single oxygen (1O2), the molecular oxygen at its excited state, plays a crucial role in the photodynamic therapy (PDT) of some diseases owing to its strong oxidizing property to destroy malignant cells. Although the fluorescent probe technique has proven its powerful application abilities for detection of 1O2 in biological systems, most of the reported fluorescent probes suffered from the interference of background autofluorescence of biological samples. It is clear that the real-time and in situ, background-free fluorescent detection of 1O2 generated in live cells, especially in some organelles, is of great significance for understanding the action mechanism of PDT drugs.

RESULTS

By introducing a lysosome-anchoring motif, a morpholine moiety, into a 1O2-specifically-reactive terpyridine polyacid ligand, [4'-(9-anthryl)-2,2':6',2″-terpyridine-6,6″-diyl] bis(methylenenitrilo) tetrakis (acetic acid) (ATTA), and chelating with lanthanide ions (Eu3+ or Tb3+), two lanthanide complex-based "turn-on" luminescent probes that can be used for the background-free time-gated luminescent (TGL) detection of lysosomal 1O2, Lyso-ATTA-Eu3+ and Lyso-ATTA-Tb3+, have been developed. The probes exhibit fast luminescence responses (within 2.5 min) towards 1O2 with high selectivity and sensitivity (<0.75 μM) in a wide pH range (4-11). And the excellent lysosome-localization performance of the probes allowed them to be used for the monitoring of endogenous 1O2 in lysosomes, which enabled the variability of lysosomal-1O2 concentrations induced by different photosensitizers to be successfully discriminated. Furthermore, by doping Lyso-ATTA-Eu3+ into the polyethylene glycol (PEG) hydrogel, the smart luminescent sensor film, PEG-Lyso-ATTA-Eu3+, was prepared, and successfully used for the detection of the on-site 1O2 production during the PDT process of psoriatic disease in model mice.

SIGNIFICANT

Two lysosome-targetable background-free TGL probes for 1O2 were firstly reported. The developed smart luminescent sensor film could be a powerful tool for the clinical monitoring of PDT on skin diseases without using sophisticated and expensive instruments.

Spirostrain-Accelerated Chemiexcitation of Dioxetanes Yields Unprecedented Detection Sensitivity in Chemiluminescence Bioassays

Chemiluminescence is a fascinating phenomenon that involves the generation of light through chemical reactions. The light emission from adamantyl-phenoxy-1,2-dioxetanes can glow from minutes to hours depending on the specific substituent present on the dioxetane molecule. In order to improve the light emission properties produced by these chemiluminescent luminophores, it is necessary to induce the chemiexcitation rate to a flash mode, wherein the bulk of light is emitted instantly rather than slowly over time. We report the realization of this goal through the incorporation of spirostrain release into the decomposition of 1,2-dioxetane luminophores. DFT computational simulations provided support for the hypothesis that the spiro-cyclobutyl substituent accelerates chemiexcitation as compared to the unstrained adamantyl substituent. Spiro-linking of cyclobutane and oxetane units led to greater than 100-fold and 1000-fold emission enhancement, respectively. This accelerated chemiexcitation rate increases the detection sensitivity for known chemiluminescent probes to the highest signal-to-noise ratio documented to date. A turn-ON probe, containing a spiro-cyclobutyl unit, for detecting the enzyme β-galactosidase exhibited a limit of detection value that is 125-fold more sensitive than that for the previously described adamantyl analogue. This probe was also able to instantly detect and image β-gal activity with enhanced sensitivity in E. coli bacterial assays.

Colloidal Quantum Dots as an Emerging Vast Platform and Versatile Sensitizer for Singlet Molecular Oxygen Generation

Singlet molecular oxygen (1O2) has been reported in wide arrays of applications ranging from optoelectronic to photooxygenation reactions and therapy in biomedical proposals. It is also considered a major determinant of photodynamic therapy (PDT) efficacy. Since the direct excitation from the triplet ground state (3O2) of oxygen to the singlet excited state 1O2 is spin forbidden; therefore, a rational design and development of heterogeneous sensitizers is remarkably important for the efficient production of 1O2. For this purpose, quantum dots (QDs) have emerged as versatile candidates either by acting individually as sensitizers for 1O2 generation or by working in conjunction with other inorganic materials or organic sensitizers by providing them a vast platform. Thus, conjoining the photophysical properties of QDs with other materials, e.g., coupling/combining with other inorganic materials, doping with the transition metal ions or lanthanide ions, and conjugation with a molecular sensitizer provide the opportunity to achieve high-efficiency quantum yields of 1O2 which is not possible with either component separately. Hence, the current review has been focused on the recent advances made in the semiconductor QDs, perovskite QDs, and transition metal dichalcogenide QD-sensitized 1O2 generation in the context of ongoing and previously published research work (over the past eight years, from 2015 to 2023).

Accurate Determination of the Photosensitizer Stern-Volmer Constant by the Oxygen-Dependent Consumption of 1,3-Diphenylisobenzofuran

Because of the absence of phosphorescence, the Stern-Volmer constant (KSV) of the photosensitizer is hard to determine accurately. Although the delayed fluorescence and correlated fluorescence methods have been proposed to determine KSV, the weak signal detection and non-uniform excitation enlarged the measurement error. In this work, a method was proposed to accurately determine KSV by oxygen-dependent consumption of 1,3-diphenylisobenzofuran. The consumption time (δ), as a measurable quantity, is introduced and could be obtained by the absorption spectrum with a high signal-to-noise ratio. Analytically, δ is linearly related to the inverse of oxygen content, and the ratio of the intercept to the slope equals KSV. Experimentally, rose Bengal was selected to perform this determination; the KSV is measured to be 43(1) kPa-1, and the error is reduced by 1 order of magnitude. In addition, metalloporphyrin was used to verify this method.

Ultrasound-Activated NIR Chemiluminescence for Deep Tissue and Tumor Foci Imaging

Fluorescence imaging requires real-time external light excitation; however, it has the drawbacks of autofluorescence and shallower penetration depth, limiting its application in deep tissue imaging. At the same time, ultrasound (US) has high spatiotemporal resolution, deep penetrability, noninvasiveness, and precise localization of lesions; thus, it can be a promising alternative to light. However, US-activated luminescence has been rarely reported. Herein, an US-activated near-infrared (NIR) chemiluminescence (CL) molecule, namely, PNCL, is designed by protoporphyrin IX as a sonosensitizer moiety and a phenoxy-dioxetane precursor containing a dicyanomethyl chromone acceptor scaffold (NCL) as the US-responsive moiety. After therapeutic US radiation (1 MHz), the singlet oxygen (1O2), as an "intermediary", oxidizes the enol-ether bond of the NCL moiety and then emits NIR light via spontaneous decomposition. Combining the deep penetrability of US with a high signal-to-background ratio of NIR CL, the designed probe PNCL successfully realizes US-activated deep tissue imaging (∼20 mm) and selectively turns on signals in specific tumor foci. Bridging US chemistry with luminescence using an "intermediary" will provide new imaging methods for accurate cancer diagnosis.

Chemiluminescent duplex analysis using phenoxy-1,2-dioxetane luminophores with color modulation

Multiplex technology is an important emerging field, in diagnostic sciences, that enables the simultaneous detection of several analytes in a single sample. The light-emission spectrum of a chemiluminescent phenoxy-dioxetane luminophore can be accurately predicted by determining the fluorescence-emission spectrum of its corresponding benzoate species, which is generated during the chemiexcitation process. Based on this observation, we designed a library of chemiluminescent dioxetane luminophores with multicolor emission wavelengths. Two dioxetane luminophores that have different emission spectra, but similar quantum yield properties, were selected from the synthesized library for a duplex analysis. The selected dioxetane luminophores were equipped with two different enzymatic substrates to generate turn-ON chemiluminescent probes. This pair of probes exhibited a promising ability to act as a chemiluminescent duplex system for the simultaneous detection of two different enzymatic activities in a physiological solution. In addition, the pair of probes were also able to simultaneously detect the activities of the two enzymes in a bacterial assay, using a blue filter slit for one enzyme and a red filter slit for the other enzyme. As far as we know, this is the first successful demonstration of a chemiluminescent duplex system composed of two-color phenoxy-1,2-dioxetane luminophores. We believe that the library of dioxetanes presented here will be beneficial for developing chemiluminescence luminophores for multiplex analysis of enzymes and bioanalytes.

Novel Near-Infrared Iridium(III) Complex for Chemiluminescence Imaging of Hypochlorous Acid

Chemiluminescence (CL) probes that possess near-infrared (NIR) emission are highly desirable for in vivo imaging due to their deeper tissue penetration ability and intrinsically high sensitivity. Herein, a novel iridium-based CL probe (NIRIr-CL-1) with direct NIR emission was reported as the result of hypochlorous acid (HClO)-initiated oxidative deoximation. To improve its biocompatibility and extend the CL time for in vivo imaging applications, this NIRIr-CL-1 was prepared as a CL nanoparticle probe (NIRIr-CL-1 dots) through encapsulation by an amphiphilic polymer Pluronic F127 (F127). All results demonstrate that the NIRIr-CL-1 dots have good selectivity and sensitivity for visualization of HClO even at the depth of 1.2 cm. Owing to these advantages, the CL imaging of exogenous and endogenous HClO in mice was achieved. This study could provide new insights into the construction of new NIR emission CL probes and expand their applications in biomedical imaging.

Visualization of Phototherapy Evolution by Optical Imaging

Phototherapy, including photodynamic therapy (PDT) and photothermal therapy (PTT), is a non-invasive and effective approach used for cancer treatment, in which phototherapeutic agents are irradiated with an appropriate light source to produce cytotoxic reactive oxygen species (ROS) or heat to ablate cancer cells. Unfortunately, traditional phototherapy lacks a facile imaging method to monitor the therapeutic process and efficiency in real time, usually leading to severe side effects due to high levels of ROS and hyperthermia. To realize precise cancer treatment methods, it is highly desired to develop phototherapeutic agents possessing an imaging ability to evaluate the therapeutic process and efficacy in real time during cancer phototherapy. Recently, a series of self-reporting phototherapeutic agents were reported to monitor PDT and PTT processes by combining optical imaging technologies with phototherapy. Due to the real-time feedback provided by optical imaging technology, therapeutic responses or dynamic changes in the tumor microenvironment could be evaluated in a timely manner, thereby achieving personalized precision treatment and minimizing toxic side effects. In this review, we focus on the advances in the development of self-reporting phototherapeutic agents for a cancer phototherapy evaluation based on optical imaging technology to realize precision cancer treatments. Additionally, we propose the current challenges and future directions of self-reporting agents for precision medicine.

Exploring the Structural Space of Chemiluminescent 1,2-Dioxetanes

Chemiluminescent molecules which emit light in response to a chemical reaction are powerful tools for the detection and measurement of biological analytes and enable the understanding of complex biochemical processes in living systems. Triggerable chemiluminescent 1,2-dioxetanes have been studied and tuned over the past decades to advance quantitative measurement of biological analytes and molecular imaging in live cells and animals. A crucial determinant of success for these 1,2-dioxetane based sensors is their chemical structure, which can be manipulated to achieve desired chemical properties. In this Perspective, we survey the structural space of triggerable 1,2-dioxetane and assess how their design features affect chemiluminescence properties including quantum yield, emission wavelength, and decomposition kinetics. Based on this appraisal, we identify some structural modifications of 1,2-dioxetanes that are ripe for exploration in the context of chemiluminescent biological sensors.

Practical Aspects in the Study of Biological Photosensitization Including Reaction Mechanisms and Product Analyses: A Do's and Don'ts Guide†

The interaction of light with natural matter leads to a plethora of photosensitized reactions. These reactions cause the degradation of biomolecules, such as DNA, lipids, proteins, being therefore detrimental to the living organisms, or they can also be beneficial by allowing the treatment of several diseases by photomedicine. Based on the molecular mechanistic understanding of the photosensitization reactions, we propose to classify them in four processes: oxygen-dependent (type I and type II processes) and oxygen-independent [triplet-triplet energy transfer (TTET) and photoadduct formation]. In here, these processes are discussed by considering a wide variety of approaches including time-resolved and steady-state techniques, together with solvent, quencher, and scavenger effects. The main aim of this survey is to provide a description of general techniques and approaches that can be used to investigate photosensitization reactions of biomolecules together with basic recommendations on good practices. Illustration of the suitability of these approaches is provided by the measurement of key biomarkers of singlet oxygen and one-electron oxidation reactions in both isolated and cellular DNA. Our work is an educational review that is mostly addressed to students and beginners.

An Aggregation-Induced Emission Nanosensor for Real-Time Chemiluminescent Sensing of Light-Independent Intracellular Singlet Oxygen

Characterizing the transient ultratrace light-independent intracellular singlet oxygen (¹O₂), which plays a vital role in multiple biological processes in living organisms, brings about tremendous help for understanding the nature of ¹O₂-mediated or related bioevents. Nevertheless, an approach to detect the light-independent intracellular ¹O₂ is hard to find. Herein, we developed a chemiluminescent nanosensor by compacting a great number of TPE-N(Ph)-DBT-PH molecules in one nanostructure via autoaggregation. Taking advantage of the aggregation-induced emission property, this TPE-N(Ph)-DBT-PH nanosensor is highly fluorescent and promises a bright red-light CL and the convenience of mapping in vivo sensor distribution. Experiments demonstrate the nanosensor's unprecedented selectivity toward ¹O₂ against other reactive oxygen species. The 3.7 nmol L^-1 limit of detection renders this nanosensor with the best-known sensitivity of ¹O₂ chemical sensors. Meanwhile, fluorescence confocal microscope imaging results suggest that our nanosensor simultaneously targets mitochondria and lysosomes in RAW 264.7 cells via the energy-dependent endocytosis pathway, thereby implying an attractive potential for the detection of intracellular ¹O₂. Such a potential is demonstrated by detecting ¹O₂ in RAW 264.7 cells during a lipopolysaccharide and phorbol myristate acetate stimulated respiration burst. This study represents the first approach to detect light-independent intracellular ¹O₂ during cell bioregulation. Thus, our nanosensor provides an effective tool for investigating the ¹O₂-related bioprocesses and pathological processes.

NIR-II Ratiometric Chemiluminescent/Fluorescent Reporters for Real-Time Monitoring and Evaluating Cancer Photodynamic Therapy Efficacy

The development of probes for early monitoring tumor therapy response may greatly benefit the promotion of photodynamic therapy (PDT) efficacy. Singlet oxygen (¹ O₂ ) generation is a typical indicator for evaluating PDT efficacy in cancer. However, most existing probes cannot quantitatively detect ¹ O₂ in vivo due to the high reactivity and transient state, and thus have a poor correlation with PDT response. Herein, a ¹ O₂ -responsive theranostic platform comprising thiophene-based small molecule (2SeFT-PEG) and photosensitizer Chlorin e6 (Ce6) micelles for real-time monitoring PDT efficacy is developed. After laser irradiation, the Ce6-produced ¹ O₂ could simultaneously kill cancer and trigger 2SeFT-PEG to produce increased chemiluminescence (CL) and decreased fluorescence (FL) signals variation at 1050 nm in the second near-infrared (NIR-II, 950-1700 nm) window. Significantly, the ratiometric NIR-II CL/FL imaging at 1050 nm could effectively quantify and monitor the concentration of ¹ O₂ and O₂ consumption or recovery, so as to evaluate the therapeutic efficacy of PDT in vivo. Hence, this ¹ O₂ activated NIR-II CL/FL probe provides an efficient ratiometric optical imaging platform for real-time evaluating PDT effect and precisely guiding the PDT process in vivo.

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.

Chemical Probes and Activity-Based Protein Profiling for Cancer Research

Chemical probes can be used to understand the complex biological nature of diseases. Due to the diversity of cancer types and dynamic regulatory pathways involved in the disease, there is a need to identify signaling pathways and associated proteins or enzymes that are traceable or detectable in tests for cancer diagnosis and treatment. Currently, fluorogenic chemical probes are widely used to detect cancer-associated proteins and their binding partners. These probes are also applicable in photodynamic therapy to determine drug efficacy and monitor regulating factors. In this review, we discuss the synthesis of chemical probes for different cancer types from 2016 to the present time and their application in monitoring the activity of transferases, hydrolases, deacetylases, oxidoreductases, and immune cells. Moreover, we elaborate on their potential roles in photodynamic therapy.

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.

Selective detection of ozone in inflamed mice using a novel activatable chemiluminescent probe

We report here an activatable chemiluminescent probe CL-O₃ for the high-contrast imaging of O₃in vivo. CL-O₃ exhibited a high selectivity toward O₃ and was able to evaluate the degree of inflammation in mice by detecting endogenous O₃ levels in acute inflamed mice.

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.

Activity-Based Diagnostics: Recent Advances in the Development of Probes for Use with Diverse Detection Modalities

Abnormal enzyme expression and activity is a hallmark of many diseases. Activity-based diagnostics are a class of chemical probes that aim to leverage this dysregulated metabolic signature to produce a detectable signal specific to diseased tissue. In this Review, we highlight recent methodologies employed in activity-based diagnostics that provide exquisite signal sensitivity and specificity in complex biological systems for multiple disease states. We divide these examples based upon their unique signal readout modalities and highlight those that have advanced into clinical trials.

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.

A two-photon fluorescence probe with endoplasmic reticulum targeting ability for turn-on sensing photosensitized singlet oxygen in living cells and brain tissues

Endoplasmic reticulum (ER) is an indispensable organelle responsible for protein synthesis, transportation, and maintenance of Ca²⁺ homeostasis in eukaryotic cells. Recent studies highlighted that ER-targeted photosensitizers with high yield of singlet oxygen (¹O₂) are effective in selectively disrupting ER function and are promising candidates for anticancer therapy. Unfortunately, no ER targetable fluorescent probes for determining ¹O₂ photosensitized in this photodynamic therapy process is available. In this work, we synthesized an ER-targetable, two-photon fluorescence probe, ER-¹O₂, for fluorescence turn-on sensing of ¹O₂. ER-¹O₂ demonstrated high sensitivity to ¹O₂ sensing with a wide detection range (0-2.75 μM) and a low detection limit (0.11 μM). ER-¹O₂ also displayed excellent selectivity toward ¹O₂ out of other ROS and metal ions. Notably, ER-¹O₂ exhibited low cytotoxicity but with specific ER targetable capability. On account of these advantageous features, fluctuations of ¹O₂ in living cells and brain tissues were effectively visualized by ER-¹O₂.

Accelerated antibacterial red-carbon dots with photodynamic therapy against multidrug-resistant Acinetobacter baumannii

The emergence of antibiotic resistance in bacteria is a major public-health issue. Synthesis of efficient antibiotic-free material is very important for fighting bacterial infection-related diseases. Herein, red-carbon dots (R-CDs) with a broad range of spectral absorption (350-700 nm) from organic bactericides or intermediates were synthesized through a solvothermal route. The prepared R-CDs not only had intrinsic antibacterial activities, but also could kill multidrug-resistant bacteria (multidrug-resistant Acinetobacter baumannii (MRAB) and multidrug-resistant Staphylococcus aureus (MRSA)) effectively by generating reactive oxygen species. Furthermore, R-CDs could eliminate and inhibit the formation of MRAB biofilms, while conferring few side effects on normal cells. A unique property of R-CDs was demonstrated upon in vivo treatment of antibiotic-sensitive MRAB-induced infected wounds. These data suggested that this novel R-CDs-based strategy might enable the design of next-generation agents to fight drug-resistant bacteria.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material is available for this article at 10.1007/s40843-021-1770-0 and is accessible for authorized users.

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.

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.

Metal enhanced chemiluminescence nanosensor for ultrasensitive bioassay based on silver nanoparticles modified functional DNA dendrimer

A novel metal enhanced chemiluminescence (MEC) nanosensor was developed for ultrasensitive biosensing and imaging, based on functional DNA dendrimer (FDD), proximity-dependent DNAzyme and silver nanoparticles (AgNPs). The FDD containing two split G-quadruplex structures was prepared through an enzyme-free and step-by-step assembly strategy, and then reacted with AgNPs and hemin molecules to form the FDD/hemin/AgNPs facilely. Such a MEC nanosensor consisted of three modules: FDD (scaffold), the generated G-quadruplex/hemin DNAzyme (signal reporter) and AgNPs (chemiluminescence enhancer). The MEC effect was achieved by controlling the length of DNA sequences between AgNPs on the periphery of FDD and DNAzymes inside it. Such nanosensor exhibited 9-fold amplification and another 6.4-fold metal enhancement in chemiluminescence intensity, which can be easily applied into trace detection of multiple protein markers using a disposable protein immunoarray. The FDD/hemin/AgNPs-based multiplex MEC imaging assay showed wide linear ranges over 5 orders of magnitude and detection limits down to 5× 10^-5 ng L^-1 and 1.8 × 10^-4 U mL^-1 for cardiac troponin T and carcinoma antigen 125, demonstrating a promising potential in application to protein analysis and clinical diagnosis. Moreover, the MEC nanosensor can be effectively delivered into cells with excellent biocompatibility and outstanding stability, offering a new tool for detection of intracellular targets and suggesting wide applications in bioassay.