Recent progress in expanding the chemiluminescent toolbox for bioimaging

A novel dual-activatable ultrasensitive chemiluminescent probe for mercury (II) monitoring: From rational design to multiple application

Real-time optical sensing of mercury has been developed rapidly in recent years but remains challenging such as bearing background interference. Herein, a Hg2+ and base dual-activatable ultrasensitive chemiluminescent probe CL-Hg based on benzothiazole-phenoxyl-dioxetane with profits of excitation light-free and minimal interference is presented. The photophysical properties study and sensing performance verified CL-Hg is coupled with unique advantages of long-term detection (more than 400 min), ultrahigh sensitivity (LOD = 0.52 nM), and high specificity to Hg2+, and visualization detection by the paper-based test strips. More importantly, CL-Hg showed the qualitative and quantitative detection capability for Hg2+ with great recyclability in real samples of water, seafood, and beverages, holding great potential for on-site monitoring of Hg2+ levels in the actual samples. To our knowledge, this is the first work achieving the detection of Hg2+ by chemiluminescence. Overall, the Hg2+-activated visualization platform offers a practical method for detecting Hg2+ in various application scenarios.

Fluorescent Protein-Based Sensors for Detecting Essential Metal Ions across the Tree of Life

Genetically encoded fluorescent metal ion sensors are powerful tools for elucidating metal dynamics in living systems. Over the last 25 years since the first examples of genetically encoded fluorescent protein-based calcium indicators, this toolbox of probes has expanded to include other essential and non-essential metal ions. Collectively, these tools have illuminated fundamental aspects of metal homeostasis and trafficking that are crucial to fields ranging from neurobiology to human nutrition. Despite these advances, much of the application of metal ion sensors remains limited to mammalian cells and tissues and a limited number of essential metals. Applications beyond mammalian systems and in vivo applications in living organisms have primarily used genetically encoded calcium ion sensors. The aim of this Perspective is to provide, with the support of historical and recent literature, an updated and critical view of the design and use of fluorescent protein-based sensors for detecting essential metal ions in various organisms. We highlight the historical progress and achievements with calcium sensors and discuss more recent advances and opportunities for the detection of other essential metal ions. We also discuss outstanding challenges in the field and directions for future studies, including detecting a wider variety of metal ions, developing and implementing a broader spectral range of sensors for multiplexing experiments, and applying sensors to a wider range of single- and multi-species biological systems.

Molecularly generated light and its biomedical applications

Molecularly generated light, referred to here as "molecular light", mainly includes bioluminescence, chemiluminescence, and Cerenkov luminescence. Molecular light possesses unique dual features of being both a molecule and a source of light. Its molecular nature enables it to be delivered as molecules to regions deep within the body, overcoming the limitations of natural sunlight and physically generated light sources like lasers and LEDs. Simultaneously, its light properties make it valuable for applications such as imaging, photodynamic therapy, photo-oxidative therapy, and photobiomodulation. In this review article, we provide an updated overview of the diverse applications of molecular light and discuss the strengths and weaknesses of molecular light across various domains. Lastly, we present forward-looking perspectives on the potential of molecular light in the realms of molecular imaging, photobiological mechanisms, therapeutic applications, and photobiomodulation. While some of these perspectives may be considered bold and contentious, our intent is to inspire further innovations in the field of molecular light applications.

Photoactivation of LOV domains with chemiluminescence

Optogenetics has opened new possibilities in the remote control of diverse cellular functions with high spatiotemporal precision using light. However, delivering light to optically non-transparent systems remains a challenge. Here, we describe the photoactivation of light-oxygen-voltage-sensing domains (LOV domains) with in situ generated light from a chemiluminescence reaction between luminol and H2O2. This activation is possible due to the spectral overlap between the blue chemiluminescence emission and the absorption bands of the flavin chromophore in LOV domains. All four LOV domain proteins with diverse backgrounds and structures (iLID, BcLOV4, nMagHigh/pMagHigh, and VVDHigh) were photoactivated by chemiluminescence as demonstrated using a bead aggregation assay. The photoactivation with chemiluminescence required a critical light-output below which the LOV domains reversed back to their dark state with protein characteristic kinetics. Furthermore, spatially confined chemiluminescence produced inside giant unilamellar vesicles (GUVs) was able to photoactivate proteins both on the membrane and in solution, leading to the recruitment of the corresponding proteins to the GUV membrane. Finally, we showed that reactive oxygen species produced by neutrophil like cells can be converted into sufficient chemiluminescence to recruit the photoswitchable protein BcLOV4-mCherry from solution to the cell membrane. The findings highlight the utility of chemiluminescence as an endogenous light source for optogenetic applications, offering new possibilities for studying cellular processes in optically non-transparent systems.

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.

Photophysical Mechanisms of Photobiomodulation Therapy as Precision Medicine

Despite a significant focus on the photochemical and photoelectrical mechanisms underlying photobiomodulation (PBM), its complex functions are yet to be fully elucidated. To date, there has been limited attention to the photophysical aspects of PBM. One effect of photobiomodulation relates to the non-visual phototransduction pathway, which involves mechanotransduction and modulation to cytoskeletal structures, biophotonic signaling, and micro-oscillatory cellular interactions. Herein, we propose a number of mechanisms of PBM that do not depend on cytochrome c oxidase. These include the photophysical aspects of PBM and the interactions with biophotons and mechanotransductive processes. These hypotheses are contingent on the effect of light on ion channels and the cytoskeleton, the production of biophotons, and the properties of light and biological molecules. Specifically, the processes we review are supported by the resonant recognition model (RRM). This previous research demonstrated that protein micro-oscillations act as a signature of their function that can be activated by resonant wavelengths of light. We extend this work by exploring the local oscillatory interactions of proteins and light because they may affect global body circuits and could explain the observed effect of PBM on neuro-cortical electroencephalogram (EEG) oscillations. In particular, since dysrhythmic gamma oscillations are associated with neurodegenerative diseases and pain syndromes, including migraine with aura and fibromyalgia, we suggest that transcranial PBM should target diseases where patients are affected by impaired neural oscillations and aberrant brain wave patterns. This review also highlights examples of disorders potentially treatable with precise wavelengths of light by mimicking protein activity in other tissues, such as the liver, with, for example, Crigler-Najjar syndrome and conditions involving the dysregulation of the cytoskeleton. PBM as a novel therapeutic modality may thus behave as "precision medicine" for the treatment of various neurological diseases and other morbidities. The perspectives presented herein offer a new understanding of the photophysical effects of PBM, which is important when considering the relevance of PBM therapy (PBMt) in clinical applications, including the treatment of diseases and the optimization of health outcomes and performance.

Organic persistent luminescence imaging for biomedical applications

Persistent luminescence is a unique visual phenomenon that occurs after cessation of excitation light irradiation or following oxidization of luminescent molecules. The energy stored within the molecule is released in a delayed manner, resulting in luminescence that can be maintained for seconds, minutes, hours, or even days. Organic persistent luminescence materials (OPLMs) are highly robust and their facile modification and assembly into biocompatible nanostructures makes them attractive tools for in vivo bioimaging, whilst offering an alternative to conventional fluorescence imaging materials for biomedical applications. In this review, we give attention to the existing limitations of each class of OPLM-based molecular bioimaging probes based on their luminescence mechanisms, and how recent research progress has driven efforts to circumvent their shortcomings. We discuss the multifunctionality-focused design strategies, and the broad biological application prospects of these molecular probes. Furthermore, we provide insights into the next generation of OPLMs being developed for bioimaging techniques.

Development of a rapid, simple, and sensitive point-of-care technology platform utilizing ternary NanoLuc

Point-of-care tests are highly valuable in providing fast results for medical decisions for greater flexibility in patient care. Many diagnostic tests, such as ELISAs, that are commonly used within clinical laboratory settings require trained technicians, laborious workflows, and complex instrumentation hindering their translation into point-of-care applications. Herein, we demonstrate the use of a homogeneous, bioluminescent-based, split reporter platform that enables a simple, sensitive, and rapid method for analyte detection in clinical samples. We developed this point-of-care application using an optimized ternary, split-NanoLuc luciferase reporter system that consists of two small reporter peptides added as appendages to analyte-specific affinity reagents. A bright, stable bioluminescent signal is generated as the affinity reagents bind to the analyte, allowing for proximity-induced complementation between the two reporter peptides and the polypeptide protein, in addition to the furimazine substrate. Through lyophilization of the stabilized reporter system with the formulated substrate, we demonstrate a shelf-stable, all-in-one, add-and-read analyte-detection system for use in complex sample matrices at the point-of-care. We highlight the modularity of this platform using two distinct SARS-CoV-2 model systems: SARS-CoV-2 N-antigen detection for active infections and anti-SARS-CoV-2 antibodies for immunity status detection using chemically conjugated or genetically fused affinity reagents, respectively. This technology provides a simple and standardized method to develop rapid, robust, and sensitive analyte-detection assays with flexible assay formatting making this an ideal platform for research, clinical laboratory, as well as point-of-care applications utilizing a simple handheld luminometer.

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.

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.

A plug-and-play platform of ratiometric bioluminescent sensors for homogeneous immunoassays

Heterogeneous immunoassays such as ELISA have become indispensable in modern bioanalysis, yet translation into point-of-care assays is hindered by their dependence on external calibration and multiple washing and incubation steps. Here, we introduce RAPPID (Ratiometric Plug-and-Play Immunodiagnostics), a mix-and-measure homogeneous immunoassay platform that combines highly specific antibody-based detection with a ratiometric bioluminescent readout. The concept entails analyte-induced complementation of split NanoLuc luciferase fragments, photoconjugated to an antibody sandwich pair via protein G adapters. Introduction of a calibrator luciferase provides a robust ratiometric signal that allows direct in-sample calibration and quantitative measurements in complex media such as blood plasma. We developed RAPPID sensors that allow low-picomolar detection of several protein biomarkers, anti-drug antibodies, therapeutic antibodies, and both SARS-CoV-2 spike protein and anti-SARS-CoV-2 antibodies. With its easy-to-implement standardized workflow, RAPPID provides an attractive, fast, and low-cost alternative to traditional immunoassays, in an academic setting, in clinical laboratories, and for point-of-care applications.

Many current immunoassays require multiple washing, incubation and optimization steps. Here the authors present Ratiometric Plug-and-Play Immunodiagnostics (RAPPID), a generic assay platform that uses ratiometric bioluminescent detection to allow sandwich immunoassays to be performed directly in solution.

The role of nitric oxide in ocular surface physiology and pathophysiology

Nitric oxide (NO) has a wide array of biological functions including the regulation of vascular tone, neurotransmission, immunomodulation, stimulation of proinflammatory cytokine expression and antimicrobial action. These functions may depend on the type of isoform that is responsible for the synthesis of NO. NO is found in various ocular tissues playing a pivotal role in physiological mechanisms, namely regulating vascular tone in the uvea, retinal blood circulation, aqueous humor dynamics, neurotransmission and phototransduction in retinal layers. Unregulated production of NO in ocular tissues may result in production of toxic superoxide free radicals that participate in ocular diseases such as endotoxin-induced uveitis, ischemic proliferative retinopathy and neurotoxicity of optic nerve head in glaucoma. However, the role of NO on the ocular surface in mediating physiology and pathophysiological processes is not fully understood. Moreover, methods used to measure levels of NO in the biological samples of the ocular surface are not well established due to its rapid oxidation. The purpose of this review is to highlight the role of NO in the physiology and pathophysiology of ocular surface and propose suitable techniques to measure NO levels in ocular surface tissues and tears. This will improve the understanding of NO's role in ocular surface biology and the development of new NO-based therapies to treat various ocular surface diseases. Further, this review summarizes the biochemistry underpinning NO's antimicrobial action.

Direct Comparison of Triggering Motifs on Chemiluminescent Probes for Hydrogen Sulfide Detection in Water

Hydrogen sulfide (H2S) is an important biomolecule and significant efforts have focused on developing chemical tools to aid different biological investigations. Of such tools, there are relatively few chemiluminescent or bioluminescent methods for H2S detection. Here we report two dioxetane-based chemiluminescent probes for H2S detection. With these probes, we directly compare the probe response to H2S-mediated azide reduction and nucleophilic displacement of 2,4-dinitrophenyl motifs and demonstrate that the SNAr cleavage of the DNP group results in a larger response and greater stability in water.

Untapped toolbox of luminol based polymers

The present state of luminol based polymers is highlighted with the challenges and the respective strategies for the advancement of this versatile class of polymers.

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

Singlet oxygen (¹O₂) plays a vital role in metabolism. However, because of its extremely high reactivity and short-lived state, the in vivo detection of ¹O₂ is challenging. To address this issue, for the first time, we herein constructed a near-infrared (NIR) chemiluminescent probe (CL-SO) by caging the precursor of phenoxy-dioxetane scaffolds and a dicyanomethylchromone acceptor for selective ¹O₂ detection. This probe can detect ¹O₂ in vitro with a tremendous turn-on chemiluminescence signal in the NIR region (700 nm) and image intracellular ¹O₂ produced by the photosensitizer during the simulated action of photodynamic therapy (PDT). Notably, ¹O₂ level changes in the abdominal cavity and tumor of the various mice model under different stimulations and PDT action were effectively monitored by CL-SO, providing a novel chemiluminescence imaging platform to explore ¹O₂ generation in PDT-associated applications.

RedquorinXS Mutants with Enhanced Calcium Sensitivity and Bioluminescence Output Efficiently Report Cellular and Neuronal Network Activities

Considerable efforts have been focused on shifting the wavelength of aequorin Ca²⁺-dependent blue bioluminescence through fusion with fluorescent proteins. This approach has notably yielded the widely used GFP-aequorin (GA) Ca²⁺ sensor emitting green light, and tdTomato-aequorin (Redquorin), whose bioluminescence is completely shifted to red, but whose Ca²⁺ sensitivity is low. In the present study, the screening of aequorin mutants generated at twenty-four amino acid positions in and around EF-hand Ca²⁺-binding domains resulted in the isolation of six aequorin single or double mutants (AequorinXS) in EF2, EF3, and C-terminal tail, which exhibited markedly higher Ca²⁺ sensitivity than wild-type aequorin in vitro. The corresponding Redquorin mutants all showed higher Ca²⁺ sensitivity than wild-type Redquorin, and four of them (RedquorinXS) matched the Ca²⁺ sensitivity of GA in vitro. RedquorinXS mutants exhibited unaltered thermostability and peak emission wavelengths. Upon stable expression in mammalian cell line, all RedquorinXS mutants reported the activation of the P2Y2 receptor by ATP with higher sensitivity and assay robustness than wt-Redquorin, and one, RedquorinXS-Q159T, outperformed GA. Finally, wide-field bioluminescence imaging in mouse neocortical slices showed that RedquorinXS-Q159T and GA similarly reported neuronal network activities elicited by the removal of extracellular Mg²⁺. Our results indicate that RedquorinXS-Q159T is a red light-emitting Ca²⁺ sensor suitable for the monitoring of intracellular signaling in a variety of applications in cells and tissues, and is a promising candidate for the transcranial monitoring of brain activities in living mice.

Synthetic protein switches: Combinatorial linker engineering with iFLinkC

Linker engineering constitutes a critical, yet frequently underestimated aspect in the construction of synthetic protein switches and sensors. Notably, systematic strategies to engineer linkers by predictive means remain largely elusive to date. This is primarily due to our insufficient understanding how the biophysical properties that underlie linker functions mediate the conformational transitions in artificially engineered protein switches and sensors. The construction of synthetic protein switches and sensors therefore heavily relies on experimental trial-and-error. Yet, methods for effectively generating linker diversity at the genetic level are scarce. Addressing this technical shortcoming, iterative functional linker cloning (iFLinkC) enables the combinatorial assembly of linker elements with functional domains from sequence verified repositories that are developed and stored in-house. The assembly process is highly scalable and given its recursive nature generates linker diversity in a combinatorial and exponential fashion based on a limited number of linker elements.

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.

Chemiluminescence for bioimaging and therapeutics: recent advances and challenges

The current progress, design principles in bioimaging and therapeutic applications, and future perspectives of various chemiluminescent platforms are reviewed.

Ultrasensitive Chemiluminescence Biosensor for Nuclease and Bacterial Determination Based on Hemin-Encapsulated Mesoporous Silica Nanoparticles

Bacterial determination, emerging as a critical step in the understanding of increasingly serious bacterial contaminations, remains a major challenge. Herein, a novel chemiluminescence biosensor was exploited for the ultrasensitive determination of nuclease activity and bacteria, in which, hemin, the chemiluminescent (CL) tag molecule was encapsulated into ordered mesopores of mesoporous silica nanoparticles with a specific DNA gate. The capped DNA could be specifically switched upon exposure to the DNA nuclease or bacterial lysate and allowed for an increased release of the encapsulated hemin, which therefore resulted in an obviously enhanced CL signal for the luminol-H₂O₂ system. Attributed to this unique behavior with the linear or sigmoidal relationship between CL intensity and DNA nuclease or bacterial concentration, the as-prepared CL biosensor could detect S1 nuclease activity in the concentration range 0.01-10.0 U with a detection limit of 0.1 mU, and Escherichia coli O157:H7 (E. coli) or Staphylococcus aureus (S. aureus) in the concentration ranges 10¹ to 10⁹ cfu mL^-1. The detection limit of E. coli and S. aureus was calculated to be 3.0 and 2.5 cfu mL^-1, respectively, which was comparable or even better than that of previous studies. Thus, this detection method could achieve detectable levels without cell enrichment overnight. Moreover, the proposed biosensing system could be conducted in the homogeneous solution without separation and washing, greatly improving the reaction efficiency and simplifying the procedure. As expected, the novel CL biosensor promised a great potential for simple and convenient detection of nuclease and bacteria in fields such as food bacterial contamination, pharmaceuticals, and clinical analysis.

Self-Illuminating Agents for Deep-Tissue Optical Imaging

Optical imaging plays an indispensable role in biology and medicine attributing to its noninvasiveness, high spatiotemporal resolution, and high sensitivity. However, as a conventional optical imaging modality, fluorescence imaging confronts issues of shallow imaging depth due to the need for real-time light excitation which produces tissue autofluorescence. By contrast, self-luminescence imaging eliminates the concurrent light excitation, permitting deeper imaging depth and higher signal-to-background ratio (SBR), which has attracted growing attention. Herein, this review summarizes the progress on the development of near-infrared (NIR) emitting self-luminescence agents in deep-tissue optical imaging with highlighting the design principles including molecular- and nano-engineering approaches. Finally, it discusses current challenges and guidelines to develop more effective self-illuminating agents for biomedical diagnosis and treatment.

Development and Applications of Bioluminescent and Chemiluminescent Reporters and Biosensors

Although fluorescent reporters and biosensors have become indispensable tools in biological and biomedical fields, fluorescence measurements require external excitation light, thereby limiting their use in thick tissues and live animals. Bioluminescent reporters and biosensors may potentially overcome this hurdle because they use enzyme-catalyzed exothermic biochemical reactions to generate excited-state emitters. This review first introduces the development of bioluminescent reporters, and next, their applications in sensing biological changes in vitro and in vivo as biosensors. Lastly, we discuss chemiluminescent sensors that produce photons in the absence of luciferases. This review aims to explore fundamentals and experimental insights and to emphasize the yet-to-be-reached potential of next-generation luminescent reporters and biosensors.

Chemiluminescence and Bioluminescence Imaging for Biosensing and Therapy: In Vitro and In Vivo Perspectives

Chemiluminescence (CL) and bioluminescence (BL) imaging technologies, which require no external light source so as to avoid the photobleaching, background interference and autoluminescence, have become powerful tools in biochemical analysis and biomedical science with the development of advanced imaging equipment. CL imaging technology has been widely applied to high-throughput detection of a variety of analytes because of its high sensitivity, high efficiency and high signal-to-noise ratio (SNR). Using luciferase and fluorescent proteins as reporters, various BL imaging systems have been developed innovatively for real-time monitoring of diverse molecules in vivo based on the reaction between luciferin and the substrate. Meanwhile, the kinetics of protein interactions even in deep tissues has been studied by BL imaging. In this review, we summarize in vitro and in vivo applications of CL and BL imaging for biosensing and therapy. We first focus on in vitro CL imaging from the view of improving the sensitivity. Then, in vivo CL applications in cells and tissues based on different CL systems are demonstrated. Subsequently, the recent in vitro and in vivo applications of BL imaging are summarized. Finally, we provide the insight into the development trends and future perspectives of CL and BL imaging technologies.

A generic approach towards afterglow luminescent nanoparticles for ultrasensitive in vivo imaging

Afterglow imaging with long-lasting luminescence after cessation of light excitation provides opportunities for ultrasensitive molecular imaging; however, the lack of biologically compatible afterglow agents has impeded exploitation in clinical settings. This study presents a generic approach to transforming ordinary optical agents (including fluorescent polymers, dyes, and inorganic semiconductors) into afterglow luminescent nanoparticles (ALNPs). This approach integrates a cascade photoreaction into a single-particle entity, enabling ALNPs to chemically store photoenergy and spontaneously decay it in an energy-relay process. Not only can the afterglow profiles of ALNPs be finetuned to afford emission from visible to near-infrared (NIR) region, but also their intensities can be predicted by a mathematical model. The representative NIR ALNPs permit rapid detection of tumors in living mice with a signal-to-background ratio that is more than three orders of magnitude higher than that of NIR fluorescence. The biodegradability of the ALNPs further heightens their potential for ultrasensitive in vivo imaging.

Afterglow luminescence is used to reduce background noise and increase sensitivity; however, biocompatible afterglow materials are limited. Here, the authors report on an approach to turn standard optical agents into afterglow nanoparticles and demonstrate the application in tumour imagining in vivo.

Synthesis and characterization of ethyl benzotriazolyl acrylate-based D-π-A fluorophores for live cell-based imaging applications

A series of eight new ethyl (Z)-benzotriazolyl acrylates 6a-d and 7a-d have been synthesized by conventional heating and microwave irradiation from ethyl benzotriazolyl acetates 3 and 4 with the corresponding aromatic aldehydes. This work reports the synthetic approach and spectroscopic characterization (1H, 13C-NMR, HRMS) of all the synthesized compounds. X-ray diffraction analyses were performed for molecules 6a, 7a and 7d. Photophysical properties of compounds were evaluated. Finally, compound 6a was tested in a human cell line and showed low to no cytotoxicity at relevant concentrations. Initial testing demonstrates its potential use as a fluid-phase fluorescent marker for live cell imaging.

Significance of PGR5-dependent cyclic electron flow for optimizing the rate of ATP synthesis and consumption in Arabidopsis chloroplasts

The proton motive force (PMF) across the chloroplast thylakoid membrane that is generated by electron transport during photosynthesis is the driving force for ATP synthesis in plants. The PMF mainly arises from the oxidation of water in photosystem II and from electron transfer within the cytochrome b₆f complex. There are two electron transfer pathways related to PMF formation: linear electron flow and cyclic electron flow. Proton gradient regulation 5 (PGR5) is a major component of the cyclic electron flow pathway, and the Arabidopsis pgr5 mutant shows a substantial reduction in the PMF. How the PGR5-dependent cyclic electron flow contributes to ATP synthesis has not, however, been fully delineated. In this study, we monitored in vivo ATP levels in Arabidopsis chloroplasts in real time using a genetically encoded bioluminescence-based ATP indicator, Nano-lantern(ATP1). The increase in ATP in the chloroplast stroma of pgr5 leaves upon illumination with actinic light was significantly slower than in wild type, and the decrease in ATP levels when this illumination stopped was significantly faster in pgr5 leaves than in wild type. These results indicated that PGR5-dependent cyclic electron flow around photosystem I helps to sustain the rate of ATP synthesis, which is important for growth under fluctuating light conditions.

Ratiometric Bioluminescent Sensor Proteins Based on Intramolecular Split Luciferase Complementation

Bioluminescent sensor proteins provide attractive tools for applications ranging from in vivo imaging to point-of-care testing. Here we introduce a new class of ratiometric bioluminescent sensor proteins that do not rely on direct modulation of BRET efficiency, but are based on competitive intramolecular complementation of split NanoLuc luciferase. Proof of concept for the feasibility of this sensor principle was provided by developing a blue-red light emitting sensor protein for the detection of anti-HIV1-p17 antibodies with a 500% change in emission ratio and a K_d of 10 pM. The new sensor design also improved the dynamic response of a sensor for the therapeutic antibody cetuximab 4-fold, allowing the direct quantification of this anti-EGFR antibody in undiluted blood plasma. The modular sensor architecture allows easy and systematic tuning of a sensor's dynamic range and should be generally applicable to allow rational engineering of bioluminescent sensor proteins.

How do Plants Keep their Functional Integrity?

Unlike animals, plants possess a non-strict and sometimes very fuzzy morphology. Mutual proportions of plant parts can vary to a much greater extent than in animals, changing according to the environmental conditions and the plant needs of nutrients, water and light. Despite the existence of this fundamental difference between plants and animals, it passes almost non-reflected in most studies on plants. In this review we make a preliminary attempt to gather together the mechanisms by which plants preserve their integrity, not loosing at the same time the physiological (and morphological) flexibility which allows them adapting to the different environments they can populate.

Bioluminescent Low-Affinity Ca2+ Indicator for ER with Multicolor Calcium Imaging in Single Living Cells

The sarco/endoplasmic reticulum (SR/ER) is the foremost intercellular Ca²⁺ store (at submillimolar concentrations), playing a crucial role in controlling intracellular Ca²⁺ levels. For the investigation of SR/ER Ca²⁺ dynamics in cells, fluorescent protein-based genetically encoded calcium indicators (GECIs) with low Ca²⁺ affinity have been used. Recently, bioluminescent protein-based GECIs with high brightness have been reported to counter the constraints of fluorescence imaging, such as phototoxicity. However, their Ca²⁺ affinity is high and limited for imaging in the cytosol, nucleus, or mitochondria. In this study, we developed a novel cyan color, low-affinity ( K_d = 110 μM) intensiometric bioluminescent GECI, which enables monitoring of the Ca²⁺ dynamics in the ER of HeLa cells and the SR of C2C12-derived myotubes. To facilitate the broad concentration range of Ca²⁺ in cellular organelles, we additionally developed an intermediate affinity ( K_d = 18 μM), orange color, and bioluminescent GECI, which enables monitoring of Ca²⁺ dynamics in the mitochondria of HeLa cells. With these indicators, in conjunction with an existing high-affinity, green, bioluminescent GECI, we succeeded in multicolor bioluminescent Ca²⁺ imaging in three distinct organelles (nuclei, mitochondria, and ER) simultaneously. The multicolor, live, bioluminescent Ca²⁺ imaging demonstrated here can be used to stably reveal the ER Ca²⁺ homeostasis and cooperative Ca²⁺ regulation among organelles. This will lead to the further understanding of Ca²⁺-related physiological functions and pathophysiological mechanisms.

Fluorescent Proteins for Investigating Biological Events in Acidic Environments

The interior lumen of acidic organelles (e.g., endosomes, secretory granules, lysosomes and plant vacuoles) is an important platform for modification, transport and degradation of biomolecules as well as signal transduction, which remains challenging to investigate using conventional fluorescent proteins (FPs). Due to the highly acidic luminal environment (pH ~ 4.5⁻6.0), most FPs and related sensors are apt to lose their fluorescence. To address the need to image in acidic environments, several research groups have developed acid-tolerant FPs in a wide color range. Furthermore, the engineering of pH insensitive sensors, and their concomitant use with pH sensitive sensors for the purpose of pH-calibration has enabled characterization of the role of luminal ions. In this short review, we summarize the recent development of acid-tolerant FPs and related functional sensors and discuss the future prospects for this field.

Ratiometric optical nanoprobes enable accurate molecular detection and imaging

Exploring and understanding biological and pathological changes are of great significance for early diagnosis and therapy of diseases. Optical sensing and imaging approaches have experienced major progress in this field. Particularly, an emergence of various functional optical nanoprobes has provided enhanced sensitivity, specificity, targeting ability, as well as multiplexing and multimodal capabilities due to improvements in their intrinsic physicochemical and optical properties. However, one of the biggest challenges of conventional optical nanoprobes is their absolute intensity-dependent signal readout, which causes inaccurate sensing and imaging results due to the presence of various analyte-independent factors that can cause fluctuations in their absolute signal intensity. Ratiometric measurements provide built-in self-calibration for signal correction, enabling more sensitive and reliable detection. Optimizing nanoprobe designs with ratiometric strategies can surmount many of the limitations encountered by traditional optical nanoprobes. This review first elaborates upon existing optical nanoprobes that exploit ratiometric measurements for improved sensing and imaging, including fluorescence, surface enhanced Raman scattering (SERS), and photoacoustic nanoprobes. Next, a thorough discussion is provided on design strategies for these nanoprobes, and their potential biomedical applications for targeting specific biomolecule populations (e.g. cancer biomarkers and small molecules with physiological relevance), for imaging the tumor microenvironment (e.g. pH, reactive oxygen species, hypoxia, enzyme and metal ions), as well as for intraoperative image guidance of tumor-resection procedures.