Activatable fluorescence sensors for in vivo bio-detection in the second near-infrared window

Fluorescence imaging-guided surgery: current status and future directions

Surgery is one of the most important paradigms for tumor therapy, while fluorescence imaging (FI) offers real-time intraoperative guidance, greatly boosting treatment prognosis. The imaging fidelity heavily relies on not only imaging facilities but also probes for imaging-guided surgery (IGS). So far, a great number of IGS probes with emission in visible (400-700 nm) and near-infrared (NIR 700-1700 nm) windows have been developed for pinpointing disease margins intraoperatively. Herein, the state-of-the-art fluorescent probes for IGS are timely updated, with a special focus on the fluorescent probes under clinical examination. For a better demonstration of the superiority of NIR FI over visible FI, both imaging modalities are critically compared regarding signal-to-background ratio, penetration depth, resolution, tissue autofluorescence, photostability, and biocompatibility. Various types of fluorescence IGS have been summarized to demonstrate its importance in the medical field. Furthermore, the most recent progress of fluorescent probes in NIR-I and NIR-II windows is summarized. Finally, an outlook on multimodal imaging, FI beyond NIR-II, efficient tumor targeting, automated IGS, the use of AI and machine learning for designing fluorescent probes, and the fluorescence-guided da Vinci surgical system is given. We hope this review will stimulate interest among researchers in different areas and expedite the translation of fluorescent probes from bench to bedside.

NIR-II Fluorescence Sensor Based on Steric Hindrance Regulated Molecular Packing for In Vivo Epilepsy Visualization

Fluorescence sensing is crucial to studying biological processes and diagnosing diseases, especially in the second near-infrared (NIR-II) window with reduced background signals. However, it's still a great challenge to construct "off-on" sensors when the sensing wavelength extends into the NIR-II region to obtain higher imaging contrast, mainly due to the difficult synthesis of spectral overlapped quencher. Here, we present a new fluorescence quenching strategy, which utilizes steric hindrance quencher (SHQ) to tune the molecular packing state of fluorophores and suppress the emission signal. Density functional theory (DFT) calculations further reveal that large SHQs can competitively pack with fluorophores and prevent their self-aggregation. Based on this quenching mechanism, a novel activatable "off-on" sensing method is achieved via bio-analyte responsive invalidation of SHQ, namely the Steric Hindrance Invalidation geNerated Emission (SHINE) strategy. As a proof of concept, the ClO--sensitive SHQ lead to the bright NIR-II signal release in epileptic mouse hippocampus under the skull and high photon scattering brain tissue, providing the real-time visualization of ClO- generation process in living epileptic mice.

A firm-push-to-open and light-push-to-lock strategy for a general chemical platform to develop activatable dual-modality NIR-II probes

Activatable near-infrared (NIR) imaging in the NIR-II range is crucial for deep tissue bioanalyte tracking. However, designing such probes remains challenging due to the limited availability of general chemical strategies. Here, we introduced a foundational platform for activatable probes, using analyte-triggered smart modulation of the π-conjugation system of a NIR-II-emitting rhodamine hybrid. By tuning the nucleophilicity of the ortho-carboxy moiety, we achieved an electronic effect termed "firm-push-to-open and light-push-to-lock," which enables complete spirocyclization of the probe before sensing and allows for efficient zwitterion formation when the light-pushing aniline carbamate trigger is transformed into a firm-pushing aniline. This platform produces dual-modality NIR-II imaging probes with ~50-fold fluorogenic and activatable photoacoustic signals in live mice, surpassing reported probes with generally below 10-fold activatable signals. Demonstrating generality, we successfully designed probes for hydrogen peroxide (H2O2) and hydrogen sulfide (H2S). We envision a widespread adoption of the chemical platform for designing activatable NIR-II probes across diverse applications.

NIR-II light in clinical oncology: opportunities and challenges

Novel strategies utilizing light in the second near-infrared region (NIR-II; 900-1,880 nm wavelengths) offer the potential to visualize and treat solid tumours with enhanced precision. Over the past few decades, numerous techniques leveraging NIR-II light have been developed with the aim of precisely eliminating tumours while maximally preserving organ function. During cancer surgery, NIR-II optical imaging enables the visualization of clinically occult lesions and surrounding vital structures with increased sensitivity and resolution, thereby enhancing surgical quality and improving patient prognosis. Furthermore, the use of NIR-II light promises to improve cancer phototherapy by enabling the selective delivery of increased therapeutic energy to tissues at greater depths. Initial clinical studies of NIR-II-based imaging and phototherapy have indicated impressive potential to decrease cancer recurrence, reduce complications and prolong survival. Despite the encouraging results achieved, clinical translation of innovative NIR-II techniques remains challenging and inefficient; multidisciplinary cooperation is necessary to bridge the gap between preclinical research and clinical practice, and thus accelerate the translation of technical advances into clinical benefits. In this Review, we summarize the available clinical data on NIR-II-based imaging and phototherapy, demonstrating the feasibility and utility of integrating these technologies into the treatment of cancer. We also introduce emerging NIR-II-based approaches with substantial potential to further enhance patient outcomes, while also highlighting the challenges associated with imminent clinical studies of these modalities.

NIR-II Fluorescent Probes for Fluorescence-Imaging-Guided Tumor Surgery

Second near-infrared (NIR-II) fluorescence imaging is the most advanced imaging fidelity method with extraordinary penetration depth, signal-to-background ratio, biocompatibility, and targeting ability. It is currently booming in the medical realm to diagnose tumors and is being widely applied for fluorescence-imaging-guided tumor surgery. To efficiently execute this modern imaging modality, scientists have designed various probes capable of showing fluorescence in the NIR-II window. Here, we update the state-of-the-art NIR-II fluorescent probes in the most recent literature, including indocyanine green, NIR-II emissive cyanine dyes, BODIPY probes, aggregation-induced emission fluorophores, conjugated polymers, donor-acceptor-donor dyes, carbon nanotubes, and quantum dots for imaging-guided tumor surgery. Furthermore, we point out that the new materials with fluorescence in NIR-III and higher wavelength range to further optimize the imaging results in the medical realm are a new challenge for the scientific world. In general, we hope this review will serve as a handbook for researchers and students who have an interest in developing and applying fluorescent probes for NIR-II fluorescence-imaging-guided surgery and that it will expedite the clinical translation of the probes from bench to bedside.

NIR-II Absorption/Fluorescence of D-A π-Conjugated Polymers Composed of Strong Electron Acceptors Based on Boron-Fused Azobenzene Complexes

Luminescence in the second near-infrared (NIR-II, 1,000-1,700 nm) window is beneficial especially for deep tissue imaging and optical sensors because of intrinsic high permeability through various media. Strong electron-acceptors with low-lying lowest unoccupied molecular orbital (LUMO) energy levels are a crucial unit for donor-acceptor (D-A) π-conjugated polymers (CPs) with the NIR-II emission property, however, limited kinds of molecular skeletons are still available. Herein, D-A CPs involving fluorinated boron-fused azobenzene complexes (BAz) with enhanced electron-accepting properties are reported. Combination of fluorination at the azobenzene ligand and trifluoromethylation at the boron can effectively lower the LUMO energy level down to -4.42 eV, which is much lower than those of conventional strong electron-acceptors. The synthesized series of CPs showed excellent absorption/fluorescence property in solution over a wide NIR range including NIR-II. Furthermore, owing to the inherent solid-state emissive property of the BAz skeleton, obvious NIR-II fluorescence from the film (up to λFL=1213 nm) and the nanoparticle in water (λFL=1036 nm, brightness=up to 29 cm-1 M-1) were observed, proposing that our materials are applicable for developing next-generation of NIR-II luminescent materials.

Counterion influence on near-infrared-II heptamethine cyanine salts for photothermal therapy

Photothermal therapy (PTT) has attracted extensive attention in cancer treatment. Heptamethine cyanine dyes with near-infrared (NIR) absorption performance have been investigated for PTT. However, they are often accompanied by poor photostability, suboptimal photothermal conversion and limited therapeutic efficacy. The photophysical properties of fluorescent organic salts can be tuned through counterion pairing. However, whether the counterion can influence the photostability and photothermal properties of heptamethine cyanine salts has not been clarified. In this work, we investigated the effects of eleven counter anions on the physical and photothermal properties of NIR-II heptamethine cyanine salts with the same heptamethine cyanine cation. The anions have great impacts on the physiochemical properties of dyes in solution including aggregation, photostability and photothermal conversion efficiency. The physical tuning enables the control over the cytotoxicity and phototoxicity of the dyes. The selected salts have been demonstrated to significantly suppress 4T1 breast tumor growth with low toxicity. The findings that the counterion has great effects on the photothermal properties of cationic NIR-II heptamethine cyanine dyes will provide a reference for the preparation of improved photothermal agents through counterion pairing with possible translation to humans.

Rotor-based image-guided therapy of glioblastoma

Glioblastoma (GBM), deep in the brain, is more challenging to diagnose and treat than other tumors. Such challenges have blocked the development of high-impact therapeutic approaches that combine reliable diagnosis with targeted therapy. Herein, effective cyanine dyes (IRLy) with the near-infrared two region (NIR-II) adsorption and aggregation-induced emission (AIE) have been developed via an "extended conjugation & molecular rotor" strategy for multimodal imaging and phototherapy of deep orthotopic GBM. IRLy was synthesized successfully through a rational molecular rotor modification with stronger penetration, higher signal-to-noise ratio, and a high photothermal conversion efficiency (PCE) up to ∼60%, which can achieve efficient NIR-II photo-response. The multifunctional nanoparticles (Tf-IRLy NPs) were further fabricated to cross the blood-brain barrier (BBB) introducing transferrin (Tf) as a targeting ligand. Tf-IRLy NPs showed high biosafety and good tumor enrichment for GBM in vitro and in vivo, and thus enabled accurate, efficient, and less invasive NIR-II multimodal imaging and photothermal therapy. This versatile Tf-IRLy nanosystem can provide a reference for the efficient, precise and low-invasive multi-synergistic brain targeted photo-theranostics. In addition, the "extended conjugation & molecular rotor" strategy can be used to guide the design of other photothermal agents.

Benzobisthiadiazole-Based Small Molecular Near-Infrared-II Fluorophores: From Molecular Engineering to Nanophototheranostics

Organic fluorescent molecules with emission in the second near-infrared (NIR-II) biological window have aroused increasing investigation in cancer phototheranostics. Among these studies, Benzobisthiadiazole (BBT), with high electron affinity, is widely utilized as the electron acceptor in constructing donor-acceptor-donor (D-A-D) structured fluorophores with intensive near-infrared (NIR) absorption and NIR-II fluorescence. Until now, numerous BBT-based NIR-II dyes have been employed in tumor phototheranostics due to their exceptional structure tunability, biocompatibility, and photophysical properties. This review systematically overviews the research progress of BBT-based small molecular NIR-II dyes and focuses on molecule design and bioapplications. First, the molecular engineering strategies to fine-tune the photophysical properties in constructing the high-performance BBT-based NIR-II fluorophores are discussed in detail. Then, their biological applications in optical imaging and phototherapy are highlighted. Finally, the current challenges and future prospects of BBT-based NIR-II fluorescent dyes are also summarized. This review is believed to significantly promote the further progress of BBT-derived NIR-II fluorophores for cancer phototheranostics.

Whole-Body and Whole-Organ 3D Imaging of Hypoxia Using an Activatable Covalent Fluorescent Probe Compatible with Tissue Clearing

Elucidation of biological phenomena requires imaging of microenvironments in vivo. Although the seamless visualization of in vivo hypoxia from the level of whole-body to single-cell has great potential to discover unknown phenomena in biological and medical fields, no methodology for achieving it has been established thus far. Here, we report the whole-body and whole-organ imaging of hypoxia, an important microenvironment, at single-cell resolution using activatable covalent fluorescent probes compatible with tissue clearing. We initially focused on overcoming the incompatibility of fluorescent dyes and refractive index matching solutions (RIMSs), which has greatly hindered the development of fluorescent molecular probes in the field of tissue clearing. The fluorescent dyes compatible with RIMS were then incorporated into the development of activatable covalent fluorescent probes for hypoxia. We combined the probes with tissue clearing, achieving comprehensive single-cell-resolution imaging of hypoxia in a whole mouse body and whole organs.

Cucurbit[8]uril Confinement-Based Secondary Coassembly for High-Efficiency Phosphorescence Energy Transfer Behavior

Aqueous supramolecular long-lived near-infrared (NIR) material is highly attractive but still remains great challenge. Herein, we report cucurbit[8]uril confinement-based secondary coassembly for achieving NIR phosphorescence energy transfer in water, which is fabricated from dicationic dodecyl-chain-bridged 4-(4-bromophenyl)-pyridine derivative (G), cucurbit[8]uril (CB[8]), and polyelectrolyte poly(4-styrene-sulfonic sodium) (PSS) via the hierarchical confinement strategy. As compared to the dumbbell-shaped G, the formation of unprecedented linear polypseudorotaxane G⊂CB[8] with nanofiber morphology engenders an emerging phosphorescent emission at 510 nm due to the macrocyclic confinement effect. Moreover, benefiting from the following secondary assembly confinement, such tight polypseudorotaxane G⊂CB[8] can further assemble with anionic polyelectrolyte PSS to yield uniform spherical nanoparticle, thereby significantly strengthening phosphorescence performance with an extended lifetime (i.e., 2.39 ms, c.f., 45.0 μs). Subsequently, the organic dye Rhodamine 800 serving as energy acceptor can be slightly doped into the polyelectrolyte assembly, which enables the occurrence of efficient phosphorescence energy transfer process with efficiency up to 80.1% at a high donor/acceptor ratio, and concurrently endows the final system with red-shifted and long-lived NIR emission (710 nm). Ultimately, the as-prepared assembly is successfully exploited as versatile imaging agent for NIR window labeling and detecting in living cells.

Functionalized Nanomaterials for Inhibiting ATP-Dependent Heat Shock Proteins in Cancer Photothermal/Photodynamic Therapy and Combination Therapy

Phototherapies induced by photoactive nanomaterials have inspired and accentuated the importance of nanomedicine in cancer therapy in recent years. During these light-activated cancer therapies, a nanoagent can produce heat and cytotoxic reactive oxygen species by absorption of light energy for photothermal therapy (PTT) and photodynamic therapy (PDT). However, PTT is limited by the self-protective nature of cells, with upregulated production of heat shock proteins (HSP) under mild hyperthermia, which also influences PDT. To reduce HSP production in cancer cells and to enhance PTT/PDT, small HSP inhibitors that can competitively bind at the ATP-binding site of an HSP could be employed. Alternatively, reducing intracellular glucose concentration can also decrease ATP production from the metabolic pathways and downregulate HSP production from glucose deprivation. Other than reversing the thermal resistance of cancer cells for mild-temperature PTT, an HSP inhibitor can also be integrated into functionalized nanomaterials to alleviate tumor hypoxia and enhance the efficacy of PDT. Furthermore, the co-delivery of a small-molecule drug for direct HSP inhibition and a chemotherapeutic drug can integrate enhanced PTT/PDT with chemotherapy (CT). On the other hand, delivering a glucose-deprivation agent like glucose oxidase (GOx) can indirectly inhibit HSP and boost the efficacy of PTT/PDT while combining these therapies with cancer starvation therapy (ST). In this review, we intend to discuss different nanomaterial-based approaches that can inhibit HSP production via ATP regulation and their uses in PTT/PDT and cancer combination therapy such as CT and ST.

An Extended NIR-II Superior Imaging Window from 1500 to 1900 nm for High-Resolution In Vivo Multiplexed Imaging Based on Lanthanide Nanocrystals

High-resolution in vivo optical multiplexing in second near-infrared window (NIR-II, 1000-1700 nm) is vital to biomedical research. Presently, limited by bio-tissue scattering, only luminescent probes located at NIR-IIb (1500-1700 nm) window can provide high-resolution in vivo multiplexed imaging. However, the number of available luminescent probes in this narrow NIR-IIb region is limited, which hampers the available multiplexed channels of in vivo imaging. To overcome the above challenges, through theoretical simulation we expanded the conventional NIR-IIb window to NIR-II long-wavelength (NIR-II-L, 1500-1900 nm) window on the basis of photon-scattering and water-absorption. We developed a series of novel lanthanide luminescent nanoprobes with emission wavelengths from 1852 nm to 2842 nm. NIR-II-L nanoprobes enabled high-resolution in vivo dynamic multiplexed imaging on blood vessels and intestines, and provided multi-channels imaging on lymph tubes, tumors and intestines. The proposed NIR-II-L probes without mutual interference are powerful tools for high-contrast in vivo multiplexed detection, which holds promise for revealing physiological process in living body.

Progress of near-infrared-II fluorescence in precision diagnosis and treatment of colorectal cancer

Colorectal cancer is a malignant tumour with high incidence and mortality worldwide; therefore, improving the early diagnosis of colorectal cancer and implementing a targeted "individualized treatment" strategy is of great concern. NIR-II fluorescence imaging is a large-depth, high-resolution optical bioimaging tool. Around the NIR-II window, researchers have developed a variety of luminescent probes, imaging systems, and treatment methods with colorectal cancer targeting capabilities, which can be visualized and image-guided in clinical surgery. This article aims to overcome the difficulties in diagnosing and treating colorectal cancer. The present review summarizes the latest results on using NIR-II fluorescence for targeted colorectal cancer imaging, expounds on the application prospects of NIR-II optical imaging for colorectal cancer, and discusses the imaging-guided multifunctional diagnosis and treatment platforms.

Site-specific albumin tagging with chloride-containing near-infrared cyanine dyes: molecular engineering, mechanism, and imaging applications

Near-infrared dyes, particularly cyanine dyes, have shown great potential in biomedical imaging due to their deep tissue penetration, high resolution, and minimal tissue autofluorescence/scattering. These dyes can be adjusted in terms of absorption and emission wavelengths by modifying their chemical structures. The current issues with cyanine dyes include aggregation-induced quenching, poor photostability, and short in vivo circulation time. Encapsulating cyanine dyes with albumin, whether exogenous or endogenous, has been proven to be an effective strategy for improving their brightness and pharmacokinetics. In detail, the chloride-containing (Cl-containing) cyanine dyes have been found to selectively bind to albumin to achieve site-specific albumin tagging, resulting in enhanced optical properties and improved biosafety. This feature article provides an overview of the progress in the covalent binding of Cl-containing cyanine dyes with albumin, including molecular engineering methods, binding sites, and the selective binding mechanism. The improved optical properties of cyanine dyes and albumin complexes have led to cutting-edge applications in biological imaging, such as tumor imaging (diagnostics) and imaging-guided surgery.

Activatable probes with potential for intraoperative tumor-specific fluorescence-imaging guided surgery

Owing to societal development and aging population, the impact of cancer on human health and quality of life has increased. Early detection and surgical treatment are the most effective approaches for most cancer patients. As the scope of conventional tumor resection is determined by auxiliary examination and surgeon experience, there is often insufficient recognition of tiny tumors. The ability to detect such tumors can be improved by using fluorescent tumor-specific probes for surgical navigation. This review mainly describes the design principles and mechanisms of activatable probes for the fluorescence imaging of tumors. This type of probe is nonfluorescent in normal tissue but exhibits obvious fluorescence emission upon encountering tumor-specific substrates, such as enzymes or bioactive molecules, or changes in the microenvironment, such as a low pH. In some cases, a single-factor response does not guarantee the effective fluorescence labeling of tumors. Therefore, two-factor-activatable fluorescence imaging probes that react with two specific factors in tumor cells have also been developed. Compared with single biomarker testing, the simultaneous monitoring of multiple biomarkers may provide additional insight into the role of these substances in cancer development and aid in improving the accuracy of early cancer diagnosis. Research and progress in this field can provide new methods for precision medicine and targeted therapy. The development of new approaches for early diagnosis and treatment can effectively improve the prognosis of cancer patients and help enhance their quality of life.

Long-term, real-time and label-free live cell image processing and analysis based on a combined algorithm of CellPose and watershed segmentation

Developing a rapid and quantitative method to accurately evaluate the physiological abilities of living cells is critical for tumor control. Many experiments have been conducted in the field of biology in an attempt to measure the proliferation and movement abilities of cells, but existing methods cannot provide real-time and objective data for label-free cells. The quantitative imaging technique, including an automatic segmentation algorithm for individual label-free cells, has been a breakthrough in this regard. In this study, we develop a combined automatic image processing algorithm of CellPose and watershed segmentation for the long-term and real-time imaging of label-free cells. This method shows strong reliability in cell identification regardless of cell densities, allowing us to obtain accurate information about the number and proliferation ability of the target cells. Additionally, our results also suggest that this method is a reliable way to assess real-time data on drug cytotoxicity, cell morphology, and cell movement ability.

"Dual-Key-and-Lock" NIR-II NSCyanines Enable High-Contrast Activatable Phototheranostics in Extrahepatic Diseases

Conventional cyanine dyes with a symmetric structure are "always-on", which can easily accumulate in the liver and display high liver background fluorescence, inevitably interfering the accurate diagnosis and therapy in extrahepatic diseases. We herein report a platform of NIR-II non-symmetric cyanine (NSCyanine) dyes by harnessing a non-symmetric strategy, which are extremely sensitive to pH/viscosity and can be activated via a "dual-key-and-lock" strategy. These NSCyanine dyes with a low pKa (<4.0) only show weak fluorescence at lysosome pH (key1), however, the fluorescence can be completely switched on and significantly enhanced by intracellular viscosity (key2) in disease tissues, exhibiting high target-to-liver ratios up to 19.5/1. Notably, high-contrast phototheranostics in extrahepatic diseases are achieved, including intestinal metastasis-imaging, acute gastritis-imaging, bacteria infected wound healing, and tumor ablation via targeted combined photothermal therapy and chemotherapy.

Biogenic Imaging Contrast Agents

Imaging contrast agents are widely investigated in preclinical and clinical studies, among which biogenic imaging contrast agents (BICAs) are developing rapidly and playing an increasingly important role in biomedical research ranging from subcellular level to individual level. The unique properties of BICAs, including expression by cells as reporters and specific genetic modification, facilitate various in vitro and in vivo studies, such as quantification of gene expression, observation of protein interactions, visualization of cellular proliferation, monitoring of metabolism, and detection of dysfunctions. Furthermore, in human body, BICAs are remarkably helpful for disease diagnosis when the dysregulation of these agents occurs and can be detected through imaging techniques. There are various BICAs matched with a set of imaging techniques, including fluorescent proteins for fluorescence imaging, gas vesicles for ultrasound imaging, and ferritin for magnetic resonance imaging. In addition, bimodal and multimodal imaging can be realized through combining the functions of different BICAs, which helps overcome the limitations of monomodal imaging. In this review, the focus is on the properties, mechanisms, applications, and future directions of BICAs.

A Gas/phototheranostic Nanocomposite Integrates NIR-II-Peak Absorbing Aza-BODIPY with Thermal-Sensitive Nitric Oxide Donor for Atraumatic Osteosarcoma Therapy

Photothermal therapy (PTT) has received increasing interest in cancer therapeutics owing to its excellent efficacy and controllability. However, there are two major limitations in PTT applications, which are the tissue penetration depth of lasers within the absorption range of photothermal agents and the unavoidable tissue empyrosis induced by high-energy lasers. Herein, a gas/phototheranostic nanocomposite (NA1020-NO@PLX) is engineered that integrates the second near-infrared-peak (NIR-II-peak) absorbing aza-boron-dipyrromethenes (aza-BODIPY,NA1020) with the thermal-sensitive nitric oxide (NO) donor (S-nitroso-N-acetylpenicillamine, SNAP). An enhanced intramolecular charge transfer mechanism is proposed to achieve the NIR-II-peak absorbance (λmax = 1020 nm) on NA1020, thereby obtaining its deep tissue penetration depth. The NA1020 exhibits a remarkable photothermal conversion, making it feasible for the deep-tissue orthotopic osteosarcoma therapy and providing favorable NIR-II emission to precisely pinpoint the tumor for a visible PTT process. The simultaneously investigated atraumatic therapeutic process with an enhanced cell apoptosis mechanism indicates the feasibility of the synergistic NO/low-temperature PTT for osteosarcoma. Herein, this gas/phototheranostic strategy optimizes the existing PTT to present a repeatable and atraumatic photothermal therapeutic process for deep-tissue tumors, validating its potential clinical applications.

Fluorescence enhancement of surface plasmon coupled emission by Au nanobipyramids and its modulation effect on multi-wavelength radiation

Surface plasmon coupled emission (SPCE), a novel surface-enhanced fluorescence technique, can generate directional and amplified radiation by the intense interaction between fluorophores and surface plasmons (SPs) of metallic nanofilms. For plasmon-based optical systems, the strong interaction between localized and propagating SPs and "hot spot" structures show great potential to significantly improve the electromagnetic (EM) field and modulate optical properties. Au nanobipyramids (NBPs) with two sharp apexes to enhance and restrict the EM field were introduced through electrostatic adsorption to achieve a mediated fluorescence system, and the emission signal enhancement was realized by factors over 60 compared with the normal SPCE. It has been demonstrated that the intense EM field produced by the NBPs assembly is what triggered the unique enhancement of SPCE by Au NBPs, which effectively overcomes the inherent signal quenching of SPCE for ultrathin sample detection. This remarkable enhanced strategy offers the chance to improve the detection sensitivity for plasmon-based biosensing and detection systems, and expand the range of applications for SPCE in bioimaging with more comprehensive and detailed information acquisition. The enhancement efficiency for various emission wavelengths was investigated in light of the wavelength resolution of SPCE, and it was discovered that enhanced emission for multi-wavelength could be successfully detected through the different emission angles due to the angular displacement caused by wavelength change. Benefit from this, the Au NBP modulated SPCE system was employed for multi-wavelength simultaneous enhancement detection under a single collection angle, which could broaden the application of SPCE in simultaneous sensing and imaging for multi-analytes, and expected to be used for high throughput detection of multi-component analysis.

Advances in optical molecular imaging for neural visualization

Iatrogenic nerve injury is a significant complication in surgery, which can negatively impact patients' quality of life. Currently, the main clinical neuroimaging methods, such as computed tomography, magnetic resonance imaging, and high-resolution ultrasonography, do not offer precise real-time positioning images for doctors during surgery. The clinical application of optical molecular imaging technology has led to the emergence of new concepts such as optical molecular imaging surgery, targeted surgery, and molecular-guided surgery. These advancements have made it possible to directly visualize surgical target areas, thereby providing a novel method for real-time identification of nerves during surgery planning. Unlike traditional white light imaging, optical molecular imaging technology enables precise positioning and identifies the cation of intraoperative nerves through the presentation of color images. Although a large number of experiments and data support its development, there are few reports on its actual clinical application. This paper summarizes the research results of optical molecular imaging technology and its ability to realize neural visualization. Additionally, it discusses the challenges neural visualization recognition faces and future development opportunities.

Self-delivery of metal-coordinated NIR-II nanoadjuvants for multimodal imaging-guided photothermal-chemodynamic amplified immunotherapy

The effectiveness of phototheranostics induced immunotherapy is still hampered by limited light penetration depth, the complex immunosuppressive tumor microenvironment (TME) and the low efficiency of immunomodulator drug delivery. Herein, self-delivery and TME responsive NIR-II phototheranostic nanoadjuvants (NAs) were fabricated to suppress the growth and metastasis of melanoma through the integration of photothermal-chemodynamic therapy (PTT-CDT) and immune remodeling. The NAs were constructed by the self-assembly of ultrasmall NIR-II semiconducting polymer dots and the toll-like receptor agonist resiquimod (R848) utilizing manganese ions (Mn2+) as coordination nodes. Under acidic TME, the NAs responsively disintegrated and released therapeutic components, which enable NIR-II fluorescence/photoacoustic/magnetic resonance imaging-guided tumor PTT-CDT. Moreover, the synergistic treatment of PTT-CDT could induce significant tumor immunogenic cell death and evoke highly efficacious cancer immunosurveillance. The released R848 stimulated the maturation of dendritic cells, which both amplified the antitumor immune response by modulating and remodeling the TME. The NAs present a promising integration strategy of polymer dot-metal ion coordination and immune adjuvants for precise diagnosis and amplified anti-tumor immunotherapy against deep-seated tumors. STATEMENT OF SIGNIFICANCE: The efficiency of phototheranostics induced immunotherapy is still limited by insufficient light penetration depth, low immune response and the complex immunosuppressive tumor microenvironment (TME). In order to improve the efficacy of immunotherapy, self-delivery NIR-II phototheranostic nanoadjuvants (PMR NAs) were successfully fabricated via the facile coordination self-assembly of ultra-small NIR-II semiconducting polymer dots and toll-like receptor agonist resiquimod (R848) utilizing manganese ions (Mn2+) as coordination nodes. PMR NAs not only enable TME responsive cargo release and NIR-II fluorescence/photoacoustic/magnetic resonance imaging mediated precise localization of tumors, but also achieve synergistic photothermal-chemodynamic therapy, evoking an effective anti-tumor immune response by ICD effect. The responsively released R848 could further amplify the efficiency of immunotherapy by reversing and remodeling the immunosuppressive tumor microenvironment, thereby effectively inhibiting tumor growth and lung metastasis.

Manipulating fluorescence by photo-switched spin-state conversions in an iron(ii)-based SCO-MOF

Manipulating fluorescence by photo-switched spin-state conversions is an attractive prospect for applications in smart magneto-optical materials and devices. The challenge is how to modulate the energy transfer paths of the singlet excited state by light-induced spin-state conversions. In this work, a spin crossover (SCO) Fe^II-based fluorophore was embedded into a metal-organic framework (MOF) to tune the energy transfer paths. Compound 1 {Fe(TPA-diPy)[Ag(CN)₂]₂}·2EtOH (1) has an interpenetrated Hofmann-type structure, wherein the Fe^II ion is coordinated by a bidentate fluorophore ligand (TPA-diPy) and four cyanide nitrogen atoms and acts as the fluorescent-SCO unit. Magnetic susceptibility measurements revealed that 1 underwent an incomplete and gradual spin crossover with T_1/2 = 161 K. Photomagnetic studies confirmed photo-induced spin state conversions between the low-spin (LS) and high-spin (HS) states, where the irradiation of 532 and 808 nm laser lights converted the LS and HS states to the HS and LS states, respectively. Variable-temperature fluorescence spectra study revealed an anomalous decrease in emission intensity upon the HS → LS transition, confirming the synergetic coupling between the fluorophore and SCO units. Alternating irradiation of 532 and 808 nm laser lights resulted in reversible fluorescence intensity changes, confirming spin state-controlled fluorescence in the SCO-MOF. Photo-monitored structural analyses and UV-vis spectroscopic studies demonstrated that the photo-induced spin state conversions changed energy transfer paths from the TPA fluorophore to the metal-centered charge transfer bands, ultimately leading to the switching of fluorescence intensities. This work represents a new prototype compound showing bidirectional photo-switched fluorescence by manipulating the spin states of iron(ii).

An Activatable Phototheranostic Nanoplatform for Tumor Specific NIR-II Fluorescence Imaging and Synergistic NIR-II Photothermal-Chemodynamic Therapy

The phototheranostics in the second near-infrared window (NIR-II) have proven to be promising for the precise cancer theranostics. However, the non-responsive and "always on" imaging mode lacks the selectivity, leading to the poor diagnosis specificity. Herein, a tumor microenvironment (TME) activated NIR-II phototheranostic nanoplatform (Ag₂ S-Fe(III)-DBZ Pdots, AFD NPs) is designed based on the principle of Förster resonance energy transfer (FRET). The AFD NPs are fabricated through self-assembly of Ag₂ S QDs (NIR-II fluorescence probe) and ultra-small semiconductor polymer dots (DBZ Pdots, NIR-II fluorescence quencher) utilizing Fe(III) as coordination nodes. In normal tissues, the AFD NPs maintain in "off" state, due to the FRET between Ag₂ S QDs and DBZ Pdots. However, the NIR-II fluorescence signal of AFD NPs can be rapidly "turn on" by the overexpressed GSH in tumor tissues, achieving a superior tumor-to-normal tissue (T/NT) signal ratio. Moreover, the released Pdots and reduced Fe(II) ions provide NIR-II photothermal therapy (PTT) and chemodynamic therapy (CDT), respectively. The GSH depletion and NIR-II PTT effect further aggravate CDT mediated oxidative damage toward tumors, achieving the synergistic anti-tumor therapeutic effect. The work provides a promising strategy for the development of TME activated NIR-II phototheranostic nanoprobes.

A novel strategy to construct activatable silver chalcogenide quantum dots nanoprobe for NIR-Ⅱ fluorescence imaging of hypochlorous acid in vivo

It is necessary to develop sensitive and selective probes for real-time in vivo monitoring of hypochlorous acid (HClO) which plays a significant role in physiological and pathological processes. The second near-infrared (NIR-Ⅱ) luminescent silver chalcogenide quantum dots (QDs) have shown great potential in developing activatable nanoprobe for HClO in terms of their outstanding imaging performance in the living organism. However, the limited strategy for the construction of activatable nanoprobes severely restricts their widespread applications. Herein, we proposed a novel strategy for developing an activatable silver chalcogenide QDs nanoprobe for NIR-Ⅱ fluorescence imaging of HClO in vivo. The nanoprobe was fabricated by mixing an Au-precursor solution with Ag₂Te@Ag₂S QDs to allow cation exchange and release Ag ions and then reducing the released Ag ions on the QDs surface to form an Ag shell for quenching of the emission of QDs. The Ag shell of QDs was oxidized and etched in the presence of HClO, resulting in the disappearance of their quenching effect on QDs and the activation of the QDs emission. The developed nanoprobe enabled highly sensitive and selective determination of HClO and imaging of HClO in arthritis and peritonitis. This study provides a novel strategy for the construction of activatable nanoprobe based on QDs and a promising tool for NIR-Ⅱ imaging of HClO in vivo.

Ratiometric optical probes for biosensing

Biosensing by optical probes is bringing about a revolution in our understanding of physiological and pathological states. Conventional optical probes for biosensing are prone to inaccurate detection results due to various analyte-independent factors that can lead to fluctuations in the absolute signal intensity. Ratiometric optical probes provide built-in self-calibration signal correction for more sensitive and reliable detection. Probes specifically developed for ratiometric optical detection have been shown to significantly improve the sensitivity and accuracy of biosensing. In this review, we focus on the advancements and sensing mechanism of ratiometric optical probes including photoacoustic (PA) probes, fluorescence (FL) probes, bioluminescence (BL) probes, chemiluminescence (CL) probes and afterglow probes. The versatile design strategies of these ratiometric optical probes are discussed along with a broad range of applications for biosensing such as sensing of pH, enzymes, reactive oxygen species (ROS), reactive nitrogen species (RNS), glutathione (GSH), metal ions, gas molecules and hypoxia factors, as well as the fluorescence resonance energy transfer (FRET)-based ratiometric probes for immunoassay biosensing. Finally, challenges and perspectives are discussed.

Fluorescent detection of emerging virus based on nanoparticles: From synthesis to application

The spread of COVID-19 has caused huge economic losses and irreversible social impact. Therefore, to successfully prevent the spread of the virus and solve public health problems, it is urgent to develop detection methods with high sensitivity and accuracy. However, existing detection methods are time-consuming, rely on instruments, and require skilled operators, making rapid detection challenging to implement. Biosensors based on fluorescent nanoparticles have attracted interest in the field of detection because of their advantages, such as high sensitivity, low detection limit, and simple result readout. In this review, we systematically describe the synthesis, intrinsic advantages, and applications of organic dye-doped fluorescent nanoparticles, metal nanoclusters, up-conversion particles, quantum dots, carbon dots, and others for virus detection. Furthermore, future research initiatives are highlighted, including green production of fluorescent nanoparticles with high quantum yield, speedy signal reading by integrating with intelligent information, and error reduction by coupling with numerous fluorescent nanoparticles.

Monitoring Hg2+ and MeHg+ poisoning in living body with an activatable near-infrared II fluorescence probe

Noninvasively imaging mercury poisoning in living organisms is critical to understanding its toxicity and treatments. Especially, simultaneous fluorescence imaging of Hg²⁺ and MeHg⁺in vivo is helpful to disclose the mysteries of mercury poisoning. The key limitation for mercury imaging in vivo is the low imaging signal-to-background ratio (SBR) and limited imaging depth, which may result in unreliable detection results. Here, we designed and prepared a near-infrared II (NIR II) emissive probe, NIR-Rh-MS, leveraging the "spirolactam ring-open" tactic of xanthene dyes for in situ visualization of mercury toxicity in mice. The probe produces a marked fluorescence signal at 1015 nm and displays good linear responses to Hg²⁺ and MeHg⁺ with excellent sensitivity, respectively. The penetration experiments elucidate that the activated NIR-II fluorescence signal of the probe penetrates to a depth of up to 7 mm in simulated tissues. Impressively, the probe can monitor the toxicity of Hg²⁺ in mouse livers and the accumulation of MeHg⁺ in mouse brains via intravital NIR-II imaging for the first time. Thus, we believe that detecting Hg²⁺ and MeHg⁺ in different organs with a single NIR-II fluorescence probe in mice would assuredly advance the toxicologic study of mercury poisoning in vivo.

Bright Tm3+-based downshifting luminescence nanoprobe operating around 1800 nm for NIR-IIb and c bioimaging

Fluorescence bioimaging based on rare-earth-doped nanocrystals (RENCs) in the shortwave infrared (SWIR, 1000-3000 nm) region has aroused intense interest due to deeper penetration depth and clarity. However, their downshifting emission rarely shows sufficient brightness beyond 1600 nm, especially in NIR-IIc. Here, we present a class of thulium (Tm) self-sensitized RENC fluorescence probes that exhibit bright downshifting luminescence at 1600-2100 nm (NIR-IIb/c) for in vivo bioimaging. An inert shell coating minimizes surface quenching and combines strong cross-relaxation, allowing LiTmF₄@LiYF₄ NPs to emit these intense downshifting emissions by absorbing NIR photons at 800 nm (large Stokes shift ~1000 nm with a absolute quantum yield of ~14.16%) or 1208 nm (NIR-II_in and NIR-II_out). Furthermore, doping with Er³⁺ for energy trapping achieves four-wavelength NIR irradiation and bright NIR-IIb/c emission. Our results show that Tm-based NPs, as NIR-IIb/c nanoprobes with high signal-to-background ratio and clarity, open new opportunities for future applications and translation into diverse fields.

First Nerve-Related Immunoprobe for Guidance of Renal Denervation through Colocalization of NIR-II and Photoacoustic Bioimaging

Nerve-related fluorophores generally locate in the visible or near-infrared region with shallow penetration depth and easy uptake by surrounding tissues. Prolonging the optical window promotes resolution by minimizing photoscattering and eliminating autofluorescence for NIR-II (second near infrared; 1000-1700 nm) and photoacoustic bioimaging. In addition, combination of the two could help in colocalization of targets at the 3D level. Catheter-based renal sympathetic denervation (RDN), an alternative treatment recently finishing its clinical evaluation for treating resistant hypertension, is highly dependent on experience and in urgent demand for in vivo guidance in locating the nerve over the renal artery. Here, an NIR-II and photoacoustic bioimaging system based on a dye-modified anti-tyrosine-hydroxylase antibody (TH-ICGM) to illustrate the peritoneal sympathetic nerve-related region are combined. With high resolution (0.15 mm) in NIR-II region for both absorbance (λ_ex = 925 nm) and fluorescence (bioimaging in λ_em ≥ 1300 nm), TH-ICGM succeeds in providing 3D coordinates of procedure position with a precision in 0.1 mm. As the first nerve-related NIR-II immunoprobe, TH-ICGM has great clinical potential as assistance for nerve-related interventions.

Real-Time, In Vivo Molecular Monitoring Using Electrochemical Aptamer Based Sensors: Opportunities and Challenges

The continuous, real-time measurement of specific molecules in situ in the body would greatly improve our ability to understand, diagnose, and treat disease. The vast majority of continuous molecular sensing technologies, however, either (1) rely on the chemical or enzymatic reactivity of their targets, sharply limiting their scope, or (2) have never been shown (and likely will never be shown) to operate in the complex environments found in vivo. Against this background, here we review electrochemical aptamer-based (EAB) sensors, an electrochemical approach to real-time molecular monitoring that has now seen 15 years of academic development. The strengths of the EAB platform are significant: to date it is the only molecular measurement technology that (1) functions independently of the chemical reactivity of its targets, and is thus general, and (2) supports in vivo measurements. Specifically, using EAB sensors we, and others, have already reported the real-time, seconds-resolved measurements of multiple, unrelated drugs and metabolites in situ in the veins and tissues of live animals. Against these strengths, we detail the platform's remaining weaknesses, which include still limited measurement duration (hours, rather than the more desirable days) and the difficulty in obtaining sufficiently high performance aptamers against new targets, before then detailing promising approaches overcoming these hurdles. Finally, we close by exploring the opportunities we believe this potentially revolutionary technology (as well as a few, possibly competing, technologies) will create for both researchers and clinicians.

Synthesis of a Dual-Color Fluorescent Dendrimer for Diagnosis of Cancer Metastasis in Lymph Nodes

Detection of cancer metastasis spread in lymph nodes is important in cancer diagnosis. In this study, a fluorescence imaging probe was designed for the detection of both lymph node and tumor cells using always-ON and activatable fluorescence probes with different colors. Rhodamine B (Rho), a matrix metalloproteinase-2 (MMP-2)-responsive green fluorescence probe, and a tumor-homing peptide were conjugated to a carboxy-terminal dendrimer that readily accumulates in lymph nodes. The activatable green fluorescence signal increased in the presence of MMP-2, which is secreted by tumor cells. Both the always-ON Rho signal and the activatable green fluorescence signal were observed from tumor cells, but only the weak always-ON Rho signal was from immune cells. Thus, this type of dendrimer may be useful for non-invasive imaging to diagnose cancer metastasis in lymph nodes.

Repurposing Cyanine Photoinstability To Develop Near-Infrared Light-Activatable Nanogels for In Vivo Cargo Delivery

The favorable properties of cyanines (e.g., near-infrared (NIR) absorbance and emission) have made this class of dyes popular for a wide variety of biomedical applications. However, many cyanines are prone to rapid photobleaching when irradiated with light. In this study, we have exploited this undesirable trait to develop NIR-nanogels for NIR light-mediated cargo delivery. NIR-nanogels feature a photolabile cyanine cross-linker (Cy780-Acryl) that can cleave via dioxetane chemistry when irradiated. This photochemical process results in the formation of two carbonyl fragments and concomitant NIR-nanogel degradation to facilitate cargo release. In contrast to studies where cyanines are utilized as photocages, our approach does not require direct chemical attachment to the cargo, thus expanding our ability to deliver molecules that cannot be covalently modified. We showcase this feature by encapsulating a palette of small-molecule chemotherapeutics that feature a structurally diverse chemical architecture. To demonstrate site-selective release in vivo, we generated a murine model of breast cancer. Relative to nonlight irradiated and drug-free controls, treatment with NIR-nanogels loaded with paclitaxel (a potent cytotoxic agent) and NIR light resulted in significant attenuation of tumor growth. Moreover, we show via histological staining of the vital organs that minimal off-target effects are observed.

Recent advances in small-molecule fluorescent probes for studying ferroptosis

Ferroptosis is an iron-dependent, non-apoptotic form of programmed cell death driven by excessive lipid peroxidation (LPO). Mounting evidence suggests that the unique modality of cell death is involved in the development and progression of several diseases including cancer, cardiovascular diseases (CVDs), neurodegenerative disorders, etc. However, the pathogenesis and signalling pathways of ferroptosis are not fully understood, possibly due to the lack of robust tools for the highly selective and sensitive imaging of ferroptosis analytes in complex living systems. Up to now, various small-molecule fluorescent probes have been applied as promising chemosensors for studying ferroptosis through tracking the biomolecules or microenvironment-related parameters in vitro and in vivo. In this review, we comprehensively reviewed the recent development of small-molecule fluorescent probes for studying ferroptosis, with a focus on the analytes, design strategies and bioimaging applications. We also provided new insights to overcome the major challenges in this emerging field.

Rational Design of NIR-II AIEgens with Ultrahigh Quantum Yields for Photo- and Chemiluminescence Imaging

Fluorescence imaging in the second near-infrared window (NIR-II, 1000-1700 nm) using small-molecule dyes has high potential for clinical use. However, many NIR-II dyes suffer from the emission quenching effect and extremely low quantum yields (QYs) in the practical usage forms. The AIE strategy has been successfully utilized to develop NIR-II dyes with donor-acceptor (D-A) structures with acceptable QYs in the aggregate state, but there is still large room for QY improvement. Here, we rationally designed a NIR-II emissive dye named TPE-BBT and its derivative (TPEO-BBT) by changing the electron-donating triphenylamine unit to tetraphenylethylene (TPE). Their nanoparticles exhibited ultrahigh relative QYs of 31.5% and 23.9% in water, respectively. By using an integrating sphere, the absolute QY of TPE-BBT nanoparticles was measured to be 1.8% in water. Its crystals showed an absolute QY of 10.4%, which is the highest value among organic small molecules reported so far. The optimized D-A interaction and the higher rigidity of TPE-BBT in the aggregate state are believed to be the two key factors for its ultrahigh QY. Finally, we utilized TPE-BBT for NIR-II photoluminescence (PL) and chemiluminescence (CL) bioimaging through successive CL resonance energy transfer and Förster resonance energy transfer processes. The ultrahigh QY of TPE-BBT realized an excellent PL imaging quality in mouse blood vessels and an excellent CL imaging quality in the local arthrosis inflammation in mice with a high signal-to-background ratio of 130. Thus, the design strategy presented here brings new possibilities for the development of bright NIR-II dyes and NIR-II bioimaging technologies.

Management of fluorescent organic/inorganic nanohybrids for biomedical applications in the NIR-II region

Biomedical fluorescence imaging in the second near-infrared (NIR-II, 100-1700 nm) window provides great potential for visualizing physiological and pathological processes, owing to the reduced tissue absorption, scattering, and autofluorescence. Various types of NIR-II probes have been reported in the past decade. Among them, NIR-II organic/inorganic nanohybrids have attracted widespread attention due to their unique properties by integrating the advantages of both organic and inorganic species. Versatile organic/inorganic nanohybrids provide the possibility of realizing a combination of functions, controllable size, and multiple optical features. This tutorial review summarizes the reported organic and inorganic species in nanohybrids, and their biomedical applications in NIR-II fluorescence and lifetime imaging. Finally, the challenges and outlook of organic/inorganic nanohybrids in biomedical applications are discussed.

Topical pH Sensing NIR Fluorophores for Intraoperative Imaging and Surgery of Disseminated Ovarian Cancer

Fluorescence-guided surgery (FGS) aids surgeons with real-time visualization of small cancer foci and borders, which improves surgical and prognostic efficacy of cancer. Despite the steady advances in imaging devices, there is a scarcity of fluorophores available to achieve optimal FGS. Here, 1) a pH-sensitive near-infrared fluorophore that exhibits rapid signal changes in acidic tumor microenvironments (TME) caused by the attenuation of intramolecular quenching, 2) the inherent targeting for cancer based on chemical structure (structure inherent targeting, SIT), and 3) mitochondrial and lysosomal retention are reported. After topical application of PH08 on peritoneal tumor regions in ovarian cancer-bearing mice, a rapid fluorescence increase (< 10 min), and extended preservation of signals (> 4 h post-topical application) are observed, which together allow for the visualization of submillimeter tumors with a high tumor-to-background ratio (TBR > 5.0). In addition, PH08 is preferentially transported to cancer cells via organic anion transporter peptides (OATPs) and colocalizes in the mitochondria and lysosomes due to the positive charges, enabling a long retention time during FGS. PH08 not only has a significant impact on surgical and diagnostic applications but also provides an effective and scalable strategy to design therapeutic agents for a wide array of cancers.

Prospects of NIR fluorescent nanosensors for green detection of SARS-CoV-2

The pandemic of the novel coronavirus disease 2019 (COVID-19) is continuously causing hazards for the world. Effective detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can relieve the impact, but various toxic chemicals are also released into the environment. Fluorescence sensors offer a facile analytical strategy. During fluorescence sensing, biological samples such as tissues and body fluids have autofluorescence, giving false-positive/negative results because of the interferences. Fluorescence near-infrared (NIR) nanosensors can be designed from low-toxic materials with insignificant background signals. Although this research is still in its infancy, further developments in this field have the potential for sustainable detection of SARS-CoV-2. Herein, we summarize the reported NIR fluorescent nanosensors with the potential to detect SARS-CoV-2. The green synthesis of NIR fluorescent nanomaterials, environmentally compatible sensing strategies, and possible methods to reduce the testing frequencies are discussed. Further optimization strategies for developing NIR fluorescent nanosensors to facilitate greener diagnostics of SARS-CoV-2 for pandemic control are proposed.

Recent Progresses in NIR-II Luminescent Bio/Chemo Sensors Based on Lanthanide Nanocrystals

Fluorescent bio/chemosensors are widely used in the field of biological research and medical diagnosis, with the advantages of non-invasiveness, high sensitivity, and good selectivity. In particular, luminescent bio/chemosensors, based on lanthanide nanocrystals (LnNCs) with a second near-infrared (NIR-II) emission, have attracted much attention, owing to greater penetration depth, aside from the merits of narrow emission band, abundant emission lines, and long lifetimes. In this review, NIR-II LnNCs-based bio/chemo sensors are summarized from the perspectives of the mechanisms of NIR-II luminescence, synthesis method of LnNCs, strategy of luminescence enhancement, sensing mechanism, and targeted bio/chemo category. Finally, the problems that exist in present LnNCs-based bio/chemosensors are discussed, and the future development trend is prospected.

NIRII-HDs: A Versatile Platform for Developing Activatable NIR-II Fluorogenic Probes for Reliable In Vivo Analyte Sensing

Small-molecule-based second near-infrared (NIR-II) activatable fluorescent probes can potentially provide a high target-to-background ratio and deep tissue penetration. However, most of the reported NIR-II activatable small-molecule probes exhibit poor versatility owing to the lack of a general and stable optically tunable group. In this study, we designed NIRII-HDs, a novel dye scaffold optimized for NIR-II probe development. In particular, dye NIRII-HD5 showed the best optical properties such as proper pK_a value, excellent stability, and high NIR-II brightness, which can be beneficial for in vivo imaging with high contrast. To demonstrate the applicability of the NIRII-HD5 dye, we designed three target-activatable NIR-II probes for ROS, thiols, and enzymes. Using these novel probes, we not only realized reliable NIR-II imaging of different diseases in mouse models but also evaluated the redox potential of liver tissue during a liver injury in vivo with high fidelity.

Nanoparticulate Photoluminescent Probes for Bioimaging: Small Molecules and Polymers

Recent interest in research on photoluminescent molecules due to their unique properties has played an important role in advancing the bioimaging field. In particular, small molecules and organic dots as probes have great potential for the achievement of bioimaging because of their desirable properties. In this review, we provide an introduction of probes consisting of fluorescent small molecules and polymers that emit light across the ultraviolet and near-infrared wavelength ranges, along with a brief summary of the most recent techniques for bioimaging. Since photoluminescence probes emitting light in different ranges have different goals and targets, their respective strategies also differ. Diverse and novel strategies using photoluminescence probes against targets have gradually been introduced in the related literature. Among recent papers (published within the last 5 years) on the topic, we here concentrate on the photophysical properties and strategies for the design of molecular probes, with key examples of in vivo photoluminescence research for practical applications. More in-depth studies on these probes will provide key insights into how to control the molecular structure and size/shape of organic probes for expanded bioimaging research and applications.

NIR-II Fluorescent Probe for Detecting Trimethylamine Based on Intermolecular Charge Transfer

A new kind of small organic NIR-II fluorophore molecule (ZS-1010) based on intermolecular charge transfer was developed as a NIR-II fluorescent probe for trimethylamine (TMA) detection, which is important for the diagnosis of cardiovascular disease, chronic kidney disease and diabetes. ZS-1010 has a strong push-pull electron system composed of electron donor unit and electron acceptor unit, exhibiting strong absorption and emission in the NIR-II region. When mixed with TMA which possesses strong electron-donating characteristics, the push-pull system of ZS-1010 will be affected along with the dipole moment change, leading to the quenching of fluorescence. This is the first example of TMA fluorescent probe in the NIR-II window showing deep penetration, fast response speed, high selectivity and pH stability.

A "Self-Checking" pH/Viscosity-Activatable NIR-II Molecule for Real-Time Evaluation of Photothermal Therapy Efficacy

We present a second near-infrared (NIR-II) self-checking molecule, LET-1052, for acidic tumor microenvironment (TME) turn-on photothermal therapy (PTT), followed by viscosity based therapeutic efficacy evaluation by itself in two independent channels, denoted as "self-checking" strategy. In acidic TME, LET-1052 was protonated and turned on NIR-II absorption for PTT under 1064 nm laser irradiation. Subsequently, PTT-induced cellular death increases intracellular viscosity, which inhibited the intramolecular rotation of LET-1052, resulting in the enhancement of NIR-I fluorescence for real-time evaluation of PTT efficacy. After PTT of tumor-bearing mice for different periods of NIR-II laser irradiation, NIR-I fluorescence in the tumor region showed positive correlation with tumor growth inhibition rate, demonstrating reliable and prompt prediction of PTT efficacy. The strategy may be expanded for instant evaluation of other therapeutic modalities for personalized medicine.

Recent Progress of Cyanine Fluorophores for NIR-II Sensing and Imaging

The cyanine fluorophores, a kind of classic organic fluorophores, are famous for their high extinction coefficient, simple synthetic route, and relatively long absorption and emission wavelengths. Moreover, the excellent biocompatibility and low toxicity in biological samples make cyanine fluorophores show excellent application value in the biomedical field, especially in Near-Infrared II (NIR-II) sensing and imaging. In this review, we briefly outline the history, characteristics, and current state of development of cyanine fluorophores. In particular, we described the application of cyanine fluorophores in NIR-II sensing and imaging. We hope this review can help researchers grab the latest information in the fast-growing field of cyanine fluorophores for NIR-II sensing and imaging.