Real-Time and High-Resolution Bioimaging with Bright Aggregation-Induced Emission Dots in Short-Wave Infrared Region

Versatile Types of Inorganic/Organic NIR-IIa/IIb Fluorophores: From Strategic Design toward Molecular Imaging and Theranostics

In vivo imaging in the second near-infrared window (NIR-II, 1000-1700 nm), which enables us to look deeply into living subjects, is producing marvelous opportunities for biomedical research and clinical applications. Very recently, there has been an upsurge of interdisciplinary studies focusing on developing versatile types of inorganic/organic fluorophores that can be used for noninvasive NIR-IIa/IIb imaging (NIR-IIa, 1300-1400 nm; NIR-IIb, 1500-1700 nm) with near-zero tissue autofluorescence and deeper tissue penetration. This review provides an overview of the reports published to date on the design, properties, molecular imaging, and theranostics of inorganic/organic NIR-IIa/IIb fluorophores. First, we summarize the design concepts of the up-to-date functional NIR-IIa/IIb biomaterials, in the order of single-walled carbon nanotubes (SWCNTs), quantum dots (QDs), rare-earth-doped nanoparticles (RENPs), and organic fluorophores (OFs). Then, these novel imaging modalities and versatile biomedical applications brought by these superior fluorescent properties are reviewed. Finally, challenges and perspectives for future clinical translation, aiming at boosting the clinical application progress of NIR-IIa and NIR-IIb imaging technology are highlighted.

Ultra-homogeneous NIR-II fluorescent self-assembled nanoprobe with AIE properties for photothermal therapy of prostate cancer

Near-infrared two-zone (NIR-II) fluorescence imaging has attracted attention as a non-invasive imaging technology that provides centimeter-level depth and micron-level resolution. However, producing a NIR-II fluorescent nanoprobe with uniform size, high bio-identical capacity, and fluorescence intensity, while being metabolizable in vivo, remains a challenge. We first produce a hydrophobic NIR-II fluorescent molecule with AIE properties, and subject it to ultrasonic and extrusion treatments to generate a DSPE-PEG-encapsulated NIR-II nanoprobe with an ultra-homogeneous particle size. The current study based on in vitro and mouse tumor-bearing model-based experiments indicate that cancer cells could efficiently take up this nanoprobe, which aggregates in tumor tissues, is susceptible to metabolization, and enables ideal photothermal therapeutic effects. Thus, this NIR-II nanoprobe with AIE properties shows great potential for precise clinical diagnosis and treatment of cancer.

Optical Imaging in the Second Near Infrared Window for Vascular Bioimaging

Optical imaging in the second near infrared region (NIR-II, 1000-1700 nm) provides higher resolution and deeper penetration depth for accurate and real-time vascular anatomy, blood dynamics, and function information, effectively contributing to the early diagnosis and curative effect assessment of vascular anomalies. Currently, NIR-II optical imaging demonstrates encouraging results including long-term monitoring of vascular injury and regeneration, real-time feedback of blood perfusion, tracking of lymphatic metastases, and imaging-guided surgery. This review summarizes the latest progresses of NIR-II optical imaging for angiography including fluorescence imaging, photoacoustic (PA) imaging, and optical coherence tomography (OCT). The development of current NIR-II fluorescence, PA, and OCT probes (i.e., single-walled carbon nanotubes, quantum dots, rare earth doped nanoparticles, noble metal-based nanostructures, organic dye-based probes, and semiconductor polymer nanoparticles), highlighting probe optimization regarding high brightness, longwave emission, and biocompatibility through chemical modification or nanotechnology, is first introduced. The application of NIR-II probes in angiography based on the classification of peripheral vascular, cerebrovascular, tumor vessel, and cardiovascular, is then reviewed. Major challenges and opportunities in the NIR-II optical imaging for vascular imaging are finally discussed.

Perfecting and extending the near-infrared imaging window

In vivo fluorescence imaging in the second near-infrared window (NIR-II) has been considered as a promising technique for visualizing mammals. However, the definition of the NIR-II region and the mechanism accounting for the excellent performance still need to be perfected. Herein, we simulate the photon propagation in the NIR region (to 2340 nm), confirm the positive contribution of moderate light absorption by water in intravital imaging and perfect the NIR-II window as 900-1880 nm, where 1400-1500 and 1700-1880 nm are defined as NIR-IIx and NIR-IIc regions, respectively. Moreover, 2080-2340 nm is newly proposed as the third near-infrared (NIR-III) window, which is believed to provide the best imaging quality. The wide-field fluorescence microscopy in the brain is performed around the NIR-IIx region, with excellent optical sectioning strength and the largest imaging depth of intravital NIR-II fluorescence microscopy to date. We also propose 1400 nm long-pass detection in off-peak NIR-II imaging whose performance exceeds that of NIR-IIb imaging, using bright fluorophores with short emission wavelength.

Second near-infrared (NIR-II) imaging: a novel diagnostic technique for brain diseases

Imaging in the second near-infrared II (NIR-II) window, a kind of biomedical imaging technology with characteristics of high sensitivity, high resolution, and real-time imaging, is commonly used in the diagnosis of brain diseases. Compared with the conventional visible light (400-750 nm) and NIR-I (750-900 nm) imaging, the NIR-II has a longer wavelength of 1000-1700 nm. Notably, the superiorities of NIR-II can minimize the light scattering and autofluorescence of biological tissue with the depth of brain tissue penetration up to 7.4 mm. Herein, we summarized the main principles of NIR-II in animal models of traumatic brain injury, cerebrovascular visualization, brain tumor, inflammation, and stroke. Simultaneously, we encapsulated the in vivo process of NIR-II probes and their in vivo and in vitro toxic effects. We further dissected its limitations and following optimization measures.

Dithieno[3,2-b:2',3'-d]silole-based conjugated polymers for bioimaging in the short-wave infrared region

The short-wave infrared window (SWIR, 900–1700 nm) fluorescence imaging has been demonstrated to have excellent imaging performance in signal/noise ratio and tissue penetration compared to the conventional NIR biological window (NIR-I, 700–900 nm). Conventional organic SWIR fluorescent materials still suffer from low fluorescence quantum efficiency. In this work, a donor unit with sp³ hybrid configuration and an acceptor unit with small hindered alkyl side chains are employed to construct donor–acceptor (D–A) type conjugated polymers P1 and P2, which were substituted with one or two fluorine atoms. These structural features can alleviate the aggregation-caused quenching (ACQ) and contribute to charge transfer, resulting in a significantly improved fluorescence quantum efficiency. The SWIR fluorescent quantum efficiencies of P1 and P2 nanoparticles are 3.4% and 4.4%, respectively, which are some of the highest for organic SWIR fluorophores reported so far. Excellent imaging quality has been demonstrated with P2 nanoparticles for SWIR imaging of the vascular system of nude mice. The results indicate that our design strategy of introducing sp³ hybrid configuration and small hindered alkyl side chains to fabricate conjugated polymers is efficient in improving the fluorescent quantum efficiency as SWIR fluorescent imaging agents for potential clinical practice.

A D–A type polymer with a SWIR fluorescence quantum efficiency of 4.4% was obtained after structural optimization.

Boosting Photoacoustic Effect via Intramolecular Motions Amplifying Thermal-to-Acoustic Conversion Efficiency for Adaptive Image-Guided Cancer Surgery

Photoacoustic (PA) imaging emerges as a promising technique for biomedical applications. The development of new strategies to boost PA conversion without depressing other properties (e.g., fluorescence) is highly desirable for multifunctional imaging but difficult to realize. Here, we report a new phenomenon that active intramolecular motions could promote PA signal by specifically increasing thermal-to-acoustic conversion efficiency. The compound with intense intramolecular motion exhibits amplified PA signal by elevating thermal-to-acoustic conversion, and the fluorescence also increases due to aggregation-induced emission signature. The simultaneously high PA and fluorescence brightness of TPA-TQ3 NPs enable precise image-guided surgery. The preoperative fluorescence and PA imaging are capable of locating orthotopic breast tumor in a high-contrast manner, and the intraoperative fluorescence imaging delineates tiny residual tumors. This study highlights a new design guideline of intramolecular motion amplifying PA effect.

Visualizing Oxidative Stress Level for Timely Assessment of Ischemic Stroke via a Ratiometric Near-Infrared-II Luminescent Nanoprobe

Ischemic stroke (IS) characterized with high morbidity and mortality rates is considered as one of the most dangerous brain diseases. The timely assessment of IS is crucial for making a clinical decision due to the severity of IS featured with time-dependence. Herein, we develop a highly reactive oxygen species (HROS)-responsive ratiometric near-infrared-II (NIR-II) nanoprobe based on a dye-sensitized system between IR-783 dye and lanthanide-doped nanoparticles. Once intravenously injected into the mice, the probe is rapidly accumulated at a lesion site by recognizing the activated endothelial cell or impaired blood-brain barrier (BBB) in the ischemic area and further responds to HROS, thereby allowing in vivo imaging of the oxidative stress level. The probe is not only able to discriminate the salvageable ischemic tissue from infarcted stroke core by visualizing the enriched degree of the probe at the lesion site but also can grade the salvageable ischemic tissue by analyzing the oxidative stress level. In addition, the ischemia area was clearly delineated by NIR-II luminescence imaging after cerebral ischemia for 30 min, which is significantly earlier than with the magnetic resonance imaging (MRI) method, thereby providing a practical tool for the timely assessing of IS.

Biomimetic Glucan Particles with Aggregation-Induced Emission Characteristics for Noninvasive Monitoring of Transplant Immune Response

Real-time monitoring of post-transplant immune response is critical to prolong the survival of grafts. The current gold standard for assessing the immune response to graft is biopsy. However, such a method is invasive and prone to false negative results due to limited tissue size available and the heterogeneity of the rejection site. Herein, we report biomimetic glucan particles with aggregation-induced emission (AIE) characteristics (HBTTPEP/GPs) for real-time noninvasive monitoring of post-transplant immune response. We have found that the positively charged near-infrared AIEgens can effectively aggregate in the confined space of glucan particles (GPs), thereby turning on the fluorescence emission. HBTTPEP/GPs can track macrophages for 7 days without hampering the bioactivity. Oral administration of HBTTPEP/GPs can specially target macrophages by mimicking yeast, which then migrate to the transplant rejection site. The fluorescence emitted from HBTTPEP/GPs correlated well with the infiltration of macrophages and the degree of allograft rejection. Furthermore, a single oral HBTTPEP/GPs dose can dynamically evaluate the therapeutic response to immunosuppressive therapy. Consequently, the biomimetic AIE-active glucan particles can be developed as a promising probe for immune-monitoring in solid organ transplantation.

Unlocking the power of optical imaging in the second biological window: Structuring near-infrared II materials from organic molecules to nanoparticles

Biomedical imaging techniques play a crucial role in clinical diagnosis, surgical intervention, and prognosis. Fluorescence imaging in the second biological window (second near-infrared [NIR-II]; 1000-1700 nm) has attracted attention recently. NIR-II fluorescence imaging offers unique advantages in terms of reduced photon scattering, deep tissue penetration, high sensitivity, and many others. A host of materials, including small organic molecules, single-walled carbon nanotubes, polymeric and rare-earth-doped nanoparticles, have been explored as NIR-II emitting fluorescent probes. Efficient and viable approaches to design and develop fluorescence probes with tunable photophysical properties without compromising other key features are of paramount importance. Various chemical strategies are explored to increase the quantum yield of these imaging agents without compromising their spatiotemporal resolution, specificity, and tissue penetration capabilities. This review summarizes the strategies implemented to design and synthesize NIR-II emitting nanoparticles and small organic molecule-based fluorescent probes for applications in the biomedical field. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Implantable Materials and Surgical Technologies > Nanoscale Tools and Techniques in Surgery.

Starlike polymer brush-based ultrasmall nanoparticles with simultaneously improved NIR-II fluorescence and blood circulation for efficient orthotopic glioblastoma imaging

Fluorescence imaging (FI) in the second near-infrared region (NIR-II, 1000-1700 nm) has attracted great attention for brain tumor imaging due to its deep penetration and high resolution. However, traditional NIR-II organic fluorescent nanoparticles (NPs) are usually hindered by uncontrolled large size (~30-100 nm), marked aggregation-caused quenching (ACQ) effect, and limited blood circulation (~1-3 h), which have great impact on efficient NIR-II FI of deep brain tumors. Herein, starlike polymer brush-based ultrasmall TQFP-10 NPs, with bright NIR-II fluorescence, prolonged blood circulation, and enhanced tumor accumulation, are facilely prepared for efficient orthotopic glioblastoma (GBM) imaging. Compared with traditional method prepared NPs (physically coated TQF@NPs and PEG modified TQF-PEG_5K NPs), the ultrasmall (~8 nm) TQFP-10 NPs display a higher NIR-II fluorescence QY (1.9%), which is 2.1- and 3.8-fold higher than TQF@NPs (0.9%) and TQF-PEG_5K NPs (0.5%), respectively. In addition, TQFP-10 NPs present a 10.6-fold higher blood circulation half-life (t_1/2 = 8.5 h) than that of TQF-PEG_5K NPs. Consequently, TQFP-10 NPs exhibit 4.2- and 33-fold higher maximal tumor to normal tissue ratio in subcutaneous and in situ NIR-II FI of GBM, respectively, than TQF@NPs and TQF-PEG_5K NPs, attractively realizing GBM imaging. This work provides a general strategy for constructing ultrasmall NIR-II fluorescent NPs with simultaneously improved NIR-II fluorescence and blood circulation for efficient brain tumor imaging.

Designing highly emissive over-1000 nm near-infrared fluorescent dye-loaded polystyrene-based nanoparticles for in vivo deep imaging

Polystyrene-based nanoparticles (PSt NPs) prepared by emulsion polymerization are promising organic matrices for encapsulating over-thousand-nanometer near-infrared (OTN-NIR) fluorescent dyes, such as thiopyrilium IR-1061, for OTN-NIR dynamic live imaging. Herein, we propose an effective approach to obtain highly emissive OTN-NIR fluorescent PSt NPs (OTN-PSt NPs) in which the polarity of the PSt NPs was adjusted by changing the monomer ratio (styrene to acrylic acid) in the PSt NPs and the dimethyl sulfoxide concentration in the IR-1061 loading process. Moreover, OTN-PSt NPs covalently modified with poly(ethylene glycol) (PEG) (OTN-PSt-PEG NPs) showed high dispersion stability under physiological conditions and minimal cytotoxicity. Notably, the optimized OTN-PSt-PEG NPs were effective in the dynamic live imaging of mice. This methodology is expected to facilitate the design of certain polar thiopyrilium dye-loaded OTN-NIR fluorescent imaging probes with high emissivity.

By changing the ratio of acrylic acid to styrene, the loading amount of fluorescent dye can be increased and the optical properties of the resulting bioimaging probe can be improved.

One-for-All Phototheranostic Agent Based on Aggregation-Induced Emission Characteristics for Multimodal Imaging-Guided Synergistic Photodynamic/Photothermal Cancer Therapy

Phototheranostics represents a promising direction for modern precision medicine, which has recently received considerable attention for cancer research. The ingenious integration of all phototheranostic modalities in a single molecule with precise spatial colocalization is a tremendously challenging task, which mainly arises from the complexity of molecular design and energy dissipation. Reports on a single molecular one-for-all theranostic agent are still very rare. Herein, we designed two novel aggregation-induced emission (AIE)-active fluorogens (AIEgens, named DPMD and TPMD) with a cross-shaped donor-acceptor structure via a facile synthetic method and constructed versatile nanoparticles (NPs) by encapsulating AIEgen with an amphiphilic polymer. The AIEgen TPMD with a twisted structure, high donor-acceptor (D-A) strength, small singlet-triplet energy gap, and abundant intramolecular rotators and vibrators was selected as an ideal candidate for balancing and utilizing the radiative and nonradiative energy dissipations. Notably, TPMD NPs simultaneously possess adequate near-infrared (NIR) fluorescence emission at 821 nm for fluorescence imaging, effective reactive oxygen species generation for photodynamic therapy (PDT), and outstanding photothermal effect for photoacoustic imaging, photothermal imaging, and photothermal therapy (PTT), which demonstrates the superior potential of AIE NPs in multimodal imaging-guided synergistic PDT/PTT therapy.

Nanoprobes-Assisted Multichannel NIR-II Fluorescence Imaging-Guided Resection and Photothermal Ablation of Lymph Nodes

Lymph node metastasis is a major metastatic route of cancer and significantly influences the prognosis of cancer patients. Radical lymphadenectomy is crucial for a successful surgery. However, iatrogenic normal organ injury during lymphadenectomy is a troublesome complication. Here, this paper reports a kind of organic nanoprobes (IDSe-IC2F nanoparticles (NPs)) with excellent second near-infrared (NIR-II) fluorescence and photothermal properties. IDSe-IC2F NPs can effectively label lymph nodes and helped achieve high-contrast lymphatic imaging. More importantly, by jointly using IDSe-IC2F nanoparticles and other kinds of nanoparticles with different excitation/emission properties, a multichannel NIR-II fluorescence imaging modality and imaging-guided lymphadenectomy is proposed. With the help of this navigation system, the iatrogenic injury can be largely avoided. In addition, NIR-II fluorescence imaging-guided photothermal treatment ("hot" strategy) can ablate those metastatic lymph nodes which are difficult to deal with during resection ("cold" strategy). Nanoprobes-assisted and multichannel NIR-II fluorescence imaging-guided "cold" and "hot" treatment strategy provides a general new basis for the future precision surgery.

Near-infrared Fluorophores for Thrombosis Diagnosis and Therapy

Thrombosis is an adverse physiological event wherein the resulting thrombus and thrombus-induced diseases collectively result in high morbidity and mortality rates. Currently, nano-medicines that incorporate fluorophores emitting in the near-infrared-I (NIR-I, 700-900 nm) spectral region into their systems have been adopted to afford thrombosis theranostics. However, several unsolved problems such as limited penetration depth and image quality severely impede further applications of such nano-medicine systems. Fortunately, the ability to incorporate fluorophores emitting in the NIR-II (1000-1700 nm) window into nano-medicine systems can unambiguously identify biological processes with high signal-to-noise, deep tissue penetration depth, and high image resolution. Considering the inherently favorable properties of NIR-II fluorophores, we believe such have enormous potential to quickly become incorporated into nano-medicine systems for thrombosis theranostics. In this review, we i) discuss the development of NIR fluorescence as an imaging modality and fluorescent agents; ii) comprehensively summarize the recent development of NIR-I fluorophore-based nano-medicine systems for thrombosis theranostics; iii) highlight the state-of-the-art NIR-II fluorophores that have been designed for the specific purpose of affording thrombotic diagnosis; iv) speculate on possible forward avenues for the use of NIR-II fluorophores towards thrombosis diagnosis and therapy; and v) discuss the potential for their clinical translation.

Noninvasive In Vivo Imaging and Monitoring of 3D-Printed Polycaprolactone Scaffolds Labeled with an NIR Region II Fluorescent Dye

Significant progress has been made in fabricating porous scaffolds with ultrafine fibers for tissue regeneration. However, the lack of noninvasive tracking methods in vivo makes it impossible to track the fate of such scaffolds in situ. The development of near-infrared region II (NIR-II, 1000-1700 nm) dyes provides the possibility of performing noninvasive visualization with deep-tissue penetration and high spatial resolution in vivo. Herein, we developed a polycaprolactone (PCL) ink containing the small organic NIR-II dye SY-1030 and the fluorescently labeled macromolecular dye SY-COO-PCL and fabricated high-resolution NIR-II active scaffolds via electrohydrodynamic jet (EHDJ) printing. All printed scaffolds subcutaneously implanted in mice were clearly imaged one week after the operation. Compared with scaffolds containing SY-1030, the fluorescence intensity emitted from scaffolds containing SY-COO-PCL can be tracked for up to three weeks. Moreover, the image quality can be optimized by adjusting the dye concentration, laser power, and exposure time. The advantage of such NIR-II active scaffolds is evidenced by the lower dye concentration, longer tracking period, and better in vivo stability. We also demonstrated the biocompatibility and biodegradability of the scaffolds containing SY-COO-PCL over a 3-month period. The developed NIR-II active scaffolds have potential applications in biopolymer implant tracking, tissue reconstruction monitoring, and target-position-based drug delivery.

Aggregation Induced Emission and Nonlinear Optical Properties of an Intramolecular Charge-Transfer Compound

Intramolecular charge transfer (ICT) compounds have attracted wide attention for their potential applications in optoelectronic materials and devices such as fluorescent sensors, dye-sensitized solar cells, organic light emitting diodes and nonlinear optics. In this work, we have synthesized a new ICT compound, dimethyl-[4-(7-nitro-benzo[1,2,5]thiadiazol-4-yl)-phenyl]-amine (BTN), and have fabricated it into low dimensional micro/nano structures with well-defined morphologies. These self-assembled nanostructures exhibit high efficiency solid state fluorescence via an aggregation induced emission mechanism, which overcomes the defect of fluorescence quenching caused by aggregation in the solid state of traditional luminescent materials. We also explored and studied the nonlinear optical properties of this material through the Z-scan method, and found that this material exhibits large third-order nonlinear absorption and refraction coefficients, which promises applications of the materials in the fields of nonlinear optics and optoelectronics.

Smart Self-Assembly Amphiphilic Cyclopeptide-Dye for Near-Infrared Window-II Imaging

Development of novel nanomaterials for disease theranostics represents an important direction in chemistry and precision medicine. Fluorescent molecular probes in the second near-infrared window (NIR-II, 1000-1700 nm) show high promise because of their exceptional high detection sensitivity, resolution, and deep imaging depth. Here, a sharp pH-sensitive self-assembling cyclopeptide-dye, SIMM1000, as a smart nanoprobe for NIR-II imaging of diseases in living animals, is reported. This small molecule assembled nanoprobe exhibits smart properties by responding to a sharp decrease of pH in the tumor microenvironment (pH 7.0 to 6.8), aggregating from small nanoprobe (80 nm at pH 7.0) into large nanoparticles (>500 nm at pH 6.8) with ≈20-30 times enhanced fluorescence compared with the non-self-assembled CH-4T. It yields micrometer-scale resolution in blood vessel imaging and high contrast and resolution in bone and tumor imaging in mice. Because of its self-aggregation in acidic tumor microenvironments in situ, SIMM1000 exhibits high tumor accumulation and extremely long tumor retention (>19 days), while being excretable from normal tissues and safe. This smart self-assembling small molecule strategy can shift the paradigm of designing new nanomaterials for molecular imaging and drug development.

Biologically Excretable Aggregation-Induced Emission Dots for Visualizing Through the Marmosets Intravitally: Horizons in Future Clinical Nanomedicine

Superb reliability and biocompatibility equip aggregation-induced emission (AIE) dots with tremendous potential for fluorescence bioimaging. However, there is still a chronic lack of design instructions of excretable and bright AIE emitters. Here, a kind of PEGylated AIE (OTPA-BBT) dots with strong absorption and extremely high second near-infrared region (NIR-II) PLQY of 13.6% is designed, and a long-aliphatic-chain design blueprint contributing to their excretion from an animal's body is proposed. Assisted by the OTPA-BBT dots with bright fluorescence beyond 1100 nm and even 1500 nm (NIR-IIb), large-depth cerebral vasculature (beyond 600 µm) as well as real-time blood flow are monitored through a thinned skull, and noninvasive NIR-IIb imaging with rich high-spatial-frequency information gives a precise presentation of gastrointestinal tract in marmosets. Importantly, after intravenous or oral administration, the definite excretion of OTPA-BBT dots from the body is demonstrated, which provides influential evidence of biosafety.

Er3+ self-sensitized nanoprobes with enhanced 1525 nm downshifting emission for NIR-IIb in vivo bio-imaging

Traditional sensitizer (Yb³⁺ or Nd³⁺) and activator (Er³⁺) co-doped lanthanide-based nanoprobes possessing emission of Er³⁺ at 1525 nm have attracted much attention in NIR-IIb bio-imaging. However, the 1525 nm fluorescence efficiency was not high enough in such co-doped systems due to the serious back energy transfer from the activator to the sensitizer, resulting in a lot of excitation energy loss. Herein, we have designed an efficient NIR-IIb nanoprobe Er³⁺ self-sensitized NaErF₄:0.5%Tm³⁺@NaLuF₄, where substantially all the excitation energy could contribute to Er³⁺ ions and most energy transfer processes were confined among Er³⁺ ions, avoiding the energy dissipation by heterogeneous sensitizer ions. The influence of the types of epitaxial heterogeneous shells, the doping effect and optimal doping concentration of Tm³⁺ ions, as well as the critical shell thickness for obtaining the surface quenching-assisted downshifting emission are systematically investigated to acquire the most efficient 1525 nm luminescence under 800 nm excitation. The quantum yield in the 1500-1700 nm region reached 13.92%, enabling high-resolution through-skull cerebrovascular microscopy imaging and large-depth in vivo physiological dynamic imaging with an extremely low excitation powder density of 35 mW cm^-2. The designed nanoprobe can be potentially used for brain science research and clinical diagnosis.

Organic Dots with Large π-Conjugated Planar for Cholangiography beyond 1500 nm in Rabbits: A Non-Radioactive Strategy

Iatrogenic extrahepatic bile duct injury remains a dreaded complication while performing cholecystectomy. Although X-ray based cholangiography could reduce the incidence of biliary tract injuries, the deficiencies including radiation damage and expertise dependence hamper its further clinical application. The effective strategy for intraoperative cholangiography is still urgently required. Herein, a fluorescence-based imaging approach for cholangiography in the near-infrared IIb window (1500-1700 nm) using TT3-oCB, a bright aggregation-induced emission luminogen with large π-conjugated planar unit, is reported. In phantom studies, TT3-oCB nanoparticles exhibit high near-infrared IIb emission and show better image clarity at varying penetrating depths. When intrabiliary injected into the gallbladder or the common bile duct of the rabbit, TT3-oCB nanoparticles enable the real-time imaging of the biliary structure with deep penetrating capability and high signal-to-background ratio. Moreover, the tiny iatrogenic biliary injuries and the gallstones in established disease models could be precisely diagnosed by TT3-oCB nanoparticle assisted near-infrared IIb imaging. In summary, we reported a feasible application for aggregation-induced emission dots as biliary contrast agent and realized high-quality cholangiography in the near-infrared IIb window with precise diagnostic ability and nonradioactive damage, which could possibly be applied for intraoperative diagnosis.

Rational Design of Dual-Emission Lanthanide Metal-Organic Framework for Visual Alkaline Phosphatase Activity Assay

The alkaline phosphatase (ALP) activity assay is very significant for disease diagnosis and biomedical research. Lanthanide metal-organic framework (Ln-MOF) based fluorescence sensors have great application potential in ALP activity assays. However, it is critical but challenging to investigate the emission law of Ln-MOFs for revealing rational design principles and selecting an appropriate MOF. Here, we describe a reasonable design strategy for dual-emission Ln-MOFs based on theoretical calculations. This strategy combines Reinhoudt empirical rule, intramolecular charge transfer theory, and aggregation/coordination-induced emission theory; reveals the luminescence law of Ln-MOFs; and provides theoretical guidance for the rational design of dual-emission Ln-MOFs. On the basis of this strategy, we create a dual-emission Tb-MOF fluorescent probe used for ALP activity assay and investigate the detection mechanism. The probe shows ultrasensitive (limit of detection 0.002 mU mL^-1) and selective response to ALP, and it suits for point-of-care visual detection coupled with a self-designed portable enzyme activity assay kit and smartphone-assisted visual device. The kit-based visual assay method can accurately quantify the activity of ALP in real serum samples (recovery >93%, and relative error is less than 6.8% compared with the results of fluorescence spectrometer-based method) and consumes only 25 μL of serum. In addition, a logical decoder based on the "dual-key unlocking strategy" is designed, providing a feasible solution for the development of intelligent ALP activity detection equipment. As far as we know, this is the first report of a theoretical calculation-guided versatile design strategy for dual-emission Ln-MOFs and a portable enzyme activity assay kit for visual detection.

Recent Progress in Nanomedicine for Melanoma Theranostics With Emphasis on Combination Therapy

Melanoma is an aggressive type of skin cancer with increasing incidence and high mortality rates worldwide. However, there is still a lack of efficient and resolutive treatment strategies, particularly in clinical settings. Currently, nanomedicine, an emerging area in the medical field, is being widely investigated in small animal models to afford melanoma theranostics. However, several problems, such as tumor heterogeneity, and drug resistance treatment with a single therapy, remain unresolved. Previous reviews have primarily focused on monotherapy for melanoma in the context of nanomedicine. In this review article, we summarize the recent progress in the application of nanomedicine for melanoma treatment, with particular attention to combination therapy based on nanomedicine to achieve optimized therapeutic output for melanoma treatment. In addition, we also highlight the fluorescence-guided strategies for intraoperative melanoma detection, especially in the near-infrared imaging window with greatly improved imaging contrast and penetration depth.

Endogenous H2S-Activated Orthogonal Second Near-Infrared Emissive Nanoprobe for In Situ Ratiometric Fluorescence Imaging of Metformin-Induced Liver Injury

Metformin as a hypoglycemic drug for antidiabetic treatment has emerged as a multipotential drug for many disease treatments such as cognitive disorders, cancers, promoting weight loss. However, overdose uptake may upregulate the hepatic H₂S level, subsequently leading to serious liver injury and toxicity. Therefore, developing intelligent second near-infrared (NIR-II) emitting nanoprobes by using endogenous H₂S as a smart trigger for noninvasive highly specific in situ monitoring of the metformin-induced hepatotoxicity is highly desirable, which is rarely explored. Herein, an endogenous H₂S activated orthogonal NIR-II emitting myrica rubra-like nanoprobe based on NaYF₄:Gd/Yb/Er@NaYF₄:Yb@SiO₂ coated with Ag nanodots was explored for highly specific in vivo ratiometrically monitoring of hepatotoxicity. The designed nanoprobes were mainly uptaken by the liver and subsequently converted to NaYF₄:Gd/Yb/Er@NaYF₄:Yb@SiO₂@Ag₂S via in situ sulfuration reaction triggered by the overexpressed endogenous H₂S in the injured liver tissues, finally leading to a turn-on orthogonal emission centered at 1053 nm (irradiation by 808 nm laser) and 1525 nm (irradiation by 980 nm laser). The designed nanoprobe presents a high detection limit down to 0.7 nM of H₂S. More importantly, the in situ highly specific ratiometric imaging of the metformin-induced hepatotoxicity was successfully achieved by using the activatable orthogonal NIR-II emitting probe. Our results provide an NIR-II ratiometric fluorescence imaging strategy for highly sensitive/specific diagnosis of hepatotoxicity levels induced by metformin.

NIR-II bioimaging of small organic molecule

In recent years, people have been actively exploring new imaging methods with high biological imaging performance because the clinical image definition and depth in vivo cannot meet the requirements of early diagnosis and prognosis. Based on the traditional near-infrared region I (NIR-I), the molecular probe of the near-infrared region II (NIR-II) is further explored and developed. In the NIR-II region due to the wavelength is longer than the NIR-I region can effectively reduce the molecular scattering, optical absorption of the organization, the organization of spontaneous fluorescence negligible, thus the NIR-II Fluorescence imaging (FI) can get deeper penetration depth, higher signal-to-background ratio (SBR) and better spatiotemporal resolution, FI in NIR-II region are an important and rapidly developing research region for future imaging. In the NIR-II fluorophore, small organic molecule fluorophore has attracted much attention because of its good biocompatibility and good pharmacokinetic properties. In this review, we briefly introduced the existing NIR-II organic small molecule fluorophores, and introduced the existing relatively mature methods for improving quantum yield and water solubility, and the small molecule dyes on FI of various improvement methods, also briefly introduces the small molecules of photoacoustic imaging (PAI), and a brief introduction of imaging-guided surgery (IGS) for some small organic molecules, finally, a reasonable prospect is made for the development of small organic molecules.

Facilitation of molecular motion to develop turn-on photoacoustic bioprobe for detecting nitric oxide in encephalitis

Nitric oxide (NO) is an important signaling molecule overexpressed in many diseases, thus the development of NO-activatable probes is of vital significance for monitoring related diseases. However, sensitive photoacoustic (PA) probes for detecting NO-associated complicated diseases (e.g., encephalitis), have yet to be developed. Herein, we report a NO-activated PA probe for in vivo detection of encephalitis by tuning the molecular geometry and energy transformation processes. A strong donor-acceptor structure with increased conjugation can be obtained after NO treatment, along with the active intramolecular motion, significantly boosting "turn-on" near-infrared PA property. The molecular probe exhibits high specificity and sensitivity towards NO over interfering reactive species. The probe is capable of detecting and differentiating encephalitis in different severities with high spatiotemporal resolution. This work will inspire more insights into the development of high-performing activatable PA probes for advanced diagnosis by making full use of intramolecular motion and energy transformation processes.

Near-Infrared-II Semiconducting Polymer Dots for Deep-tissue Fluorescence Imaging

Fluorescence imaging, particularly in the NIR-II region (1000-1700 nm), has become an unprecedented tool for deep-tissue in vivo imaging. Among the fluorescent nanoprobes, semiconducting polymer nanoparticles (Pdots) appear to be a promising agent because of their tunable optical and photophysical properties, ultrahigh brightness, minimal autofluorescence, narrow-size distribution, and low cytotoxicity. This review elucidates the recent advances in Pdots for deep-tissue fluorescence imaging and the facing future translation to clinical use.

Polarity-active NIR probes with strong two-photon absorption and ultrahigh binding affinity of insulin amyloid fibrils

Amyloid fibrils are associated with many neurodegenerative diseases. In situ and in vivo visualization of amyloid fibrils is important for medical diagnostics and requires fluorescent probes with both excitation and emission wavelengths in the far-red and NIR region, and simultaneously with high binding-affinity to amyloid fibrils and the ability to cross the blood-brain barrier, which, however, remain a challenge. Here, we rationally design and synthesize an excellent polarity-sensitive two-photon excited NIR fluorophore (TZPI) based on a donor (D)-acceptor (A)-ion compound. The electron-rich carbazole group and the ionic pyridinium bromide group, linked by an electron-poor π-conjugated benzothiadiazole group, ensure strong near infrared (NIR) emission. Furthermore, the lipophilic carbazole together with the benzothiadiazole group facilitates docking of the probe in the hydrophobic domains of amyloid aggregates with the dissociation constant K d = 20 nM and 13.5-fold higher binding affinity to insulin fibrils than the commercial probe ThT. On association with the amyloid fibrils, the tiny decrease in polarity leads to a large increase in its NIR emission intensity with an on-off ratio > 10; meanwhile, the TZPI probe exhibits a quantum yield of up to 30% and two-photon absorption cross-section values of up to 467.6 GM at 890 nm. Moreover, the application of TZPI in two-photon imaging is investigated. The ultrahigh binding affinity, the strong NIR emission, the good two-photon absorption properties, the high photo-stability, the appropriate molecular mass of 569 Da and the lipophilicity with log P = 1.66 ± 0.1 to cross the BBB make TZPI promising as an ideal candidate for detecting amyloid plaques in vivo.

Whole-Body Fluorescence Imaging in the Near-Infrared Window

Fluorescence imaging is one of the most widely used in vivo imaging methods for both fundamental research and clinical practice. Due to the reduced photon scattering, absorption, and autofluorescence in tissues, the emerging near-infrared (NIR) imaging (650-1700 nm) can afford deep tissue imaging with high spatiotemporal resolution and in vivo report the anatomical structures as well as the physiological activities in a whole-body level. Here, we give a brief introduction to fluorescence imaging in the first NIR (NIR-I, 650-950 nm) and second NIR (NIR-II, 1000-1700 nm) windows, summarize the recently developed NIR fluorophores and their applications in whole-body vascular system imaging, precision cancer theranostics, and regenerative medicine. Finally, the clinical applications and future prospects of in vivo NIR fluorescence imaging are also discussed.

AIE-active two-photon fluorescent nanoprobe with NIR-II light excitability for highly efficient deep brain vasculature imaging

Aggregation induced emission (AIE)-active bright two-photon fluorescent probes with second near-infrared (NIR-II) light excitability can be used for efficient brain bioimaging studies, wherein the fabrication of water-dispersible nanoparticles by encapsulating the hydrophobic probes with amphiphilic polymer holds the key to ensuring biocompatibility and in vivo adaptability. However, barely any study has evaluated the structural requirements that can substantially affect the water-dispersible nanoparticle formation ability of an organic AIE-active dye with amphiphilic polymers. The present study systematically assessed the structural dependency of a well-known acrylonitrile based AIE system/fluorogenic core upon the formation of water-dispersible nanoparticles and elucidated how the structural modifications can impact the in vivo two-photon imaging.

Methods: A total of four acrylonitrile-based aggregation induced emission (AIE)-active two-photon (TP) fluorescent probes (AIETP, AIETP C1, AIETP C2 and AIETP C3) have been judiciously designed and synthesized with structural variations to realize how the structural alterations could substantially influence the water-dispersible nanoparticle formation ability (with amphiphilic polymers) and photo-stability to impact the in vivo imaging.

Results: It has been found that the incorporation of the phenyl-thiazole unit in AIETP, AIETP C2 and AIETP C3 facilitated the formation of water-dispersible nanoparticles (NPs) with amphiphilic polymers (Pluronic F127) whereas the presence of only phenyl moiety instead in AIETP C1 could not meet the suitable condition to form the NPs with good aqueous dispersibility. Rationally designed AIETP NPs that exhibited higher brightness, improved photostability and good two-photon absorption cross section was successfully employed for in vivo brain vasculature imaging.

Conclusions: Robust noninvasive 2D and 3D two-photon (NIR-II light, 1040 nm) brain vasculature imaging with beneficial attributes such as outstanding penetration depth (800 µm) and exceptional spatial resolution (1.92 µm), were achieved by utilizing AIETP NPs in this study.

Preclinical techniques to investigate exercise training in vascular pathophysiology

Atherosclerosis is a dynamic process starting with endothelial dysfunction and inflammation and eventually leading to life-threatening arterial plaques. Exercise generally improves endothelial function in a dose-dependent manner by altering hemodynamics, specifically by increased arterial pressure, pulsatility, and shear stress. However, athletes who regularly participate in high-intensity training can develop arterial plaques, suggesting alternative mechanisms through which excessive exercise promotes vascular disease. Understanding the mechanisms that drive atherosclerosis in sedentary versus exercise states may lead to novel rehabilitative methods aimed at improving exercise compliance and physical activity. Preclinical tools, including in vitro cell assays, in vivo animal models, and in silico computational methods, broaden our capabilities to study the mechanisms through which exercise impacts atherogenesis, from molecular maladaptation to vascular remodeling. Here, we describe how preclinical research tools have and can be used to study exercise effects on atherosclerosis. We then propose how advanced bioengineering techniques can be used to address gaps in our current understanding of vascular pathophysiology, including integrating in vitro, in vivo, and in silico studies across multiple tissue systems and size scales. Improving our understanding of the antiatherogenic exercise effects will enable engaging, targeted, and individualized exercise recommendations to promote cardiovascular health rather than treating cardiovascular disease that results from a sedentary lifestyle.

Synthesizing Ag+: MgS, Ag+: Nb2S5, Sm3+: Y2S3, Sm3+:Er2S3, and Sm3+:ZrS2 Compound Nanoparticles for Multicolor Fluorescence Imaging of Biotissues

Development of the fluorophores whose fluorescence bands can be flexibly selected is of great interest for biotissue imaging. Compounds of Ag⁺:MgS, Ag⁺:Nb₂S₅, Sm³⁺:Y₂S₃, Sm³⁺:Er₂S₃, and Sm³⁺:ZrS₂ were obtained through new chemical synthesis. They were characterized by X-ray photoelectron spectroscopy, X-ray diffraction spectroscopy, and transmission electron microscopy. They revealed polychromatic-photoluminescence spectra when excited by 280, 380, 480, 580, 680, and 785 nm light. Especially, near-infrared emission ranging from 800-1100 nm was found upon 785 nm light excitation. A band model was proposed to explain transitions responsible for the observed components of emission. Their broad fluorescence spectra cover from the ultraviolet to near-infrared spectral range. Their ability of emitting wide-range fluorescence was utilized for multicolor fluorescence imaging of biotissues, as demonstrated by pig-kidney tissue samples.

Protein-Enhanced NIR-IIb Emission of Indocyanine Green for Functional Bioimaging

Fluorescence imaging performed in the 1500-1700 nm spectral range (labeled near-infrared IIb, NIR-IIb) promises high imaging contrast and spatial resolution for its little photon scattering effect and minimum autofluorescence. Though inorganic and organic probes have been developed for NIR-IIb bioimaging, most are in the preclinical stage, hampering further clinical application. Herein, we showed that indocyanine green (ICG), a US Food and Drug Administration (FDA)-approved agent, exhibited a remarkable amount of NIR-IIb emission when dissolved into different protein solutions, including human serum albumin, rat bile, and fetal bovine serum. We performed fluorescence imaging in the NIR-IIb window to visualize structures of the lymph system, extrahepatic biliary tract, and cerebrovascular. The results demonstrated that proteins promoted NIR-IIb emission of ICG in vivo and that NIR-IIb imaging with ICG preserved a higher signal-to-background ratio and spatial resolution compared with the conventional NIR-II fluorescence imaging. Our findings confirm that NIR-IIb fluorescence imaging can be successfully performed using the clinically approved agent ICG. Further clinical application in the NIR-IIb region would hopefully be carried out with appropriate ICG-protein solutions.

Lighting up forensic science by aggregation-induced emission: A review

Forensic science requires a fast, sensitive, and anti-interfering imaging tool for on-site investigation and bio-analysis. The aggregation-induced emission (AIE) phenomenon exhibits remarkable luminescence properties (large Stokes shift, diverse molecular structures, and high photo-stability), which can provide a viable solution for on-site analysis, while at the same time overcoming the problem of aggregation-caused quenching (ACQ). Based on the outstanding performance in chemical analysis and bio-sensing, AIE materials have great prospects in the field of forensic science. Therefore, the application of AIE in forensic science has been summarized for the first time in this article. After a brief introduction to the concept and development of AIE, its applications in the determination of toxic or hazardous substances, based on data on poisoning deaths, has been summarized. Subsequently, besides the bio-imaging function, other applications of AIE in analyzing markers related to forensic genetics, forensic pathology, (focusing on the corpse) and clinical forensics (focusing on the living) have been discussed. In addition, applications of AIE molecules in criminal investigations, including recognition of fingerprints and blood stains, detection of explosives and chemical warfare agents, and anti-counterfeiting have also been presented. It is hoped that this review will light up the future of forensic science by stimulating more research work on the suitability of AIE materials in advancing forensic science.

Incorporation of Planar Blocks into Twisted Skeletons: Boosting Brightness of Fluorophores for Bioimaging beyond 1500 Nanometer

The brightness of organic fluorescence materials determines their resolution and sensitivity in fluorescence display and detection. However, strategies to effectively enhance the brightness are still scarce. Conventional planar π-conjugated molecules display excellent photophysical properties as isolated species but suffer from aggregation-caused quenching effect when aggregated owing to the cofacial π-π interactions. In contrast, twisted molecules show high photoluminescence quantum yield (Φ_PL) in aggregate while at the cost of absorption due to the breakage in conjugation. Therefore, it is challenging to integrate the strong absorption and high solid-state Φ_PL, which are two main indicators of brightness, into one molecule. Herein, we propose a molecular design strategy to boost the brightness through the incorporation of planar blocks into twisted skeletons. As a proof-of-concept, twisted small-molecule TT3-oCB with larger π-conjugated dithieno[3,2-b:2',3'-d]thiophene unit displays superb brightness at the NIR-IIb (1500-1700 nm) than that of TT1-oCB and TT2-oCB with smaller thiophene and thienothiophene unit, respectively. Whole-body angiography using TT3-oCB nanoparticles presents an apparent vessel width of 0.29 mm. Improved NIR-IIb image resolution is achieved for femoral vessels with an apparent width of only 0.04 mm. High-magnification through-skull microscopic NIR-IIb imaging of cerebral vasculature gives an apparent width of ∼3.3 μm. Moreover, the deeply located internal organ such as bladder is identified with high clarity. The present molecular design philosophy embodies a platform for further development of in vivo bioimaging.

Visible-near infrared-II skull optical clearing window for in vivo cortical vasculature imaging and targeted manipulation

Skull optical clearing window permits us to perform in vivo cortical imaging without craniotomy, but mainly limits to visible (vis)-near infrared (NIR)-I light imaging. If the skull optical clearing window is available for NIR-II, the imaging depth will be further enhanced. Herein, we developed a vis-NIR-II skull optical clearing agents with deuterium oxide instead of water, which could make the skull transparent in the range of visible to NIR-II. Using a NIR-II excited third harmonic generation microscope, the cortical vasculature of mice could be clearly distinguished even at the depth of 650 μm through the vis-NIR-II skull clearing window. The imaging depth after clearing is close to that without skull, and increases by three times through turbid skull. Furthermore, the new skull optical clearing window promises to realize NIR-II laser-induced targeted injury of cortical single vessel. This work enhances the ability of NIR-II excited nonlinear imaging techniques for accessing to cortical neurovasculature in deep tissue.

Centimeter-Deep NIR-II Fluorescence Imaging with Nontoxic AIE Probes in Nonhuman Primates

Fluorescence probes with aggregation-induced emission (AIE) characteristics are of great importance in biomedical imaging with superior spatial and temporal resolution. However, the lack of toxicity studies and deep tissue imaging in nonhuman primates hinders their clinical translation. Here, we report the blood chemistry and histological analysis in nonhuman primates treated with AIE probes over tenfold of an intravenous dose of clinically used indocyanine green (ICG) during a study period of 36 days to demonstrate AIE probes are nontoxic. Furthermore, through bright and nontoxic AIE probes and fluorescence imaging in the second window (NIR-II, 1,000–1,700 nm), we achieve an unprecedented 1.5-centimeter-deep vascular imaging in nonhuman primates, breaking the current limitation of millimeter-deep NIR-II fluorescence imaging. Our important findings, i.e., nontoxic features of AIE probes and centimeter-deep NIR-II vascular imaging in nonhuman primates, may facilitate successful translation of AIE probes in clinical trials.

Self-assembled AIEgen nanoparticles for multiscale NIR-II vascular imaging

In the recent decades, fluorogens with aggregation-induced emission (AIEgens) have been intensively explored in biomedical applications. One main strategy to bring these hydrophobic AIEgens into the aqueous biological environment is to encapsulate them in nanoparticles with functionalized polymeric matrices. However, exploration of reliable strategies that can afford AIE nanoparticles with uniform size and stable loading efficiency with minimized variation still remains a challenge. Here, we rationally designed amphiphilic AIEgens, constructed by a hydrophobic donor-acceptor-donor (D-A-D) core and hydrophilic polyethylene glycol (PEG) chain. The afforded amphiphilic AIEgens can self-assemble into uniform nanoparticles with average sizes of ~35 nm, showing an emission maximum beyond 1000 nm and quantum yields (QYs) above 10%. We then used the bright AIE nanoparticles for multiscale intravital vascular fluorescence imaging in the second near-infrared window (NIR-II, 1000-1700 nm) in mouse and rabbit models with a high-resolution of ~38 μm and a penetration depth of ~1 cm. As such, our results demonstrate an efficient self-assembly strategy to construct advanced AIE nanoparticles for angiography.

NIR-II bioluminescence for in vivo high contrast imaging and in situ ATP-mediated metastases tracing

Bioluminescence imaging has been widely used in life sciences and biomedical applications. However, conventional bioluminescence imaging usually operates in the visible region, which hampers the high-performance in vivo optical imaging due to the strong tissue absorption and scattering. To address this challenge, here we present bioluminescence probes (BPs) with emission in the second near infrared (NIR-II) region at 1029 nm by employing bioluminescence resonance energy transfer (BRET) and two-step fluorescence resonance energy transfer (FRET) with a specially designed cyanine dye FD-1029. The biocompatible NIR-II-BPs are successfully applied to vessels and lymphatics imaging in mice, which gives ~5 times higher signal-to-noise ratios and ~1.5 times higher spatial resolution than those obtained by NIR-II fluorescence imaging and conventional bioluminescence imaging. Their capability of multiplexed imaging is also well displayed. Taking advantage of the ATP-responding character, the NIR-II-BPs are able to recognize tumor metastasis with a high tumor-to-normal tissue ratio at 83.4.

Conventional bioluminescence imaging usually operates in the visible region and its performance is limited by strong tissue absorption and scattering. Here, the authors present bioluminescence probes (BPs) with emission in the second near infrared (NIR-II) region, and show the NIR-II-BPs could sensitively recognize tumor metastasis with a high tumor-to-normal tissue ratio.