Near-Infrared-II Cyanine/Polymethine Dyes, Current State and Perspective

Supramolecular dye nanoassemblies for advanced diagnostics and therapies

Dyes have conventionally been used in medicine for staining cells, tissues, and organelles. Since these compounds are also known as photosensitizers (PSs) which exhibit photoresponsivity upon photon illumination, there is a high desire towards formulating these molecules into nanoparticles (NPs) to achieve improved delivery efficiency and enhanced stability for novel imaging and therapeutic applications. Furthermore, it has been shown that some of the photophysical properties of these molecules can be altered upon NP formation thereby playing a major role in the outcome of their application. In this review, we primarily focus on introducing dye categories, their formulation strategies and how these strategies affect their photophysical properties in the context of photothermal and non-photothermal applications. More specifically, the most recent progress showing the potential of dye supramolecular assemblies in modalities such as photoacoustic and fluorescence imaging, photothermal and photodynamic therapies as well as their employment in photoablation as a novel modality will be outlined. Aside from their photophysical activity, we delve shortly into the emerging application of dyes as drug stabilizing agents where these molecules are used together with aggregator molecules to form stable nanoparticles.

Exploring flavylium-based SWIR emitters: Design, synthesis and optical characterization of dyes derivatized with polar moieties

Imaging in the shortwave infrared (SWIR, 1000-1700 nm) region is gaining traction for biomedical applications, leading to an in-depth search for fluorophores emitting at these wavelengths. The development of SWIR emitters, to be used in vivo in biological media, is mostly hampered by the considerable lipophilicity of the structures, resulting from the highly conjugated scaffold required to shift the emission to this region, that limit their aqueous solubility. In this work, we have modulated a known SWIR emitter, named Flav7, by adding hydrophilic moieties to the flavylium scaffold and we developed a new series of Flav7-derivatives, which proved to be indeed more polar than the parent compound, but still not freely water-soluble. Optical characterization of these derivatives allowed us to select FlavMorpho, a new compound with improved emission properties compared to Flav7. Encapsulation of the two compounds in micelles resulted in water-soluble SWIR emitters, with FlavMorpho micelles being twice as emissive as Flav7 micelles. The SWIR emission extent of FlavMorpho micelles proved also superior to the tail-emission of Indocyanine Green (ICG), the FDA-approved reference cyanine, in the same region, by exciting the probes at their respective absorption maxima in phosphate buffered saline (PBS) solution. The availability of optical imaging devices equipped with lasers able to excite these dyes at their maximum of absorption in the SWIR region, could pave the way for implemented SWIR imaging results.

Albumin-seeking dyes with adjustable assemblies in situ enable programmable imaging windows and targeting tumor imaging

Cyanine dyes are widely used organic probes for in vivo imaging due to their tunable fluorescence. They can form complexes with endogenous albumin, resulting in enhanced brightness and photostability. However, this binding is uncontrollable and irreversible, leading to considerable nonspecific background signals and unregulated circulation time. Methods: Here, we connect varying numbers of 4-(4-iodophenyl) butanoic acid (IP) as albumin-binding moieties (ABM) to the cyanine dye, enabling dynamic and controllable binding with albumin. Meanwhile, we provide a blocking method to completely release the dye from covalent capture with albumin, resulting in specific targeting fluorescence. Furthermore, we evaluate the pharmacokinetics and tumor targeting of the developed dyes. Results: The engineered dyes can dynamically and selectively bind with multiple albumins to change the in situ size of assemblies and circulation time, providing programmable regulation over the imaging time window. The nucleophilic substitution of meso-Cl with water-soluble amino acids or targeting peptides for IP-engineered dye further addresses the nonspecific signals caused by albumin, allowing for adjustable angiography time and efficient tumor targeting. Conclusion: This study rationalizes the binding modes of dyes and proteins, applicable to a wide range of near-infrared (NIR) dyes for improving their in vivo molecular imaging.

Confinement of a Styryl Dye into Nanoporous Aluminophosphates: Channels vs. Cavities

Styryl dyes are generally poor fluorescent molecules inherited from their flexible molecular structures. However, their emissive properties can be boosted by restricting their molecular motions. A tight confinement into inorganic molecular sieves is a good strategy to yield highly fluorescent hybrid systems. In this work, we compare the confinement effect of two Mg-aluminophosphate zeotypes with distinct pore systems (the AEL framework, a one-dimensional channeled structure with elliptical pores of 6.5 Å × 4.0 Å, and the CHA framework, composed of large cavities of 6.7 Å × 10.0 Å connected by eight-ring narrower windows) for the encapsulation of 4-DASPI styryl dye (trans-4-[4-(Dimethylamino)styryl]-1-methylpyridinium iodide). The resultant hybrid systems display significantly improved photophysical features compared to 4-DASPI in solution as a result of tight confinement in both host inorganic frameworks. Molecular simulations reveal a tighter confinement of 4-DASPI in the elliptical channels of AEL, explaining its excellent photophysical properties. On the other hand, a singular arrangement of 4-DASPI dye is found when confined within the cavity-based CHA framework, where the 4-DASPI molecule spans along two adjacent cavities, with each aromatic ring sitting on these adjacent cavities and the polymethine chain residing within the narrower eight-ring window. However, despite the singularity of this host-guest arrangement, it provides less tight confinement for 4-DASPI than AEL, resulting in a slightly lower quantum yield.

Rational Design and Biological Application of Hybrid Fluorophores

Fluorophores are considered powerful tools for not only enabling the visualization of cell structures, substructures, and biological processes, but also making for the quantitative and qualitative measurement of various analytes in living systems. However, most fluorophores do not meet the diverse requirements for biological applications in terms of their photophysical and biological properties. Hybridization is an important strategy in molecular engineering that provides fluorophores with complementarity and multifunctionality. This review summarizes the basic strategies of hybridization with four classes of fluorophores, including xanthene, cyanine, coumarin, and BODIPY with a focus on their structure-property relationship (SPR) and biological applications. This review aims to provide rational hybrid ideas for expanding the reservoir of knowledge regarding fluorophores and promoting the development of newly produced fluorophores for applications in the field of life sciences.

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.

Doubly Strapped Zwitterionic NIR-I and NIR-II Heptamethine Cyanine Dyes for Bioconjugation and Fluorescence Imaging

Heptamethine cyanine dyes enable deep tissue fluorescence imaging in the near infrared (NIR) window. Small molecule conjugates of the benchmark dye ZW800-1 have been tested in humans. However, long-term imaging protocols using ZW800-1 conjugates are limited by their instability, primarily because the chemically labile C4'-O-aryl linker is susceptible to cleavage by biological nucleophiles. Here, we report a modular synthetic method that produces novel doubly strapped zwitterionic heptamethine cyanine dyes, including a structural analogue of ZW800-1, with greatly enhanced dye stability. NIR-I and NIR-II versions of these doubly strapped dyes can be conjugated to proteins, including monoclonal antibodies, without causing undesired fluorophore degradation or dye stacking on the protein surface. The fluorescent antibody conjugates show excellent tumor-targeting specificity in a xenograft mouse tumor model. The enhanced stability provided by doubly strapped molecular design will enable new classes of in vivo NIR fluorescence imaging experiments with possible translation to humans.

Photoisomerization of Heptamethine Cyanine Dyes Results in Red-Emissive Species: Implications for Near-IR, Single-Molecule, and Super-Resolution Fluorescence Spectroscopy and Imaging

Photoisomerization kinetics of the near-infrared (NIR) fluorophore Sulfo-Cyanine7 (SCy7) was studied by a combination of fluorescence correlation spectroscopy (FCS) and transient state (TRAST) excitation modulation spectroscopy. A photoisomerized state with redshifted emission was identified, with kinetics consistent with a three-state photoisomerization model. Combining TRAST excitation modulation with spectrofluorimetry (spectral-TRAST) further confirmed an excitation-induced redshift in the emission spectrum of SCy7. We show how this red-emissive photoisomerized state contributes to the blinking kinetics in different emission bands of NIR cyanine dyes, and how it can influence single-molecule, super-resolution, as well as Förster resonance energy transfer (FRET) and multicolor readouts. Since this state can also be populated at moderate excitation intensities, it can also more broadly influence fluorescence readouts, also readouts not relying on high excitation conditions. However, this additional red-emissive state and its photodynamics, as identified and characterized in this work, can also be used as a strategy to push the emission of NIR cyanine dyes further into the NIR and to enhance photosensitization of nanoparticles with absorption spectra further into the NIR. Finally, we show that the photoisomerization kinetics of SCy7 and the formation of its redshifted photoisomer depend strongly on local environmental conditions, such as viscosity, polarity, and steric constraints, which suggests the use of SCy7 and other NIR cyanine dyes as environmental sensors. Such environmental information can be monitored by TRAST, in the NIR, with low autofluorescence and scattering conditions and on a broad range of samples and experimental conditions.

Xanthene-Based Nitric Oxide-Responsive Nanosensor for Photoacoustic Imaging in the SWIR Window

Shortwave infrared (SWIR) dyes are characterized by their ability to absorb light from 900 to 1400 nm, which is ideal for deep tissue imaging owing to minimized light scattering and interference from endogenous pigments. An approach to access such molecules is to tune the photophysical properties of known near-infrared dyes. Herein, we report the development of a series of easily accessible (three steps) SWIR xanthene dyes based on a dibenzazepine donor conjugated to thiophene (SCR-1), thienothiophene (SCR-2), or bithiophene (SCR-3). We leverage the fact that SCR-1 undergoes a bathochromic shift when aggregated for in vivo studies by developing a ratiometric nanoparticle for NO (rNP-NO), which we employed to successfully visualize pathological levels of nitric oxide in a drug-induced liver injury model via deep tissue SWIR photoacoustic (PA) imaging. Our work demonstrates how easily this dye series can be utilized as a component in nanosensor designs for imaging studies.

Albumin-based nanoparticle for dual-modality imaging of the lymphatic system

The lymphatic system is a complex network of lymphatic vessels, lymph nodes, and lymphoid organs. The current understanding of the basic mechanism and framework of the lymphatic system is relatively limited and not ideal for exploring the function of the lymphatic system, diagnosing lymphatic system diseases, and controlling tumor metastasis. Imaging modalities for evaluating lymphatic system diseases mainly include lymphatic angiography, reactive dye lymphatic angiography, radionuclide lymphatic angiography, computed tomography, and ultrasonography. However, these are insufficient for clinical diagnosis. Some novel imaging methods, such as magnetic resonance imaging, positron emission computed tomography, single-photon emission computed tomography, contrast-enhanced ultrasonography, and near-infrared imaging with agents such as cyanine dyes, can reveal lymphatic system information more accurately and in detail. We fabricated an albumin-based fluorescent probe for dual-modality imaging of the lymphatic system. A near-infrared cyanine dye, IR-780, was absorbed into bovine serum albumin (BSA), which was covalently linked to a molecule of diethylenetriaminepentaacetic acid to chelate gadolinium Gd3+. The fabricated IR-780@BSA@Gd3+ nanocomposite demonstrates strong fluorescence and high near-infrared absorption and can be used as a T1 contrast agent for magnetic resonance imaging. In vivo dual-modality fluorescence and magnetic resonance imaging showed that IR-780@BSA@Gd3+ rapidly returned to the heart through the lymphatic circulation after it was injected into the toe webs of mice, facilitating good lymphatic imaging. The successful fabrication of the new IR-780@BSA@Gd3+ nanocomposite will facilitate the study of the mechanism and morphological structure of the lymphatic system.

Microbiota in vivo imaging approaches to study host-microbe interactions in preclinical and clinical setting

In vivo imaging in preclinical and clinical settings can enhance knowledge of the host-microbiome interactions. Imaging techniques are a crucial node between findings at the molecular level and clinical implementation in diagnostics and therapeutics. The purpose of this study was to review existing knowledge on the microbiota in the field of in vivo imaging and provide guidance for future research, emphasizing the critical role that molecular imaging plays in increasing understanding of the host-microbe interaction. Preclinical microbiota animal models lay the foundation for the clinical translatability of novel microbiota-based therapeutics. Adopting animal models in which factors such as host genetic landscape, microbiota profile, and diet can be controlled enables investigating how the microbiota contributes to immunological dysregulation and inflammatory disorders. Current preclinical imaging of gut microbiota relies on models where the bacteria can be isolated, labelled, and re-administered. In vivo, optical imaging, ultrasound and magnetic resonance imaging define the bacteria's biodistribution in preclinical models, whereas nuclear imaging investigates bacterial metabolic activity. For the clinical investigation of microbe-host interactions, molecular nuclear imaging is increasingly becoming a promising approach. Future microbiota research should develop selective imaging probes to investigate in vivo microbiota profiles and individual strains of specific microbes. Preclinical knowledge can be translated into the molecular imaging field with great opportunities for studying the microbiome.

Conjugated/nonconjugated alternating copolymers for enhanced NIR-II fluorescence imaging and NIR-II photothermal-ferrotherapy

Conjugated polymers hold great promise for NIR-II fluorescence imaging (FI)-guided NIR-II photothermal therapy (PTT) due to the advantages of easy modification of chemical structures and adjustable NIR absorption. However, to make use of these advantages, it is of paramount importance to formulate conjugated polymers with excellent solubility in organic solution, great NIR-II photothermal conversion efficiency, and high NIR-II fluorescence quantum yield. Herein, a new class of conjugated/nonconjugated alternating copolymers (CNACPs) is reported by introducing nonconjugated linkers into a conjugated backbone to modulate the extinction coefficient at 1064 nm and NIR-II fluorescence quantum yield. The NIR-II absorption, NIR-II emission, and NIR-II photothermal properties of the new CNACPs were studied. Interestingly, it is observed that longer nonconjugated linkers in CNACPs result in higher NIR-II fluorescence intensity with sufficient NIR-II absorption and NIR-II photothermal ability. With these newly developed CNACPs (BBT-C6), phototheranostic nanoparticles (BBTD6/Fe@PMA) are prepared through facile nanoprecipitation using PMA-AD-PEG as an iron ion chelator for NIR-II FI-guided NIR-II PTT/ferrotherapy synergistic therapy. In vitro and in vivo, BBTD6/Fe@PMA effectively inhibited 4T1 cells and tumor progression under 1064 nm laser irradiation. Consequently, this work provides new CNACPs by incorporating nonconjugated linkers into a conjugated backbone to design more effective NIR-II fluorescence imaging and NIR-II photothermal therapy agents.

The pursuit of xanthenoid fluorophores with near-infrared-II emission for in vivo applications

As fluorescence imaging in the second near-infrared window (NIR-II, 1000-1700 nm) has gained increasing attention, it is inevitable that NIR-II fluorophores, the cornerstone of NIR-II imaging, have come to the middle of the stage. NIR-II xanthenoid fluorophores with good stability, high brightness, and fluorescence adjustability are becoming popular. We here reviewed the recent progress of xanthenoid fluorophores with NIR-II emission for in vivo applications. Especially, we focus on the strategies used for longer wavelength and fluorescence regulation to construct OFF-ON or ratiometric NIR-II fluorescent probes.

D-A Type NIR-II Organic Molecules: Strategies for the Enhancement Fluorescence Brightness and Applications in NIR-II Fluorescence Imaging-Navigated Photothermal Therapy

NIR-II fluorescence imaging (NIR-II FI) and photothermal therapy (PTT) have received broad attentions in precise tumor diagnosis and effective treatment attributed to high-resolution and deep tissue imaging, negligible invasivity, and high-efficiency treatment. Although many fluorescent molecules have been designed and conducted for NIR-II FI and PTT, it is still an enormous challenge for researchers to pioneer some rational design guidelines to improve fluorescence brightness. Organic D-A-type molecules, including small molecules and conjugated polymers, can be designed and developed to improve fluorescence brightness due to their tunable and easy functionalized chemical structures, allowing molecules tailored photophysical properties. In this review, some approaches to the development and design strategies of D-A type small molecules and conjugated polymers for the enhancement of fluorescence brightness are systemically introduced. Meanwhile, some applications of PTT and PTT-based combination therapy (such as PDT, chemotherapy, or gas therapy) assisted by NIR-II FI-based single or multiimaging technologies are classified and represented in detail as well. Finally, the current issues and challenges of NIR-II organic molecules in NIR-II FI-navigated PTT are summarized and discussed, which gives some guidelines for the future development direction of NIR-II organic molecules for NIR-II FI-navigated PTT.

J-aggregates albumin-based NIR-II fluorescent dye nanoparticles for cancer phototheranostics

Phototheranostics, relying on energy conversions of fluorophores upon excitation, integrating diagnostic fluorescence imaging and photo-driven therapy, represents a promising strategy for cancer precision medicine. Compared with the first near-infrared biological window (NIR-I), fluorophores imaged in the second window (NIR-II, 1000–1700 ​nm) exhibit a higher temporal and spatial resolution and tissue penetration depth. Polymethine cyanine-based dye IR1061 is a typical NIR-II small-molecule organic fluorophore, but its low water solubility and short circulation time limiting its biological applications. Therefore, human serum albumin (HSA) nanoparticles with great biocompatibility and biosafety were employed to fabricate hydrophobic IR1061, which exhibited red-shifted absorption band as typical for J-aggregates. Moreover, IR1061@HSA nanoparticles can be successfully used for NIR-II imaging to noninvasively visualize the tumor vascular networks, as well as real-time intraoperative image-guided tumor resection. Interestingly, benefiting from the high photothermal conversion efficiency brought by J-aggregates, IR1061@HSA nanoparticles were also explored for photothermal therapy (PTT) and cause efficient thermal ablation of tumors. Overall, IR1061@HSA, as a novel J-aggregates albumin-based NIR II dye nanoparticle with high biocompatibility, provides an integrated versatile platform for cancer phototheranostics with promising clinical translation prospects.

Water-soluble near-infrared fluorescent heptamethine dye for lymphatic mapping applications

The identification of sentinel lymph node (SLN) is an important method for prognostic evaluation and minimally invasive staging of metastatic tumors. Here, we report a series of near-infrared fluorescent heptamethylamine dyes (series A, B and C) with central cycloalkene ring modified by tyrosine or N-Boc tyrosine via ether linkage. N-Boc tyrosine/tyrosine modification provided enhanced absorption coefficient and fluorescence quantum yield in DMSO, however with slight hypsochromic shift compared to the mother dyes in DMSO. In PBS, series A and B were found to be more fluorescent than ICG and showed brighter images. Compound A1 was found to exhibit the most favorable imaging performance among all the dyes investigated and was selected for in vivo sentinel lymph node mapping experiments in mice. A1 showed faster response and stronger fluorescence emission than FDA-approved ICG. The lymph node tracing with A1 could be assisted by MB staining. Ex vivo imaging of harvested organs indicated that similar metabolic characteristics of A1 and ICG. Overall, A1 is advantageous over ICG and is very promising for non-invasive lymph node imaging.

Thiadiazoloquinoxaline derivatives-based NIR-II organic molecules for NIR-II fluorescence imaging and photothermal therapy

NIR-II FI have emerged as a promising imaging tool for in vivo precise diagnosis and visualization towards various diseases, ascribed to its merits of attenuated light scattering and tissue absorption...

Organic Fluorophores for 1064 nm Excited NIR-II Fluorescence Imaging

Second near-infrared window (NIR-II) fluorescence imaging has shown great potential in the field of bioimaging. However, the excitation wavelengths of most NIR-II fluorescence dyes are in the first near-infrared (NIR-I) region, which leads to limited imaging depth and resolution. To address such issue, NIR-II fluorescence dyes with 1,064 nm excitation have been developed and applied for in vivo imaging. Compared with NIR-I wavelength excited dyes, 1,064 nm excited dyes exhibit a higher tissue penetration depth and resolution. The improved performance makes these dyes have much broader imaging applications. In this mini review, we summarize recent advances in 1,064 nm excited NIR-II fluorescence fluorophores for bioimaging. Two kinds of organic fluorophores, small molecule dye and semiconducting polymer (SP), are reviewed. The general properties of these fluorophores are first introduced. Small molecule dyes with different chemical structures for variety of bioimaging applications are then discussed, followed by the introduction of SPs for NIR-II phototheranostics. Finally, the conclusion and future perspective of this field is given.

Force-Induced Near-Infrared Chromism of Mechanophore-Linked Polymers

A near-infrared (NIR) mechanophore was developed and incorporated into a poly(methyl acrylate) chain to showcase the first force-induced NIR chromism in polymeric materials. This mechanophore, based on benzo[1,3]oxazine (OX) fused with a heptamethine cyanine moiety, exhibited NIR mechanochromism in solution, thin-film, and bulk states. The mechanochemical activity was validated using UV-vis-NIR absorption/fluorescence spectroscopies, gel permeation chromatography (GPC), NMR, and DFT simulations. Our work demonstrates that NIR mechanochromic polymers have considerable potential in mechanical force sensing, damage detection, bioimaging, and biomechanics.

Unsymmetrical cyanine dye via in vivo hitchhiking endogenous albumin affords high-performance NIR-II/photoacoustic imaging and photothermal therapy

Herein, an unprecedented synergistic strategy for the development of high-performance NIR-II fluorophore is proposed and validated. Based on an unsymmetrical cyanine dye design strategy, the NIR-II emissive dye NIC was successfully developed by replacing only one of the indoline donors of symmetrical cyanine dye ICG with a fully conjugated benz[c,d]indole donor. This minor structural change maximally maintains the high extinction coefficient advantage of cyanine dyes. NIC-ER with endogenous albumin-hitchhiking capability was constructed to further enhance its in vivo fluorescence brightness. In the presence of HSA (Human serum albumin), NIC-ER spontaneously resides in the albumin pocket, and a brilliant ~89-fold increase in fluorescence was observed. Due to its high molar absorptivity and moderate quantum yield, NIC-ER in HSA exhibits bright NIR-II emission with high photostability and significant Stokes shift (>110 nm). Moreover, NIC-ER was successfully employed for tumor-targeted NIR-II/PA imaging and efficient photothermal tumor elimination. Overall, our strategy may open up a new avenue for designing and constructing high-performance NIR-II fluorophores.

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.