A hemicyanine-based dual-responsive fluorescent sensor for the detection of lithium and cyanide ions: application in living cells

A hemicyanine-based colorimetric and fluorometric sensor, 2-(2-(2,3,5,6,8,9-hexahydrobenzo[b][1,4,7,10]tetraoxacyclododecin-12-yl)vinyl)-3,3-dimethyl-1-propyl-3H-indol-1-ium iodide (MH-5), was developed and synthesized to detect Li+ and CN- ions in DMSO-PBS buffer solution (10 mM, pH 7.25, v/v 1:9). MH-5 displayed a rapid and highly selective colorimetric response to both Li+ and CN-, indicated by a distinct color change from pink to pale pink in the presence of Li+ and to colorless upon CN- detection, without interference from other cations or anions. The interaction mechanisms of MH-5 with Li+ and CN- ions were investigated using various analytical techniques, including 1H NMR, ESI-MS, FT-IR spectroscopy, and Job's plot analysis. These studies suggest that CN- is detected through nucleophilic addition to the indolium moiety of MH-5, while Li+ detection occurs via coordination with oxygen atoms in the crown ether structure. The fluorescence-based detection limits for Li+ and CN- were determined to be 0.150 µM and 0.154 µM, respectively. Additionally, MH-5 was evaluated in living cells, demonstrating effective cell penetration and reliable detection of Li+ and CN- ions for potential bio-imaging applications.

A self-calibrated tri-emission fluorescent probe based on carbon dots assembly for in vitro pepsin detection

A novel self-calibrating triple-emission fluorescent probe (CDs/BSA-SQ-Cy7) was engineered through the strategic integration of blue-emitting carbon dots (CDs) with covalently assembled near-infrared fluorescent dyes (BSA-SQ and BSA-SQ-Cy7) for accurate in vitro pepsin detection. To validate the enhanced performance of the tri-emissive design, a dual-emission control probe (CDs/BSA-SQ) was developed. BSA-SQ-Cy7 undergoes hydrolysis upon interaction with pepsin, resulting in concentration-dependent quenching of the red fluorescence, where the blue CDs serve as a reference emission during the process. The CDs/BSA-SQ-Cy7 probe demonstrated a robust linear correlation with pepsin concentrations ranging from 0.018 to 30 μg/mL (determined by the lowest detection limit). The detection limits were found to be 0.182 μg/mL (I495/I659), 0.033 μg/mL (I495/I808), and 0.018 μg/mL ((I495-I808)/I659), respectively. Additionally, the probe displayed excellent stability and reproducibility over a 72-h period, and its three-channel emission signal ratios can be utilized for self-calibration, thereby improving assay accuracy. This self-calibrated tri-emission probe is distinguished by its stability, reproducibility and high selectivity, offering an effective method for the accurate in vitro detection of pepsin.

A Microwave-Assisted Method for the Synthesis of Symmetrical and Unsymmetrical Cyanine Dyes

Cyanine dyes are widely used fluorophores with a range of labeling applications. However, there exists a lack of common, general methods for their synthesis that are applicable to dyes with a broad range of emission wavelengths. Herein, we describe a rapid, microwave-assisted, one-pot procedure for the synthesis of symmetrical trimethine, pentamethine, and heptamethine cyanine dyes in near-quantitative yields. A variation of the procedure allows the moderate-to-high-yielding synthesis of unsymmetrical cyanine dyes bearing -COOH substituents for functionalization.

Data-Driven Discovery of a New Fluorescent BASHY Dye for Bioimaging

Fluorescent molecules play a crucial role in biomedicine by facilitating the visualization and tracking of biological processes with sensitivity and specificity. However, tailoring their structure to meet the demands of live cell and in vivo imaging presents a significant challenge due to the intricate interplay of factors governing their structural and photophysical properties. In this study, we explored the potential of using multivariate linear free-energy relationships (mLFER) to optimize a multicomponent fluorescent platform. We prepared a small library of 20 fluorescent boronic-acid-derived salicylidenehydrazone (BASHY) complexes using a versatile reaction protocol and characterized their chemical stability in water-containing media. The obtained data served as input for the development of an mLFER model, enabling the prediction of a new BASHY dye and unraveling previously unknown mechanisms governing the stability of this unique platform of fluorescent dyes. The optimized dye was successfully employed in live cell experiments and in zebrafish larvae.

Fluorescent Water-Soluble Polycationic Chitosan Polymers as Markers for Biological 3D Imaging

Over the last decades, various tissue-clearing techniques have broken the ground for the optical imaging of whole organs and whole-organisms, providing complete representative data sets of three-dimensional biological structures. Along with advancements in this field, the development of fluorescent markers for staining vessels and capillaries has offered insights into the complexity of vascular networks and their impact on disease progression. Here we describe the use of a modified water-soluble chitosan linked to cyanine dyes in combination with ethyl cinnamate-based optical tissue clearing for the 3D visualization of tissue vasculature in depth. The water-soluble fluorescent Chitosans have proven to be an optimal candidate for labeling both vessels and capillaries ex vivo thanks to their increased water solubility, high photostability, and optical properties in the near-infrared window. In addition, the nontoxicity of these markers broadens their applicability to in vitro and in vivo biological applications. Despite the availability of other fluorescent molecules for vascular staining, the present study, for the first time, demonstrates the potential of fluorescent chitosans to depict vessels at the capillary level and opens avenues for advancing the diagnostic field by reducing the complexity and costs of the currently available procedures.

Multivalent supramolecular fluorescent probes for accurate disease imaging

Optical imaging is a powerful tool for early disease detection and effective treatment planning, but its accuracy is often compromised by the uptake of imaging materials by the mononuclear phagocyte system (MPS). Herein, we leverage multivalent host-guest interactions between cyanine dyes and β-cyclodextrin polymers to develop supramolecular probes with enhanced stability, optical, and transport profiles for accurate in vivo imaging. These multivalent interactions not only ensure the stability of the probes but also enhance fluorescence efficiency by minimizing nonradiative decay. Our self-assembly approach effectively modulates probe size and surface properties, enabling evasion of MPS clearance and promoting prolonged bloodstream circulation, thereby improving the signal-to-background ratio for imaging. The effectiveness of our design is demonstrated by substantial advancements in the early diagnosis of acute kidney injury and by providing high-contrast imaging and precise surgical navigation across various tumor models. Our strategy not only advances optical imaging materials toward clinical translation but also establishes a versatile platform applicable to multiple imaging modalities.

Engineering Central Substitutions in Heptamethine Dyes for Improved Fluorophore Performance

As a major family of red-shifted fluorophores that operate beyond visible light, polymethine dyes are pivotal in light-based biological techniques. However, methods for tuning this kind of fluorophores by structural modification remain restricted to bottom-up synthesis and modification using coupling or nucleophilic substitutions. In this study, we introduce a two-step, late-stage functionalization process for heptamethine dyes. This process enables the substitution of the central chlorine atom in the commonly used 4'-chloro heptamethine scaffold with various aryl groups using aryllithium reagents. This method borrows the building block and designs from the xanthene dye community and offers a mild and convenient way for the diversification of heptamethine fluorophores. Notably, this efficient conversion allows for the synthesis of heptamethine-X, the heptamethine scaffold with two ortho-substituents on the 4'-aryl modification, which brings enhanced stability and reduced aggregation to the fluorophore. We showcase the utility of this method by a facile synthesis of a fluorogenic, membrane-localizing fluorophore that outperforms its commercial counterparts with a significantly higher brightness and contrast. Overall, this method establishes the synthetic similarities between polymethine and xanthene fluorophores and provides a versatile and feasible toolbox for future optimizing heptamethine fluorophores for their biological applications.

Recent advancements of fluorescent biosensors using semisynthetic probes

Fluorescent biosensors are crucial experimental tools for live-cell imaging and the quantification of different biological analytes. Fluorescent protein (FP)-based biosensors are widely used for imaging applications in living systems. However, the use of FP-based biosensors is hindered by their large size, poor photostability, and laborious genetic manipulations required to improve their properties. Recently, semisynthetic fluorescent biosensors have been developed to address the limitations of FP-based biosensors using chemically modified fluorescent probes and self-labeling protein tag/peptide tags or DNA/RNA-based hybrid systems. Semisynthetic biosensors have unique advantages, as they can be easily modified using different probes. Moreover, the self-labeling protein tag, which labels synthetically developed ligands via covalent bonds, has immense potential for biosensor development. This review discusses the recent progress in different types of fluorescent biosensors for metabolites, protein aggregation and degradation, DNA methylation, endocytosis and exocytosis, membrane tension, and cellular viscosity. Here, we explain in detail the design strategy and working principle of these biosensors. The information presented will help the reader to create new biosensors using self-labeling protein tags for various applications.

Bifunctional Tumor-Targeted Bioprobe for Phototheranosis

Background: Near-infrared (NIR) phototheranostics provide promising noninvasive imaging and treatment for head and neck squamous cell carcinoma (HNSCC), capitalizing on its adjacency to skin or mucosal surfaces. Activated by laser irradiation, targeted NIR fluorophores can selectively eradicate cancer cells, harnessing the power of synergistic photodynamic therapy and photothermal therapy. However, there is a paucity of NIR bioprobes showing tumor-specific targeting and effective phototheranosis without hurting surrounding healthy tissues. Methods: We engineered a tumor-specific bifunctional NIR bioprobe designed to precisely target HNSCC and induce phototheranosis using bioconjugation of a cyclic arginine-glycine-aspartic acid (cRGD) motif and zwitterionic polymethine NIR fluorophore. The cytotoxic effects of cRGD-ZW800-PEG were measured by assessing heat and reactive oxygen species (ROS) generation upon an 808-nm laser irradiation. We then determined the in vivo efficacy of cRGD-ZW800-PEG in the FaDu xenograft mouse model of HNSCC, as well as its biodistribution and clearance, using a customized portable NIR imaging system. Results: Real-time NIR imaging revealed that intravenously administered cRGD-ZW800-PEG targeted tumors rapidly within 4 h postintravenous injection in tumor-bearing mice. Upon laser irradiation, cRGD-ZW800-PEG produced ROS and heat simultaneously and exhibited synergistic photothermal and photodynamic effects on the tumoral tissue without affecting the neighboring healthy tissues. Importantly, all unbound bioprobes were cleared through renal excretion. Conclusions: By harnessing phototheranosis in combination with tailored tumor selectivity, our targeted bioprobe ushers in a promising paradigm in cancer treatment. It promises safer and more efficacious therapeutic avenues against cancer, marking a substantial advancement in the field.

Heptamethine Cyanine-Based Molecule Release Triggered by Mitochondrial ROS

Conditionally activated molecule release in live cells would provide spatiotemporal control for the study and intervention of biological processes, e.g., bioactive molecule monitoring and controlled drug release. Mitochondria are the main sites of reactive oxygen species (ROS) production in cells. Here, we report an ROS-triggered molecule release strategy in mitochondria. A molecule IRTO with dual targeting groups was designed by covalently linking IR-780 (a mitochondrial targeted heptamethine cyanine) and 4-aminobutyl-thiazole orange (NH2-TO, a nuclear dye). IRTO diffused into live cells and first accumulated in mitochondria. As the cyanine moiety reacted with mitochondrial ROS directly or with the help of mitochondrial cytochromes, NH2-TO was released, escaped from mitochondria, and finally located in the nucleus. This process could be visualized by fluorescent imaging, i.e., red fluorescence (from the cyanine moiety of IRTO) first located in mitochondria, and green fluorescence (from NH2-TO) appeared and gradually enhanced in the nucleus with the increase of incubation time. The addition of H2O2 or lipopolysaccharide (LPS, an ROS accelerator) could accelerate the release of NH2-TO, whereas N-acetyl-l-cysteine (NAC, an ROS inhibitor) and mitoquinone mesylate (MitoQ, a mitochondrial ROS scavenger) could obviously decrease the release of NH2-TO. These results suggest that IRTO could serve as a fluorescent probe for monitoring ROS in mitochondria and that IR-780 might be a promising endogenous ROS-triggered molecule release platform.

Chronological development of functional fluorophores for bio-imaging

Functional fluorophores represent an emerging research field, distinguished by their diverse applications, especially in sensing and cellular imaging. After the discovery of quinine sulfate and subsequent elucidation of the fluorescence mechanism by Sir George Stokes, research in the field of fluorescence gained momentum. Over the past few decades, advancements in sophisticated instruments, including super-resolution microscopy, have further promoted cellular imaging using traditional fluorophores. These advancements include deciphering sensing mechanisms via photochemical reactions and scrutinizing the applications of fluorescent probes that specifically target organelles. This approach elucidates molecular interactions with biomolecules. Despite the abundance of literature illustrating different classes of probe development, a concise summary of newly developed fluorophores remains inadequate. In this review, we systematically summarize the chronological discovery of traditional fluorophores along with new fluorophores. We briefly discuss traditional fluorophores ranging from visible to near-infrared (NIR) in the context of cellular imaging and in vivo imaging. Furthermore, we explore ten new core fluorophores developed between 2007 and 2022, which exhibit advanced optical properties, providing new insights into bioimaging. We illustrate the utilization of new fluorophores in cellular imaging of biomolecules, such as reactive oxygen species (ROS), reactive nitrogen species (RNS), and proteins and microenvironments, especially pH and viscosity. Few of the fluorescent probes provided new insights into disease progression. Furthermore, we speculate on the potential prospects and significant challenges of existing fluorophores and their potential biomedical research applications. By addressing these aspects, we intend to illuminate the compelling advancements in fluorescent probe development and their potential influence across various fields.

Second Generation of Near-Infrared Cyanine-Based Photocatalysts for Faster Organic Transformations

A second generation of cyanine-based near-infrared photocatalysts has been developed to accelerate organic transformations. Cyanines were prepared and fully characterized prior to evaluation of their photocatalytic activities. Catalyst efficiency was determined by using two model oxidation and reduction reactions. For the aza-Henry reaction, cyanines bearing an amino group on the heptamethine chain led to the best results. For trifluoromethylation, the stability of the photocatalyst was found to be the key parameter for efficient and rapid conversion.

Enhanced Lifetime of Cyanine Salts in Dilute Matrix Luminescent Solar Concentrators via Counterion Tuning

Organic luminophores offer great potential for energy harvesting and light emission due to tunable spectral properties, strong luminescence, high solubility, and excellent wavelength-selectivity. To realize their full potential, the lifetimes of luminophores must extend to many years under illumination. Many organic luminophores, however, have a tendency to degrade and undergo rapid photobleaching, leading to the perception of intrinsic instability of organic molecules. In this work we demonstrate that by exchanging the counterion of a heptamethine cyanine salt the photostability and corresponding lifetime of dilute cyanine salts can be enhanced by orders of magnitude from 10 hours to an extrapolated lifetime of greater than 65,000 hours under illumination. To help correlate and comprehend the underlying mechanism behind this phenomenon, the water contact angle and binding energy of each pairing were measured and calculated. We find that increased water contact angle, and therefore increasing hydrophobicity, generally correlate to improved lifetimes. Similarly, a lower absolute binding energy between cation and anion correlates to increased lifetimes. Utilizing the binding energy formalism, we predict the stability of a new anion and experimentally verify with good consistency. Moving forward, these factors could be used to rapidly screen and identify highly photostable organic luminophore salt systems for a range of energy harvesting and light emitting applications.

Orchestrated Strategies for Developing Fluorophores for NIR-II Imaging

Fluorescence imaging (FLI), a non-invasive, real-time, and highly sensitive imaging modality, allows for investigating the molecular/cellular level activities to understand physiological functions and diseases. The emergence of the second near-infrared window (NIR-II, 1000-1700 nm) has endowed fluorescence imaging with deeper tissue penetration and unprecedented clarity. Among the various NIR-II imaging fluorophores, the organic fluorescent probes have occupied a pivotal position in bioimaging due to their higher biocompatibility, safety, and potential for clinical applications compared with those of the inorganic probes. To obtain high-quality organic dyes, diverse strategies have been taken. In this review, different strategies for optimizing NIR-II organic fluorophores are summarized, including traditional chemical modifications, and emerging bioengineering operations, which have not previously been elaborated on and summarized. Moreover, the bioengineering strategies are highlighted using endogenous serum proteins and even exogenous gene-editing proteins, which would provide fresh insights to design good-performance dyes and help develop NIR-II probes with clinical translation potential in the future. A critical perspective on the direction of the design strategies of NIR-II dyes for disease imaging is also proposed.

Achieving photostability in dye-sensitized upconverting nanoparticles and their use in Fenton type photocatalysis

Dye sensitization is a promising approach to enhance the luminescence of lanthanide-doped upconverting nanoparticles. However, the poor photostability of near-infrared dyes hampers their use in practical applications. To address this, commercial IR820 was modified for improved photostability and covalently bonded to amine-functionalized silica-coated LnUCNPs. Two methods of covalent linking were investigated: linking the dye to the surface of the silica shell, and embedding the dye within the silica shell. The photostability of the dyes when embedded in the silica shell exhibited upconversion emissions from NaGdF4:Er3+,Yb3+/NaGdF4:Yb3+ nanoparticles for over four hours of continuous excitation with no change in intensity. To highlight this improvement, the photostable dye-embedded system was successfully utilized for Fenton-type photocatalysis, emphasizing its potential for practical applications. Overall, this study presents a facile strategy to circumvent the overlooked limitations associated with photodegradation, opening up new possibilities for the use of dye-sensitized lanthanide-doped upconverting nanoparticles in a range of fields.

Bright NIR-Emitting Styryl Pyridinium Dyes with Large Stokes' Shift for Sensing Applications

Two NIR-emitting donor-π-acceptor (D-π-A) type regioisomeric styryl pyridinium dyes (1a-1b) were synthesized and studied for their photophysical performance and environment sensitivity. The two regioisomers, 1a and 1b, exhibited interesting photophysical properties including, longer wavelength excitation (λex ≈ 530-560 nm), bright near-infrared emission (λem ≈ 690-720 nm), high-fluorescence quantum yields (ϕfl ≈ 0.24-0.72) large Stokes' shift (∆λ ≈ 150-240 nm) and high-environmental sensitivity. Probe's photophysical properties were studied in different environmental conditions such as polarity, viscosity, temperature, and concentration. Probes (1a-1b) exhibited noticeable changes in absorbance, emission and Stokes' shift while responding to the changes in physical environment. Probe 1b exhibited a significant bathochromic shift in optical spectra (∆λ ≈ 20-40 nm) compared to its isomer 1a, due to the regio-effect. Probes (1a-1b) exhibited an excellent ability to visualize bacteria (Bacillus megaterium, Escherichia coli), and yeast (Saccharomyces cerevisiae) via fluorescence microscopy.

Photoactivatable Heptamethine-Based Carbonic Anhydrase Inhibitors Leading to New Anti-Antibacterial Agents

With the aim to propose innovative antimicrobial agents able to not only selectively inhibit bacterial carbonic anhydrases (CAs) but also to be photoactivated by specific wavelengths, new heptamethine-based compounds decorated with a sulfonamide moiety were synthesized by means of different spacers. The compounds displayed potent CA inhibition and a slight preference for bacterial isoforms. Furthermore, minimal inhibitory and bactericidal concentrations and the cytotoxicity of the compounds were assessed, thus highlighting a promising effect under irradiation against S. epidermidis. The hemolysis activity test showed that these derivatives were not cytotoxic to human red blood cells, further corroborating their favorable selectivity index. This approach led to the discovery of a valuable scaffold for further investigations.

Dual-Responsive Benzo-Hemicyanine-Based Fluorescent Probe for Detection of Cyanide and Hydrogen Sulfide: Real-Time Application in Identification of Food Spoilage

Colorimetric and fluorescent probes have received a lot of attention for detecting lethal analytes in realistic systems and in living things. Herein, a dual-approachable Benzo-hemicyaninebased red-emitting fluorescent probe PBiSMe, for distinct and instantaneous detection of CN^- and HS^- was synthesized. The PBiSMe emitted red fluorescence (570 nm) can switch to turn-off (570 nm) and blue fluorescence (465 nm) in response to CN^- and HS^-, respectively. Other nucleophilic reagents, such as reactive sulfur species (RSS) and anions, have no contact or interference with the probe; instead, a unique approach is undertaken to exclusively interact with CN^- and HS^- over a wide pH range. The measured detection limits for CN^- (0.43 μM) and HS^- (0.22 μM) ions are lower than the World Health Organization's (WHO) recommended levels in drinking water. We confirmed 1:1 stoichiometry ratio using Job's plot and observed good quantum yield for both analytes. The probe-coated paper strips were used to detect the H₂S gas produced by food spoilage (such as eggs, raw meat, and fish) via an eye-catching visual response. Moreover, fluorescence bioimaging studies of living cells was done to confirm the probe's potential by monitoring the presence of CN^- and HS^- in a living system.

Virus-like nanoparticles as a theranostic platform for cancer

Virus-like nanoparticles (VLPs) are natural polymer-based nanomaterials that mimic viral structures through the hierarchical assembly of viral coat proteins, while lacking viral genomes. VLPs have received enormous attention in a wide range of nanotechnology-based medical diagnostics and therapies, including cancer therapy, imaging, and theranostics. VLPs are biocompatible and biodegradable and have a uniform structure and controllable assembly. They can encapsulate a wide range of therapeutic and diagnostic agents, and can be genetically or chemically modified. These properties have led to sophisticated multifunctional theranostic platforms. This article reviews the current progress in developing and applying engineered VLPs for molecular imaging, drug delivery, and multifunctional theranostics in cancer research.

P800SO3-PEG: a renal clearable bone-targeted fluorophore for theranostic imaging

BACKGROUND

Due to the deep tissue penetration and reduced scattering, NIR-II fluorescence imaging is advantageous over conventional visible and NIR-I fluorescence imaging for the detection of bone growth, metabolism, metastasis, and other bone-related diseases.

METHODS

Bone-targeted heptamethine cyanine fluorophores were synthesized by substituting the meso-carbon with a sulfur atom, resulting in a bathochromic shift and increased fluorescence intensity. The physicochemical, optical, and thermal stability of newly synthesized bone-targeted NIR fluorophores was performed in aqueous solvents. Calcium binding, bone-specific targeting, biodistribution, pharmacokinetics, and 2D and 3D NIR imaging were performed in animal models.

RESULTS

The newly synthesized S-substituted heptamethine fluorophores demonstrated a high affinity for hydroxyapatite and calcium phosphate, which improved bone-specific targeting with signal-background ratios > 3.5. Particularly, P800SO3-PEG showed minimum nonspecific uptake, and most unbound molecules were excreted into the urinary bladder. Histological analyses demonstrated that P800SO3-PEG remained stable in the bone for over two weeks and was incorporated into bone matrices. Interestingly, the flexible thiol ethylene glycol linker on P800SO3-PEG induced a promising photothermal effect upon NIR laser irradiation, demonstrating potential theranostic imaging.

CONCLUSIONS

P800SO3-PEG shows a high affinity for bone tissues, deeper tissue imaging capabilities, minimum nonspecific uptake in the major organs, and photothermal effect upon laser irradiation, making it optimal for bone-targeted theranostic imaging.

A Renal-Clearable PEGylated Semiconducting Oligomer for the NIR-II Fluorescence Imaging of Tumor

Second near-infrared window fluorescence imaging (NIR-II FI) has attracted tremendous attention in bioimaging. Until now, most probes for NIR-II FI are nanomaterials that are metabolized via hepatobiliary metabolism. Such a metabolic pathway may take several months, causing long-term toxicity. Herein, we design and synthesize a renal-clearable PEGylated semiconducting oligomer (PSO) for the NIR-II FI of tumor. PSO is composed of a semiconducting oligomer (SO) backbone as an NIR-II fluorescence reporter and four poly(ethylene glycol) (PEG) side chains as water-soluble enhancers. PSO can emit an NIR-II fluorescence signal with the maximum emission at 1000 nm under the excitation of 808 nm light. PSO shows good biocompatibility and can be partially cleared out of body via renal clearance. PSO can be utilized for the NIR-II FI of tumor as it can effectively accumulate into tumor.

Sterically Shielded Hydrophilic Analogs of Indocyanine Green

A modular synthetic process enables two or four shielding arms to be appended strategically over the fluorochromes of near-infrared cyanine heptamethine dyes to create hydrophilic analogs of clinically approved indocyanine green. A key synthetic step is the facile substitution of a heptamethine 4'-Cl atom by a phenol bearing two triethylene glycol chains. The lead compound is a heptamethine dye with four shielding arms, and a series of comparative spectroscopy studies showed that the shielding arms (a) increased dye photostability and chemical stability and (b) inhibited dye self-aggregation and association with albumin protein. In mice, the dye cleared from the blood primarily through the renal pathway rather than the biliary pathway for ICG. This change in biodistribution reflects the much smaller hydrodynamic diameter of the shielded hydrophilic ICG analog compared to the 67 kDa size of the ICG/albumin complex. An attractive feature of versatile synthetic chemistry is the capability to systematically alter the dye's hydrodynamic diameter. The sterically shielded hydrophilic ICG dye platform is well-suited for immediate incorporation into dynamic contrast-enhanced (DCE) spectroscopy or imaging protocols using the same cameras and detectors that have been optimized for ICG.

Triazole-derivatized near-infrared cyanine dyes enable local functional fluorescent imaging of ocular inflammation

Near-infrared (NIR) chemical fluorophores are promising tools for in-vivo imaging in real time but often succumb to rapid photodegradation. Indocyanine green (ICG) is the only NIR dye with regulatory approval for ocular imaging in humans; however, ICG, when employed for applications such as labelling immune cells, has limited sensitivity and does not allow precise detection of specific inflammatory events, for example leukocyte recruitment during uveitic flare-ups. We investigated the potential use of photostable novel triazole NIR cyanine (TNC) dyes for detecting and characterising activated T-cell activity within the eye. Three TNC dyes were evaluated for ocular cytotoxicity in-vitro using a MTT assay and optimised concentrations for intraocular detection within ex-vivo porcine eyes after topical application or intracameral injections of the dyes. TNC labelled T-cell tracking experiments and mechanistic studies were also performed in-vitro. TNC-1 and TNC-2 dyes exhibited greater fluorescence intensity than ICG at 10 μM, whereas TNC-3 was only detectable at 100 μM within the porcine eye. TNC dyes did not demonstrate any ocular cell toxicity at working concentrations of 10 μM. CD4⁺T-cells labelled with TNC-1 or TNC-2 were detected within the porcine eye, with TNC-1 being brighter than TNC-2. Detection of TNC-1 and TNC-2 into CD4⁺T-cells was prevented by prior incubation with dynole 34-2 (50 μM), suggesting active uptake of these dyes via dynamin-dependent processes. The present study provides evidence that TNC dyes are suitable to detect activated CD4⁺T-cells within the eye with potential as a diagnostic marker for ocular inflammatory diseases.

Supramolecular Mitigation of the Cyanine Limit Problem

Currently, there is a substantial research effort to develop near-infrared fluorescent polymethine cyanine dyes for biological imaging and sensing. In water, cyanine dyes with extended conjugation are known to cross over the "cyanine limit" and undergo a symmetry breaking Peierls transition that favors an unsymmetric distribution of π-electron density and produces a broad absorption profile and low fluorescence brightness. This study shows how supramolecular encapsulation of a newly designed series of cationic, cyanine dyes by cucurbit[7]uril (CB7) can be used to alter the π-electron distribution within the cyanine chromophore. For two sets of dyes, supramolecular location of the surrounding CB7 over the center of the dye favors a nonpolar ground state, with a symmetric π-electron distribution that produces a sharpened absorption band with enhanced fluorescence brightness. The opposite supramolecular effect (i.e., broadened absorption and partially quenched fluorescence) is observed with a third set of dyes because the surrounding CB7 is located at one end of the encapsulated cyanine chromophore. From the perspective of enhanced near-infrared bioimaging and sensing in water, the results show how that the principles of host/guest chemistry can be employed to mitigate the "cyanine limit" problem.

Dual Probes for Positron Emission Tomography (PET) and Fluorescence Imaging (FI) of Cancer

Dual probes that possess positron emission tomography (PET) and fluorescence imaging (FI) capabilities are precision medicine tools that can be used to improve patient care and outcomes. Detecting tumor lesions using PET, an extremely sensitive technique, coupled with fluorescence-guided surgical resection of said tumor lesions can maximize the removal of cancerous tissue. The development of novel molecular probes is important for targeting different biomarkers as every individual case of cancer has different characteristics. This short review will discuss some aspects of dual PET/FI probes and explore the recently reported examples.

On the photostability and luminescence of dye-sensitized upconverting nanoparticles using modified IR820 dyes

Dye sensitization is a promising route to enhance luminescence from lanthanide-doped upconverting nanoparticles (LnUCNPs) by improving the photon harvesting capability of LnUCNPs through the use of dye molecules, characterized by higher absorption coefficients. The literature does not fully address the poor photostability of NIR dyes, hindering solution-based applications. The improvements achieved by dye-sensitized LnUCNPs are usually obtained by comparison with non-dye sensitized LnUCNPs. This comparison results in exciting the LnUCNPs at different wavelengths with respect to the dye-sensitized LnUCNPs or at the same wavelengths, where, however, the non-dye sensitized LnUCNPs do not absorb. Both these comparisons are hardly conclusive for a quantification of the improvements achieved by dye-sensitization. Both shortcomings were addressed by studying the photodegradation via thorough spectroscopic evaluations of a 4-nitrothiophenol-modified and unmodified IR820-LnUCNP system. The modified IR820 dye system exhibits a 200% enhancement in the emission of NaGdF₄:Er³⁺,Yb³⁺/NaGdF₄:Yb³⁺ nanoparticles relative to the unmodified IR820-sensitized LnUCNPs and emits for over twice the duration, demonstrating a substantial improvement over previous dye-LnUCNP systems. Upconversion dynamics between the dyes and Er³⁺ establish the importance of back-transfer dynamics in modulating the dye-LnUCNP luminescence. Quantum yield measurements further illustrate the mechanism of sensitization and increased efficiency of this new nanosystem.

Tailor-made aromatic porphyrinoids with NIR absorption

The highlight of this article is the recent progress in the state-of-the-art synthetic design and isolation of artificial porphyrinoids by swapping pyrrole component(s) with diverse functionalized pyrrolic(heterocyclic)/carbacycle building block(s) to compare the impact on the electronic absorption spectra and aromaticity of the incorporated isomeric/expanded porphyrinoids. Attention has been directed towards five distinct criteria of utilizing functionalized pyrrolic(heterocyclic)/aromatic hydrocarbons as synthons for NIR absorbing aromatic isomeric (N-confusion)/expanded porphyrinoids (with five/six heterocycles): (i) fused or annelated pyrrole (heterocycle), (ii) functionalized bi-pyrrole/bi-thiophene/bi-furan building blocks, (iii) azulene based carbacycle building block, (iv) vinylogous aromatic carbacycle/heterocycle(s) building block and (v) N-confused pyrrole ring(s), and N-confused fused pyrrole ring(s) leading to π-extension. These hybrid porphyrinoids are ideal candidates for basic research into macrocyclic aromaticity and for many potential applications owing to NIR absorption.

Rational design of stable heptamethine cyanines and development of a biomarker-activatable probe for detecting acute lung/kidney injuries via NIR-II fluorescence imaging

Heptamethine cyanines exhibiting high photo- and chemostability have been developed. And an activatable probe was developed for H2O2 to visualize acute lung and kidney injuries via NIR-II fluorescence imaging.

Targeted Fluorogenic Cyanine Carbamates Enable In Vivo Analysis of Antibody-Drug Conjugate Linker Chemistry

Antibody-drug conjugates (ADCs) are a rapidly emerging therapeutic platform. The chemical linker between the antibody and the drug payload plays an essential role in the efficacy and tolerability of these agents. New methods that quantitatively assess the cleavage efficiency in complex tissue settings could provide valuable insights into the ADC design process. Here we report the development of a near-infrared (NIR) optical imaging approach that measures the site and extent of linker cleavage in mouse models. This approach is enabled by a superior variant of our recently devised cyanine carbamate (CyBam) platform. We identify a novel tertiary amine-containing norcyanine, the product of CyBam cleavage, that exhibits a dramatically increased cellular signal due to an improved cellular permeability and lysosomal accumulation. The resulting cyanine lysosome-targeting carbamates (CyLBams) are ∼50× brighter in cells, and we find this strategy is essential for high-contrast in vivo targeted imaging. Finally, we compare a panel of several common ADC linkers across two antibodies and tumor models. These studies indicate that cathepsin-cleavable linkers provide dramatically higher tumor activation relative to hindered or nonhindered disulfides, an observation that is only apparent with in vivo imaging. This strategy enables quantitative comparisons of cleavable linker chemistries in complex tissue settings with implications across the drug delivery landscape.

A NIR Emitting Cyanine with Large Stokes' Shift for Mitochondria and Identification of their Membrane Potential Disruption

An NIR emitting (λ_em ≈730 nm) cyanine probe ExCy was synthesized in good yields by extending the π-conjugation length (i. e., with furan moiety) to the donor-accepter system. ExCy exhibited a large Stokes' shift (Δλ≈100 nm) due to strong intramolecular charge transfer (ICT), and high fluorescence quantum yield (Φ_fl ≈0.47 in DCM). Due to its low fluorescence in an aqueous environment (Φ_fl ≈0.007 in H₂ O), the probe exhibited the potential of achieving a large fluorescence turn-on upon entering a hydrophobic cellular environment. Fluorescence confocal microscopy studies revealed that ExCy was readily excitable with a far-red laser line (i. e., 640 nm) while the corresponding emission was collected in the NIR region. ExCy exhibited excellent selectivity towards live cell mitochondria according to the co-localization studies. The probe also exhibited high photostability, long-term imaging ability and wash-free staining ability, when being applied to live cells. Our studies indicated that the mitochondrial localization of ExCy was dependent on the membrane potential of the mitochondria. ExCy was successfully utilized as a mitochondrial membrane potential dysfunction indicator to visually identify cells with mitochondrial dysfunction via fluorescence confocal microscopy. ExCy was further examined for potential in vivo imaging of zebrafish.

Mechanism of Cyanine5 to Cyanine3 Photoconversion and Its Application for High-Density Single-Particle Tracking in a Living Cell

Cyanine (Cy) dyes are among the most useful organic fluorophores that have found a wide range of applications in single-molecule and super-resolution imaging as well as in other biophysical studies. However, recent observations that blueshifted derivatives of Cy dyes are formed via photoconversion have raised concerns as to the potential artifacts in multicolor imaging. Here, we report the mechanism for the photoconversion of Cy5 to Cy3 that occurs upon photoexcitation during fluorescent imaging. Our studies show that the formal C₂H₂ excision from Cy5 occurs mainly through an intermolecular pathway involving a combination of bond cleavage and reconstitution while unambiguously confirming the identity of the fluorescent photoproduct of Cy5 to be Cy3 using various spectroscopic tools. The carbonyl products generated from singlet oxygen-mediated photooxidation of Cy5 undergo a sequence of carbon-carbon bond-breaking and -forming events to bring about the novel dye-to-dye transformation. We also show that the deletion of a two-methine unit from the polymethine chain, which results in the formation of blueshifted products, commonly occurs in other cyanine dyes, such as Alexa Fluor 647 (AF647) and Cyanine5.5. The formation of a blueshifted congener dye can obscure the multicolor fluorescence imaging, leading to misinterpretation of the data. We demonstrate that the potentially deleterious photoconversion, however, can be exploited to develop a new photoactivation method for high-density single-particle tracking in a living cell without using UV illumination and cell-toxic additives.

A Near-Infrared Organic Fluorescent Probe for Broad Applications for Blood Vessels Imaging by High-Throughput Screening via 3D-Blood Vessel Models

Blood vessels are present in all of the organs, reflecting their importance for oxygen and nutrient delivery to the cells. Until now, no organic fluorophore has been reported for the live imaging of endothelium although the layer is the key to blood vessel functions. Here, the discovery of a blood vessel organic probe at near-infrared (NIR) wavelength range (BV-NIR) through an engineered blood capillary-based screening system, which is a more physiological model than a conventional cell culture condition, is reported. This selected Cy5 based probe shows the highest specific adsorption property out of 240 candidates on the endothelium and is equivalent to an anti-CD31 antibody in terms of intensity. The BV-NIR probe indicating strong and stable in vitro, ex vivo, and in vivo imaging of the endothelium even after histological immunostaining processes shows potential as a convenient tool for live imaging as well as for covisualization with a specific antibody.

Borisov S. Carbazole- and Fluorene-Fused Aza-BODIPYs: NIR Fluorophores with High Brightness and Photostability

Three new aza-BODIPY dyes incorporating fused fluorene or carbazole moieties have been prepared. The dyes show significant enhancement of photophysical properties compared to the parent 1,3,5,7-tetraphenyl aza-BODIPY (TPAB): a bathochromic shift of the absorption maximum (up to 2700 cm-1 ) and emission maximum (up to 2270 cm-1 ); an almost threefold increase in molar absorption coefficients (to ca. 230 000 M-1  cm-1 ) and a significant increase in the fluorescence quantum yield to 49-66 %. Owing to the combination of these properties, the new aza-BODIPY dyes belong to the brightest NIR dyes reported. The dyes also show excellent photostability. Due to their outstanding properties, the new dyes represent a promising platform for further exploration in biomedical research. A pH indicator containing only one fused carbazole unit was also prepared and shows absorption and emission spectra that are bathochromically shifted by about 110 and 100 nm, respectively, compared to the indicator dye based on the TPAB chromophore.

ZW800-PEG: A Renal Clearable Zwitterionic Near-Infrared Fluorophore for Potential Clinical Translation

Near-infrared (NIR) fluorescence imaging has advanced medical imaging and image-guided interventions during the past three decades. Despite tremendous advances in imaging devices, surprisingly only a few dyes are currently available in the clinic. Previous fluorophores, ZW800-1A and ZW800-1C, significantly improved the poor performance of the FDA-approved indocyanine green. However, ZW800-1A is not stable in serum and ZW800-1C induces severe stacking in aqueous media. To solve such dilemmas, ZW800-PEG was designed by introducing a flexible yet stable thiol PEG linker. ZW800-PEG shows high solubility in both aqueous and organic solvents, thus improving renal clearance with minimal binding to serum proteins during systemic circulation. The sulfide group on the meso position of the heptamethine core improves serum stability and physicochemical properties including the maximum emission wavelength shift to 800 nm, enabling the use of ZW800-PEG for image-guided interventions and augmenting photothermal therapy.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Supramolecular ensembles modified by near-infrared dyes and their biological applications

Near-infrared dyes possess the qualities of lower interference with biological autofluorescence, low photon scattering, and deep tissue penetration, and are being increasingly involved in the development of biomaterials for sensing and precision medicine. However, dyes usually suffer from the disadvantages of poor water solubility and photobleaching, factors that limit their application in vivo. The introduction of supramolecular ensembles can provide an ideal solution. This review presents recently developed supramolecular ensembles modified by near-infrared dyes. Compared with small-molecule fluorophores, the specific size of a supramolecular-based fluorophore endows it with longer circulation time in the bloodstream, increasing its chances of reaching a specific target. In addition, the construction of supramolecule-based fluorophores with versatile functions can be achieved by simple encapsulation or doping, instead of by complicated chemical synthesis. Thus, supramolecular-complex-based fluorophores offer high potential in diagnosis and therapy. This review outlines four different species of near-infrared dye based ensembles in terms of their method of formation, including simple encapsulation or doping and copolymerisation. Recently, a new technology has employed modified fluorophores for in situ self-assembly that form supramolecular ensembles at a specific position, thus solving the problem of poor uptake of nanoparticles by cells, and is included in this review. Finally, the future of this field is considered.

Smart J-aggregate of cyanine photosensitizer with the ability to target tumor and enhance photodynamic therapy efficacy

Photodynamic therapy (PDT) has been demonstrated to be effective for cancer treatment. Design of photosensitizers (PS) featuring tumor targeting, long excitation wavelengths, and high singlet oxygen quantum yields (Φ(¹O₂)) is the crucial point of highly efficient PDT. However, it is especially difficult to integrate above features into one photosensitizer. Herein, we put forwards a PS J-aggregation method, simultaneously endowing PS with the ability of tumor targeting, the red-shift of spectra, and the increase of Φ(¹O₂). Cyanine PS 4-((E)-3-((E)-5-iodo-1,3,3-trimethylindolin-2-ylidene)prop-1-en-1-yl)-1-methylquinolin-1-ium iodide (IDMQ) is firstly designed and synthesized by us. IDMQ can form smart response-type J-aggregates under the control of negatively charge in microenvironments. In blood, IDMQ J-aggregates target tumor via "enhanced permeability and retention (EPR)" effect. Then the intracellular IDMQ J-aggregates assembled by RNA facilitate the red shift of absorption peak with deeper penetration, and compared to free state IDMQ, improve the Φ(¹O₂), resulting in the bathochromic shift of optimal PDT wavelength from 630 (free state IDMQ) to 700 nm (IDMQ J-aggregates). This intelligent J-aggregation method establishes a promising way for endowing photosensitizer with the ability of tumor-targeting and the synchronous improvement of PDT efficacy and further boosts the development of PDT.

Near-infrared fluorescent probes: a next-generation tool for protein-labeling applications

The development of near-infrared (NIR) fluorescent probes over the past few decades has changed the way that biomolecules are imaged, and thus represents one of the most rapidly progressing areas of research. Presently, NIR fluorescent probes are routinely used to visualize and understand intracellular activities. The ability to penetrate tissues deeply, reduced photodamage to living organisms, and a high signal-to-noise ratio characterize NIR fluorescent probes as efficient next-generation tools for elucidating various biological events. The coupling of self-labeling protein tags with synthetic fluorescent probes is one of the most promising research areas in chemical biology. Indeed, at present, protein-labeling techniques are not only used to monitor the dynamics and localization of proteins but also play a more diverse role in imaging applications. For instance, one of the dominant technologies employed in the visualization of protein activity and regulation is based on protein tags and their associated NIR fluorescent probes. In this mini-review, we will discuss the development of several NIR fluorescent probes used for various protein-tag systems.

Cyanine-Like Boronic Acid-Derived Salicylidenehydrazone Complexes (Cy-BASHY) for Bioimaging Applications

Boronic acid-derived salicylidenehydrazone complex (BASHY) dyes with a polymethine backbone were designed to yield efficient red-emitting and two-photon absorbing fluorophores that can be used as markers for astrocytes. The dyes are chemically stable in aqueous solution and do not undergo photodecomposition. Their photophysical properties can be electronically fine-tuned and thereby adapted to potentially different imaging situations and requirements.

Cyanine-Flavonol Hybrids for Near-Infrared Light-Activated Delivery of Carbon Monoxide

Carbon monoxide (CO) is an endogenous signaling molecule that controls a number of physiological processes. To circumvent the inherent toxicity of CO, light-activated CO-releasing molecules (photoCORMs) have emerged as an alternative for its administration. However, their wider application requires photoactivation using biologically benign visible and near-infrared (NIR) light. In this work, a strategy to access such photoCORMs by fusing two CO-releasing flavonol moieties with a NIR-absorbing cyanine dye is presented. These hybrids liberate two molecules of CO in high chemical yields upon activation with NIR light up to 820 nm and exhibit excellent uncaging cross-sections, which surpass the state-of-the-art by two orders of magnitude. Furthermore, the biocompatibility and applicability of the system in vitro and in vivo are demonstrated, and a mechanism of CO release is proposed. It is hoped that this strategy will stimulate the discovery of new classes of photoCORMs and accelerate the translation of CO-based phototherapy into practice.

Sterically Shielded Heptamethine Cyanine Dyes for Bioconjugation and High Performance Near-Infrared Fluorescence Imaging

The near-infrared window of fluorescent heptamethine cyanine dyes greatly facilitates biological imaging because there is deep penetration of the light and negligible background fluorescence. However, dye instability, aggregation, and poor pharmacokinetics are current drawbacks that limit performance and the scope of possible applications. All these limitations are simultaneously overcome with a new molecular design strategy that produces a charge balanced and sterically shielded fluorochrome. The key design feature is a meso-aryl group that simultaneously projects two shielding arms directly over each face of a linear heptamethine polyene. Cell and mouse imaging experiments compared a shielded heptamethine cyanine dye (and several peptide and antibody bioconjugates) to benchmark heptamethine dyes and found that the shielded systems possess an unsurpassed combination of photophysical, physiochemical, and biodistribution properties that greatly enhance bioimaging performance.