Water-Soluble NIR Absorbing and Emitting Indolizine Cyanine and Indolizine Squaraine Dyes for Biological Imaging

Water-Soluble Small Organic Fluorophores for Oncological Theragnostic Applications: Progress and Development

Cancer is one of the major noncommunicable diseases, responsible for millions of deaths every year worldwide. Though various cancer detection and treatment modalities are available today, many deaths occur owing to its late-stage detection and metastatic nature. Noninvasive detection using luminescence-based imaging tools is considered one of the promising techniques owing to its low cost, high sensitivity, and brightness. Moreover, these tools are unique and valuable as they can detect even the slightest changes in the cellular microenvironment. To achieve this, a fluorescent probe with strong tumor uptake and high spatial and temporal resolution, especially with high water solubility, is highly demanded. Recently, several water-soluble molecules with emission windows in the visible (400-700 nm), first near-infrared (NIR-I, 700-1000 nm), and second near-infrared (NIR-II, 1000-1700 nm) windows have been reported in literature. This review highlights recently reported water-soluble small organic fluorophores/dyes with applications in cancer diagnosis and therapeutics. We systematically highlight and describe the key concepts, structural classes of fluorophores, strategies for imparting water solubility, and applications in cancer therapy and diagnosis, i.e., theragnostics. We discuss examples of water-soluble fluorescent probes based on coumarin, xanthene, boron-dipyrromethene (BODIPY), and cyanine cores. Some other emerging classes of dyes based on carbocyclic and heterocyclic cores are also discussed. Besides, emerging molecular engineering methods to obtain such fluorophores are discussed. Finally, the opportunities and challenges in this research area are also delineated.

Switch-on near infrared emission in albumin behind dark fabric: toward application in forensic latent bloodstain detection

Latent bloodstain detection remains imperative for crime scene investigators. Widely used luminol offers high sensitivity to human blood, but can produce untrustworthy results from a bleach-cleaned crime scene or in a room not dark enough. Furthermore, dark pigments impede imaging bloodstains covered by dark materials with previously reported bloodstain detection agents. A novel on/off human albumin-sensing dye (SO3C7) is reported herein with a longer emission wavelength (942 nm) than previous materials that allows imaging behind ∼5 mm of black fabric. The switch-on emission of SO3C7 is selective and sensitive to human albumin and lasts longer than luminol (24-48 hours). Emission studies, transient absorption spectra (TAS), and near-infrared (NIR) photographs herein describe the albumin sensing properties of the dye.

Latent bloodstain detection using a selective turn-on NIR fluorescence dye responsive to serum albumin

Bloodstain detection can provide crucial information and evidence at a crime scene; however, the ability to selectively detect bloodstains in a non-destructive manner with high sensitivity and low background is limited. This study reports a fluorescent dye (sulfonate indolizine squaraine, SO3SQ) for bloodstain visualization under near-infrared (NIR) irradiation. While the dye itself is minimally fluorescent in aqueous solution, it exhibits a "turn-on" mechanism upon binding with human serum albumin (HSA) as the fluorescence intensity increases over 160 times with strong absorption and emission at 693 nm and 758 nm, respectively. Bloodstains can be visualized on a surface even after being diluted 1000 times, and washed latent bloodstains can be detected with high sensitivity. Further, the turn-on fluorescent emission lasts for a minimum of seven days, allowing adequate time for detection. This study also indicates that the SO3SQ can sensitively detect bloodstain after the bloodstain aged for one week. Furthermore, the detection of latent blood fingerprint patterns from colorful backgrounds is demonstrated using this non-destructive method. The selective turn-on fluorescent dye with NIR excitation and emission is highly suitable in forensic science for bloodstain visualization.

Selective Near-Infrared Blood Detection Driven by Ionic Liquid-Dye-Albumin Nanointeractions

Due to its abundance in blood, a great deal of research has been undertaken to develop efficient biosensors for serum albumin and provide insight into the interactions that take place between these biosensing molecules and the protein. Near-infrared (NIR, >700 nm) organic dyes have been shown to be effective biosensors of serum albumin, but their effectiveness is diminished in whole blood. Herein, it is shown that an NIR sulfonate indolizine-donor-based squaraine dye, SO3SQ, can be strengthened as a biosensor of albumin through the addition of biocompatible ionic liquids (ILs). Specifically, the IL choline glycolate (1:1), at a concentration of 160 mM, results in the enhanced fluorescence emission ("switch-on") of the dye in the presence of blood. The origin of the fluorescence enhancement was investigated via methods, including DLS, ITC, and molecular dynamics. Further, fluorescence measurements were conducted to see the impact the dye-IL system had on the fluorescence of the tryptophan residue of human serum albumin (HSA), as well as to determine its apparent association constants in relation to albumin. Circular dichroism (CD) spectroscopy was used to provide evidence that the dye-IL system does not alter the secondary structures of albumin or DNA. Our results suggest that the enhanced fluorescence of the dye in the presence of IL and blood is due to diversification of binding sites in albumin, controlled by the interaction of the IL-dye-albumin complex.

Photosensitizers with multiple degradation modes for efficient and postoperatively safe photodynamic therapy

Photodynamic therapy (PDT) is emerging as a powerful tool for cancer treatment due to its unique advantages in terms of noninvasive and spatiotemporal selectivity. However, the residue of photosensitizers (PSs), which usually lead to thorny post-treatment side effects after photodynamic therapy (PDT), is one of bottlenecks for clinical translation. Herein, PSs with multiple degradation modes are developed to solve this issue. Upon 660 nm laser excitation, PSs can produce different types of reactive oxygen species (ROS), in which 1O2 and O2·- could kill the cancer cells, while ·OH could oxide the PSs themselves for photodegradation. After PDT, the residual few number of PSs could be further oxidized by endogenous ROS for biodegradation, and the degradation products could be further excreted by urine. This process therefore solves the slow-metabolism issue of traditional PSs. Among them, SQSe demonstrates the highest killing efficiency with best degradation ability, as confirmed by both in vitro and in vivo results. The postoperative safety of SQSe is further verified by assessment on in vivo artificially induced post-operative side effects.

An indolizine squaraine-based water-soluble NIR dye for fluorescence imaging of multidrug-resistant bacteria and antibacterial/antibiofilm activity using the photothermal effect

The majority of nosocomial infections are caused by bacteria with antimicrobial resistance and the formation of biofilms, such as implant-related bacterial infections and sepsis. There is an urgent need to develop new strategies for early-stage screening, destruction of multidrug-resistant bacteria, and efficient inhibition of biofilms. Organic dyes that absorb and emit in the near-infrared (NIR) region are potentially non-invasive, high-resolution, and rapid biological imaging materials. In this study, a non-toxic and biocompatible indolizine squaraine dye with water-solubilizing sulfonate groups (SO₃SQ) is studied for bacterial imaging and photothermal therapy (PTT). PTT is efficient in eliminating microorganisms through local hyperthermia without the risk of developing drug-resistant bacteria. The optical properties of SO₃SQ are studied extensively in phosphate-buffered saline (PBS). UV-Vis-NIR absorption spectra analysis shows a strong absorption between 650 nm - 1000 nm. SO₃SQ allows for the wash-free fluorescence imaging of drug-resistant bacteria via NIR fluorescence imaging due to a "turn-on" fluorescence property of the dye when interacting with bacteria. Although SO₃SQ exhibits no toxicity against both Gram-positive bacteria and Gram-negative bacteria, the PTT property of SO₃SQ is efficient in killing bacteria as well as inhibiting and eradicating biofilms. PTT experiments demonstrate that SO₃SQ reduces 90% of cell viability in bacterial strains under NIR radiation with a minimum inhibition concentration (MIC₉₀) of >450 μg/mL. The PTT property of SO₃SQ can also inhibit biofilms (BIC₉₀ = 1000-2000 μg/mL) and eradicate both preformed young and mature biofilms (MBEC₉₀ = 1500-2000 μg/mL) as observed by crystal violet assays.

Photothermal bleaching of nickel dithiolene for bright multi-colored 3D printed parts

HP's Multi Jet Fusion is a powder bed fusion 3D printing technology that utilizes a carbon-based radiation absorber in combination with a near infrared (NIR) light source to facilitate the fusion of polymer powder in a layer-by-layer fashion to generate 3D parts. Most available carbon-based and NIR radiation absorbers have an intrinsic dark color, which as a result will only produce black/gray and dark colored parts. However, there are many applications that require variable color, including prosthetics, medical models, and indicators, among others. To create white, bright colored, and translucent parts with MJF, a visibly transparent and colorless radiation absorber is required. In this paper, we designed an activating fusing agent (AFA) that contains a red, strong NIR absorbing dye that turns colorless after harvesting irradiation energy during the MJF 3D printing process and provide a bright colored part when working with other color agents.

Acid-Triggered Switchable Near-Infrared/Shortwave Infrared Absorption and Emission of Indolizine-BODIPY Dyes

Fluorescent organic dyes that absorb and emit in the near-infrared (NIR, 700-1000 nm) and shortwave infrared (SWIR, 1000-1700 nm) regions have the potential to produce noninvasive high-contrast biological images and videos. BODIPY dyes are well known for their high quantum yields in the visible energy region. To tune these chromophores to the NIR region, fused nitrogen-based heterocyclic indolizine donors were added to a BODIPY scaffold. The indolizine BODIPY dyes were synthesized via microwave-assisted Knoevenagel condensation with indolizine aldehydes. The non-protonated dyes showed NIR absorption and emission at longer wavelengths than an aniline benchmark. Protonation of the dyes produced a dramatic 0.35 eV bathochromic shift (230 nm shift from 797 nm to 1027 nm) to give a SWIR absorption and emission (λ_max^emis = 1061 nm). Deprotonation demonstrates that material emission is reversibly switchable between the NIR and SWIR.

Indolizin-1-ols with Charged Electron Acceptors: A Direct Way to 3H-Indolizinium-1-olates with Donor Functions

The reaction of cyclopropenones with pyridines, having an attached integer-charged electron-withdrawing group (pyridinium, imidazolium, and phosphonium) was discovered to afford novel indolizin-1-ol derivatives in high yields with no chromatographic purification required. While being stable as solids, these indolizin-1-ols have a limited lifetime in solution. The study of reasons for such instability uncovered an aerobic oxidative pathway, eventually resulting in indolizine-1,7-dione dimers. The exploration of N-(1-hydroxyindolizin-7-yl)pyridinium salts' chemistry led to a reaction discovery, affording a new type of rare pseudo-cross-conjugated mesomeric betaines (3H-indolizin-4-ium-1-olates with an electron-donating function at C7 position) inaccessible by other means. In this reaction, a sequential introduction of nucleophiles takes place: the first one (Nu₁) is represented by simple anilines, whereas Nu₂ extends to primary, secondary, aliphatic, aromatic amines, and phenols. For the obtained betaines having unsymmetrical aliphatic amino groups at C7 position an increased order of the C7-Nu₂ bond resulting in existence of amide type E/Z-forms (∼1:1 at room temperature) was demonstrated. For aryl amino groups, with typically reduced nitrogen's lone-pair donation, the barrier of rotation around the C7-Nu₂ bond was lower, and for the C7-oxy betaines, no such E/Z-isomerism was revealed. Although primary amines (as Nu₂) introduce a hydrogen atom in the conjugated betaine system, allowing prototropic tautomerism in this way, non-zwitterionic tautomers (3-amino-7-iminoindolizin-1-ones) were rejected by nuclear Overhauser effect spectroscopy experiments.

Recent Progress of Squaraine-Based Fluorescent Materials and Their Biomedical Applications

Squaraines (SQs) are unusual cyanine dyes with a unique resonance-stabilized zwitterionic structure. These dyes have attracted significant attention in the fields of organic electronics and organic photonics, due to their facile synthesis, intense and narrow visible and near-infrared absorption/emission, high photostability, low biotoxicity, etc. In this review, we summarize the recent progress of SQ-based fluorescent materials and their biomedical applications. After a brief introduction to SQs, general synthetic routes and design principles of SQ-based fluorescent materials, as well as their aggregation-induced luminescence behaviors, are discussed. Subsequently, their biomedical applications for cell imaging and as fluorescent sensors and therapeutic agents are introduced. Finally, a summary and perspective are given for promoting the development of SQs-based fluorescent materials.

Mechanochemically Induced Dehydrogenation Coupling and [3+2] Cycloaddition of Indolizines with Allenes Using Piezoelectric Materials

A wide range of indolizines with allenes proceeded smoothly under mechanochemically induced conditions via a [3+2] annulation process, affording various substituted pyrrolo[2,1,5-cd]indolizines in good yields. The reaction efficiency was greatly improved by using piezoelectric material as the charge transfer catalyst. The photophysical properties of the resulting pyrrolo[2,1,5-cd]indolizine were characterized.

Donor–acceptor strategy to construct near infrared AIEgens for cell imaging

A donor–acceptor strategy was applied to construct NIR AIEgens. Six new AIEgens were obtained and among them, DMNIC exhibited the longest emission maximum at 694 nm and was successfully applied for NIR cell imaging.

Interest of novel N-alkylpyridinium-indolizine hybrids in the field of Alzheimer's disease: Synthesis, characterization and evaluation of antioxidant activity, cholinesterase inhibition, and amyloid fibrillation interference

A small library of molecules combining indolizine and N-alkyl pyridinium was synthesized and evaluated in a multi-target-directed-ligand strategy for Alzheimer's disease (AD) treatment. The new compounds were classified in three series depending on the number of methylene residues linking the two heterocycles (Ind-PyCx with x = 0, 2 or 3). The molecules were synthesized from the corresponding bis-pyridines by two-step formation of the indolizine core including mono-alkylation of pyridine and 1,3-dipolar cycloaddition with an alkylpropiolate. Their activities against AD's key-targets were evaluated in vitro: acetyl- and butyrylcholinesterase (AChE and BChE) inhibition, antioxidant properties and inhibition of amyloid fibril formation. None of the three series showed significant activities against all the targets. The Ind-PyC2 and Ind-PyC3 series are active on eeAChE and hAChE (µM IC₅₀ values). Most of the positively charged molecules from these two series also appeared active against eqBChE, however they lost their activity on hBChE. Comparative molecular modeling of 13 and 15 docked in hAChE and hBChE highlighted the importance of the substituent (p-methoxybenzoyl or methyloxycarbonyl, respectively) located on the indolizine C-3 for the binding. The larger molecule 13 fits more tightly at the active site of the two enzymes than 15 that shows a larger degree of freedom. The Ind-PyC2 and Ind-PyC3 hybrids displayed some antioxidant activity when tested at 750 µg/mL (up to 95% inhibition of DPPH radical scavenging for 10). In both series, most hybrids were also able to interact with amyloid fibers, even if the inhibitory effect was observed at a high 100 µM concentration. The Ind-PyC0 molecules stand out completely due to their spectroscopic properties which prevent their evaluation by Ellman's and ThT assays. However, these molecules showed interesting features in the presence of preformed fibers. In particular, the strong increase in fluorescence of 3 in the presence of amyloid fibers is very promising for its use as a fibrillation fluorescent reporter dye.

Metal-Free Catalyzed Synthesis of Fluorescent Indolizine Derivatives

A mild and high efficient method to prepare indolizines by two-component reaction with the acid as the catalyst was developed. In this reaction, a new ring efficiently formed in one-step reaction. A wide range of substrates could be applied and the desired products were obtained in 8-95% yields under metal-free conditions. Different indolizine derivatives (compounds 3a-3n) were synthesized by general conditions and microwave irradiation conditions, and compound 3a gave the best results with an isolated yield of 95% and 82%, respectively. The structures of synthesized compounds were characterized by spectral analysis, and compound 3m was confirmed by single crystal X-ray analysis. UV-vis absorption and fluorescence properties of these compounds were correlated with substituent groups on indolizine rings.

SWIR emissive RosIndolizine dyes with nanoencapsulation in water soluble dendrimers

Shortwave infrared (SWIR) emission has great potential for deep-tissue in vivo biological imaging with high resolution. In this article, the synthesis and characterization of two new xanthene-based RosIndolizine dyes coded ^PhRosIndz and ^tolRosIndz is presented. The dyes are characterized via femtosecond transient absorption spectroscopy as well as steady-state absorption and emission spectroscopies. The emission of these dyes is shown in the SWIR region with peak emission at 1097 nm. ^TolRosIndz was encapsulated with an amphiphilic linear dendritic block co-polymer (LDBC) coded 10-PhPCL-G3 with high uptake yield. Further, cellular toxicity was examined in vitro using HEK (human embryonic kidney) cells where a >90% cell viability was observed at practical concentrations of the encapsulated dye which indicates low toxicity and reasonable biocompatibility.

A xanthene–indolizine dye is shown to emit in the shortwave infrared spectral region (∼1100 nm maximum) in water when nanoencapsulated.

Squaraine dyes as serum albumins probes: Synthesis, photophysical experiments and molecular docking studies

Three barbiturate squaraine dyes derived from indolenine or benzothiazole, with different barbituric acid derivatives were prepared, characterized and photophysically evaluated by standard spectroscopic methods. As expectable for squaraines, these dyes showed narrow and intense absorption and emission bands in the Vis/NIR region. The interaction of synthesized dyes with bovine and human serum albumins (BSA and HSA) was also evaluated in phosphate buffer (PB). The results revealed that upon the addition of BSA or HSA the complex dye-protein emit more fluorescence, and the emission intensity is directly proportional to the concentration of protein used (0-3.5 µM). The titration tests allowed to calculate the binding constants, in an order of magnitude of 10⁴-10⁶ M, as well as the limits of detection and quantification in the nanomolar tens range. All dyes showed a good response to the interaction with both proteins, but the most pronounced envisioning their use as protein labeling was observed for the squaraine dye derived from the indolenine with a 1,3-dimethylbarbituric acid moiety. The molecular docking studies revealed the existence of a binding between the compounds and four sites on the HSA molecule, where one of these four locations is a new binding site with which this series of dye interacts.

Radical and Ionic Reactions of Indolizin-1-ols: Synthesis of 3-Arylsulfanyl-, 3-(Tropon-2-yl)- and 3-(Tropolon-5-ylazo)-1-hydroxyindolizines from 3,3-Difluorocyclopropenes

An aerobic multicomponent reaction between monoalkyl-3,3-difluorocyclopropenes, pyridines, and arylthiols has been discovered to afford 3-arylsulfanyl-1-hydroxyindolizines. This reaction proceeds via intermediate C3-free indolizin-1-ols, easily forming free radicals in air. In the presence of arylthiols, potent radical traps, incorporation of arylsulfanyl substituent occurs at the C3 position of indolizin-1-ols by radical recombination. On the contrary, in an inert atmosphere, intermediate 1-hydroxyindolizines react with C- and N-electrophiles in a one-pot fashion. Novel, intensively colored 3-(tropon-2-yl)-indolizin-1-ols and high absorption coefficient 3-(tropolon-5-ylazo)-indolizin-1-ol dyes were synthesized in a multicomponent manner in 50-80% yields. The presence of an O-uncapped indolizin-1-ol moiety modulates the redox properties of the whole molecule, facilitating free radical formation, which is susceptible to further transformations. Three such examples were demonstrated: oxidative recyclization of 3-(2-hydroxyphenylsulfanyl)-indolizin-1-ol, auto-oxidation of substituted 3,3'-biindolizine-1,1'-diol, and diacetoxyiodobenzene (DAIB)-mediated dehydrogenation of 3-(tropolon-5-ylazo)-indolizin-1-ol. The latter reaction affords 3-((4,5-dioxocyclohepta-2,6-dien-1-ylidene)hydrazono)-3H-indolizin-4-ium-1-olate, a mesomeric betaine, strongly absorbing light on the borders of the visible range and showing a solvatochromic effect.

Development of biotin-tagged near-infrared fluorescence probes for tumor-specific imaging

Near-infrared (NIR) probes are applicable for tumor imaging due to deep tissue penetration and low background signal. And cyanine dyes with long emission wavelength are excellent fluorophores to develop NIR probes. However, the aggregation of cyanine dyes in aqueous solution reduces the utilization of light. To solve this problem, polyethylene glycol (PEG) was introduced into the probes to reduce their aggregation. In our work, two new NIR probes G₁ and G₂ were designed and prepared by conjugating the cyanine dye G₀ with Biotin-PEG5-Azide. The conjugated biotin could enhance the target specificity of probes. And the photophysical and photochemical parameters demonstrated that G₁ and G₂ had a reduced aggregation tendency. In vitro fluorescence imaging proved that these two probes could be specifically taken up by HeLa cells, and in vivo imaging demonstrated that these two probes could specifically target tumors with large tumor-to-muscle (T/M) ratios. All these results indicated that G₁ and G₂ are promising NIR fluorescent contrast agents for tumor-specific imaging.

Access to 6-hydroxy indolizines and related imidazo[1,5-a]pyridines through the SN2 substitution/condensation/tautomerization cascade process

A simple and efficient cascade reaction was developed for the construction of hydroxy substituted indolizines from pyrrole-2-carbaldehydes and commercially available 4-halogenated acetoacetic esters.

Synthesis of near-infrared absorbing and fluorescent bis(pyrrol-2-yl)squaraines and their halochromic properties

1,2-Squaraines (partially conjugated neutral structure) showed blue-shifted λmax compared to the corresponding 1,3-squaraines (fully conjugated zwitterionic structure) and the mono-protonated thiophene-fused 1,3-squaraine showed λmax at 1007 nm.

Enantioselective Synthesis of 3-Allylindolizines via Sequential Rh-Catalyzed Asymmetric Allylation and Tschitschibabin Reaction

The first highly regio- and enantioselective synthesis of 3-allylindolizines has been developed by the sequential Rh-catalyzed asymmetric allylation and Tschitschibabin reaction. Above the 20:1 branch/linear ratio, up to a 96% yield and 99% ee could be obtained with the help of tert-butyl-substituted chiral bisoxazolinephosphine ligand. In situ generated highly nucleophilic 2-alkylpyridinium ylides are utilized to undergo the asymmetric alkylation reaction before cyclization.

Driving high quantum yield NIR emission through proquinoidal linkage motifs in conjugated supermolecular arrays

High quantum yield NIR fluorophores are rare. Factors that drive low emission quantum yields at long wavelength include the facts that radiative rate constants increase proportional to the cube of the emission energy, while nonradiative rate constants increase in an approximately exponentially with decreasing S₀-S₁ energy gaps (in accordance with the energy gap law). This work demonstrates how the proquinoidal BTD building blocks can be utilized to minimize the extent of excited-state structural relaxation relative to the ground-state conformation in highly conjugated porphyrin oligomers, and shows that 4-ethynylbenzo[c][1,2,5]thiadiazole (E-BTD) units that terminate meso-to-meso ethyne-bridged (porphinato)zinc (PZnn) arrays, and 4,7-diethynylbenzo[c][1,2,5]thiadiazole (E-BTD-E) spacers that are integrated into the backbone of these compositions, elucidate new classes of impressive NIR fluorophores. We report the syntheses, electronic structural properties, and emissive characteristics of neoteric PZn-(BTD-PZn)n, PZn2-(BTD-PZn2)n, and BTD-PZnn-BTD fluorophores. Absolute fluorescence quantum yield (ϕ _f) measurements, acquired using a calibrated integrating-sphere-based measurement system, demonstrate that these supermolecules display extraordinary ϕ _f values that range from 10-25% in THF solvent, and between 28-36% in toluene solvent over the 700-900 nm window of the NIR. These studies underscore how the regulation of proquinoidal conjugation motifs can be exploited to drive excited-state dynamical properties important for high quantum yield long-wavelength fluorescence emission.