Oligo(ethylene glycol)-Functionalized Squaraine Fluorophore as a Near-Infrared-Fluorescent Probe for the In Vivo Detection of Diagnostic Enzymes

Exploration of NIR Squaraine Contrast Agents Containing Various Heterocycles: Synthesis, Optical Properties and Applications

Squaraine dye is a popular class of contrast near-infrared (NIR) dyes. Squaraine dyes have shown the ability to be modified with various heterocycles. The indole moiety is the most notable heterocycle incorporated in squaraine dyes. A tremendous amount of work has gone into developing indole-based squaraine dyes and determining their applications. The optical properties of squaraine dyes containing an indole moiety facilitate high quantum yields and molar absorptivity, but the absorbance maxima is capped near 700 nm. This is the major limitation of indole-based squaraine dyes. In comparison, other heterocycles with larger conjugated systems such as quinoline and perimidine have demonstrated promising optical properties and immense potential for modifications, albeit with limited development. Quinoline- and perimidine-based squaraine dyes have molar extinction coefficients over 100,000 M-1 cm-1 and absorbances over 800 nm. This report will look at indole-, quinoline-, and perimidine-based squaraine dyes. Due to the sheer number of reported dyes, the search for indole-based squaraine dyes has been limited to reports from the past five years (2018-2023). For quinoline- and perimidine-based squaraine dyes, a holistic search was performed to analyze the optical properties and applications, due to the abovementioned limitation. This report will evaluate the three different classes of squaraines: indole-, quinoline-, and perimidine-based, to evaluate their optical properties and applications, with the goal of encouraging the exploration of other heterocycles for use in squaraine dyes.

Recent Progress of Activity-Based Fluorescent Probes for Imaging Leucine Aminopeptidase

Leucine aminopeptidase (LAP) is an important protease that can specifically hydrolyze Leucine residues. LAP occurs in microorganisms, plants, animals, and humans and is involved in a variety of physiological processes in the human body. In the physiological system, abnormal levels of LAP are associated with a variety of diseases and pathological processes, such as cancer and drug-induced liver injury; thus, LAP was chosen as the early biochemical marker for many physiological processes, including cancer. Considering the importance of LAP in physiological and pathological processes, it is critical that high-efficiency and dependable technology be developed to monitor LAP levels. Herein, we summarize the organic small molecule fluorescence/chemiluminescence probes used for LAP detection in recent years, which can image LAP in cancer, drug-induced liver injury (DILI), and bacteria. It can also reveal the role of LAP in tumors and differentiate the serum of cirrhotic, drug-induced liver injury and normal models.

Water-soluble single molecular probe for simultaneous detection of viscosity and hydrazine

Hydrazine (N₂H₄) can cause serious damage to human health, while intracellular viscosity is highly associated with many diseases and cellular dysfunctions. Herein, we report the synthesis of a dual-responsive organic molecule-based fluorescent probe with excellent water solubility being capable of detection of N₂H₄ and viscosity through dual-fluorescence channels in "turn on" manner for both. Besides sensitive detection of N₂H₄ in aqueous solution with detection limit of 0.135 μM, this probe could be used for vapor N₂H₄ detection in colorimetric and fluorescent manners. In addition, the probe demonstrated viscosity-dependent fluorescence enhancement behavior, and as high as 150-fold enhancement could be obtained at 95% glycerol aqueous solution. Cell imaging experiment revealed that the probe could be used for the discriminating of living and dead cells.

Enhancing the Selectivity of Leucine Aminopeptidase Near-Infrared Fluorescent Probes for Assisting in Surgical Tumor Resection

Selective fluorescence imaging of analytes is a challenge for monitoring diseases as homologues interfere with the imaging agents. Leucine aminopeptidase (LAP), a kind of protease, is related to tumor pathogenesis. The known LAP fluorescent probes based on leucine recognition have limited selectivity. Herein, a selective t-butyl-alanine recognition unit for LAP through the ligand regulation strategy is prepared as a new near-infrared (NIR) fluorescent probe (DCM-LAP) having a large Stokes shift of 214 nm and a high sensitivity with a detection limit of 168 mU/L. DCM-LAP has an enhanced response toward LAP with NIR fluorescence at 656 nm based on intramolecular charge transfer. The probe is selective without being interfered with by biological enzymes including the aminopeptidase N (APN). DCM-LAP can image LAP activity in living cells. It can also visualize the cell invasion and migration processes. DCM-LAP is employed in the real-time imaging of LAP in tumor-bearing nude mice and guides in the accurate resection of breast tumors. It also distinguishes tumor tissues from normal with a high tumor-to-normal ratio (9.8). The DCM-LAP probe can thus assist in the investigations of LAP-associated clinical disease.

A ratiometric fluorescence probe for selective and sensitive detection of leucine aminopeptidase in lysosome

We designed a novel ratiometric fluorescent probe P1-Leu with a donor-acceptor-donor fluorophore for the detection of leucine aminopeptidase in lysosomes. P1-Leu exhibits a lower detection limit than the ratiometric donor-π-acceptor probe, due to the low ratiometric background. Besides, P1-Leu has good lysosome-targeting ability and realizes the distinction of LAP levels in different cells.

Squaraine Dyes Derived from Indolenine and Benzo[e]indole as Potential Fluorescent Probes for HSA Detection and Antifungal Agents

Four squaraine dyes derived from 2,3,3-trimethylindolenine and 1,1,2-trimethyl-1H-benzo[e]indole with different combinations of barbituric groups attach to the central ring, having ester groups and alkyl chains in the nitrogen atoms of heterocyclic rings were synthesized. These dyes were fully characterized and their photophysical behavior was studied in ethanol and phosphate-buffered saline solution. Absorption and emission bands between 631 and 712 nm were detected, with the formation of aggregates in aqueous media, which is typical of this class of dyes. Tests carried out with 1,3-diphenylisobenzofuran allowed us to verify the ability of the dyes to produce singlet oxygen. The interaction of synthesized dyes with human serum albumin (HSA) was also evaluated, being demonstrated a linear correlation between fluorescence intensity and protein concentration. The antifungal potential of the dyes against the yeast Saccharomyces cerevisiae was evaluated using a broth microdilution assay. In order to test the photosensitizing capacity of the synthesized dyes, tests were carried out in the dark and with irradiation, using a custom-built light-emitting diode that emits close to the absorption wavelength of the studied dyes. The results showed that the interaction of dyes with HSA and the antifungal activity depends on the different structural modifications of the dyes.

Molecular design of near-infrared (NIR) fluorescent probes targeting exo-peptidase and application for detection of dipeptidyl peptidase 4 (DPP-4) activity

Monitoring the activities of proteases in vivo is an important requirement in biological and medical research. Near-infrared (NIR) fluorescent probes are particularly useful for in vivo fluorescence imaging, due to the high penetration of NIR and the low autofluorescence in tissue for this wavelength region, but most current NIR fluorescent probes for proteases are targeted to endopeptidase. Here, we describe a new molecular design for NIR fluorescent probes that target exopeptidase by utilizing the >110 nm blueshift of unsymmetrical Si–rhodamines upon amidation of the N atom of their xanthene moiety. Based on this molecular design, we developed Leu-SiR640 as a probe for leucine amino peptidase (LAP). Leu-SiR640 shows a one order of magnitude larger fluorescence increment (669-fold) upon reaction with LAP than existing NIR fluorescent probes. We similarly designed and synthesized EP-SiR640, a NIR fluorescent probe that targets dipeptidyl peptidase 4 (DPP-4). We show that this probe can monitor DPP-4 activity not only in living cells but also in mouse organs and tumors. This probe could also detect esophageal cancer in human clinical specimens, based on the overexpression of DPP-4 activity.

We developed a new molecular design for NIR fluorescent probes that target exopeptidase by utilizing the >110 nm blueshift of unsymmetrical Si–rhodamines.

Near-infrared (NIR) fluorescence-emitting small organic molecules for cancer imaging and therapy

We discuss recent advances made in the development of NIR fluorescence-emitting small organic molecules for tumor imaging and therapy.

A fluorescent probe for bioimaging of Hexosaminidases activity and exploration of drug-induced kidney injury in living cell

Hexosaminidases (Hexs) as an exoglycosidase participates in the catalytic hydrolysis of non-reducing end of glycoconjugates in the biological system. The fluctuation of Hexs level could cause many hereditary neurodegenerative diseases such as Tay-Sachs and Sandhoff. The Hexs activity is significantly up-regulated in colorectal cancer and kidney injury tissue so that it is particularly important to construct a fluorescent probe with significant signal change to understand its physiological role. In this work, DyOH was selected as fluorophore scaffolds to synthesize probe Hex-1 for detection of Hexs with good water solubility, high specificity, large stokes shift and quick response. Hex-1 can sensitively detect Hexs with the low detection limit (0.025 mU mL^-1) in vitro by "naked eye" due to superior spectral properties of DyOH. Furthermore, Hex-1 was not only employed for imaging Hexs in living cells with low toxicity, but also successfully applied to evaluate the fluctuation of Hexs activity during drug induced kindey injury in living HK-2 cells. These results indicated that Hex-1 could be used as a potential image tool to further explore the pathogenesis of kidney disease and cancer.

In vivo imaging via a red-emitting fluorescent probe to diagnosing liver cancer or drug-induced liver disease

Leucine aminopeptidase (LAP) as an important proteolytic enzyme, has been mainly found in hepatobiliary cells, and overexpressed in hepatoma cells. Herein, a new highly selective red-emitting fluorescent probe (DCDHF-Ala) for LAP has been synthesized based on 2-dicyanomethyldiene-3-cyano-2, 5-dihydrofuran (DCDHF) as fluorophore, and alanine (Ala) as the detection group. More importantly, it's the first time to use Ala as a reactive group for LAP. DCDHF-Ala has a low detection limit (0.20 U/L), excellent water solubility and cell membrane permeability. In addition, the probe has been successfully applied to fluorescent imaging in cells and zebrafish. It's especially worth mentioning that, DCDHF-Ala has a high biosafety and enables a real-time detection of LAP levels in mice model. What's the most important is that DCDHF-Ala may be an effective tool to qualitatively monitor the upregulation of LAP induced by liver injury and liver cancer.

Synthesis of Small Fluorescent Molecules and Evaluation of Photophysical Properties

A series of aniline-based fluorophores were newly synthesized. To increase their fluorescence quantum yields, it was particularly important to substitute 3,3,3-trifluoroprop-1-enyl (TFPE) groups next to the amino group to benefit from an extended π-electron delocalization. Among these, 5-CN-2-TFPE-aniline was found to behave as an excellent fluorophore with a reasonable fluorescence quantum yield of 0.89 even in aqueous solution. l-Alanine peptide, a nonfluorescent analogue of 5-CN-2-TFPE-aniline, was synthesized and successfully employed as an enzyme probe to detect aminopeptidase N activity.

Lysosome-Targeting Red-Emitting Aggregation-Induced Emission Probe with Large Stokes Shift for Light-Up in Situ Visualization of β-N-Acetylhexosaminidase

Lysosomal β-N-acetylhexosaminidase (Hex) has been reported to possess unique physiological performances. Detection and visualization of Hex in lysosome will be favorable to reveal the basis of its functions. However, Hex-specific fluorescent probes are rarely reported. In this study, we reported the first lysosome-targeting Hex-lighting-up aggregation-induced emission (AIE)-active fluorescent probe (GlcNAc-TPE) with remarkably large Stokes shift and high sensitivity and selectivity. GlcNAc-TPE can selectively locate in lysosome and visualize endogenous Hex in live HCT116 cells and in live mice with high stability and good biocompatibility, providing a useful AIE probe for real-time visualization of Hex in live samples.

An Activatable Near-Infrared Chromophore for Multispectral Optoacoustic Imaging of Tumor Hypoxia and for Tumor Inhibition

Hypoxia is a key hallmark of solid tumors and tumor hypoxia usually contributes to cancer progression, therapeutic resistance and poor outcome. Accurately detecting and imaging tumor hypoxia with high spatial resolution would be conducive to formulating optimized treatment plan and thus achieving better patient outcome. Methods: Tumor hypoxia can cleave the azo linker and release a NIR fluorophore (NR-NH2) and release the active drug as well. NR-NH2 shows a strong absorption band at around 680 nm and a strong fluorescence band at 710 nm, allowing for both multispectral optoacoustic tomography imaging (MSOT) and fluorescent imaging of tumor hypoxia in a tumor-bearing mouse model. Results: Liposome encapsulated with the activatable chromophore (NR-azo) for detecting/imaging tumor hypoxia and for tumor inhibition was demonstrated. For this chromophore, a xanthene-based NIR fluorophore acts as the optoacoustic and fluorescent reporter, an azo linker serves as the hypoxia-responsive moiety and a nitrogen mustard as the therapeutic drug. NR-azo shows an absorption at around 575 nm but exhibits negligible fluorescence due to the existence of the strong electron-withdrawing azo linker. Conclusion: We demonstrated an optoacoustic and fluorescent system for not only imaging tumor hypoxia in vivo but also achieving tumor inhibition.

Near-infrared fluorescence probe for hydrogen peroxide detection: design, synthesis, and application in living systems

Using fluorescent probes to detect endogenous hydrogen peroxide, which is associated with many diseases in the human body, remains an essential technique. Cyanine fluorochromes are a class of dyes that have attracted much attention and are widely used in the synthesis of fluorescent probes. In this article, a novel near-infrared (NIR) fluorescence probe for the detection of hydrogen peroxide was constructed and successfully applied to imaging endogenous hydrogen peroxide in vivo. Notably, probe 1 was designed by connecting 4-(bromomethyl)benzeneboronic acid pinacol ester as the sensing unit to the IR-780 hemicyanine skeleton, which exhibits excellent properties like NIR fluorescence emission over 700 nm. Probe 1 has satisfactory sensitivity to hydrogen peroxide with a low detection limit of 0.14 μM (S/N = 3), attributed to a responding mechanism that leads to the oxidation of phenylboronic acid pinacol ester and thereby releases fluorophore 2. Moreover, probe 1 displays excellent selectivity towards hydrogen peroxide over other substances. Taking advantage of these properties, the probe proved to be cell-permeable. Based on the results of N-acetylcysteine and rotenone together, probe 1 is capable of clearly visualizing endogenously produced hydrogen peroxide in living HepG2 cells and mice. The superior performance of the probe, as a reliable chemical tool, makes it of great potential application for exploring the role played by hydrogen peroxide in biological systems.

An enzyme-activatable probe liberating AIEgens: on-site sensing and long-term tracking of β-galactosidase in ovarian cancer cells

Development of fluorescent probes for on-site sensing and long-term tracking of specific biomarkers is particularly desirable for the early detection of diseases. However, available small-molecule probes tend to facilely diffuse across the cell membrane or remain at the activation site but always suffer from the aggregation-caused quenching (ACQ) effect. Here we report an enzyme-activatable aggregation-induced emission (AIE) probe QM-βgal, which is composed of a hydrophilic β-galactosidase (β-gal)-triggered galactose moiety and a hydrophobic AIE-active fluorophore QM-OH. The probe is virtually non-emissive in aqueous media, but when activated by β-gal, specific enzymatic turnover would liberate hydrophobic AIE luminogen (AIEgen) QM-OH, and then highly fluorescent nanoaggregates are in situ generated as a result of the AIE process, allowing for on-site sensing of endogenous β-gal activity in living cells. Notably, taking advantage of the improved intracellular retention of nanoaggregates, we further exemplify QM-βgal for long-term (∼12 h) visualization of β-gal-overexpressing ovarian cancer cells with high fidelity, which is essential for biomedicine and diagnostics. Thus, this enzyme-activatable AIE probe not only is a potent tool for elucidating the roles of β-gal in biological systems, but also offers an enzyme-regulated liberation strategy to exploit multifunctional probes for preclinical applications.

Detection of carboxylesterase by a novel hydrosoluble near-infrared fluorescence probe

A novel hydrosoluble near-infrared fluorescence off–on probe has been developed for detecting carboxylesterase activity.

A red lysosome-targeted fluorescent probe for carboxylesterase detection and bioimaging

A red lysosome-targeting probe for carboxylesterase activity has been successfully applied in complex biological samples.

Sensitive and accurate detection of ALP activity using a fluorescence on–off–on switch and mass barcode signal amplification

Alkaline phosphatase (ALP) is an important biomarker for many diseases. Therefore, the sensitive and accurate detection of ALP activity is essential for fundamental biochemical processes and clinic diagnosis. Herein, we design a fluorescent on–off–on switch for sensitive and visual detection of ALP activity. Meanwhile, mass barcode-modified quantum dots (QDs) amplified the LC-MS/MS detection signal in complex biological samples. Firstly, the QDs were modified with phosphorylated Gly-Gly-Phe-Phe-Tyr (OPO₃H₂) peptide (GGFFYp) and the mass barcode. The fluorescence of QDs-SS-Yp was quenched by fluorescence resonance energy transfer (FRET) between QDs-SS-Yp and dansyl chloride (DNS). ALP can hydrolyze the phosphorylated peptide to form peptide self-assemblies on the QDs-SS-Yp surfaces. The effective separation distance between the QDs-SS-Yp donor and DNS acceptor becomes larger, restricting FRET between the QDs-SS-Yp and DNS. At this point, the obvious QDs-SS-Yp fluorescence signal can be restored. However, the absence of ALP results in no peptide self-assembly on the QDs-SS-Yp surface and no obvious QDs-SS-Yp fluorescence signal was detected. Therefore, the ALP activity can be analyzed according to the degree of fluorescence restoration by the fluorescence on–off–on switch. Finally, the small tag molecules obtained by cleaving the disulfide bond of the QDs-SS-Yp as a mass barcode were used to amplify the LC-MS/MS detection signal. The proposed approach shows a good linear relationship (from 0.01 to 2.4 U L⁻¹) and has the significant advantage of a low detection limit of 0.001 U L⁻¹.

The sensitive and accurate detection of ALP activity using a fluorescence on–off–on switch and mass barcode signal amplification.