A high-resolution mitochondria-targeting ratiometric fluorescent probe for detection of the endogenous hypochlorous acid

Recent progress of self-immobilizing and self-precipitating molecular fluorescent probes for higher-spatial-resolution imaging

Flourished in the past two decades, fluorescent probe technology provides researchers with accurate and efficient tools for in situ imaging of biomarkers in living cells and tissues and may play a significant role in clinical diagnosis and treatment such as biomarker detection, fluorescence imaging-guided surgery, and photothermal/photodynamic therapy. In situ imaging of biomarkers depends on the spatial resolution of molecular probes. Nevertheless, the majority of currently available molecular fluorescent probes suffer from the drawback of diffusing from the target region. This leads to a rapid attenuation of the fluorescent signal over time and a reduction in spatial resolution. Consequently, the diffused fluorescent signal cannot accurately reflect the in situ information of the target. Self-immobilizing and self-precipitating molecular fluorescent probes can be used to overcome this problem. These probes ensure that the fluorescent signal remains at the location where the signal is generated for a long time. In this review, we introduce the development history of the two types of probes and classify them in detail according to different design strategies. In addition, we compare their advantages and disadvantages, summarize some representative studies conducted in recent years, and propose prospects for this field.

Orderly Self-Assembly of Organic Fluorophores for Sensing and Imaging

Fluorescence imaging utilizing traditional organic fluorophores is extensively applied in both cellular and in vivo studies. However, it faces significant obstacles, such as low signal-to-background ratio (SBR) and spurious positive/negative signals, primarily due to the facile diffusion of these fluorophores. To cope with this challenge, orderly self-assembled functionalized organic fluorophores have gained significant attention in the past decades. These fluorophores can create nanoaggregates via a well-ordered self-assembly process, thus prolonging their residency time within cells and in vivo settings. The development of self-assembled-based fluorophores is an emerging field, and as such, in this review, we present a summary of the progress and challenges of self-assembly fluorophores, focusing on their development history, self-assembly mechanisms, and biomedical applications. We hope that the insights provided herein will assist scientists in further developing functionalized organic fluorophores for in situ imaging, sensing, and therapy.

Construction of HPQ-based activatable fluorescent probe for peroxynitrite and its application in ferroptosis and mice model of LPS-induced inflammation

As one of the important members of reactive oxygen species, ONOO- plays a crucial role in signal transduction, immune response, and other physiological activities. Aberrant changes in ONOO- levels in the living organism are usually associated with many diseases. Therefore, it is important to establish a highly selective and sensitive method for the determination of ONOO- in vivo. Herein, we designed a novel ratio near-infrared fluorescent probe for ONOO- by directly conjugating dicyanoisophorone (DCI) to hydroxyphenyl-quinazolinone (HPQ). Surprisingly, HPQD was unaffected by environmental viscosity and responded rapidly to ONOO- within 40 s. The linear range of ONOO- detection was from 0 μM to 35 μM. Impressively, HPQD did not react with reactive oxygen species and was sensitive to exogenous/endogenous ONOO- in live cells. We also investigated the relationship between ONOO- and ferroptosis and achieved in vivo diagnosis and efficacy evaluation of mice model of LPS-induced inflammation, which showed promising prospects of HPQD in ONOO--related studies.

Comparative studies on the absorption and fluorescence responses of hemicyanine to HSO3-, CN-, HS- and ClO. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2022;269:120781. [PMID: 34968839 DOI: 10.1016/j.saa.2021.120781]

A fluorescent probe Hcy-Im bearing an indolium-hemicyanine structure was designed and synthesized to compare its responses to four anions, namely HSO₃^-, CN^-, HS^- and ClO^-. The results disclosed that Hcy-Im reacted with all these four anions in 5% DMSO-PBS buffer with different speeds and spectral changes. Hcy-Im responded to HSO₃^- markedly quicker than CN^- and HS^-, and it responded to CN^- a little quicker than HS^- while the response to ClO^- was much slower than the other three anions. The detection limits for these four anions were calculated to be 0.15 μM, 1.32 μM, 2.07 μM and 2.29 μM, respectively. The characteristic conjugated CN⁺ and CC bonds in Hcy-Im were responsible for the responses towards these four anions via a Michael addition-rearrangement reaction, a 1, 2-addition reaction or an oxidation reaction. These different sensing mechanisms were verified by ¹HNMR and HRMS. Thus, it could be inferred that hemicyanine-based fluorescent probe could detect HSO₃^- sensitively and selectively while the interference of HSO₃^- should not be neglected when it was used for the detection of CN^-, HS^- and ClO^-. Moreover, as HSO₃^-, HS^- and ClO^- are anions endogenously generated in human bodies, enough attention should be paid to the presence of physiological level of these three anions in certain tissues when hemicyanine-based fluorescent probe is applied for the detection of biorelevant analytes in biological samples.

A POSS-assisted fluorescent probe for the rapid detection of HClO in mitochondria with a large emission wavelength in dual channels

Hypochlorous acid (HClO) is closely related to many diseases and is an inevitable part of the physiological processes. It is significant to detect HClO in mitochondria for getting meaningful physiological and pathological information. However, adequate tools to detect HClO with emissions in two channels are rarely reported. To achieve this target, in this work, a "turn-off" visual and near infrared (NIR) fluorescent dual emission probe D6 based on polyhedral oligomeric silsesquioxanes (POSS) was successfully designed and synthesized. D6 showed high selectivity and sensitivity to HClO. Notably, the emission wavelength of D6 reached 820 nm due to the assistance of the POSS cage. In addition, bioimaging experiments clearly showed that probe D6 promoted the visualization of exogenous and endogenous HClO in living HepG2 cells and zebrafish models.

Precipitated Fluorophore-Based Probe for Accurate Detection of Mitochondrial Analytes

Mitochondria-targeted fluorescent probes are highly important to obtain mitochondrial function information. However, the accuracy of the current mitochondria-targeted fluorescent probes is unsatisfactory owing to the following two reasons. In the first case, some probes that always have a mitochondria-targeting group, thus, would react with the analytes outside of mitochondria and enter mitochondria with the generated fluorophore signal, which leads to a false-positive result. In the other case, after response to the analytes in mitochondria, some probes could diffuse from mitochondria to other organelles, thus triggering a false-negative result. To avoid the two problems, herein, we develop a precipitated fluorophore-based probe, which precipitates in situ after reacting with analytes, for the accurate detection of mitochondrial analytes. The probe was modified with HQPQ, a novel solid-state fluorophore that is insoluble in water. As a proof of concept, we designed and synthesized a probe (HQPQ-B) for H₂O₂ detection. Based on the different mitochondria-targeting capacities of quinoline salts and quinolone, HQPQ loses the mitochondria-targeting ability after reacting with analytes outside of mitochondria, thus avoiding a false-positive result. On the contrary, when the probe first localized in mitochondria and then reacted with analytes, HQPQ would precipitate and remain in mitochondria without diffusing to other sites, thus avoiding a false-negative result. Therefore, HQPQ enables the accurate detection of mitochondrial analytes. We believe that the novel strategy based on HQPQ will be a general strategy for accurate detection of mitochondrial analytes without interference from other sites, which enables an accurate study on mitochondrial function.

Sensitive and selective fluorescent probe for hypochlorite in 100% aqueous solution and its application for lysosome-targetable cell imaging

A morpholine-functionalized pyrrole-cyanine probe was synthesized via a simple condensation reaction in high yield. This probe exhibits high selectivity toward ClO^- on fluorescence and UV-vis spectra in neat aqueous solution. The strong green emission of the probe solution was quenched and the yellow color faded immediately upon the addition of ClO^-. The detection limit of the probe for ClO^- was 0.165 μM. The mechanism of hypochlorite-induced CC breakage was supposed on the basis of EIS-MS, NMR, and density functional theory (DFT) calculation. Finally, the probe was utilized to image ClO^- in lysosomes of living cells.

Fast detecting hypochlorous acid based on electron-withdrawing group promoted oxidation and its biological applications in cells and root tips of plants

Hypochlorous acid, a type of reactive oxygen species, has been shown to play an important role in organisms. Nowadays, there are many kinds of fluorescence detecting mechanisms to detect hypochlorous acid in vivo. Due to the high selectivity, the mechanism of using the strong oxidation of hypochlorous acid to break carbon‑carbon double bonds has been favored by many scientists. However, the reported probes of breaking carbon‑carbon double bonds still had drawback such as slow response. Based on this, we introduced electron-withdrawing group malonitrile to accelerate the oxidation of hypochlorous acid, resulting in reaction time less than 150 s. Meanwhile, the probe exhibited excellent selectivity, optical stability, high sensitivity and the detection limit as low as 0.19 μM. More importantly, we also successfully proved the potential application of the probe for the detection of intracellular ClO^- living cells and Arabidopsis root tip by fluorescence imaging.

Detecting and Imaging of γ-Glutamytranspeptidase Activity in Serum, Live Cells, and Pathological Tissues with a High Signal-Stability Probe by Releasing a Precipitating Fluorochrome

γ-Glutamytranspeptidase (GGT) is a significant tumor-related biomarker that overexpresses in several tumor cells. Accurate detection and imaging of GGT activity in serum, live cells, and pathological tissues hold great significance for cancer diagnosis, treatment, and management. Recently developed small molecule fluorescent probes for GGT tend to diffuse to the whole cytoplasm and then translocate out of live cells after enzymatic reaction, which make them fail to provide high spatial resolution and long-term imaging in biological systems. To address these problems, a novel fluorescent probe (HPQ-PDG) which releases a precipitating fluorochrome upon the catalysis of GGT is designed and synthesized. HPQ-PDG is able to detect GGT activity with high spatial resolution and good signal-stability. The large Stokes shift of the probe enables it to detect the activity of GGT in serum samples with high sensitivity. To our delight, the probe is used for imaging GGT activity in live cells with the ability of discriminating cancer cells from normal cells. What's more, we successfully apply it for pathological tissues imaging, with the results indicating that the potential application of HPQ-PDG in histopathological examination. All these results demonstrate the potential application of HPQ-PDG in the clinic.

Mitochondria and lysosome-targetable fluorescent probes for HOCl: recent advances and perspectives

Hypochlorous acid (HOCl), as one of the reactive oxygen species (ROS), plays an important role in the destruction of pathogens in the immune system. However, abnormal concentration of biogenic HOCl can also damage host tissues, and it has been shown to be associated with many diseases. Accordingly, detection of HOCl at the subcellular level is important for understanding inflammation and cellular apoptosis. Toward this end, in the past few years, a wide variety of fluorescent HOCl probes have been engineered and applied for imaging of HOCl in subcellular organelles. In this review, we highlight the representative cases of the fluorescent HOCl probes with mitochondria and lysosome-targetable ability. The discussion includes their design strategies, sensing mechanisms, and applications in bio-imaging of HOCl in mitochondria and lysosomes.

A new ratiometric fluorescent probe for detecting endogenous HClO in living cells

A new benzimidazole-hemicyanine-based ratiometric fluorescent probe (ZBM-H) was developed, which showed high selectivity and sensitivity for detecting HClO.

Mitochondria-Targeted Triphenylphosphonium-Based Compounds: Syntheses, Mechanisms of Action, and Therapeutic and Diagnostic Applications

Mitochondria are recognized as one of the most important targets for new drug design in cancer, cardiovascular, and neurological diseases. Currently, the most effective way to deliver drugs specifically to mitochondria is by covalent linking a lipophilic cation such as an alkyltriphenylphosphonium moiety to a pharmacophore of interest. Other delocalized lipophilic cations, such as rhodamine, natural and synthetic mitochondria-targeting peptides, and nanoparticle vehicles, have also been used for mitochondrial delivery of small molecules. Depending on the approach used, and the cell and mitochondrial membrane potentials, more than 1000-fold higher mitochondrial concentration can be achieved. Mitochondrial targeting has been developed to study mitochondrial physiology and dysfunction and the interaction between mitochondria and other subcellular organelles and for treatment of a variety of diseases such as neurodegeneration and cancer. In this Review, we discuss efforts to target small-molecule compounds to mitochondria for probing mitochondria function, as diagnostic tools and potential therapeutics. We describe the physicochemical basis for mitochondrial accumulation of lipophilic cations, synthetic chemistry strategies to target compounds to mitochondria, mitochondrial probes, and sensors, and examples of mitochondrial targeting of bioactive compounds. Finally, we review published attempts to apply mitochondria-targeted agents for the treatment of cancer and neurodegenerative diseases.

A new fluorogenic sensing platform for salicylic acid derivatives based on π-π and NH-π interactions between electron-deficient and electron-rich aromatics

A novel simple fluorescent probe was designed for the recognition of electron-rich salicylic acid derivatives (SAs). The imidazole-appended aminomethyl perylene probe 1 selectively differentiated between electron-rich amino-SAs and electron-deficient nitro-SAs in EtOH, exhibiting the highest selectivity and sensitivity toward 5-aminosalicylic acid (5-ASA) and showing strong 1:1 binding (K_a=1.37×10⁷M^-1). This high selectivity and sensitivity resulted from the synergistic multiple hydrogen bonding interactions of secondary amine and imidazole units and π-π interactions between electron-rich and electron-deficient rings, along with the unusual NH-π interactions between 5-ASA and the perylene moiety of 1. The limit of detection (LOD) for 5-ASA in EtOH was 0.012ppb.

Selective and sensitive fluorimetric determination of carbendazim in apple and orange after preconcentration with magnetite-molecularly imprinted polymer

In this study, magnetite-molecularly imprinted polymer has been used for the first time as selective adsorbent before the fluorimetric determination of carbendazim. Adsorption capacity of the magnetite-molecularly imprinted polymer was found to be 2.31±0.63mgg^-1 (n=3). Limit of detection (LOD) and limit of quantification (LOQ) of the method were found to be 2.3 and 7.8μgL^-1, respectively. Calibration graph was linear in the range of 10-1000μgL^-1. Rapidity is an important advantage of the method where re-binding and recovery processes of carbendazim can be completed within an hour. The same imprinted polymer can be used for the determination of carbendazim without any capacity loss repeatedly for at least ten times. Proposed method has been successfully applied to determine carbendazim residues in apple and orange, where the recoveries of the spiked samples were found to be in the range of 95.7-103%. Characterization of the adsorbent and the effects of some potential interferences were also evaluated. With the reasonably high capacity and reusability of the adsorbent, dynamic calibration range, rapidity, simplicity, cost-effectiveness and with suitable LOD and LOQ, the proposed method is an ideal method for the determination of carbendazim.

A novel (3,6-di-tert-butylcarbazol-9-yl) triphenylamine–BODIPY–tricyanofuran conjugated dye: synthesis and rapid naked-eye detection of hypochlorite

A colorimetric and ratiometric probe, BCPA–BODIPY–TCF, for hypochlorite (ClO⁻) based on a D–π–A structure was synthesized through the Knoevenagel reaction.

A New Two-Photon Ratiometric Fluorescent Probe for Detecting Alkaline Phosphatase in Living Cells

Alkaline phosphatase (ALP) is an important diagnostic indicator of many human diseases. To quantitatively track ALP in biosystems, herein, for the first time, we report an efficient two-photon ratiometric fluorescent probe, termed probe 1 and based on classic naphthalene derivatives with a donor-π-acceptor (D-π-A) structure and deprotection of the phosphoric acid moiety by ALP. The presence of ALP causes the cleave of the phosphate group from naphthalene derivatives and the phosphate group changes the ability of the intramolecular charge transfer (ICT) and remarkably alters the probe's photophysical properties, thus an obvious ratiometric signal with an isoemissive point is observed. The fluorescence intensity ratio displayed a linear relationship against the concentration of ALP in the concentration range from 20 to 180 U/L with the limit of detection of 2.3 U/L. Additionally, the probe 1 is further used for fluorescence imaging of ALP in living cells under one-photon excitation (405 nm) or two-photon excitation (720 nm), which showed a high resolution imaging, thus demonstrating its practical application in biological systems.