Tunable heptamethine-azo dye conjugate as an NIR fluorescent probe for the selective detection of mitochondrial glutathione over cysteine and homocysteine

A mitochondria-targeted iridium(III) complex-based sensor for endogenous GSH detection in living cells

Glutathione (GSH) plays an important role in maintaining redox homeostasis in biological systems. Development of reliable glutathione sensors is of great significance to better understand the role of biomolecules in living cells and organisms. Based on the advantages of the photophysical properties of iridium complexes, we proposed a "turn-on" phosphorescent sensor. Ir-DNFB has the characteristics of a large Stokes shift, high sensitivity for GSH detection, low cytotoxicity, and extremely short response time, and can specifically analyze glutathione in living cells and highly target endogenous glutathione in mitochondria. The N-H group on the imidazole ring of Ir-DNFB could form a new electrostatic interaction with the α-carboxyl group on the glutamate moiety of glutathione. The nucleophilic attack reaction was regulated by the sulfhydryl group on GSH, following which the ether bond linking the 2,4-dinitrobenzene to probe Ir-DNFB was broken, accompanied with a phosphorescence enhancement. Most importantly, the process of recognizing glutathione was not affected by other amino acids. Overall, this work provided a very useful tool for rapidly distinguishing between normal, inflammatory, and progressive tumor cells.

Rational engineering of a recognition group to construct a two-photon reaction-based fluorescent probe for rapid and selective sensing of cysteine

It is highly required to rationally design fluorescent probes via a molecular engineering strategy with desired analytical performance for applications in sensing and imaging. Reaction-based fluorescent probes for highly selective sensing of cysteine (Cys) are mainly based on the participation of Cys in reactions such as, addition-cyclization with acrylates, cyclization with aldehydes, coordination displacement, Michael addition reactions, and cleavage reactions. Cys-triggered reactions with the O atoms of ether bonds has also been used to construct reaction-based fluorescent probes based on the substitution of the ether with the nucleophilic thiolate of Cys. However, many of the developed probes still suffer from long response times, interference from homocysteine (Hcy) and glutathione (GSH), high background fluorescence, and a lack of two-photon absorption (TPA) properties. Herein, we successfully design a Cys-sensitive two-photon fluorescent probe (F-BTD) using a donor-acceptor-donor (D-A-D) type π-extended benzothiadiazole framework as the fluorophore, with nitrobenzofuran (NBD) as the recognition unit. The proposed F-BTD probe displays some advantages over other probes including a rapid response time, high selectivity, low background fluorescence, and two-photon imaging capability. The F-BTD probe is applied to the two-photon fluorescence imaging of endogenous and exogenous Cys in HeLa cells with satisfactory results. For comparison, commonly used biothiol recognition groups including 2,4-dinitrobezensulfonyl and 2,4-dinitrophenyl are also used to construct S-BTD and N-BTD probes, respectively. The response mechanism of F-BTD to Cys is studied in detail through kinetic studies and transition-state analysis. This study may provide an example of how to design fluorescent probes with desired analytical performance by considering the recognition group as an important index of the design.

Simultaneous detection of cysteine and glutathione in food with a two-channel near-infrared fluorescent probe

L-Cysteine (Cys) and glutathione (GSH) are closely related biological species that widely exist in food and living cells. To simultaneously detect Cys and GSH from different emission channels, we developed a fluorescent probe (BDP-NBD) based on near-infrared BODIPY and 7-nitrobenzofurazan (NBD). Upon nucleophilic substitution reaction with GSH, BDP-NBD generated an emission band at 713 nm, which can be used to determine GSH (0-100 μM) with a low detection limit (34 nM). Different from GSH, BDP-NBD underwent a nucleophilic substitution-rearrangement reaction with Cys, affording two emission bands at 550 nm and 713 nm, respectively. BDP-NBD was successfully employed to quantify Cys and GSH in various food samples with good recoveries (86.6%-104.6%). Besides, BDP-NBD can image Cys and GSH in living cells from two emission channels. Therefore, this work developed a tool for the simultaneous determination of Cys and GSH in both food and living cells so as to ensure food safety and human health.

A mitochondria-targeted nitric oxide probe with large Stokes shift for real-time imaging and evaluation of inflammatory bowel disease in situ

BACKGROUND

Inflammatory bowel disease (IBD) is a prevalent inflammatory disorder, and the abnormal expression of nitric oxide (NO) produced by biocatalysis of iNOS enzyme in mitochondria is directly associated with the occurrence and progression of IBD. Activatable fluorescent probes offer promising tools for early diagnosis of IBD, however, inadequate biodistribution and limited targeting properties of these probes in vivo severely impede accurate diagnosis of IBD and real-time evaluation of inflammatory levels in situ. Therefore, it is necessary to design a highly efficient fluorescent probe towards NO to overcome inadequate biodistribution and achieve accurate diagnosis and evaluation of IBD in situ.

RESULTS

We designed a highly efficient mitochondria-targeted "turn-on" NIR fluorescent probe Cy-OMe which has excellent targeting properties and imaging ability. The response mechanism is probe Cy-OMe rapidly undergoes N-nitrosation reaction resulting in "turn-on" NIR fluorescence signal when exposed to NO. Cy-OMe exhibits high sensitivity and specificity in detecting NO content in vitro, owing to its large Stokes shift. Furthermore, the probe Cy-OMe not only efficiently targets mitochondria but also enables precise assessment of fluctuations in endogenous NO concertation across various cell types. Importantly, by virtue of large Stokes shift and excellent mitochondrial targeting ability, Cy-OMe has the capability to specifically evaluate dynamic fluctuations of NO in lipopolysaccharide (LPS)-stimulated IBD mouse models in situ and Cy-OMe was achieved high-contrast imaging and precision diagnosis of intestinal inflammation diseases.

SIGNIFICANCE

Cy-OMe can accurately assess fluctuations in NO levels and show high signal fidelity in the diseased intestine region, which has prospects in the non-invasive diagnosis of intestinal inflammation in vivo. At the same time, it is expected to serve as a potential diagnose platform for investigating the physiological processes underlying NO-related inflammatory diseases and promoting understanding of the pathological functions of NO across diverse inflammatory diseases.

NIR-II Protein-Escaping Dyes Enable High-Contrast and Long-Term Prognosis Evaluation of Flap Transplantation

Real-time vascular positioning, postoperative flap monitoring, and vascular reconstruction assessment are of great importance in flap transplantation. Cyanine dyes offer the advantage of high resolution in the Near-infrared-II (NIR-II) imaging window. However, the nonspecific binding of many cyanine dyes to endogenous albumin leads to high organ accumulation and skin absorption, resulting in low-quality imaging and poor reproducibility of contrast during long-term (e.g., 7 days) postoperative monitoring. Here, a novel strategy is proposed that can be widely applied to prevent protein binding for NIR-I/II Cl-containing cyanine dyes. This strategy produces protein-escaping dyes, ensuring high fluorescence enhancement in the blood with rapid clearance and no residual fluorescence, allowing for short-term repeatable injections for vascular imaging. This strategy in the perioperative monitoring of pedicle perforator flap models in mice and rats is successfully applied. Furthermore, leveraging the universality of this strategy, multiple nonoverlapping protein-escaping probes that achieve dual-excitation (808 and 1064 nm) interference-free imaging of nerve-vessel and tumor-vessel simultaneously are designed and synthesized. These protein-escaping dyes enable long-term repeatable dual-color imaging of tumor localization, resection, and tumor-vessel reconstruction at the wound site.

Dual-channel luminescent Ir(III) complex for detection of GSH and Hcy/Cys in cells

Glutathione (GSH), homocysteine (Hcy) and cysteine (Cys) play important roles in many physiological processes. However, due to their structural and functional similarities, it is still a challenge to develop a probe that can differentiate between GSH and Hcy/Cys simultaneously. In this work, a luminescent probe Ir-NBD was designed and synthesized, which emit weakly due to the presence of photo induced electron transfer (PET) interaction. When it reacted with the three biothiols, NBD dissociated and luminescence of Ir-OH was enhanced in the near-infrared (NIR) region due to the disappearance of the PET effect. On the other hand, the products obtained from the reaction of GSH with NBD were hardly luminescent, but the products from the reaction of Hcy/Cys with NBD could undergo an intramolecular rearrangement, resulting in an enhanced luminescence of the solution in the visible region. Ir-NBD enabled highly selective and sensitive detection of GSH and Cys/Hcy in a relatively short time (15 min). The two luminescent colors were clearly differentiated without spectral interference and the detection limit reached 1.32 μM (GSH), 0.42 μM (Hcy) and 0.51 μM (Cys), respectively. Ir-NBD also had low cytotoxicity, it realized the simultaneous detection of GSH and Hcy/Cys by dual-channel luminescence, and also provided ideas for the design of multifunctional luminescent probes.

A dual-response NIR fluorescent probe for separately and continuously recognizing H2S and Cys with different fluorescence signals and its applications

Given the significant interactions between hydrogen sulfide (H2S) and cysteine (Cys) in organisms, a dual-site multi-purpose fluorescent probe (Cy-NP) for H2S and Cys was synthesized. Cy-NP is composed of two fluorophores: naphthalimide that emits in the visible region of 500-600 nm, and cyanine dye that emits in the NIR region of 700-800 nm. Cy-NP showed admirable sensitivity and selectivity for identifying H2S and Cys by fluorescent signals with limits of detection as low as 0.15 μM and 1.4 μM, respectively. Furthermore, other biological thiols (especially GSH and Hcy) showed no positive response to Cy-NP compared with H2S and Cys. The chemical mechanism of Cy-NP with H2S and Cys in DMF/PBS (1/1, v/v, pH = 7.4) solution was verified by HRMS and DFT calculations. Further, Cy-NP was successfully applied to monitor H2S released in raw meat and adapted to detect H2S and Cys in MCF-7 cells independently and continuously.

Glutathione displacement assay based on a fluorescent Au(I) complex

Glutathione (GSH) is an essential molecule that plays a pivotal role in maintaining intracellular redox homeostasis, as well as other physiological processes. However, the chemical mechanisms underlying the GSH-induced processes remain insufficiently understood due to the lack of appropriate detection tools. Fluorescence GSH imaging can serve as a useful principle for the rapid, convenient, and non-destructive detection of GSH in living organisms. In this study, we developed a fluorescent GSH probe based on a linear, homoleptic Au(I) complex with two 1,3-diphenylbenzimidazolium carbene ligands. The Au(I) complex produced a fluorescence turn-on response to GSH. Fluorescence GSH signaling was characterized with a short response time of a few seconds. The rapid response was attributed to the displacement of the carbene ligand with GSH, which involved a labile inner-sphere coordination interaction. Finally, we demonstrated the biological utility of our GSH probe by unambiguously discriminating between different GSH levels in normal and senescent preadipocytes.

Sensitive and discriminative detection of cysteine by a Nile red-based NIR fluorescence probe

Cysteine (Cys) is a significant biological mercaptan that achieves key roles in several important physiological processes, such as reversible redox homeostasis in living organisms. Abnormal levels of Cys in the human body are directly related to many diseases. In this work, we constructed a sensitive sensor (Cys-NR) by connecting a Cys recognition group to a Nile red derivative. Due to photo-induced electron transfer (PET), the Cys-NR probe showed little fluorescence at 650 nm. With the addition of Cys to the assay solution, the chlorine unit of the probe was substituted by the thiol group of Cys. Further, the amino and sulfhydryl groups in cysteine underwent an intramolecular rearrangement, which led to the Cys-NR probe water solution turning from colorless to pink with an enhancement in fluorescence. The red fluorescence at 650 nm increased about 20 times. Based on the turn-on signal, a selective Cys detection method is developed. The probe signal is not affected by various potential interferences or other competing biothiols and the limit of detection (LOD) is determined to be 0.44 μM. In addition, the probe is further employed for imaging of Cys in live cells, revealing good biological imaging ability that could provide a new way of intracellular Cys detection.

Near-infrared fluorophore IR-61 delays postovulatory aging of mouse oocytes through suppressing oxidative stress mediated by mitochondrial protection

Postovulatory aging can trigger deterioration of oocyte quality and subsequent embryonic development, and thus reduce the success rates of assisted reproductive technology (ART). The molecular mechanisms underlying postovulatory aging, and preventative strategies, remain to be explored. The near-infrared fluorophore IR-61, a novel heptamethine cyanine dye, has the potential for mitochondrial targeting and cell protection. In this study, we found that IR-61 accumulated in oocyte mitochondria and reduced the postovulatory aging-induced decline in mitochondrial function, including mitochondrial distribution, membrane potential, mtDNA number, ATP levels, and mitochondrial ultrastructure. In addition, IR-61 rescued postovulatory aging-caused oocyte fragmentation, defects in spindle structure, and embryonic developmental potential. RNA sequencing analysis indicated that the postovulatory aging-induced oxidative stress pathway might be inhibited by IR-61. We then confirmed that IR-61 decreased the levels of reactive oxygen species and MitoSOX, and increased GSH content in aged oocytes. Collectively, the results indicate that IR-61 may prevent postovulatory aging by rescuing oocyte quality, promoting successful rate in ART procedure.

A multi-bioresponsive self-assembled nano drug delivery system based on hyaluronic acid and geraniol against liver cancer

Herein, a multi-bioresponsive self-assembled nano-drug delivery system (HSSG) was constructed by conjugating the anticancer drug Geraniol (GER) to hyaluronic acid (HA) via a disulfide bond. The HSSG NPs displayed a uniform spherical shape with an average diameter of ∼110 nm, maintained high stability, and realized controlled drug release in the tumor microenvironment (pH/glutathione/hyaluronidase). Results of fluorescence microscopy and flow cytometry verified that HSSG NPs were selectively uptaken by human hepatocellular carcinoma cell lines HepG2 and Huh7 via CD44 receptor-mediated internalization. Studies on H22 tumor-bearing mice demonstrate that HSSG NPs could effectively accumulate at the tumor site for a long period. In vitro and in vivo studies show that HSSG NPs significantly promoted the death of cancer cells while reducing the toxicity as compared to GER. Therefore, the HSSG NPs have great potential in the treatment of tumors.

Organelle-targeting ratiometric fluorescent probes: design principles, detection mechanisms, bio-applications, and challenges

Biological species, including reactive oxygen species (ROS), reactive sulfur species (RSS), reactive nitrogen species (RNS), F-, Pd2+, Cu2+, Hg2+, and others, are crucial for the healthy functioning of cells in living organisms. However, their aberrant concentration can result in various serious diseases. Therefore, it is essential to monitor biological species in cellular organelles such as the cell membrane, mitochondria, lysosome, endoplasmic reticulum, Golgi apparatus, and nucleus. Among various fluorescent probes for species detection within the organelles, ratiometric fluorescent probes have drawn special attention as a potential way to get beyond the drawbacks of intensity-based probes. This method depends on measuring the intensity change of two emission bands (caused by an analyte), which produces an efficient internal referencing that increases the detection's sensitivity. This review article discusses the literature publications (from 2015 to 2022) on organelle-targeting ratiometric fluorescent probes, the general strategies, the detecting mechanisms, the broad scope, and the challenges currently faced by fluorescent probes.

Fluorescent probe for highly selective detection of cysteine in living cells

Cys play an important physiological role in the human body. Abnormal Cys concentration can cause many diseases. Therefore, it is of great significance to detect Cys with high selectivity and sensitivity in vivo. Because homocysteine (Hcy) and glutathione (GSH) have similar reactivity and structure to cysteine, few fluorescent probes have been reported to be specific and efficient for cysteine. In this study, we designed and synthesized an organic small molecule fluorescent probe ZHJ-X based on cyanobiphenyl, which can be used to specifically recognize cysteine. The probe ZHJ-X exhibits specific selectivity for cysteine, high sensitivity, short reaction response time, good anti-interference ability, and has a low detection limit of 3.8 × 10^-6 M. The probe ZHJ-X was successfully applied to the visualization of Cys in living cells and had great application prospects in cell imaging and detection.

Manipulating the monomer-dimer transformation of a heptamethine cyanine ligand: near infrared chromogenic recognition of biothiols

A novel absorbance recovery method has been developed for the determination of biothiols with a near-infrared reagent. This method employs a two-reagent system composed of cation heptamethine cyanine (CyL) and Hg²⁺. The absorbance of CyL, with a maximum peak at 760 nm, was decreased due to addition of Hg²⁺, but recovered when biothiols were added. Under optimal conditions, the reciprocal extent of recovered absorbance was proportional to the concentration of biothiols. The calibration curves are linear over the range of (0.3-7.0) × 10^-6 M for cysteine, (1.0-10.0) × 10^-6 M for homocysteine and (1.0-9.0) × 10^-6 M for glutathione. Because of the specific affinity of Hg²⁺for biothiols, there is minimal interference from other amino acids. This method has been successfully applied to the determination of homocysteine in human urine samples with satisfactory results.

A Near-Infrared Fluorescent and Photoacoustic Probe for Visualizing Biothiols Dynamics in Tumor and Liver

Biothiols, including glutathione (GSH), homocysteine (Hcy) and cysteine (Cys), play crucial roles in various physiological processes. Though an array of fluorescent probes have been designed to visualize biothiols in living organisms, few one-for-all imaging agents for sensing biothiols with fluorescence and photoacoustic imaging capabilities have been reported, since instructions for synchronously enabling and balancing every optical imaging efficacy are deficient. Herein, a new near-infrared thioxanthene-hemicyanine dye (Cy-DNBS) has been constructed for fluorescence and photoacoustic imaging of biothiols in vitro and in vivo. Upon treatment with biothiols, the absorption peak of Cy-DNBS shifted from 592 nm to 726 nm, resulting in a strong NIR absorption as well as a subsequent turn-on PA signal. Meanwhile, the fluorescence intensity increased instantaneously at 762 nm. Then, Cy-DNBS was successfully utilized for imaging endogenous and exogenous biothiols in HepG2 cells and mice. In particular, Cy-DNBS was employed for tracking biothiols upregulation in the liver of mice triggered by S-adenosyl methionine by means of fluorescent and photoacoustic imaging methods. We expect that Cy-DNBS serves as an appealing candidate for deciphering biothiols-related physiological and pathological processes.

Targeted implementation strategies of precise photodynamic therapy based on clinical and technical demands

With the development of materials science, photodynamic-based treatments have gradually entered clinics. Photodynamic therapy is ideal for cancer treatment due to its non-invasive and spatiotemporal properties and is the first to be widely promoted in clinical practice. However, the shortcomings resulting from the gap between technical and clinical demands, such as phototoxicity, low tissue permeability, and tissue hypoxia, limit its wide applications. This article reviews the available data regarding the pharmacological and clinical factors affecting the efficacy of photodynamic therapy, such as photosensitizers and oxygen supply, disease diagnosis, and other aspects of photodynamic therapy. In addition, the synergistic treatment of photodynamic therapy with surgery and nanotechnology is also discussed, which is expected to provide inspiration for the design of photodynamic therapy strategies.

Near-Infrared Probes for Biothiols (Cysteine, Homocysteine, and Glutathione): A Comprehensive Review

Biothiols (cysteine, homocysteine, and glutathione) are an important class of compounds with a free thiol group. These biothiols plays an important role in several metabolic processes in living bodies when present in optimum concentration. Researchers have developed several probes for the detection and quantification of biothiols that can absorb in UV, visible, and near-infrared (NIR) regions of the electromagnetic spectrum. Among them, NIR organic probes have attracted significant attention due to their application in in vivo and in vitro imaging. In this review, we have summarized probes for these biothiols, which could work in the NIR region, and discussed their sensing mechanism and potential applications. Along with focusing on the pros and cons of the reported probes we have classified them according to the fluorophore used and summarized their photophysical and sensing properties (emission, response time, limit of detection).

A water-soluble fluorescent probe for monitoring mitochondrial GSH fluctuations during oxidative stress

In this research, we constructed a styrylpyridine derivative-based fluorescent probe MITO-PQDNs to monitor mitochondrial glutathione (GSH). The probe MITO-PQDNs could react rapidly (20 min) with GSH in PBS buffer and exhibited a strong fluorescence signal (586 nm) as well as a significant Stokes shift (200 nm). Moreover, MITO-PQDNs could quantitatively detect GSH with high sensitivity (LOD = 253 nM). Meanwhile, MITO-PQDNs possessed favorable biocompatibility and could detect both endogenous and exogenous GSH in MCF-7 cells. Above all, MITO-PQDNs enabled the detection of fluctuations in mitochondrial GSH concentrations during oxidative stress.

Nitro-Containing Self-Immolative Systems for Biological Applications

Since its introduction in 1981, the chemistry of self-immolative systems has received increasing attention in different application areas, such as analytical chemistry, medicinal chemistry, and materials science. This strategy is based on a stimulation that triggers a cascade of disassembling reactions leading to the release of smaller molecules. The particular reactivity of the nitro group, due to its powerful electron-withdrawing nature, has been exploited in the field of self-immolative chemistry. In this context, the present review describes the major role of the nitro group in self-immolative processes depending on its position.

Fully Reversible Ratiometric Nanosensors for Continuously Quantifying Mitochondrial Glutathione Concentration in Living Cells

Mitochondrial glutathione (mGSH) is both the cause of the oxidative damage and a mechanism for maintaining the redox homeostasis in mitochondria. To effectively measure mGSH dynamics in living cells, we have developed a new FRET-based nanosensor by immobilizing rhodamine B into dendritic mesoporous silica nanoparticles and installing GSH probes and mitochondria-targeting motifs onto the surface of nanoparticles. The result shows that these nanosensors show efficient FRET and a full reversibility and rapid response (<10 s) to GSH in the range of 0.5-20 mM, due to their unique nanostructure and well-overlapped spectra. The excellent photostability and low cytotoxicity make them an effective means for monitoring mGSH concentration in real time. When the mGSH nanosensors are used for quantitatively measuring mGSH variations under glucose deprivation stimulation in HeLa cells, they successfully prove themselves a useful tool for mitochondrial redox activity studies.

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.

Two-Photon Small-Molecule Fluorogenic Probes for Visualizing Endogenous Nitroreductase Activities from Tumor Tissues of a Cancer Patient

Nitroreductase (NTR), a common enzymatic biomarker of hypoxia, is widely used to evaluate tumor microenvironments. To date, numerous optical probes have been reported for NTRs detection. Approaches capable of concisely guiding the probe design of NTRs suitable for deep-tissue imaging, however, are still lacking. As such, direct optical imaging of endogenous NTR activities from tumors derived from cancer patients is thus far not possible. Herein, aided by computational calculations, the authors have successfully developed a series of two-photon (TP) small-molecule fluorogenic probes capable of sensitively detecting general NTR activities from various biological samples; by optimizing the distance between the recognition moiety and the reactive site of NTRs from different sources, the authors have discovered and experimentally proven that X4 displays the best performance in both sensitivity and selectivity. Furthermore, X4 shows excellent TP excited fluorescence properties capable of directly monitoring/imaging endogenous NTR activities from live mammalian cells, growing zebrafish, and tumor-bearing mice. Finally, with an outstanding TP tissue-penetrating imaging property, X4 is used, for the first time, to successfully detect endogenous NTR activities from the liver lysates and cardia tissues of a cancer patient. The work may provide a universal strategy to design novel TP small-molecule enzymatic probes in future clinical applications.

Mitochondria-Targeted Fluorescence/Photoacoustic Dual-Modality Imaging Probe Tailored for Visual Precise Diagnosis of Drug-Induced Liver Injury

The multispectral optoacoustic tomography (MSOT) technique can be used to perform high-resolution molecular imaging under deep tissues, which gives the technology significant prospective for clinical application. Here, we developed a superoxide anion (O₂^•-)-activated MSOT and fluorescence dual-modality imaging probe (APSA) for early diagnosis of drug-induced liver injury (DILI). APSA can respond quickly to O₂^•-, resulting in an absorption peak blueshift from 845 to 690 nm, which also leads to the photoacoustic (PA) signal at 690 nm and the fluorescence signal at 748 nm increases linearly with increasing O₂^•- concentration, which can be utilized to assess the extent of liver damage. The developed MSOT imaging method can eliminate background interference from hematopoietic tissue by collecting the PA signals excited at 680, 690, 740, 760, 800, 845, and 900 nm wavelengths to achieve noninvasive in situ visual diagnosis of DILI. The developed fluorescence imaging method can be used for the imaging of endogenous O₂^•- in living cells and anatomic diagnosis of liver injury. The developed probe has broad application prospects in the early diagnosis of DILI.

A cysteine and Hg2+ detection method based on transformation supramolecular assembly of cyanine dye by AGRO100

ETC (3,3'-di(3-sulfopropyl)-4,5,4',5'-dibenzo-9-ethylthiacarbocyanine triethylammonium salt), as a derivative of thiazole, is capable of forming various aggregates by the short-range noncovalent interaction forces under specific conditions, accompanying with significant absorbance and fluorescence characteristics. In this work, a label-free probe (ETC) for the detection of Cys (Cysteine) and Hg²⁺ was developed based on transformation between monomers and J-aggregations by AGRO100. AGRO100 can transform between single-stranded DNA and G-quadruplex to realize recognition of Cys and Hg²⁺ in dual-channel mode. These recognitional signals can be captured by UV-visible absorption spectra and fluorescence spectroscopy. ETC exhibits high sensitivity and selectivity with the detection limit of 0.197 nM in a wide range of 0-15 μM, which can apply of Cys and Hg²⁺ detection in human serum.

Up-Converting Nanocrystals Modified With Fluorescent Markers for the Detection of Amino Acids: Preparation, Characterization, and Sensing Performance

The present work was devoted to developing rhodamine-like chemosensing systems for cysteine (Cys) optical recognition. Aiming at low background light and minimal photobleaching effect, up-converting nanocrystals were firstly synthesized and latterly coated by α-cyclodextrin, and finally used as an exciting host. An energy transfer procedure from these nanocrystals and rhodamine sensors was established via their spectroscopic analysis and emissive decay dynamics comparison. The binding dynamics of our chemosensors for Cys were revealed to have uncomplicated recognition with a stoichiometric ratio of 1 vs. 1. The addition of cysteine increased the emission intensity of the chemosensors. As a consequence, the luminescence off-on effect with sensing selectivity and linear sensing behavior for Cys was demonstrated. Sulfur modification on our chemosensors was shown to be effective in improving their selectivity and photostability.

Activatable fluorescence molecular imaging and anti-tumor effects investigation of GSH-sensitive BRD4 ligands

Overexpression of bromodomain 4 (BRD4) is closely correlated with a variety of human cancers by regulating the histone post-translational modifications, which renders BRD4 a promising target for pharmacological discoveries of novel therapeutic agents for cancer therapy. We herein present the design, chemical synthesis, cellular imaging and biological assessment of a novel tumor-sensitive BRD4 ligand (compound 4) by introducing anticancer BRD4 inhibitor into naphthalimide moiety (fluorescent reporter) via a sulfonamide unit as glutathione (GSH)-specific cleavable linker. Upon reaction with abundant intramolecular GSH in cancer cells or free GSH in aqueous solution (pH = 7.4), sulfonamide cleavage of 4 occurs, leading to the release of BRD4 inhibitor and concomitant fluorescence-on. This activatable fluorescence molecular imaging was demonstrated to preferentially occur in tumor cells. Moreover, towards cancer cell lines MGC-803 cells and THP-1, compound 4 was identified to show better antitumor efficacy than net BRD4 inhibitor. Collectively, this study presents a drug delivery strategy, wherein the drug release can be directly monitored in the cellular content by fluorescence imaging, and provides a valuable compound 4 as a potential antitumor agent. Compound 4 may represent a useful tool for explorative studies of BRD4 inhibition, such as an improved understanding of BRD4 inhibitor release-related information.

A coumarin-based reversible two-photon fluorescence probe for imaging glutathione near N-methyl-D-aspartate (NMDA) receptors

Glutathione (GSH) is known to play a key role in the modulation of the redox environment in N-methyl-d-aspartate (NMDA) receptors. Coumarin derivative 1 bearing cyanoacrylamide and ifenprodil moieties was synthesized and reported to monitor GSH near NMDA receptors. The cyanoacrylamide moiety allows probe 1 to monitor GSH reversibly at pH 7.4 and the ifenprodil group acts as a directing group for NMDA receptors. Two-photon fluorescence microscopy allows probe 1 to successfully sense endogenous GSH in neuronal cells and hippocampal tissues with excitation at 750 nm. Furthermore, the addition of H₂O₂ and GSH induced a decrease and an increase in fluorescence emission. Probe 1 can serve as a potential practical imaging tool to get important information on GSH in the brain.

Long wavelength emission fluorescent probe for highly selective detection of cysteine in living cells

We developed a fluorescent probe, named 2-(4-(acryloyloxy) phenyl)-4-(2-carboxyphenyl)-7-(diethylamino) chromenylium (PA-A), for detecting Cys using the -OH protection/deprotection strategy, which can react with Cys to form a red-emitting anthocyanidin derivative fluorophore. The probe has high selectivity to Cys over Hcy and GSH in phosphate buffer solution (PBS, 10 mM, pH = 7.4), high sensitivity, a low detection limit of 4.48 × 10^-8 mol/L, and it can be recognized with the naked eye. Fluorescence imaging experiment of Cys with PA-A at the cellular successfully showed excellent tissue penetration.

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.

Activatable NIR-II Fluorescence Probe for Highly Sensitive and Selective Visualization of Glutathione In Vivo

Visualization of glutathione (GSH) enables us to understand GSH-related pathophysiological processes in living subjects. Currently, in vivo visualization methods of GSH are based on visible or first near-infrared (NIR-I) window fluorescence (FL) probes, which possess limitations due to their low tissue penetration depth and strong tissue autofluorescence. Herein, we developed a GSH-activatable second near-infrared (NIR-II) window FL probe (denoted as LET-7) for highly sensitive and selective visualization of GSH in vivo. LET-7, composed of an anionic polymethylcyanide skeleton linked with a FL quenching group of 3,5-bis(trifluoromethyl)benzenethiol, can be specifically activated by GSH, thus triggering a significant NIR-II FL emission enhancement with excellent photostability, which enables us to efficiently distinguish GSH from closely related low-molecular-weight biothiols. The limit of detection of LET-7 for GSH was determined to be as low as 85 nM. Most intriguingly, the in vivo studies demonstrated that LET-7 showed high sensitivity and good selectivity toward GSH. Therefore, our study provides a solution to design activatable NIR-II FL probes for in vivo imaging of GSH and other disease-related biomarkers.

Boosting and Activating NIR-IIb Luminescence of Ag2Te Quantum Dots with a Molecular Trigger

Near-infrared (NIR) excited and NIR-IIb emissive Ag₂Te quantum dots (QDs) display significant advantages in luminescence bioimaging and biosensing due to their unique photophysical properties. However, the poor luminescence intensity and limited strategy for constructing activatable probes severely restrict the wide bioapplications of Ag₂Te QDs. Herein, we proposed a NIR dye-sensitization strategy to solve these two problems. First, we used IR-780 as the antenna for Ag₂Te QDs to improve the ability of harvesting excitation light, obtaining 21-fold luminescence enhancement at 1620 nm under an 808 nm laser irradiation. Subsequently, by further functionalizing the heptamethine cyanine with a recognition unit of glutathione (GSH), Cy-GSH with target-triggered emission was yielded, which served as the potential sensitizer for Ag₂Te QDs to fabricate an activatable ratiometric NIR-IIb nanoprobe for visualizing GSH in vivo with high contrast. This new strategy is expected as a powerful tool to promote the bioapplication of NIR-IIb QDs in the future.

Polymeric nanostructures based on azobenzene and their biomedical applications: synthesis, self-assembly and stimuli-responsiveness

Amphiphilic polymers can self-assemble to form nanoparticles with different structures under suitable conditions. Polymer nanoparticles functionalized with aromatic azo groups are endowed with photo-responsive properties. In recent years, a variety of photoresponsive polymers and nanoparticles have been developed based on azobenzene, using different molecular design strategies and synthetic routes. This article reviews the progress of this rapidly developing research field, focusing on the structure, synthesis, assembly and response of photo-responsive polymer assemblies. According to the molecular structure, photo-responsive polymers can be divided into linear polymers containing azobenzene in a side chain, linear polymers containing azobenzene in the main chain, linear polymers containing azobenzene in an end group, branched polymers containing azobenzene and supramolecular polymers containing azobenzene. These systems have broad biomedical application prospects in the field of drug delivery and imaging applications.

All-in-One Nanomedicine: Multifunctional Single-Component Nanoparticles for Cancer Theranostics

The development of cancer diagnostic imaging and treatment is a major concern worldwide. By integrating imaging and therapy into one theranostic nanoplatform for simultaneously detecting tumors, evaluating the targeting ability and timely monitoring therapeutic responses provide more opportunities for precision medicine. Among various theranostic nanosystems, a series of single-component nanoparticles (NPs) have been developed for "all-in-one" theranostics, which presents the unique properties of facile preparation, simple composition, defined structure, high reproducibility, and excellent biocompatibility. Specifically, utilizing single-component NPs for both diagnostics and therapeutics can reduce the possible numerous untoward side effects and risks to the living body. In this review, the recent progress of multifunctional single-component NPs in the applications of cancer theranostics is systematically summarized. Notably, the structure design, categories of NPs, targeted strategies, biomedical applications, potential barriers, challenges, and prospects for the future clinical practice of this rapidly growing field are discussed.

Exciton Delocalization Counteracts the Energy Gap: A New Pathway toward NIR-Emissive Dyes

Exciton coupling between the transition dipole moments of ordered dyes in supramolecular assemblies, so-called J/H-aggregates, leads to shifted electronic transitions. This can lower the excited state energy, allowing for emission well into the near-infrared regime. However, as we show here, it is not only the excited state energy modifications that J-aggregates can provide. A bay-alkylated quaterrylene was synthesized, which was found to form J-aggregates in 1,1,2,2-tetrachloroethane. A combination of superradiance and a decreased nonradiative relaxation rate made the J-aggregate four times more emissive than the monomeric counterpart. A reduced nonradiative relaxation rate is a nonintuitive consequence following the 180 nm (3300 cm^-1) red-shift of the J-aggregate in comparison to the monomeric absorption. However, the energy gap law, which is commonly invoked to rationalize increased nonradiative relaxation rates with increasing emission wavelength, also contains a reorganization energy term. The reorganization energy is highly suppressed in J-aggregates due to exciton delocalization, and the framework of the energy gap law could therefore reproduce our experimental observations. J-Aggregates can thus circumvent the common belief that lowering the excited state energies results in large nonradiative relaxation rates and are thus a pathway toward highly emissive organic dyes in the NIR regime.

Recent Strategies to Develop Innovative Photosensitizers for Enhanced Photodynamic Therapy

This review presents a robust strategy to design photosensitizers (PSs) for various species. Photodynamic therapy (PDT) is a photochemical-based treatment approach that involves the use of light combined with a light-activated chemical, referred to as a PS. Attractively, PDT is one of the alternatives to conventional cancer treatment due to its noninvasive nature, high cure rates, and low side effects. PSs play an important factor in photoinduced reactive oxygen species (ROS) generation. Although the concept of photosensitizer-based photodynamic therapy has been widely adopted for clinical trials and bioimaging, until now, to our surprise, there has been no relevant review article on rational designs of organic PSs for PDT. Furthermore, most of published review articles in PDT focused on nanomaterials and nanotechnology based on traditional PSs. Therefore, this review aimed at reporting recent strategies to develop innovative organic photosensitizers for enhanced photodynamic therapy, with each example described in detail instead of providing only a general overview, as is typically done in previous reviews of PDT, to provide intuitive, vivid, and specific insights to the readers.

Synthesis of 9,9-bis(4-hydroxyphenyl) fluorene catalyzed by bifunctional ionic liquids

Through structural design, a series of bifunctional ionic liquids (BFILs) containing sulfonic acid (-SO3H) and sulfhydryl groups (-SH) were synthesized and characterized by NMR and MS. The acidity of these BFILs was measured by the Hammett acidity (H 0) and the effective sulfhydryl molar content of BFILs was determined by Ellman's method. Moreover, BFIL's catalytic properties in the condensation reaction of 9-fluorenone and phenol were studied. BFIL catalyst 6c can achieve nearly 100% conversion of 9-fluorenone with a high selectivity of 9,9-bis(4-hydroxyphenyl) fluorene (95.2%).

Molecular-Dimension-Dependent ESIPT Break for Specific Reversible Response to GSH and Its Real-Time Bioimaging

Glutathione (GSH) plays many important roles in maintaining intracellular redox homeostasis, and determining its real-time levels in the biological system is essential for the diagnosis, treatment, and pathological research of related diseases. Fluorescence imaging has been regarded as a powerful tool for tracking biomarkers in vivo, for which specificity, reversibility, and fast response are the main issues to ensure the real-time effective detection of analytes. The determination of GSH is often interfered with by other active sulfur species. However, in addition to the common features of nucleophilic addition, GSH is unique in its large molecular scale. 2-(2-Hydroxyphenyl) benzothiazole (HBT) was often formed in the ESIPT process. In this study, HBT was installed with α,β-unsaturated ketone conjugated coumarin derivates or nitrobenzene, which were used to adjust the reactivity of α,β-unsaturated ketone. Experimental and theoretical calculations found ESIPT to be favorable in HBT-COU but not HBT-COU-NEt₂ or HBT-BEN-NO₂ due to the higher electronic energies in the keto form. Thus, for HBT-COU, in the presence of GSH, the hydrogen-bonding interaction between C═N of the HBT unit and carboxyl of GSH would inhibit the process, simultaneously promoting the Michel addition reaction between α,β-unsaturated ketone and GSH. As a consequence, probe HBT-COU could exhibit a rapid reversible ratiometric response to GSH. Small structures of Hcy and Cys are passivated for such reactions. Cell imaging demonstrated the specific response of the probe to GSH, and the probe was successfully used to monitor fluctuations in GSH concentration during cells apoptosis in real-time.

Research on endoplasmic reticulum-targeting fluorescent probes and endoplasmic reticulum stress-mediated nanoanticancer strategies: A review

Subcellular localization of organelles can achieve accurate drug delivery and maximize drug efficacy. As the largest organelle in eukaryotic cells, the endoplasmic reticulum (ER) plays an important role in protein synthesis, folding, and posttranslational modification; lipid biosynthesis; and calcium homeostasis. Observing the changes in various metal ions, active substances, and the microenvironment in the ER is crucial for diagnosing and treating many diseases, including cancer. Excessive endoplasmic reticulum stress (ERS) can have a killing effect on malignant cells and can mediate cell apoptosis, proper modulation of ERS can provide new perspectives for the treatment of many diseases, including cancer. Therefore, the ER is used as a new anticancer target in cancer treatment. This review discusses ER-targeting fluorescent probes and ERS-mediated nanoanticancer strategies.

Fluorescence Probe for Imaging N-Methyl-d-aspartate Receptors and Monitoring GSH Selectively Using Two-Photon Microscopy

N-Methyl-d-aspartate (NMDA) is an excitotoxic amino acid used to identify a specific subset of glutamate receptors. The activity of NMDA receptors is closely related to the redox level of the biological system. Glutathione (GSH) as an antioxidant plays a key role with regard to modulation of the redox environment. In this work we designed and developed a GSH-specific fluorescent probe with the capability of targeting NMDA receptors, which was composed of a two-photon naphthalimide fluorophore, a GSH-reactive group sulfonamide, and an ifenprodil targeting group for the NMDA receptor. This probe exhibited high selectivity toward GSH in comparison to other similar amino acids. Two-photon fluorescence microscopy allowed this probe to successfully monitor GSH in neuronal cells and hippocampal tissues with an excitation at 750 nm. It could serve as a potential practical imaging tool to explore the function of GSH and related biological processes in the brain.