Biochemistry of mobile zinc and nitric oxide revealed by fluorescent sensors

Molecular Imaging of Nitric Oxide Surrogates with Organelle-Specific Fluorescent Probes

Nitric oxide is an important signalling molecule responsible for maintaining body's homeostasis. Any dysregulation in NO can lead to many pathological conditions like atherosclerosis, cancers, neurodegenerative disorders, hypertension and inflammation. Several, sensing technologies are used for sensing NO. Among these, fluorescent imaging is considered to be one of the most efficient. Till date, approximately 123 fluorescent probes are reported related to nitric oxide (NO) sensing fluorescent probes for the sensitive, selective, and real-time detection of NO at both the cellular and subcellular levels. In the past five years, around 41 fluorescent probes and four review articles have been published, specifically focusing on the detection of nitric oxide. Despite considerable advancements in this area, no systematic review has summarized various organelle-targeting NO-sensing fluorescent probes. Herein, we summarized last five years from 2019 to 2024 along with the key pioneering research in this field covering divergent roles of NO across various cellular organelles. We have included 41 probes by classifying into different organelle targeting sections. We strongly believe this review will provide an advanced summary of NO specific fluorescent probes and their applications for monitoring the progression of diseases in in vitro to in vivo models such as drosophila, zebrafish, mouse models.

Antioxidant-related enzymes and peptides as biomarkers of metallic nanoparticles (eco)toxicity in the aquatic environment

Increased awareness of the impact of human activities on the environment has emerged in recent decades. One significant global environmental and human health issue is the development of materials that could potentially have negative effects. These materials can accumulate in the environment, infiltrate organisms, and move up the food chain, causing toxic effects at various levels. Therefore, it is crucial to assess materials comprising nano-scale particles due to the rapid expansion of nanotechnology. The aquatic environment, particularly vulnerable to waste pollution, demands attention. This review provides an overview of the behavior and fate of metallic nanoparticles (NPs) in the aquatic environment. It focuses on recent studies investigating the toxicity of different metallic NPs on aquatic organisms, with a specific emphasis on thiol-biomarkers of oxidative stress such as glutathione, thiol- and related-enzymes, and metallothionein. Additionally, the selection of suitable measurement methods for monitoring thiol-biomarkers in NPs' ecotoxicity assessments is discussed. The review also describes the analytical techniques employed for determining levels of oxidative stress biomarkers.

Vitamin B6 Cofactor Pyridoxal 5'-phosphate Conjugated Papain-Stabilized Fluorescent Gold Nanoclusters for Switch-on Detection of Zinc(II)

In this study, fluorescent gold nanoclusters (AuNCs) conjugated with pyridoxal-5-phosphate (PLP) were synthesized, characterized, and used for Zn2+ fluorescence turn-on sensing. PLP was conjugated over the surface of papain-stabilized fluorescent gold nanoclusters (pap-AuNCs; λex = 380 nm, λem = 670 nm) by forming imine linkage. Due to this modification, the red color emitting pap-AuNCs changed to orange color emitting nanoclusters PLP_pap-AuNCs. The nano-assembly PLP_pap-AuNCs detect Zn2+ selectively by showing a notable fluorescence enhancement at 477 nm. Zn2+ detection with PLP_pap-AuNCs was quick and easy, with an estimated detection limit of 0.14 µM. Further, paper strips and cotton buds coated with PLP_pap-AuNCs were developed for affordable on-site visual detection of Zn2+. Finally, the detection of Zn2+ in actual environmental water samples served as validation of the usefulness of PLP_pap-AuNCs.

Activity-Based Fluorescent Probes for Hydrogen Sulfide and Related Reactive Sulfur Species

Hydrogen sulfide (H2S) is not only a well-established toxic gas but also an important small molecule bioregulator in all kingdoms of life. In contemporary biology, H2S is often classified as a "gasotransmitter," meaning that it is an endogenously produced membrane permeable gas that carries out essential cellular processes. Fluorescent probes for H2S and related reactive sulfur species (RSS) detection provide an important cornerstone for investigating the multifaceted roles of these important small molecules in complex biological systems. A now common approach to develop such tools is to develop "activity-based probes" that couple a specific H2S-mediated chemical reaction to a fluorescent output. This Review covers the different types of such probes and also highlights the chemical mechanisms by which each probe type is activated by specific RSS. Common examples include reduction of oxidized nitrogen motifs, disulfide exchange, electrophilic reactions, metal precipitation, and metal coordination. In addition, we also outline complementary activity-based probes for imaging reductant-labile and sulfane sulfur species, including persulfides and polysulfides. For probes highlighted in this Review, we focus on small molecule systems with demonstrated compatibility in cellular systems or related applications. Building from breadth of reported activity-based strategies and application, we also highlight key unmet challenges and future opportunities for advancing activity-based probes for H2S and related RSS.

Cellular zinc metabolism and zinc signaling: from biological functions to diseases and therapeutic targets

Zinc metabolism at the cellular level is critical for many biological processes in the body. A key observation is the disruption of cellular homeostasis, often coinciding with disease progression. As an essential factor in maintaining cellular equilibrium, cellular zinc has been increasingly spotlighted in the context of disease development. Extensive research suggests zinc's involvement in promoting malignancy and invasion in cancer cells, despite its low tissue concentration. This has led to a growing body of literature investigating zinc's cellular metabolism, particularly the functions of zinc transporters and storage mechanisms during cancer progression. Zinc transportation is under the control of two major transporter families: SLC30 (ZnT) for the excretion of zinc and SLC39 (ZIP) for the zinc intake. Additionally, the storage of this essential element is predominantly mediated by metallothioneins (MTs). This review consolidates knowledge on the critical functions of cellular zinc signaling and underscores potential molecular pathways linking zinc metabolism to disease progression, with a special focus on cancer. We also compile a summary of clinical trials involving zinc ions. Given the main localization of zinc transporters at the cell membrane, the potential for targeted therapies, including small molecules and monoclonal antibodies, offers promising avenues for future exploration.

Design and Synthesis of Poly(2,2'-Bipyridyl) Ligands for Induction of Cell Death in Cancer Cells: Control of Anticancer Activity by Complexation/Decomplexation with Biorelevant Metal Cations

Chelation therapy is a medical procedure for removing toxic metals from human organs and tissues and for the treatment of diseases by using metal-chelating agents. For example, iron chelation therapy is designed not only for the treatment of metal poisoning but also for some diseases that are induced by iron overload, cancer chemotherapy, and related diseases. However, the use of such metal chelators needs to be generally carried out very carefully, because of the side effects possibly due to the non-specific complexation with intracellular metal cations. Herein, we report on the preparation and characterization of some new poly(bpy) ligands (bpy: 2,2'-bipyridyl) that contain one-three bpy ligand moieties and their anticancer activity against Jurkat, MOLT-4, U937, HeLa S3, and A549 cell lines. The results of MTT assays revealed that the tris(bpy) and bis(bpy) ligands exhibit potent activity for inducing the cell death in cancer cells. Mechanistic studies suggest that the main pathway responsible for the cell death by these poly(bpy) ligands is apoptotic cell death. It was also found that the anticancer activity of the poly(bpy) ligands could be controlled by the complexation (anticancer activity is turned OFF) and decomplexation (anticancer activity is turned ON) with biorelevant metal cations. In this paper, these results will be described.

Expression and Distribution of Free Zinc in Penile Erectile Tissue

PURPOSE

Several studies have shown that zinc has a significant influence on erectile function. However, no studies evaluating the cellular distribution of free zinc in penile erectile tissue have been performed. Therefore, this study aimed to test whether free zinc is present in penile tissue and whether it may be involved in the electrical stimulation (ES)-induced penile erection.

MATERIALS AND METHODS

The subjects for this study were 26 young (8-week-old) male C57BL/6J mice. After the cavernous nerve was exposed through a midline stomach incision, 14 mice received ES of the cavernous nerve (ES group), whereas 12 mice did not (control group). Intracavernous pressure (ICP) (consisting of 10 V at a duration of 1 min, frequency of 12 Hz and a pulse width of 1 m/s) was recorded during ES. Immediately after ICP was recorded, penile tissues were harvested for histological and biochemical analysis, including analysis of zinc transporter 3 (ZnT3) and intracellular free zinc levels.

RESULTS

The expression of neuronal nitric oxide synthase (nNOS) and endothelial NOS (eNOS) in penile tissue was significantly greater in the ES group than in the control group (p=0.036 and 0.016, respectively). And then, ZnT3 and intracellular free zinc were present in the penile tissue of both groups. However, ZnT3 immunofluorescence in the ES group was more intense in the dorsal nerve bundle (22% increase, p=0.032). The ES group also showed higher intensity N-(6-methoxy-8-quinolyl)-para-toluenesulfonamide (TSQ) fluorescence signals indicative of intracellular free zinc level in the penile tissue compared to the control group (49% increase in dorsal nerve bundle, p=0.001; 50% increase in corpus cavernosum, p=0.001).

CONCLUSIONS

The results of the study supported the expression and distribution of free zinc in penile tissue and increased levels after penile erection. Therefore, this study provides anatomical evidence for the potential role of free zinc in penile erection.

Triphenylamine-embedded copper(II) complex as a "turn-on" fluorescent probe for the detection of nitric oxide in living animals

Nitric oxide (NO) is one of three major signaling molecules, which is involved in a large amount of physiological and pathological processes in biological systems. Furthermore, more and more evidence indicates that NO levels are closely associated with several aspects of human health. Accordingly, it is of great significance to develop a convenient and reliable detection method for NO in biological systems. In this work, a novel triphenylamine-embedded copper(II) complex (NZ-Cu²⁺) has been developed to be used as a fluorescence probe for the detection of NO in living animals. The proposed sensing mechanism of NZ-Cu²⁺ towards NO has been confirmed by high-resolution mass spectrometry, spectroscopic titration and density functional theory calculation. NO induced the conversion of paramagnetic Cu²⁺ to diamagnetic Cu⁺, which blocked the photoinduced electron transfer process of NZ-Cu²⁺, resulting in a remarkable enhancement of the emission spectra. The NZ-Cu²⁺ probe possesses several advantages including high selectivity, low detection limit (12.9 nM), long emission wavelength (640 nm), large Stokes shift (201 nm), fast response time (60 s) and low cytotoxicity. More importantly, NZ-Cu²⁺ has been successfully applied to detect NO in vivo by fluorescence imaging.

Probing nitric oxide signaling using molecular MRI

Wide field measurements of nitric oxide (NO) signaling could help understand and diagnose the many physiological processes in which NO plays a key role. Magnetic resonance imaging (MRI) can support particularly powerful approaches for this purpose if equipped with molecular probes sensitized to NO and NO-associated targets. In this review, we discuss the development of MRI-detectable probes that could enable studies of nitrergic signaling in animals and potentially human subjects. Major families of probes include contrast agents designed to capture and report integrated NO levels directly, as well as molecules that respond to or emulate the activity of nitric oxide synthase enzymes. For each group, we outline the relevant molecular mechanisms and discuss results that have been obtained in vitro and in animals. The most promising in vivo data described to date have been acquired using NO capture-based relaxation agents and using engineered nitric oxide synthases that provide hemodynamic readouts of NO signaling pathway activation. These advances establish a beachhead for ongoing efforts to improve the sensitivity, specificity, and clinical applicability of NO-related molecular MRI technology.

A sensitive zinc probe operating via enhancement of excited-state intramolecular charge transfer

Novel highly sensitive fluorescent probes for zinc cations based on the diketopyrrolopyrrole scaffold were designed and synthesized. Large bathochromic shifts (≈80 nm) of fluorescence are observed when the Zn²⁺-recognition unit (di-(2-picolyl)amine) is bridged with the fluorophore possessing an additional pyridine unit able to participate in the coordination process. This effect originates from the dipolar architecture and the increasing electron-withdrawing properties of the diketopyrrolopyrrole core upon addition of the cation. The new, greenish-yellow emitting probes, which operate via modulation of intramolecular charge transfer, are very sensitive to the presence of Zn²⁺. Introduction of a morpholine unit in the diketopyrrolopyrrole structure induces a selective six-fold increase of the emission intensity upon zinc coordination. Importantly, the presence of other divalent biologically relevant metal cations has negligible effects and typically even at a 100-fold higher concentration of Mg²⁺/Zn²⁺, the effect is comparable. Computational studies rationalize the strong bathochromic shift upon Zn²⁺-complexation. Decorating the probes with the triphenylphosphonium cation and morpholine unit enables selective localization in the mitochondria and the lysosome of cardiac H9C2 cells, respectively.

Engineering CuMOF in TiO2 Nanochannels as Flexible Gas Sensor for High-Performance NO Detection at Room Temperature

As a marker molecule in respiratory gases for the pulmonary disease asthma, nitric oxide (NO) has attracted much attention for real-time gas monitoring. However, low sensitivity, poor selectivity, and high operating temperature limit the practical applications of metal oxide semiconductor (MOS) based chemiresistor gas sensors. Herein, by deliberately introducing metal-organic frameworks (MOFs) in free-standing TiO₂ nanochannels (NCs), a chemiresistor gas sensor with excellent detection ability and outstanding selective traits is developed for sensing NO at room temperature (RT). The precisely engineered Cu(II)-based MOF Cu-TCA (H₃TCA = tricarboxytriphenyl amine) induces more active surface in the NCs, causing the buildup of CuTCA/TiO₂ p-n heterojunctions that improve the sensing response at RT just via a simple UV irradiation (λ = 365 nm). Importantly, the specialized reductive reaction of Cu(II) by NO enables a remarkable selectivity toward NO analysis. Owing to the synergistic large active surface and chemical sensitization effects from Cu-TCA, the resulting Cu-TCA/TiO₂ NCs show outstanding sensing performance; i.e., the response ((R_gas - R_air)/R_air) reaches 124% at 50 ppm of NO with a detection limit of 140 ppb at RT. In addition, the response time decreases to 25.6% if the system is subjected to UV irradiation. The as-formed sensing membrane is also demonstrated to be practically effective for flexible and wearable sensing devices for quantitative NO analysis. This study facilitates the use of MOFs to achieve synergistically enhanced selectivity and sensitivity to develop high-performance gas sensors.

Overcoming Major Barriers to Developing Successful Sensors for Practical Applications Using Functional Nucleic Acids

For many years, numerous efforts have been focused on the development of sensitive, selective, and practical sensors for environmental monitoring, food safety, and medical diagnostic applications. However, the transition from innovative research to commercial success is relatively sparse. In this review, we identify four scientific barriers and one technical barrier to developing successful sensors for practical applications, including the lack of general methods to (a) generate receptors for a wide range of targets, (b) improve sensor selectivity to overcome interferences, (c) transduce the selective binding to different optical, electrochemical, and other signals, and (d) tune dynamic range to match thresholds of detection required for different targets; and the costly development of a new device. We then summarize solutions to overcome these barriers using sensors based on functional nucleic acids that include DNAzymes, aptamers, and aptazymes and how these sensors are coupled to widely available measurement devices to expand their capabilities and lower the barrier for their practical applications in the field and point-of-care settings.

Fluorescent-based nanosensors for selective detection of a wide range of biological macromolecules: A comprehensive review

Thanks to their unique attributes, such as good sensitivity, selectivity, high surface-to-volume ratio, and versatile optical and electronic properties, fluorescent-based bioprobes have been used to create highly sensitive nanobiosensors to detect various biological and chemical agents. These sensors are superior to other analytical instrumentation techniques like gas chromatography, high-performance liquid chromatography, and capillary electrophoresis for being biodegradable, eco-friendly, and more economical, operational, and cost-effective. Moreover, several reports have also highlighted their application in the early detection of biomarkers associated with drug-induced organ damage such as liver, kidney, or lungs. In the present work, we comprehensively overviewed the electrochemical sensors that employ nanomaterials (nanoparticles/colloids or quantum dots, carbon dots, or nanoscaled metal-organic frameworks, etc.) to detect a variety of biological macromolecules based on fluorescent emission spectra. In addition, the most important mechanisms and methods to sense amino acids, protein, peptides, enzymes, carbohydrates, neurotransmitters, nucleic acids, vitamins, ions, metals, and electrolytes, blood gases, drugs (i.e., anti-inflammatory agents and antibiotics), toxins, alkaloids, antioxidants, cancer biomarkers, urinary metabolites (i.e., urea, uric acid, and creatinine), and pathogenic microorganisms were outlined and compared in terms of their selectivity and sensitivity. Altogether, the small dimensions and capability of these nanosensors for sensitive, label-free, real-time sensing of chemical, biological, and pharmaceutical agents could be used in array-based screening and in-vitro or in-vivo diagnostics. Although fluorescent nanoprobes are widely applied in determining biological macromolecules, unfortunately, they present many challenges and limitations. Efforts must be made to minimize such limitations in utilizing such nanobiosensors with an emphasis on their commercial developments. We believe that the current review can foster the wider incorporation of nanomedicine and will be of particular interest to researchers working on fluorescence technology, material chemistry, coordination polymers, and related research areas.

A chromone-based multi-selective sensor: applications in paper strips and real sample

A chromone-based multi-selective sensor: applications in a paper strip and real sample.

CuO/Cu-MOF nanocomposite for highly sensitive detection of nitric oxide released from living cells using an electrochemical microfluidic device

The integration of large surface area and high catalytic profiles of Cu-MOF and CuO nanoparticles is described toward electrochemical sensing of nitric oxide (NO) in a microfluidic platform. The CuO/Cu-MOF nanocomposite was prepared through hydrothermal method, and its formation was confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDS). The CuO/Cu-MOF nanostructured modified Au electrodes enabled electrocatalytic NO oxidation at 0.6 V vs. reference electrode, demonstrating linear response over a broad concentration range of 0.03-1 μM and 1-500 μM with a detection limit of 7.8 nM. The interference effect of organic molecules and common ions was negligible, and the sensing system demonstrated excellent stability. Finally, an electrochemical microfluidic NO sensor was developed to detect of NO released from cancer cells, which were stimulated by L-arginine. Furthermore, in the presence of Fe³⁺, the stressed cells produced more NO. This work offers considerable potential for its practical applications in clinical diagnostics through determination of chemical symptoms in microliter-volume biological samples. Electrochemical microfluidic NO sensor was developed for detection of NO released from cancer cells. This miniaturized device consumes less materials and provides the basis for greener analytical chemistry.

Facile Green Approach for Developing Electrochemically Reduced Graphene Oxide-Embedded Platinum Nanoparticles for Ultrasensitive Detection of Nitric Oxide

Nitric oxide (NO) plays a crucial and important role in cellular physiology and also acts as a signaling molecule for cancer in humans. However, conventional detection methods have their own limitations in the detection of NO at low concentrations because of its high reactivity and low lifetime. Herein, we report a strategy to fabricate Pt nanoparticle-decorated electrochemically reduced graphene oxide (erGO)-modified glassy carbon electrode (GCE) with efficiency to detect NO at a low concentration. For this study, Pt@erGO/GCE was fabricated by employing two different sequential methods [first GO reduction followed by Pt electrodeposition (SQ-I) and Pt electrodeposition followed by GO reduction (SQ-II)]. It was interesting to note that the electrocatalytic current response for SQ-I (184 μA) was ∼15 and ∼3 folds higher than those of the bare GCE (11.7 μA) and SQ-II (61.5 μA). The higher current response was mainly attributed to a higher diffusion coefficient and electrochemically active surface area. The proposed SQ-I electrode exhibited a considerably low LOD of 52 nM (S/N = 3) in a linear range of 0.25-40 μM with a short response time (0.7 s). In addition, the practical analytical applicability of the proposed sensor was also verified.

Exploring the binding interactions of janus green blue with serum albumins from spectroscopic and calorimetric studies aided by in silico calculations

Binding interactions of the phenazinium dye Janus green blue (JGB) with human and bovine serum albumins (BSA - and BSA) have been explored for the first time from multi-spectroscopic and calorimetric measurements aided by in silico calculations. The formation of ground state complexes between JGB and the respective serum albumins have been suggested from the UV-Vis and steady-state fluorescence spectroscopic studies. The nonlinear Stern Volmer (SV) plots at higher concentrations of JGB primarily indicate the formation of more than one ground state complexes in BSA -/BSA-JGB systems. Modified SV plots and isothermal titration calorimetry (ITC) studies however signify the possibilities of one type of binding complexes between HSA/BSA - JGB systems. Binding constants and the thermodynamic parameters associated with the HSA/BSA-JGB complexes have also been estimated from the ITC studies. Förster distances (R₀) for HSA-JGB and BSA-JGB complexes are estimated from Förster resonance energy transfer (FRET) results. Variations in the micro-environment of the Tyr and Trp residues of the serum proteins in presence of JGB have been observed from the synchronous fluorescence measurements. The conformational changes in the protein structures induced by the dye JGB have been revealed from 3 D fluorescence and circular dichroism (CD) studies. The experimental observations are supported by in silico calculations. This in depth investigation on the interactions of serum albumins with JGB may provide the fundamental information toward exploring the therapeutic efficacy of JGB as a potent drug molecule. Communicated by Ramaswamy H. Sarma.

Recent progress on the organic and metal complex-based fluorescent probes for monitoring nitric oxide in living biological systems

Nitric oxide (NO) is an important gaseous signaling molecule related to various human diseases. To investigate the biological functions of NO, many strategies have been developed for real-time monitoring the NO levels in biological systems. Among these strategies, fluorescent probes are considered to be one of the most efficient and applicable methods owing to their excellent sensitivity and selectivity, high spatiotemporal resolution, noninvasiveness, and experimental convenience. Therefore, great efforts have been paid to the design, synthesis, and fluorescence investigation of novel NO fluorescent probes in the past several years. However, few of them exhibit practical applications owing to the low concentration, short half-life, and rapid diffusion characteristics of NO in biological systems. Rational design of NO fluorescent probes with excellent selectivity and sensitivity, low cytotoxicity, long-lived fluorescent emission, and low background interference is still a challenge for scientists all over the word. To provide spatial-temporal information, this article focuses on the progress made in the organic and metal complex-based NO fluorescent probes during the past five years. The key structural elements and sensing mechanisms of NO fluorescent probes are discussed. Some novel ratiometric, luminescence, and photoacoustic probes with low background interference and deep tissue penetrating ability are mentioned. All these probes have been used for imaging exogenous and endogenous NO in cells and animal models. More importantly, this article also describes the development of multi-functional NO fluorescent probes, such as organelle targeting probes, dual-analysis probes, and probe-drug conjugates, which will inspire the design of various functional fluorescent probes.

Molecular Magnetic Resonance Imaging of Nitric Oxide in Biological Systems

Detection of nitric oxide (NO) in biological systems is challenging due to both physicochemical properties of NO and limitations of current imaging modalities and probes. Magnetic resonance imaging (MRI) could be applied for studying NO in living tissue with high spatiotemporal resolution, but there is still a need for chemical agents that effectively sensitize MRI to biological NO production. To develop a suitable probe, we studied the interactions between NO and a library of manganese complexes with various oxidation states and molecular structures. Among this set, the manganese(III) complex with N,N'-(1,2-phenylene)bis(5-fluoro-2-hydroxybenzamide) showed favorable changes in longitudinal relaxivity upon addition of NO-releasing chemicals in vitro while also maintaining selectivity against other biologically relevant reactive nitrogen and oxygen species, making it a suitable NO-responsive contrast agent for T₁-weighted MRI. When loaded with this compound, cells ectopically expressing nitric oxide synthase (NOS) isoforms showed MRI signal decreases of over 20% compared to control cells and were also responsive to NOS inhibition or calcium-dependent activation. The sensor could also detect endogenous NOS activity in antigen-stimulated macrophages and in a rat model of neuroinflammation in vivo. Given the key role of NO and associated reactive nitrogen species in numerous physiological and pathological processes, MRI approaches based on the new probe could be broadly beneficial for studies of NO-related signaling in living subjects.

Double-detecting fluorescent sensor for ATP based on Cu2+ and Zn2+ response of hydrazono-bis-tetraphenylethylene

Although all kinds of sensors with unique detecting ability for one guest were reported, the fluorescence sensor with multiple detecting abilities was seldom presented. This work designed and synthesized a novel AIE fluorescence probe bearing double detecting for ATP based on Cu²⁺ and Zn²⁺ response of hydrazono-bis-tetraphenylethylene (Bis-TPE). Bis-TPE was prepared in 82% yield with simple procedure. It exhibited strong red AIE fluorescence based on the large conjugated electron effect in aqueous media. It showed outstanding selective sensing abilities for Cu²⁺ by strong fluorescence quenching and for Zn²⁺ by red-orange fluorescence change. The sensing mechanism of 1:1 stoichiometric ratios was confirmed by ¹H NMR and MS study. The strong red fluorescence of Bis-TPE + Cu²⁺ system could be recovered by adding ATP. The orange fluorescence of Bis-TPE + Zn²⁺ system could be quenched by adding Cu²⁺ and then was recovered by adding ATP. These double detecting abilities for ATP with the "off-on" red fluorescence in Bis-TPE + Cu²⁺ system and "allochroic-off-on" orange fluorescence in Bis-TPE + Zn²⁺+Cu²⁺ system were successfully applied to test Cu²⁺, Zn²⁺ and ATP in test paper and living cell imaging, displaying the good application prospects for sensing Cu²⁺, Zn²⁺ and double detecting ATP in the complicated environment.

Visualizing mitochondria and mouse intestine with a fluorescent complex of a naphthalene-based dipolar dye and serum albumin

A fluorophore–protein complex for the visualization of mitochondria and the mouse intestine was developed. The complex formation of a naphthalene-based dipolar dye and serum albumin was identified and its imaging applications were investigated.

Metal-Dependent DNAzymes for the Quantitative Detection of Metal Ions in Living Cells: Recent Progress, Current Challenges, and Latest Results on FRET Ratiometric Sensors

Many different metal ions are involved in various biological functions including metallomics and trafficking, and yet there are currently effective sensors for only a few metal ions, despite the first report of metal sensors for calcium more than 40 years ago. To expand upon the number of metal ions that can be probed in biological systems, we and other laboratories employ the in vitro selection method to obtain metal-specific DNAzymes with high specificity for a metal ion and then convert these DNAzymes into fluorescent sensors for these metal ions using a catalytic beacon approach. In this Forum Article, we summarize recent progress made in developing these DNAzyme sensors to probe metal ions in living cells and in vivo, including several challenges that we were able to overcome for this application, such as DNAzyme delivery, spatiotemporal control, and signal amplification. Furthermore, we have identified a key remaining challenge for the quantitative detection of metal ions in living cells and present a new design and the results of a Förster resonance energy transfer (FRET)-based DNAzyme sensor for the ratiometric quantification of Zn2+ in HeLa cells. By converting existing DNAzyme sensors into a ratiometric readout without compromising the fundamental catalytic function of the DNAzymes, this FRET-based ratiometric DNAzyme design can readily be applied to other DNAzyme sensors as a major advance in the field to develop much more quantitative metal-ion probes for biological systems.

Interrogating Intracellular Zinc Chemistry with a Long Stokes Shift Zinc Probe ZincBY-4

Previous work has shown that fluctuations in zinc content and subcellular localization play key roles in regulating cell cycle progression; however, a deep mechanistic understanding requires the determination of when, where, and how labile zinc pools are concentrated into or released from stores. Labile zinc ions can be difficult to detect with probes that require hydrolysis of toxic protecting groups or application at high concentrations that negatively impact cell function. We previously reported a BODIPY-based zinc probe, ZincBY-1, that can be used at working concentrations that are 20-200-fold lower than concentrations employed with other probes. To better understand how zinc pools can be visualized at such low probe concentrations, we modulated the photophysical properties via changes at the 5-position of the BODIPY core. One of these, ZincBY-4, exhibits an order of magnitude higher affinity for zinc, an 8-fold increase in brightness in response to zinc, and a 100 nm Stokes shift within cells. The larger Stokes shift of ZincBY-4 presents a unique opportunity for simultaneous imaging with GFP or fluorescein sensors upon single excitation. Finally, by creating a proxy for the cellular environment in spectrometer experiments, we show that the ZincBY series are highly effective at 50 nM because they can pass membranes and accumulate in regions of high zinc concentration within a cell. These features of the ZincBY probe class have widespread applications in imaging and for understanding the regulatory roles of zinc fluxes in live cells.

The nitric oxide suppressed Arabidopsis mutants- Atnoa1 and Atnia1nia2noa1-2 produce nitric oxide in MS growth medium and on uranium exposure

The mutants Atnoa1 and Atnia1nia2noa1-2 having a defective chloroplast developmental process, showed enhanced chlorophyll levels when they were grown on Murashige and Skoog (MS) medium and on exposure with uranium (U) on Hoagland medium. Thus we hypothesized that these mutants probably produced NO in MS medium and on exposure with U. Wild-type Col-0, Atnoa1, Atnia1nia2noa1-2 plants were cultured on modified Hoagland and 1/10 MS media and NO generation in the roots of these mutants was monitored using NO selective fluorescent dyes, DAF-2DA and Fl2E. Both Atnoa1 and Atnia1nia2noa1-2 triple mutants produced NO as observed by increases in DAF-2T and Fl2E fluorescence when these mutants were grown on MS medium but not on Hoagland medium. In presence of NO scavenger, methylene blue (MB, 200 μM), DAF-2T and Fl2E fluorescence was completely abolished. On the other hand treatment of the plants with 25 μM U triggered NO generation. U-treated Atnoa1 and Atnia1nia2noa1-2 plants upregulated genes (POR B, POR D, CHL D) involved in the chlorophyll biosynthesis. From these results it was concluded that Atnoa1 and Atnia1nia2noa1-2 are conditional NO producers and it appears that NO generation in plants substantially depends on growth medium and NIA1, NIA2 or NOA1 does not appear to be really involved in NO generation in MS medium or after U exposure.

Low Molecular Weight Fluorescent Probes (LMFPs) to Detect the Group 12 Metal Triad

Fluorescence sensing, of d-block elements such as Cu2+, Fe3+, Fe2+, Cd2+, Hg2+, and Zn2+ has significantly increased since the beginning of the 21st century. These particular metal ions play essential roles in biological, industrial, and environmental applications, therefore, there has been a drive to measure, detect, and remediate these metal ions. We have chosen to highlight the low molecular weight fluorescent probes (LMFPs) that undergo an optical response upon coordination with the group 12 triad (Zn2+, Cd2+, and Hg2+), as these metals have similar chemical characteristics but behave differently in the environment.

Detecting Zn(II) Ions in Live Cells with Near-Infrared Fluorescent Probes

Two near-infrared fluorescent probes (A and B) containing hemicyanine structures appended to dipicolylamine (DPA), and a dipicolylamine derivative where one pyridine was substituted with pyrazine, respectively, were synthesized and tested for the identification of Zn(II) ions in live cells. In both probes, an acetyl group is attached to the phenolic oxygen atom of the hemicyanine platform to decrease the probe fluorescence background. Probe A displays sensitive fluorescence responses and binds preferentially to Zn(II) ions over other metal ions such as Cd2+ ions with a low detection limit of 0.45 nM. In contrast, the emission spectra of probe B is not significantly affected if Zn(II) ions are added. Probe A possesses excellent membrane permeability and low cytotoxicity, allowing for sensitive imaging of both exogenously supplemented Zn(II) ions in live cells, and endogenously releases Zn(II) ions in cells after treatment of 2,2-dithiodipyridine.

Development of a Nitric Oxide-Responsive Labeling Reagent for Proteome Analysis of Live Cells

Nitric oxide (NO) is a pleiotropic signaling molecule involved in the regulation of diverse physiological and pathophysiological mechanisms in cardiovascular, nervous, and immunological systems. To understand the biological functions of NO in detail, comprehensive characterization of proteins found in high-NO concentration environments is crucial. Herein, we describe the design of NO-responsive protein labeling reagents based on N-alkoxyacyl- o-phenylenediamine as an optimal reactive scaffold. The designed molecules can label proteins in murine macrophage cells in response to endogenously produced NO. The combination of NO-responsive protein labeling and liquid chromatography-tandem mass spectrometry technology allowed the characterization of the proteome under NO-generated conditions. Moreover, we demonstrated that our reagent was able to selectively mark and be used to fluorescently visualize NO-producing cells in a mixed cell culture system.

An azine-based carbothioamide chemosensor for selective and sensitive turn-on-off sequential detection of Zn(II) and H2PO4-, live cell imaging and INHIBIT logic gate

Hydrazino-carbothioamide, 1, serves as a turn-on fluorescent chemosensor to Zn²⁺ and the mixture shows green emission (λ_em 492) in presence of large number of ions with impressive limit of detection (LOD), 0.59 nM. Intense fluorescence of Zn-complex is selectively turn-off upon adding H₂PO₄^- only to the limit of detection, 26 μM, while other anions remain silent. The sensing mechanism of 1 is established by suppressing the ESIPT of 1 and complexation with Zn²⁺ (CHEF) is supported by Job's plot, ¹H NMR, and HR-MS data. Turn-on-off sequential detection of Zn²⁺ and H₂PO₄^- also have successfully been employed for the engineering of INHIBIT molecular logic gate. The sensor is non-toxic and has been used in identifying Zn²⁺ in the intracellular region of African green monkey kidney cells (Vero cells).

In Vitro and in Cellulo Sensing of Transition Metals Using Time-Resolved Fluorescence Spectroscopy and Microscopy

In this work we demonstrate that time domain techniques can be used successfully to monitor realtively weak modulations of the fluorescence in sensing applications. The metal sensing complex Newport Green DCF™ can detect selected transition metals in vivo as well as in vitro. Incremental addition of Ni and/or Zn (in vitro) lead to a substantial reduction in the yield of the fast component in a bi-exponential fluorescence decay (τ₁ = 150-250 ps) from 60% to 30-35%. This is rationalised as an inhibition of intra-molecular electron transfer in the NPG sensing complex due to metal complexation. In order to explore this effect in cellulo, NIH 3 T3 mouse skin fibroplast cells were pre-incubated with set levels of Ni and Zn, at a constant concentration of NPG. The fluorescence modulation in cellullo was subsequently studied employing both time-resolved fluorescence microscopy and confocal fluorescence microscopy. In correlation with the in vitro observations, similar effects were observed on the fluorescence decay in cellulo.

Roles for endothelial zinc homeostasis in vascular physiology and coronary artery disease

The discovery of the roles of nitric oxide (NO) in cardiovascular signaling has led to a revolution in the understanding of cardiovascular disease. A new perspective to this story involving zinc (Zn) is emerging. Zn and its associated Zn transporter proteins are important for the integrity and functions of both the large conduit vessels and the microvascular resistance vessels. The Zn and NO pathways are tightly coordinated. Zn ions are required for the dimerization of endothelial nitric oxide synthase and subsequent generation of NO while generation of NO leads to a rapid mobilization of endothelial Zn stores. Labile Zn may mediate important downstream actions of NO including vascular cytoprotection and vasodilation. Several vascular disease risk factors (including aging, smoking and diabetes) interfere with Zn homeostatic mechanisms and both hypozincaemia and Zn transporter protein abnormalities are linked to atherosclerosis and microvascular disease. Some vegetarian diets and long-term use of certain anti-hypertensives may also impact on Zn status. The available evidence supports the existence of a Zn regulatory pathway in the vascular wall that is coupled to the generation and actions of NO and which is compromised in Zn deficiency with consequent implications for the pathogenesis and therapy of vascular disease.

Measuring zinc in biological nanovesicles by multiple analytical approaches

Exosomes are nanovesicles known to mediate intercellular communication. Although it is established that zinc ions can act as intracellular signaling factors, the measurement of zinc in circulating nanovesicles has not yet been attempted. Providing evidence of the existence of this zinc fraction and methods for its measurement might be important to advance our knowledge of zinc status and its relevance in diseases. Exosomes from 0.5 ml of either fresh or frozen human plasma were isolated by differential centrifugation. A morphological and dimensional evaluation at the nanoscale level was performed by atomic force microscopy (AFM) and Transmission Electron Microscopy (TEM). Energy Dispersive X-Ray Microanalysis (EDX) revealed the elemental composition of exosomes and their respective total Zinc content on a quantitative basis. The zinc mole fraction (in at%) was correlated to the phosphorous mole fraction, which is indicative for exosomal membrane material. Both fresh (Zn/P 0.09 ± 0.01) and frozen exosomes (Zn/P 0.08 ± 0.02) had a significant zinc content, which increased up to 1.09 ± 0.12 for frozen exosomes when treated with increasing amounts of zinc (100-500 μM; each p < 0.05). Interestingly, after zinc addition, the Calcium mole fractions decreased accordingly suggesting a possible exchange by zinc. In order to estimate the intra-exosomal labile zinc content, an Imaging Flow Cytometry approach was developed by using the specific membrane permeable zinc-probe Fluozin-3AM. A labile zinc content of 0.59 ± 0.27 nM was calculated but it is likely that the measurement may be affected by purification and isolation conditions. This study suggests that circulating nano-vesicular-zinc can represent a newly discovered zinc fraction in the blood plasma whose functional and biological properties will have to be further investigated in future studies.

Emerging Roles of Carbonyl Sulfide in Chemical Biology: Sulfide Transporter or Gasotransmitter?

SIGNIFICANCE

Carbonyl sulfide (COS) is the most prevalent sulfur-containing gas in the Earth's atmosphere, and it plays important roles in the global sulfur cycle. COS has been implicated in origin of life peptide ligation, is the primary energy source for certain bacteria, and has been detected in mammalian systems. Despite this long and intertwined history with terrestrial biology, limited attention has focused on potential roles of COS as a biological mediator. Recent Advances: Although bacterial COS production is well documented, definitive sources of mammalian COS production have not been confirmed. Enzymatic COS consumption in mammals, however, is well documented and occurs primarily by carbonic anhydrase (CA)-mediated conversion to hydrogen sulfide (H2S). COS has been detected in ex vivo mammalian tissue culture, as well as in exhaled breath as a potential biomarker for different disease pathologies, including cystic fibrosis and organ rejection. Recently, chemical tools for COS delivery have emerged and are poised to advance future investigations into the role of COS in different biological contexts.

CRITICAL ISSUES

Possible roles of COS as an important biomolecule, gasotransmitter, or sulfide transport intermediate remain to be determined. Key advances in both biological and chemical tools for COS research are needed to further investigate these questions.

FUTURE DIRECTIONS

Further evaluation of the biological roles of COS and disentangling the chemical biology of COS from that of H2S are needed to further elucidate these interactions. Chemical tools for COS delivery and modulation may provide a first avenue of investigative tools to answer many of these questions. Antioxid. Redox Signal. 28, 1516-1532.

Theoretical Design of a Two-Photon Fluorescent Probe for Nitric Oxide with Enhanced Emission Induced by Photoninduced Electron Transfer

In the present work, we systematically investigate the sensing abilities of two recently literature-reported two-photon fluorescent NO probes, i.e., the o-phenylenediamine derivative of Nile Red and the p-phenylenediamine derivative of coumarin. The recognition mechanisms of these probes are studied by using the molecular orbital classifying method, which demonstrates the photoinduced electron transfer process. In addition, we have designed two new probes by swapping receptor units present on fluorophores, i.e., the p-phenylenediamine derivative of Nile Red and the o-phenylenediamine derivative of coumarin. However, it illustrates that only the latter has ability to function as off-on typed fluorescent probe for NO. More importantly, calculations on the two-photon absorption properties of the probes demonstrate that both receptor derivatives of coumarin possess larger TPA cross-sections than Nile Red derivatives, which makes a better two photon fluorescent probe. Our theoretical investigations reveal that the underlying mechanism satisfactorily explain the experimental results, providing a theoretical basis on the structure-property relationships which is beneficial to developing new two-photon fluorescent probes for NO.

Critical Role of Zinc as Either an Antioxidant or a Prooxidant in Cellular Systems

Zinc is recognized as an essential trace metal required for human health; its deficiency is strongly associated with neuronal and immune system defects. Although zinc is a redox-inert metal, it functions as an antioxidant through the catalytic action of copper/zinc-superoxide dismutase, stabilization of membrane structure, protection of the protein sulfhydryl groups, and upregulation of the expression of metallothionein, which possesses a metal-binding capacity and also exhibits antioxidant functions. In addition, zinc suppresses anti-inflammatory responses that would otherwise augment oxidative stress. The actions of zinc are not straightforward owing to its numerous roles in biological systems. It has been shown that zinc deficiency and zinc excess cause cellular oxidative stress. To gain insights into the dual action of zinc, as either an antioxidant or a prooxidant, and the conditions under which each role is performed, the oxidative stresses that occur in zinc deficiency and zinc overload in conjunction with the intracellular regulation of free zinc are summarized. Additionally, the regulatory role of zinc in mitochondrial homeostasis and its impact on oxidative stress are briefly addressed.

Rhodol Derivatives as Selective Fluorescent Probes for the Detection of HgII Ions and the Bioimaging of Hypochlorous Acid

Two sensors, 1 with a spirolactone group and 2 with a spirolactam group containing a phenyl isothiocyanate moiety, based on rhodol, were designed and synthesized in order to obtain materials with excellent optical properties for the detection of environmentally and biologically important Hg2+ and hypochlorous acid (HClO) ions. The crystal structure of 1 revealed two moieties, a rhodamine-like portion with a spirolactone and a fluorescein-like portion without a spirolactone. In the absence of analyte, 1 produced an optical output with a maximum absorption and emission at 475 and 570 nm, respectively, which was attributed to the fluorescein-like moiety without a spirolactone. In contrast, the rhodamine-like moiety containing a spirolactone was activated by the addition of H+ or Hg2+ ions, and 1 yielded new absorption and emission peaks at 530 and 612 nm, respectively. Further functionalization with a phenyl isothiocyanate group afforded 2, a fluorescent probe for HClO. High selectivity and sensitivity towards the hypochlorite ion were anticipated, owing to the stoichiometric and irreversible formation of a thiosemicarbazide group, which led to dramatic fluorescence responses. With good functionality at physiological pH, probe 2 was successfully used to image HClO in HeLa cells.

A multi-analyte responsive chemosensor vanilinyl Schiff base: fluorogenic sensing of Zn(ii), Cd(ii) and I−

A single fluorescence probe recognizes multiple ions and grabs the great attention of scientists.

Highly selective and sensitive recognition of Zn(ii) by a novel coumarinyl scaffold following spectrofluorometric technique and its application in living cells

A biocompatible coumarinyl Schiff base scaffold (H2L), a nontoxic probe and a fluorescent sensor to Zn²⁺, with an LOD of 11 nM, was used for living cell imaging.

Combined experimental and theoretical studies on selective sensing of zinc and pyrophosphate ions by rational design of compartmental chemosensor probe: Dual sensing behaviour via secondary recognition approach and cell imaging studies

A compartmental chemosensor probe HL was designed and synthesized for dual sensing of zinc ions and PPi via secondary recognition approach.

A biomimetic approach to conjugate vitamin B6 cofactor with the lysozyme cocooned fluorescent AuNCs and its application in turn-on sensing of zinc(II) in environmental and biological samples

This communication focusses on the synthesis of red fluorescent lysozyme cocooned gold nanoclusters (Lyso-AuNCs) that have been successfully applied for the selective and specific recognition of the vitamin B₆ cofactor pyridoxal-5'-phosphate (PLP). The red fluorescence of Lyso-AuNCs showed remarkable color change to yellow upon conjugation with PLP due to the formation of a Schiff base between the free -NH₂ present in the lysozyme and the -CHO group of PLP. The developed PLP conjugated Lyso-AuNCs (PLP_Lyso-AuNCs) was applied for the selective turn-on recognition of Zn²⁺ ions in aqueous medium. The yellow fluorescence of PLP_Lyso-AuNCs exhibited significant enhancement at 475 nm in the presence of Zn²⁺ producing bluish-green fluorescence attributed to the complexation-induced aggregation of nanoclusters. The nanoprobe exhibits nanomolar limit of detection for Zn²⁺ ions (39.2 nM) and the practicality of the nanoprobe was validated in various environmental water samples and biological plasma, urine, and beetroot extract, with fairly good recovery percent. Also, the system was successfully implemented for the intracellular detection and monitoring of Zn²⁺ in live HeLa cells. Graphical abstract Applications of red emitting lysozyme cocooned gold nanoclusters (Lyso-AuNCs) for the selective recognition of the vitamin B₆ cofactor pyridoxal-5'-phosphate (PLP) and the conjugated nano-assembly PLP_Lyso-AuNCs for turn-on detection of Zn²⁺ ions in various environmental and biological samples.