Fluorescent probes for iron, heme, and related enzymes

A novel fluorescent probe based on coumarin derivatives-grafted cellulose for specific detection of Fe3+ and its application

Fe3+ is one of the most important ions for maintaining the normal growth of plants and animals. However, imbalance and accumulation of Fe3+ can lead to serious damage to the environmental system. Hence, it is considerably crucial to monitor the concentration of Fe3+. In this paper, a high-performance fluorescent probe CA-NCC for specifically detecting Fe3+ was obtained by grafting cellulose acetate (CA) with coumarin derivative (NCC). The resulted probe displayed a bright blue fluorescence in THF solution and showed a distinct "turn-off" fluorescence response to Fe3+, while the small molecule compound NCC could not realize the detection of Fe3+. CA-NCC displayed excellent response performance to Fe3+ including excellent selectivity and sensitivity, rapid reaction time (2.5 min), wide pH detection range (6-11), and low detection limit (0.178 µM). More importantly, CA-NCC was successfully fabricated into fluorescent film based on the good processability of cellulose acetate, and achieved highly selective recognition of Fe3+ from various metal ions.

Click Coupling of Flavylium Dyes with Plasmodione Analogues: Towards New Redox-Sensitive Pro-Fluorophores

The development of redox-sensitive molecular fluorescent probes for the detection of redox changes in Plasmodium falciparum-parasitized red blood cells remains of interest due to the limitations of current genetically encoded biosensors. This study describes the design, screening and synthesis of new pro-fluorophores based on flavylium azido dyes coupled by CuAAC click chemistry to alkynyl analogues of plasmodione oxide, the key metabolite of the potent redox-active antimalarial plasmodione. The photophysical and electrochemical properties of these probes were evaluated, focusing on their fluorogenic responses. The influence of both the redox status of the quinone and the length of the PEG chain separating the fluorophore from the electrophore on the photophysical properties was investigated. The fluorescence quenching by photoinduced electron transfer is reversible and of high amplitude for probes in oxidized quinone forms and fluorescence is reinstated for reduced hydroquinone forms. Our results demonstrate that shortening the PEG chain has the effect of enhancing the fluorogenic response, likely due to non-covalent interactions between the two chromophores. All these systems were evaluated for their antiparasitic activities and fluorescence imaging suggests the efficacy of the fluorescent flavylium dyes in P. falciparum-parasitized red blood cells, paving the way for future parasite imaging studies to monitor cellular redox processes.

Development of a NIR Iridium(III) Complex-Based Probe for the Selective Detection of Iron(II) Ions

As a commonly used metal ion, iron(II) (Fe2+) ions pose a potential threat to ecosystems and human health. Therefore, it is particularly important to develop analytical techniques for the rapid and accurate detection of Fe2+ ions. However, the development of near-infrared (NIR) luminescence probes with good photostability for Fe2+ ions remain challenging. In this work, we report a novel iridium(III) complex-based luminescence probe for the sensitive and rapid detection of Fe2+ ions in a solution based on an Fe2+-mediated reduction reaction. This probe is capable of sensitively detecting Fe2+ ions with a limit of detection (LOD) of 0.26 μM. Furthermore, this probe shows high photostability, and its luminescence remains stable under 365 nm irradiation over a time period of 30 min. To our knowledge, this is first iridium(III) complex-based NIR probe for the detection of Fe2+ ions. We believe that this work provides a new method for the detection of Fe2+ ions and has great potential for future applications in water quality testing and human monitoring.

Benzothiazole-Quinoline-Based Fluorescent Probe for Fe3+ and its Applications in Environmental and Biological Samples

Due to the its high abundance, iron ion contamination and toxicity is one of the most challenging issue for living beings. Although, iron is extremenly important for several body functions, excess amount of iron in the body can also be fatal. In last century, rapid industrialization, iron extraction and mismanagement of industrial waste disposal leads to iron contamination in water bodies. Therefore, versatile iron sensors needs to be develop which can be employed for detection in biological as well as real water samples. 8-hydroxyquinoline is well-known for its strong affinity towards transition metals including Fe3+. In this regard, we have synthesised benzothiazole-quinoline derived 1,2,3- triazole (4HBTHQTz), in which 4-(benzo[d]thiazol-2-yl)phenolic (4-HBT) group acts as a fluorophore. 4HBTHQTz showed high fluorescence and induced a selective decrease in fluorescence with Fe3+ at 380 nm (λex. = 320 nm). The detection limit of 4HBTHQTz with Fe3+ is calculated as 0.64 μM, which is lower than the WHO recommended limit in drinking water. 4HBTHQTz works over the 5-8 pH range and has shown promising results for quantitative detection of Fe3+ in water samples collected from tap, river and seawater. 4HBTHQTz can also detect the Fe3+ in biological samples which is confirmed by fluorescence cell imaging using L929 mouse fibroblast cells. Overall, 4HBTHQTz showed advantages such as high selectivity, quick detection, and good limit of detection (LOD) for Fe3+.

A fluorescent nanoprobe and paper-based nanofiber platform for detection and imaging of Fe3+ in actual samples and living cells

In this study, a novel fluorescent nanoprobe (ZIF-90@FSS) was constructed using a zeolite imidazolium ester skeleton (ZIF-90) incorporating sodium fluorescein within its porous structure. Notably, this nanoprobe exhibited regular fluorescence "off" detection performance of Fe3+ in actual samples and living cells. The concentration range of 0-150 ng/mL exhibited a lowest detection limit of 0.26 ng/mL. A nanofiber paper-based platform (VL78/ZIF-90@FSS) was further developed by coupling the prepared nanoprobe to a multi-dimensional fiber paper via CN bonds, enabling rapid visual white light colorimetric and fluorescence imaging of Fe3+ within 2 min. The constructed nanoprobe and its paper-based detection platforms demonstrated a stable recovery range in tap water, beer, and soy sauce samples during spiking-recovery assessments. The recovery rates ranged from 98.46 % to 108.24 % for the nanoprobe and from 91.75 % to 108.71 % for the nanofiber paper-based platform. Therefore, the developed nano-fluorescent sensor and paper-based nanofiber sensing platform offer a promising strategy for the visual detection of Fe3+, while also presenting novel and valuable methods to investigate the regulatory mechanisms of Fe3+ in living cells.

Elucidating Iron Metabolism through Molecular Imaging

Iron is essential for many physiological processes, and the dysregulation of its metabolism is implicated in the pathogenesis of various diseases. Recent advances in iron metabolism research have revealed multiple complex pathways critical for maintaining iron homeostasis. Molecular imaging, an interdisciplinary imaging technique, has shown considerable promise in advancing research on iron metabolism. Here, we comprehensively review the multifaceted roles of iron at the cellular and systemic levels (along with the complex regulatory mechanisms of iron metabolism), elucidate appropriate imaging methods, and summarize their utility and fundamental principles in diagnosing and treating diseases related to iron metabolism. Utilizing molecular imaging technology to deeply understand the complexities of iron metabolism and its critical role in physiological and pathological processes offers new possibilities for early disease diagnosis, treatment monitoring, and the development of novel therapies. Despite technological limitations and the need to ensure the biological relevance and clinical applicability of imaging results, molecular imaging technology's potential to reveal the iron metabolic process is unparalleled, providing new insights into the link between iron metabolism abnormalities and various diseases.

Rapid, Sensitive, and Selective "ON-OFF" Detection of Fe3+ Ions Using Novel Acetalophanes and Their Applications in Real Samples

Three novel acetalophanes 1a-c have been designed, synthesized and characterized. The receptors 1b-c, featuring bulky anthracene groups, displayed significant selectivity for Fe3+ ions, resulting in a turn-off fluorescence mode in a DMF-buffer solution. Conversely, the non-steric probe 1a could serve as a versatile sensor for the simultaneous detection of Fe3+ and Cu2+ ions in MeOH-buffer solution. The sensing mechanism for the capability of 1a was demonstrated to be different, as evidenced by the addition of cyanide ions. The probes with Fe3+ exhibited a sensing mechanism that resulted in the deprotection of acetals to the corresponding starting materials, as confirmed by 1H NMR, IR spectra and TLC analysis. The attractive features of these practical and efficient sensors are selectivity, sensitivity (limit of detection = 0.15 µM by 1a, 0.16 µM by 1b and 0.14 µM by 1c), rapid response (less than 5 s). The on-site monitoring of various real samples, including well water, apricot, and green tea, proved to be successful for the quantitative and cost-effective detection of Fe3+. The method demonstrated good precision, even in the presence of other interfering materials.

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.

Optical Imaging Opportunities to Inspect the Nature of Cytosolic Iron Pools

The chemical nature of intracellular labile iron pools (LIPs) is described. By virtue of the kinetic lability of these pools, it is suggested that the isolation of such species by chromatography methods will not be possible, but rather mass spectrometric techniques should be adopted. Iron-sensitive fluorescent probes, which have been developed for the detection and quantification of LIP, are described, including those specifically designed to monitor cytosolic, mitochondrial, and lysosomal LIPs. The potential of near-infrared (NIR) probes for in vivo monitoring of LIP is discussed.

Assessing metal ion transporting activity of ZIPs: Intracellular zinc and iron detection

Zrt/Irt-like proteins (ZIPs or SLC39A) are a large family of metal ion transporters mainly responsible for zinc uptake. Some ZIPs have been shown to specifically transport zinc, whereas others have broader substrate specificity in divalent metal ion trafficking, notably those of zinc and iron ions. Measuring intracellular zinc and iron levels helps assess their molecular and physiological activities. This chapter presents step-by-step methods for evaluating intracellular metal ion concentrations, including direct measurement using inductively coupled plasma-mass spectrometry (ICP-MS), chemical staining, fluorescent probes, and indirect reporter assays such as activity analysis of enzymes whose activities are dependent on metal ion availability.

Recent advances in application of heterogeneous electro-Fenton catalysts for degrading organic contaminants in water

Over the last decades, advanced oxidation processes (AOPs) have been widely used in surface and ground water pollution control. The heterogeneous electro-Fenton (EF) process has gained much attention due to its properties of high catalytic performance, no generation of iron sludge, and good recyclability of catalyst. As of October 2022, the cited papers and publications of EF are around 1.3 × 10^-5 and 3.4 × 10^-3 in web of science. Among the AOP techniques, the contaminant removal efficiencies by EF process are above 90% in most studies. Current reviews mainly focused on the mechanism of EF and few reviews comprehensively summarized heterogeneous catalysts and their applications in wastewater treatment. Thus, this review focuses on the current studies covering the period 2012-2022, and applications of heterogeneous catalysts in EF process. Two kinds of typical heterogeneous EF systems (the addition of solid catalysts and the functionalized cathode catalysts) and their applications for organic contaminants degradation in water are reviewed. In detail, solid catalysts, including iron minerals, iron oxide-based composites, and iron-free catalysts, are systematically described. Different functionalized cathode materials, containing Fe-based cathodes, carbonaceous-based cathodes, and heteroatom-doped cathodes, are also reviewed. Finally, emphasis and outlook are made on the future prospects and challenges of heterogeneous EF catalyst for wastewater treatments.

Azo-tagged C4N4 fluorophores: unusual overcrowded structures and their application to fluorescent imaging

The C4N4 fluorophore is an intense fluorescence emitter featuring a 2,5-diaminopyrimidine core comprising four carbon and four nitrogen atoms. A series of C4N4 derivatives was photochemically dimerized at the 5-amino group, furnishing overcrowded ortho-tetraaryl-substituted diaryl azo compounds with a characteristic skewed structure revealed by X-ray crystallography. The photoquenched azo-C4N4s are useful for fluorescently visualizing cells under hypoxic conditions.

Indirect spectrophotometric determination of aqueous ferrate(VI) based on its reaction with iodide in acidic media

Ferrate(VI) (Fe(VI)) is utilized as an efficient and environmentally friendly water treatment agent that can be widely used for degradation of (in)organic pollutants in practical applications. However, only a few spectrophotometric methods for Fe(VI) determination were reported. In this study, a novel method for determining trace levels of aqueous Fe(VI) was developed based on the fact that Fe(VI) reacts with iodide at acidic pH to form iodine, which subsequently is treated with starch to yield the blue starch-iodine complex measured spectrophotometrically at 590 nm. The key measurement parameters, including acidic medium, starch dosages, temperature, time, and addition order were optimized to improve the sensitivity of detection. The increase in absorbance at 590 nm was linear with respect to Fe(VI) added (0.022-50 µM). Its sensitivity was determined as (4.61 ± 0.05) × 10⁴ M^-1 cm^-1, which was higher than that of existing spectrophotometric methods. The principle for Fe(VI) determination was studied by investigating stoichiometry, kinetics, and mechanism of Fe(VI) reaction with iodide. The molar stoichiometry of Fe(VI) with I₃^- species was determined to be 1:2. The reaction of Fe(VI) with iodide followed a second-order rate law with first order in each reactant and displayed apparent anti-Arrhenius kinetics, then its reaction pathway was proposed as well. Furthermore, the established method was successfully applied to measure Fe(VI) in various environmental water samples. The results show that the proposed approach is simple, convenient, highly reproducible and extremely sensitive, and is also expected to be of use for kinetic studies of Fe(VI) reaction with (in)organic compounds under acidic conditions.

Bioactive nutraceutical ligands and their efficiency to chelate elemental iron of varying dynamic oxidation states to mitigate associated clinical conditions

The natural bioactive or nutraceuticals exhibit several health benefits, including anti-inflammatory, anti-cancer, metal chelation, antiviral, and antimicrobial activity. The inherent limitation of nutraceuticals or bioactive ligand(s) in terms of poor pharmacokinetic and other physicochemical properties affects their overall therapeutic efficiency. The excess of iron in the physiological compartments and its varying dynamic oxidation state [Fe(II) and Fe(III)] precipitates various clinical conditions such as non-transferrin bound iron (NTBI), labile iron pool (LIP), ferroptosis, cancer, etc. Though several natural bioactive ligands are proposed to chelate iron, the efficiency of bioactive ligands is limited due to poor bioavailability, denticity, and other related physicochemical properties. The present review provides insight into the relevance of studying the dynamic oxidation state of iron(II) and iron(III) in the physiological compartments and its clinical significance for selecting diagnostics and therapeutic regimes. We suggested a three-pronged approach, i.e., diagnosis, selection of therapeutic regime (natural bioactive), and integration of novel drug delivery systems (NDDS) or nanotechnology-based principles. This systematic approach improves the overall therapeutic efficiency of natural iron chelators to manage iron overload-related clinical conditions.

Visual Monitoring of Nucleic Acid Dynamic Structures during Cellular Ferroptosis Using Rationally Designed Carbon Dots with Robust Anti-Interference Ability to Reactive Oxygen Species

Ferroptosis triggered by an iron-dependent accumulation of lipid reactive oxygen species (ROS) has drawn widespread attention. Directly visualizing the dynamic structures of nucleic acids during the ferroptosis of cells is of great importance considering their vital roles in numerous biological functions. However, direct imaging remains challenging, largely due to the extremely high concentrations of ROS generated during ferroptosis, which can affect the imaging of nucleic acid targeted fluorescent probes. To overcome this challenge, nucleic acid-responsive carbon dots (CDs) providing favorable optical properties together with high chemical stability were synthesized. Furthermore, the CDs penetrated the cell membrane quickly and accumulated in the nuclei of cells. The robust anti-interference ability to ROS allows the CDs to visualize the dynamic structures of nucleic acids during ferroptosis. Moreover, the CDs were successfully employed in the imaging of nucleic acids during cell division. The nuclei-targeting CDs show great potential as a powerful tool for imaging nuclei in ferroptosis-related biological and clinical research.

The selective detection of Fe3+ ions using citrate-capped gold nanoparticles

Citrate is a ubiquitous biological molecule that functions as Fe³⁺ chelators in some bacteria and the blood plasma of humans. Inspired by the strong affinity between citrate and Fe³⁺, a colorimetric Fe³⁺ probe based on citrate-capped AuNPs without any additional modification was designed. Citrate-capped AuNPs with a diameter of 22 nm were applied to detect Fe³⁺ without other reagents' assistance. This easily-prepared and low-cost colorimetric sensor exhibited good selectivity towards Fe³⁺ among common metal ions, a good linear relationship in the range of 0.1-0.8 μM of Fe³⁺ and quick response time of 10 min.

An out of box thinking: the changes of iron-porphyrin during meat processing and gastrointestinal tract and some methods for reducing its potential health hazard

Iron-porphyrin is a very important substance in organisms, especially in animals. It is not only the source of iron in human body, but is also the catalytic center of many reactions. Previous studies suggested that adequate intake of iron was important for the health of human, especially for children and pregnant women. However, associated diseases caused by iron over-intake and excessive meat consumption suggested its potential harmfulness for human health. During meat processing, Iron-porphyrin will cause the oxidation of proteins and fatty acids. In the gastrointestinal tract, iron-porphyrin can induce the production of malondialdehyde, fats oxidation, and indirectly cause oxidation of amino acids and nitrates etc. Iron-porphyrin enters the intestinal tract and disturbs the balance of intestinal flora. Finally, some common measures for inhibiting its activity are introduced, including the use of chelating agent, antioxidants, competitive inhibitor, etc., as well as give the hypothesis that sodium chloride increases the catalytic activity of iron-porphyrin. The purpose of this review is to present an overview of current knowledge about the changes of iron-porphyrin in the whole technico- and gastrointesto- processing axis and to provide ideas for further research in meat nutrition.