Small-Molecule Fluorescent Probes for Binding- and Activity-Based Sensing of Redox-Active Biological Metals

Although transition metals constitute less than 0.1% of the total mass within a human body, they have a substantial impact on fundamental biological processes across all kingdoms of life. Indeed, these nutrients play crucial roles in the physiological functions of enzymes, with the redox properties of many of these metals being essential to their activity. At the same time, imbalances in transition metal pools can be detrimental to health. Modern analytical techniques are helping to illuminate the workings of metal homeostasis at a molecular and atomic level, their spatial localization in real time, and the implications of metal dysregulation in disease pathogenesis. Fluorescence microscopy has proven to be one of the most promising non-invasive methods for studying metal pools in biological samples. The accuracy and sensitivity of bioimaging experiments are predominantly determined by the fluorescent metal-responsive sensor, highlighting the importance of rational probe design for such measurements. This review covers activity- and binding-based fluorescent metal sensors that have been applied to cellular studies. We focus on the essential redox-active metals: iron, copper, manganese, cobalt, chromium, and nickel. We aim to encourage further targeted efforts in developing innovative approaches to understanding the biological chemistry of redox-active metals.

A new class of teraryl-based AIEgen for highly selective imaging of intracellular lipid droplets and its detection in advanced-stage human cervical cancer tissues

Lipid droplets (LDs) have drawn much attention in recent years. They serve as the energy reservoir of cells and also play an important role in numerous physiological processes. Furthermore, LDs are found to be associated with several pathological conditions, including cancer and diabetes mellitus. Herein, we report a new class of teraryl-based donor-acceptor-appended aggregation-induced emission luminogen (AIEgen), 6a, for selective staining of intracellular LDs in in vitro live 3T3-L1 preadipocytes and the HeLa cancer cell line. In addition, AIEgen 6a was found to be capable of staining and quantifying the LD accumulation in the tissue sections of advanced-stage human cervical cancer patients. Unlike commercial LD staining dyes Nile Red, BODIPY and LipidTOX, AIEgen 6a showed a high Stokes shift (195 nm), a good fluorescence lifetime decay of 12.7 ns, and LD staining persisting for nearly two weeks.

Synthesis and bioimaging of a BODIPY-based fluorescence quenching probe for Fe3

Iron is the main substance for maintaining life. Real-time determination of ferric ion (Fe3+) in living cells is of great significance for understanding the relationship of Fe3+ concentration changes with various physiological and pathological processes. Fluorescent probes are suitable for the detection of trace metal ions in cells due to their low toxicity and high sensitivity. In this work, a boron-dipyrromethene-based fluorescent probe (BODIPY-CL) for selective detection of Fe3+ was synthesized. The fluorescence emission of BODIPY-CL was determined at 516 nm. In a pH range of 1 to 10, the probe BODIPY-CL exhibits a quenching response to Fe3+. Meanwhile, BODIPY-CL showed a highly selective response to Fe3+ compared with 16 kinds of metal ions. The stoichiometry ratio of BODIPY-CL bound to Fe3+ was nearly 2 : 1. The fluorescence quenching response obtained by the sensor was linear with the Fe3+ concentration in the range of 0-400 μM, and the detection limit was 2.9 μM. BODIPY-CL was successfully applied to image Fe3+ in cells. This study provides a promising fluorescent imaging probe for further research on the physiological and pathological effects of Fe3+.

An Electrochemical Sensor Based on Gold and Bismuth Bimetallic Nanoparticles Decorated L-Cysteine Functionalized Graphene Oxide Nanocomposites for Sensitive Detection of Iron Ions in Water Samples

In this work, gold and bismuth bimetallic nanoparticles decorated L-cysteine functionalized graphene oxide nanocomposites (Au-BiNPs/SH-GO) were prepared and applied to selective detection of Fe(III) in lake and seawater samples by modifying onto glassy carbon electrodes. Bimetallic nanoparticles have various excellent properties and better catalytic properties because of the unique synergistic effect between metals. The modified electrode was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. Under optimized conditions, current peak intensity increased linearly with increasing Fe(III) concentration over the range of 0.2-50 μM and a detection limit of 0.07 μM (S/N = 3). The Au-BiNPs/SH-GO/GCE was used for the determination of Fe(III) in lake and seawater samples with recoveries ranged from 90 to 103%. Those satisfactory results revealed the potential application of the Au-BiNPs/SH-GO electrochemical sensor for heavy metals detection in environmental monitoring.

Epigenomic regulation by labile iron

Iron is an essential micronutrient metal for cellular functions but can generate highly reactive oxygen species resulting in oxidative damage. For these reasons its uptake and metabolism is highly regulated. A small but dynamic fraction of ferrous iron inside the cell, termed intracellular labile iron, is redox-reactive and ready to participate multiples reactions of intracellular enzymes. Due to its nature its determination and precise quantification has been a roadblock. However, recent progress in the development of intracellular labile iron probes are allowing the reevaluation of our current understanding and unmasking new functions. The role of intracellular labile iron in regulating the epigenome was recently discovered. This chapter examine how intracellular labile iron can modulate histone and DNA demethylation and how its pool can mediate a signaling pathway from cAMP serving as a sensor of the metabolic needs of the cells.

Recent Endeavors on Molecular Imaging for Mapping Metals in Biology

Abstract

Transition metals such as zinc, copper and iron play vital roles in maintaining physiological functions and homeostasis of living systems. Molecular imaging, including two-photon imaging (TPI), bioluminescence imaging (BLI) and photoacoustic imaging (PAI), could act as non-invasive toolkits for capturing dynamic events in living cells, tissues and whole animals. Herein, we review the recent progress in the development of molecular probes for essential transition metals and their biological applications. We emphasize the contributions of metallostasis to health and disease, and discuss the future research directions about how to harness the great potential of metal sensors.

Graphic Abstract

High throughput fluorimetric assessment of iron traffic and chelation in iron-overloaded Caenorhabditis elegans

The nematode Caenorhabditis elegans (C. elegans) is a convenient tool to evaluate iron metabolism as it shares great orthology with human proteins involved in iron transport, in addition to being transparent and readily available. In this work, we describe how wild-type (N2) C. elegans nematodes in the first larval stage can be loaded with acetomethoxycalcein (CAL-AM) and study it as a whole-organism model for both iron speciation and chelator permeability of the labile iron pool (LIP). This model may be relevant for high throughput assessment of molecules intended for chelation therapy of iron overload diseases.

Fluorescent Benzoselenadiazoles: Synthesis, Characterization, and Quantification of Intracellular Lipid Droplets and Multicellular Model Staining

In this work, we described the synthesis of 10 new fluorescent 2,1,3-benzoselenadiazole small-molecule derivatives and their chemical- and photocharacterizations. The new derivatives could, for the first time, be successfully applied as selective live cell imaging probes (at nanomolar concentrations) and stained lipid-based structures preferentially. Density functional theory (DFT) calculations were used to help in understanding the photophysical data and the intramolecular charge-transfer (ICT) processes of the synthesized dyes. Some derivatives showed impressive cellular responses, allowing them to be tested as probes in a complex multicellular model (i.e., Caenorhabditis elegans). When compared with the commercially available dye, the new fluorescent compounds showed far better results both at the cellular level and inside the live worm. Inside the multicellular complex model, the tested probes also showed selectivity, a feature not observed when the commercial dye was used to carry out the bioimaging experiments.

Synthetic ratiometric fluorescent probes for detection of ions

Metal cations and anions are essential for versatile physiological processes. Dysregulation of specific ion levels in living organisms is known to have an adverse effect on normal biological events. Owing to the pathophysiological significance of ions, sensitive and selective methods to detect these species in biological systems are in high demand. Because they can be used in methods for precise and quantitative analysis of ions, organic dye-based ratiometric fluorescent probes have been extensively explored in recent years. In this review, recent advances (2015-2019) made in the development and biological applications of synthetic ratiometric fluorescent probes are described. Particular emphasis is given to organic dye-based ratiometric fluorescent probes that are designed to detect biologically important and relevant ions in cells and living organisms. Also, the fundamental principles associated with the design of ratiometric fluorescent probes and perspectives about how to expand their biological applications are discussed.

A ratiometric iron probe enables investigation of iron distribution within tumour spheroids

Iron dysregulation is implicated in numerous diseases, and iron homeostasis is profoundly influenced by the labile iron pool (LIP). Tools to easily observe changes in the LIP are limited, with calcein AM-based assays most widely used. We describe here FlCFe1, a ratiometric analogue of calcein AM, which also provides the capacity for imaging iron in 3D cell models.

Live-Cell Imaging of Physiologically Relevant Metal Ions Using Genetically Encoded FRET-Based Probes

Essential biochemical reactions and processes within living organisms are coupled to subcellular fluctuations of metal ions. Disturbances in cellular metal ion homeostasis are frequently associated with pathological alterations, including neurotoxicity causing neurodegeneration, as well as metabolic disorders or cancer. Considering these important aspects of the cellular metal ion homeostasis in health and disease, measurements of subcellular ion signals are of broad scientific interest. The investigation of the cellular ion homeostasis using classical biochemical methods is quite difficult, often even not feasible or requires large cell numbers. Here, we report of genetically encoded fluorescent probes that enable the visualization of metal ion dynamics within individual living cells and their organelles with high temporal and spatial resolution. Generally, these probes consist of specific ion binding domains fused to fluorescent protein(s), altering their fluorescent properties upon ion binding. This review focuses on the functionality and potential of these genetically encoded fluorescent tools which enable monitoring (sub)cellular concentrations of alkali metals such as K⁺, alkaline earth metals including Mg²⁺ and Ca²⁺, and transition metals including Cu⁺/Cu²⁺ and Zn²⁺. Moreover, we discuss possible approaches for the development and application of novel metal ion biosensors for Fe²⁺/Fe³⁺, Mn²⁺ and Na⁺.

When the strategies for cellular selectivity fail. Challenges and surprises in the design and application of fluorescent benzothiadiazole derivatives for mitochondrial staining

Design, synthesis, molecular architecture and the unexpected behavior of fluorescent benzothiadiazole for selective mitochondrial and plasma membrane staining are investigated.

A new “ON–OFF–ON” fluorescent probe for sequential detection of Fe3+ and PPi based on 2-pyridin-2-ylethanamine and benzimidazo [2,1-a]benz[de]isoquinoline-7-one-12-carboxylic acid

A new fluorescent probe X based on 2-pyridin-2-ylethanamine and benzimidazo[2,1-a]benz[de]isoquinoline-7-one-12-carboxylic acid was designed and synthesized for the detection of Fe³⁺ and PPi.

Multifunctional fluorescent leucomalachite green derivatives for chemodosimetric detection of Fe3+, specific imaging of lipid droplets and intracellular pH monitoring

Fluorescent leucomalachite green derivatives detect micromolar concentrations of Fe³⁺, specifically stain lipid droplets in live cells and monitor intracellular pH variations in microorganisms.

A dual-responsive colorimetric and fluorescent chemosensor based on diketopyrrolopyrrole derivative for naked-eye detection of Fe3+ and its practical application

A novel dual-responsive colorimetric and fluorescent chemosensor L based on diketopyrrolopyrrole derivative for Fe³⁺ detection was designed and synthesized. In presence of Fe³⁺, sensor L displayed strong colorimetric response as amaranth to rose pink and significant fluorescence enhancement and chromogenic change, which served as a naked-eye indicator by an obvious color change from purple to red. The binding constant for L-Fe³⁺ complex was found as 2.4×10⁴ with the lower detection limit of 14.3nM. The sensing mechanism was investigated in detail by fluorescence measurements, IR and ¹H NMR spectra. Sensor L for Fe³⁺ detection also exhibited high anti-interference performance, good reversibility, wide pH response range and instantaneous response time. Furthermore, the sensor L has been used to quantify Fe³⁺ ions in practical water samples with good recovery.

Base-Promoted Selective Synthesis of 2H-Pyranones and Tetrahydronaphthalenes via Domino Reactions

A highly efficient domino protocol has been developed for the synthesis of 6-aryl-4-(methylthio/amine-1-yl)-2-oxo-2H-pyran-3-carbonitriles and 4-aryl-2-(amine-1-yl)-5,6,7,8-tetrahydronaphthalene-1-carbonitriles from simple and readily available α-aroylketene dithioacetals, malononitrile, secondary amines, and cyclohexanone. This elegant domino process involved consecutive addition-elimination, intramolecular cyclization, and ring opening and closing sequences. Notably, in situ generated 2-imino-4-(methylthio/amine-1-yl)-6-aryl-2H-pyran-3-carbonitrile plays multiple roles in the construction of various novel polyaromatic hydrocarbons.

A Tetraphenylethene-Naphthyridine-Based AIEgen TPEN with Dual Mechanochromic and Chemosensing Properties

Synthesis of new tetraphenylethene (TPE) conjugates via an innocuous route led to the revelation of a unique TPE-based aggregation-induced emissive fluorogen 3 (TPEN), which showed an interesting mechanochromic property when the emission was changed from blue to green upon grinding and green to blue upon fuming. The mechanochromic property of TPEN has been explored to prepare ink-free rewritable paper for security documentation. A detailed photophysical investigation of the TPE-naphthyridine scaffold led to the discovery of its high sensitivity to silver ions (Ag⁺) over other metal ions with a detection limit of 0.25 μM in an aqueous system. The stoichiometry of the complex of TPEN and silver ion was established to be 2:1 (TPEN:Ag⁺) on the basis of photophysical studies, mass analysis, and high-resolution mass spectrometry analysis.

New visible light excitable donor-acceptor 7-hydroxy-coumarins as blue fluorescent probes for selective staining of vacuoles in yeasts and L. donovani

In order to address the existing limitations of the commercially available fluorescent probe CMAC (7-amino-4-chloromethylcoumarin), a new series of highly fluorescent donor-acceptor 7-hydroxy-coumarin derivatives was prepared and these derivatives were used as vacuole specific fluorescent probes for live cell imaging of unicellular parasitic protozoa L. donovani promastigotes and yeast cells S. pombe and S. cerevisiae. The synthesized 7-hydroxy-coumarins exhibited interesting photophysical properties and have advantages such as excitation in the visible region, good water solubility, photo-stability, good quantum yield and low cytotoxicity.

A highly selective fluorescent probe for Fe3+ in living cells: a stress induced cell based model study

A rhodamine–phenanthroline conjugated fluorescent probe 4 has been designed and synthesized for selective sensing and imaging of endogenous Fe³⁺ ions in living cells under different stress conditions.

Chemical tools for detecting Fe ions

Owing to its distinctive electrochemical properties with interconvertible multiple oxidation states, iron plays a significant role in various physiologically important functions such as respiration, oxygen transport, energy production, and enzymatic reactions. This redox activity can also potentially produce cellular damage and death, and numerous diseases are related to iron overload resulting from the dysfunction of the iron regulatory system. In this case, “free iron” or “labile iron,” which refers to iron ion weakly bound or not bound to proteins, causes aberrant production of reactive oxygen species. With the aim of elucidating the variation of labile iron involved in pathological processes, some chemical tools that can qualitatively and/or quantitatively monitor iron have been utilized to investigate the distribution, accumulation, and flux of biological iron species. Since iron ions show unique reactivity depending on its redox state, i.e., Fe²⁺ or Fe³⁺ (or transiently higher oxidative states), methods for the separate detection of iron species with different redox states are preferred to understand its physiological and pathological roles more in detail. The scope of this review article covers from classical chromogenic to newly emerging chemical tools for the detection of Fe ions. In particular, chemical tools applicable to biological studies will be presented.

Preparation of Gold-Carbon Dots and Ratiometric Fluorescence Cellular Imaging

In this study, we synthesized novel gold-carbon dots (GCDs) with unique properties by microwave-assisted method. The characterization of high-resolution transmission electron microscope (HRTEM), XRD, high-angle annular dark field scanning transmission electron microscope (HAADF-STEM), and energy dispersive spectrometer demonstrates that GCDs are composed of carbon and Au. Tiny Au clusters are dispersed in a 2 nm-size carbon skeleton, which integrates the properties of typical CDs and gold nanoclusters (AuNCs), displaying fascinating peroxidase-like activity and single excitation/dual emission. Dual emission of the GCDs exhibits different fluorescent response to the target species and enables the GCDs to be exploited for sensing and bioimaging. The highly photostable and biocompatible GCDs were applied to dual fluorescent imaging for breast cancer cells and normal rat osteoblast cells under a single excitation. Moreover, ratiometric fluorescence imaging was used to monitor Fe(3+) level in normal rat osteoblast cells.

Fluorescent probes for the selective detection of chemical species inside mitochondria

This feature article systematically summarizes the development of fluorescent probes for the selective detection of chemical species inside mitochondria.

A new type of biocompatible fluorescent probe AFN for fixed and live cell imaging of intracellular lipid droplets

We discovered a new type of nontoxic, highly fluorogenic and biocompatible probe AFN for selective staining of intracellular Lipid Droplets (LDs) in both fixed and live human cervical cancer cells (HeLa) and 3T3-L1 pre-adipocytes without any background artifacts.

Pyrano[3,2-c]julolidin-2-ones: a novel class of fluorescent probes for ratiometric detection and imaging of Hg2+ in live cancer cells

A novel pyrano[3,2-c]julolidin-2-one based fluorescent molecular rotor PYJO4 has been designed and developed for selective ratiometric detection, quantification and imaging of intracellular Hg²⁺ in live cells.

2-(1-Pyrenyl) benzimidazole as a ratiometric and “turn-on” fluorescent probe for iron(iii) ions in aqueous solution

A fluorescent probe for Fe³⁺, 2-(1-pyrenyl) benzimidazole (L), was synthesized. In aqueous solution, L shows a highly selective and sensitive ratiometric probe for Fe³⁺ in emission spectra, and it can also be used as a fluorescent “turn-on” probe for Fe³⁺ in excitation spectra.

A new fluorescent chemosensor for highly selective and sensitive detection of inorganic phosphate (Pi) in aqueous solution and living cells

A new fluorescein-based chemosensor, FP-Fe³⁺, was developed for the detection of inorganic phosphate (Pi) in aqueous solution and living cells.