A turn-on fluorescent sensor for imaging labile Fe(3+) in live neuronal cells at subcellular resolution

A new six-membered spiro-rhodamine probe for Cu2+ and its imaging in mitochondria and lysosomes of Hela cells

Different from five-membered rhodamine spirolactam (Rh-OH), a new six-membered spiro-rhodamine probe (SRh-OH) bearing urea structure has been developed in this paper. Compared with five-membered Rh-OH, six-membered SRh-OH exhibited higher selectivity and sensitivity to Cu²⁺ in aqueous solution. Upon addition of 10 equiv. Cu²⁺, the molar extinction coefficient of SRh-OH at 563 nm can be up to 4.73 × 10⁴ Lmol^-1cm^-1. In the range of Cu²⁺ from 0 to 28 μM, there was an excellent linear relationship between the absorption or emission intensity and Cu²⁺ concentration. The detection limit of SRh-OH was as low as 26 nM (S/N = 3). The reversible binding mode of SRh-OH with Cu²⁺ was further confirmed by S^2- and EDTA addition. Bio-imaging showed SRh-OH can map the distribution of Cu²⁺ not only in mitochondria but also in lysosomes of Hela cells.

Development of a switching-type fluorescence sensor for the detection of boronic acid-containing agents

Since the therapeutic effect of boron neutron capture therapy is influenced by the intracellular distribution profile of boronoagents containing ¹⁰B atoms, it is necessary to establish a method that can determine the intracellular distribution profile of boronoagents. We aimed to develop a small molecule-based fluorescence sensor that changes its fluorescence properties upon complexation with the boronic acid moiety of a boronoagent. Thus, we designed a 2-(2-pyridyl)phenol derivative PPN-1 by introducing a N,O ligand substructure into a zinc sensor probe with excellent fluorescence properties. To investigate the effectiveness of PPN-1, we synthesized PPN-1 and evaluated its fluorescence properties compared to DAHMI, a current available boronic acid sensor. Consequently, PPN-1 showed favorable off/on fluorescence switching ability with a large Stokes shift after the addition of p-boronophenylalanine (BPA). Notably, after adding BPA, PPN-1 exhibited a rapid increase and reached a fluorescence plateau within 5 min, which is much shorter than the 2 h needed for DAHMI. Further, PPN-1 has excellent selectivity and detection and quantification limits similar to those of ICP-OES. These results demonstrated that PPN-1 is a practical scaffold for the detection and quantification of boronic acids and will provide essential insights for the development of boronic acid-targeted fluorescent sensors in the future.

Facile synthesis of a novel genetically encodable fluorescent α-amino acid emitting greenish blue light

We report the facile synthesis and characterization of a novel fluorescent α-amino acid 4-phenanthracen-9-yl-l-phenylalanine (Phen-AA) (5) that emits greenish blue light in the visible region. This genetically encodable l-α-amino acid has excellent photostability with a 75% quantum yield. It readily gets into human cells, being clearly imaged upon 405 nm laser excitation. The synthetic procedure is resistant to racemization and only involves three simple steps which use mild conditions and generate the Phen-AA in reasonably good yield. It may find broad applications in research, biotechnology, and the pharmaceutical industry.

A novel near-infrared turn-on and ratiometric fluorescent probe capable of copper(ii) ion determination in living cells

A near-infrared ratiometric fluorescent probe CR-Ac based on a coumarin-benzopyrylium platform has been developed for selective detection of Cu2+. The cell imaging data revealed the capabilities of CR-Ac in monitoring the dynamic changes of subcellular Cu2+ and the quantification of Cu2+ levels in living cells.

Two Novel BODIPY-Functional Magnetite Fluorescent Nano-Sensors for Detecting of Cr(VI) Ions in Aqueous Solutions

In this study, we developed two different very sensitive magnetite fluorescent Fe₃O₄@SiO₂-TPED-BODIPY and Fe₃O₄@SiO₂-TMPTA-BODIPY nano-sensors for the selective detection of Cr(VI) ions. The Cr(VI) metal ions sensing is based on the fluorescent quenching of BODIPY functionalized with Fe₃O₄@SiO₂-TPED and Fe₃O₄@SiO₂-TMPTA nanoparticles in the ethanol-water environment. Characterization of the newly synthesized fluorescent BODIPY compound was performed on a ¹H and ¹³C-NMR spectrometer. The morphology, chemical and physical properties of the sensing nano-sensors were studied by transmission thermogravimetric analysis (TGA), X-ray diffraction (XRD), energy dispersive X-ray (EDX), scanning electron microscopy (SEM), FT-IR spectroscopy, and transmission electron microscopy (TEM). UV-visible and fluorescent spectroscopy were used to characterize BODIPY functionalized magnetite fluorescent nano-sensors. Characterization measurements revealed that the mean particle diameter of magnetite fluorescent Fe₃O₄@SiO₂-TPED-BODIPY and Fe₃O₄@SiO₂-TMPTA-BODIPY nano-sensors was 18.5 and 19 nm, respectively. The magnetite fluorescent Fe₃O₄@SiO₂-TPED-BODIPY and Fe₃O₄@SiO₂-TMPTA-BODIPY nano-sensors (0.1 gL^-1 in EtOH/H₂O, v/v (3/7)) showed fluorescence quenching responses towards Cr(VI) ions in the medium at pH:1. The fluorescence quenches of the magnetite fluorescent Fe₃O₄@SiO₂-TPED-BODIPY and Fe₃O₄@SiO₂-TMPTA-BODIPY nano-sensors by Cr(VI) were completed in first 5 and 3 min. Respectively. These features provide potential uses of BODIPY functionalized magnetite fluorescent nano-sensors (Fe₃O₄@SiO₂-TPED-BODIPY and Fe₃O₄@SiO₂-TMPTA-BODIPY) as a new class of non-toxic sensors for environmental applications. Graphical Abstract.

Ethanol-Precipitation-Assisted Highly Efficient Synthesis of Nitrogen-Doped Carbon Quantum Dots from Chitosan

Nitrogen-doped carbon quantum dots (NCQDs) were prepared from chitosan through a hydrothermal reaction. When ethanol precipitation was used as the purification method, a high product yield of 85.3% was obtained. A strong blue fluorescence emission with a high quantum yield (QY) of 6.6% was observed from the NCQD aqueous solution. Physical and chemical characteristics of the NCQDs were carefully investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), Raman spectra, X-ray photoelectron spectroscopy (XPS), and transient fluorescence spectra. Experimental results showed that diameters of the NCQDs were in the range of 2-10 nm. The carbon quantum dots possess good water dispersibility and precipitation by ethanol. When used for metal ion detection, the detection limit of the NCQDs for Fe3+ was as low as 1.57 μM. This work proposed a facile method to synthesize NCQDs from chitosan with high yield and demonstrated that carbon quantum dots derived from chitosan were promising for ion detection.

Recent Advances on Iron(III) Selective Fluorescent Probes with Possible Applications in Bioimaging

Iron(III) is well-known to play a vital role in a variety of metabolic processes in almost all living systems, including the human body. However, the excess or deficiency of Fe3+ from the normal permissible limit can cause serious health problems. Therefore, novel analytical methods are developed for the simple, direct, and cost-effective monitoring of Fe3+ concentration in various environmental and biological samples. Because of the high selectivity and sensitivity, fast response time, and simplicity, the fluorescent-based molecular probes have been developed extensively in the past few decades to detect Fe3+. This review was narrated to summarize the Fe3+-selective fluorescent probes that show fluorescence enhancement (turn-on) and ratiometric response. The Fe3+ sensing ability, mechanisms along with the analytical novelties of recently reported 77 fluorescent probes are discussed.

Mitochondrial Iron in Human Health and Disease

Mitochondria are an iconic distinguishing feature of eukaryotic cells. Mitochondria encompass an active organellar network that fuses, divides, and directs a myriad of vital biological functions, including energy metabolism, cell death regulation, and innate immune signaling in different tissues. Another crucial and often underappreciated function of these dynamic organelles is their central role in the metabolism of the most abundant and biologically versatile transition metals in mammalian cells, iron. In recent years, cellular and animal models of mitochondrial iron dysfunction have provided vital information in identifying new proteins that have elucidated the pathways involved in mitochondrial homeostasis and iron metabolism. Specific signatures of mitochondrial iron dysregulation that are associated with disease pathogenesis and/or progression are becoming increasingly important. Understanding the molecular mechanisms regulating mitochondrial iron pathways will help better define the role of this important metal in mitochondrial function and in human health and disease.

Two colorimetric fluorescent turn-on chemosensors for detection of Al3+ and N3 - : Synthesis, photophysical and computational studies

Two new rhodamine derivative L1 and L2 bearing 2-methoxy-1-naphthaldehyde and 5-bromo-3-methoxy salicylaldehyde units were designed and synthesized using microwave-assisted organic synthesis and utilized towards sequential fluorescence detection of aluminum ion (Al3+ ) and azide (N3 - ) in aqueous acetonitrile solution. Aluminum ion (Al3+ ) triggers the formation of highly fluorescent ring-open spirolactam. The fluorescence and colorimetric response of the L1 -Al3+ and L2 -Al3+ complexes were quenched by the addition of N3 - , which extracting the Al3+ from the complexes and turn-off the sensors, confirming that the recognition process is reversible. The recognition ability of the sensors was investigated by fluorescence titration, Job's plot, 1 H-NMR spectroscopy and density functional theory (DFT) calculations.

(E)-3′,6′-bis(Diethylamine)-2-[(2-methoxynaphthalen-1-yl)methyleneamino]spiro[isoindoline-1,9′-xanthen]-3-one

The title compound, (E)-3′,6′-bis(diethylamine)-2-[(2-methoxynaphthalen-1-yl)methyleneamino]spiro[isoindoline-1,9′-xanthen]-3-one, was synthesized in 92% isolated yield using microwave-assisted organic synthesis. This new rhodamine derivative was fully characterized by 1H-NMR, 13C-NMR, FTIR and high resolution MS.

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.

Lysosomal tracking with a cationic naphthalimide using multiphoton fluorescence lifetime imaging microscopy

A naphthalimide-based chemosensing motif capable of turning on the fluorescence emission in solution and in vitro is reported.

Synthesis of poly(p-phenylene) containing a rhodamine 6G derivative for the detection of Fe(iii) in organic and aqueous media

A poly(p-phenylene) (PPP) containing rhodamine 6G (R6G) was synthesized by the Suzuki-coupling reaction, in which PPP acted as a blue-emitting energy donor and R6G acted as a ligand for Fe(iii) as well as the energy acceptor for Förster resonance energy transfer.

Highly Sensitive and Selective Detection of Nanomolar Ferric Ions Using Dopamine Functionalized Graphene Quantum Dots

The good stability, low cytotoxicity, and excellent photoluminescence property of graphene quantum dots (GQDs) make them an emerging class of promising materials in various application fields ranging from sensor to drug delivery. In the present work, the dopamine-functionalized GQDs (DA-GQDs) with stably bright blue fluorescence were successfully synthesized for low level Fe(3+) ions detection. The as-synthesized GQDs are uniform in size with narrow-distributed particle size of 4.5 ± 0.6 nm and high quantum yield of 10.2%. The amide linkage of GQDs with dopamine, confirmed by using XPS and FTIR spectra, results in the specific interaction between Fe(3+) and catechol moiety of dopamine at the interfaces for highly sensitive and selective detection of Fe(3+). A linear range of 20 nM to 2 μM with a detection limit of 7.6 nM is obtained for Fe(3+) detection by DA-GQDs. The selectivity of DA-GQDs sensing probe is significantly excellent in the presence of other interfering metal ions. In addition, the reaction mechanism for Fe(3+) detection based on the complexation and oxidation of dopamine has been proposed and validated. Results obtained in this study clearly demonstrate the superiority of surface functionalized GQDs to Fe(3+) detection, which can pave an avenue for the development of high performance and robust sensing probes for detection of metal ions and other organic metabolites in environmental and biomedical applications.

A selective fluorescent 'turn-on' sensor for recognition of Zn(2+) in aqueous media

A new rhodamine-based fluorescent probe 'RhAP' was synthesized and successfully characterized using FT-IR, (13)C NMR and (1)H NMR spectroscopies, LC-MS/MS spectrometry and elemental analysis. The RhAP, a colorless and non-fluorescent compound, showed a selective fluorescent response and colorimetric change for Zn(2+) in HEPES buffer (10mM, EtOH:water, 2:1, v/v, pH7.2). Upon the addition of two equivalents of Zn(2+) to a solution of RhAP, a nearly 35-fold enhancement of the fluorescence intensity, with an emission maximum at 578 nm, was observed in comparison to the sensor alone under the same experimental conditions. The complex formation between RhAP and Zn(2+) was found to have a 1:1 ratio based on calculations obtained from Job's plot and the mole ratio plot methods. The results showed that RhAP can be used as an effective fluorescent probe for selective detecting of Zn(2+) in an aqueous medium.

Cell Staining by Photo-activated Dye and Its Conjugate with Chitosan

Photo-activated or "Caged" rhodamine dyes are the most useful for microscopic investigation of biological tissue by various fluorescent techniques. Novel precursor of the fluorescent dye (PFD813) has been studied for photosensitive staining of numerous animal cells. The functional rhodamine dye (Rho813) with intensive fluorescence has been obtained after photoactivation of its precursor PFD813 inside cells. The dye Rho813 has been successfully used for the optical detection of particular features in biological objects (HaCaT cells, HBL-100, MDCK, lymphocytes). Moreover, the chitosan conjugate with PFD molecules ("Chitosan-PFD813″) has been obtained and studied for the first time. The developed procedures and obtained data are important for further applications of novel precursors of fluorescent dyes ("caged" dyes) for microscopic probing of biological objects. As example, the synthesized "Chitosan-PFD813″ has been successfully applied in this study for intracellular transport visualization by fluorescent microscopy.

A dual colorimetric-ratiometric fluorescent probe NAP-3 for selective detection and imaging of endogenous labile iron(iii) pools in C. elegans

The first dual colorimetric and ratiometric fluorescent probe NAP-3 for selective visualization of labile iron(iii) pools in Caenorhabditis elegans is reported.

Reaction-based turn-on fluorescent probes with magnetic responses for Fe2+ detection in live cells

A highly selective reaction-based “turn-on” fluorescent sensor is capable of detecting Fe²⁺ in mitochondria with distinct EPR responses.

Conjugates of a photoactivated rhodamine with biopolymers for cell staining

Conjugates of the photoactivated rhodamine dyes with biopolymers (proteins, polysaccharides, and nucleic acids) are important tools for microscopic investigation of biological tissue. In this study, a precursor of the photoactivated fluorescent dye (PFD) has been successfully used for staining of numerous mammalian cells lines and for conjugate formation with chitosan (“Chitosan-PFD”) and histone H1 (“Histone H1.3-PFD”). The intensive fluorescence has been observed after photoactivation of these conjugates inside cells (A431, HaCaT, HEK239, HBL-100, and MDCK). Developed procedures and obtained data are important for further application of novel precursors of fluorescent dyes (“caged” dyes) for microscopic probing of biological objects. Thus, the synthesized “Chitosan-PFD” and “Histone H1-PFD” have been successfully applied in this study for intracellular transport visualization by fluorescent microscopy.

Review: iron metabolism and the role of iron in neurodegenerative disorders

Iron plays a role for the biogenesis of two important redox-reactive prosthetic groups of enzymes, iron sulphur clusters (ISC) and heme. A part of these biosynthetic pathways takes plays in the mitochondria. While several important proteins of cellular iron uptake and storage and of mitochondrial iron metabolism are well-characterized, limited knowledge exists regarding the mitochondrial iron importers (mitoferrins). A disturbed distribution of iron, hampered Fe-dependent biosynthetic pathways and eventually oxidative stress resulting from an increased labile iron pool are suggested to play a role in several neurodegenerative diseases. Friedreich's ataxia is associated with mitochondrial iron accumulation and hampered ISC/heme biogenesis due to reduced frataxin expression, thus representing a monogenic mitochondrial disorder, which is clearly elicited solely by a disturbed iron metabolism. Less clear are the controversially discussed impacts of iron dysregulation and iron-dependent oxidative stress in the most common neurodegenerative disorders, i.e. Alzheimer's disease (AD) and Parkinson's disease (PD). Amyotrophic lateral sclerosis (ALS) may be viewed as a disease offering a better support for a direct link between iron, oxidative stress and regional neurodegeneration. Altogether, despite significant progress in molecular knowledge, the true impact of iron on the sporadic forms of AD, PD and ALS is still uncertain. Here we summarize the current knowledge of iron metabolism disturbances in neurodegenerative disorders.

A dansyl-rhodamine chemosensor for Fe(III) based on off-on FRET

A novel fluorescent chemosensor bearing a rhodamine and a dansyl moiety was developed for highly selective detection of Fe(3+) based on fluorescence resonance energy transfer (FRET) mechanism. Binding of Fe(3+) to the chemosensor induced spirolactam ring opening in the rhodamine moiety and subsequent off-on FRET from the dansyl energy donor to the rhodamine energy acceptor due to the spectral overlap between the emission of the dansyl moiety and the absorption of the ring opened rhodamine moiety. Job's plot analysis indicated a 1:1 binding stoichiometry between the chemosensor and Fe(3+). The association constant was estimated to be 2.72×10(3) M(-1) according to the Benesi-Hildebrand method. With the feature of easy synthesis, simple structural skeleton and excellent sensing ability, the newly synthesized chemosensor provided the potential for applying as a highly selective fluorescent probe in complex samples containing various competitive metal ions and developing other metal ion chemosensors to fulfill various needs of biological and environmental field.

Molecular imaging of labile iron(II) pools in living cells with a turn-on fluorescent probe

Iron is an essential metal for living organisms, but misregulation of its homeostasis at the cellular level can trigger detrimental oxidative and/or nitrosative stress and damage events. Motivated to help study the physiological and pathological consequences of biological iron regulation, we now report a reaction-based strategy for monitoring labile Fe(2+) pools in aqueous solution and living cells. Iron Probe 1 (IP1) exploits a bioinspired, iron-mediated oxidative C-O bond cleavage reaction to achieve a selective turn-on response to Fe(2+) over a range of cellular metal ions in their bioavailable forms. We show that this first-generation chemical tool for fluorescence Fe(2+) detection can visualize changes in exchangeable iron stores in living cells upon iron supplementation or depletion, including labile iron pools at endogenous, basal levels. Moreover, IP1 can be used to identify reversible expansion of labile iron pools by stimulation with vitamin C or the iron regulatory hormone hepcidin, providing a starting point for further investigations of iron signaling and stress events in living systems as well as future probe development.

Rhodamine-based "turn-on" fluorescent probe with high selectivity for Fe(2+) imaging in living cells

A rhodamine-based "turn-on" fluorescent probe 1 was synthesized with high yield. The recognizing behavior displays high selectivity of 1 toward Fe(2+) with a 2:1 complex, and 1 exhibits a stable response for Fe(2+) over a concentration range from 2 μM to 24 μM. Most importantly, probe is hardly interfered by other transition metal ions. Their fluorescent enhancement is observed in the presence of Fe(2+) because of the ring-open interactions of spirocyclic. All measurements are made in PBS buffer environments simulating biological conditions to make them suitable candidates for fluorescent labeling of biological systems. Confocal laser scanning microscopy experiments have proven that probe can be used to monitor Fe(2+) in living cells.

Ironing out the issues: integrated approaches to understanding iron homeostasis in plants

Plants initialize responses to environmental changes at all levels, from signaling to translation and beyond. Such is the case for fluctuations in the availability of iron (Fe), one of the most critical micronutrients for plants. The results of these responses are physiological and morphological changes that lead to increased iron uptake from the rhizosphere, and recycling and reallocation of Fe, which must be properly localized within specific cells and cellular compartment for use. The use of reductionist approaches, in combination with in vivo and in situ Fe localization tools, has been able to shed light on critical signaling molecules, transcriptional regulators, transporters and other proteins involved in Fe homeostasis. Recent advances in elemental distribution and speciation analysis now enable detection and measurement of Fe and other elements at resolutions never seen before. Moreover, increasing use of systems biology approaches provide a substantially broader perspective of how Fe availability affects processes at many levels. This review highlights the latest in vivo and in situ iron localization approaches and some of the recent advances in understanding mechanisms that control Fe translocation. A broad perspective of how Fe localization data might one day be integrated with large-scale data to create models for Fe homeostasis is presented.

A DNAzyme-gold nanoparticle probe for uranyl ion in living cells

DNAzymes have shown great promise as a general platform for detecting metal ions, as many metal-specific DNAzymes can be obtained using in vitro selection. While DNAzyme-based metal sensors have found many applications in the extracellular environment, no intracellular application of DNAzyme sensors has yet been reported. Here, we demonstrate a novel type of metal ion sensor for intracellular metal ion detection. The probe consists of a 13 nm gold nanoparticle (AuNP) core functionalized with a shell consisting of a uranyl-specific 39E DNAzyme whose enzyme strand contains a thiol at the 3' end for conjugation to the AuNP, and whose substrate strand is modified with a Cy3 fluorophore at the 5' end and a molecular quencher at the 3' end. In the absence of uranyl, the fluorescence of the Cy3 is quenched by both AuNP and the molecular quencher. In the presence of uranyl, the DNAzyme cleaves the fluorophore-labeled substrate strand, resulting in release of the shorter product strand containing the Cy3 and increased fluorescence. We demonstrate that this DNAzyme-AuNP probe can readily enter cells and can serve as a metal ion sensor within a cellular environment, making it the first demonstration of DNAzymes as intracellular metal ion sensors. Such a method can be generally applied to the detection of other metal ions using other DNAzymes selected through in vitro selection.