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 dimedone-phenylalanine-based fluorescent sensor for the detection of iron (III), copper (II), L-cysteine, and L-tryptophan in solution and pharmaceutical samples

The development of fluorescence molecules for the fast and effective detection of L-tryptophan (L-Trp) has attracted a lot of attention because it is an important amino acid for baby growth, nitrogen equilibrium in adults, improving sleep, and mood regulation. A dimedone-phenylalanine-based chiral sensor (SDPA) was synthesized and exhibited a strong fluorescence quenching by Fe³⁺ and Cu²⁺ in a water/DMSO (3/7) solution with a detection limit of 2.29 × 10^-6 M and 6.37 × 10^-6 M, respectively. The factors affecting fluorescence sensings, such as the pH and competing cations, were studied. The sensor can be reused at least five times after being treated with EDTA. The Job plot, ESI-MS spectra, ¹H NMR spectra, absorbance, and fluorescence titration experiments were investigated to study the mechanism of SDPA-Fe³⁺ and SDPA-Cu²⁺ complexation. The SDPA-Cu²⁺ complex can detect L-tryptophan and L-cysteine at trace levels by turn-on fluorescence with a detection limit of 9.35 × 10^-6 M and 8.86 × 10^-6 M, respectively. Moreover, applying the SDPA-Cu²⁺ complex for quantitative analysis of L-tryptophan in real sleep-improving capsules resulted in good recovery. The L-tryptophan level of the Elining capsule was determined at 190.8 ± 10.5 mg/g (mg L-tryptophan/g medicine), which is close to the announced quantity of 180 mg/g. Besides, the SDPA-Cu²⁺ complex can selectively detect free L-Try molecules and L-Try residues in proteins.

A selective and sensitive near-infrared fluorescent probe for real-time detection of Cu(i)

The disruption of copper homeostasis (Cu+/Cu2+) may cause neurodegenerative disorders. Thus, the need for understanding the role of Cu+ in physiological and pathological processes prompted the development of improved methods of Cu+ analysis. Herein, a new near-infrared (NIR) fluorescent turn-on probe (NPCu) for the detection of Cu+ was developed based on a Cu+-mediated benzylic ether bond cleavage mechanism. The probe showed high selectivity and sensitivity toward Cu+, and was successfully applied for bioimaging of Cu+ in living cells.

Pyrene-imidazole conjugate as a fluorescent sensor for the sequential detection of iron(III) and histidine in aqueous solution

We developed PIM, a pyrene-based fluorescence sensor bearing an imidazole moiety and a carbonyl group as the binding sites for Fe³⁺ ions. The pyrene-based control compounds 1 and 2 were synthesized to demonstrate the structure-activity relationships. Compound 1, which contained a thiazoline moiety and a carbonyl group, displayed high selectivity for Cu²⁺ ions. This property indicated that heterocycles play an important role in the metal ion selectivity modulation. Compound 2, which lacked a carbonyl group, did not display metal ion selectivity. This characteristic demonstrated that introducing an additional recognition unit (cooperative recognition strategy) should be an effective way to improve metal ion selectivity. Furthermore, the PIM-Fe³⁺ ensemble can serve as a fluorescent sensor for histidine (His) detection via the removal of Fe³⁺ from the ensemble by His and the release of PIM. The sequential detection of Fe³⁺ and His exhibited on-off-on phenomenon, and the Fe³⁺ and His detection limits were 0.11 and 3.06 μM, respectively. These results will help in the further enhancement or modulation of metal ion selectivity in the development of fluorescent sensor systems. Moreover, the organic-metal ensemble provides an effective platform for detecting amino acids through the displacement strategy.

Smart probe for simultaneous detection of copper ion, pyrophosphate, and alkaline phosphatase in vitro and in clinical samples

Wilson's disease (WD), which might lead to acute liver failure, is an inherited disorder characterized by accumulation of copper (Cu2+) in the brain, the liver, and other vital organs. In the clinic, decreased serum alkaline phosphatase (ALP) concentration is used for WD diagnosis. But to the best of our knowledge, using a fluorescent probe to simultaneously detect multiple factors in WD (e.g., Cu2+, pyrophosphate (PPi), and ALP) has not been reported. Herein, we rationally designed a fluorescent switch (E)-8-((4-methylbenzylidene)amino)napthalen-1-amine (L) and successfully applied it for sequential and selective detections of Cu2+, PPi, and ALP in vitro, in living cells and synovial fluid samples with "Off," "On," and "Off" fluorescence signals, respectively. Considering the obvious correlations among Cu2+, PPi, and ALP in WD, we envision that our fluorescent probe L could be applied to in vitro diagnosing WD in the near future.

A dansyl-appended N-heterocycle for Cu2+ and S2- recognition via a displacement mode

A novel l-proline based heterocycle 3 of C₂ symmetry has been designed and synthesized for cation and anion recognition in aqueous solution. Ligand 3 shows a strong affinity to Cu²⁺ ion, and their interaction induces a remarkable fluorescence quenching in DMSO:H₂O=9:1 (HEPES buffer, 0.01M, pH7.4) among various metal ions. Both the in-situ generated and isolated 3-Cu²⁺ complex exhibit specific fluorescence recovery upon addition of S^2-, even in the presence of S₂O₃^2-, L-histidine, and thiol-containing amino acids. For this dual functional switch, a combination of ¹H NMR titration, ESI mass and FT-IR spectra suggest that its sensing behavior is via a displacement mode. Sequential "on-off-on" fluorescence bio-imaging of the heterocycle 3 to Cu²⁺ and S^2- was carried out in HeLa cells.

Rational design of an "on-off-on" fluorescent switch for Cu2+ and histidine based on chiral macrocyclic dioxopolyamine

A novel chemosensor 1 having a chiral macrocyclic dioxopolyamine of C₂ symmetry as a receptor and anthracene as a signal unit has been designed and synthesized for cations and α-amino acids recognition in DMSO-HEPES buffer (1:9, v/v, pH 7.2). The ligand exhibited selective response to Cu²⁺ even in the presence of other metal ions with a fluorescence "switch-off" behavior. Additionally, the in situ generated 1-Cu²⁺ ensemble displayed specific recognition to histidine by a "switch-on" fluorescence response. For this dual functional switch, its sensing behavior via a displacement mode was confirmed by ¹H NMR titration and ESI mass spectroscopy. Sequential "on-off-on" fluorescence responses of 1 to Cu²⁺ and histidine are successfully applied in HeLa cells.

Design and application of a fluorogenic receptor for selective sensing of cations, small neutral molecules, and anions

An unprecedented single multi-analyte fluorogenic receptor, a sodium salt of N-(methyl-2-thiophenyl)-tyrosine (NaHTyrthio), is reported for the selective sensing of cations (Cu²⁺), small neutral molecules (nitrobenzene and aniline) and anions (F⁻) by variable spectral responses.

Green and Red Fluorescent Dyes for Translational Applications in Imaging and Sensing Analytes: A Dual-Color Flag

Red and green are two of the most-preferred colors from the entire chromatic spectrum, and red and green dyes are widely used in biochemistry, immunohistochemistry, immune-staining, and nanochemistry applications. Selective dyes with green and red excitable chromophores can be used in biological environments, such as tissues and cells, and can be irradiated with visible light without cell damage. This critical review, covering a period of five years, provides an overview of the most-relevant results on the use of red and green fluorescent dyes in the fields of bio-, chemo- and nanoscience. The review focuses on fluorescent dyes containing chromophores such as fluorescein, rhodamine, cyanine, boron-dipyrromethene (BODIPY), 7-nitobenz-2-oxa-1,3-diazole-4-yl, naphthalimide, acridine orange, perylene diimides, coumarins, rosamine, Nile red, naphthalene diimide, distyrylpyridinium, benzophosphole P-oxide, benzoresorufins, and tetrapyrrolic macrocycles. Metal complexes and nanomaterials with these dyes are also discussed.

Hydrazide d-luciferin for in vitro selective detection and intratumoral imaging of Cu(2.)

Copper is an essential micronutrient involved in fundamental life processes but using a bioluminescence (BL) probe to selectively sense Cu(2+)in vitro or image Cu(2+)in vivo is still unavailable. Herein, a latent BL probe hydrazide d-luciferin (1) was rationally designed and successfully applied it for selective detection of Cu(2+)in vitro and imaging Cu(2+) in living cells and in tumors. Upon the catalysis of Cu(2+), 1 was converted to d-luciferin and turned on the BL in the presence of firefly luciferase (fLuc). In vitro tests indicated that 1 could be applied for highly selective sensing Cu(2+) within the range of 0-80μM with a limit of detection (LOD) of 39.0nM. Cell and animal experiments indicated that 1 could be applied for specific BL imaging of Cu(2+) in living cells and tumors and the BL signal of 1 was more stable and longer than that of d-luciferin. We envision that this unique probe 1 might serve as an elucidative tool for further exploration of the biological roles of Cu(2+) in physiological and pathological processes in the near future.

Nitrogen-doped carbon nanoparticle modulated turn-on fluorescent probes for histidine detection and its imaging in living cells

In this work, nitrogen-doped carbon nanoparticle (N-CNP) modulated turn-on fluorescent probes were developed for rapid and selective detection of histidine. The as synthesized N-CNPs exhibited high fluorescence quantum yield and excellent biocompatibility. The fluorescence of N-CNPs can be quenched selectively by Cu(II) ions with high efficiency, and restored by the addition of histidine owing to the competitive binding of Cu(II) ions and histidine that removes Cu(II) ions from the surface of the N-CNPs. Under the optimal conditions, a linear relationship between the increased fluorescence intensity of N-CNP/Cu(II) ion conjugates and the concentration of histidine was established in the range from 0.5 to 60 μM. The detection limit was as low as 150 nM (signal-to-noise ratio of 3). In addition, the as-prepared N-CNP/Cu(II) ion nanoprobes showed excellent biocompatibility and were applied for a histidine imaging assay in living cells, which presented great potential in the bio-labeling assay and clinical diagnostic applications.

Remarkably selective and enantiodifferentiating sensing of histidine by a fluorescent homochiral Zn-MOF based on pyrene-tetralactic acid

A highly luminescent and water-stable homochiral Zn-MOF, i.e., Zn-PLA, developed based on pyrene-tetraacetic, selectively senses histidine amongst all other amino acids and also differentiates d and l isomers, as revealed by quenching of the fluorescence of its aqueous suspension.

A highly luminescent and water-stable homochiral Zn-MOF, i.e., Zn-PLA, has been developed based on a pyrene-tetralactic acid, which inherently features concave shapes for guest inclusion, to explore sensing of amino acids by fluorescence quenching; the solid-state fluorescence quantum yield of the MOF was found to be 46%. The fluorescence of an aqueous suspension of Zn-PLA was shown to be quenched specifically by histidine amongst all the other amino acids. Selective sensing of histidine is of prime importance due to its relevance in a variety of biological functions. The lack of quenching of fluorescence of Zn-PLA by all the amino acids other than histidine has been rationalized based on the exchange of the cationic dimethylammonium species in the MOF crystals with histidine that is protonated in water; the latter is envisaged to quench the fluorescence via charge transfer in the excited state. Furthermore, the homochiral crystals of Zn-PLA were found to permit enantiodiscrimination in the quenching by the d- and l-forms such that the ratio of enantioselectivity, i.e., K_d/K_l, is 1.8, as determined by Stern–Volmer quenching plots. The highly selective as well as enantiodifferentiating sensing of amino acids by MOFs is unprecedented for any sensor type.

Highly selective ratiometric fluorescent recognition of histidine by tetraphenylethene–terpyridine–Zn(ii) complexes

Fluorescent detection of histidine is achieved with distinctive color change from yellow to blue by using the Zn(ii) complexes of the terpyridine–tetraphenylethene conjugates.

A new Schiff base and its metal complex as colorimetric and fluorescent–colorimetric sensors for rapid detection of arginine

A new Schiff base (L) and its Pb²⁺-complex have been utilized for rapid detection of arginine in aqueous medium.Lexhibits an excellent selective colorimetric response whereas its Pb²⁺-complex exploits fluorescent-colorimetric response towards arginine with very low detection limits.

Fluorescence turn-on for the highly selective detection of nitric oxide in vitro and in living cells

A water-soluble, biocompatible, small molecular fluorescent turn-on probe was developed for the highly selective and sensitive detection of NO in vitro and in living cells.

A diarylethene-based fluorescent chemosensor for the sequential recognition of Fe3+ and cysteine

A new fluorescent chemosensor based on a diarylethene was designed and synthesized for sequential recognition of Fe³⁺ and cysteine.

Selective fluorescence sensing of Cu(II) and Zn(II) using a simple Schiff base ligand: naked eye detection and elucidation of photoinduced electron transfer (PET) mechanism

A simple Schiff base compound 2-((cyclohexylmethylimino)-methyl)-naphthalen-1-ol (2CMIMN1O) has been synthesized and characterized by (1)H NMR, (13)C NMR and FT-IR spectroscopic techniques. A significantly low emission yield of the compound has been rationalized in anticipation with photo-induced electron transfer (PET) from the imine receptor moiety to the naphthalene fluorophore unit. Consequently, an evaluation of the transition metal ion-induced modification of the fluorophore-receptor communication reveals the promising prospect of the title compound to function as a chemosensor for Cu(2+) and Zn(2+) ions selectively, through remarkable fluorescence enhancement as well as visual changes. While perturbation of the PET process has been argued to be the plausible mechanism behind the fluorescence enhancement, the selectivity for these two metal ions has been interpreted on the grounds of an appreciably strong binding interaction. Particularly notable aspects regarding the chemosensory activity of the compound is its ability to detect the aforesaid transition metal ions down to the level of micromolar concentration (detection limit being 2.74 and 2.27ppm respectively), along with a simple and efficient synthetic procedure.

Pyridine-biquinoline-metal complexes for sensing pyrophosphate and hydrogen sulfide in aqueous buffer and in cells

Herein, we report a new pyridine-biquinoline-derivative fluorophore L for effectively sensing pyrophosphate (PPi) and monohydrogen sulfide (HS(-)) in aqueous buffer and in living cells. L could selectively coordinate with metal ions (M(n+)) in Groups IB and IIB to form L-M(n+) complexes with 1:1 stoichiometry, resulting in fluorescence quenching via photoinduced electron transfer (PET) mechanism. L-Zn(2+) complex was applied to competitively coordinate with PPi to form a new "ate"-type complex, turning on the fluorescence by a 21-fold-increase. The limit of detection (LOD) of this assay for PPi detection in aqueous buffer is 0.85 μM. L-Cu(2+) complex was applied for highly selective detection of HS(-) with an excellent sensitivity by 25-fold decomplexation-induced fluorescence increase. LOD of L-Cu(2+) complex for HS(-) detection in aqueous buffer is 2.24 μM. With the in vitro data obtained, we successfully applied these two complexes for sequential imaging Zn(2+) and PPi, Cu(2+) and HS(-) in living cells, respectively. Since PPi and HS(-) occur in vascular calcification in positive correlation, our multifunctional probe L might help doctors to more precisely diagnose this disease in vivo in the future. For example, we could use radioactive tracer L-(64)Cu for qualitative and quantitative positron emission tomography/computed tomography (PET/CT) imaging of HS(-) in vivo.

Highly selective and sensitive recognition of histidine based on the oxidase-like activity of Cu2+ions

A simple, highly sensitive and selective PL sensing platform for histidine has been developed, based on the oxidase-like activity of Cu²⁺ions.

A colorimetric chemosensor for the sequential detection of copper ion and amino acids (cysteine and histidine) in aqueous solution

A simple and selective colorimetric chemosensor was reported for the sequential detection of Cu²⁺, cysteine and histidine.