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 chromogenic diarylethene-based probe for the detection of Cu2+ in aqueous medium in Drosophila for early diagnosis of Alzheimer

A diarylethene-based probe (Z)-N'-((2-amino-5-chlorophenyl)(phenyl)methylene)-2-hydroxy benzohydrazide (KBH) has been proficiently developed and its structure has been confirmed by single crystal X-ray diffraction technique. It displays a selective and sensitive colorimetric sensing of Cu2+ ions in aqueous medium with a naked eye colour change from colourless to yellow. It exhibits a significantly low limit of detection as 1.5 nM. A plausible binding mechanism has been proposed using Job's plot, FT-IR, 1H NMR titration, HRMS and DFT studies. The chemosensor is effectively reversible and reusable with EDTA. Test strip kit and real water sample analysis have been shown to establish its practical applicability. Further, the potential of KBH for the early diagnosis of Cu2+ ion-induced amyloid toxicity has been investigated in eye imaginal disc of Alzheimer's disease model of Drosophila 3rd instar larvae. The in-vivo interaction of KBH with Cu2+ in gut tissues of Drosophila larvae establishes its sensing capability in biological system. Interestingly, the in-vivo detection of Cu2+ has been done using bright field imaging which eliminates the necessity of a fluorescent label, hence making the method highly economical.

Azo-hydrazone tautomerism in a simple coumarin azo dye and its contribution to the naked-eye detection of Cu2+ and other potential applications

A new tailor-made azo dye of coumarin connected to phenolic derivative is presented herein. Azo-hydrazone tautomerism in aqueous solution of the dye was observed and studied using spectroscopic assays such as ¹H NMR, absorption and emission assays, and theoretical studies. Tautomerism was attributed to the presence of a labile phenolic hydrogen in the ortho position to the azo functionality and the hydrazone was found to be the more dominant tautomer. Influence of metal ions on the azo-hydrazone chemical equilibrium and how the accompanying colour and spectroscopic changes can be exploited for various functions, especially the detection and quantification of Cu²⁺ in aqueous environments was explored. The presence of Cu²⁺ affects the azo-hydrazone equilibrium resulting in visual appearance and spectroscopic changes and the likely binding sites for Cu²⁺ were evaluated. Cu²⁺ pushes the azo-hydrazone equilibrium towards the more conjugated form and the presence of other metal ions does not have any perceivable impact on this mechanism. The dye showed potential applications as a sensor in colorimetric and spectroscopic detection and quantification of Cu²⁺ in domestic and environmental water samples, photo-imprinting and as a logic gate. The limits of detection (LOD) and quantification (LOQ) for Cu²⁺ were found to be 0.0779 mg/L and 0.236 mg/L, respectively, much lower than the World Health Organization (WHO) guideline limit for Cu²⁺ levels in drinking water.

Light-triggered theranostic hydrogels for real-time imaging and on-demand photodynamic therapy of skin abscesses

The management of wound infection remains the major challenges in real-time diagnosis, effective bacterial elimination and rapid wound healing. Herein, we developed injectable theranostic hydrogels to achieve long-term visual imaging of infected wounds and possible infection recurrence and to launch an on-demand bactericidal effect without using any antibiotics. Antimicrobial peptide ε-polylysine (ePL)-derived hydrogels were prepared through the copolymerization of methacrylated ePL (mPL) and the conjugates with tetrakis(4-carboxyphenyl) porphyrin (mPL-TCPP) and phenol red (mPL-Pr). Light illumination of mPL-TCPP produces reactive oxidative species (ROS) to initiate free radical crosslinking into PL@Pr-TCPP hydrogels without using any additional photoinitiators and concurrently exhibits antibacterial photodynamic therapy (PDT). PL@Pr-TCPP hydrogels experience quick color changes from yellow to orange and finally to red when pH values change from 5.0 to 9.0. The actual pH and related bacterial levels in the wounds could be read from G/B signal ratios of hydrogel colors captured by a smart phone. The conjugation of phenol red and TCPP into hydrogels affords a robust bacterial infection diagnosis and persistent bactericidal effect after cycled light illumination. The bacterial capture by ePL hydrogels strengthens PDT effect through alleviating the short lifetime and action distance of ROS. On a Staphylococcus aureus-infected abscess model, light illumination of the pregel solutions achieves in situ formation of hydrogel dressings. The synergistic bactericidal performance significantly relieves inflammatory status, accelerates collagen deposition, and promotes neovascularization, leading to full recovery of the infected wounds with regeneration of skin accessories. PL@Pr-TCPP hydrogels on the wound bed show color changes upon the recurrence of bacterial infection, which could also be totally eliminated after light illumination. Therefore, this study demonstrates a feasible strategy to develop theranostic hydrogel dressings for life-cycle diagnosis and on-demand treatment of wound infections. STATEMENT OF SIGNIFICANCE: Over 30% of skin and soft tissue infections become chronic even after appropriate antibacterial treatment, and recurrent infections are commonly reported after initial infection. Challenges remain in the development of theranostic wound dressings having the capability of point-of-care diagnosis, life-cycle monitoring and on-demand elimination of bacterial infection. Herein, light-triggered gelation is used to develop theranostic hydrogels for reversible naked-eye diagnosis and on-demand photodynamic therapy of wound infections. Light illumination plays a "one-stone-two-birds" role, i.e., photodynamically produced reactive oxidative species enable bactericidal effect without using any antibiotics, and the generated free radicals initiate crosslinking of hydrogels without using any additional photoinitiators. Bacterial infection-activated color changes of hydrogels could be captured with a smart phone for on-site and persistent monitoring of bacterial infection and wound healing process.

BODIPY-Pyridylhydrazone Probe for Fluorescence Turn-On Detection of Fe3+ and Its Bioimaging Application

A novel pyridylhydrazone-tethered BODIPY (BODIPY-PH) was synthesized, fully characterized via nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopic (FTIR), and single-crystal X-ray diffraction (SC-XRD) techniques, and developed for the selective detection of Fe3+ through fluorescent enhancement process. This derivative showed 1:1 binding with Fe3+ in an acetonitrile-water mixture (1:9 v/v) with the binding constant (K) of 5.4 × 104 M−1 and the limit of detection of 0.58 µM. The Fe3+ complexation reaction has been proved to be a reversible process and could be effectively repeated up to three cycles. The electronic properties of BODIPY-PH and its Fe3+ complex modeled by the density functional theory (DFT) method suggested the presence of chelation-enhanced fluorescence (CHEF) effect in the Fe3+ binding reaction. The X-ray absorption spectroscopy (XAS) probed at Fe K-edge confirmed the complex formation between BODIPY-PH and the Fe3+ in an octahedral geometry. Finally, bioimaging against human embryonic kidney (Hek293) cell, through confocal fluorescence microscopic technique indicated that the BODIPY-PH displayed good permeability and low toxicity toward the tested cell lines and showed enhanced fluorescent signal in the cells incubated with Fe3+ proving its capability for Fe3+ analysis in cellular matrix.

Solid state emissive azo-Schiff base ligands and their Zn(ii) complexes: acidochromism and photoswitching behaviour

Two novel azo Schiff base ligands L1 and L2 and Zn(ii) complexes C1 and C2 exhibiting reversible acidochromic behavior and photoswitching properties have been described.

Three AIE-ligand-based Cu(i) coordination polymers: synthesis, structures and luminescence sensing of TNP

Cu(i) CPs were constructed for the first time with the AIE ligand DPMF; they exhibited distinct architectures and efficient detection of TNP.

Paper based field deployable sensor for naked eye monitoring of copper (II) ions; elucidation of binding mechanism by DFT studies

The study demonstrates the fabrication of test strips made from newly synthesized ortho-Vanillin based colorimetric chemosensor (probe P) that could be employed as field deployable tool for rapid and naked eye detection of Cu²⁺. Upon addition of Cu²⁺ to the chemosensor, it exhibits rapid pink color from colorless and can be easily seen through the naked eye. This probe exhibits a remarkable colorimetric "ON" response and the absorbance intensity of the probe enhances significantly in presence of Cu²⁺. The sensing mechanism has been deduced using FTIR, XPS, LCMS and DFT studies. The binding mechanism of the probe to Cu²⁺ was substantiated by DFT studies. HOMO of the probe suggests that a high electronic density resides on O, N atoms and thus these are the favorable binding site for the metal ions. Study revealed that the P + Cu²⁺ complex is -35.64 eV more stable than individual reactants. The Cu²⁺ binds to the probe in 1:1 stoichiometry with a binding constant of 2.6 × 10⁴ M^-1 as calculated by Job's plot and Benesi-Hildebrand plot. The chemosensor shows 1.8 × 10^-8 M detection limit, which is considerably lesser than that of the WHO admissible limit of [Cu²⁺] in drinking water. Possible interfering ions namely Ca²⁺, Mg²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cd²⁺, Hg²⁺, Mn²⁺, Al³⁺ and Cr³⁺ do not show any appreciable interference in the colorimetric response of the probe towards Cu²⁺. Particularly, the colorimetric "ON-OFF-ON" responses are proved to be repeated over 5 times by the sequential inclusion of Cu²⁺ and S^2-. Sensitivity of the probe in real-time water and blood samples is found at par with results with AAS and ICP-OES techniques. Further, the reversibility of the probe and the easy fabrication of deployable strips for real-field naked eye detection of Cu²⁺ suggest importance of synthesized probe.

A novel chromone based colorimetric sensor for highly selective detection of copper ions: Synthesis, optical properties and DFT calculations

In this work, a new chromone based colorimetric sensor (ChrCS) was developed for highly selective detection of copper ions in semi-aqueous media. Evaluation of color and spectral changes displayed by the developed sensor shows that the sensor can be applied to detect copper ions in the presence of other competing metal ions and anions. The developed sensor, which contains biologically active chromone ring, shows excellent selectivity at microlevel for Cu²⁺ with a color change from colorless to yellow. Job's plot based on spectroscopic data showed the complex formation between ChrCS and Cu²⁺ ions has the stoichiometric ratio of 1:1 (ChrCS-Cu²⁺ complex). In addition, the binding constant of the ChrCS to Cu²⁺ was determined using the Benesi-Hildebrand equation. Furthermore, the test papers of the developed ChrCS were successfully prepared and employed to detect different concentration Cu²⁺ (10^-3 M to 10^-7 M) in aqueous solution. Importantly, sensor ChrCS was applied to detect Cu²⁺ ions in real water samples. To better understand the optical character of ChrCS and the effect of metal ion titration, density functional theory (DFT) calculations at the B3LYP/6-31 + G(d,p) level were performed for ChrCS and its complex ChrCS-Cu²⁺. Furthermore, on the basis of the Job's plot analysis DFT calculations, and reversible nature of the developed sensor, the sensing mechanism was demonstrated.

Diaminomaleonitrile-based Fluorophores as Highly Selective Sensing Platform for Cu2

A colorimetric and turn-on fluorescent chemodosimeter 1 based on diaminomaleonitrile was synthesized for Cu²⁺ detection. It showed high selectivity and sensitivity towards Cu²⁺ over the other tested metal ions. Probe 1 in acetonitrile exhibited a strong absorption band at 530 nm and weak fluorescence emission when excited at 480 nm, while the addition of Cu²⁺ could lead to a 30-nm blue shift of the absorption band and a remarkable fluorescence enhancement. Moreover, the detection limit of probe 1 for Cu²⁺ was calculated to be 28 nM. Quite different from the reported mechanism based on a metal-complexation induced fluorescence enhancement, the sensing mechanism was proved to be based on the Cu²⁺-promoted hydrolysis reaction, which was confirmed by ¹H NMR, ¹³C NMR and mass spectrum analysis. Studies on probe 2 were carried out to verify the universality of this sensing mechanism.

A novel fluoro-chromogenic Cu2+ probe for living-cell imaging based on rhodamine 6G-pyridine conjugation

A novel fluoro-chromogenic rhodamine spirolactam probe (RP) has been prepared through the condensation of rhodamine hydrazine and 2-acetylpyridine, which displayed the detection of Cu²⁺ with high selectivity over a large number of other common metal ions. It shows a "turn-on" response to paramagnetic Cu²⁺ with an about 12-fold enhancement, and a color change from colorless to red that is observable by the naked eye. These changes are ascribed to the ring-opening of the spirolactam in RP, and subsequent host-guest coordination. The 2:1 binding stoichiometry of RP to Cu²⁺ was confirmed by Job's and B-H plots. The resulting fluorescence enhancement can be used to detect Cu²⁺ at concentrations from 2.0 to 20.0 μM with a limit of detection of 0.21 μM, which was lower than the maximum allowable Cu²⁺ level set by the WHO. Finally, RP has been utilized to monitor Cu²⁺ in living cells and natural water. Graphical abstract.

Two mixed-ligand Cd(ii)–organic frameworks with unique topologies: selective luminescence sensing of TNP and Cu2+ ions with recyclable performances

Two luminescent Cd(ii)–organic frameworks exhibit unprecedented (4,4,5,5)-c and (4,4,4,6,7)-c topologies, and highly sensitive and selective sensing of 2,4,6-trinitrophenol and Cu²⁺.

Nanomolar-level selective dual channel sensing of Cu2+and CN−from an aqueous medium by an opto-electronic chemoreceptor

A novel chromogenic and fluorogenic chemoreceptor exhibiting a proclivity towards Cu²⁺and CN⁻, with applications in bioimaging and molecular electronics, was developed.

A versatile quinoxaline derivative serves as a colorimetric sensor for strongly acidic pH

A strongly acidic colorimetric pH sensor induced by the acidity of [Fe(H₂O)₆]³⁺, and single crystal to single crystal transformation between the protonated and deprotonated form.