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 Coumarin-azo Derived Colorimetric Chemosensor for Hg2+ Detection in Organic and Aqueous Media and its Extended Real-world Applications

Pollution caused by the release of toxic heavy metals into the environment by industrial and farming processes has been regarded as a major problem worldwide. This has attracted a great deal of attention into restoration and remediation. Mercury is classified as a toxic heavy metal which has posed significant challenges to public and environmental health. To date, conventional methods for mercury detection rely on expensive, destructive, complex, and highly specialized methods. Evidently, there is a need to develop systems capable of easily identifying and quantifying mercury within the environment. In this way, organic-based colorimetric chemosensors are gaining increasing popularity due to their high sensitivity, selectivity, cost-effectiveness, ease of design, naked-eye, and on-site detection ability. The formation of coumarin-azo derivative AD1 was carried out by a conventional diazotization reaction with coumarin-amine 1c and N,N-dimethylaniline. Sensor AD1 displayed remarkable visual colour change upon mercury addition with appreciable selectivity and sensitivity. The detection limit was calculated as 0.24 µM. Additionally, the reversible nature of AD1 allowed for the construction of an IMPLICATION type logic gate and Molecular Keypad Lock. Chemosensor AD1 displayed further sensing applications in real-world water samples and towards on-site assay methods. Herein, we describe a coumarin-derived chemosensor bearing an azo (N = N) functionality for the colorimetric and quantitative determination of Hg²⁺ in organic and aqueous media.

Elastase inhibitory activity of quinoline Analogues: Synthesis, kinetic mechanism, cytotoxicity, chemoinformatics and molecular docking studies

Herein, we have synthesized quinoline united various Schiff base derivatives (Q1-Q13) and systematically characterized them using diverse analytical practices such as ¹H NMR, ¹³C NMR, FT-IR and LC-MS respectively. All of the target compounds that have been synthesized were tested for elastase inhibition, and the findings were compared to the standard drug oleanolic acid. Among the entire series, compound Q11 (IC₅₀ = 0.897 ± 0.015 µM) exhibit most promising elastase inhibitory activity than oleanolic acid (Standard) having an IC₅₀ value of 13.426 ± 0.015 µM. Also, the utmost effectivecompound Q11 was used for kinetic mechanism investigation based on in-vitro data, from which it has been concluded that compound Q11 inhibits elastase competitively. Furthermore, utilizing the MTT test approach, the most effective compounds were assessed for cytotoxicity on B16F10 melanoma cells. From the cytotoxicity experiment, the most potent compound did not display any hazardous response against B16F10 melanoma cells despite being treated at high concentrations. Additionally, the molecular docking study was settled to govern the binding interaction pattern among an enzyme and inhibitors.

Aggregation-Induced Emission of Quinoline Based Fluorescent and Colorimetric Sensors for Rapid Detection of Fe3+ and 4-Nitrophenol in Aqueous Medium

New quinoline based fluorescent sensors 4 and 5 were rationally synthesized that exhibited excellent aggregation induced emission (AIE) in an aqueous medium. High fluorescence emission of sensors was accompanied by a noticeable redshift in their absorption and emission spectra that corresponds to the formation of J-aggregates. An AIE feature of sensors 4 and 5 was used for selective detection of Fe³⁺ and 4-NP in an aqueous medium that is attributed to the involvement of intermolecular charge transfer (ICT). The interaction mechanism of sensors with Fe³⁺ and 4-NP was investigated through ¹H NMR titration, Jobs plots, dynamic light scattering (DLS), and DFT analysis. The fluorescence quenching response of sensors 4 and 5 displayed distinguished linear behavior with the concentrations of Fe³⁺ and limits of detection (LOD) were calculated to be 15 and 10 nM, respectively. Further, LOD of sensors 4 and 5 for 4-NP (7.3 and 4.1 nM, respectively) was very low compared to previously reported sensors. Moreover, sensors' coated test strips were fabricated for solid-supported detection of Fe³⁺ and 4-NP. Sensors were successfully applied for the detection and quantification of Fe³⁺ and 4-NP in real water samples. Additionally, sensors were used for the determination of trace amounts of Fe³⁺ in the human serum sample.

Fluorescence "Turn-Off" and Colorimetric Sensor for Fe2+, Fe3+, and Cu2+ Ions Based on a 2,5,7-Triarylimidazopyridine Scaffold

Two cyanoimidazopyridine-based sensors (SS1 and SS2) were explored for the colorimetric and fluorometric detection of Fe2+, Fe3+, and Cu2+ ions in the semi-aqueous medium. The "turn-off" fluorescence response of both sensors to these ions was due to the restriction in internal charge transfer. Job's plot and semi-empirical calculations revealed that SS1 and SS2 complexed with Cu2+ ions in a 1:1 ratio and Fe2+/3+ ions in a 2:1 ratio, respectively. The sensors were found to have high binding constant (K a) values and low detection limit values. FMO analysis using the semi-empirical quantum mechanics method revealed the decrease in energy gap after complexation with metal ions. Sensor-coated filter paper strips were prepared and analyzed, where the color changes in the strips could be utilized for the real-time detection of Fe2+, Fe3+, and Cu2+ ions.