Dual detection of Hg2+ and Pb2+ by a coumarin-functionalized Schiff base in environmental and biosystems

Fluorescent chemosensors are often preferred for tracking toxic ions because of their non-destructive measurement and ease of use in environmental real samples and biosystems. Exploring high selectivity, great sensitivity, and biocompatible fluorophores with facile, accessible and dual-responsive features is currently highly demanding. A coumarin-based naphthol hydrazone Schiff base chemosensor, NaChro, is designed and synthesized in a two-step process to detect toxic metal ions with strong emission. Fluorescence spectra analysis demonstrates that the probe binds to Hg2+ and Pb2+ ions with a 1:1 and a 2:1 stoichiometry, respectively, with high sensitivity, short response time and minimal interference from other metal ions. The observed reversible turn-on reaction was attributed to the inhibition of C = N isomerization and excited-state intramolecular proton transfer (ESIPT) processes once the ions were introduced. The practical applications of NaChro are successfully addressed in paper strips, various water samples, HeLa cells and Zebrafish, demonstrating that the probe can detect and track Hg2+ and Pb2+ ions in environmental samples and biosystems.

An overview on recent advances of reversible fluorescent probes and their biological applications

Due to the simplicity and low detection limit, fluorescent probes are widely used in both analytical sensing and optical imaging. Compared to conventional fluorescent probes, reversibility endows the reversible fluorescent probe outstanding advantages and special properties, making reversible fluorescent probes with capable of quantitative, repetitive or circulatory. Reversible fluorescent probes can also monitor the concentration dynamics of target analytes in real time, such as metal ions, proteins and enzymes, as well as intracellular redox processes, which have been widely applied in various fields. This review summarized the types and excellent properties of reversible fluorescent probes designed and developed in recent years. It also summarized the applications of reversible fluorescent probe in fluorescence imaging, biological testing, monitoring redox cycles, and proposed the remaining challenges and future development directions of the reversible fluorescent probe. This review provided comprehensive overview of reversible fluorescent probe, which may provide valuable references for the design and fabrication of the reversible fluorescent probe.

Imidazoles as Serotonin Receptor Modulators for Treatment of Depression: Structural Insights and Structure-Activity Relationship Studies

Serotoninergic signaling is identified as a crucial player in psychiatric disorders (notably depression), presenting it as a significant therapeutic target for treating such conditions. Inhibitors of serotoninergic signaling (especially selective serotonin reuptake inhibitors (SSRI) or serotonin and norepinephrine reuptake inhibitors (SNRI)) are prominently selected as first-line therapy for the treatment of depression, which benefits via increasing low serotonin levels and norepinephrine by blocking serotonin/norepinephrine reuptake and thereby increasing activity. While developing newer heterocyclic scaffolds to target/modulate the serotonergic systems, imidazole-bearing pharmacophores have emerged. The imidazole-derived pharmacophore already demonstrated unique structural characteristics and an electron-rich environment, ultimately resulting in a diverse range of bioactivities. Therefore, the current manuscript discloses such a specific modification and structural activity relationship (SAR) of attempted derivatization in terms of the serotonergic efficacy of the resultant inhibitor. We also featured a landscape of imidazole-based development, focusing on SAR studies against the serotoninergic system to target depression. This study covers the recent advancements in synthetic methodologies for imidazole derivatives and the development of new molecules having antidepressant activity via modulating serotonergic systems, along with their SAR studies. The focus of the study is to provide structural insights into imidazole-based derivatives as serotonergic system modulators for the treatment of depression.

Comprehensive Insights into Medicinal Research on Imidazole-Based Supramolecular Complexes

The electron-rich five-membered aromatic aza-heterocyclic imidazole, which contains two nitrogen atoms, is an important functional fragment widely present in a large number of biomolecules and medicinal drugs; its unique structure is beneficial to easily bind with various inorganic or organic ions and molecules through noncovalent interactions to form a variety of supramolecular complexes with broad medicinal potential, which is being paid an increasing amount of attention regarding more and more contributions to imidazole-based supramolecular complexes for possible medicinal application. This work gives systematical and comprehensive insights into medicinal research on imidazole-based supramolecular complexes, including anticancer, antibacterial, antifungal, antiparasitic, antidiabetic, antihypertensive, and anti-inflammatory aspects as well as ion receptors, imaging agents, and pathologic probes. The new trend of the foreseeable research in the near future toward imidazole-based supramolecular medicinal chemistry is also prospected. It is hoped that this work provides beneficial help for the rational design of imidazole-based drug molecules and supramolecular medicinal agents and more effective diagnostic agents and pathological probes.

Naphthalimide-Piperazine Derivatives as Multifunctional "On" and "Off" Fluorescent Switches for pH, Hg2+ and Cu2+ Ions

Novel 1,8-naphthalimide-based fluorescent probes NI-1 and NI-2 were designed and screened for use as chemosensors for detection of heavy metal ions. Two moieties, methylpyridine (NI-1) and hydroxyphenyl (NI-2), were attached via piperazine at the C-4 position of the napthalimide core resulting in a notable effect on their spectroscopic properties. NI-1 and NI-2 are pH sensitive and show an increase in fluorescence intensity at around 525 nm (switch "on") in the acidic environment, with pK_a values at 4.98 and 2.91, respectively. Amongst heavy metal ions only Cu²⁺ and Hg²⁺ had a significant effect on the spectroscopic properties. The fluorescence of NI-1 is quenched in the presence of either Cu²⁺ or Hg²⁺ which is attributed to the formation of 1:1 metal-ligand complexes with binding constants of 3.6 × 10⁵ and 3.9 × 10⁴, respectively. The NI-1 chemosensor can be used for the quantification of Cu²⁺ ions in sub-micromolar quantities, with a linear range from 250 nM to 4.0 μM and a detection limit of 1.5 × 10^-8 M. The linear range for the determination of Hg²⁺ is from 2 μM to 10 μM, with a detection limit of 8.8 × 10^-8 M. Conversely, NI-2 behaves like a typical photoinduced electron transfer (PET) sensor for Hg²⁺ ions. Here, the formation of a complex with Hg²⁺ (binding constant 8.3 × 10³) turns the green fluorescence of NI-2 into the "on" state. NI-2 showed remarkable selectivity towards Hg²⁺ ions, allowing for determination of Hg²⁺ concentration over a linear range of 1.3 μM to 25 μM and a limit of detection of 4.1 × 10^-7 M.

Fluorescence Variation in Selective Sensing of Hg2+and Cu2+ Ions By Coumarin-xanthene Fused Optical Probe

The fluorescent moieties coumarin and xanthene (R6GCP) combined in a single molecule was designed and synthesized. The colorimetric and fluorescent variation of the probe towards the copper and mercury ions sensing is examined. With the added copper/mercury ions to the solution of R6GCP in DMF:H2O (2:8, v/v), the probe showed deep red color from yellow color. The probe showed turn-off and turn-on fluorescence for copper and mercury ion respectively. In the presence of other competing metal ions, the probe showed better sensitivity towards copper and mercury ions. The probe's detection limit found to be 5.29 × 10^-6 M and 1.24 × 10^-5 M for Cu²⁺ and Hg²⁺ ion respectively by the UV-visible spectral measurement. Fluorescence measurement, the detection limit for the Cu²⁺ and Hg²⁺ ions detection by this probe is 1.91 × 10^-7 M, and 1.32 × 10^-8 M respectively. 1:1 binding stoichiometry was confirmed between the probe and Cu²⁺/Hg²⁺ ions from jobs plot by UV-visible spectral technique. Moreover, R6GCP combined filter paper were prepared. These test paper containing probe could detect Cu²⁺/Hg²⁺ ions in real-time with a spontaneous color change.

Recent Progress in Small-Molecule Fluorescent Probes for Detecting Mercury Ions

Mercury is a highly toxic and non-essential element that is found in every corner of the globe. The small amount of mercury produced by various pathways eventually enters freshwater and marine ecosystems, circulating through the food chain (especially fish) and causing various environmental problems in aspects including plants, animals, and human. There are several traditional quantitative methods developed for mercury ions (II) analysis in water samples. However, due to the complexity of the detection process, high cost and strong technical expertise, it is difficult to detect mercury ions in real-time. Therefore, in recent years, a large number of researchers have developed small-molecule fluorescent probes for Hg ions detection. Fluorimetry has the advantages of convenient detection, short response time, high sensitivity and good selectivity. This review summarized the small-molecule fluorescent probes for mercuric ion detection developed in recent years according to the chemical structural classification, compared their performances and elaborated the mechanism. We hope that the review will help the researches for the designs of metal ions fluorescent probes and their applications with certain reference value.

Fluorescent, colourimetric, and ratiometric probes based on diverse fluorophore motifs for mercuric(II) ion (Hg2+) sensing: highlights from 2011 to 2019

Though it has not been shown to deliver any biological importance, mercuric(II) ion (Hg²⁺) is a deleterious cation which poses grievous effects to the human body and/or the ecosystem, hence, the need for its sensitive and selective monitoring in both environmental and biological systems. Over the years, there has been a great deal of work in the use of fluorescent, colourimetric, and/or ratiometric probes for Hg²⁺ recognition. Essentially, the purpose of this review article is to give an overview of the advances made in the constructions of such probes based on the works reported in the period from 2011 to 2019. Discussion in this review work has been tailored to the kinds of fluorophore scaffolds used for the constructions of the probes reported. Selected examples of probes under each fluorophore subcategory were discussed with mentions of the typically determined parameters in an analytical sensing operation, including modulation in fluorescence intensity, optimal pH, detection limit, and association constant. The environmental and biological application ends of the probes were also touched where necessary. Important generalisations and conclusions were given at the end of the review. This review article highlights 196 references.

An indolizine–rhodamine based FRET fluorescence sensor for highly sensitive and selective detection of Hg2+ in living cells

An indolizine–rhodamine-based ratiometric fluorescent probe was designed and successfully synthesized.

Rhodamine 6G hydrazone with coumarin unit: a novel single-molecule multianalyte (Cu2+ and Hg2+) sensor at different pH value

An effective colourimetric and ratiometric fluorescent probe for Cu²⁺ in neutral aqueous media was synthesised. Moreover, the probe could sense Hg²⁺ with fluorescence enhancement at high pH.