Ferrocene and Triazole-Appended Rhodamine Based Multisignaling Sensors for Hg2+ and Their Application in Live Cell Imaging

Ferrocenyl-triazole complexes and their use in heavy metal cation sensing

Complexes tris((1-ferrocenyl-1H-1,2,3-triazol-4-yl)methyl)amine (3), bis((1-ferrocenyl-1H-1,2,3-triazol-4-yl)methyl)amine (6), bis((1-ferrocenyl-1H-1,2,3-triazol-4-yl)methyl)ether (7), and 1-ferrocenyl-1H-1,2,3-triazol-4-yl)methanamine (9) were synthesized using the copper-catalyzed click reaction. Complexes 3, 6, 7, and 9 were characterized using NMR (1H and 13{1H}) and IR spectroscopy, elemental analysis, and mass spectrometry. Structures of 3, 7, and 9 in the solid state were determined using single-crystal X-ray diffraction. It was found that the triazole rings were planar and slightly twisted with respect to the cyclopentadienyl groups attached to them. Chains and 3D network structures were observed due to the presence of π⋯π and C-H⋯N interactions between the cyclopentadienyl and triazole ligands. A reversible redox behavior of the Fc groups between 239 and 257 mV with multicycle stability was characteristic for all the compounds, revealing that the electrochemically generated species Fc+ remained soluble in dichloromethane. Electrochemical sensor tests demonstrated the applicability of all the complexes to enhance the quantification sensing behavior of the screen-printed carbon electrode (SPCE) toward Cd2+, Pb2+, and Cu2+ ions.

A Study of Small Molecule-Based Rhodamine-Derived Chemosensors and their Implications in Environmental and Biological Systems from 2012 to 2021: Latest Advancement and Future Prospects

Rhodamine-based chemosensors have sparked considerable interest in recent years due to their remarkable photophysical properties, which include high absorption coefficients, exceptional quantum yields, improved photostability, and significant red shifts. This article presents an overview of the diverse fluorometric, and colorimetric sensors produced from rhodamine, as well as their applications in a wide range of fields. The ability of rhodamine-based chemosensors to detect a wide range of metal ions, including Hg+2, Al3+, Cr3+, Cu2+, Fe3+, Fe2+, Cd2+, Sn4+, Zn2+, and Pb2+, is one of their major advantages. Other applications of these sensors include dual analytes, multianalytes, and relay recognition of dual analytes. Rhodamine-based probes can also detect noble metal ions such as Au3+, Ag+, and Pt2+. They have been used to detect pH, biological species, reactive oxygen and nitrogen species, anions, and nerve agents in addition to metal ions. The probes have been engineered to undergo colorimetric or fluorometric changes upon binding to specific analytes, rendering them highly selective and sensitive by ring-opening via different mechanisms such as Photoinduced Electron Transfer (PET), Chelation Enhanced Fluorescence (CHEF), Intramolecular Charge Transfer (ICT), and Fluorescence Resonance Energy Transfer (FRET). For improved sensing performance, light-harvesting dendritic systems based on rhodamine conjugates has also been explored for enhanced sensing performance. These dendritic arrangements permit the incorporation of numerous rhodamine units, resulting in an improvement in signal amplification and sensitivity. The probes have been utilised extensively for imaging biological samples, including imaging of living cells, and for environmental research. Moreover, they have been combined into logic gates for the construction of molecular computing systems. The usage of rhodamine-based chemosensors has created significant potential in a range of disciplines, including biological and environmental sensing as well as logic gate applications. This study focuses on the work published between 2012 and 2021 and emphasises the enormous research and development potential of these probes.

Synthesis of a novel hydrazone-based compound applied as a fluorescence turn-on chemosensor for iron(iii) and a colorimetric sensor for copper(ii) with antimicrobial, DFT and molecular docking studies

Hydrazone-hydrazide-based linkers perform a crucial role in environmental as well as biological fields. Such linkers are employed to detect exact metal ions at a minute level; hence, numerous probes are available. Even though thiophene-based molecules have a unique position in the medicinal arena, only very few chemosensors are reported based on such a moiety. In this current work, a novel hydrazide-hydrazone-based fluorogenic molecule 5-bromo-2-hydroxy-N'-[(1E)-1-(thiophen-2-yl)ethylidene]benzohydrazide (L) has been successfully designed and synthesized. The sensing studies of L demonstrated a ratio metric as well as turn-on-enhanced fluorescence and colorimetric response toward Fe3+ and Cu2+ ions, respectively and it was observed to be insensitive toward various metal ions. The Job plots revealed that the binding stoichiometry of L and metal ions is 2 : 1. In addition, density functional theory (DFT) results strongly suggested that L can be used as a powerful colorimetric sensor for the detection of Cu2+ ions. In vitro antimicrobial activities of L were evaluated by disk diffusion and results revealed good antibacterial activities against E. coli. Further, molecular docking was executed with DNA gyrase (PDB ID: 1KZN) of E. coli and the calculated interaction energy value was found to be -7.7 kcal mol-1. Finally, molecular docking, fluorescence, colorimetry and the HOMO-LUMO energy gap of the compound can provide new insights into developing drugs and detecting metals in biomolecules.

[3 + 2] Cycloadditions in Asymmetric Synthesis of Spirooxindole Hybrids Linked to Triazole and Ferrocene Units: X-ray Crystal Structure and MEDT Study of the Reaction Mechanism

Derivatization of spirooxindole having triazole and ferrocene units was achieved by the [3 + 2] cycloaddition (32CA) reaction approach. Reacting the respective azomethine ylide (AY) intermediate generated in situ with the ethylene derivative produced novel asymmetric cycloadducts with four contiguous asymmetric carbons in an overall high chemical yield with excellent regioselectivity and diastereoselectivity. X-Ray single-crystal structure analyses revealed, with no doubt, the success of the synthesis of the target compounds. The 32CA reaction of AY 5b with ferrocene ethylene 1 has been studied within MEDT. This 32CA reaction proceeds via a two-stage one-step mechanism involving a high asynchronous transition state structure, resulting from the nucleophilic attack of AY 5b on the β-conjugated position of ferrocene ethylene 1. The supernucleophilic character of AY 5b and the strong electrophilic character of ferrocene ethylene 1 account for the high polar character of this 32CA reaction. Further, Hirshfeld analyses were used to describe the molecular packing of compounds 4b, 4e, 4h and 4i.

A Molecular Electron Density Theory Study of the [3+2] Cycloaddition Reaction of an Azomethine Ylide with an Electrophilic Ethylene Linked to Triazole and Ferrocene Units

The [3+2] cycloaddition (32CA) reaction of an azomethine ylide (AY) with an electrophilic ethylene linked to triazole and ferrocene units has been studied within the Molecular Electron Density Theory (MEDT) at the ωB97X-D/6-311G(d,p) level. The topology of the electron localization function (ELF) of this AY allows classifying it as a pseudo(mono)radical species characterized by the presence of two monosynaptic basins, integrating a total of 0.76 e, at the C1 carbon. While the ferrocene ethylene has a strong electrophilic character, the AY is a supernucleophile, suggesting that the corresponding 32CA reaction has a high polar character and a low activation energy. The most favorable ortho/endo reaction path presents an activation enthalpy of 8.7 kcal·mol^-1, with the 32CA reaction being exergonic by -42.1 kcal·mol^-1. This reaction presents a total endo stereoselectivity and a total ortho regioselectivity. Analysis of the global electron density transfer (GEDT) at the most favorable TS-on (0.23 e) accounts for the high polar character of this 32CA reaction, classified as forward electron density flux (FEDF). The formation of two intermolecular hydrogen bonds between the two interacting frameworks at the most favorable TS-on accounts for the unexpected ortho regioselectivity experimentally observed.

Organic Molecules Containing N, S and O Heteroatoms as Sensors for the Detection of Hg(II) Ion; Coordination and Efficiency toward Detection

Rapid detection of potentially toxic heavy metals like Hg(II) has attracted great attention in the last few decades due to the importance to maintain a safe and sustainable environment for human beings. Coordination chemistry and concepts therein, play an important role in the detection of Hg(II). Size, charge, and nature of the donor atom and the respective cation (metal ion), are crucial in selective interactions between the sensor and metal ions. The sensors designed for the purpose, coordinate to Hg(II) ion through various donor sites, coordination causes a change in the electron density in organic molecules and results in either visible color change or enhancing/quenching fluorescence intensity. Since Hg(II) is soft metal, with d10 electron system, so majority of the sensors have soft donor sites which prefer to coordinate with Hg(II). Oxygen is also present in some chelating ligands which is least preferred coordination site, due to its hard nature. There are several reports of replacing other ligating sites by sulfur for enhanced mercury sensing. In some cases, desulfurization is being detected as clear change in spectral behavior during the sensing process. Efforts are still in progress to design and introduce a sensor with utmost sensitivity and selectivity. In this review, we made an attempt to explain the coordination aspects of Hg(II) detectors, reasons for poor efficiency and possible suggestions to improve the selection criterion of various compounds. It will help researchers to know about important concepts in designing more sensitive and selective sensors for detection of Hg(II) in environmental and biological samples.

Synthesis and Structure Elucidation of Novel Spirooxindole Linked to Ferrocene and Triazole Systems via [3 + 2] Cycloaddition Reaction

In the present work, a novel heterocyclic hybrid of a spirooxindole system was synthesized via the attachment of ferrocene and triazole motifs into an azomethine ylide by [3 + 2] cycloaddition reaction protocol. The X-ray structure of the heterocyclic hybrid (1″R,2″S,3R)-2″-(1-(3-chloro-4-fluorophenyl)-5-methyl-1H-1,2,3-triazole-4-carbonyl)-5-methyl-1″-(ferrocin-2-yl)-1″,2″,5″,6″,7″,7a″-hexahydrospiro[indoline-3,3″-pyrrolizin]-2-one revealed very well the expected structure, by using different analytical tools (FTIR and NMR spectroscopy). It crystallized in the triclinic-crystal system and the P-1-space group. The unit cell parameters are a = 9.1442(2) Å, b = 12.0872(3) Å, c = 14.1223(4) Å, α = 102.1700(10)°, β = 97.4190(10)°, γ = 99.1600(10)°, and V = 1484.81(7) ų. There are two molecules per unit cell and one formula unit per asymmetric unit. Hirshfeld analysis was used to study the molecular packing of the heterocyclic hybrid. H···H (50.8%), H···C (14.2%), Cl···H (8.9%), O···H (7.3%), and N···H (5.1%) are the most dominant intermolecular contacts in the crystal structure. O···H, N···H, H···C, F···H, F···C, and O···O are the only contacts that have the characteristic features of short and significant interactions. AIM study indicated predominant covalent characters for the Fe–C interactions. Also, the electron density (ρ(r)) at the bond critical point correlated inversely with the Fe–C distances.

Fluorescent chemosensors containing redox-active ferrocene: a review

The redox-active ferrocene containing two cyclopentadienyl rings and iron was extensively employed in the field of sensing, catalysis, medicine, biotechnology etc., due to the structural stability, solubility in common solvents and easy structural modification to make a wide variety of ferrocene derivatives. The ferrocene moiety can be linked suitably with fluoro-chromogenic units and applied for the multichannel (fluorescent, chromogenic and redox) sensing of various bioactive and toxic analytes. This review was narrated to compile some important ferrocene based fluorescent chemosensors developed for the detection of metal ions, anions and neutral analytes. The analytical novelty and sensing mechanisms of the summarized chemosensors are discussed to open new scopes for future research.

Similar, Yet Different: Long-Range Metal-Metal Coupling and Electron-Transfer Processes in Metal-Free 5,10,15,20-Tetra(ruthenocenyl)porphyrin

The electronic structure, redox properties, and long-range metal-metal coupling in metal-free 5,10,15,20-tetra(ruthenocenyl)porphyrin (H₂TRcP) were probed by spectroscopic (NMR, UV-vis, magnetic circular dichroism (MCD), and atmospheric pressure chemical ionization (APCI)), electrochemical (cyclic voltammetry, CV, and differential pulse voltammetry, DPV), spectroelectrochemical, and chemical oxidation methods, as well as theoretical (density functional theory, DFT, and time-dependent DFT, TDDFT) approaches. It was demonstrated that the spectroscopic properties of H₂TRcP are significantly different from those in H₂TFcP (metal-free 5,10,15,20-tetra(ferrocenyl)porphyrin). Ruthenocenyl fragments in H₂TRcP have higher oxidation potentials than the ferrocene groups in the H₂TFcP complex. Similar to H₂TFcP, we were able to access and spectroscopically characterize the one- and two-electron oxidized mixed-valence states in the H₂TRcP system. DFT predicts that the porphyrin π-system stabilizes the [H₂TRcP]⁺ mixed-valence cation and prevents its dimerization, which is characteristic for ruthenocenyl systems. However, formation of the mixed-valence [H₂TRcP]²⁺ is significantly less reproducible than the formation of [H₂TRcP]⁺. DFT and TDDFT calculations suggest the ruthenocenyl fragment dominance in the highest occupied molecular orbital (HOMO) energy region and the presence of the low-energy MLCT (Rc → porphyrin (π*)) transitions in the visible region with energies higher than the predominantly porphyrin-centered Q-bands.

A novel near-infrared optical and redox-active receptor for the multi-model detection of Hg2+ in water and living cells

A key issue for constructing optical and redox-active receptors is how to conjugate a specific sensing kernel with a multi-signal-responsive system to carry out multi-feature analysis. Mercury is considered to be highly toxic to human health and ecological security. In this work, we present a novel near-infrared optical and redox-active receptor that can sense Hg²⁺ at ppb level in aqueous media via multi-model monitors with a low detection limit of 8.4 × 10^-9 M (1.68 ppb). This receptor features a visible detection, 'off-on' fluorescence response, and efficient electrochemistry assessment, as well as pH-insensitivity to Hg²⁺ with high sensitivity. In view of its marked near-infrared emission and fluorescence enhancement, we successfully applied this receptor to visualize Hg²⁺ in live cells. Furthermore, a possible sensing model was established and rationalized with theoretical studies.

New Pyridine-Bridged Ferrocene-Rhodamine Receptor for the Multifeature Detection of Hg2+ in Water and Living Cells

A challenge in the design of optical and redox-active receptors is how to combine a specific recognition center with an efficient responsive system to facilely achieve multifeature detection in biological and environmental analyses. Herein, a novel ferrocene-rhodamine receptor conjugated with a pyridine bridge was designed and synthesized. This receptor can sensitively sense Hg²⁺ in aqueous media via chromogenic, fluorogenic, and electrochemical multisignal outputs with a low detection limit and fast response time. Moreover, it can be qualified as a fluorescent probe for effectively monitoring Hg²⁺ in living cells. A plausible recognition mode was proposed and rationalized with theoretical calculations.

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.

A novel pyrazole bearing imidazole frame as ratiometric fluorescent chemosensor for Al3+/Fe3+ ions and its application in HeLa cell imaging

New pyrazole bearing imidazole derivative was successfully synthesized and thoroughly characterized by various spectroanalytical techniques. The sensor DIBI shows a highly selective and sensitive fluorescent response with the addition of Al³⁺/Fe³⁺ ions in acetonitrile-water mixture. The strong fluorescent molecule exhibits a notable ratiometric emissions at 462 nm and 470 nm for Al³⁺ and Fe³⁺ ions, respectively (λ_ex = 280 nm). Job's plot studies conclude that the coordination between DIBI with Al³⁺/Fe³⁺ was 1:1 binding stoichiometry. The limit of detection of DIBI with Al³⁺/Fe³⁺ was calculated as 2.12 × 10^-7 M and 1.73 × 10^-6 M, respectively. The TD-DFT calculations further supported the photonics performances of the free probe and its complexes. The reversibility and reusability of the sensor molecule are studied using EDTA. The probe was used to track Al³⁺/Fe³⁺ in cancer cells via fluorescence microscopy.

New ferrocene-triazole derivatives for multisignaling detection of Cu2+ in aqueous medium and their antibacterial activity

Ferrocene-based naphthalene or quinoline receptors 1-4 linked by triazole were designed and synthesized. Their recognition properties of metal cations have been investigated systematically in aqueous environment. Upon addition 1 equiv. of Cu²⁺ ion, receptors 1 (C₂₃H₁₉FeN₃O) and 2 (C₂₂H₁₈FeN₄O) showed fluorescent turn-off, enhanced absorption and color variations. At the same time, receptor 1 also caused the perturbation of redox potential after addition 1 equiv. of Cu²⁺ ion. Therefore, receptors 1 and 2 behaved as naked-eye chemosensors and fluorescent probes for Cu²⁺ without interference by other ions and with low detection limit. In addition, receptor 1 could also be considered electrochemical sensor for Cu²⁺ having excellent sensitivity and selectivity. However, increasing the molecules flexibility resulted in the lower selectivity of ion recognition in the case of receptors 3 (C₂₄H₂₁FeN₃O) and 4 (C₂₃H₂₀FeN₄O). Furthermore, this series of compounds were nontoxicity and receptor 1 exhibited certain antibacterial activity.

A reaction-type receptor for the multi-feature detection of Hg2+ in water and living cells

The first optical and redox-active receptor of reaction-type has been developed for efficiently multi-model survey of Hg²⁺.

Squaramide based ion pair receptors possessing ferrocene as a signaling unit

Ferrocene offers a convenient reporter in squaramide based ion pair sensors able to recognize anions more strongly in the presence of cations.

A highly sensitive and selective fluorescent "off-on-off" relay chemosensor based on a new bis(salamo)-type tetraoxime for detecting Zn2+ and CN. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2019;222:117209. [PMID: 31200268 DOI: 10.1016/j.saa.2019.117209]

A new naphthalenediol-based bis(salamo)-type fluorescent probe H₄L for Zn²⁺ and CN^- was reported. Probe H₄L showed a highly selective fluorescence enhancement toward Zn²⁺ over other metal ions including Cd²⁺, and obtained the L-Zn²⁺complex can only detect CN^- in various anions. Their recognition mechanisms were explained by Job plots, fluorescent and UV-vis titrations, and theoretical calculations. The L-Zn²⁺complex has been synthesized and structurally characterized using Hirshfeld surface analysis, elemental analyses, IR, UV-Vis and fluorescent spectra. Additionally, the relay probe with the wide adaptability of pH range and excellent stability showed highly selectivity for CN^- recognition.

Dual chemosensor for the rapid detection of mercury(ii) pollution and biothiols

A new benzothiazole azo dye [(E)-1-((6-methoxybenzo[d]thiazole-2-yl)diazenyl)naphthalene-2,6-diol] (also known as “BAN”), has been synthesised and used as a chemosensor for the rapid and selective detection of mercury(ii) ions in water.

Anthracene-Based Highly Selective and Sensitive Fluorescent "Turn-on" Chemodosimeter for Hg2

Three π-extended anthracene-bearing thioacetals (1-3) have been synthesized, and their fluorescence "turn-on" responses to Hg2+ ions are studied. The chemodosimetric fluorescence-sensing behavior and their resulting hydrolysis via a desulfurization reaction mechanism leads to the formation of highly fluorescent respective aldehyde substitutions. Furthermore, this mechanism was supported by increase in the quantum yields of their resulting aldehydes and is correlated to their molecular substitution. The chemosensors 1-3 have exhibited to be promising receptors toward Hg2+ ions in the presence of other competitive metal ions. Moreover, the detection limits of 1-3 have been found to be in the nanomolar range (94, 59, and 235, respectively). Fluorescence microscopic imaging studies show that 1-2 have been found to be effective for fluorescence imaging in live cells. Moreover, compounds 1-3 act as potential candidates for the detection of Hg2+ in environmental and biological systems as well as real samples.

Synthesis and characterisation of calix[4]arene based bis(triazole)-bis(hexahydroquinoline): Probing highly selective fluorescence quenching towards mercury (Hg2+) analyte

In the present study, we are reporting the synthesis of a triazoles incorporated fluorescent hexahydroquinoline appended calix[4]arene 7 and its highly selective optical recognition ability towards Hg²⁺. The optical sensor 7 was synthesized via two different synthetic pathways and unambiguously characterized by mass spectrometry and NMR spectroscopy. The stoichiometric ratio between 7 and Hg²⁺ was determined as 1:1 based on Job's plot and ESI-MS analysis. The chemosensor 7 selectively recognized Hg²⁺ in the presence of competitive cations such as Cu²⁺, Cu¹⁺, Co³⁺, Ni²⁺, Ag¹⁺, Fe²⁺, Fe³⁺, Cr³⁺, Pb²⁺, Cd²⁺, Zn²⁺, Sn²⁺, Ti⁴⁺, Sb³⁺, In³⁺, Ba²⁺, Ca²⁺, and K¹⁺. The limit of detection (LOD) was established as 0.5 μM on spectrofluorimetric analysis. According to world health organization (WHO) in guidelines for drinking-water quality, the mercury level in natural occurring ground and surface water is about ∼0.5 μM (μg/L). The supra molecule 7 with LOD (0.5 μM) could potentially serve effective receptors for Hg²⁺ cation. The probe 7 was applied effectively for detection of Hg²⁺ in real water samples (i.e. tap and deionized water), spiked with Hg²⁺ (10 μM) solution. Moreover, the compound 7 showed non-toxicity during cytotoxic assay against human B cells as they retain their morphology. The supramolecule 7 in living cells showed selectivity towards Hg²⁺ in cytotoxic assay with T lymphocytes, evident by morphological changes observed via AFM analysis.

Imine ligands based on ferrocene: synthesis, structural and Mössbauer characterization and evaluation as chromogenic and electrochemical sensors for Hg2+

Synthesis of imine ligands based on ferrocene and their evaluation as chromogenic and electrochemical sensors for Hg²⁺.