A highly efficient dinuclear Cu(II) chemosensor for colorimetric and fluorescent detection of cyanide in water

Urea- and Thiourea-Based Receptors for Anion Binding

ConspectusOver the past five decades, significant progress has been made in the field of anion recognition with a diverse variety of synthetic receptors because of the fundamental importance of anions in chemical, environmental, and biological processes. In particular, urea- and thiourea-based molecules offering directional binding sites are attractive receptors for anions due to their ability to bind anions employing primarily hydrogen-bonding interactions under neutral conditions and have gained a recent paramount attention in the area of supramolecular chemistry. The presence of two imine (-NH) groups on each urea/thiourea functionality in these receptors gives them potential for excellent binding of an anion, mimicking the natural binding process in living cells. The increased acidity offered by thiocarbonyl groups (C═S) in a thiourea-functionalized receptor could enhance its anion binding ability as compared to its analogous urea-based receptor containing a carbonyl (C═O) group. During the last several years, our group has been involved in exploring a wide variety of synthetic receptors, and we have studied them with anions experimentally and computationally. In this Account, we will highlight the overall summary of our group's efforts focusing on anion coordination chemistry of urea- and thiourea-based receptors with varying linkers (rigid and flexible), dimensions (dipodal and tripodal), and functionalities (bifunctional, trifunctional, and hexafunctional). Depending on the linkers and attached groups, bifunctional-based dipodal receptors can bind anions forming 1:1 or 1:2 complexes. A dipodal receptor with flexible aliphatic or rigid m-xylyl linkers forms a cleft to bind a single anionic species in the pocket. However, a dipodal receptor with p-xylyl linkers binds anions in both 1:1 and 1:2 binding modes. As compared to a dipodal receptor, a tripodal receptor provides a more organized cavity for an anion, forming predominantly a 1:1 complex, while the binding strength and selectivity are influenced by linking chains and terminal groups. A hexafunctional-based tripodal receptor bridged with o-phenylene groups provides two clefts that can host two small anions or one large anion. However, a hexafunctional receptor with p-phenylene groups as linkers binds two anions, one at an inner pocket and the other at an outer pocket. It was shown that the presence of suitable chromophores at the terminal groups makes the receptor useful for the naked-eye detection for certain anions (e.g., fluoride, acetate) in solution. The field of anion binding chemistry is rapidly growing, and this Account aims to provide fundamental aspects influencing the binding strength and selectivity of anionic species with abiotic receptors which might eventually be useful for the development of new devices for binding, sensing, and separating biologically and environmentally important anions.

Selective Turn-On Fluorescence Sensing of Cyanide Using the Pyridoxal Platform of a Ni(II) Complex

Cyanide is a very toxic pollutant to aquatic life and the environment. Analytical methods for the quantitative assay of cyanide, which are rapid, sensitive (low limit of detection), and cost-effective, are in great demand. Colorimetric and fluorometric methods are ideally suited for this purpose. In this report, we describe a Ni(II) complex containing a pyridoxal platform for the rapid and sensitive fluorometric estimation of cyanide. The square-planar Ni(II) complex, [Ni(L)(N₃)]·3H₂O, where the ligand LH = 4-[(2-dimethylamino-ethylimino)-methyl]-5-hydroxymtheyl-2-methyl-pyridin-3-ol, a Schiff base formed between pyridoxal and (2-dimethylamino)ethyl amine, was synthesized and characterized by various spectroscopic techniques as well as by single-crystal X-ray structure determination. The complex was found to selectively bind CN^- in the presence of other biologically important anions such as F^-, Cl^-, Br^-, I^-, OAc^-, S^2-, NO₃ ^-, PO₄ ^3-, SO₄ ^2-, and H₂PO₄ ^- in tris-HCl/NaCl buffer [pH = 7.4], and it can be monitored by fluorescence turn-on or by UV-visible spectroscopy. The binding constant of the complex with CN^- was estimated to be 2.046 × 10¹⁴ M^-2 and the limit of detection (LOD) was 9 nM, the LOD being considerably lower than the maximum permissible level of cyanide ions (1.9 μM) in drinking water, as recognized by the World Health Organization (WHO). The effects of pH and temperature on the sensing are also investigated. The Ni(II) complex is also found to bind to calf-thymus DNA very strongly, and the apparent binding constant (K _app) was determined to be 1.33 × 10⁷ M^-1 by the fluorescence quenching of the ethidium bromide-DNA adduct by the complex.

Optical sensing of anions by macrocyclic and interlocked hosts

This review summarises recent developments in the use of macrocyclic and mechanically-interlocked host molecules as optical sensors for anions.

A new disulfide Schiff base as a versatile “OFF–ON–OFF” fluorescent–colorimetric chemosensor for sequential detection of CN− and Fe3+ ions: combined experimental and theoretical studies

A new disulfide Schiff base as a versatile “OFF–ON–OFF” fluorescent–colorimetric chemosensor has been synthesized for sequential detection of CN⁻ and Fe³⁺ ions.

The presence of a single-nucleotide mismatch in linker increases the fluorescence of guanine-enhanced DNA-templated Ag nanoclusters and their application for highly sensitive detection of cyanide

Fluorescence of DNA-templated silver nanoclusters can be enhanced by more than 100-fold by placing the nanoclusters in proximity to guanine-rich DNA sequences after hybridization. We found that the fluorescence of the guanine-enhanced silver nanoclusters is not increased with the guanine-rich DNA sequence closer to the silver nanoclusters. By studying the different numbers of mismatches in the linker sequences, we found that the presence of a single-nucleotide mismatch in the linker increases fluorescence more than the complementary nucleotide. Further study indicated the mismatch position of the linker sequence also affects the fluorescence of the hybridized DNA-Ag NCs. The evidence reported here indicated that the mismatch of the linker sequence affects the fluorescence enhancement of guanine-enhanced silver nanoclusters. We also found that DNA-Ag NCs is an excellent fluorescence sensor for cyanide, as cyanide effectively quenches the fluorescence of NCs at a very low concentration with high selectivity. Cyanide in the range from 0.10 μM to 0.35 μM could be linearly detected, with a detection limit of 25.6 nM.

Highly selective and sensitive macrocycle-based dinuclear foldamer for fluorometric and colorimetric sensing of citrate in water

The selective detection of citrate anions is essential for various biological functions in living systems. A quantitative assessment of citrate is required for the diagnosis of various diseases in the human body; however, it is extremely challenging to develop efficient fluorescence and color-detecting molecular probes for sensing citrate in water. Herein, we report a macrocycle-based dinuclear foldamer (1) assembled with eosin Y (EY) that has been studied for anion binding by fluorescence and colorimetric techniques in water at neutral pH. Results from the fluorescence titrations reveal that the 1·EY ensemble strongly binds citrate anions, showing remarkable selectivity over a wide range of inorganic and carboxylate anions. The addition of citrate anions to the 1·EY adduct led to a large fluorescence enhancement, displaying a detectable color change under both visible and UV light in water up to 2 μmol. The biocompatibility of 1·EY as an intracellular carrier in a biological system was evaluated on primary human foreskin fibroblast (HF) cells, showing an excellent cell viability. The strong binding properties of the ensemble allow it to be used as a highly sensitive, detective probe for biologically relevant citrate anions in various applications.

Anion Complexation Studies of 3-Nitrophenyl-Substituted Tripodal Thiourea Receptor: A Naked-Eye Detection of Sulfate via Fluoride Displacement Assay

A thiourea-based tripodal receptor L substituted with 3-nitrophenyl groups has been synthesized, and the binding affinity for a variety of anions has been studied by ¹H NMR titrations and nuclear Overhauser enhancement spectroscopy experiments in dimethyl sulfoxide-d₆. As investigated by ¹H NMR titrations, the receptor binds an anion in a 1:1 binding mode, showing the highest binding and strong selectivity for sulfate anion. A competitive colorimetric assay in the presence of fluoride suggests that the sulfate is capable of displacing the bound fluoride, showing a sharp visible color change. The strong affinity of L for sulfate was further supported by UV-vis titrations and density functional theory (DFT) calculations. Time-dependent DFT calculations indicate that the fluoride complex possesses a different optical absorption spectrum (due to charge transfer between the fluoride and the surrounding ligand) than the sulfate complex, reflecting the observed colorimetric change in these two complexes. The receptor was further tested for its biocompatibility on primary human foreskin fibroblasts and HeLa cells, exhibiting an excellent cell viability up to 100 μM concentration.

Supramolecular Assembly of Uridine Monophosphate (UMP) and Thymidine Monophosphate (TMP) with a Dinuclear Copper(II) Receptor

Understanding the intermolecular interactions between nucleotides and artificial receptors is crucial to understanding the role of nucleic acids in living systems. However, direct structural evidence showing precise interactions and bonding features of a nucleoside monophosphate (NMP) with a macrocycle-based synthetic molecule has not been provided so far. Herein, we present two novel crystal structures of uridine monophosphate (UMP) and thymidine monophosphate (TMP) complexes with a macrocycle-based dinuclear receptor. Structural characterization of these complexes reveals that the receptor recognizes UMP through coordinate-covalent interactions with phosphates and π-π stackings with nucleobases and TMP through coordinate-covalent interactions with phosphate groups. Furthermore, the receptor has been shown to effectively bind nucleoside monophosphates in the order of GMP > AMP > UMP > TMP > CMP in water at physiological pH, as investigated by an indicator displacement assay.

An Ideal C3-Symmetric Sulfate Complex: Molecular Recognition of Oxoanions by m-Nitrophenyl- and Pentafluorophenyl-Functionalized Hexaurea Receptors

The anion-binding properties of two tripodal-based hexaureas appended with the m-nitrophenyl (1) and pentafluorophenyl (2) groups have been studied both experimentally and theoretically, showing strong affinities for sulfate over other inorganic oxoanions such as hydrogen sulfate, dihydrogen phosphate, bicarbonate, nitrate, and perchlorate. The structural analysis of the sulfate complex with 1 reveals that the receptor organizes all urea-binding sites toward the cavity at precise orientations around a tetrahedral sulfate anion to form an ideal C₃-symmetric sulfate complex that is stabilized by 12 hydrogen-bonding interactions. The receptor and the encapsulated sulfate are located on the threefold axis passing through the bridgehead nitrogen of 1 and the sulfur atom of the anionic guest. The high-level density functional theory calculations support the crystallographic results, demonstrating that the C₃-symmetric conformation of the sulfate complex is achieved due to the complementary NH···O between the receptor and sulfate.

Synthetic Receptors Induce Anti Angiogenic and Stress Signaling on Human First Trimester Cytotrophoblast Cells

The cytotrophoblast (CTB) cells of the human placenta have membrane receptors that bind certain cardiotonic steroids (CTS) found in blood plasma. One of these, marinobufagenin, is a key factor in the etiology of preeclampsia. Herein, we used synthetic receptors (SR) to study their effectiveness on the angiogenic profile of human first trimester CTB cells. The humanextravillous CTB cells (Sw.71) used in this study were derived from first trimester chorionic villus tissue. Culture media of CTB cells treated with ≥1 nM SR level revealed sFlt-1 (Soluble fms-like tyrosine kinase-1) was significantly increased while VEGF (vascular endothelial growth factor) was significantly decreased in the culture media (* p < 0.05 for each) The AT₂ receptor (Angiotensin II receptor type 2) expression was significantly upregulated in ≥1 nM SR-treated CTB cells as compared to basal; however, the AT₁ (Angiotensin II receptor, type 1) and VEGFR-1 (vascular endothelial growth factor receptor 1) receptor expression was significantly downregulated (* p < 0.05 for each). Our results show that the anti-proliferative and anti-angiogenic effects of SR on CTB cells are similar to the effects of CTS. The observed anti angiogenic activity of SR on CTB cells demonstrates that the functionalized-urea/thiourea molecules may be useful as potent inhibitors to prevent CTS-induced impairment of CTB cells.

A Highly Selective Chemosensor for Cyanide Derived from a Formyl-Functionalized Phosphorescent Iridium(III) Complex

A new phosphorescent iridium(III) complex, bis[2',6'-difluorophenyl-4-formylpyridinato-N,C4']iridium(III) (picolinate) (IrC), was synthesized, fully characterized by various spectroscopic techniques, and utilized for the detection of CN(-) on the basis of the widely known hypothesis of the formation of cyanohydrins. The solid-state structure of the developed IrC was authenticated by single-crystal X-ray diffraction. Notably, the iridium(III) complex exhibits intense red phosphorescence in the solid state at 298 K (ΦPL = 0.16) and faint emission in acetonitrile solution (ΦPL = 0.02). The cyanide anion binding properties with IrC in pure and aqueous acetonitrile solutions were systematically investigated using two different channels: i.e., by means of UV-vis absorption and photoluminescence. The addition of 2.0 equiv of cyanide to a solution of the iridium(III) complex in acetonitrile (c = 20 μM) visibly changes the color from orange to yellow. On the other hand, the PL intensity of IrC at 480 nm was dramatically enhanced ∼5.36 × 10(2)-fold within 100 s along with a strong signature of a blue shift of the emission by ∼155 nm with a detection limit of 2.16 × 10(-8) M. The cyanohydrin formation mechanism is further supported by results of a (1)H NMR titration of IrC with CN(-). As an integral part of this work, phosphorescent test strips have been constructed by impregnating Whatman filter paper with IrC for the trace detection of CN(-) in the contact mode, exhibiting a detection limit at the nanogram level (∼265 ng/mL). Finally, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed to understand the electronic structure and the corresponding transitions involved in the designed phosphorescent iridium(III) complex probe and its cyanide adduct.

Silica nanoparticles functionalized with polyamidoamine (PAMAM) dendrimers as platforms for photoluminescence (PL) sensing of copper and cyanide ions

Functionalized nanoparticles for photoluminescence (PL) applications are a promising technology for biomedical imaging and as sensors for small molecules. This work presents a new method to modify silica nanoparticles (SNP) using the bifunctional linker 1,1'-carbonyldiimidazole (CDI) with a series of polyamidoamine (PAMAM) dendrimer molecules followed by grafting of fluorescein isothiocyanate (FITC) or rhodamine B isothiocyanate (RITC) to create platforms for photoluminescence (PL) sensors. A dendrimer size and charge-variable response to only copper(II) ions confirmed the prediction of a selective turn-off sensor via proximity quenching. Both dye density and Cu(2+) quenching efficiency peaked with SNP-dendrimer generation 4 (64 terminal amines). In addition, changing the terminal dendrimer arms to carboxylic acid end groups increased the copper quenching suggesting that more metal ion binding sites were created in close proximity to the dyes. Of the small anions tested for a turn-off sensor, only cyanide ion fully restored the PL when reaching a 2:1 CN(-):Cu(2+) ratio, while EDTA was not as effective at the same ratio. Therefore, dendrimer size and surface charge on the nanoparticles controlled the dye loading and copper quenching efficiency, while creating multiple binding sites for cyanide over other metal binding anions.

Colorimetric and Optical Discrimination of Halides by a Simple Chemosensor

A thiophene-based tripodal copper(II) complex has been synthesized as a new colorimetric and optical chemosensor for naked-eye discrimination of halides in acetonitrile and an acetonitrile-water mixture. The binding interactions of the new receptor with several anions were analyzed by UV-Vis titrations, electrospray ionization mass spectrometric (ESI-MS) experiments and density functional theory (DFT) calculations. The results from UV-Vis titrations indicate that the coordinative unsaturated copper(II) complex strongly binds a halide at its vacant copper(II) centre via a metal-ligand bond forming a 1:1 complex, exhibiting binding affinities in the order of fluoride > chloride > bromide > iodide. The interactions of the receptor with halides were further confirmed by ESI-MS, showing a distinct signal corresponding to a 1:1 complex for each halide, suggesting that the noncovalent interactions also exist in the gas phase. In addition, time-dependent DFT (TD-DFT) calculations were also carried out to understand the excited-state properties of the chemosensor complexes. A detailed analysis of the TD-DFT calculations shows a consistent red-shift in the first optically-allowed transition, consistent with the observed colorimetric experiments.