Peroxidase activity-structure relationship of the intermolecular four-stranded G-quadruplex-hemin complexes and their application in Hg ion detection

The peroxidase activities of the complexes of hemin and intermolecular four-stranded G-quadruplexes formed by short-stranded X_nG_mX_p sequences (X = A, T or C), especially T_nG_mT_p sequences, were compared. The results, combining with those of circular dichroism (CD) spectra and acid-base transition study for DNA-hemin complexes, provide some important information about DNAzymes based on G-quadruplex-hemin complexes, such as the formation of a G-quadruplex structure is an important factor for determining whether a DNA sequence can enhance the catalytic activity of hemin; both intramolecular parallel G-quadruplexes and intermolecular four-stranded parallel G-quadruplexes can enhance the catalytic activity of hemin; the addition of T nucleotides to the 5′-end of a G-tract confers corresponding G-quadruplex greatly enhanced catalytic activity, whereas the addition of T nucleotides to the 3′-end of the G-tract has little effect; the high catalytic activity of hemin in the presence of some short-stranded G-rich sequences may be a result of the reduction of the acidity of the bound hemin cofactor. These studies provide more information for the DNA-hemin peroxidase model system, may help to elucidate the structure-function relationship of peroxidase enzymes and to develop novel, highly efficient peroxidase-liking DNAzymes. As a sequence of such an investigation, a new Hg²⁺ detection method was developed.     

Colorimetric detection of sodium ion in serum based on the G-quadruplex conformation related DNAzyme activity

There has been a big challenge in developing the Na⁺ sensor that can be practically used in the physiological system with the interference of large amounts of K⁺. In this research, a novel Na⁺ sensor has been designed based on the G-quadruplex-conformation related DNAzyme activity. The sensor exhibits high selectivity and sensitivity with the detection limit of 0.6 μM, which enables the sensor to be practically used in determination of the Na⁺ level in serum. The research not only provides a simple Na⁺ sensor but also opens a new way for developing the detection technology of Na⁺.     

Synthetic multivalent DNAzymes for enhanced hydrogen peroxide catalysis and sensitive colorimetric glucose detection

A peroxidase-mimic DNAzyme is a G-quadruplex (G₄) DNA–hemin complex, in which the G₄-DNA resembles an apoenzyme, and hemin is the cofactor for hydrogen peroxide (H₂O₂) catalysis. Twenty-one-mer CatG4 is a well-proven G₄-DNA as well as a hemin-binding aptamer for constituting a DNAzyme. This work studied if a multivalent DNAzyme with accelerated catalysis could be constructed using a multimeric CatG4 with hemin. We compared CatG4 monomer, dimer, trimer, and tetramer, which were prepared by custom oligo synthesis, for G₄ structure formation. According to circular dichroism (CD) analysis, we found that a CatG4 multimer exhibited more active G₄ conformation than the sum effect of equal-number CatG4 monomers. However, the DNAzyme kinetics was not improved monotonically along with the subunit number of a multimeric CatG4. It was the trivalent DNAzyme, trimeric CatG4:hemin, resulting in the rapidest H₂O₂ catalysis instead of a tetravalent one. We discovered that the trivalent DNAzyme’s highest catalytic rate was correlated to its most stable hemin-binding G₄ structure, evidenced by CD melting temperature analysis. Finally, a trivalent DNAzyme-based colorimetric glucose assay with a detection limit as low as 10 μM was demonstrated, and this assay did not need adenosine 5′-tri-phosphate disodium salt hydrate (ATP) as a DNAzyme boosting agent.     

Visual and trap emission spectrometric detections of Ag (I) ion with mesoporous ZnO/CdS@SiO2 core/shell nanocomposites

A new method for the preparation of mesoporous ZnO/CdS@SiO₂ core/shell nanostructure (CSN) has been developed. The mesoporous silica shells allow Ag⁺ to enter into the interior of the nanostructures to contact with ZnO/CdS core, accordingly causes the quenching of its band edge emission (475 nm) along with a simultaneous enhancement of red emission at around 595 nm. So, a novel visual fluorescence detection strategy for Ag⁺ ion is proposed based on a common core/shell Quantum dots nanostructure. Under optimal conditions, the enhanced fluorescence intensity at 595 nm increased linearly with the concentration of Ag⁺ ion ranging from 0.03 μM to 0.24 μM with a detection limit (3σ) of 3.3 nM.     

Sorption properties of oxides VI. Silver ion impregnated mixed oxides

Hydrous bismuth-thorium mixed oxide coprecipitated with silver ion is shown to be a useful sorbent for the removal of I^- from alkaline solutions. Required capacity of the sorbent for I^- could be achieved by controlling the amount of Ag⁺ in the mixed oxide.     

Synthesis of crownophanes possessing bipyridine moieties: bipyridinocrownophanes exhibiting perfect extractability toward Ag ion

Novel crownophanes with two bipyridine moieties (bipyridinocrownophanes 1a and 1b) were conveniently prepared by means of intramolecular [2+2] photocycloaddition of vinylbipyridine derivatives. In the liquid-liquid extraction of heavy metal cations, 1a and 1b exhibited perfect selectivity toward Ag⁺ with high efficiency. It was found that the ethereal oxygen atoms and the four nitrogen atoms in 1a and 1b acted as ligating sites, according to the high extractability and complexing stability constant for Ag⁺ compared to those of the corresponding pyridinocrownophanes 4a and 4b. ¹H NMR and ESI-MS analyses suggested that the crownophanes formed a 1:1 complexes with the Ag⁺ ion.     

Preparation of an intelligent hydrogel sensor based on g-C3N4 nanosheets for selective detection of Ag+

In this study, an “intelligent” graphitic carbon nitride nanosheets/ polyacrylamide/ polyacrylic acid (CNNS/ PAM/ PAA) composite hydrogel was prepared by radical polymerization method. The structure, swelling and fluorescent properties of the gels were investigated. The results showed that the mixed gel not only had pH sensitivity in aqueous solution, but also had specific concentration-dependent fluorescent intensities in the presence of Ag⁺ over the range (0–100) μM with a detection limit of 6.31 μM. The mixed gel can be applied to be fluorescence probe for detection of Ag⁺.     

i-Motif-modulated fluorescence detection of silver(I) with an ultrahigh specificity

A novel Ag⁺ sensor has been designed based on the mechanism that i-motif formation induced by Ag⁺ was sensitively recognized by a cyanine dye. The sensor exhibited an over 130–16,000 fold selectivity toward Ag⁺ than that toward other metal ions. This research not only provides a step forward toward the development of Ag⁺ detection but also represents a new application for i-motif DNA.     

An intramolecular charge transfer fluorescent probe: synthesis and selective fluorescent sensing of Ag+

An intramolecular charge transfer (ICT) fluorescent probe, in which the thiourea derivative moiety is linked to the fluorescent 4-(dimethylamino) benzamide, has been designed and synthesized. The ions-selective signaling behaviors of the probe were investigated. Upon the addition of Ag+, an overall emission enhancement of 14-fold was observed. Compound 1 displayed highly selective chelation enhanced fluorescence (CHEF) effect with Ag+ over alkali, alkali earth metal ions and some transition metal ions in aqueous methanol solutions. The prominent selective and efficient fluorescent enhancing behavior could be utilized as a new chemosensing probe for the analysis of Ag+ ion in aqueous environment.     

Electronic structures of organo-transition-metal complexes I. Silver(I)-Olefin complexes

By using the closed-shell SCF-MO method with the CNDO type approximation for all valence electron systems, the electronic structures of some Ag⁺-olefin complexes are investigated. The calculated values of -H increase with the increasing number of methyl groups on the double bond and this trend agrees with the experimental result. Also calculation reproduces many experimental results, such as the infrared, Raman, and¹³C NMR spectra. These experimental results are discussed on the basis of the calculated electronic structures of Ag⁺-olefin complexes.     

Single column ion chromatography of ammonium ion,alkali metals and alkyl amines

Summary Comparative efficiencies of oxalic, dichloroacetic and sulfamic acids as eluents in ion chromatographic determination of ammonium ion, alkali ions and alkyl amines have been evaluated. In all cases these eluents compared favorably to the popular nitric acid eluent employed for the ion chromatography of these species. Sulfamic acid has been found to be particularly superior for the separation and quantitation of alkali ions and several alkyl amines in ppm and sub ppm range.     

Paper-based scanometric assay for lead ion detection using DNAzyme

A facile and simple paper-based scanometric assay was developed to detect Pb²⁺ using GR5-DNAzyme. Magnetic beads (MBs) and gold nanoparticles (AuNPs) were used as a signal collector and a signal indicator, respectively. They were linked together by GR5-DNAzyme, comprising an enzyme and a substrate strand pairing up with each other. In the presence of Pb²⁺, the substrate strand is cut into two pieces, resulting in the disassembly of AuNPs from the MBs. These AuNPs were spotted on predefined areas on a chromatography paper, where signal is amplified through silver reduction. This sensing platform exhibits high sensitivity and selectivity toward Pb²⁺, giving a detection limit of 0.3 nM and a linear fitting range from 0.1 to 1000 nM. Testing of this biosensor in river water and synthetic urine samples also showed satisfying results. Besides offering simultaneous and multi-sample analysis, this paper-based sensing platform presented here could be potentially applied and served as a general platform for on-site, naked eyes, and low-cost monitoring of other heavy metal ions in environmental and body fluid samples.     

Sensitive pseudobienzyme electrocatalytic DNA biosensor for mercury(II) ion by using the autonomously assembled hemin/G-quadruplex DNAzyme nanowires for signal amplification

Herein, a novel sensitive pseudobienzyme electrocatalytic DNA biosensor was proposed for mercury ion (Hg²⁺) detection by using autonomously assembled hemin/G-quadruplex DNAzyme nanowires for signal amplification. Thiol functionalized capture DNA was firstly immobilized on a nano-Au modified glass carbon electrode (GCE). In presence of Hg²⁺, the specific coordination between Hg²⁺ and T could result in the assembly of primer DNA on the electrode, which successfully triggered the HCR to form the hemin/G-quadruplex DNAzyme nanowires with substantial redox probe thionine (Thi). In the electrolyte of PBS containing NADH, the hemin/G-quadruplex nanowires firstly acted as an NADH oxidase to assist the concomitant formation of H₂O₂ in the presence of dissolved O₂. Then, with the redox probe Thi as electron mediator, the hemin/G-quadruplex nanowires acted as an HRP-mimicking DNAzyme that quickly bioelectrocatalyzed the reduction of produced H₂O₂, which finally led to a dramatically amplified electrochemical signal. This method has demonstrated a high sensitivity of Hg²⁺ detection with the dynamic concentration range spanning from 1.0 ng L⁻¹ to 10 mg L⁻¹ Hg²⁺ and a detection limit of 0.5 ng L⁻¹ (2.5 pM) at the 3S_blank level, and it also demonstrated excellent selectivity against other interferential metal ions.     

Excellent Ag+ selective receptors: Syntheses and complexation properties of novel biscalix[4]arene with benzalazine groups

By reacting mono-substituted or 1,3-bi-substituted [2-(p-formylphenyloxy)ethyloxy]-p-tert-butylcalix[4]arene (3 or 4) with hydrazine hydrate in ‘1+2’ or ‘2+2’ condensation mode, novel benzalazine-bridging biscalix[4]arenes 5 and 7 were conveniently obtained in the yields of 76 and 81%, respectively. Condensation of compound 4 and salicylide hydrazone gave a novel calix[4]arene benzalazine derivative 6 in the yield of 85%. The structures and conformations of all new compounds were characterised by elemental analyses, ESI-MS, ¹H NMR and ¹H–¹H COSY techniques. Biscalix[4]arene 7 adopts a symmetrical cone conformation with tube cavity. The liquid–liquid extraction experiment showed that all new hosts possessed excellent complexation abilities towards soft metal cations. Compound 7 exhibited high complexation selectivity towards Ag⁺. The Ag⁺/Na⁺ and Ag⁺/Hg²⁺ extraction percentages of host 7 were as high as 73.1 and 54.9, respectively. The UV–vis spectra complexation experiments revealed that the complexation constant of receptor 7 with Ag⁺ was 1.9 × 10⁵ M^? 1 and the 1:1 stoichiometry of receptor 7–Ag⁺ complex was formed. The ¹H NMR spectra complexation experiments suggested that Ag⁺ was bound in a cavity composed of two benzalazine groups on bridging chains.     

Electrochemiluminescence Enhancement of CdTe Quantum Dots by the Addition of Silver(I) Ions

Water-soluble CdTe quantum dots (QDs) were synthesized by using a 3-mercaptopropionic acid (MPA) capped method. Stable electrochemiluminescence (ECL) was obtained when the CdTe QDs were immobilized onto a glassy carbon electrode (GCE) by Layer-By-Layer (LBL) assembly of CdTe QDs and polydiallyldimethylam-monium chloride (PDDA) by using 2-(dibutylamino)-ethanol (DBAE) as a co-reactant. The ECL enhancement of CdTe QDs by the addition of silver(I) ions was also investigated. The maximum enhancement factor about 4 was obtained on a GCE in the presence and absence of the co-reactant. The enhancement was observed in phosphate-citric acid and phosphate buffer solutions (PBS), but not in borate buffer solution (BBS). This was newly formed Ag nanoparticles or silver(I) complex with large surface area and high catalytic activity in the phosphate-citric acid and phosphate buffer solutions, thus resulting in ECL enhancement.     

Quantum Chemical Calculations on the Structure and Adsorption Properties of NO and N2O on Ag+ and Cu+ Ion-Exchanged Zeolites

Ab initio cluster quantum chemical calculations at the Hartree–Fock (HF/Lanl2dz) and correlated second-order Moller–Plesset perturbation theory (MP2/Lanl2dz) levels were performed for NO and N₂O interactions with Ag⁺ and Cu⁺ ion-exchanged zeolites. The interaction energies were estimated in a conventional way and also corrected for basis set superposition errors. It was shown that the highly dispersed Ag⁺ counterions establish twofold coordination to the lattice oxygens on the zeolite surface, similar to the case of Cu⁺ ions. However, both NO and N₂O bind relatively strongly to the Cu active sites of Cu⁺ ion-exchanged zeolites than those of the Ag⁺ site of the Ag⁺ ion-exchanged zeolites. Based on the results of these calculations, the two different forms of adsorption for these molecules on the catalyst surface, the nature of their binding and characteristics of the adsorption properties have been discussed. Finally, some comparisons with the results obtained by a variety of density functional theory calculations on target systems have been presented.     

Proton NMR study of the stoichiometry, stability and thermodynamics of complexation of Ag+ ion with octathia-24-crown-8 in binary dimethylsulfoxide–nitrobenzene mixtures

Proton NMR was used to study the complexation reaction of Ag⁺ with octathia-24-crown-8 (OT24C8) in a number of binary dimethylsulfoxide (DMSO)–nitrobenzene (NB) mixtures at different temperatures. In all cases, the exchange between free and complexed OT24C8 was fast on the NMR time scale and only a single population average ¹H signal was observed. The formation constants of the resulting 1:1 complexes in different solvent mixtures were determined by computer fitting of the chemical shift-mole ratio data. There is an inverse relationship between the complex stability and the amount of DMSO in the solvent mixtures. The enthalpy and entropy values for the complexation reaction were evaluated from the temperature dependence of formation constants. In all solvent mixtures studied, the resulting complex is enthalpy stabilized but entropy destabilized. The TΔS° versus ΔH° plot of all thermodynamic data obtained shows a fairly good linear correlation indicating the existence of enthalpy–entropy compensation in the complexation reaction.     

Ion-exchange chromatography using vanadyl and vandate salts as mobile phases with indirect detection

Summary Vanadyl sulfate, VOSO₄, was characterized as the mobile phase for the ion exchange separation of Li⁺, Na⁺, NH ₄ ⁺ , and K⁺ using indirect photometric detection at 254 nm. Detection limits ranged from 0.2 ppm for Li⁺ to 1 ppm for K⁺. Indirect electrochemical detection of these separated cations by reduction of VO (II) to V³⁺ was compared to spectrophotometric detection. The potential of the vanadate species, HVO ₄ ^2– , for the separation of F^–, Cl^–, and SO ₄ ^2– , with indirect photometric detection was also demonstrated.     

Crystal Structure of 1,3-Alternate 25,26,27,28-Tetra(tert-butoxyethoxy)calix[4]arene: a Selective Ag+ Extractant

Alkylation of calix[4]arene by 2-tert-butoxyethyl bromide led to the tetraalkylatedcalix[4]arene in the 1,3-alternate, the conformation of which has been established byX-ray crystallography. This spatial structure included a cavity potentially useful forhost–guest complexes achieved with metal cations, especially with Ag⁺. The titlecompound crystallizes in the monoclinic space group Cc with cell constants a = 29.901(2),b = 8.139(1), c = 22.264(3) Å, = 90°, = 117.08(1)°and = 90°. This conformer represents an example for Ag⁺-tunnelingacross an aromatic cavity. This behaviour could lead to important implications with regardto the metal cation- interaction expected for metal transport through ion channels,metal inclusion in fullerenes, intercalation of metal cations into graphites, etc.