Layer‐by‐layer Assembled Multilayers of Graphene/Mono‐(6‐amino‐6‐deoxy)‐β‐cyclodextrin for Detection of Dopamine

Graphene/mono‐(6‐amino‐6‐deoxy)‐β‐cyclodextrin multilayer films composed of graphene sheet (GS) and mono‐(6‐amino‐6‐deoxy)‐β‐cyclodextrin (NH₂‐β‐CD) were fabricated easily by two steps. First, negatively charged graphene oxide (GO) and positively charged mono‐(6‐amino‐6‐deoxy)‐β‐cyclodextrin (NH₂‐β‐CD) were layer‐by‐layer (LBL) self‐assembled on glassy carbon electrode (GCE) modified with a layer of poly(diallyldimethylammonium chloride) (PDDA). Then graphene/mono‐(6‐amino‐6‐deoxy)‐β‐cyclodextrin (GS/NH₂‐β‐CD) multilayer films were built up by electrochemical reduction of graphene oxide/mono‐(6‐amino‐6‐deoxy)‐β‐cyclodextrin (GO/NH₂‐β‐CD). Combining the high surface area of GS and the active recognition sites on β‐cyclodextrin (β‐CD), the GS/NH₂‐β‐CD multilayer films show excellent electrochemical sensing performance for the detection of DA with an extraordinary broad linear range from 2.53 to 980.05 µmol·L^?1. This study offers a simple route to the controllable formation of graphene‐based electrochemical sensor for the detection of DA.     

Direct cytochrome c electrochemistry at boron-doped diamond electrodes

Highly boron-doped diamond electrodes are characterized voltammetrically employing Ru(NH₃)₆^3+/2+, Fe(CN)₆^3−/4−, benzoquinone/hydroquinone, and cytochrome c redox systems. The diamond electrodes, which are polished to nanometer finish, are initially `activated' electrochemically and then pretreated by oxidation, reduction, or polishing. All electrodes give reversible cyclic voltammetric responses for the reduction of Ru(NH₃)₆³⁺ in aqueous solution.Redox systems other than Ru(NH₃)₆^3+/2+ show characteristic electrochemical behavior as a function of diamond surface pretreatment. In particular, the horse heart cytochrome c redox system is shown to give reversible voltammetric responses at Al₂O₃ polished boron-doped diamond electrodes. No voltammetric response for cytochrome c is detected at anodically pretreated diamond electrodes. The observations are attributed to preferential interaction of the polished diamond surface with the reactive region of the cytochrome c molecule and low interference due to a lack of protein electrode fouling.     

A sensitive DNA biosensor based on a facile sulfamide coupling reaction for capture probe immobilization

A novel DNA biosensor was fabricated through a facile sulfamide coupling reaction. First, the versatile sulfonic dye molecule of 1-amino-2-naphthol-4-sulfonate (AN-SO₃⁻) was electrodeposited on the surface of a glassy carbon electrode (GCE) to form a steady and ordered AN-SO₃⁻ layer. Then the amino-terminated capture probe was covalently grafted to the surface of SO₃⁻-AN deposited GCE through the sulfamide coupling reaction between the amino groups in the probe DNA and the sulfonic groups in the AN-SO₃⁻. The step-by-step modification process was characterized by electrochemistry and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Using Ru(NH₃)₆³⁺ as probe, the probe density and the hybridization efficiency of the biosensor were determined to be 3.18 × 10¹³ strands cm⁻² and 86.5%, respectively. The hybridization performance of the biosensor was examined by differential pulse voltammetry using Co(phen)₃^3+/2+ (phen = 1,10-phenanthroline) as the indicator. The selectivity experiments showed that the biosensor presented distinguishable response after hybridization with the three-base mismatched, non-complementary and complementary sequences. Under the optimal conditions, the oxidation peak currents of Co(phen)₃^3+/2+ increased linearly with the logarithm values of the concentration of the complementary sequences in the range from 1.0 × 10⁻¹³ M to 1.0 × 10⁻⁸ M with a regression coefficient of 0.9961. The detection limit was estimated to be 7.2 × 10⁻¹⁴ M based on 3σ.     

Voltammetric Characterization of Grafted Polymer Modified with ZnO Nanoparticles on Glassy Carbon Electrode

In this study, a grafted polymer (GP) with ZnO nanoparticles (GP/ZnO NPs) was attached on the surface of glassy carbon electrode (GCE), in order to produce a new modified electrode (GP/ZnO NPs-GCE). The gamma irradiation method was used to grafted polystyrene (polymer) with acrylonitrile (monomer), while slow evaporation process was used to prepare the new modified electrode. The cyclic voltammetry (CV) of K₄[Fe(CN)₆] was used to study the electrochemical properties GP/ZnO NPs-GCE. The peak separation (ΔE_pa-c) was 500 mV between the redox peaks of Fe(II)/Fe(III) in an aqueous solution of 1 M KCl and the current ratio of redox current peaks (I_pa/I_pc) was ≈ 1 for the modified electrode. This indicated that the modified electrode has s good reversibility and conductivity, wherefore; it was applied in the voltammetric filed. It was found that the modified electrode GP/ZnO NPs-GCE have a reasonable solubility and stability at various pH medium. Additionally, the sensitivity of the electrochemical analysis by cyclic voltammetric (CV) method is extensively subjected to the pH medium and the scan rate (SR). A couple of redox current peaks of K₄[Fe(CN)₆] in KCl solution was observed with a reversible process: Fe³⁺/Fe²⁺. Finally a good diffusion coefficient of electroactive species (D) for the new modified electrode was found in this study by chronoamperometry method using Cottrell equation.     

Copper hexacyanoferrate multilayer films on glassy carbon electrode modified with 4-aminobenzoic acid in aqueous solution

4-Aminobenzoic acid (4-ABA) was covalently grafted on a glassy carbon electrode (GCE) by amine cation radical formation during the electrooxidation process in 0.1 M KCl aqueous solution. X-ray photoelectron spectroscopy (XPS) measurement proves the presence of 4-carboxylphenylamine on the GCE. Electron transfer processes of Fe(CN)₆³⁻ in solutions of various pHs at the modified electrode are studied by both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Changing the solution pH would result in the variation of the terminal group's charge state, based on which the surface pK_a values were estimated. The copper hexacyanoferrate (CuHCF) multilayer films were formed on 4-ABA/GCE prepared in aqueous solution, and which exhibit good electrochemical behavior with high stability.     

Sensitive Nucleic Acid Detection at NH2‐MWCNTs Modified Glassy Carbon Electrode and its Application for Monitoring of Gemcitabine‐DNA Interaction

An electrochemical dsDNA nanobiosensor was fabricated using amino‐functionalized multi walled carbon nanotubes modified glassy carbon electrode (NH₂fMWCNTs/GCE) for the sensitive detection of DNA bases and electrochemical monitoring of drug‐DNA interaction. The influence of functional groups on MWCNT was studied by MWCNT functionalized with NH₂ (NH₂fMWCNTs) and COOH (COOHfMWCNT) groups based on the signal of DNA bases. The modified electrodes were characterized by scanning electron microscopy. One layer of calf thymus double stranded deoxyribonucleic acid (ct‐dsDNA) was immobilized onto the NH₂fMWCNTs/GCE (dsDNA/NH₂fMWCNTs/GCE). The dsDNA/NH₂fMWCNTs/GCE were used to investigate the interaction between the dsDNA and the anticancer drug gemcitabine by differential pulse voltammetry in acetate buffer of pH 4.70. For the confirmation of interaction, the lowering in intensity of the current signals of guanine and adenine was considered as an indicator. Electrochemical impedance spectroscopy studies were performed for the comparison of the modified surfaces. In order to define and visualize the interaction mechanism between gemcitabine and dsDNA/NH₂fMWCNTs/GCE at the molecular level, in silico methods including docking and molecular dynamics simulations were employed.     

Electrochemical Determination and in silico Studies of Fludarabine on NH2 Functionalized Multiwalled Carbon Nanotube Modified Glassy Carbon Electrode

A sensitive voltammetric technique has been developed for the determination of Fludarabine using amine‐functionalized multi walled carbon nanotubes modified glassy carbon electrode (NH₂‐MWCNTs/GCE). Molecular dynamics simulations, an in silico technique, were employed to examine the properties including chemical differences of Fludarabine‐ functionalized MWCNT complexes. The redox behavior of Fludarabine was examined by cyclic, differential pulse and square wave voltammetry in a wide pH range. Cyclic voltammetric investigations emphasized that Fludarabine is irreversibly oxidized at the NH₂‐MWCNTs/GCE. The electrochemical behavior of Fludarabine was also studied by cyclic voltammetry to evaluate both the kinetic (k_s and E_a) and thermodynamic (ΔH, ΔG and ΔS) parameters on NH₂‐MWCNTs/GCE at several temperatures. The mixed diffusion‐adsorption controlled electrochemical oxidation of Fludarabine revealed by studies at different scan rates. The experimental parameters, such as pulse amplitude, frequency, deposition potential optimized for square‐wave voltammetry. Under optimum conditions in phosphate buffer (pH 2.0), a linear calibration curve was obtained in the range of 2×10^?7 M–4×10^?6 M solution using adsorptive stripping square wave voltammetry. The limit of detection and limit of quantification were calculated 2.9×10^?8 M and 9.68×10^?8 M, respectively. The developed method was applied to the simple and rapid determination of Fludarabine from pharmaceutical formulations.     

Self-assembly of the light-harvesting complex of photosystem II (LHCII) on alkanethiol-modified gold electrodes

A light-harvesting complex of photosystem II (LHCII), isolated from spinach, was immobilized onto a gold electrode modified with self-assembled monolayers (SAMs) of alkanethiols, NH₂–(CH₂) _n –SH, n = 2, 6, 8, 11; HOOC–(CH₂)₇–SH; and CH₃–(CH₂)₇–SH; and a bare electrode. The extent of LHCII complex adsorption according to surface treatment decreased in the order amino groups > carboxylic acid groups > methyl groups and increased with the methylene chain length in NH₂–(CH₂) _n –SH. Interestingly, the photocurrent density depended on the terminal group and the methylene chain length in NH₂–(CH₂) _n –SH and decreased in the order amino groups > methyl groups > carboxylic acid groups. An efficient photocurrent response of the LHCII complex on SAMs of NH₂–(CH₂) _n –SH, n = 8 was observed upon illumination at 680 nm. These results indicated that the LHCII complexes were well organized on the cationic surfaces of the gold electrodes modified with amino alkanethiols. The quantum yield depended on the methylene chain length (n), where the maximum photocurrent response was observed at n = 8, which corresponded to a distance of 1.7 nm between the terminal amino group in NH₂–(CH₂)₈–SH and the gold surface.     

Attachment of NH3 clusters to Hg(6P0)

The ultraviolet luminescence from the Hg-photosensitised reaction of ammonia was investigated at pressures up to 10 atmospheres. From a variation of the wavelength distribution on the [NH₃], it was concluded that Hg(6³P₀) can attach clusters of NH₃ molecules to form Hg(NH₃)_n^* with n up to at least 5. The emission profiles of the stabilized complexes with n = 1–4 were determined, and also the profile from unstabilised HgNH₃^* formed in a bimolecular encounter of Hg(6³P₀) with NH₃. Dissociation constants for complexes with n = 2, 3 and 4 were measured.     

Interlayer reactions of the silver molybdate Ag6Mo10O33

Silver molybdate Ag₆Mo₁₀O₃₃ exchanges silver ions for organic cations, particularly surface-active agents such as long-chain n-alkylammonium ions C_nH_2n+1NH⁺₃. The alkylammonium ions penetrate between the layers and aggregate as bimolecular structures. The alkyl chains in the interlayer are not in all-trans conformation but are isomerized into conformers with gauche-bonds. These chains aggregate as gauche-blocks because the polar chain ends (NH⁺₃ and NH₂ groups) interacting with the molybdate layer cannot be close-packed. The specially favored formation and pronounced stability of gauche-blocks impede the quantitative exchange of the silver ions. No more than 20% of the silver ions are exchanged by alkylammonium nitrate. The gauche-blocks are stabilized by additional uptake of alkylamine molecules. Silver molybdate also reacts with alkylamine and forms long-spacing complexes with long segments of the alkyl chains perpendicular to the layers.     

Sensitive Phenol Detection Using Tyrosinase‐Based Phenol Oxidation Combined with Redox Cycling of Catechol

This paper reports sensitive phenol detection using (i) tyrosinase (Tyr)‐based oxidation of phenol to catechol, combined with (ii) electrochemical‐chemical‐chemical (ECC) redox cycling involving Ru(NH₃)₆³⁺, catechol, and tris(2‐carboxyethyl)phosphine (TCEP). Phenol is converted into catechol by Tyr in the presence of dissolved O₂. Catechol then reacts with Ru(NH₃)₆³⁺, generating o‐benzoquinone and Ru(NH₃)₆²⁺. o‐Benzoquinone is reduced back to catechol by TCEP, and Ru(NH₃)₆²⁺ is accumulated over the course of the incubation. When Ru(NH₃)₆²⁺ is electrochemically oxidized to Ru(NH₃)₆³⁺, ECC redox cycling occurs. For simple phenol detection, bare ITO electrodes are used without modifying the electrodes with Tyr. The detection limit for phenol in tap water using Tyr‐based oxidation combined with ECC redox cycling is ca. 10^?9 M, while that using only Tyr‐based oxidation is ca. 10^?7 M.     

Synthesis, physical and electrochemical characterization of mono- and heterobinuclear m-functionalized iron(III) Schiff base complexes

Mo(NO)T _p ^* Cl₂ (T _p ^* ?=?3,5-dimethyl pyrazole) when reacted with m-functionalized Fe(III) Schiff base complexes; the Schiff base ligands being derived from condensation of 2,4-dihydroxybenzaldehyde or salicylaldehyde with a variety of ??,??-diamines [1,2-C₆H₄(NH₂)₂, NH₂(CH₂) _n NH₂; n?=?2?C4] affords bimetallic complexes containing two potential reduction centers. The compounds were characterized by physicochemical and spectroscopic methods. It is shown that as the polymethylene carbon chain of the Schiff base backbone increases, the physicochemical and spectroscopic properties also change gradually. Electrochemical data show that the m-functionalized complexes reduce at potentials less cathodic than their p-substituted analogues. It is also shown that the redox potentials are solvent dependent.     

Amperometric determination of sulfide ion by glassy carbon electrode modified with multiwall carbon nanotubes and copper (II) phenanthroline complex

Electrocatalytic oxidation of sulfide ion on a glassy carbon electrode (GCE) modified with multiwall carbon nanotubes (MWCNTs) and a copper (II) complex was investigated. The Cu(II) complex was used due to the reversibility of the Cu(II)/Cu(III) redox couple. The MWCNTs are evaluated as a transducer, stabilizer and immobilization matrix for the construction of amperometric sensor based on Cu(II) complex adsorbed on MWCNTs immobilized on the surface of GCE. The modified GCE was applied to the selective amperometric detection of sulfide at a potential of 0.47 V (vs. Ag/AgCl) at pH 8.0. The calibration graph was linear in the concentration range of 5 µM–400 µM; while the limit of detection was 1.2 µM, the sensitivity was 34 nA µM^?1. The interference effects of SO₃ ^2?, SO₄ ^2?, S₂O₃ ^2?, S₄O₆ ^2?, Cysteine, and Cystein were negligible at the concentration ratios more than 40 times. The modified electrode is more stable with time and more easily restorable than unmodified electrode surface. Also, modified electrode permits detection of sulfide ion by its oxidation at lower anodic potentials.      

Electron transfer through single-crystal silver electrode/<Emphasis Type="Italic">n</Emphasis>-decanthiol monolayer/NaNO<Subscript>3</Subscript> solution interfaces

The kinetics of Ru[(NH₃)₆]³⁺ reduction in 1 M NaNO₃ solution at Ag(210) and Ag(111) singlecrystal electrodes modified by n-decanthiol monolayer is studied by electrochemical impedance spectroscopy and cyclic voltammetry. By using these two methods, standard rate constants of the redox reaction involving Ru[(NH₃)₆]^3+/2+ redox couple in the absence and in the presence of the n-decanthiol film were estimated. The equivalent circuit describing the experimental data in the presence of the self-assembled organic monolayer and in the absence of redox reaction is an electrical circuit comprising a large resistance (∼10⁶ Ω) connected in parallel with a capacitance (∼10⁻⁸ F). Analysis of kinetic data and extrapolation of Tafel lines resulted in the determination of the rate constant at unmodified Ag-electrode, which is characteristic of very fast heterogeneous electron transfer. The calculated rate constants for n-decanthiol-modified silver singlecrystal faces (210) and (111) in 1 M NaNO₃ solution (pH 6.3) equal 4.63 × 10⁻⁵ and 3.05 × 10⁻⁵ cm/s, respectively. The results are compared with the data at hand reported by different authors for gold electrodes in indifferent electrolyte solution in the absence and in the presence of self-assembled monolayer.     

Chemically modified glassy carbon electrode for electrochemical sensing paracetamol in acidic solution

Different organic molecules were covalently grafted on glassy carbon electrodes (GCEs) by an electrochemical reduction or potentiostatic process of several in situ-generated diazonium cations in acidic aqueous solution containing NaNO₂. The cyclic voltammetry implemented in 0.1?M KCl aqueous solution containing 5?mM Fe(CN) ₆ ^3? or Ru(NH₃) ₆ ³⁺ confirmed the blocking properties of the modified GCEs. The electrochemical impedance spectroscopy (EIS) performed in 0.1?M KCl aqueous solution containing 5?mM Fe(CN) ₆ ^3?/4? was used to measure the surface coverage of the modifiers on GCE; the results showed that the modified layers on GCEs are very compact. The linear sweep voltammetry (LSV) was employed to investigate the electrochemical sensing properties of the bare and modified GCEs toward paracetamol (PCT) in sulfuric acid solution of pH?1.02, and the corresponding calibration plots were obtained, respectively. The results indicated there is an oxidation peak of PCT in the linear sweep voltammograms on the bare and modified GCEs with the active terminal groups such as ?OPO₃H₂, ?SO₃H, ?COOH, and so on, but do not appear on GCEs modified with the inert terminal groups such as ?NO₂ and ?Br. These imply that the GCEs modified with the active terminal groups display an electrochemical behavior like bulk GCE; however, those with the inert terminal groups present an electrochemical behavior like microelectrode. The varying electrochemical sensitivity of all the electrodes toward PCT was explained according to electronegativity and pK _a of the terminal groups of the modifiers on the electrodes and hydrogen bond between the modifiers and PCT. Apparent standard rate constants of PCT oxidation reaction on the bare and modified GCEs were obtained from the Laviron’s approach.     

Ammonium oxofluorosilicates

The chemical and phase composition of the sublimates and nonvolatiles obtained as a result of the vaporization at 280°C of a mixture of silicon dioxide with (NH₄)₂SiF₆ at ratios of (0.25–10): 1 (mol/mol) is studied. Ammonium oxofluorosilicates of the general formula (NH₄)₂SiF₆ · nNH₄SiOF₃, where n = 1, 2, 4, or 8, are isolated from the gas phase and analyzed. These oxofluorosilicates have the structure of cubic (NH₄)₂SiF₆, which allows them to be treated as solid solutions. The sublimate components (NH₄)₂SiF₆ and NH₄SiOF₃ are in the ratio of 1: 2ⁿ, where n = 0, 1, 2, or 3. In the nonvolatile residue, the starting SiO₂ undergoes deep chemical destruction.     

Zur kenntnis der chemie der metallcarbonyle und der cyanokomplexe in flüssigem ammoniak : XXXIII. Über die darstellung neuer carbamoylcarbonyl-komplexe aus bekannten und neuen kationischen carbonylverbindungen des mangans und rheniums

The covalent carbamoyl carbonyl compounds Re(CO)₅COHN₂, cis-M(CO)₄(L)CONH₂, M(CO)₃(L)₂CONH₂ and M(CO)₃(D)CONH₂ (M = Mn, Re; L = PPh₃, PEt₃; D = bipy, phen) are formed by reactions of the cationic complexes [Re(CO)₆]⁺, [M(CO)₅L]⁺, [M(CO)₄L₂]⁺ and [M(CO)₄D]⁺ (M = Mn, Re; L = PPh₃, PEt₃; D = bipy, phen) with liquid NH₃ with concomitant deprotonation: [M(CO)_6?nL_n]⁺ + 2 NH₃ → M(CO)_5?nL_nCONH₂ + NH₄⁺ (n = 0, 1, 2) and [M(CO)₄D]⁺ + 2 NH₃ → M(CO)₃(D)CONH₂ + NH₄⁺ The stability of the above-mentioned carbamoyl carbonyl complexes increases from the penta- to the tetra- to the tri-carbonyl derivatives. In all cases the rhenium compounds are much more stable than the corresponding manganese complexes. Whereas the carbamoyl compound Re(CO)₄(PEt₃)CONH₂ can be isolated by reaction of [Re(CO)₅PEt₃]⁺ with NH₃, the corresponding manganese complex undergoes Hofmann degradation of amides even at ?70°C to form HMn(CO)₄PEt₃ and NH₄NCO. The IR and some mass and ¹H NMR spectra of the new hexacoordinated carbamoyl carbonyl complexes are discussed and the reactions of these compounds with liquid NH₃, HCl and CH₃OH are described.     

An electrochemical chiral sensor based on amino-functionalized graphene quantum dots/β-cyclodextrin modified glassy carbon electrode for enantioselective detection of tryptophan isomers

An electrochemical chiral sensing platform based on amino-functionalized graphene quantum dots/β-cyclodextrin modified glassy carbon electrode (NH₂-GQDs/β-CD/GCE) was developed for enantioselective detection of tryptophan (Trp) isomers. NH₂-GQDs/β-CD/GCE showed high electrocatalytic activity and good analytical behavior toward the oxidation of Trp isomers. The oxidation peak potentials and oxidation peak currents of Trp isomers at NH₂-GQDs/β-CD/GCE surface were observed by differential pulse voltammetry. NH₂-GQDs/β-CD nanocomposite exhibited different binding ability for two Trp isomers and selectively bonded with d-Trp, resulting in the higher oxidation peak current of d-Trp at NH₂-GQDs/β-CD/GCE surface. Trp isomers exhibited different oxidation peak potentials at NH₂-GQDs/β-CD/GCE surface, and the peak potential separation between l-Trp and d-Trp was around 0.022 V, which was used for the enantioselective detection of Trp isomers. Under the optimum experimental conditions, the oxidation peak currents were linearly dependent on the concentrations of Trp isomers. The linear ranges of l-Trp and d-Trp were all from 1.0 to 30.0 μM with correlation coefficients of 0.9886 and 0.9800, respectively. The detection limits of l-Trp and d-Trp were 0.65 and 0.12 μM (3σ/K), respectively. Such NH₂-GQDs/β-CD/GCE displayed high anti-interference against some physiological substances, good reproducibility and excellent long-term stability toward Trp isomers detection in biomedical applications.     

Multiphoton ionization mass spectroscopic detection of ammonia clusters

Ammonia cluster ions (NH₃)_n?2NH⁺₄ (n = 2?8), havevbeen detected by multiphoton ionization (MPI) mass spectroscopy, incorporating both a supersonic nozzle source and an ArB excimer laser. The MPI method reveals that the intensity ratio of NH⁺₄/NH⁺₃ is more than an order of magnitude greater than that in the electron impact mass spectrum.     

Click chemistry electrode modification using 4-ethynylbenzyl substituted cobalt phthalocyanine for applications in electrocatalysis

In this work, we report on the synthesis and applications of a new cobalt tetrakis 4-((4-ethynylbenzyl) oxy) phthalocyanine (3) for the detection of hydrazine. The glassy carbon electrode (GCE) was first grafted through diazotization, providing the GCE surface layer with azide groups. Thereafter, the 1,3-dipolar cycloaddition reaction, catalyzed by a copper(I) catalyst was used to “click” complex 3 to the grafted surface of GCE. The new platform was then characterized using cyclic voltammetry (CV), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). This work shows that 3 is an effective sensor with sensitivity of 91.5 μA mM^?1 and limit of detection of 3.28 μM which is a great improvement compared to other reported sensors for this analyte.