Electrochemical detection of dopamine using arrays of liquid-liquid micro-interfaces created within micromachined silicon membranes

The detection of protonated dopamine by differential pulse voltammetry (DPV) and square wave voltammetry (SWV) at arrays of micro-interfaces between two immiscible electrolyte solutions (μITIES) is presented. Microfabricated porous silicon membranes (consisting of eight pores, 26.6 μm in radius and 500 μm pore-pore separation, in a hexagonal layout) were prepared by photolithographic and etching procedures. The membrane pores were fabricated with hydrophobic internal walls so that the organic phase filled the pores and created the liquid interface at the aqueous side of the membrane. These were used for harnessing the benefits of three-dimensional diffusion to the interface and for interface stabilisation. The liquid-liquid interface provides a simple method to overcome the major problem in the voltammetric detection of dopamine at solid electrodes due to the co-existence of ascorbate at higher concentrations. Selectivity for dopamine over ascorbate was achieved by the use of dibenzo-18-crown-6 (DB18C6) for the facilitated ion transfer of dopamine across the μITIES array. Under these conditions, the presence of ascorbate in excess did not interfere in the detection of dopamine and the lowest concentration detectable was ca. 0.5 μM. In addition, the drawback of current signal saturation (non-linear increase of the peak current with the concentration of dopamine) observed at conventional (millimetre-sized) liquid-liquid interfaces was overcome using the microfabricated porous membranes.     

Serum-protein effects on the detection of the β-blocker propranolol by ion-transfer voltammetry at a micro-ITIES array

In this work, the effect of the serum protein, bovine serum albumin (BSA), on the detection of propranolol in artificial serum by ion-transfer voltammetry at an array of micro-interfaces between two immiscible electrolyte solutions (μITIES) is presented. Cyclic voltammetry (CV), differential pulse voltammetry (DPV), and differential pulse stripping voltammetry (DPSV) were examined for the detection of low concentrations of propranolol. Both CV and DPV had an interference effect from BSA, manifested as lower currents in the presence of the protein. DPSV proved to be the most effective technique, enabling the detection of 0.05 μM propranolol in the presence of BSA. The DPSV method employed a preconditioning step as well as a preconcentration step followed by the analytical signal generation step. The latter was based on the back-transfer of the drug across the μITIES. The preconcentration step was crucial to prevention of the adverse effects of BSA on the voltammetric detection. These results demonstrate that serum-protein effects on drug detection at low concentrations can be eliminated by use of DPSV at arrays of μITIES. CVs of propranolol with increasing concentrations of BSA revealed the influence of the drug-protein binding interaction, with decreases in current but no change in transfer potential. Therapeutic concentrations of propranolol were detected, demonstrating the viability of this approach for bioanalytical investigations.     

Design of single chain magnets through cyanide-bearing six-coordinate complexes

The design and preparation of stable cyanide-bearing six-coordinate complexes of formula [M^III(L)(CN)_x]^{(x + l − m)−} (M = trivalent transition metal ion and L = polydentate blocking ligand) are summarized here. Their use as ligands towards either fully hydrated metal ions or coordinatively unsaturated preformed species, to afford a wide variety of low-dimensional metal assemblies whose nuclearity, dimensionality and magnetic properties can be tuned, is also reviewed. Special emphasis is put on the appropriate choice of the end-cap ligand L whose denticity determines the number of coordinated cyanide groups in the mononuclear precursors. Among the different new spin topologies obtained through this rational synthetic strategy, ferromagnetically coupled 4,2-ribbon like bimetallic chains which exhibit slow magnetic relaxation and hysteresis effects (chain as magnets) are one of the most appealing and constitute the heart of the present contribution.     

Acyclic tertiary diamines and 1,4,7,10-tetraazacyclododecane with fluorine-containing β-diketones: Leading to low melting ionic adducts

N,N,N′,N′-Tetramethylmethanediamine (1a), N,N,N′,N′-tetramethylethanediamine (1b), N,N,N′,N′-tetramethyl-1,3-propanediamine (1c), and N,N,N′,N′-tetramethyl-1,6-hexanediamine (1d) were reacted at 25 °C with 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (2a), 2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedione (2b), 2-thenoyltrifluoroacetone (2c), and 4,4,4-trifluoro-1-(2-furyl)-1,3-butanedione (2d) to form the ionic adducts 3-18. 1,4,7,10-Tetraazacyclododecane (1e) reacted at 25 °C with β-diketones (2a-d) and 1,1,1-trifluoro-2,4-pentanedione (2e) to give ionic solids 19-23 in good yields. Some of the products are liquid at 25 °C and are thermally stable over long liquid ranges as determined by thermal gravimetric analyses. Single-crystal X-ray structure determinations show that compounds 9 and 21 crystallize in the monoclinic space groups P2(1)/c and P2(1)/n, respectively. All the new compounds were characterized by ¹H, ¹⁹F and ¹³C NMR, electrospray MS and/or elemental analyses.     

Selectively light scattering spectrometric detection of copper (II) based on a new synthesized oxamide ligand

Light scattering (LS) signals have been applied for analytical detections, but the selectivity is poor. In order to improve the selectivity, pre-separation or new machines are generally considered. Differing from these methods, we synthesized a highly selective oxamide ligand, N′,N′-bis (2-aminophenyl) oxamide (NAPO). It was found that the LS signals of NAPO, measured with a common spectrofluorometer, could be selectively enhanced by copper ion in neutral medium. Thus, a new highly selective detection method for copper ion could be developed over the range of 0.9-31.0 μM with the limit of determination of 97.6 nM (3σ). Foreign ions including Cd(II), Al(III) could be allowed even if present at the level of 7-fold more than that of Cu²⁺, avoiding pre-separation procedures from complicated samples such as real wastewater samples. Mechanism studies showed that the reaction between NAPO and copper ion could form some kinds of clusters and induce the enhanced LS signals.     

N,N′-Bis(aryl)pyridine-2,6-dicarboxamide complexes of ruthenium: Synthesis,structure and redox properties

Reaction of five N,N′-bis(aryl)pyridine-2,6-dicarboxamides (H₂L-R, where H₂ denotes the two acidic protons and R (R = OCH₃, CH₃, H, Cl and NO₂) the para substituent in the aryl fragment) with [Ru(trpy)Cl₃](trpy = 2,2′,2″-terpyridine) in refluxing ethanol in the presence of a base (NEt₃) affords a group of complexes of the type [Ru^II(trpy)(L-R)], each of which contains an amide ligand coordinated to the metal center as a dianionic tridentate N,N,N-donor along with a terpyridine ligand. Structure of the [Ru^II(trpy)(L-Cl)] complex has been determined by X-ray crystallography. All the Ru(II) complexes are diamagnetic, and show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ru^II(trpy)(L-R)] complexes shows a Ru(II)–Ru(III) oxidation within 0.16–0.33 V versus SCE. An oxidation of the coordinated amide ligand is also observed within 0.94–1.33 V versus SCE and a reduction of coordinated terpyridine ligand within −1.10 to −1.15 V versus SCE. Constant potential coulometric oxidation of the [Ru^II(trpy)(L-R)] complexes produces the corresponding [Ru^III(trpy)(L-R)]⁺ complexes, which have been isolated as the perchlorate salts. Structure of the [Ru^III(trpy)(L-CH₃)]ClO₄ complex has been determined by X-ray crystallography. All the Ru(III) complexes are one-electron paramagnetic, and show anisotropic ESR spectra at 77 K and intense LMCT transitions in the visible region. A weak ligand-field band has also been shown by all the [Ru^III(trpy)(L-R)]ClO₄ complexes near 1600 nm.     

Synthetic,structural and spectroscopic studies of complexes derived from the copper(II) perchlorate/fumaric acid/N,N′-chelates tertiary reaction systems

The synthetic investigation of the Cu(ClO₄)₂·6H₂O/fumaric acid (H₂fum)/N,N’-chelates (1,10-phen, 2,2′-bpy) tertiary reaction systems has yielded mononuclear, dinuclear and tetranuclear complexes, and three coordination polymers. The chemical and structural identity of the products depends on the solvent, the absence or presence of external hydroxides in the reaction mixtures and the N,N’-donor. Three fumarato(−2) complexes, i.e. compounds [Cu₂(fum)(phen)₄](ClO₄)₂·2H₂O (1·2H₂O), [Cu(fum)(phen)(H₂O)]_n (3) and [Cu₂(fum)(bpy)₂(H₂O)₂]_n(ClO₄)_2n (6), were isolated and structurally characterized, and four non-fumarato complexes, i.e. compounds [Cu₄(μ₃-ΟΗ)₂(μ₂-ΟΗ)₂(phen)₄(H₂O)₂](ClO₄)₄·2H₂O (2·2H₂O), [Cu(ClO₄)(phen) (MeCN)₂(H₂O)](ClO₄) (4), [Cu(ClO₄)(phen)(MeCN)₂]_n(ClO₄)_n (5) and [Cu(ClO₄)₂(bpy)(MeCN)₂] (7), were simultaneously obtained from the reaction systems investigated. The coordination versatility of the fumarato(−2) ligand is reflected to the three different coordination modes observed in 1·2H₂O, 3 and 6; the monodentate bridging μ₂-κO:κO′ mode in 3, the asymmetric chelating bridging μ₂-κO:κO′:κO′′:κO′′′ mode in 1·2H₂O and 3, and the syn,syn bridging μ₄-κO:κO′:κO′′:κO′′′ mode in 6. The crystal structures of the complexes are stabilized by intra- and inter-molecular hydrogen bonding and π–π stacking interactions leading to interesting supramolecular architectures. Characteristic IR bands of the complexes are discussed in terms of the known structures, and the coordination modes of the fum²⁻ ligands.     

Dimolybdenum complexes containing anions of N,N′-bis(pyrimidine-2-yl)formamidine and N-(2-pyrimidinyl)formamide

The reactions of Mo₂(O₂CCH₃)₄ with different equivalents of N,N′-bis(pyrimidine-2-yl)formamidine (HL1) and N-(2-pyrimidinyl)formamide (HL2) afforded dimolybdenum complexes of the types Mo₂(O₂CCH₃)(L1)₂(L2) (1) trans-Mo₂(L1)₂(L2)₂ (2) cis-Mo₂(L1)₂(L2)₂ (3) and Mo₂(L2)₄ (4). Their UV–Vis and NMR spectra have been recorded and their structures determined by X-ray crystallography. Complexes 2 and 3 establish the first pair of trans and cis forms of dimolybdenum complexes containing formamidinate ligands. The L1 ligands in 1–3 are bridged to the metal centers through two central amine nitrogen atoms, while the L2 ligands in 1–4 are bridged to the metal centers via one pyrimidyl nitrogen atom and the amine nitrogen atom. The Mo–Mo distances of complexes 1 [2.0951(17) Å], 2 [2.103(1) Å] and 3 [2.1017(3) Å], which contain both Mo?N and Mo?O axial interactions, are slightly longer than those of complex 4 [2.0826(12)–2.0866(10) Å] which has only Mo?O interactions.     

Complexes derived from the copper(II)/succinamic acid/N,N′,N″-chelate tertiary reaction systems: Synthesis,structural and spectroscopic studies

The use of succinamic acid (H₂sucm) in Cu^II/N,N′,N″-donor [2,2′:6′,2″-terpyridine (terpy), 2,6-bis(3,5-dimethylpyrazol-1-yl)pyridine (dmbppy)] reaction mixtures yielded compounds [Cu(Hsucm)(terpy)]_n(ClO₄)_n (1), [Cu(Hsucm)(terpy)(MeOH)](ClO₄) (2), [Cu₂(Hsucm)₂(terpy)₂](ClO₄)₂ (3), [Cu(ClO₄)₂(terpy)(MeOH)] (4), [Cu(Hsucm)(dmbppy)]_n(NO₃)_n·3nH₂O (5^.3nH₂O), and [CuCl₂(dmbppy)]·H₂O (6·H₂O). The succinamate(−1) ligand exists in four different coordination modes in the structures of 1–3 and 5, i.e., the μ₂-κO:κO′:κO″ in 1 and 5 which involves asymmetric chelating coordination of the carboxylato group and ligation of the amide O-atom leading to 1D coordination polymers, the μ₂-κ²O:κO′ in 3 which involves asymmetric chelating and bridging coordination of the carboxylato group, and the asymmetric chelating mode in 2. The primary amide group, either coordinated in 1 and 5, or uncoordinated in 2 and 3, participate in hydrogen bonding interactions, leading to interesting crystal structures. Characteristic IR bands of the complexes are discussed in terms of the known structures and the coordination modes of the Hsucm⁻ ligands. The thermal decomposition of complex 5·3nH₂O was monitored by TG/DTG and DTA measurements.     

The acetyl CoA synthase paradigm for hybrid bio-organometallics: Quantitative measures for resin-bound Ni-Rh complexes

The active site of Acetyl CoA Synthase utilizes a square planar NiN₂S₂ complex in the form of Ni^II(CGC)²⁻ (CGC = the cysteine-glycine-cysteine tripeptide motif within the protein) to serve as a bidentate sulfur-donor ligand to chelate a second, catalytically active Ni atom responsible for the C-C and C-S coupling reactions for the production of Acetyl CoA. Metalloenzymes, such as this, which house stable catalytic complexes within intricately designed pockets accessible by solvent channels, have inspired design of resin-bound complexes. Through the use of TentaGel S-RAM^® resin beads, the O-Ni(CGC)²⁻ ligand has been synthesized and derivatized with the Rh^I(CO)₂ moiety. The identification of the adduct on these resin beads is afforded by attenuated total reflectance FTIR spectroscopy in the ν(CO) region and compared to solution analogues. The goal of this study is to establish a quantitative measure of the loading of nickel and rhodium on the tripeptide modified resin beads, O-(CGC). The extent of CGC derivatization was determined by Fmoc cleavage of the Fmoc protected O-(CGC). Nickel and rhodium loading were determined by Neutron Activation Analysis. This work provides evidence that the TentaGel S-RAM^® resin beads greatly decrease the air sensitivity of the Ni-Rh complex as compared to the unsupported solution phase analogue. The derivatized beads have also been studied for their ability to withstand a number of physical stresses, i.e., for leaching.     

Probing trace Pb(2+) using electrodeposited N,N'-(o-phenylene)-bis-benzenesulfonamide polymer as a novel selective ion capturing film

A novel film modified electrode for the determination of trace lead was developed in this work. The modified electrode was prepared by the electropolymerization of N,N′-(o-phenylene)-bis-benzenesulfonamide (PBSA) as the ion capturing reagent to create the functional film. The modified electrode shows a high selectivity towards Pb²⁺ over interfering cations, e.g. Cu²⁺, Cd²⁺, Co²⁺, Ni²⁺, Zn²⁺, Cr²⁺, and the calibration curve was linear in the concentration range of 2.0 × 10⁻⁹ to 1.0 × 10⁻⁷ M with correlation coefficient of 0.999. For 20 min accumulation, detection limit of 1.0 × 10⁻⁹ M was obtained at the signal to noise ratio of 3. Analytical availability of the modified electrode was demonstrated by the application for samples from pond water.     

Synthesis,spectral characterization of the zinc(II) mixed-ligand complexes of N(4)-allyl thiosemicarbazones and N,N,N′,N′-tetramethylethylenediamine,and crystal structure of the novel [ZnL2(tmen)] compound

Mixed-ligand zinc complexes with N,N,N′,N′-tetramethylethylenediamine (tmen) and R-salicylaldehyde N(4)-allyl thiosemicarbazones (R: 3-OCH₃ (L₁), 5-Br(L₂)), [ZnL_1,2(tmen)], were synthesized and the complexes were characterized by elemental analysis, atomic absorption spectrometer, magnetic susceptibility, molar conductivity, electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) mass spectra and IR, UV–Vis, ¹H NMR and ¹⁵N spectroscopies. Crystal of [ZnL₂(tmen)] have a slightly distorted square pyramid involving O, N, S atoms of thiosemicarbazone and one N atom of tmen in basal plane and the other N atom of tmen in apex of the pyramid. The non-coordinated allyl group is disordered.     

Doubly hydrogen-bridged 1,2-diphenylenediboranes derived from 9-chloro-9-borafluorene and ligand exchange reactions

Cyclic 1,2-diphenylenediboranes containing a doubly hydrogen-bridged structure, including 1,2-(2,2^′biphenylylene)diborane(I) and 1,2-(2,2^′biphenylylene)-1,2-diethyldiborane (II), are conveniently prepared by treating 9-chloro-9-borafluorene with NaBH₄ and Na(Et)₃BH, respectively. The reaction mechanism involves an initial Cl-H exchange to form 9-borafluorene containing a reactive 5-member ring diarylborane moiety, which subsequently engages in a facile ring expansion with the in situ formed B-H containing residue (BH₃ or HBEt₂) to result in cyclic 1,2-diphenylenediboranes compounds. The doubly hydrogen-bridged structure shows good thermal stability up to 50 °C. Upon thermal cleavage at higher temperature, all free B-H groups become very reactive involving hydroboration with α-olefin. The complexization study also reveals that this intradiborane moiety forms a 1:2 complex with a strong base, such as pyridine.     

Gas analyzer for continuous monitoring of trace level methanethiol by microchannel collection and fluorescence detection

The highly odorous compound methanethiol, CH₃SH, is commonly produced in biodegradation of biomass and industrial processes, and is classed as 2000 times more odorous than NH₃. However, there is no simple analytical method for detecting low parts-per-billion in volume ratio (ppbv) levels of CH₃SH. In this study, a micro gas analysis system (μGAS) was developed for continuous or near real time measurement of CH₃SH at ppbv levels. In addition to a commercial fluorescence detector, a miniature high sensitivity fluorescence detector was developed using a novel micro-photomultiplier tube device. CH₃SH was collected by absorption into an alkaline solution in a honeycomb-patterned microchannel scrubber and then mixed with the fluorescent reagent, 4-(N,N-dimethylaminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole (DBD-F). Gaseous CH₃SH was measured without serious interference from other sulfur compounds or amines. The limits of detection were 0.2 ppbv with the commercial detector and 0.3 ppbv with the miniature detector. CH₃SH produced from a pulping process was monitored with the μGAS system and the data agreed well with those obtained by collection with a silica gel tube followed by thermal desorption–gas chromatography–mass spectrometry. The portable system with the miniature fluorescence detector was used to monitor CH₃SH levels in near-real time in a stockyard and it was shown that the major odor component, CH₃SH, presented and its concentration varied dynamically with time.     

Development, validation and application of Micellar Electrokinetic Capillary Chromatography method for routine analysis of catechins, quercetin and thymol in natural samples

Natural matrices such as several vegetables, fruits or beverages are the origin of different natural antioxidants with great interest to be used instead of synthetic antioxidants in several applications, as a result of their health-promoting and disease-preventing properties. In this work, a capillary electrophoresis method capable of separating and quantifying some of these antioxidants (seven catechins with the same or similar molecular weight and relation q/z) is showed. Short analysis time and precision parameters equivalent to those obtained by liquid chromatography were obtained. This method was developed using the micellar electrokinetic chromatography (MEKC) mode with ultraviolet–visible detection. Quality parameters were established, obtaining low instrumental detection and quantification limits (0.6–2.0 mg L^{− 1} and 2.0–6.5 mg L^{− 1}, respectively), good precision (relative standard deviation in the intermediate repeatability lower than 7% for every compound) and short analysis time. The developed method also showed good performance for the determination of two more natural antioxidants, quercetin and thymol. The applicability of the method to the analysis of catechins in several natural samples was tested.     

Free-radical crosslinking copolymerization of acrylamide and N,N-methylenebis acrylamide by used Ce(IV)/polyethylene glycol and Ce(IV)/diethylmalonate redox initiator systems

Acrylamide and N,N^′-methylenebis (acrylamide) (AAm-Bis) copolymerization has been investigated in water at a total monomer concentration of the 0.5 M. Conversion of monomer was measured as a function of the reaction time up to the onset of macrogelation. Experimental results indicate that the critical conversion at the gel point shows a minimum at 6 mol.% Bis when the monomer concentration was kept constant at 0.5 M and it was found that polymer formed with different Bis% are not dissolved in water, acetic acid, toluene or chloroform.     

Complex formation equilibria between Ag(I) and thioureas in n-propanol

Complex formation equilibria between Ag(I) and thiourea or N-alkyl-substituted thioureas have been investigated in n-propanol by potentiometry at 10 °C intervals from 5 to 50 °C. Stepwise formation of tris-coordinated AgL_n (n = 1-3) complexes has been found for the majority of the ligands. ΔH and ΔS values for the complex formation reactions have been evaluated from the dependence of ln β_n on temperature. The alkyl-substituents affect the ligand affinities in different ways in relation with the coordination level n.The reactions are exothermic with few exceptions. Enthalpy favoured complex formation with negative dependence of ΔG on temperature (ΔS > 0) have been found.The enthalpy and entropy changes for the stepwise complex formation equilibria are correlated by two linear compensative relationships with the same isoequilibrium temperature 50-51 °C.     

Highly Sensitive Spectrofluorometric Determination of Nucleic Acid Based on the Aggregation of two Oppositely Charged Kinds of Water-Soluble Porphyrins

Abstract

When cationic 5, 10, 15, 20-tetrakis(4-N-trimethylaminophenyl)porphine (TTMAPP) and anionic 5, 10, 15, 20-tetrakis(4-sulfophenyl)porphine (TSPP) were mixed in aqueous solution, these compounds were rapidly aggregated due mainly to their electrostatic interaction. The fluorescences of both porphyrins were quenched. However, if small amounts of a bulky molecule such as a polymer electrolyte coexisted in this reaction system, the aggregation of TTMAPP and TSPP was inhibited and their fluorescences reappeared. Based on these findings, a new highly sensitive spectrofluorometric determination of nucleic acids (DNA, RNA) was developed. Each calibration curve was linear in the concentration range of 0.03–0.4 μg/ml (DNA) and 0–05–0.6 μg/ml (RNA). Further, the detection limit (S/N = 3) was 0.016 μg/ml for DNA and for RNA, 0.020 μg/ml. The relative standard deviations were DNA: 1.70% and RNA: 1.83% (5 determinations). When the proposed method was applied to the determination of DNA originating from a bacteriophage, the results were satisfactory.     

Ion-transfer voltammetry at silicon membrane-based arrays of micro-liquid-liquid interfaces

Microporous silicon membranes, fabricated by lithographic patterning and wet and dry silicon etching processes, were used to create arrays of micro-scale interfaces between two immiscible electrolyte solutions (muITIES) for ion-transfer voltammetry. These membranes served the dual functions of interface stabilization and enhancement of the rate of mass-transport to the interface. The pore radii were 6.5 microm, 12.8 microm and 26.6 microm; the pore-pore separations were ca. 20- to 40-times the pore radii and the membrane thickness was 100 microm. Deep reactive ion etching (DRIE) was used for pore drilling through the silicon, which had been previously selectively thinned by potassium hydroxide etching. DRIE produces hydrophobic fluorocarbon-coated internal pore walls. The small pore sizes and large pore-pore separations used resulted in steady-state voltammograms for the transfer of tetramethylammonium cation (TMA(+)) from the aqueous to the organic phase, whereas the reverse voltammetric sweeps were peak-shaped. These asymmetric voltammograms are consistent with the location of the ITIES at the aqueous side of the silicon membrane such that the organic phase fills the micropores. Comparison of the experimental currents to calculated currents for an inlaid disc micro-interface revealed that the interfaces were slightly recessed, up to 10 microm (or 10% of the pore length) in one case. Facilitated ion transfer, with an organic-phase ionophore, confirmed the location of the organic phase within the pores. These microporous silicon membranes offer opportunities for various analytical operations, including enhancing the rate of mass transport to ITIES-based sensing devices and stabilization of the ITIES for hydrodynamic applications.