Control of the Properties of Carbon Nanotubes Synthesized by CVD for Application in Electrochemical Biosensors

Interest in carbon nanotubes (CNT) has grown at a very rapid rate in the last decade. Their interesting physical and chemical properties open attractive possibilities in many application areas. These properties depend on the process conditions during synthesis and on subsequent purification steps. Recent studies have demonstrated that CNT can promote the electron transfer of biomolecules. These exceptional properties make them attractive for use in electrochemical biosensors. Multi walled nanotubes have been synthesized by the Chemical Vapor Deposition (CVD) method using methane as a carbon source and Ni–Al₂O₃–SiO₂ as the catalyst. The influence of the variation of certain reaction parameters such as feed gas composition, catalyst mass, temperature and reaction time in the yield of the CVD process has been established. In addition, the structural and chemical characteristics of the CNTs have been studied and a purification process to eliminate the catalyst and amorphous carbon has been developed that involves a gaseous oxidative process and acid treatment. The efficiency of the purification step has been determined by analytical techniques. Atomic force microscopy, Raman scattering, thermogravimetric analysis, inductively coupled plasma atomic spectroscopy are the characterization techniques employed in this work.     

The organocatalytic enantioselective [3+2] cycloaddition reaction of α,β-unsaturated aldehydes with azomethine ylides applied to the asymmetric synthesis of densely substituted pyrroloisoquinolines

The synthesis of densely functionalized pyrrolo[2,1-a]isoquinolines was accomplished using the organocatalytic [3+2] cycloaddition between enals and azomethine ylides under iminium catalysis developed in our group some years ago as the key step. The cycloaddition proceeds smoothly yielding the corresponding pyrrolidine as a single endo diastereoisomer with enantiopurity in the range of 94–96% and further manipulations of the obtained cycloadducts using high yielding reaction protocols and a final mesylation/intramolecular N-alkylation sequence allowed the synthesis of target compounds in which the integrity of all stereocenters installed during the first step is maintained.     

N‐vinylcaprolactam‐based microgels: Synthesis and characterization

Poly(N‐vinylcaprolactam) (PVCL) is well known for its thermoresponsive behavior in aqueous solutions. PVCL combines useful and important properties; it is biocompatible polymer with the phase transition in the region of physiological temperature (32–38 °C). This combination of properties allows consideration of PVCL as a material for design biomedical devices and use in drug delivery systems. In this work, PVCL based temperature‐sensitive crosslinked particles (microgels) were synthesized in a batch reactor to analyze the effect of the crosslinker, initiator, surfactant, temperature, and VCL concentration on polymerization process and final microgels characteristics. The mean particle diameters at different temperatures and the volume phase‐transition temperature of the final product were analyzed. To obtain information about the inner structure of microgel particles, semicontinuous polymerizations were carried out and the evolution of the hydrodynamic average particle diameters at different temperatures of the microgel synthesized was investigated. In the case of microgel particles obtained in a batch reactor the size and the swelling‐de‐swelling behavior as a function of the temperature of the medium can be tuned by modulating the reaction variables. When the microgel particles were synthesized in a semibatch reactor different swelling‐de‐swelling behaviors were observed depending on particles inner structure. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2510–2524, 2008     

Iridium-catalyzed hydrogenation of N-heterocyclic compounds under mild conditions by an outer-sphere pathway

A new homogeneous iridium catalyst gives hydrogenation of quinolines under unprecedentedly mild conditions-as low as 1 atm of H(2) and 25 °C. We report air- and moisture-stable iridium(I) NHC catalyst precursors that are active for reduction of a wide variety of quinolines having functionalities at the 2-, 6-, and 8- positions. A combined experimental and theoretical study has elucidated the mechanism of this reaction. DFT studies on a model Ir complex show that a conventional inner-sphere mechanism is disfavored relative to an unusual stepwise outer-sphere mechanism involving sequential proton and hydride transfer. All intermediates in this proposed mechanism have been isolated or spectroscopically characterized, including two new iridium(III) hydrides and a notable cationic iridium(III) dihydrogen dihydride complex. DFT calculations on full systems establish the coordination geometry of these iridium hydrides, while stoichiometric and catalytic experiments with the isolated complexes provide evidence for the mechanistic proposal. The proposed mechanism explains why the catalytic reaction is slower for unhindered substrates and why small changes in the ligand set drastically alter catalyst activity.     

A concise and efficient route to the Alzheimer's therapeutic agent (R)-arundic acid

A short and efficient procedure for the preparation of (R)-arundic acid, a therapeutic agent for the treatment of Alzheimer's disease, has been developed. Based on cheap and commercially available (1R)-(+)-camphor as the source of chiral information, (R)-arundic acid is synthesized in a four-step cyclic sequence with 55% overall yield and high optical purity, ≥98% ee. Alkyl halide and acetylene constitute the only consumable carbon sources of this method, which allows obtaining of both enantiomers and recycling of chiral auxiliary.     

Influence of temperature on performance of an anaerobic sequencing biofilm batch reactor with circulation applied to treatment of low-strength wastewater

The effect of temperature on the performance of an anaerobic sequencing biofilm batch reactor (ASBBR) with liquid-phase recirculation was assessed. Assays were performed using a recirculation velocity of 0.20 cm/s, 8-h cycles, and an average treated synthetic wastewater volume of 2 L/cycle with a concentration of 500 mg of Chemical Oxygen Demand (COD)/L. Operation temperatures were 15, 20, 25, 30, and 35°C. At 25, 30, and 35°C, organic matter removal efficiencies for filtered samples ranged from 81 to 83%. At lower temperatures, namely 15 and 20°C, removal efficiency decreased significantly to 61 and 65%, respectively. A first-order model could be fitted to the experimental concentration profile values. The first-order kinetic parameter value of this model varied from 0.46 to 0.81 h¹ considering the lowest and highest temperature studied. Moreover, analysis of the removal profile values allowed fitting of an Arrhenius-type equation with an activation energy of 5715 cal/mol.     

Tetrahydroxy-p-benzoquinone as a source of polydentate O-donor ligands. Synthesis, crystal structure, and magnetic properties of the [Cu(bpy)(dhmal)]2 dimer and the two-dimensional [{SiW12O40}{Cu2(bpy)2(H2O)(ox)}2].16H2O inorganic-metalorganic hybrid

The use of tetrahydroxy-p-benzoquinone as a slow source of dihydroxymalonato and oxalato ligands led to the isolation under open-air mild reaction conditions of five different compounds, two of them prepared for the first time: [Cu(bpy)(dhmal)]2 (1) and [{SiW12O40}{Cu2(bpy)2(H2O)(ox)}2].16H2O (5) (bpy, 2,2'-bipyridine; dhmal, dihydroxymalonate; ox, oxalate). A possible mechanism for the oxidation of the benzoquinone to give the croconate dianion, which undergoes further ring-opening oxidation to decompose into dihydroxymalonate and oxalate, is proposed. All compounds have been characterized by elemental analysis, thermogravimetry, infrared spectroscopy, and powder X-ray diffraction. Single-crystal X-ray diffraction, electron paramagnetic resonance, and magnetic susceptibility measurements have been performed for compounds 1 and 5. A complete band assignment of the experimental FT-IR spectra is given through comparison with the ones calculated using density functional theory (DFT). The neutral dimer 1 constitutes the first structurally characterized example of a transition metal-dhmal complex, and it contains two copper atoms bridged by two dihydroxymalonato ligands acting in a mu2-kappa3O,O',O":kappa1O coordination fashion, so that an equatorial-axial Cu2(mu2-O)2 rhomboid core is formed. On the other hand, the inorganic-metalorganic hybrid compound 5 shows a two-dimensional arrangement of Keggin polyanions linked by one of the Cu atoms of the oxalate cationic dimers to give layers parallel to the (10) plane, the remaining ox-Cu-bpy fragments acting as interlamellar separators. In both cases, magnetic and EPR results are discussed with respect to the crystal structure of the compounds and, for compound 1, also with respect to DFT calculations of the exchange coupling constant.     

A critical analysis of the cyclic and open alternatives of the transmetalation step in the stille cross-coupling reaction

The transmetalation step of the Stille cross-coupling reaction catalyzed by PdL(2) (L = PH(3), AsH(3)) has been analyzed by means of DFT methods for PhBr as the electrophile and CH(2)=CHSnMe(3) as the nucleophile. Both experimentally proposed mechanisms (cyclic and open) were theoretically studied. For the case of the cyclic mechanism, the associative and dissociative ligand substitution alternatives were both analyzed. For the case of the open mechanism, the cis and the trans pathways were evaluated. All the reaction pathways were also studied taking into account the solvent effects by means of continuum models, for THF and PhCl as solvents. In selected cases, explicit solvent molecules were introduced to account for their potential role as ligands. Theoretical analysis indicates that the open reaction mechanism is preferred for organotriflate systems, whereas the cyclic mechanism is favored for the reaction with organohalide systems.     

Site selectivity and reversibility in the reactions of titanium hydrazides with Si-H, Si-X, C-X and H+ reagents: Ti=N(α) 1,2-silane addition, Nβ alkylation, Nα protonation and σ-bond metathesis

We report a combined experimental and computational comparative study of the reactions of the homologous titanium dialkyl- and diphenylhydrazido and imido compounds Cp*Ti{MeC(N(i)Pr)(2)}(NNR(2)) (R = Me (1) or Ph (2)) and Cp*Ti{MeC(N(i)Pr)(2)}(NTol) (3) with silanes, halosilanes, alkyl halides and [Et(3)NH][BPh(4)]. Compound 1 underwent reversible Si-H 1,2-addition to Ti=N(α) with RSiH(3) (experimental ΔH ca. -17 kcal mol(-1)), and irreversible addition with PhSiH(2)X (X = Cl, Br). DFT found that the reaction products and certain intermediates were stabilised by β-NMe(2) coordination to titanium. The Ti-D bond in Cp*Ti{MeC(N(i)Pr)(2)}(D){N(NMe(2))SiD(2)Ph} underwent σ-bond metathesis with BuSiH(3) and H(2). Compound 1 reacted with RR'SiCl(2) at N(α) to transfer both Cl atoms to Ti; 2 underwent a similar reaction. Compound 3 did not react with RSiH(3) or alkyl halides but formed unstable Ti=N(α) 1,2-addition or N(α) protonation products with PhSiH(2)X or [Et(3)NH][BPh(4)]. Compound 1 underwent exclusive alkylation at N(β) with RCH(2)X (R = H, Me or Ph; X = Br or I) whereas protonation using [Et(3)NH][BPh(4)] occurred at N(α). DFT studies found that in all cases electrophile addition to N(α) (with or without NMe(2) chelation) was thermodynamically favoured compared to addition to N(β).     

Determination of additives in an electrolytic zinc bath by q1H-NMR spectroscopy

The use of proton nuclear magnetic resonance (¹H-NMR) for the quantification of additives in an electrolytic Zn bath is reported. A simple and quick method is described that does not need any prior sample preparation. Contrary to other analytical methods, the three additives in the bath, benzylidene acetone (BDA), benzoic acid (BA) and poly(ethylene glycol) (PE400), can be quantified. Two calibration methods were tried: integration of NMR signals with the use of an internal standard and partial least squares (PLS) regression applied to the characteristic NMR peaks. Both methods are compared and the univariate method was preferred because of simplicity, accuracy and precision. The following limits of detection were found: 0.30 g L⁻¹ BA, 0.08 g L⁻¹ BDA and 0.7 g L⁻¹ PE400 with dynamic ranges of at least 1.0–6.0, 0.1–0.6 and 3.0–18.0 g L⁻¹ respectively. Those concentration ranges are suitable to follow the concentration of additives in the bath in real time. ¹H-NMR spectra provide evidence for the BDA degradation pattern.