Voltammetric response of phenylboronic acid monolayer-modified gold electrode to sugars.

The surface of a gold (Au) disk electrode was modified with a self-assembled monolayer consisting of phenylboronic acid moiety to fabricate a voltammetric sensor sensitive to sugars. The modified Au electrode exhibited a voltammetric response to sugars in the presence of Fe(CN)6(3-) ion in the sample solution at neutral pH. The peak current of the cyclic voltammograms decreased depending on the type and concentration of sugars. The dynamic range of the electrode is 3 - 100 mM for glucose and mannose and 1 - 30 mM for fructose. The sugar sensor can be used repeatedly after rinsing in 10 mM acetate buffer (pH 4.5).     

Phenylboronic acid functionalized SBA-15 for sugar capture

The synthesis and characterization of organic-inorganic hybrid materials that selectively capture sugars from model biomass hydrolysis mixtures are reported. 3-Aminophenylboronic acid (PBA) groups that can reversibly form cyclic esters with 1,2-diols, and 1,3-diols including sugars are attached to mesoporous SBA-15 via different synthetic protocols. In the first route, a coupling agent is used to link PBA and SBA-15, while in the second route poly(acrylic acid) brushes are first grafted from the surface of SBA-15 by surface-initiated atom transfer radical polymerization and PBA is then immobilized. The changes in pore structure, porosity, and pore size due to the loading of organic content are measured by powder X-ray diffraction and nitrogen porosimetry. The increase in organic content after each synthesis step is monitored by thermal gravimetric analysis. Fourier transform infrared spectroscopy and elemental analysis are used to characterize the chemical compositions of the hybrid materials synthesized. D-(+)-Glucose and D-(+)-xylose, being the most commonly present sugars in biomass, are chosen to evaluate the sugar adsorption capacity of the hybrid materials. It is found that the sugar adsorption capacity is determined by the loading of boronic acid groups on the hybrid materials, and the hybrid material synthesized via route two is much better than that through route one for sugar adsorption. Mathematical modeling of the adsorption data indicates that the Langmuir model best describes the sugar adsorption behavior of the hybrid material synthesized through route one, while the Freundlich model fits the data most satisfactorily for the hybrid material prepared via route two. The adsorption kinetics, reusability, and selectivity toward some typical chemicals in cellulose acidic hydrolysis mixtures are also investigated.     

Single tungsten nanowires as pH sensitive electrodes

The electrochemical potentials of tungsten nanowire samples, covered with their own oxide, were measured in dependence of the pH value. The samples were prepared by selective etching of a directionally solidified eutectic NiAl–W alloy. Directional solidification in a Bridgman-type crystal growth furnace yields nanostructured two-phase materials. Electrochemical processing allows selective etching of the phases exposing the nanoscale structures. In this work, pointed samples with a single wire 200 nm in diameter protruding from the tip were produced. Subsequently the tungsten oxide layer on these single nanowires was electrochemically modified to optimize their pH sensing capabilities. The method has a potential for further downsizing since the wire diameter and exposed length can be controlled by the process parameters during solidification and during electrochemical processing. The advantages of these nanowire pH probes along with possible applications such as the pH measurement in ultra small cavities and other small systems of interest such as corrosion pits and biological cells are discussed.     

Phenylboronic acid mediated triple condensation reactions of phloroglucinol and unsaturated carbonyl compounds

[reaction: see text] A remarkable phenylboronic acid mediated triple condensation reaction of phloroglucinol (1,3,5-trihydroxybenzene) with a series of alpha,beta-unsaturated carbonyl compounds is reported. This experimentally simple reaction afforded novel C3-symmetric 2H-chromene derivatives. These derivatives represent structural analogues of the natural product xyloketal A, which has been reported to be a potent inhibitor of acetylcholine esterase.     

Phenylboronic acid as a ligand for biospecific chromatography of serine proteinases.

Via attachment of p-(omega-aminoethyl)phenylboronic acid to CH-Sepharose in the presence of water-soluble carbodiimide, a new sorbent for the biospecific chromatography of serine proteinases was obtained. The sorbent was shown to be suitable for the purification of subtilisn, alpha-chymotrypsin and trupsin. It is assumed that the serine hydroxyl group at the active site of the enzyme forms, with the boronic acid moiety of the ligand, a structure that imitates transition enzyme--substrage complex. The presence of glycerol selectively improves the binding of serine proteinases, presumably because of stabilization of the tetrahedral state of the boron atom. Direct isolation of subtilisin from a Bacillus subtilis cultural filtrate on phenylboronic acid-containing sorbent gives a virtually homogenous enzyme (42-fold purification) in a nearly-quantitative yield.     

Interactions of doxorubicin with self-assembled monolayer-modified electrodes: electrochemical, surface plasmon resonance (SPR), and gravimetric studies

We present the results on the partitioning of doxorubicin (DOX), a potent anticancer drug, through the model membrane system, self-assembled monolayers (SAMs) on gold electrodes. The monolayers were formed from alkanethiols of comparable length with different ω-terminal groups facing the aqueous electrolyte: the hydrophobic -CH(3) groups for the case of dodecanethiol SAMs or hydrophilic -OH groups of mercaptoundecanol SAMs. The electrochemical experiments combined with the surface plasmon resonance (SPR) and gravimetric studies show that doxorubicin is likely adsorbed onto the surface of hydrophilic monolayer, while for the case of the hydrophobic one the drug mostly penetrates the monolayer moiety. The adsorption of the drug hinders further penetration of doxorubicin into the monolayer moiety.     

Phenylboronic acid as a mild and efficient catalyst for Biginelli reaction

The synthesis of 3,4-dihydropyrimidinone derivatives was achieved in good to excellent yields using phenylboronic acid as catalyst to promote the Biginelli three-component condensation of a diversity of aromatic aldehydes, ethyl acetoacetate and urea or thiourea.     

Membrane electrodes sensitive to doubly charged surfactants. Application to a cationic gemini surfactant

The response of a membrane electrode is not always identical to that of a redox electrode. Indeed, in the case of membrane electrode the response is not due to a redox equilibrium but to a cross-membrane potential. So, the membrane electrode's response depends mainly on the carrier system and the nature of the membrane. The properties of the membrane can favour several reactions giving rise to different ionic species diffusing in the membrane. The expression of the cross-membrane potential thus depends on the number and quantities of these ionic species. To illustrate this, we established the equations for the case of a two-charge cation detected by a univalent charged carrier. We show that the Nernstian response is not applicable to membrane electrodes. This approach allowed us to interpret results obtained with a cationic gemini surfactant-selective electrode prepared in the laboratory. To prove the well working of this electrode, we determined the critical micelle concentration in water and several NaBr solutions (0.004, 0.006, 0.01 and 0.02 M) from which the counterion binding has been determined.     

Sulpha-drug sensitive membrane electrodes and their analytical applications

Cetyltrioctylammonium sulphathiazole, sulphadoxine and sulphadimethoxine membrane electrodes are proposed for the potentiometric determination of sulpha-drugs. The construction and basic characteristics of the electrodes are discussed.     

Phenylboronic acid and dopamine as probe set for electrochemical detection of saccharides

A novel electrochemical approach for detection of saccharides via indicator displacement assay was presented. In this system, 2-fluorophenylboronic acid and dopamine (DA) were performed as probe set. The electrochemical properties of DA and the binding to 2-fluorophenylboronic acid in phosphate buffer at different pH values were investigated by cyclic voltammetry. After addition of fructose to the solution, a competition for the binding 2-fluorophenylboronic acid occurred that led to the release of the DA. The regenerate oxidation current of DA increased with increasing fructose concentration. Under optimized experimental conditions, the peak current was linearly related to fructose concentration in the range of 0.3-5.0 mmol/L with a detection limit of 0.1 mmol/L. In addition, the interaction between 2-fluorophenylboronic acid and other cis-diol compounds such as glucose, galactose and mannose was investigated.     

Phenylboronic acid self-assembled layer on glassy carbon electrode for recognition of glycoprotein peroxidase

We have successfully fabricated a phenylboronic acid self-assembled layer on glassy carbon electrodes (GCE), where 3-aminophenylboronic acid (APBA) is covalently bound to the electrochemical pretreated GCE surface with glutaraldehyde linkage. The specific binding of glycoprotein peroxidase with the self-assembled layer has been studied using horseradish peroxidase (HRP) as a model glycoprotein. Cyclic voltammetric, electrochemical impedance studies and photometric activity assays show that the affinity interaction of HRP with the APBA modified GCE surface includes specific and nonspecific bonding. The specific binding is attributed to the boronic acid–diols interaction where the boronic acid specifically binds the glycosylation sites of the HRP. This specific binding is reversible and can be split by sorbitol and glucose or released in an acidic buffer. The catalytic current of the HRP-loaded electrode, due to the catalytic oxidation of thionine in the presence of hydrogen peroxide, is proportional to HRP concentrations of the incubation solution. This work offers a new way to build novel sensors by specific binding of glycoproteins to a boronic acid self-assembled layer for determination of glycated proteins.     

Phenylboronic acid immunoaffinity reactor coupled with flow injection chemiluminescence for determination of α-fetoprotein

A reusable and sensitive immunoassay based on phenylboronic acid immunoaffinity reactor in combination with flow injection chemiluminescence (CL) for determination of glycoprotein was described. The reactor was fabricated by immobilizing 3-aminophenylboronic acid (APBA) on glass microbeads with γ-glycidoxypropyltrimethoxysilane (GPMS) as linkage. The α-fetoprotein (AFP) could be easily immobilized on the APBA coated beads through sugar-boronic interaction. After an off-line incubation, the mixture of the analyte AFP with horseradish peroxidase-labeled AFP antibody (HRP-anti-AFP) was injected into the reactor. This led the trapping of free HRP-anti-AFP by the surface coated AFP on glass beads. The trapped HRP-anti-AFP was detected by chemiluminescence due to its sensitizing effect on the reaction of luminol and hydrogen peroxide. Under optimal conditions, the chemiluminescent signal was proportional to AFP concentration in the range of 10-100 ng mL⁻¹. The whole assay process including regeneration of the reactor could be completed within 31 min. The proposed system showed acceptable detection and fabrication reproducibility, and the results obtained with the present method were in acceptable agreement with those from parallel single-analyte test of practical clinical sera. The described method enabled a low-cost, time saving and was potential to detect the serum AFP level in clinical diagnosis.     

Electrocatalytic oxidation of sugars on silver-UPD single crystal gold electrodes in alkaline solutions

A highly catalytic system for sugar oxidation in alkaline media is presented, for the first time, in which glucose oxidation takes place at ca. −0.44 V (vs. Ag|AgCl). Modification of Au(1 1 1) single crystal surface by under potential deposition (UPD) was carried out for a variety of metals and catalytic effect for sugar oxidation has been studied in 0.1 M NaOH. UPD of Ag ad-atoms on Au electrodes were of the best catalytic activity compared to other metals (Cu, Co, Ru, Cd, Ir, and Pt, etc.). For aldose type monosaccharide studied (glucose, mannose and xylose) as well as for aldose-containing disaccharides (maltose and lactose), one significant oxidation peak was obtained, however, no significant oxidation current was observed for disaccharides like sucrose. Gluconolactone and mannolactone gave no oxidation current at negative potentials at which glucose was oxidized, indicating no more than two-electron oxidation took place. With Ag ad-atoms coverage of ca. 0.3 monolayer leads to a positive catalytic effect expressed through a negative shift of ca. 0.14 V (glucose case) on the oxidation potential and a slight increase in peak current. At the Au(1 0 0) surface similar results to those at an Au(1 1 1) electrode were also observed.     

Microcylinder electrodes as sensitive detectors for high-efficiency, high-speed liquid chromatography

A carbon fiber microcylinder electrode (r = 3.5 microns) is used as a detector for high-efficiency and high-speed liquid chromatography. The microcylinder has a detection volume of a few picoliters, and can be placed directly at the outlet frit of the column. With proper positioning equipment, the electrode can be placed at the region of the outlet frit where the separation efficiency is highest. Such selective sampling results in greatly increased measured efficiency over a conventional electrochemical detector when short (4 cm) columns of conventional diameter (3.2 mm) are used. The microcylinder detector is sensitive and subpicomole detection limits are obtained in less than 30 s for norepinephrine. The need for expensive positioning equipment is eliminated by mounting the electrode into a fitting which can be mated directly with the column.     

Phenylboronic acid catalyzed-cyanide promoted, one-pot synthesis of 2-(2-hydroxyphenyl)benzoxazole derivatives

A novel, one-pot, phenylboronic acid catalyzed, cyanide promoted synthesis of 2-(2-hydroxyphenyl)benzoxazoles from salicylaldehydes and o-aminophenols is described. The synthesis is characterized by mild conditions, short reaction times, and simple workup of crystalline, high purity products.     

Amino acid analysis using amine-sensitive membrane electrodes

A method for determining specific amino acids is described, based on the detection of the amine formed by the enzymatic reaction of an amino acid with decarboxylase, using an amine-sensitive membrane electrode. l-Tyrosine and l-phenylalanine are considered as examples. The corresponding amines tyramine and phenethylamine, respectively, were selectively detected by using a poly(vinyl chloride)-based membrane electrode containing sodium tetrakis[3,5- bis(trifluoromethyl)phenyl]borate as an ion exchanger and tricresyl phosphate as a solvent mediator. The detection limits of l-tyrosine and l-phenylalanine were 20 and 50μM, respectively. The response characteristics of electrodes were compared by changing ion exchangers and solvent mediators.     

Covalent assembly and micropatterning of functionalized multiwalled carbon nanotubes to monolayer-modified Si(111) surfaces

Multiwalled carbon nanotubes (MWNTs) covalently bound to monocrystalline p-type Si(111) surfaces have been prepared by attaching soluble amine-functionalized MWNTs onto a preassembled undecanoic acid monolayer using carbodiimide coupling. SEM analysis of these functionalized surfaces shows that the bound MWNTs are parallel to the surface rather than perpendicular. The voltammetric and electrochemical impedance spectroscopy measurements reveal that the electron transfer at the MWNT-modified surface is faster than that observed at a MWNT-free alkyl monolayer. We have also demonstrated that it is possible to prepare MWNT micropatterns using this surface amidation reaction and a "reagentless" UV photolithography technique. Following this approach, MWNT patterns surrounded by n-dodecyl areas have been produced and the local electrochemical properties of these micropatterned surfaces have been examined by scanning electrochemical microscopy. In particular, it is demonstrated that the MWNT patterns allow a faster charge transfer which is consistent with the results obtained for the uniformly modified surfaces.