Flow injection analysis of zinc pyrithione in hair care products on a cobalt phthalocyanine modified screen-printed carbon electrode

Zinc pyrithione (ZPT) is an antibacterial and antifungal reagent that is often utilized for the antidandruff activity in hair-care shampoos with a composition level up to 1% in the formulation. It has some adverse effects to human and animal if consumed orally. A disposable type of cobalt phthalocyanide modified screen-printed carbon electrode (CoPc/SPE) in couple with flow injection analysis (FIA) was developed for easy and selective analysis of ZPT in commercial hair-care products. Under the optimized FIA conditions, the CoPc/SPE yielded a linear calibration plot in the window of 6–576 μM with sensitivity and detection limit of 1.65 nA μM⁻¹ and 0.9 μM (i.e. 1.42 pg in 5 μl sample loop), respectively, in 0.1 M KOH solution at an applied potential of 0.3 V versus Ag/AgCl. Since the approach is simple, easy, selective, and inexpensive, it offers a potential application of daily ZPT analysis in hair-care products.     

Amperometric lactate biosensor for flow injection analysis based on a screen-printed carbon electrode containing Meldola's Blue-Reinecke salt, coated with lactate dehydrogenase and NAD

A biosensor for the measurement of lactate in serum has been developed, which is based on a screen-printed carbon electrode, modified with Meldola's Blue-Reinecke Salt (MBRS-SPCE), coated with the enzyme lactate dehydrogenase NAD⁺ dependent (from Porcine heart), and NAD⁺. A cellulose acetate layer was deposited on the top of the device to act as a permselective membrane. The biosensor was incorporated into a commercially available, thin-layer, amperometric flow cell operated at a potential of only +0.05 V vs. Ag/AgCl. The mobile phase consisted of 0.2 M phosphate buffer pH 10 containing 0.1 M potassium chloride solution; a flow rate of 0.8 ml min⁻¹ was used throughout the investigation. The biosensor response was linear over the range 0.55-10 mM lactate; the former represents the detection limit. The precision of the system was determined by carrying out 10 repeat injections of 10 mM l(+)lactic acid standard; the calculated coefficient of variation was 4.28%. It was demonstrated that this biosensor system could be applied to the direct measurement of lactate in serum without pre-treatment; therefore, this would allow high throughput-analysis, at low cost, for this clinically important analyte.     

Amperometric simultaneous sensing system for d-glucose and l-lactate based on enzyme-modified bilayer electrodes

An amperometric biosensor system which uses screen-printed electrodes to simultaneously detect d-glucose and l-lactate has been developed and applied for simple and rapid determination of d-glucose and l-lactate levels in lactic fermenting beverages. The system was constructed from three-dimensionally layered electrodes. Taking into consideration the effects of easily oxidized substances contained in the samples, ferricyanide ions, which are electrochemically oxidized at a lower voltage, were chosen as a mediator. A linear relationship between steady-state current and concentration was found over a range of 1-100 mM (d-glucose) and 1-50 mM (l-lactate); the variation coefficients were 1.43% (n = 10) and 3.50% (n = 10) for the d-glucose and l-lactate sensors, respectively. When applied to lactic fermenting beverages, there was good agreement between the results obtained by the proposed sensing system and those obtained by the HPLC method. Using the proposed method, assays were completed within 5 min.     

Amperometric l-glutamate biosensor based on bacterial cell-surface displayed glutamate dehydrogenase

A novel l-glutamate biosensor was fabricated using bacteria surface-displayed glutamate dehydrogenase (Gldh-bacteria). Here the cofactor NADP⁺-specific dependent Gldh was expressed on the surface of Escherichia coli using N-terminal region of ice nucleation protein (INP) as the anchoring motif. The cell fractionation assay and SDS-PAGE analysis indicated that the majority of INP-Gldh fusion proteins were located on the surface of cells. The biosensor was fabricated by successively casting polyethyleneimine (PEI)-dispersed multi-walled carbon nanotubes (MWNTs), Gldh-bacteria and Nafion onto the glassy carbon electrode (Nafion/Gldh-bacteria/PEI-MWNTs/GCE). The MWNTs could not only significantly lower the oxidation overpotential towards NAPDH, which was the product of NADP⁺ involving in the oxidation of glutamate by Gldh, but also enhanced the current response. Under the optimized experimental conditions, the current–time curve of the Nafion/Gldh-bacteria/PEI-MWNTs/GCE was performed at +0.52 V (vs. SCE) by amperometry varying glutamate concentration. The current response was linear with glutamate concentration in two ranges (10 μM–1 mM and 2–10 mM). The low limit of detection was estimated to be 2 μM glutamate (S/N = 3). Moreover, the proposed biosensor is stable, specific, reproducible and simple, which can be applied to real samples detection.     

Determination of glutamate pyruvate transaminase activity in clinical specimens using a biosensor composed of immobilized l-glutamate oxidase in a photo-crosslinkable polymer membrane on a palladium-deposited screen-printed carbon electrode

The determination of glutamate pyruvate activity (GPT) is of particular clinical importance. A portable GPT sensor for both diagnostic and home-care purpose is highly expected. A highly sensitive and stable l-glutamate sensor was fabricated for the rapid detection of the GPT activity in serum. The sensor is composed of immobilized l-glutamate oxidase in a photo-crosslinkable polymer (PVA-SbQ) membrane on a palladium-deposited screen-printed carbon electrode. The sensor exhibited high sensitivity (detection limit of 50 nM for monosodium glutamate), remarkable long-term stability in storage (5 months in the dry dark state and 1 month in buffer solution) and good reproducibility (R.S.D.=2.6%, n=100). The electrode-to-electrode reproducibility was found to depend on the composition of the polymeric matrix. The optimal substrate composition for the detection of GPT activity was 1 mM α-ketoglutarate and 100 mM l-alanine. The GPT activity in serum can be determined within 3 min. The response of the sensor to GPT activity is linear over the range of 8-250 U/l. Good correlation between the sensor and the Sigma GPT assay kit was achieved (R²=0.9958). The sensor is potentially applicable to a home-care purpose when a portable measuring device is adapted.     

Electrochemical determination of L-Tryptophan, L-Tyrosine and L-Cysteine using electrospun carbon nanofibers modified electrode

A novel and simple method for the direct and quantitative determination of L-tryptophan (Trp), L-tyrosine (Tyr) and L-cysteine (Cys) was proposed in this work. Carbon nanofibers (CNFs), made by electrospinning technique, were used to modify carbon paste electrode (CPE) without any treatment to study the electrochemical behaviors of the three amino acids using cyclic voltammetry (CV) and constant potential amperometric method. The results demonstrated that the CNFs modified carbon paste electrode (CNF-CPE) exhibited high electrocatalytic activity and good analytical performance towards the oxidation of the three amino acids. The linear ranges of Trp, Tyr and Cys were 0.1-119, 0.2-107 and 0.15-64 μM with correlation coefficients of 0.9994, 0.9985 and 0.9996, respectively. All the detection limits of the analytes were 0.1 μM (S/N = 3). In addition, the CNF-CPE displayed good reproducibility, high sensitivity and good selectivity towards the determination of the amino acids, making it suitable for the determination of Trp, Tyr and Cys in clinical and medicine.     

New low-molecular weight gelators based on l-valine and l-isoleucine with various terminal groups

New low-molecular weight gelators based on l-valine and l-isoleucine, which have various terminal groups such as ester, carboxyl, and carboxylate, function as a good organogelator that form an organogel in many organic solvents. In addition, the sodium salt compounds form not only organogels but also a hydrogel in the presence of a cationic surfactant.     

Monitoring of malolactic fermentation in wine using an electrochemical bienzymatic biosensor for l-lactate with long term stability

l-lactic acid is monitored during malolactic fermentation process of wine and its evolution is strongly related with the quality of the final product. The analysis of l-lactic acid is carried out off-line in a laboratory. Therefore, there is a clear demand for analytical tools that enabled real-time monitoring of this process in field and biosensors have positioned as a feasible alternative in this regard. The development of an amperometric biosensor for l-lactate determination showing long-term stability is reported in this work. The biosensor architecture includes a thin-film gold electrochemical transducer selectively modified with an enzymatic membrane, based on a three-dimensional matrix of polypyrrole (PPy) entrapping lactate oxidase (LOX) and horseradish peroxidase (HRP) enzymes. The experimental conditions of the biosensor fabrication regarding the pyrrole polymerization and the enzymes entrapment are optimized. The biosensor response to l-lactate is linear in a concentration range of 1 × 10⁻⁶–1 × 10⁻⁴ M, with a detection limit of 5.2 × 10⁻⁷ M and a sensitivity of – (13500 ± 600) μA M⁻¹ cm⁻². The biosensor shows an excellent working stability, retaining more than 90% of its original sensitivity after 40 days. This is the determining factor that allowed for the application of this biosensor to monitor the malolactic fermentation of three red wines, showing a good agreement with the standard colorimetric method.     

Electrochemical oxidation of l-cysteine mediated by a fullerene-C60-modified carbon electrode

Use of a glassy carbon electrode modified by adhered microcrystals of fullerene-C₆₀ mediates the oxidation of cysteine in the presence of aqueous potassium-containing electrolytes. Under conditions of cyclic voltammetry, the potential for the oxidation of cysteine is lowered by approximately 100 mV and current is enhanced significantly relative to the situation prevailing when a bare glassy carbon electrode is used. Additional mediation occurs when the potential range covered includes that of C₆₀/C₆₀ⁿ⁻ redox couples. The sensitivity under condition of cyclic voltammetry is significantly dependent on pH, temperature and C₆₀ dosage. Excellent analytical and/or recovery data are obtained with vitamin pill, cassamino acid (hydrolyzed casein) and for a range of beverages.     

Ring-opening polymerization of l-lactide in supercritical carbon dioxide using PDMS based stabilizers

Ring-opening suspension polymerization of l-lactide in supercritical CO₂ (scCO₂) was investigated in the presence of different stabilizer architectures based on poly(dimethyl siloxanes) (PDMS). Two amphiphilic AB type block copolymers, a graft copolymer, and an ester-capped PDMS were selected to find their efficacy as stabilizers for the synthesis of poly(l-lactide) (PLLA) in scCO₂. The stabilizer’s efficiency was analyzed in terms of the molecular weight, yield, and particle morphology of PLLA. The block copolymers, poly(dimethylsiloxane)-b-poly(acrylic acid) (PDMS-b-PAA) and poly(dimethylsiloxane)-b-poly(methacrylic acid) (PDMS-b-PMA) were found to be effective, leading to the formation of fine, discrete PLLA microparticles. On the other hand, the graft copolymer, poly(dimethylsiloxane-g-pyrrolidonecarboxylic acid) (PDMS-g-PCA) and acetylated PDMS (PDMS-OAc) failed to give an enough stabilization to the PLLA due to their short polymer-philic chains, resulting in hard agglomerates.     

Electrochemical deoxyribonucleic acid biosensor based on carboxyl functionalized graphene oxide and poly-l-lysine modified electrode for the detection of tlh gene sequence related to vibrio parahaemolyticus

A carboxyl functionalized graphene oxide (GO-COOH) and electropolymerized ploy-l-lysine (PLLy) modified glassy carbon electrode (GCE) was fabricated and used for the construction of an electrochemical deoxyribonucleic acid (DNA) biosensor. The NH₂ modified probe ssDNA sequences were immobilized on the surface of GO-COOH/PLLy/GCE by covalent linking with the formation of amide bonds, which was stable and furthur hybridized with the target ssDNA sequence. Differential pulse voltammetry (DPV) was used to monitor the hybridization events with methylene blue as electrochemical indicator, which gave a sensitive reduction peak at −0.287 V (vs. SCE). Under the optimal conditions the reduction peak current was proportional to the concentration of tlh gene sequence in the range from 1.0 × 10⁻¹² to 1.0 × 10⁻⁶ mol L⁻¹ with a detection limit as 1.69 × 10⁻¹³ mol L⁻¹ (3σ). The polymerase chain reaction products of tlh gene from oyster samples were detected with satisfactory results, indicating the potential application of this electrochemical DNA sensor.     

Mediated amperometric biosensors for d-galactose,glycolate and l-amino acids based on a ferrocene-modified carbon paste electrode

Direct-current cyclic voltammetry is used to investigate the suitability of a ferrocene derivative as a mediator with galactose, glycolate and l-amino acid oxidases. The three enzymes coupled catalytically to ferrocene monocarboxylic acid exhibiting homogeneous second-order rate constants in the range 0.4 × 10⁵ to 8.5 × 10⁵ l mol^?1 s^?1. Enzyme electrodes which responded to d-galactose, glycolate or l-amino acids were constructed. The appropriate oxidase was retained behind a dialysis membrane at a carbon paste electrode containing the poorly soluble derivative 1,1′-dimethylferrocene. All the electrodes responded rapidly to millimolar concentrations of their respective substrates producing 95% of the steady-state current response in <2 min. This general method of biosensor construction should be widely applicable to oxidases and other oxidoreductase enzymes.     

Prussian blue-glutamate oxidase modified glassy carbon electrode: A sensitive l-glutamate and β-N-oxalyl-α,β-diaminopropionic acid (β-ODAP) sensor

A sensitive biosensor has been developed for the neurotoxin β-N-oxalyl-α,β-diaminopropionic acid (β-ODAP) contained in the seeds of grass pea (Lathyrus sativus) and for l-glutamate based on glutamate oxidase (GlOx) and a Prussian blue (PB) modified glassy carbon (GC) electrode. The configuration of the system is so as to detect the hydrogen peroxide released during the enzymatic cycle at a low applied potential, −50 mV versus Ag|AgCl, in the flow injection mode. For this purpose GlOx was coupled to PB electrodeposited onto a glassy carbon electrode and stabilised by treatment with tetrabutylammonium toluene-4-sulfonate (TTS) during one of the steps in the electrodeposition. GlOx was cross-linked with glutaraldehyde (GA), bovine serum albumin (BSA) and Tween-20 on the surface of the PB modified GC electrodes. Addition of 0.01% and 0.001% polyethyleneimine (PEI) to the immobilisation mixture resulted in an enhancement of the response signal with about 35% and 62% for glutamate and β-ODAP, respectively, when using 0.01% PEI and with 164% and 200% for glutamate and β-ODAP, respectively, when using 0.001% PEI. The linear response range for β-ODAP was extended from 0.05-0.5 mM to 0.01-1 mM, when 0.001% PEI was used. However, a higher concentration of PEI, 0.1%, caused a decrease in the sensitivity of the biosensor.     

Electrocatalytic properties of [Ru(bpy)(tpy)Cl]PF6 at carbon ceramic electrode modified with nafion sol-gel composite: application to amperometric detection of l-cysteine

A two-step sol-gel technique was used here to prepare a carbon ceramic electrode modified with nafion and [Ru(bpy)(tpy)Cl]PF₆. This involves two steps: first, forming a bulk-modified carbon ceramic electrode with nafion, and then immersing the electrode into a Ru-complex solution (electroless deposition) for a short period of time (5-25 s). Cyclic voltammograms of the resulting surface-modified carbon ceramic electrode show stable and a well-defined redox couple due to Ru(II)/Ru(III) system with surface-confined characteristic. l-Cysteine (CySH) has been chosen as a model to elucidate the electrocatalytic ability of Ru-complex nafion sol-gel composite electrode. Not only the modified electrode shows excellent catalytic activity toward l-cysteine electrooxidation in pH range 3-9, but the antifouling effect of nafion film also increases the reproducibility of results in comparison with CCE modified with homogeneous mixing of graphite powder and Ru-complex (one step sol-gel method). Under the optimized conditions in amperometry method, the concentration calibration range, detection limit and sensitivity were 0.1-100 μM, 20 nM and 50 nA/μM, respectively. The advantages of this modified electrode are good reproducibility, excellent catalytic activity, simplicity of preparation and especially its antifouling properties towards l-cysteine and its oxidation products. Additionally, it is promising as a detector in flow system or chromatography systems.     

Influence of the nature of the ligand on the microstructure of poly d,l-lactides prepared with organoaluminum initiators

The polymerization of d,l-lactide with organoaluminum initiators bearing two different ligands, acetylacetone (1) and achiral Schiff’s base (2), has been studied. The microstructure of the obtained polymers analyzed by high resolution ¹³C NMR (125 MHz) is strongly influenced by the nature of the ligand. Al(acac)₃ (1) leads to stereocopolymers having random repartition of configurational units while polylactides prepared with aluminum methoxide derivatives bearing as ligand Schiff’s base (2) present some degree of stereoregularity with preferential formation of isotactic sequences.Variation of the ligand linked to the initiator appears as one of the best tools for the moulding of the microstructure of obtained polylactides.     

Degradation studies on segmented polyurethanes prepared with poly (d, l-lactic acid) diol, hexamethylene diisocyanate and different chain extenders

In order to investigate the effect of different chain extenders on degradation properties of segmented polyurethanes (SPUs), three types of segmented polyurethanes (SPU-P, SPU-O and SPU-A) based on poly (d, l-lactic acid) diol, hexamethylene diisocyanate (HDI), were synthesized with three chain extenders: peperazine (PP), 1, 4-butanediol (BDO) and 1, 4-butanediamine (BDA), respectively. Thermogravimetric analysis, activation energy and in vitro degradation were used to characterize the obtained polymers, quantitatively. The results revealed that chain extender played an important role in thermal degradation and biodegradation of polyurethanes. Thermogravimetric analysis and activation energy demonstrated that SPU-O, SPU-P and SPU-A presented best, second and weakest thermostability, respectively, and the thermal degradation mechanism of three SPUs was the same and regarded as a two-stage degradation. Data of hydrolytic degradation of the polymers during 12 weeks indicated that the in vitro degradation stability of SPU-A and SPU-P was similar, but both were better than that of SPU-O. The reason for the differences among three types of SPUs was discussed in this paper.     

Synthetic studies on thiostrepton family of peptide antibiotics: synthesis of the cyclic core portion containing the dehydropiperidine, dihydroquinoline, l-valine, and masked dehydroalanine segments

The cyclic core portion containing the dehydropiperidine, dihydroquinoline, l-valine, and masked dehydroalanine (i.e., β-phenylselenoalanine) segments of the thiostrepton family of peptide antibiotics was synthesized via the consecutive coupling of these four segments followed by cyclization at the amide bond between the dehydropiperidine and masked dehydroalanine segments.