Oxygen cathode based on a layer-by-layer self-assembled laccase and osmium redox mediator

Trametes trogii laccase has been studied as biocatalyst for the oxygen electro-reduction in three different systems: (i) soluble laccase was studied in solution; (ii) an enzyme monolayer was tethered to a gold surface by dithiobis N-succinimidyl propionate (DTSP), with a soluble osmium pyridine-bipyridine redox mediator in both cases. The third case (iii) consisted in the sequential immobilization of laccase and the osmium complex derivatized poly(allylamine) self-assembled layer-by-layer (LbL) on mercaptopropane sulfonate modified gold to produce an all integrated and wired enzymatic oxygen cathode. The polycation was the same osmium complex covalently bound to poly-(ally-lamine) backbone (PAH-Os), the polyanion was the enzyme adsorbed from a solution of a suitable pH so that the protein carries a net negative charge. The adsorption of laccase was studied by monitoring the mass uptake with a quartz crystal microbalance and the oxygen reduction electrocatalysis was studied by linear scan voltammetry.While for the three cases, oxygen electrocatalysis mediated by the osmium complex was observed, for tethered laccase direct electron transfer in the absence of redox mediator was also apparent but no electrocatalysis for the oxygen reduction was recorded in the absence of mediator in solution.For the fully integrated LbL self-assembled laccase and redox mediator (case iii) a catalytic reduction of oxygen could be recorded at different oxygen partial pressures and different electrolyte pH. The tolerance of the reaction to methanol and chloride was also investigated.     

Electrochemical evaluation of electron transfer kinetics of high and low redox potential laccases on gold electrode surface

Laccases and other multicopper oxidases are reported to be able to carry out direct electron transfer reactions when immobilized onto electrode surface. This allows detailed research of their electron transfer mechanisms. We have recently characterized the kinetic properties of four laccases in homogenous solution and immobilized onto an electrode surface with respect to a set of different redox mediators. In this paper we report the direct electron transfer of four purified laccases from Trametes hirsuta (ThL), Trametes versicolor (TvL), Melanocarpus albomyces (r-MaL) and Rhus vernicifera (RvL), by trapping the proteins within an electrochemically inert polymer of tributylmethyl phosphonium chloride coating a gold electrode surface. In particular, we have characterized the steps involved in the laccases electron transfer mechanism as well as the factors limiting each step. During the voltammetric experiments, non-turnover Faradic signals with midpoint potential of about 790 and 400 mV were observed for high potential laccases, ThL and TvL, corresponding to redox transformations of the T1 site and the T2/T3 cluster of the enzyme, respectively, whereas low redox potential laccases r-MaL and RvL shown a redox couple with a midpoint potential around 400 mV. The electrocatalytic properties of these laccase modified electrodes for the reduction of oxygen have been evaluated demonstrating significative direct electron transfer kinetics. The biocatalytic activity of laccases was also monitored in the presence of a well known inhibitor, sodium azide. On the basis of the experimental results, a hypothesis about the electronic pathway for intramolecular electron transfer characterizing laccases has been proposed.     

Surface modification of gold electrode with gold nanoparticles and mixed self-assembled monolayers for enzyme biosensors

This paper describes the development and evaluation of a generic method for the immobilization of enzymes onto a gold electrode and its application to amperometric biosensors. The surface of the gold electrode was modified with gold nano-particles (AuNP) and mixed self-assembled monolayers (SAMs) to form an enzyme biosensor matrix. Horseradish peroxidase (HRP) was immobilized on the modified surface to form a biosensor matrix on a gold electrode. After the deposition of gold nano-particles on a bare gold surface, the AuNP-deposited gold electrode and a bare electrode were compared for the surface area and electric current using AFM and cyclic voltammetry (CV). The AuNP strongly adhered to the surface of the gold electrode, had uniform distribution and was very stable. A mixed SAM, composed of two different monolayer molecules, dithiobis-N-succinimidyl propionate (DTSP) and inert tetradecane-1-thiol (TDT), was formed using reductive desorption technique and cyclic voltammetry was used to verify the formation of mixed deposition. First, 3-mercaptopropionic acid (MPA) and TDT were deposited with a specified deposition ratio between the two components. Then, MPA was desorbed by applying electric potential to the surface. Finally, DTSP was deposited where MPA was. The ratios of 20: 80 and 50: 50 between MPA and TDT were examined, and differences in the CV responses were discussed. HRP was immobilized on the mixed SAM surface. The investigated method is regarded as an effective way for stable enzyme attachment, while the presence of gold nanoparticles provides enhanced electrochemical activity; it needs very small amounts of samples and enzymes and the SAM matrix helps avoid enzyme leaking. It is interesting that the mixed SAM shows unique CV characteristics compared to the uni-molecular SAMs. The reaction kinetics of the SAM-immobilized enzyme is discussed with the CV results and is observed to obey the Michaelis-Menten equation.     

An Artificial Supramolecular Nanozyme Based on β-Cyclodextrin-Modified Gold Nanoparticles

An artificial nanozyme model was developed by the supramolecular complexation of a β-cyclodextrin-modified gold nanoparticle and metal catalytic centers. The cyclodextrin-based monolayer was first constructed on the surface of gold nanoparticle by using the thiol modified cyclodextrin, subsequently the cyclodextrin-modified gold nanoparticle was utilized as a backbone to install metal catalytic centers by supramolecular assembly of the copper complex of triethylnetetramine-adamantane and β-cyclodextrin receptors immobilized on the surface of gold nanospheres via hydrophobic interaction. The catalytic behaviors of β-cyclodextrin-modified gold nanoparticles with adjacent multi-metal catalytic centers were investigated as an esterase mimic. Strong hydrolase activities for catalyzing the cleavage of an active ester 4,4′-dinitrodiphenyl carbonate (DNDPC) were observed. A detailed kinetic study on nanozyme-catalyzed hydrolysis of ester DNDPC has been described.     

Surface characterization and direct bioelectrocatalysis of multicopper oxidases

Multicopper oxidases (MCO) have been extensively studied as oxygen reduction catalysts for cathodic reactions in biofuel cells. Theoretically, direct electron transfer between an enzyme and electrode offers optimal energy conversion efficiency providing that the enzyme/electrode interface can be engineered to establish efficient electrical communication. In this study, the direct bioelectrocatalysis of three MCO (Laccase from Trametes versicolor, bilirubin oxidase (BOD) from the fungi Myrothecium verrucaria and ascorbate oxidase (AOx) from Cucurbita sp.) was investigated and compared as oxygen reduction catalysts. Protein film voltammetry and electrochemical characterization of the MCO electrodes showed that DET had been successfully established in all cases. Atomic force microscopy imaging and force measurements indicated that enzyme was immobilized as a monolayer on the electrode surface. Evidence for three clearly separated anodic and cathodic redox events related to the Type 1 (T1) and the trinculear copper centers (T2, T3) of various MCO was observed. The redox potential of the T1 center was strongly modulated by physiological factors including pH, anaerobic and aerobic conditions and the presence of inhibitors.     

Nanostructured carbon electrodes for laccase-catalyzed oxygen reduction without added mediators

Reduction of dioxygen catalyzed by laccase was studied at carbon electrodes without any added mediators. On bare glassy carbon electrode (GCE) the catalytic reduction did not take place. However, when the same substrate was decorated with carbon nanotubes or carbon microcrystals the dioxygen reduction started at 0.6 V versus Ag/AgCl, which is close to the formal potential of the laccase used. Four different matrices: lecithin, hydrophobin, Nafion and lipid liquid-crystalline cubic phase were employed for hosting fungal laccase from Cerrena unicolor. The carbon nanotubes and nanoparticles present on the electrode provided electrical connectivity between the electrode and the enzyme active sites. Direct electrochemistry of the enzyme itself was observed in deoxygenated solutions and its catalytic activity towards dioxygen reduction was demonstrated. The stabilities of the hosted enzymes, the reduction potentials and ratios of catalytic to background currents were compared. The boron-doped diamond (BDD) electrodes prepolarized to high anodic potentials exhibited behavior similar to that of nanotube covered GCE pointing to the formation of nanostructures during the anodic pretreatment. BDD is a promising substrate in terms of potential of dioxygen reduction, however the catalytic current densities are not large enough for practical applications, therefore as shown in this paper, it should be additionally decorated with carbon particles being in direct contact with the electrode surface.     

The influence of gold nanoparticle modified electrode on the structure of mercaptopropionic acid self-assembly monolayer

The comparison of assembly structure and property of mercaptopropionic acid (MPA) self-assembled monolayers (SAMs) on gold nanoparticle modified electrode (nanogold electrode) with that on planar gold electrode was studied by cyclic voltammetry (CV). The electron transfer of through the MPA SAMs and Cu underpotential deposition (UPD) on MPA-covered electrode indicated that MPA molecules assemblied on the planar gold electrode could form a very compact layer, which could surpass the electron transfer K₃Fe(CN)₆ greatly, whereas on the surface on the nanogold, which curvature make the compact packing loose. The reductive desorption in 0.5 M KOH and oxidative desorption in phosphate buffer solution (PBS) (pH 6.8) showed that gold nanoparticles could enhance the Au-S bond and stabilize the MPA molecules.     

Hydrophilic carbon nanoparticle-laccase thin film electrode for mediatorless dioxygen reduction: SECM activity mapping and application in zinc-dioxygen battery

Laccase from Cerrena unicolor was adsorbed on hydrophilic carbon nanoparticles (diameter = ca. 7.8 nm) modified with phenyl sulfonate groups and immobilized on an ITO electrode surface in a sol-gel processed silicate film. As shown by scanning electron and atomic force microscopies, the nanoparticles are evenly distributed on the electrode surface forming small aggregates of tens of nanometers in size. The mediator-free electrode exhibits significant and pH-dependent electrocatalytic activity towards dioxygen reduction. The maximum catalytic current density (95 μA cm⁻²) is obtained at pH 4.8 corresponding to maximum activity of the enzyme. Under these conditions dioxygen electroreduction commences at 0.575 V vs. Ag|AgCl_sat, a value close to the formal potential of the T1 redox centre of the laccase. The scanning electrochemical microscopy images obtained in redox competition mode exploiting mediatorless electrocatalysis show that the laccase is evenly distributed in the composite film. The obtained electrode was applied as biocathode in a zinc-dioxygen battery operating in 0.1 M McIlvaine buffer (pH 4.8). It provides 1.48 V at open circuit and a maximum power density 17.4 μW cm⁻² at 0.7 V.     

Surface chemistry and electrocatalytic behaviour of tetra-carboxy substituted iron, cobalt and manganese phthalocyanine monolayers on gold electrode

Surface chemistry and electrocatalytic properties of self-assembled monolayers of metal tetra-carboxylic acid phthalocyanine complexes with cobalt (Co), iron (Fe) and manganese (Mn) as central metal ions have been studied. These phthalocyanine molecules are immobilized on gold electrode via the coupling reaction between the ring substituents and pre-formed mercaptoethanol self-assembled monolayer (Au-ME SAM). X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy confirmed chemisorption of mercaptoethanol via sulfur group on gold electrode and also coupling reaction between phthalocyanines and Au-ME SAM. Electrochemical parameters of the immobilized molecules show that these molecules are densely packed with a perpendicular orientation. The potential applications of the gold modified electrodes were investigated towards l-cysteine detection and the analysis at phthalocyanine SAMs. Cobalt and iron tetra-carboxylic acid phthalocyanine monolayers showed good oxidation peak for l-cysteine at potentials where metal oxidation (M^III/M^II) takes place and this metal oxidation mediates the catalytic oxidation of l-cysteine. Manganese tetra-carboxylic acid phthalocyanine monolayer also exhibited a good catalytic oxidation peak towards l-cysteine at potentials where Mn^IV/Mn^III redox peak occurs and this redox peak mediates l-cysteine oxidation. The analysis of cysteine at phthalocyanine monolayers displayed good analytical parameters with good detection limits of the orders of 10⁻⁷ mol L⁻¹ and good linearity for a studied concentration range up to 60 μmol L⁻¹.     

Electrocatalytic oxidation of ascorbic acid using a single layer of gold nanoparticles immobilized on 1,6-hexanedithiol modified gold electrode

This paper describes the electrocatalytic oxidation of ascorbic acid (AA) in phosphate buffer solution by the immobilized citrate capped gold nanoparticles (AuNPs) on 1,6-hexanedithiol (HDT) modified Au electrode. X-ray photoelectron spectrum (XPS) of HDT suggests that it forms a monolayer on Au surface through one of the two SH groups and the other SH group is pointing away from the electrode surface. The free SH groups of HDT were used to covalently attach colloidal AuNPs. The covalent attachment of AuNPs on HDT monolayer was confirmed from the observed characteristic carboxylate ion stretching modes of citrate attached with AuNPs in the infra-red reflection absorption spectrum (IRRAS) in addition to a higher reductive desorption charges obtained for AuNPs immobilized on HDT modified Au (Au/HDT/AuNPs) electrode in 0.1 M KOH when compared to HDT modified Au (Au/HDT) electrode. The electron transfer reaction of [Fe(CN)₆]^4−/3− was markedly hindered at the HDT modified Au (Au/HDT) electrode while it was restored with a peak separation of 74 mV after the immobilization of AuNPs on Au/HDT (Au/HDT/AuNPs) electrode indicating a good electronic communication between the immobilized AuNPs and the underlying bulk Au electrode through a HDT monolayer. The Cottrell slope obtained from the potential-step chronoamperometric measurements for the reduction of ferricyanide at Au/HDT/AuNPs was higher than that of bare Au electrode indicating the increased effective surface area of AuNPs modified electrode. The Au/HDT/AuNPs electrode exhibits excellent electrocatalytic activity towards the oxidation of ascorbic acid (AA) by enhancing the oxidation peak current to more than two times with a 210 mV negative shift in the oxidation potential when compared to a bare Au electrode. The standard heterogeneous electron transfer rate constant (k_s) calculated for AA oxidation at Au/HDT/AuNPs electrode was 5.4 × 10⁻³ cm s⁻¹. The oxidation peak of AA at Au/HDT/AuNPs electrode was highly stable upon repeated potential cycling. Linear calibration plot was obtained for AA over the concentration range of 1–110 μM with a correlation coefficient of 0.9950. The detection limit of AA was found to be 1 μM. The common physiological interferents such as glucose, oxalate ions and urea do not show any interference within the detection limit of AA. The selectivity of the AuNPs modified electrode was illustrated by the determination of AA in the presence of uric acid.     

Phthalocyanines and porphyrins linked to gold adatoms and their catalytic property towards hydroxide oxidation

This paper describes the electrochemical formation and detection of gold adatoms, by recording successive cyclic voltammograms of a gold electrode in base, and their ability to function as an anchor for phthalocyanine or porphyrin adsorption. The values of the redox potential of the adatom reactions are linked to the redox potential of the adsorbed central metal ion, cobalt or copper, of the phthalocyanine or porphyrin compound. In addition, when using a phthalocyanine or porphyrin immobilized on a gold electrode, the detection of hydroxide can be improved by electrocatalysis. The catalytic current was found to vary linearly with the hydroxide concentration and a detection limit of 11 × 10⁻⁶ mol L⁻¹ for a 5,10,15,20-tetrakis-(4-carboxyphenyl)-porphyrin-Co(II) modified gold electrode could be calculated.     

A carboxyalkanethiol monolayer modified electrode for blocking of ascorbic acid oxidation and its application to a biosensor

Upon the application of amperometric biosensor to the biological fluid, ascorbic acid interferes the amperometric determination of analytes, because the oxidative potential of ascorbic acid is lower than that of electro active substances such as H₂O₂ produced by the enzymatic reaction. In this study we propose a method to block ascorbic acid based on the electrostatic interaction with self-assembled monolayer (SAM) and its application of the surface modified electrode to biosensor. In order to form SAM on the gold electrode with carboxyl group, 7-carboxy-heptanethiol (7-CHT) was used. The 7-CHT modified electrode did not show anodic response to ascorbic acid, but oxidized phenanthroline cobalt complex [Co(phen)₃²⁺], which can be used as a mediator of biosensor. Thus, the 7CHT-modified electrode was applied to biosensor mediated with Co(phen)₃²⁺. Fructose dehydrogenase (FDH) was immobilized to the 7-CHT modified electrode. Fructose was determined selectively with the FDH/7-CHT modified electrode at the range of 0.2-2 mM.     

Construction of layer-by-layer self-assemblies of glucose oxidase and cationic polyelectrolyte onto glassy carbon electrodes and electrochemical study of the redox-mediated enzymatic activity

Build-up of enzyme-polyelectrolyte multilayer onto glassy carbon (GC) surfaces by electrostatic self-assembling method has been investigated. In order to functionalize GC surface by a starting negatively charged layer, two approaches have been carried out: (i) covalent linkage of phenyl acetic acid through electroreduction of 4-phenylacetic diazonium salt (GC_A surface), and (ii) formation of a glucose oxidase (GOD) monolayer through an affinity reaction between a GOD conjugated antibody and a previously adsorbed antigen monolayer (GC_B surface). GC_A and GC_B surfaces have been modified by a precursor film (PF) composed of one layer of poly(styrenesulfonate) (PSS) sandwiched between two layers of poly(dimethyldiallylammonium) (PDDA), which improves the further assembling of enzymes. GOD, used as a model enzyme, has been self-assembled with PDDA onto these GC/PF surfaces. Enzymatic activity of immobilized GOD has been electrochemically assessed layer-by-layer during the build-up of (GOD-PDDA)_n multilayers. Cyclic voltammetry (CV) performed in the presence of excess glucose and using ferrocene derivatives as artificial redox mediators allowed to quantify the amount of electrically wired enzyme on the basis of kinetic models reported in literature. By using three mediators bearing different electrical charges we conclude that electrostatic interactions between the redox mediator and enzyme microenvironment play a key role in determining the rate of enzyme active site regeneration.     

Covalent and embedment immobilization of macrocyclic polyamines on gold electrodes and their voltammetric responses towards ethene dicarboxylic acids

In the paper presented, we report on the electrochemical signals generated upon interaction between polyamine host molecules immobilized on the surface of gold electrodes and anions of ethene dicarboxylic acids existing in the aqueous phase.Two methods of gold electrode modification will be compared: covalent (macrocyclic polyamine molecules with -SH groups were bound directly on the gold surface) and embedment (polyamines with long alkyl chains were adsorbed into the monolayer of 1-dodecanethiol deposited on the gold surface).The signals generated due to formation of supramolecular complexes between macrocyclic polyamines and anions of cis- and trans-isomers of ethene dicarboxylic acids at the electrode interface were measured by cyclic voltammetry and Osteryoung square-wave voltammetry with [Ru(NH₃)₆]³⁺ as an electroactive marker.The selectivity and sensitivity of the presented sensors were compared with those ion selective electrodes (ISEs) incorporated with the same and similar macrocyclic polyamines.     

Immobilization of Pycnoporus sanguineus laccase by metal affinity adsorption on magnetic chelator particles

BACKGROUND: Immobilized enzymes provide many advantages over free enzymes including repeated or continuous reuse, easy separation of the product from reaction media, easy recovery of the enzyme, and improvement in enzyme stability. In order to improve catalytic activity of laccase and increase its industrial application, there is great interest in developing novel technologies on laccase immobilization. RESULTS: Magnetic Cu²⁺‐chelated particles, prepared by cerium‐initiated graft polymerization of tentacle‐type polymer chains with iminodiacetic acid (IDA) as chelating ligand, were employed for Pycnoporus sanguineus laccase immobilization. The particles showed an obvious high adsorption capacity of laccase (94.1 mg g^?1 support) with an activity recovery of 68.0% after immobilization. The laccase exhibited improved stability in reaction conditions over a broad temperature range between 45 °C and 70 °C and an optimal pH value of 3.0 after being adsorbed on the magnetic metal‐chelated particles. The value of the Michaelis constant (K_m) of the immobilized laccase (1.597 mmol L^?1) was higher than that of the free one (0.761 mmol L^?1), whereas the maximum velocity (V_max) was lower for the adsorbed laccase. Storage stability and temperature endurance of the immobilized laccase were found to increase greatly, and the immobilized laccase retained 87.8% of its initial activity after 10 successive batch reactions. CONCLUSION: The immobilized laccase not only can be operated magnetically, but also exhibits remarkably improved catalytic capacity and stability properties for various parameters, such as pH, temperature, reuse, and storage time, which can provide economic advantages for large‐scale biotechnological applications of laccase. Copyright © 2007 Society of Chemical Industry     

Poly-o-phenylenediamine as redox mediator for laccase

Electrocatalytic activity towards oxygen reduction of fungal laccase entrapped in poly-o-phenylenediamine (POPDA) matrix on glassy carbon electrodes was studied. Cyclic voltammetry and amperometric responses to dissolved oxygen were investigated in pH range from 2 to 6. POPDA reveals a unique ability to serve as a redox mediator for laccase and immobilizing matrix at the same time. The entrapped enzyme efficiently catalyzes reduction of molecular oxygen without any additional mediators. The electrocatalytic current reaches 0.1 mA/cm⁻² per 1 μg of immobilized enzyme on cyclic voltammograms recorded at 1 mV/s in a not stirred electrolyte. Current density values are comparable with those revealed by dissolved laccase (60 μg/ml) mediated by hydroquinone and greatly higher than by the mediator less laccase/glassy carbon system. The potential of oxygen reduction is determined by the polymer redox couple. Consequently, the onset of the oxygen reduction shifts from −0.15 V versus Ag/AgCl in pH 6 to +0.05 V versus Ag/AgCl in pH 2. The laccase-POPDA layers immersed in the deaerated solution show fast amperometric responses to addition of the oxygen containing solution. The observed current values depend linearly on the oxygen concentration. Factors affecting the electrocatalytic activity of the laccase-POPDA system, including the layer thickness and the pH value, are studied. The electrodeposited laccase-POPDA films are characterized by infrared spectra. The results prove that the enzyme secondary structure remains unchanged during the entrapment procedure. POPDA matrix structure consists of the phenazine-type polymer according to infrared spectra.     

Fractal and percolation properties of active layer structure at oxygen electrode based on nanocomposite material of dispersed carbon carrier/laccase

The properties of the oxygen electrode active layer with laccase immobilized on highly dispersed colloid graphite (HCG) or carbon black AD 100 are studied. It was assumed that such a composite material provides the optimum conditions for bioelectrocatalytic oxygen reduction and allows enhancing the measured current values referred to the electrode geometric surface. These expectations were not wholly justified: the employment of HCG as a carrier allowed enhancing the specific activity per laccase molecule up to 29.8 μA/pmol of the enzyme, which is five times higher than the specific activity of laccase applied to AD 100 (5.9 μA/pmol of the enzyme); however no success was achieved in forming the active layers of any considerable thickness both in the first and in the second case, as the activity decreased drastically at the increase of the layer thickness. The nature of these problems can be caused by a number of reasons: the physico-chemical and structural characteristics of the carbon materials determining, in their turn, the adsorption value and the orientation of the enzyme molecules towards the carrier and the regularities of the active layer structure consisting of the fractal clusters of carbon particles. With the help of computer modeling, the fractal and percolation properties of the active layer structure are studied. The models of the active layer structure are suggested, in which carbon particles form agglomerates of fractal cluster type. This allows to suggest a number of considerations that can help to explain the fractal and percolation effects and the effect of the carbon carrier particle size on the immobilized enzyme coverage.     

Electron transport through alkanethiolate films decorated with monolayer protected gold clusters

Monolayers of 1,9-nonanedithiol or 1-decanethiol self-assembled directly on the gold substrate transfer charge to the K₄[Fe(CN)₆] molecule in solution more efficiently when they are decorated with 1-butanethiol protected gold nanoclusters. The clusters bound to the electrode by means of dithiol are more uniformly distributed on the monolayer and their efficiency in transferring electrons between the electrode and the redox couple is higher than when they are simply adsorbed through weak van der Waals interactions with the methyl groups of 1-decanethiol. With increasing adsorption time in the solution of clusters, the capacitance of the cluster decorated electrode significantly increases compared to the constant value of capacitance observed after prolonged immersion in pure toluene. This difference is explained assuming that gold clusters act as an array of capacitors on the monolayer modified electrode surface.     

Simultaneous determination of paracetamol and ascorbic acid using tetraoctylammonium bromide capped gold nanoparticles immobilized on 1,6-hexanedithiol modified Au electrode

Tetraoctylammonium bromide stabilized gold nanoparticles (TOAB-AuNPs) attached to 1,6-hexanedithiol (HDT) modified Au electrode was used for the simultaneous determination of paracetamol (PA) and ascorbic acid (AA) at physiological pH. The attachment of TOAB-AuNPs on HDT modified Au surface was confirmed by attenuated total reflectance (ATR)-FT-IR spectroscopy and atomic force microscope (AFM). The ATR-FT-IR spectrum of TOAB-AuNPs attached to the HDT monolayer showed a characteristic stretching modes corresponding to -CH₂ and -CH₃ of TOAB, confirming the immobilization of AuNPs with surface-protecting TOAB ions on the surface of the AuNPs after being attached to HDT modified Au electrode. AFM image showed that the immobilized AuNPs were spherical in shape and densely packed to a film of ca. 7 nm thickness. Interestingly, TOAB-AuNPs modified electrode shifted the oxidation potential of PA towards less positive potential by 70 mV and enhanced its oxidation current twice when compared to bare Au electrode. In addition, the AuNPs modified electrode separated the oxidation potentials of AA and PA by 210 mV, whereas bare Au electrode failed to resolve them. The amperometry current of PA was increased linearly from 1.50 × 10⁻⁷ to 1.34 × 10⁻⁵ M with a correlation coefficient of 0.9981 and the lowest detection limit was found to be 2.6 nM (S/N = 3). The present method was successfully used to determine the concentration of PA in human blood plasma and commercial drugs.     

Laccase functionalized cellulose acetate for the removal of toxic combustion products

The ability of Trametes villosa laccase immobilized on cellulose acetate to reduce/eliminate combustions toxicants was investigated using a model enzyme filter design. In the initial stages, various strategies of grafting laccase onto cellulose acetate polymers including partial deacetylated cellulose acetate followed by generation of reactive groups using either periodate or 2,2,6,6-tetramethylpiperidinyl-1-oxy radical (TEMPO) and the use of different spacer arms [1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDAC); 1,4-butanediol diglycidyl ether (BDGE)] and 3-aminopropyltriethoxysilane was investigated. The best process for effective immobilization of laccase onto both cellulose acetate powders and tows were those involving partial deacetylation, TEMPO activation to generate carboxylic groups, treatment with EDAC as a spacer arm followed by adding the enzyme. This procedure resulted in 45 units/mg laccase activity (28% increase in activity of immobilized enzyme) measured using ABTS as substrate as compared to the other strategies used to immobilize laccase. Further, the immobilized enzyme was able to oxidize > 60% of toxicants resorcinol, hydroquinone and methylcatechol passing through the model enzyme filter. This study therefore demonstrates the great possibility of immobilizing laccase onto modified cellulose acetate and the great potential application of immobilized laccase to remove toxicants during combustion.