Shishan Zhao

Enzyme reactions in the presence of cyclodextrins : biosensors and enzyme assays

Cyclodextrins, macrocyclic carbohydrates with apolar internal cavities, can form complexes with, and solubilize many normally water-insoluble compounds. Ferrocene and its derivatives, tetrathiafulvalene and tetramethylbenzidine, can function as redox mediators, but are insoluble in water; when they are complexed with cyclodextrins, they can be used in enzymatic assays and in the construction of mediated biosensors. In addition, the solubilization of polynuclear aromatic hydrocarbons (PAHs), including the potent carcinogen benzo[a]pyrene, by cyclodextrins has enabled the detection of these important environmental contaminants.

Bioelectrocatalysis of a water-soluble tetrathiafulvalene2-hydroxypropyl-β-cyclodextrin complex

Abstract Water-soluble 2-hydroxypropyl-β-cyclodextrin (hp-β-CyD), a cyclic and non-reducing oligosaccharide, was used to enclose tetrathiafulvalene (TTF) to from a water-soluble complex. The molar ratio of hy-β-CyD to TTF in TTF-saturated solutions was about 2.8 and the complex was much more stable than free TTF in aqueous solutions. The cyclic voltammogram of the TTF-hp-β-CyD complex exhibited two oxidation waves with E p values of 210 mV (TTF → TTF + ) and 415 mV (TTF + → TTF 2+ ) at a glassy carbon electrode vs. Ag/AgCl. The former was relatively reversible and had a pH-independent E p , while the latter was not reversible due to the decomposition of TTF 2+ and had a strongly pH-dependent E p . Application of this novel complex as a homogeneous mediator for the enzyme-catalyzed electrooxidation of glucose, hypoxanthine and lactate, respectively at a glassy carbon electrode was successfully demonstrated. A well-defined catalytic wave was also obtained from NADH,illustrating the feasibility of the TTF-hp-β-CyD complex for bioreactions involving NADH dehydrogenases. The complex for bioelectrocatalysis was then investigated as membrane electrodes of glucose oxidase and xanthine oxidase. Fast response, high sensitivity and a large range of linearity were observed.

A cyclodextrin–porphyrin assembly as chemosensor for pentachlorophenol

A quantitative change in the absorption profile of the assembly prepared from the torus-shaped 2-hydroxypropyl-cycloheptaamylose (hp-β-cyclodextrin, hp-β-CD) and planar porphyrin 4,4′,4″,4‴-(21H,23H-porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid)(POR) occurs highly selectively in response to a guest–host complexation of pentachlorophenol.

An electrocatalytic approach for the measurement of chlorophenols

Abstract An electrocatalytic approach using an enzymatic reaction for the regeneration of analytes has been developed for the measurement of chlorophenols. Chlorophenols were oxidized to quinoid compounds by ceric sulfate or chloroperoxidase and the resulting oxidation products were measured in an amperometric system using glucose oxidase immobilized on a glassy carbon electrode. In this system, glucose oxidase was readily reduced by its substrate, glucose, to provide a non-rate-limiting source of electron flow towards the electrode. The quinoid products of chlorophenols then recycled the reduced enzyme to its original active form, i.e., mediating the rate-limiting electron transfer from the enzyme to the electrode. Analytical selectivity resulted from both the production of quinoid products and their electrocatalytic specificity for glucose oxidase, while high sensitivity was obtained from the amplified redox cycling of the analyte electrocatalyst. While this method was able to differentiate para-chlorinated chlorophenols from other chlorophenols, its selectivity for 2,4,6-trichlorophenol was uniquely high. The detection limit of this method was about 1 nM and the detection principle was easily integrated into flow injection analysis for a sample throughput as high as one per min.

A novel assembly of cyclodextrins with 4,4′4″,4‴-(21H,23H-porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid) through hydrogen bonds

Semi-closed assemblies are formed from cyclodextrins (CyDs) and 4,4′,4″,4‴-(21H,23H-porphine-5,10,15,20-tetrayl)tetrakis benzoic acid (PTTKBA); the assemblies are a new group of host molecules that possess inclusion properties different from those of the component CyDs.

Stable mediated amperometric biosensors using a graphite electrode embedded with tetrathiafulvalene in silicone oil

Abstract A new method has been developed to incorporate the mediator, tetrathiafulvalene (TTF), to the electrode/solution interface of an amperometric biosensor. TTF was dissolved in methylphenyl polysiloxane (silicone oil) and embedded in a graphite disc electrode. The mediator was able to diffuse to the electrode surface at an electrocatalytically significant speed. The storage of TTF in the inert polysiloxane provided a long-lasting and stable mediator supply. TTF-silicone oil electrodes with immobilized glucose oxidase, xanthine oxidase, or amino acid oxidase exhibited sensitive, fast and reproducible responses. The glucose oxidase electrode was very stable for at least 2 months when stored at 4°C. Together with flow injection analysis (FIA), the enzyme electrodes were reused for at least 500 repeated analyses during a 25 h operation without losing their initial activity.

Novel Approaches to the Development of Mediated Biosensors and Enzyme Assay

2-Hydroxypropyl-β-cyclodextrin (hp-β-CyD), a cyclic and non-reducing Oligosaccharide was used to enclose the hydrophobic guest molecules, ferrocene (FeCp2) and 1,1’-dimethylferrocene (DMFeCp2) to form a water-soluble yellow complex. At 300 mM, hp-β-CyD enclosed up to 100 mM FeCp2 or DMFeCp2. The cyclic voltammogram of the DMFeCp2-hp-β-CyD complex exhibited one single electron transfer process with a half-wave potential of 170 mV. The oxidation potential of the DMFeCp2-β-CyD complex was not affected by pH ranging from pH 4.7 to 8.8 and the difference between the anodic and cathodic peak potentials was observed to be 95 mV.