The polycrystalline fluoride ion-selective electrode and horseradish peroxidase—an alternative electrochemical biosensor?

Tetra(p-tolyl)borate-functionalized solvent polymeric membrane: a facile and sensitive sensing platform for peroxidase and peroxidase mimetics

The determination of peroxidase activities is the basis for enzyme-labeled bioaffinity assays, peroxidase-mimicking DNAzymes- and nanoparticles-based assays, and characterization of the catalytic functions of peroxidase mimetics. Here, a facile, sensitive, and cost-effective solvent polymeric membrane-based peroxidase detection platform is described that utilizes reaction intermediates with different pKa values from those of substrates and final products. Several key but long-debated intermediates in the peroxidative oxidation of o-phenylenediamine (o-PD) have been identified and their charge states have been estimated. By using a solvent polymeric membrane functionalized by an appropriate substituted tetraphenylborate as a receptor, those cationic intermediates could be transferred into the membrane from the aqueous phase to induce a large cationic potential response. Thus, the potentiometric indication of the o-PD oxidation catalyzed by peroxidase or its mimetics can be fulfilled. Horseradish peroxidase has been detected with a detection limit at least two orders of magnitude lower than those obtained by spectrophotometric techniques and traditional membrane-based methods. As an example of peroxidase mimetics, G-quadruplex DNAzymes were probed by the intermediate-sensitive membrane and a label-free thrombin detection protocol was developed based on the catalytic activity of the thrombin-binding G-quadruplex aptamer.

Optical fluoride sensor based on monomer-dimer equilibrium of scandium(III)-octaethylporphyrin in a plasticized polymeric film

A fluoride-selective optical sensor based on scandium(III)-octaethylporphyrin (Sc(III)OEP) as an ionophore within a plasticized PVC film is described. The presence of fluoride ion in the aqueous sample phase increases the formation of a difluoro-bridged Sc(III)OEP dimer species in the polymer film. The ability of the Sc(III) porphyrin to form the dimeric structure in the presence of fluoride is confirmed by UV-vis spectroscopy and X-ray crystallography. For more practical sensing applications, a pH chromoionophore (ETH 7075) is added to the plasticized PVC film along with Sc(III)OEP and the observed optical response is based on coextraction of protons with sample phase fluoride to create the dimeric porphyrin and a protonated chromoionophore species. The selectivity pattern observed is F- >> ClO4(-), SCN-, NO3(-) > Br-, Cl-. Only organic salicylate is a significant interferent. Fast and reversible fluoride response is observed over the range of 10(-4) to 10(-2) M fluoride, allowing use of the sensing film in a waveguide configuration for flow-injection measurements.

Thermostabilized chemical derivatives of horseradish peroxidase

Horseradish peroxidase finds a variety of uses in analysis, immunology, organic synthesis, and biosensors. Although moderately stable, its applicability to biosensors and other fields would be greatly enhanced if it could be made yet more stable. Appropriate chemical modification can substantially stabilize enzymes. Here we describe the use of bis-imidates and of bis-succinimides to modify free amino groups of commercial horseradish peroxidase under mild conditions of pH and temperature. Imidates yielded a marginal stabilization. Some of the succinimide derivatives, however, are much more thermostable than the native enzyme. Apparent half-lives indicate stabilizations of 6- to 23-fold, depending on the bis-succinimide used. These modifications preserve the carbohydrate side chains for subsequent reaction or immobilization.

The rapid potentiometric detection of catalase positive microorganisms

The rate of fluoride ion release from the enzymatic cleavage of fluoride ion from 4-fluorophenol by horseradish peroxidase, in the presence of hydrogen peroxide, was measured using a fluoride ion selective electrode. Monitoring the utilisation of hydrogen peroxide by catalase (intracellularly present in almost all aerobic microorganisms) in the presence of 4-fluorophenol demonstrated the inhibition of the enzyme. Horseradish peroxidase appeared to impart a partial protective mechanism of this inhibition. The development of a sequential assay demonstrated the applicability of the proposed method in the assessment of aerobic microorganism numbers. The judicious variation of three parameters, the length of incubation, the concentration of the primary substrate (hydrogen peroxide) and the indicator enzyme activity (horseradish peroxidase), affected both the detection limit and the sensitivity of the assay. Typically with a 15 minute incubation, a detection limit for catalase activity of 1.5 x 10(-6) Uml-1 was obtained together with a sensitivity of 2.42 mumol l-1 s-1 per decade change in activity. Application of the developed catalase assay to the detection of Escherichia coli achieved a detection limit of 1 x 10(2) colony forming units (cfu) ml-1 with a sensitivity of 3.26 mumol l-1 s-1 per decade change in intact microorganisms. By lysis of the microorganisms the detection limit was further reduced to less than 10 cfu ml-1, indicating the future possibilities of the assay.