Influence of graphite powder, additives and enzyme immobilization procedures on a mediatorless HRP-modified carbon paste electrode for amperometric flow-injection detection of H2O2

Cr2O3-TiO2-Modified Filter Paper-Based Portable Nanosensors for Optical and Colorimetric Detection of Hydrogen Peroxide

In the present approach, a Cr₂O₃-TiO₂-modified, portable, and biomimetic nanosensor was designed to meet the requirement of a robust and colorimetric sensing of hydrogen peroxide. Cr₂O₃-TiO₂ nanocomposites prepared via the hydrothermal method were fabricated as a transducer surface on the filter paper using the sol-gel matrix. The color on the filter paper sensor changed from green to blue upon the addition of hydrogen peroxide in the presence of TMB. This change in the color intensity was linear with the concentration of H₂O₂. RGB software was used as a color analyzing model to evaluate the optical signals. This paper-based colorimetric platform provided us with an improved analytical figure of merit with a linear range of 0.005-100 μM with 0.003 μM limit of detection. The real sample analysis and excellent anti-interference potential results proved the good analytical performance of the proposed design, providing a more promising tool for colorimetric H₂O₂ detection. Introducing Cr₂O₃-TiO₂ nanocomposite-based paper sensors, being a novel method for optical and colorimetric detection, can pave the way for the development of other sensing devices for the detection of different analytes.

Direct electrochemistry and electrocatalysis based on film of horseradish peroxidase intercalated into layered titanate nano-sheets

Intercalation of horseradish peroxidase (HRP) into layered titanate by assembling it with titanate nano-sheets (TNS) was firstly used for fabrication of enzyme electrode (HRP-TNS electrode). XRD result revealed that HRP-TNS film featured layered structure with HRP monolayer intercalated between the titanate layers. UV-vis spectra result indicated the intercalated HRP in TNS film well retained its native structure. The HRP-TNS film was uniform with porous structures which were confirmed by SEM. The immobilized HRP in the TNS film exhibited fast direct electron transfer and showed a good electrocatalytic performance to H2O2 with high sensitivity, wide linear range and low detection. The excellent electrochemical performance of the HRP-TNS electrode was attributed to biocompatibility of the titanate sheets, porous architectures of the HRP-TNS film which retained activity of HRP to large extent, avoided aggregation of HRP, provided better mass transport and allowed more HRP loading per unit area. Thus, the simple method described here provides a novel and effective platform for immobilization of enzyme in realizing direct electrochemistry and has a promising application in fabrication of the third-generation electrochemical biosensors.

Enhancement of stability of immobilized glucose oxidase by modification of free thiols generated by reducing disulfide bonds and using additives

Stability of glucose oxidase (GOD) immobilized with lysozyme has been considerably enhanced by modification of free thiols generated by reducing disulfide bonds using beta-mercaptoethanol and N-ethylmaleimide in conjunction with additives like antibiotics and salts. Thermal stability of immobilized GOD was quantified by means of the transition temperature, Tm and the operational stability by half-life t1/2 at 70 degrees C. Modification of the free thiols in the enzyme coupled with the presence of kanamycin, NaCl, and K2SO4, led to increase in Tm, to 80, 82 and 84 degrees C (compared to 75 degrees C in control) and t1/2 by 7.7-, 11- and 22-fold, respectively, indicating that this method can be effectively used for enhancing the stability of enzymes.

Optimisation of the composition of a screen-printed acrylate polymer enzyme layer with respect to an improved selectivity and stability of enzyme electrodes

Glucose oxidase (GOD) was immobilized on screen-printed platinum electrodes by entrapment in a screen printable paste polymerized by irradiation with UV-light. The influences of different additives, in particular polymers and graphite, on the sensitivity and stability of the sensor and the permeability of the enzyme layer for a possible electrochemical interferent were investigated. The chosen additives were Gafquat 755N, poly-L-lysine, bovine serum albumin (BSA), sodium dodecylsulfate (SDS), polyethylene glycol (PEG), Nafion and graphite. All additives led to increases of glucose signals, i.e. improved the sensitivity of glucose detection with Gafquat 755N, poly-L-lysine, SDS and graphite showing the strongest influences with increases by a factor 4, 6.5, 5 and 10, respectively. Ascorbic acid was used as a model interferent showing the influence of the enzyme layer composition on the selectivity of the sensor. The addition of Gafquat 755N or poly-L-lysine led to higher signals not only for glucose, but also for ascorbic acid. SDS addition already reduced the influence of ascorbic acid, which was almost completely eliminated when Nafion (5%) and PEG (10%) were added. A comparable beneficial effect on the selectivity of the sensors was also observed for the addition of 0.5% graphite. Thus, the enzyme electrodes with PEG, Nafion or graphite as additives in the enzyme layer were applied to glucose determinations in food samples and samples obtained from E. coli cultivations.

Stabilization of immobilized glucose oxidase against thermal inactivation by silanization for biosensor applications

An important requirement of immobilized enzyme based biosensors is the thermal stability of the enzyme. Studies were carried out to increase thermal stability of glucose oxidase (GOD) for biosensor applications. Immobilization of the enzyme was carried out using glass beads as support and the effect of silane concentration (in the range 1-10%) during the silanization step on the thermal stability of GOD has been investigated. Upon incubation at 70 degrees C for 3h, the activity retention with 1% silane was only 23%, which increased with silane concentration to reach a maximum up to 250% of the initial activity with 4% silane. Above this concentration the activity decreased. The increased stability of the enzyme in the presence of high silane concentrations may be attributed to the increase in the surface hydrophobicity of the support. The decrease in the enzyme stability for silane concentrations above 4% was apparently due to the uneven deposition of the silane layer on the glass bead support. Further work on thermal stability above 70 degrees C was carried out by using 4% silane and it was found that the enzyme was stable up to 75 degrees C with an increased activity of 180% after 3-h incubation. Although silanization has been used for the modification of the supports for immobilization of enzymes, the use of higher concentrations to stabilize immobilized enzymes is being reported for the first time.

Liquid chromatographic determination of residual hydrogen peroxide in pharmaceutical excipients using platinum and wired enzyme electrodes

Hydrogen peroxide (H(2)O(2)) is a chemically reactive reagent that can oxidize and degrade many pharmaceutical compounds under normal conditions. Unfortunately, H(2)O(2) is often introduced into pharmaceutical excipients during manufacturing and it may significantly affect the chemical stability of drugs in formulations. Thus, a sensitive analytical method for determination of residual H(2)O(2) in excipients is of importance in formulation development and product quality control. A liquid chromatographic system with a dual channel electrochemical detector (LCEC) was equipped with either a platinum electrode or a wired peroxidase electrode for determination of H(2)O(2). The excipient (0.1 g) was dissolved in 10 ml of mobile phase and 5 microl of the dissolved solution was directly injected. The chromatographic run time for each sample was 1 min with a detection limit of 10 ng/ml (S/N=5) using the platinum electrode and 1 ng/ml (S/N=5) using the wired enzyme coated electrode, respectively. The peak purity was assured by comparing the peak ratios at different potentials for both the standard and the samples. The H(2)O(2) levels in different batches of PVP, PEG, and other surfactants from different manufacturers were determined and the values ranged from 0 to 244 ppm. The LCEC method is exceptionally fast, accurate and convenient for quantitation of low levels of residual H(2)O(2) in pharmaceutical formulation excipients.

Mechanistic and molecular investigations on stabilization of horseradish peroxidase C

The enzyme horseradish peroxidase (HRP) shows a decreasing activity when the enzyme's substrate hydrogen peroxide is present with the degree of inactivation being dependent on the incubation time and the hydrogen peroxide concentration. Incubation times of some minutes do not inactivate the enzyme independent of the H2O2 concentration. After several hours, only 50% of the activity is found for a medium H2O2 excess, and a >100-fold excess of H2O2 completely inactivates the enzyme. Polymeric additives, in particular Gafquat, lead to higher residual activities, whereas stabilizers, such as aminopyrine, preserve the full activity. Circular dichroism (CD) measurements reveal that the enzyme structure remains more or less unchanged when hydrogen peroxide is added. Only when a 1000-fold excess of hydrogen peroxide is present are structural changes observed. UV spectra highlight that the heme group in the enzyme is affected by hydrogen peroxide in a first step. Without any prolonged incubation, a decrease of the Soret band to approximately 50% is found for low hydrogen peroxide concentrations (HRP/H2O2 from 1:1 to 1:100). Higher H2O2 concentrations lead to the formation of catalytically inactive HRP forms. Preincubation of Gafquat, which is a copolymer from vinylpyrrolidone and derivatized methyl methacrylate, with hydrogen peroxide shifts the influence of hydrogen peroxide to higher concentrations, the shift being dependent on the Gafquat concentration. This effect is not observed for other polymers, such as dextrans, but it is also found for the stabilizer aminopyrine. Extended incubation times (24 h) of HRP together with H2O2, however, lead to an at least partial recovery of the Soret band for lower H2O2 concentrations (H2O2/HRP from 1:1 to 1:100). When hydrogen peroxide is used in a >100 fold excess, the heme group is irreversibly destroyed, and even the characteristic band of cpd III is not found. Here, the presence of Gafquat only reduces the degree of destruction. Computer modeling of the interaction between the polymers and the enzyme shows no specific binding sites for the functional groups of the vinylpyrrolidone-methacrylate copolymer Gafquat or of DEAE-dextran on the enzyme, whereas for the only activating polymer, polyethylenimine clustering of binding sites is observed.

Discrete analysis of serum uric acid with immobilized uricase and peroxidase

Commercially available uricase and peroxidase have been immobilized onto alkylamine glass and arylamine glass beads respectively. A discrete method has been developed to determine uric acid in serum using immobilized uricase and peroxidase. The method is based on generation of H2O2 from serum uric acid by immobilized uricase and its measurement by a colour reaction catalyzed by immobilized peroxidase. The minimum detection limit of the method was 8 microg/0.1 ml sample. The mean analytical recovery of added uric acid in serum was 87.5%. The within and between assay coefficient of variation (C.V.) were <6.58% and <10.77% respectively. The serum uric acid in apparently healthy adults and persons suffering from different disease was found to be 25-55 microg/ml, 32+/-2.25 (range, mean+/-S.D.) and 55-200 microg/ml; 52+/-6.4 (range, mean+/-S.D.) respectively by our method. A good correlation (r = 0.8170) was obtained between the serum urate values by this method and with those obtained by commercial Enzo-kit method.

Rapid alcohol determination in plasma and urine by column liquid chromatography with biosensor detection

An enzyme based amperometric biosensor used as a selective and sensitive detection unit in column liquid chromatography for the determination of ethanol and methanol in biological fluids such as plasma and urine is described. The reagentless enzyme electrode is based on the co-immobilisation of alcohol oxidase and horseradish peroxidase in carbon paste. The selectivity of the biosensor was found to vary when four various alcohol oxidase enzyme preparations from Candida boidinii, Pichia pastoris, and Hansenula polymorpha were used in the biosensors described. High sensitivity could be obtained for a number of alcohols, organic acids, and aldehydes. Optimisation regarding the sensitivity and selectivity of the four alcohol oxidase co-immobilised biosensors are outlined. A fast and reliable liquid chromatographic separation system with a PLRP-S polymer based separation column used with a phosphate buffer as the mobile phase was optimised using the best biosensor which was based on alcohol oxidase from P. pastoris and which showed the highest turnover rate for alcohols, as the detector for the determination of ethanol and methanol in human urine and plasma samples. The selectivity and stability of the biosensor were retained by working at an applied potential of -50 mV versus Ag/AgCl, the optimal operational potential, and by the casting of a protective membrane on the electrode surface. High selectivity of the enzyme electrode was also found towards other easily oxidisable interfering species normally present in biological fluids. It was found that stable and reliable determinations of ethanol and methanol in plasma and urine could be performed with only a simple dilution and centrifugation step prior to injection into the liquid chromatographic system. An analysis time of 4 min was required for the assay, with a sample throughput of 13 samples h(-1).