Peroxidase-based amperometric sensor of hydrogen peroxide generated in oxidase reaction: Application to creatinine and creatine assay

Sensitive Detection of Serum Creatinine Based on β-Cyclodextrin-Ferrocenylmethanol Modified Screen-printed Electrode

Ferrocenylmethanol (Fc-OH) is included in β-cyclodextrin (β-CD) to form the β-CD-Fc-OH complex by host-guest supramolecular interaction. β-CD dissociates from the β-CD-Fc-OH complex due to the conversion of Fc-OH to Fc⁺-OH under a stimulus of oxidant. In our study, Fc-OH is oxidized after a series of enzymatic reactions of creatinine, which blocks the other means for oxidation of Fc-OH. And the background noise is reduced for testing for serum creatinine (sCr). The chronoamperometry signal for creatinine (with a constant potential -0.3 V vs. Ag/AgCl) increases linearly in the 1 - 1000 μM range, with a limit of detection as low as 0.5 μM. The amperometric potential of -0.3 V greatly prevents the interference of various redox substances in serum. The biosensor was used to test 120 clinical specimens and the results showed a linear correlation with the biochemical analyzer (R² = 0.9885). The biosensor could be applied to clinical trials and offers good prospects for clinical sCr detection.

Biosensing methods for determination of creatinine: A review

Creatinine is a metabolic product of creatine phosphate in muscles, which provides energy to muscle tissues. Creatinine has been considered as indicator of renal function specifically after dialysis, thyroid malfunction and muscle damage. The normal level of creatinine in the serum and its excretion through urine in apparently healthy individuals is 45-140 μM and 0.8-2.0 gm/day respectively. The level of creatinine reaches >1000 μM in serum during renal, thyroid and kidney dysfunction or muscle disorder. A number of conventional methods such as colorimetric, spectrophotometric and chromatographic are available for determination of creatinine. Besides the advantages of being highly sensitive and selective, these methods have some drawbacks like time-consuming, requirement of sample pre-treatment, high cost instrumental set-up and skilled persons to operate. The sensors/biosensors overcome these drawbacks, as these are fast, easy, cost effective and highly sensitive. This review article describes the classification, operating principles, merits and demerits of various creatinine sensors/biosensors, specifically nanomaterials based biosensors. Creatinine biosensors work optimally within 2-900 s, potential range 0.1-1.0 V, pH range 4.0-10.0, temperature range 25-35 °C and had linear range, 0.004-30000 µM for creatinine with the detection limit between 0.01.01 µM and 520 µM. These biosensors measured creatinine level in sera and urine samples and had storage stability between 4 and 390 days, while being stored dry at 4 °C. The future perspective for further improvement and commercialization of creatinine biosensors are discussed.

Flat-chip microanalytical enzyme sensor for salivary amylase activity

It is considered that measurement of salivary alpha-amylase activity is a useful tool for evaluating the sympathetic nervous system. The purpose of this research is to demonstrate a new design of a flat-chip microanalytical enzyme sensor (flat-chip sensor) for salivary amylase activity as a Micro-Electro-Mechanical Systems (MEMS), which may be used for wearable analytical systems. To meet this purpose, the biosensor needs to be miniaturized and to possess high-sensitivity. A pre-column and a flat-enzyme electrode were incorporated in a flow cell of volume 25.7 ml. In order to miniaturize the flow cell, two enzymatic membranes containing maltose phosphorylase obtained from Enterococcus hirae (MP membrane) and glucose oxidase and peroxidase (GOD-POD membrane) were immobilised on the same planar surface. As a result, a flat-chip sensor incorporating a flow cell as small as a C battery was produced. The optimum conditions of three parameters of the fabricated flat-chip sensor, the immobilising method of the enzymatic membrane, dropping volume of the mixed enzymatic solution and flow rate of the sample solution, were examined. An analytical system for 0-190 kU/l amylase activity with R(2) of 0.97 was fabricated with a sample volume of 50 microl. This research indicates the possibility of a wearable biosensor for continuous monitoring of salivary amylase activity.

Unusual response in mediated biosensors with an oxidase/peroxidase bienzyme system

Mediated biosensors consisting of an oxidase and peroxidase (POx) have attracted increasing attention because of their wide applicability. However, since most of oxidases utilize artificial electron acceptors in place of dioxygen, the competition between O2 and the electron acceptor in the mediated sensors is anticipated. This has been evidenced with a glucose oxidase (GOx)- and POx-coentrapped and ferrocene-embedded carbon paste electrode, which exhibits peak-shaped current-time curves at increased concentrations of glucose and also gives a peak-shaped calibration curve. Digital simulation has been applied to clarify the cause of such unusual responses, by taking into account the ping-pong enzyme kinetics on two- and three-substrate models for POx and GOx, respectively. The simulation has well reproduced such unusual responses and has clearly revealed that the depletion of O2 in the enzyme layer is the most important factor responsible for such unusual responses. To overcome such a drawback of oxidase/POx bienzyme sensors, it is expected to be essential to decrease the rate of the oxidase reaction. In contrast, increase in the POx activity is useful to improve the sensitivity. According to the simulation-based expectation, the GOx and POx concentrations in the bienzyme sensor are adjusted to exhibit normal behavior with high sensitivity.

Kinetic analysis and mechanistic aspects of autoxidation of catechins

A peroxidase-based bioelectrochemical sensor of hydrogen peroxide (H(2)O(2)) and a Clark-type oxygen electrode were applied to continuous monitoring and kinetic analysis of the autoxidation of catechins. Four major catechins in green tea, (-)-epicatechin, (-)-epicatechin gallate, (-)-epigallocatechin, and (-)-epigallocatechin gallate, were used as model compounds. It was found that dioxygen (O(2)) is quantitatively reduced to H(2)O(2). The initial rate of autoxidation is suppressed by superoxide dismutase and H(+), but is independent of buffer capacity. Based on these results, a mechanism of autoxidation is proposed; the initial step is the one-electron oxidation of the B ring of catechins by O(2) to generate a superoxide anion (O(2)(*-)) and a semiquinone radical, as supported in part by electron spin resonance measurements. O(2)(*-) works as a stronger one-electron oxidant than O(2) against catechins and is reduced to H(2)O(2). The semiquinone radical is more susceptible to oxidation with O(2) than fully reduced catechins. The autoxidation rate increases with pH. This behavior can be interpreted in terms of the increase in the stability of O(2)(*-) and the semiquinone radical with increasing pH, rather than the acid dissociation of phenolic groups. Cupric ion enhances autoxidation; most probably it functions as a catalyst of the initial oxidation step of catechins. The product cuprous ion can trigger a Fenton reaction to generate hydroxyl radical. On the other hand, borate ion suppresses autoxidation drastically, due to the strong complex formation with catechins. The biological significance of autoxidation and its effectors are also discussed.

A planar amperometric creatinine biosensor employing an insoluble oxidizing agent for removing redox-active interferences

A planar microchip-based creatinine biosensor employing an oxidizing layer (e.g., a PbO2 film), where interfering redox-active substances are broken (i.e., oxidized) to redox-inactive products, was developed to facilitate the microfabrication of the sensor and to provide improved, reliable determination of creatinine in physiological samples. The feasibility of using hydrophilic polyurethanes in permselective barrier membranes for creatinine biosensors and the effect of adding a silanizing agent (adhesion promoter) on the sensor performance (e.g., sensitivity, stability, and lifetime) are described. The proposed creatinine microsensor with a three-layer configuration, i.e., enzyme, protecting, and oxidizing layers, exhibits good electrochemical performance in terms of response time (t95% = 98 s at 100-->200 microM creatinine change), linearity (1-1000 microM, r = 0.9997), detection limit (0.8 microM), and lifetime (approximately 35 days). The creatinine biosensor devised in a differential sensing arrangement that compensates the erroneous results from creatine is considered to be suitable for assay of serum specimens.

Comparison of photovoltaic behaviors for horseradish peroxidase and its mimicry by surface photovoltage spectroscopy

Surface photovoltage spectroscopy (SPS) was chosen to study the photovoltaic behavior of horseradish peroxidase (HRP), hemin and immobilized hemin (poly(NIPAAm/MBA/hemin)). Different photovoltaic behaviors were observed in these three systems. In air, similar SPS curves were found for HRP and poly(NIPAAm/MBA/hemin) with different response intensities. However, poly(NIPAAm/MBA/hemin) showed a wider changing range upon increasing the positive and negative bias to 1.0 V. The SPS of hemin showed a total different behavior when an external positive potential was applied. In vacuum, clearly different photovoltaic behaviors were found. Moreover, the response value decreased when HRP was exposed to O2, the SPS intensity was different from that in air, and could be altered by changing the external biases. On the other hand, the SPS could not be changed before and after poly(NIPAAm/MBA/hemin) was exposed to O2. These differences may result from different chemical microenvironments for hemin in HRP versus that in poly(NIPAAm/MBA/hemin). It could be concluded that H2O and O2 were important factors affecting the photovoltage response in HRP, but only H2O played this important role in poly(NIPAAm/MBA/hemin).

Biomedical application of immobilized enzymes

Reports on chemical immobilization of proteins and enzymes first appeared in the 1960s. Since then, immobilized proteins and enzymes have been widely used in the processing of variety of products and increasingly used in the field of medicine. Here, we present a review of recent developments in immobilized enzyme use in medicine. Generally speaking, the use of immobilized enzyme in medicine can be divided into two major categories: biosensors and bioreactors. A brief overview of the evolution of the biosensor and bioreactor technology, of currently existing applications of immobilized enzymes, of problems that researchers encountered, and of possible future developments will be presented.

Role of 2-amino-3-carboxy-1,4-naphthoquinone, a strong growth stimulator for bifidobacteria, as an electron transfer mediator for NAD(P)(+) regeneration in Bifidobacterium longum

2-Amino-3-carboxy-1,4-naphthoquinone (ACNQ) is a novel growth stimulator for bifidobacteria. The role of ACNQ as a mediator of the electron transfer from NAD(P)H to dioxygen (O(2)) and hydrogen peroxide (H(2)O(2)), proposed in our previous paper, was examined using the cell-free extract and whole cells of Bifidobacterium longum. Continuous monitoring of ACNQ, O(2) and H(2)O(2) by several amperometric techniques has revealed that ACNQ works as a good electron acceptor of NAD(P)H diaphorase and that the reduced form of ACNQ is easily autoxidized and also acts as a better electron donor of NAD(P)H peroxidase than NAD(P)H. The generation of H(2)O(2) by B. longum under aerobic conditions is effectively suppressed in the presence of ACNQ. These ACNQ-mediated reactions would play roles as NAD(P)(+)-regeneration processes. The accumulation of ACNQ in the cytosol has been also suggested. These characteristics of ACNQ seem to be responsible for the growth stimulation of bifidobacteria. Vitamin K(3), which has an extremely low growth-stimulating activity and was used as a reference compound, exhibits much lower activity as an electron transfer mediator. The difference in the activity is discussed in terms of the redox potential and partition property of the quinones.

Amperometric determination of high-density lipoprotein cholesterol using polyethylene glycol-modified enzymes and a peroxidase-entrapped electrode

A peroxidase-entrapped and ferrocene-embedded carbon paste (POD-Fc-CP) electrode allows a highly sensitive detection of H2O2 at levels as low as 10 nmol/L with practically no interference by coexisting substances, turbidity or coloration of samples. The electrode was applied to the amperometric determination of high-density lipoprotein (HDL)-cholesterol in a very small volume (1-2 microL) using polyethylene glycol (PEG)-modified cholesterol esterase and cholesterol oxidase without prior precipitation or separation of HDL. PEG-modified enzymes exhibit a selective activity toward HDL-cholesterol in the presence of dextran sulphate and MgCl2 to generate H2O2. The HDL-cholesterol concentrations of human serum samples determined by this method showed a good correlation with those determined by an ordinary spectrophotometric method using PEG-modified enzymes and peroxidase or by a conventional precipitation method.