Short communication

A solid-state immobilized enzyme polymer membrane microelectrode for measuring lactate-ion concentration

We present a brief overview of bioelectrocatalytic devices for in vitro health applications, including food safety and environmental analysis, focusing on microelectrode- and microfluidic-based biosensors, paper-based point-of-care devices and wearable biosensors. The main hurdles and future perspectives are discussed. We then consider the role of electron transfer between a biocatalyst and an electrode in biosensor design. Brief descriptions of indirect, direct and mediated mechanisms are given. The principal strategies, as well as recent developments for modulation of electron transfer in biocatalytic systems are summarised. In conclusion, we highlight some of the challenges associated with improving these redox systems.

The mediation of oxidases glucose oxidase (GOx), lactate oxidase (LOx) and cholesterol oxidase (ChOx) by a new electron shuttling mediator, unsubstituted phenothiazine (PTZ), was studied. Cyclic voltammetry and rotating-disk electrode measurements in nonaqueous media were used to determine the diffusion characteristics of the mediator and the kinetics of its reaction with GOx, giving a second-order rate constant of 7.6×10³–2.1×10⁴ M⁻¹ s⁻¹ for water–acetonitrile solutions containing 5–15% water. These values are in the range reported for commonly used azine-type mediators, indicating that PTZ is able to function as an efficient mediator. PTZ and GOx, LOx and ChOx were successfully co-immobilised in sol–gel membrane on a screen-printed electrode to construct glucose, lactate and cholesterol biosensors, respectively, which were then optimised in terms of stability and sensitivity. The electrocatalytic oxidation responses showed a dependence on substrate concentration ranging from 0.6 to 32 mM for glucose, from 19 to 565 mM for lactate and from 0.015 to 1.0 mM for cholesterol detection. Oxidation of substrates on the surface of electrodes modified with PTZ and enzyme membrane was investigated with double-step chronoamperometry and the results showed that the PTZ displays excellent electrochemical catalytic activities even when immobilised on the surface of the electrode.

This chapter examines the development of enzymatic biofuel cells over the last four decades. Like traditional PEM fuel cells, biofuel cells consist of an anode and cathode separated by a membrane. The difference is that biofuel cells eliminate the dependence on precious metal catalysts by replacing them with biological catalysts. Biofuel cells can be categorized as microbial fuel cells and/or enzymatic biofuel cells. Microbial fuel cells utilize living microorganisms to oxidize the fuel, whereas enzymatic fuel cells employ isolated enzymes. A comparison of enzymatic biofuel cells to traditional fuel cells is presented and the types of enzymes employed at the anode and cathode of biofuel cells, along with strategies for immobilization of those enzymes at electrode surfaces are discussed. A detailed comparison of mediated electron transfer and direct electron transfer, along with discussion of the advantages and disadvantages of both types of electron transfer mechanisms are also discussed in the chapter. Furthermore, the chapter describes the importance of metabolic pathways in enzymatic biofuel cell development and fuel cell design and engineering issues for enzymatic biofuel cells.

This chapter examines the development of enzymatic biofuel cells over the last four decades. Like traditional PEM fuel cells, biofuel cells consist of an anode and cathode separated by a membrane. The difference is that biofuel cells eliminate the dependence on precious metal catalysts by replacing them with biological catalysts. Biofuel cells can be categorized as microbial fuel cells and/or enzymatic biofuel cells. Microbial fuel cells utilize living microorganisms to oxidize the fuel, whereas enzymatic fuel cells employ isolated enzymes. A comparison of enzymatic biofuel cells to traditional fuel cells is presented and the types of enzymes employed at the anode and cathode of biofuel cells, along with strategies for immobilization of those enzymes at electrode surfaces are discussed. A detailed comparison of mediated electron transfer and direct electron transfer, along with discussion of the advantages and disadvantages of both types of electron transfer mechanisms are also discussed in the chapter. Furthermore, the chapter describes the importance of metabolic pathways in enzymatic biofuel cell development and fuel cell design and engineering issues for enzymatic biofuel cells.

The principle of the signal amplification based on substrate recycling and mediated regeneration of uricase was applied to a novel disposable amperometric uric acid (UA) strip. The sensitivity was much improved over a batch method using dithiothreitol and the detection limit of uric acid was found to be down to 3 × 10⁻⁶ mol L⁻¹. Thinking of using electrochemically active electron acceptors (potassium ferricyanide) to which the enzyme can donate electrons, the UA strip can apply an electrode potential as low as possible to eliminate the interference. This chemically amplified system is very useful for the direct assay of uric acid in whole blood without complicated pretreatment of the samples, because this sensor has the potential to detect the excellent sensitivity and selectivity. Good correlation was observed between this strip and conventional spectrophotometry.

A disposable amperometric biosensor for monitoring the blood ketones has been developed. It can be used to determine the concentration of β-hydroxybutyrate without pretreatment of blood samples. Enzymes are immobilized on screen-printed carbon electrodes. For immobilizing the enzymes, first a layer of hydrophilic gel CMC was printed on the screen-printed carbon electrodes. β-Hydroxybutyrate dehydrogenase, cofactor NAD and electron transfer mediator, potassium ferricyanide are immobilized on the electrode surface by adsorption. Experimentally the compositions of β-hydroxybutyrate dehydrogenase, potassium ferricyanide, NAD and other components are optimized. Amperometry performed by a handheld meter is used for the determination of β-hydroxybutyrate concentration. Two microliters of whole blood or serum is needed for one test, and the response time of the sensor is 50 s. Experimental results show that the responses of the sensors have relevant good linearity in the range 1.5–550 mg L⁻¹. The testing results by the biosensors and the routine hospital blood analyzer are agreed closely.