Electroactive layers principles

This three-enzyme scheme suffers interference from endogenous creatine in the sample, requiring correction. Low concentrations of creatinine found in blood (<100p,moI/L) must be measured in the presence of oxidizable interfering substances, sometimes present at higher concentrations than the analyte. Special electroactive layers within the biosensor have been proposed to remove redox-active interfering substances. Since the useful life of the creatinine biosensor based on these reactions requires three enzymes to retain activity, reusable commercial biosensors for creatinine based on this measurement principle typically suffer from a short useful life of only a few days. 

Chronopotentiometry. Paunovic and Oechslin (8) measured the adsorption of peptone on lead-tin alloy electrodes using chronopotentiometric and double-layer measurements. This case is different from the adsorption of HCOOH because peptone is not an electroactive species in the conditions smdied but only blocks the surface used for the electrodeposition of lead-tin alloys from solutions containing Sn and Pb ions. Chronopotentiometric analysis is based on the following principles (7). In the absence of adsorption, the relationship between the transition time r (for reduction of Sn and Pb in this case), the bulk concentration c° of the substance reacting at the electrode, and the current I is given by the equation... 

In connection with the foregoing, double layers of course also play an important role in electroanalysis. Transfer of. say, electroactive ions through the polarized mercury-solution interface is preceded by passage through the double layer. Therefore current-potential plots depend in principle on double layer properties. Historically, it was his interest In charge-transfer and corrosion problems that induced Grahame to start his seminal double layer Investigations. 

In principle, electrochemical transducers can be used to detect the formation of a surface-bound affinity complex when the affinity-binding reaction is associated with a change in electrical properties (e.g., ion permeability or capacitance) of the layer immobilized onto the electrode surface. For example, the so-called ion-chemnel sensors detect permeabilily changes of a film immobilized on an electrode surface to an electroactive molecule, which is used as a redox marker. The formation of a surface-bound affinity complex results in a permeability change, which can be monitored by the change of cyclic voltammetric response of the redox marker. 

As previously noted (see Section 2.5), the depth reached in a given experiment can, in principle, be approached by the advance of the diffusion layer if the diffusion coefficient for the rate-determining charge transport is known. Then, at short times, only the electrochemical response of the guest molecules externally adsorbed or located in the more external layers of the crystals will be electroactive. At longer times, the observed response will reflect the contribution of molecules located deeper in the host crystals. 

Schematic diagram of an enzymatically coupled potentiometric sensor is shown in Fig.6. Its basic operating principle is simple an enzyme (a catalyst) is immobilized inside a layer into which the substrate(s) diffuse. As it does it reacts according to the Michaelis-Menten mechanism and the product(s) diffuse out of the layer, into the solution. Any other species which participate in the reaction must also diffuse in and out of the layer. Because of the combined mass transport and chemical reaction this problem is often referred to as diffusion-reaction mechanism. It is quite common in electrochemical reactions where the electroactive...

We will first outline the scope of this chapter. The emphasis is on principles and problems associated with the use of these materials rather than on providing an exhaustive review of the literature. The examples quoted are intended only as illustrations of specific aspects. In the first section we will address the general aspects of preparation of selective layers based on EPs and their interactions with chemical species present in the sample, i.e. the issue of selectivity. The specialized use of polymers as barriers is also included here. The use of electroactive polymer layers in three principal transduction sensing modes is then discussed. Each of these sections contains examples of typical applications of electroactive polymers for chemical sensing. The overview, comparisons and future directions are discussed in section 10.6. 

A bilayer-coated electrode has two electroactive films, each having different reduction potentials. The inner layer is in direct contact with the carrier electrode surface and acts as a mediator to the outer layer which is mainly in contact with the solution. Provided that the redox levels in the two layers are appropriate, the interface between the two polymer films acts analogously to a semiconductor junction as a charge rectifying junction. The method of preparing first the inner layer on the electrode and then, in a second step, the outer layer may in principle be deduced from the conventional methods already described. 

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