Amperometric sensors diffusion layer

By changing the enzyme and mediator, the amperometric sensor in Figure 11.39 is easily extended to the analysis of other substrates. Other bioselective materials may be incorporated into amperometric sensors. For example, a CO2 sensor has been developed using an amperometric O2 sensor with a two-layer membrane, one of which contains an immobilized preparation of autotrophic bacteria. As CO2 diffuses through the membranes, it is converted to O2 by the bacteria, increasing the concentration of O2 at the Pt cathode. 

Amperometric sensors — A class of electrochemical sensors based on amperometry. A - diffusion-limited current is measured which is proportional to the concentration of an electrochemically active analyte. Preferred technique for - biosensors with or without immobilized enzymes (biocatalytic sensors). The diffusion layer thickness must be kept constant, either by continuous stirring or by means of an external diffusion barrier. Alternatively, micro electrodes can be... 

Amperometric sensors are based on the electrochemical reduction of oxygen, and are governed by the diffusion of the electroactive species through a barrier - that is, holes or porous layers 81,82], The coulometric sensors are based on a similar concept, and were developed for closed-chamber systems. Although very few amperometric and coulometric sensors have been studied to date, some relevant examples are summarized in Chapter 13. 

It is evident from the equation that potentiometric CO2 electrodes as well as amperometric O2 or H2O2 electrodes can be used as transducers. Both potentiometric and amperometric sensors have been covered by a layer of oxalate oxidase protected by a dialysis membrane (Bradley and Rechnitz, 1986 Rahni et al.f 1986a). The sensors had a pH optimum at pH 3.5-4. Diffusion control was reached at 1 U oxalate oxidase per electrode. Oxalate determination was not affected by ascorbic acid or amino acids. The hydrogen peroxide-detecting sensor (Rahni et al., 1986a) has been used to measure oxalate in urine diluted 1 40. 

The substrate generation/tip collection (SG/TC) mode with an ampero-metric tip was historically the first SECM-type measurement performed (32). The aim of such experiments was to probe the diffusion layer generated by the large substrate electrode with a much smaller amperometric sensor. A simple approximate theory (32a,b) using the well-known c(z, t) function for a potentiostatic transient at a planar electrode (33) was developed to predict the evolution of the concentration profile following the substrate potential perturbation. A more complicated theory was based on the concept of the impulse response function (32c). While these theories have been successful in calculating concentration profiles, the prediction of the time-de-pendent tip current response is not straightforward because it is a complex function of the concentration distribution. Moreover, these theories do not account for distortions caused by interference of the tip and substrate diffusion layers and feedback effects. 

Diffusion barriers are typically used where the chemical reaction responsible for sensing is slow relative to the diffusion rate of the analyte to the active portion of the sensor. As shown above, a gas-permeable membrane can act as a diffusion barrier in some cases for gas sensors. Diffusion barriers are also often used in amperometric enzyme sensors which exploit the native selectivity of an enzyme-substrate reaction to measure the concentration of the substrate. If the substrate is at a high concentration, the enzyme may become saturated, especially if the enzyme turnover rate is low. In this condition, the signal will plateau and no longer be dependent upon the substrate concentration. A diffusion barrier between the enzyme layer and the sample reduces the flux of the substrate to the enzyme and, thus, prevents saturation of the enzyme and increases the linear range of the sensor. 

R.H.-independent signal output has been achieved in thefour-probe type sensor shown in Fig. 36.4, where two additional Ag probes are inserted in the proton conductor bulk (AA) beneath the Pt electrodes. One of the Pt electrodes is covered by a layer of AA sheet, which acts as a sort of gas diffusion layer. The short-circuit current flowing between the two Pt electrodes is proportional to H2 concentration but dependent on R.H., just as in the previous amperometric sensor. On the other hand, the difference in potential between the two Ag probes (inner potential difference, AE g) with the outer Pt electrodes short-circuited is shown to be not only proportional to H2 concentration but also independent of R.H. as shown in Fig. 36.3b and Table 36.2. This mode of sensing has no precedence, and is noted as a new method to overcome the greatest difficulty in using proton conductor-based devices, i.e. their R.H. dependence. 

Amperometric sensors are small electrochemical cells consisting of two or three electrodes that are usually combined in a single body. A constant potential is applied, i.e., the sensor operates as a Faradaic cell, and a dependence of the measured current on the analyte concentration in the sample is obtained. As in ordinary amperometry, this requires a diffusion layer on the surface of the working electrode. This diffusion layer, in which the analyte concentration is depleted, arises because the analyte is consumed in the electrode reaction. In order for this depletion to occur, the electrode kinetics has to be faster than the... 

If two such electrodes are separated by a thin layer of only zirconia, the application of a potential will lead to the pumping of oxygen from the cathode to the anode. This device can be used as an amperometric sensor for oxygen if a diffusion barrier restricts the flux of oxygen to the cathode. Note that similar devices are also often used as potentiometric sensors according to the Nernst equation (i.e., the lambda-probe in cars with catalytic converters). In this case one side of the cell has to act as a reference, e.g., by using ambient air. 

The observed decrease in current is due to a slow spread of the diffusion layer out into the bulk solution, with a concomitant decrease in the concentration gradient. In practice, this process continues for ca. 100 s. after which random convection processes in the solution take over, putting an end to further movement of the diffusion layer. Waiting nearly two minutes to obtain a steady-slate current is not a particularly attractive alternative. Accordingly, in amperometric measurements for sensor applications the spread of the diffusion layer is controlled by invoking one or more of the following mechanisms ... 

Amperometric sensors are based on electrochemical reactions which are governed by the diffusion of the electroactive species through a barrier. 129 The barrier usually consists of a hole (see Figure 10.11a) or a porous neutral layer. The control of the gas inflow by an electrochemical method was proposed recently. 2 The voltage is fixed on the diffiision plateau of the I(U) curve (Figure 10.1 lb). For a reaction limited by the mass transport process, the general flux equation in a one-dimensional model is... 

In a SG/TC experiment, there are two different distance scales one scale is determined by the tip size and another one by the substrate size. If a la 1, the diffnsion layer generated by the substrate is much thicker than that at the tip electrode. The theory assnming no feedback (i.e., the substrate current nnaffected by the tip process) is applicable either at d/a 1 or when the product of the tip process does not react at the substrate. The rigorous theoretical description is difficult because (i) the moving tip stirs the substrate diffusion layer distnrbances are especially significant when the tip is an amperometric sensor and has its own diffusion layer (ii) when the substrate is large, no true steady state can be achieved and (iii) the tip blocks the diffusion to the substrate surface, and this screening effect is hard to take into acconnt. 

In amperometric sensors (Fig. 19.2 right) " raie electrode is covered by a diffusion-limiting layer (also a chamber with small holes can be used) so that the transport of oxygen to the electrode is the rate-limiting step. This is schematically shown in Fig. 19.3. When the electrode is polarised by an external voltage, the current increases as long as enough electrochemicaUy active species are present. 

In Fig. 2.10, the boundary between the enzyme-containing layer and the transducer has been considered as having either a zero or a finite flux of chemical species. In this respect, amperometric enzyme sensors, which have a finite flux boundary, stand apart from other types of chemical enzymatic sensors. Although the enzyme kinetics are described by the same Michaelis-Menten scheme and by the same set of partial differential equations, the boundary and the initial conditions are different if one or more of the participating species can cross the enzyme layer/transducer boundary. Otherwise, the general diffusion-reaction equations apply to every species in the same manner as discussed in Section 2.3.1. Many amperometric enzyme sensors in the past have been built by adding an enzyme layer to a macroelectrode. However, the microelectrode geometry is preferable because such biosensors reach steady-state operation. 

A sensor for dissolved oxygen is based on a similar principle as the C02, employing a gas-permeable membrane which selectively allows diffusion of oxygen into a thin layer of—usually—phosphate buffer. However, this sensor typically operates in amperometric mode, i.e., the signal is generated by reduction of oxygen at the platinum or gold cathode with silver chloride used as the anode. 

This type of diffusion/reaction mechanism has been treated semi-analyti-cally by Albery et al. [42, 44, 45], under steady-state conditions and its applications to amperometric chemical sensors has been described by Lyons et al. [46]. In both models, only diffusion and reaction within a boundary layer is considered, while the effect of concentration polarisation in the solution is neglected. Thus, to apply the model to an experimental system it is necessary to be able to accurately determine the concentration of substrate at the polymer/solution interface. Assuming that the system is in the steady state, the use of the rotating disc electrode allows simple determination of the substrate concentration at the interface from the bulk concentration and the experimentally determined flux using [47]... 

The Clark electrodes described above are not suitable for oxygen determination in dry gaseous samples such as air because the thin layer of electrolyte solution contained behind the membrane is prone to rapid drying. A different arrangement is therefore used for such applications. Amperometric gas sensors for oxygen (and sensors for other electroactive species in the vapor phase) usually consist of a porous PTFE membrane that bears a precious metal electrode deposited, also in porous form, directly on the backside. This keeps the diffusion length short while... 

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