Stability, amperometric sensors

In general, traditional electrode materials are substituted by electrode superstructures designed to facilitate a specific task. Thus, various modifiers have been attached to the electrode that lower the overall activation energy of the electron transfer for specific species, increase or decrease the mass transport, or selectively accumulate the analyte. These approaches are the key issues in the design of chemical selectivity of amperometric sensors. The long-term chemical and functional stability of the electrode, although important for chemical sensors as well, is typically focused on the use of modified electrodes in energy conversion devices. Examples of electroactive modifiers are shown in Table 7.2. 

Figure 14. Long-term stability of the modified amperometric sensor at room temperature.

One of the major deterrents to the successful application of electroanalytical sensors has been the lack of long-term stability of the polymer films. At least three factors effect the stability of these amperometric sensors. These factors are the mode of polymer film attachment to the electrode surface (adsorption vs. covalent bonding), solubility of the film in the contacting solution, and finally, the mode of attachment of the catalyst in the polymer film (electrostatic vs. covalent). 

Glucose sensors are mainly used to determine blood glucose in clinical chemistry and diabetes home monitoring. The detection limit is around 1 pM and increases by additional diffusion barriers for undiluted whole-blood measurement and in-vivo application. Typical response times are below one min. Screen-printed electrodes are often made for single use. Entrapment of GOD in polyurethane and amperometric indication of hydrogen peroxide may result in a massively stabilized glucose sensor which may be reused more than 1000 times. 

Amperometric sensors monitor current flow, at a selected, fixed potential, between the working electrode and the reference electrode. In amperometric biosensors, the two-electrode configuration is often employed. However, when operating in media of poor conductivity (hydroalcoholic solutions, organic solvents), a three-electrode system is best (29). The amperometric sensor exhibits a linear response versus the concentration of the substrate. In these enzyme electrodes, either the reactant or the product of the enzymatic reaction must be electroactive (oxidizable or reducible) at the electrode surface. Optimization of amperometric sensors, with regard to stability, low background currents, and fast electron-transfer kinetics, constitutes a complete task. 

Miura, N., Lu, G., Ono, M. and Yamazoe, N. (1999) Selective detection of NO by using an amperometric sensor based on stabilized zirconia and oxide electrode. Solid State Ionics, 117 (3-4), 283—90. 

An amperometric biosensor for OPC pesticides based on a CNT-modified transducer and an OPH biocatalyst is described. A bilayer approach with the OPH layer on top of the CNT film was used for preparing the CNT/OPH biosensor. The CNT layer leads to a greatly improved anodic detection of the enzymatically generated p-nitrophenol product, including higher sensitivity and stability. The sensor performance was optimized with respect to the surface modification and operating conditions. Under the optimal conditions the biosensor was used to measure as low as... 

These results demonstrate that electrodes which are modified with a mediator-containing inorganic film have the combination of stability, conductivity and kinetic characteristics toward important analytes to serve as practical amperometric sensors in flow systems. As... 

Then, the electrode was used as amperometric sensor for 2-ME at —0.15 V exhibiting a linear concentration range of 30 pM-6 mM and a calculated LOD (S/N = 3) of 25 pM. No smdy of electrode long-term stability and of selectivity was performed at this stage. 

R. Garjonyte and A. Malinauskas, Operational stability of amperometric hydrogen peroxide sensors, based on ferrous and copper hexacyanoferrates. Sens. Actuators, B B56, 93—97 (1999). 

Agents for chemical bleaching rely on different types of peroxides. Potentiometric or amperometric biosensors that detect the highly specific and sensitive reaction of enzymes like katalases with their corresponding substrates can be used for on-line measurement [84]. The sensors can be manufactured with simple technologies at moderate cost, but their stability is not sufficient for integration in household appliances. 

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