Amperometric sensors long-term stability

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). 

Long Term Performance of Sensors. The sensors showed excellent long term stability stored in buffer at 4 °C. Table 1 shows the long term performance of four typical biosensors. The usual current response to 6 mM glucose for a new biosensor is about 2 fiA. The precision of the absolute current response to 6 mM glucose remains within 15 % for up to four months. This level of precision is unusual for amperometric biosensors. Normally, variations in current are compensated by calibration. The sensitivity of the biosensors, as indicated by the slope of the response, is also stable. This stability is due to the film which maintains the ferrocene within the sensing layer. Biosensors with adsorbed mediator and immobilized enzyme but without film are not stable for any period of... 

For the stationary registration, long-term stability of the sensor or analytical unit is the dominant criterion, while moving sensors have to be optimized for minimum response times. For example, the fast response time of membrane covered amperometric oxygen sensors and long-term stability are mutually exclusive. The sensors can only be optimized for one or the other criterion. 

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. 

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. 

---