Protein changes, sensor response

Reproducibility problems in the production of polymer layers as described above can sometimes occur. Since washing steps and drying procedures are often incorporated to remove loosely bound material, and changes in intensity or duration of these steps can result in different layers. Thus the amoimt of the immobilised protein will be different and reduce the reproducibility of the sensors response. The use of thick organic films produced by this method can also lead to frequency instabihty and loss of sensitivity. It is also difficult to control film thickness and homogeneity, which also effect the reproducibility of the surface produced. Weak adhesion between the polymer and the substrate can also be a problem. Methods, such as plasma polymerisation and electropolymerisation, have been developed to overcome these problems and to gain more control over the layer parameters. 

Enzymes can be used not only for the determination of substrates but also for the analysis of enzyme inhibitors. In this type of sensors the response of the detectable species will decrease in the presence of the analyte. The inhibitor may affect the vmax or KM values. Competitive inhibitors, which bind to the same active site than the substrate, will increase the KM value, reflected by a change on the slope of the Lineweaver-Burke plot but will not change vmax. Non-competitive inhibitors, i.e. those that bind to another site of the protein, do not affect KM but produce a decrease in vmax. For instance, the acetylcholinesterase enzyme is inhibited by carbamate and organophosphate pesticides and has been widely used for the development of optical fiber sensors for these compounds based on different chemical transduction schemes (hydrolysis of a colored substrate, pH changes). 

Figure 9 Response of running mean of sensor to change in protein.

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