Response in the steady state

For the measurement to be carried out properly, it is necessary to know the response dme of die biosensor. This is the time taken to reach a steady state fixim die instant of die variation in the concentration under investigation. In principle, the response time is infinitely long because of the exponential nature of the curve. A reasonable value can be found, however, by fixing an interval between the instantaneous value of the response and the theoretical final value, as a function of the required precision. The steady state is considered as obtained when this predefined interval has been covered. The response time of the biosensor gives a measure of how quickly it responds to a variation in concentration. It also defines a minimum delay, after which data can be collected with a given precision. The response time is not always used. It is often quicker to obtain the same information fiom the derivative of the response curve. In systems that use flow injection analysis, only the peak height is taken into account and the achievement of a steady state is unnecessary, also the rate of sample throughput is increased. 

ITie information decoded by the bioreceptor is converted into an electrical signal by the transducer using measuring techniques like potentiometry, amperometry, thermometry, or photometry, all of which are based on the variation of physical quantities. The mediod chosen must be simple and of a reasonable size, so that it is cheap and easy to use. This is why potentiometric and amperorttetric electrodes have been so extensively adopted. 

Potentiometry measures the difference in potential between two electrodes immersed in a solution. One of the electrodes probes the solution, while the other serves as a reference. The reference electrode has a constant and reproducible potential which is independent of its environmenL The potential of the probe electrode is the potential at the interface between the solid and liquid phases, where the oxidation and reduction reactions occur. For example, at the interface between a conducting wire and a redox system, there is an exchange of electrons between the wire and the compounds being oxidized and reduced. Equilibrium is achieved when the rates of oxidation and reduction are equal, and the composition of the solution surrounding the electrode is constant. The equilibrium potential is then given by the Nemst Law  

If the glass membrane of a pH electrode is covered with a gas-permeable hydrophobic membrane, then the electrode is sensitive to gases. An acidic or basic gas (CO2 or NH3) can cross the hydrophobic membrane and its presence in the inner solution causes variations in pH which depend upon the gas concentration. A biological component may be attached to this hydrophobic monbrane to create a biosensor. 

The use of semiconducKrr structures, such as field-effect transistors (ISFET), means that the results of electrochemical techniques can be exploited, and the problem of the strong impedances exhibited by potentiometric transducers can be solved. The pH and the concentration of ions or even gaseous compounds can be detomined by measurement of the drain current in the presence of a reference electrode. Here, the biological system is immobilized on an Si02 oxide layer, which forms the sensitive component of the transistor. 

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