5 - Measuring parameters

According to the Michaelis-Menten kinetic model of biocatalytic reactions, the reaction rate (speed) depends on the concentration of the enzyme substrate (glucose in the given example). Thus, there are two main possibilities for the electrochemical transducer application: either measuring the H₂O₂ evaluation amperometrically by its direct decomposition on the electrode surface (see animation), or measuring the O₂ consumption of the biocatalytic reaction when the glucose diffuses through a membrane containing the immobilized enzyme. The latter process can be realized using a dual Clark-type amperometric oxygen sensor, as shown in the Figure.

DNA chip (Biochip)

DNA-sensors are the most sophisticated transducers with molecular receptors. DNA molecules are specific to their counterparts and to proteins built-up of amino-acid sequences according to the DNA "encoding". The basis for the nucleic-acid hybridization is the DNA base pairing namely the strong interaction between two complementary nucleic acid strands. Once fabricated, the DNA chip probe arrays are ready for hybridization. The nucleic acid to be analyzed - the target - is isolated and labeled with a fluorescent reporter group. The labeled target is then incubated with the array using the fluidics station. After the hybridization reaction is complete, the array is inserted into the scanner, where patterns of hybridization are detected. The hybridization data are collected as light emitted from the fluorescent reporter groups already incorporated into the target, which is now bound to the probe array. Probes that perfectly match the target produce stronger signals than those that have mismatches. Since the sequence and position of each probe on the array are known, by complementarity, the identity of the target nucleic acid applied to the probe array can be determined.

Immunosensors

Immunosensors are processing sequences used in analytical chemistry for detecting objects by means of immune reactions. A useful method of the immunosensors is to immobilize receptor (antigen or antibody) molecules on a solid state surface and follow the effect of AB-AG complex formation. This is also adapted for immunosensors: the receptors are immobilized on the transducer surface and the complex formation gives the transduction effect. Immunosensors can be developed by incorporating either antigens or antibodies on the surface. The labeled antigens added to the measurand analyte in a well known concentration are competing for the receptor sites with nonlabeled antigens, the concentration of which has to be determined.

Microbial biosensors incorporate microorganisms (algae, bacteria, yeast, and funghi) as sensing elements that specifically recognizes species of interest. The concentration of these species is related to the assimilation capacity of the microorganisms, measured as a change in respiration activity (metabolism) of them. The transducers follow these changes, similarly to the enzymatic sensors, by measuring oxygen consumption or reaction product formation. A number of different transducer types can be applied, as it was shown already in connection with enzymatic biosensors.