Enzymatic Mass Sensors

It is generally true that catalytic selectivity is not a good basis for mass sensors. Propose a scheme in which a highly selective interaction between the enzyme and the substrate could be used to detect interaction as a change in mass. 

---

The geometry shown here corresponds to a semi-infinite planar diffusion. Other geometries (e.g., radial geometries) typical for microsensors can be used. The enzyme-containing layer is usually a hydrogel, whose optimum thickness depends on the enzymatic reaction, on the operating pH, and on the activity of the enzyme (i.e., on the Km). Enzymes can be used with nearly any transduction principle, that is, thermal, electrochemical, or optical sensors. They are not, however, generally suitable for mass sensors, for several reasons. The most fundamental one is the fact... 

The interplay of mass transfer, partition and enzymatic substrate conversion determines the dynamic measuring range, response time, and accessibility towards interferences of enzyme sensors. New principles for designing the analytical performance by coupled enzyme reactions are presented in this paper ... 

The type of molecular recognition reaction determines the form of the transducer used (Table 5.3). Enzymatic reactions often involve an electron transfer. This electrical activity can be measured with amperometric, potentiometric or conductometric sensors. If the bioreaction includes the generation of H+ or OH ions, then a pH sensitive dye in combination with an optical device can be used. For antibody-antigen binding, the mass change on the surface of the transducer can be detected with a piezoelectric device. Exothermic or endothermic reactions can be followed with a temperature sensor. 

Schematic diagram of an enzymatically coupled potentiometric sensor is shown in Fig.6. Its basic operating principle is simple an enzyme (a catalyst) is immobilized inside a layer into which the substrate(s) diffuse. As it does it reacts according to the Michaelis-Menten mechanism and the product(s) diffuse out of the layer, into the solution. Any other species which participate in the reaction must also diffuse in and out of the layer. Because of the combined mass transport and chemical reaction this problem is often referred to as diffusion-reaction mechanism. It is quite common in electrochemical reactions where the electroactive...

Detection limits for enzyme-based potentiometric biosensors are governed by the iimate detection limits of the membrane electrode toward the product of the enzyme reaction, as well as the kinetics of the enzymatic reaction (Michaelis-Menten constant and turnover number of the enzyme), and mass transfer rate of substrate into the enzymatic layer. Carr and Bowers developed in detail the theory of how such parameters affect the response curves of such sensors [48]. Generally, enzyme-based potentiometric biosensors respond to target substrates over the concentration range of 0.01 mM to 10 mM. In the case of the urea example mentioned earlier, limited selectivity of the nonactin-based ammonium membrane electrode over potassium ions k ... 

E-tongues can be found in different versions according to the detectors employed electrochemical (potentiometric, voltammetric, amperometric, impedimetric, or conduc-timetric), optical, mass and enzymatic sensors (biosensors). One example is a taste sensor made by potentiometric electrodes coated with a lipid layer, another one is a taste analyzer made by ChemFET. 

A schematic representation of an enzyme sensor is given in Figure 4.1. The sensitive surface of the transducer is in contact with an enzymatic layer, and it is assumed that there is no mass transfer across this interface. The external surface of the enzymatic layer is immersed in a solution containing the substrate under study. The substrate migrates towards the interior of the layer and is converted into reaction products when it reacts with the immobilized enzyme. In order to achieve rapid equilibrium of concentrations, the enzymatic membrane must be as thin as possible. The solution must also be well stirred to ensure a constant supply of substrate. In summary, the different steps during the operation of an enzyme sensor are ... 

---