Capacitive affinity sensor

A concept for a generic biosensor is introduced that is capable of measuring small molecules in environmental matrices. This sensor, the capacitive affinity sensor, makes use of a combination of antibody and microelectronic technologies. Sensor operation is demonstrated using sensors for hydrocortisone and pentachlorophenol as examples. Because of the mature nature of the critical technologies, sensors based on this design should be commercially available in the near future. 

Much of the current agricultural abundance of the United States is due to the availabilty of chemical means of pest control. However, pesticides also represent a considerable threat to the environment when they are used improperly. For this reason the ability to measure pesticide residues at low concentrations in environmental matrices such as surface and groundwaters and soils is of great importance. In this paper we will describe a concept for a generic biosensor, the capacitive affinity sensor, capable of rapidly determining the concentration of many types of small molecules in the environment. 

As discussed above the key to any biosensor is in the transduction mechanism. The mechanism of the capacitive affinity sensor is shown in Figure 2. The sensor consists of a planar capacitor composed of interdigi-tated metal fingers on an insulating substrate. This structure is then coated with a thin layer of a passivation material. The role of the passivation material is to protect the metal structure from deterioration from contact with the solution. A sample of the analyte or an analog retaining the ability to bind an antibody to the analyte is covalently bound to the... 

Figure 2. Transduction mechanism of the capacitive affinity sensor.

Figure 3. Decrease in capacitance on antibody binding to a pentachlorophenol capacitive affinity sensor.

Figure 4. Increase in capacitance when a hydrocortisone capacitive affinity sensor is exposed to hydrocortisone.

In conclusion, we have presented a concept for a generic sensor, the capacitive affinity sensor, based on a marriage of antibodies and microelectronics. Development of this technology is continuing with the expectation of producing commercially viable sensors for a wide variety of analytes in the near future. 

In principle, electrochemical transducers can be used to detect the formation of a surface-bound affinity complex when the affinity-binding reaction is associated with a change in electrical properties (e.g., ion permeability or capacitance) of the layer immobilized onto the electrode surface. For example, the so-called ion-chemnel sensors detect permeabilily changes of a film immobilized on an electrode surface to an electroactive molecule, which is used as a redox marker. The formation of a surface-bound affinity complex results in a permeability change, which can be monitored by the change of cyclic voltammetric response of the redox marker. 

During the last decade the number of application of MIP-based sensors has increased dramatically. The high selectivity and affinity of MIPs for target analytes make them ideal recognition elements in the development of sensors. Capacitive (Panasyuk etal., 2001), conductimetric (Piletsky et al., 1995), field effect (Lahav et al., 2004), amper-ometric (Kritz and Mosbach, 1995), and voltammetric (Pizzariello et al., 2001), electrochemical transduction systems have been used. Sensors based on conductimetric transduction have been developed by Piletsky et al. (1995) for the analysis of herbicides. A system using a TiC>2 sol-gel system, and with a linear range of 0.01-0.50 mg L-1 for atrazine, without interference of simazine, and chloroaromatic acids has been described by Lahav et al. (2004). 

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