Optical biosensor inhibition

Biosensors based on optical fibers as transduction element have been recently reviewed by Wolfbeis [97]. Optical biosensors based on miniattaized SPR or on evanescent field monitoring are not as often found in miniaturized biosensors, especially in comparison to miniaturized electrochemical transducers, yet. Two examples will be given here a miniaturized SPR biosensor by Cullen and co-workers [98] and an evanescent based microchip biosensor by Borchers and co-workers [99]. The best-known SPR biosensor is the BIAcore device from Pharmacia Company, Sweden. It has been on the market for over a decade and is routinely used for hybridization kinetic analyses, specificity analyses, etc. Cullen and co-workers have incorporated a commercially available miniaturized SPR transducer into a field analyzer and developed a competition and inhibition assay for an estrogenic compound in water samples that function as endocrine disrupting compounds (EDCs). 

Figure 6. Standard response curve for the TNT competitive immunoassay in buffer. TNT (1-860 ng/ml) solutions were assayed using the fiber optic biosensor. The percent inhibition of the reference signal for Cy5-TNB only for each concentration is shown. The 95% confidence intervals are shown. A minimum of 3 assays were performed for each concentration with the exception of 200 ng/ml TNT.

Pesticides are another important group of pollutants that can be detected by fiber-optic chemical sensors. Since pesticides are designed to interact with biological molecules, fiber-optic biosensors are mostly used for their detection. One example is the detection of organophosphate and carbamate pesticides by monitoring their inhibition effect on the enzymatic reaction of acetylcholinesterase (AChE) with its substrate, acetylcholine. The enzyme is coimmobilized at the distal end of the fiber together with... 

The design and implementation of a portable fiber-optic cholinesterase biosensor for the detection and determination of pesticides carbaryl and dichlorvos was presented by Andreou81. The sensing bioactive material was a three-layer sandwich. The enzyme cholinesterase was immobilized on the outer layer, consisting of hydrophilic modified polyvinylidenefluoride membrane. The membrane was in contact with an intermediate sol-gel layer that incorporated bromocresol purple, deposited on an inner disk. The sensor operated in a static mode at room temperature and the rate of the inhibited reaction served as an analytical signal. This method was successfully applied to the direct analysis of natural water samples (detection and determination of these pesticides), without sample pretreatment, and since the biosensor setup is fully portable (in a small case), it is suitable for in-field use. 

Aptamer-based biosensors, also called aptasensor have gain a wide interest in the last years due to the advantages of aptamers compared to antibodies. Similar to antibodies, a variety of immobilization methods is available to bind aptamers to the sensor element. Aptasensors can be coupled to an electrochemical, optical or mass-sensitive transducer [13]. One of the successful examples for aptasensor was the detection of thrombin which was widely investigated [14]. Xiao et al. [15] have made an interesting development a redox compound (methylene blue) was inserted into the thrombin aptamer. When the target bound to the aptamer, the induced conformation change inhibited the electron transfer from the methylene blue to the electrode. This change could be detected amperometrically. 

Monitoring the amount of pesticides in water and soil is an effective way to detect the abuse of pesticides in agriculture. Because pesticides can inhibit the activity of many enzymes, such as acetylcholinesterase (AChE), butyrylcholi-nesterase (BCliE), organophosphate hydrolase (OPH), and tyrosinase (Tyr), various inhibition biosensor systems emerged in recent years as promising alternatives for in situ detection of pesticides. Modem methods for the detection of pesticides usually involve liquid or gas chromatography coupled to mass spectrometric detection (HPLC-MS, GC-MS), requiring an appropriate sample preparation (as seen in Table 8.1). However, optical and electrochemical detection methods were also developed for this purpose. 

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