FUTURE TRENDS Immunoassay is a mature analytical technology with broad application to pesticide
4.5 FUTURE TRENDS Immunoassay is a mature analytical technology with broad application to pesticide
analysis. Extensive fundamental investigations as well as technical improvements will make immunoassay methods more powerful tools for the identification and determination of a variety of pesticides. New breakthroughs in the development and application of immunoassays will result from the integration of future state- of-the-art research in several key areas including antibody production, new platforms and detection systems, and nanotechnology.
Future research that may enhance the use of immunoassays and immunosensors for pesticide analysis is the development of novel antibodies for individual pesticide compounds. This includes the design and synthesis of new haptens using the latest concepts and techniques, better understanding and control of the combination of hapten molecules and macromolecular carriers, and improving the efficiency of
116 Analysis of Pesticides in Food and Environmental Samples existing laboratory procedures to increase the yield of antibodies having the desired
characteristics. Molecularly imprinted polymers (MIPs) and aptamers are emerging as possible reagents (i.e., artificial antibodies) for pesticide immunoassays and immunosensors. These reagents have the potential to provide large amounts of reagents for the development of methods and to support their widespread use. Some MIP-based affinity separation methods and biosensors have already been developed for the extraction and determination of pesticides in aqueous samples. Aptamers are artifi- cial nucleic acid ligands that can be generated to detect biomacromolecules, such as proteins, and small molecules, such as amino acids, drugs, and pesticides. Currently, aptamer-based bioanalytical methods are mainly employed for clinical applications. Additional studies of molecular recognition-based MIPs and aptamers could facili- tate the development of more cost-effective methods including immunoaffinity separation techniques for pesticides.
Future research may also be directed to new immunoassay formats. The devel- opment of microimmunoassays, using compact discs (CDs) as an analytical plat- form, has recently drawn much attention from researchers. An indirect competitive procedure is conducted on the polycarbonate surface of a CD and a modified CD reader performs as a laser scanner for the detection of microscopic reaction products [93 –95]. These test systems hold promise for the simultaneous determination of multiple pesticide residues in environmental samples in a rapid and cost-effective format. New platforms may also be integrated with new labels such as more robust enzymes or highly sensitive visualization techniques, such as laser-induced fluores- cence detection (LIF) to produce even lower limits of detection.
Nanotechnology is a rapidly growing discipline of scientific research and is applied to a wide variety of fields. Nanomaterials with dimensions of <100 nm have physical and chemical properties that make them attractive for many applica- tions requiring high strength, conductivity, durability, and reactivity. The application of nanotechniques in immunoassays is also of great interest to researchers [93,96]. New detection strategies based on gold and silver particles have been successfully demonstrated for immunoassay labeling to meet the needs of diverse detection methods. These particles have been used for various techniques such as scanning and transmission electron microscopy, Raman spectroscopy, and sight visualization due to their easily controlled size distribution, and long-term stability and compati- bility with biomacromolecules.
Initial studies on nanoparticle-labeled microfluidic immunoassays have shown their unique advantages over conventional immunoassay formats for the detection of small molecules, macromolecules, and microorganisms. Submicron-sized striped metallic rods intrinsically encoded through differences in reflectivity of adjacent metal stripes have been used in autoantibody immunoassays. These bar-coded particles act as supports with antigens attached to the surface providing a permanent tag for the tracking of analyte [97].
Nanomaterials including gold, zirconia (ZrO 2 ), and carbon nanotubes have been applied as biosensors for monitoring OP pesticides [76,77,98]. An optical sensor based on fumed silica gel functionalized with gold nanoparticles has also been reported for OP pesticides [98]. Nanoparticles possess extraordinary optical
Immunoassays and Biosensors 117 properties that may offer alternative strategies for the development of optical sensors.
An electrochemical sensor for detection of OP pesticides has been developed using ZrO 2 nanoparticles as selective sorbents, possessing a strong affinity for the phos- phoric group. The nitroaromatic OPs strongly bind to the ZrO 2 surface. A square- wave voltammetric analysis was used to monitor the amount of bound OP pesticide. Another sensitive flow-injection amperometric biosensor for OP pesticides and nerve agents was developed using self-assembled acetylcholinesterase (AchE) on a carbon nanotube (CNT)-modified glassy carbon electrode [77]. The CNTs have two main functions for the biosensor; first, as platforms for AchE immobilization by providing
a microenvironment that can maintain the bioactivity of AchE, and second, as a transducer for amplifying the electrochemical signal of the product of the enzymatic reaction. The integration of nano- and biomaterials could be extended to other biological molecules for future biosensor or immunoassay research.
Advancements in biosensor technology will continue with expansion of multi- analyte detection and more rapid analytical capability. For example, a chip contain- ing 92,000 electrodes with a 30 ms read is already investigated. With a 30 ms read time, enzymatic kinetic reads could be performed directly on the chip. However, the capability of 92,000 electrodes 3 1000 reads presents storage, data acquisition, and conversion issues. The limiting factor at this time is computer capability. Other technologies such as a 40 s kinetic read of 12,000 electrodes with 4 or 8 electrodes discharged at one time in microsecond intervals are near realization.
Through future research, immunoassays and biosensors for pesticides may find critical applications related to in vitro and in vivo studies in the diverse field of environmental science and human exposure.