Label free optical immunoprobes for pesticide detection

In environmental analysis immunological methods based on non covalent selective molecular interactions can be used as a sensitive tool. The label free detection of these interactions in real time allows simple, fast, and elegant approaches. Optical transducers are used for direct, label free immunoprobes with considerable success. For the detection of low molecular weight environmental analytes binding inhibition assays are common. Antibodies are mixed with the sample and antibody binding sites are blocked by the analyte. Subsequently the concentration of free antibodies is quantified by binding to a transducer modified with a derivative of the analyte. The basic effects monitored by the transducers are an increase in refractive index or changes in surface adlayers. Accordingly the transducers can be described as micro-refractometers or micro-reflectometers. A large number has been published in recent years [G. Gauglitz, Opto-Chemical and Opto-Immuno Sensors, in: H. Baltes, W. Gopel, J. Hesse (Eds.), Sensor Update, VCH Verlagsgesellschaft, Weinheim, 1996.] Results from four optical transducers out of this variety (grating coupler, channel waveguide interferometer, waveguide surface plasmon resonance, thin film reflectometry) applied to pesticide detection are compared. Test cycles below 15 min can be reached. Performance is limited by drift and noise of the transducers. Limits of detection reached are comparable for all of the transducers and reach values between 0.05 and 0.15 ppb under laboratory conditions. Application to environmental samples reveals problems with the sample matrix. The performance of these four devices and the potential for further application is discussed.