716 K. Campbell et al. Figure 5. Trends in carbamate toxin response at a fixed concentration of 10 ng ml 1 with increasing percentage of NEO antibody in STX/NEO antibody binder mix. Source: Campbell et al., unpublished data. limited numbers of available epitopes that are not already encapsulated in the receptor pocket. A differ- ent approach that could be investigated is using a ligand of a high molecular weight that would complete and dissociate appropriately with the toxin for the receptor site when the receptor is immobilised on the surface (Karlsson 1994). This ligand would provide the molecular mass necessary for the SPR response, but sourcing a suitable competitor to PSP toxins maybe difficult. The limited availability of the sax- iphillin receptor to only a few research groups has inhibited the characterisation of this binder in the role of PSP toxin detection by SPR. Research is on-going into the synthesis of cloned receptors that would prove beneficial in both functional assays and as more stable alternative binders to antibodies for sensor technologies. Antibodies Antibodies unlike receptors are biochemical binders and bind selectively to chemical moieties on the structure of the toxin to which they are produced. These binders do not show a similar correlation between the toxicity of a toxin and binding affinity in the same manner as receptors for PSP toxins. The difficulty in trying to detect such small molecular weight compounds using antibodies is that there are limited functional groups to conjugate a protein for antibody production based on site specificity and, therefore, few chemical reactions can be utilised for immunogen synthesis. Restricted and limited availability of toxins makes the synthesis of alternative derivatives impractical unless sufficient yield can be achieved for immunogen production. The formation of the toxin protein conjugate must also reduce the toxicity of the toxin or when immunised the animal may be affected with PSP symptoms. The first antibodies produced against PSP toxins were reported by Johnston et al. (1964). For most PSP toxin antibodies produced since then either a modification of the Mannich reaction or reductive alkylation chemistry (Bu¨ rk et al. 1995) has been employed for the synthesis of the protein conjugate. As highlighted by Usleber et al. (2001) during these reactions there are three potential reactive sites for the carbamate toxins. These include the amino, guanidine and imine groups, but it is difficult to characterise which is the predom- inant orientation of the toxin to the protein conjugate or the ratio of protein conjugation to all three sites. Immunochemical methods are frequently used for qualitative, semi-quantitative and quantitative mea- surements of many chemical substances with high biological impact. These methods are well established in clinical chemistry and endocrinology and have become in recent years more sophisticated in their use for food and environmental analysis and toxicol- ogy screening due to their ability to function effectively in complex biological matrices without major purifi- cation. From the production of the first antibodies for PSP toxins, it was decades later when immunoassays were actually utilised to monitor PSP toxins in shellfish. Immunoassay methods for PSP toxins have been reviewed in the past (Usleber et al. 2001) and several antibody-based approaches for detecting these