720 K. Campbell et al. Table 3. Sensitivity and specificity of the STX/NEO antibody mix for each PSP toxin (Campbell et al., unpublished data). Antibody dilution in HBS-EP Antibody mix Antibody ratio to standard Flow rate (ml min 1) Injection volume (ml) Contact time (min) PSP toxin analogue Saxitoxin dihydrochloride Neosaxitoxin Gonyautoxin 1/4 Gonyautoxin 2/3 Decarbamoyl saxitoxin Decarbamoyl neosaxitoxin Decarbamoyl gonyautoxin 2/3 C1/C2 Gonyautoxin 5 C3/C4 1/500 1/50 100 90.3 70 96.6 933 43.8 41.1 255 9333 70 Toxin concentration as STXdiHCl equivalents (ng ml 1) Dynamic range, 0.20–68.8 0.87–10.8 0.72–22.8 0.54–16.8 0.07–0.9 1.19–34.2 1.23–37.7 0.15–7.7 0.01–0.1 1.03–8.5 2.8 2.8 4.4 4.9 0.4 9.1 17.1 14.5 0.4 91.8 100 100 63.6 57.1 700 30.7 16.4 19.3 700 3.0 STX NEO Toxin concentration (ng ml 1) 20% STX/80% NEO 1:1 12 60 5 Dynamic range, IC(n20g tmolIC1)80 0.2–68.8 0.8–9.9 0.8–25.3 0.9–28.0 0.1–1.2 1.7–48.7 3.1–94.7 2.0–102.7 0.1–1.5 24.0–198 2.8 3.1 4.0 2.9 0.3 6.4 6.8 1.1 0.03 4.0 (ngICm5l0 1) Percentage cross-reactivity (ngICm5l0 1) Percentage cross-reactivity IC(n20g tmolIC1)80 the cross-reactivity profile has been amalgamated to produce a potential single-assay format for all PSP toxins. With the exceptions of GTX5 and dcSTX, which will overestimate based on this cross-reactivity profile, this antibody mix correlates much better than either binder individually with the mouse bioassay toxicity factors (Figure 1) for each of the toxins. This is illustrated in Table 3 as the cross-reactivity profile in STXdiHCl equivalents. Consequently, by using optical SPR biosensor analysis, there is the potential to resolve cross- reactivity issues that arise using a single binder. Combining the two polyclonal antibodies STX and NEO with a common denominator of the NEO surface enabled the design of a single-assay format for both hydroxylated and non-hydroxylated toxins. In addi- tion, as the cross-reactivity is more in line with the toxicity profile for the mouse bioassay, this is an advantage in having one screening test for all PSP toxins compared with two ELISA tests. This approach of mixing antibodies raised in the same species could be used for other families of chemical contaminants or toxins where there is significant variability in the cross- reactivity profile as long as there is a common denominator in the family for chip surface immobilisation. Potential binders for future optical SPR analysis of PSP toxins A wide range of non-antibody-based binders have been reviewed previously as alternative molecules for the detection of pathogens (Ngundi et al. 2006). The most promising non-antibody binders worth investigating in their role for PSP toxin analysis by SPR are chemosensors (Bell and Hext 2004), molecular imprinted polymers (MIPs) (Hall et al. 2005) and aptamers (Tombelli et al. 2005). The advantages and disadvantages of these particular binders are high- lighted in Table 4. Chemosensors Chemists have used a variety of approaches to design artificial receptors or chemosensors capable of selec- tively binding analytes. Chemosensors are molecules specifically designed for the qualitative and quantita- tive monitoring of analytes and are being researched in the areas of biological and analytical chemistry, medicine and environmental sciences (Bell and Hext 2004; Minkin et al. 2008). These molecules bind to the target analyte through non-covalent interactions and produce a change in light absorption or fluorescence. Molecular structures that have displayed some poten- tial as chemosensors are polyalcohols, crown ethers, calixarenes, helicenes, sterically geared tripods, metal complexes, pinwheels, porphyrins and fused ring het- erocycles. Most chemosensors described do not display sufficient sensitivity or selectivity to their targets for practical applications, and they usually require syn- thetic modification for immobilisation onto a sensor surface. However, chemosensors such as crown ether sensors like calix[4]arene have been investi- gated as alternative binders for STX detection