Food Additives and Contaminants 721 Table 4. Advantages and disadvantages of potential binders for SPR analysis of PSP toxins. Binder Chemosensors Molecular imprinted polymers (MIPS) Aptamers Advantages Non-animal-based binders Can be incorporated into self-assembly monolayers for sensing on a surface Non-animal-based binder Target analyte defines its own recogni- tion site Stable at extreme temperature and pH Show specificity in natural systems Adaptable and flexible in their use Low cost and ease of manufacture May be used in non-aqueous media or aggressive environments Nucleic acid aptamers are non-animal- based binders produced by chemical synthesis resulting in limited batch to batch variation Selectively bind to low molecular weight compounds Binding affinities of aptamers are in the mM to nM range and kinetic parameters can be altered on demand Toxins that do not illicit a good immune response can be used to generate high-affinity aptamers Aptamers are stable for long-term storage and can be transported at ambient temperature Selection conditions can be manipu- lated to obtain aptamers with prop- erties desirable for in vitro assays, e.g. non-physiological buffers, solvents, etc. Reporter molecules can be attached to aptamers at precise locations so as not to affect binding Disadvantages Lower binding affinity than antibodies No correlation between the binding intensity and biological response or toxicity May be sensitive to small alterations in target analyte structure and properties Show non-specific binding Diversity of binding sites Template bleeding requires suitable template analogue for imprinting and affects quantitative applications Generally lower binding affinities than antibodies Aptamers are costly to generate and long aptamer sequences are difficult to achieve DNA aptamers have a smaller range of three-dimensional structures obtain- able compared with RNA aptamers but they can bind their target to the entire sequence DNA and RNA aptamers are suscep- tible to enzymatic degradation by nucleases, thus requiring highly pure environments. This can be overcome Peptide aptamers are constrained to the scaffold protein so they are less flexible, which may affect their effectiveness. Only the variable range is used for binding Peptide aptamers require biological systems for selection purposes (Chen et al. 2007). These binders are showing consid- erable promise in detecting STX using fluorescence, but as yet the sensitivity is five orders of magnitude lower than that achievable using the mouse bioassay (Gawley et al. 2002, 2007). For naturally contaminated shellfish samples there was no correlation between the intensity of absorption and toxicity of the sample. However, SPR spectroscopic detection using these binders increased the sensitivity up to 1000-fold (Chen et al. 2007). Shellfish matrices have not as yet been evaluated. Molecular imprinted polymers (MIPs) MIPs have received controversial exposure over the past 10 years from researchers and industries in their potential role in the environmental and food analysis of chemical contaminants and toxins (Lotierzo et al. 2004; Henry et al. 2005; Baggiani et al. 2008). They are artificially created ligand binding sites that can selec- tively recognise a target molecule. The synthesis involves the polymerisation of functional and cross- linking monomers around the target of interest or a structural mimic template through non-covalent inter- actions or reversible covalent bonds or both with the functional groups of the template. These three-dimen- sional polymer-based systems can be optimised to yield recognition sites highly specific and selective to the target of interest in the same manner as either a polyclonal or a monoclonal antibody. However, the mechanism of interaction or recognition of a particular analyte to the polymer is still not completely compre- hended. Unlike antibodies, these are fairly rigid mac- roscopic structures are insoluble in any solvent. MIPs therefore are more commonly marketed and used for