Noncovalent ligand sensors

The sensors discussed so far are based on ligands covalently bound to the polymer backbone. Other methods of detection - often referred to as mix and detect methods - work by simple noncovalent incorporation of the polymer with the ligand of interest. Reichert et al. generated liposomes of polydiacetylene with sialic acid for the same purpose of detection as Charych s surface-bound polymers, but realized that covalent functionalization of the polymer was not necessary [17]. Through simple mixing of the lipid-bound sialic acid with the polymer before sonication and liposome formation, they were able to form a functional colorimetric recognition system (Fig. 8). 

In both of these cases, the ligand (sialic acid) for the analyte of interest (influenza vims) was covalently linked to the PDA backbone generated upon photopolymerization. Functional sensors based on ligands that are noncovalently incorporated into liposomes have also been reported (Charych et al. 1996 Pan and Charych 1997). Mixed liposomes as well as mixed thin films on glass containing a combination of the ganglioside GMl and diacetylene lipids detect the presence of cholera toxin, a protein that binds to GMl. 

Artificial receptors can be converted into sensors by covalent attachment of a signaling unit such as a fluorescent dye. An interesting alternative are soindicator displacement assays (IDAs) [9]. These assays are based on receptors that are bound to dyes (or fluorescent ligands) via noncovalent interactions. Upon addition of an analyte, the dyes are displaced, which results in a change of their optical properties. These changes can be used to identify and/or quantify the analyte. 

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