Developing liquid crystal-based immunoassay for melamine detection

Abstract

We have developed a liquid crystal (LC)-based immunoassay for melamine detection by using direct, sandwich and competitive formats. The detection mechanism was based on the specific immunobinding of melamine with the anti-melamine immobilized on the glass substrate, which disrupted the orientation of the LCs and resulted in changes in the optical images of the LCs under polarized light. In the direct immunoassay where the melamine was bound to the anti-melamine on the substrate, no change in the optical images of LC was observed. In contrast, in the sandwich immunoassay where the immunocomplex of melamine and secondary anti-melamine linked with streptavidin were bound to the primary anti-melamine on the substrate, a dark-to-bright transition of the LC images was observed when the concentration of melamine was 2.5 ng/mL or above. Moreover, in the competitive immunoassay where both melamine and melamine-streptavidin conjugate were bound to the anti-melamine on the substrate, a bright-to-dark transition of the LC images was observed when the concentration of melamine was 5 ng/mL or above. We found that the larger the molecules bound to the substrate, the more easily was the orientation of the LCs disrupted which resulted in the change in the optical images of the LCs. This result provides a good strategy for developing LC-based immunoassays for the detection of small molecules by using a large auxiliary molecule attached to the target molecules. To the best of our knowledge, our work demonstrates for the first time the detection of small molecules by using a LC-based immunoassay.