A principle of detecting and differentiating dialdehydes from monoaldehydes by using surface reactions and liquid crystals

In this Letter, we report a principle of discriminating dialdehydes from monoaldehydes by using optical responses of liquid crystals (M) triggered by a surface reaction between aldehydes and surface amine groups. When a solid surface is decorated with a mixture of long hydrocarbons and primary amines, a surface reaction between dialdehydes and the surface amines generates an aldehyde-terminated surface (which can be tested by using a triglycine-copper complex as an aldehyde marker). This type of surface is able to trigger an ordering transition of 5CB supported on the surface. In contrast, the surface reaction between monoaldehydes and the surface amines leads to a hydrocarbon-terminated surface which does not influence the orientations of 5CB on the surface. Because LCs are birefringent materials, the changes of orientations can cause distinct optical textures visible to the naked eye. The methodology reported herein provides a simple optical method of detecting and distinguishing dialdehydes from monoaldehydes with high sensitivity (similar to 10 mu M for glutaraldehyde and phthaldialdchyde) and specificity.