Tuning the sensitivity of a liquid crystal-based chemical/biological sensor using a novel double layer alignment film

Abstract

Liquid crystal-based detectors of bio/chemical molecules based on a surface alignment transition of the liquid crystal director have been demonstrated. This type of detector could be made more sensitive if the alignment transition was more abrupt and tunable to be at a ‘trigger’ point for the desired level of concentration of the target molecule. In this study, we investigate the use of ‘double layer alignment films’ to cause a more abrupt change in the bulk surface alignment of a liquid crystal-based sensor of lecithin. We show that the detection level of the percentage of lecithin dissolved in the liquid crystal host can be controlled from greater than 4% for a conventional single layer alignment film, to less than 0.1% using the double layer alignment film method. This result verifies an earlier theory, which predicted that a double layer alignment film should provide an abrupt surface alignment transition. The utility of this controllable surface alignment transition as a sensor lies in the anisotropy of liquid crystal materials and the amplification of a detectable signal arising from this anisotropy that results from the alignment of the bulk liquid crystal material in contact with it. For example, the surface alignment change would be expected to cause a large change in capacitance (×2) of a micro-capacitor on an integrated circuit. This work provides a direction for enhancement of liquid crystal-based sensors of other biomolecules, drugs and chemicals.

Keywords:

• chemical sensor
• biosensor
• liquid crystal
• alignment film
• sensitivity
• detection limit
• fluoropolymer
• polyimide
• anchoring transition
• pretilt
• lecithin

Acknowledgements

This work was supported by DuPont. The authors would like to thank Brian Auman at DuPont for discussions on fluorinated polyimide synthesis.