Selective and direct detection of free amino acid using the optical birefringent patterns of confined nematic liquid crystals

ABSTRACT

Detection of amino acid (AA) is an essential step to understand various biological processes. In this study, we used innovative optical birefringent patterns of nematic liquid crystals (LCs) for the detection of several AAs. We attempted to use capillary-confined nematic LCs as sensor for AA analysis because their three-dimensional micro-scale architecture of LCs allowed better performance than that of mainly reported LC thin film sensors owing to the effect on the formation and dynamics of point defect. The sensing system was built by disrupting the alignment of a nematic LC, 4-cyano-4′-pentylbiphenyl (5CB), using the dopant of dodecyl aldehyde. Detection principle is based on the chemical analytical method of aldehyde titration for AAs, wherein the reaction between AAs and aldehyde group generates Schiff bases that could alter the configuration of nematic LCs at the aqueous/LC interface. The patterns generated in the reaction are captured by polarising optical microscopy (POM) and are visible to the naked eye. The functionalised LCs detected glycine at concentrations as low as 1 pM. There was a surprising result that it can selectively detect D-AAs against their L-isomers, however, further efforts are required to explain the mechanism.

GRAPHICAL ABSTRACT

KEYWORDS:

• Nematic liquid crystals
• 4-cyano-4′-pentylbiphenyl (5CB)
• birefringence
• amino acids (AAs)
• micro-capillary

Disclosure statement

The authors declare that they have no conflict of interest.

Additional information

Funding

This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning [NRF-2013R1A1A1A05008333] and a grant from the Korean Health Technology R&D Project, Ministry of Health & Welfare and Republic of Korea [HI13C0891].