Takuya Ohzono (Leader) of Soft Mechanics Group, Takahiro Yamamoto (Senior Researcher) of Smart Materials Group, and Jun-ichi Fukuda (Senior Researcher) of Soft Matter Modeling Group, the Nanosystem Research Institute (Director: Tomohiko Yamaguchi) of the National Institute of Advanced Industrial Science and Technology (AIST; President: Ryoji Chubachi), discovered the utility of self-organized periodic orientational structures of a liquid crystal confined within microwrinkle grooves for sensing chiral molecules in gas samples and their chiral handedness.

The developed method utilizes a novel phenomenon in which a liquid crystal in microwrinkles alters its orientational structures in response to the chirality of dissolved gas molecules. This structural change can be easily observed by a polarizing microscope alone as soon as a gas sample is blown onto the liquid crystal, enabling the construction of a sensor system for quick evaluation of the chirality of a tiny amount of gas samples at ambient temperatures and pressures. This method is expected to be applicable to the analysis of volatile chemicals in perfumes etc. and to environmental monitoring.

The details of this research will be published online in a scientific journal, Nature Communications, on April 30, 2014 (Japan Standard Time).

Sensing of chiral gas molecules by the change of the periodic orientational structures of a liquid crystal in microwrinkle grooves

Many materials, in particular organic compounds, involved in a diverse range of industries including pharmaceutical industry, perfumery, chemical industry, and agrichemical industry, can have chirality. It is known that the presence of chirality and its handedness in volatile substances affect scent and the physiological functions of pheromones, and are useful as indices of environmental conditions in forests. However, chiral sensing and resolution of volatile gas samples usually require expensive and complicated analytical methods, which urges the need of simple techniques for chiral discrimination. Such techniques, once practicalized, will be applicable to simple screening in developing perfumes and pharmaceutical products, monitoring of living and natural environments, and analysis of volatile molecules in forensic science and clinical diagnosis, and will be useful in industries and people's lives.

Precise analytical methods including artificial gas sensors mimicking biological olfactory systems and chromatography have been developed for the analysis of the chirality of gas samples. However, these methods require in advance the preparation of a complementary chiral environment that interacts selectively with one of the enantiomers of the molecules to be analyzed. Molecular design, synthesis, and stabilization of such environments for the detection of various molecules are not easy. Moreover, the disadvantage of these methods is that they require complex and expensive apparatuses such as quartz resonators, semiconductor devices, light-reflecting devices, and mass spectrometers.

AIST has been conducting research and development of functional materials utilizing self-organized shape-tunable microwrinkles that can be produced easily with low costs. In this study, the researchers applied the orientational structures of a liquid crystal confined in microwrinkle grooves (AIST press release on April 19, 2012) to a simple analysis of the chirality of gas samples. This study was carried out in Research Party for Emergent Functions of Anisotropic Media (Leader: Takahiro Yamamoto), a cross-organizational team initiated for the development of new functions of liquid crystals. This study was partly supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) via a Grant-in-Aid for Scientific Research on Innovative Areas and JSPS Grant-in-Aid for Scientific Research (C).

The developed method utilizes microwrinkles as a substrate of a liquid crystal. The microwrinkles, self-organized undulating surface structure of a hard polymer film on a rubber substrate, are formed by tangential uniaxial compression. In this study, a spontaneously-formed periodic structure of a nematic liquid crystal confined in the microwrinkle grooves (Fig. 1, left) is utilized as a chirality sensor. Regions with left-handed and right-handed local twist distortions alternate in this periodic structure of the liquid crystal. Initially, the length of a region with left-handed twist is approximately the same as that with right-handed twist. When chiral molecules in the gas sample dissolve into the liquid crystal after blown onto it, one region with specific twist handedness is energetically more favorable than the other. As a result, the length of the former becomes larger than that of the latter. This phenomenon can be utilized for visual detection of the existence of chiral molecules and their chiral handedness. The mechanism of this novel phenomenon can be explained qualitatively by the kinetics of the dissolution of the gas molecules, and numerical calculations based on a theoretical model. Typical response time is as small as a few seconds, owing to rapid diffusion and homogenization of the gas molecules dissolved from the vapor into the liquid crystal whose effective thickness is as small as one micrometer. It is important to note that this method enables direct analysis of gas samples, and does not require condensation or dissolution of the samples in advance.

Regions with right-handed and left-handed local twist are colored in blue and orange, respectively, under a polarizing microscope with a sensitive tint plate (Fig. 1, right). This coloring facilitates the detection of the above-mentioned phenomenon by the change in the lengths of colored regions (hereafter denoted by R and L). Moreover, the observation of the color of the longer region enables the determination of the major enantiomer in a sample of a specific chiral compound, if one determines in advance which region (blue or orange) becomes longer in response to an enantiomer of this compound with (+) (or (-)) optical activity.

Figure 1: Sensing of chiral gas molecules by the change of the periodic orientational structures of a liquid crystal in microwrinkle grooves

Furthermore, quantitative measurement of the change in the lengths R and L from a microscope image makes it possible to quantify the enantiomeric excess and the concentration of a chiral compound. Figure 2 shows z, a quantity proportional to the difference between R and L, versus the enantiomeric excess (ee%) of the mixture of (+)-limonene and (-)-limonene (it is known that different enantiomers of limonene yield different aroma, and different physiological stress reduction in rats). Under the same experimental conditions (e.g., total concentration of the chiral compound and blow speed), z varies according to the enantiomeric excess. Therefore, the enantiomeric excess of a gas sample can be determined by measuring z.

Figure 2 : Relation between the enantiomeric excess (ee%) of limonene gas and z, an index of the difference between R and L

Figure 3 shows the variation of z when the relative concentration of an enantiopure (ee% = ±100) compound with respect to its saturated concentration (horizontal axis) is varied. These dependencies of z on concentration can be used for the estimation of the concentration of a specific compound. These results demonstrate that the sensitivity depends on compounds. Various unknown parameters including saturated concentration, solubility, and twisting power in the liquid crystal affect the sensitivity. The actual detection limits are small enough, indicating high sensitivity; 3 ppm for β-pinene and 5 ppm for carvone. The curves in Fig. 3 are convex upwards, which is reproduced qualitatively by numerical calculations based on a theoretical model.

Figure 3 : Relation between the relative concentration of each gas and z, an index of the difference between R and L.

The advantages of the developed method for chirality detection include the following:
(i) The sensor part is very inexpensive; its preparation requires only the coating of a nematic liquid crystal widely used in current display technology onto self-organized microwrinkles.
(ii) Its operation is quite easy, because gas samples can be blown onto the sensor part directly at ambient temperatures, pressures, and humidities. Condensation or thermal pre-treatment of the sample is not necessary.
(iii) Chirality can be detected visually by a polarizing microscope alone. A sensitive tint plate further clarifies the structural change of the liquid crystal in response to chiral molecules.
(iv) The response is fast enough, within a few seconds.
(v) In most cases, the sensor part is reusable because the dissolved molecules evaporate, and the structure of the liquid crystal itself is stable for at least a few months.

Therefore the developed method can be used as a simple and economical chirality sensor for gas samples that allows its outdoor use. This method makes use of a novel response of a topological defect of a liquid crystal, and can be regarded as an example of an ingenious application of soft materials that respond sensitively to external stimuli.

The researchers will expand the versatility of the developed method by choosing a suitable host nematic liquid crystal and optimizing experimental conditions, and thus by increasing the number of detectable chiral compounds. They will investigate the possibility of practical applications of self-organized structures of liquid crystals other than nematic in microwrinkles, aiming at novel unique sensors and functional materials.