Food colourant: Sensor tech

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  • Published: Apr 1, 2017
  • Author: David Bradley
  • Channels: Raman
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Novel sensor

A material that has been used as a food dye for decades in corn chips and ice cream could be used in novel sensor technology, according to researchers at Georgia Institute of Technology. (Credit: PNAS/Srinivasarao et al)

A material that has been used as a food dye for decades in corn chips and ice cream could be used in novel sensor technology, according to researchers at Georgia Institute of Technology.

Karthik Nayani, Jinxin Fu, Rui Chang, Jung Ok Park, and Mohan Srinivasarao describe in the Proceedings of the National Academy of Sciences (USA) how a class of water soluble liquid crystals, more specifically lyotropic chromonic liquid crystals, exhibit unexpected characteristics that could be exploited in new types of chemical sensor and in applications.

"We were seeking to understand the aggregation and phase behaviour of these plank-like molecules as a function of temperature and concentration," explains Nayani, a former student at GATech who worked on the problem. "When observed under crossed polarisers in an optical microscope, liquid crystals can exhibit beautiful textures that hint toward how the molecules themselves are arranged."

Fundamental questions

In order to answer various fundamental questions about the phase behaviour of these materials, the researchers used a microscope to observe the materials' texture when the molecules were confined to droplets known as tactoids.

"Surprisingly, we found a configuration that hasn’t been seen before in the 70 years that people have been studying liquid crystals," explains team leader Srinivasarao. "Historically, liquid crystals in tactoids conform to what is known as a bipolar and a bipolar configuration with a twist. At lower concentrations, we found that these liquid crystals arrange in a concentric fashion, but one that appears to be free of a singular defect."

The team built a simplified model of the phenomenon but then turned to polarized Raman microscopy to help them focus on the aggregation behaviour of these molecules in more detail so that they might explain the surprising results. These new findings add to our growing understanding of how chromonic liquid crystals might be used in sensing applications, Srinivasarao adds. Such crystals are soluble in water but respond in a dramatic way to being confined to certain patterns – such as the aforementioned tactoidal droplets. They also reveal different behaviour depending on environmental conditions, concentration and temperature, for instance. Fundamentally, the material's responsiveness to changes in its environment might even be used to detect the chirality, or handedness, of other molecules and important and well-known property of countless biomolecules, pharmaceuticals and agrochemicals, Srinivasarao suggests.

Chirality

"These materials don’t have a chiral centre but they exhibit a chiral structure," Srinivasarao explains further. That in itself is very interesting and is a phenomenon that might be useful in answering the kinds of questions that have perplexed scientists for many decades. For instance, we do not yet know why biochemistry biased to one hand and not the other at the molecular level, amino acids, for instance being used by living things only in one chiral form. There have been many hypotheses and even the odd theory, but none satisfactorily explains life's chirality. An explanation as to the origin of this bias might take us a step towards understanding the origin of life itself and perhaps give us new clues as to what to look for in the search for extraterrestrial life.

"We are looking to see if all other known forms of lyotropic liquid crystals show similar effects," Srinivasarao told SpectroscopyNOW. "This will take some time but we are amazed at the beauty of what we found. We are also looking to study the chiral amplification of such structures and understand whether this pathway might provide a clue in the quest for homochirality in the context of "origins of Life"."

Related Links

Proc Natl Acad Sci 2017, online: "Using chiral tactoids as optical probes to study the aggregation behavior of chromonics"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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