One of the fastest ways to qualitatively analyze chemical reactions is visually; it’s hard to mistake an abrupt color change or the appearance of new precipitates in a solution. These methods, however, are restricted to only a few processes, and normally do not provide an understanding of advanced phenomena such as chirality — the tendency for certain compounds to occur in mirror-image forms. The two chiral forms, called left- and right-handed enantiomers, can have very different properties, such as pharmaceutical effects. Yet physically, the two forms can only be distinguished through specialized and time-consuming techniques.

Researchers led by Xiao-Qi Yu from Sichuan University in China and Lin Pu from the University of Virginia in the USA1 have now discovered an innovative system that allows chirality to be detected visually. Their method involves a gel that collapses when it interacts with enantiomers of opposite ‘handedness’.

Molecular gels have recently gained attention as sensing platforms because their structures at the boundary between liquid and solid states are extremely responsive to changes in their environment. Originally, Yu, Pu and their colleagues had been studying the reaction between a derivative of binaphthol — a chiral aromatic compound — and copper to develop a fluorescent sensor. But when the binaphthol derivative was mixed with copper ions in an ultrasonic bath, an opaque green gel quickly formed. The formation of such a gel was unexpected because ultrasound usually breaks up gel networks.

Fig. 1: Electron microscopy images of the dried gel (left) and solution (right) forms of binaphthol–copper.From Ref. 1. Reproduced with permission. © 2010 ACS

The team found that their gel consisted of a stabilized three-dimensional network (Fig. 1). Because chiral compounds such as binaphthol act differently when mixed with other chiral substances, the researchers decided to investigate the gel’s stability towards chiral amino alcohols — molecules known to displace copper from aromatic binding sites.

Pu and his colleagues found that the binaphthol–copper gel was particularly sensitive to enantiomer type. When the gel and the amino alcohol had the same chirality, the gel remained stable even after ultrasonication. If the chiral sites were different, however, the gel collapsed as soon as it was agitated.

The researchers are now working on fully understanding the mechanism behind their collapsible detection system. “This will allow us to develop new gel materials for recognition of diverse chiral molecules with rapid assays,” says Pu.