Multilayer photonic gel detects substances via color change

Houston, TX--An inexpensive gel-based optical metamaterial developed by researchers at Rice University and the Massachusetts Institute of Technology (Cambridge, MA), when exposed to ions in a solution or in the environment, changes color depending on the ions’ ability to infiltrate the gel's hydrophilic (water-loving) layers.¹ Rice materials scientist Ned Thomas says the micron-thick material is so cheap to make that “We could cover an area the size of a football field with this film for about a hundred dollars.”

Substances detectable via color change in the gel include ions such as perchlorate (ClO₄-), thiocyanate (SCN-), acetate (CH₃CO₂-), nitrate (NO₃-), and others. After detecting a substance, the gel can be washed and is then ready again to detect different types of ions.

Like a multilayer optical coating
The films are made of nanoscale layers of hydrophobic polystyrene and hydrophilic poly(2-vinyl pyridine). In the liquid solution, the polymer molecules are diffused, but when the liquid is applied to a surface and the solvent evaporates, the block copolymer molecules self-assemble into a layered structure. The result is akin to a multilayer optical coating, but with certain layers that change thickness in the presence of certain ions.

The researchers progressively turned a clear film to blue (with SCN-), to green (iodine), to yellow (NO₃-), to orange (bromine), and finally to red (ClO₄-). In each case, the changes were reversible. The colors can be characterized by eye or with a spectrophotometer.

The structures could also be used as color-changing mirrors or filters.

Co-authors of the paper are Rice research scientist Jae-Hwang Lee and MIT postdoctoral researchers Ho Sun Lim and Joseph Walish. The work was supported by the U.S. Army Research Office, the U.S. Air Force, and the Korea Research Foundation, funded by the Korean government.

REFERENCE:

1. Ho Sun Lim et al., ACS Nano (2012); DOI: 10.1021/nn302949n.