Photonic gels are colorful sensors
Rice, MIT researchers create thin-film polymer metamaterial with potential for many uses
Alternating, nano-sized layers of hydrophilic and hydrophobic molecules self-assemble into a block copolymer
called a photonic gel, developed at Rice University and the Massachusetts Institute of Technology. It changes color depending
on the amount of water absorbed by the hydrophilic layers, which can be tuned by the solvent used. (Thomas Lab/Rice University)
October 10, 2012
HOUSTON – (Oct. 10, 2012) – Materials scientists at Rice University and the Massachusetts Institute of Technology
(MIT) have created very thin color-changing films that may serve as part of inexpensive sensors for food spoilage or security,
multiband optical elements in laser-driven systems and even as part of high-contrast displays.
The new work led by Rice materials scientist Ned Thomas combines polymers into a unique, self-assembled metamaterial that,
when exposed to ions in a solution or in the environment, changes color depending on the ions’ ability to infiltrate the
hydrophilic (water-loving) layers.
The research was published in the American Chemical Society journal
The micron-thick material called a photonic gel, far thinner than a human hair, is so inexpensive to make that, Thomas said,
“We could cover an area the size of a football field with this film for about a hundred dollars.”
A photonic gel developed at Rice University and the Massachusetts Institute of Technology self-assembles from long polymer
molecules. Polystyrene and poly(2-vinyl pyridine) are mixed in a solution that, when evaporated, allows the polymers to
quickly form into nanosized layers. The layers can be tuned to reflect specific colors when exposed to particular chemicals.
But for practical applications, much smaller pieces would do. “Suppose you want a food sensor,” said Thomas, the William and
Stephanie Sick Dean of Rice’s
George R. Brown School of Engineering
and former chair of the Department of Materials Science
and Engineering at MIT. “If it’s inside a sealed package and the environment in that package changes because of contamination
or aging or exposure to temperature, an inspector would see that sensor change from blue to red and know immediately the food
One photonic gel developed at Rice University was put through a series of color changes when repeatedly washed and exposed
to new compounds. The gels show potential for inexpensive sensors and filters. (Thomas Lab/Rice University)
Such visual cues are good, he said, “especially when you need to look at a lot of them. And you can read these sensors with
low tech, either with your own eyes or a spectrophotometer to scan things.”
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 polystyrene molecules clump together to keep water molecules out, while the poly(2-vinyl pyridine), P2VP for short, forms its
own layers between the polystyrene. On a substrate, the layers form into a transparent stack of alternating “nano-pancakes.”
“The beauty of self-assembly is that it’s simultaneous, all the layers forming at once,” Thomas said.
The researchers exposed their films to various solutions and found different colors depending on how much solvent was taken up by
the P2VP layers. For example with a chlorine/oxide/iron solution that is not readily absorbed by the P2VP, the film is transparent,
Thomas said. “When we take that out, wash the film and bring in a new solution with a different ion, the color changes.”
The researchers progressively turned a clear film to blue (with thiocyanate), to green (iodine), to yellow (nitrate), to orange
(bromine) and finally to red (chlorine). In each case, the changes were reversible.
Thomas explained that the direct exchange of
from the solution to the P2VP expands those layers and creates a photonic band gap — the light equivalent of a semiconducting
band gap — that allows color in a specific wavelength to be reflected. “The wavelengths in that photonic band gap are forbidden
to propagate,” he said, which allows the gels to be tuned to react in specific ways.
“Imagine a solid in which you create a band gap everywhere but along a 3-D path, and let’s say that path is a narrowly defined
region you can fabricate within this otherwise photonic material. Once you put light in that path, it is forbidden to leave because
it can’t enter the material, due to the band gap.
“This is called molding the flow of light,” he said. “These days in photonics, people are thinking about light as though it were
water. That is, you can put it in these tiny pipes. You can turn light around corners that are very sharp. You can put it where
you want it, keep it from where you don’t want it. The plumbing of light has been much easier than in the past, due to photonics,
and in photonic crystals, due to band gaps.”
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