Scientists at the university have enhanced their laser-induced graphene (LIG) technique and is investigating ways to write graphene patterns onto food and other materials.
The idea is that the team will be able to quickly embed conductive identification tags and sensors into the products themselves.
“This is not ink,” James Tour of the Rice lab, said. “This is taking the material itself and converting it into graphene.”
The Tour lab has previously turned Girl Scout cookies into graphene and this development is an extension of the lab’s contention that anything with the proper carbon content can be turned into graphene. In recent years, the lab has developed and expanded upon its method to make graphene foam by using a commercial laser to transform the top layer of an inexpensive polymer film.
The foam consists of microscopic, cross-linked flakes of graphene, the two-dimensional form of carbon. According to the team, LIG can be written into target materials in patterns and used for applications such as a supercapacitor, an electrocatalyst for fuel cells, radio-frequency identification antennas (RFID) and biological sensors.
This latest development, reported in the American Chemical Society journal ACS Nano, is said to have demonstrated that laser-induced graphene can be burned into paper, cardboard, cloth, coal and certain foods.
“Very often, we don’t see the advantage of something until we make it available,” Tour said. “Perhaps all food will have a tiny RFID tag that gives you information about where it’s been, how long it’s been stored, its country and city of origin and the path it took to get to your table.”
Tour added that LIG tag could also be sensors that detect E. coli or other microorganisms on food. “They could light up and give you a signal that you don’t want to eat this,” he continued. “All that could be placed not on a separate tag on the food, but on the food itself.”
Multiple laser passes with a defocused beam allowed the researchers to write LIG patterns into cloth, paper, potatoes, coconut shells, cork and toast – the bread was toasted first to ‘carbonise’ the surface. The process happens in air at ambient temperatures.
“In some cases, multiple lasing creates a two-step reaction,” Tour explained. “First, the laser photothermally converts the target surface into amorphous carbon. Then on subsequent passes of the laser, the selective absorption of infrared light turns the amorphous carbon into LIG. We discovered that the wavelength clearly matters.”
The researchers discovered that increasing the laser’s power didn’t improve the graphene on organic materials, so they turned to multiple lasing and defocusing. Adjusting the process allowed them to make a micro supercapacitor in the shape of a Rice “R” on their twice-lased coconut skin.
Acording to the scientists, defocusing the laser sped the process for many materials. They explained this was because the wider beam allowed each spot on a target to be lased many times in a single raster scan. Tour also explained that this allowed for fine control over the product and defocusing enabled them to turn previously unsuitable polyetherimide into LIG.
“We also found we could take bread or paper or cloth and add fire retardant to them to promote the formation of amorphous carbon,” said Rice graduate student and co-lead author of the paper, Yieu Chyan. “Now we’re able to take all these materials and convert them directly in air without requiring a controlled atmosphere box or more complicated methods.”
Tour said that the common element of all the targeted materials appeared to be lignin. An earlier study relied on lignin, a complex organic polymer that forms rigid cell walls, as a carbon precursor to burn LIG in oven-dried wood. Cork, coconut shells and potato skins have even higher lignin content, which Tour explained, made it easier to convert them to graphene.
Tour concluded that flexible, wearable electronics may be an early market for the technique: “This has applications to put conductive traces on clothing, whether you want to heat the clothing or add a sensor or conductive pattern.”
