Scientists who introduced laser-induced graphene (LIG) enhanced their technique to produce what may become a new class of edible electronics.
The chemists, who once turned Girl Scout cookies into graphene, are investigating ways to write graphene patterns onto food and other materials to quickly embed conductive identification tags and sensors into the products themselves.
“This is not ink,” says James Tour, chair of chemistry and professor of computer science and of materials science and nanoengineering at Rice University. “This is taking the material itself and converting it into graphene.”
The process is an extension of the idea that anything with the proper carbon content can be turned into graphene. In recent years, the lab 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. LIG can be written into target materials in patterns and used as a supercapacitor, an electrocatalyst for fuel cells, radio-frequency identification (RFID) antennas, and biological sensors, among other potential applications.
The new work demonstrates 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 says. “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.”
LIG tags could also be sensors that detect E. coli or other microorganisms on food, Tour says. “They could light up and give you a signal that you don’t want to eat this. All that could be placed not on a separate tag on the food, but on the food itself.”
The researchers used multiple laser passes with a defocused beam to write LIG patterns into cloth, paper, potatoes, coconut shells, and cork, as well as toast. (The bread is toasted first to “carbonize” the surface.) The process happens in air at ambient temperatures.
“In some cases, multiple lasing creates a two-step reaction,” Tour says. “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 turned to multiple lasing and defocusing when they discovered that simply turning up the laser’s power didn’t make better graphene on a coconut or other organic materials. But adjusting the process allowed them to make a micro supercapacitor in the shape of a Rice “R” on their twice-lased coconut skin.
Defocusing the laser sped the process for many materials as the wider beam allowed each spot on a target to be lased many times in a single raster scan. That also allowed for fine control over the product, Tour says. Defocusing allowed 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,” says Yieu Chyan, graduate student and co-lead author. “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.”
The common element of all the targeted materials appears to be lignin, Tour says. 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 made it easier to convert them to graphene.
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,” Tour says.
Rice alumnus Ruquan Ye is co-lead author of the study in ACS Nano. Other coauthors are from Rice and from Ben-Gurion University. The Air Force Office of Scientific Research supported the research.
Source: Rice University