Laser-induced graphene (LIG) has been used to write graphene patterns onto food and other materials to quickly embed conductive identification tags and sensors into the products themselves. The method previously was used to make graphene foam with 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.
The new work demonstrated that laser-induced graphene can be burned into paper, cardboard, cloth, coal, and certain foods. 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. Food could have a RFID tag that provides information about where it has been, how long it has been stored, its country and city of origin, and the path it took to get to the consumer's table.
LIG tags could also be sensors that detect E. coli or other microorganisms on food. The tags would light up and provide a signal that the food is not safe to eat. That information could be placed not on a separate tag on the food, but on the food itself.
Multiple laser passes with a defocused beam allow the LIG patterns to be written 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. 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.
Defocusing the laser speeds the process for many materials, as the wider beam allows each spot on a target to be lased many times in a single raster scan. That also allows for fine control over the product. Defocusing allows previously unsuitable polyetherimide to be turned into LIG.
Bread, paper, or cloth can have fire-retardant added to them to promote the formation of amorphous carbon. The materials can be converted 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. 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.