LEDs made from sugary drinks could reduce waste and replace other light sources made with toxic elements. A process for turning food, beverages and even combustion exhaust into light-emitting quantum dots (QDs), which are subsequently integrated into LEDs, has been developed by researchers at the University of Utah. The luminescence of carbon dots can be seen when irradiated with UV light. Courtesy of Prashant Sarswat. Many types of QD are based on rare earth materials, making them expensive to synthesize and potentially harmful to dispose of. QDs derived from waste, on the other hand, are carbon-based. That means they have lower toxicity, better biocompatibility and a broader variety of potential applications. "QDs derived from food and beverage waste are not based on common toxic elements such as cadmium and selenium, which makes their processing and disposal more environmentally friendly than it is for most other QDs," said professor Michael Free. "In addition, the use of food and beverage waste as the starting material for QDs allows for reduced waste and cost to produce a useful material." The researchers created carbon dots via solvothermal synthesis, where waste was placed into a solvent under pressure and high temperature for 30 to 90 minutes. In this case, the raw materials were soft drinks and pieces of bread and tortilla. A small LED fabricated from carbon dots. Courtesy of Prashant Sarswat. To measure the optical and material properties of the resulting carbon dots, the researchers used Fourier transform IR spectroscopy, x-ray photoelectron spectroscopy, Raman imaging and atomic force microscopy. Sugars dissolved in the soft drinks (sucrose and D-fructose) were found to be the most effective sources. "Synthesizing and characterizing [carbon dots] derived from waste is a very challenging task," said professor Prashant Sarswat. "We essentially have to determine the size of dots, which are only 20 nm or smaller in diameter, so we have to run multiple tests to be sure [carbon dots] are present and to determine what optical properties they possess." After synthesis and testing, the carbon dots were suspended in epoxy resins, heated and hardened to create practical LEDs. "The ultimate goal is to do this on a mass scale and to use these LEDs in everyday devices," Sarswat said. "To successfully make use of waste that already exists, that's the end goal." The research was published in Physical Chemistry Chemical Physics (doi: 10.1039/C5CP04782J).
