From bread to bulb


Wednesday, 11 November, 2015


Used in a wide variety of applications — from Christmas lights to televisions — light-emitting diodes (LEDs) have been a popular, more efficient alternative to fluorescent and incandescent bulbs for the past few decades.

Researchers have now found a way to create LEDs from food and beverage waste, which could reduce potentially harmful waste from LEDs made from toxic elements.

Unlike fluorescent and incandescent bulbs, which direct 80% of the energy consumed to producing heat, LEDs direct 80% of the energy consumed to producing light. This is made possible by the fact that LEDs do not require a filament to be heated as incandescent and fluorescent bulbs do.

LEDs can be produced by using quantum dots, or tiny crystals that have luminescent properties, to produce light. Quantum dots (QDs) can be made with numerous materials, some of which are rare and expensive to synthesise, and even potentially harmful to dispose of. Some research over the past 10 years has focused on using carbon dots (CDs), or simply QDs made of carbon, to create LEDs instead.

Compared to other types of quantum dots, CDs have lower toxicity and better biocompatibility, meaning they can be used in a broader variety of applications.

Researchers from The University of Utah have successfully turned food waste, such as discarded pieces of tortilla, into CDs and, subsequently, LEDs. The results were published in Physical Chemistry Chemical Physics, a journal of the Royal Society of Chemistry.

To synthesise waste into CDs, Professor Prashant Sarswat and Professor Michael Free employed a solvothermal synthesis, in which the waste was placed into a solvent under pressure and high temperature until CDs were formed. In this experiment, the researchers used soft drinks and pieces of bread and tortilla.

“Synthesising and characterising CDs derived from waste is a very challenging task. We essentially have to determine the size of dots which are only 20 nanometres or smaller in diameter, so we have to run multiple tests to be sure CDs are present and to determine what optical properties they possess,” said Sarswat.

For comparison, a human hair is around 75,000 nanometres in diameter.

The various tests Sarswat and Free ran first measured the size of the CDs, which correlates with the intensity of the dots’ colour and brightness. The tests then determined which carbon source produced the best CDs. For example, sucrose and D-fructose dissolved in soft drinks were found to be the most effective sources for production of CDs.

Finally, the CDs were suspended in epoxy resins, heated and hardened to solidify the CDs for practical use in LEDs.

An environmentally sustainable alternative

Currently, one of the most common sources of QDs is cadmium selenide, a compound comprising two toxic elements. The ability to create QDs in the form of CDs from food and beverage waste would eliminate the need for concern over toxic waste, as the food and beverages themselves are not toxic.

“Cadmium selenide is also expensive, compared with food and beverage waste that is essentially free,” said Sarswat.

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