Luminescent nanodots are useful materials for cell imaging, optoelectronics and solar cells; however, highly toxic chemicals are needed to prepare these materials, raising concerns over their safety. Now, scientists from Taiwan have developed a less toxic way to create luminescent carbon nanodots by using waste coffee grounds.

Photoluminescent quantum dots (QDs) such as CdS and CdSe are good for imaging and sensing because of their high quantum yields, but toxic cadmium species leaching into biological systems is a big concern. Gold and silver nanodots have also shown potential for use as sensors, but their quantum yields are lower and they are more expensive. The advantage of using carbon nanodots (C-dots) over other types of QDs are that they are biocompatible and have good chemical and photostability. The method developed by Huan-Tsung Chang and co-workers from the National Taiwan University, Taipei, takes advantage of a waste feedstock to produce these potentially useful luminescent materials.

The team produced the C-dots by grinding the coffee grounds followed by heating and simple separation. 'Unlike most reported approaches, strong acid is not required in our approach,' explains Chang. 'It has the advantage of simplicity, large-scale preparation and biocompatibility.' They characterised the C-dots by a variety of analytical techniques and then, they used them to image cells and in surface-assisted laser desorption/ionisation mass spectrometry to detect angiotensin I (a peptide that causes blood vessels to constrict, driving up blood pressure).  

Rafael Luque, an expert in converting biomass and waste into useful chemicals and materials at the University of Cordoba in Spain, tells Chemistry World: 'This is a very nice example that illustrates the potential of biomass and waste valorisation for valuable chemicals and materials. While the potential of carbon nanodots in sensing, imaging, solar cells and related areas still needs to be fully demonstrated, these results and materials may constitute a promising starting point as alternatives to other more developed systems such as semiconductor quantum dots like CdS and CdSe.' 

Chang and the team will be working on preparing high quality C-dots from a variety of organic compounds in the future. 'We are also interested in preparing functionalised C-dots for sensing, targeting and suppression of cancer cells,' he says. 

Emma Eley 

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