Until recently perhaps the best use for used coffee grounds was the lowest of low-tech: to top up the compost pile. Now, researchers in Taiwan have used the waste material as a novel carbon source for synthesizing high-tech nanodots. The developers suggest that approach could be a viable alternative to semiconductor quantum dots that might be used as photoluminescent materials for sensing, imaging, in photovoltaic solar cells and in optoelectronic devices [P-C Hsu et al., Green Chem (2012) doi: 10.1039/c2gc16451e].
Huan-Tsung Chang of the National Taiwan University, Taiwan and colleagues have obtained carbon-dots with an average diameter of about 5 nanometres, which provide a quantum yield of 3.8 %. The team has tested the viability of these materials in cell imaging studies and surface-assisted laser desorption/ionization-mass spectrometry (SALDI-MS) of important proteins, including insulin and angiotensin I.
Quantum dots have gained enormous visibility in recent years as photostable materials for use in putative optoelectronics devices and other applications, including sensing and imaging for biomedical research, environmental monitoring and medical diagnostics. However, the toxic heavy metal cadmium is the usual base for selenide and sulfide semiconductors in this field. As such, there are numerous safety and environmental concerns regarding these early quantum dots not least in medical applications.
Various approaches, such as coating quantum dots with a polymer or developing gold and silver alternatives, have been investigated as alternatives to cadmium semiconductors. Finding alternatives that retain the strong emission profile, are stable, and inexpensive remains high on the agenda. Of course, little can be as cost effective as re-using waste material and so it was that Chang and colleagues turned to used coffee grounds to make carbon nanodots. Other researchers have exploited graphite powders, oxidation of nanotubes and other materials for making C-dots. The Taiwanese team found that grinding, dehydration, polymerization, and carbonization of coffee grounds were sufficient to produce C-dots in large quantities.
Spectroscopic and x-ray analysis as well as high-resolution electron microscopy revealed the nature of their C-dots and the broad range of sizes produced (averaging 5 nm +/− 2 nm. The emission spectra of the C-dots range from blue (400 nm wavelength) to red (600 nm), depending on the wavelength of the incident excitatory light.
The team describes their approach to C-dots as “green” not only because it recycles a waste materials but also because it avoids the strong acid and passivation steps needed in other approaches to these materials.
“The C-dots can be further used to prepare sensors, drugs, and catalysts,” Change told Materials Today. “For example, we can conjugate C-dots with various aptamers for sensing of metal ions, proteins, and DNA.” He adds that functionalised C-dots might also be used to target cancer cells for imaging and therapy.