Fluorescent carbon dots can be made by plasma pyrolysis and used as printer ink. Image: Wiley-VCH
carbon nanoparticles based on carbon exhibit advantageous optical
properties. They are also biocompatible, and therefore better suited for
imaging procedures in the biosciences than metal-based semiconductor
quantum dots. A variety of processes have thus been developed to make
these miniature objects known as carbon dots or C-dots. Chinese
researchers have now introduced a new method in the journal Angewandte Chemie,
by which C-dots can be produced particularly quickly and inexpensively.
In addition, they have demonstrated the use of these luminescent dots
as printer ink.
new process is based on plasma-induced pyrolysis. A plasma is a gas
whose components have been partially or completely separated into ions
and electrons. This plasma is highly reactive and energetic; its uses
include plasma welding, for which it serves as the heat source. A team
led by Su Chen at Nanjing University of Technology directed a special
plasma beam onto a small amount of egg yolk or egg white in order to
carbonize the material. Within a few minutes, they formed C-dots with a
yield of about 6 %.
C-dots obtained from the egg yolk have a crystalline structure and a
diameter of about 2.2 nm; those from the egg white are amorphous and
measure about 3.4 nm. They contain mainly graphitic structures. Oxygen
and nitrogen atoms are also bound to the surface in various ways,
allowing for good solubility in a broad range of aqueous and organic
solvents. The C-dots are not sensitive to acids or bases. Under UV
light, they fluoresce bright blue. It is presumed that the luminescence
comes from passivated surface defects that “trap” the excitatory UV
light like antennae.
researchers were able to follow the pyrolytic process by means of
thermogravimetric analysis and IR spectroscopy: it includes the
uncoiling and breaking down of proteins as well as various chemical
reactions. Towards the end, primarily carbon dioxide, ammonia, and water
“This process is not limited to eggs,” explains Chen, “it works with many inexpensive natural carbon sources, including sugars.”
scientists fabricated inks based on the luminescent C-dots and printed
glowing patterns on a variety of surfaces by both inkjet and silk-screen
printing processes. The addition of small amounts of organic dyes or
semiconducting quantum dots allows the color of the ink to be varied.
fluorescent inks may be useful in optoelectronic applications,” says
Chen, “for example in forgery-proof labeling and optoelectronic sensor