Quantum dots are tiny photoluminescent particles that have great potential in biosensing and biomedical imaging applications. Unfortunately, many quantum dots are made using toxic compounds such as cadmium, raising questions about their safety. Zhenhui Kang from Soochow University in Jiangsu, China, and co-workers have now found a way to synthesize carbon quantum dots (CQDs) — biocompatible nanoscale materials with tunable optical properties and remarkable energy-conversion capabilities.1

Fig. 1: Fluorescence microscopy image of highly fluorescent carbon quantum dots. © 2010 Z. Kang, Y. Liu

The researchers prepared their CQDs by immersing graphite electrodes in a strong base electrolyte and applying an electric current, a technique that ‘cuts’ the graphite into fragments as small as 4 nm in diameter. Fluorescence microscopy revealed that the fragments emitted various colors — blue, green, yellow and brown — depending on size, symmetry and defects (Fig. 1).

By varying the current density during the cutting process, the team was able to tune the size of the particles, and they confirmed that this induced the formation of particles with different colors of emission: lower currents favored larger, reddish-brown light-emitting particles, while higher currents reduced the size of the CQDs and shifted their light emission towards blue.

Kang and his co-workers also found that the CQDs could be easily dispersed in water, and were remarkably stable, retaining their fluorescence even after storage at ambient temperature for one year. The particles were successfully separated by size using standard chromatography techniques. The batches of small CQDs emitted ultraviolet light, while the medium-sized and larger dots emitted visible and near-infrared light.

The CQDs exhibited strong ‘upconversion photoluminescence’ — a phenomenon in which the light energy emitted by the dots exceeds the excitation energy. This is critical for many energy-transfer applications, including the harnessing of solar power.

The researchers also tested the energizing abilities of the dots by attaching them to titanium dioxide (TiO2) nanoparticles, catalysts that can break down pollutants in water and air if they can capture light energy efficiently. The team found that a CQD–TiO2 complex could almost completely degrade a sample pollutant by exposing the mixture to visible light, a feat unachievable with pure TiO2. They also observed a similar increase in efficiency with CQD-doped silica nanoparticles.

The researchers are currently planning to further exploit the upconversion photoluminescence of the dots to design new photocatalysts. “We are trying to use the CQDs for the design of in vivo bioimaging applications, solar cells, light-emitting diodes and photocatalysts,” says Kang.