A technique for synthesizing nanocrystals with highly improved brightness is reported in a paper published online this week in Nature Nanotechnology.
Upconversion nanocrystals are a promising material for applications in biodetection, bioimaging, solar cells and 3D display technologies as they convert infrared radiation to visible light. To improve the functioning of the nanocrystals, two ion species - a sensitizer and an activator - can be added. These are known as dopants. Sensitizer ions absorb infrared radiation and transfer their excitation to activator ions, which then emit visible light. The more dopants there are the higher the emission brightness; however, at concentrations of activator ions above a relatively low threshold, the brightness diminishes. The brightness of upconversion nanocrystals is therefore intrinsically limited.
Dayong Jin and colleagues investigated ways to overcome this limitation and found that high levels of infrared radiation, combined with higher activator concentrations in excess of the threshold, led to significantly enhanced luminescence signals by up to a factor of 70. These single nanocrystals were bright enough to be visible to the naked eye, could be recorded by a low-cost digital camera and were used to provide high-contrast biolabels for the tracking of individual cells.