Quantum dots are usually coated (or functionalized) with hydrophilic monolayers to make them more soluble in the water-based environment found in living cells and with functional groups so that they recognize and attach to specific biomolecules. Quantum dots are bright and photostable and their fluorescence emission can be tuned by varying the size of the dots. The fluorescence can be monitored using laser scanning microscopy, for example, and so be used to image biological cells in either in vitro or in vivo.

However until now, no one had studied how stable these dots actually were in biological environments. A team led by Richard Vachet and Vincent Rotello has now done just this – with some unexpected results.

The team found that the quantum dots' stability depends on how big they are and what they are coated with. For example, a coating becomes less stable as the dot it is coating becomes bigger. The coating is very important because if it is lost, the dots can become toxic to living cells. The results also show that monolayer anchor groups (such as monothiol) attached to the dots are less stable than dithiolate ligands and fall off more easily.

Losing layers
The researchers obtained their results using inductively coupled plasma mass spectrometry (ICP-MS) to analyse the core material of the quantum dots and laser desorption/ionization (LDI) to quantify the amount of coating attached to the dots. "Since we know how much coating should be attached, any differences between the quantities measured by ICP-MS and LDI-MS reflect the amount of coating layer that has been lost," explained Vachet.

"Our results indicate that quantum dots are not as stable as people think," he told nanotechweb.org. "Many scientists coat quantum dots with biomolecules, such as proteins, so that they can visualize the location of that protein in a cell or so that they can target the quantum dots to some sub-cellular location. If the quantum dots are not stable, then the resulting information could be misleading or even totally incorrect."

Our work provides a caveat to researchers who routinely use quantum dots as labelling agents, he added, but the technique is also a general way to measure how stable quantum dots, and indeed other nanoparticles, are in biological cells.

Analyses in this study were performed on HeLa cancer cell lines, and the researchers now hope to extend their technique to test the stability of quantum dots in in vivo.

The current work is detailed in Nature Chemistry.