Quantum dots (QDs) are semiconducting crystals possessing optical properties that are absent in the bulk material due to the confinement of electron-hole pairs in a region of a few nanometres. As the dots decrease in size, the emitted light becomes shorter in wavelength, moving to the blue. A rainbow of colours can be emitted from a single material simply by changing the dot size.

Due to their small diameter, water-soluble QDs with only one layer of a small thioalkyl acid capping ligand (generally thioglycolic acid TGA or 3-mercaptopropanoic acid MPA) have demonstrated broad applications as biolabels. However, for some applications like the study of biomolecular interactions at the cellular environment, the preparation of QDs with various surface hydrophobic properties is required.

In a study recently published in Nanotechnology, we explored the influence of three thioalkyl acids (MPA, 6-mercaptohexanoic acid MHA and 11-mercaptoundecanoic acid MUA) on the growth and photo-physical properties of CdTe and CdSe QDs prepared under hydrothermal conditions (high temperature liquid water). Using MPA as a stabilizer, CdTe and CdSe QDs with controllable photoluminescence wavelengths from 510 to 650 nm were prepared within one hour. The use of the more hydrophobic MHA and MUA ligands allowed the preparation of very small QDs (0.6–2.5 nm) regardless of the reaction conditions.

The current investigations not only reveal the crucial role played by the ligand during the synthesis of QDs in aqueous solution but also demonstrate that the number of carbons in the thioalkyl acid chain influences the growth rates, the fluorescence intensities and the surface coverage of QDs.

The ultra-fine particle sizes, tunable fluorescence wavelengths, good quantum yields and stabilities in physiological conditions should make these QDs attractive for bioimaging and biolabelling.