Quantum dots might become more widely used in biological applications if better methods for making them water-soluble were available. Chemically coating quantum dots to improve their water solubility often makes them so large that they interfere with Förster resonance energy transfer or normal cellular processes. Silica-coated quantum dots are small and have a high surface area and a large pore size, the latter of which may be useful for biosensing and photodynamic therapy (PDT). However, silica coating often yields quantum dots that quench fluorescence because they aggregate, thereby reducing the quantum yield. Researchers at the National Institute for Advanced Industrial Science and Technology in Tosu, Japan, have modified the silica-coating method to enable the encapsulation of one quantum dot in a silica sphere by consecutively using three different silica precursors without exchanging hydrophobic coordinating ligands. The product can be conjugated with biological molecules. In the Aug. 15 issue of Analytical Chemistry, they said that their silica-coating method yielded quantum dots of uniform size — 17.42 ±2.12 nm in diameter with about a 12 percent size distribution. The quantum dots had a high quantum yield and sharp photoluminescence spectra with a full width half maximum of about 30 nm. Ninety-two percent of the silica shells contained a single quantum dot, as the researchers desired. Before silica-shelling, they coated the quantum dots with detergent micelles, which enable the incorporation of other hydrophobic molecules, such as lipid-soluble photosensitizers, and hydrophobic oxidizing and contrast agents. As a result, this technique can be used to fabricate biological probes that have multiple functions. The researchers demonstrated that ability by creating a probe with paramagnetic properties that enable its operation in magnetic resonance and fluorescence imaging. Experiments in which the quantum dots were incubated with cell lines for 48 hours showed that they were potentially nontoxic. Their small size and hydrophilicity could enable them to easily travel through the urinary system, which the investigators are currently verifying. In addition, initial studies showed that PDT performed with silica micelles containing quantum dots and phthalocyanine could kill cancer cells.