Quantum dots are luminescent nanoparticles whose optical properties are dependent on size. They can be used in noninvasive medical imaging and targeting; however they usually contain heavy metals, so a biocompatible, polymeric shell surrounding the particles is critical for in vivo applications.
Silica coatings are common for quantum dots that need biocompatibility; this is not surprising considering that amorphous silica is a common food additive. Silica-coated quantum dots have been made in various ways, but the preparations typically involve organometallic routes followed by a solubilization process to make the particles water-soluble. The nontrivial synthetic procedures make it difficult to modify existing syntheses for testing new coatings, so optimizing the properties of the coated particles for imaging can be challenging.
Now, in new work, Nan Ma et al. at Stanford University prepare CdTe nanoparticles with multilayered silica shells of highly negative charge using a simple and mild synthetic route. With these nanoparticles, they also succeed for the first time in evaluating how silica-coated quantum dots are distributed within a living animal.
Both the nanoparticle preparation and the silanization are carried out in air using water or alcohol as the solvents. A silanization process is unnecessary as the particles are already water-soluble. Compared with other silica-encapsulated quantum dots, the researchers discovered that they could achieve smaller particle sizes. This means that the particles can fluoresce at higher energies than previously reported silica-covered particles. Additionally, the new particles exhibit higher quantum yields than the silica-coated particles produced using conventional methods.
To investigate the biocompatibility of their silica-coated quantum dots, the researchers ran in vivo experiments to compare the in vivo behaviour of the new particles with that of commercial quantum dots bearing a carboxylate outer ligand coating, but having similar size and luminescence characteristics. Relative to the commercial particles, the silica-coated quantum dots are found to remain within the blood of living mice for longer periods of time. A common problem in imaging and targeting applications is that the particles are too quickly eliminated from the body by the immune system. The longer blood retention time suggests that these particles would be more useful for in vivo applications. Furthermore, a higher kidney uptake is observed, indicating that the quantum dots will likely be cleared by the renal system. Despite the higher biocompatibility, the particles should eventually be removed from the body, and renal clearance would be the preferred way, allowing the particles just enough time in the system to assist in imaging. These newly developed silica-coated quantum dots should help in the establishment of the routine use of quantum dots for in vivo applications in imaging and targeting.