Diffusion Behaviors of Water-Soluble CdSe/ZnS Core/Shell Quantum Dots Investigated by Single-Particle Tracking

Abstract

As is known to us all, quantum dots (QDs), the fluorescent semiconductor nanocrystals, have many excellent optical properties which make them attractive fluorescent tags in single-molecule tracking in live cells. Because the intracellular environment is so complex, this paper aimed at simulating the intracellular solution environment in vitro and investigated the influence of the solution environment on the diffusion of water-soluble core/shell CdSe/ZnS QDs. Single-particle tracking (SPT) was applied to measure the diffusion coefficients of two water-soluble core/shell QDs, CTAB-modified CdSe/ZnS QDs (CTAB-QDs) and octylamine-modified poly(acrylic acid)-modified CdSe/ZnS QDs (OPA-QDs). The exposure time was optimized to be 29.95 ms. Then the paper was focused on the effects of pH value, salt concentration, and solution viscosity on the diffusion coefficients of the two water-soluble QDs. The results demonstrated that the pH value had a great influence on the diffusion coefficient of CTAB-QDs but little on that of OPA-QDs. The difference should be mainly due to the distinguishment of the charge and structure of surface ligands on the two water-soluble QDs. The diffusion coefficient of either CTAB-QDs or OPA-QDs was hardly affected by the salt concentration of the solution. Furthermore, for both CTAB-QDs and OPA-QDs, the diffusion coefficients decreased as the solution viscosity increased, which obeyed the Stokes−Einstein relation. In summary, OPA-QDs have more promising applications in single-molecule tracking in live cells, as compared with CTAB-QDs. The obtained results would benefit the further applications of QDs in single-molecule tracking in live cells. This system could also serve as a model system for studying the diffusion behavior of nanoparticles.