The ζ-potential of a colloidal quantum dot (QD) in solution has a strong impact on its photoluminescence emission quantum yield as well as the population lifetime. In this study we show that varying the surface charged groups on CdSe/ZnS QDs allows one to tune the ζ-potential and, with it, to control the quantum yield of emission as well as the recombination dynamics. We infer that the net charge density within the slipping plane around the QD in the solution strongly affects the nonradiative recombination processes, depending on the surface charge sign and value. For zwitterionic surface groups it is possible to tune the ζ-potential and the quantum yield by pH. As a general trend, QDs with zwitterionic surface groups produce a low (absolute) ζ-potential value and exhibit the highest quantum yield. Our results pave the way to, for example, future intracellular, time-resolved pH sensing applications with similar systems.

Since CdSe nanoplatelets were reported to have a ten-fold higher two-photon (2P) absorption coefficient as compared to quantum dots, we examined their applicability for cell labeling and 2P imaging. CdSSe/ZnCdS core-shell nanoplatelets and CdSe/ZnS quantum dots, both emitting at 585 nm were encapsulated with an amphiphilic zwitterionic polymer having slightly positive zeta potential. As measured with flow cytometry, glioma C6 cells demonstrated equally efficient uptake of nanoplatelets and quantum dots, despite the different sizes of these two types of nanoparticles. 2P fluorescence microscopy revealed ca. two orders of magnitude higher fluorescence response from nanoplatelets thus offering a chance to use them as highly efficient 2P fluorescent labels in biomedicine.