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.
We prepared water-soluble quantum dots (QDs) with tunable ξ-potential varied from À 38 to + 20 mV and controlled sensitivity to pH level. Poly(maleic anhydride-alt-1tetradecene) (PMAT) was modified with bifunctional molecules containing sulfate, sulfonate, phosphate, phosphonate, and quaternary ammonium in different proportions. The sulfonate shell of QDs leads to a stable negative ξ-potential in the biological pH range from 4.5 to 9.5, while modification with quaternary ammonium groups gives QDs with positive and pH-independent charge. If nearly half of carboxyl groups of PMAT were modified with quaternary ammonium groups, ξ-potential begins pH-dependent with the isoelectric point at pH � 5.5. To obtain pH-independent, neutrally charged QDs, the rest of carboxyl groups can be converted into sulfonate groups at a 1 : 1 ratio; sulfonate-quaternary ammonium QDs show high colloidal stability in solutions with high ionic strength. In addition, encapsulated QDs show an unusual correlation between their hydrodynamic size and ζ-potential: when ζ-potential approaches zero, the hydrodynamic size markedly increases due to a decrease in the electrostatic repulsion component of the QDs diffusion coefficient.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.