Poly(maleic anhydride) Shell Modified with Negatively and Positively Charged Groups to Control Zeta Potential and Hydrodynamic Size of Encapsulated Quantum Dots at Variable pH

Abstract

AbstractWe prepared water‐soluble quantum dots (QDs) with tunable ξ‐potential varied from −38 to +20 mV and controlled sensitivity to pH level. Poly(maleic anhydride‐alt‐1‐tetradecene) (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.