Noncovalent Control of the Electrostatic Potential of Quantum Dots through the Formation of Interfacial Ion Pairs

This paper describes the role of tetraalkylammonium counterions [NR₄⁺, R = -CH₃, -CH₂CH₃, -(CH₂)₂CH₃, or -(CH₂)₃CH₃] in gating the electrostatic potential at the interface between the 6-mercaptohexanoate (MHA) ligand shell of a PbS quantum dot (QD) and water. The permeability of this ligand shell to a negatively charged anthraquinone derivative (AQ), measured from the yield of electron transfer (eT) from the QD core to AQ, increases as the steric bulk of NR₄⁺ increases (for a given concentration of NR₄⁺). This result indicates that bulkier counterions screen repulsive interactions at the ligand/solvent interface more effectively than smaller counterions. Free energy scaling analysis and molecular dynamics simulations suggest that ion pairing between the ligand shell of the QD and NR₄⁺ results from a combination of electrostatic and van der Waals components, and that the van der Waals interaction promotes ion pairing with longer-chain counterions and more effective screening. This work provides molecular-level details that dictate a nanoparticle's electrostatic potential and demonstrates the sensitivity of the yield of photoinduced charge transfer between a QD and a molecular probe to even low-affinity binding events at the QD/solvent interface.