Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States.
Department of Materials Science and Engineering, Northwestern University , 2220 Campus Drive, Evanston, Illinois 60208-3108, United States.
J Am Chem Soc. 2017 Jul 26;139(29):10126-10132. doi: 10.1021/jacs.7b05501. Epub 2017 Jul 12.
This paper describes the role of tetraalkylammonium counterions [NR, R = -CH, -CHCH, -(CH)CH, or -(CH)CH] 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 NR increases (for a given concentration of NR). 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 NR 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.
本文描述了四烷基铵抗衡离子[NR，R = -CH、-CHCH、-(CH)CH 或 -(CH)CH]在调节 PbS 量子点（QD）的巯基己酸酯（MHA）配体壳与水之间的静电势中的作用。通过从 QD 核到 AQ 的电子转移（eT）产率来测量这种配体壳对带负电荷的蒽醌衍生物（AQ）的渗透性，随着 NR 空间位阻的增加（对于给定浓度的 NR）而增加。这一结果表明，较大的抗衡离子比较小的抗衡离子更有效地屏蔽配体/溶剂界面上的排斥相互作用。自由能标度分析和分子动力学模拟表明，QD 配体壳与 NR 之间的离子对形成是静电和范德华分量的组合，范德华相互作用促进了与长链抗衡离子的离子对形成和更有效的屏蔽。这项工作提供了决定纳米颗粒静电势的分子水平细节，并证明了 QD 和分子探针之间光诱导电荷转移的产率对 QD/溶剂界面上甚至低亲和力结合事件的敏感性。
