NMR and Structure Analysis Unit, Ghent University, Krijgslaan 281 S4, B-9000 Ghent, Belgium.
J Am Chem Soc. 2010 Jul 28;132(29):10195-201. doi: 10.1021/ja104351q.
We use solution NMR techniques to analyze the organic/inorganic interface of CdSe quantum dots (Q-CdSe) synthesized using oleic acid as a surfactant. It is shown that the resulting Q-CdSe are stabilized by tightly bound oleic acid species that only exchange upon addition of free oleic acid. The NMR analysis points toward a two-step exchange mechanism where free ligands are initially physisorbed within the ligand shell to end up as bound, chemisorbed ligands in a second step. Importantly, we find that every ligand is involved in this exchange process. By addition of oleic acid with a deuterated carboxyl headgroup, we demonstrate that the bound ligands are oleate ions and not oleic acid molecules. This explains why a dynamic adsorption/desorption equilibrium only occurs in the presence of excess free oleic acid, which donates the required proton. Comparing the number of oleate ligands to the excess cadmium per CdSe quantum dot, we find a ratio of 2:1. This completes the picture of Q-CdSe as organic/inorganic entities where the surface excess of Cd(2+) is balanced by a double amount of oleate ligands, yielding overall neutral nanoparticles.
我们使用溶液 NMR 技术来分析使用油酸作为表面活性剂合成的 CdSe 量子点（Q-CdSe）的有机/无机界面。结果表明，所得的 Q-CdSe 被紧密结合的油酸物种稳定，只有在添加游离油酸时才会发生交换。NMR 分析表明存在两步交换机制，其中游离配体最初在配体壳中物理吸附，最终在第二步中成为结合的化学吸附配体。重要的是，我们发现每个配体都参与了这个交换过程。通过添加带有氘代羧基头的油酸，我们证明了结合的配体是油酸根离子而不是油酸分子。这解释了为什么只有在存在过量游离油酸的情况下才会发生动态吸附/解吸平衡，因为油酸提供了所需的质子。将油酸配体的数量与每个 CdSe 量子点上的过量镉进行比较，我们发现其比值为 2:1。这就完成了 Q-CdSe 作为有机/无机实体的描述，其中表面过剩的 Cd(2+) 被两倍量的油酸根配体平衡，从而产生整体中性纳米颗粒。
