Department of Chemistry, University of Tennessee, Knoxville, TN 37996, United States.
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States.
J Colloid Interface Sci. 2019 Mar 1;537:389-395. doi: 10.1016/j.jcis.2018.11.011. Epub 2018 Nov 7.
Controlling nanomaterial interfaces for emerging technologies has driven the need to understand the molecular species located there; however, challenges arise using traditional analytical techniques to directly characterize the molecular structure and local environments of these interfacial species due to their low relative populations. We hypothesized that vibrational sum frequency generation (vSFG) spectroscopy would be uniquely sensitive to the chemical modification of nanoparticle surfaces that is obscured using traditional bulk sensitive methods.
Octadecylamine ligands were removed from model CdSe quantum dot surfaces using a common precipitation-resuspension procedure with polar protic and aprotic nonsolvents. Vibrational spectra of the ligands at the surface were collected with vSFG to directly probe the ligand ordering and coverage. Photoluminescence (PL), optical absorption, NMR, and mass spectrometry measurements were conducted for comparison.
vSFG was found to be sensitive to subtle changes in ligand disorder over multiple precipitation-resuspension washes, and a limit to the number of ligand molecules removed from the surface and subsequent amount of disorder introduced to their packing was clearly observed. We also find that nonsolvents do not remain associated with the surface after washing.
控制新兴技术中的纳米材料界面，推动了人们对位于界面处的分子种类的理解需求；然而，由于这些界面物种的相对丰度较低，传统的分析技术在直接表征这些界面物种的分子结构和局部环境方面存在挑战。我们假设振动和频产生（vSFG）光谱对纳米粒子表面的化学修饰具有独特的敏感性，而传统的体相敏感方法则掩盖了这种化学修饰。
使用常见的极性质子和非质子去溶剂沉淀-再悬浮程序，从模型 CdSe 量子点表面去除十八胺配体。用 vSFG 收集表面配体的振动光谱，以直接探测配体的有序性和覆盖率。进行光致发光（PL）、光吸收、NMR 和质谱测量进行比较。
发现 vSFG 对配体无序的细微变化非常敏感，在多次沉淀-再悬浮洗涤过程中，可以明显观察到从表面去除的配体分子数量的限制，以及它们的堆积无序程度的增加。我们还发现，在洗涤后，非溶剂不再与表面结合。
