Charge Transfer as a Probe of the Permeability of Organic Adlayers on Colloidal Semiconductor Quantum Dots

在由大分子、超分子和纳米化学项目资助的研究中，西北大学的艾米丽·韦斯正在进行研究，以寻找在被称为纳米颗粒的小物体上覆盖有机物质的方法，以便将它们转化为更有用的材料。由金属或半导体物质组成的纳米粒子在各种技术中都有应用，包括能源、医学领域以及化学和生物传感器。然而，这些非常小的颗粒的表面本质上是不稳定的，所以为了使它们更有用，它们通常必须在表面覆盖一层有机分子，使表面在化学和电子上都是均匀的。这一有机层还形成了一道物理屏障，阻碍了其他分子的接近，限制了它们的吸附。因此，有机层起到了半透膜的作用，保护了纳米颗粒，使其更有用。这项研究通过帮助研究人员改善纳米颗粒在分析、治疗和能源应用中的使用，正在产生更广泛的影响。潜在的应用包括更好的耐腐蚀性手段，化学和生物物质的特定检测，以及更好地将药物靶向人体所需位置的新方法。这项工作通过参与本科生和历史上在科学界代表性不足的群体的成员的研究，正在产生进一步的广泛影响。作为该项目的一部分，一名本科生还在帮助西北大学重新设计普通化学的课程，使所有学生都更容易接触到它。这项研究正在开发将有机包覆纳米颗粒(NP)转化为功能材料的方法，方法是促进表面化学反应的设计，精确控制纳米颗粒在各种环境中发生的化学反应、氧化还原反应和吸附事件的类型。为了做到这一点，研究人员正在寻找方法来控制1)NP与感兴趣的邻近分子的相互作用，同时最小化非特异性或非生产性相互作用，以及2)有机单层在各种化学环境中的稳定性。本研究项目的一个具体目的是通过测量胶体半导体量子点(QD)和分子氧化还原探针之间的界面电荷转移(CT)来确定不同环境条件下胶体半导体量子点(QD)上有机附着层的化学组成与这些附着层对小分子的渗透性之间的关系。纳米粒子的化学功能化是控制纳米粒子反应性的最通用、精确可调的方法，因为自组装单分子膜(SAM)已被证明是分子识别层。这项工作的智力价值在于它定量地表征了附着层的化学结构与其对小分子的渗透性之间的关系，并决定了我们可以通过粒子的表面化学来控制QD-分子相互作用的精度。本研究探讨了附着层在调节其稳定性和渗透性方面的四个性质：(I)天然配体的结合常数，(Ii)天然配体壳的分子间有序性，(Iii)配体壳与溶剂界面的电荷分布，以及(Iv)配体壳的疏水性/疏油性。

In research funded by the Macromolecular, Supramolecular and Nanochemistry Program, Emily Weiss of Northwestern University is carrying out research to find ways to coat small objects known as nanoparticles with organic substances so that they can be converted into more useful materials. Nanoparticles composed of metal or semiconductor substances find application in a variety of technologies, including energy, the medical field, and in chemical and biological sensors. The surfaces of these very small particles are inherently unstable, though, so to make them more useful they must typically be coated with a layer of organic molecules that make the surface chemically and electronically homogeneous. This organic layer also presents a physical barrier that impedes the approach of other molecules and limits their adsorption. The organic layer, therefore, acts like a semi-permeable membrane, protecting the nanoparticle and making it more useful. This research is having a broader impact by helping investigators improve the use of nanoparticles in analytical, therapeutic and energy applications. Potential applications include better means of corrosion resistance, specific detection of chemical and biological substances and new ways to better target drugs to the desired location in the body. The work is having a further broad impact through the involvement in the research of undergraduates and members of groups historically under-represented in science. As part of the project, an undergraduate is also helping to redesign the curriculum for General Chemistry at Northwestern to make it more accessible to all students.This research is developing ways to convert organic-coated nanoparticles (NPs) into functional materials by facilitating the design of surface chemistries that precisely control the types of chemical reactions, redox reactions, and adsorption events that nanoparticles undergo in a variety of environments. To do this, the investigators are finding ways to control 1) the interaction of the NP with proximate molecules of interest while minimizing non-specific or unproductive interactions, and 2) the stability of the organic monolayer in various chemical environments. A specific aim of this research project is to determine the relationship between the chemical composition of organic adlayers on colloidal semiconductor quantum dots (QDs) and the permeability of these adlayers to small molecules, under various environmental conditions, using measurements of interfacial charge transfer (CT) between the QD and molecular redox probes. Chemical functionalization of NPs is the most versatile, precisely tunable method for controlling the reactivity of a NP, because self-assembled monolayers (SAMs) have been shown to act as molecular recognition layers. The intellectual merit of this work is that it quantitatively characterizes the relationship between the chemical structure of the adlayer and its permeability to small molecules, and determines the precision with which we can control QD-molecule interactions through the surface chemistry of the particle. This study explores four properties of the adlayer in tuning its stability and permeability: (i) the binding constant of the native ligands, (ii) the intermolecular order of the native ligand shell, (iii) the charge distribution at the interface between the ligand shell and the solvent, and (iv) the hydrophobicity/oleophobicity of the ligand shell.