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

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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.