Investigation of the Ligand–Nanoparticle Interface: A Cryogenic Approach for Preserving Surface Chemistry

• Published in: Journal of physical chemistry. C 2018-02, Vol.122 (6), p.3582-3590
• Main Authors: Karakoti, Ajay S, Yang, Ping, Wang, Weina, Patel, Vaishwik, Martinez, Abraham, Shutthanandan, Vaithiyalingam, Seal, Sudipta, Thevuthasan, Suntharampillai
• Format: Article
• Language: English
• Subjects: Environmental Molecular Sciences Laboratory
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.7b09930

Ligand-functionalized nanoparticles have replaced bare nanoparticles from most biological applications. These applications require tight control over size and stability of nanoparticles in aqueous medium. Understanding the mechanism of interaction of nanoparticle surfaces with functional groups of different organic ligands such as carboxylic acids is confounding despite the two decades of research on nanoparticles because of the inability to characterize their surfaces in their immediate environment. Often the surface interaction is understood by correlating the information available, in a piecemeal approach, from surface sensitive spectroscopic information on ligands and the bulk and surface information on nanoparticles. In present study we report the direct interaction of 5–7 nm cerium oxide nanoparticles surface with acetic acid. An in-situ XPS study was carried out by freezing the aqueous solution of nanoparticles to liquid nitrogen temperatures. Analysis of data collected concurrently from the ligands as well as functionalized frozen cerium oxide nanoparticles show that the acetic acid binds to the ceria surface in both dissociated and molecular state with equal population over the surface. The cerium oxide surface was populated predominantly with Ce4+ ions consistent with the thermal hydrolysis synthesis. DFT calculations reveal that the acetate ions bind more strongly to the cerium oxide nanoparticles as compared to the water molecules and can replace the hydration sphere of nanoparticles resulting in high acetate/acetic surface coverage. These findings reveal molecular level interaction between the nanoparticle surfaces and ligands, giving a better understanding of how materials behave in their immediate solution environment. This study also proposes a simple and elegant methodology to directly study the surface functional groups attached to nanoparticles in their immediate solution environment.