Deciphering Mixed Self-Assembled Monolayers on Metal Oxide Nanoparticle Surfaces

Self-assembled monolayers (SAMs) composed of various chemical moieties have been widely employed to engineer the surface functionality of nanomaterials. The incorporation of binary mixtures of SAM-forming ligands with distinct chemical structures and grafting behaviors enables the development of sophisticated surface properties. Precise control and characterization of the surface state are essential, and thus accessible analytical tools are needed to determine the composition of mixed SAMs. Fourier-transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) is a widely used technique for investigating mixed SAMs on nanoparticles (NPs), providing qualitative insights into their chemical structure. Furthermore, quantitative analysis can be achieved by examining the IR absorbance peak intensity ratios. In this study, we present a comprehensive investigation of 15 different phosphonic acid (PA)-based ligands forming mixed SAMs on metal oxide NPs, aiming to promote the use of the IR absorbance peak ratio as a quantitative analytical method. Additionally, we establish a correlation between the ligand stoichiometry in solution during functionalization and the resulting surface composition in mixed SAMs, as governed by their grafting behavior. Our findings indicate a direct translation of the solution ratio to surface ratio for ligands with similar grafting behavior. However, in mixtures of ligands with different grafting behaviors, the ligand with a higher grafting density is overrepresented on the surface.