Combining Surface Plasmon Resonance and Quartz Crystal Microbalance To Determine Hydration of Dendrimer Monolayers

Polyelectrolytes are abundant in nature and crucial due to their versatility in biological systems. The controlled assembly of polyelectrolytes has potential applications in the formation of nanostructured and microstructured materials of desired structure and functionality. Dendrimers are a special class of polyelectrolytes, which are characterized by their densely branched and well-defined spherical geometry. Amino-terminal dendrimers resemble spheres, whose uniform surface charge densities can be continuously modulated by pH or ionic strength. The present study focuses on the dendrimer monolayer structure on gold surfaces. We used multiparametric surface plasmon resonance (MP-SPR) and a quartz crystal microbalance with dissipation energy monitoring (QCM-D) to investigate the conformational behavior of the sixth generation of PAMAM molecules. Both the kinetics of dendrimer deposition and the maximum surface concentration were determined. The dependence of the maximum coverage on the pH, ionic strength, and the experimental kinetic runs were quantitatively interpreted in terms of the random sequential adsorption (RSA) model using the concept of effective hard particles. It is shown that the MP-SPR measurements can be used to determine the mechanisms of dendrimer adsorption, e.g., the reversibility and orientation of molecules at interfaces. Additionally, this method can be used for a precise determination of dendrimer coverage, hence, its concentration in the bulk solution at levels of 0.05 ppm or less. The extent of hydration of dendrimer films was estimated from the combination of the QCM-D and MP-SPR data, with the assumption that the excess mass measured in QCM-D compared to MP-SPR mass is due to trapped water molecules. The structure of the resulting films is strongly dependent on the deposition conditions.