Ordered Structures of Functionalized Silica Nanoparticles on Gold Surfaces: Correlation of Quartz Crystal Microbalance with Structural Characterization.

Quartz crystal microbalance (QCM) is frequently used to investigate adsorption of nanometer-sized objects such as proteins, viruses, or organic as well as inorganic nanoparticles from solution. The interpretation of the data obtained for heterogeneous adsorbate layers is not straightforward in particular if the systems exhibit sizable amounts of dissipation. In this study we investigate the deposition of monodisperse, amine functionalized silica nanoparticles on gold surfaces using QCM with dissipation (QCM-D) to obtain frequency and dissipation changes during adsorption from the liquid phase. These investigations are combined with ex situ scanning electron microscopy (SEM) measurements to study both coverage as well as lateral arrangement of the particles. An ordered layer of particles is found at saturation coverage due to the charged particle surface resulting in a repulsive interaction between the particles. The repulsion ensures a minimal distance between the particles, which leads to a saturation coverage of 15% for particles of 137 nm diameter. The frequency shift is shown to be a linear function of coverage which is a behavior expected for an elastic medium according to the Sauerbrey equation. However, the system shows a strong dependence of the normalized frequency shift on the overtones as well as a large dissipation, which is a clear indication for a system with viscoelastic properties. The analysis of the data show that a reliable determination of the adsorbed mass solely on the basis of QCM-D results is not possible, but additional information as determined by SEM in the present case is required to determine the coverage. From a correlation of the QCM-D results with the structural characterization it is possible to infer that the dissipation is a long ranged phenomenon. A lower boundary of the interaction length could be derived being twice the particle diameter for the particles studied here. In contrast to that the frequency response behaves like local phenomenon.