Manipulated wettability of a superhydrophobic quartz crystal microbalance through electrowetting

The liquid phase response of quartz crystal microbalances (QCM) with a thin coating (~9 µm) of epoxy resin with and without a carbon nanoparticles top layer is reported. The nanoparticles convert the epoxy surface to a superhydrophobic one with a high static contact angle (~151º-155º) and low contact angle hysteresis (~1º-3.7º) where droplets of water are in the suspended Cassie-Baxter state. The frequency decrease of the fully immersed QCM with the superhydrophobic surface is less than with only epoxy layer, thus indicating a decoupling of the QCM response. A wettability transition to a liquid penetrating into the surface roughness state (for droplets a high contact angle hysteresis Wenzel state) was triggered using a molarity of ethanol droplet test (MED) and electrowetting; the MED approach caused some surface damage. The electrowetting induced transition caused a frequency decrease of 739 Hz at a critical voltage of ~100 V compared to the QCM in air. This critical voltage correlates to a contact angle decrease of 26º and a high contact angle hysteresis state in droplet experiments. These experiments provide a proof-of-concept that QCMs can be used to sense wetting state transitions and not only mass attachments or changes in viscosity-density products of liquids

NMR characterization of ligand binding and exchange dynamics in triphenylphosphine-capped gold nanoparticles

Triphenylphosphine (PPh3)-capped 1.8 nm diameter gold nanoparticles (AuNPs) are characterized by a combination of 1H, 2H, and 31P solution- and solid-state NMR. The 31P{1H} NMR resonance associated with the surface-bound PPh3 is clearly identified and is present as a broad peak centered at 56 ppm. 31P and 1H hole burning NMR experiments show that the line broadening associated with the surface-bound PPh3 is primarily due to a variety of different chemical shift environments at the surface of the nanoparticles. The surface bound PPh3 can be displaced with either d15-PPh3 or Au(d15-PPh3)Cl in CD2Cl2 solution. In both cases, exchange results in loss of Au(PPh3)Cl from the nanoparticle surface, with no evidence for loss of the PPh3 ligand alone. Solution-state NMR was used to determine the room temperature rate constants for these exchange processes, with values of 0.17 and 0.20 min-1, respectively. Thus, essentially the same rate is observed for displacement of Au(PPh3)Cl from the surface with either d15-PPh3 or Au(d15-PPh3)Cl. The observed 31P chemical shift of surface-bound PPh3 is consistent with mixed valence Au(0) and Au(I) at the nanoparticle surfaces, suggesting the presence of surface-bound Au complexes