Self-assembled monolayers and thin films: Structure, interfacial interactions, and electrowetting

Vibrational spectroscopy and computational chemical methods have been used to characterize the first inorganic self-assembled monolayers (SAMs), comprised of [closo-B₁₂H₁₁S] 3−, and SAMs of L-cysteine and L-cystine on gold.; Surface-enhanced Raman spectroscopy (SERS) was used to determine the conditions for forming stable [closo-B₁₂H ₁₁S]3− SAMs on polycrystalline and nanoparticulate gold. [Closo-B₁₂H₁₁S]3− does not form monolayers on silver, but instead forms a precipitating silver salt. [Closo-B₁₂H₁₁S] 3− SAMs on gold are stable in air, in various solvents, withstand heating and are more stable to UV exposure than alkanethiolate SAMs. Density functional theory (DFT) was used to determine the electronic structure of [B₁₂H₁₂]2−, [B₁₂H ₁₁SH]2−, and [B₁₂H₁₁S] 3−, to assign vibrational modes, to predict 11B NMR chemical shifts for related borane and borate compounds, and to explain the relative adsorptivities of [B₁₂H₁₁S]3− , hexanethiolate, and benzenethiolate on gold. These inorganic monolayers offer improved oxidative stability compared with alkanethiolate monolayers since the aromatic borane cages are unusually robust.; Our previous studies had shown that L-cysteine dimerizes and that L-cystine remains intact at the water-gold interface. The goal of this project was to elucidate the driving force for L -cysteine dimerization. Molecular mechanics (MM) calculations and thermodynamic analysis were used to model the conformation and packing of L-cystine molecules on gold surfaces. It was found that hydrogen-bonding between adsorbate molecules and adsorbate-solvent interactions can account for the stability of disulfide bond of L-cystine at the water-gold interface.; The third project involved determining the conditions for moving aqueous salt droplets on hydrophobic surfaces using the electrowetting-on-dielectric (EWOD) phenomena, in which electric field was applied to improve the wettability of the dielectric surface between the electrodes. A combination of thin SiO ₂ and a very thin Teflon layer as the dielectric gave the largest contact angle change with the smallest applied voltage. This makes it possible to use EWOD to move liquids in biofluidic chips.