Molecular thin films: Phospholipid bilayers and biosensors

New insights into the physics and chemistry of molecular thin films were achieved by applying advanced surface science methods, microfabrication techniques and chemical methods. We used a whole host of analytical surface tools to investigate suspended phospholipids bilayers, organically passivated and atomically flat silicon surfaces and an electrochemically programmed protein chip.; Second harmonic generation microscopy (SHGM) was employed to interrogate suspended phospholipid bilayers. SHGM is a non-linear optical technique that requires a breaking of the inversion symmetry, which naturally occurs at an interface. A suspended bilayer is a hydrocarbon interface between two water layers. SHGM on this model system allows the observation of the evolution of the bilayer as a function of time, substrate dependence of bilayer formation, and visualizes the interaction between the phospholipids and incorporated dye molecules.; Temperature-dependent scanning tunneling microscopy (STM) was used to study alkane molecules covalently attached to unoxidized silicon surfaces as a function of attachment mechanism, alkane length, and silicon crystallographic orientation. Alkane molecules form ordered monolayers on Si through highly stable C-Si bonds. Two chemical attachment chemistries were explored, including UV-assisted free radical addition of alkenes to the Si surface and the displacement of chlorines at the surface using Grignard chemistry. The simplest alkane adlayer to examine is the methyl-terminated surface which was shown to have a 1:1 registry with the underlying Si(111) lattice, as observed in high resolution atomic scale STM images.; These Si surface chemical approaches were extended to the preparation of protein chips via electrochemically activated, site-directed functionalization of silicon micro- or nanowire electrodes. We utilized fluorescence microscopy, electrochemical attachment of biological probe molecules, and x-ray photoelectron spectroscopy to study these wires. Monolayers of electrochemically active hydroquinone molecules were covalently attached to silicon surfaces and then oxidized to the benzoquinone state for subsequent reaction by either cycloaddition with a cyclopentadiene biotin complex or Michael's addition with a thiolated biotin. The selective functionalization was verified by coupling either fluorescently labeled or nanoparticle complexed streptavidin molecules to the biotinylated sites.