Water compatible self assembled monolayers on gold and iron oxide surfaces: Applications and adsorption kinetics.

摘要

Although the formation of thiol and disulfide self-assembled monolayers (SAMs) onto Au surfaces has been extensively studied, many contradictions still exist in the literature regarding their adsorption kinetics. The work at hand provides answers to controversial questions regarding the assembly of thiols vs. the respective disulfides and the effect of the size and charge of the end group on film formation kinetics. It also carefully determines whether the latter is controlled by the mass transport of adsorbates in solution and/or by the reaction rate at the surface. The adsorption studies were done using short chain water soluble thiols onto amorphous Au thin films (∼ 14 nm in thickness) from aqueous solutions. The film assembly of a neutral thiol, 2-mecrcaptoethanol was compared to the respective disulfide. The adsorption of a negatively charged thiol, 2-mercaptoethane sulfonic acid (MESA) which has the same hydrocarbon chain length as the neutral thiol, was investigated with counterions of different sizes and valencies. The kinetics and extent of adsorption were monitored using four point conductivity measurements, which can detect the formation of a fraction of a monolayer, with a high signal-to-noise ratio. This technique is sensitive enough (detection limit of 0.24% of monolayer) to precisely measure the adsorption rate for low coverage, where complications from lateral interactions are minimized. The data showed that the neutral mercaptan adsorbs at the same rate as that of the respective disulfide following a transport limited/Langmuir kinetics model. The rate of film formation and the net coverage were higher for the neutral adsorbates compared to the charged thiols due to a difference in the size of the terminal group. The counterion had a negligible effect on the adsorption profiles for monovalent cations. On the other hand, MESA having a divalent counterion exhibited faster film assembly kinetics than the other charged thiols. This work was the first to measure the flux of adsorbates towards the Au substrate while monitoring the assembly of thiols/disulfides. Adsorption rate equations including both mass transport and Langmuir kinetics showed that systems limited by mass transport must also be slowed by Langmuir kinetics.;One of the major applications of SAMs is decorating and stabilizing colloidal particles. Since colloids are very promising in targeted delivery of therapeutic or diagnostic agents to a cell of interest, there is a need to produce nanoscale shuttles capable of surviving the stringent in vivo environment.;The colloidal stability challenge was tackled by functionalizing iron oxide nanospheres (ca. 8 nm in diameter) with a short zwitterionic siloxane SAM, in aqueous media. Superparamagnetic nanoparticles are particularly interesting for biomedical uses since they can be directed towards the organ of interest using an external magnetic field. They are also good magnetic resonance imaging (MRI) contrast agents and are tremendously promising in the treatment of malignant tumors (hyperthermia). The stabilization procedure yielded nanoscale shuttles (ca. 10 nm in diameter) that resisted aggregation at physiological salt concentration, temperature and pH. The morphology and structure of the stabilized magnetite nanoparticles were studied using dynamic light scattering (DLS), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and powder X-ray diffraction (pXRD). The organic shell was probed using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, energy dispersive X-Ray spectroscopy (TEM-EDS) and thermal gravimetric analysis (TGA). Magnetizability measurements were also performed in order to check for the magnetic properties of the end product.;In the same direction, gold nanoparticles having an average diameter of 5 nm were passivated. Ligand exchange reactions were utilized in the production of aggregation-resistant colloids, comprising a gold core capped with a mixed shell of citrate and newly synthesized thiolated zwitterionic ligands. These nanoparticles showed a remarkable stability in saline media with salt concentrations as high as 3.0 M. Likewise, the gold nanoparticles did not precipitate out of solution when positively charged polyelectrolytes, or biopolymers were added, indicating the absence of nonspecific interactions. The modified colloids were characterized using various experimental techniques such as UV-Vis absorbance spectroscopy, quasi-elastic light scattering and surface enhanced Raman spectroscopy.