In-situ characterization of self-assembled monolayers of water-soluble oligo(ethylene oxide) compounds

In-situ spectroscopic ellipsometry (SE) was utilized to examine the formation of the self-assembled monolayers (SAMs) of the water-soluble oligo(ethylene oxide) [OEO] disulfide [S(CH2CH2O)6CH3]2 {[S(EO)6]2} and two analogous thiols – HS(CH2CH2O)6CH3 {(EO)6} and HS(CH2)3O(CH2CH2O)5CH3 {C3(EO)5} – on Au from aqueous solutions. Kinetic data for all compounds follow simple Langmuirian models with the disulfide reaching a self-limiting final state (d = 1.2 nm) more rapidly than the full coverage final states of the thiol analogs (d = 2.0 nm). The in-situ ellipsometric thicknesses of all compounds were found to be nearly identical to earlier ex-situ ellipsometric measurements suggesting similar surface coverages and structural models in air and under water. Exposure to bovine serum albumin (BSA) shows the self-limiting (d = 1.2 nm) [S(EO)6]2 SAMs to be the most highly protein resistant surfaces relative to bare Au and completely-formed SAMs of the two analogous thiols and octadecanethiol (ODT). When challenged with up to near physiological levels of BSA (2.5 mg/mL), protein adsorption on the final state [S(EO)6]2 SAM was only 3% of that which adsorbed to the bare Au and ODT SAMs.