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

Abstract

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(CH 2CH 2O) 6CH 3] 2 {[S(EO) 6] 2} and two analogous thiols – HS(CH 2CH 2O) 6CH 3 {(EO) 6} and HS(CH 2) 3O(CH 2CH 2O) 5CH 3 {C 3(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.