Pragmatic Studies on Protein-Resistant Self-Assembled Monolayers

Summary.

The present study describes new synthetic routes to oligo(ethylene glycol)-terminated alkanethiols (OEG-ATs), starting from α,ω-dibromoalkanes, which are reacted either with OEG or with trityl mercaptan in the first step. In addition to these ether conjugates of OEG and AT, analogous ester and amide conjugates were prepared by established procedures. All thiols were used to form self-assembled monolayers (SAMs) on cleaned gold surfaces and these were stored for 1–2 weeks under water at 4°C before the extent of nonspecific protein adsorption was tested with IgG, BSA, and lysozyme at 1 mg cm⁻³ protein concentration in phosphate-buffered saline. Under these practice-oriented testing conditions, SAMs with tri(ethylene glycol) chains (EG ₃) exhibited nonsatisfactory protein resistance, in sharp contrast to EG ₄ or longer OEG chains. The effectiveness of EG ₃ was partially restored when they were linked to a long acyl chain (16-mercaptohexadecanoic acid) instead of 12-mercaptododecane or 11-mercaptoundecane. Furthermore it was found that (i) SAM formation at 20 μM thiol versus 500 μM OEG-AT gave identical results, (ii) gel-filtered proteins were much less adsorbed than the unpurified commercial products, and (iii) the method for gold-precleaning was very critical. In conclusion, this study offers convenient synthetic routes to OEG-AT and helps to choose molecules and procedures for reliable preparation of protein-resitant SAMs with prolonged stability during storage.