Hydrolytic, Thermal, and Electrochemical Stability of Thiol- and Terminal Alkyne-Based Monolayers on Gold: A Comparative Study

The terminal alkyne-Au interaction is emerging as a promising adsorbing bonding motif for organic monolayers, allowing it to be used for installing antifouling layers and/or recognition elements on gold surfaces for biosensing applications. In contrast to the well-known thiol-on-gold monolayers, the long-term hydrolytic, thermal, and electrochemical stability of the alkyne-Au bond remains relatively unexplored. Insight into these is, however, essential to deliver on the promise of the alkyne-Au bond for (bio)sensing applications, and to see under which conditions they might replace thiolate-gold bonds, if the latter are insufficiently stable due to, e.g., biological thiol exchange. Therefore, these stabilities were investigated for monolayers on Au substrates formed from 1-octadecanethiol and 1-octadecyne. Additionally, monodentate and tridentate alkyne-based adsorbates were designed to investigate the effect of multivalency on the stability. The hydrolytic stability over time in four aqueous media and the thermal stability in air were evaluated using static water contact angle measurements and X-ray photoelectron spectroscopy. Electrochemical oxidative desorption potentials were also assessed by cyclic voltammetry. All three tests indicate that the monovalent terminal alkyne monolayers on gold are slightly less stable than their thiolate analogs, which we could attribute to a lower packing density but still sufficiently stable to be applied in biosensing in the gut, while multivalency can further improve this. Our work provides insight into the stability of terminal alkynes under different conditions, better enabling the use of terminal alkyne-Au interactions in biosensors.