Organic thiosulfates (Bunte salts): Novel surface-active sulfur compounds for the preparation of self-assembled monolayers on gold

In this paper, we demonstrate that organic thiosulfates (Bunte salts) with the general formula R-SSO3M, where R is either an aliphatic or aromatic gro up and M a monovalent cation, constitute a novel class of surface-active co mpounds with a sulfur-containing headgroup. Bunte salts form self-assembled monolayers (SAMs) on gold under anaerobic conditions and chemisorb forming a Au-S bond, in which the chemical nature of sulfur is indistinguishable b y X-ray photoelectron spectroscopy (XPS) from gold thiolate formed upon che misorption of thiols and disulfides. The S-SO3 bond in the thiosulfate is c leaved during adsorption on the gold surface and the sulfite moiety is rele ased. We have prepared one alkyl thiosulfate (sodium S-dodecylthiosulfate, C12SSO3Na) and two aromatic redox-active thiosulfates (potassium S-(2,5-dih ydroxyphenyl)thiosulfate, QSSO(3)K, and dipotassium S,S'-(3,6-dihydroxy-1,2 -phenylene)bisthiosulfate, Q(SSO3K)(2)) and compared the formation and prop erties of the SAMs prepared from these Bunte salts and the corresponding th iols (1-dodecylmercaptan, C12SH, and 1,4-dihydroxy-2-mercaptobenzene, QSH) using XPS, cyclic voltammetry, and ac impedance spectroscopy. The chemisorp tion of Bunte salts takes place 1-2 orders of magnitude slower than the ads orption of thiols. The SAMs formed from aromatic Bunts salts QSSO(3)K or Q( SSO3K)(2) have lower surface coverage than those prepared using QSH. With a liphatic compounds, the films prepared from Bunte salts are either slightly or relatively well-blocking, although they do not reach the quality achiev ed with thiol-based SAMs. The differences in the adsorption time scale and surface coverage are attributed to the bulky thiosulfate headgroup. A major advantage of using Bunte salts derives from the general synthetic pathway to organic thiosulfates, generally involving a one-pot synthesis starting f rom the corresponding halides and yielding the products as odorless crystal line compounds. This offers a synthetically feasible way of introducing a s ulfur-containing surface-active headgroup into several redox-active or othe r functional molecules, allowing their incorporation in SAMs. This facilita tes the preparation of functional monolayers for applications in sensor tec hnology and molecular electronics.