Organic Thiosulfates (Bunte Salts):  Novel Surface-Active Sulfur Compounds for the Preparation of Self-Assembled Monolayers on Gold

Abstract

In this paper, we demonstrate that organic thiosulfates (Bunte salts) with the general formula R−SSO3M, where R is either an aliphatic or aromatic group and M a monovalent cation, constitute a novel class of surface-active compounds 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 by X-ray photoelectron spectroscopy (XPS) from gold thiolate formed upon chemisorption of thiols and disulfides. The S−SO3 bond in the thiosulfate is cleaved during adsorption on the gold surface and the sulfite moiety is released. We have prepared one alkyl thiosulfate (sodium S-dodecylthiosulfate, C12SSO3Na) and two aromatic redox-active thiosulfates (potassium S-(2,5-dihydroxyphenyl)thiosulfate, QSSO3K, and dipotassium S,S ‘-(3,6-dihydroxy-1,2-phenylene)bisthiosulfate, Q(SSO3K)2) and compared the formation and properties of the SAMs prepared from these Bunte salts and the corresponding thiols (1-dodecylmercaptan, C12SH, and 1,4-dihydroxy-2-mercaptobenzene, QSH) using XPS, cyclic voltammetry, and ac impedance spectroscopy. The chemisorption of Bunte salts takes place 1−2 orders of magnitude slower than the adsorption of thiols. The SAMs formed from aromatic Bunte salts QSSO3K or Q(SSO3K)2 have lower surface coverage than those prepared using QSH. With aliphatic compounds, the films prepared from Bunte salts are either slightly or relatively well-blocking, although they do not reach the quality achieved 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 from the corresponding halides and yielding the products as odorless crystalline compounds. This offers a synthetically feasible way of introducing a sulfur-containing surface-active headgroup into several redox-active or other functional molecules, allowing their incorporation in SAMs. This facilitates the preparation of functional monolayers for applications in sensor technology and molecular electronics.

Synthesis, surface characterization, and platelet reactivity evaluation for the self-assembled monolayer of alkanethiol with sulfonic acid functionality

“…Moreover, a recent study indicated that oxidized sulfur species on a gold surface are only weakly bound if they adsorb, and will be easily displaced, based on both theoretical and experimental evidence. 44 This finding further supports the chemical structure model in which the SO 3 H end group will be at the outermost layer of the SAM, whereas the thiolate is kept bound with the Au substrate. Figure 4 shows typical scanning electron micrographs of adherent platelets from the same donor on four different SAMs and the Au control.…”

Metal ion recognition and molecular templating in self-assembled monolayers of cyclic and acyclic polyethers

“…These results indicate that the interaction of the metal ion with the dithia-crown ether must be responsible for the potential shifts. The recently published metal ion complexation studies of [60]fullerene derivatives of TTF-crown-ethers confirm this hypothesis (51). Although the C 60 -based reduction potentials are not affected by metal ion complexation because of the long distance between these centers, shifts were observed always for the TTF-centered oxidations, and these, as expected, were always anodic (more positive) (51).…”

“…9B shows the impedance response of a monolayer of 8 grown on a gold electrode in the absence of K ϩ (curve b) and that of a monolayer of 8 grown in the presence of K ϩ (curve a). The R CT values obtained are 82.9 and 49.9 K⍀ for parts a and b, respectively, indicating a reasonably effective blocking monolayer in both cases (60). The impedance response corresponding to the monolayer alone (b) contains no diffusional part.…”

Nanoarray‐Surfaces by Reconstitution of the Porin MspA into Stabilized Long‐Chain‐Lipid‐Monolayers at a Gold‐Surface

“…3) Organic thiosulfates have been described as a method for cautious deposition of thiols on gold surfaces, in the literature [34]. This process can be described in four steps: (a) physisorption of the organic thiosulfate on gold; (b) hydrolysis of the gold-associated organic thiosulfate; (c) chemisorption of the formed organic thiol on gold and (d) co-adsorption of the generated sulfate on gold [35].…”