SELF-ASSEMBLED MONOLAYERS CONTAINING OMEGA-MERCAPTOALKYLBORONIC ACIDSADSORBED ONTO GOLD FORM A HIGHLY CROSS-LINKED, THERMALLY STABLE BORATE GLASS-SURFACE

This paper describes the preparation and properties of self-assembled monolayers (SAMs) obtained by the adsorption of 11-mercaptoundecanyl-1 -boronic acid, HS(CH2)11B(OH)2, 1, onto the surface of gold substrates . In solution in dry hydrocarbon solvents, 11-mercaptoundecanyl-1-boro nic acid reversibly forms a tri-w-mercaptoalkylboroxine that adsorbs o nto gold and forms a hydrophobic boroxine bilayer: this bilayer is mar ginally stable in air; it can be completely hydrolyzed to a monolayer in aqueous ethanol, yielding a hydrophilic, 17-angstrom-thick SAM that presents boronic acid groups at its surface. Under water-saturated cy clooetane, SAMs exposing the boronic acid groups do not titrate apprec iably over a pH range of 0 to 11-that is, the contact angle is indepen dent of the pH of the contacting aqueous solution. At pH 13 to 14, the re is a sharp increase in the hydrophilicity of the SAM, indicating an onset of ionization of the boronic acid groups. In dry hydrocarbon so lvents or under vacuum, the boronic acid tail groups dehydrate rapidly and reversibly and form a cross-linked borate glass at the surface. A t 147-degrees-C in hexadecane, SAMs having this cross-linked borate gl ass as a surface are at least 5 times more stable to thermal desorptio n than SAMs of the structurally analogous 11-hydroxyundecane-1-thiol. The boronic acid/anhydride groups of SAMs of 1 can be derivatized with solutions of certain cis-diols (especially pinanediol), catechols, an d alkyltrichlorosilanes to yield hydrophobic structures consisting of an inner monomer derived from 1 and an outer partial monolayer of the boronate ester or the alkylsiloxane.