No abstract

The formation of self-assembled chemisorbed layers of cystamine, cysteamine, and 4-aminothiophenol on gold has been studied by XPS and voltammetry. These compounds, often used in the preparation of biosensors and modified electrodes, are shown to yield surface coverages of approximately 80% of that of a octadecanethiol monolayer within 5 min in millimolar aqueous and ethanolic solutions. The results of the XPS experiments reveal that a shoulder on the S 2p3/2 peak (situated at 162.1 eV) develops at 161.3 eV upon increasing the adsorption time from minutes to 1 week and that the initial rate of formation of the shoulder is higher for cystamine than for cysteamine. This shoulder is believed to be due to the presence of a sulfur species with a higher coordination number with respect to gold. Increased adsorption times also give rise to increased amounts of oxidized carbon and sulfur in the films. The oxidation of the sulfur in the thiols results in a detachment of the molecules from the gold surface, as indicated by XPS experiments with different takeoff angles. The main N 1s peak for cystamine is shifted toward higher binding energies for increasing adsorption times while two prominent nitrogen peaks are generally seen for cysteamine. For cysteamine, increasing adsorption times result in an increase of the main nitrogen component at the higher binding energy, yielding an apparent shift in the nitrogen peak with time similar to that seen for cystamine. Possible explanations for these experimental findings are discussed. Cystamine, cysteamine, and 4-aminothiophenol films on gold are shown to be irreversibly oxidized in the gold oxide formation region. On the basis of evaluation of the oxidation charge, surface coverages of approximately 1 × 10-9 mol/cm2 were obtained for adsorption times between 5 min and 1 week.

α-Functionalized terthiophenes containing disulfide (−S−T3−H)2 and alkanethiol (HS−(CH2)11−T3−H) anchoring groups have been synthesized for direct immobilization onto gold. Monolayer structures of these compounds are prepared by spontaneous assembly from ethanol solutions on evaporated gold substrates and thoroughly characterized by ellipsometry, contact angle goniometry, infrared and X-ray photoelectron spectroscopy, and cyclic voltammetry. The two molecules coordinate to the gold substrate exclusively via the anchoring groups upon formation of gold−thiolate bonds. The kinetics of monolayer formation vary dramatically for the two compounds. The alkanethiol analogue assembles rapidly, within a few minutes, and forms a densely packed and highly organized monolayer, with the alkyl chains in an almost perfect all-trans conformation and the Cα−Cα axis of the α-T3 units tilted about 14° away from the surface normal. The assembly process is much slower for the disulfide, but an organized monolayer with an average α-T3 chain tilt of about 33° will eventually form when the assembly is allowed to equilibrate with a solution containing the disulfide for at least 1 day. Moreover, the two monolayer assemblies also display a remarkably different electrochemical behavior. The heterogeneous electron-transfer rate at the disulfide-covered gold substrate is almost indistinguishable from that at bare gold, suggesting that the assembly contains a large number of easily accessible defects. An alternative mechanism for explaining the large electron-transfer rate involving electronic coupling via the conjugated π-system of the α-T3 units is also proposed. The electrochemical response is significantly reduced for the HS−(CH2)11−T3−H assembly, but another type of defects, the so-called “shallow defects” originating from sparsely populated areas on the electrode surface, can be identified.

Molecular gradients of ω-substituted alkanethiols HS(CH2)15X (X:CH3, CH2OH, CO2CH3, CH2C⋮N) were prepared on sputtered gold films by using a novel cross-diffusion methodology. Ellipsometry, contact angle measurements, infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used to monitor the coverage, chain conformation and gradient profiles. The XPS analysis were performed with improved lateral resolution as compared to our earlier infrared study (Langmuir 1995, 11, 3821) by imaging a 160 μm wide segment of the gradient surface on the slit of the XPS analyzer and by using a step size of 0.5 mm. The so-obtained gradient profiles are expected to be more representative of the true composition profiles and were for the three gradients investigated found to be 5−6 mm, about 2−3 mm shorter than the corresponding profiles obtained by scanning infrared spectroscopy. The coverages (alkyl chain densities) were found to be constant for the gradients prepared from thiols with relatively small tail groups OH, C⋮N and CH3, whereas a minor reduction in coverage, ≈ 3−4%, was observed in a narrow range for the gradient containing the CO2CH3-terminated alkanethiol. This change is, however, too small to be detected by ellipsometry and infrared spectroscopy and supports a model where the gradient assemblies approach the limiting structure of densely packed all trans polymethylenes.