16040
J. Phys. Chem. B 2005, 109, 16040-16046
Structure of tert-Butyl Carbamate-Terminated Thiol Chemisorbed to Gold Rodrigo M. Petoral, Jr. and Kajsa Uvdal* DiVision of Molecular Physics, Department of Physics, Chemistry and Biology (IFM), Linko¨ping UniVersity, SE-581 83 Linko¨ping, Sweden ReceiVed: May 20, 2005; In Final Form: June 17, 2005
Monolayers of tert-butyl carbamate-terminated thiol were formed by adsorption of the molecules onto polycrystalline gold substrate. The adsorbates were studied using techniques as X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and infrared reflection-absorption spectroscopy (IRAS). The results provide the electronic structure, composition, characteristic fingerprint, and orientation of the molecular adsorbate. XPS verified that the thiolate group is chemically bonded to the gold surface and that a complete chemisorption of the molecule occurs. Elemental depth profiling by varying the excitation energy in XPS supports the angle-dependent XPS results. Both techniques showed that the tert-butyl group is oriented away from the gold surface. A nearly parallel orientation of the carbonyl group relative to the gold surface is deduced from the IRAS results. The main molecular axis is estimated to have an average tilt angle of about 38° relative to the gold surface normal on the basis of the NEXAFS results. Cyclic voltammetry indicates a less blocking capability of the adsorbates. Overall, the molecules are oriented in an upright manner with indications of presence of pinholes and/or defects possibly due to steric hindrance of the bulky tert-butyl group. This molecular system is envisioned to be of use for surface-based organic synthesis on gold substrates.
1. Introduction Self-assembled monolayers (SAMs) of thiolates have increasingly been studied for different applications in biorelated fields such as biochemistry, biomedicine, biosensor and biomaterial technology, and bioelectronics.1-4 Monolayers composed of bifunctional linking molecules, with headgroups for surface attachments and reactive tail groups for further derivatization, can create distinctive organic sites for specific affinities and reactions to take place. An example is immobilization or linkage of peptides, proteins, and other biomolecules to the monolayers for particular biological experiments. Nucleophilic substitution, free radical halogenation, and redox reactions are among simple organic reactions that are commonly performed easily on the bulk but can also be executed on a surface.5,6 Peptide synthesis is one category of reaction that is carried out on substrates that are covalently attached to a polymer resin.7 Studying amino acids and peptides immobilized on surfaces and exploiting the capabilities for sensing purposes, such as electrochemical metal ion sensors,8 optical biosensors to probe interactions with molecules,9-11 and piezoelectric sensors,12 have been continuously explored. Synthesis of such peptides, using the conventional method and later on immobilizing them onto a surface (e.g. metal electrode), is demanding and time-consuming. Synthesizing peptides directly on the sensor surface is an efficient way to execute the procedure and is worth exploring. Recently, Gooding and his group reported on a stepwise synthesis of a tripeptide on gold surfaces with mixed SAMs as starting material.13 Immobilizing alternative molecules as a starting material, not only for peptide synthesis purposes but also for general organic synthesis on solid surfaces, is indeed of interest. Molecules such as carbamates, serving as * Corresponding author. E-mail:
[email protected]. Fax: +46-13-288-969.
protecting groups during peptide synthesis,7 are good candidates as starting materials. Surface functionalization using thiol chemistry is a wellknown technique to chemically adsorb the molecules on the surface. Subjecting the adsorbates to further chemical reactions could modify their surface properties. For specific reactions to be realized, access of an external reagent to the reaction center present in the monolayer should be introduced or be available. During solid-phase synthesis (chemical reactions/synthesis at a surface), there is a high risk of poor yields due to steric obstruction of surrounding terminal functional groups (e.g. COOH, NH2, and OH). It has earlier been found that the reaction rate (i.e. SN2 reaction with monolayers deposited on Au colloids) is substantially diminished with bulky nucleophiles or if the monolayer on the gold surface has a short alkyl chain important in reactions.14 On the part of the monolayer, steric hindrances due to bulky terminal functional groups result in formation of monolayer defects. A less packed monolayer or sparsely adsorbed molecules, however, can be partly an advantage if reactions on the molecules are to take place on the organic interphase. It is beneficial in a sense that reagents can easily permeate through the monolayer. Studying these adsorbates will be useful for further reaction processes on the surface for possible organic or more specifically for peptide synthesis applications. In this study, structural investigation of a thiol-functionalized tert-butyl carbamate adsorbate on gold, envisioned to be a starting molecule for surface-based organic synthesis, is given focus. Composition, molecular binding, and molecular orientation of the adsorbate are investigated by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and infrared spectroscopy (IR). Null ellipsometry, water contact angle goiniometry, and cyclic voltammetry are used as supporting measurement techniques.
10.1021/jp0526445 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/04/2005
t-Bu Carbamate-Terminated Thiol Chemisorbed to Au 2. Experimental Section The tert-butyl N-(2-mercaptoethyl)carbamate (t-BMC, 97%) (Sigma-Aldrich) was used without further purification. The t-BMC adsorbates were prepared from ethanol solutions with a concentration of 2 mM. The gold substrates used were prepared by electron beam evaporation of 2000 Å thick Au at a rate of 10 Å/s onto a clean single-crystal Si(100) wafers. The silicon wafers were precoated with 20-25 Å thick of Ti layer at a rate of 2 Å/s before gold film evaporation. The base pressure was always
