Self-Assembled Monolayers (SAMs)

1. Self-Assembled Monolayers (SAMs) ——————————————————————————————————————— By Jingpeng Wang CHEM*7530 Feb 7. 2006

2. What is SAMs? ——————————————————————————————————————— • Definition:SAMs are organic assemblies that are formed spontaneously by the adsorption of molecular constituents from solution or gas phase onto a substrate with a specific affinity of its headgroup. • Interactions between substrate and adsorbate: • Physisorption - the enthalpies of interactions are rather low (∆H < 10 kcal/mol, typically from van der Waals forces) • Chemisorption - the formation of covalent bonds, more stable than their physisorbed counterparts (∆H > 10 kcal/mol) • Other forms: hydrogen bonding, donor–acceptor and ion pairing, etc.

3. History and Models ——————————————————————————————————————— • In 1980, SAMs of alkyltrichlorosilane on glass • In order to form a complete monolayer, the silane groups condense with surface hydroxyl groups to form a thin layer of polysiloxane. • In 1983, SAMs of dialkyldisulfides on Au • The assembly is held together by the bonds between the sulfur headgroups and the gold surface as well as van der Waals interactions between neighboring hydrocarbon chains. • In 1985, SAMs of alkanoic acids on Al2O3 • Others: other sulfur head compounds, like disulfides and sulfides, on metals (especially Au, but also Ag, Cu, and even Pt, Fe, and Ni) and semiconductors (GaAs); trialkyl-, trichloro-, or trialkoxysilanes on SiO2/Si, Al2O3/Al, mica, glass; fatty acids on metal oxides (Al2O3, AgO); hydrocarbons on Si.

4. Adsorbates and Substrates that Form SAMs ———————————————————————————————————————

5. Why n-alkanethiolate SAMs on Au ——————————————————————————————————————— • Well-ordered SAMs can be formed from a variety of sulfurcontaining species (i.e., thiols, sulfides, disulfides). • Covalent bond strength between gold and thiolate = 44 kcal/mol, one of the highest between a non-metal and a metal, forms rapidly, typically within seconds to minutes. • The gold surface is relatively chemically inert; it does not readily form a surface oxide nor keep a strong hold of adventitiously adsorbed material, and therefore SAMs can easily be prepared in ambient conditions. • At low surface coverage, the alkanethiolate molecules lie flat with their hydrocarbon backbones parallel to the gold surface. • At higher surface coverages, the molecules begin to stand up, with the hydrocarbon tails tilting approximately 30˚ from the surface normal and nominally in the all-trans configuration so as to maximize van der Waals interactions. [sqrt(3)×sqrt(3)]R30°alkanethiolate lattice on Au(111); the alternating orientation of the alkane chains defines a c(4 x 2) superlattice structure.

6. Various Functional Groups for Thiol-based SAMs ———————————————————————————————————————

7. Mixed SAMs and Molecular Gradients ——————————————————————————————————————— • By mixing two differently terminated thiols in the preparation solution • The relative proportion of the two functionalities in the assembled SAM will then depend upon several parameters: • the mixing ratio in solution; • the alkane chain lengths; • the solubilities of the thiols in the solvent used; • the properties of the chain-terminating groups; Schematic illustration of the preparation of two-component alkanethiolate gradients. (a)The two different thiols, represented by X and O, are injected into glass filters. (b) They diffuse slowly through the polysaccharide gel and attach to the gold substrate. (c) Top view showing the placement of the gold substrate between the filters. (d) Schematic illustration of a fully assembled gradient

8. Characterization of SAMs ——————————————————————————————————————— • Contact angle goniometry(CAG): has been often used to examine the general hydrophilicity or hydrophobicity of a surface. • X-ray photoelectron spectroscopy (XPS): testify the bond types between the headgroup and the substrate, define the chemical species and oxidation states of constituent atoms in the SAM, and demonstrate that the film is of single monolayer thickness. • Fourier-transform infrared spectroscopy (FT-IR): has long been used to measure the vibrational frequencies of bonds within molecules. The alkyl tails vibrate at characteristic frequencies (in the region of ~2800–3000 cm-1; both the breadth of these peaks as well as the frequencies of the vibrations themselves yield a picture of the relative order and fraction of chain defects within the SAM. • Electrochemistry: as electrons can be moved controllably through a SAM, electrochemistry can also be used to reduce or to oxidize pendant groups at the solution–film interface that may be used for further reaction. • Scanning probe microscopes: STM;AFM;LFM - greatly assisted in the patterning of SAMs by analyzing the spatial distribution of adsorbates across a surface.

9. Patterning Self-assembled Monolayers ——————————————————————————————————————— • The selective removal of particular adsorbates • The selective placement of adsorbates • The selective reaction of adsorbates, • Their destruction with energetic beams • Their deliberate removal with scanning probe microscopes moving in a determined rastering pattern • The application of force or delivery of low energy beams

10. SAMs in Biology and Sensoring ——————————————————————————————————————— • Microcontact printing and electrochemistry are two particular methods of patterning SAMs which have found exceptional utility in making SAMs that are selectively activated to study biological events. • Immobilisation of Enzymes onto SAMs Schematic diagram showing the covalently attaching proteins to SAMs is by using carbodiimide coupling which couples amines to carboxylic acids. In the reaction N-ethyl-N-[dimethylaminopropyl] carbodiimide (EDC) converts the carboxylic acid into a reactive intermediate which is susceptible to attack by amines.

11. References ——————————————————————————————————————— • "Patterning self-assembled monolayers" R.K. Smith et al. Progress in Surface Science 75 (2004) 1–68 • "Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology" Love et al., Chemical Reviews, 2005, Vol. 105, No. 4 • "Precision chemical engineering: integrating nanolithography and nanoassembly." P.M. Mendes, J.A. Preece Current Opinion in Colloid & Interface Science 9 (2004) 236–248 • "Self-assembled monolayers of alkanethiols on Au(111): surface structures, defects and dynamics" C. Vericat, M. E. Vela and R. C. Salvarezza; Phys . Chem. Chem. Phys . , 2005, 7, 3258 – 3268 Thank you for your attention!