Self-Assembled Monolayers (SAMs)

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By Jingpeng Wang

CHEM*7530

Feb 7. 2006

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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 physi

sorbed counterparts (∆H > 10 kcal/mol)

Other forms: hydrogen bonding, donor–acceptor and ion pairing, etc.

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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 betw

een the sulfur headgroups and the gold surface a

s well as van der Waals interactions between neig

hboring hydrocarbon chains.

In 1985, SAMs of alkanoic acids on Al2O3

Others: other sulfur head compounds, like disulfides a

nd sulfides, on metals (especially Au, but also Ag, Cu,

and even Pt, Fe, and Ni) and semiconductors (GaAs); t

rialkyl-, trichloro-, or trialkoxysilanes on SiO2/Si, Al2O3/

Al, mica, glass; fatty acids on metal oxides (Al2O3, Ag

O); hydrocarbons on Si.

———————————————————————————————————————History and Models

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Adsorbates and Substrates that Form SAMs———————————————————————————————————————

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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 bet

ween 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 n

or keep a strong hold of adventitiously adsorbed material, and therefore SAMs can easi

ly be prepared in ambient conditions.

At low surface coverage, the alkanethiolate molecules lie flat with their hydrocarbon bac

kbones parallel to the gold surface.

At higher surface coverages, the molecules begin to stand up, with the hydrocarbon tail

s tilting approximately 30˚ from the surface normal and nominally in the all-trans configu

ration so as to maximize van der Waals interactions.

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[sqrt(3)×sqrt(3)]R30°alkanethiolate lattice on

Au(111); the alternating orientation of the alk

ane chains defines a c(4 x 2) superlattice str

ucture.

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

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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;

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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

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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.

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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

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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

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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.

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References• "Patterning self-assembled monolayers" R.K. Smith et al. Progress i

n 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

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Thank you for your attention!