Transcript 幻灯片 1 - University of Guelph

Self-Assembled Monolayers (SAMs)
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By Jingpeng Wang
CHEM*7530
Feb 7. 2006
What is SAMs?
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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.
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History and Models
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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
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.
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Adsorbates and Substrates that Form SAMs
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Why n-alkanethiolate SAMs on Au
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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
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structure.
Various Functional Groups for Thiol-based SAMs
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Mixed SAMs and Molecular Gradients
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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 chainterminating 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)
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Schematic illustration of a fully assembled gradient
Characterization of SAMs
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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
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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
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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 Nethyl-N-[dimethylaminopropyl] carbodiimide (EDC) converts the carboxylic acid
into a reactive intermediate which is susceptible to attack by amines.
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References
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• "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.
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• "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!
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