Post on 31-Jul-2020
Self-assembled monolayers
(SAMs)
The constituents and their
functions in a SAM
Silanization
SiR
RR
SiR
OH O O OHOH OHOH OH OH OHOH OHDry organic
solvent
Silanes spontaneously form covalent bonds with e.g. hydroxylated surfaces.
+ x RH
OH OH OH OHOH OH
SiR
RR
OSi
OH O O OHOH OH
OH
SiR
R O Si
Water will result in polymerization,rather than grafting to the surface!
H2O H2OH2O Dry organic
solvent
Silanization agents
Chlorosilanes Alkoxy silanes
R'
SiCl
ClCl
R'
SiCl
ClCH3
R'
SiCl
CH3
CH3
Very reactive!
R’ must be a non-polar group to avoidreactions with the silane.
Yields robust monolayers, the best silanelayers with respect to structure, stabilityand homogeneity.
R'
SiOMe
OMeOMe
R’ can vary arbitrarily!
Great flexibility in the preparation ofsurfaces with variation in chemicalidentity and properties.
Binding requires heat treatment at100°-150° C.
Common silanes, ”coupling agents”
Bonding through silanes by interdiffusion.: regions of coupling agent,: regions of polymer.
Industrial applications, ”glues”
Langmuir’s surface balance
The pressure in the surface film can be varied by moving the barriers defining the film area.
The surface pressure is measured using a wettable plate immersed into the film (Wilhelmy’s method).
By repeatedly bringing a substrates up and down through the surface film, ordered multilayers, Langmuir-Blodgett films, can be prepared.
Requires amphiphiles which are insolublein water (to stay between the barriers!)
Comparing Langmuir-Blodgett
and silanizationLangmuir-Blodgett:+ Strongly organized 2D films which can be deposited onto many different types of substrates, polar as well as non-polar.+ Mono- and multilayers and alternating structures may be prepared.
- The stability relies primarily on lateral van der Waals forces, and possibly weak electrostatic interaction with the substrate.- Does not permit formation of statistically mixed layers due to phase separation on the water subphase.
Silanization:+ Gives robust and usually strongly organized films. + Considerable chemical flexibility.
- Requires hydrophilic substrates (e.g. oxides, glass)- Alkoxysilanes must be heat treated.- Chlorosilanes are very reactive, restricting their use.
Layer-by-layer assembly
Decher, Science, 277, 1232-1237 (1997).DOI: 10.1126/science.277.5330.1232
Electrostatically attach alternating cationic and anionic components to form multilayers.
LBL assemblymethods
Chem. Rev., 116, 14828 (2016)10.1021/acs.chemrev.6b00627
Spin coating
Spray coating
Centrifugation
Large-scaleapplication
Chem. Lett.,43, 36 (2014),10.1246/cl.130987
Jpn. J. Appl. Phys. 44, L126 (2005),10.1143/JJAP.44.L126
Roll-to-roll
Spray coating
LBL films for optimizing fluorescence
enhancement
Fluorescent molecules are quenched in the presence of gold surfaces, but may be enhanced in the presence of gold nanoparticles – this is distance dependent!
(Diploma work by Ming-Tao Lee 2010)
The distance between a fluorophore and gold nanoparticles can be accurately controlled by LBL to optimize the enhancement, making possible more sensitive fluorescent detection in sensors.
Epoxy resin
DOI: 10.1016/j.apsusc.2011.07.080
A coupling agent between copper and epoxy is crucial for adhesion. An epoxy film was successfully deposited on amine-terminated alkylthiol and dithiol SAMs. The resulting coating is homogeneous and adherent on both surfaces.
Lou, Langmuir 2011, 27, 3436
Molecular anchors for self-assembled monolayers on ZnO: A direct comparison of the thiol and phosphonicacid moieties
Perkins, J. Phys. Chem. C, 113, 18276
Alkyl phosphonates readily form monolayerson most transition metals!
On titanium oxide and borosilicate glass, monolayers prepared from hexadecylarsonic acid provide significantly greater surface protection than surfaces reacted under similar conditions with hexadecylphosphonic acid, a common modifying agent for oxide substrates.
Adv. Funct. Mater. 2012, DOI: 10.1002/adfm.201202566
Borosilicate glass substrates are soaked in a 1 mM solution of hexadecylarsonic acid in tetrahydrofuran for 48 h at 40 ° C to achieve an arsonic acid self-assembled monolayer (SAM).
Self-organization of thiols on gold
from solution
Au
Coverage ≈ 1
”Pinning”, rapid process ~ s(Diffusion controlled)
Au100-150 kJ/mol
0.2-0.5 kJ/mol CH2
0.2-1 kJ/mol
Au-S
(CH2)n
µM-mM
Coverage <1
Time
Gold-coated surface,Au(111) dominating
Slow organization of the chains ~ h
Thiol SAM architectures
O
NH
OOO
OHOH
OO
OHOH
OHO
OHOH
OH
OH OH
O
O
NH
H
O
NTA-chelators Four-helix bundles Lipid bilayers
Ethylene glycols Chemoattractants Carbohydrates
O
NH
H
O
O
NH
OOO
OHOH
OO
OHOH
OHO
OHOH
OH
OH OH
O
Dissociative chemisorption of thiols to Au(111)
R-SH + Au(0) → R-S--Au(I) + ½ H2
R-S-H H
Free electron pairs
Hydrogen abstraction”transition state”
”Trapping”,physisorption
Chemisorption3-fold symmetrysp3-like structure
Overlayer structure of the thiol layer
Differences in the latticeparameter of Au(111) and the size of the thiols creates somespace between the chains.
Tilting of the chains increasespacking density, and thus alsointerchain van der Waalsinteractions.
Adsorption sites on Au(111)
J. Phys. Chem. B, 107, 3803-3807 (2003)DOI: 10.1021/jp021989+
Mechanisms of film formation
”Substrate decoupled” ”Substrate coupled”
OH O O OH
RSi O SiR
O
SiO
O
Molecules bind to the surface independent of any substrate structure.
E.g. silanization, LB-films
Molecules chemisorb to specific surface sites, and form an overlayer structure.
E.g. thiols on gold
Ideal ”statistically” mixed SAMs
Complexmixtures
Pairwise interactionB-B >> A-B
A
B
1:1 mixture
Mixed monolayers- Do alkane thiols in a SAM phase separate
upon adsorption from a binary solution?- Is the surface composition the same as
the solution composition?
Randommixture
Phaseseparation
HS-(CH2)n-X / HS-(CH2)m-Y
Mixed thiol layers
HS-(CH2)16-OH
HS-(CH2)16-OH/HS-(CH2)11-CH3 (1:1)
HS-(CH2)16-OH/HS-(CH2)16-CH3 (1:1)
Methyl- and hydroxyl terminated thiols- Ideal mixture or phase separation?
Fraction of OH groups in the monolayer, vs the fraction in the solution.
Bertilsson, Langmuir 9, 141 (1993)
Free OH vibration
H-bondedOH-groups
Phase separation i mixed thiol systems
75% HS-(CH2)15-COOCH3
25% HS-(CH2)15-CH3
Stranick et al., J. Phys. Chem., 98, 7636 (1994)
STM image500 x 390 Å2
Methyl thiols
Methyl esters
Defect(”pinhole”)
Kinetic and thermodynamic
control of mixed SAMs
Kinetically determinedstructure
Lateraldiffusion
Phase separation via solution exchange
Phase separationvia surface diffusion
(extremely slow)
Thermodynamically determinedstructure
Exchange withthe solution
Rapid adsorptionfrom solution tothe surface
Au Au
Au Au
Without thepresence of
molecues in solution
Interactionenergy
Strong
Moderate
Weak
For example,
= Thiols withshort chains
= Thiols withlong chains
Exchange withthe solution
Molecular gradients
Kinetically controlled processat low concentrations
mm
HS-(CH2)n-CH3 HS-(CH2)m-OH
Liedberg, Langmuir, 11, 3821 (1995)
TPD studies of alkanethiols on gold
Stettner, Langmuir, 26, 9659 (2010)
Alkanethiol withassociatedgold (ad)atom (!)
Reversed self-organzation of gold on thiol
layersGold atoms form clusters with Au(111) orientation onto an LB film
of octadecanethiol [ CH3-(CH2)17-SH ] at the air/water interface.
Uysal, Phys Rev Lett 107, 115503 (2011)
Upon adsorption of gold atoms, the degree of order in the LangmuirFilm also increases, as the gold atoms arrange themselves in an orderedstructure.
Optimization of biosensors
Effects of the presentation and distribution of ligands
Distance between ligands
Motion restrictionsof the ligands.
Au (111)
22 Å22°
S
OH
S
OH
S
OH
S
OH
S
OHOH
S
OH
S
O
NH
O
O
O
S
O
OH
OH O
OH
OHO
O
OH
OH
O
O
OH
OH
OH
OH
OOH
OH
O
OHOH
O
O
OHOH O
O
OHOH OH
OH
S
OOH
OH
O
OHOHO
O
OHOH O
O
OHOH OH
OH
S
5 Å
S
OH
S
OH
S
OH
S
OH
S
OH
S
OH
5 Å
OH-terminatedSAM
Globotriose
OEG - spacer
Svedhem, Langmuir, 18, 2848 (2002)
Mixed monolayer of globotriose
on gold
S
OOO
OHOH
OO
OHOH
OHO
OHOH
OH
OH OH
S
OH
O
NH
OOO
OHOH
OO
OHOH
OHO
OHOH
OH
OH OH
O
S
O
OOO
OHOH
OO
OHOH
OHO
OHOH
OH
OH OH
O
O
NH
O
S
O
O
NH
O
S
O
OH
O
NH
H
O
S
Au
2 1 4
Antibody response to
globotriose/OH monolayer
Fraction (%) of molecule 2 in a mixturewith molecule 1 (solution composition)
Re
sp
on
se
(R
U)
0
2000
4000
0 1 100
Re
sp
on
se
(R
U)
10
O
OOO
OHOH
O
O
OHOH
OH
O
OHOH
OH
OH OH
O
O
NH
O
SH
OH
SH
1
4
0
2000
4000
0 1 10010
SH
OOO
OHOH
O
O
OHOH
OH
O
OHOH
OH
OH OH
SH
OH
1
2
IgG MAHI 5 IgM MAHI 419
, = Non-specific controls (MAHI 4, MAHI 10)Svedhem, Langmuir, 18, 2848 (2002)
Fraction (%) of molecule 4 in a mixturewith molecule 1 (solution composition)
Biofunctional surfaces – “wish list”
• Intert surfaces which repel macromolecules, microorganisms and cells.
Anti-fouling surfaces in biomedical, industrial and marine
environments, biosensors.
• “Stealth surfaces” which are invisible to the immune system, and which are not considered as foreign objects by cells.
In vivo sensors, implants, drug delivery vehicles.
• Surfaces which stimulate cell growth and enable differentiation of cells.
Cell culture, tissue engineering, wound healing, biomedical
analysis, regenerative medicine.
What is biocompatibility?
IUPAC definition:Ability to be in contact with a living system without producing an adverse effect.
Vert, Pure Appl. Chem., 84(2), 377-410, (2012).
The idea of what an “adverse effect” is has changed over the years, with increasing understanding and appreciation of the complexity of living tissues.
From ‘not causing too much damage’ to‘complete invisibility by the immune system’.
The canonical protein-resistant
surface: PEG
However, PEG has limited stability, and is not suitable for long-term use, and we need to find more stable materials with the antifouling properties of PEG.
Some comments on PEG properties:
• The resistance of PEG-coated surfaces increases withincreasing surface density and chain length.
• At moderate temperatures, PEG is water-soluble in allproportions, and for Mw up to over 100 kDa.
• Closely related polyethers such as poly(methylene oxide) orpoly(propylene oxide) are generally insoluble in water.
– CH2 – CH2 – O –n
But what makes PEG protein-resistant?
Materials presenting poly(ethylene glycol) (PEG)chains on their surface resist non-specific adsorptionof proteins.
PEGs are widely used in biomedical applications.
Why is PEG protein resistant?For a flexible, non-charged, hydrated polymer (such as PEG) we expect steric forces to prevent protein adsorption.
This system is protein resistant. How can we explain this, and which parameters are relevant? Two main strategies:
The physical view – based in polymer theory, not taking interfacial chemistry or molecular structure into account, but entropic contributions.
The chemical view – explanation in terms of molecular structure of water and the polymer, effects of water orientation and molecular conformation.
The physical viewTreats proteins and water molecules as hard spheres,and the polymers as random coils.
Assumes that entropic contributions are muchmore significant than enthalpic terms.
Chain compressionincreases local concentration
Expulsion of solventincreases local
osmotic pressue.
Equilibrium is attained byinflux of solvent.
Jeon, JCIS, 142, 149 (1991); Halperin, Langmuir, 15, 2525 (1999)
This is sufficient to explain the critical
dependence on e.g.
• Grafting density• Brush thickness
SAMs in protein adsorption studies
Prime, Science, 252, 1164 (1991);Pale-Grosdemange, J. Am. Chem. Soc., 113, 12 (1991)
Beside large molecular-weight polymers, hydrophilicSAMs were found to be protein resistant.
SAM-bound oligo(ethylene glycol) or sugars do nothave the conformational freedom of long-chain PEG.
The “physical view” cannot explainthe protein resistance in these cases!
-(EG)6
-Maltose
-OH
The ”chemical view” explains protein resistance in terms of interfacial forces, hydrogen bonding and molecular conformations, although there is not yet consensus about general criteria for protein-resistance.
M. Grunze (Heidelberg): The coatings must:• Display unique conformation (like helical OEGs, see later!)• Bind water to specific sites on the coating.• Satisfy the ”Berg limit” (i.e. water adhesion tension > 30 mN/m,
or water contact angle θ < 65°)
G.M. Whitesides (Harvard): These are the important characteristics:• Hydrophilic surface• Contain hydrogen bond acceptors (but not donors)• Overall electrical neutrality
Two versions of the chemical view
To what extent is generalization possible?
Substrate effects on adsorption
onto SAMs
Conclusions:- The conformation of the OEG chains is important for their ability to resist protein adsorption.- The underlying crystal lattice is also important...
Harder, J. Phys. Chem. B, 102, 426 (1998)
Mixtures of EGx-terminated
alkylthiols
EG6 Helical Amorphous EG4 all-trans
EG6 - rich EG4 - rich
Au
Riepl, Langmuir, 21, 1042 (2005)
5-10 mm
EG6
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4.0
Thi
ckne
ss (n
m)
0.000
0.005
0.010
0.015
0.020
0.025 Integrated peakcm
-1
EG4
0.2
0.4
0.6
0.8
∆d(n
m)
0.04
0.08
0.12
0.16
0.20
Integrated area amide I, cm
-1
EG6 EG4
EllipsometryFT-IRAS
Thickness &integrated IRAS intensity
Fibrinogen adsorption1 mg/ml, pH 7.4, 1.5 h
0.0
~ 0.25 ML ≈ 1 nm
Tuning the protein rejecting
properties
0 20 40 60 80 100 0 20 40 60 80 100
~ 3.3 nm
~ 3.9 nm
Why is EG6 protein resistant, while EG4 is not?
Helical EG-chains (like EG6) adsorb water strongly, acting as templates for water nucleation, while all-trans chains (EG6) interact weakly with water molecules:
Strong hydration prevents protein adsorption.Harder, J. Phys. Chem. B, 102, 426 (1998)
Riepl, Langmuir, 21, 1042 (2005)χ EG4
Water binding to OEG conformers
Solvated EG moieties with non-uniform gauche rotations are by far the most stable.
Binding of interfacial water by OEG is important for the protein resistance!
Total energies and binding energy of water (in brackets) of the inequivalent groups of conformers, without attached water, with one and with two water molecules.
Wang, PCCP, 2, 3613 (2000)
No water One watermolecule
Two watermolecules
A different example: Saccharide SAMs
Saccharide-terminated SAMs on gold:
1. Hydroxylated2. Methylated3. Partly methylated4. A mixed SAM of CH3- and
OH-terminated alkylthiolsmatching the wettability of 3.
1
2
3
4
AdvancingCA
(degrees)
RecedingCA
(degrees)
SurfaceEnergy (mJ/m2)
1 < 10 < 10 46
2 76 60 29
3 50 24 38
4 52 22 43
Ederth, ACS Appl. Mater. Interfaces 2011, 3, 3890–3901
Protein adsorptionFor mixtures of 1 and 2 (increasingproportion of 2 to the right), proteinadsorption is reduced to a minimumfor certain composition ranges!
The behaviour is reproduced with themonomethylated saccharide 3 (squares).
No adsorption! No adsorption!
1
2
Hederos et al., Langmuir 2005, 21, 2971-2980
Is this generally applicable?
Hypothesis: The monomethylated sugar SAM 3has ’universal’ antifouling properties,i.e. will perform better than 1 and 2.
Lab assays: Algal spores: Ulva linzaBarnacle cyprids: Balanus amphitriteBacteria: Cobetia marina
Marinobacter hydrocarbonoclasticus
3
O
HOOH
HO
OMe
O
1
O
HOOH
HO
OH
O
2
O
OMeOMe
MeO
OMe
O
02000400060008000
100001200014000
1 2 3 40
2000400060008000
100001200014000
1 2 3 4
Biofilm formation
After flow
0
100
200
300
400
500
1 2 3 4
( Spo
res
(mm
2 NFCFC
Marine biofouling lab assaysUlva linza spore settlement
Cobetia marina
Barnacle larva settlement
0
10
20
30
40
50
60
PS 1 2 3 4
Mea
n %
set
tlem
ent
24 h48 h
Bacterial biofilm - Marinobacter
hydrocarbonoclasticus Bacterial biofilm - Cobetia marina
4 results in more fouling than 3 in every assay!
What is different between 3 and 4?
This is not different:
WettabilityExposed functional groupsRatio of functional groupsSubstrate roughness
This is different:
Film thicknessNanoscale topography
What about...
Surface free energy?Surface energy...components?Hydrogen bonding?
...Water structure?
3
MeO
HO
HO
HO
O
HOOH
HO
OMe
O
4
Surface energy
Youngs equation: cos sv sl
lv
γ γθγ−=
γlv
γsvγsl θ
If the contact angles are the same for two surfaces,can their surface free energies be different?
As long as (γsv – γsl ) is the same, the contact angle willremain the same irrespective of the absolute valuesof γsv or γsl .
Yes, same θ does not necessarily imply γsv is the same!
Surface energy componentsOwens and Wendt geometric mean approximation:
The acid-base model by van Oss, Chaudhury & Good:
( )(1 cos ) 2LW LW
l s l s l s lγ θ γ γ γ γ γ γ+ − − ++ = + +
(1 cos ) 2 2d d p p
l s l s lγ θ γ γ γ γ+ = + γ d = Dispersiveγ p = Polar
γ LW = Lifshitz-van der Waalsγ AB = Acid-baseγ + = Acceptor (Lewis acid) γ − = Donor (Lewis base)
W = WaterDi = DiiodomethaneEG = Ethylene glycol Whoever is right, the surfaces are different, but how?
Contributions to thesurface free energy:
Owens & Wendt van Oss, Chaudhury & Good
θW
θDi
θEG
γd
γp
γTot
γLW
γ +
γ-
γAB
γ Tot
1 5.8 26.5 13.3 29.3 40.3 69.7 45.4 0.0 66.3 0.0 45.4
2 73.6 63.1 68.4 18.6 12.7 31.3 26.1 0.0 20.0 0.0 26.1
3 48.7 46 41.7 26.0 24.1 50.0 36.5 0.01 39.4 1.26 37.7
4 52 45.1 32.3 30.2 19.8 50.0 37.0 0.34 29.6 6.34 43.3
Contact angle (°)
SummaryThe hypothesis that protein resistance results in reduced marine biofouling is not contradicted by the assays:
Better protein resistance sometimes results inless marine biofouling (...at least in these assays).
However, although 3 and 4 have similar wetting properties, expose the same functional groups (at the same ratio), the assay results are consistently different!
Detailed information at the molecular level is required to explain the biofouling results (…and antifouling properties in general)!
OH OH Me OH
43θa = 50θr = 24
θa = 52θr = 22