afm studies of high generation pamam dendrimers at the liquid/solid interface structural distortion...
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AFM Studies of High Generation
PAMAM Dendrimers
at the Liquid/Solid Interface
Structural Distortion of Isolated Features
and Aggregate Formation
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Polyamidoamine (PAMAM) Dendrimers Polyamidoamine (PAMAM) Dendrimers
# amines# amines33oo 11oo
G2G2 1414 1616G4G4 6262 6464G6G6 254254 256256......G10G10 40944094 40964096
pKapKa 3-6 3-6 7-97-9
G2G229 Å29 Å
G4G445 Å45 Å
G6G667 Å67 Å
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Polyamidoamine (PAMAM) Dendrimers Polyamidoamine (PAMAM) Dendrimers
N
N
O
NH
H2N
G=0
O
HN
H2N1o
O
NH
N
O
HN
N
3o
3o
(( CCHH22 -- CCHH22 -- CCOO -- NNHH -- CCHH22 -- CCHH22 -- NN ))
Repeating (monomer) unit :Repeating (monomer) unit :
3o
O
O
NH
NH
NH2
NH2
G=1
1o
1o
3o
O
O
NH
NH
NH2
NH2
1o
1o
pKa = 7-9
pKa = 3-6
N
N
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Structure & ApplicationsStructure & Applications
G7 PAMAM DendrimerG7 PAMAM DendrimerOrdered Dendrimer FilmOrdered Dendrimer Film
High density of functional groups,High density of functional groups,branched structure, spherical shape for gen. ≥ 5,branched structure, spherical shape for gen. ≥ 5,empty “container” space (micelle mimic),empty “container” space (micelle mimic),size: diameter ~10 nm for G9size: diameter ~10 nm for G9
Many transport applications,Many transport applications,catalysis / reaction vessels,catalysis / reaction vessels,molecular antennaemolecular antennae
Self-assembly at interfaceSelf-assembly at interface
Useful for chemical sensing devices,Useful for chemical sensing devices,modifies size & shape of dendrimersmodifies size & shape of dendrimers
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• • Focus: dried adsorbate on hydrophilic surfacesFocus: dried adsorbate on hydrophilic surfaces
• • Amine-terminated dendrimers readily adsorbAmine-terminated dendrimers readily adsorb
• • Observed single/packed dendrimers & smooth filmsObserved single/packed dendrimers & smooth films
• • Compression along surface normal & lateral spreadingCompression along surface normal & lateral spreading
(G5: d = 15nm, h = 1nm / G10: d = 25nm, h = 5nm)(G5: d = 15nm, h = 1nm / G10: d = 25nm, h = 5nm)
• • Surface hydrophobicity & neighboring dendrimersSurface hydrophobicity & neighboring dendrimers
lessen dendrimer distortionlessen dendrimer distortion
Previous StudiesPrevious Studies
• • Evolution of conditionsEvolution of conditions during drying process ?during drying process ?
• • Influence of Influence of residual waterresidual water (& capillary forces) ? (& capillary forces) ?
Investigate adsorption Investigate adsorption in situin situ (in the presence of solvent) (in the presence of solvent)
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G7 PAMAM at H2O/HOPG interface (pH~7): G7 PAMAM at H2O/HOPG interface (pH~7):
Extensive Aggregation PossibleExtensive Aggregation Possible
100 100 g/mlg/ml10 10 g/mlg/ml
1 1 g/mlg/ml
100 ng/ml100 ng/ml1 ng/ml1 ng/ml
0.01 ng/ml0.01 ng/ml
(all images 10(all images 10m scan size)m scan size)
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G9 PAMAM, 1 G9 PAMAM, 1 g/ml, on HOPGg/ml, on HOPG
pH = 2.2, 10pH = 2.2, 10m scan sizem scan size
Moderate Accumulation Moderate Accumulation
at Step Edgesat Step Edges Domain FormationDomain Formation
pH = 10.7, 10pH = 10.7, 10m scan sizem scan size
Other Characteristics of Dendrimer AggregationOther Characteristics of Dendrimer Aggregation
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G9 PAMAM Dendrimers in MeOH
Section analysis of small featuresSection analysis of small features
Many features with h < 5 nm, d < 20 nmMany features with h < 5 nm, d < 20 nm
Single dendrimers, small sizeSingle dendrimers, small size
Very different than in HVery different than in H22O …O …
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High Polarity & Autoprotolysis of Water:High Polarity & Autoprotolysis of Water:
Highly Charged PAMAM DendrimersHighly Charged PAMAM Dendrimers
Autoprotolysis Const. Dielectric Const.
H2O 10-14 78.5
MeOH 10-17 32.6
EtOH 10-19 24.3
BuOH 10-21 17.1
n-Alkanes << 10-21 ~ 2De
cre
asin
g p
ola
rity
& p
roto
na
tio
n
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0
50
100
150
200
250
300
350
400
450
500
0 1 2 3 4 5 6 7 8 9 10
G
u p ¢ Measured
Ï SEC
r SANS
¯ Rh
o Rg (sim)
™ 35 Rg
Dendrimer GenerationDendrimer Generation
QuartzQuartzCrystalCrystal
Kang, Durning,Rahman
PAMAM Dendrimers on Gold Surface PAMAM Dendrimers on Gold Surface
EthanolEthanol
WaterWater
Adsorbed Mass at liquid/solid Interface from QCM Measurements
More evidence that water is special as dendrimer solvent … More evidence that water is special as dendrimer solvent …
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Solvent Influence on Aggregation
David Knapp’s Study of PAMAM Dendrimers in BuOH
• • Confirms absence of aggregates in less polar solventsConfirms absence of aggregates in less polar solvents
• • Stronger adsorption on mica than HOPGStronger adsorption on mica than HOPG
[see also Macromol. Symp. [see also Macromol. Symp. 167167, 257 (2001)], 257 (2001)]
• • Determination of single dendrimer sizes Determination of single dendrimer sizes in situin situ challenging challenging
(possibly less dendr. distortion in less polar solvents)(possibly less dendr. distortion in less polar solvents)
• • Doubts about aggregation in aqueous solvent …Doubts about aggregation in aqueous solvent …
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Mica, 5Mica, 5m, pH~7, 0.05% NaNm, pH~7, 0.05% NaN33
Less Aggregation on Hydrophilic SubstratesLess Aggregation on Hydrophilic Substrates
Azide (bacteriostatic) & pH made no difference for aggregationAzide (bacteriostatic) & pH made no difference for aggregation
More coverage at lower pHMore coverage at lower pH
Mica, 1.5Mica, 1.5m, pH 3.3, no NaNm, pH 3.3, no NaN33
G9 PAMAM on mica: mostly isolated dendrimersG9 PAMAM on mica: mostly isolated dendrimers
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HOPG, 10HOPG, 10m, pH~7, no LiClm, pH~7, no LiCl
Web-Formation on Hydrophobic SubstratesWeb-Formation on Hydrophobic Substrates
Web-formation: signature of loose surface bonding ?Web-formation: signature of loose surface bonding ?
Consistent with dragging effects in small scale scansConsistent with dragging effects in small scale scans
& with studies of dried films [Macromol. Symp. & with studies of dried films [Macromol. Symp. 167167, 257 (2001)], 257 (2001)]
LiCl (“water structure breaker”) has no effectLiCl (“water structure breaker”) has no effect
HOPG, 5HOPG, 5m, pH~7, 0.05M LiClm, pH~7, 0.05M LiCl
G9 PAMAM on HOPG without AggregatesG9 PAMAM on HOPG without Aggregates
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Aggregates & WebsAggregates & Webs
Web-formation & aggregates up to d~100nmWeb-formation & aggregates up to d~100nm
Acidification may promote aggregationAcidification may promote aggregation
G5 PAMAM on HOPG: More aggregation than G9G5 PAMAM on HOPG: More aggregation than G9
HOPG, 5HOPG, 5m, pH~7m, pH~7
HOPG, 1HOPG, 1m, pH~7m, pH~7
HOPG, 2HOPG, 2m, pH 2.0m, pH 2.0
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In Situ Isolated DendrimersIn Situ Isolated Dendrimers
In dried films: w~20nmIn dried films: w~20nm
Governed by HGoverned by H22O surface tension ?O surface tension ?
G9 PAMAM on MicaG9 PAMAM on Mica
w~11–13nmw~11–13nm
h~4-5nmh~4-5nm
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In Situ Isolated DendrimersIn Situ Isolated Dendrimers
Acidification may increase dendrimer volumeAcidification may increase dendrimer volume
G9 PAMAM on Mica, pH 3.3G9 PAMAM on Mica, pH 3.3
w~16-18nmw~16-18nm
h~6nmh~6nm