AFM Studies of High Generation

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AFM Studies of High Generation PAMAM
Dendrimers at the Liquid/Solid Interface Structura
l Distortion of Isolated Features and Aggregate
Formation
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Polyamidoamine (PAMAM) Dendrimers
G2 29 Å
G4 45 Å
G6 67 Å
amines 3o 1o G2 14 16 G4 62 64 G6 254 25
6 ... G10 4094 4096 pKa 3-6 7-9
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Polyamidoamine (PAMAM) Dendrimers
N
N
G0
Repeating (monomer) unit
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Structure Applications
High density of functional groups, branched
structure, spherical shape for gen. 5, empty
container space (micelle mimic), size diameter
10 nm for G9
Many transport applications, catalysis / reaction
vessels, molecular antennae
G7 PAMAM Dendrimer
Ordered Dendrimer Film
Self-assembly at interface
Useful for chemical sensing devices, modifies
size shape of dendrimers
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Previous Studies
Focus dried adsorbate on hydrophilic
surfaces Amine-terminated dendrimers readily
adsorb Observed single/packed dendrimers
smooth films Compression along surface normal
lateral spreading (G5 d 15nm, h 1nm /
G10 d 25nm, h 5nm) Surface hydrophobicity
neighboring dendrimers lessen dendrimer
distortion
Evolution of conditions during drying process
? Influence of residual water ( capillary
forces) ? ? Investigate adsorption in situ (in
the presence of solvent)
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G7 PAMAM at H2O/HOPG interface (pH7) Extensive
Aggregation Possible
(all images 10mm scan size)
100 mg/ml
10 mg/ml
1 mg/ml
100 ng/ml
1 ng/ml
0.01 ng/ml
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Other Characteristics of Dendrimer Aggregation
Moderate Accumulation at Step Edges
Domain Formation
pH 2.2, 10mm scan size
pH 10.7, 10mm scan size
G9 PAMAM, 1 mg/ml, on HOPG
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G9 PAMAM Dendrimers in MeOH
Very different than in H2O
Section analysis of small features
Many features with h lt 5 nm, d lt 20 nm Single
dendrimers, small size
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High Polarity Autoprotolysis of Water Highly
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 ltlt 10-21 2
Decreasing polarity protonation
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PAMAM Dendrimers on Gold Surface
More evidence that water is special as dendrimer
solvent
Adsorbed Mass at liquid/solid Interface from QCM
Measurements
Quartz Crystal ??
Water
Ethanol
Kang, Durning, Rahman
Dendrimer Generation
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Solvent Influence on Aggregation David Knapps
Study of PAMAM Dendrimers in BuOH
Confirms absence of aggregates in less polar
solvents Stronger adsorption on mica than HOPG
see also Macromol. Symp. 167, 257 (2001)
Determination of single dendrimer sizes in situ
challenging (possibly less dendr. distortion
in less polar solvents) Doubts about
aggregation in aqueous solvent
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Less Aggregation on Hydrophilic Substrates
G9 PAMAM on mica mostly isolated dendrimers
Mica, 5mm, pH7, 0.05 NaN3
Mica, 1.5mm, pH 3.3, no NaN3
Azide (bacteriostatic) pH made no difference
for aggregation More coverage at lower pH
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Web-Formation on Hydrophobic Substrates
G9 PAMAM on HOPG without Aggregates
HOPG, 10mm, pH7, no LiCl
HOPG, 5mm, pH7, 0.05M LiCl
Web-formation signature of loose surface bonding
? Consistent with dragging effects in small scale
scans with studies of dried films Macromol.
Symp. 167, 257 (2001) LiCl (water structure
breaker) has no effect
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Aggregates Webs
G5 PAMAM on HOPG More aggregation than G9
Web-formation aggregates up to
d100nm Acidification may promote aggregation
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In Situ Isolated Dendrimers
G9 PAMAM on Mica
w1113nm h4-5nm
In dried films w20nm Governed by H2O surface
tension ?
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In Situ Isolated Dendrimers
G9 PAMAM on Mica, pH 3.3
w16-18nm h6nm
Acidification may increase dendrimer volume