AFM Studies of High Generation

PAMAM Dendrimers

at the Liquid/Solid Interface

Structural Distortion of Isolated Features

and Aggregate Formation

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

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

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

• • 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)

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)

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

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 …

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

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 …

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 …

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

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

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

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

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