Scanning Probe Investigations Scanning Probe Investigations of Physisorption of Physisorption

and Chemical Reactivityand Chemical Reactivity

Tapping Mode AFM Studies Tapping Mode AFM Studies of PAMAM Dendrimersof PAMAM Dendrimers

T. MüllerT. Müller, D. Yablon, M. Kleinman,, D. Yablon, M. Kleinman,R. Karchner, H. Fang, and G. FlynnR. Karchner, H. Fang, and G. Flynn

& collaborators: & collaborators: S. Jockusch and N. Turro, S. Jockusch and N. Turro,

K. Rahman, and C. DurningK. Rahman, and C. Durning

Range over whichRange over whichtunneling probabilitytunneling probability

is non-zero is is non-zero is 10 Å10 Å

[Resolution [Resolution 0.5 Å] 0.5 Å]

Organic Pollutants (e.g., CCl4)

Natural Hematite -Fe2O3 (0001)

X,YScan Circuit

PiezoFeedback

Electronics

Images

Vbias

PCZ

motion

ItTip

Probing the Surface Morphology of Iron Oxides in UHV

Schematic of Scanning Tunneling Microscope

Pollutants (e.g., Uranyl, Chromate, Selenate)

X,YScan Circuit

PiezoFeedback

Electronics

Images

Vbias

PCZ

motion

ItTip

Schematic of Scanning Tunneling Microscope

Redox Reactions on Iron Foil Liquid-Solid Interfaces

Iron Foil

Liquid-Solid Interface

Atomic Force MicroscopyAtomic Force Microscopy

Operation in Tapping ModeOperation in Tapping ModeMinimizes Lateral (Shear) ForcesMinimizes Lateral (Shear) Forces

Tip Engaged

Free Oscillation

Cantilever drivennear resonance frequency

Sample contact reduces oscillation amplitude

Scanned Sample Setup withScanned Sample Setup withBeam Deflection DetectorBeam Deflection Detector

position-sensitive detector

lens

mirror mirror

piezotube

scanner

cantilever

& tip

controller

computer

scan X,Ycontrol Z

driveoscillator

nor

mal

ized

dif

fere

nce

sig

nal

laser

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

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

high density of functional groupshigh density of functional groupsbranched structure, spherical shape for gen. ≥ 5branched structure, spherical shape for gen. ≥ 5empty “container” space (micelle mimic)empty “container” space (micelle mimic)size: diameter ~ 10nm for G9size: diameter ~ 10nm for G9

many transport applications,many transport applications,catalysis / reaction vessels,catalysis / reaction vessels,molecular antennaemolecular antennae

G7 PAMAM DendrimerG7 PAMAM Dendrimer

Ordered Dendrimer FilmOrdered Dendrimer Film

Self-assembly at interfaceSelf-assembly at interface

useful for chemical sensing devicesuseful for chemical sensing devicesmodifies size & shape of dendrimersmodifies size & shape of dendrimers

Structure & ApplicationsStructure & Applications

• • Focus: dried adsorbate on hydrophilic surfacesFocus: dried adsorbate on hydrophilic surfaces

• • Amine-terminated dendrimers readily adsorbAmine-terminated dendrimers readily adsorb

• • Observed single dendrimers and smooth filmsObserved single dendrimers and 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)

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) ?

• • Influence of Influence of charge interactionscharge interactions between dendrimer between dendrimer

(+) and surface (-) ?(+) and surface (-) ?

Dried Films on Hydrophobic SurfacesDried Films on Hydrophobic Surfaces

PAMAM Dendrimers on dry HOPGPAMAM Dendrimers on dry HOPG

G9, 10 g/ml, pH~8 G9, 1 g/ml, pH~7

• • Adsorption onto (hydrophobic) HOPGAdsorption onto (hydrophobic) HOPG

from dilute (0.001% w/w) solutionfrom dilute (0.001% w/w) solution

• • AFM-induced lateral motion avoidableAFM-induced lateral motion avoidable

• • Few surface contact-minimizing aggregatesFew surface contact-minimizing aggregates

Compression along surface normal but limited lateral spreadingCompression along surface normal but limited lateral spreading ~ 45% smaller molecular volume than on mica~ 45% smaller molecular volume than on mica

Conformational change due to absence of polar medium ?Conformational change due to absence of polar medium ?

G9, 100 g/ml, pH = 7

PAMAM Dendrimers on dry HOPGPAMAM Dendrimers on dry HOPG

Cross Section of Single Dendrimers Cross Section of Single Dendrimers

FWHM = 18 nm, height = 4.8 nm

In SituIn Situ Studies of Self-Assembly Studies of Self-Assemblyat the Liquid-Solid Interfaceat the Liquid-Solid Interface

drying drying SupernatantSupernatant

SolidSolid SolidSolid

AirAir

? ? ? ?? ? ? ?

In Situ AFM Studies of PAMAM Dendrimers In Situ AFM Studies of PAMAM Dendrimers at the Liquid-Solid Interfaceat the Liquid-Solid Interface

Self-Assembly in the Presence of the Supernatant Self-Assembly in the Presence of the Supernatant

Aggregates form and reside Aggregates form and reside

exclusively at interface exclusively at interface

• • EPR & fluorescence probes find noEPR & fluorescence probes find no evidence of aggregation in solution evidence of aggregation in solution [Turro Group] [Turro Group]

G9, 1 g/ml, pH~2, 8m

• • Formation of Oblate AggregatesFormation of Oblate Aggregates G9, pH ~ 7: d ~ 200 nm, d/h ~ 10G9, pH ~ 7: d ~ 200 nm, d/h ~ 10

In Situ AFM Studies of PAMAM Dendrimers In Situ AFM Studies of PAMAM Dendrimers at the Liquid-Solid Interfaceat the Liquid-Solid Interface

Self-Assembly in the Presence of the Supernatant Self-Assembly in the Presence of the Supernatant

The Solution-Adsorption EquilibriumThe Solution-Adsorption Equilibrium

Emission Wavelength (nm)Emission Wavelength (nm)

RelativeRelativeEmissionEmissionIntensityIntensity

Fluorescence of PAMAM dendrimers remaining in solutionFluorescence of PAMAM dendrimers remaining in solution

After Inserting HOPGAfter Inserting HOPG( ~1cm( ~1cm22 surface surface per ml solution )per ml solution )

Initial Solution:Initial Solution:2x102x10-8-8 M G6 PAMAM. M G6 PAMAM.

(Fluorescein labeled (Fluorescein labeled G6 PAMAM dend.,G6 PAMAM dend.,pumped at 480nm)pumped at 480nm)

AFM studies: Solution depleted of dendrimers !AFM studies: Solution depleted of dendrimers !

Extensive Concentration Study for G9 PAMAM, pH~7Extensive Concentration Study for G9 PAMAM, pH~7

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)

Concentration Study for G9Concentration Study for G9Parameterization of Aggregate Size DistributionParameterization of Aggregate Size Distribution

AggregateAggregate

FWHMFWHM

[nm][nm]

Concentration in Supernatant [Concentration in Supernatant [g/ml]g/ml]

pH-Dependence of Dendrimer Aggregation pH-Dependence of Dendrimer Aggregation

on HOPGon HOPG

G9 PAMAM, 1 G9 PAMAM, 1 g/ml, 10 g/ml, 10 m scan sizem scan size

pH = 2.2pH = 2.2 pH = 10.7pH = 10.7

G9 PAMAM, 1 G9 PAMAM, 1 g/ml, 5 g/ml, 5 m scan sizem scan size

pH = 3.1pH = 3.1 pH = 6.2pH = 6.2

pH-Dependence of Dendrimer Aggregation pH-Dependence of Dendrimer Aggregation

on Micaon Mica

Acidification favors increased aggregationAcidification favors increased aggregation

Protonation of G9 PAMAM dendrimers:Protonation of G9 PAMAM dendrimers:

• • 2048 outer (primary) amines with pKa ≈ 7-92048 outer (primary) amines with pKa ≈ 7-9

• • 2046 inner (tertiary) amines with pKa ≈ 3-62046 inner (tertiary) amines with pKa ≈ 3-6

all within ~ 5 nm radiusall within ~ 5 nm radius

dramatic changes of charge & H-bonding with pHdramatic changes of charge & H-bonding with pH

pH 3pH 3 pH 6 pH 6 pH 9 pH 9

… … control ionic strength of supernatant ?control ionic strength of supernatant ?

Self-Assembly and Ionic StrengthSelf-Assembly and Ionic Strength

G9 PAMAM on HOPG, 10 G9 PAMAM on HOPG, 10 g/ml, 4.5 g/ml, 4.5 m scan sizem scan size

0.001 M Na0.001 M Na22HPOHPO44 0.1 M Na0.1 M Na22HPOHPO44

Film Formation and Ionic StrengthFilm Formation and Ionic Strength

Section Analysis of Film FragmentsSection Analysis of Film Fragments

0.001 M Na0.001 M Na22HPOHPO44 0.1 M Na0.1 M Na22HPOHPO44

Ions in supernatant lessen compression along surface normalIons in supernatant lessen compression along surface normal

Substrate Dependence of Self-AssemblySubstrate Dependence of Self-AssemblyG5 PAMAM Dendrimers on Si and HOPGG5 PAMAM Dendrimers on Si and HOPG

pH = 5.4pH = 5.4 pH = 9.2pH = 9.2

HOPGHOPG

SiSi

10 10 mm

10 10 mm

10 10 mm

5 5 mm

Less aggregation on hydrophilic substrates (?)Less aggregation on hydrophilic substrates (?)

Summary & ConclusionsSummary & Conclusions• Dendrimers exhibit rich behavior at surfaces & interfacesDendrimers exhibit rich behavior at surfaces & interfaces

• Adsorption to hydrophobic surfaces despite strong interaction with waterAdsorption to hydrophobic surfaces despite strong interaction with water

• Significant compression along surface normal upon physisorptionSignificant compression along surface normal upon physisorption

•• Future Directions:Future Directions:

Imaging in nonpolar solvents (e.g., phenyloctane)Imaging in nonpolar solvents (e.g., phenyloctane)

Submolecular resolution (low-current STM)Submolecular resolution (low-current STM)

Dendrimers with enclosed guest molecules (FeODendrimers with enclosed guest molecules (FeOxx nanoparticles ?) nanoparticles ?)

• • Formation of dried films:Formation of dried films:

Drying process breaks up aggregates (isolated dendrimers / film)Drying process breaks up aggregates (isolated dendrimers / film)

Important role of residual water (flattening & expansion)Important role of residual water (flattening & expansion)

• Investigated self-assembly in solution:Investigated self-assembly in solution: Near-universal formation of large, oblate aggregatesNear-universal formation of large, oblate aggregates

Aggregates form & reside exclusively at interfaceAggregates form & reside exclusively at interface

Weak dependence on solution parametersWeak dependence on solution parameters