Scanning Probe Investigations of Physisorption and Chemical Reactivity

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Scanning Probe Investigations of Physisorption
and Chemical Reactivity
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Tapping Mode AFM Studies of PAMAM Dendrimers
T. Müller, D. Yablon, M. Kleinman, R. Karchner,
H. Fang, and G. Flynn
collaborators S. Jockusch and N. Turro, K.
Rahman, and C. Durning
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Range over which tunneling probability is
non-zero is ?10 Å Resolution ? 0.5 Å
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Schematic of Scanning Tunneling Microscope
Probing the Surface Morphology of Iron Oxides in
UHV
Organic Pollutants (e.g., CCl4)
Natural Hematite ?-Fe2O3 (0001)
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Schematic of Scanning Tunneling Microscope
Redox Reactions on Iron Foil Liquid-Solid
Interfaces
Liquid-Solid Interface
Pollutants (e.g., Uranyl, Chromate, Selenate)
Iron Foil
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Atomic Force Microscopy
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Polyamidoamine (PAMAM) Dendrimers
N
N
G0
Repeating (monomer) unit
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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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Structure Applications
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 dendrimers and smooth
films Compression along surface normal
lateral spreading (G5 d 15nm, h 1nm /
G10 d 25nm, h 5nm)
Evolution of conditions during drying process
? Influence of residual water ( capillary
forces) ? Influence of charge interactions
between dendrimer () and surface (-) ?
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Dried Films on Hydrophobic Surfaces
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PAMAM Dendrimers on dry HOPG
G9, 10 mg/ml, pH8
G9, 1 mg/ml, pH7
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PAMAM Dendrimers on dry HOPG
G9, 100 mg/ml, pH 7
Cross Section of Single Dendrimers
FWHM 18 nm, height 4.8 nm

• Compression along surface normal but limited
  lateral spreading
• 45 smaller molecular volume than on mica
• Conformational change due to absence of polar
  medium ?

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In Situ Studies of Self-Assemblyat the
Liquid-Solid Interface
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Supernatant
Air
drying
? ? ? ?
Solid
Solid
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EPR fluorescence probes find no
evidence of aggregation in solution Turro
Group

• Aggregates form and reside exclusively at
  interface

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The Solution-Adsorption Equilibrium
Fluorescence of PAMAM dendrimers remaining in
solution
(Fluorescein labeled G6 PAMAM dend., pumped at
480nm)
Initial Solution 2x10-8 M G6 PAMAM.
Relative Emission Intensity
After Inserting HOPG ( 1cm2 surface per ml
solution )
Emission Wavelength (nm)
AFM studies Solution depleted of dendrimers !
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Extensive Concentration Study for G9 PAMAM, pH7
(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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Concentration Study for G9
Parameterization of Aggregate Size Distribution
Aggregate FWHM nm
Concentration in Supernatant mg/ml
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pH-Dependence of Dendrimer Aggregation on HOPG
G9 PAMAM, 1 mg/ml, 10 mm scan size
pH 2.2
pH 10.7
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pH-Dependence of Dendrimer Aggregation on Mica
G9 PAMAM, 1 mg/ml, 5 mm scan size
pH 3.1
pH 6.2
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Acidification favors increased aggregation Proton
ation of G9 PAMAM dendrimers 2048 outer
(primary) amines with pKa 7-9 2046 inner
(tertiary) amines with pKa 3-6 all within 5
nm radius ? dramatic changes of charge
H-bonding with pH pH 3 pH 6
pH 9 control ionic strength of
supernatant ?
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Self-Assembly and Ionic Strength
G9 PAMAM on HOPG, 10 mg/ml, 4.5 mm scan size
0.001 M Na2HPO4
0.1 M Na2HPO4
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0.001 M Na2HPO4
0.1 M Na2HPO4

• Ions in supernatant lessen compression along
  surface normal

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• Less aggregation on hydrophilic substrates (?)

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Summary Conclusions

• Dendrimers exhibit rich behavior at surfaces
  interfaces
• Adsorption to hydrophobic surfaces despite
  strong interaction with water
• Significant compression along surface normal
  upon physisorption

• Investigated self-assembly in solution
• ? Near-universal formation of large, oblate
  aggregates
• ? Aggregates form reside exclusively at
  interface
• ? Weak dependence on solution parameters

Formation of dried films ? Drying
process breaks up aggregates (isolated dendrimers
/ film) ? Important role of residual water
(flattening expansion)
Future Directions ? Imaging in nonpolar
solvents (e.g., phenyloctane) ?
Submolecular resolution (low-current STM) ?
Dendrimers with enclosed guest molecules (FeOx
nanoparticles ?)