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Scanning Probe Investigations of Physisorption and Chemical Reactivity

Scanning Probe Investigations of Physisorption and Chemical Reactivity. 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. Range over which

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Scanning Probe Investigations of Physisorption and Chemical Reactivity

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  1. Scanning Probe Investigations of Physisorption and Chemical Reactivity

  2. 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

  3. Range over which tunneling probability is non-zero is 10 Å [Resolution  0.5 Å]

  4. Piezo Feedback Electronics X,Y Scan Circuit PC Z motion It Images Tip Vbias Schematic of Scanning Tunneling Microscope Probing the Surface Morphology of Iron Oxides in UHV Organic Pollutants (e.g., CCl4) Natural Hematite -Fe2O3 (0001)

  5. Piezo Feedback Electronics X,Y Scan Circuit PC Z motion It Images Tip Vbias Schematic of Scanning Tunneling Microscope Redox Reactions on Iron Foil Liquid-Solid Interfaces Liquid-Solid Interface Pollutants (e.g., Uranyl, Chromate, Selenate) Iron Foil

  6. Scanned Sample Setup with Beam Deflection Detector Operation in Tapping Mode Minimizes Lateral (Shear) Forces laser Free Oscillation position- sensitive detector mirror mirror drive oscillator lens cantilever & tip Cantilever driven near resonance frequency scan X,Y control Z normalized difference signal Tip Engaged piezo tube scanner controller Sample contact reduces oscillation amplitude computer Atomic Force Microscopy

  7. o H N 1 2 N H 2 O pKa = 3-6 N H N H o 1 O O N H 2 o o o 3 3 3 N N N H N H O ( C H - C H - C O - N H - C H - C H - N ) 2 2 2 2 o H 1 N O H N pKa = 7-9 2 O H N O N N N o 3 H N H 2 o 1 O N H N H 2 o 1 G=1 Polyamidoamine (PAMAM) Dendrimers N N G=0 Repeating (monomer) unit :

  8. Polyamidoamine (PAMAM) Dendrimers G2 29 Å G4 45 Å G6 67 Å # amines 3o 1o G2 14 16 G4 62 64 G6 254 256 ... G10 4094 4096 pKa 3-6 7-9

  9. high density of functional groups branched structure, spherical shape for gen. ≥ 5 empty “container” space (micelle mimic) size: diameter ~ 10nm for G9 many transport applications, catalysis / reaction vessels, molecular antennae G7 PAMAM Dendrimer Structure & Applications Ordered Dendrimer Film Self-assembly at interface useful for chemical sensing devices modifies size & shape of dendrimers

  10. 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 conditionsduring drying process ? • Influence of residual water (& capillary forces) ? • Influence of charge interactions between dendrimer (+) and surface (-) ?

  11. Dried Films on Hydrophobic Surfaces

  12. • Adsorption onto (hydrophobic) HOPG from dilute (0.001% w/w) solution • Few surface contact-minimizing aggregates • AFM-induced lateral motion avoidable PAMAM Dendrimers on dry HOPG G9, 10 mg/ml, pH~8 G9, 1 mg/ml, pH~7

  13. 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 ?

  14. In Situ Studies of Self-Assemblyat the Liquid-Solid Interface

  15. In Situ AFM Studies of PAMAM Dendrimers at the Liquid-Solid Interface Self-Assembly in the Presence of the Supernatant Supernatant Air drying ? ? ? ? Solid Solid

  16. In Situ AFM Studies of PAMAM Dendrimers at the Liquid-Solid Interface Self-Assembly in the Presence of the Supernatant G9, 1 mg/ml, pH~2, 8mm • Formation of Oblate Aggregates G9, pH ~ 7: d ~ 200 nm, d/h ~ 10 • EPR & fluorescence probes find no evidence of aggregation in solution [Turro Group] • Aggregates form and reside exclusively at interface

  17. 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 !

  18. Extensive Concentration Study for G9 PAMAM, pH~7 (all images 10mm scan size) 100 mg/ml 10 mg/ml 1 mg/ml 100 ng/ml 1 ng/ml 0.01 ng/ml

  19. Concentration Study for G9 Parameterization of Aggregate Size Distribution Aggregate FWHM [nm] Concentration in Supernatant [mg/ml]

  20. pH-Dependence of Dendrimer Aggregation on HOPG G9 PAMAM, 1 mg/ml, 10 mm scan size pH = 2.2 pH = 10.7

  21. pH-Dependence of Dendrimer Aggregation on Mica G9 PAMAM, 1 mg/ml, 5 mm scan size pH = 3.1 pH = 6.2

  22. Acidification favors increased aggregation Protonation 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 ?

  23. 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

  24. Film Formation and Ionic Strength Section Analysis of Film Fragments 0.001 M Na2HPO4 0.1 M Na2HPO4 • Ions in supernatant lessen compression along surface normal

  25. Substrate Dependence of Self-Assembly G5 PAMAM Dendrimers on Si and HOPG pH = 5.4 pH = 9.2 HOPG 10 mm 10 mm Si 10 mm 5 mm • Less aggregation on hydrophilic substrates (?)

  26. 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 ?)

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