Scanning Probe Investigations of Physisorption and Chemical Reactivity

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]

5. 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)

6. 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

9. 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

10. 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 :

11. 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

12. 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

13. 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 (-) ?

14. Dried Films on Hydrophobic Surfaces

15. • 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

16. 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 ?

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

18. 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

19. 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

20. 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 !

21. 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

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

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

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

25. 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 ?

26. 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

27. 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

28. 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 (?)

29. 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 ?)