pH Responsive Polymer Thin Films

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pH Responsive Polymer Thin Films
Dan Regan
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Outline

• Background
• Surface chemistry
• Covalent immobilization
• Photoactive linker
• Procedure
• Piranha cleaning
• Silanization
• Immobilization
• Protonation

• Analysis
• Ellipsometry
• Contact angle measurements
• Atomic Force Microscopy
• Results
• Thickness vs. Molar ratio
• Swelling
• Hydrophilicity
• Reversibility
• Conclusion

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Miniaturization - Surface Matters!
Increased surface/volume ratio

• Surface property
• Compatibility with reagents
• Reaction yield
• Fluid flow

Smart surface - surface that provides functions
surface energy gradient
molecular recognition
biocompatibility
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Covalent Immobilization of Polymer Thin Films
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Immobilization
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Piranha cleaning

• 73 ratio by volume of H2SO4 to H2O2
• 80o to 90oC for 1.5 hours
• Hot water bath (80o to 90oC) for 1 hour

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Silanization

• PFPA-silane PTMS

• Prepare solutions at different molar ratios of
  PFPAPTMS
• 10, 110, 130, 170, 1100, 1500
• Relation between number of attachment points and
  degree of swelling observed
• Soak samples for 4 hours

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Spin coating

• 3 or 4 droplets of P4VP in n-butanol per wafer
• Spin coat
• 2000 rpm
• 60 sec

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UV irradiation and Solvent extraction

• Place 280nm optical filter over sample
• Irradiate 5 minutes
• Sonicate in
• n-butanol 5 minutes
• Extracts unattached polymer

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Protonation

• Prepare dilute solution of HCl
• pH 2
• Soak sample 10 minutes

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Ellipsometry

• Ellipsometer used to measure thickness of

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• Molar ratio refers to PTMSPFPA-silane

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• The lower the surface density of PFPA-silane, the
  thinner the film

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• Covalent immobilization allows film to withstand
  fairly strong acidic conditions (pH 2)

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• Immobilized polymer films swell at pH 2
• The amount of swelling is independent of the
  surface density of the azide

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AFM topography image of immobilized film (at
molar ratio 1500) before and after protonation

• Scale 500nm x 500nm
• Surface structure changed after protonation

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Reversibility

• The protonation process is reversible

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Contact angle measurements

• Use syringe to place a drop of water on the
  surface
• 2 microliters
• Measure contact angle using contact angle viewer

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• Films become hydrophilic after protonation

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Conclusion

• Covalently immobilized P4VP exhibits a pH
  responsive behavior that is reproducible
• Swelling
• Hydrophilicity
• Reversibility
• Future work
• Investigate why the molar ratio of 10 does not
  fit the trend for thickness
• Investigate salt effects on immobilized thin films

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Protonation with Phosphate Buffer (pH 2)

• Salt on surface made ellipsometry difficult
• When surface was rinsed, apparent that film
  thickness decreased

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Acknowledgements

• Thanks to
• Dr. Mingdi Yan
• Dr. Li Liu
• Yan Lab John Gann
• Sailaja Chada
• Suji Uppalapati
• Kai Wang
• Melissa Taylor
• Aye Hlaing
• Dr. Raymond Bard

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Perfluorophenyl Azides (PFPAs)
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Surface Modification Chemistry
Silica-based substrates
Metallic substrates