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Super-resolution Methods

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  1. Super-resolution Methods I - PALM

  2. Detecting A Single Fluorescent Molecule? • Size: ~ 1nm • Absorption Cross-section: ~ 10-16 cm2 • Quantum Yield: ~1 Absorbance of 1 molecule = ? How many fluorescence photons per excitation photons?

  3. Single Molecule “Blinks”

  4. Myosin V -- a motor protein.

  5. De-convolution Microscopy Thompson, RE; Larson, DR; Webb, WW, Biophys. J. 2002,

  6. Paul Selvin

  7. Photo-activation De-convolution Accuracy # of photons

  8. Photo-switchable Fluorescent Protein Gurskaya NG et al. 2006 Nat. Biotechnol.

  9. Photo-activation Localization Microscopy (PALM)

  10. stochastic optical reconstruction microscopySTORM

  11. Ground-State Depletion (GSDIM)

  12. What Next? • Z-resolution • Better fluorescent proteins • Multiple-color labeling • Cryo-temperature imaging

  13. II. NSOM

  14. Super-Resolution: Beyond Diffraction Limit of λ/2: Near-Field: Distance <<Optical Wavelength Light not yet diffracted at sample Resolution not diffraction Limited, no diffraction, Limited by aperture size Aperture Diameter<<Wavelength: 50-100 nm Aperture-Surface distance<<Wavelength: 20 nm Probes made from pulled fiber-optics

  15. Experimental Geometries with Fiber-based Probes trans epi • Transmission mode most common (far-field collection) • Epi-illumination good for two-photon excitation • Far-field excitation, Near field Collection mode good for SHG • (not shown here)

  16. Fabrication of Tapered Fiber tips: cannot with standard pipette puller for electrophysiology CO2 Laser Pull-solenoid Pull down to 30-100 nm diameter Very fragile, fabrication not highly reproducible

  17. EM of Uncoated Tip Hallen lab, NC State Uncoated tips do not confine light well for one photon excitation Good for NLO modes (intrinsic peak power confinement) Much higher transmission than coated tips

  18. Coating tips with Evaporated aluminum Coating confines light Rotate at magic angle For even coverage Bell Jar Hallen lab, NC State

  19. Signal Strength vs Resolution Resolution only depends on aperture, not wavelength Theoretical: 1/r6 scaling 50 nm practical limit: 106 throughout loss of laser Hallen lab, NC State

  20. Measuring forces Scanning Probe Feedback Mechanism: AFM and NSOM same implementation Need constant tip-specimen distance for near-field Use second NIR laser and 2-4 Sectored position sensitive diode Probe has mirror on top

  21. Experimental Geometry with AFM type Feedback Tapered fibers use same Feedback as AFM Control piezo for Axial control

  22. Nanonics Design Sits on Inverted Microscope Far-field collection

  23. Nonlinear excitation and NSOM with probe collection • Use uncoated probes: • Higher efficiency • Metals can interact with • Strong laser field, • perturb sample • (e.g. quench fluorescence) • Confinement from NLO • Don’t need coating Far-field excitation, NSOM collection Saykally, J. Phys. Chem. B, (2002)

  24. Shear force (topography), transmission NSOM, and fluorescence NSOM images of a phase separated polymer blend sample (NIST)

  25. Limitations • Shallow depth of view. • Weak signal • Very difficult to work on cells, or other soft samples • Complex contrast mechanism – image interpretation not always straightforward • Scanning speed unlikely to see much improvement

  26. Practical Concerns Hallen lab, NC State ↑ - Coating can have small pinholes: Loss of confinement - Easily damaged in experiment

  27. Aperture vs Apertureless NSOM

  28. Principle of the Apertureless NSOM Sharp tip of a electric conductor enhance (condense) the local electric field.

  29. Raman spectrum (SERRS) of Rh6G with and without AFM tip

  30. Apertureless NSOM Probes

  31. III. STED

  32. Stimulated Emission Rate: Absorption Rate: Number of atoms or molecules in lower energy level (Unit: per cm3) Number of atoms or molecules in lower energy level (Unit: per cm3) -σ12FN1 -σ21FN2 Absorption Cross-Section Units → cm2 Photon Flux Units → #/cm2sec Stimulated emission Cross-Section Units → cm2 (typical value ~ 10-19 to 10-18 cm2) Photon Flux Units → #/cm2sec σ12 = σ21

  33. Stimulated Emission Depletion (STED) Drive down to ground state with second “dump”pulse, Before molecule can fluoresce Quench fluorescence and Combine with spatial control to make “donut”, achieve super-resolution in 3D (unlike NSOM)

  34. Setup

  35. STED Experimental Setup and PSF’s 100 nm Axial and lateral PSFs Need two tunable lasers, Overlapped spatially, temporally And synchronized Hell et al

  36. Resolution increase with STED microscopy applied to synaptic vesicles

  37. The real physical reason for the breaking of the diffraction barrier is not the fact that fluorescence is inhibited, but the saturation (of the fluorescence reduction). Fluorescence reduction alone would not help since the focused STED-pulse is also diffraction-limited.

  38. RESOLFT: Extending the STED Idea • Triplet – Singlet • PAFP • Photochromic Dye

  39. 4-pi Microscopy

  40. 4pi Microscopy: Improves Axial Resolution Excite high NA top and bottom

  41. Standing Wave interference makes sidelobes Need deconvolution to remove sidelobes from image

  42. The resolution is largely given by the extent of the effective 4Pi-spot, which is 3-5 times sharper than the spot of a regular confocal microscope

  43. ~100 nm Axial Resolution 2-photon confocal 2-photon 4pi 2-photon 4pi With sidelobes gone