The AFM Probe - Fundamentals,
Selection, and Applications
Introduction
• Appropriate selection of AFM probe can be one of the most
important decisions to be made towards a successful AFM
experiment
• As there are many probes to chose from, this selection may appear
daunting
• The purpose of this webinar is to enable the AFM researcher to
select an AFM probe like an expert.
Outline
• Probes overview
•
Key metrics for probe choice
• Applications based probe decisions
•
Very High Resolution Imaging
•
Quantitative Nanomechanical
•
Fast Scanning in Air and in Fluid
•
Biological Applications and Force Spectroscopy
•
Nano Electrical – KPFM
•
Nano Optical - TERS
• Probe Preparation and Cleaning
• Bruker AFM Website
• Conclusion
Basic Operation of the SPM:
Simplified Schematic
AFM Probe
Bruker Confidential
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2: Sensitivity on the
scale of atoms
3: Forces commensurate
with atomic bonds.
1: Apex on size
scale of atoms
Bruker Confidential
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1: The Probe Apex:
The critical factor for determining AFM resolution
•
The microscope maker’s rule: you can’t visualize anything smaller than what you
are interrogating it with.
•
This is true for photons, electrons, and AFM and why the Apex size and geometry
are so important.
RTESPA
OTESPA
NCHV-A
TESP-HAR
TESP-SS
CDR50C
MCNT-500
VITA-DM-GLA-1
DPT10
MLCT (Dec’13)
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2: AFM Sensitivity:
From the cantilever’s perspesctive
• Simple explanation of optical lever:
smaller cantilevers deflect the spot
more, creating more signal:
• However, its not so simple. Optics
and detection electronics come into
play:
• But the same scaling holds true,
shorter levers create greater signal.
See Fukuma and Jarvis, RSI, 77 (2006)
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•
But there is a tradeoff. . .
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3: Spring Constant
Forces must be commensurate with surface
• As you continue to reduce length, the spring constant (k) of the
lever increases to the 3rd power.
• To maintain low tip sample forces the thickness of the cantilever has
to be scaled at the same rate.
• This is fundamentally why AFM cantilevers are small.
•
L must be short enough to get good signal – 200um or less
•
T must then be thin enough to apply a force “soft” compared to surface & tip
• See brukerprobes.com for to see probes ranging from:
•
L - less than 20um to greater than 300um
•
t – less than 0.2um to greater than 7um
Sarid, “Scanning Force Microscopy”,
ISBN 9780195344691, Oxford University Press
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Cantilever Dynamics and Beyond
• For many modes resonant frequency also plays a critical role in the
AFM’s speed and resolution.
• The probe’s frequency scales with geometry and in general a higher
frequency is better.
• So additionally, one cannot simply scale length and thickness
together for many applications.
• As cantilevers get smaller, tip mass and
cantilever drag are become very important and
add significant complexity to cantilever design.
SCANASYST-AIR-HR
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Point of Order. . .
“AFM’ers” will routinely use relative terms expecting them to be
interpreted in the context of the Application, not absolutely.
• Cantilever Coatings – both simply called coatings
•
Front (Tip) Side Coatings: Used for electrical/conductive measurements
•
Backside Coatings: Used to increase laser reflectivity. These are generally
prefered, but they can cause thermal drift (bimetallic effect) or contamination in
biological.
• Spring Constant – “normal levers” span 4-orders of magnitude
•
In Bio Applications: Soft < 0.05 N/m , Hard > .5 N/m
•
In PeakForce Tapping: Soft < 0.1N/m, Hard > 20 N/m
•
In Tapping Mode: Soft < 5 N/M, Hard > 50 N/m
• Resonant Frequency – “normal levers” span
•
Fluid Tapping: low < 5kHz, high > 50kHz
•
Air Tapping: low < 20kHz, high > 500kHz
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Bruker Confidential
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VERY HIGH RESOLUTION
IMAGING
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AFM probes for very high resolution
TappingMode imaging of Mica in Water
Localize interaction with very small
amplitude (<2nm)
Localize interaction with very sharp tip
•
May need to ‘cherry pick’ probe
•
Preserve tip: moderate spring constant
(1-10 N/m)
•
Use the maximum Amplitude Setpoint
possible (reducing peak force)
Recommended probes
•
ScanAsyst-Fluid+ or FastScan B
AFM probes for very high resolution
PeakForce QNM of Polydiacetylene (PDA) in Air
Height
0.5 nm
Stiffness
Adhesion
1.4 nm
lattice defect
Peak Force Tapping provides much higher
resolution than TappingMode in air & provides
property maps
Localize interaction with very sharp tip
•
May need to ‘cherry pick’ probe tip
•
Preserve tip: low spring constant <1N/m & setpoint <500pN
Recommended probes
•
ScanAsyst-Fluid+ or FastScan C
b
A.Enkelmann, Adv.Pol. Sci., 1984, 63, 91
AFM probes for very high resolution
PeakForce QNM for sub-nm resolution calcite in liquid
DMTModulus channel
provides a qualitative
stiffness map
Height
•
Alternate rows of atoms have
significantly increased contact
stiffness, rows switch over step
•
Need moderate spring constant
for modulus contrast
2 nm
Localize interaction with
very sharp tip
•
May need to ‘cherry pick’ probe tip
•
Preserve tip: moderate spring
constant <5N/m & setpoint <5nN
Recommended probe
Stiffness
•
ScanAsyst-Fluid+ or FastScan B
Data collected in collaboration with Dr. Daniel Ebeling, U. Maryland
AFM probes for very high resolution
PeakForce Tapping to Resolve the DNA Double Helix
PeakForce Tapping images reveal
corrugation corresponding to widths
of major and minor grooves
MAJOR
MINOR
•
DNA loosely bound to mica surface
(~1mM NiCl2) to minimize
conformational effects
Challenge for high-resolution
imaging – requires very low force
and high sensitivity
•
Short cantilever provides sensitivity
•
Low spring constant, setpoint protect tip
and sample from damage
Recommended Probe
•
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FastScan-Dx: 18um length, k~0.25N/m
Bruker Nano Surfaces Division
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AFM probes for very high resolution
Guidelines for probe selection
Resolution determined by localization of interaction
•
Smaller tips resolve smaller features without influence from neighboring structures
•
Peak Force Tapping mode localizes interaction by default while Tapping Mode
requires small amplitudes to localize force
•
Lateral forces can quickly damage the tip, leading to larger interaction areas
Shorter (<125um) probes are recommended
•
Shorter cantilevers are more sensitive, higher frequency, and have smaller viscous
background in liquid
Spring constant should be selected to avoid tip and sample damage
•
Keep peak force low (typically <500pN)
•
Larger spring constants are often more stable, especially in TappingMode (less
concern about adhesion), so very soft (<0.5N/m) cantilevers are not recommended
Consider cantilever resonant frequency (f0)
•
For Peak Force Tapping, resonant frequency f0>10*modulation frequency
•
For TappingMode performance is usually better f0>10kHz
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QUANTITATIVE
NANOMECHANICAL
MEASUREMENTS
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AFM Quantitative Nanomechanics (QNM)
Calculate sample properties directly from force curves
Probe selection for Nanomechanical Measurement key parameters
•
Tip shape (R and/or half angle)
•
Spring Constant (k)
•
Resonant Frequency
•
Length
•
Coating
~
Remember: Quantitative force measurement requires calibration of
spring constant and deflection sensitivity. Modulus also requires
calibration of tip shape. Nominal values are not accurate.
(ii)
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Barrier layer
Nylon
AFM probes for QNM
Strength & gas impermeability
High resolution PF-QNM of a
heat-sealed bag
Tie layer
ULDPE
Preserves layer adhesion
DMTModulus
Sealant layer
Metallocene PE/LDPE blend
Adheres to itself when heated
(a)
Study of interphase between Tie and
Sealant layers requires high resolution and
good modulus contrast
(b)
•
Sharp tip (R<25nm) needed to provide
resolution to separately measure individual
lamella
•
Moderate spring constant (k~2-10N/m) needed
for good match to material stiffness ~100300MPa
•
Backside coating to reduce optical interference
(c)
(a)
(b)
Recommended probe:
•
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TAP150A also known as MPP-12120 (k~5N/m,
R~8nm, Al backside coating)
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AFM probes for QNM
Expanding PeakForce QNM to softer samples
Tip half angle
~18°
Cells
& gels
Tip radius ~20nm
Real AFM tip
Real AFM tips are not a simple sphere or cone
•
Choose the deformation model (Hertz/DMT (spherical) or Sneddon (conical)) that best
represents your tip shape at a given deformation depth
By choosing spring constant, tip shape, and model carefully, the widest
range of properties can be studied
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AFM probes for QNM
Guidelines for probe selection
Tip shape (end radius, half angle) determines resolution
•
Larger tips: shape matches models better and are more stable
•
Smaller tips: resolve smaller features without influence from neighboring structures
Spring constant and tip shape: select to match sample stiffness
•
Most important for modulus & deformation measurements
Resonant frequency >10*modulation freq. to avoid cantilever resonance
•
Not generally an issue in air, but can be a problem in liquid with soft cantilevers
Shorter is usually better
•
More sensitive, higher freq, and have smaller viscous background
•
Longer cantilevers are easier to align and less prone to optical interference
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AFM probes for QNM
PeakForce QNM and Force Volume of Agarose gels
Sneddon model works well over
the biologically relevant kPaMPa range when coupled with
soft cantilever in liquid
•
Relatively blunt tip provides
better repeatability and is more
gentle for very soft samples
•
Soft cantilever (k~2-10N/m)
needed for good match to
material stiffness ~10-500kPa
•
Backside coating to improve
reflectivity of silicon nitride
cantilever
•
Similar results obtained with PFQNM and FV at ramp rates from
1Hz to 500Hz
A stiffer probe would
be better here
Recommended probe:
•
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MLCT-E (k~0.1N/m, R~20nm, half angle
~18deg, Ti/Au backside coating)
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AFM probes for QNM
PeakForce QNM of live cells on glass bottom Petri
dish
Spring constant should match modulus
•
Ecell=kPa << Eglass=GPa
•
MLCT-D has k ~0.03N/m
•
Good for Ecell (see cell structures)
•
Too small for Eglass (out of range)
Glass Surface
Tip Shape should match Fit Model
•
MLCT has pyramidal tip
•
Indentation on soft cell ≥100nm
•
Indentation too deep for DMT (sphere)
•
Sneddon better modulus estimation
DMT Modulus = 50kPa
Sneddon Modulus = 37kPa
Recommended probe for cell:
•
MLCT-D (k~0.03N/m, R~20nm, half
angle~35deg, Ti/Au backside coating)
Modulus image of B16 mouse carcinoma cell. Force curves show fit
for DMT and Sneddon modulus. Images obtained on the BioScope
Catalyst AFM in PeakForce QNM using MLCT-D probes in fluid.
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Bruker Nano Surfaces Division
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FAST SCANNING IN AIR
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Why Fast Scanning SPM?
• Fast Scanning allows real-time study of dynamic
processes at the Nanoscale
• Allow rapid collection of thousands of images for
meaningful statistics
• Collect large (~25MPixel) images in reasonable time for
examination of both fine detail and superstructure in a
single image
AFM probes for Fast Scanning in air
Guidelines for probe selection
Consider cantilever resonant frequency (f0) and quality factor (Q)
•
For TappingMode, cantilever bandwidth is approximately f0/Q
•
Cantilever Q depends on the fluid environment (air, liquid) and on the length of the tip
•
For Peak Force Tapping, resonant frequency f0>10*modulation frequency and modulation
frequency should be high enough to have at least one tap per pixel
Shorter (<50um) probes are recommended
•
Shorter cantilevers are higher frequency, have smaller viscous background in liquid, and are more
sensitive
Resolution determined by localization of interaction
•
Smaller tips resolve smaller features without influence from neighboring structures
•
Peak Force Tapping mode localizes interaction by default while Tapping Mode requires small
amplitudes to localize force
•
Lateral forces can quickly damage the tip, leading to larger interaction areas
Spring constant & setpoint should be selected to avoid tip and sample damage
•
Keep peak force as low as possible, but larger forces may be required for good tracking
•
Larger spring constants are often more stable, especially in TappingMode (less concern about
adhesion), so very soft (<0.5N/m) cantilevers are not recommended
Bottom line
•
PeakForce Tapping is not very fast in air. For Tapping Mode, the best probe is the FastScan A!
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Image
Specifications:
Size: 2.2um
ScanRate: 100Hz
Pixels:256 x 256
TipV: 440um/s
Frame Rate: 2.5s
Real Time Video
Duration: ~4min
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Sample Courtesy:
Dr. Jamie Hobbs
27
University
of Sheffield
Bruker
Nano Confidential
HIGH SPEED AFM – FLUID
IMAGING
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AFM Probes for HS-AFM Imaging in Fluid
Imaging Dynamic Biological Processes
Choosing a probe for high-speed imaging of molecular/cellular dynamics
in TappingMode in fluid. (1 of 2)
•
Small or Ultra-Short Cantilever – length ≤50nm.
•
•
•
Provides low spring constant probes with high resonance frequency (high bandwidth).
Soft Cantilever/Low Spring Constant (k).
•
Soft samples (modulus :~kPa-MPa range.)
•
Samples weakly attached to supporting substrate.
•
Typically Si3N4 cantilevers. Some made of ‘new material’ (quartz-like material).
High Resonance Frequency – f ≥80kHz.
•
Facilitates high-resolution imaging at high scan speeds (secs/frame – frames/sec).
•
Traditional TappingMode probes have lower resonance frequency (eg. SNL-C f~13kHz in fluid.)
•
Challenging probes to design – optimizing spring constant and resonance frequency.
•
IMPORTANT: Most resonance frequencies stated by manufacturer are for operation in air. This
resonance frequency will decrease 1/2 to 2/3 of this value when used in fluid.
•
Use (fast) thermal tune to identify the correct resonance frequency of a probe in fluid.
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Bruker Nano Surfaces Division
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AFM Probes for HS-AFM Imaging in Fluid
Imaging Dynamic Biological Processes
Choosing a probe for high-speed imaging of molecular/cellular dynamics
in TappingMode in fluid. (2 of 2)
•
Tip Sharpness.
•
Small cantilevers =
Force sensitivity =
•
FastScan-Dx has a silicon tip (R ~8nm, ≤12nm )
11Hz
100nm
100nm
Imaging force = Ability to use sharper tips (w/o damage).
22Hz
100nm
43Hz
100nm
Images of DNA origami obtained on the FastScan-Bio AFM in TappingMode using a FastScan-Dx probe in fluid
(512 pixels/image) . Sample courtesty of P. Rothemond and L. Qian, CalTech.
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AFM Probes for HS-AFM Imaging in Fluid
High-Speed Imaging of DNA or DNA-Protein
Dynamics
High-Speed TappingMode in Fluid.
•
Short Cantilever – length ≤50nm.
•
Soft Cantilever – k ~0.1N-0.3N/m.
•
•
Stiff enough to oscillate in fluid.
•
Too stiff – damage to sample.
High Resonance Frequency – f ≥70kHz.
•
•
1.0fps
2.0fps
3.0fps
Higher f = faster imaging speeds.
Sharp tip – small radius.
•
Resolve molecular structure.
Recommended AFM Probes.
•
0.5fps
FastScan-Dx (l ~18µm; k ~0.25N/m; f
~110kHz; R ~8nm)
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High-speed imaging of Lambda Digest DNA. Parts of the DNA
strand loosely bound to the surface appear to move from frameto-frame (circle). Images were obtained on the FastScan-Bio AFM
in TappingMode using FastScan-Dx probes in fluid.
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AFM Probes for HS-AFM Imaging in Fluid
High-Resolution and High-Speed Imaging of Cell
Membrane Dynamics
High-Speed TappingMode in Fluid.
•
Short Cantilever – length ≤50nm.
•
Soft Cantilever – k ~0.1N-0.3N/m.
•
•
High Resonance Frequency – f ≥70kHz.
•
•
Too stiff = damage to sample.
Higher f = faster imaging speeds.
Sharp tip – smallest radius.
•
Resolve structures on cell surface.
•
Small cantilever =
imaging force.
100nm
Recommended AFM Probes.
•
FastScan-Dx (l ~18µm; k ~0.25N/m; f
~110kHz; R ~8nm)
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High-speed imaging of the effects of CM15 (AmP) on the outer
membrane of live E. coli cells. Ordered structures in the
membrane are believed to be 2D-organized porin molecules.
Images were obtained at 8 sec/frame on the FastScan-Bio AFM
in TappingMode using FastScan-Dx probes in fluid.
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BIOLOGICAL
APPLICATIONS
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AFM Probes for Biological Samples
Molecular and Live Cell Imaging
Choosing a probe for imaging biomolecules or live cells in fluid.
•
•
•
Soft Cantilever/Low Spring Constant (k)
•
Soft samples (modulus :~kPa-MPa range.)
•
Samples weakly attached to supporting substrate.
•
Typically use Si3N4 cantilevered probes (vs. Silicon).
Gold (Au) Coated or Non-Coated Cantilever (Backside Coating).
•
Al coating (air probes) can contaminate biological samples in fluid.
•
Use non-coated only if absolutely necessary (see force spectroscopy section).
Tip Sharpness.
•
Sharper tips with small half angle provide high-resolution for single molecule imaging.
•
Blunter tips with larger half angle provide low pressure needed for live cell imaging.
•
Silicon tips tend to be sharper (smaller tip radius/half angle) than Si3N4 tips.
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AFM Probes for Molecular Imaging
Contact Mode Imaging of Membrane
Proteins
Contact Mode Imaging in Fluid.
• Very Soft Cantilever – low k.
•
sample distortion/damage.
• Gold Coated (backside) Cantilever.
•
Non-coated only if necessary (drift).
20nm
• Sharp tip – small radius.
•
Resolve molecular structure.
Recommended AFM Probes.
10nm
10nm
• MSNL-C (k ~0.01N/m; R ~2nm)
• SNL-D (k ~0.06N/m; R ~2nm)
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Images of aquaporin OmpF (top) and bacteriorhodopsin
(bottom) obtained on the MultiMode AFM operated in
Contact Mode using a SNL-A probe in fluid. Image courtesy
of D. Mueller, ETH D-BSSE, Basel.
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AFM Probes for Molecular Imaging
PeakForce Tapping Mode Imaging of DNA
PeakForce Tapping in Fluid.
• Soft Cantilever – intermediate k.
•
Stiff enough to oscillate in fluid.
• Gold Coated (backside) Cantilever.
• Sharp tip – small radius.
•
Resolve molecular structure.
Recommended AFM Probes.
• ScanAsyst Fluid+ (k ~0.7N/m; R ~2nm)
• SNL-A (k ~0.56N/m; R ~2nm)
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Image of plasmid DNA obtained on the MultiMode AFM
operated in PeakForce Tapping Mode using a ScanAsyst
Fluid+ probe in fluid. Image courtesy of A. Pyne and B.
Hoogenboom, UCL, London.
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AFM Probes for Live Cell Imaging
Contact Mode Imaging of Live Cells
Contact Mode Imaging in Fluid.
• Very Soft Cantilever – low k.
•
Sample distortion/damage.
• Gold Coated (backside) Cantilever.
•
Non-coated only if necessary (drift).
• Blunt tip – larger tip radius.
•
Low pressure won’t damage cell.
Recommended AFM Probes.
• MLCT-C (k~ 0.01N/m; R ~20nm)
• DNP-A (k~ 0.06N/m; R ~20nm)
45 m
Image of live endothelial cells obtained on the BioScope Catalyst
AFM operated in Contact Mode using an MLCT-C probe in fluid.
• OBL (k ~0.006-0.03N/m; R ~30nm)
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AFM Probes for Live Cell Imaging
PeakForce Tapping Mode Imaging Live Cells
PeakForce Tapping in Fluid.
• Soft Cantilever – intermediate k.
•
Stiff enough to oscillate in fluid.
• Gold Coated (backside) Cantilever.
• Sharp tip – small radius.
•
Resolve molecular structure.
Recommended AFM Probes.
• DNP-A (k~ 0.56N/m; R ~20nm)
• MLCT-F (k~ 0.6N/m; R ~ 20nm)
• ScanAsyst Fluid (k~ 0.7N/m; R ~20nm)
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5 m
Sneddon Modulus data overlaid on 3D topography image of
live E. coli cells. Dividing cell (right) has lower modulus than
single cell (left). A single flagellum is also observed. Imaging
obtained on the BioScope Catalyst AFM operated in
PeakForce Tapping Mode using a DNP-A probe in fluid.
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FORCE SPECTROSCOPY
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AFM Probes for Force Spectroscopy
Localized Measurements of Modulus, Molecular
Unfolding, and Binding Interactions.
Choosing a probe for single point force measurements of modulus
(pushing) and unfolding/unbinding (pulling). (1 of 3)
•
Cantilever Spring Constant (k).
•
Modulus/Indentation – based on expected sample modulus (kPa–MPa = softer probes;
MPa–GPa = stiffer probes).
•
Unfolding/Unbinding – based on forces being measured (pN = softer probes; nN =
stiffer probes).
•
Measured cantilever deflection should be within 2-3V of noncontact deflection value.
•
•
•
Probe too stiff = deflection too small (below noise floor – cannot measure).
Probe too soft = deflection too large (photodetector response non-linear at large deflections).
Important to calculate deflection sensitivity and spring constant (thermal tune).
Nominal values are not accurate.
•
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Note: For very soft probes sometimes easier to calculate k in air and then re-measure the
deflection sensitivity in fluid (k does not change).
Bruker Nano Surfaces Division
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AFM Probes for Force Spectroscopy
Localized Measurements of Modulus, Molecular
Unfolding, and Binding Interactions.
Choosing a probe for single point force measurements of modulus
(pushing) and unfolding/unbinding (pulling). (2 of 3)
•
Coated or Non-Coated Cantilever (Backside Coating).
•
Al coating (air probes).
•
Au coating (fluid probes) – Al can contaminate biological samples.
•
Non-coated (most probes are coated)
•
•
•
•
Backside coating can introduce drift, especially in fluid or at elevated temp (bimetallic effect).
Use non-coated cantilever only if absolutely necessary (can introduce other issues)
Beware of optical interference with non-coated probes (observed in baseline of force curves).
Also ensure adequate sum signal (force sensitivity) – typically want >1.5V.
Optical interference causes
regular pattern in force curve.
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Bruker Nano Surfaces Division
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AFM Probes for Force Spectroscopy
Localized Measurements of Modulus, Molecular
Unfolding, and Binding Interactions.
Choosing a probe for single point force measurements of modulus
(pushing) and unfolding/unbinding (pulling). (3 of 3)
•
•
•
Tip Shape.
•
Very important for modulus measurements.
•
Based on modulus fit model (indentation) eg. DMT/Hertz = sphere; Sneddon = cone.
Tip Sharpness.
•
Modulus/Indentation –blunt/dull probes minimize pressure (tip or sample damage).
•
Unfolding/Unbinding – larger R increases chances of molecule attached to end of tip.
Tip Functionalization.
•
Modification of tip with chemical group (eg. CH3/COOH) or molecules/ligands.
•
Cleaning probe is first step in functionalization process (refer to previous slides).
•
See AN#130 “Common Approaches to Tip Functionalization for AFM-Based Molecular
Recognition Measurements”.
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AFM Probes for Force Spectroscopy
Optically Targeted Cell Modulus Measurements
Modulus Measurements (in fluid).
•
Soft Cantilever – k ~0.01-0.06N/m.
•
•
Gold Coated (backside) Cantilever.
•
•
•
Cells have low modulus (kPa).
Non-coated if drift issues.
Conical or Spherical Tip.
•
Soft cells =
indentation depth.
•
Sneddon = conical, Hertz = spherical.
Blunt Tip (or Spherical) – larger radius.
•
Won’t damage cell membrane.
Recommended AFM Probes.
•
Spherical (k ~0.01-0.06N/m; R ~50nm-5µm)
•
MLCT-C (k ~0.01N/m; R ~20nm)
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Force curves performed on live endothelial cells
fluorescently labeled with a membrane potential indicating
dye (bis-oxonol). Cells were found to be stiffer in
hyperpolarized state. Studies conducted on the BioScope
Catalyst AFM using spherical probes (k ~0.01N/m) in fluid.
Images courtesy of H. Oberleithner, Munster, Germany.
Bruker Nano Surfaces Division
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AFM Probes for Force Spectroscopy
Single Molecule (Un)Folding Studies
Molecular Unfolding Studies (in fluid).
•
Soft Cantilever – k ~0.01-0.06N/m.
•
•
Gold Coated (backside) Cantilever.
•
•
Non-coated if drift issues.
Blunt Tip – larger radius (R).
•
•
Forces measured are in pN range.
>R =
chances of ‘catching’ molecules.
Tip Functionalization.
•
Au-coated tips if molecule has thiol groups.
Recommended AFM Probes.
•
MLCT-C (k ~0.01N/m; R ~20nm)
•
MLCT-D (k ~0.03N/m; R ~20nm)
•
DNP-D (k ~0.03N/m; R ~20nm)
10/28/2013
AFM pulling/unfolding curve for a single titin 8-mer
construct. The retract portion of the curve show the classic
‘sawtooth’ pattern with ~200pN force required to unfold
each of the 8 repeated subunits. Force curves obtained on
the MultiMode AFM using a MLCT-C probe in fluid.
Bruker Nano Surfaces Division
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KELVIN PROBE FORCE
MICROSCOPY (KPFM)
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Bruker Confidential
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KPFM Modes and
Probe Selection Guide
AN140-RevA1-PeakForce_KPFM-AppNote
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Bruker Nano Surfaces Division
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Spatial Resolution
Geometry
L
l
W
∆L
 The tip needs to be sharp
 Tip height and cantilever width, length:
 Taller, Higher aspect ratio
 Smaller Cantilever
10/28/2013
Bruker Nano Surfaces Division
47
Sensitivity
Cantilever Spring Constant and Q
Thermal Tune
• Higher Q (on surface) and
lower k gives higher
sensitivity for KPFM
measurements, this is
vitally important for FMKPFM.
• Probe must also have
clean resonance peak.
10/28/2013
Q
Bruker Nano Surfaces Division
48
Q/k of Some Probes Types
Other factors must also be accounted for
PFQNE-AL
#6
#7
#8
f
311
312
305
k
0.689
0.669
0.648
Q
58
60
59
Q/k
84
89
92
SCM-PIT
f
k
Q
Q/k
Q(Engaged)
Q/k(Engaged)
#2
#3
#4
65
66
68
2.344
2.712
2.995
182
197
195
78
73
65
116
114
120
49
42
40
ScanAsyst-Air-HR
#2
#3
#4
f
112
117
116
k
0.346
0.335
0.436
Q
59
57
63
Q/k
172
172
144
Q(Engaged) Q/k(Engaged)
26
75
19
57
30
69
AC40
f
k
Q
Q/k
Q(Engaged)
Q/k(Engaged)
#2
#3
#4
119
117
123
0.111
0.114
0.101
42
46
46
378
400
451
20
26
26
176
232
259
10/29/2013
Bruker Nano Surfaces Division
Q(Engaged) Q/k(Engaged)
27
39
30
45
35
54
49
NANO OPTICAL
MICROSCOPY
10/28/2013
Bruker Confidential
50
Tip Enhanced Raman Spectroscopy probes
Application: nanoChemical identification
System specific: IRIS
Key considerations
•
•
•
Feedback method
•
STM = gets closer but needs conducting sample
•
AFM (tuning fork) = any sample, less signal
Material
•
Etched Gold yields best field enhancement
•
Silver probes promising, but less consistent
Consistent Performance
•
• NearIR Deflection Laser
• Improved Optical Access
See App Note 136 for more details
October 28, 2013
TERS contrast must be large AND reproducible
100X tip
enhancement!
TERS Probes: STM vs. AFM
• STM
•
STM feedback
•
Requires conductive substrate
•
Optimal TERS performance
•
10X contrast
• AFM (tuning fork) - NEW
•
5X contrast
•
Force feedback using tuning fork
•
Works on any substrate
•
Coming Soon
Tuning Fork
Probe
October 28, 2013
Zoomed-in, showing the
etched gold wire attached
Prototype Tuning
Fork cartridge
Examples of TERS spectra for ChemID
Methylene Blue
Extremely low power
Sensitivity – fragile samples
Nile Blue
Extreme enhancement (600!)
Outstanding probe performance
•
Innova-IRIS with IRIS TERS-STM probes
•
Consistent Performance!
•
Detecting molecular films that are
undetectable in micro-Raman
October 28, 2013
AFM-Raman Solutions
Thiophenol
No far field (shown: p/s)
Sensitivity - low Raman x-section
53
Consistency of Bruker TERS
probes
• Making probes consistently is a costly art to master
• Our testing has shown we reach >10x enhancement on
average for over 80% of the tips
30
Contrast
25
20
15
10
5
0
1
2
3
October 28, 2013
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
PROBE PREPARATION
AND CLEANING
10/28/2013
Bruker Confidential
55
Tip cleaning “tips”
For specialized experiments
•
•
Example of functionalized tip
Probe cleaning NOT a recipe for cost savings
•
Main purpose is for functionalizing probes or eliminating
contamination in liquid
•
Only in rare circumstances (i.e. DNISP) are probes recyclable
Pre-experiment probe prep options
•
Ethanol/Isopropanol – simple, good for atomic resolution
•
UV light + DI water – better for removing organics (gelpak)
•
Plasma etching – good for functionalization, oxidized tip
•
Acidic piranha – functionalization, warning:attacks metal
•
Ultrasonication - used by some but difficult to hold tip
Please refer to App Note 44 for more information
October 28, 2013
A biased view of tip “recovery”
Weighing your options
• Cost benefit analysis
•
Can you change probes without disrupting your experiment?
•
If at all possible change probes!
• Last ditch efforts if probe change is not practical
•
Retract probe, run AC drive on high, run frequency sweep several
times (occasionally works in fluid)
•
Approach sample, while in contact increase feedback to cause tip to
oscillate
•
“Controlled crash”, using low setpoint
•
Scan clean glass slide in contact mode
•
Press (i.e. perform force curve) into soft sample such as BOPP
• Special cases
•
Nanoident probes – very expensive, durable, recoverable by making
forceful indentation on Gold or Rubber.
•
STM probes –possible to make Nano-asperity w/ hard contact. Low
success rate.
October 28, 2013
BRUKER AFM PROBES
WEBSITE
October 29, 2013
Bruker AFM Probes Website:
www.brukerafmprobes.com
October 29, 2013
Bruker AFM Probes Website:
www.brukerafmprobes.com
October 29, 2013
Bruker AFM Probes Website:
www.brukerafmprobes.com
October 29, 2013
Conclusion
• Appropriate selection of AFM probe can be one of the most
important decisions to be made towards a successful AFM
experiment
• Though there are many probes to chose from and this selection
may appear daunting the applications requirements usually point
to a clear path for probe selection.
• The Bruker website has distilled this information for the user to
help make their probe selection as the AFM experts do
• The Bruker applications and Probes teams are always available to
assist our customers in making the best probe selection for their
needs.
October 29, 2013
www.bruker.com
October 29,
©2013
Copyright Bruker Corporation. All rights reserved.