3.052 Nanomechanics of

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3.052 Nanomechanics of Materials and
Biomaterials
LECTURE 9 QUANTITATIVE TREATMENT OF INTRA-
AND INTERMOLECULAR FORCES
Prof. Christine Ortiz DMSE, RM 13-4022 Phone
[phone] Email cortiz_at_mit.edu WWW
http//web.mit.edu/cortiz/www
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Review Lecture 4 Experimental Aspects of Force
Spectroscopy III
I. Comparison of high-resolution force
spectroscopy techniques atomic force
microscopy (AFM), surface forces apparatus (SFA),
optical tweezers (OT), biomembrane surface
probe (BSP) II. Conversion of raw data in a
high-resolution force spectroscopy experiment
sensor output, s ? transducer displacement,
d ? force, F z-piezo deflection, z ?
tip-sample separation distance, D III. Typical
force spectroscopy data for a weak cantilever on
stiff substrate (ksamplegtgtkcantilever) APPROACH
(sample and tip come together) A tip and
sample out of contact, no interaction,
cantilever undeflected, zero force (set F0)
B/C attractive interaction pulls tip down
to surface and tip jumps to contact, cantilever
exhibits mechanical instability D contact,
constant compliance regime, no sample
indentation, tip and sample move in unison
(Ds/Dz1) RETRACT (sample and tip move apart)
D repulsive contact, constant compliance
Regime, tip deflected up E attractive force
(adhesion) keep tip attached to surface, tip
deflected down F tip pulls off from surface,
cantilever instability G same as region
A
Fkd
ds/m
Dz?d
D
D
D
D
A
A
B/C
B/C
G
F
G
E
E
F
Adhesive Interaction
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Types of Intra- and Intermolecular Interactions
in Different Materials
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Biomolecular Adhesion
controlled by bonds between molecular ligands
and cell surface receptors which exhibit the
lock-n-key principle (e.g. biotin-streptavidin)
Grubmüller, et al, Science 1996 (http//www.mp
ibpc.gwdg.de/abteilungen/ 071/strept.html)
(http//www.amber.ucsf.edu/amber/tutorial/strepta
vidin/index.html)
complex, multiatomic, relatively weak formed
by an assembly of multiple, weak non-covalent
interactions (e.g. H-bonding, coulombic, van der
Waals, hydrophilic / hydrophobic,
electrostatic) complementary,
sterically-contrained geometric considerations
specificity
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BRIDGING THE GAP BETWEEN LENGTH SCALES
Force, F (nN)
kc
0
Tip-Sample Separation Distance, D (nm)
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Characterizing an Individual Intra- and
Intermolecular Interaction
interaction distance (nm)
interaction energy (kJ/mol)
interaction force (electromagnetic in origin)
(nN)
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Characterizing an Individual Intra- and
Intermolecular Interaction
interaction distance (nm)
interaction energy (kJ/mol)
interaction force (electromagnetic in origin)
(nN)
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Steric Repulsion Interaction Potentials
Due to overlap of negatively charged electron
clouds (e.g. Pauli Exclusion principle) and ()
charged nuclei, quantum mechanical in origin
short-range, i.e. takes place over the order of
distances of bond lengths 0.1 nm
Soft Repulsion
Hard-Core Repulsion n?
s
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Attractive Interaction Potentials
longer range gt 1 nm A is a constant
determined by the polarizability or ease of
distortion of electron cloud
London dispersion interaction
A10-77Jm6 m6
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Net or Complete Interaction Potential The
Lennard-Jones or 6-12 Potential
r(nm)
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Interaction Strength
DEB
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Equilibrium Interaction Distance, re
regtre
re
re
r (nm)
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Force Profile for The Lennard-Jones or 6-12
Potential
ro
re
rs
Frupture
r(nm)
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More Complicated Interaction Potentials
Grubmüller, et al, Science 1996 (http//www.mpi
bpc.gwdg.de/abteilungen/071/strept.html)
R. MERKEL, P. NASSOY, A. LEUNG, K. RITCHIE
E. EVANS, Nature 397, 50 - 53 (1999)