Biomedical Engineering Reference
In-Depth Information
the.liquid.environment.surrounding.a.biological.sample.affects.the.creation.of.eddy.cur-
rents,.and.how.its.potential.interference.can.be.minimized.to.extract.details.of.the.internal.
structures.of.the.specimens.
Mechanical. excitation. is. another. way. in. which. AFM. can. reveal. a. range. of. subsurface.
information.about.a.sample..MSAFM,.as.introduced.by.Tetard.et.al..(2010),.is.one.of.these.
developments. for.high-resolution. subsurface. imaging.. In. general,. MSAFM. is. performed.
by.exerting.multiharmonic.forces.on.both.the.sample.and.the.AFM.probe..Those.forces.on.
the.sample.generate.waves.that.travel.through.and.are.scattered.by.the.internal.structures.
and.properties.of.the.sample..The.waves.emerge.from.the.sample.surface.and.affect.the.
interaction.between.the.AFM.probe.and.the.sample.in.the.form.of.AFM.probe.frequency.
and.phase.shifts..Although.MSAFM.can.generate.information.about.structures.up.to.50.nm.
beneath.the.sample.surface,.it.remains.a.challenge.to.interpret.and.extract.the.frequency.
and.phase.shift.data.into.information.about.subsurface.sample.properties.(Garcia.2010).
SNFUH. is. another. subsurface. AFM. imaging. technique. based. on. mechanical. excita-
tion. (Shekhawat. and. Dravid. 2007).. In. SNFUH. applications,. a. high-frequency. acoustic.
wave. is. launched. from. under. the. sample. stage. and. propagates. through. the. sample;. a.
second,. slightly. different,. high-frequency. acoustic. wave. is. launched. on. the. AFM. canti-
lever..Materials.with.different.elastic.moduli.embedded.in.the.sample.dictate.the.phase.
and.amplitude.of.the.propagating.acoustic.waves..These.modulations.are.relected.on.the.
acoustic.interference.that.occurs.at.the.cantilever.tip.and,.consequently,.are.relected.on.
the. cantilever. oscillation. around. its. resonant. frequency.. Shekhawat. et. al.. demonstrate.
how. the. SNFUH. technique. has. the. sensitivity. capable. of. detecting. gold. nanoparticles.
up. to. 500.nm. below. a. polymer. surface. and. how. the. resolution. is. capable. of. identifying.
malaria.parasites.within.infected.red.blood.cells.(Shekhawat.and.Dravid.2007).
16.4 AFM as a Force Sensor
One.of.the.most.powerful.capabilities.of.the.AFM.is.its.ability.to.measure.forces.between.
the. probe. and. the. sample;. this. feature. is. called. force. spectroscopy.. Instead. of. laterally.
scanning. the. AFM. cantilever. and. probe. over. the. sample,. this. mode. requires. the. AFM.
cantilever. to. be. brought. down. (approached). vertically. onto. the. sample. and. then. pulled.
back.(retracted)..The.height.“Z”.of.the.cantilever.and.the.speed.with.which.it.approaches/
retracts.from.the.sample.can.be.user.deined..The.delection.of.the.cantilever.as.it.bends.
during. interaction. with. the. sample. is. monitored. as. a. function. of. its. “Z”. height,. giving.
rise.to.a.“force.curve.”.Analysis.of.the.force.curves.can.yield.valuable.information.about.
tip-sample.interaction.forces.and.mechanical.properties.of.the.underlying.sample..Often.
the.force.spectroscopy.of.a.sample.is.coupled.with.image.acquisition.via.light.microscopy.
or.AFM.imaging.to.map.out.the.speciic.locations.on.the.sample.where.force.curves.are.
acquired..Figure.16.3.highlights.force.measurement.capabilities.of.the.AFM.
16.4.1  Molecular and Cellular Adhesion and recognition
One.of.the.most.popular. uses.of.AFM.force.spectroscopy.is.to.evaluate. the.adhesion.or.
recognition.between.two.biomolecules..In.this.case,.one.binding.partner.is.adhered.to.the.
AFM.probe.while.the.second.binding.partner.is.on.the.substrate..As.the.“functionalized”.
AFM.probe.interacts.with.the.sample,.the.force.of.adhesion.between.the.probe.molecule.
 
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