Biomedical Engineering Reference
In-Depth Information
“contact”.mode.of.AFM,.the.cantilever.grazes.the.sample.like.the.record-player.needle.in.
a.gramophone;.the.delections.of.the.cantilever.are.recorded.as.it.traces.lateral.and.vertical.
changes.in.the.sample..A.second.widely.used.mode.is.dynamic.mode.(also.called.“tapping”.
or. “intermittent-contact”. mode),. in. which. the. AFM. cantilever. is. made. to. oscillate. and.
intermittently. touch. the. sample.. In. dynamic. mode,. changes. in. the. cantilever. resonance.
frequency.or.amplitude.can.be.recorded.giving.rise.to.frequency.modulation.(FM)-AFM.
(Albrecht.et.al..1991).or.amplitude.modulation.(AM)-AFM.(Holscher.and.Schwarz.2007),.
respectively..Both.static.and.dynamic.modes.are.widely.used.on.most.commercial.AFM.
systems.for.biological.samples.in.air.and.luid.environments.
The. lateral. (X-Y). movement. of. the. AFM. cantilever. and/or. probe. enables. imaging. of. a.
sample.similar.to.a.scanning.microscope..The.dimensions.and.geometry.of.the.probe.are.
responsible.for.the.spatial.resolution.in.AFM.images,.while.the.type.of.cantilever.and.mode.
of.imaging.and.data.collection.employed.dictate.the.information.present.in.AFM.images..
The.vertical,.or.Z-delection,.of.the.AFM.cantilever.enables.measurement.of.sample.height.
as.well.as.forces.between.the.AFM.probe.and.the.sample,.a.feature.widely.used.in.AFM.
force.spectroscopy..Similar.to.a.light.microscope.(LM),.the.AFM.can.be.conigured.to.be.
an.upright. or.an.inverted. microscope. depending. on. whether. the.sample. is. moved. with.
respect.to.the.cantilever.or.vice.versa..The.movement.of.the.sample.and/or.the.probe.in.the.
X.and.Y.directions.(or.Z.direction.in.force.measurement.applications).is.accomplished.by.
means.of.piezoelectric.transducers..All.these.features.of.AFM,.including.the.type.of.mode,.
cantilever.and.probe.properties,.lateral.or.vertical.movement.of.the.cantilever,.inverted.or.
upright.conigurations,.and.feedback.electronics,.can.be.exploited.and/or.adjusted.to.serve.
a.desired.application..We.summarize.the.major.applications.of.AFM.in.nanobiotechnology.
in.the.following.sections.
16.3 AFM as an Imaging Tool
AFM.is.unique.compared.to.typical.microscopes,.especially.in.the.way.an.image.is.formed..
The.LM.and.the.transmission.electron.microscope.(TEM).both.form.an.image.primarily.
based. on. the. diffraction. of. an. incident. beam.. Contrarily,. the. AFM. employs. an. X-Y. scan.
to. construct. an. image,. much. more. like. the. laser. scanning. confocal. microscope. (LSCM).
and.the.scanning.electron.microscope.(SEM)..Additionally,.the.AFM.gathers.information.
pertaining.to.the.sample's.height.(Z).at.each.X.and.Y.position..The.AFM,.therefore,.maps.
the. 3D. surface. topography. of. the. sample. to. form. images. called. AFM. height. images. or.
topographs
.. Both. qualitative. as. well. as. quantitative. information. about. the. samples. can.
be. ascertained. by. analyzing. these. AFM. images.. Figure. 16.2A. shows. a. schematic. of.
AFM. modes. and. highlights. the. imaging. capabilities. of. AFM. for. biological. samples. and.
nanotechnology.
AFM.is.an.attractive.imaging.tool.for.biological.specimens.because.it.enables.analysis.
of.samples.in.near-physiological.conditions.(i.e.,.luid.environments).with.minimal.sample.
preparation.and.without.the.use.of.ixatives.or.stains..AFM.imaging.enables.a.lateral.reso-
lution.of.∼1.nm.and.a.vertical.resolution.of.0.1.nm,.with.one.of.highest.contrasts.among.all.
microscopy.techniques..A.major.drawback.of.AFM.is.that.a.single.image.acquisition.can.
take.30.s.to.a.minute.or.more,.which.can.be.much.longer.than.other.microscopic.imaging.
methods..Table.16.1.summarizes.the.various.capabilities.of.AFM.compared.to.other.stan-
dard.microscopy.techniques.
