Biomedical Engineering Reference
In-Depth Information
ligand. nanostructures. could. be. used. to. mimic. the. binding. location. of. HIV. viruses. (Yu.
et.al..2005)..Different.patterns.of.HIV-binding.ligands.were.created,.from.which.this.group.
could.determine.the.optimal.spacing.and.pattern.design.required.for.maximal.HIV-virus.
binding.. Further. discussions. of. nanografting. applications. for. manipulation. of. proteins.
(Liu. and. Amro. 2002). and. DNA. (Castronovo. and. Scaini. 2011). can. be. found. in. the. cited.
literature.
16.6 Recent Developments in AFM
The.two.major.challenges.in.advancing.AFM.imaging.include.(1).improving.the.resolution.
of.AFM.images.and.(2).increasing.the.scanning.speed.to.enable.the.observation.of.single-
molecule. events. in. real. time.. The. past. decade. has. witnessed. signiicant. advances. along.
both. of. these. directions.. Speciically,. the. lateral. resolution. of. an. AFM. image. is. limited.
by.the.diameter.of.the.AFM.probe..Advancements.in.nanotechnology.have.played.a.sub-
stantial.role.in.manufacturing.AFM.probes.with.smaller.diameters.and.ultrahigh.aspect.
ratios,.both.of.which.can.enhance.the.lateral.resolution.in.AFM.images..Microfabrication.
techniques.like.reactive.ion.etching.(RIE).and.deep.RIE.(DRIE).have.been.used.to.micro-
machine.high-aspect-ratio.(HAR).silicon.tips.speciically.with.heights.>40.μm.and.aspect.
ratios. of. 7. (Wang. and. van. der. Weide. 2005).. Single-. and. multiwalled. carbon. nanotubes.
(SWCNT,. MWCNT). attached. to. AFM. probes. have. enabled. a. resolution. better. than. the.
nanotube. probe. diameter. and,. in. some. cases,. a. resolution. better. than. 1.nm. (Wade. et. al..
2004)..Quartz.micropipette.and.optical.iber-based.structures.have.additionally.facilitated.
sharper,. multifunctional. AFM. probes. (Lieberman. et. al.. 1994).. Recently,. long. polymeric.
needles.of.various.materials.have.also.been.attached.onto.an.AFM.probe.or.cantilever.as.
additional.means.for.improving.lateral.AFM.resolution.(Kang.et.al..2010).
Secondly,. although. AFM. is. highly. capable. of. imaging. single. molecules,. it. has. been. a.
challenge. to. advance. the. AFM. technique. to. capture. the. progression. of. single-molecule.
events. in. real. time.. This. is. because. the. rate. of. these. events,. such. as. the. movement. of. a.
motor.protein,.occurs.on.the.order.of.milliseconds.or.less..The.typical.scan.rate.of.a.com-
mercial.AFM,.however,.is.∼1.to.2.Hz,.requiring.at.least.30.s.to.1.min.(or.longer).to.acquire.
a. single. image.. Over. the. past. decade,. methods. to. achieve. faster. scanning. for. biological.
samples. have. been. developed. to. enable. real-time. imaging. of. single-molecule. processes..
Ultrafast.AFM.scanning.has.been.achieved.by.means.of.shorter.cantilevers.and.a.special.
optical.delection.detector.(ODD)..Recent.years.have.witnessed.more.comprehensive.AFM.
systems.for.ultrafast.scanning,.which.include.enhanced.hardware.and.software.features.
like. high-speed. scanners,. fast. amplitude. detectors,. drift. compensators,. active. damping.
techniques,.and.faster.data.acquisition.systems.(Ando.2012).
The. fast. AFM. techniques. mentioned. earlier. have. enabled. imaging. of. several. single-
molecule.events,.including.the.capturing.of.DNA.molecules.in.0.17.s.(Schäffer.et.al..1996;.
Viani.et.al..1999),.the.formation.and.dissociation.of.a.streptavidin-biotinylated.DNA.complex.
(Kobayashi.et.al..2007),.and.the.1D.diffusion.of.a.restriction.enzyme.along.a.DNA.strand.
(Yokokawa.et.al..2006b)..A.variety.of.dynamic.protein-protein.interactions.or.conformation.
changes.have.also.been.imaged.in.real.time.using.high-speed.AFM..These.include.imaging.
of. conformation. changes. of. dynein. C. (Ando. et. al.. 2006),. GroEL-GroES. association-
dissociation. cycles. controlled. by. the. ATPase. reaction. (Yokokawa. et. al.. 2006a),. moving.
