Biomedical Engineering Reference
In-Depth Information
(SAW).that.was.coated.with.a.16-mercaptohexadecanoic.acid.(MHDA).SAM.(Wu.et.al..2011,.
2012)..Inclusion.of.the.MHDA.monolayer.on.the.sensor.surface.also.improved.eficiency.
of.immobilization.and.stability.of.the.membrane-bound.receptors.(Table.10.1).(Wu.et.al..
2011,.2012).
Membrane. proteins. can. also. directly. be. coupled. to. the. solid. surface. by. membrane.
linkers,.such.as.a.streptavidin-biotin.link.or.antibody.binding,.increasing.the.amount.of.
receptor.binding.on.the.surface,.as.well.as.aiding.in.directionality.of.receptor.placement.
(Castellana. and. Cremer. 2006,. Hou. et. al.. 2007,. Cook. et. al.. 2009).. For. example,. Goldsmith.
et. al.. immobilized. recombinant. mouse. mOR174-9,. mOR203-1,. and. mOR256-17. proteins.
containing.a.terminal.Histidine.tag.to.carbon.nanotubes.(CNTs).functionalized.with.nickel-
nitrilotriacetic. acid. (Ni-NTA). (Goldsmith. et. al.. 2011).. ORs. were. maintained. in. digitonin.
micelles.or.embedded.in.protein-lipid.particle.“nano-discs”.(Bayburt.and.Sligar.2010).that.
enabled. correct. protein. folding. and. signiicantly. enhanced. the. stability. of. the. receptor..
The. nanodisc-contained. ORs. maintained. their. ability. to. consistently. sense. volatiles. for.
1.month,.suggesting.that.this.approach.may.be.a.stable.alternative.for.solid.lipid.bilayer.
adsorption.(Goldsmith.et.al..2011).
10.5 Physical Sensor Platforms
Unlike. channel. proteins,. which. transmit. ions. following. conformational. changes. in.
response. to. ligand. binding,. GPCRs. do. not. have. this. electrochemical. signature. and. the.
sensor,.therefore,.needs.to.be.sensitive.enough.to.be.able.to.detect.an.alternate.event..The.
majority.of.examples.described.here.depend.on.receptor-ligand.binding.induced.protein.
changes.
Here,. a.number. of.techniques. are.described. that.have. recently.been.employed. for.OR.
biosensor.applications..These.do.not.rely.on.labeling.techniques. for.visualization.of.the.
binding. event.. Label-free. assays. are. attractive. as. they. remove. an. additional. component.
that.might.have.stability,.cost,.and.decreased.signal-to-noise-ratio-related.issues..The.fol-
lowing.techniques.will.be.discussed.in.the.context.of.cell-free.systems..However,.it.should.
be. pointed. out. that. these. have. also. been. successfully. applied. to. whole. olfactory. neu-
rons.and.tissues,.serving.as.an.alternative.to.traditional.electrophysiological.techniques,.
such.as.patch.clamp..For.example,.by.coupling.the.antenna.of.the.Colorado.potato.beetle.
(
Leptinotarsa decemlineata
). to. a. ield. effect. transistor. (FET). device,. researchers. were. able.
to. detect. (Z)-3-hexen-1-ol,. a. chemical. released. when. plants. are. damaged. in. response. to.
insect.bites.(Schoning.et.al..1998,.Schütz.et.al..2000)..Olfactory.responses.have.also.been.
measured.in.intact.olfactory.epithelial.tissue,.olfactory.neurons,.or.cells.expressing.recom-
binant. ORs,. with. a. variety. of. sensor. devices. including. the. use. of. microelectrode. arrays.
(Marrakchi.et.al..2007,.Lee.et.al..2009,.Ling.et.al..2010,.Liu.et.al..2010b),.SPR.(Lee.et.al..2006),.
and.quartz.crystal.microbalance.(Ko.and.Park.2005).
While.there.are.a.number.of.sensor.platforms,.the.techniques.highlighted.in.the.subse-
quent.section(s).appear.particularly.promising.for.nanoscale,.multiplex.applications,.ulti-
mately.leading.to.large-scale.device.manufacture..These.sensors.have.been.developed.over.
many.years.and.applied.to.a.number.of.different.devices..We.refer.the.reader.to.referenced.
materials. for. further. detail. on. each. technique.. Most. devices. described. in. the. following.
section(s). are. enclosed. in. a. chamber. to. allow. for. low. of. odorants. over. the. sensor. sur-
face..Delivery.of.the.odorants.in.liquid.form.allows.for.accurate.determination.of.dosage.
