Biomedical Engineering Reference
In-Depth Information
platform.has.been.used.extensively.for.traditional.e-nose.applications.(Pearce.et.al..2006),.
and.shown.to.be.a.sensitive.method.for.use.in.odor.recognition.by.ORs..This.technique.is.
based.on.changes.in.resonance.frequency.of.piezoelectric.materials,.such.as.quartz.crystals,.
following.binding.of.a.molecule.to.their.surface..It.is.believed.that.this.technique.not.only.
senses. changes. in. mass. but. may. also. detect. conformational. changes. in. immobilized.
molecules. (Ferreira. et. al.. 2009).. A. QCM. sensor. was. irst. utilized. in. conjunction. with.
ORs,.when.crude.protein.extracts.from.bullfrog.olfactory.epithelium.were.adsorbed.onto.
the. quartz. crystal. and. shown. to. respond. to. a. number. of. different. odorants. (Wu. 1999)..
Interestingly,.separating.the.crude.protein.extract.into.ive.different.fractions.based.on.the.
size.of.protein.using.gel.chromatography,.the.investigator.was.able.to.show.that.different.
volatiles. tested. had. distinct. proiles. by. differentially. affecting. the. resonance. frequency.
of. the. crystal. surfaces. coated. with. different. protein. fractions. (Wu. 1999).. More. recently,.
investigators.have.focused.on.identifying.the.ability.to.use.this.technique.for.sensing.of.
compounds.by.speciic.ORs,.following.protein.expression.in.
E. coli
.(Sung.et.al..2006).
Compared.to.the.QMC,.where.a.wave.propagates.in.the.complete.piezoelectric.material,.
SAW.propagates.along.the.surface.of.the.substrate.(Gronewold.2007,.Lange.et.al..2008)..SAW.
has.been.effectively.used.to.assess.cell-free.OR.response.to.volatiles.with.high.speciicity.
and.afinity.(Wu.et.al..2011,.2012).
Currently,. QCM. sensors. are. considered. to. be. less. sensitive. than. SPR.. This. can. be.
improved.by.decreasing.the.thickness.and.changes.in.type.of.the.piezoelectric.material..
The.QCM.and.SAW.methods.are.considered.to.be.less.expensive.than.SPR,.mainly.due.to.
the.less.complicated.nature.of.the.instrumentation.required.for.the.former..While.SAW.is.
less.widely.used.for.sensor.applications,.both.piezoelectric.techniques.lend.themselves.to.
adaptation.at.the.micro-.and.nanoscale.
10.6 Conclusions and Perspectives
Using.natural.receptor.proteins.as.the.sensor.component.in.biosensors.is.the.next.leap.in.
making.high.afinity,.sensitive.sensors.for.biologically.relevant.applications.in.ields.such.
as.medicine,.food.industry,.drug.screening,.and.toxin.detection..Given.the.speciicity.of.
ORs.to.their.ligands,.receptor-based.biosensor.application.should.improve.the.signal-to-
noise.ratio,.which.is.often.a.problem.in.nonbiological.sensor.platforms..However,.unlike.
ion.channels,.which.have.the.advantage.of.naturally.conducting.an.electrochemical.signal,.
transduction.of.the.binding.event.of.a.ligand.to.a.non-ionophore.receptor.protein.is.not.
easy. to. achieve.. Effectively. coupling. a. biological. component. to. a. physical. transduction.
method.still.remains.one.of.the.most.challenging.aspects.of.biosensor.research.
Given. our. large. repertoire. of. ORs,. the. challenge. will. be. to. identify. and. facilitate. the.
immobilization. of. the. most. relevant. type. of. ORs. for. biosensor. applications.. Variability.
between.receptor.subtypes.may.exist.in.their.ability.to.be.easily.expressed.as.recombinant.
proteins.in.large.enough.quantities,.protein.functionalization.in.lipid.bilayers.and/or.onto.
sensor.surfaces,.protein.stability,.and.ability.to.effectively.transmit.a.signal.to.the.physical.
transducer.system.upon.ligand.binding.
Protein.and.lipid.bilayer.stability.is.a.major.issue.for.biosensor.applications..Ideally,.
a. sensor. should. be. able. to. withstand. a. number. of. environmental. factors. that. are.
considered. “extreme”. compared. to. the. natural. cellular. (or. lab). environment.. These.
include. humidity. (or. lack. thereof). and. temperature.. It. is,. therefore,. imperative. that.
