biosensors, a survey report by saraju p mohanty(2001) 10.1.1.19.2093
TRANSCRIPT
Biosensors: A Survey Report
SarajuP. MohantyDept.of Comp.Sc.andEngg.
Universityof SouthFloridaTampa,FL 33620,[email protected]
November24,2001
Abstract
A biosensoris a sensingdevicemadeupof a com-bination of a specificbiological elementand atransducer. The”specificbiological element”rec-ognizesa specificanalyteand the changesin thebiomoleculeare usuallyconvertedinto electricalsignal (which is in turn calibrated to a certainscale)by a transducer. Aim of this survey work isto discussthevariousbiosensorsavailablefor dif-ferentbiosensingapplications.Initially, thesurveyfocuseson thebasicsof biosensingdeviceswhichcan serveas introductorytutorial for the readerswho are new to this field. Later, the survey high-lights the technicalitiesof few biosensors in greatdetail. The survey endswith brief discussiononthemajor difficultiesthebiosensorresearch com-munitiesnormallyencounter.
1 Introduction
Thehistoryof biosensorsstartedin theyear1962with the developmentof enzymeelectrodesbythe scientistLelandC. Clark. Sincethen the re-searchcommunitiesfrom variousfieldslikeVLSI,physics,chemistry, material science,and so on,havecometogetherto developmoresophisticated,reliableandmaturedbiosensingdevicesfor appli-
cationsin the fields of medicine,agriculture,andbiotechnology, etc.
Whatis a biosensor? Variousdefinitionsandter-minologiesareuseddependingon thefield of ap-plications.Dependingon thefield of applicationsbiosensorsare known as : immunosensors, op-trodes, chemicalcanaries, resonantmirrors, glu-cometers, biochips, biocomputers, andsoon. Twogeneralizeddefinitionsof biosensorscanbefoundin [2, 4]. Authors in [2] defineit as: ”a biosen-sor is a chemicalsensingdevice in which a bio-logically derivedrecognitionentity is coupledto atransducer, to allow the quantitative developmentof somecomplex biochemicalparameter”. Ac-cordingto the authors[4] : ”a biosensoris a an-alytical device incorporatinga deliberateandinti-matecombinationof a specificbiologicalelement(thatcreatesarecognitionevent)andaphysicalel-ement(thattransducestherecognitionevent)”.
Thename”biosensor”signifiesthatthedevice is acombinationof two parts:
� bio-element
� sensor-element.
The bio-elementmay be an enzyme, antibody,antigen,living cells,tissues,etc.Thelargevarietyof sensor-elementsincludeselectriccurrent,elec-
1
tric potential,intensity andphaseof electromag-netic radiations,mass,conductance,impedance,temperature,viscosity, andsoon. Thebasiccon-ceptsof biosensorcanbeillustratedby thehelpofFig. 1. A specific”bio” element(say, enzyme)recognizesa specificanalyteandthe ”sensor”el-ementtransducesthe changein the biomoleculeinto electricalsignal.Thebio elementis veryspe-cific to theanalyteto which it is sensitive. It doesnot recognizesotheranalytes.
Signal
Bio Element TransducerAnalyte
Biosensor
Electrical
Figure1: BasicConceptsof Biosensor
Dependingonthetransducingmechanismusedthebiosensorscanbeof many typessuchas:
� Resonantbiosensors
� Optical-Detectionbiosensors
� Thermal-Detectionbiosensors
� Ion-SensitiveFETs(ISFETs)biosensors
� Electrochemicalbiosensors
Details of all thesedifferent types will be dis-cussedin this survey work. The electrochemicalbiosensorsbasedon the parametermeasuredcanbefurtherclassifiedas[1]:
� conductimetric
� amperometric
� potentiometric
Thebiosensorscanhavevarietyof biomedicalandindustryapplications.Someof thepossibleappli-cationsareshown in Fig. 2. The major applica-
Figure 2: PotentialApplications of Biosensors,Source: [2]
tion so far is in blood glucosesensingbecauseofabundantmarket potential. But, biosensorshavetremendousopportunityfor commercializationinotherfields of applicationaswell. Fig.3 shows aneedle-typeglucosebiosensorimplantedin subcu-taneousfatty tissue.Somecommerciallyavailableglucosebiosensorsproductsfrom Medisenseareshown in Fig. 4 andFig. 5. A handheldbiodetec-tor developedby G. Kovacs[19] is shown in Fig.6. Eventhoughbiosensorshavegotverygoodap-plicationpotentialit hasnot beenhighly commer-cialisedbecauseof several difficulties, for exam-ple,dueto thepresenceof biomoleculesalongwithsemiconductormaterialsthebiosensorcontamina-tion is amajorissue[2, 6].
This survey paperis organizedas follows. Sec-tion 2 discussesthe fundamentalmechanismsofbiosensors.Dif ferent typesof biosensorsarede-tailed in Section3. Section4 discussesdetailsofa biosensorthat canbe usedto monitor cell mor-phologyin tissuecultureenvironment. A biosen-sor basedon usinghologramto detectpancreaticdisordersis discussedin section5. Section6 dis-cusesDNA detection-on-a-chip.Variousglucosebiosensorsarediscussedin section7.
2
Figure 3: A needle-typeglucosebiosensorim-plantedin subcutaneousfatty tissue
Figure 4: Medisenseglucose biosensor Pen,Source:http://www.medisense.com
Figure 5: Medisense glucose biosen-sor with Big Digital Display, Source:http://www.medisense.com
Figure6: A handheldbiodetectordevelopedby G.Kovacs,Source:[20]
2 Basic Concepts of Biosensors
We have alreadyseenthat a biosensorconsistsof a bio-elementanda sensor-element. The bio-elementof themaybeanenzyme,antibody, livingcells,tissue,etc.,andthesensingelementmaybeelectric current,electric potential,and so on. Adetailedlist of differentpossiblebio-elementsandsensor-elementsis in Fig. 7.
The ”bio” andthe ”sensor”elementscanbe cou-pled togetherin one of the four possiblewayslistedbelow [19] (referFig. 8).
� MembraneEntrapment
� PhysicalAdsorption
� Matrix Entrapment
� CovalentBonding
3
Biosensors
Sensor-ElementBio-Element
Enzyme
Antibody
Tissue
Microbial
Polysaccharide
Nucleic Acid
Electric Potential
Electric Current
Electric Conductance
Electric Impedance
Intensity and phase of em radiation
Mass
Temperature
Viscocity
Figure7: Elementsof aBiosensor
In the membraneentrapmentscheme,a semiper-meablemembraneseparatesthe analyteand thebioelement, and the sensor is attachedto thebioelement.Thephysicaladsorptionschemeis de-pendentonacombinationof vanderWaalsforces,hydrophobicforces, hydrogenbonds, and ionicforcestoattachthebiomaterialto thesurfaceof thesensor. Theporousentrapmentschemeis basedonforming a porousencapsulationmatrix aroundthebiological materialthat helpsin binding it to thesensor. In thecaseof thecovalentbondingthesen-sorsurfaceis treatedasa reactivegroupsto whichthebiologicalmaterialscanbind.
A typically usedbioelementis anenzyme.Thesearelargeproteinmoleculesthatactascatalystsinchemicalreactions,but remainunchangedat theendof reaction.Fig. 9 shows theworking princi-ple of enzymes.Theenzymesareextremelyspe-cific in their action. Meaning,an enzymeX willchangea specificsubstanceA ( not C ) to anotherspecificsubstanceB ( notD ). This is illustratedintheFig. 10. Thisextremelyspecificityactionof theenzymesis thebasisof biosensors.
3 Types of biosensors
In this sectionwewill discussthevarioustypesofpossiblebiosensors.We will analyzetheworkingmechanismof eachoneof them.
BB
BB
BB
B
B B B B BBBB
Sensor
Sensor
SemipermeableMembrane
Membrane
(a)
(b)
Membrane Entrapment
Physical Adsorption
Sensor
Sensor
(c) Matrix Entrapment
(d) Covalent Bonding
B B B B BB BB
B B BB BBBB Porous Encapsulation
Covalent Bond
Figure8: Couplingof Bio-Materialwith theSen-sor, Source: [19]
3.1 Resonant biosensors
In this typeof biosensorsanacousticwave trans-ducer is coupled with antibody (bio-element).Whentheanalytemolecules(antigen)getattachedto themembrane,themembranemasschanges,re-sultingin a subsequentchangein theresonantfre-quency of the transducer. This frequency changeis measuredout [19].
3.2 Optical-detection biosensors
The output transducedsignal that is measuredis light signal for this type of biosensors. Thebiosensorscanbe madebasedon optical diffrac-tion or electrochemilluminence.In opticaldiffrac-tion baseddevices,a silicon wafer is coatedwitha protein via covalent bonds. The wafer is ex-posedto UV light throughaphotomaskandthean-tibodiesmadeinactivatedin the exposedregions.The dicedwafer chipswhenincubatedin analyte
4
Figure9: WorkingPrincipleof Enzymes,Source:[5]
antigen-antibodybinding is formed in active re-gions,thuscreatingdiffractiongrating. This grat-ing producesdiffraction signal when illuminatedwith alight sourcesuchaslaser. Thissignalcanbemeasuredor canbefurtheramplifiedbeforemea-suringfor improving sensitivity [19].
3.3 Thermal-detection biosensors
This typeof biosensorsareconstructedcombiningenzymeswith temperaturesensors.Whentheana-lyte comesin contactwith theenzyme,theheatre-actionof theenzymeis measuredandis calibratedagainsttheanalyteconcentration[19].
3.4 Ion-Sensitive biosensors
Theseare basically semiconductorFETs havingion-sensitivesurface[7, 19]. Thesurfaceelectricalpotentialchangeswhentheionsandthesemicon-ductorinteract.Thispotentialchangecanbemea-sured. The Ion Sensitive Field Effect Transistor(ISFET) canbe constructedby covering the sen-sorelectrodewith a polymerlayer. This polymerlayeris selectively permeableto analyteions.Theionsdiffusethroughthepolymerlayerandin turncausea changein theFET surfacepotential. Fig.11 shows anISFEThaving enzymeenzymelayer
Figure10: Specificityof Enzymes,Source: [5]
placedon it; also called ENFET (EnzymeFieldEffect Transistor)[7]. This type of biosensorareprimarily usedfor pH detection.
3.5 Electrochemical biosensors
Electrochemicalbiosensorsare mainly used fordetectionof hybridisedDNA, DNA-bindingdrugs,glucoseconcentration,etc.Theunderlyingprinci-ple for thisclassof biosensorsis thatmany chemi-cal reactionsproduceor consumeionsor electronswhich in turn causesomechangein the electri-cal propertiesof thesolutionwhich canbesensedout andusedasmeasuringparameter[1, 19]. Theelectrochemicalbiosensorscanbeclassifiedbasedon the measuringelectrical parametersas : (1)conductimetric,(2) amperometricand (3) poten-tiometric [1]. A comparative discussionof thesethreetypesis give in Table1.
5
Figure 11: EnzymeField Effect Transistor(EN-FET),Source: [7]
3.5.1 Conductimetric biosensors
The measuredparameteris the electrical con-ductance/resistanceof the solution. When elec-trochemicalreactionsproduceions or electronsthe overall conductivity/resistivity of the solutionchanges.This changeis measuredandcalibratedto a properscale. Conductancemeasurementhasrelatively low sensitivity. The electric field gen-eratedusingsinusoidalvoltage(AC) which helpsin minimizingundesirableeffectssuchasFaradaicprocess,doublelayer charging andconcentrationpolarization[1].
3.5.2 Amperometric biosensors
Thishighsensitivity biosensorcabdetectelctroac-tive speciespresentin biological test samples.Sincethebiologicaltestsamplesmaynotbeintrin-sically electro-active, enzymesneededto catalyzeproductionof radio-activespecies.In thiscase,themeasuredparameteris current[1].
ElectrochemicalSensingCharacteristics Conductimetric Amperometric Potentiometric
Measured Conductance/ Current Potential/Parameter Resistance VoltageApplied Sinusoidal Constant RampVoltage (AC) Potential(DC) Voltage
Sensitivity Low HighGoverning Incremental Cottrell NesrtEquation Resistance Eqn. Eqn.
Fabrication FET+Enzyme FET+Enzyme FET+Enzyme2 elctrodes oxideelectrode
Table1: Dif ferentElectrochemicalSensing
3.5.3 Potentiometric biosensors
In this type of sensorsthe measuredparameterisoxidation/reductionpotential(of anelectrochemi-calreaction).Theworkingprincipleof thatwhenarampvoltageis appliedto anelectrodein solutionthe currentflow occursbecauseof electrochemi-cal reaction.Thevoltageat which thesereactionsoccursindicateaparticularreactionandparticularspecies[1].
4 A biosensor to monitor cellmorphology
Keeseand Giaever [3] have designeda biosen-sor that can be used to monitor cell morphol-ogy in tissueculture environment. The sensingprincipleusedis known asElectricCell-substrateImpedanceSensing(ECIS). In this process,asmallgold electrodeis immersedin tissueculturemedium. Whencells get attachedandspreadontheelectrodes,theimpedancemeasuredacrosstheelectrodeschange.This changingimpedancecanbeusedfor understandingcell behavior in culturemedium. This is the key themebehindthe pro-posedbiosensorwhich is well projectedby thehelpof Fig. 12.
The attachment and spreading behavior of thecellsareimportantfactorsfor this biosensor. Thecancerouscells usually can grow and reproduce
6
Figure 12: Electric Cell-substrateImpedanceSensing(ECIS),Source: [3]
(mitosis) freely in a medium without being at-tachedto any substrate/surface.But, normalcellsneedto beattachedto a surfacebeforethey grow.After attachmentthe shapeof the cells becomesflat andno longerremainssphericalshaped.Fig.13 demonstratesthis cell behavior in a tissuecul-turemedium.
The principle of measurementis schematicallyrepresentedby the help of Fig. 14. The cells aregrown on gold electrodes.Theelectrodesareim-mersedin tissueculturemediumwhich works aselectrolyte.Theappliedvoltageis
��������� �. The
voltageis appliedthrougha�����
resistance.Tomeasuremagnitudeand phaseof the voltagethelock-in-amplifieris used.Since,thecurrentis con-stant,the measuredmagnitudeandphasecanbeassumedasproportionalto impedance(resistanceandcapacitance).Fig. 15 shows capacitanceandresistancemeasurementsovera timeperiod.Aftersometimeit is foundthattheR, C valuesfluctuatevery often. This happenswhencellsarealive andmoving.
Theelectrodesusedabove is fabricatedby thefol-lowing processsteps:
Figure13: A cell in tissueculturemedium,Source: [3]
� a thin gold film is sputteredon a polycarbon-atesubstrate
� film is patternedandinsulatedby lithographytechniques.
The small electrodemadeis of ��������� diameter.A completesix well unit with ECIS electrodesisshown in Fig. 16.
Theadvantagesof this biosensorare:
� The biosensoris lesstime consumingcom-paredto theconventionalmethods.
� It is possibleto automateand quantify cellmorphologymeasurement.
� Thefluctuatingpatterncanbeusedassigna-turefor acell.
Thepossibledisadvantagesof this biosensormaybe:
� The accuracy of the biosensoris doubtful, itmayhappentwo cellscanhavealmostsimilarpattern.
7
Figure14: ECISschematicdiagram,Source: [3]
� If the averageimpedanceis to be taken asa measurethen it is possiblethat two en-tirely differentpatternscanhave sameaver-agevalue.
� It is not clear if the biosensoris useful fornonmammaliancellsandplantcells.
5 A holographic biosensor forscreening pancreatic disor-ders
Hologramsarephotographsof 3D impressionsonthesurfaceof light. Tomakeahologramoneneedsto photographlight waves. Whenanobjectwavemeetsareferencewave,astandingwavepatternofinterferenceis createdwhichcanbephotographed;thus creatinga hologram. A hologramis gener-ally recordedon silver hallidefilm. Thefilm con-sistsof a basematerialof glassor plastic. Thenthereis a photoactive layercalledemulsion. Thisemulsionlayer is madeup of gelatin (a colorless/ yellowish protein). Silver andhallide materialsfloat in the gelatin layer. They chemicallyreactto form silver hallide molecule. When light en-ergy goesinto thegelatinit is transferedto thesil-
Figure 15: Resistanceand capacitancemeasure-mentover time,Source: [3]
(a) Six electrode array
(b) Cross section of one electrode
Figure16: Electrodestructure,Source: [3]
8
ver hallide molecule. For moredetailsof holog-raphyreadersarerecommendedto visit theURL :http://www.holography.ru/ .
Millington, et al. [8] have developeda biosen-sor which uses hologram as sensing element.This biosensorcanhave potentialapplicationsinscreeningpancreaticdisordersat lowerprice. Thebioelementusedis bovine pancreatic trypsin in-hibitor (BPTI) which is an enzyme. To screenpancreaticdisorderstrypsin needsto be detectedin duodenealfluid or stoolsample.By properuseof BPTI trypsindetectioncanbemadepossible.
Whenthe hologramis illuminatedby white lightconstructive interference gives a characteristicspectrumhaving spectral peak and wavelengthpeakdescribedby ”Bragg equation”.Thecharac-teristic spectrumis dependenton the gelatinma-trix of thehologram.If gelatinmoleculesof holo-gram film are proteasedegradedthe characteris-tic spectrumchanges.This changesis specifictothe type of degradation. The authorsstudiedthespectrumafter degradingthe gelatinwith trypsinandBPTI. The reflectedlight from the hologramwas detectedby spectrographand CCD detectorat intervals of 1 or 2 minutesandwereanalyzedfor peakwavelengthandreflectivity changewithtime. Fig. 17 shows thepeakwavelengthandre-flectivity response.The major advantageof thisbiosensoris that very small trypsin levels canbedetectedwithin 60 minutesperiod.
6 DNA detection-on-a-chip(Lab-on-a-chip system)
The category of biosensorsusedfor DNA detec-tion are also known as biodetectors. The biode-tectorsareusedto identify a small concentrationof DNA (of microorganismlike virus or bacteria)is largesample.This relieson comparingsampleDNA with DNA of known microorganism(probeDNA). Sincethesamplesolutionmaycontainonly
(a) Peak wavelength response
(b) Peak reflectivity response
Figure 17: Peakwavelengthand reflectivity re-sponse,Source: [8]
asmallnumberof biorganismmolecules,multiplecopiesof thesampleDNA needsto becreatedforproperanalysis. This is achieved by the help ofpolymerasechain reaction(PCR).PCRstartsbysplitting sample’s of double-helixDNA into twopartsby heatingit about ������� . If the reagentscontainpropergrowth enzymes,theneachof thesestrandswill grow thecomplementarymissingpartandform double-helixstructureagain. This hap-penswhen temperatureis lowered. This in oneheating/coolingcycle theamountof sampleDNAis doubled(onecycle time is oneaboutminute).Thus,for � cycles �! copiesaremade.Typically,25-40cyclesareneededto produceapproximatelya billion copies.This amountis sufficient enoughto be detectedoptically. While the PCR is busyin copying DNA identificationalsocouldbemade
9
(b) Magnified view of chamber unit
(a) Peterson’s microfluidic device
Figure18: Biodetectordevelopedby K. Peterson,Source: [20]
possibleusingfluorescentDNA probes.
In general,PCRis veryverypowerconsumingbe-causeof heating/coolingcycle which takesabout30 minutes. So it was previously not possibleto fabricateportablebatteryoperatedbiodetectorswhich cando PCR.But, usingMEMS suchkindbiodectorswhich arebasicallylab-on-a-chipsys-temshave beendeveloped.In theseMEMS baseddevicestheamountof reagentusedis scaleddown.Fig. 18 shows a microfludicdevice developedbyK. Peterson[13, 20]. This lab-on-a-chipsystem
containschannels,valvesandchambersasshownin thefigure. To detectmicroorganismDNA stepsfollowedare:
� Some milliliters of sample solution arepumpedinto thechamber.
� Thesampleis concentratedto a volumeof amicroliter.
� SampleDNA arenow extractedfrom samplesolution.
� PCRis performed. A small thin-film heaterheatstheDNA andfanhelpsin cooling. Thecycle time is 25seconds.
� Flouroscenceprobe DNAs bind the sampleDNA.
� WhentheLEDscausetheprobeDNAs to flu-orescentthe glow is capturedby photodiodeandhencecanbedetected.
In [20] anotherbiodetectorhas beendiscussed.Thisbiodetectorusesmagneticfield insteadof op-tics or flouresence.This biodetectorwith thehelpof magneticsensorsandmicrobeadsis ableto de-tectpresenceandconcentrateof bioagents.Fig. 19showsa magneticbiodetectordevelopedby NavalResearchLaboratory(NRL). Themagneticsensoror groupof sensorsis coatedwith single-stranded(i.e. onepart of double-helix)DNA probesspe-cific for abioagentor sampleDNA. Onceasingle-strandof DNA probeanda single-strandof sam-ple DNA combinethey form a double stranded(double-helix) structure. The resulting double-helix structurebindsasinglemagneticmicrobead.Whena magneticbeadis presentabove a sensor,the sensor’s resistancedecreaseswhich is the de-tectableentity. The magneticbeadsare of-the-shelfproductsof diameter�#"%$���� which arecom-monly usedfor biochemicalseparation.Themorebeads,largeris thedecreasein resistance.
The MEMS based microreactor developed byNorthrup,et al. [10] have cycle timesasshortas
10
Figure 19: Magnetic biodetectordeveloped byNRL, Source: [20]
several secondsand35 cycles(for full amplifica-tion of DNAs) takesseveralminutes.Theadvan-tagesof thisbiosensorare:
� �'& � � timesfasterthanconventionalPCRs
�)( ��* moreefficient in numberof DNA copiesproduced
� easilydesignedto usesmallvolumes
� economical.
Figure20: Biodetectordevelopedby Northrupetal., Source: [10]
Fig. 20 illustratestheschematicdetectionprocessafter the PCR-amplificationbeingcompletedin areactionchamber. Thenylon basedteststrip con-tainsthespecificDNA probe.Thesample(ampli-fied) DNA is put into a reagent.If the two typesof DNAs bind each-otherthen the DNA-biotin-steptavidin-enzyme complex will changecolor,thusdetectingthesampleDNA. ThePCRreactionchambercanbedesignedof severaldifferentsizes.Thecross-sectionof a reactionchamberis shownin Fig. 21. The IC-fabricationstepsusedto de-
Figure 21: PCR cross-sectiondeveloped byNorthrupetal., Source: [13]
velopthis PCRarelistedbelow.
� A (�+ � diameter, �#",���-� thick singlecrystalsiliconwaferis taken.
� Silicon nitride ( of thickness� &)���.� ) is de-
positedon to entirewaferby LPCVD.
� Photolithographicpatternis donefor reactionchamber.
� Silicon nitride is etchedby RIE processoverthechamberarea.
� Silicon is etchedto silicon nitride backsidedefiningthechambervolume.
11
� Thewafer is patternedandetcheddependinguponreactionchamberdesign.
� Silicon nitride alongwith polysilicon de-positedby LPCVD.
� Polysilicon is dopedwith boronup to resis-tivity of ���/&0����� �21�3�4!576�8�9 .
� Aluminum contactdepositedthatdefinestheheatergeometry.
� Polyethyleneinput andoutputtubesarecon-structed.
� Glasscoversealingis done.
7 Glucose biosensors
The most successfulcommercialbiosensorsareamperometricglucosebiosensors.Thesebiosen-sorshavebeenmadeavailablein themarketin var-ious shapesandforms suchasglucosepens,glu-cosebig display, andsoon asdiscussedin section1. Theaimof thissectionis to doandetailedstudyof someglucosebiosensors.
Thefirst historic experimentthat servedasoriginof glucosebiosensorswascarriedout by LelandC. Clark [5]. He usedplatinum (Pt) electrodesto detectoxygen. The enzymeglucoseoxidase(GOD) was placedvery close to the surface ofplatinumby physicallytrappingit againsttheelec-trodesby a pieceof dialysismembrane.The en-zymeactivity changesdependingonthesurround-ing oxygenconcentration.Fig. 22shows thereac-tion catalysedby GOD. Glucosereactswith glu-coseoxidase(GOD) to form gluconicacid.At thesametime producingtwo electronsand two pro-tons,thusreducingGOD.This reducedGOD,sur-roundingoxygen,electronsandprotons(producedabove) react to form hydrogenperoxide(
;:�</:)
andoxidizedGOD (theoriginal form). This GODcanagainreactwith moreglucose.More theglu-cosecontent,more the oxygenconsumptionand
henceless is the detection. On the other hand,more the glucose, more the
;:�</:production.
Hence,either the consumptionof</:
or the pro-duction of
=:></:can be detectedby the help of
platinumelectrodeswhich canserve asmeasuresfor glucoseconcentration.
Figure22: Clark’sExperiment,Source: [5]
P. Yu andS.Dong[9] havedevelopedadisposableamperometricbiosensorfor detectionof glucose.Thebiosensoris button-shapedhaving overall di-ameterof $��-� andthicknessof ��"%$�$��-� . Fig. 23shows theconstructionof thebiosensor. Thedis-posablesensorconsistsof following layers:
� layer1: metallicsubstrate
� layer2: graphitelayer
� layer3: isolatinglayer
� layer4: mediatormodifiedmembrane
� layer5 : immobilized enzyme membrane(GOD)
� layer6: celluloseacetatemembrane
12
(b) Botton-shaped biosensor
(a) Construction Layers
Figure23: Detailsof a disposableglucosebiosen-sor, Source: [9]
This biosensorusesgraphiteelectrodeinsteadofplatinum electrode(usedin caseof Clark). Theisolatinglayeris placedon thegraphiteelectrodesthat can filter out certain interfering substances(ascorbicacid, uric acid) while allowing passageof
;:></:and
</:. The mediatormodified mem-
branehelps in keepingthe GOD membraneat-tachedwith the graphiteelectrodewhen electro-chemicalreactiontakesplaceat appliedpotential( ?�" ( � � ). The celluloseacetateouter layer placedover theGOD membranealsoprovidesbarrierforinterferingsubstances.Theamperometricreadingof the biosensor(current Vs glucoseconcentra-tion) is shown in Fig. 24. Therelationshipis linearuptoglucoseconcentrationof ����� �A@!BC1�B +ED 9F8 .
Figure24: Calibrationcurve of a disposableglu-cosebiosensor, Source: [9]
8 Conclusions
In this survey report we have discussedvariousbiosensorin detail. The survey initially, brieflyintroducesthe basic conceptson the biosensor.Highl-evel view of different types of biosensorsalsogiven. Working principles,constructions,ad-vantagesand disadvantagesof many biosensorsgiven.Theauthorwould like to mentionthatthereare variousdifficulties for which somesolutionsexist, but still more researchefforts needto begivento find betteralternatives.Few of themmen-tionedbelow :
� contamination : bioelementsandchemicalsusedin the biosensorsneedto be preventedfrom leakingout of the biosensorover time(seriousissuefor nondisposableones).
� immobilisation of biomolecules : to avoidcontamination,biomoleculesare attachedtothetransducerasstronglyaspossible,but theproblemwith this is that the behavior of en-zymeswhenabsorbedon surfaceis lessun-
13
derstood(reactionof enzymesin freesolutionis betteruderstood.
� sterilization : if a sterilized probe is usedsome sensor’s biomolecules may be de-stroyed whereasif non-sterileprobesusedsomecompromisesneeded.
� uniformity of biomolecule preparation :fabricationof biosensorsthat can reproduceresultsneedsuchuniformity.
� selectivity and detection range : shouldbe more selective and more detectionrangeshouldbelarge.
� cost :researchshouldbe focussedfor devel-opmentof low-costbiosensors.
At present,when people talk about bioterror-ism, thedevelopmentof faster, reliable,accurate,portableandlow-costbiosensorshasbecomemoreimportantthanever.
Acknowledgment
The authoracknowledgesDr. ShekharBhansaliwho gave him the opportunity to do this surveywork as a part of ”Introduction to MicrosystemsTechnology (MEMs)” course requirement inthe Departmentof Electrical Engineering,USF,Tampa,USA.
Disclaimers
Someof the figures usedin this paperare bor-rowed from othersourcesandusedhereonly foracademicspurposeand the authordoesn’t claimany originality for thesame.
References
[1] R. S. SethiandC. R. Lowe, ”Electrochemi-calMicrobiosensors”,IEE ColloqiumonMi-
crosensors, 1990,pp.911-915.
[2] S. P. J. Higson, S. M. Reddy and P. M.Vadgama, ”Enzyme and other biosensors: Evolution of a technology”, EngineeringScienceand EducationJournal, Feb 1994,pp.41-48.
[3] C. R. Keeseand I. Giaever, ”A BiosensorthatMonitorsCell Morphologywith ElectricFields”, IEEE Engineeringin MedicineandBiology, June/July1994,pp.435-445.
[4] D. M. Fraser, ”Biosensors: MakingSenseofThem”, Medical Device Technology, Vol.5,No.8,pp.38-41,1994.
[5] D. M. Fraser, ”Glucose biosensors- theSweetSmell of Success”,Medical DeviceTechnology, Vol.5,No.9,pp.44-47,1994.
[6] D. M. Fraser, ”BiosensorMarkets-Oppertu-nities andObstacles”Medical Device Tech-nology, 1995,Vol.6,No.2,pp.32-37.
[7] D. M. Fraser, ”Biosensorsin Critical Care”Medical Device Technology, 1995, Vol.6,No.3,pp.36-40.
[8] R. B. Millington, A. G. Mayes, J. Blyth,C. R. Lowe, ”A Holographic Biosensor”,Proc.of the8th InternationalConferenceonSolid-StateSensors and Actuators, and Eu-rosensors IX, TRANSDUCERS’95, June25-29,1995,pp.509-512.
[9] P. Yu and S. Dong, ”A DisposableSensorandIts Applicationsin theMeassurementsofGlucoseand Lead”, Proc. of the 8th Inter-national Conferenceon Solid-StateSensorsandActuators,andEurosensors IX, TRANS-DUCERS’95, June25-29,1995,pp.513-516.
[10] M. A. Northrup, C. Gonzalez,et. al., ”AMEMS-BasedMiniatureDNA AnalysisSys-tem”, Proc. of the 8th InternationalConfer-enceon Solid-StateSensors and Actuators,
14
and Eurosensors IX, TRANSDUCERS’95,June25-29,1995,pp.764-767.
[11] C. Podaru,C. Boston,et. al., ”An Amper-ometricGlucoseBiosensor”,Proc. of Inter-national SemiconductorConference, 1996,Vol.1,pp.101-102.
[12] Jean-MarieORY, F. JacquesandY. Boudey,”A biosensorfor waterquality monitoring”,Proc.of IEEEInstrumentationandMeasure-mentTechnology Conference, Brussels,Bel-gium,June4-6,1996,Vol.2,pp.1354-1359.
[13] K. Peterson,”Biomedical Applications ofMEMS”, Electron devices meeting, 1996,pp.239-242.
[14] D. R. Baselt, G. U. Lee, et. al., ”A High-Sensitivity Micromachined Biosensor”,Proc. of the IEEE, Vol.85, No.4, Apr 1997,pp.672-680.
[15] P. J. Hesketh, S. Zivanovic, et. al., ”Micro-fabricated Biosensorsand Microsystems”,Procof 21stInternationalConferenceonMi-croelectronics (MIEL’97), Sep1997, Vol.1,pp.63-69.
[16] L. SellamiandR. W. Newcomb, ”A MOS-FET Bridge Fluid Biosensor”, Proc. ofthe IEEE International Symposiumon Cir-cuits and Systems, 1999, ISCAS’99, Vol.5,pp.140-143.
[17] M. C. Hsieh,Y. K. Fang,et.al., ”A Contact-TypePiezoresistive Micro-ShearStressSen-sorfor Above-KneeProsthesisApplication”,IEEE Journal of MicroelectromechanicalSystems, Vol.10, No.1, Mar 2001, pp.121-127.
[18] J.H. Kim, B. G. Kim, et.al., ”A New Mono-lithic Micro Biosenserfor Blood Analysis”,Proc.of 14thIEEEInternationalConferenceon Microelectromechanical Systems, 2001,MEMS’2001,pp.443-446.
[19] GregoryKovacs,”MicromachinedTransduc-ers: Sourcebook”,WCB/McGraw Hill, Inc.
[20] C. Aston, ”Biological Warfare Canaries”,CoverStory: IEEESpectrum,October2001.
15