Biosensors: A Survey Report

SarajuP. MohantyDept.of Comp.Sc.andEngg.

Universityof SouthFloridaTampa,FL 33620,USA.smohanty@csee.usf.edu

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

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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.

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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.

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� 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.

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