1微机电基础技术简介2000国科会北区微机电系统研究中心.pdf
1微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬電化學分析方法與感測器El e c t r o c h e m i c a l Se n s o r林啟萬臺灣大學醫工所2微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬電化學分析方法與感測器I.IntroductionII.Basics of Electrochemical measurementIII.Types of Electrochemical SensorsIV.Applications of Electrochemical SensorsV.Future developments3微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬I.化學感測器之應用臨床及生醫研究藥物測試，醫療儀器之診斷與治療用工業藥廠品管，發酵製程，廢水廢棄物監測農業成熟度、儲藏、農藥與運輸過程之檢測安全防衛毒性物質檢測環保空氣、水質污染機器人自動化檢測應用4微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬I.化學感測器之種類 Electrochemicala.ConductimetricConductanceEnzyme-catalysed reactionsb.Enzyme electrodeAmperometric Enzyme and Ab-Agc.Field effect transistorsPotentiometricIons,gases,enzymes,Ab-Ag(FET)d.Ion-selective electrodesPotentiometricIons,enzyme,Ab-Ag(ISE)e.Gas-sensing electrodesPotentiometricGases,enzymes,organelle,cell,tissue,enzyme,Ab-Ag Piezo-electric crystalsMass changeVolatile gases,vapours,and Ab-Ag OptoelectricOpticalpH,enzyme,Ab-Ag ThermistorsHeatEnzyme,organele,cell or tissuegases,pollutants,antibiotic,vitamins5微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬I.臨床非破壞性診斷之現況方法空間解析度時間解析度監測能力新陳代謝參數X 光0.1 m m0.1 s e c有限無斷層掃瞄 0.1 m m即時有無磁核共振影像 10-6 10-9 mol/l?Easy manufacture with high yield ratesBulk process in mind?Nondestructive,reliable,inexpensive quality control procedure for each piece producedTotal quality control9微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬II.Basics of Electrochemical measurementII.1.EC as an energy source for perturbation:Electrochemical measurement on chemical systems can provide thermodynamic data about a reaction,generate an unstable intermediate ion and study its rate of decay or its spectroscopic properties.These applications require an understanding of the fundamental principles of electrode reactions and the electrical properties of electrode-solution interfaces.Ref:A.J.Bard,L.R.Faulkner,“Electrochemical Methods Fundamentals and Applications”,John Wiely&Sons,Inc.198010微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬II.2.Electrochemical cells and reactionsWe primarily are concerned the processes and factors that affects the charge across interfaces between chemical phases in electrochemical reactions.There are two phases in an interface,electrolyte and electrode.Electrolyte is a phase that charge iscarried by the movement of ions.It can be liquid solutions,fused saltsorionicallyconducting solids.Electrode is a phase through which charge is carried by electronic movement.Electrodes can be metals,semiconductors,in either solid or liquid phase.An electrochemical system is defined most generally as two electrode separated by at least one electrolyte phase.There is a measurable potential difference between the two electrodes whether the cell is passing current or not.11微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬This potential is the collected differences in electricalpotetnial between all of the various phases in the current path,especially at the interface.The sharpness of the transition implies that a very high electric field exists at the interface,and thus exert great effects on the kinetic behavior of charge carriers(electrons or ions)in the interfacial region.The magnitude of the potential difference at an interface affects the direction of charge transfer.Thus the measurement and control of cell potentials is one of the most important aspects of experimental electrochemistry.Notation:/:a phase boundary,:separation between two components in the same phaseFor example:Pt/H2/H+,Cl-/AgCl/Ag12微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬The overall chemical reactions taking place in a cell is made up of two independent half-reactions,which describe the real chemical changes at the two electrodes.Each half-reaction responds to the interfacial potential difference at the corresponding electrode.Most of the time one is interested in only one of these reactions,and the electrode at which it occurs is called the working(or indicator)electrode.To standardize the other half of the cell,one uses an electrode made up of phases having constant composition as a reference electrode.The internationally accepted primary reference is the standardhydrogen electrode(SHE,NHE),which has all components at unit activity.To simplify experiment setup,one often use the saturated calomel electrode(SCE,Hg/Hg2Cl2/KCl),which has 0.242 V vs.NHE.(Ag/AgCl for in vivo experiments)13微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬Therefore,we observe or control the potential of the working electrode with respect to the reference one.It is equivalent to observing or controlling the energy of the electrons within the working electrode.By driving the electrode to more negative potentials the energy of the electrons is raised,and they will eventually reach a level high enough to occupy vacant states on species in the electrolyte(electrons flow from electrode to solution,a reduction current).Similarly,the energy of the electrons can be lowered by imposing a more positive potential,and at some point electrons on solutes in the electrolyte will find a more favorable energy on the electrode and will transfer there(electrons flow from solution to electrode,a oxidation current).The critical potentials at which these processes occur are related to the standard potentials,E0,for the specific chemical substances in the system.*So,we need to know the relative potential level of species in the reaction to verify the relative order of oxidation/reduction during the potential manipulation.14微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬The above mentioned electron transfer processes at the metal-solution interface of electrode is termed Faradaic processes(Faradays law:the amount of chemical reaction caused by the flow of current is proportional to the amount of electricity passed).There are times that a given electrode-solution interface will show a range of potentials where no charge transfer reactions occur because such a reaction is thermodynamically or kinetically unfavorable.However,adsorption anddesorption can still occur,and the structure of the electrode-solution interface can change with changing potential or solution composition.These processes are termed nonfaradaic processes(capacitance).Although charge does not cross the interface under these conditions,external currents can still flow(transiently)when the potential,electrode area,or solution composition changes.15微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬II.3.The Nature of Electrode-Solution Interface(One electrode)II.3.1 The Ideal Polarized Electrode and NonfaradaicprocessesAn ideal polarized electrode(IPE)is an electrode,which has no charge transfer across the metal-solution interface regardless the potential imposed by an outside source of voltage(good for reference).There is no such electrode that can ideally function as an IPE in the whole potential ranges.The SCE can function as an IPE in the range of 1.5 V.Outside this range,Hg will oxidize(Hg+Cl-1/2Hg2Cl2+e-,0.25 V vsNHE)or K+will be reduced(K+Hg+e-K(Hg),2.1V vs NHE).Small currents can still occur on this electrode due to reduction of water or trace of impurities.An IPE behaves like a capacitor,which accumulates charge at the interface.At a given potential,there will exist equal amount of charges on the metal electrode and in the solution.The polarity of the charge on the metal with respect to the solution depends on the potential across the interface and the composition of the solution.The charges in the vicinity of the electrode(0.1um),expressed in charge density(=q/A,A:electrode area),forms the electrical double layer.At a given potential,this double layer can be characterized by a double-layer capacitance,Cd,in the range of 10 to 40 uF/cm2.Unlike the real capacitor,Cdis a function of potential.16微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬II.3.2 Electrical Double Layer:There are several layers in the solution side of double layer.An inner Helmholtz plane(IHP)is formed by specifically adsorbed solvent molecules and other species(ions or molecules)with the thickness of x1,which is the locus of the electrical centers of the adsorbed ions.An outer Helmholtz plane(OHP)is formed by the solvated ions with the distance of x2,which is the locus of centers of these nearest solvated ions.The interaction of this OHP with metal electrode is due to long-range electrostatic forces,which is independent of the chemical properties of the ions,andforms a diffusion layer extended in to the solution.The total charges density in the double layer of solution is equal to the sum of the accumulated charges in these two layers,S=i+d=-M.The thickness of the diffuse layer depends on the total ionic concentration in the solution,which is around few hundreds.The structure of the double layer can affect the rates of electrode processes due to the potential drop in this diffuse layer and thus reduced the effective applied potential on the electroactive species.This layer also contributes to the nonfadaradic current during the measurement of EC.17微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬II.3.3 Equivalent circuit model of double layer:An EC cell consisting of an IPE(mercury drop electrode)and an ideal reversible electrode(SCE)can be approximated by an electrical circuit of a resistor,Rs(solution resistance),a capacitor,Cd(double layer capacitance),and capacitor,CSCE(capacitance at SCE).With CSCECd,CT=Cdfor a series connection.It is instructive to consider the behavior of working electrode as a RC circuit to several types of electrical perturbation,voltage step,current step,and voltage ramp.II.3.3.1 Voltage step:RC circuit after applying a constant voltage step.Remembering,and by KVL,With initial condition q=0,the behavior of the current,I,with time,t,after applying a potential step of magnitude E,is.The current response is an exponentially decaying curve with a time constant,=RSCd.(For a RS=1,Cd=20 uF,=20 usec,the charging of double layer through solution will reach 95%complete in 60 usec)CdECq=dSCRCqiREEE/+=+=SdSRECRqdtdqi/+=dSCRtSeREi/=18微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬II.3.3.2 Current step:RC circuit after applying a constant current stepSince,Again by KVL,Or II.3.3.3 Voltage ramp:Potential sweep in volts per secondThe superposition rule holds for such a system with the analysis given above.E=Ei+vt=Rsdq/dt+q/Cd.Results in current,.The current now contains a transient part,which dies away with a time constant,RsCd,and a steady state current,vCd(a way to measure Cd).If we apply a triangular wave,+v to v at some potential to the RC circuit,then the steady-state current changes from vCdto v Cdduring the forward and reverse scan(Cycle Voltametry).+=tdSdtCiiRE0=idtq)/(dSCtRiE+=)/exp()(dsdsidCRtvCREcCi+=19微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬II.4.Types and definitions of EC cells:Two electrodesEC cells can be classified as either galvanic or electrolytic cells.A galvanic cell is one in which reactions occur spontaneously at the electrodes when they are connected externally by a conductor.These cells are often used in converting chemical energy into electrical energy.An electrolytic cell is one in which reactions are effected by the imposition of an external voltage.These cells are often used to carry out desired chemical reactions by expending electrical energy.However,for a single electrode,its reactions and behavior is independent of whether this electrode is part of a galvanic or electrolytic cell.We thus will define the electrode at which reductions occur the cathode and the electrode at which oxidations occur the anode.A current in which electron cross the interface from metal to a species in solution is termed a cathodic current while electron flow from a solution species into the metal is called an anodic current.20微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬II.4.1 Variables and Factors in EC CellsEC cells can be considered as a perturbed system by electrical energy,just as the photons in a spectrophotometric system.In order to study the system transfer function(thermodynamic,kinetic,analytical,etc),one can perturb the system with certain excitation function(e.g.,pulse,step,sweep.)and measure the system response function(e.g.,the resulting current with time).In such a system,one needs to control certain variables of the EC cell and observe how the others vary with changed ones.These include(1)external variables(temperature,pressure,time),(2)electrode variables(material,surface area,geometry,surface condition),(3)electrical variables(potential,current,charge),(4)mass transfer variables(diffusion,convection,surface concentration,adsorption),(5)solution variables(bulk concentration of electroactive species,concentration of other possible interference species,solvent).For a homogeneous reaction,the electrolysis rate and faradaiccurrent is proportional to a constant,N.,.However,for a heterogeneous reaction with a rate that depends on mass transfer and various surface effects,the reaction rates are usually described in units of mol/sec per unit area,i.e.,nFjnFAiv=)()/()(rolyzedmoleselectNmolecoulombsCoulombsnFQ=21微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬22微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬Information regarding the electrode reaction can be determined from the I-V curve.The departure of the electrode potential from the reversible(nerstian)value upon passage of faradiac current is termed polarization.So,for an ideal polarized electrode,it shows a very large change in potential upon the passage of current.While for an ideal nonpolarizable electrode,its potential does not change upon passage of current.The deviation from the equilibrium potential is measured by theoverpotential,which is equal to E Eeq.II.4.1.1 Factors affecting the rate of an electrode reactionAs mentioned above,the electrode reaction rate on the surface of an electrode with the conversion of dissolved oxidized species,O,and reduced species,R,in solution,O+ne R,is governed by:(1)Mass transfer,(2)Electron transfer at the surface,(3)Chemical reactions preceedingor following the electron transfer(protonation,dimerization,catalytic decomposition),(4)Other surface reactions(adsorption,crystallization).It depends on the chemical system that we are investigated.The reactions can range from simple to very complex for multiple parallel pathways.23微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬24微機電基礎技術簡介2000國科會北區微機電系統研究中心林啟萬One of these reaction steps will be the rate-limiting step,which puts a upper limit on the steady state current.Even with this rate-limiting current that flows in the complete circuit of EC cell,there will be a certain overpotential,.One can characterize the reaction step with series of resistance components.The effective potential on the electrode can be presented as Eappl=Eeq+iRs,where Rs is theohmic potential drop along the solution path.It can be minimized by proper cell and instrumentation design.If the passage of current does not affect the potential of the reference electrode(ideal nonpolarized electrode),the potential of working electrode can be given by above equation.With small iRs(2 mV,say for I 10 uA,Rs H+K+NH4+Na+Li+Solid-state:The sensor is a thin layer of a single or mixed crystal or precipitate which is an ion conductor.(Silicone rubber,PVC)Liquid ion-exchange:Dissolving an organic ion exchanger in an appropri
