f E ~- 2.4 Conductivity

Transcription

1 36 Chemial Sensors and Biosensors Transdution Elements 37 A detailed analysis of this situation is ompliated, although this is not needed for understanding the operation of a biosensor. The effet of this reation is that the redox proess yles round many times. The reverse oxidation of R is not seen but the forward redution peak is enhaned many times (see Figure 2.17). n this figure, the reversible yli voltammogram is of ferroene (diylopentadieneiron(rn)). The atalyti wave is aused by interation with gluose oxidase in the presene of gluose (desribed in more detail below in Chapter 5). SAQ 2.11 What is the essential differene between a atalytially limited wave and a kinetially limited wave? N R =R 3 x R 1 R; L =1- = R2 R R 1 R 3 Figure 2.18 A ondutivity bridge. From Eggins, B. R., Biosensors: An ntrodution, Copyright John Wiley & Sons Limited. Reprodued with permission. 2.4 Condutivity Condutivity is the inverse of resistane. t is a measure of the ease of passage of eletri urrent through a solution. Ohm's law gives the following relationship: E = R and for the ondutane, L [in siemens (S), where 1 S = 1 ohm- 1 ]: and therefore: L = lr The measurement of ondutane involves an alternating urrent, as in the.lassial ondutane bridge. Varying the frequeny of the alternating urrent an extend this. The quantity measured is then the admittane = l/impedane), whih not only depends on simple ondutane but also on the apaitane and indutane of the system. These omponents an be separated as imaginary omponents, in partiular by using a frequeny response analyser, and then dispjaying in an Argand diagram, as shown in Figure Suh diagrams are sometimes alled admittane (impedane) spetra. This approah has not so far been used to any great extent in devejoping sensors and biosensors, but is now reeiving inreased attention. E = l/l Condutane is related to the dimensions of a ell in a similar way to resistane. For a ell of length l and ross-setional area A, the ondutane L = K All, where K is the speifi ondutivity (S em-). This is often further normalized by dividing by the molality of the solute to give the molar ondutivity, 1\ = K/C (C in mol m- 3 ), so the units of 1\ are S mol- m- 2. Condutivity is fairjy simple to measure, being diretly proportional to the onentration of ions in the solution. Figure 2. J8 shows a general ondutivity bridge iruit. n the traditional bridge, the resistane R3 is adjusted to balane the bridge and a ell onstant is then used to onvert ondutane into (speifi) ondutivity. n modern instruments, this is arried out automatially to give a digital read-out. The ondutivity varies aording to the harge on the ion, the mobility of the ion and the degree of dissoiation of the ion. These all introdue ompliations. n itself the tehnique has no seletivity. t an be used in ontrolled situations but really needs to have seletivity superimposed by means of a membrane or oating. Mass Kineti transfer ontrol ontrol f E ~- R n R = Ret Rn+R et ZRen 2 p' lh~u~e 2.19 A typial Argand diagram, showing the frequeny dependene of RaU~rnagtnary' impedane against the 'real' impedane. From Bard, A. J. and 1980 ~r, L. R, Eletrohemial Methods: Fundamentals and Appliations, Wiley,. epnnted by permission of John Wiley & Sons, n.

2 38 Chemial Sensors and Biosensors Transdution Elements 39 EnZyme/POly~-i layer ~:, : '.' Blank Solution potentiometri signals, produed by a potentiometri sensor proess on the gate of the FET. A separate referene eletrode is also needed. Ciruit wiring is minimized, so that in addition to miniaturization, eletroni noise is greatly redued and sensitivity is inreased. The FET devie an be part of an integratediruit system leading to the read-out, or to the proessing of the analytial data. However, as yet, no partiularly satisfatory miniaturized referene eletrodes exist. Aording to Janata (see Bibliography), most of the proposed versions violate some of the basi priniples of referene eletrodes. Despite tbis, a number of possibilities have been proposed and used, varying from a 'pseudo-referene eletrode', onsisting of a single platinum or silver wire, to the sreen-printed type made with silver-silver hloride ink. Perhaps a more satisfatory approah is to avoid the problem by operating in a differential mode with two FETs, i.e. one being a blank with a gate having negligible response to the analyte and the other oated with the analyte-seletive membrane. Figure 2.20 S)1emati of a differential type of ondutivity ell, as used in biosensors. From Eggins, B. R., Biosensors: An ntrodution, Copyright John Wiley & Sons Limited. Reprodued with permission. SAQ 2.12 Why an diret urrent not be used in a ondutivity bridge? n priniple, a hange in ondutane an be used to follow any reation that produes a hange in the number of ions, the harge on the ions, the dissoiation of the ions or the mobility of the ions. Usually a differential type of ell is used, as shown in Figure DQ 2.6 Disuss fators, whih would enable one to use ondutane devies as transduers. Answer Any reation or hange that involves a hange in the number of ions, the harge on the ions or the mobilities of the different ions will produe a hange in the ondutivity of the solution, whih ould therefore be used as the transduer. This is a relatively simple, although a somewhat under-used method. 2.5 Field-Effet Transistors Field-effet transistors (FETs) are devies in whih a transistor amplifier is adapted to be a miniature transduer for the detetion and measurement of Semiondutors - ntrodution Materials an be lassified as metals, non-metals or semiondutors. Generally metals are good ondutors of eletriity, while non-metals are bad ondutors i.e. they behave as insulators. Semiondutors ome somewhere in between. The differenes an be seen in the way that they form energy levels. Non-metal atoms form disrete bonding and anti-bonding moleular orbitals when they ombine to form moleules. The bonding orbitals ontain the bonding eletrons, while the anti-bonding orbitals are empty, unless eletrons are promoted into these by exitation. The energy spae between these levels is 'forbidden' and is therefore unoupied. n metals, there are overlapping energy bands and so there is no forbidden region. Eletrons an move freely throughout the bands, thus leading to their high ondutivities. Semiondutors form energy bands, although in this ase they are separated by a forbidden region. The lower band is known as the valene band (VB), while the upper band is alled the ondution band (CB). The energy gap between the two is alled the band gap. These features are shown n Figure f small amounts of dopants are added to a semiondutor, it may aquire an exess of eletrons to give a p-type semiondutor, or a defiit of eletrons (ex~ess of holes) giving an n-type semiondutor. Fifth-row elements (in the Penodi Table), suh as arseni, form p-type semiondutors, while third-row elements, like gallium, will form n-type semiondutors. b The Fermi level ( ) is the point where the probability of filling the (energy) band is 0.5. For an undoped (intrinsi) semiondutor, this will be half-way t~tween the VB and the CB, while for a doped semiondutor, lies nearer to ~VB in p-doped materials and nearer to the CB in n-doped materials. met ~rnmon arrangement of semiondutors for sensor appliations is the Per a!.-lusulator-semiondutor (MS) system. f no potential is applied, the nu level is the same aross the metal to the semiondutor. However,

3 40 Chemial Sensors and Biosensors Transdution Elements 41 (a) i -0 o. 0 :;:; U ::J -0 0 Vaant (a) Metal nsulator p-type semiondutor E EF E y (b) -0 o. Q) u Q) ~ > ~ Q) UJ!! Metals_' ~! Semiondutors Filled solated ~ n~eratomi atoms paing, (b) () '-- E ', E; l E (VG<O) F ~ E y Aumulation EF E E; VG>O EF Depletion Ey Forbidden region E g (d) E i t; > 0 Ey Figure 2.21 (a) Classifiation of a material aording to energy bands and interatomi spaing. (b) The semiondutor band gap energy model. From Hall, E. A. H., Biosensors, Copyright John Wiley & Sons Limited. Reprodued with permission. when a potential is applied the levels on the two sides separate. The system then behaves like a apaitor and harges build up on eah side. Figure 2.22(a) shows the energy levels aross a p,type semiondutor. t also shows the effet of applying a potential (a gate voltage, Va) aross the MS system. With a small negative potential (Va < 0) Figure 2.22(b), there is an aumulation of eletrons at the metal/insulator (Mil) interfae, and of holes (positive harges) at Figure 2.22 Energy bands through an MS system as a funtion of the applied voltage. From Hall, E. A. H., Biosensors, Copyright Jobn Wiley & Sons Limited. Reprodued with permission. :e semiondutor/insulator (S/!) interfae. EF is shifted towards the VB lower an the value in the metal by an amount equal to Va and the energy levels ~ear the semiondutor beome bent upwards to ompensate for this. With a rn~l~ positive potential (Va> 0) Figure 2.22(), there is a depletion effet as PoSltlVe be does h 1 are repelled from the Sl.. mterfae. n this ase, the VB and CB then h downwards to ompensate for this. f Va is further inreased, eventually Per?le and eletron onentrations near the interfae beome equal. Now, the 1lj level is again midway between the VB and the CB - equivalent to the

4 42 Chemial Sensors and Biosensors Transdution Elements 43 intrinsi level (Figure 2.22(d». Further inreases in potential beyond this lead to an exess eletron onentration, thus ausing the semiondutor to invert and beome n-type in nature. The potential required to ause inversion is known as the threshold potential (VT) Semiondutor-Solution Contat When an n-type semiondutor is in ontat with a solution ontaining a redox ouple (Ox/R), the Fermi level is related to the redox potential EO. f the of the semiondutor lies above that of the solution, there will be a net flow of eletrons from the former into the solution and the CB and VB will be bent upwards (as shown in Figure 2.23). fthe interfae region is illuminated with light of energy greater than the band-gap energy (EG), there will be a separation of the eletron-hole pairs. The holes migrate to the surfae with a potential equivalent to the VB and ause oxidation of R to Ox. The eletrons move into the bulk: semiondutor and to the external iruit or reat with an eletron-aeptor (Ox) speies, thus asing redution. This phenomenon is known as photoatalysis. Titanium dioxide is used extensively as a photoatalyst material. SAO 2.13 Explain how inversion ours in a field-effet transistor. (a) E (b) Ev Semiondutor _ Ox R Solution Field-Effet Transistor This is an arrangement to monitor and ontrol hanges in the MS system. nversion at a p-type S system an be monitored by two n-type sensors plaed on, either side of the p-type layer. The basi type of field-effet transistor (FET) is the insulated-gate FET (GFET). This is shown in Figure A soure region (4), onsisting of n-type silion, is separated from a similar drain region (5), also of n-type silion, by p-type silion (1), with the insulator (2) onsisting of silion dioxide. The soure is eletrially biased with respet to the drain by the applied potential, V D. The gate (3) is a metal. insulated from the rest, so that it _n~,:-x---- ~- Semiondutor Solution () ~ Ox E O /R 4 5 Semiondutor Solution Figure 2.23 Formation of a juntion between an n-type semiondutor and a solution ontaining a redox ouple OxlR: (a) before ontat; (b) at equilibrium in the dark; () after irradiation, where hv > EG. From Hall, E. A. H., Biosensors, Copyright John Wiley & Sons Limited. Reprodued with permission. p-type silion p. Si;gu re 2.24 Shemati of the insulated-gate field-effet transistor (GFET): 1, p-type lcon o substrate; 2, insulator; 3, gate metal; 4, n-type soure; 5, n-type drain; 6, metal 19~~ts to SOure and drain. From Eggins, B. R., Biosensors: An ntrodution, Copyright. John Wiley & Sons Limited. Reprodued with permission.

5 44 Chemial Sensors and Biosensors Transdution Elements 45 MS»l~'...'...',',' ~:~=,',' :;:; ~;:; :':.....'..' :;:; :;~; 7 i Referene 8 eletrode VG ---. Figure 2.25 Shemati of the gate in an GFET: M, metal;, insulator; S, semiondutor. From Eggins, B. R., Biosensors: An ntrodution, Copyright John Wiley & Sons Limited. Repro~ued with pennission. forms a apaitor sandwih, a metal/insulator/semiondutor (MS) arrangement, as shown in Figure 2.25 This gate region is harged with a bias potential Vo,. The urrent from the drain (5) to the soure (4), D, is measured. There is also a threshold potential, VT, at whih silion hanges from p-type to n-type, and inversion ours. With a small positivevd and Vo < V T, silion (1) remains in the p-state, and there is no drain urrent; n-si is biased positive with respet to p-si. When Vo > V T, there is surfae inversion, and p-si beomes n-si. Now urrent an pass from drain to soure, without rossing the reversed-bias p-n juntion. Vo now modulates the number of eletrons from the inversion layer and so ontrols the ondutane. n flows from soure to drain, and is proportional to both the eletrial resistane of the surfae inversion layer and V D. n order to onvert this devie into a sensor, the metal of the gate is replaed by a hemially sensing surfae. This general onformation is known as a CHEMFET and is shown in Figure n this arrangement, the hemially sensitive membrane (3) is in ontat with the analyte solution (7). A referene eletrode (8) ompletes the iruit via the Vo bias. The membrane potential minus the solution potential has the effet of orreting for this bias. The urrent may be measured diretly at onstant Vo by using a iruit suh as that shown in Figure Alternatively, one an keep D onstant by hanging Vo and measuring the latter by using a iruit suh as the arrangement shown in Figure Suh a system is used in a number of sensor modes. The general CHEMFET mode has already been mentioned. A further mode is the ion-seletive mode (SFET), whih uses the FET as an ion-seletive eletrode. Following on from this, the ENFET is a form of biosensor in whih the gate ontains an enzyme system. 4 5 p-type silion ~--VD Figure 2.26 Shemati of a field-effet transistor with a hemially sensing gate surfae (CHEMFET): 1, silion substrate; 2, insulator; 3, hemially sensitive membrane; 4, soure; 5, drain; 6, insulating enapsulant; 7, analyte solution; 8, referene eletrode. From Eggins, B. R., Biosensors: An ntrodution, Copyright John Wiley & Sons Limited. Reprodued with permission. Ref Sol F' vl~ure 2.27 Shemati of the iruit used for measuring G at a onstant gate 0 A tage: A, operational amplifier; R j, 1 kq; R 2, 470 Q. From Eggins, B. R., Biosensors: p: ntrodution, Copyright John Wiley & Sons Limited. Reprodued with Trttission. -

6 46 V o Chemial Sensors and Biosensors Transdution Elements pv substrate Condutive Working silver trak eletrode 47 Ref Sol R 2 V set 1 '>', 1 Condutive arbon trak Dieletri layer Ag-AgC referene eletrode Figure 2.30 Shemati of the ExaTeh biosensor disposable eletrode strip. From Hildith, P. 1. and Green, M. J., Analyst, 116, (1991). Reprodued with permission of The Royal Soiety of Chemistry. Figure 2.28 Shemati of the iruit used for measuring hanges in VG at a onstant drain urrent: A and A 2, operational amplifiers; R" KQ; R2 = R3, 100 kq; R4, 20 KQ; R j, 470 Q;, 10 pf. From Eggins, B. R., Biosensors: An ntrodution, Copyright John Wiley & Sons Limited. Reprodued with permission. 2.6 Modified Eletrodes, Thin-Film Eletrodes and Sreen-Printed Eletrodes Modified eletrodes will be disussed in detail in Chapter 3. A major aspet in the manufature of sensors is miniaturization. Three developments, whih have assisted this, are thik-film eletrodes formed by sreen-printing, thin-film eletrodes and miroeletrodes Thik-Film - Sreen-Printed Eletrodes Here, the working eletrode is usually a graphite-powder-based 'ink' printed on to a polyester material. The referene eletrode is usually silver-silver hloride ink. A typial layout is shown in Figure Working eletrode Referene eletrode 4 Figure 2.29 A sreen-printed eletrode. From Wang, J., Analyst, 119, (199 ). Reprodued with permission of The Royal Soiety of Chemistry. Appropriate modifying omponents an be inorporated into the arbon ink, suh as gold, merury, helating agents (for use in stripping voltammetry), mediators suh as phthaloyanines and ferroenes to atalyse eletron transfer, or enzymes suh as gluose oxidase, asorbi aid oxidase, glutathione oxidase or uriase. The proedure has the advantages of miniaturization, versatility and heapness, and in partiular lends itself to the mass prodution of disposable eletrodes. A version is marketed ommerially in the 'ExaTeh' biosensor for gluose (Figure 2.30). SAQ 2.14 How ould a sreen-printed eletrode be made by using a plant tissue material suh as that of a banana? Miroeletrodes Miroeletrodes, also alled ultra-rniroeletrodes, having dimensions in the range.1-10 j..lm, have greatly extended the range of sample environments and exper mental time-sales that an be used for eletroanalysis. Suh eletrodes have ~urfae areas whih are many times smaller than the ross-setional area of a,tuman hair. They operate with small urrents in the pa to na range, and have ~ eady-state responses and short response times. Eletrodes have been made in the e:~ Of. diss, bands, ylinders, rings and arrays. A simple dis an be made by the t ddmg a platinum wire or arbon fibre in glass or epoxy resin and exposing B ross-setional) dis to the solution. the ; aus e of their small dimensions, the double-layer apaitane is low, so that Due aradai urrent is large when ompared to the bakground apaitive urrent. to the small urrent magnitudes, the R drop is very muh redued (or