The fast chemical response of the pH-ISFET makes the device an excellent detector in analytical chemistry. The time response of ISFETs, with Al2O3 at the pH-sensitive gate insulator, is determined in a flow injection analysis system. Application of an ISFET and a glass electrode are compared in rapid acid-base titrations and in a coulometric system for stable pH control. Finally, a method is described that combines both the stability of the glass electrode and the fast response of the ISFET

Bergveld, P.

Bos, M.

Bousse, L.J.

van der Schoot, B.H.

English

NARCIS 

The isfet in analytical chemistry

University of Twente Research Information

Sensors and Actuators, 4 (1983) 267 - 272 267 THE ISFET IN ANALYTICAL CHEMISTRY* B H VAN DER SCHOOT, P BERGVELD, M BOS and L J BOUSSE Twente Unrverslty of Technology, P 0 Box 217, 7500 AE Enschede (The Netherlands) Abstract The fast chemical response of the pH-ISFET makes the device an excel- lent detector m analytical chemistry The time response of ISFETs, mth A1203 as the pH-senwtlve gate insulator, IS determmed m a flow mjectlon analysis system Apphcatlon of an ISFET and a glass electrode are compared m rapid acid-base tltratlons and m a coulometrlc system for stable pH control Finally, a method 1s described that combmes both the stabMy of the glass electrode and the fast response of the ISFET Introduction The pH-sensitive mechanism of A1203-ISFETs 1s based on surface reac- tions [l] This leads to a very fast response compared to the glass electrode, where the potential-determmmg process takes place m the hydrated gel layer This property makes the ISFET a useful detector m analytical chemistry, especially m fast automated analysis systems Flow qectlon One appllcatlon of fast pH sensors could be m flow mJect.lon analysis [2] In this technique a sample solution is inJected mto a carrier solution and transported to a suitable detector We used a flow mJectlon system to create a stepwise change m pH m order to determine the response time of an ISFET [3] This method proved to be very useful as it offers an umnterrupted change of solutions The experimental set-up IS shown m Fig 1 Two buffer solutions of different pH are contmuously pumped through a four-way rotary valve which selects the solution that streams mto the detector block In this perspex block the ISFET 1s mounted mth its ion- sensltlve surface perpendicular to the fluid stream, while a reference elec- trode IS placed downstream *Based on a Paper presented at Solld-State Transducers 83, Delft, The Netherlands, May 31 June 3,1983 0250~6874/83/$3 00 0 Elsevler Sequola/Prmted m The Netherlands 268 PH 8 I * 1= / \\ISFET \ Ref el L!-_l I pump selectlon valve waste Fig 1 Diagram of the experimental set up to determine the response time of an ISFET Of course, the response time of the system depends on the steepness of the pH gradient that 1s offered to the detector We found that this response time depends on the composltlon of the two buffer solutions When two solutions of approximately the same buffer capacity are used, the response time 1s conslderabfy higher than m the case where the second buffer has a greater capacity, thus reducmg mlxmg effects of the solutions The relatlon- ship between response time and Inverse flow rate IS shown m Fig 2 150-_ 01 02 03 04 Inverse flow [ml-l mln] > Fig 2 Response time of an A1203 ISFET The mltlal pH IS defmed by a pH 8 buffer by Merck (0) Second solution 1s a buffer by Merck, pH = 5, buffer capacrty 0 = 0 03 (0) Second solution 1s a 1M acettc acld/lM sodmm acetate buffer, pH = 4 7, fl= 1 15 Since this linearity 1s vahd for both combmatlons of buffer solutions, it suggests that the response time of the system 1s not determined by the elec- trode nor by dlffuslon through the stagnant layer at the electrode surface The hmltmg factor IS here the non-ldeahty of the stepwlse pH change Extrapolation of the linear relatlonshlps m the Figure makes It probable that the mtrmslc response speed of the A1203-ISFET IS at least of the order 269 of 1 mllhsecond This experiment Justifies the assumption that the response mechanism 1s based on surface reactions rather than on ddfuslon-limited changes m the bulk of the surface insulator Fast titrations The speed of automated acid-base tltratlons 1s normally limited by the slow response of the glass electrode Use of an ISFET as a pH monitor permits titration speeds that are considerably higher We studied the titration of trls(hydroxymethyl)ammo-methane (Trls) with hydrochloric acid [ 41 Experimental results with a glass electrode and an ISFET are shown m Fig 3 Allowmg a 1% tltratlon error, we see that this limit 1s reached at a speed of 1 ml/mm for the glass electrode Using the ISFET, overshoot 1s below 1% even at the highest tltratlon speed of 5 ml/ mm The hmltmg factor m the latter case 1s again not the pH sensor but the mlxmg of the solution This 1s shown m the lower curve of Fig 3, where stlrrmg was improved by mounting two glass strrps on the wall of the tltra- tlon vessel, causing better turbulence overshoot 4 + tltratlon speed [ml /mln] Fig 3 Overshoot m the titration of 0 5 mmol Trls with 0 1000 M hydrochloric acid (A) Glass electrode, (n) ISFET, (0) ISFET with improved stlrrmg Coulometrlc control system We designed a coulometrlc control system m order to achieve accurate control of the pH m an unbuffered solution [5] This method ensures control of a constant pH or deared pH gradients without altering the total volume or ionic strength of the solution The dynamic propertles of the control system are determmed by the stu-rmg of the solution and the response time of the pH sensor Therefore the use of an ISFET 1s agam favourable compared to a glass electrode The system (Fig 4) controls the pH of a c&am volume of solution Its pH 1s measured and fed mto a microcomputer where it 1s compared to a desired pH value Devlatlons between actual and desired pH are corrected by 270 counter electrode - reference electrode pH electrode working electrode Fig 4 Experimental set up of the coulometrw control system coulometrlc generatlons of protons or hydroxyl ions by means of the hydrolysis of water A stable and rapid operation of the system requires that the design complies with certain control engrneermg criteria In contrast to the tltratlon example, we now consider rather small changes in pH This allows us to linearize the response of pH to reagent addition over small regions, thus faclhtatmg the description of a model Our description of this model contains four different time constants Two of these are caused by the transportation lag and mlxmg of the generated ions m the solution The pH LS sampled once every second, mtroducmg a delay of half this sampling interval The fourth time constant IS due to the pH electrode and its accessory ampllfler In the case of the glass electrode this time constant IS about 3 seconds, but for the ISFET It 1s so small that it can be neglected The difference m response time of the electrodes allows a closed-loop amphficatlon of the system which IS, m the case of the ISFET, twice as high as for the glass electrode This higher ampllflcatlon results m a faster and more stable pH control, as can be seen m Fig 5 This Figure shows the recovery of the system to pH 7 after addition of a drop of 0 01 N hydro- chloric acid ISFET PH 7 gloss electrode PH 7 I 01 pH 6s W Fig 5 Recovery of the pH after disturbance by lr-qectlon of 0 03 ml 0 01 N HCl 271 Parallel operation of a glass electrode and an ISFET Although the ISFET 1s a very useful sensor m analytical apphcatlons, it 1s inferior to the glass electrode when long-term stability 1s considered Modern glass electrodes drift as little as 0 002 pH/day, whereas ISFETs can show a drift of about 0 5 mV/h or approximately 0 1 pH/day The causes of this dnft are not yet fully understood To get the best of both worlds, we constructed a pH measuring system that combines the favourable properties of both the glass electrode and the ISFET The signals of the two electrode amplifiers are summated after fllter- mg From the glass electrode we take only the low frequency portion of Its response, while the ISFET adds the higher frequency components of a given change m pH We use simple first-order filters that can be easily matched to give an over-all linear frequency response To obtam the correct pH readmg at the output of the system, the sensltlvltles of both electrode amphflers have to be matched The d c level of the output 1s provided by the glass electrode-pH meter, which 1s calibrated against two standard buffer solutions The offset of the ISFET amplifier 1s not important since it IS blocked at the high pass filter Figure 6 shows a schematic diagram of the system as well as a typical output registration The dashed lme m the Figure represents the conespond- mg output from the glass electrode only glass electrode reference electrode I SFET -,y -3 low pass fflter ’ @_y--- /-) high pass filter Fig 6 Parallel operation of a glass electrode and an ISFET, together with a typlcal output reglstratlon - Glass electrode + ISFET - - - Glass electrode only Conclusions Summarlzmg the results of the various experunents described m this paper, it can be said that the pH sensor need no longer be the llmitmg factor when rt comes to speed of analysis Further improvement can be expected from better mlxmg of the solutions, for mstance through mmlaturlzatlon of the various experimental set-ups Here the ISFET offers excellent posslbll- ltles because of its small dlmenslons 272 References L J Bousse, The chemtcal sensltlvlty of electrolyte/msulator/sillcon structures, Theszs, Twente Umverslty of Technology, The Netherlands, 1982 A U Ramsmg, J Janata, J Ruzlcka and M Levy, Mmiaturlzation in analytical chemistry - a combmatlon of flow mlectlon analysis and ion sensltlve field effect transistors for determmatlon of pH and potassium and calcium Ions, Anal Chzm Acta, 118 (1980) 45 52 L J Bousse, P Bergveld and W E van der Linden, ISFETs m flow nqectlon analysis, Proc Sczentzfzc sesszon on electrochemzcal detectzon zn flow znjectzon analyszs, Matrafured, Hungary, October 17 - 20, 1982, to be published M Bos, P Bergveld and A M W van Veen Blaauw, The ion sensltlve field effect transistor in rapid acid-base titrations, Anal Chzm Acta, 109 (1979) 145 148 P Bergveld, B H van der Schoot and J H L Onoklewlcz, Development of a micro processor controlled coulometry system for stable pH control, Anal Chzm Acta, 151 (1983) 143 - 151 

The isfet in analytical chemistry

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