The development of durable anion selective CHEMFET micro sensors is described. Selectivity in these sensors is either obtained from differences in hydration energy of the anions (the Hlofmeister series, giving nitrate selectivity) or by introduction of a new class of uranyl salophene ionophores (bicarbonate selectivity). The durability of the nitrate sensor was enhanced by using polysiloxane membranes in which 1 cationic tetraalkylammonium sites were covalently bound to the membrane matrix

Antonisse, Martijn M.G.

Engbersen, Johan F.J.

Reinhoudt, David N.

English

Antonisse, M.M.G.

Engbersen, Johannes F.J.

Reinhoudt, David

University of Twente Research Information

220 - PD6 NITRATE AND BICARBONATE SELECTIVE CHEMFETS Martijn M.G. Antonisse, Johan F.J. Engbersen and David N. Reinhoudt Laboratory of Organic Chemistry and MESA Research Institute, University of Twente P.OBox 217, NL-7500 AE ENSCHEDE, The Netherlands SUMMARY The development of durable anion selective CHEMFET micro sensors is described. Selectivity in these sensors is either obtained from differences in hydration energy of the anions (the Hlofmeister series, giving nitrate selectivity) or by introduction of a new class of uranyl salophene ionophores (bicarbonate selectivity). The durability of the nitrate sensor was enhanced by using polysiloxane membranes in which 1 cationic tetraalkylammonium sites were covalently bound to the membrane matrix. INTRODUCTION We have studied various Chemically Modified Field Effect Transistors (CHEMFETs, Figure 1) as microsensors for sensing of cations, like for example sodium [I], potassium [2], and heavy metal ions like lead, copper, silver, and cadmium [3]. In these devices a membrane with an ion selective calix[4]arene derivative was placed over the gate oxide of the CHEMFET. These CHEMFETs differ from the earlier reported Membrane Field Effect Transistors (MEMFETs) in that a hydrophilic polyhydroxyethyl methacrylate (polyHEMA) layer is positioned between the gate oxide of the CHEMFET and the ion selective membrane. I I B  Figure 1 .  Chemically Modified Field Effect Transistor (CHEMFET); I :  Ilydrophobic membrane, 2:  PolyIIEMA hydrogel, 3: SiO, gate oxide, 4 :  Channel, 5 :  Insdating resin, 6: Reference electrode, B :  Bulk, S :  Source, D :  Drain. This polyHEMA layer, is conditioned in a pH buffered solution of the primary ion that will be detected, and eliminates undesired pH response and CO, interference. In general, the selective detection of anions is more demanding than the detection of cations, because of their greater differences in hydration energies. Therefore it is more difficult to attract anions into the membrane. A simple anion selective membrane, containing positively charged ionic sites, like for example a plasticized PVC membrane with tetraoctylammonium bromide, shows a selectivity governed by the relative hydration energies, as is given in the well-known Hofmeister series: Clod > I‘ > SCN- > NO,’ > Br- > NO; > C1- >> A c O  > CO,”, SO,“, HJ’O, Because of the selectivity of nitrate over other common anions like nitrite, chloride, sulfate, and phosphate, various nitrate sensors have been made based on this principle. To  change the selectivity for other ions, it is necessary to add an ionophore to the membrane which can selectively bind to the target anion. Uranyl salophenes are a promising new class of neutral anion binding molecules that can be used as ionophore. URANYL SALOPHENES AS IONOPHORES Uranyl (UO,2’) has a pentagonal bipyramidal coordination structure with the two oxygen atoms at the apical positions. When bound by a salophene, four coordination site are occupied by the nitrogen and oxygen atoms of the salophene unit. The fifth coordination site is available for anion binding [4]. Besides the selectivity of the uranyl binding site based upon the Lewis acid character of uranyl, it is possible to introduce additional binding sites at the phenolic part of the salophene that enhance the selectivity. Compound 1 (Chart 1) possesses two extra amide moieties that can intensify anion binding by the formation of hydrogen bonds. CHEMFET measurements using membranes incorporating 1 showed a high selectivity of this ionophore for bicarbonate (Figure 2 and Table 1). TRANSDUCERS ‘95 . EUROSENSORS IX The 8th International Conference on Solid-state Sensors and Actuators, and Eurosensors IX. Stockholm, Sweden, June 25-29, 1995 867 H N  Q Me Chart 1 10 T o  [mv -10 -20 -30 4 -50 60 -70 -5 4 -3 -2 -1 *CO,> Figure 2 .  CI-IEMFET response f o r  NCO; in  the presence of 0.1 M .  SO:. Table 1. Selectivity data ( F I M )  of CIIEMFET with ionophore 1. NEW M E M ~ R A N E  MATERIALS The use of PVC as membrane material in ion sensors has some major disadvantages. Upon prolonged contact with the analyte solution the plasticizer slowly dissolves into the contacting aqueous solution, causing an increase in noise and finally resulting in a failure of the sensor. Also the ionophore and lipophilic cationic sites in the membrane will slowly leak into the sample solution causing lower seIectivity and sensitivity. Therefore it is very difficult to make sensors based on PVC membranes that show a required longterm stability. Besides these problems, the use of PVC is not very compatible with IC manufacturing methods. Covalent binding of the membrane with the silicon chip is not well possible, neither as patterning of the membrane with photolithographic techniques. Polysiloxane membranes appear to be very promising in sensing devices. Simple polysiloxanes, however, like poly(dimethylsiloxane), are too apolar to be used for ion selective membranes. By introducing polar side chains (e.g. cyanopropyl groups) to the polysiloxane backbone the polarity is enhanced. Recently a reproducible synthesis for well-defined siloxane copolymers was published [5]. Anionic copolymerization of cylotetrasiloxanes monofunctionalized with either methyl, cyanopropyl, or methacroylpropyl resulted in polymers with various amounts of cyanopropyl groups (Scheme 1). 0 Scheme 1. The methacrylate moiety is in principle available for cross-linking of the polymer, but can also be used for anchoring of the membrane to the (surface methcrylated) polyHEMA layer. Via the polyHEMA layer the ion selective membrane is covalently bound to the CHEMFET silicon surface. This enhances the durability of the sensor. It was also shown that ionophores for cations and lipophilic anions can be bound via the methacrylate moieties, leading to a further increase in durability [6]. I. Chart 2. For the development of a durable nitrate selective CHEMFET, the quaternary ammonium compound 2 (Chart 2) was synthesized. This compound was covalently bound in the polysiloxane membrane by photopolymerization. The CHEMFET response of a polysiloxane membrane with 25 % of cyanopropyl moieties and 0.5 wt% 2 is shown in Figure 2. This sensor shows good nitrate selectivity, governed by the Hofmeister series, and is stable upon prolonged exposure to a flow of tap water. ACKNOWLEDGEMENT The research described was carried out with financial support of the Netherlands Technology Foundation (STW). TRANSDUCERS '95 . EUROSENSORS IX The 8th International Conference on Solid-State Sensors and Actuators, and Eurosensors IX. Stockholm, Sweden, June 25-29, 1995 868 220 - PD 6 0 CHEMPET =Po= [mq -50 -100 -150 -200 -5 4 -3 -2 -1 m WOJ Figure 3 .  Nitrate selective CHEMFET with polysiloxane membrane and covalently bound ammonium site. REFERENCES [ l ]  Brunink, J.A.J.; Haak, J.R.;Bomer, J.G.; McKervey, M.A.; Harris, S.J.; Reinhoudt, D N .  Anal. Chim. Acta 254 [2] Brzozka, 2.; Lammerink, B.; Reinhoudt, DN.; Ghidini, E.; Ungaro, R. J. Chem. Soc., Perkin Trans. 2 131 Cobben. P.L.H.M.; Egberink, R.J.M.; Bomer, J.G.; Bergveld, P.; Verboom, W.; Reinhoudt, D N .  J. Am. Chem. Soc. 114 (1992) 10573-10582. [4] Rudkevich, D.M.; Verboom, W.; Brzozka, 2.; Palys, M.J.; Stauthamer, W.P.R.V.; Van Hummel, GJ.; Franken, S.M.; Harkema, S. ;  Engbersen, J.F.J.; Reinhoudt, D N .  J. Am. Chem. Soc. 116 (1994) 4341-4351. [5] Gankema, H.; Lugtenberg, RJ.W.; Engbersen, J.F.J.; Reinhoudt, DN.; Moller, M. Adv. Mater. 6 (1994) 944- 947. [6] Brunink, J.A.J.; Lugtenberg, R.J.W.; Brzozka, Z.; Engbersen, J.F.J.; Reinhoudt, D N .  J. Electroanal. Chem. (1991) 75-80. (1993) 1037-1040. 378 (1994) 185-200. TRANSDUCERS '95 EUROSENSORS IX The 8th International Conference cn Solid-State Sensors and Actuators, and Eurosensors IX. Stockholm, Sweden, June 25-29, 1995 869 

Nitrate and bicarbonate selective CHEMFETs

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Nitrate And Bicarbonate Selective Chemfets

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