Membrane Characterization of Anion-Selective CHEMFETs by

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Anal. Chem. 2000, 72, 343-348

Membrane Characterization of Anion-Selective CHEMFETs by Impedance Spectroscopy Martijn M. G. Antonisse,† Bianca H. M. Snellink-Rue 1 l, Ronny J. W. Lugtenberg,‡ Johan F. J. Engbersen, Albert van den Berg, and David N. Reinhoudt*

Department of Supramolecular Chemistry and Technology, MESA Research Institute, University of Twente, P.O. Box 217, 7500 AE Enschede, Netherlands

Impedance spectroscopy can be used to determine the influence of several membrane parameters on the membrane resistance of anion selective CHEMFETs. The concentration of the ammonium sites in the membrane, the anion-receptor complex stoichiometry, and the polarity of the membrane matrix are of particular importance. In general the resistance of polysiloxane membranes is higher than that of PVC membranes. However, in polysiloxane membranes the membrane polarity can be influenced by the type or concentration of polar substituents on the polysiloxane chain. Polysiloxane ion-exchange membranes with 25 mol % of polar sulfone substituents exhibit the same conductance as NPOE plasticized PVC membranes. Remarkably, the membrane resistance of cation-selective polysiloxane membranes is much lower and is much less dependent on the substituents. Impedance spectroscopy is a versatile technique which has been used to study the influence of the various parameters in ionselective membranes (i.e., the membrane matrix, the plasticizer, receptor molecules, and lipophilic counterions) on the electrical characteristics, such as resistance and capacitance. For cationselective electrodes, impedance spectroscopy revealed that the addition of receptor,1 lipophilic borate salts,2 or combinations of lipophilic cations and anions3 results in a decrease of the membrane resistance compared with that of membranes without these components. Leaching of the electroactive species from the membrane induces a time-dependent increase in the resistance,4,5 and this could be related to the durability of the ion-selective membrane.6 The presence of plasticizing solvents in polymeric * Corresponding author: (tel.) 31-53-4892981; (fax) 31-53-4894645; (e-mail) d.n. [email protected]. † Present address: DSM-Resins, Zwolle, The Netherlands. ‡ Present address: OCE, Venlo, The Netherlands. (1) Horvai, G.; Gra´f, E.; To´th, K.; Pungor, E.; Buck, R. P. Anal. Chem. 1986, 58, 2735-2740. (2) (a) Gehrig, P.; Morf, W. E.; Welti, M.; Pretsch, E.; Simon, W. Helv. Chim. Acta 1990, 73, 203-212; (b) Rosatzin, T.; Bakker, E.; Suzuki, K.; Simon, W. Anal. Chim. Acta 1997, 280, 197-208. (3) Ammann, D.; Pretsch, E.; Simon, W.; Lindner, E.; Bezegh, A.; Pungor, E. Anal. Chim. Acta 1985, 171, 119-129. (4) Covington, A. K.; Zhou, D.-M. Electrochim. Acta 1992, 37, 2691-2694. (5) Armstrong, R. D.; Lockhart, J. C.; Todd, M. Electrochim. Acta 1986, 31, 591-594. (6) (a) Diamond, D.; Regan, F. Electroanalysis 1990, 2, 113-117; (b) Lugtenberg, R. J. W.; Egberink, R. J. M.; van den Berg, A.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Electroanal. Chem. 1998, 452, 69-86. 10.1021/ac990721k CCC: $19.00 Published on Web 12/17/1999

© 2000 American Chemical Society

cation-selective membranes reduces the membrane resistance due to the increased mobility of the charge carriers and improved ionpair separation.1,4,7-9 Impedance spectroscopy has been used to investigate novel membrane materials for cation-selective electrodes, e.g., poly(dimethylsiloxane).10-12 Impedance spectroscopy has hardly been used to investigate anion-selective sensors. This is mainly due to the limited number of available potentiometric anion-selective sensors with selectivity that deviates from the well-known Hofmeister series. However, because of the poor solvation of anions in apolar membranes, strong effects on the impedance characteristics of anion-selective membranes can be expected of the membrane matrix and of the presence of anion receptors. In the past five years we have developed a number of anion sensors based on the CHEMFET (chemically modified field effect transistor). These sensors can be made selective for different types of anions (NO3-, NO2-, H2PO4-, F-)13-15 depending on the receptor present in the membrane. Both plasticized PVC and the more durable polysiloxane membranes have been used, and the influence of the membrane matrix and additives on the sensor characteristics has been discussed.16-19 Moreover, the novel H2PO4- and F- selective receptors appeared to bind the anions in a 2:1 receptor-anion stoichiometry.19 This paper describes the electrical characteristics (7) (a) Buck, R. P.; Toth, K.; Graf, E.; Horvai, G.; Pungor, E. J. Electroanal. Chem. 1987, 223, 51-66; (b) Verpoorte, E. M. J.; Harrison, D. J. J. Electroanal. Chem. 1992, 325, 153-166. (8) Armstrong, R. D.; Covington, A. K.; Proud, W. G. J. Electroanal. Chem. 1988, 257, 155-160. (9) To´th, K.; Gra´f, E.; Horvai, G.; Pungor, E.; Buck, R. P. Anal. Chem. 1986, 58, 2741-2744. (10) Lindner, E.; Niegreisz, Zs.; Pungor, E.; Berube, T. R.; Buck, R. P. J. Electroanal. Chem. 1989, 259, 67-80. (11) Van der Wal, P. D.; Sudho¨lter, E. J. R.; Boukamp, H. J. M.; Boumeester, H. J. M.; Reinhoudt, D. N. J. Electroanal. Chem. 1991, 317, 153-168. (12) Oh, B. K.; Kim, C. Y.; Lee, H. J.; Rho, K. L.; Cha, G. S.; Nam, H. Anal. Chem. 1996, 68, 503-508. (13) Malinowska, E.; Oklejas, V.; Hower, R. W.; Brown, R. B.; Meyerhoff, M. E. Sens. Actuators, B 1996, 33, 161-167. (14) Ho ¨gg, G.; Lutze, O.; Cammann, K. Anal. Chim. Acta 1996, 335, 103-109. (15) Antonisse, M. M. G.; Snellink-Rue¨l, B. H. M.; Yigit, I.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Org. Chem. 1997, 62, 9034-9038. (16) Antonisse, M. M. G.; Snellink-Rue¨l, B. H. M.; Engbersen, J. F. J.; Reinhoudt, D. N. Sens. Actuators B 1998, 47, 9-12. (17) Antonisse, M. M. G.; Snellink-Rue¨l, B. H. M.; Ion, A. C.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Chem. Soc., Perkin Trans. 2, in press. (18) Stauthamer, W. P. R. V.; Engbersen, J. F. J.; Verboom, W.; Reinhoudt, D. N. Sens. Actuators, B 1994, 17, 197-201. (19) Antonisse, M. M. G.; Lugtenberg, R. J. W.; Egberink, R. J. M.; Engbersen, J. F. J.; Reinhoudt, D. N. Anal. Chim. Acta 1996, 332, 123-129.

Analytical Chemistry, Vol. 72, No. 2, January 15, 2000 343

Figure 1. Equivalent circuit of a membrane-coated ISFET (MEMFET).

of these anion sensors as studied by impedance spectroscopy. The influence of cationic sites, receptor molecules, and the nature of the membrane matrix are described. Impedance Spectroscopy with CHEMFETs. In the literature, only a few studies of the impedance behavior of ion-selective membranes deposited on top of ISFET devices or semiconductor electrodes have been described.20,21 In these studies, the MEMFET (an ISFET with a physically adhered membrane on top of the gate22) is generally represented by an equivalent electronic circuit as depicted in Figure 1.21 In this circuit, the membrane impedance is represented by the bulk impedance contributions in series with the surface contributions. The membrane bulk impedance is described by a bulk resistance (Rmem) in parallel with a bulk capacitance (Cmem). The interfacial part of the membrane impedance consists of the double-layer capacitance (CDL), the resistance of the charge transfer at the interface (RCT), and a contribution from the diffusion of ions, described by a Warburg impedance (W). When interfacial effects on the membrane impedance are detectable, this is generally at low frequencies (e5 Hz),23 but the rapid exchange of the ions at the interface results in a charge-transfer resistance which is too small to be observed.24 The underlying ISFET contributes to the MEMFET impedance by the silicon-electrode resistance (RSi) in series with a space-charge capacitance (CSC), and the capacitance of the oxide layer (Cox). When the ISFET is operated in inversion, the spacecharge capacitance can be neglected in comparison with the oxidelayer capacitance.25 Since the electrode resistance of the counter electrode and the resistance of the electrolyte solution (Rsol) are small (100 Ω or less) compared with the membrane resistance (105 Ω), this term can also be neglected. In CHEMFETs, an intermediate polyHEMA layer is present between the membrane and the ISFET surface which results in additional terms in the description of the CHEMFET impedance, i.e., similar interfacial contributions as those at the outer membrane-electrolyte inter(20) Antonisse, M. M. G.; Snellink-Rue¨l, B. H. M.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Chem. Soc., Perkin Trans. 2 1998, 773-777. (21) (a) Smith, R. L.; Janata, J.; Huber, R. J. J. Electrochem. Soc. 1980, 127, 1599-1603; (b) Kruise, J.; Ripens, J. G.; Bergveld, P.; Kremer, F. J. B.; Starmans, J. R.; Haak, J.; Feijen, J.; Reinhoudt, D. N. Sens. Actuators, B 1992, 6, 101-105; (c) Friebe, A.; Lisdat, F.; Moritz, W. Sens. Mater. 1993, 5, 65-82. (22) Demoz, A.; Verpoorte, E. M. J.; Harrison, D. J. J. Electroanal. Chem. 1995, 389, 71-78. (23) The nomenclature in this chapter generally follows the IUPAC recommendations 1994, Covington, A. K. Pure Appl. Chem. 1994, 66, 565-569. However, the chemically modified ISFETs will be referred to as CHEMFETs, whereas devices where the membrane is only physically attached to the ISFET are referred to as MEMFETs. (24) Armstrong, R. D.; Horvai, G. Electrochim. Acta 1990, 35, 1-7 and references therein. (25) Lugtenberg, R. J. W. Durable Chemically Modified Field Effect Transistors; Microsensors for Selective Cation Detection. Ph.D. Thesis, University of Twente, The Netherlands, 1997, p 111.

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Figure 2. Measurement setup for CHEMFET transconductance measurements.

face and a double-layer capacitance at the interface between the polyHEMA layer and the ISFET surface.21 However, the interfacial contributions do not affect the impedance characteristics above 5 Hz, whereas the double-layer capacitance is only small compared with the oxide-layer capacitance. From the above-mentioned assumptions it follows that at frequencies g5 Hz, where interfacial processes do not play an important role, the CHEMFET can be described with the simplified equivalent circuit in which the capacitance Cox is in series with a resistance Rmem and the capacitance Cmem is in parallel. The relaxation time, τ, which is the time required to redistribute the charge and to establish electrical profiles in the membrane after application of an electrical field, is therefore determined by these electrical properties of the material. The relaxation time τ can be determined from the frequency dependence of the transconductance (gm ) δID/δVGS) when a sinusoidal signal is applied to the CHEMFET.26 Using the measuring setup depicted in Figure 2, the small variations in the drain current (iD) can be measured as a function of small (sinusoidal) variations in the gate-source voltage (vGS). The output voltage, vout, is related to iD, vGS, and gm as given by eq 1 (R is an electrical resistance in the measurement circuit).

vout ) -RiD ) -RgmvGS

(1)

When a membrane is deposited on top of the gate oxide of the ISFET, the applied voltage vGS will be divided over the polymer and the oxide. Therefore, the effective potential difference between gate and source will be smaller than the applied voltage and the transfer function H(jω) has to be introduced as a multiplication factor of vGS (eq 2). The transfer function, given by eq 3, depends on the electrical properties defined in the simplified equivalent circuit discussed above and on the angular frequency ω of the applied voltage vGS. Consequently, the transfer function can be used to determine the electrical properties of CHEMFETs.

vout ) -RgmH(jω)vGS H(jω) )

1 + jωRmemCmem 1 + jωRmem(Cmem + Cox)

(2) (3)

In Figure 3 the relation between |H(jω)| and the frequency ω is depicted. At the intersections of the linear parts of the curve (26) Bergveld, P.; van den Berg, A.; van der Wal, P. D.; Skowronska-Ptasinska, M.; Sudho ¨lter, E. J. R.; Reinhoudt, D. N. Sens. Actuators, B 1989, 18, 309327.

Figure 3. Theoretical transfer function |H(jω)| of CHEMFETs.

Figure 4. Transconductance measurements of NO3- selective CHEMFETs with o-NPOE plasticized PVC membrane with 0 (a), 0.1 (b), or 1 (c) wt % of TOAB.

the time constants τ1 and τ2 of the circuit, given by eqs 4 and 5, can be determined.27

τ1 ) Rmem(Cmem + Cox)

(4)

τ2 ) RmemCmem

(5)

The time constant τ1 can be used as an indication for the membrane resistance Rmem. Because Cox is normally about 100 times larger than Cmem, and therefore τ1 ≈ RmemCox, an increase in τ1 corresponds to a higher membrane resistance.28 This method was used to determine the effect of the several membrane components on the membrane resistance of anion-selective CHEMFETs. The time constant τ2 is the relaxation time of the polymeric membrane. EXPERIMENTAL SECTION Fabrication of CHEMFETs. The preparation of anion-selective CHEMFETs with PVC13 or polysiloxane17,18 membranes has been described previously. The Na+ selective CHEMFETs were prepared analogously to the F- and NO2- selective CHEMFETs, but calix[4]arene 429 was used as receptor and 50 mol % (relative to the receptor) of potassium tetrakis-(3,5-bis(trifluoromethyl)phenyl)borate (Fluka) was added as the lipophilic counterion. Impedance Measurements. Before measuring the impedance characteristics, the CHEMFETs were conditioned for one night in 0.1 M solutions of the primary ion (sodium salt for anions, and NaCl for Na+ selective CHEMFETs, Fluka, analytical-grade). For the impedance measurements, the same solution as for the conditioning was used. For impedance spectroscopy of CHEMFETs, the CHEMFET was connected to the electrical circuit depicted in Figure 2 with a resistance R of 100 kΩ. Without applying an AC potential, the reference electrode potential, VGS, was adjusted to result in IDS ) 400 µA. The AC potential (vGS ) 0.5 V) with a frequency ω ) 1 Hz - 100 kHz of the internal function generator of the Stanford Research System SR 830 DSP lock-in amplifier was added to VGS. Vout was compared with VAC (27) An alternative way to obtain τ1 is to determine the frequency at which the output potential is half the value obtained at low frequency (|H(jω)| ) 0.5). (28) Van der Wal, P. D.; van den Berg, A.; de Rooij, N. F. Sens. Actuators, B 1994, 18-19, 200-207. (29) Arnaud-Neu, F.; Collins, E.; Deasy, M.; Ferguson, G.; Harris, S. J.; Kaitner, B.; Lough, A. J.; McKervey, M. A.; Marques, E.; Ruhl, B. L.; Schwing-Weill, M. J.; Seward, E. M. J. Am. Chem. Soc. 1989, 111, 8681-8691.

using this lock-in amplifier. The radius signal, normalized at 1 Hz, gives the transfer function |H(jω)|. RESULTS AND DISCUSSION Effect of Cationic Sites and Anion-Selective Receptor Molecules in PVC Membranes. For CHEMFETs with a plasticized PVC polymeric membrane on top of the polyHEMA layer, a frequency dependent transconductance is observed (Figure 4) that shows the general features represented in Figure 3. NPOE plasticized PVC membranes without any additives show a relaxation time τ1 of 7.3 ms. The relaxation time of the membrane bulk (τ2 ) 0.2 ms) is comparable with values reported in the literature for free-standing membranes23 and is due to the presence of anionic (sulfate or sulfonate) impurities present in PVC.30 The addition of lipophilic quaternary ammonium sites, creating an NO3- selective ion-exchange membrane, enlarges the number of charge carriers in the membrane and lowers the membrane resistance reflected by the lower value of τ1 (0.2 ms) of CHEMFETs with PVC/NPOE membranes containing 0.1 wt % of TOAB (tetraoctylammonium bromide). A further increase of the concentration of ionic sites to 1 wt % (the amount commonly used for NO3- selective CHEMFETs) causes a 10-fold reduction in the membrane resistance (τ1 ) 0.03 ms). The presence of anion receptor molecules in the membrane of CHEMFETs lowers the concentration of free anions in the membrane. However, the transconductance measurements of H2PO4- selective CHEMFETs13,14 based on the uranyl salophene receptor 1 depicted in Figure 5, show that the anion complexation has only minor effects on the relaxation time (Table 1). The relaxation time τ1 of the H2PO4- selective CHEMFETs with the uranyl salophene receptor and 20 mol % (0.11 wt %) of TOAB (Figure 5, curve b, τ1 ) 0.3 ms) is slightly higher than that observed for an ion-exchange membrane without ion receptor but with a comparable concentration of 0.1 wt % of TOAB (Figure 4, curve b, τ1 ) 0.2 ms). This is of the same order of magnitude as that observed for cation-selective membranes.31 Like for ionexchange membranes, the transconductance of the H2PO4(30) Van den Berg, A.; van der Wal, P. D.; Skowronska-Ptasinska, M.; Sudho ¨lter, E. J. R.; Reinhoudt, D. N.; Bergveld, P. Anal. Chem. 1987, 59, 2827-2829. (31) (a) Armstrong, R. D.; Nikitas, P. Electrochim. Acta 1985, 30, 1627-1629; (b) Armstrong, R. D.; Ashassi-Sorkhabi, H. Electrochim. Acta 1987, 32, 135-137.

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345

Figure 5. Transconductance measurements of H2PO4- selective CHEMFETs with o-NPOE plasticized PVC membrane with 1 wt % of receptor 1 and 10 (a), 20 (b), 30 (c), 40 (d), or 50 (e) mol % of TOAB. Table 1. Effect of the Relative Amount of TOAB on the Relexation Time τ1 of PVC-NPOE Membranes for NO3-, H2PO4-, F-, or NO2- Selective CHEMFETs (Receptor TOAB, 1, 2, or Co-Porphyrin Respectively) mol % TOABa receptorb TOAB 1 2 Co-porf d

10

20

25

30

40

50

75

0.2 0.1

0.08 0.08

0.08 0.08 0.2

0.2c 0.6 0.4 0.6

100 0.03d

0.3 0.4 0.2

a See text for equivalent wt %. 1.0 wt % TOAB.

b

1 wt % receptor.

c

0.1

0.05

0.10 wt % TOAB.

selective CHEMFETs strongly depends on the concentration of cationic sites in the membrane. Lowering of the concentration of TOAB to 10 mol % (0.055 wt %), with respect to the receptor (Figure 5, curve a), results in an increase of the resistance (τ1 ) 0.6 ms), whereas membranes with increased concentrations of TOAB (40 or 50 mol %, i.e., 0.22 or 0.28 wt %, curves d and e) show a decreased membrane resistance (τ1 ) 0.08 ms). Since receptor 1 forms a 2:1 receptor-H2PO4- complex, at 40-50 mol % the concentration of ammonium sites exceeds the maximum concentration of anions which can be bound by the receptor, and the H2PO4- response and selectivity are lost.19 The membrane starts to function as an ion-exchange membrane, and this results in an increase of the concentration of free anions in the membrane.32 The higher mobility of the free anions compared with the anion-receptor complex results in a lower membrane resistance. In contrast with receptor 1, the NO2- selective Co(III)porphyrin binds as a neutral receptor to NO2- in a 1:1 stoichiometry.33 The different stoichiometry is reflected in the transconductance measurements of CHEMFETs with varying concentrations of TOAB. With 50 mol % (0.23 wt %) of TOAB, the relaxation time τ1 is 0.2 ms, which is 2.5 times larger than that observed for H2PO4- selective CHEMFETs with 50 mol % of TOAB (Table 1). Because of the difference in complex stoichiometry, in membranes with the porphyrin receptor, a large number of receptor sites is (32) Antonisse, M. M. G.; Snellink-Rue¨l, B. H. M.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Org. Chem. 1998, 63, 9776-9781. (33) Antonisse, M. M. G. Anion Recognition and Sensing. Ph.D. Thesis, University of Twente, The Netherlands, 1998.

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still unoccupied at this concentration of cationic sites, whereas in the case of the uranyl salophene receptors these are almost completely converted into complex. Between 25 and 75 mol % (0.11 and 0.34 wt %) of TOAB, a relatively constant value is observed for the relaxation time of the NO2- selective CHEMFETs (0.10.2 ms). At 100 mol % (0.45 wt %) of TOAB, the membrane resistance decreases again (τ1 ) 0.05 ms) due to an increased concentration of free anions. The transconductance measurements with F- selective CHEMFETs13 based on the uranyl salophene derivative with acetamido substituents (receptor 2) show the same trend as that observed for the H2PO4- selective CHEMFETs represented in Figure 5. An increase of the concentration of cationic sites results in a decrease in the relaxation time, indicating a lower membrane resistance (τ1 ) 0.4, 0.4, 0.1. and 0.08 ms for membranes with 10, 20, 30, or 40 mol % (0.045, 0.090, 0.14, 0.18 wt %) of TOAB, respectively, Table 1). Effect of the Membrane Matrix. Ion-pair formation reduces the concentration of free charge carriers and therefore influences the transconductance of the CHEMFET. The polarity of the membrane matrix determines the extent of ion-pair formation. Polysiloxanes are relatively apolar materials and generally result in a high membrane resistance. Recently we have developed polysiloxane membrane materials 3a-e which enable the polarity to be influenced by the type and/or amount of polar substituents attached to the siloxane backbone.17,18 Experiments with ionexchange membranes or ion-selective membranes with anion receptors have previously illustrated the large effect of the substituents on the slope and selectivity of these anion-selective CHEMFETs. Moreover, the substituents strongly influence the impedance of the membrane. Polysiloxane membranes with 10 mol % of the commonly used cyanopropyl moieties as polar substituents (polymer 3a[10]34) have an increase in their membrane resistance (τ1 ) 0.2 s, Figure 6, curve a) compared with those of the plasticized PVC membranes. However, this material has been successfully applied for NO3- selective CHEMFETs that operate with acceptable noise levels (1000 >1000 0.2

2 28 16 0.3

2 28 25 0.4

300 ∼1000 200 0.3

>1000 >1000 >1000 0.5

2b Co-porfc 4d

a 0.4 wt %. b 0.4 wt % receptor and 20 mol % TOAB. c 0.4 wt % receptor and 50 mol % TOAB. d 0.4 wt % receptor and 50 mol % borate.

The relative concentration of polar substituents at the polysiloxane backbone has a large influence on the membrane resistance. Transconductance measurements with polysiloxane membranes that contain 25 mol % of cyanopropylsiloxane units (polymer 3a[25], Figure 6, curve f) show strongly reduced resistances (τ1 ) 1.3 ms, τ2 ) 0.05 ms) compared with those for membranes based on polymer 3a[10] (curve a). Similarly, an increase of the relative number of the phenylsulfonylpropyl substituents from 10 to 25 mol % (polymer 3b[25], curve g) reduces the resistance of the membrane. Because of the polarity and water-binding character of the substituents, the τ1 relaxation time (0.1 ms) is as low as that observed for PVC membranes plasticized with the polar NPOE. This once more emphasizes the importance of the polar substituents in polysiloxane membranes and the role these substituents can fulfill in optimizing the membrane characteristics and polarity.

The solubility of the F- selective uranyl salophene derivative 2 is rather low in polysiloxane membranes with the nonaromatic substituents such as the cyano- and acetylpropyl substituents. Crystallization of the receptor in the membrane of these CHEMFETs is reflected in the increased membrane resistance observed in the transconductance measurements (for CHEMFETs with 3a[10] or 3e[10] τ1 > 1 s, Figure 7 curves a and e). The resistance of membranes based on polysiloxane 3d[10] is somewhat lower than that observed for polysiloxanes with nonaromatic substituents, but the lowest resistance is again observed for polysiloxane membranes with phenylsulfonylpropyl or benzoylaminopropyl substituents (polymers 3b[10] and 3c[10], curves b and c). The higher polarity of these membrane materials might result in better solvation of the anions and, consequently, in less strong complex formation which might account for the better sensor characteristics of F- selective CHEMFETs with membranes based on polysiloxane 3b[10]18 compared with membranes based on polysiloxane 3d[10]. The relaxation time of the F- selective CHEMFETs (τ1 ) 28 ms) with membranes of polysiloxanes 3b[10] and 3c[10] is about a factor of 10 higher than that observed for CHEMFETs with ion-exchange membranes based on the corresponding material and 0.5 wt % of TOAB (Table 2). This is similar to the difference observed between CHEMFETs with PVC membranes, with or without an anion receptor. The NO2- selective Co(III)-porphyrin has a better solubility than the uranyl salophene receptors and therefore the resistance of CHEMFETs with this receptor in membranes based on Analytical Chemistry, Vol. 72, No. 2, January 15, 2000

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Figure 8. Effect of the polar substituents of the polysiloxane membrane material on the transconductance measurements of Na+ selective CHEMFETs. (a) 3a[10]; (b) 3b[10]; (c) 3c[10]; (d) 3d[10]; (e) 3e[10].

polysiloxane 3e[10] is decreased, although the relaxation time is still too high to be determined accurately. Compared to the Fselective CHEMFETs, a slight decrease is observed in the membrane resistance of the NO2- selective membranes with aromatic substituents (τ1 ) 16 ms, 25 ms, and 0.2 s for polysiloxanes 3b[10], 3c[10], and 3d[10], respectively, Table 2), probably due to the higher lipophilicity of the NO2- anion.

As a reference experiment for the anion-selective CHEMFETs with the different polysiloxane membranes, the influence of the polar substituents has been investigated for cation-selective CHEMFETs based on polysiloxane membranes 3a-e with Na+ receptor 4. Remarkably, these transconductance measurements show a much lower membrane resistance than observed for the anion-selective CHEMFET membranes described above (Figure 8). Because of the low membrane resistance, variation of the polar substituent at the polysiloxane chain has almost no effect on the membrane resistance. Only CHEMFETs with siloxane 3e[10], having acetylpropyl substituents, show a slightly larger relaxation time τ1 (0.5 ms, curve e) compared with CHEMFETs with membranes based on polymers 3a[10]-3d[10] (0.2-0.3 ms). The lower membrane resistance for cation-selective CHEMFETs relative to that of the anion-selective membranes might be related to the generally better solvation of cations in organic media

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compared with anions.35 This results in higher free-ion concentrations in the membrane. Furthermore, the relatively high mobility of the lipophilic borate ions in the cation-selective membranes can lower the membrane resistance. Previous studies have shown that tetrakis(3,5-bis(trifluoromethyl)phenyl) borate results in a much lower membrane resistance than other, less lipophilic borate salts.9 In summary, the transconductance measurements reveal several aspects of the impedance characteristics of anion-selective CHEMFETs. A large reduction in the membrane resistance is observed with increasing concentration of ammonium sites in the membrane. Compared with this, the addition of receptors has only a minor effect. However, when the concentration of ammonium sites exceeds the maximum concentration of anion-receptor complex, which is related to the receptor concentration and the complex stoichiometry, a decrease in the membrane resistance is observed. Although the low polarity and, consequently, the relatively high membrane resistance of polysiloxanes is often mentioned as a disadvantage for their application as membrane matrix of ion-selective sensor devices, the measurements show that this resistance can be lowered in different ways. Both by an increase of the percentage of polar substituents or the incorporation of more polar, hydrogen-bonding substituents, the membrane resistance is improved. Remarkably, the membrane resistance of cation-selective CHEMFETs is less dependent on the type of polar substituent of the polysiloxane membrane and is probably due to the lower absolute level of the membrane resistance of the cationselective membranes. This might point to an intrinsic problem in the development of anion-selective membrane sensors. High association constants are required to overcome the high free energy of transfer of the hydrophilic anions from the aqueous environment to the membrane and to obtain selectivity over less hydrophilic anions, e.g. NO3-. This strong complexation results in a low free-anion concentration in the membrane, reflected by the high membrane resistance. For proper functioning of the potentiometric sensor this concentration should remain constant at varying ion concentrations of the sample due to a fixed ratio between free receptor and the anion receptor complex. Because of the high association constants, at higher concentrations anions are extracted into the membrane and relatively lipophilic cations are co-extracted to keep electroneutrality. This disturbs the ionbuffering mechanism in the membrane, and consequently, poor slopes or Donnan exclusion failure will often be observed with anion sensors for highly hydrophilic anions such as H2PO4- or F-. ACKNOWLEDGMENT The Technical Foundation (STW), Technical Science Branch of The Netherlands Organization for Scientific Research (NWO) is gratefully acknowledged for financial support. Received for review June 30, 1999. Accepted October 12, 1999. AC990721K (35) Marcus, Y. Ion solvation; John Wiley: New York, 1985; pp 166-169.