Anal. Chem. 1995, 67, 3589-3595
Photocurable Polymer Matrices for Potassium-SensitiveIon-Selective Electrode Membranes Andrey Bratov,t Nataliia Abramova,t Javier MuAoz,* Carlos Dominguez,*l* Salvador Alegret,t and Jodi Bartrolit
Sensors and Biosensors Group, Depattment of Chemistty, Universitat Autonoma de Barcelona, and Centre Nacional de Microelectrbnica, Campus U.A.B., 08 193 Bellaterra, Barcelona, Spain
Photocurable oligomers based on urethane and Bisphenol A (epoxy) &acrylates have been studied as an altemative polymer matrix for ion-selective membranes of ion sensors. The compatibility of the polymers with various plasticizers commonly used in PVC-based ion-selective membrane formulations has been tested, and it has been shown that the urethane-based membranes may hold more than 50%of a plasticizer, while epoxy-based compositions are not MIy compatible with them. Potassiumsensitive electrodes with an inner reference solution and a membrane formed by photocured urethane containing valinomycin as the ionophore have been investigated. The influence of the amount and the type of photoinitiator, cross-linkingagent, and plasticizer on ion response of the ion-selective electrode has been studied. Ion-selective electrodes with an optimized membrane composition show a sensitivity of 57 mV/decade, a range of linear moVL of KCl, and a limit of response from 1 to detection around moVL. The developed polymer matrix was used to make an ISFET-based potassium sensor. Though coated wire electrodes (CWE) in which a polymer incorporating a suitable ionophore is deposited directly onto a metallic conductor have been known for more than a decade,' interest in the development of ion sensors based on polymer membranes with solid-state contacts has renewed considerably in recent year^,^-^ mainly due to advaptages offered by modem microelectronic technology. These advantages include, among others, sensor miniaturization and its integration with microfabricated analytical systems, multisensor array fabrication, on-chip integration with signal processing circuits, and low-cost mass production. Attempts to use poly(viny1 chloride) ( W C ) as a polymer matrix for ion sensors with a solid contact faced certain daculties due to the poor adhesion of this material to a solid substrate surface. Several approaches were made to solve this problem including chemical anchoring of W C membranes containing OH groups
' Universitat Autonoma de Barcelona.
* Centre Nacional de Microelectrbnica. (1) Cattrall, R W.; Hamilton, I. C. Ion-Sei. Electrode Reu. 1984,6,125-172. (2) Cosofret, V. V.; Erdosy, M.; Lindner, E.; Johnson, T. A; Buck, R P. Anal. Lett. 1994,27, 3039-3063. (3) Cadogan, A; Gao, Z.; Lewenstam, A; Ivaska, A Anal. Chem. 1992,64, 2496-2501. (4) Van den Berg, A; Van der Wal, P. D.; Van der Schoot, B. H.; de Rooij, N. F. Sens. Mater. 1994,6,23-43. 0003-2700/95/0367-3589$9.00/0 0 1995 American Chemical Society
to an oxide surface5s6via cross-linking with Sic&. Utilization of photo-cross-linkable7and high molecular weight plasticizerss to improve the adhesion of the WC layers must also be noted. Several other polymer materials including silicon rubber?JOpoly(methyl methacrylate) ,I1 and polyurethane12were tested as possible ion-sensitivemembrane matrices for solid-state ion sensors. Among these, polyurethane seems to be more feasible for the development of sensors for biomedical applications due to its biocompatibility.12 Two main methods compatible with microelectronic technology that can be used for ion-selective membrane deposition and patterning are screen printing and photolithography, the latter being the most promising approach from a technological point of view. In this case, an ion-sensitive membrane (or several membranes of different types) may be formed on an integrated chip by a standard photolithography process. Among possible photocurable polymers that can be applied as ion-selective membrane matrices, acrylate- and methacrylate-based compositi0ns,~~J3 methacrylated siloxane resins,'OJ4 epo~yacrylates,~~J~ and poly~tyrene'~ were studied. It must be noted that most of the investigation done on new polymer matrices for ion sensors was carried out using ISFETslOJ1 or ion-selective electrodes with a solid inner contact.12J8In both cases, poorly defined from a thermodynamic point of view, the (5) Harrison, D. J.; Cunningham, L. L.; Li, X.; Teclemariam, A; Permann, D. 1. Electrochem. SOC.1988,135,2473-2478. (6) Moody, G. J.; Thomas, J. D. R; Slater, M. J. M. Analyst 1988,113,17031707. (7) Harrison, D. J.; Teclemariam, A; Cunningham, L. L. Anal. Chem. 1989, 61,246-251. (8) Igarashi, I.; Ito, T.; Taguchi, T.; Tabata, 0.; Inagaki, H. Sens. Actuaton B 1990,1,8-11. (9) Van der Wal, P. D.; Skowronska-Ptasinska, M.; Van den Berg, A; Sudholter, E.J. R.; Reinhoudt, D. N. Anal. Chim. Acta 1990,231,41-52. (10) Van der Wal, P. D.; Van den Berg, A; de Rooij, N. F. Sens. Actuaton B 1994,18-19,200-207. (11) Moody, G. J.; Slater, M. J. M.; Thomas, J. D. R Analyst 1988,113,103108.
(12)Cha, G. S.; Liu, D.; Meyerhoff, M. E. Anal. Chem. 1991,63,1666-1672. (13) Levichev, S. S.; Bratov, A V.; Vlasov, Yu.G. Sem. Actuaton B 1994,1819,625-628. (14) Reinhoudf D. N.; Engbersen, J.; Brzozka, Z.; Van den Vlekkert, H. H.; Honig, G.; Holterman, H.; Verkerk, U. Anal. Chem. 1994,66,3618-3623. (15) Cardwell, T. J.; Cattrall, R W.; Iles, P. J.; Hamilton, I. C. Anal. Chim.Acta 1985,177, 239-242. (16) Cardwell, T. J.; Cattrall, R W.; Iles, P. J.; Hamilton, I. C. Anal. Chim. Acta 1989,219,135-140. (17) Tietje-Girault, J.; MacInnes, I.; Schroder, M.; Tennat, G.; Girault, H. H. Electrochim. Acta 1990,35,777-783. (18) Cardwell, T. J.; Cattrall, R W.; Iles, P. J.; Hamilton, I. C. Anal. Chim.Acta 1988,204, 329-332.
Analytical Chemistry, Vol. 67,No. 19, October 1, 1995 3589
interface between the polymer membrane and the electronic conductor or insulator (ISFET) does not permit the separation of processes associated solely with the ion-selective membrane. Earlier we reported1gon an ammonium-sensitive ISFET with a photocurable polymer ion-selective membrane. To obtain more detailed information on electrochemical properties of the membranes with different compositions, a conventional ion-selective electrode configuration with liquid inner contact was used in this work. EXPERIMENTAL SECTION
Reagents. Valinomycin, bis(2ethylhexyl) sebacate @OS), bis(2-ethylhexyl) phthalate (DOP), dibutyl phthalate (DBP), di5nonyl adipate (DNA), dioctyl phenylphosphonate (DOPP), 2-nitrophenyl octyl ether (NPOE), Cnitrophenyl phenyl ether (NPPE), tetraundecylbenzhydrol-3,3',4,4'-tetracarboxylate (ETH 2112), potassium tetrakis(pchloropheny1)borate (K-TpClPB), and tetradodecylammonium bromide were purchased from Fluka. Aliphatic urethane diacrylates (oligomers Ebecryl230 andd 270 of molecular weights 5000 and 1500, respectively), epoxy acrylates (oligomers Ebecryl 150 and 600, molecular weight 500), amine functional acrylate (Ebecryl7100), unsaturated copolymerizable benzophenone derivative (Ebecryl P-36), tripropylene glycol diacrylate (TPGDA), and hexanediol diacrylate (HDDA) were donated by UCB Chemicals. Photoinitiators 2,T-dimethoxyphenylacetophenone (Irgacure 651) and 2-hydroxy-Zmethyl-1-phenylpropan-1-one (Darocure 1173) were from Ciba-Geigy. All other chemicals were analytical-reagentgrade. Standard solutions were prepared with deionized water. Preparation of Ion-SelectiveMembranes. The exact compositions of photocurable polymer membranes of potassiumsensitive electrodes studied in this work are presented later in the text. The typical procedure for membrane formation consisted of preparing the main polymer composition by mixing together the acrylated oligomer, reactive diluent (HDDA or TPGDA), and photoinitiator (Irgacure 651 or Darocure 1173). A 0.3 g sample of the main polymer composition was then dissolved in 0.2 mL of tetrahydrofuran, and to this solution plasticizer were added valinomycin and potassium tetrakis(pchloropheny1)borate. The mixture was thoroughly stirred in an ultrasonic bath until homogeneous and then left for several hours to evaporate the solvent. Ion-selective membranes 8 mm in diameter and 0.3-0.4 mm thick were formed by pipeting several drops of the membrane cocktail into a special cylindrical holder. This layer was exposed to UV using standard mask aligner equipment with an irradiance of 22 mW*cm-zat wavelength 365 nm. Typical exposure time was 2 min. After being rinsed with ethanol, the membranes were glued to the end of an acrylate plastic tube (8 mm o.d., 6 mm id.) using the main polymer composition without a plasticizer and with exposure to W for 10 s. Methods of Membrane Electrochemical Characterization. The membranes were characterized in the conventional ionselective electrode USE) configurationwith an inner liquid contact based on a 0.1 mol/L KCl solution. At least three electrodes were prepared to characterize each membrane composition. Chlorinated silver wire served as an inner reference electrode. Doublejunction Ag/AgCl reference electrode (Orion 9@02)was used as an external electrode. To prevent interference due to leaching
of the solution from the salt bridge of the reference electrode, the salt bridge was filled with a 0.1 mol/L solution of lithium acetate. Calibration curves were obtained in the concentration range 10-7-10-1 mol/L in pure solutions of KCI by adding a known amount of prepared stock solutions under stining. The activities of the ions were calculated using the Debye-Huckel approximation.20 The emf values were measured on a Crison micropH 2002 digital potentiometer with f0.1 mV accuracy. All the measurements were carried out at 25 f 1'C in a thermostated laboratory room. Between measurements, the electrodes were mol/L solution of KCI. stored in a To obtain selectivitycoefficients,the separate solution method as well as the mixed solution method were used. In the second method, the concentration of interfering ion remained constant (0.1 mol/L) while the concentration of the primary ion was changed. Electric resistivity of the membranes was evaluated by impedance measurements carried out in 0.1 mol/L KCI solution with a platinum plate as an auxiliary electrode in a 50 kHz -0.01 Hz frequency range using an AutoLab electrochemical system (Eco Chemie B.V., Utrecht, The Netherlands). RESULTS AND DISCUSSION
Photopolymerization of Acrylic Oligomers and Their Compatibility with Plasticizers. Acrylic monomers and oligomers are among the most widely used UV-curable systemsz1 because of their high reactivity and low volatility. A variety of structures with different properties can be obtained, depending upon the prepolymer backbone and the reactive diluent used. There is a large choice of commercially available multifunctional acrylates based on urethanes, Bisphenol A (epoxy) derivatives, polyethers, etc., with high cross-link density which, after exposure to W, give coatings that are insoluble in organic solvents. This property can be used in lithographic applications of these systems. For acrylic compounds, light-induced polymerization that transforms a monomer into a macromolecule by a chain reaction is initiated by radicals formed by dissociation of a photoinitiator under UV irradiation. Because of the high viscosity of the oligomers, reactive diluents must be introduced in the formulation. For these purposes, mono- or multifunctional acrylates are used that provide good application of the resin film to a substrate and at the same time increase the cure speed. The exact structure of the commercial oligomers used in this work is not available, but typical structural formulas of compounds of these classes are presented in Figure 1. For the Bisphenol A derivative, a typical value of n lays between 1 and 2. Urethane diacrylates used in photocurable composition usually have n between 5 and 10, depending on the chain length of the aliphatic substituents R and R'. Two types of reactive diluents used were hexanediol diacrylate and tripropylene glycol diacrylate (Figure 1). As recommended by the manufacturer, Darocure 1173 and Irgacure 651 have been used as photoinitiators for epoxy acrylates and for urethane acrylates, respectively. One of the roles of a polymer matrix of an ion-selective membrane is to hold a large amount (up to 60%)of plasticizer in which the ionophore is dissolved. To test the compatibility of different oligomers with plasticizers most commonly used for ISE fabrication, a 0.3-0.5 mm thick layer of a mixture composed of (20) Meier, P. C. Anal. Chim.Acta 1982,136, 363-368. (21) Chandra, R.; Soni, R K Prog. Polym. Sci. 1994,19,137-169.
3590 Analyfical Chemistty, Vol. 67, No. 79, October 1, 1995
Bisphenol A (epoxy) diacrylate
Aliphatic urethane diacrylate
t
Cy=
0
0
OC?€j-CH,-O-C-CH II
=CH,
0
TriProPYleneglYcol diacryiate (TPGDA) Hexanedlol dlacrylate (HDDA) CHI= CH- C -0-(CHJ,-0 -. C -CH =CH, I1
0
II
0
0 Figure 1. Chemical structures of the oligomers and monomers used for the preparation of photocured polymers. Table 1. Compatibility and PhotopolymerizationAbility of Different Polymer Compositions wlth Plasticizers
no. A
B C
D Eb-7100 (15%)
E
F
polymeld Eb-600 (59%) HDDA (39%) DAR (2%) Eb-150 (59%) HDDA (39%) DAR (2%) Eb-600 (50%) Eb-7100 (10%) Eb-P36 (15%) HDDA (25%) Eb-150 (50% opaque EbP36 (5%) HDDA (30%) Eb-230 (81%) HDDA (17%) IRG (2%) Eb-270 (81%) HDDA (17%) IRG (2%)
dioctyl
phenylphosphonate
diisooctyl sebacate dinonyl adipate
dibutyl
phthalate
NPhPh ether NPhO ether
diisooctyl
no polymer
opaque
phthalate 1 min
1 min
1 min
opaque
transp
2 min white
brittle 1 min white
soft 1min
solid 10 min
transp no adhes 1min
white solid
white
2 min white
solid 1 min opaque solid
porous 1 min
transp soft
1min
opaque solid
porous 2 min
1 min
transp
white
soft
solid
porous
1 min transp
1min transp
transp
soft 2 min
soft 1 min transp soft
transp soft
solid 1 min
1 min
soft
1 min
white porous 1 min transp low adh. 1 min
1 min transp
transp
soft
soft
DAR and IRG are photoinitiators Darocure 1173 and Irgacure 651, respectively.
the main polymer composition and the plasticizer in a 1:l ratio was applied to a glass substrate. After UV exposure to remove the upper tacky layer, the polymers were rinsed with ethanol. Properties of the resulting polymers are presented in Table 1. Formulations A-D are based on epoxy acrylates and E and F on aliphatic urethane diacrylates. Instead of low molecular weight photoinitiators,a copolymerizable photoinitiator (Ebecryl P36)was used in compositions C and D. In order to raise the efficiency of the photoinitiator, amine functional acrylate (Ebecryl 7100) was also introduced into these compositions. Several seconds of exposure to UV was enough to achieve complete polymerization of the main polymer compositions without plasticizer. The introduction of a plasticizer led to longer exposure times necessary to complete polymerization due to dilution of the oligomers and also to additional absorption of UV by the plasticizer. No polymerization occurred when NPOE or NPPE were used as plasticizers. This fact can be attributed to the inhibition of radical polymerization due to the presence of nitro groups.15
From the results in Table 1it follows that epoxy acrylatebased compositionswith most plasticizers except dibutyl phthalate give white or opaque solid films after rinsing with ethanol. We ascribe this factto the inhomogeneity of the polymer-plasticizer composition. During the polymerization, small globules of a plasticizer are formed in the polymer network. Rinsing with ethanol leads to the extraction of the plasticizer from the polymer, thus giving a pore structure. Urethane acrylate-based compositions (E, F) gave soft transparent films with all the plasticizers used except those with a nitro group. Polymer Matrix Permselectivity Measurements. The role of a polymer matrix in ion-selective electrodes is not only a mechanical one, to hold a suitable plasticizer; its chemical properties play an important role in the proper functioning of a sensor. Though a polymer matrix is thought to be inert from the chemical point of it contains different impuritiesz3that can drastically affect the properties of a sensor such as sensitivity, Analytical Chemistry, Vol. 67, No. 19, October 1, 1995
3591
Table 2. Composition. and Properties of Studied ion-Selective Membranes
polymer, a t (%)
cross-linker,
no.
a t (%I
photoinitiator
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Eb270, 52.4 Eb230, 50.6 Eb270, 77.8 Eb230, 77.8 Eb270, 48.1 Eb270, 50.9 Eb270,46.2 Eb270, 53.4 Eb270, 45.7 Eb270,48.2 Eb270, 42.9 Eb270, 30.9 Eb270, 47.8 Eb270, 46.8 Eb270, 49.5 Eb270. 46.9
HDDA, 10.9 HDDA, 10.6 HDDA, 11.3 HDDA, 11.3 HDDA, 10.1 HDDA, 10.6 HDDA, 9.6
1.3 1.3 1.9 1.9 1.1 0.5 0.2 1.6 1.1 1.1 1.0 0.8 1.2 1.2 1.2 1.2
TPGDA, 9.6 HDDA, 10.1 HDDA, 9.0 HDDA, 6.5 HDDA, 10.0 HDDA, 9.8 HDDA, 1040 HDDG 9.8
plasticizer, mat (%)
slope (mVlpaK)
DOS, 35.3 DOP, 37.5 DOS, 38.5 DOS,35.5 DOS, 41.7 DOS, 42.8 DOS, 40.8 TPGDA, 38.5 DOS, 44.8 DOS, 59.2 DNA, 38.4 DOP, 39.4 ETH 2112, 36.0 DOS, 43.0
-log KWN~
47 42 40 40 57.4 54.8 54.9 54.4 57.5
3.2 2.8 2.2 3.1 2.1
57.3 57.8 56.8 56.1 54.8 45.0
3.2 3.1 3.3 2.7 2.0 2.0
All the compositions contained additionally 2%valinomycin and 0.5% K-TpCIPB except for 16, which was prepared without K-TpCIPB.
limit of detection, and selectivity. The latter parameter is the most sensitive one. Impurities in a polymer (or its own functional groups) may form anionic or cationic sites that take part in an ion-exchange process between membrane and solution phases, thus altering the parameters of a sensor. To achieve better selectivity, the polymer matrice of a cation-sensitive membrane must have some anionic sites. The intrinsic selectivity of a polymer is called permselectivityz4and can be determined using ISEs with blank membranes containiig only plasticizer without ionophore. The membranes studied were composed of the main polymer composition based on urethane diacrylate and a plasticizer DOS or DOP (compositions 1 and 2 in Table 2). Ion response was measured in KCl solutions after 2-3 h of electrode preconditioning in a mol/L solution of KCl. As seen from the calibration curve presented in Figure 2, cation permselectivity is characteristic for this type of polymer matrices. One way to estimate the concentrationof ionic impurities is to add to the membrane small amounts of lipophilic anion exchangers, such as tetradodecylammonium bromide, that act as lipophilic counterions of the anionic impurities. Various amounts of TDAB were added to composition 1 before the photopolymerization was carried out, and the response of the electrodes toward the increase of KCl concentration was measured. From the results presented in Figure 2, the concentration of intrinsic anionic impurities in the studied membrane matrix is low enough and the addition of 0.05% of TDAB changes the response from cationic to anionic at high salt concentrations in the solution. Characterizationof Polyurethane Matrices without Plasticizer. It is known that some polymers, such as silicone resinlo may function well as an ion-selective membrane without any additional plasticizer. According to Ruzicka,2jpolyurethane-based membranes with valinomycin also may be used without plasticizer. To test this point, four electrodes based on urethane acrylate (22) Morf, W. De principles of ionselective electrodes and of membrane transport;
Akademiai Kiado: Budapest, 1981. (23) Van den Berg, A; Van der Wal, P. D.; Skowronska-F’tasinska,M.: Sudholter, E. J. R; Reinhoudt, D. N. Anal. Chem. 1987,59,2827-2829. (24) Buck, R. P. Ionselective electrodes in analytical chemistry; Freiser, H., Ed.: Plenum Press: New York, 1978 pp 1-142. (25) Fiedler, U.; Ruzicka, J. Anal. Chim. Acta 1973,67, 179-193.
3592 Analytical Chemistry, Vol. 67, No. 79, October 7, 7995
GW!
i i
i
JY’
1
3
sE W
4
w
-log BK+
5
7
Figure 2. Response curves of ion-selective electrodes with photocured polyurethane membranes plasticized with DOS that contain different amounts of anion exchanger (tetradodecylammonium bromide). The response is measured in KCI solutions.
compositions containii 4% of the ionophore and no plasticizer (compositions 3 and 4 in Table 2) were prepared. It was found that ion response of these membranes is not linear and the sensitivity in a 10-2-10-4 mol/L concentration range of KC1 is low (40-45 mV per concentration decade). Influence of the Polymer Ma& Composition on ISE Properties. Results reported above reveal that the photocured polyurethane may be regarded as a feasible material for ionselective membrane formation. To narrow the possible set of polymer membrane formulations, Eb 270 was chosen for further investigation as having better adhesion to a solid support.1g ISE properties depend strongly on a polymer membrane composition. Taking into consideration that the polymer under investigation is composed of different components, it is necessary to optimize the composition of the membranes. To study the influence of photoinitiator, cross-linker, plasticizer, and lipophilic additives on the properties of the electrodes, different membrane formulations presented in Table 2 were tested.
Photoinitiator. Introduction of a photoinitiator into the mixture of acrylated oligomers and monomers is necessary to start the reaction of radical polymerization. It is well-knownz6that the photoinitiator is not totally consumed in the reaction, so the concentration of this component needs to be optimized in order to attain the best compromise with regard to cure speed and low unreacted residues. Though the photoinitiator concentration in the membranes is not very high, it is comparable with the concentration of ionophore and so may affect the ion-selective properties of the resulting membrane. Characterization of ISE based on compositions 5-7 (Table 2) with the concentration of photoinitiator varying in the range from 0.2 to 1.1%showed that the response slope and the selectivity of potassium-selective membranes with a low amount of photoinitiator are inferior to those with high amounts. Taking also into consideration the properties of the electrodes based on composition 12 with 0.8% photoinitiator, it may be assumed that the optimum concentration range is 0.8-1.2%. The intluence of the concentration of photoinitiator on the polymer structure is not straightforward, but it is generally admittedz7that in the processes of radical polymerization the average chain length of the resulting polymer increases as both the initiator efficiency and concentration decrease. Cross-Linker. Low molecular weight monomers are usually introduced into photocurable polymer compositions to reduce the viscosity of the mixture, but being incorporated into the polymer network they also alter the mechanical and chemical properties of the polymer. Difunctional acrylates give additional cross-linkage to a polymer network, resulting in more rigid films. To test the influence of the cross-link density on the electrode characteristics, the polymer matrix of electrode 8 was prepared without HDDA and so must have a less dense structure. The selectivity of the electrode did not change but the sensitivity was affected and became lower in comparison to electrode 5. Another cross-linking agent that is commonly used in acrylic compositions is TPGDA (Figure l), which is more hydrophilic than HDDA Introduction of TPGDA instead of HDDA into the membrane composition (electrode 9) reduced the selectivity of the sensor. Plasticizer. For PVC-based electrodes, a typical polymer/ plasticizer ratio in membrane formulations is 1:2. Usually, lower plasticizer loading leads to the distortion of electrode characteristics of PVC-based sensors.z8 For a potassium sensor with polyurethane photocured membranes the influence of the amount of plasticizer @OS) in the membrane was studied and it was found that in the range of 34-60% of the plasticizer content (electrodes 5,11, and 12) the parameters of the sensor are not affected. This is in agreement with the most recent data reported on the potassium sensor based on a carboxylated PVC polymer membrane.Z9 Figure 3 shows the chemical response of the sensors with different plasticizers. Substitution of DOS by DNA in potassiumselective membranes resulted in nearly identical electrode behavior including slope, h i t of detection, and selectivity. Introduction of DOP and especially M*H 2112 gives a reduction in the (26) Pappas, S. P. Radiation Curing: Science and Technology;Plenum Press: New York, 1992. (27) Stevens, M. P. Polymer Chemisty, An Introduction, 2nd ed.; Oxford University Press: New York, 1990. (28) Horvai, G.; Graf, E.; Toth, K; Pungor, E. Anal. Chem. 1986, 58, 27352740. (29) Cosofret, V. V.; Erdosy, M.; Buck, R P.; Kao, W. J.; Anderson, J. M.Analyst 1994, 119, 2283-2292.
\
t
-200'
3
1
7
5
-log aK. Figure 3. Response curves of ion-selective electrodes with photocured polyurethane membranes containing different plasticizers. loo
,
I
t
I -200 1
3
5
-log aK+
7
Figure 4. Influence of the addition of 0.5% of KTClPhB into the membrane formulation on the response of the potassium ion-selective electrode.
sensitivity and the selectivity of the sensors. ETH 2112 was designed as a highly lipophilic plasticizer to prevent the leaching of organic phase into the analyte, but unfortunately, its application seriously affects the parameters of ion-selective electrodes.30 K-TpClPB. Addition of mobile cation-exchange sites based on K-TpClPB into the polymer membranes of ISE was shown to be favorable in many producing reduction of interferences by lipophilic sample anions, reduction of response time and electric membrane resistance, and reduction of the activation barrier for the cationexchangereaction at the membrane/solution interface. All the compositions in Table 2 except 16 contained 0.5%K-TpClPB. Data presented in Figure 4 reveal that without lipophilic salt the sensitivity of the sensor is far from ideal. The selectivity is also affected, -log K W N = ~ 2.0, comparing to the value of 3.2-3.3 for the electrodes with K-TpClPB. (30) Huser, M.; Gehrig, P.; Mod, W.;'Simon, W.; Lindner, E. Anal. Chem. 1991, 63.1380-1386.
Analytical Chemistry, Vol. 67, No. 19, October 1, 1995
3593
The results obtained permit us to formulate the optimized membrane composition for the potassium ion sensor with a polymer membrane based on photocurable polyurethane: urethane diacrylate (Eb-270) 30-45%; hexandiol diacrylate 10%; photoinitiator 0.8-1.2% plasticizer @OS or DNA) 35-55% valinomycin 2 %K-TpClPB 0.5%.The properties of the resulting ionselective electrodes may be characterized by the following parameters: sensitivity 57.4 mV/decade; range of linear response from 1 to 10j mol& detection limit 2.5 x mol/L. The selectivity coefficients obtained by the separate solution method and the mixed solution method were nearly identical: log KK” = -3.2, log K ~ L-3.4, log K ~ =H-1.8, log Kwca = -4.2, and log K ~ =M-3.3.~ The impedance spectrum of the membrane measured in a symmetrical cell with a 0.1 mol/L KCl solution forms a semicircle. Intersection of the low-frequency part of the semicircle with the axis of the real part of the impedance gives the value of the membrane resistance, which is equal to 2.6 MB. The parameters of the sensors stored between measurements in a mol/L KCl solution at room temperature were studied in time. After the first month, the changes in the sensitivity and the detection limit were negligible, and after three months, they amounted to 54.3 mV/decade and 1 x 10-j mol/L, respectively. These changes are usually associated with the leaching of plasticizer, ionophore, or both from the membrane. It is necessary to recognize that the selectivitycoefficientsK W N ~ for the obtained membrane are inferior to the best reported for PVC-based membrane ion-selective electrodes with valinomycin as though they have the same order of magnitude as reported for ISE with modifted PVC matri~es.2~ It must be noted that not only cations interfere; the presence of lipophilic anions in solution may also strongly affect ISE r e s p o n ~ e . This ~ ~ , ~type ~ of interference manifests itself as deviations from Nernstian response and even transition into an anionic response at high salt concentrations. For neutral carrier-based ion-selective membranes, the sequence of anions according to their interfering ability correlates with their energy of hydration and follows the well-known Hofmeister series (organic anions > Clod- > SCN- > I- > Nos- > Br- > C1- > Soh2-). The effect of the anions present in the solution on the response of the electrodes is presented in Figure 5. For K+ sensors, anion interference reveals itself as a decline of the response from linearity that starts at concentrationshigher than 4.3 x mol/ L. In the presence of thiocyanate and iodide anions, saturation on the response curve is evident at high salt concentrations but the slope does not become anionic. Results published in the literature24131,32 reveal that anion interference in the case of PVG based potassium-selective electrodes is much higher than for the studied polyurethane membranes. One of the problems that may arise in photocurable polymer membranes is photobleaching of the membrane components upon extended W illumination. it is known that the stability of tetraphenylborate ion is limited, especially in the presence of acids and oxidizing agents and under ill~mination.~~ Decomposition of tetraphenylborate derivatives goes with the consumption of protons, giving as a result neutral products. This means that the concentration of tetrakis (pchloropheny1)borate ion in a membrane (31) Morf, W. E Simon, W. Ionselective electrodes in analytical chemistty; Freiser, H., Ed.; Plenum Press: New York, 1978; Vol. 1, pp 211-286. (32) Mikhelson, K N. Sens. Actuators B 1994,18-19,31-37. (33) Rosatzin, T.; Bakker. E Suzuki. K; Simon, W. Anal. Chim. Acta 1993,280, 197-208.
3594 Analytical Chemistry, Vol. 67, No. 19, October 7, 1995
1
P E
W
w
-250 0
’
’ 1
I 2
3
4
5
6
7
-log aK+ Figure 5. Influence of different anions on the response of the potassium ion-selective electrode with optimized membrane composition.
may depend upon the time of exposure to UV. The extent of anion interference may be used as an indicator for the K-TpCPB content in the membrane. To test the influence of illumination, electrodes with membrane composition 5 were exposed to UV 10 and 20 min. The measurements carried out in KSCN solutions showed that the deviation from linear response expressed as the difference between the experimental value of emf obtained in a 1 mol/L KSCN solution and the value determined by linear extrapolation at the same concentration depends upon the exposure time. For the electrode with membrane 5 polymerized for 2 min, this value is equal to -48 mV. Electrodes illuminated for 10 and 20 min showed deviations of -90 and -101 mV, respectively. For comparison, the same parameter was measured for electrode 16 without K-TpClPB. For this electrode, the slope in the 1-0.1 mol/L concentration range was anionic and the deviation in a 1 mol/L KSCN solution amounted to -145 mV. For thick (300400 pm) polymer layers used for the fabrication of ISE, exposure time lower than 2 min leads to the formation of very soft layers with a large amount of exuded plasticizer. Characterization of K+-SensitiveISFET. The final goal of the development of a photocurable polymer matrix is its application as an ion-sensitive membrane in miniature chemical sensors produced with the help of microelectronic technology. Among the requirements imposed on the properties of the material are good adhesion to a solid substrate and sufficient resolution of the image obtained after exposure through a mask and subsequent development in a suitable solvent. A preliminary studylgshowed that the composition tested in this work satisfies these requirements, thus being suitable for the fabrication of potassiumsensitive ISFETs. Sensors were made using ISFETs with a SiOd TazOs gate insulator, To enhance the adhesion of the polymer, the wafers were preliminarily silylated in a toluene solution of (methacry1oxy)propyltrimethoxysilane. The mixture with the optimized composition for membrane formation was applied to a wafer by spin-coating and exposed to W through the mask for 15 s. After development during several seconds in ethanol, 1030 pm thick membranes were formed over the gate region of
ISFETs on a wafer. After scribing, ISFETs were wire bonded and encapsulated using photocurable resins by a method reported elsewhere.34 The membranes showed excellent adhesion, and no peeling occurred after 5 months of contact with the solution. The sensitivity and the detection limit were found to be the same as for ISE with the corresponding membrane, but the selectivity coefficients were inferior (log K W N=~ -1.8, log KWLi= -2.1, log = -3.5, and log KK/M~ = -3.2). During 2 months of constant contact with the solution, the sensitivity of ISFETs remained unchanged and decreased up to 53 mV/decade after 4 months. Large differences in the selectivity coefficients between the thick membranes of ISE and the thii membranes of ISFET may be attributed to a loss of membrane components during the development process in ethanol. Further investigation is necessary to clarify whether it is possible to solve this problem simply by applying a thicker membrane or whether additional changes in the membrane composition must be introduced. CONCLUSIONS The results reveal that a polyurethane-based photocurable polymer may be regarded as an appropriate alternative matrix for ion-selectivemembranes. It contains a small amount of the anionic impurities that cause intrinsic cationic permselectivity. Being compatible with various plasticizers traditionally used in ionselective membrane formulations, it may be used to make other sensors selective to cations as well as anions. For proper functioning, the membrane with neutral carrier as an ionophore should contain at least 35% of a suitable plasticizer and small amounts of a salt with a highly lipophilic anion. Addition of photoinitiator in a concentration close to that of the ionophore (34) Bratov, A: Muiioz. J.: Dominguez, C.; Bartroli, J. Sens. Actuators B, in press. (35) Moody, G. J.: Saad, B.; Thomas, J. R D. Analyst 1987,112, 1142-1147. (36) Ionophoresfor ionselectiue electrodes and optodes; Fluka Chemie AG: Buchs, 1991.
does not affect the properties of the potassium ion sensor. As in the case of WGbased ion-selective membranes,35 the parameters of the sensor depend upon the slncture of the membrane polymer network. Photocured polymer matrices with low cross-linkdensity based on compositions without low molecular weight crosdinker HDDA or with a low amount of initiator give electrodeswith lower selectivity and/or sensitivity. Analytical parameters, such as sensitivity, concentration range of linear response, and limit of detection of the potassium sensor with the optimized membrane composition, are comparable to those of WC. Though the selectivity coefficients of the polyurethane membrane are higher than those of WGbased ISE, they satisfy the requirements for measurements in Low interference from lipophilic anions and a long lifetime during constant contact with solution are characteristic for the membranes studied. This polymer has excellent adhesion to a silylated oxide suface and being photosensitive can be directly patterned using conventional photolithographic techniques. These features permit the use of highly productive technology for sensor fabrication in which an ion-sensitivelayer is deposited directly on a wafer with several devices on it. Moreover, application of photocurable polymers of the same class for encapsulation of miniature sensors34resolves the problem of compatibility between the encapsulating and the membrane materials. ACKNOWLEDGMENT The authors are grateful to UCB Chemicals for the supply of acrylated oligomers. Financial support from CIIUT, Barcelona, CICYT, Madrid (program BI093-0635), and INTAS, Brussels (Project 93-1418) is also acknowledged. Received for review March 13, 1995. Accepted June 21, 1995.B AC950249C @Abstractpublished in Advance ACS Abstracts, August 1, 1995.
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