Anal. Chem. 2001, 73, 1605-1609
Sol-Gel Modification of pH Electrode Glass Membranes for Sensing Anions and Metal Ions Keiichi Kimura,*,† Setsuko Yajima,† Hironori Takase,‡ Masaaki Yokoyama,‡ and Yoshiaki Sakurai§
Department of Applied Chemistry, Faculty of Systems Engineering, Wakayama University, Sakae-dani 930, Wakayama, Wakayama 640-8510, Japan, Department of Material and Life Science, Graduate School of Engineering, Osaka University, Yamada-oka 2-1, Suita, Osaka 565-0871, Japan, and Technology Research Institute of Osaka Prefecture, 7-1, Ayumino 2-Chome, Izumi, Osaka 594-1157 Japan
To obtain glass membrane electrodes selective for anions and metal ions, pH electrode glass membranes were modified by a sol-gel method using a quaternary ammonium salt and a bis(crown ether). A chloride ionsensing glass membrane was designed, in which a pH electrode glass membrane was modified chemically by an alkoxysilyl quaternary ammonium chloride. X-ray photoelectron spectroscopy confirmed the chemical bonding of the quaternary ammonium moiety to the starting glass surface, which afforded the first example of glass-based “anion”-sensing membranes. A neutral carrier-type sodium ion-selective glass membrane was also fabricated which encapsulates a bis(12-crown-4) derivative in its sol-gel-derived surface. Both sol-gel-modified anion and metal ion-selective glass electrodes exhibited high sensitivity to their ion activity changes. The present sol-gel modification paves the way for designing glass-based ion sensors with tailor-made ion selectivities toward anions as well as cations. Glass membrane electrodes are typical laboratory tools, which are generally employed as pH electrodes due to their excellent H+ selectivity. Although some glass electrodes can be used as ion-selective electrodes for alkali metal ions such as Na+, they are not very selective, especially exhibiting a terrible H+ interference. Alkali-metal ion assay by glass electrodes is feasible only under limited conditions, that is, high-pH conditions. Anion assay by glass electrodes is, of course, impossible at the moment. Glass electrodes are not toxic because of their chemical inactivity and would be therefore applicable to assay of various ions in biological systems if glass electrodes were equipped with their ion selectivities. Considerable attention has been focused on neutral carriertype ion-selective electrodes because of their high selectivities that depend on the molecular design of the neutral carriers.1,2 However, since plasticized poly(vinyl chloride) (PVC) membranes are * Corresponding author: (e-mail)
[email protected]. † Wakayama University. ‡ Osaka University. § Technology Research Institute of Osaka Prefecture. (1) Koryta, J. Ion-Selective Electrodes; Cambridge University Press: Cambridge, U.K., 1974 and references therein. (2) Ammann, D.; Morf, W. E.; Anker, P.; Meier, P. C.; Pretsch, E.; Simon, W. Ion-Sel. Rev. 1983, 5, 3-92 and references therein. 10.1021/ac001434f CCC: $20.00 Published on Web 03/02/2001
© 2001 American Chemical Society
generally applied for the neutral carrier-type ion-sensing membranes,1-3 ion assay by neutral carrier-type electrodes in biological systems, especially in situ assay, is quite difficult due to their high toxicity and low biocompatibility.4-6 To prevent exudation of neutral carriers out from ion-sensing membranes, attempts have been made to immobilize neutral carriers to PVC membranes by covalent bonding.7-9 However, unless the neutral carrier-type PVC membranes are well plasticized, they do not show very good ion sensor property, i.e., nonNernstian response and slow potential response. It is probably because PVC and the chemically modified PVC themselves do not allow mobility enough for the chemically bonded neutral carriers to exchange ions due to their high glass transition temperatures. Covalent bonding of neutral carriers bearing an alkoxysilyl group to silicone rubber is feasible by simple reaction of silicone rubber precursors with alkoxysilylated neutral carriers.10 Another way to immobilize neutral carriers to silicone rubber by covalent bonding may be copolymerization of a vinyl-modified polysiloxane and a neutral carrier vinyl monomers induced by UV irradiation, and the resultant neutral carrier-based membranes were applied to K+- and Na+-sensitive field-effect transistors (ISFET).11-13 Modification of chemically inactive glass membranes with highly ion-selective neutral carrier-type layers might afford neutral carrier-type ion-sensing “glass” membranes that have a potential to realize nontoxic ion sensors for biological systems. Modification of glass membranes with anion-sensing layers might also realize (3) Buhlmann, P.; Pretsch, E.; Bakker, E. Chem. Rev. 1998, 98, 1593-1687 and references therein. (4) Jenny, H.-B.; Riess, C.; Ammann, D.; Magyar, B.; Asper, R.; Simon, W. Mikrochim. Acta 1980, II, 309-315. (5) Kimura, K.; Tsujimura, Y.; Yokoyama, M. Pure Appl. Chem. 1995, 67, 10851089. (6) Espadas-Torre, C.; Meyerhoff, M. E. Anal. Chem. 1995, 67, 3108-3114. (7) Tietje-Girault, J.; MacInnes, I.; Schroder, M.; Tennant, G.; Girault, H. H. Electrochim. Acta 1990, 35, 777-783. (8) Daunert, S.; Bachas, L. G. Anal. Chem. 1990, 62, 1428-1431. (9) Cross, G. G.; Fyles, T. M.; Suresh, V. V. Talanta 1994, 41, 1589-1595. (10) Tsujimura, Y.; Sunagawa, T.; Yokoyama, M.; Kimura, K. Analyst (London) 1996, 121, 1705-1709. (11) Reinhoudt, D. N.; Engbersen, J. F. J.; Brzozka, Z.; v. d. Vlekkert, H. H.; Honig, G. W. N.; Holterman, H. A. J.; Verkerk, U. H. Anal. Chem. 1994, 66, 3618-3623. (12) Brunink, J. A. J.; Lugtenberg, R. J. W.; Brzozka, Z.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Electroanal. Chem. 1994, 378, 185-200. (13) Lugtenberg, R. J. W.; Egberink, R. J. M.; v. d. Berg, A.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Electroanal. Chem. 1998, 452, 69-86.
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“anion-sensitive” glass electrodes. We had already applied solgel-derived membranes containing neutral carriers and quaternary ammonium salts to ISFETs.14,15 This idea induced us to modify glass membranes for pH electrodes with a quaternary ammonium salt and a neutral carrier by a sol-gel method. We here report the sol-gel modification of glass membrane electrodes for anion and metal-ion sensing, their characterization, and their ion sensor properties. EXPERIMENTAL SECTION Chemicals. The starting chemicals for the sol-gel processing, tetraethoxysilane (TEOS), diethoxydimethylsilane (DEDMS), and triethoxychlorosilane, were purchased from Shin-Etsu Silicon Chemicals. Tetradecyldimethyl(3-trimethoxysilylpropyl) ammonium chloride (1) (50% methanol solution) and bis(12-crown-
4-methyl) dodecylmethylmalonate16 (2) and were obtained from Chisso Co. Ltd. and Dojindo Lab., respectively. Sodium chloride and other sodium salts with different counteranions were of analytical reagent grade. Water was deionized. Sol-Gel Procedure. The glass substrate (tube) of the pH electrode (Toko Chemical Lab., type MG101) for sol-gel modification was kindly supplied by the company. The membrane surface of the glass substrate was pretreated by soaking in concentrated HCl at 65 °C for 2 h to promote the formation of silanol groups. For ethoxysilylation of the glass surface, 2 or 3 drops of triethoxychlorosilane was placed on the glass surface, which was then heated by a heat gun for several minutes. TEOS (60 µL, 2.7 × 10-4 mol), DEDMS (112 µL, 6.3 × 10-4 mol), ethanol (148 µL), 0.1 M HCl aqueous solution (46 µL), and an ion-sensing material [29 mg (∼30 wt %) of 1 or 7.4 mg (10 wt %) of 2] were mixed in a sample tube, and the mixture was then allowed to stand at 85 °C for ∼1 h to afford a viscous sol-gel solution. The glass membrane surface was coated by a portion (100-150 µL) of the sol-gel solution and was then allowed to stand at room temperature for 1 day. Thereafter, the modified glass substrate was heated at 65 °C for 1 day for aging. In the case of the sol-gel modification by 1, the coating-drying procedure was repeated once more. The modified glass membranes have a sol-gel-derived layer with about 1-2-mm thickness, unless otherwise stated. X-ray Photoelectron Spectroscopy (XPS). The XPS measurements for surface analysis were carried out by using a Shimadzu ESCA 3300 system with an Mg KR radiation source. The pressure in the vacuum chamber during the measurements (14) Kimura, K.; Sunagawa, T.; Yajima, S.; Miyake, S.; Yokoyama, M. Anal. Chem. 1998, 70, 4309-4313. (15) Kimura, K.; Takase, H.; Yajima, S.; Yokoyama, M. Analyst (London) 1999, 124, 517-520. (16) Shono, T.; Okahara, M.; Ikeda, I.; Kimura, K.; Tamura, H. J. Electroanal. Chem. Interfacial Electrochem. 1982, 132, 99-105.
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was lower than 1.0 × 10-7 Pa. Depth profiles for elements consisting of chemical species involved in the surface film were obtained by using an argon ion sputtering (2 kV) with an etching rate of ∼3 nm min-1. The binding energy was referenced to the C1s line at 285.4 eV. EMF Measurements. Into the sol-gel-modified glass tube was placed 1 × 10-3 M NaCl aqueous solution as the internal solution. A Ag-AgCl internal electrode was then set inside. Conditioning of the electrodes was made by soaking in NaCl solutions of 1 × 10-3 M for more than 6 h. EMF measurements were made at 25 °C using a pH/mV meter (Toko, TP-1000). The external reference electrode was double-junction-type Ag-AgCl electrode with a 3 M KCl internal solution and a 1 M CH3CO2Li external solution. The electrochemical cell was Ag | AgCl | 1 × 10-3 M NaCl | membrane | sample solution || 1 M CH3CO2Li || 3 M KCl | AgCl | Ag. The measuring ion activities were changed by injection of high-concentration solutions to the testing solutions, while stirring with a magnetic stir bar. The EMF readings were made after the potential reached a constant value. The activity coefficients (γ) were calculated according to the Davies equation,17 log γ ) -0.51z2I1/2/(1 + 0.33RI1/2) + 0.1z2I, where z, R, and I stand for electric charge, ionic parameter (3 for K+, 4 for Na+, 3 for Cl-, 6 for Li+, 8 for Mg2+, 6 for Ca2+, 5 for CH3CO2-, 4 for SO42-, 4 for HPO42-, 4 for HCO3-, 3 for F-, 3 for Br-, 3 for NO3-), and ionic strength, respectively. Selectivity coefficients with respect to other ions were determined by a mixed-solution method (FIM). The background ion concentrations for Na+ selectivity were 1 × 10-1 M for Li+, Mg2+, and Ca2+ and 1 × 10-2 M for K+. Those for Cl- selectivity were 1 × 10-2 M for F-, SO42-, CH3CO2-, HCO3-, and HPO42- and 1 × 10-3 M for NO3- and Br-. Response times (t90) were determined on changing the ion activity of sample solution from 1 × 10-3 to 3 × 10-3 M. RESULTS AND DISCUSSION Chemical Modification with Sol-Gel-Derived Layer Containing Quaternary Ammonium Salt. Using sol-gel process, we tried to modify pH electrode glass membranes with an alkoxysilylated quaternary ammonium salt 1 by covalent bonding. The sol-gel process using alkoxysilanes such as TEOS and DEDMS consists of their hydrolysis followed by condensation of the resulted silanol groups. The glass surface, pretreated with hydrochloric acid, has a considerable amount of silanol groups that are expected to participate in the sol-gel reaction. An attempt was first made to modify the bare glass membrane (Scheme 1a) directly by the sol-gel process using a starting solution containing a TEOS/DEDMS(1:2) mixture and 1. However, the resultant solgel-derived membranes came off easily from the original glass membrane. This means that any silanol groups on the bare glass membrane hardly participate in the sol-gel reaction and that the covalent bonding of the quaternary ammonium salt did not proceed very effectively. As shown schematically in Scheme 1, we therefore decided to treat the bare glass membrane (a) with triethoxychlorosilane to incorporate triethoxysilyl groups that can take part in the subsequent sol-gel reaction readily. The solgel reaction on the alkoxylsilylated glass surface (b) proceeded smoothly and the modified glass surface was quite stable without (17) Davies, C. E. J. Chem. Soc. 1938, 1938, 2093-2098.
Scheme 1
Figure 1. Si2s peaks in XPS spectra obtained just after sol-gel processing steps a-c (see Scheme 1).
easy elimination of the sol-gel-derived layer. Similar sol-gel modification was carried out with starting solutions with TEOS/ DEDMS ratios of 1:1, 1:2, and 1:3. The starting sol-gel solutions of 1:1 and 1:2 TEOS/DEDMS afforded hard modified surfaces, but the 1:3 solution of TEOS/DEDMS formed only a sticky modified surface. The sol-gel process on the glass membrane was followed by surface analysis with XPS. Figure 1 shows XPS peaks assigned to Si2s electron for sol-gel steps a-c. From the steps a to c through step b, the Si2s peak shifted to the lower energy. These peak shifts are probably attributed to the formation of Si-O-C bonding in step b and that of Si-C bonding in step c. Two peaks were found in the C1s XPS peaks (the top spectrum in Figure 2). The lower-energy peak is based on C atom-containing contamination, which is always seen in any XPS observation as a reference peak. The higher-energy peak can be assigned to the C atoms for the Si(OEt)3 moiety for step b. Argon ion etching of the modified surface for step b changed the spectrum from the upper one to the lower one in Figure 2. There is only one peak for the contamination at the lower spectrum, although the peak includes some of the peak of the Si(OEt)3 moiety at its base. This clearly indicates that the treatment of the bare glass membrane (a) with
triethoxychlorosilane led to triethoxysilylation of the glass silanol groups. Step b exhibited only peaks assigned to the C1s atoms in the range of binding energy from 150 to 450 eV, as illustrated in the upper spectrum of Figure 3. On the other hand, tiny but clear peaks assigned to N1s and Cl2p electrons were observed in step c (the lower spectrum). Even continuous extraction of the surface of (c) with hot methanol hardly changed the spectrum. Definitely, the quaternary ammonium chloride moiety was bonded covalently to the surface for step b by the sol-gel reaction containing alkoxysilyl derivative 1. Coating of Sol-Gel-Derived Membrane Encapsulating Bis(crown ether). An attempt was also made to coat the pH electrode glass membranes by a sol-gel method using bis(12crown-4) 2, which might afford Na+-selective glass membranes, instead of 1. A sol-gel starting solution of a TEOS/DEDMS (1: 2) mixture and 2 brought about a stable modification of pH electrode glass membranes by the Na+ neutral carrier, although the sol-gel modification is not by covalent bonding but by simple encapsulation, unlike the above-mentioned system of 1. Ion Sensor Property of Sol-Gel-Modified Glass Electrodes. The glass electrodes modified chemically by alkoxysilyl quaternary ammonium chloride 1 surely respond to the activity change of anions. Figure 4a shows a typical potential response for the anion-selective electrodes based on glass membranes modified by a sol-gel starting solution containing a 1:2 TEOS/ DEDMS mixture and 1. The anion-selective glass electrode shows Nernstian response (∼59 mV decade-1) to the Cl- activity changes in a wide activity range of 3 × 10-4 - 1 × 10-1 M. This is the first example for anion-selective electrodes based on glass membranes, to the best of our knowledge. Ideally, a bare glass membrane should be modified by a very thin sol-gel-derived layer, if possible, with a monolayer thickness. The anion electrodes based on glass membranes modified by very thin sol-gel layers, however, did not show Nernstian responses to Cl- activity changes, possibly due to the cation response of the underlying glass surface. Solgel layers thicker than ∼1 mm were required for Nernstian responses toward the anion. This observation implies that the ionic Analytical Chemistry, Vol. 73, No. 7, April 1, 2001
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Figure 2. Changes of C1s peaks in XPS spectra by argon ion etching of step b. The top and bottom spectra were taken before and after the etching, respectively. The black bar denotes C atom-containing contamination.
Figure 3. XPS spectra obtained just after sol-gel processing steps b and c in the binding energy range of 150-450 eV.
equilibrium at the very interface between the sol-gel layer and a sample aqueous phase governs the EMF response of the solgel-modified glass electrodes. The EMF response for the anion electrode that showed a Nernstian response to Cl- activity changes is quite fast, as seen in the response time profile (Figure 5a). The response time (t90) is several seconds, being similar to that for conventional ionselective electrodes. The anion selectivity for the glass electrode of 1 is as shown in Figure 6a. The anion selectivity essentially obeys the Hofmeister series.18 The glass electrodes modified by a sol-gel solution of 1:1 TEOS/DEDMS exhibited a little lower sensitivity (50 mV decade-1 for the Cl- calibration graph slope) than those modified by the sol-gel solution of 1:2 TEOS/DEDMS. The glass electrodes modified by sol-gel-derived membranes encapsulating bis(crown ether) 2 responded to Na+ activity changes as expected. Figure 4b exemplifies the Na+ response for (18) Hofmeister, F. Arch. Exp. Pathol. Pharmakol. 1888, 24, 247-260.
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Figure 4. Typical potential response for Cl--selective (a) and Na+selective (b) glass membrane electrodes obtained by sol-gel modification with 1 and 2.
Figure 5. Time course changes of potential response for Cl-selective (a) and Na+-selective (b) glass membrane electrodes obtained by sol-gel modification with 1 and 2 on changing ion concentrations from 1 × 10-3 to 3 × 10-3 M.
Figure 6. Selectivities for Cl--selective (a) and Na+-selective (b) glass membrane electrodes obtained by sol-gel modification with 1 and 2.
the 2-modified glass electrodes, showing high sensitivity with a Nernstian slope to the Na+ activity change. The 2-based Na+selective electrodes are almost the same in the electrode response time as the 1-based Cl- electrodes (Figure 5b). The Na+ selectivity for the electrodes based on glass membranes modified by 2 is quite different from the conventional glass electrodes that respond to cation activity changes by SiO- residues, as shown in Figure 6b. The ion selectivity for the Na+-selective electrodes is reflected in that for the neutral carrier 2 itself, although the Na+ selectivity against K+ is lower than that for plasticized PVC membrane electrodes based on 2.16 CONCLUSION In this work, the sol-gel modification of pH electrode glass membranes has been successfully made, using an anion exchanger and a cation neutral carrier. The present process has modified pH electrode glass membranes to afford a new type of ion-selective electrodes of which selectivities are reflected in those for the ion carrier employed. The anion-selective electrodes are
the first example for membrane electrodes. The sol-gel modification is a promising process that may realize a variety of ionselective electrodes based on nontoxic and chemically inactive glass membranes. Specifically, in the case that ion carriers can be immobilized powerfully to glass membranes by covalent bonding without any water-soluble components, the present procedure might afford nontoxic ion-sensing membranes for in situ ion sensing in biological systems. ACKNOWLEDGMENT This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture.
Received for review December 6, 2000. Accepted January 25, 2001. AC001434F
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