Anal. Chem. 1997, 69, 4490-4494
Sol-Gel-Derived Gold Composite Electrodes Joseph Wang* and Prasad V. A. Pamidi
Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003
The preparation, characterization, and analytical utility of sol-gel-derived gold composite electrodes is described. The new metal-ceramic electrodes are comprised of gold powder homogeneously dispersed in a modified silica matrix. They couple the favorable electron-transfer kinetics common to gold surfaces with the regeneration, bulk modification, and versatility features of sol-gel-derived composite materials. The voltammetric characteristics of the composite gold-silica electrodes are explored and compared with conventional gold electrodes. Sol-gelderived gold biosensors have been prepared by incorporating an oxidase enzyme within the sol-gel gold solution. Analogous thick-film enzyme strips, based on a new screen-printable gold biogel ink, have also been fabricated. To our knowledge, the above represent the first examples of metal-ceramic sensing electrodes and of bulk modification of metallic working electrodes. Considerable research efforts are being devoted toward the development of chemical sensors and biosensors based on the sol-gel technology.1-3 The sol-gel process involves a lowtemperature production of ceramic materials through the hydrolysis of an alkoxide precursor, followed by condensation and polycondensation of the hydroxylated monomer.4 Such chemistry provides a convenient route for incorporating chemical and biological recognition species relevant to chemical sensing. In particular, because of its low-temperature preparation, the solgel process represents an attractive avenue for the immobilization of biological entities in connection with the development of new biosensors. Lev and co-workers5-7 introduced carbon ceramic electrodes based on the combination of sol-gel-derived organically modified silicate and graphite powder. We have developed biogel-based carbon inks that display compatibility with the screen-printing microfabrication process.8,9 The dispersed carbon thus provides the electrical conductivity essential for electrochemical measurements. Non-silicate materials have also been employed for designing sensors based on sol-gel technology.10 Recently Cox and co-workers reported the development of solid state amperometric sensors for gaseous analytes based on vanadium oxide xerogel-coated microelectrodes.11,12 (1) Dave, B. C.; Dunn, B.; Valentine, J. S.; Zink, J. I. Anal. Chem. 1994, 66, 1120A. (2) Lev, O.; Tsionsky, M.; Rabinovich, L.; Glezer, V.; Sampath, S.; Pankratov, I.; Gun, J. Anal. Chem. 1995, 67, 22A. (3) Ingersoll, C. M.; Bright, F. V. CHEMTECH 1997, 27 (1), 26. (4) Buckley, A.; Greenblatt, M. J. Chem. Educ. 1994, 71, 599. (5) Tsionsky, M.; Gun, G.; Glezer, V.; Lev, O. Anal. Chem. 1994, 66, 1747. (6) Sampath, S.; Lev, O. Anal. Chem. 1996, 68, 2015. (7) Sampath, S.; Lev, O. Electroanalysis 1996, 8, 1112. (8) Wang, J.; Pamidi, P. V. A.; Park, D. S. Anal. Chem. 1996, 68, 2705. (9) Wang, J.; Pamidi, P. V. A. ; Park, D. S. Electroanalysis 1997, 9, 52.
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We report here the preparation, characterization, and analytical utility of a new sol-gel-derived composite material, consisting of gold powder homogeneously dispersed in the modified silica matrix. Such percolation of the gold powder through the porous silicate network provides the desired electrical conductivity. To our knowledge, these are the first metal-ceramic electrodes for electroanalytical applications. Gold-encapsulated sol-gel glass monoliths were used by Zink’s group as substrates in surfaceenhanced Raman spectroscopy.13 Solid composite gold electrodes based on a network of gold particles pervading the Kel-F polymeric matrix were described by Petersen and Tallman.14 While offering high conductivity, Kel-F-gold electrodes are not amenable for the reagent encapsulation due to their high-temperature preparation process. In addition, the new metal-ceramic material offers numerous analytical attributes with respect to surface renewability (by polishing), bulk modification (including doping with bioactive molecules), compatibility with thick-film microfabrication, low cost (compared to the pure gold counterpart), mechanical rigidity, and versatility. Its voltammetric characteristics are similar to those of conventional gold electrodes. These properties and advantages, along with two types of biosensors based on renewable and disposable metal bioceramics, are described in the following sections. EXPERIMENTAL SECTION Apparatus. Voltammetric and amperometric measurements were carried out with Bioanalytical Systems (BAS) voltammetric analyzer CV-27 in connection with a BAS X-Y-t recorder and a 10 mL voltammetric cell. The sol-gel-derived gold composite (3.17 mm diameter) or a commercial gold (BAS, 1mm diameter) working electrode, the Ag/AgCl (in 3M NaCl) reference electrode, and platinum wire counter electrodes were connected through holes in the cell cover. A stirring bar and magnetic stirrer provided convective transport during the amperometric measurements. Surface images and X-ray measurements were obtained at the NMSU Electron Microscopy Laboratory using a Hitachi S-3200N scanning electron microscope in connection with a KevexSigma-3 X-ray analysis system. Reagents. All the chemicals used were of reagent grade unless otherwise mentioned. Methyltrimethoxysilane (MTMOS), tetraethoxysilane (TEOS), tetraethylammonium perchlorate (TEAP), and glucose oxidase (GOx, EC 1.1.3.4, 150 units/mg) were purchased from Fluka. Gold powder (5 µm or less) and gold powder (-65 mesh) were obtained from Alfa AESAR. Potassium (10) Dunuwila, D. D.; Togerson, B. A.; Chang, C. K.; Berglund, K. A. Anal. Chem. 1994, 66, 2739. (11) Cox, J. A.; Alber, K. S.; Tess, M. E.; Cummings, T. E.; Gorski, W. J. Electroanal. Chem. 1995, 396, 485. (12) Cox, J. A.; Alber, K. S. J. Electrochem. Soc. 1996, 143, L126. (13) Akbarian, F.; Dunn, B. S. ; Zink, J. I. J. Raman Spectrosc. 1996, 27, 775. (14) Petersen, S. L.; Tallman, D. E. Anal. Chem. 1990, 62, 459. S0003-2700(97)00680-X CCC: $14.00
© 1997 American Chemical Society
ferrocyanide, ferrocene, and acetonitrile (HPLC grade, Aldrich) and ascorbic acid, acetaminophen, uric acid, dopamine, glucose, and hydrogen peroxide (Sigma) were used as received. All solutions were prepared in doubly distilled deionized water. Ethanol (200 proof) was supplied by Quantum Chemical Co. A phosphate buffer solution (0.05 M, pH 7.4) served as supporting electrolyte. Electrode Preparation. The silica sol was prepared by mixing 1.0 mL of MTMOS, 1.5 mL of ethanol, and 0.05 mL of 11 M hydrochloric acid, followed by a 1 min sonication. Usually, 0.025 mL of the sol was dispersed into 0.160 g of gold powder (usually of 5 µm or less) and mixed thoroughly to ensure uniform composition. The resulting composite contained 94% metal by weight and 6% silica (corresponding to 52.5 and 47.5% by volume, respectively). An active gold area of 0.041 cm2 was estimated by multiplying the geometric area (0.079 cm2) with the metal volume fraction. The silica sol for the enzyme-dispersed electrodes was prepared by mixing 1.5 mL of TEOS, 1.0 mL of water, 1.0 mL of ethanol, and 0.1 mL of 1.0 M HCl. Glucose oxidase (30 mg, dispersed in 0.2 mL water) was added to 0.5 mL of this silical sol. Gold powder (0.15 mg, 5 µm or less) was mixed with 0.05 mL of this sol-gel GOx. The resulting composite was packed tightly into the cavity of a commercial carbon paste electrode (BAS, containing no carbon paste) and was allowed to dry under ambient conditions for 48 h. Newly prepared electrodes were polished on a 400-grit silicon carbide polishing pad (BAS) and further on a very fine grit polishing pad (MF-1043, BAS), to yield shiny surfaces with strong adhesion of gold composite to the Kel-F body of the carbon paste electrode. Between measurements, the solgel gold electrodes or BAS gold electrodes were polished on using coarse diamond polish and were washed with copious amounts of water. Thick-Film Fabrication of Enzyme Electrodes. A sol-gel stock solution was prepared by mixing 1.5 mL of TEOS, 1.0 mL of water, 1.0 mL of ethanol, and 0.1 mL of 1.0 M HCl. After 1 h, a clear solution was obtained. To 0.5 mL of sol-gel stock solution was added 0.03 g of GOx (dispersed in 0.2 mL of water). The sol-gel gold-composite ink was prepared by adding 0.350 mL of the sol-gel enzyme mixture to 0.5 g of gold powder (5 µm or less). The resulting gold paste was screen-printed immediately using a semiautomatic screen printer (Model TF-100, MPM Inc., Franklin, MA). The printing proceeded with freshly made inks since the sol-gel gold ink was dried within 15 min. The ink was printed on alumina ceramic substrate (3.4 × 10 cm) through a patterned stencil to yield 10 strips of 0.15 × 2.85 cm working electrode. The sol-gel gold ink was cured at 4 °C for 3 h. An insulation layer was placed on the center of the gold surface (by using nail polish) leaving 0.5 × 0.15 cm areas on both sides for the working electrode and electrical contact. Procedure. All measurements were performed under ambient conditions using nondeaerated samples. Amperometric measurements were carried out after the decay of transient current. It was necessary to clean the BAS gold electrode prior to each measurement by cycling the potential between -1.5 V and +1.5 V in 1.0 M H2SO4 for 5 cycles followed by cycling in phosphate buffer. RESULTS AND DISCUSSION Microscopy. Figure 1 shows scanning electron micrographs of typical surface regions of the sol-gel-derived gold composite disk (A) and enzyme strip (B) electrodes, obtained with 1000×
Figure 1. Typical electron micrograph images of the surface of solgel gold composites in the form of polished disk (A) and thick-film (B) electrodes. Magnification: 1000× (A) and 3000× (B).
and 3000× magnifications, respectively. The polished disk electrode is characterized with a smooth surface, except for ripples introduced during the polishing step. The surface of the thickfilm strip electrode is characterized by two different gold particles (5 µm gold flakes and 1 µm gold spheres), covered with a thin coating of the sol-gel enzyme solution. These types of gold particles are characteristic of the commercial gold powder used, as was verified separately using SEM and X-ray analyses. Energy-dispersive X-ray analysis (EDX) was employed for chemical characterization of the new composites. The goldceramic disk revealed two gold peaks at 1.60 and 2.07 keV. The silica peak, appearing at 1.70 keV, was relatively weak, as expected for the low silicate content (∼6% w) in the final composite. Estimation of the silica peak area was difficult due to its severe overlap with the gold peaks. Background Currents. Figure 2 compares background cyclic voltammograms at the conventional gold (A) and sol-gel goldceramic (B) electrodes recorded over the -1.0 to +1.5 V range in various aqueous and nonaqueous solutions. Both electrodes display similar potential windows and background profiles characteristic of gold electrodes in these media.15 These include (in the aqueous solutions) small oxide formation peaks and a large solvent decomposition signal during the positive scan and a sharp oxide dissolution peak during the subsequent negative run. An additional cathodic peak, associated with the reduction of dissolved (15) Woods, R. In Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1977; Vol. 9, p 119.
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Figure 3. Scan rate dependence on the cyclic voltammetric response to 1 × 10-3 M ferrocyanide (A) and ferrocene (B) at solgel gold composite electrodes. Electrolyte: phosphate buffer (0.05 M, pH 7.4) (A) and acetonitrile containing 0.08 M TEAP (B). Scan rates: 5 (a), 25 (b), 50 (c), 100 (d), 200 (e), 300 (f), and 400 (g) mV/s. Also shown are plots of anodic peak current vs square root of scan rate. Figure 2. Background cyclic voltammograms of sol-gel gold composite (A) and continuous gold (B) electrodes in different supporting electrolytes: (a) 0.05 M phosphate buffer (pH 7.4); (b) 0.1M H2SO4; (c) 0.1 M NaOH; (d) acetonotrile containing 0.08 M TEAP. Scan rate, 100 mV/s.
oxygen, is observed in the alkaline solution (c). The background current density of the gold-silicate electrode is similar to that of the conventional gold surface (considering the different surface areas and current scales). This is attributed to the use of the hydrophobic MTMOS precursor that prevents creeping of water into the interior of the electrode. A plot of the charging current (at +0.3V) vs the scan rate over the 5-400 mV/s range was used for estimating the double-layer capacitance. A value of 210 µF/ cm2 was estimated from the slope of the resulting linear plot (not shown). The continuous gold electrode yielded a value of 175 µF/cm2. Faradaic Electrochemical Response. Figure 3 shows cyclic voltammograms for ferrocyanide (A) and ferrocene (B), obtained at different scan rates (5-400 mV/s) using aqueous and nonaqueous media, respectively. Defined anodic and cathodic peaks, proportional to the solute concentration, are observed for both couples over the entire range of scan rates. The peak-shaped response indicates conditions of linear diffusion. Such diffusion characteristics are attributed to the relatively low loading of the silicate insulator in the final gold-silicate composite. To ensure well-defined cyclic voltammograms, the conventional gold electrodes require electrochemical pretreatment prior to each measurement. In contrast, the sol-gel counterparts display reversible voltammograms even without any pretreatment. Such an attractive behavior is attributed to greater immunity to electrode fouling associated with the presence of a thin silica coating. Both systems exhibit a nearly reversible response at the slowest (5 mV/s) scan rate, with peak potential separations of 57 (A) and 65 (B) mV. The peak separation increases with the scan rate, up to 75 (A) and 98 (B) mV at 50 mV/s, and 220 (A) and 240 (B) at 400 mV/s. A similar trend and values were observed at the continuous gold electrode (not shown). The insets of Figure 3 display the resulting 4492 Analytical Chemistry, Vol. 69, No. 21, November 1, 1997
plots of peak current vs the square root of the scan rate. The peak current is proportional to the square root of the scan rate, indicating again conditions of linear diffusion. Proportionality to the concentration of both solutes was also observed over the 1 × 10-3-1 × 10-2 M range (not shown). No apparent change in the response was observed following exposure of the electrodes to the organic medium (of Figure 3B) or to aqueous solutions of extreme pH. Figure 4 compares cyclic voltammograms at 50 mV/s for hydrogen peroxide (a), dopamine (b), ascorbic acid (c), uric acid (d), and acetaminophen (e), obtained at the sol-gel gold composite (A) and conventional gold (B) electrodes. The shape of the voltammograms and the overvoltage for all five solutes at the composite and continuous gold electrodes are quite similar. While the voltammograms for dopamine and ascorbic acid are more defined at the metal-ceramic electrode, the hydrogen peroxide one is slightly sharper with the conventional gold surface. The anodic peak potentials for the various solutes at the gold-ceramic electrode [0.65 (a), 0.28 (b), 0.40 (c), 0.55 (d), and 0.55 (e) V] compare favorably with those of the conventional gold electrode [0.50 (a), 0.25 (b), 0.30 (c), 0.57 (d), and 0.60 (e) V]. These and other cyclic voltammetric data indicate that the metal-ceramic surface has favorable electron-transfer kinetics, similar to those of conventional gold electrodes. The size of the gold particles has a significant effect on the physical characteristics of the resulting composite, but little effect upon its electrochemical behavior. For example, by using 250 µm particles, the resulting composite was significantly softer than that prepared from the 5 µm particles. The larger particles also failed off the surface upon repeated polishing. Such softness is attributed to defects in the interparticle packing and/or to binding of the sol-gel. Yet, these composites displayed an electrochemical behavior similar to their smaller particle counterparts, as was indicated from the background and ferrocyanide voltammetric data (not shown). Bulk-Modified Gold-Ceramics. It is possible to dope the metal-ceramic composite with a plethora of chemical or biological components relevant to chemical sensing. Unlike common
Figure 4. Cyclic voltammograms for 1 × 10-3 M H2O2 (a), dopamine (b), ascorbic acid (c), uric acid (d), and acetaminophen (e) at solgel gold composite (A) and conventional gold (B) electrodes. Scan rate, 50 mV/s; supporting electrolyte, 0.05 M phosphate buffer. Dotted lines correspond to the backround response.
Figure 5. Current vs time recording for the successive additions of 1 × 10-3 M glucose at GOx-dispersed sol-gel gold composite electrodes (B). Also shown (A) is the corresponding response of the unmodified metal-ceramic electrode. Applied potential, 0.6 V; stirring rate, 600 rpm; supporting electrolyte, 0.05 M phosphate buffer. The corresponding calibration plot is shown in the inset.
surface-modified gold electrodes, such three-dimensional bulk modification represents the first example of tailoring the interior of metal working electrodes. For example, glucose biosensors can be readily prepared by adding the enzyme GOx to the solgel solution during the hydrolysis step, prior to the dispersion of the gold powder. Such preparation of bioactive metal-ceramics relies on the use of lower amount of acid (with pH of 4.5 for preserving the biocatalytic activity) and on the use of the TEOS precursor (instead of the TMOS one) that offers improved encapsulation of biomolecules due to the slower condensation reaction. Figure 5 shows typical amperometric responses of the conventional (A) and GOx-containing (B) gold-ceramic electrodes held at +0.6 V for successive 1 × 10-3 M additions of glucose. As expected, the unmodified electrode does not respond to these glucose additions. In contrast, the biogel-based gold electrode responds favorably and rapidly to these millimolar additions. The resulting calibration plot (also shown) displays linearity up to 8 × 10-3 M, with a curvature thereafter. The corresponding EadieHofstee plot was highly linear (r2 ) 0.999) and yielded an apparent Km value of 41.5 mM. These data indicate that the sensor response is controlled essentially by enzyme kinetics and that mass transport plays only a minor role in determining the sensitivity. Similar to carbon-ceramic biosensors, the sol-gel-derived gold composite bioelectrode can be easily renewed by mechanical polishing with good repeatability. The bulk of the metal-ceramic electrode thus serves as a “reservoir” for the biological entity. The ability to polish the surface of the biogel-metal electrode and expose a fresh inner reactive layer was demonstrated in measurement of 1 × 10-3 M glucose at five different freshly polished gold-ceramic bioelectrodes. This series yielded reproducible signals with similar sensitivity (0.1015 ( 0.008 µA/mM)
or dynamic properties and a relative standard deviation of 8.7% (not shown). Such data indicate that the enzyme retains its biocatalytic activity upon confinement in the interior of the ceramic-gold composite and that it is uniformly dispersed within this matrix. Thick-Film Gold Composite Bioelectrodes. Sol-gelderived enzyme-containing carbon inks have been recently shown to be compatible with the screen-printing microfabrication process.8 Such coupling of the sol-gel and thick-film technologies offers a one-step fabrication of disposable biocatalytic strip electrodes and addresses the enzyme deactivation problem encountered during the thermal curing of conventional thick-film biosensors. Similarly, it is possible to prepare enzyme-containing metal inks by adding the gold powder to the sol-gel solution. Unlike sol-gel carbon strips that require an addition of a cellulosic binder,8 the gold-ceramic sensors can be fabricated without such binder. This is attributed to the strong affinity of the sol-gel solution to the gold particles, which results in a fine and homogeneous metal dispersion. Electrodes fabricated in this manner are mechanically robust, possess good adhesion to the ceramic substrate, and have smooth edges with minimal defects. Figure 6 displays current-time amperometric (A) and chronoamperometric (B) recordings at the GOx/sol-gel gold strips on 10 successive 1 × 10-3 M additions of glucose. The new thickfilm biogel sensors result in well-defined glucose signals in connection with these different measurement modes. While the amperometric response displays some curvature above 7 × 10-3 M glucose, the chronoamperometric one is linear over the entire range (indicating greater mass transport control, as expected from the use of a quiescent solution). The long term stability was tested over a period of 3 months, using the same strip with intermittent usage (every 3 days) and dry storage at 4 °C. The electrode Analytical Chemistry, Vol. 69, No. 21, November 1, 1997
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between the 5th and 90th day. A slow shrinkage of the sol-gel microstructure may account for the initial decrease in the sensitivity. Conclusions. Gold-ceramic electrodes have been shown to couple the favorable electron-transfer kinetics of gold surfaces, with the regeneration, modification, and versatility features of solgel-derived composite materials. Such combination makes the gold-ceramic composite an attractive electrode material for electroanalytical measurements. Even though the concept of modified metal-ceramics is presented in connection with the enzyme glucose oxidase, it could be readily expanded to other biomaterials and chemical entities applicable to chemical sensing. Such a bulk modification strategy permits the simultaneous incorporation of several modifiers, as needed for producing reagentless devices. The ability to regenerate the modifier layer makes the bulk-modified metal-ceramic electrodes favorable for many practical applications, in comparison to commonly used surface-modified gold electrodes. Other precious metal-ceramic composites should lead to similar improvements. Work in these areas is in progress.
Figure 6. Amperometric (A) and chronoamperometric (B) currenttime recordings for the successive additions of 1 × 10-3 M glucose at the sol-gel gold composite thick-film enzyme electrodes. (a) in part B represents the chronoamperometric recording for the blank solution. Other conditions, as in Figure 5.
response to glucose decreased over the first few days (3-5 days) by 20-25% and remained stable for 3 months with an RSD of 5.5%
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ACKNOWLEDGMENT The financial support of the U.S. Department of Energy (Grant DE-FG07 96ER62306) is gratefully acknowledged. Received for review June 30, 1997. Accepted August 19, 1997.X AC970680X X
Abstract published in Advance ACS Abstracts, October 1, 1997.
