Amperometric Bifunctional Enzyme

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Anal. Chem. 2005, 77, 5063-5067

Fabrication of a Potentiometric/Amperometric Bifunctional Enzyme Microbiosensor K. Ravi Charan Reddy,†,‡ Florin Turcu,† Albert Schulte,† Arvind M. Kayastha,‡ and Wolfgang Schuhmann*,†

Analytical Chemistry, Elektroanalytik & Sensorik, Ruhr-Universita¨t Bochum, Universita¨tsstrasse 150, D-44780 Bochum, Germany, and School of Biotechnology, Faculty of Science, Banaras Hindu University, Varanasi, 221005, India

We report the fabrication and functional characterization of a needle-type bifunctional enzyme microbiosensor that has, as technical novelty, simultaneously integrated a potentiometric and amperometric detection of an enzymecatalyzed reaction at the tip of a pulled glass micropipet. The construction involved immobilizing an enzyme onto the platinized outer tip surface using the precipitation of electrodeposition paint with direct entrapment of the biocomponent in the slowly growing polymer film. Products of enzyme-substrate reaction could then be targeted in a dual-detection mode on one hand with the covered Pt layer at the tip region as amperometric detector and on the other hand with a proton-selective liquid membranebased potentiometric sensor inside the open pipet tip. Completing and testing bifunctional glucose microsensors demonstrated the functionality of the proposed strategy. Synchronized amperometric and potentiometric detection of the addition of a glucose standard to a buffer solution became evident by observing stepwise increases in the amperometric H2O2 oxidation current and corresponding increases in the potential of the pH-selective sensor, which translates to a local pH decrease around the tip due to hydrolysis of enzymatically formed gluconic acid. Taking advantage of the specificity of immobilized enzymes, antibodies, or protein receptor molecules that are kept in direct contact with highly sensitive physicochemical transducers, biosensors1-4 have been proven well capable of detecting their associated targets at low concentrations and as part of multicomponent systems. Motivated by the demand on reliable and accurate biosensors suitable for miniaturization, current research and development in biosensor technology is directed toward optimizing the immobilization of biological recognition elements on * To whom correspondence should be addressed. Tel.: +49-234-3226200. Fax: +49-234-3214683. E-mail: [email protected]. † Ruhr-Universita¨t Bochum. ‡ Banaras Hindu University. (1) Schuhmann, W. Bonsen, E. In Encyclopedia of Electrochemistry, Vol. 3: Instrumentation and Electroanalytical Chemistry; Bard, A. J., Stratmann, M., Unwin, P., Eds.; Wiley-VCH: Weinheim; 2003; pp 350-384. (2) Castillo, J.; Gaspar, S.; Leth, S.; Niculescu, M.; Mortari, A.; Bontidean, I.; Soukharev, V.; Dorneanu, S. A.; Ryabov, A. D.; Cso ¨regi, E. Sens. Actuators, B 2004, 102, 179-194. (3) O’Connel, P. J.; Guilbault, G. G. Anal. Lett. 2001, 34, 1063-1078. (4) Ziegler, C.; Go ¨pel, W. Curr. Opin. Chem. Biol. 1998, 2, 585-591. 10.1021/ac048073e CCC: $30.25 Published on Web 07/06/2005

© 2005 American Chemical Society

defined transducer surfaces5,6 and improving the transducer systems for converting initially occurring molecular recognition events into a quantifiable electrical output signal.7,8 The long-term stability of a sensor and its specificity and selectivity in complex samples and in the presence of interfering species are other properties that have to be improved.9,10 Moreover, it would be advantageous if a sensor offered possibilities for in situ quality tests thus allowing improvement of the confidence in concentration values derived from the sensor signal. The latter is of special importance if the sensor signal will form the input for a closedloop control system such as, for example, the delivery of a related amount of insulin in an artificial pancreas for diabetes control.11 Numerous enzyme biosensors have been developed with integrated amperometric, potentiometric, conductometric, or optical transducers. Normally, either one or the other of the transduction principles is implemented in a specific sensor arrangement. However, integrating a combination of at least two transduction schemes in the same sensor platform and acquisition of their independent analytical signals would potentially increase the gathered information. For example, a dual transducer flow injection biosensor detection system was recently introduced for synchronized amperometric and potentiometric measurements of organophosphorus (OP) neurotoxins.12 Manufactured by means of thin-film microfabrication technology, a silicon-based pHsensitive electrolyte-insulator-semiconductor (EIS) device operated in the constant-capacitance mode for potentiometry and a Au electrode for amperometry were made part of two closely spaced electrochemical thin-layer flow cells. Both sensor surfaces were coated with polymer films entrapping OP hydrolase (OPH), which made them sensitive to alterations in OP level, with no apparent cross reactivity. While the Au electrode displayed distinct signals only for OP substrates (pesticides) liberating electrochemically oxidizable p-nitrophenol, the potentiometric EIS biosensor responded favorably to all OP compounds, reflecting pH changes related to OPH activity. (5) Pal, P. S.; Sarkar, P. J. Indian Chem. Soc. 2002, 79, 211-218. (6) Scouten, W. H.; Luong, J. H. T.; Brown, R. S. Trends Biotechnol. 1995, 13, 178-185. (7) Braguglia, C. M. Chem. Biochem. Eng. Q. 1998, 12, 183-190. (8) Heller, A. Curr. Opin. Biotechnol. 1996, 7, 50-54. (9) Schuhmann, W. Rev. Mol. Biotechnol. 2002, 82, 425-441. (10) Lau, C.; Reiter, S.; Schuhmann, W.; Gru ¨ ndler P. ABC-Anal. Bioanal. Chem. 2004, 379, 255-260. (11) Wilson, G. S.; Hu, Y. Chem. Rev. 2000, 100, 2693-2704. (12) Wang, J.; Krause, R.; Block, K.; Musameh, M.; Mulchandani, A.; Mulchandani, P.; Chen, W.; Scho ¨ning, M. J. Anal. Chim. Acta 2002, 469, 197-203.

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In the present work, we demonstrate the fabrication and characterization of needle-type bifunctional glucose biosensors with integrated amperometric and potentiometric detection schemes simultaneously available at the tapered tips of pulled glass micropipets. The analytical signals with the proposed bifunctional glucose microsensor arise from local reaction of glucose with glucose oxidase (GOx) in one and the same immobilization layer. EXPERIMENTAL SECTION Chemicals and Materials. Triple-distilled water was used for preparing all electrolyte solutions. Unless stated otherwise, chemicals were from Sigma Aldrich Chemie (Deisenhofen, Germany). GOx, type X-S from Aspergillus niger (EC 1.1.3.4) with an activity of 179,000 units/g of solid, was used for the preparation of glucose biosensors. Hydrogen ionophore II-cocktail A (Selectophore) and hexamethyldisilazane (HMDS) were obtained from Fluka (Buchs, Switzerland) and H2PtCl6 was from Merck (Darmstadt, Germany). The cyanide-based Au plating electrolyte (Aurocor K24HF) was kindly provided by Atotech (Berlin, Germany). Ag and Pt wires were purchased from Goodfellow (Bad Nauheim, Germany). Resydrol AY498w/34WA, an anodic electrodeposition paint, was purchased from Vianova Resins (Kastel, Germany). Borosilicate glass capillaries (L ) 100 mm, o.d. ) 1.5 mm, i.d. ) 0.75 mm) were from Hilgenberg (Malsfeld, Germany). Sodium acetate (pH 4), disodium succinate (pH 5), 2-(N-morpholino)ethanesulfonic acid hemisodium salt (pH 6), N-(2-hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid) sodium salt (HEPES, pH 7), tris(hydroxymethyl)aminomethane (pH 8), and sodium tetraborate (pH 10) were used together with NaCl for preparing buffer solutions of adjusted pH values and for calibrating the obtained potentiometric pH microsensors. Fabrication of Platinized Glass Micropipets. With a homemade vertical pipet puller, glass capillaries were drawn to tapered pipets with a tip length and opening of about 10 mm and 20 µm, respectively. Avoiding tip damage, the outside of the pipets was polished on fine emery paper, producing a roughened surface that was preferred for the following metallization owing to improved metal adhesion. As described recently for preparing coaxial Pt/ Ag electrodes,13 electroless plating using Tollen’s reaction was applied for coating the outer surface of abraded pipets with welladhering Ag layers. A stable electrical connection was formed between the Ag coating and a thin copper wire by means of a heat-shrinkable tube of suitable diameter. The thickness of the chemical deposited Ag was then increased by means of galvanic deposition of Ag at room temperature and a current density of ∼0.5 A/dm2 applied in a thiosulfate-based plating solution containing in 40 g/L Ag2SO4, 380 g/L Na2S2O3‚5H2O, 20 g/L Na2B4O7‚ 10H2O, and 20 g/L Na2SO4‚10H2O. Afterwards, a compact Au film was electrodeposited on the Ag coating using the commercial Au plating solution at room temperature and a current density of 0.5 A/dm2. In a potential sweep mode,14 Au-coated pipets were finally platinized through reductive deposition of Pt from an oxygen-free aqueous solution of H2PtCl6 (2 mM) using three potential cycles between + 500 and - 400 mV relative to a Ag/AgCl pseudoreference electrode at slow scan rates of 10 mV s-1. (13) Turcu, F.; Schulte, A.; Schuhmann, W. ABC-Anal. Bioanal. Chem. 2004, 380, 736-741. (14) Schuhmann, W. In Immobilized Biomolecules in Analysis: A Practical Approach; Cass, T., Ligler, F. S., Eds.; Oxford University Press: Oxford, 1998; pp 187-210.

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Immobilization of GOx to Platinized Pipet Tips. A mixture of GOx solution (5 mg mL-1 in water) and the anodic electrodeposition paint (EDP) Resydrol AY498w/34WA (70 µL/mL GOx stock solution) was filled into a small electrochemical cell15 that was equipped with a Pt wire counter electrodes, a Ag/AgCl pseudoreference electrode, and the platinized pipet as working electrode. A slight overpressure was applied to the inner channel of the pipet during the electrochemically induced EDP precipitation in order to prevent the suspension from entering the microcapillary. Following previous protocols,16-18 repeated application of a sequence of potential pulses (2200 mV for 0.2 s, 800 mV for 1 s, and 0 mV for 5 s; 20 times) and the related evolution of protons through anodic splitting of water induced the precipitation of Resydrol at Pt-coated regions with the slowly growing polymer film simultaneously entrapping GOx. After rinsing with water and 1 mM phosphate buffer (pH 7.0), the enzyme-modified pipets were dried in air. Placing H+-Selective Membranes inside GOx/ResydrolModified Pipets. Although slight overpressure was applied in the previous step during EDP application, the orifice of the pipet tip was often blocked with Resydrol. Therefore, the very end of the tip had to be chopped off to reexpose the inner pipet channel, a procedure that was carefully carried out using an inverted microscope and a sharp scalpel. Only tips with smooth transections and clean openings were silanized and filled with ionophore cocktail. For silanization of the inner glass surface with HMDS, the pipet was fixed tip-down and connected from the back to a syringe and HMDS was pumped in and out of the micropipet leading to silanization of the glass walls. Silanized tips were then dipped for short time into the H+-selective liquid membrane solution, Selectophore hydrogen ionophore II-cocktail A, to fill their very end with the ionophore cocktail by capillary forces. Backfilling ionophore-loaded pipets with buffer solution (0.5 M KCl, 10 mM NaCl, 100 mM HEPES, pH 5.5), insertion, and fixation of a Ag/AgCl wire as internal reference electrode completed the potentiometric pH microsensor. Instrumentation. Potentiometric pH measurements were performed at room temperature employing a potentiometer (Wenking model PPT 85, Bank Elektronik, Pohlheim, Germany) in two-electrode configuration with H+-selective pipets as working electrode and a Ag/AgCl/3M KCl reference electrode. Amperometric measurements were obtained in a three-electrode electrochemical cell connected to a model 1000 potentiostat from CH Instruments (Austin, TX). In the dual-detection mode, bifunctional glucose microbiosensors were operated using the above-mentioned potentiometer and a PG 310 potentiostat from HEKA Elektronik (Lamprecht, Germany). The potentiometric and amperometric responses were recorded simultaneously over time using a computer-controlled data acquisition system built with a PCM-DAS 16S/16 analog-to-digital and PCM-DAC 08 digital-toanalog converter card (Plug-In, Eichenau, Germany) and software (15) Habermu ¨ ller, K.; Schuhmann, W. Electroanalysis 1998, 10, 1281-1284. (16) Kurzawa, C.; Hengstenberg, A.; Schuhmann, W. Anal. Chem. 2002, 74, 355-361. (17) Reiter, S.; Ruhlig, D.; Ngounou, B.; Neugebauer, S.; Janiak, S.; Vilkanauskyte, A.; Erichsen, T.; Schuhmann, W. Macromol. Rapid Commun. 2004, 25, 348-354. (18) Ngounou, B.; Neugebauer, S.; Frodl, A.; Reiter, S., Schuhmann, W. Electrochim. Acta 2004, 49, 3855-3863.

Figure 1. (A) Design of a bifunctional needle-type glucose microbiosensor with an amperometric and potentiometric detection of enzyme-catalyzed glucose oxidation available at the tip of a pulled micropipet. (B) Photograph and SEM image of a metallized pipet tip as used as platform for enzyme immobilization and amperometric detection of H2O2. (C) Schematic of the cross section through the tip of a complete glucose microbiosensor showing the multilayer approach for establishing the dual-detection mode.

written in Microsoft Visual Basic 3.0 (Microsoft, Unterschleissheim, Germany). RESULTS AND DISCUSSION Aiming at miniaturized probes for measurements in small volumes or at spatially restricted interfacial test sites, pulled glass micropipets with microscopic tip dimensions were used as precursors for bifunctional enzyme microsensors. Prototypes for tests trials were made with GOx, given that this enzyme is relatively low cost, robust, and well-characterized in biosensors. To establish the desired dual-detection mode, a GOx/polymer coating and an amperometric transducer for H2O2 oxidation were attached one upon the other to the outer surface of the micropipet tip while a potentiometric transducer in the form of an ionophorebased pH sensor was placed inside the opening of the microcapillary. This enabled successfully modified pipets to detect jointly the evolution of protons and H2O2, both of which are products of the reaction of immobilized GOx with its substrate glucose. The design of needle-type bifunctional glucose microbiosensors is shown in Figure 1. A compact layer of Pt was needed on the tips of pulled pipets as a chemically stable, catalytically active interface for the electrooxidative detection of enzymatically generated H2O2. Thus, a simple and reproducible strategy had to be developed for depositing Pt films exclusively on the cylindrical outer surface of the pipet tip. A multilayer deposition of different metal layers was applied starting with the electroless plating of the roughened glass surface with Ag using Tollen’s reagent, followed by an increase of the Ag layer thickness through galvanic Ag deposition. The latter was needed to protect the thin layer of chemically deposited Ag against dissolution in the cyanide-based Au electrolyte. To obtain a pinhole-free metal coating, the Ag layer was further modified with a layer of Au by means of a commercial Au plating solution. Because the oxidation of H2O2 is less favored at smooth Au, a Pt layer was finally electrodeposited (see the photograph and cross section in Figure 1B and C). The intermediate layer of Au was necessary because the electrodeposited Pt adhered much better to Au than Ag. Of course, sputter coating

and chemical vapor deposition of Pt could be alternatives for platinizing the tips of the microcapillaries; however, the multilayer electrodeposition procedure is a much simpler approach not involving expensive vacuum and evaporation equipment. A precise, reproducible, and easy-to-use nonmanual procedure was required for locally immobilizing GOx onto platinized micropipet tips and was offered by the electrodeposition of paint. Originally developed in industry to provide corrosion-protective coatings for domestic metal wares19-21 and later introduced by Schulte22-26 as an easy, nonmanual approach for effectively insulating carbon fiber microelectrodes as well as etched Pt/Ir and W tips for electrochemical scanning tunneling microscopy, an anodic EDP precipitates after discharging the carboxylate functions at the polyacrylate-based resin using H3O+ produced by water oxidation in vicinity of the anode surface. As expected from previous work,16,17 the application of Resydrol, an isobutylacrylate-based anodic EDP, led to the formation of uniform paint layers at the surface of Pt-coated micropipets. Depositions from GOx-containing Resydrol suspensions allowed the entrapment of GOx in the resultant paint coverage and its proper fixation close to the Pt surface. The preparation of bifunctional glucose microsensors included pulling of pipets, applying a multilayer metal coating to their tips, immobilizing enzyme on top of the outer Pt film, loading tip openings with ionophore, and finally backfilling with electrolyte and establishing electrical contact. The greatest challenge faced throughout the multiple steps was not breaking the fragile glass tips, and successful completion of probes required patience and sleight of hand. However, with practice, the procedure finally could be carried out with ∼60% success rate. To evaluate their functional efficiency and temporal stability, the response of Resydrol-GOx/Pt/Au/Ag-modified micropipets to glucose was investigated over a wide range of concentration using amperometry at a working potential of 600 mV versus Ag/AgCl/3 M Cl-. Virtually no drop in the sensitivity toward glucose was observed in calibration plots that were derived from two sets of measurements using the same microsensors, however, with a time interval of 24 h (see Figure 2). The linear detection range could be estimated to extend up to ∼25 mM, with a sensitivity of 1.63 ( 0.19 µA/mM (mean ( sd; n ) 2). The potentiometric pH sensor at the apex of Resydrol-GOx/ Pt/Au/Ag-modified micropipets was established by filling its tip with Selectophore, a H3O+-selective liquid membrane. Completed pH microsensors were calibrated in buffer solutions with the pH ranging from 4 to 10. As expected from the application of an optimized commercial filling solution, their response to pH changes was fast and the equilibrium potential was stable over time. Also, the typical linear dependence of the microelectrode potential on pH (not shown) was observed with an almost Nernstian slope of about -58.9 ( 2.9 mV per decade and the (19) (20) (21) (22) (23)

(24) (25) (26)

Beck, F. Prog. Org. Coat. 1976, 4, 1-60. Beck, F. Electrochim. Acta 1988, 33, 839-850. PCT/US00/16247 BASF, 2001; EP 0256521 B1 BASF, 1991. Schulte, A. Ph.D. Thesis, Westfaelische Wilhelms-Universitaet (WWU) Mu ¨ nster, Mu ¨ nster, Germany, 1993. Bach, C. E.; Nichols, R. J.; Beckmann, W.; Meyer, H.; Schulte, A.; Besenhard, J. O.; Jannakoudakis, P. D. J. Electrochem. Soc. 1993, 140, 1281-1284. Schulte, A.; Chow, R. H. Anal. Chem. 1996, 68, 3054-3058. Schulte, A.; Chow, R. H. Anal. Chem. 1998, 70, 985-990. Schulte, A. SPIE Proc. 1998, 3512, 353-357.

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Figure 2. Glucose calibration curve of a glucose microbiosensor obtained by entrapping glucose oxidase in a thin film of the anodic electrodeposition paint Resydrol that was deposited on the platinized tips of pulled glass micropipets (constant-potential amperometry at 600 mV vs Ag/AgCl/3 N Cl- in 1 mM phosphate buffer pH 7.0).

potential decreasing from 506 ( 6.7 mV at pH 4 to 157.7 ( 9.4 mV at pH 10 (values given as mean ( sd; n ) 3). The results of the calibration experiments demonstrated the independent function of the amperometric and potentiometric assays of the bifunctional microbiosensor. The dual-detection capability, however, had to be confirmed by operating the two transducers simultaneously for glucose measurements and recording the amperometric signal offered by the oxidation of H2O2 along with the pH change caused in front of the tip due to the hydrolysis of enzymatically generated δ-gluconolactone under liberation of protons. Figure 3A shows the result of such a dualdetection experiment. A stepwise increase in the amperometric H2O2 current caused by the addition of multiple aliquots of glucose standard solution and the onset of enzymatic reaction typically was accompanied by corresponding increases in the potential of the pH sensor, which translates to a pH decrease around the tip. The observed match in the curve shapes provided evidence that the bifunctional glucose microsensor indeed followed the GOxinduced conversion of glucose parallel amperometrically and potentiometrically. Displaying the amplitude of the H2O2 currents as a function of the concentration of glucose led to calibration curves (not shown) comparable to the one in Figure 2, with practically the same sensitivity and linear range. A calibration plot of the potential of the pH-selective tip of the glucose microsensor (Etip) against the negative logarithm of the glucose concentration (pG) is shown in Figure 3B. One would have expected a linear relationship between pG and the sensor readout; however, Etip was exponentially increasing with decreasing pG values. This may be due to the influence of a number of parameters that are responsible for the pH shift around the tip of an “active” microsensor. During enzymatic glucose oxidation, a H3O+ gradient will be established in the vicinity of the enzyme-containing polymer matrix, an effect that depends on the turnover rate of the immobilized GOx and the buffering capacity of the buffer through which the protons are diffusing. The deviation from linearity in the plot of Etip versus pG may therefore be related to a complex convolution between the functions of the enzyme and the buffer. This is additionally supported by the fact that in slightly stronger phosphate buffers () 10 mM) the pH-selective tip of bifunctional glucose microsensors did not respond any longer with a change in potential when subjected to the addition of glucose (data not 5066 Analytical Chemistry, Vol. 77, No. 15, August 1, 2005

Figure 3. (A) Simultaneous amperometric (1, at 600 mV vs Ag/ AgCl/3 M Cl-) and potentiometric (2) detection of the addition of aliquots of glucose stock solution (3 M) to a 1 mM phosphate buffer (pH 7.0; 20 mL). Volumes of the stock solutions that were added: (from the left to the right) 10, 20, 30, 50, 100, 200, 500, 1000, and 1000 µL. The amperometric signal offered by the oxidation of enzymegenerated H2O2 was recorded together with the pH value change taking place in front of the tip due to the hydrolysis of the enzymatically generated δ-gluconolactone. (B) Potentiometric glucose calibration curve derived from trace 2 with the potential of the pH-selective tip electrode plotted vs the negative logarithm of the glucose concentration in solution, pG.

shown). Apparently, at higher concentrations the buffer is strong enough to neutralize the released protons before they can interact with the ion-selective membrane and change its surface potential. Nevertheless, the exponential shape of the potentiometric glucose calibration curve could be of advantage for quantifying higher concentrations of glucose since especially at elevated concentrations the rapid rise of the E versus pG plot offered increased sensitivity. Also, the potentiometric sensor can be tuned independently from the amperometric one, for example, by varying the buffer capacity of the electrolyte solution. Thus, although the response of both sensors is related to the same enzymatic reaction layer, the simultaneously acquired information is complementary. In control measurements in the dual-detection mode (see Figure 4), multiple aliquots of H2O2 were added to the buffer before glucose and finally NaOH was injected. Six sequential injections of H2O2 were clearly detected amperometrically but were not visible in the potentiometric trace. In contrast, as expected, the addition of glucose led to simultaneous appearance of an amperometric and potentiometric response from both parts of the bifunctional microbiosensor. Finally, addition of NaOH caused a clear drop in the potential of the pH sensor but no signal at the amperometric sensor. These observations proved unequivocally that the changes in the pH value measured at the potentiometric

the pH-dependent activity of the enzyme. Obviously, the two independent calibration functions can be used for a plausibility check of the dual microsensor status, allowing one to trace variations in the sensor response at an early stage.

Figure 4. Simultaneous amperometric (at 600 mV vs Ag/AgCl/3 M Cl-, trace 1) and potentiometric (trace 2) recordings with a tapered bifunctional glucose microsensor in a 1 mM phosphate buffer solution (20 mL, pH 7.0) to which sequentially six aliquots of 10 mM H2O2 solution (from left to right: 10, 10, 20, 50, 100, 200 µL), 3 M glucose stock solution (500 µL), and 100 mM NaOH solution (50 µL) were added.

tip during glucose injections indeed originate from the liberation of H3O+ ions due to the dissociation of enzymatically produced δ-gluconolactone at the stem of the sensor in the Resydrol-GOxpolymer layer. Due to the inherent differences in the parameters defining the actual sensor signal, the integrated bifunctional microbiosensor can be potentially used for the determination of glucose in complex samples and even in the presence of interfering compounds. For example, direct co-oxidation of, for instance, ascorbic acid would cause an increased current at the amperometric tip without any influence of the potentiometric response. Thus, hints may be obtained on the possibility to use the sensor values for quantification. In addition, the pH value close to the amperometric sensor can be permanently monitored, which may allow prediction about

CONCLUSION A bifunctional needle-type glucose microsensor was presented simultaneously holding a pH-sensitive potentiometric tip and a platinized surface as platform for the addressable immobilization of GOx within an electrodeposition paint and amperometric sensor. Calibration experiments in the dual-detection mode underlined the functionality of the new type of glucose microsensors by providing the amperometric signal offered by the oxidation of enzymatically generated H2O2 at the same time as the potentiometric detection of the pH change in front of the tip. The derived amperometric and potentiometric calibration curves displayed a significant difference in their dependence on the glucose concentration. Currently work is in progress to explore in more detail the opportunities offered by the bifunctional tapered sensor design and to use the dual-detection mode for interference elimination and plausibility checks of the status of glucose sensors. ACKNOWLEDGMENT The European Commission and the Ministry of Education and Research, Germany (BMBF) financially supported part of the work in the framework of the programs “CellSens” (QLK3-2001-00244) and “Nanobiotechnologie” (AZ NBT066), respectively. The authors are grateful to Bertrand Ngounou for helpful discussions concerning the deposition of Resydrol.

Received for review December 30, 2004. Accepted June 2, 2005. AC048073E

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