Fabrication and characterization of a miniaturized planar voltammetric sensor array for use in an electronic tongue

Sensors and Actuators B 140 (2009) 532–541

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Fabrication and characterization of a miniaturized planar voltammetric sensor array for use in an electronic tongue K. Twomey ∗ , E. Alvarez de Eulate, J. Alderman, D.W.M. Arrigan Tyndall National Institute, University College, Lee Maltings, Prospect Row, Cork, Ireland

a r t i c l e

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Article history: Received 20 January 2009 Received in revised form 21 May 2009 Accepted 26 May 2009 Available online 6 June 2009 Keywords: Electronic tongue Voltammetric sensor array Cyclic voltammetry Microfabrication Photolithography Electrochemical Adhesion

a b s t r a c t A planar voltammetric sensor array for use in an electronic tongue was fabricated using a combination of microfabrication techniques. The techniques of e-beam evaporation and pulsed laser deposition were applied to prepare a device that contained all of the electrodes integrated on a silicon die (6 mm × 6 mm). The working electrodes were metals gold, platinum, iridium and rhodium. They were characterized by SEM and EDX, and by electrochemical investigation of the packaged dies with cyclic voltammetry in solutions of sulfuric acid and of ferrocene carboxylic acid in aqueous buffer solution. The robustness and reproducibility of the devices were assessed by potential cycling in acid solution. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Electronic tongues are multi-sensor systems, which incorporate low-selectivity sensors and pattern recognition-based signal processing routines [1] for analysis of complex solutions. Many types of sensors have been developed in the past two decades for these systems. The first of these sensors can be attributed to Toko, who in 1990 introduced a sensor that could distinguish between the five basic tastes salt, sweet, sour, bitter and umami [2]. The sensor consisted of eight types of lipid/polymer membranes. Other sensors followed, with potentiometric [3], voltammetric [4–6] and surface acoustic wave array sensors [7] being developed. In general, the emphasis has been on the exploratory use of these sensors in different application areas, but in more recent times the miniaturization of these devices has been investigated. Different techniques can be applied to develop miniaturized sensors arrays. For example, screen-printing is a fast, inexpensive thick-film technique that can be used to develop a range of sensor arrays [8–10]. Thin film techniques such as electron beam (e-beam) evaporation, thermal vacuum deposition, and pulsed laser deposition are also used in sensor array development [11–13]. The present paper describes the miniaturization of a voltammetric sensor array based on a previously developed sensor array of Winquist et al. [6].

∗ Corresponding author. E-mail address: [email protected] (K. Twomey). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.05.031

It consists of an array of different noble metal electrodes, which use voltammetry to probe the solution properties at the metal electrode surfaces. There has been a recent attempt to miniaturize this sensor array [14] through the use of smaller diameter electrodes, which results in an overall reduction in the sensor area size; however, this still relies upon wires being immobilized in epoxy. To the best of our knowledge, there has been no attempt to fabricate an integrated planar version of such a sensor array using appropriate microfabrication techniques. Using such techniques, the reproducibility from sensor to sensor can be controlled exactly, and the production of a large batch of sensors is much quicker when compared to the established method of producing one sensor probe at a time, which is labour-intensive and prone to error. In addition, in microfabrication methods, the metals are deposited as a very thin film to form the required sensing area, in comparison to the alternative of manually positioning wires into the required location and allowing sufficient metal wire to make an electrical connection [5]. Such sensor array fabrication presents a number of challenges. Firstly, a variety of metals must be deposited onto a common substrate. This is not a routine process step in microfabrication and so requires careful investigation and optimization. Secondly, because each metal is deposited consecutively, the process must prevent the degradation or contamination of the previously deposited metal. The aim of the work reported in this paper was to develop a microfabrication strategy for preparation of a multi-metal electrode sensor array. A suitable multi-step process has been developed to deposit gold, platinum, iridium and rhodium on a common sub-

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strate. The techniques of e-beam evaporation and pulsed laser deposition were applied to develop this planar sensor array. The geometry of the device was defined via photolithography masks and stainless steel stencils. The fabrication details and characterization of the subsequent devices are presented in the following sections. 2. Experimental 2.1. Fabrication of sensor array dies The sensor array device consisted of a 6 mm × 6 mm silicon die, which contained a counter electrode (CE) and an array of working electrodes (WE). Five die layouts were examined, as illustrated in Fig. 1. These are photographs of the first deposited layer (Au deposition) which defined the different die types, and are labelled dies 1–5. In order to simplify the fabrication process and reduce the number of steps and materials used, it was decided to use this first Au layer to combine the connection areas for other metal electrodes, the connection tracks and the formation of the Au WE. Die 1 had four 1.1 mm diameter connection areas. One area formed the Au WE, and the remaining deposited areas formed connections to the Pt, Ir and Rh electrodes that were deposited subsequently using appropriate process steps. These connection areas were arranged around a central 2.1 mm diameter connection area, which eventually formed the CE. In this design, the CE area was ca. four times the total WE area. The distance between the centre of each of the WEs to the centre of the CE was 1 mm. The electrodes were connected to the bondpads by 50 ␮m wide metal tracks. The bondpads were 100 ␮m, 300 ␮m or 500 ␮m in width. The wafer processing was carried out at the Central Fabrication Facility at Tyndall National Institute. A Temescal e-beam evaporator was used to carry out the Au and Pt processing, similar to the processing of previously developed devices [15–17]. A combination of photolithography masks and stainless steel stencils was used for patterning. Initially, the wafers had a 5000 Å oxide layer grown on the silicon, and after this step, the wafers were ready for the metal deposition processes. Table 1 details the processing carried out at the wafer level.

Fig. 1. Layout of the various sensor dies developed. (a) Die 1: 1 mm distance from centre of CE to centre of WE, 2.1 mm diameter CE, 1.1 mm diameter WE. (b) Die 2: 0.5 mm diameter from centre of CE to centre of WE, 2.1 mm diameter CE, 1.1 mm diameter WE. (c) Die 3: 0.5 mm diameter from centre of CE to centre of WE, 2.1 mm diameter CE, 1.1 mm diameter WE. (d) Die 4: 1 mm distance between CE and WE, 2.1 mm diameter CE and 0.6 mm diameter WE. (e) Die 5: 1 mm distance between CE and WE, 3.1 mm diameter CE and 1.1 mm diameter WE.

The first four steps of the process, steps 1–4, were applied to wafers 1 and 2. These steps used two photolithography masks, ‘Metal 1’ and ‘Passivation’. Using ‘Metal 1’, a combination of 50 nm chromium and 250 nm Au was deposited in the required pattern

Table 1 The processing steps used in the deposition of the gold, platinum, iridium and rhodium electrodes and gold electrical connections. Process applied

Mask used

Wafer 1

Wafer 2

Wafer 3

Wafer 4

Wafer 5

Wafer 6

Wafer 7

1. Pattern for lift-off lithography

Metal 1: first layer is deposited to form the connection areas connection tracks, bond pads and gold working electrode

×

×

×

×

×

×

×

×

×

×

×

×

×

×

× ×

× ×

× ×

× ×

× ×

× ×

× ×

×

×

× × ×

× × ×

×

×

2. 50 nm Cr/250 nm gold deposition and lift-off 3. Deposition 500nm PECVD nitride 4. Pattern and dry etch 5. Pattern for lift-off lithography

6. Pattern for lift-off lithography

7. Use stencil 1 8. Deposit 250 nm platinum 9. Lift-off lithography 10. Use stencil 2 and Deposit 250 nm iridium by pulsed laser deposition 11. Use stencil 3 and deposit 250 nm rhodium by pulsed laser deposition

Passivation: opens up certain areas in the passivation layer Platinum: deposits the platinum to form the working and counter electrodes Open Sensor: opens up exact dimension areas for the platinum, indium and rhodium electrodes Stencil 1: used to deposit platinum by stencil

Stencil 2: used to deposit iridium by stencil Stencil 3: used to deposit rhodium by stencil

×

×

×

×

× ×

× ×

×

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Fig. 2. Flow diagram of the microfabrication process employed in sensor preparation.

using the lift-off lithography technique. The Cr acted to improve the adhesion of the Au to the oxide layer. The next step was the deposition of a plasma enhanced chemical vapour deposition (PECVD) silicon nitride layer of thickness 500 nm. This coated the entire wafer. The ‘Passivation’ mask was then used to open up (i.e. remove the nitride layer from) certain areas of each die. The electrode on the upper leftmost side of the die, see Fig. 1, was opened up to expose 1 mm diameter of the Au. This electrode formed the Au WE. The remaining electrodes had smaller openings of 0.5 mm diameter. These openings were used as connections to the remaining WEs, which were prepared from different metals, and the counter electrode. Wafers 3–5 underwent additional processing steps to deposit Pt. Pt was deposited on wafer 3 using the photolithography lift-off process, similar to the Au deposition. Using the ‘Platinum’ mask, a 0.5 mm or 1 mm diameter Pt WE and a 2 mm or 3 mm diameter Pt CE was deposited. Wafer 4 underwent Pt deposition using the mask ‘Open Sensor’ and Stencil 1. The role of ‘Open Sensor’ was to open up the exact dimensions for each of the electrodes prior to the Pt deposition patterned with the stencil. Wafer 5, on the other hand, had Pt deposited using only stencil 1. As a result, the Pt on this wafer

was deposited both on the inner Au connection and an outer-ring of silicon nitride passivation. The difference between depositing directly on Au and depositing on Au and nitride was observed from these wafers. If an improvement was seen during comparison of the resulting dies, a similar process step was implemented in the Ir and Rh process; the outcome of this work is discussed later. Wafers 6 and 7 underwent Ir and Rh deposition once the Au and Pt deposition process was finalized. The process flow is shown diagrammatically in Fig. 2. The Ir and Rh electrodes were deposited by Axyntec (Augsburg, Germany). Pulsed laser deposition was used to deposit the electrodes with the appropriate stencils. This allowed a thin layer of target metal to be deposited using a high power pulsed laser beam that was focused on the target. 250 nm of Ir and Rh were deposited using appropriate targets and stencils 2 and 3, as indicated in Table 1. 2.2. Characterization of sensor array dies Scanning electron microscope (SEM) and energy dispersive Xray (EDX) analysis was carried out using an Hitachi S 4000 F SEM.

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information on the behaviour of the electrodes was obtained. Cyclic voltammetry (CV) was used to characterize the die. 3. Results and discussion 3.1. Metal deposition

Fig. 3. Photograph of a packaged die (PCB dimensions 10 mm × 35 mm) attached to a test dipstick using wirebonding technology.

The scotch tape test was used to evaluate the adhesion of the metal deposits. This involved the application of a pressure-sensitive tape to the metal. The adhesion was considered adequate if the metal was not detached when the tape was removed. Electrochemical testing was carried out to determine the functional behaviour of the final device, which integrated the four different metals on a single die. All electrochemical experiments were performed using a CH Instruments 620B computer-controlled potentiostat. A threeelectrode setup was employed with the on-chip WEs and the Pt CE, and a standalone Ag/AgCl reference electrode (CH Instruments). Prior to electrochemical experimentation, the dies were packaged onto printed circuit board (PCB) dipsticks. Each dipstick contained a large bondpad, upon which the die was attached using an electrically conductive adhesive ABLEBOND 84-1LMISR4, which was sourced from Emerson Cuming. This adhesive was cured in a conventional oven at 150 ◦ C for 1 h. Wirebonding technology was used to attach the row of bondpads on the die to the connection pads on the PCB. An epoxy coating was then dispensed around the die and over the wirebonds (Fig. 3), using a glob-top encapsulant material AMICON 50300HT from Emerson Cuming and a Camalot automatic dispensing machine. This coating ensured adequate protection of the brittle wirebonds, and prevented any leakage between the die and PCB when in the test solutions. The epoxy was cured under the same conditions as the die attach adhesive, with additional care taken to reach the maximum temperate slowly to reduce epoxy splatter on the die. All of the packaged dies were exposed to oxygen plasma for 5 min at a power of 70 W. This process cleaned the die surface, and removed any splatters of epoxy from the die surface, which otherwise would affect the electrochemistry. This dipstick with die was dipped into the test solution, and electrochemical

The Au electrodes were successfully prepared using photolithography and the lift-off technique. The images of the die at 10× magnification are shown in Fig. 4a. EDX confirmed the presence of the Au through the dominant Au peak present in the spectrum (Fig. 4b). The preparation of the Pt electrodes using photolithography was also successful (Fig. 4c), as was indicated by the clear, dominant Pt peak in the EDX spectrum (Fig. 4d). The SEM images showed that there was no peeling away at the edges of the Pt deposition from the underneath layers. It should be noted that this deposition did not incorporate a titanium adhesion layer. Ir and Rh electrodes were prepared with the use of stainless steel stencils. Preliminary trials were carried out on wafer 4 and 5 to investigate the best process for stencil use. Deposition of Pt using the ‘Open Sensor’ mask did not result in significant adhesion improvement. The deposition did not seem to differ when directly on Au, or on a combination of Au and silicon nitride passivation. Therefore, the use of this mask was discontinued. The following process was determined to be best for the Ir and Rh deposition. The wafer was firstly diced into rows to allow a closer positioning of the stencil over the wafer, and the wafer row then underwent a plasma clean technique to improve adhesion. Images of the Ir deposition including SEM and EDX analysis are shown in Fig. 5. The SEM showed uneven deposition, and EDX illustrated the dominance of Au in some areas of the die, along with peaks due to Ir presence. Further investigation by electrochemical methods was used to determine if the Au surface was exposed, as this could not be confirmed by EDX. These electrochemical experiments are discussed in Sections 3.2 and 3.3. The deposition of Rh was carried out using the procedure identified above. The wafer was coated and patterned with Ir at this stage, and another stencil was used to deposit the Rh in the correct location of each die. Each wafer row underwent a plasma clean step to improve the adhesion prior to the Rh deposition. Preliminary trials using the scotch tape test for adhesion measurement

Fig. 4. (a) Image of Au WE at 10× magnification. The exposed Au area is 1 mm diameter. (b) EDX of Au WE showing a dominant gold peak. (c) Images of Pt WE. The exposed Pt is 1 mm diameter. (d) EDX of Pt WE showing a dominant Pt peak.

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Fig. 5. Images of the Ir deposition: (a) 10× image, (b) SEM image 350× of electrode surface, (c) EDX of two areas on the electrode showing a dominant Au peak in area 1 and a dominant Ir peak in area 2.

showed that the quality of the Ir deposition was not affected by this subsequent plasma clean. Images of the Rh deposit (at 10× magnification) showed an uneven surface (Fig. 6). SEM and EDX offered further information to support this. The SEM, magnification 400×, showed an uneven surface. EDX showed dominant peaks for both Rh and Au. Therefore, electrochemical testing was again necessary to see if the Au was protruding through the Rh layer and thus possibly interfering with the electrochemical behaviour of the Rh. This testing is discussed in Sections 3.2 and 3.3. A comparison of the metal deposits patterned either by the photolithography masks or the stencils has shown that the resulting deposits varied from mask to stencil. Use of the photolithography masks gave a very exact electrode diameter. The diameter of the 0.5 mm Au WE was measured on three separate dies, and an average diameter of 0.480 mm with a standard deviation of ±0.001 mm was obtained. For the 0.5 mm Pt WE, a measurement of 0.485 (±0.002) mm was obtained, and for the 2 mm Pt CE, 1.950 (±0.010) mm was measured. In general, use of the stencils increased the electrode diameter from 0.5 mm to, on average, 0.645 (±0.008) mm for Ir and 0.687 (±0.010) mm for Rh. This was due

to the slight gap between stencil and wafer, which resulted in an isotropic deposition under the edges of the stencil. 3.2. Electrochemical characterization The electrochemical behaviour of the metal WEs in aqueous 0.1 M H2 SO4 was determined by CV (cyclic voltammetry) at each of the electrodes. Single-cycle CVs are shown in Fig. 7. The behaviour of Au and Pt electrodes in sulfuric acid solution is well documented, and the CVs obtained here were in agreement with the literature [18–22]. The Au electrode exhibited a broad oxidation wave at ca. 1.3 V during the positive potential sweep and a corresponding reduction peak at ca. 0.9 V on the subsequent negative sweep. These were due to Au oxide formation and reduction, respectively. The Pt electrode exhibited an oxidation wave at ca. 0.7 V when the potential was scanned from −0.2 V to 1.2 V, and a reduction peak was observed at 0.4 V during the negative scan, due to Pt oxide formation and reduction, respectively. Peaks for hydrogen adsorption–desorption on Pt were observed between −0.2 V and 0 V. For the Ir electrode, the voltammogram obtained when

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Fig. 6. Images of the Rh deposition: (a) 10× image, (b) SEM image 400×, (c) EDX showing a dominant peak for both Au and Rh.

scanning over the range −0.3 V to 1.3 V is shown. There were no well-defined oxidation or reduction peaks over the first cycle; however during continuous cycling in acidic solution, it was found that a non-reversible oxide layer formed, similar to that described in the

537

literature [23]. This was investigated further, as discussed in Section 3.3, where prolonged cycling is used to determine the stability of the dies. For the Rh electrode, the hydrogen adsorption–desorption peaks were observed in the 0.1 V to −0.3 V range [22], but there were no peaks due to formation/reduction of any rhodium oxides. The response of the WEs in 0.5 mM ferrocene carboxylic acid in phosphate buffered saline (PBS) as a supporting electrolyte, over a range of scan rates, from 10 mV s−1 to 500 mV s−1 , is shown in Fig. 8. The Au electrode showed characteristic curves for each scan rate with an oxidation peak at ca. 0.35 V and a reduction peak at ca. 0.25 V, attributable to oxidation and reduction, respectively, of the ferrocene moiety of this redoxactive compound. Pt showed well-defined redox peaks, with an oxidation peak at ca.0.3 V and a reduction peak at 0.25 V. Rh also had well-defined peaks with oxidation at 0.35 V and reduction at 0.25 V. In contrast, the Ir electrode did not exhibit such well-defined peaks, although slight peaks were seen at similar potentials of 0.2 V and 0.3 V. The oxidation peak currents increased with increasing scan rate. Plots of peak current versus the square root of the scan rate (Fig. 8, insets) exhibited linear relationships for the Au, Pt and Rh electrodes, which are indicative of linear diffusion-controlled processes at these electrodes as described by the Randles-Sevcik equation [24]. The peak-topeak separation over the increasing scan rates averaged 85 mV for Au, 70 mV for Pt and 80 mV for Rh. The values are close to the 60 mV expected for a one-electron electrode reaction, with the slight increases being due to residual uncompensated resistance in the cell. The poor CV behaviour observed at the Ir electrode can be due to oxide coating on the surface, which affects the metallic surface properties and results in a decrease of the conductivity. This behaviour of Ir in acid solutions is discussed in the literature [23,25–29]. This is due to the formation of an oxide via a mechanism of double injection of protons and electrons. During the oxidation stage, electrons flow from the oxide layer to the metal, and a corresponding proton flow occurs from the oxide to the acid solution to maintain electroneutrality of the oxide films. During the reduction stage, electrons flow from the metal to the oxide, and protons flow from the acid solution to the oxide layer, again maintaining electroneutrality inside the oxide film. The sensitivity of the WEs to three different analyte solutions, 0.1 M H2 SO4 , 0.5 mM ferrocene carboxylic acid in PBS, and 1 M HCl is

Fig. 7. Cyclic voltammograms of 0.1 M H2 SO4 at (a) Au, (b) Pt, (c) Ir and (d) Rh. Scan rate: 200 mV s−1 .

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Fig. 8. Cyclic voltammograms of 0.5 mM FeCOOH in PBS: (a) Au, (b) Pt, (c) Ir and (d) Rh. Scan rates: 10–500 mV s−1 . Insets: peak current as a function of square root of the scan rate.

summarized in Fig. 9. The sensitivity plots were obtained from measurements of the oxidation current from each electrode in the array when immersed in each of these solutions. Three different patterns were obtained using the sensor array, which highlights the ability of the device to distinguish between these solutions. The behaviour of the WEs in the first two solutions was already described above. In the HCl solution, both Au and Pt exhibited dominant oxidation peaks, which can be seen in the relatively large current magnitudes of 400 ␮A and 600 ␮A, attributed to formation of the metal oxides [30,31]. In comparison, the Ir and Rh exhibited minimal oxidation behaviour [26,32], with correspondingly smaller current magnitudes at these electrodes (<50 ␮A). The behaviour of the device in more complex multi-analyte solutions will be assessed in future work. Based on the above electrochemical data, it can be stated that the electrochemical behaviour of the four WE metal electrodes was in accord with literature data, and in the case of the Pt, Ir and Rh electrodes, there was no electrochemical interference from the Au layer underneath each of these metals. This was deduced from observation of the CVs obtained at the Pt, Ir and Rh in solutions of sulfuric acid, ferrocene carboxylic acid and hydrochloric acid. In the case of each electrode, no redox peak that could be attributed to the presence of Au (i.e. Au oxide formation and reduction) was observed for the Pt, Ir and Rh electrodes. This supported the conclusion that the Au layer beneath each of these metals was not exposed to the electrolyte solution. 3.3. Long-term stability As a further investigation, the long-term adhesion of the metal layers on the devices was examined. Each of the electrodes was

continuously cycled in 0.5 M H2 SO4 at a scan rate of 2 V s−1 . Despite being subjected to 300 cycles, the influence of the Au did not appear in the voltammograms for Pt, Ir and Rh (Fig. 10). The voltammograms did not change very much between the 1st and 300th cycles, with the exception of the Ir electrode. The Ir electrode was seen to form an oxide coating during the continuous cycling in acid solutions (Fig. 9c). This oxide formation was irreversible, i.e. it did not get reduced to the metal during the negative potential sweep. Thus, the oxide layer thickness grew on subsequent cycles. This can be seen from the CV plots for Ir showing the 1st and 300th cycle. Furthermore, the surface of the Ir turned a blue/black colour. During the positive potential sweep the iridium oxide started to change from a dark grey metallic colour to a blue colour; this blue colour became increasingly darker as the potential increased to a maximum of 1.3 V. A description of this colour change can be sourced from the literature [28], and it has been attributed to the double proton–electron injection mechanism [29] described in Section 3.2. Overall, the observed electrochemical response of the electrodes during the continuous cycling in acid solution was a good indicator of the robustness of the device. Besides the oxide formation on Ir, no degradation or peeling-off of the other metallic layers was evident, indicating their good stability and suitability for repeated measurements. 3.4. Reproducibility The reproducibility of the different electrodes from four sensor arrays was investigated. Both the adhesion after continuous cycling in acid solution, and the variation in electrochemical response was observed. The adhesion of the sensor arrays was tested from the same batch by continuous cycling of each electrode in sulfuric acid

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Table 2 The variationa between four sensor arrays after repeated cyclic voltammetry in 0.1 M H2 SO4 . Electrode

Variation (␮A)

Ir Au Rh Pt

53 0.3 0.2 0.03

a Determined from the difference in measured current values from each electrode when oxidation occurred or, in the case of negligible oxidation occurring, the measured current value at a set potential value.

Fig. 9. Sensitivity plots of Au, Pt, Ir and Rh to three analyte solutions: (a) 0.1 M H2 SO4 , (b) 0.5 mM FeCOOH and (c) 1 M HCl.

solution. All four metals on each device exhibited good stability. Similar electrochemical profiles were obtained from the four electrodes of each device after 300 continuous cycles were applied. For the four Au electrodes, clear oxidation and reduction peaks were

observed over the applied potential range. The four Pt electrodes exhibited clear hydrogen adsorption–desorption peaks at the negative potential range, and for each electrode, an oxidation wave was observed when an increasing positive potential was applied. The Ir electrodes from each of the four devices exhibited a continually increasing peak in the positive potential range, and this was attributed to the formation of a non-reversible oxide layer, which was described earlier. The Rh electrodes each showed the formation of hydrogen adsorption–desorption peaks in the negative potential range. The variation of the different electrodes from the four sensor arrays was also determined after repeated cycling in 0.1 M H2 SO4 . The current measured when oxidation occurred or, in the case of negligible or no oxidation occurring, at a set potential, was recorded and is summarized in Table 2. The Ir electrode showed the maximum variation of around 50 ␮A. The rate of iridium oxide formation with increasing cycling was examined, see Fig. 11. In general, all of the electrodes exhibited a steady increase in growth over the initial 100 cycles, followed by a slower growth rate with continued cycling. These growth rates differed from electrode to electrode, which lead to the observed 50 ␮A variation. In comparison, the other electrodes exhibited a minimal variation, with Pt having the lowest variation of the four metals, 0.03 ␮A, followed by Au and Rh having a 10-fold variation of approximately 0.3 ␮A. These measured variations were not significantly large enough to result in a difference in electrochemical response from the electrodes. Overall,

Fig. 10. Cyclic voltammograms of 0.5 M H2 SO4 : (a) Au, (b) Pt, (c) Ir and (d) Rh. Scan rate: 2 V s−1 , continuously cycled 300 times, 1st cycle (—–), 300th cycle (

).

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ity at Tyndall National Institute for wafer level processing and access to facilities, and Axyntec (Dünnschichttechnik GmbH) for pulsed laser deposition of metals.

References

Fig. 11. Iridium oxide growth rate for four Iridium electrodes during repeated cyclic voltammetry in 0.1 M H2 SO4 .

the good adhesion and minimal variation recorded from the sensor arrays determines that the fabrication process results in devices with good reproducibility. 4. Conclusion A sensor array suitable for use in a voltammetric electronic tongue has been prepared using microfabrication techniques. Previously, such sensor arrays have consisted of wires of different metals encased in a suitable packaging material. This slow, labourintensive method has been difficult to reproduce exactly and has proven expensive due to the amount of metals utilized in the production of the required sensing area. Using microfabrication techniques, electrodes of Au, Pt, Ir and Rh, with the same size sensing area, were successfully deposited together on a silicon wafer. Significantly less metal materials were used in the fabrication of the electrodes compared to established techniques. The metals formed the working and counter electrodes of the sensor array, and were designed to fit on a single 6 mm × 6 mm die. The die was easily packaged into a dipstick format, which allowed for straightforward testing in different solutions. A preliminary electrochemical investigation showed that the resulting devices were operating well. The behaviour of each of the working electrodes was characterized in sulfuric acid solution and ferrocene carboxylic acid solution. In sulfuric acid, well-defined oxidation and reduction peaks were seen for Au and Pt. Hydrogen adsorption–desorption peaks were also observed at the Pt electrode. Rh did not exhibit oxidative behaviour but hydrogen adsorption–desorption behaviour was seen. Ir formed a non-reversible oxide, which resulted in significant increases in the observed oxidation and reduction peaks, and a physical colour change on the electrode surface. In ferrocene carboxylic acid solution, Au, Pt and Rh behaved according to the Randles-Sevcik equation, with a linear relationship seen between peak current and the square root of applied scan rate. The behaviour of Ir was not governed by this equation, and this has been attributed to the formation of an oxide coating on the electrode surface, which reduced the electrode conductivity. Long-term cycling in sulfuric acid solution of five sensor arrays showed that the adhesion of the metals was good and did not deteriorate during the cycling period. Good reproducibility between different sensor arrays was also observed. Future investigation for this device will involve characterization of the sensor device in different application areas such as monitoring of food and environmental quality. The integration of a miniaturized reference electrode on-chip [33,34] will also be investigated, and this will eliminate the external reference electrode employed in the present study. Acknowledgements The authors thank Enterprise Ireland for funding of this work (grants ATRP/02/41 and CFTD/05/112), the Central Fabrication Facil-

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Biographies Karen Twomey obtained her degree in electrical and electronic engineering from University College Cork in 1999. She obtained her PhD in portable sensing systems from University of Limerick in 2002. She is a Staff Researcher at Tyndall National Institute, Cork. Her research interests include portable and miniaturized sensor sys-

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tems, electrochemical microsensor fabrication and characterization, and methods for signal processing and data interpretation. Eva Alvarez de Eulate was born in Pamplona, Spain, in 1983. She graduated in chemistry from the Universidad de Navarra, in 2005, and received a MSc degree from University College of Cork, Ireland, in 2008. From 2006 to 2007, she worked at Tyndall National Institute as a MSc research student. At the beginning of 2008, she joined the IBEC group at the University of Barcelona as a technician. Her current work activity is focused on biosensors devices for nanobiotechnology. John C Alderman has a BSc (Chemistry) from Bristol University (1978) and an MSc (Microelectronics) from Bangor College, University of Wales (1982), obtained while working as a Research Scientist with Plessey Research Caswell, UK (1978–1990). He joined the then National Microelectronics Research Centre (now Tyndall National Institute) in 1990 as Senior Research Scientist and is now Group Head for Bionics (2008). His current research interests are focused on at the possibilities to be derived from direct interaction between the cellular and microelectronic worlds. He has published over 100 research papers as sole or joint author in the various fields of work and has several patents filed. Damien Arrigan is Group Head of the Molecular Microsystems Group at Tyndall National Institute. He completed his PhD at University College Cork in 1992, then was a postdoctoral researcher in Cork and at the Southampton Electrochemistry Group before taking up a lectureship at University of Salford (UK). He returned to Cork and Tyndall National Institute in 2001. His research interests encompass bio-inspired molecular measurement systems which combine electrochemistry with micro- and nano-technology tools.