a multisensor for electrochemical sequential autonomous...

Chem. Anal. (Warsaw), 54, 3 (2009)

* Corresponding author. E-mail: [email protected]

Key words: Multisensor; Solid amalgam electrodes; Solid composite electrodes; Glassycarbon electrode; Gold electrode; Platinum electrode; Voltammetry;Chronopotentiometry; Nucleic acid bases

A Multisensor for Electrochemical Sequential AutonomousAutomatic Measurements

by Tomáš Navrátil*1, Bogdan Yosypchuk1 and Jiøí Barek2

1 J. Heyrovský Institute of Physical Chemistry of ASCR, v.v.i.,Dolejškova 3, 182 23 Prague 8, Czech Republic

2 Charles University in Prague, Faculty of Science, Department of Analytical Chemistry,UNESCO Laboratory of Environmental Electrochemistry,

Albertov 6, CZ-128 43 Prague 2, Czech Republic

A newly constructed electrochemical multisensor containing 8 different sensors (5 differentsolid amalgam electrodes, platinum, gold and glassy carbon electrodes) for analysis of smallsample volumes (5–200 µL) was developed. The multisensor was connected to the spe-cially constructed and developed electrochemical multichannel device for sequential auto-nomous measurements. This device consisted of a computer (PC or notebook), an interfacecard, and a control box; it was able to control up to 8 sensors (i.e. electrode sets) ina selected sequence, in which each sensor could utilize different detection mode (e.g. DCV,DPV, coulometry, chronopotentiometry, square-wave voltammetry) and operate at differentparameters. The analysis of a mixture of ferrocene and nucleic acid bases: adenine andguanine was used to demonstrate the possibilities of this device.

Opracowano nowy sensor sk³adaj¹cy siê z 8 elektrod, w tym 5 ze sta³ego amalgamatu i pojednej z platyny, z³ota i wêgla szklistego. Sensor jest przewidziany do analiz próbek o ma³ejobjêtoœci (5–200 µL). Sensor zosta³ pod³¹czony do specjalnie skonstruowanego, wielo-kana³owego urz¹dzenia elektrochemicznego, które umo¿liwi³o sekwencyjne, niezale¿nepomiary na wszystkich elektrodach. Urz¹dzenie sk³ada³o siê z komputera (PC lub note-book), karty interfejsowej i czêœci kontrolnej. Ka¿d¹ elektrodê mo¿na by³o u¿yæ w po³¹czeniuz wybran¹ technik¹ elektrochemiczn¹ (np. woltamperometria sta³opr¹dowa, woltampe-rometria pulsowa ró¿nicowa, woltamperometria prostok¹tnej fali, kulometria i chronoam-perometria). W celu zademonstrowania mo¿liwoœci nowego uk³adu przeprowadzono analizêmieszaniny ferocenu, adenicy i guaniny.

4 T. Navrátil, B. Yosypchuk and J. Barek

Main advantages of electrochemical methods, especially voltammetric, are: highsensitivity of measurements, possibility of determination of analytes at various oxida-tion states and their speciation, applicability in wide concentration ranges, possibilityof multicomponent analysis, and low investment and running costs. However, thereare some limitations in wider application of voltammetry and related methods, bothin research and industrial laboratories, e.g. strong matrix effect on determinationresults and the corresponding necessity of thorough sample preparation or decompo-sition, unfounded mercuro-phobia (nowadays even considered in the resolutions ofthe European parliament [1]), higher demand on the operator’s qualifications (in com-parison to spectral methods), and relatively low efficiency of measurements. Modernelectrochemical instrumentation, mostly computerized, enables one to significantlyreduce the effect of the abovementioned limitations (except for the mercury preju-dices).

Many attempts were made to increase the efficiency of the voltammetric measu-rements. The attention was mostly focused on „hardware” automation. Probably oneof the first attempts in this field was made 80 years ago and concerned the construc-tion of a carousel-like device with active and resting electrodes [2]. Most probably,this device existed only as a prototype. Further development of automation was faci-litated by computerization in the nineties of 20th century. Some attempts were madeto connect and control more than one voltammetric devices using one computer [3].Unfortunately, the number of free slots for insertion of interface cards in a commonPC is limited (usually up to 4). Other variant [3] is to drive a parallel signal to allstands connected to the interface card, and to process sequentially the output signalsusing a multiplexer. The problem is that the signal evaluation can be affected bytransient effects in case of rapid scans. Such device is controlled using a single soft-ware and by a single operator, but it is necessary to utilize a number of interface cardsand voltammetric stands; therefore, the cost is practically comparable to the cost ofthe same number of separate devices.

A commercially available handheld electrochemical sensor interface Palmsens(Palm Instruments, the Netherlands) [4] can be extended by an optional multiplexer.Up to 8 sensors can be multiplexed, leaving the not-selected sensors at open circuit.In some sensor configurations there is a possibility to apply the same potential to allsensors. Such a device can be employed for control and evaluation of signals from anelectrode array.

The computer-controlled instrument, which is based on the scanning electroche-mical microscope technology, is suitable for a multi-sample analysis. It is employedin the miniaturized combinatorial electrosynthesis of localized compound collections[5–8]. A 96-well microtiter plate is moved horizontally (x, y) by two stepper motorsand an electrode bundle is moved in a vertical direction (z) by the third stepper motor.The electrodes are introduced into the microtiter plate wells through a hole in a glass

5A Multisensor for electrochemical sequential autonomous automatic measurements

plate. The abovementioned technology was used for automated microelectrosynthesisof different organic compounds [6], for on-line monitoring of electrolysis by steady-state cyclic voltammetry at microelectrodes [5], and for voltammetric redox screen-ing of homogeneous ruthenium(II) hydrogenation catalysts [7]. However, this deviceused only one electrode set, which could not be automatically changed during theanalysis.

For the construction of a multisensor, i.e. an array of electrodes, it is necessary touse the proper solid electrode material. The development of such electrodes is one ofthe long-term trends in electroanalytical research (probably also due to mercuro-pho-bia). Solid platinum, gold, silver and carbon electrodes are most frequently used.

The electrodes, which were utilized in the construction of a newly designed multi-sensor described in this paper, belong to the group of non-toxic solid amalgam electro-des introduced by our research group [9–17]. The amalgam used in their constructionis a purer form of non-toxic dental amalgams. These electrodes exhibit very similarproperties to these of mercury electrodes. Solid surfaces of these amalgam electrodescan be polished (p-MeSAE = polished solid amalgam electrode, where Me = Ag, Au,Ir, Cu, Bi, etc.), covered with mercury meniscus (m-MeSAE) or mercury film (MF-MeSAE), or modified in many other ways. Using fine powder of solid amalgams ofvarious compositions it is possible to prepare paste amalgam electrodes [18] or solidamalgam composite electrodes [19, 20]. Similarly, liquid mercury can be replacedwith a solid amalgam in the calomel reference electrode [14] and was successfullyused in this paper.

Composite solid electrodes (CSE) are suitable for the construction of a multisensor.CSE are composed, according to their definition, of at least one conductor and oneinsulator phase, which particles are mixed [21, 22]. The insulator phase is usuallyrepresented by a polymeric or monomeric material. The conductor phase can be formedby either metallic (silver, gold, etc.) or nonmetallic conducting material (e.g. graphitepowder), or by their mixture [21–27]. Some specific components, required for deter-mination of selected analytes in a specific matrix, can be added to the bulk of theelectrode materials. Surfaces of these electrodes can be also modified e.g. with Nafion,catalysts, or enzymes [28, 29].

Another electrode material potentially applicable in a multisensor construction isa solid amalgam composite. Solid amalgam composite electrodes (SA–CE) representa combination of the above solid amalgam and solid composite electrodes [19]. Theyare made of a solid amalgam powder and an insulator phase (e.g. epoxide resin) [19],and are a reliable and environmentally friendly substitute for liquid mercury electro-des in electrochemical analyses.

A renewal of the solid surface is often complex in comparison to mercury electro-des. It results in worse reproducibility and sensitivity of measurements. Electrochemi-cal pretreatment of polished electrodes – SAEs, CSEs, or SA–CEs consists of three


steps: polishing on alumina, electrochemical activation, and electrochemical regene-ration of the electrode surface before each measurement. Simple preparation of theseelectrodes and easy renewal of their surface, which requires only electrochemicalpretreatment between the measurements, appears promising for their applications inbatch analysis and integration into the flow-through systems. Renewal of the menis-cus-covered amalgam or composite electrodes can be realized by formation of a newmeniscus (once a week) or a film (once a day) and by application of a suitable elec-trochemical pretreatment between the measurements [11–15, 19].

The main goal of this work was to construct the multisensor containing 8 inde-pendent working solid electrodes and one auxiliary platinum electrode. This array ofelectrodes represents a prototype of a more complex system applicable to the practi-cal analysis of complex samples. The number of electrodes (here: 8) was selectedwith respect to the possible number of multiplexer outputs used in the testing device,which is usually 2n, where n is an integer.

For the purposes of testing the proposed multisensor, a computer-controlled8-channel electrochemical device with mutually independent channels was develo-ped and described in detail in this paper. This system should enable sequential pro-cessing of current, potential, and time signals from up to 8 electrodes in the describedmultisensor-multiplexer system.

EXPERIMENTAL

Procedures

Differential pulse voltammetry (DPV) with pulse amplitude of 50 mV, pulse width of 100 ms, and scanrate of 20 mV s–1 was applied in test measurements (if not stated otherwise). Before inserting the analyzedsolution into the multisensor cell, purified nitrogen gas was passed through this solution for 300 s.The analyzed solution was not stirred. All measurements were made at the room temperature (22 ± 2°C).The experiments were performed using three-electrode configuration (similar results were obtained witha two-electrode arrangement).

Reagents

All chemicals were of the highest purity (Merck; suprapur Merck, Sigma Aldrich) or of analyticalreagent grade (Lachema Brno, Merck). Standard solutions of metals were purchased from Astasol (Analytica,Prague, Czech Republic). High-purity doubly distilled water (conductivity < 1 µS cm–1) and a glasswarepreviously soaked in diluted HNO3 were used throughout.


Apparatus

Electrodes. The newly developed multisensor comprised an array of small sensors – 8 working elec-trodes: m-AgSAE (disc diameter Ø 0.8 mm), MF-AgSAE (Ø 0.8 mm), p-AgSAE (Ø 0.8 mm), copper (CuE;Ø 0.6 mm), m-CuSAE (Ø 0.8 mm), gold (AuE; Ø 0.4 mm), platinum (PtE; Ø 0.6 mm), and glassy carbon(GCE; Ø 2.0 mm), and one platinum auxiliary electrode (Ø 0.8 mm) (Fig. 1B and 1C) (where „SAE” meanssolid amalgam electrode, „m” stays for mercury meniscus modified, „p” denotes polished, MF is mercuryfilm). The electrodes prepared from solid amalgams were prepared by placing metal powders into the appro-priate holes in the Teflon body (Fig. 1). The platinum contacts were then introduced and the powders weresaturated with metallic mercury. After solidification the amalgam expanded. This guaranteed tight connec-tion between Teflon and amalgam and the sample solution could not penetrate the Teflon-electrode bodyboundary. The electrodes made of metal wires and glassy carbon were fixed in the sensor body using epoxyresin, which expanded after polymerization and thus isolated the analyzed solution from other parts ofthe electrode. The array had the shape of a Teflon disc of 11 mm in diameter. The working electrodes werelocated at the circumference of the disc. The platinum electrode (PtE; Ø 0.8 mm), used as a common auxi-liary electrode, was placed in the centre of the disc (Fig. 1B and 1C). Instead of it, a 1 mm platinum wirecould be inserted into the analyzed solution and applied as the auxiliary electrode. 1–2 mm high walls ofthe multisensor formed a small cell for the analyzed solution. A sample volume analyzed in this cell was50–100 µL. Such small amounts of liquids are very suitable for the analysis of biological samples.

Figure 1. A) A multielchem (8-chanel voltammetric/potentiometric computer-controlled) device; B) 8-work-ing electrode multisensor with Pt auxiliary electrode (in the centre) and 8 working electrodes(clockwise): m-AgSAE, MF-AgSAE, p-AgSAE, CuE, m-CuSAE, AuE, PtE and GCE; C) A schemeof the 8-working electrode multisensor; 1) A sensor for the analysis of small sample volumes;2) A sensor with a tube-cell for the analysis of larger sample volumes


In other version of the sensor construction (Fig. 1C–2), its body and the electrode surfaces (before theirmodification with meniscuses) were in the same plane. The electrode cell was made of elastic transparenttube of the corresponding diameter and length and was put on the sensor body. It was possible to increasethe analyzed volume to the required value by changing the tube length. For the adsorptive transfer strippingvoltammetry (AdTSV) small sample volume (a few µL) was placed on the active parts of the electrodesto allow the analyte adsorb for a given time (1–2 min); then, the sensor was washed and the proper suppor-ting electrolyte was added [30, 31]. An indisputable advantage of AdTSV is that 5–10 µL of the appliedsample is sufficient.

Regeneration of electrode surfaces was realized as follows. Firstly, the electrodes with solid surfacewere polished using the Teflon rod connected to the stirrer spindle (small electric motor) and aqueous sus-pension of alumina. The sensor was thoroughly washed with a distilled water and dried using a filtrationpaper. A small mercury drop, formed at the capillary mouth of the hanging mercury electrode (HMDE),was transferred onto a MF-AgSAE surface. The excess of mercury was removed by gentle shaking ofthe sensor and only a thin mercury film stayed on the AgSAE surface. The similar procedure withoutremoval of excessive mercury was used for meniscus formation on m-AgSAE and m-CuSAE.

In case of AdTSV, only one measurement on each electrode was performed. It was sufficient to immersethe working electrode surfaces in the smallest sample volume. In case of repeatable measurements it wasrecommended to put a longer tube as a reservoir on the electrode body and to use larger sample volumesto avoid a decrease in the analyte’s concentration caused by electrode reactions progress. Reproducibility ofthe measurements was assured by the proper electrochemical regeneration process, usually specific for eachelectrode, analyte, and matrix. No interference or interaction between closely localized electrodes wasobserved. The effectiveness of the electrochemical regeneration was checked by repeating the measurementswith the analyzed sample.

Any proper reference electrode could be inserted into the cell. The calomel reference electrode based onsolid silver amalgam (AgSA-SCE), which exhibits practically the same potentials as saturated calomelelectrode [14], was used. It was prepared in the form of a disc, complementary to the above describedmultisensor disc. Into the bottom of the reference electrode body the porous material Vycor (BioanalyticalSystems Inc., USA) was built in and the inner part was filled with the homogenized mixture of calomel,silver amalgam powder, and saturated KCl solution. A platinum wire, which served as the electric contact,was immersed into this suspension. After both parts (sensor and reference disc) were connected, a free spacefor the analyzed liquid was formed (ca 50 µL).

All potentials given in the text and figure captions are referred to the potential of this miniaturized SCE.A platinum disc placed in the centre of the multisensor disc was used as an auxiliary electrode.

Multichannel device. In order to evaluate and control the input/output signals in the above describedmultisensor, a special multichannel analyzer „Multielchem” was developed (J. Heyrovský Institute of Physi-cal Chemistry of AS CR, v.v.i. and Institute of Biophysics of AS CR, v.v.i, Czech Republic) . The analyzerwas fully computer-controlled. It consisted of three main parts: an interface card, a controlling box(Fig. 1A), and a sensory part (e.g. Fig. 1B). Its total mass was about 0.5 kg, dimensions of the controllingbox were 13 × 26 × 4.5 cm. The box could be located at a distance of up to 2 m from the controlling computer(depending on the cable length). A high-resolution 16-bit multifunctional card PCI–1716 (Advantech, USA)was used as the interface card. This card, responsible for data transfer, was inserted into a common PCor into an industrial computer. It was connected to the controlling box (equipped with galvanostat, potentiostat,and circuits for managing signals). The measured analog signals were amplified in the control box, subse-quently digitalized, and directed to the control computer, where the data were stored in a digital form,forwarded to other devices and peripheries (monitors, printers, etc.), or processed in other ways, or stored.


The controlling signals, according to the controlling software instructions, were processed by the inter-face card and the control box (Fig. 1A). Figure 2 presents a block circuit diagram of the described device.In the control computer (1), equipped with the controlling and evaluation software (2), the signal was gene-rated and converted to the analog signal using a D/A converter (3); from here, it was driven directly tothe potentiostat (4) or galvanostat (5) input. From there, the signal was transferred to the input of the multi-plexer (6), which connected the selected set of sensors (selected working, reference, and auxiliary elec-trodes) according to the control signal. The prototype of the used Multielechem device contained 8 sets ofelectrodes (8 working and 8 reference electrodes) (S1, S2, …, S8) (equal to the number of electrodes inthe developed multisensor). In this device only one connector for the auxiliary electrode was built in;this electrode was common for all sets of electrodes. The current passing through these electrodes was – afterleaving the multiplexer – brought to the current-voltage converter (7) and to the control sensitivity circuits(8). The transformed current signal, as well as the voltage signal, entered the amplifier (9). The output signalfrom the amplifier went to the A/D converter (3), and the digitalized signal was processed by the evaluationsoftware (2) in the control computer (1). Some simple circuits, which performed only auxiliary functions(e.g. electrode switch circuits, control circuits for multiplexer, etc.), are not included in Figure. 2. For fastscans the device was equipped with a special switch, which enabled decreasing the capacity of the system.The device could be connected to the ground using a special connector (e.g. using a Faraday cage).

Figure 2. Block circuit diagram of a Multielchem device: (1) control computer, (2) control and evaluationsoftware, (3) D/A–A/D converter, (4) potentiostat, (5) galvanostat, (6) multiplexer, (S1 … Sn) setsof electrodes, (7) current–voltage converter, (8) sensitivity control circuits, (9) amplifier

The controlling software selected one of 8 sets of electrodes, which should be active at a given moment(others were not connected), and set the multiplexer correspondingly to insert and to read the proper signalsfrom the selected set of electrodes. The signals from the electrode sets were measured sequentially in theprogrammed order (at one time only one sensor was active, others were at open circuit). The applied tech-nique, measurement parameters, regeneration parameters, resolution, etc. could be different for each elec-trode set.

The Multielchem device was constructed in such a way that it was possible to extend the number ofconnected sets of electrodes (this number was limited only by the multiplexer capacity 8, 16, 256, …).The parameters of this device are summarized in Table 1. It was controlled via the „MultiElChem“ v. 2.0software (first experiments were realized using v. 1.0); the overview of the software parameters is givenin Table 1. The software was user-friendly, easy to install and control, and suitable for design of newmethods. It was operated under Windows XP/2000 (drivers for Vista will be available very soon) andenabled application of a large spectrum of methods: direct current (DC) voltammetry and polarography,


DP voltammetry, potentiometric stripping analysis (PSA), differential potentiometric stripping analysis(Dif-PSA), chronoamperometry, coulometry, and square wave voltammetry (SWV). The selected methodcould be easily replaced with another one using the software option „MultiElchem Design Utility”, accor-ding to the user’s requirements (e.g. DC scan for electrode surface renewal or sample pretreatment; determi-nations performed using DPV or potentiometric stripping analysis). The software included a unit for pro-cessing of the registered curves by elimination voltammetry with a linear scan [15, 27, 33–35]. Data (para-meters, curves, results) were saved in an ASCII format and were easily exportable as e.g. MS Excel worksheets.On the other hand, the software enabled importing and processing the data from other electrochemicaldevices (e.g. Eco-Tribo Polarograph; Polaro-Sensors, Czech Republic). The maximum available scan rate(ca 10 V s-1) and the highest frequency (ca 500 Hz) were limited by the built-in potentiostat power (0.04 W)and damping time constants (5 ms). The maximum number of the saved curves in one file was 48(the number of curves recorded in one measurement could be much higher, but maximally 48 curves could besaved, e.g. in case of electrochemical pretreatment of a solid electrode 50 or 100 potential cycles wereapplied, but no curve was recorded). The maximum number of evaluated species in one files was 8.

Table 1. Parameters of the developed Multielchem device


An order and a sequence of repetitions of the used sensors, which could be connected to the controllingbox through 8 universal connectors (jacks), was programmable using a user-friendly procedure (at one timeonly one sensor was active, the others were at the open circuit). The applied technique, measurement para-meters, regeneration parameters, resolution, etc. could be different for each sensor.

The software enabled the analysis of each sample either by standard addition method or calibrationcurve method (both linear and quadratic). In both methods the peak (wave) height or peak area could beappropriately plotted. Moreover, the software enabled evaluation of the peak width (measured at the 1/2 ofits maximal height), what could be useful for determination of the number of electron transferred in theelectrode process. The measurements could be realized in two- or three-electrode circuits (each sensor equippedwith its own reference electrode, whereas only one auxiliary electrode, common for all working electrodes,was used; the on/off switching of the auxiliary electrode was controlled by the software as well).

The proposed instrument was designed for the analysis of small samples (mostly of biological origin),which could not be stirred or bubbled with an inert gas. Moreover, since the intention was to use only solidelectrodes, the instrument contained neither the controlling circuits for a stirrer and a gas inlet, nor thecircuits for mercury drop formation and removal. The described device enabled connection of any sensor,e.g. a carbon paste electrode [36], a solid composite electrode (e.g. silver [23], gold [37], or solid graphite[38] composite electrode), a screen printed electrode [39], or a solid amalgam electrode [11–15].

RESULTS AND DISCUSSION

The developed multisensor (Fig. 1B, C) was tested for determination of variousorganic and inorganic compounds as nucleic acids bases (guanine (Gua), adenine(Ade)), ascorbic acid, ferrocene, heavy metals, etc. Oxidation of Gua and Ade isoften performed in the studies of DNA [40, 41]. Determination of their mixture usingvarious voltammetric methods is depicted in Figure 3. For silver amalgam electrode(m-AgSAE, MF-AgSAE and p-AgSAE) direct current voltammetry (DCV) in thecathodic stripping voltammetry (CSV) mode was used. The overlapping CSV peaksof Gua and Ade recorded using the above mentioned three electrodes and m-CuSAEenabled highly sensitive determination of these nucleic acid bases. The principle ofsuch determination was utilized earlier for DNA hybridization sensors [42, 43]. Nosignal of either Gua or Ade was observed on the copper cathode. CSV peak of Guaand Ade (Ep = –546 mV) and reduction peak of Ade (Ep = –1454 mV) were recordedin the cathodic part of the cyclic voltammogram using m-CuSAE. Noteworthy, Guaalone yielded the reduction signal at m-CuSAE (Ep = –1598 mV), in contrast to theearlier experiments with mercury electrode. It can be seen in Figure 3 that the use ofDPV on glassy carbon electrode seems to be the most suitable for the studied analyti-cal purposes. Gold and platinum electrodes were not suitable due to the easy oxida-tion of their surfaces. Figure 3 shows that the described system enables one to findthe optimal method and the best working electrode, or to perform a complex analysisof the sample using various methods and up to 8 electrodes by only one keystroke.Simple and intuitive setup for electrochemical regeneration of each working elec-trode independently enables achieving good reproducibility of determination.


Figure 3. Voltammograms registered using 8-working electrode multisensor (as described in the legend forFigure 1) in one measuring cycle. Curve 1: supporting electrolyte: 0.1 mol L–1 acetate buffer ofpH 4.8; curve 2: supporting electrolyte, 5 × 10–5 mol L–1 Gua and 5 × 10–5 mol L–1 Ade. For DCVand CV: v = 500 mV s–1, for DPV: v = 20 mV s–1. Electrochemical regeneration before each scanfor m-AgSAE, MF-AgSAE, p-AgSAE, and m-CuSAE: potential of regeneration Ereg = –1300 mV,time of regeneration treg = 30 s; for CuE: Ereg = –900 mV, treg = 30 s; for AuE: Ereg = –400 mV,treg = 30 s; for PtE: Ereg 1 = –200 mV, treg 1 = 30 s, Ereg 2 = +200 mV, treg 2 = 30 s; for GCE: Ereg 1 =–200 mV, treg 1 = 30 s, Ereg 2 = +400 mV, treg 2 = 30 s. For m-AgSAE: DCV, accumulation potentialEaoc = –100 mV, time of accumulation tacc = 60 s, scan rate v = 500 mV s–1; for MF-AgSAE: DCV,Eacc = –150 mV, tacc = 60 s, v = 500 mV s–1; for p-AgSAE: DCV, Eacc = –200 mV, tacc = 60 s,v = 500 mV s–1; for CuE: DCV, Eacc = –250 mV, tacc = 60 s, v = 500 mV s–1; for m-CuSAE: CV,Eacc = –250 mV, tacc = 60 s, initial potential Ein = –250 mV, reverse potential Erev = –1700 mV,v = 500 mV s–1; for AuE: DPV, Ein = +500 mV, final potential Efin = +1400 mV, v = 20 mV s–1;for PtE: DPV, Ein = +300 mV, Efin = +1300 mV, v = 20 mV s–1; for GCE: DPV, Ein = +300 mV,Efin = +1300 mV, v = 20 mV s–1. (Continuation on the next page)


Figure 3. (Continuation)

Obviously, it was possible to perform reduction or oxidation of an analyte usingonly one selected method (e.g. DPV) and various working electrodes. As an example,oxidation of ferrocene in 0.1 mol L–1 HClO4 solution using the described multisensoris shown in Figure 4. It is obvious that the registered voltammetric signals differ fordifferent electrodes. The highest and most symmetric peak was obtained on GCE.On CuE and m-CuSAE in turn no signal was registered. The complete informationon suitability of the electrode material could be obtained in one set of measurement.


Figure 4. Differential pulse anodic voltammograms of 10–4 mol L–1 ferrocene in 0.1 mol L–1 HClO4–48%ethanol mixture obtained using the 8-working electrode multisensor. Types of electrodes usedin the multisensor are described in the legend for Figure 1

It was proven that though all electrodes were placed in the same electrode cell,the processes occurring at them proceeded separately and independently, i.e. the poten-tial imposed on currently active electrodes did not influence other electrodes.

For the scientific and practical purposes, it was desirable to perform the measure-ments on the electrodes of the same type applying different parameters. In sucha case, all electrodes in the multisensor could be of the same type, e.g. oxidation of1-naphthol on 8 equivalent carbon electrodes in one multisensor. This method hasbeen utilized in the electrochemical detection of DNA triplet repeats expansion [44].

The multisensor in combination with the analyzer should increase substantiallythe efficiency of the electrochemical measurements. The number of the analyzedsamples in a time unit is comparable to that in atomic absorption spectroscopy (AAS)and many times higher than that in AAS with electrothermal atomization (ET–AAS).The requirements on the qualifications of the operator can be lowered due to the factthat the largest part of the routine work with the device and electrodes is realizedtotally automatically using intuitive, user-friendly software. These advantages in com-bination with the use of non-toxic electrode materials could contribute to the improve-ment of the wide-spreading electrochemical methods in research and routine labora-tories.

The designed instrument is only a prototype, appropriate for testing the construc-tion possibilities of the devices of such type. It can be operated in combination with8 electrode sets only. For the purposes of DNA analysis, at which this device wasaimed, larger samples have to be analyzed. It is possible, however, to extend the


device by a larger number of the connected sets of electrodes (this number is limitedonly by the multiplexer capacity – 8, 16, 256, …).

CONCLUSIONS

The newly designed sensor is suitable for the analysis of small sample volumes(50 to 200 μL). Therefore, it seems to be especially suitable for DNA analysis.The tested multisensor contains 8 different working electrodes (4 solid amalgam,gold, copper, platinum, and glassy carbon electrodes) and one platinum auxiliaryelectrode. The electrochemical cell is covered with a special calomel amalgam elec-trode. Therefore, the analyzed liquid sample is separated from the outer area. Suc-cessful tests with the 8-electrode multisensor were performed in connection with thecomputer-controlled Multielchem device, which enabled one to set up different mea-surement methods for each sensor (e.g. DCP, DCV, CV, DPV, PSA, DPSA, SVW,chronoamperometry, coulometry), 2- or 3-electrode mode, different parameters ofmeasurement and electrochemical regeneration of each working electrode before everyscan, independently for each channel. During determination, 1 to 8 identical or diffe-rent working electrodes can be connected to the Multielchem device. They can betotally independent or arranged in the 8-electrode multisensor. Theoretically, eachworking electrode can be accompanied by its own reference electrode, or only onecommon reference electrode can be used for all channels.

The main advantage of the proposed device (multisensor in connection with Multi-elchem) is that it offers a possibility of selection of a different method and differentoperational, pretreatment, regeneration and measurement parameters for each sensor(the built-in function). The electrodes in the tested multisensor are fully autonomousand the measurements with particular sensors are realized in the programmedsequence (at one time only one sensor is active; the others are at open circuit).

The application of 8-working electrode multisensor in combination with 8-chan-nel voltammetric/potentiometric computer-controlled Multielchem device, describedin detail in this paper, substantially simplifies complex electrochemical measure-ments, increases the efficiency of the measurements, and – due to the intuitive andeasily controllable software – decreases the demands concerning the operator’s quali-fications. The electrode materials used are non-toxic and can be applied practicallyin any laboratory, or even in field measurements.


Acknowledgements

This work was financially supported by the Grant Agency of the Academy of Sciences of the CzechRepublic (project No. IAA 400400806), by the Grant Agency of the Czech Republic (project No. 203/07/1195), and by the Ministry of Education, Youth, and Sports of the Czech Republic (projects LC 06035 andMSM 0021620857).

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Received June 2008Revised December 2008Accepted February 2009

a multisensor for electrochemical sequential autonomous...

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