simultaneous detection of cyanide and heavy metals for environmental analysis by means of µises

Phys. Status Solidi A 207, No. 4, 817–823 (2010) / DOI 10.1002/pssa.200983303 p s sa

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applications and materials science

Simultaneous detection of cyanideand heavy metals for environmentalanalysis by means of mISEs

Monika Turek1,2, Wolfgang Heiden3, Sharon Guo1,2, Alfred Riesen3, Jurgen Schubert2, Willi Zander2,Peter Kruger4, Michael Keusgen5, and Michael J. Schoning*,1,2

1 Institute of Nano- and Biotechnologies, Aachen University of Applied Sciences, Ginsterweg 1, 52428 Julich, Germany2 Institute of Bio- and Nanosystems, Research Centre Julich GmbH, Leo-Brandt-Straße, 52425 Julich, Germany3 Bonn-Rhein-Sieg University of Applied Sciences, Grantham-Allee 20, 53757 Sankt Augustin, Germany4 Institute of Biochemistry and Molecular Biology, RWTH Aachen University, Pauwelsstraße 30, 52074 Aachen, Germany5 Institute for Pharmaceutical Chemistry, Philipps-University Marburg, Wilhelm-Roser-Straße 2, 35037 Marburg, Germany

Received 2 September 2009, revised 24 November 2009, accepted 14 December 2009

Published online 18 March 2010

Keywords electrochemical sensor, heavy metal, cyanide, ion-selective electrodes

* Corresponding author: e-mail [email protected], Phone: þ49 241 6009 53215, Fax: þ49 241 6009 53235

In environmental analysis, cyanide and heavy metals play an

important role, because these substances are highly toxic for

biological systems. They can lead to chronic and acute diseases.

Due to the chemical properties of cyanide it is frequently used

for industrial processes such as extraction of silver and gold.

Heavy metals can be found as trace elements in nature and are

often applied in industries e.g., galvanization processes. Up to

now, cyanide and heavy metals can be detected by several

sensors separately and their detection is often limited to

laboratory investigations. In this publication, with regard to an

in situ analysis, a new miniaturized silicon-based sensor system

for the simultaneous detection of cyanide and heavy metals in

aqueous solutions is presented that is based on chalcogenide

glass-based micro ion-selective electrodes (mISEs). The mISEs

are incorporated into a specially designed measuring system for

the simultaneous detection of heavy metals and cyanide in

solutions and validated by simultaneous measurements of

Cu2þ- and CN�-ions, Cd2þ- and CN�- ions and Pb2þ- and

CN�-ions. The particular sensor system has shown good sensor

properties in the m-molar ion-concentration range. For simulta-

neous measurements in complex heavy metal and cyanide

solutions an intelligent software using fuzzy logic is discussed.

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction In environmental monitoring, cya-nide and heavy metals play an important role, because thesesubstances are toxic for biological systems: Already afew mg cyanide/kg human weight (average lethal dose:1.5 mg/kg human weight) can lead to death. Hereby, cyanidereversibly inhibits the enzyme cytochrome C oxidase, whichis a key enzyme in the respiration cycle. This metabolicpathway triggers the synthesis of adenosine triphosphate(ATP), the chemical fuel of cells. Mostly, the nervoussystem, heart and lung are affected due to their highrequirement of ATP and oxygen. Acute poisonings canoccur in a few seconds to several hours. Cyanide occursnaturally in more than 2,500 plants as cyanogenic glycosideor liposides e.g., in roots, stone fruits and bitter almonds.Furthermore, due to the relatively high affinity of cyanide to

metal components and the chelating character it is frequentlyused for industrial processes such as the extraction of silverand gold, electroplating-, agricultural- and pharmaceuticalindustries. The maximum limit of cyanide in drinking wateris 1.1mmol/l according to the world health organization(WHO) [1–3].

Heavy metals, such as copper, cadmium and lead, canaffect organs (e.g., liver and kidney) and block severalenzymes in organs of the human body, which are responsiblefor essential biochemical processes. For example, in 1950cadmium in food and drinking water led to the ‘Itai-Itai’disease in Japan. Hereby, cadmium affected the kidneyresulting in a decreased reabsorption of calcium and thus, in adeformation and destruction of bones. Heavy metals havealready been used for hundreds of years by humans. Typical


818 M. Turek et al.: Simultaneous detection of cyanide and heavy metals by means of mISEsp

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industrial application fields are metallic and electronicindustries, galvanization processes, fabrication of pigmentsand antimicrobial agents. The upper limit of heavy metals indrinking water lies between 0.012 and 11mmol/l (for Cu2þ-,Cd2þ- and Pb2þ-ions) according to the WHO [1, 4–6].

Up to now, cyanide and heavy metals can be detected byseveral sensors separately and their detection is often limitedto laboratory investigations [7, 8]. A well-established andstandardized method for cyanide detection in environmentalanalysis is based on the spectrophotometric detection ofcyanide by means of the pyridine/barbituric acid reactionwith prior diffusion or distillation treatment [9].Commercially available sensors for cyanide determinationin liquids are potentiometric sensors with cyanide-sensitivesilver-halide membranes (e.g., AgI or AgI/Ag2S). Themechanism of these electrodes is based on the complexationreaction of cyanide with silver-halides (Eq. (1)) resulting in apartial corrosive destruction of the membrane [10].

� 20

AgIþ 2CN� ! AgðCNÞ�2 þ I� (1)

Due to the diffusion mechanism of CN�, I� andAg(CN)�2 in the membrane, the theoretically expectedsensitivity corresponds to the Nernstian sensitivity of59 mV/pCN at standard conditions. The Nernstian sensi-tivity describes the linear relation between the logarithm ofion activity (or ion concentration) and the sensor signal forpotentiometric measurements. At standard conditions(25 8C) for monovalent and bivalent ions the sensitivity is59.2 and 29.6 mV/decade, respectively. Furthermore, poten-tiometric silver-based chalcogenide glass sensors forcyanide detection have been introduced [11]. The mechan-ism has been explained by Morf et al. by means of acomplexation reaction between free interstitial Agþ

�-ions

(Frenkel ions) in the solid membrane and CN�-ions in thesolution to silverdicyano-complexes (Eq. (2)). This is due tothe low solubility constants of silver-chalcogenides [12].

Agþ� þ 2CN� ! AgðCNÞ�2 (2)

The expected sensitivity hereby is a double-Nernstiansensitivity of 118 mV/pCN at standard conditions. Recently,Neshkova et al. have introduced miniaturized Ag-basedchalcogenide glass electrodes for cyanide detection, whichhave been fabricated by electrochemical deposition [13, 14].The advantages of such Ag-based chalcogenide glasselectrodes are the high sensitivity and sufficient stabilitytowards aggressive cyanide analyte solutions.

On the other hand, for the detection of heavy metals inenvironmental analysis, well established and standardizedmethods are based on the simultaneous spectrophotometricdetermination of up to 20 different heavy metal ions. Forexample, the atomic adsorption spectroscopy (AAS), or theatomic emission spectroscopy (AES) in combination withthe inductively coupled plasma (ICP) method are applied[15]. Commercially available potentiometric sensors for theheavy metal detection in liquids utilize a particularchalcogenide glass membrane as sensitive membrane. Due

10 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

to the modified surface layer, particular heavy metal ions inanalyte solution dynamically interact with the heavy metalions in the modified surface layer at the boundary ‘analyte/membrane’. This results in a change of the heavy metal ionconcentration in the membrane, which is dependent on theion concentration in the analyte solution, serving as sensorsignal [16]. Recently, miniaturized heavy metal ion-selective electrodes (mISEs) have been introduced withthin-film chalcogenide glass membranes. Those sensorsshowed good stability in liquid media, high long-termstability in operation, low detection limit and compatibilityto silicon technology [17, 18].

In this work, silicon-based chalcogenide glass mISEshave been applied for the simultaneous detection of cyanideand heavy metals in aqueous solutions for the first time.Physical and electrochemical investigations towards thehomogeneity, morphology and topology as well as towardsthe intrinsic sensor properties, like sensitivity, responsebehavior and linear measuring range of such mISEs incyanide and heavy metal analyte solutions will beperformed. First measurements in a specifically designed,portable diffusion measuring cell for the simultaneousdetection of different heavy metal ions, like Cu2þ-, Cd2þ-,Pb2þ-ions, and CN�-ions will be presented and discussed.

2 Experimental2.1 Fabrication of chalcogenide glass mmISEs For

the detection of CN�-ions Ag-based chalcogenide glassmISEs and for the detection of Cu2þ-, Cd2þ-, Pb2þ-ions, Cu-,Cd- and Pb-based chalcogenide glass mISEs, respectively,have been fabricated by means of silicon- and thin-filmplanar technology. Each sensor is made of a p-doped Si layer(specific resistance >1,000Vcm) with a 500 nm thick SiO2

layer for electrical insulation and a metal contact consistingof 15 nm Ti, 30 nm Pt and 250 nm Au on the sensor substrate.In order to realize the particular miniaturized CN-, Cu-, Cd-and Pb-sensors, chalcogenide glass material systems ofAgAsSeTe, CuAgAsSe, CdSAgIAs2S3 and PbSAgIAs2S3

have been used, respectively. The deposition of thesechalcogenide glass materials in a thin-film state onto thesubstrates has been performed by means of pulsed laserdeposition (PLD) technique. The sensing area is approxi-mately 40 mm2. For more detailed information of sensorfabrication and PLD process, see Refs. [19–21].

2.2 Characterization of single mmISEs: Physicaland electrochemical The physical characterization of thefabricated chalcogenide glass mISE has been performed bydigital video-microscopy (VHX-digital microscope, Z25/Z500, Keyence) and scanning electron microscopy(GEMINI 1550, Zeiss). Ion-selective potentiometry hasbeen applied to investigate the electrochemical behavior ofthe chalcogenide glass sensors in cyanide and heavy metalsolutions. The measurement set-up includes the particularmISEs and a conventional double-liquid junction Ag/AgClreference electrode, both immersed in the analyte solutionand connected via a highly ohmic multimeter (2,700,

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Phys. Status Solidi A 207, No. 4 (2010) 819

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Figure 2 Digital microscopy(a)andscanningelectronmicroscopy(b) picture of the Ag-mISE for CN�-ion detection.

Keithley, 10 GV) to close to the electrical circuit. The outerand inner electrolyte of the reference electrode is 0.1 mol/lKNO3 and 0.1 mol/l KCl, respectively. As backgroundsolution for the investigation of cyanide and heavy metals0.1 mol/l KNO3, pH 11.5, and 0.1 mol/l KNO3, pH 3, havebeen applied. 0.1 mol/l KCN in deionized water and 0.1 mol/lCu(NO3)2, 0.1 mol/l Cd(NO3)2 and 0.1 mol/l Pb(NO3)2 in0.1 M KNO3, pH 3.0, have been prepared as stock solutions.The electrodes have been characterized in the particularbackground solution with cyanide or heavy metal ionconcentration ranging from 0.1mmol/l to 1.1 mmol/l.

2.3 Simultaneous detection of heavy metalsand cyanide : Measurement set -up andmethod For the simultaneous detection of heavy metalsand cyanide in liquids a specifically designed measuring cellhas been developed. Figure 1 shows a photo (with schematicof measuring principle) of the measuring cell containing thetwo compartments (diameter 23 mm and height 50 mm),which are separated by a hydrophobic, gas-permeablemembrane (TF-200, 47 mm, Pall Life Science).

For the simultaneous detection of heavy metals andcyanide, stock solutions (heavy metal and cyanide) havebeen added into the lower chamber. Heavy metal ions havebeen directly detected by means of the particular heavy metalmISE in the lower chamber. Furthermore, due to the low pHvalue of the solution (pH 3.0), cyanide has been immediatelyconverted into gaseous HCN (pKa¼ 9.31), which hasdiffused through the gas-permeable membrane into theupper chamber. Due to the high pH value (pH 11.5) of theKNO3 solution in the upper chamber, HCN has been changedback to CN�-ions and could be detected by means of the Ag-based chalcogenide glass mISE. During the measurementboth solutions have been kept at 30 8C (boiling point of HCNis 26 8C) and stirred constantly by a miniaturized magneticstirrer (not shown in Fig. 1).

Figure 1 (online colour at: www.pss-a.com) Photo of the measur-ing cell including a schematic of the measuring principle for thesimultaneous detection of Cu2þ- and CN�-ions.

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3 Results and discussion3.1 Characterization of chalcogenide glass-

based mmISEs Prior to the simultaneous detection of heavymetals and cyanide, the fabricated mISEs have beenphysically and electrochemically characterized. Thephysical investigation of the chalcogenide glass-basedmISEs has been done by means of digital microscopy andSEM to study the homogeneity, morphology and topology ofthe sensitive thin-film chalcogenide glass membrane.Figure 2 exemplarily shows a digital microscopy (a) andSEM (b) picture of the Ag-basedmISE for cyanide detection.The sensor surface is very homogeneous, which is indicatedby the same colour of the membrane in Fig. 2(a). Thedistribution of so-called droplets (see Fig. 2(b)) on the sensorsurface is typical for the applied PLD process, but does notnegatively affect the electrochemical sensor properties ofthese electrodes. Furthermore, the sensor membrane iscompletely dense and closed even at high resolutions. Bothaspects are crucial for a defined and good functionality of thechalcogenide glass-based mISEs. Similar results have beenobserved for the Cu-, Cd- and Pb-mISEs.

In order to examine the intrinsic properties of thesensors, like sensitivity, response behavior, linear measuringrange and detection limit, electrochemical characterizationhas been performed by means of ion-selective potentiome-try. The Ag-basedmISE and the Cu-, Cd- and Pb-mISEs havebeen investigated in cyanide and heavy metal solutions,respectively. Figure 3 presents typical measurement curvesof the Ag-, Cu-, Cd- and Pb-mISEs in CN�-, Cu2þ-, Cd2þ-and Pb2þ-ion solutions. Cyanide measurements have beenperformed at pH 11.5 and measurements with heavy metalions have been done at pH 3.0 due to the chemical properties(solubility, ionic form) of these components. As shown inFig. 3, the response signal of the Ag-mISE towards CN�-ionsin solution was decreasing with increasing CN�-ionconcentration. The resulted sensitivity was 100 mV/pCN inthe cyanide concentration range between 1.1mmol/l and1.1 mmol/l and the lower detection limit was 0.5mmol/lcyanide. The obtained values correspond well with thesensor properties of chalcogenide glass-based mISEs withsimilar membrane composition from the literature;Neshkova et al. obtained an average sensitivity of 101 mV/pCN in the concentration range of 5mmol/l to 1 mmol/l



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Figure 3 (online colour at: www.pss-a.com) Calibration measure-ments of the Cu-, Cd- and Pb-mISE in the particular heavy metal ionsolution and of the Ag-mISE in the CN�-ion solution.

Figure 4 (online colour at: www.pss-a.com) Measurement curvesfor simultaneous determination of Cu2þ- and CN�-ions.

Figure 5 (onlinecolour at:www.pss-a.com) Calibration curves forsimultaneous determination of Cu2þ- and CN�-ions.

cyanide with a thin-film sensor of Ag-Se-Te membranecomposition [13]. The calibration measurements of theheavy metal mISE resulted in a fast and stable, increasingsensor response with increasing heavy metal ion concen-tration. The sensitivities of the Cu-, Cd- and Pb-mISE were28 mV/pCu, 20 mV/pCd and 23 mV/pPb, respectively. Thelower detection limits for the Cu-mISE, Cd-mISE and Pb-mISE were 0.2mmol/l, 3.8mmol/l and 5mmol/l, respect-ively. The response time (t90%) was less than 30 s for allinvestigatedmISEs. The obtained results from the calibrationmeasurements of the heavy metal mISEs correspond wellwith data in literature [22]. Furthermore, it is in goodagreement with the theoretically expected half-Nernstiansensitivity of 29.6 mV/decade for divalent ions. The intrinsicparameters of the mISEs are summarized in Table 1.

3.2 Simultaneous detection of heavy metalsand cyanide The presented chalcogenide glass mISEshave been applied to the measuring system for thesimultaneous detection of heavy metals and cyanide. Themeasuring procedure and corresponding evaluations will bepresented exemplarily by means of the simultaneousdetection of Cu2þ- and CN�-ions with a Cu-mISE and anAg-mISE, respectively. Figure 4 shows the measurementcurves and Fig. 5 the corresponding calibration graphs.

For simultaneous measurements, a stock solution of therespective heavy metal and cyanide is added into the

Table 1 Summary of the intrinsic sensor parameters.

Ag-mISE

sensitivity in mV/dec 100lower detection limit in mmol/l 0.5linear range in mmol/l 1.1-1,100


background solution in the lower chamber of the measuringcell. Here, in the fist step, eight times 50ml of Cu2þ-ion stock(and diluted stock) solution have been added. In the secondstep, for the simultaneous measurement of Cu2þ- and CN�-ions at a fixed Cu2þ-ion concentration, stepwise a particularamount of cyanide stock (and diluted stock) solution hasbeen added. The resulted concentration range for Cu2þ- andCN�-ions has been from 2 to 87mmol/l and from 2 to85mmol/l, respectively.

With increasing concentration of Cu2þ-ions the responsesignal of the Cu-mISE increased with fast and stable steps,

Cu-mISE Cd-mISE Pb-mISE

28 20 230.2 3.8 50.5-1,100 1.1-1,100 1.1-1,100

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Figure 6 (online colour at: www.pss-a.com) Comparison betweencalibration curves from simultaneous measurements of Cu2þ- andCN�-ions, Cd2þ- and CN�-ions, and Pb2þ- and CN�-ions.

while the sensor signal of the Ag-mISE in the upper part ofthe measuring cell remained nearly constant. At Cu2þ-ionconcentration of 87mmol/l, different CN�-ion concen-trations have been added into the lower chamber. Due tothe low pH value of the background solution in this chamberand the applied temperature, cyanide as HCN diffused intothe upper chamber, where CN�-ions could be detected bymeans of the Ag-mISE. Therefore, with increasing cyanideconcentration in the lower chamber the sensor response ofthe Ag-mISE in the upper chamber of this diffusionmeasuring cell was decreasing. At a CN�-ion concentrationup to 8mmol/l the sensor response was fast and stable. Due tolonger diffusion times at higher cyanide concentrations theresponse time shifted to higher values. The sensor signalfrom the Cu-mISE remained constant. That means that theCu-mISE was not affected by CN�-ions in the test sample atthe utilized cyanide concentrations.

The linear measuring range and sensitivity of the mISEsfor the applied simultaneous measurement of the heavymetal and cyanide could be achieved from the correspondingcalibration curves. As can be seen in Fig. 5, the Cu-mISEshowed in the whole applied Cu2þ-ion concentration range alinear behavior with a slope of 31 mV/pCu. This is in goodagreement with the theoretically expected value and with theachieved values from the calibration measurement (Fig. 3).In the CN�-ion concentration range from 2 to 27mmol/l, theAg-mISE resulted in a sensitivity of 24 mV/pCN and forhigher concentrations a slope of 130 mV/pCN was observed.The decreased sensitivity for very small cyanide concen-trations could be explained by means of possible complexa-tion reactions between the copper and cyanide ions, beforeCN�-ions were converted to HCN in the lower chamber ofthe measuring cell. Copper cyanide complexes, likeCu(CN)�2 , Cu(CN)2�

3 and Cu(CN)3�4 with stability constants

(KS) of 1016 (mol/l)�2, 1022 (mol/l)�3 and 1023 (mol/l)�4,respectively, might be formed [23, 24]. Cyanide from thesecomplexes was not available to diffuse into the upperchamber and thus, the real CN�-ion concentration in theupper chamber was smaller than the defined ion concen-tration in the lower chamber. For higher cyanide concen-trations, the obtained sensitivity fits well with thetheoretically expected sensitivity for Ag-based chalcogenideglass sensors for cyanide detection of 118 mV/pCN.

Furthermore, the simultaneous detection of Cd2þ- andCN�-ions and Pb2þ- and CN�-ions has been performed.Therefore, the same measuring procedure as presented forthe simultaneous measurement of Cu2þ- and CN�-ions hasbeen applied. The comparison of the calibration curves fromsimultaneous detection of Cu2þ- and CN�-ions, Cd2þ- andCN�-ions and Pb2þ- and CN�-ions is shown in Fig. 6. Thesensitivity of the Cu-, Cd- and Pb-mISE towards therespective heavy metal ions was 31 mV/pCu, 15 mV/pCdand 17 mV/pPb. For cyanide concentrations higher than27mmol/l, the Ag-mISE showed similar response behaviorfor all three different simultaneous measurements ofheavy metals and cyanide with an average sensitivity of117 mV/pCN. This corresponds very well to the theoretically

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expected sensitivity value for Ag-chalcogenide glass-basedsensors for cyanide detection of 118 mV/pCN. At lowercyanide concentrations, the obtained slopes of the Ag-mISEwere 24, 16 and 14 mV/pCN for simultaneous measurementsof Cu2þ- and CN�-ions, Cd2þ- and CN�-ions and Pb2þ- andCN�-ions, respectively. The differences in the sensorbehavior at low cyanide concentrations may be explainedby means of different heavy metal cyanide complexationreactions to e.g., Cd(CN)2 (KS¼ 1011 (mol/l)�2) orPb(CN)2�

4 (KS¼ 1042(mol/l)�4) [24]. However, furtherinvestigations are necessary for the complete understandingof this mechanism.

3.3 Fuzzy logic for simultaneous measurementsof different heavy metal ions In multi-componentsolutions potentiometric chalcogenide glass chemical sen-sors show cross-sensitivities towards other/interfering ionsin the test sample due to the nature of the complex sensingmaterial. Based on cross-sensitivities of chemical sensors incombination with intelligent data analysis software, differentelectronic tongues for the detection of e.g., heavy metals[25], different kinds of wines [26], beverages [27] and eventomatoes [28], have been developed. However, suchelectronic tongues consist of either pattern or complexrecognition tools. These tools may be artificial neuronalnetworks (ANN), principal component analysis (PCA),partial least squares regression (PLS) and soft-independentmodelling of class analogy (SIMCA). Such electronictongues usually include a huge number of sensors resultingin a complex, time-consuming and laborious calibration.Therefore, another approach has been followed for acombined analysis of heavy metal sensor data. Fuzzy logicas intelligent data recognition software can offer a relatively‘simple’ and fast approach for qualitative and quantitativedetection of multi-component heavy metal solutions. Fuzzylogic, with the concept of ‘linguistic variables’ in particularhas been developed as a means to mimic human decision



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processes by mathematical structures and procedures[29, 30]. It is based on fuzzy sets as a generalization of theclassical set theory, associating each variable with acorresponding membership function value, which depicts adegree of membership to the fuzzy set. Linguistic variablesare groups of fuzzy sets with (partially) overlappingmembership functions over a common (crisp) basic variable.Linguistic variables have been used for segmentation ofmolecular surfaces by means of several physicochemicalparameters [31].

Linguistic variables are formed linking sensor data withtype and concentration of components in an aqueous solutionby generating membership functions from the calibrationcurves of the different sensors according to the solutioncomponents that influence these sensors. The composition ofunknown multi-component solutions can then be analyzedwith good accuracy by a simple fuzzy inference procedure[32]. For a combinatory approach, calibration curves foreach sensor are generated with a set of solutions of knowncomposition and concentration, resulting in a set ofcharacteristic curves for each measurable component asrelated to a particular sensor. Fuzzy sets are calculated foreach sensor from the intersection of a constant meanpotential measured in a solution of unknown compositionand concentration with the sensor’s characteristic curves.The fuzzy sets for all sensors are then integrated in alinguistic variable, where a fuzzy intersection operator(fuzzy AND connective) constitutes a membership functionfor the unknown solution to each of the known solutions usedfor calibration. Finally, through an analysis of all linguisticvariables a likely conclusion on the composition of the

Figure 7 (online colour at: www.pss-a.com) Schematic of deter-mination of an unknown heavy metal solution by means of fuzzylogic and a chalcogenide glass sensor array.


unknown heavy metal solution can be made. The most likelycomposition is the linguistic variable with the highestmembership value of its fuzzy AND connective membershipfunction. Figure 7 illustrates this procedure with measure-ment of a two-component solution of unknown types andconcentration of heavy metal ions (as published in Ref. [32]).The example was taken from a set of measurements not usedfor calibration of the system.

The procedure could be shown to work well with single-and two-component solutions of heavy metal ions.Examinations of multi-component solutions are in progress.In order to investigate multi-component heavy metalsolutions containing cyanide ions, the already developedfuzzy logic algorithm can be applied to the presentedsimultaneous detection of heavy metals and cyanide as arelatively ‘simple’ evaluation tool.

4 Conclusions Miniaturized ISEs on the basis ofchalcogenide glass materials have been applied for thesimultaneous detection of heavy metals and cyanide. Prior tothe simultaneous determination of heavy metals and cyanide,electrochemical investigations in Cu2þ-, Cd2þ- and Pb2þ-ionsolutions and CN�-ion solution with a Cu-, Cd-, Pb- and Ag-mISE, respectively, have resulted in good sensor propertiesin a concentration range over three decades. The Cu-mISEand Ag-mISE have presented lower detection limits of0.2mmol/l copper and 0.5mmol/l cyanide ions, respectively,which are even lower than the upper limits of such toxiccomponents in drinking water of 11mmol/l copper and1.1mmol/l cyanide ions defined by the WHO. Simultaneousmeasurements of heavy metals and cyanide in liquids havebeen performed in a specially designed, portable measuringsystem. This system consists of two chambers which areseparated by a diffusion membrane. The functionality of thismeasuring system has been proven and validated by means ofsimultaneous measurements of Cu2þ- and CN�-ions, Cd2þ-and CN�-ions, and by Pb2þ- and CN�-ions for different ionconcentrations in the m-molar concentration range.

Future work will deal with both ‘following’ fuzzy logicdata processing for the simultaneous multi-heavy metalanalysis together with cyanide detection and extension of themISEs to sensor array arrangements. Here, different field-effect-based sensor platforms such as capacitive electrolyte-insulator-semiconductor (EIS) structures [33], light-addres-sable potentiometric sensors (LAPS) [34] or ion-sensitivefield-effect transistors (ISFETs) [35] can advantageouslycombine the chemical recognition (receptor) layer togetherwith a first-stage microelectronic processing on the samechip. Moreover, alternative and highly long-term stable gateinsulator materials, like Ta2O5 or Al2O3 [36, 37], mightallow to improve the sensor’s behavior with regard to drift inthe short as well as the long-term scale.

Acknowledgements The authors gratefully thank theFederal Ministry for Education and Research (BMBF) within theproject ‘SAFE’, ‘K2 Forschungsforderung’ and the PhD fellowshipof FH Aachen for partial financial support.

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