Sensors and Actuators A 120 (2005) 589–595

A multisensor in thick-film technology for water quality control

Ramon Martınez-Maneza, Juan Sotoa,∗, Eduardo Garcıa-Breijob,∗, Luis Gilb,Javier Ibanezb, Elena Gadeab

a GDDS, Departamento de Qu´ımica, Centro de Investigaci´on en Qu´ımica Molecular Aplicada, Universidad Polit´ecnica de Valencia,Camino de Vera s/n, 46021 Valencia, Spain

b Grupo de Microelectr´onica Hıbrida, Departamento de Ingenier´ıa Electronica, Centro de Investigaci´on en Qu´ımica Molecular Aplicada,Universidad Politecnica de Valencia, Camino de Vera s/n, 46021 Valencia, Spain

Received 8 November 2004; received in revised form 3 March 2005; accepted 4 March 2005Available online 11 April 2005

Abstract

A set of sensors for water quality measurement has been developed. It consists of a multisensor designed to measure the following parameters:pH, temperature, dissolved oxygen, conductivity, redox potential and turbidity. The system has been built-up in thick-film technology, usingw umentationf urization,r apabilities.©

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oliFannmptsto

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menttru-ntrolherewa-asureure,andand

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hen possible commercially available serigraphic pastes. The multisensor can be implemented in in situ portable or stationary instror water quality monitoring. The technology used in the multisensor implementation provides fundamental properties of miniateasonable precision and low cost. The complete system also includes electronic acquisition, signal conditioning and computing c

2005 Elsevier B.V. All rights reserved.

eywords:Water quality; Thick-film electrodes

. Introduction

Traditionally the control of water quality has been carriedut by means of sampling, transport and further analysis at the

aboratory. Although this is a current methodology compris-ng some advantages, it also shows a number of drawbacks.or instance, there usually is a long gap between samplingnd detection of a certain kind of contamination; data areot obtained continuously; data are collected only at a smallumber of fixed sampling locations and these methods mightiss small-scale variations. Additionally, some water qualityarameters can only be determined in situ, and the sampling-

ransport-analysis protocol is, in general, accurate but expen-ive. Many of these disadvantages can be solved by using wa-er quality remote sensing technologies via the developmentf in situ and/or stand-alone water quality instrumentation. In

∗ Correspondence to: Juan Soto, Tel.: +34 963877343;ax: +34 963877349 and Eduardo Garcıa-Breijo, Tel.: +34 963877608;ax: +34 963877609.

E-mail addresses:jsotoca@qim.upv.es (J. Soto), egarciab@eln.upv.esE. Garcıa-Breijo).

this respect there is an increasing interest in the developof single, compact multiparametric integrated flexible insmentation capable of meeting changing water quality coneeds. Following this timely area of research we showthe development of multiparametric instrumentation forter quality measurement. The system is prepared to mein situ the following parameters of interest: pH, temperatdissolved oxygen, conductivity, ORP (redox potential)turbidity. The developed system is simple and compacthas been manufactured using thick-film technology.

2. Experimental

Although the development of sensors in thick-film tenology is not new[1–9], the development of novel innvative multiparametric instrumentation might be of inteas thick-film technology is an appropriate tool for sostate manufacturing which allows flexible designs offerent configurations and the fabrication of large numbesensors.

924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2005.03.006

590 R. Martınez-Manez et al. / Sensors and Actuators A 120 (2005) 589–595

Following the above-mentioned ideas, one central goalin our research was the development of easy-to-use, low-costand accurate enough instrumentation for water quality controlin thick-film technology, using when possible commerciallyavailable serigraphic pastes. The instrumentation we havedeveloped consists of a certain number of sensors in hybridtechnology that can be implemented in portable or stationaryinstrumentation for water quality monitoring. A descriptionof the set of thick film water sensors is given below.

2.1. Manufacturing process

The system containing a set of water quality sensors hasbeen prepared using thick-film technology procedures on aRUBALIT 708S (supplied by Ceramic Tec) alumina substratewith an area of 2 in.× 1 in. and a thickness of 0.64 mm. Inone side of the alumina substrate the electrodes for conductiv-ity, pH, ORP, dissolved oxygen and reference were located,whereas on the back the semiconductor sensors for temper-ature and turbidity were sited. Also for research purposesthe system also includes two individual electrodes (graphite)for the possible incorporation of certain ion-selective mem-branes. In order to prepare the above-mentioned set of elec-trodes, three screens were used corresponding to the conduc-tive layer, working as a conductor of the signal, the activelayer, which is different for different electrodes, and a pro-t /PtC wasm para-t dif-f pliedb byH itya r-a( -d 711( 5SPm catedf . Thes f 230m

CDF3) for final layers thickness of 10�m after firing. Forthe protective layer a screen of 180 mesh (Saatilane Hitech71/55) and a film of 50�m (Ulano CDF5) were used for athickness after firing of 15�m. The pastes were serigraphiedusing a semiautomatic serigraphic machine (AUREL model1010).

The firing process for the Ag/Pd/Pt, Ag and Au pasteswas carried out at 850◦C in a cycle of 60 min with a peak of10 min. For the resistive RuO2 paste, the firing was at 700◦Cin a cycle of 30 min with a 10 min peak. The protective pastewas fired at 550◦C in a 45 min cycle and a peak of 2 min.The graphite paste heated at 200◦C for 1 h and the Ag/AgClpaste was heated 10 min at 80◦C.

2.2. pH measurement

Traditionally, the measurement of pH has been made bymeans of the use of the well-known glass electrode. Nev-ertheless, although this electrode is very accurate it is rela-tively large and high in cost. As an alternative, we have usedhere, pH electrodes based on solid-state thick-film technol-ogy. Studies with different metal oxide pastes were carriedout and finally a thick-film electrode by screen printing ofruthenium(IV) oxide resistive paste for potentiometric pHmeasurements has been characterized. In the multiparamet-ric instrumentation the resistive pastes HERAEUS R8921 ofr p-i a of6 y,d ar tot es ofs en-s uptv -i pHr atert

sul-f redoxa cen-t -

; (a) co

ective layer (seeFig. 1). The conductive paste was Ag/Pd4081T (supplied by Heraeus). The layout of the tracksade to join the board to a flat cable connector with a se

ion of 3 mm between terminals. The active pastes for theerent electrodes were; (i) redox electrode: Au 8081 (supy ESL), (ii) reference electrode: Ag C11075S (suppliederaeus) and Ag/AgCl (supplied by GEM), (iii) conductivnd pH: resistive pastes of RuO2 R8921 (supplied by Heeus), (iv) dissolved oxygen: resistive pastes of RuO2 R8921supplied by Heraeus) and TiO2 (supplied by Ferro), (v) conuctive materials for ion-selective electrodes: graphitesupplied by ACP). The protective paste was the IP902odel (supplied by Heraeus). The electrodes were fabri

ollowing serigraphic methods, using polyester screenscreen for the conductive and the active areas was oesh (Saatilane Hitech 90/48) and a film of 30�m (Ulano

Fig. 1. Layout of the three layers

esistivity 99.5� sq−1 fired at 700◦C has been used. Tycally the pH-active surface has approximately an aremm2 and a thickness of 30�m. It presents a sensitivitynamic response and influence to interferences simil

he results obtained by other authors using different typensor materials[6,7]. Typically, the electrode showed a sitivity of −57± 3 mV/pH in a linear range of responseo 12 unities of pH and a standard potential of 650± 20 mVersus Ag/AgCl reference electrode (seeFig. 2). The pH testng was carried out using acid and alkali solutions. Theesponse time is lower than 5 s and the life time is grehan 6 months.

Interferences of common anions, cations (halides,ate, phosphate, sodium, potassium, calcium, etc.) andgents (sulfite, hydrogen peroxide) were studied in con

rations up to 0.01 M for the thick-film RuO2 electrode (elec

nductive, (b) active and (c) protective.

R. Martınez-Manez et al. / Sensors and Actuators A 120 (2005) 589–595 591

Fig. 2. Voltage variation vs. pH for a RuO2 electrode.

trode A). For the sake of comparison two other electrodeswere also prepared; a thick-film epoxy resin-RuO2 electrode(electrode B) and an oxidized metallic Ru bar (electrode C).Electrode A and C showed a very stable response and only se-rious interferences were observed in the presence of specieswith redox character such as iodide, sulfite or hydrogen per-oxide. For the electrode B, cationic interferences were ob-served.

2.3. Dissolved oxygen (DO) sensor

New designed dissolved oxygen potentiometric sensorsin thick film technology based in the use of RuO2 as ac-tive material and TiO2 as membrane have been used in thismultiparameter equipment. A complete study of this elec-trode has been recently published by us[10]. TiO2-coatedRuO2 electrodes showed a linear response as a function ofthe logarithm of the dissolved oxygen concentration in the0.5–8 ppm range. The electrode displays a Nernstian slopeof 59.4 mV/decade at 25◦C (seeFig. 3). The value of thisslope suggests the presence of a reaction involving one elec-tron per oxygen molecule that was tentatively attributed tothe formation of super-oxide ions at the electrode surface.Studies on TiO2-coated RuO2 electrodes as a function of theproton concentration showed a relatively low electromotive

Fa 5

Fig. 4. Scheme of conductivity measuring circuit and signals applied to theelectrode.

force variation at neutral and basic pH indicating that theTiO2 coating appears to be a suitable layer capable of effec-tively isolating the RuO2 surface from ionic species from thesolution.

2.4. Conductivity measurement

Electrolytic conductivityχ is defined as the conductanceG between two electrodes of an areaA separated by a dis-tanced. The cell thus formed is characterized by a constant:k=d/A. The conductivity is then expressed as:χ =Gk. Fora certain cell and for invariant measurement conditions,k isconstant and then the conductivity is directly proportional tothe conductance. For the conductivity measurement a capac-itive system has been implemented in thick-film by means ofthe use of an alternating square wave voltage that producesa fall of voltage between two electrodes proportional to theconductivity of the liquid. To achieve that, the applied signalis absent of a constant voltage, in order to avoid electrolysisof the solution, and a positive voltage between V1 and V2 isapplied and vice versa with the aim of get a periodical signalwith a zero charge mean value (seeFig. 4).

The conductance is calculated using the cell constant andthe temperature. The latter affects the conductivity as follows:

k

wm

e thee hitea alityr 511)o ellsa

timeo wereo

2

liq-u heni iedt tiono olved

ig. 3. Voltage response of the potentiometric TiO2-coated RuO2 electrodes a function of the logarithm of the oxygen concentration in water at 2◦C.

25 = km

1 + 0.02(tm − 25)

herek25 is the conductivity at 25◦C andkm the conductivityeasured at temperaturetm.Different serigraphic pastes have been tested to mak

lectrodes for the conductivity cell: gold, platinum, grapnd ruthenium(IV) oxide. We have found that the best quesponse/cost ratio was for resistive graphite (ESL RS-1r RuO2 pastes.Fig. 5shows the response of these two cs a function of the conductivity.

In the system developed a relatively short responsef less than 2 min and no significant hysteresis effectsbserved.

.5. Turbidity measurement

The turbidity is the reduction of the transparency of aid due to the presence of non-dissolved materials. W

nfrared optical radiation through a cloudy liquid is applwo effects can take place; (i) it might cause an attenuaf the radiation as consequence of the presence of diss

592 R. Martınez-Manez et al. / Sensors and Actuators A 120 (2005) 589–595

Fig. 5. Conductivity comparison for graphite and ruthenium(IV) oxide elec-trodes.

compounds and (ii) non-dissolved materials might cause adiffusion of the radiation in all directions.

The measurement of turbidity is regulated by the EuropeanNorm EN 27027 of December of 1999 that adopts the Interna-tional standard ISO 7027:1999. This norm specifies diversemethods for the determination of turbidity. Among them, itis described the optical method based on determining therelation between the dispersed and the attenuated radiation.Based on this standard, a turbidimeter has been constructedwith an infrared light transmitter (E) and two receivers (R90◦)and (R0◦) separated 1 cm from the transmitter. One of the re-ceivers is located opposite the transmitter and gathers thedirect transmission of the radiation measuring the absorptionand the other receiver has been located at a direction of 90◦with respect to the emitting beam and gathers the radiationdispersed by non-dissolved particles (seeFig. 6).

Calibration of the turbidimeter was carried out by usingformazine (C2H4N2) solutions in water. The turbidity is thenmeasured in Formazine Attenuation Units (FAU).

A superficial assembly in thick-film hybrid technologyhas been set for the assembly of the turbidimeter. For theemitting element an electroluminescent diode of GaAlAs(model SHF426) by Siemens was used. For the receiving el-ement a phototransistor of infrared NPN by Siemens (modelSHH325FA) was employed. In order to make the measure-ment in the receivers, a current converter has been built using

.

Fig. 7. Results of turbidity measures in FAU.

a operational amplifier of very low current of polarizationOPA15.Fig. 7shows a typical calibration curve. A slope of2.5× 10−4 FAU−1 was observed.

2.6. Reference electrode

Probably one of the most popular reference electrodes isthat made with the redox couple Ag/AgCl and certain at-tempts to use it in thick film configurations have alreadybeen published[11]. The reference electrode incorporatedin the sensor device has also been made using thick filmtechnology. In our case the reference electrode was built upwith three active layers; the first one was the commerciallyavailable polymeric thick-film silver-containing ESL1109-S paste, the second layer was a C50672R1, GEM Ag/AgClpaste and finally a membrane containing KCl was placed.For this final layer different membranes containing finely di-vided crystalline KCl (concentrations higher than 15%) weretested. Among several potential membranes studied those thatgave suitable results were the dielectric paste D2020529D1of GEM, the encapsulating resin EPOTEK H77 and a simplemembrane made with polyisophthalamide diphenylsulfone(PIDS). In the three cases, a reference electrode in thick-filmconfiguration was obtained with a standard voltage of ap-proximately 0.2 V versus NHE at 25◦C. Fig. 8 shows thee tain-i er a

F branev

Fig. 6. Layout of the optical sensors in the turbidity measurement

lectrode potential versus time when the electrode conng the membrane PIDS in water was monitorized. Aft

ig. 8. Voltage of the reference electrode containing the PIDS mems. time.

R. Martınez-Manez et al. / Sensors and Actuators A 120 (2005) 589–595 593

period of some few days (from 4 to 5 days) the potential re-mains constant. A lifetime higher than 6 month, with a driftlower 10 mV per day was observed. A complete study of theresponse of these references electrodes will be published indue course.

2.7. Temperature and redox potential

It is possible to make temperature sensors with thick-film technology by using standard resistive pastes contain-ing ruthenium oxide or platinum (thermometric resistances).However, although temperature sensors based on thermomet-ric resistances are very precise, they are also excessively highin cost and need electronic circuits of adaptation. As an al-ternative we have used a semiconductor temperature sensor(LM50D with encapsulation SOT-23) in superficial assemblytechnology. This sensor has a sensitivity of 10 mV/◦C a pre-cision of±2◦C at 25◦C and can work within a 40–125◦Ctemperature range. Also as an advantage the semiconductortemperature sensor is rather small in size and low in cost.

The electrode for measuring the redox potential (ORP)was made using an ESL 5715 gold paste.

3. Results and discussion

3

beenp thec elec-t uctorsF trodes( em-b s-t ea oft stemi ronic

Fig. 10. Stationary unit for water quality monitoring.

acquisition system by means of a flat multisignal connec-tor. This housing can be immersed directly in the liquid orlodged in a small, portable stand-alone integrated system ascan be seen inFigs. 10 and 11. This instrumentation containsa power block that controls a set of electrovalves and pumpsthat were designed for automatic water sampling, electrodecleaning and calibration. The system is also capable of remotetransmission of the data.

3.2. Data acquisition system

The measurement system is formed by a printed circuitboard of surface mount technology (SMT), containing the cir-cuits for the signal conditioning of each sensor, and a generalpurpose analog and digital acquisition card, model ADLINKPCI9112, inserted in one PCI bus slot of a personal computer.A total of 12 analog inputs are used for the signals providedby each sensor.

The software has been developed in the visual program-ming language VEE Pro, providing a suitable graphic userinteractive interface and with information on screen (seeFig. 12). The advantage of this programming language is thatit allows, with minimum time and with no need to be an ex-pert programmer, quick modifications in the acquisition and

water

.1. Multisensor

The set of water quality sensors described above havelaced on a 2 in.× 1 in. alumina substrate. In one faceonductivity, pH, ORP, dissolved oxygen and referencerodes are located, whereas on the back the semicondensors for temperature and turbidity are sited (seeFig. 9).or research purposes the system also has two elecgraphite) for the incorporation of certain ion-selective mranes (see Section2 for fabrication details). The whole sy

em is covered with a transparent resin, except the arhe electrodes that must be in contact with water. The sys lodged in a PVC housing and connected to the elect

Fig. 9. Set of

quality sensors.

594 R. Martınez-Manez et al. / Sensors and Actuators A 120 (2005) 589–595

Fig. 11. Acquisition control system.

Fig. 12. Data acquisition system screen.

graphical representations, as well as in the calculation anddata management. The program makes a sequential sweepingof all inputs, accepting the selection of sampling frequency atany time (1 s by default). It also makes the necessary calcu-lations to obtain the parameters (pH, temperature, turbidity,etc.) for the voltage readings. The final values, correspond-ing to each one of the water quality parameters are recordedcontinuously and automatically in an ASCII file, organizedin columns, easily importable by other programs or spread-sheets such as EXCEL. In order to facilitate the analysis ofresults a temporal mark, indicating the day, hour and secondof the acquisition is inserted in the mentioned file.

4. Conclusions

In summary, a set of sensors including temperature, con-ductivity, redox potential, pH and dissolved oxygen in thickfilm technology for water quality measurements has beendeveloped. The whole system has been made using thickfilm fabrication protocols employing, when possible, com-mercially available serigraphic pastes. The system lodgedon a PVC housing can be used as simple multiparamet-ric probe or can be incorporated in a small, portable andstand-alone integrated system for in situ water quality mon-itoring. The reduced size and low cost of the set of sen-

R. Martınez-Manez et al. / Sensors and Actuators A 120 (2005) 589–595 595

sors makes them suitable for a wide range of applica-tions.

Acknowledgements

We thank the Spanish Ministerio de Ciencia y Tec-nologıa (AMB99-0504-C02-01 and MAT2000-1387-C02-02) for support. We also thank the Polytechnic Universityof Valencia for support.

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[3] W.F. Chu, V. Leonhard, Thick-film chemical sensors, Sens. ActuatorsB 4 (1991) 321–324.

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ncefilmeters,

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Biographies

Ramon Mart ınez-Manezwas graduated in Chemistry from the Universi-tat de Valencia in 1986 and received his PhD in 1990 in the same Univer-sity. After a postdoctoral period at Cambridge (UK), he joined the Depart-ment of Chemistry at the Universidad Politecnica de Valencia. He becamefull professor in 2002. His research interest comprises the developmentof new electrochemical, fluorogenic or chromogenic chemosensors andmolecular probes for anions, cations and neutral chemical species.

Juan Soto Caminowas graduated in Chemistry from the Universitat deValencia in 1981 and received his PhD in 1986 in the same University.He is currently associate professor in the Department of Chemistry atthe Universidad Politecnica de Valencia. His main areas of interest arethe development of chemical chemosensors and probes, especially thosebased on electrochemical processes.

Eduardo Garcıa-Breijo was graduated in Electronic Engineering fromthe Universitat de Valencia (Spain) in 1997, and received his PhD in2004 from the Universidad Politecnica de Valencia (UPV). His thesis wasfocused on the development of multisensor for water quality parameters inthick-film technology. He is assistant professor of Electronic Technologicin the Electronic Engineering Department of the UPV. His is a memberof the Hybrid Microelectronic Laboratory at the UPV.

Luis Gil was graduated in Electronic Engineering from the Universi-tat de Valencia (Spain) in 1998. He is assistant professor of ElectronicTechnologic in the Electronic Engineering Department of the UniversidadPolitecnica de Valencia (UPV). His is a member of the Hybrid Microelec-tronic Laboratory of UPV. His main areas of interest are the chemicalsensors, instrumentation systems and pattern recognition for electronict

J ni-v essoro t oft theH resta

E theU BAi as-s eringD ry andC ent oft part-m

[7] J.K. Atkinson, A.W.J. Cranny, An investigation of the performacharacteristics and operational lifetimes of multi-element thicksensor arrays used in the determination of water quality paramSens. Actuators B 54 (1999) 215–231.

[8] J.A. Mihell, J.K. Atkinson, Planar thick-film pH electrodes baon ruthenium dioxide hydrate, Sens. Actuators B 48 (1998)511.

[9] W. Glasspool, J.K. Atkinson, A screen-printed amperometricsolved oxygen sensor utilising an immobilised electrolyte gelmembrane, Sens. Actuators B 48 (1998) 308–317.

10] R. Martinez-Manez, J. Soto, J. Lizondo-Sabater, E. Garcıa-Breijo, L.Gil, J. Ibanez, I. Alcaina, S. Alvarez, New potentiometric dissoloxygen sensors in thick film technology, Sens. Actuators B(2004) 295–301.

11] A.W. Cranny, J.K. Atkison, Thick film silver–silver chloride refeence electrodes, Meas. Sci. Technol. 9 (1998) 1557–1565.

ongues.

avier Ib anez is Matrisse in Power Electronic and Control from the Uersite Pierre et Marie Curie (Paris VI) in 1994. He is assistant proff Electronic Technologic in the Electronic Engineering Departmen

he Universidad Politecnica de Valencia (UPV). His is a member ofybrid Microelectronic Laboratory of UPV. His main areas of intere water organic contamination devices.

lena Gadea Morant was graduated in Telecommunications fromniversidad Politecnica de Valencia (UPV) in 1994, and received a M

n 2002 from the Escuela de Organizacion Industrial. She has beenistant professor of Electronic Technology in the Electronic Engineepartment, and at present she is assistant professor of Signal Theoommunication Systems Planning in the Communications Departm

he UPV. She is preparing her PhD in the Electronic Engineering Deent.