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Sensors and Actuators A 169 (2011) 1– 11

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

jo u rn al hom epage: www.elsev ier .com/ locate /sna

eview

flexible pH sensor based on the iridium oxide sensing film

en-Ding Huanga,∗, Hung Caoa, Sanchali Deba, Mu Chiaob, J.C. Chiaoa

Department of Electrical Engineering, University of Texas at Arlington, Arlington, TX 76019-0016, USADepartment of Mechanical Engineering, University of British Columbia, Vancouver, BC V6T1Z4, Canada

r t i c l e i n f o

rticle history:eceived 26 August 2010eceived in revised form 6 April 2011ccepted 12 May 2011vailable online 20 May 2011

a b s t r a c t

We have developed a novel flexible pH sensor based on a polymeric substrate by low-cost sol–gel fabri-cation process of iridium oxide (IrOx) sensing film. A pair of miniature IrOx/AgCl electrodes on a flexiblesubstrate generated electrical potentials in solutions by electrochemical mechanisms responding to theirpH levels. Our flexible IrOx pH sensors exhibited promising sensing performance with a near-Nernstianresponse in sensitivity repeatedly and reversibly between −51.1 mV/pH and −51.7 mV/pH in the pH range

◦

eywords:

ridium oxideiniature pH sensor array

olyimide flexible substrateol–gel processeformable sensors

between 1.5 and 12 at 25 C. The fabrication processes including sol–gel deposition, thermal oxidation,and AgCl electro-plating on polymeric substrates were reported. The performance and characterizationof the flexible pH sensors in sensitivity, response time, stability, reversibility, repeatability and selectiv-ity were also discussed. Our IrOx pH electrodes on a deformable substrate demonstrated their sensingcapability while they were conformed to a curved surface inside a limited space with distinct respondingpotentials at various pH levels similar to the traditional glass-rod pH electrodes.

Published by Elsevier B.V.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Sensor fabrication and measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1. Sensor fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.1.1. Sol–gel process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.1.2. Electroplating process of the AgCl reference electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2. Measurement procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.1. pH sensing mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.2. Sensor performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2.1. Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.2.2. Response time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2.3. Stability and repeatability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2.4. Reversibility and hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2.5. Selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.2.6. Temperature dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.2.7. pH sensor array tube test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

. Introduction

pH sensor is a common tool used in chemical laboratories andndustrial factories since many biological and chemical reaction

∗ Corresponding author. Tel.: +1 469 487 8060.E-mail address: hendiauhuang@yahoo.com (W.-D. Huang).

924-4247/$ – see front matter. Published by Elsevier B.V.oi:10.1016/j.sna.2011.05.016

mechanisms are pH dependent. Conventional glass-type electrodeshave been widely used, however; still have certain disadvantagesin specific applications. The glass rod sensor configuration has dif-ficulty to be used for in vivo biomedical, clinical or food monitoring

applications due to the brittleness of glass, size limitation and thelack of deformability. To achieve small sizes and robust design, ion-sensitive field-effect transistor (iSFET) pH sensors [1,2], optical fiberpH sensors [3,4], hydrogel film pH sensors [5], and solid-state pH

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ensors [6,7] have been proposed. iSFET sensors have power con-umption concerns due to the FET operation requirements [8] whensed for in vivo applications. Hydrogel film pH sensors utilize thehysical properties of the pH-responding swelling and shrinkingolymer to measure electrical resistance changes [8]. The sensortructure design and polymer layer fabrication processes could beomplicated and expensive [5]. The electromechanical responsesre usually slow. Optical pH sensors also have power consumptionssues due to the use of light sources. The system including opticalevices could be expensive or unsuitable for long-term implanta-ion [1].

In many biomedical in vivo applications, reduction of power con-umption and elimination of wire connection are important issues.n order to achieve passively powered and wireless signal trans-uction functionalities in applications which require pH sensorso conform to curved surfaces, with cost effectiveness and simplerabrication procedures, a thermally treated sol–gel metal-oxide pHensing film, which can be integrated with electrical circuits for sig-al transduction, on a flexible substrate to provide deformabilityas proposed in our work.

Various solid-state metal oxides have been investigated for pHensing electrodes [1,9,10] including PtO2, IrOx, RuO2, OsO2, Ta2O5,hO2, TiO2 and SnO2 as the pH sensing films. Typically, the pHensitivity, selectivity, working range, and hysteresis indicate sens-ng performance. IrOx, RuO2 and SnO2 have been demonstrated

ith more advantages in sensor performance for various applica-ions compared to others [9]. RuO2 [7,10] and SnO2 [11] showedear-Nernstian responses in wide pH ranges, however, presentedysteresis and drift problems leading to potential calibration issuesnd unstable responses [10,11]. Iridium oxide film has performedutstanding stability over wide pH ranges, rapid responses, lessotential drift and high durability, which have also been tested atigh temperatures up to 250 ◦C [12].

There are different fabrication methods of iridium oxide filmIROF) including sputtering deposition [12], electrochemical depo-ition [13,14], thermal oxidation [12], and sol–gel [15,16] processes.he sputtering iridium oxide film (SIROF) deposition process pro-uces uniform films with good quality, however, is costly due to thearget costs. The oxygen and argon pressure ratios, position of thearget, deposition rates, and RF powers during the fabrication pro-esses all affect the pH sensing parameters such as potential driftsnd redox interferences [9]. Anodic iridium oxide thin film (AIROF)rocess is based on electrolysis of a solution containing iridiumomplexes and can be an economical way for iridium oxide thin filmabrication. The iridium tetrachloride compound has been widelysed as a deposition agent such as the commonly used Yamanakaolution [13]. The pH value of the deposition solution, solutionemperature and current density control may easily affect the depo-ition efficiency [13]. A precise power supply system as potentiostats required in the electro-deposition process for thickness and filmuality control. For thermal oxidation process, it requires a highemperature ranging from 500 to 800 ◦C [6,9]. The film made byhermal oxidation can be thicker than the AIROF providing moretable potentials [9,12]. However, the film surface has a tendencyo crack due to the high temperature treatment. The adhesion prop-rty of the cracked film then becomes an issue for sensing. The highemperature treatment also presents a limitation during sensorabrication, especially for the use of polymer and photoresist as sac-ificial materials for defining film electrode patterns, which oftenannot survive at a temperature above 200 ◦C, and the integrationf integrated circuits, especially CMOS circuitry with the sensinglm. The sol–gel IROF deposition process has been demonstrated

ith dip coating [16] and heat treatment [15,16] techniques which

ould provide a simpler and economical fabrication approach.In this paper, we utilized the sol–gel process previously reported

y Nishio and Tsuchiya [16], which has only been demonstrated

ctuators A 169 (2011) 1– 11

before on rigid substrates, to make amorphous uniform IROFpH-sensing films on flexible polyimide substrates with lower-temperature thermal oxidation. Our flexible IROF pH sensorperformed with high pH sensitivity, good potential stability, lowpotential drift, low ion-interference, fast time response, excellentreversibility and repeatability in pH sensing tests. Furthermore, theadvantages of our sensor also include the simpler and potentiallylower cost fabrication processes. The sensor device architecturebased on deformable flexible substrates will enable new pH sensorapplications such as in vivo biomedical, biological, clinical [17–19],food monitoring [20] and lubricant applications [21].

2. Sensor fabrication and measurements

2.1. Sensor fabrication

Our pH sensor was fabricated by standard photolithographyand lift-off processes. All of the metal layers were deposited byelectron-beam evaporation. The complete fabrication procedureis illustrated in Fig. 1(a)–(f). First, a layer of 7-nm thick Cr wasdeposited on a piece of polyimide substrate, followed by a 0.1-�m thick layer of Au. A 0.4-�m thick iridium oxide sensing filmwas then formed by the sol–gel process [16] which will be intro-duced in the following discussion. 7-nm thick Cr and 3-nm thickPt were evaporated for adhesion. A 30-nm thick silver layer thenwas deposited and silver chloride (AgCl) reference electrodes wereformed by electroplating on silver. Fig. 1(g) shows the fabricatedsensors on a flexible polyimide substrate with 3-pair electrodes.The sizes of working and reference electrodes in our prototypedevices were 2 × 2 mm2, as shown in Fig. 1(h). The working andreference electrodes are connected with metal lines to probingcontacts. The connection lines and contacts were designed in mil-limeter scales for ease of handling. However, the electrode sizes andconnection lines could be in submicron- or micron-scales, which isdetermined by the photolithography and photo-mask resolutions,while the contact pads will be eliminated in the future practicaldevices.

2.1.1. Sol–gel processIrOx films were selectively deposited with sol–gel processes

onto the gold electrodes. The electrodes were exposed throughsmall windows in the SU-8 sacrificial layer, as shown in Fig. 1(b).The sol–gel solution was based on the recipe described in [16] withmodification. One gram of anhydrous iridium chloride (IrCl4) wasdissolved in 42 ml of ethanol (C2H5OH) and 10 ml of acetic acid(CH3COOH) was added in the solution. The coating solution wasstirred with a magnetic rod for 1 h. Thin film was then formed bydip coating at a 2.0-cm/min withdrawing rate in the solution. Afterdip coating, the sample was thermally treated with a heating pro-file starting at 25–300 ◦C in a 2-h period. The temperature stayed at300 ◦C for 5 h. The furnace was then cooled down in a 10-h periodto 25 ◦C.

In order to obtain an iridium oxide sensing film, the surface wassuggested to be heated at above 300 ◦C [15,16] for oxidation. How-ever, the crystallization of materials will introduce surface crackson the IrOx films which normally begin at 400 ◦C. For sensing andelectrical conduction, a uniform and robust film surface is required.After experimenting at different temperatures for annealing, wefound that heat treatment at 300 ◦C provided a good film quality asindicated by the iridium oxide thin film that is close to amorphousand fine grain structures. The morphology of the IrOx surfaces pro-

duced at peak treatment temperatures of 300 ◦C and 550 ◦C with thesame heating profile is shown and compared in scanning electronmicroscope (SEM) photos (ZEISS Supra 55 VP) in Fig. 2 (a) and (b),respectively. At 300 ◦C peak temperature treatment, the surfaces

W.-D. Huang et al. / Sensors and Actuators A 169 (2011) 1– 11 3

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ig. 1. Fabrication processes: (a) Cr and Au deposition on a polyimide substrate, (b) Se) Cr, Pt and Ag deposition, and (f) AgCl electroplating. (g) A photo of a fabricated d

id not produce cracks, even at the boundaries between metal andrOx where the thermal expansion difference between two mate-ials usually induced cracks, while multiple cracks were observedn the IrOx surface like the ones shown in Fig. 2(b).

.1.2. Electroplating process of the AgCl reference electrodeElectrochemical anodization process was used on the anodic sil-

er electrode pattern along with a platinum cathode electrode in.1-M HCl solution. An electrical current of 0.5 mA was applied onlectrodes in HCl solution for 5 s. During the electrolysis, a brownilver chloride layer was formed on the silver surface of the poly-

mide substrate. The electrode surface was rinsed by DI water andhen immersed in 3 M KCl solution for 24 h to saturate and stabi-ize potentials. The AgCl thickness was measured as 1.5 �m by aLA-Tencor profilometer.

00 deposition for the sacrificial layer, (c) IrCl4 sol–gel process, (d) thermal treatment, containing three pairs of sensor electrodes. (h) The dimensions of the device.

2.2. Measurement procedures

An Agilent 34401A digital multi-meter with a GPIB interfaceoperated in a LabVIEW-based program was used for real-timepotential recording. A unit-gain amplifier (Texas InstrumentTLC074 op-amp) as a buffer providing an appropriate impedancematch was used. During the experiment, our pH sensors weretested using acid- or alkaline-based diluted solutions. The differ-ent pH levels of solution were prepared by mixing deionized (DI)water with different concentrations of hydrochloride (HCl) acid orpotassium hydroxide (KOH). The pH level of each test solution was

confirmed by a commercial pH glass-electrode (HI98128, HANNAInstruments) sensor. The potentials and pH values were displayedand recorded in the digital multi-meter and computer simultane-ously.

4 W.-D. Huang et al. / Sensors and Actuators A 169 (2011) 1– 11

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. Results and discussion

.1. pH sensing mechanism

Three possible mechanisms were involved for pH dependentedox intercalation equilibrium between two oxidation states ofhe iridium oxide [22] as

r2O3 + 6H+ + 6e− ↔ 2Ir + 3H2O (1)

rO2 + 4H+ + 4e− ↔ Ir + 2H2O (2)

IrO2 + 2H+ + 2e− ↔ Ir2O3 + H2O (3)

nd the redox potential is determined by

= E0 − 2.303RT

FpH = E0 − 0.05916 pH (4)

here E0 is the standard electrode potential with a value of 926 mVor a standard hydrogen electrode (SHE) or 577 mV for a Ag/AgCleference electrode. F is the Faraday’s constant with a value of6,487 coul/equiv, and R is the gas constant with a value of 8.314 J/◦.T/F is equal to 25.688 at 25 ◦C. Thus, the pH potential sensitivityill be −59 mV/pH if all of the space charges are formed, which

s called the Nernstian response [9,22,23]. In Fig. 3(a), near idealernstian slopes were obtained experimentally with our flexible

H sensor. Three tests were performed with the same sensor andhe fitting slopes were −51.7, −51.6 and −51.1 mV/pH with r2 val-es larger than 0.95. The intercepts at pH = 0 indicated that E0 was12, 523, or 521 mV for the Ag/AgCl reference electrode on the flex-

Fig. 3. Measured sensitivity of our IrOx flexible pH sensor from (a) pH = 1.5 topH = 12.1, and (b) from pH = 12.1 to pH = 1.5.

ible polyimide substrate in our sensor which are slightly differentfrom the that of a standard hydrogen electrode (SHE), 577 mV, fora Ag/AgCl reference electrode on a glass substrate.

There are some factors which may be accounted for the discrep-ancies from the theoretical values of potential in the pH sensingmechanism. The variations of E0 may be due to the variations inthe stoichiometry of the oxide compounds, and the difference inoxidation states of iridium oxides [9]. In addition to the solid phaseand intercalation equilibriums, Fog and Buck [24] presented thatpH response variations could also be due to the ion exchanges inthe surface layer of electrode film containing OH− groups. The gen-eral equilibriums investigated by Burke et al. [25] and Olthuis et al.[26] for proton exchange were

[–IrIV–(OH)x–IrIV–]n + 2ne− + 3nH+ ↔ [–IrIII–(OH)x−3·3H2O–IrIII–]n (5)

Ir2O(OH)3O33− + 3H+ + 2e− ↔ 2Ir(OH)2O− + H2O (6)

The equilibrium of (6) indicated that the theoretical sensitivitymay be different from −59 mV/pH given more electrons could beproduced. On the other hand, the surface layer of electrodes usu-ally is not perfectly smooth and the micro- or nano-scale pores in

the film surface may also influence the ion exchanges and reducethe ion production. Both factors that are related to the mechanicaland chemical properties of the sensing film, along with three redoxequilibriums in the solution that are related to hydrodynamics,

and Actuators A 169 (2011) 1– 11 5

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Table 1Comparison of the sensitivity for different types of pH electrodes [44].

pH electrode type Sensitivity(mV/pH)

Glass (Microelectrodes Co.) 54.9Glass (Radiometer Co.) 55.1Antimony electrode 47.6

W.-D. Huang et al. / Sensors

nevitably affect the potential sensitivity and create discrepancies.hese discrepancy factors then will also occur in each experimentnd in different fabrication batches.

.2. Sensor performance

.2.1. SensitivityThe sensitivity of our flexible IrOx pH sensor was validated by

ipping the sensor in different pH levels of solutions at the roomemperature. 0.1 M of HCl and 0.1 M of KOH were used to adjusthe pH levels of the testing solutions. We tested each sensor elec-rode three times in the same solution in order to demonstratehe near-Nernstian response and the consistency in sensing for ourlectrodes. The sensors were immersed in the solution for 5 s, takenut to be washed by DI water, dried by compressed air and testedgain. The tests were conducted by measuring the sensor in theost acidic solution (pH = 1.5) first and gradually increasing the pH

evels of solution to the most alkaline level (pH = 12.1). After threeests, the tests were then conducted in the reversed order fromlkaline to acid for three times.

Fig. 3(a) shows the potential responses with eight differentH levels of solution from 1.5 to 12.1. The results showed con-istent sensitivity of −51.1, −51.6, and −51.7 mV/pH with highorrelation coefficient r2 values between 0.95 and 0.959. Fig. 3(b)

hows the pH responses tested in the reversed order with eightifferent solutions from 12.1 to 1.5. The sensitivity results were48.9 mV/pH, −50.1 mV/pH and −51.1 mV/pH. The r2 values of

hese linear regressions were between 0.95 and 0.953.

Fig. 4. Temporal response in titration (a) from pH = 3.9 to pH = 11, (b) from pH = 12 t

Flexible IrOx pH electrode 51.1

Comparing with the sensitivity of an anodic iridium oxide film(AIROF), the sensitivity of the IrOx film made by sol–gel thermaloxidation process is naturally less according to literatures [9,26].The main reason is that AIROF has more porous surfaces whichintroduce the presence of many hydroxyl groups to ensure highionic conduction [26,27]. The anhydrous IrOx films made by sput-tered iridium oxide film (SIROF) or sol–gel [15,16,28] usually havesmaller numbers of hydrophilic sites, due to low porosity, whichare responsible for proton and electron transfer during redox pro-cesses in the oxide films. Our flexible pH electrodes based on theanhydrous IrOx film, although did not produce the highest sensi-tivity, yet showed a consistent and comparable sensitivity to AIROFand SIROF [9,29,30].

Our experimental sensitivity was compared with the sensitiv-ity of commercial glass-based pH electrodes in Table 1 [31]. The

comparison shows that our flexible pH electrode achieves simi-lar performance as the conventional electrodes yet with additionaladvantages of the smaller sizes and being deformable.

o pH = 3.5, and (c) from the dry condition to a solution droplet with pH = 4.01.

6 and Actuators A 169 (2011) 1– 11

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W.-D. Huang et al. / Sensors

.2.2. Response timeThe response time of our flexible pH sensor was measured in

hree different tests. The first test was from the acid to alkaline con-ition by quickly dripping 0.1 M KOH into an acidic solution wherehe sensor was. The second one was from alkaline to acid by quicklyripping 0.1 M HCl into an alkaline solution with the sensor in theolution. The third one was tested by dripping diluted HCl dropletsirectly on the dry sensing electrode surface of the sensor. In gen-ral, the pH sensor response time has not been universally definedn literatures or product specification sheets since the environmen-al parameters are quite different under different sensing scenarios.n our work, the response time is defined as the time needed forhe potential change to reach 90% within the equilibrium value ofotential [9].

Fig. 4 shows typical results for the response–time tests. Fig. 4(a)hows a response time of 0.9 s with a measured potential stephange in our flexible pH electrode from pH = 3.9–11. Fig. 4(b)hows the result from pH = 12–3.5. The response time is about 2 s.n both tests, the pH values of the solutions were verified 15 s afterhe events by the commercial pH sensor (HANNA Instruments) inhe beaker since the reading of the HANNA sensor also varied withime. Fig. 4(c) shows the result from the dry surface condition toH = 4.01. The response time is about 0.8 s. After 10 similar exper-

ments were conducted, we concluded that our flexible pH sensoresponds within 2 s.

Compared with the response times of 5–15 s reported in liter-ture [32], the response time of our IrOx flexible pH sensor washorter and consistent with different pH level changes. This maye due to the better quality of the IrOx film with appropriate coat-

ng and annealing processes in our sol–gel fabrication, as suggestedy Olthuis et al. [26] that the response time is mostly affected by theorous properties of the sensing film. The bulk pH solution needso equilibrate the liquid in the pores of iridium oxide film in whichhe process increases the response time. Thus, although our sol–gelrOx film has lower porosity, which provides less sensitivity thanorous AIROF and SIROF, the sensor responds quicker.

.2.3. Stability and repeatabilityIn the previous sensitivity test, the electrode exhibited distinct

otential responses at each different pH level. The measurementsere carried out by placing the sensor in individual solutions of

arious pH values. The sensor was washed with DI water betweenhe experiments. The potential generated in each buffer solutionith static pH was not affected by environmental parameters from

ther solutions.However, practical issues such as stability, repeatability and

eversibility in applications should be considered in which the sen-or encounter environmental factors. Different literatures definedtability, repeatability and reversibility differently for their respec-ive applications and general terms could not be found, to the bestf our knowledge. For clarification purpose, the related terms areefined and summarized in Fig. 5. In the following measurements,ur sensor was tested continuously, without DI water wash, in aolution to identify potential fluctuation (�V), potential deviationıV), and potential drift (V′), shown in Fig. 5(a), and hysteresis (dV),hown in Fig. 5(b).

The �V is defined as a small and non-random voltage fluctuatingange which happens after the detected electrochemical potentialas reached a stable condition. The potential fluctuation may beaused by the noises in the recording instrument or interferenceuch as the global or local motion in liquid during tests. The poten-ial deviation (ıV) is defined as the potential difference among the

ests, after the potentials stabilize, with the same electrode testedn the same solution. The potential deviation may be caused by fac-ors such as the oxidation states and the surface OH− ion exchangesn iridium oxide. These phenomena may generate new equilibrium

reversibility.

every time when the solution is disturbed, which causes surfacecharge variations with time [9,25,26]. The potential drift (V′) isdefined as the difference between the peak potential value andthe 90% value of the saturated potential. The potential drift maymainly be caused by the reasons such as dynamic processes of ionneutralization called the effect of liquid junction potential [33]. Thesensor age and different fabrication methods may also be the factorsaffecting surface oxidation states and porosity [9]. These parame-ters of potential fluctuation (�V), deviation (ıV) and drift (V′) are allrelated to pH measurement errors which impact the measurementresolution and accuracy.

Our flexible pH electrode was immersed in eight different pHbuffer solutions at the room temperature for three times, and eachtime for 4 min to test sensor stability and repeatability. The sen-sor was cleaned by DI water and dried between the tests. Fig. 6shows the potential changes with time. The potential values mostlystayed constant for each pH level with small potential fluctuations(�V) between ±0.3 mV and ±1 mV. The potential fluctuations werecomparable to the results shown in literature [34] which rangedmore than ±1 mV in a pH = 4 buffer solution. The �V of our sensorwas small enough compared with potential changes to recognizedistinct pH levels.

For the potential deviations (ıV), the values were less than 5 mVin all cases showing stable outputs in each experiment, which wereacceptable compared to the results in literatures [9,35] and pro-vided a minimum resolution of potential to distinguish pH valuesduring measurements. With output potentials between −0.07 V

and +0.46 V for solutions in the range between pH = 1.5 and 12.1,the ideal resolution should be 0.02 pH/mV. However, with a max-

W.-D. Huang et al. / Sensors and Actuators A 169 (2011) 1– 11 7

Fr

is

3tnttr[ttdsadiottwvi

ommawvod

3

sdcwsasdp

Fig. 7. Measured results for reversibility and repeatability experiments with pH

ig. 6. Measured potentials in eight different pH levels to demonstrate stability andepeatability.

mum potential deviation of 5 mV, the minimum resolution of pHensing then becomes 0.1 unit of pH change.

For the measured potential drifts (V′), the drifts ranged from mV to 10 mV in different tests and the potentials stabilized fromhe drift peaks in all cases within 5 s. The potential drift phe-omenon was common in other ion selective pH sensors [36] andhe 3–10 mV drift was considered small. It should be noted thathe potential drifts mainly depend on the test scenarios which areelated to liquid junction potential or diffusion potential effects33]. The phenomenon usually occurs at the solution containingwo different ion concentrations. The higher concentration solutionends to diffuse into the lower concentration one. During the ioniciffusion, if anions diffuse faster than the cations, it will produceeparation of electrical charges at the junction between the sensornd solution [33]. Therefore, a temporary differential potential iseveloped. In our experiments, we stirred the solution constantly

n each test with a magnetic rod in the bottom of the beaker inrder to reduce the effect of junction potential. The ion concen-ration around the sensing film reaches equilibrium quicker thanhe one without stirring as the junction potential stabilizes quickerith mixing. In practical scenarios, the potential drifts (V′) could

ary significantly since the stabilization of junction potential variesn different environments.

Moreover, �V, ıV and V′ may be also due to the variationsf solution temperature since the test solutions were not ther-ally stabilized in our tests [9], and the random hydrodynamicicro-environment where micro-scale air bubbles might be cre-

ted around or in the micro/nano-pores on the sensing film surfacehen the sensor was immersed into the solutions. With the small

ariation values in �V, �V, and V′, the stability and repeatability ofur flexible IrOx pH sensor were demonstrated to be able to identifyistinct pH values.

.2.4. Reversibility and hysteresisIn the previous measurements, stability and repeatability of the

ensor was demonstrated with the sensor cleaned in DI water andried with compressed air between each test. However, in a practi-al environment, the pH sensor will be used in a long period of timeith randomly varied pH levels. To demonstrate reversibility, our

ensor was tested in a pH titrated cycle with pH = 1.5 to pH = 13.1

nd back to pH = 1.5 continuously without cleaning and drying theurface of electrode. During the titration process, KOH or HCl wasripped into the base solution in order to increase or decrease theH level of the base solution. A commercial pH electrode based on

varying from 1.5 to 13.1, and back to 1.5 in titration. The experiments were repeatedby three cycles.

glass substrate (HI98128, HANNA Instruments) was used to ref-erence the pH values of the titrated base solution. The one-timereading from the HANNA sensor was taken 10 s after the titra-tion event. The titration and measurement cycles were repeated bythree times, and the results were plotted with the cycles overlappedin Fig. 7. The distinct and constant potentials clearly responded andrecognized eight different pH levels in a complete titration cycle.In each step, stability, repeatability and reversibility (�V, ıV andV′) could be indentified and their values were within the rangesmentioned in the previous discussion.

However, during the titration process, the electrochemicalpotentials at the same pH level may be different. The phenomenonis called hysteresis [10,26] which is defined by us as the value dVshown in Fig. 5(b). Hysteresis commonly exists in IROF and othermetal oxide pH-sensing films [26]. The phenomenon may be due tovarious factors such as different oxidation states and the degree ofhydration on the film surfaces which may establish a new equilib-rium of ions every time in the three redox reactions at differenttimes of testing [9,26]. These factors are strongly related to themicro structure changes in the electrode surface which are affectedby the thermal treatment during the electrode fabrication and theage of the electrodes [37]. Moreover, Preocanin et al. [38] proposedthat the hysteresis errors may also depend on different titrationrates. The phenomenon is related to electrode surface reactionswith factors such as surface charge density [39] and electro-kinetics[40]. Thus, the hysteresis logically depends on the material and fab-rication processes. It should be noted that the commercial sensormay also contain hysteresis. During the tests, the pH value of solu-tion was sampled once and recorded in each step after 10 s as thepH value stabilized. Our sensor, however, continued to record thepotentials with a sampling rate of 1 Hz during the titration cycles.

In Fig. 7, the hysteresis could be identified by comparing thepotentials in the same pH levels (assuming that the sampled valuesby the HANNA pH sensor were accurate). For example, the poten-tials in the first half of the third test cycle with pH = 1.50, 2.81, 3.75,6.28, 7.86, 9.52, 10.50 were 483.4, 321, 280.1, 154.9, 124.5, 19.6,and −34.6 mV, respectively, while they were 459.7, 311.5, 279.8,140.4, 108, 26.3, and −23.1 mV, respectively, in the second half ofthe same third cycle. The differences were 23.7, 9.5, 0.3, 14.5, 16.5,

6.7 and 11.5 mV in their respective titration steps.

In Fig. 8, the standard error measurement (SEM) values of poten-tials at each individual pH level in three test cycles, shown in Fig. 7,were calculated to represent the hysteresis (dV). The SEMs were

8 W.-D. Huang et al. / Sensors and Actuators A 169 (2011) 1– 11

Fd

brmrataip

3

HHimtaswipt

E

wtaact

K

w

p0a1bt

One example of applications for our flexible sensor is for the invivo reflux detection in human esophagus [49] for which the sen-sor needs to fit into a small confined tunnel and conform onto thecurved surface. It is difficult to use conventional glass pH electrodes

Table 2The selectivity coefficients of our sensing electrode.

pot pot

ig. 8. Standard errors obtained from potentials measured at different pH levelsuring the reversibility tests.

etween ±1.9 mV and ±15.3 mV in the measured pH levels. Theesults are comparable with the hysteresis values of 26–30 mVeasured in the IrOx films made with other fabrication processes on

igid substrates such as glass [34,35], or in different sensing materi-ls such as WO2 and RuO2 [41,42]. To minimize hysteresis, the key iso produce high-quality iridium oxide films in terms of thickness,morphousness, and porosity [9]. The relatively small hysteresisndicated that our sol–gel fabrication process on a flexible substraterovided sufficiently good quality of iridium oxide sensing films.

.2.5. SelectivityIn the previous experiments, our pH sensor was tested using

Cl- or KOH-base solution without the existence of other ions.owever, various ions and compounds typically present randomly

n environments. We tested possible cation interferences in pHeasurements for sensor selectivity. Katube et al. [43] discussed

he effects of certain cations on the IrOx pH sensing electrodesnd found the presence of metal cations could induce potentialhifts up to 8 mV. The selectivity tests of our flexible pH sensorsere conducted with the fixed interference method (FIM) and the

nterference effects from metal cations were evaluated with theotentiometric selective coefficients, which were calculated withhe Nikolskii–Eisenman formula as [42]:

= E0I + RT

zIFln(aI + Kpot

IJ × azI/zJJ ) (7)

here E is the experimentally measured potential, E0I is a constant

hat includes the standard sensing potential of the primary ion, aI

nd aJ are the concentrations of the primary ion Iz1+, such as H+,nd interfering ion Jzj+ such as Na+, K+ or Ca2+. zI and zJ are theharge numbers of the principal ion I and interfering ion J. Kpot

IJ ishe Nikolskii (potentiometric selective) coefficient as

potIJ = exp

(zIF

RT(E0

J − E0I )

)(8)

here E0J is the sensing potential of the interfering ion.

The interference effects of three cations were compared inotentiometric selective coefficient measurements. 0.1 M of NaCl,.1 M of KCl, and 0.1 M of MgCl2 were used as the interference cation

gents and added separately into the HCl or KOH based pH = 2, 4, 7,0 and 12 solutions. The sensitivity of our pH sensor was measuredefore and after the interference agent was added. The presence ofhe cations Na+, K+ and Mg2+ changed the sensitivity slopes from

Fig. 9. The effects of interference cations on the pH sensitivity.

−51.5 mV/pH to −50.4, −55.4, and −50.1 mV/pH, respectively, asshown in Fig. 9. The sensitivity did not vary significantly. The selec-tivity coefficients of our flexible IrOx pH sensor were calculatedand summarized in Table 2. The selectivity coefficients for potas-sium, sodium and magnesium cations were 5.6 × 10−3, 8 × 10−3

and 8.02 × 10−4. They were acceptable in comparison to the resultsfound in literatures [45,46] which stated that typically an accept-able range of Kpot

IJ is less than 10−2 for ion-selective electrodes.

3.2.6. Temperature dependenceIntrinsically, the Nernstian potential and interference factor are

temperature dependent, as shown in Eq. (4), that the potentialE − E0 is directly proportional to the temperature T in the Nerns-tian equation. The temperature dependence of our flexible IrOx pHsensor was investigated in four different buffer solutions of pH = 2,pH = 4, pH = 7, and pH = 10 at three temperatures 3.8 ◦C, 21.2 ◦C, and50.8 ◦C. The sensor was cleaned with DI water and air dried betweentests in different solutions. The pH sensor is expected have a tem-perature dependence corresponding to the Nernstian equation (Eq.(4)). The theoretical dependence is shown in Fig. 10, with calcu-lated temperature coefficients of 0.3, 0.8, 1.3 and 2 mV/◦C at pH = 2,4, 7, and 10, respectively. The measured temperature coefficients,shown in Fig. 11, were 0.3, 0.7, 0.9, and 2 mV/◦C at pH = 2, 4, 7,and 10, respectively, which matched with the theory. The resultsfor our sol–gel electrodes on a flexible substrate were accept-able in comparison to the 1.27 mV/◦C of temperature dependenceusing other IrOx sensing films [47], and the dependence of 2 mV/◦Cusing palladium oxide electrodes [48]. The predictable tempera-ture dependence in our flexible IrOx pH electrode could eliminatethe need of calibration for individual sensors.

3.2.7. pH sensor array tube test

0.1 M of interference ions KIJ

(selectivity coeff.) Log KIJ

K+ 5.6 × 10−3 −2.24Na+ 8.3 × 10−3 −2.07Mg2+ 8.02 × 10−4 −3.09

W.-D. Huang et al. / Sensors and Actuators A 169 (2011) 1– 11 9

Fig. 10. Theoretical temperature dependence for our IrOx flexible pH sensor.

Fs

iepaea3rstapatpr

otept

ig. 11. Experimental results of temperature dependence for our IrOx flexible pHensor.

n such applications. To demonstrate the capability for our sensinglectrodes on a deformed substrate and in a small volume, an IrOx

H sensor array on a flexible polyimide substrate was rolled up with curvature radius of 1 cm and inserted into a long tube with a diam-ter of 2 cm, as shown in Fig. 12(a). The sensor electrode array wasrranged vertically, and three electrodes with a separation gap of

mm between electrode pairs, in which both the working IrOx andeference Ag/AgCl electrodes have a width of 2 mm, gave a sensingpatial resolution of 5 mm for localized pH measurement. Duringhe experiment, the tube was first filled with DI water verified with

pH of 7, then 0.1 M KOH and 1 M HCl solutions were drippedseudo-randomly into the tube. A magnetic rod stirrer was usedt the bottom of the tube, as shown in Fig. 12(b), to mix the solu-ion. In this tube experiment, the commercial pH glass electroderobe (HANNA) was too large to be inserted inside the tube as aeference reading for pH values.

Fig. 13 shows the potential changes with pseudo-random dripsf acidic or alkaline solutions. The potential variations in each elec-rode responded to the titration quickly and coordinately. With the

lectrode #3 at the top and the electrode #1 at the bottom, theotential peak values of the electrode #3 were always higher thanhose in the electrode #1. This was due to the spatial distributions

Fig. 12. (a) Photo and (b) the setup configuration of the tube test. The tube has adiameter of 2 cm.

of ion concentrations during titration. This phenomenon was moreobvious in the beginning of the experiment when HCl was firstadded into the DI water. As more titration solutions were added,

more ions existed in the tube and so the differences of potentialsamong three electrodes became less. This tube experiment demon-strated the capabilities of using our flexible sensor in a small volume

10 W.-D. Huang et al. / Sensors and A

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[ing convergence of standard electrode potential values of thermally oxidized

ig. 13. Potential responses with the flexible sensor array conformed onto the wallnside a small tube.

hile sensor electrodes conform onto the curved wall. The devicerchitecture of our electrode allows for an array configuration ofensors providing spatial resolution of pH value variations across amall space and showing time-lapsed electrical potentials respond-ng to localized pH information. Due to the space limitation, theocalized pH values could not be verified by a conventional glass-ased sensor probe, although the potentials were distinct in eachitration steps. In the near future, we will investigate a means toetermine and verify the spatial resolution of pH measurements inur flexible pH sensors.

. Conclusions

In this paper, we developed a new iridium oxide film based pHensor on a flexible polyimide substrate and investigated its per-ormance. The sol–gel fabrication process on the flexible substraterovides advantages of lower costs, simpler processes and deviceexibility. A miniature pH sensor has been tested for sensitivity,esponse time, stability, repeatability, reversibility, selectivity, andemperature dependence. The sensitivity matched with the theo-etical Nernstian response. The sensor performed rapid responseso pH level variations statically and dynamically. The potential fluc-uation, deviation, drifts and hysteresis of the flexible sensor werenvestigated showing good stability, repeatability and reversibil-ty. Our flexible sensor showed low ion interference indicatingigh selectivity, as well as low and predictable temperature depen-ence. A sensor array was tested in a small tube with its substrateeformed to the curved surface. The sensor electrodes produce dis-inct potentials responding to various titration steps. With theseeatures and advantages, our miniature IrOx pH sensors on flexibleubstrates are suitable for practical uses on curved surfaces and willnable many new applications.

cknowledgement

The authors would like to give sincere appreciation to the Airorce Office of Scientific Research (AFOSR) for support.

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