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aDepartment of Chemistry, Faculty of Sci

65000, ThailandbDepartment of Chemistry, Faculty of Scie

10330, Thailand. E-mail: tthawatc@chula.a

7643cMaterials Chemistry Research Unit, Dep

Excellence for Innovation in Chemistry, Fa

Khon Kaen 40002, Thailand

† Electronic supplementary information (silver ink sintering at RT in air, CVs ofpotential response of the coated-wire Ag-Idata of the coated-wire Ag-ISE and thparameters of the proposed coated-wirepreviously. See DOI: 10.1039/c3an01385e

Cite this: DOI: 10.1039/c3an01385e

Received 21st July 2013Accepted 7th September 2013

DOI: 10.1039/c3an01385e

www.rsc.org/analyst

This journal is ª The Royal Society of

Development of coated-wire silver ion selectiveelectrodes on paper using conductive films of silvernanoparticles†

Wanwisa Janrungroatsakul,a Chutiparn Lertvachirapaiboon,b Wittaya Ngeontae,c

Wanlapa Aeungmaitrepirom,b Orawon Chailapakul,b Sanong Ekgasitb

and Thawatchai Tuntulani*b

Films of silver nanoparticles are used for the first time as an electrical conductor and ion-to-electron

transducer to fabricate coated-wire ion selective electrodes (ISEs) on paper. The film of nano silver ink

(nano silver film), synthesized from the reduction of AgNO3 by NaBH4, was screen printed on paper.

Transmission electron microscopy showed that the synthesized silver nanoparticles (AgNPs) possessed a

spherical shape with diameter ca. 5 nm. Energy-dispersive X-ray spectroscopy supported the purity and

good stability of the synthesized AgNPs. Nano silver films were sintered at room temperature, 100 �Cand 200 �C. Upon increasing the sintering temperature, atomic force microscopy showed that the size

of AgNPs of nano silver films increased, but the sheet resistivity decreased. Silver ISEs were then

fabricated from nano silver films and o-NPOE-plasticized polymeric membranes containing

benzothiazolyl calix[4]arene (CU1) as ionophore and KTpClPB as anionic site. The performance of the

developed Ag-ISEs was investigated by potentiometric measurements, potentiometric water layer tests,

current reversal chronopotentiometry and electrochemical impedance spectroscopy. The coated-wire

electrode fabricated from the nano silver film sintering at room temperature showed the best

characteristics of Ag-ISEs giving a near Nernstian response slope of 59.7 � 1.0 mV per decade, 10�6 to

10�2 M linear range, detection limit of 4.5 � 10�7 M, long-term potential stability and good reversibility.

Introduction

Potentiometric techniques such as ion selective electrodes (ISEs)show a leading role in sensing technology as low cost and easy touse devices.1 Conventional ISEs consist of an ion selectivemembrane and an inner lling solution which maintainsconstant inner interfacial potential. Inner lling solution,however, is a major restriction for construction of miniaturizedISEs to detect small volumes of samples in clinical and envi-ronmental applications. One way to reduce the size of an

ence, Naresuan University, Phitsanulok

nce, Chulalongkorn University, Bangkok

c.th; Fax: +66 2218 7598; Tel: +66 2218

artment of Chemistry and Center of

culty of Science, Khon Kaen University,

ESI) available: SEM images of the nanoAgNPs colloids and AgNO3 solution,SE at various solution pH, reversibilitye comparison of the potentiometricAg-ISE with other Ag-ISEs reported

Chemistry 2013

electrode is to eliminate the inner lling solution from conven-tional ISEs leading to the development of solid contact ISEs2 andcoated-wire ISEs which are easier and cheaper to construct thanthe conventional ones. Conducting polymers are introduced tosolid contact ISEs as ion-to-electron transducers.1,2 Along with therecent development of nanotechnology, nano-materials such asPt nanoparticles,3 Au nanoparticles,4 carbon nanotubes5 andgraphenes6 are used as transducer layers in the fabrication ofsolid contact ISEs.

Coated-wire ISEs, however, are prepared by placing selectivemembranes directly on conductive wires such as platinum,copper, silver wires and graphite rods. Recently, Andrade andcoworkers have used carbon nanotubes (CNTs) ink to produce aconductive paper that can act as a wire and an ion-to-electrontransducer in a planar ISE.7 The selective membrane wasdeposited on the conductive paper, and paper-based NH4

+, K+

and pH sensors were obtained. Thus far, there is no report onother paper-based coated-wire ISEs.

Recently, nano silver ink made from silver nanoparticles(AgNPs) has been applied in inkjet printing.8 The inkjet printednano silver ink has been used as a conductive layer on substratesto produce electronic applications such as thin lm transis-tors.9,10 Chailapakul and coworkers have demonstrated the use of

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screen printed electrodes to detect Au(III) in gold rening wastesolutions by using square wave voltammtery.11

Due to growing demands in the use of AgNPs, simple andeffective ways to determine silver contents in AgNPs solutionshave been developed.12 Recently, our group has fabricatedconventional Ag-ISEs and demonstrated their applications indetecting free Ag+ in AgNPs solutions by direct potentiometry13

and to indirectly detect glucose in beverages.14 Herein, we haveprepared and fully characterized a nano silver ink and used it asa conductive layer and an ion-to-electron transducer for devel-opment of paper-based coated-wire Ag-ISEs. The Ag-ISEs werefabricated by simply drop casting o-NPOE-plasticized PVCmembrane containing benzothiazolyl calix[4]arene (CU1)13 asionophore for Ag+ and KTpClPB as anionic site on the nanosilver lm that was screen printed on paper. Since it is knownthat sintering phenomenon enhanced conductivity of nano-particles,10,15 the nano silver lms are sintered at differenttemperatures and their morphology are examined by atomicforce microscopy (AFM). The characteristics of paper-basedcoated-wire Ag-ISEs prepared from these lms were examinedby potentiometry and impedance spectroscopy.

ExperimentalMaterials and methods

Potassium tetrakis[4-chlorophenyl]borate (KTpClPB), o-nitro-phenyl octyl ether (o-NPOE), high molecular weight poly(vinylchloride) (PVC), and tetrahydrofuran (THF) were purchased inselectophore� or puriss quality from Fluka. Nitrate salts ofcations were of analytical grade obtained from Merck, Fluka,Sigma-Aldrich, Reidel and Carlo Erba. Citric acid and sodiumborohydride (NaBH4) were purchased from Merck. The iono-phore CU1 was prepared using the previous published proce-dure.13 All solutions were prepared with deionized water withthe specic resistivity of 18.2 MU cm (Milli-Q water puricationsystem; Bedford, MA, USA).

A transmission electron microscope (TEM) image of conduc-tive nano silver ink was recorded with a Hitachi, H-7650 analyt-ical transmission electron microscope. Atomic force microscopywas carried out on a SPA-400 atomic force microscope (SeikoInstrument, Inc., Japan) with a calibrated 20 micrometer XY-scanand 10 micrometer Z-scan range PZT-scanner. Scanning electronmicroscope (SEM, JEOL JSM-6510A) and energy-dispersive X-rayspectroscopy (EDX, JEOL JED 2300) were used to characterize thesize and to analyze the composition of the synthesized nanosilver ink, respectively.

Preparation of the nano silver ink

Citric acid solution (0.07 M, 25 mL) was added into the silvernitrate solution (0.37 M, 25 mL), and silver citrate complex wasformed. Subsequently, 50 mL of silver citrate complex andsodium borohydride solution (0.19 M, 50 mL) were loaded intothe syringe which connected with a reactor. Silver citratecomplex and sodium borohydride solution were injected intothe reactor, and the redox reaction occurred. Aer the reactioncompleted, the supernatant was separated by centrifugation

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and silver nanoparticles were repeatedly washed with distilledwater to eliminate nitrate (NO3

�), borate (BO33�) and sodium

(Na+) ions. The silver nanoparticles were redispersed in deion-ized water. Aer ultrasonicating, the conductive nano silver inkwas obtained in 10% (w/v) or 100 000 ppm.

Preparation of coated-wire Ag-ISEs on paper

Preparation of ISE membranes. A silver ion selectivemembrane contained 1 wt% (10 mmol kg�1) ionophore CU1,0.25wt%(5mmolkg�1)KTpClPB,33wt%PVCand66wt%o-NPOE.All components (220 mg in total) were dissolved in 1 mL of THF.

Preparation of conductive nano silver lms. The nano silverink was screen printed on the ne printing paper (100 g) and leto dry on paper having 1 mm thickness and 3 cm length. TheAgNPs conductive paper was covered with plastic masks on bothsides. The front mask had two circular orices with an area ofapproximately 12.6 mm2. The membrane cocktail (10 mL) wasdrop casted on one orice, while the other one was used toconnect to a potentiometer (Fig. 1). The polymeric layer was leto dry overnight at room temperature. The paper-based Ag-ISEwas then conditioned in 0.01 M solution of AgNO3 overnightprior to use.

To investigate the sintering effect of AgNPs to potentiometricresponse, three printed nano silver lms were dried overnight atroom temperature, the second one was heated in an oven at100 �C and the third one at 200 �C for 30 min. The preparationof electrodes was carried out as mentioned above.

Potentiometric measurements

EMF measurements were carried out with a 16-channel elec-trode monitor (Lawson Labs Inc., Malvern, PA 19355, USA) atambient temperature. The reference electrode Ag/AgCl withdouble junction was used (type 6.0726.100, MetrohmAG,CH-9010Herisau, Switzerland).

Impedance spectroscopy measurements

A conventional three-electrode cell was used in the EIS studies: acoated-wire Ag-ISE as the working electrode, a Ag/AgCl referenceelectrode and a platinum counter electrode. All experiments wereconducted at room temperature. The coated-wire Ag-ISE wasconditioned in 0.01 M AgNO3 overnight. Impedance measure-ments were carried out using a potentiostat/galvanostat instru-ment (Autolab PG STAT 30, Eco Chemie B.V., Utrecht, theNetherlands) monitored by Frequency Response Analyser. Thespectra were recorded within a frequency range 80–0.01 kHz withan AC voltage amplitude of 0.01 V in 0.01 M AgNO3 solution.

Chronopotentiometry

Constant current chronopotentiometry was performed by usingthe same instruments described in impedance measurements.A constant current of +1 nA was applied on the working elec-trode for 100 s followed by a current of �1 nA for 100 s withsimultaneous recording of the electrode potential. Themeasurements were performed in 0.01 M AgNO3 at roomtemperature.

This journal is ª The Royal Society of Chemistry 2013

Fig. 2 TEM images of conductive nano silver ink.

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Results and discussionPreparation and characterization of conductive lms of thenano silver ink

Silver nanoparticles (AgNPs) were synthesized by the chemicalreduction of silver nitrate with sodium borohydride. The UV-vis spectrum of the synthesized AgNPs shows a strongabsorption band at 400 nm, which is a characteristic of AgNPs.In order to use as a conductive silver ink, AgNPs must have noresidual ion such as nitrate, borate, and sodium ions. Whenthe nano silver ink is dried on substrate, these ions can formsalt crystals, which preferably absorb the moisture that canlead to short circuit when the electrical circuit wasclosed. These ions can be isolated by washing with distilledwater. Aer purication, AgNPs are redispersed into deionizedwater with ultrasonic mixture to adjust the nal concentrationof AgNPs. The optimum concentration of AgNPs forapplying as a conductive nano silver ink is 100 000 ppm or10% w/v.

The morphology of synthesized nano silver ink was charac-terized by TEM. Fig. 2 shows TEM images of spherical AgNPswith diameter approximately 5 nm. The energy-dispersive X-rayanalysis was performed to determine the chemical compositionof the nano silver ink. The EDX spectrum (Fig. 3) shows a sharpsignal at approximately 3 keV corresponding to the absorptionof metallic silver. The signal from oxygen was not observed.Furthermore, there was no signal from carbon that came fromthe stabilizer because the metallic silver content in the nanosilver ink was much more than that of the stabilizer. Eventhough the nano silver lm was le in air for one week, therewas still no signal from oxygen. Therefore, the synthesized nanosilver ink does not form silver oxide and is stable at ambienttemperature.

The nano silver ink was screen printed on 100 g ne printingpaper having thickness layer around 1 mm as observed by SEMcross section (Fig. 4). The nano silver lm on paper showedconductive properties and had particle size of about 50 nm asshown in Fig. 5. The particle size of nano silver ink on paper waslarger than that in solution due to the sintering phenomenon atroom temperature. However, the particle size remained at50 nm aer the nano silver lm was le for one week at room

Fig. 1 Scheme of fabricating a coated-wire AgISE on paper: (a) an ion selectivemembrane, (b) a plastic mask, (c) a conductive nano silver film and (d) an area forelectrical contact.

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temperature (Fig. S1 in the ESI†). According to the sinteringmechanism, each particle moved toward to the center ofanother particle and then two particles were merged together.As a result, the neckings between the particles were formedleading to larger particles.

Generally, sintering phenomenon is temperature depen-dent. Moon and Schubert have developed conductive nanosilver ink by thermal treatments.10,15 To investigate the rela-tionship between the conductivity of the prepared nano silverink and the sintering temperature, three nano silver lms onpaper were rst le drying at room temperature and then thesecond lm was heated at 100 �C and the third one at 200 �Cfor 30 min. Changes in morphology of these lms wereobserved by AFM. The results in Fig. 5 show that the shapeof particles was still spherical but particle size was largerwith increasing temperature. The average particle size aersintering on the substrate grew up to 50, 100 and 200 nm forroom temperature, 100 �C and 200 �C, respectively. Largeparticle size of AgNPs aer sintering gave low sheetresistivity (Table 1) implying good electrical conductivity ofnano silver lms due to increasing of the contact amongmetallic particles.

Fig. S2 (in the ESI†) shows the cyclic voltammograms of aglassy carbon electrode (GCE) in silver nitrate solution andsilver nanoparticles colloid. Standard potentials16 of Ag+/Ag andAg/AgCl with respect to standard hydrogen electrode (SHE) are0.79 V and 0.22 V, respectively. The anodic peak of AgNPs/Ag+

with respect to Ag/AgCl was found to be 0.39 V as shown inFig. S2.† Thus, the potential of AgNPs/Ag+ with respect to SHE is

Fig. 3 Energy-dispersive X-ray spectrum of the synthesized nano silver ink.

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Fig. 4 SEM cross section of thickness conductive nano silver ink printed onpaper.

Fig. 5 AFM images of conductive nano silver ink after sintering at variousconditions: (a) room temperature, (b) 100 �C for 30 min, and (c) 200 �C for 30 min.The area of the top images (1) is 100 mm2 and the area of the bottom images (2) is360 000 nm2.

Table 1 Sheet resistivity of nano silver ink after sintering at various temperatures

Sintering condition Sheet resistivity (U square�1)

Room temperature 0.613100 �C, for 30 min 0.021200 �C, for 30 min 0.005

Fig. 6 Time trace line response of the coated-wire Ag-ISE fabricated from sin-tering AgNPs at different temperature (a) to increasing level of Ag+ solution and(b) the corresponding calibration plot.

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0.61 V, which is less than that of Ag+/Ag. The result implied thatAgNPs could be oxidized more easily than a Ag wire employed ina conventional ISE.17

Due to good conductivity and ease of oxidation of AgNPs,they were utilized as both conductive wire and ion-to-electrontransducer for the construction of paper-based coated-wire Ag-ISEs. Themembrane cocktail (10 mL) was directly drop casted ona circular orice of the plastic covered nano silver lm. Thecomposition of polymeric membrane was 50 mol% of KTpClPB(relative to ionophore CU1) using o-NPOE as plasticizer.13

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Stability of EMF measurements

Time trace lines and calibration plots of paper-based coated-wire Ag-ISEs fabricated from conductive nano silver lmssintered at different temperatures are shown in Fig. 6. Thecharacteristic of the coated-wire Ag-ISE from the nano silverlm sintering at room temperature shows a close to Nernstianresponse, but electrodes fabricated from nano silver lms sin-tering at 100 �C and 200 �C gave super-Nernstian slopes whenincreasing Ag+ concentration probably resulting from theexcellent conductivity whichmade the electrode over respond toAg+. It was seen that sintering AgNPs at 100 �C and 200 �C gavelow bulk resistance (Table 1) compared to sintering at roomtemperature. Therefore, a better electrical conductor of largeparticle size AgNPs from sintering at higher temperatures gavepoorer characteristics of potentiometry. The coated-wire Ag-ISEfabricated from the nano silver lm sintering at room temper-ature showed a near Nernstian response (59.7 � 1.0 mV perdecade) with a wide linear range from 10�6 to 10�2 M, a limit ofdetection of 4.5 � 10�7 M and a fast response time of 5 s.

Water layer tests of paper-based coated-wire Ag-ISEs fabri-cated from nano silver lms sintering at different temperatureshave been carried out, and the results are compared in Fig. 7.Firstly, the potential was measured in 0.01 M AgNO3 and thepotentials of all electrodes were stable. The analyte solution was

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Fig. 8 Chronopotentiograms for paper-based coated-wire Ag-ISEs fabricatedfrom nano silver films sintering at room temperature (blue curve), 100 �C (redcurve) and 200 �C (green curve) recorded in 0.01 M AgNO3.

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then changed from 0.01 M AgNO3 to 0.01 M NaNO3 leading to anegative potential response. It indicated that the electrodeswere selective towards Ag+ over interfering Na+. However, thepotentials in 0.01 M NaNO3 of electrodes fabricated from nanosilver lms sintering at 100 �C and 200 �C were not stable uponthe formation of water layer between the sensing membraneand the conductive substrate.18 The potential of all Ag-ISEsbecame positive again with stable potential when the solutionwas changed back to 0.01 M AgNO3.

Constant-current chronopotentiometry was used to examinethe potential stability of the paper-based coated-wire electrodes.Fig. 8 shows changes in potential when current of �1 nA wasapplied to the paper-based coated wire electrodes. The potentialdri of electrodes can be derived from the ratio of EMF andtime, DE/Dt.19 The resulting values of electrodes fabricated fromnano silver lms sintering at room temperature, 100 �C and200 �C were 6.10, 5.64 and 5.03 mV s�1, respectively. These lowpotential dris of electrodes suggested the capability of nanosilver lms to maintain the equilibrium of transition from ionicto electronic conductivity.

The impedance spectroscopy (Fig. 9) shows semicircleNyquist plots assigned to the bulk membrane resistance. Thebulk resistances of Ag-ISEs were 2.52, 1.47 and 1.22 MU forelectrodes fabricated from sintering AgNPs at room tempera-ture, 100 �C and 200 �C, respectively. Bulk resistances obtainedare in good agreement with potential dris from chro-nopotentiometry as illustrated in Fig. 8 and sheet resistivityshown in Table 1.

Although Ag-ISEs fabricated from nano silver lms sinteringat 100 �C and 200 �C gave low resistances and potential driswhen compared to the Ag-ISE fabricated from the lm sinteringat room temperature, they possessed poorer potentiometriccharacteristics. Therefore, the conductive lm sintering at roomtemperature was used to construct an optimal paper-basedcoated-wire Ag-ISE.

Selectivity of the paper-based coated-wire Ag-ISE

Selectivity coefficients of the optimal paper-based coated-wireAg-ISE were determined using the separate solution method

Fig. 7 Water layer test of paper-based coated-wire Ag-ISEs fabricated fromnano silver films sintering at room temperature (blue curve), 100 �C (red curve)and 200 �C (green curve).

This journal is ª The Royal Society of Chemistry 2013

(SSM).20,21 Table 2 shows the selectivity coefficients of thecoated-wire electrodes and conventional macroelectrodes usingthe same membrane composition. It was seen that the paper-based coated-wire electrodes show a poorer discriminatingability to alkali and alkaline earth metal ions than the conven-tional one. However, the paper-based coated-wire electrode wasstill highly selective toward Ag+ over other metal ions.

Effect of pH and reversibility

The effect of pH of test solution was examined in 10�4, 10�3 and10�2 M AgNO3 on the response of electrode, pH in solutions wasadjusted by diluted nitric acid and sodium hydroxide. Theresults in Fig. S3 (in the ESI†) show that the electrode can beused in the range of 2.5–8.0. However, there was the EMFchanges at lower and higher pH values due to interference fromH+ and formation of silver hydroxide, respectively.

The reversibility of electrode was performed in alternateconcentration of AgNO3 between 10�4 and 10�3 M. The resultsin Fig. S4 (in the ESI†) show that the potential was stable at eachconcentration. When solution of AgNO3 was changed from highto low concentration, EMF signals were restored.

Fig. 9 Impedance spectra of the coated-wire Ag-ISEs fabricated from nano silverfilms sintering at room temperature (blue), 100 �C (red) and 200 �C (green)recorded in 0.01 M AgNO3.

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Table 2 Selectivity coefficients of conventional and coated wire Ag-ISEs basedon CU1

Interfering ion

Log KpotAgþ ;j

Conventional13 This work

Na+ �6.78 �4.00K+ �5.06 �3.38Mg2+ �8.59 �5.59Ca2+ �8.20 �5.38Ni2+ �8.37 �5.30Cu2+ �4.09 �5.15Zn2+ �8.13 �5.84Cd2+ �6.02 �8.26Pb2+ �4.63 �6.30Hg2+ �2.43 �2.73

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Comparison with conventional, solid contact and othercoated-wire electrodes

The comparison of characteristics of conventional,13,22,23 solidcontact24,25 and other coated-wire electrodes26–30 with theproposed paper-based electrode are summarized in Table S1 (inthe ESI†). It can be seen that the detection limit, working rangeand response time of our proposed coated-wire Ag-ISE arecomparable to those of other electrodes. The advantage of thiswork is to use conductive layers from nano silver ink to simplyconstruct the paper-based coated-wire Ag-ISE. Recently,Andrade and coworkers have demonstrated that polymericmembranes can be prepared on cotton yarns using carbonnanotubes as electrical conducting and signal transducingmaterials.31 We optimistically foresee that our synthesized nanosilver ink may also be applied to fabrics and used as wearablesensors as well.

Conclusions

We have demonstrated that a conductive layer of silver nano-particles can be used as both conductive wire and ion-to-elec-tron transducer in a paper-based coated-wire Ag-ISE. The newelectrode fabricated from the nano silver lm sintering at roomtemperature provided a long-term potential stability, revers-ibility and good Nernstian response to Ag+ (59.7 � 1.0 mV perdecade) with detection limit of 4.5 � 10�7 M. This paper-basedcoated-wire Ag-ISEs can be prepared with ease and low cost.Furthermore, the synthesized nano silver ink can possibly beused in the future development of wearable sensors.

Acknowledgements

The authors gratefully acknowledge nancial support from theThailand Research Fund (RTA5380003).

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This journal is ª The Royal Society of Chemistry 2013

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31 T. Guinovart, M. Parrilla, G. A. Crespo, F. X. Rius andF. J. Andrade, Analyst, 2013, 138, 5208–5215.

Analyst