Electrochemistry Communications 50 (2015) 60–63

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Electrochemistry Communications

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Short communication

Using sp2-C dominant porous carbon sub-micrometer spheres as solidtransducers in ion-selective electrodes

Junjin Ye, Fenghua Li ⁎, Shiyu Gan, Yuanyuan Jiang, Qingbo An, Qixian Zhang, Li Niu ⁎Engineering Laboratory for Modern Analytical Techniques, c/o State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,Changchun 130022, Jilin, China

⁎ Corresponding authors. Tel.: +86 431 8526 2425; faxE-mail addresses: fhli@ciac.ac.cn (F. Li), lniu@ciac.ac.cn

http://dx.doi.org/10.1016/j.elecom.2014.10.0141388-2481/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 September 2014Received in revised form 13 October 2014Accepted 27 October 2014Available online 31 October 2014

Keywords:Solid-contact ISEsTransducersPorous carbon sub-micrometer spheresPotential stability

Potential stability is one of the most intractable challenges for solid-contact ion-selective electrodes (SC-ISEs)with regard to the classic inner-reference ISEs. Among the well-established electrode structures for SC-ISEs,solid-contact transducer layer is the most crucial component. Herein, we introduce porous carbon sub-micrometer spheres (PC-SMSs) as a promising solid-contact material for fabrication of highly stable K+-SC-ISE.Systematic characterizations including contact angle, electrochemical impedance spectroscopy and cyclicvoltammetry demonstrate that the PC-SMSs disclose high hydrophobicity and large interfacial capacitance,which efficiently eliminate the water-layer and stabilize the potential of K+-SC-ISEs. The developed PC-SMSselectrode also exhibits good long-term stability as well as anti-interferences from O2 and CO2.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Ion selective electrodes (ISEs) featured by low-cost and ion sensingare frequently used in environmental monitoring, research laboratoriesand clinical analysis [1–3]. Conventional inner-reference ISEs with goodpotential stability remain the popular ion sensor devices. However, sev-eral frequently mentioned drawbacks, such as leaching of inner refer-ence solution, susceptibility to the temperature and miniaturizationchallenging have limited the development of conventional ISEs [4].Wire coated electrode (CWE) prepared by directly coating the ion-selective membrane (ISM) on a platinum wire pioneered the firstsolid-contact ion-selective electrodes (SC-ISEs) [5]. CWE opens theresearch area of SC-ISEs, but poor potential stability becomes the mainobstacle for its practical applications. Lately, detailed studies demon-strate that the lack of “block” interfaces between the electrode andISM layer dominates the factor of potential instability [6]. The crucialcharacteristics for the “block” interface have been distinguished includ-ing sufficiently reversible and fast ion-to-electron transduction, high ex-change currents and anti-interferences [7]. Conducting polymers (CPs),for example, the representative poly(3,4-ethylenedioxythiophene)with large redox capacitance and fast ion-to-electron transductionhave been extensively investigated during the past decades [8–10].Further developed more lipophilic and solution prepared CPs, such aspolyoctylthiophene and other alkylthiophenes have further improved

: +86 431 8526 2800.(L. Niu).

the potential stability [11–14]. Nevertheless, light and gas sensitivitiesand redox interferences as well as inadequate hydrophobicity remainthe challenges for the CPs based SC-ISEs [7,15,16].

With regard to the redox-type transducers, another attractivetransducer is the carbon-based nanomaterial based on the electricdouble layer capacitance. For example, Buehlmann and coworkersfirst proposed a three-dimensionally ordered macroporous carbon(3DOM) as the solid-contact layer, which exhibits high ionic andelectric conductivity. The long-term potential drift is only11.7 μV h−1 and also displays few responses to the gas and light[17]. Recently, carbon nanotubes (CNTs) and graphene have alsobeen utilized to be effective and promising transducers [18,19].Their remarkable capacitances based on large surface area result inhigh exchange current density and thus a closely nonpolarizableinterface because of the asymmetric ion-to-electron transductionprocess. Very recently, mesoporous carbon transducers by a colloidimprinted way further exhibit the promising scale-up fabrication ofSC-ISEs based on carbon based materials [20].

In this work, we introduce another carbon based materials,i.e., porous carbon sub-micrometer spheres (PC-SMSs) as a solid-contact transducer for fabrication of a highly stable K+-SC-ISE. Theprepared PC-SMSs show uniform structural size distribution and highhydrophobicity (137° contact angle). PC-SMSs also exhibit good electricconductivity as well as high capacitance (12 mF), which meets thecrucial requirements for the development of SC-ISEs with superiorperformances. Additionally, simple preparation process would makePC-SMSs as an effective solid-contact material in scale-up fabricationof SC-ISEs.

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2. Experimental

Valinomycin was obtained from Acros. Bis(2-ethylhexyl)sebacate(DOS), potassium tetrakis(4-chlorophenyl) borate(KTClPB), high mo-lecular weight poly(vinyl chloride) (PVC) and dopamine hydrochloridewere purchased from Fluka. Ammonia, methanol, and tetrahydrofuran(THF, 99.8%) were obtained from Shanghai Chemicals, China.

PC-SMSs were prepared using themethod proposed by Ai et al. [21].Briefly, in a typical synthesis of SMSs with size of ca. 620 nm, 0.5 mLNH4OH (28–30%) was mixed with 40 mL ethanol and 90 mL waterunder mild stirring at room temperature for 0.5 h. Then 10 mLdopamine water solution (0.05 g mL−1) was injected into the mixturesolution and the reaction was proceeded for 30 h to obtain the PDASMSs. After centrifugation and washed with water for 3 times, the re-sulted PDA SMSs were carbonized at 800 °C for 1 h in Ar at a rate of5 °C min−1 to obtain the carbon SMSs. Finally, carbon SMSs weremixedwith KOH at a weight ratio of 1:4, followed by thermal treatmentat 700 °C for 1 h in Ar at a rate of 5 °C min−1. After cooling down, theresulting powder was washed with 1 M HCl for 3 times and thenwashed with water for 3 times to remove the excess KOH. Scanningelectron microscopy (SEM) images were performed by a field emissionscanning electron microscopy (FE-SEM, XL30ESEM-FEG). Transmissionelectron microscopy (TEM) images were obtained by using a JEOL2000 transmission electron microscope operating at 200 kV. Contactangle measurements combined with an optical CCD were performedat a contact angle apparatus (Changchun optics and fine mechanicinstrument).

Themembrane cocktail was a THF solution containing ca. 15% (w/w)potassium ion-selective membrane (K+-ISM) component, whichconsisted of 1.0% valinomycin, 0.5% KTClPB, 65.2% DOS and 33.3%PVC (w/w). PC-SMSs (5 mg) were dispersed in a mixture containingethanol (0.49 mL) and nafion (11 μL, 5%) respectively, resulting in aconcentration of ca. 10 mg·mL−1. An aliquot of 4 μL was dropped

Fig. 1. a) Schematic illustration of the synthesis of PC-SMSs. b) SEM image of PC-SMSs. c) Lowm5 μL water droplet on the surface of PC-SMSs film.

onto the surface of a polished glassy carbon electrode (GCE, 3mmdiam-eter), dried in the air. The procedure was repeated four times. PC-SMSs/K+-ISE was prepared by depositing K+-ISM (ca. 30 μL) onto the PC-SMSs film. For comparison, K+-CWE was fabricated by directly coatingthe ISM onto a GC electrode. All the prepared electrodes were dried inair over night and then conditioned in 0.01 M KCl solution for 24 h be-fore and between the measurements.

Electrochemical measurements including cyclic voltammograms,electrochemical impedance spectroscopy, chronopotentiometry andopen circuit potentials were performed using the CHI 660A electro-chemical workstation with a three-electrode electrochemical cell. Theworking electrodes are modified GCE electrode, and the auxiliary elec-trode is a Pt wire. The reference electrode is Ag/AgCl electrode (3 MKCl). The analytical performance of the K+-SC-ISEs (PC-SMSs/K+-ISE)was studied in the concentration range of 10−9 to 10−1 M KCl. The ac-tivity coefficients were calculated by using the extended Debye–Hückelequation.

3. Results and discussion

Fig. 1a schematically illustrates the preparation process of PC-SMSs,which just consists of three simple steps including the synthesis of PDASMSs, carbonizing the PDA SMSs and the synthesis of PC-SMSs. The PC-SMSswhich used dopamine as a carbon precursor contain little sp3-C inthematrix, exhibiting enhanced conductivity. As can be seen froma typ-ical SEM image (Fig. 1b), the resulting PC-SMSs are uniformly sphericalin shapewith diameters of ca. 650 nm. Both high resolution TEM images(Fig. 1c and d) manifest the porous surface that provides a highly largesurface area after etching by KOH. Solid-contact materials with highhydrophobicity are expected to be beneficial for suppressing the unde-sirable water layer at the SC|ISM interface [22,23]. After coating the PC-SMSs dispersion on a cleaned GC electrode, the wetting property of thePC-SMSs film was checked by a contact angle (CA) measurement. The

agnification and d) highmagnification HRTEM image of PC-SMSs. Inset: optical image of a

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advancing CA was obtained by forming a 5 μL drop of water on the sur-face. As shown in the inset of Fig. 1d, the PC-SMSs film has a CA of 137°between a 5 μL water droplet and the film, and the droplet stayedstatically over 30 min, indicating high hydrophobicity and stability ofthe film.

Electrochemical investigations of the PC-SMSs film were carried outby electrochemical impedance spectroscopy (EIS) and cyclic voltamme-try (CV) in 0.1M KCl solution. Fig. 2a illustrates the complex impedanceplot for GC/PC-SMSs where the spectrum is dominated by a ca. 90°capacitive line extending down to low frequencies. The value fordouble-layer capacitance estimated by an equivalent electrical circuit(not shown) was 12.01 mF, which agrees with the calculated result ofthe CV well (~12.8 mF, represented in the inset). The remarkableresults would derive from the morphology of PC-SMSs. The absence ofa high-frequency semicircle indicates a fast charge transfer across theGC|PC-SMSs and the PC-SMSs|KCl interfaces aswell as fast charge trans-port in PC-SMSs layer, confirming the excellent electroconductivity ofsp2-C dominant structure. To further verify the excellent electricalcharacterization of developed ISE, impedance measurements for thePC-SMSs/K+-ISE and K+-CWE were also recorded. As can be seen inFig. 2b, each electrode shows a high-frequency semicircle representedby total bulk resistance and the result is 7.5 MΩ and 6.0 MΩ for thePC-SMSs/K+-ISE and the K+-CWE, respectively. The slight differenceof resistance of the two electrodesmay have arisen from the uncertaintyin making membranes of the same thickness. By contrast, the low-frequency parts of two electrodes are obviously distinct. The K+-CWEexhibits a second semicircle due to a large charge transfer resistancein parallel with the small double-layer capacitance at the GC|ISM inter-face, whereas the low-frequency resistance of the PC-SMSs/K+-ISE inthe impedance spectrum sticks to the real impedance axis and doesn'tchange with frequencies, indicating a high low-frequency capacitance.Current-reversed chronopotentiometry was found to be a fast and

Fig. 2. a) Electrochemical impedance plots of GCE/PC-SMSs in 0.1M KCl. Frequency range, 10 kH0.01 V·s−1. b) Electrochemical impedance plots of the PC-SMSs/K+-ISE andK+-CWE. Frequencypart: PC-SMSs/K+-ISE; applied current: 10 nA for 60 s and−10 nA for 60 s; bottom part: KPC-SMSs/K+-ISE compared to previous SC-ISEs based on chronopotentiometric results.

convenient method to critically evaluate the potential stability of SC-ISEs. We used it to quantify the low-frequency capacitance of the PC-SMSs/K+-ISE and the K+-CWE, thus examining their potential stability.As shown in Fig. 2c, a potential jump (IR) related to the resistance (R) ofthe ISM is followed by a potential drift. A drastic potential drift appearswhen a current of ±1 nA was applied to the K+-CWE, indicating thepoor capacitance. The PC-SMSs/K+-ISE, however, shows little potentialdrift under more harsh conditions with a current of ±10 nA. The calcu-lated ΔE/Δt value for the PC-SMSs/K+-ISE is 4.8 μV·s−1 correspondingto a capacitance value of 2083 μF from the fundamental capacitorequation, I= C·ΔE/Δt. The decrease of capacitance value after coveragewith the ISM can be attributed to the difference of the contactingmedium that has an effect on ion-to-electron process [7]. In spite ofthat, the value was still much higher than those measurement resultsfor other well-established transducers [18,19,24–28], which can beseen clearly in Fig. 2d. Additionally, the resistance values estimatedfrom the potential jump for the PC-SMSs/K+-ISE and K+-CWE are7.67 MΩ and 6.14 MΩ respectively, in agreement with the resultsfrom the impedance measurements.

Open-circuit potentiometric measurements were used to check themain analytical performance of the developed electrode. Firstly, waterlayer testwas applied to check the aqueous layer at the SC|ISM interface.The PC-SMSs/K+-ISEwas sequentially exposed to 0.1M KCl, 0.1MNaCl,and then back to 0.1 M KCl. As shown in Fig. 3a, there is no noticeablepositive or negative potential drift during the whole test, suggestingthe absence of an undesirable water layer, which resulted from thehighly hydrophobic characterization of the PC-SMSs. Besides, theintermediate-term stability was calculated by the last step of the testand the value obtained corresponds to 14.9 μV·h−1 for 20 h. The inter-ference of O2 and CO2 on the potential response was tested by immers-ing the developed electrode in 0.1 M KCl solution and alternatingsaturation with N2, O2 and CO2 in the sequence shown in the inset of

z to 0.01 Hz; Edc, 0.2 V;△Eac, 10mV. Inset: cyclic voltammograms in 0.1M KCl; scan rate:range, 100 kHz to 0.3 Hz; Edc, 0.2 V;△Eac, 100mV. c) Chronopotentiometric results. Upper

+-CWE; applied current: 1 nA for 60 s and−1 nA for 60 s; solution: 0.1 M KCl. d) CLF of

Fig. 3. a)Water layer test for PC-SMSs/K+-ISE. Inset: CO2 and O2 interference test. b) Timeresponse of the PC-SMSs/K+-ISE. Inset: calibration curve for the PC-SMSs/K+-ISE in thevaried concentration of KCl solution.

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Fig. 3a. Scarce potential drift was observed upon O2 exposure over30 min while an instantly increasing potential of ca. 2 mV was foundafter switching to CO2. This result is similar with that of the 3DOMcarbon-based SC-ISE [17].

Secondly, electromotive force (emf) measurement for the PC-SMSs/K+-ISE was recorded as a function of time at different concentrations ofK+. Fig. 3c shows that the emf values stabilized within 10 s when eachsolution concentration was changed throughout the measurement.The calibration curve of the PC-SMSs/K+-ISE exhibits a Nernst slope of57.8 mV and a linear range from 10−5 to 10−1 M and a low detectionlimit of 10−5.7 M. The selectivity coefficients (logKpot

ij) of the electrodes(n = 3) were obtained by the separated solution method (SSM) [29]logKpot

KLi = −3.9 ± 0.3, logKpotKNa = −4.3 ± 0.2, logKpot

KNH4 =−2.1 ± 0.3, logKpot

KCa = −3.9 ± 0.2, and logKpotKMg = −4.5 ± 0.3.

These values are quite in accord with that of other well-established SC-ISEs, suggesting that the solid-contact doesn't perturb the response char-acteristics of the developed ISEs [18,19]. Finally, the PC-SMSs/K+-ISEshowed a remarkable long-term potential stability. By conditioning in0.01 M KCl, the calculated standard potentials (E0) of the PC-SMSs/K+-ISE were 633 mV for 24 h, 629 mV for 1 week and 626 mV for 2 months,respectively. Meanwhile, the slopes of calibration curves and the low de-tection limit were slightly changed after such long conditioning time. Thelarge capacitance and the high hydrophobicity of the PC-SMSs layer were

supposed to account for such a promising long-term potential stability ofthe developed electrode [30].

4. Conclusions

Herein we developed a new type of SC-ISE by using sp2-C dominantPC-SMSs as promising transducers. The porous structure of PC-SMSsprovides great interface area for ion-to-electron transfer while thehigh proportion of sp2-C benefits the electroconductivity. Because ofthe high hydrophobicity of PC-SMS film, the undesirable interfacialwater layer was effectively suppressed. Thus the developed ISEs exhib-ited excellent long-term potential stability and remarkable interferenceimmunity. Furthermore, the simplicity in synthesis and construction ofISEsmakes the PC-SMSs based electrodes promising for miniaturizationand mass fabrication.

Conflict of interest

The authors declare that there are no conflicts of interest.

Acknowledgments

The authors aremost grateful to the NSFC (no. 21205112, 21105096,21175130 and 21225524), Ministry of Science and Technology of thePeople's Republic of China (2012YQ170003) and Department of Scienceand Technology of Jilin Province (no. 201215091 and 20120308).

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