Electrochemistry Communications 11 (2009) 1285–1288

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Electrochemistry Communications

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Towards surface acid–base property of the carboxylic multi-walled carbonnanotubes by zero current potentiometry

Wei Gao *, Junfeng SongInstitute of Analytical Science, Northwest University, Xi’an 710069, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 March 2009Received in revised form 13 April 2009Accepted 24 April 2009Available online 6 May 2009

Keywords:Carboxylic MWNTsZero current potentialInterface potentialAcid–base propertySurface pKa

1388-2481/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.elecom.2009.04.026

* Corresponding author. Tel./fax: +86 29 8830 3448E-mail address: gaowei2003425@163.com (W. Gao

The surface acid–base property of carboxylic multi-walled carbon nanotubes (MWNTs) is investigated byzero current potentiometry with a new electrochemical measurement system. The pH dependent inter-face potential variation at the interface of carboxylic MWNTs/solution is investigated by measuring zerocurrent potential Ezcp. In the pH range of 1–11, the pH response of carboxylic MWNTs exhibits two linearrelationships according to the following equations: Ezcp = 0.791–0.0535 pH (pH 1–5.1) and Ezcp = 0.643–0.0241 pH (pH 5.1–11), respectively. The intersection at pH 5.1 of two regions indicates the surface pKa

value of carboxylic group terminated MWNTs.� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Introducing carboxylic groups to the ends or side-wall of thecarbon nanotubes represents a primary functionalizing procedure[1–3]. Due to the acid–base property of the carboxylic groups,the solubility of carbon nanotubes is intensively promoted, andsome immobilizations of carbon nanotubes with polymer [4],nanoparticles [5] and biomolecule [6] can be easily conducted viaelectrostatic interaction. This features opens up more excitingopportunities for applications of carbon nanotubes in some fields,such as composite material [7,8], nanoelectronic devices [9], chem-ical sensors and biosensors [10–12]. To optimize these applica-tions, it is of significance to understand the surface acid–baseproperty of carboxylic carbon nanotubes.

A few techniques have been used to investigate the surfaceacid–base property of carboxylic carbon nanotubes. Yang et al.[13] used atomic force microscopy to investigate the dissociationproperties of carboxylic groups at the end of single-walled carbonnanotubes (SWNTs). The surface pK1/2 was determined to be about9.0. Nikosi and Ozoemena [14] studied the impedance response ofAu–Cys–SWNTs at different pH of the [Fe(CN)6]3�/[Fe(CN)6]4�

solutions by electrochemical impedance spectroscopy, the surfacepKa of carboxylic SWNTs was estimated to be ca. 5.5. Furthermore,several groups have studied the pH-sensitive properties of carbox-ylic carbon nanotubes using the IR and Raman spectroscopy [15],cyclic voltammetry [10], differential pulse voltammetry [16], and

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conductivity measurement [17]. These studies reported the pro-tonation and deprotonation of carboxylic groups terminated car-bon nanotubes in a certain extent, but the surface pKa value wasnot determined.

Here, we design a novel electrochemical measurement systemfor measuring zero current potential. By using the proposed zerocurrent potentiometry, the surface acid–base property of carbox-ylic groups terminated multi-walled carbon nanotubes (MWNTs)is investigated. The surface pKa value of carboxylic MWNTs isdetermined to be 5.1, which is consistent with that of carboxylicSWNTs [14].

2. Experimental

2.1. Apparatus and reagents

Electrochemical measurements were carried out by using a CHI660 electrochemical workstation (CH Instruments, USA). A satu-rated calomel electrode (SCE) was used as the reference electrode.The pH value of the buffers was measured using a pH-211 pH-me-ter (Hanna, Italy). Fourier transform (FT) IR spectra were recordedon a Nicolet FTIR-5700 spectrometer.

MWNTs were obtained from Shenzhen Nanotech Port Co. Ltd.,China with typical diameter 10–20 nm, length 5–15 lm and purityabove 95%. The MWNTs was purified and functionalized by oxida-tion in a 2.2 M nitric acid under sonication for 20 h, then washedwith double-distilled water to neutrality and dried in an oven in37 �C. The as-treated MWNTs were terminated by carboxylicgroups as indicated by FTIR measurement [18]. Other reagents

(A) (B)

A

solution

potentiostat

computera

b

c

d

TWE TCE TRE

SCEe

E

Eref

Eref

TCE TWE

I

ψ

(C)

solution

Fig. 1. (A) Schematic diagram of the carboxylic MWNTs film coated sensor: (a) Teflon tube body; (b) plastic cap; (c) copper wire; (d) pristine MWNTs paste; (e) carboxylicMWNT film. (B) Schematic diagram of the set-up. TWE, TCE, TRE are the terminal points of the working electrode, counter electrode and the reference electrode of thepotentiostat, respectively. (C) Electronic circuit diagram. E, Eref and w indicate the applied potential, the reference potential of the reference electrode and the interfacepotential of the MWNTs film/solution interface, respectively. I is the circuit current.

-1.0 -0.5 0.0 0.5 1.0 1.5 2.0-0.003

-0.002

-0.001

0.000

0.001

0.002

Cur

rent

I/A

Potential E/V (vs SCE)

Ezcp

Fig. 2. Curve I–E of carboxylic MWNTs film coated sensor in 0.1 mol l�1 KCl solution(pH 7).

1286 W. Gao, J. Song / Electrochemistry Communications 11 (2009) 1285–1288

were of analytical grade, and double-distilled water was usedthroughout the experiment.

2.2. Sensor preparation

The design of the carboxylic MWNTs film coated sensor wasillustrated in Fig. 1A. This sensor combined two different sensormethodologies: MWNTs paste and casting film. Firstly, MWNTspaste was prepared by mixing pristine MWNTs and mineral oil inthe ratio 60%:40% (w:w). The paste was carefully hand-mixed ina mortar and then packed into a Teflon tube (3 mm diameter,10 mm depth). A plastic cap was fixed at one end of the tube.Two copper wires were inserted vertically into MWNTs paste at4 mm depth through two holes drilled in the end cap. The surfaceof the resulting paste were polished on a weighting paper andrinsed carefully with double-distilled water.

The casting film technique was then carried out. One milligramof carboxylic MWNTs was dispersed with the aid of ultrasonic agi-tation in 10 ml of DMF to give a 0.1 mg ml�1 black suspension. Anappropriate quantity of DMF suspension was dropped on thesmoothed surface of the MWNTs paste. Finally, it was placed atroom temperature to allow the solvent to evaporate.

2.3. Electrochemical measurement

Fig. 1B illustrated the basic measurement set-up of zero currentpotentiometry. A carboxylic MWNTs film coated sensor was con-nected in series between the terminal joints of the working elec-trode and the counter electrode of a potentiostat, and thenimmersed into the electrolyte solution together with a SCE as ref-erence electrode, establishing a new electrochemical measurementsystem. The electronic circuit diagram of the system was shown inFig. 1C. When an external potential was applied on the sensor, acurrent flowed through the terminal joint of counter electrode,the sensor and the terminal joint of working electrode to the vir-tual earth. The current rested with not only the applied potentialand reference potential but also the interface potential at the sen-sor/solution interface.

By using cyclic voltammetry, the curves I–E of current versuspotential were recorded in different pH solutions. Plotting zerocurrent potential versus the pH, the surface acid–base propertyof carboxylic MWNTs was studied.

3. Results and discussion

3.1. Measurement principle of zero current potentiometry

The curves I–E of the carboxylic MWNTs film coated sensor wererecorded using cyclic voltammetry over the pH range of 1–11. At a

given pH, the anodic and cathodic curves I–E completely over-lapped, and exhibited a non-linear S-type shape (Fig. 2). Whenthe pH increased, the curves shifted negatively along the potentialaxis while the non-linear shape remained unchanged. Apparently,these phenomena were different from the conventional cyclic vol-tammograms, indicating a different current–potential relationshipof the new electrochemical measurement system.

In the system, the sensor is connected in series between the ter-minal points of the working electrode and counter electrode of thepotentiostat. Such a distinct connection avoids the current passingthrough the interface of the sensor/solution and the electrolytesolution. Thus, the current just flows inside the sensor, and its va-lue rests with the electronic property the sensor and the potentialof the sensor. The electronic property of the carboxylic MWNTsfilm coated sensor was investigated using a multimeter. The resultshowed that the sensor simultaneously exhibited the properties ofelectric capacitor and resistance.

Upon the increase of the pH values, the non-linear curvesshifted negatively along the potential axis. This suggested that be-sides the applied potential and the reference potential, the inter-face potential at the interface of the sensor/solution, a keyparameter for describing the charge status in the interfacial region,also made a contribution to the potential of the sensor.

In view of all these factors, the fundamental current–potentialcharacteristic of the system is given.

I ¼ w� ðEþ ErefÞRs

þ ddt½Cs½w� ðEþ ErefÞ�� ð1Þ

where I is the circuit current, E and Eref are the applied potential andthe reference potential, respectively. w is the interface potential at

0 1 2 3 4 5 6 7 8 9 10 11 120.3

0.4

0.5

0.6

0.7

0.8

Ezc

p/ V

pH

B

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0-200.0-150.0-100.0-50.00.050.0100.0150.00200.0

Cur

rent

I/μ

A

Potential E/ V(vs. SCE)

Ezcp

pH

A

Fig. 3. (A) Curves I–E of carboxylic MWNTs film in the range of pH 1–11. (B) Plot ofzero current potential Ezcp as a function of pH.

W. Gao, J. Song / Electrochemistry Communications 11 (2009) 1285–1288 1287

the sensor/solution interface; Rs and Cs are the resistance and thecapacitance of the sensor, respectively.

In the Eq. (1), the first term reflects the direct current from theresistance of the sensor; the second term represents the chargingcurrent due to capacitance property of the sensor. When the samesensor is used, Rs and Cs of the sensor are two constants. Thus, thecurrent variation is codetermined by the interface potential andthe applied potential. The interface potential depends strongly onthe property of the sensing surface and the solution composition.When the solution is fixed, the interface potential w also has a con-stant value. In this case, the current is completely determined bythe applied potential. When a linear sweep potential is applied,the first term is linear and the second term is non-linear. The com-bination of these two currents leads to the appearance of non-lin-ear curve I–E.

Alternatively, when the solution pH changes, the interface po-tential is a function of the solution pH due to the surface acid–baseproperty of the carboxylic groups terminated MWNTs as expressedin the Eq. (2) [19].

Dw ¼ �a2:303RT

FDpH ð2Þ

where a is the pH sensitivity factor, R is the universal gas constant, Tis the temperature (K), and F is the Faraday constant.

In this case, the current will vary with the change of the pHdependent interface potential. Hence, the non-linear curve I–Eshifts negatively with increasing the pH as the same linear sweeppotential is applied. Apparently, we can investigate the surfaceacid–base property of carboxylic MWNTs by measuring the poten-tial shift of the curve I–E with the pH at the same current level.Among numerous detectable points, the point when the currentis zero is of particular interest. At this point, the Eq. (1) is rear-ranged as

Ezcp ¼ w� Eref ð3Þ

Here the sign of the current reverses and no net charge flowsthrough the sensor, the corresponding applied potential is thustermed as zero current potential Ezcp. Because both of the formsof the current and the electronic property of the sensor are elimi-nated in the Eq. (3), measuring zero current potential should be eas-ier and more accurate than measuring the potential variations ofother points of the curves I–E. The technique of measuring zero cur-rent potential is defined as zero current potentiometry.

Combination the Eqs. (2) and (3) leads to the primary expres-sion of the measurement principle of zero current potentiometry.

DEzcp ¼ �a2:303RT

FDpH ð4Þ

3.2. Surface acid–base property of carboxylic MWNTs

The surface acid–base property of carboxylic MWNTs film wasinvestigated using zero current potentiometry. In order to improvethe accuracy and efficiency of measurement, the curves I–E wererecorded with a profile of ±1.0 V and ±200 lA (Fig. 3A). The plotof Ezcp against pH was detailed in Fig. 3B. The plot was split intotwo linear regions which intersected at about pH 5.1 each other.The corresponding linear equations were Ezcp = 0.791–0.0535pH(pH 1.0–5.1, R = �0.999) and Ezcp = 0.643–0.0241pH (pH 5.1–11,R = �0.998), respectively.

The intersection at pH 5.1 compares favorable with the surfacepKa of carboxylic SWNTs [14] and other carboxylic groups termi-nated SAM [20], indicating the surface pKa of carboxylic MWNTs is5.1. Both the surface pKa values of carboxylic MWNTs (5.1) and car-boxylic SWNTs (5.5) are larger than the pKa value (4.20) of benzoicacid in solution, and lower than the surface pKa (6.45) of carboxylic

acids on graphite [21]. This suggests that the hydrophobicity of car-bon nanotubes surface is relatively less compared to graphitesurface.

The surface pKa is a good indicator for surface acid–base equilib-rium of carboxylic MWNTs. According to the equation pH = pKa + log([MWNTs-COO�]/[MWNTs-COOH]), MWNT-COOH dominates onthe sensing surface at lower pH than pKa, while MWNT-COO� dom-inates on the sensing surface at higher pH than pKa. The MWNT-COOH and MWNTs-COO� have different abilities to deliver or takeup protons, thereby leading to the difference of the pH-sensitivity(53.5 mV/pH and 24.1 mV/pH) in two regions.

4. Conclusions

The surface acid–base property of carboxylic groups terminatedMWNTs is investigated using zero current potentiometry. The sur-face pKa value is determined to be 5.1. The pH sensitivities are�53.5 mV/pH in the pH range of 1–5.1, and �24.1 mV/pH in thepH range of 5.1–11, respectively. Zero current potentiometry pro-vides a powerful tool to study the surface acid–base property offunctionalized carbon nanotubes.

Acknowledgement

The present work was supported by the National Natural Sci-ence Foundation of PR China (Grant No. 20475043).

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