Journal of Electroanalytical Chemistry 519 (2002) 130–136

Carbon ceramic electrodes modified with sub-micron particles ofnew methylene blue (NMB) intercalated �-zirconium phosphate

Peng Wang 1, Yi Yuan, Guoyi Zhu *State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,

Changchun 130022, People’s Republic of China

Received 18 September 2001; received in revised form 15 November 2001; accepted 23 November 2001

Abstract

New methylene blue-intercalated �-zirconium phosphate (NMBZrP) was synthesized in the presence of n-butylamine andcharacterized by powder XRD, FTIR, TEM and elemental analysis. Sub-micron particles of NMBZrP in deionized water wereapt to deposit onto the surface of graphite powder to yield graphite powder-supported NMBZrP, which was subsequentlydispersed into methyltrimethoxysilane-derived gels to fabricate surface-renewable, stable, rigid carbon ceramic electrodes contain-ing new methylene blue. Cyclic voltammetric studies revealed that peak currents of the NMBZrP-modified electrode weresurface-confined at low scan rates but diffusion-controlled at high scan rates. In addition, NMBZrP immobilized in a carbonceramic matrix presented a two-electron, three-proton redox process in acidic aqueous solution in the pH range from 0.52 to 3.95.© 2002 Elsevier Science B.V. All rights reserved.

Keywords: New methylene blue intercalated �-zirconium phosphate; Sol–gel; Carbon ceramic electrode

www.elsevier.com/locate/jelechem

1. Introduction

The sol–gel process provides a versatile means forthe production of inorganic and inorganic–organic hy-brid materials via the hydrolysis and condensation ofsuitable metal alkoxides at room temperature [1,2]. Therecent explosive applications of sol–gels in electro-chemistry may be clearly seen from several excellentreview articles [3–7]. Sol–gel derived carbon ceramiccomposite electrodes (CCEs) were first introduced byLev and coworkers [8] in 1994, and many efforts havebeen devoted to fabricating various chemically modifiedCCEs and using them as sensors for metal ions, glu-cose, and other important chemical and biological sub-stances in recent years [9]. An interesting feature ofchemically modified CCEs is that the active section ofthe electrodes is not clogged upon repeated polishingdue to the brittleness of the sol–gel derived silicate

backbone, thus the electrodes can be renewed by amechanical polish if scratch, leakage, contaminationand passivation arise. Several strategies have beenadopted to prepare highly stable CCEs bulk-modifiedelectroactive species, including directly bonding elec-troactive organic groups with organosilicate, dopinghydrophobic organic or insoluble inorganic electroac-tive species in the silicate matrix and strongly adsorbingsome electroactive species onto graphite powder.

Organic phenothiazine, phenazine and phenoxazinedyes have been widely used to construct modified elec-trodes by adsorption or entrapment in polymer ma-trices, immobilization in carbon paste electrodes andelectropolymerization because they can act as mediatorsfor the electrocatalysis of important biomolecules suchas NADH, hemoglobin and myoglobin [10–15]. In1998, Wang et al. [16] and Sampath and Lev [17] bothprepared Meldola blue modified CCEs and studiedtheir electrocatalytic oxidation of NADH. However,our preliminary experiments [18] showed that somephenothiazine and phenazine dyes directly doped in acarbon ceramic matrix were prone to leak into aqueoussolutions because of the weak interaction between dyesand the matrix. It seems imperative to explore and

* Corresponding author. Tel.: +86-431-5262069; fax: +86-431-5685653.

E-mail address: zhuguoyi@ns.ciac.jl.cn (G. Zhu).1 Present address: Institute of Photonics and Interfaces, Swiss

Federal Institute of Technology, CH-1015, Lausanne, Switzerland.

0022-0728/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 -0728 (01 )00736 -7

P. Wang et al. / Journal of Electroanalytical Chemistry 519 (2002) 130–136 131

Fig. 1. Chemical structure of NMB.

water purification system was used throughout the ex-periments. Solutions with different pH were preparedby mixing the 0.1 mol l−1 K2SO4 aqueous solution with0.1 mol l−1 K2SO4+0.5 mol l−1 H2SO4 aqueous solu-tion. Solutions were deaerated by argon bubbling priorto the experiments and the electrochemical cell was keptunder an argon atmosphere throughout theexperiments.

2.2. Apparatus

IR spectra were recorded on a Bio-Rad FTS-7 in-frared spectrophotometer in KBr pellets. Powder X-raydiffraction (XRD) patterns were measured with a com-puter controlled Rigaku D/max-IIB diffractometer us-ing Ni-filtered Cu–K� radiation to monitor all newphases and to determine their interlayer spacings. Zir-conium and phosphorous contents were determinedwith a TJA POEMS ICP atomic emission spectrometerafter dissolving a weighed amount of the sample in HFaqueous solution. Microanalytical data (C, N and S)were obtained using a GmbH VarioEL elemental analy-sis system. The transmission electron micrography(TEM) was performed with a JEOL JEM-2010 trans-mission electron microscope, operating at 200 kV. Elec-tron spectroscopy for chemical analysis (ESCA) ofgraphite powder-supported NMBZrP was carried outon a VG ESCALAB MK-II photoelectron spectrome-ter. The 29Si CP MAS (cross polarization magic anglespinning) NMR spectrum was measured with a VarianUnity-400 NMR spectrometer using a standard pulseprocedure. A computer controlled CHI 660 electro-chemical workstation was used for voltammetric mea-surements. A three-electrode cell, consisting of anNMBZrP modified CCE as the working electrode, anAg/AgCl/KClsat reference electrode (RE) and a plat-inum flag counter electrode (CE), was used. A Cole–Parmer 3™ pH meter was employed for pHmeasurements. All the experiments were conducted at20�0.2 °C.

2.3. Synthesis of NMBZrP

�-ZrP was prepared according to the literature proce-dure [28,29] and characterized by powder XRD, FTIRand elemental analysis. NMBZrP was synthesized asfollows: 1.5 g NMB, 5 g �-ZrP and 1 ml n-butylaminein 100 ml deionized water were stirred at 20 °C for 72h. The reaction mixture was filtered through a sinteredglass funnel, washed with water and dried overnight at60 °C to yield a deep blue solid. The contents ofzirconium, phosphorous, carbon, nitrogen and sulfurwere 23.6, 14.9, 17.3, 3.8 and 2.7%, respectively. Paleblue NMB exchanged �-ZrP was obtained using theprocedure for NMBZrP except that n-butylamine wasnot used.

develop a more reliable method to immobilize andstabilize small molecular electroactive species includingdyes in sol–gel materials.

�-Zirconium phosphate (�-ZrP), which has a layeredstructure, behaves not only as an inorganic ion-ex-changer, but also as a host for intercalation reactions[19–21]. In 1997, Tsuhako and coworkers [22] discov-ered that methylene blue (MB) could not be readilyintercalated into layered �-ZrP due to the weak electro-static force and steric hindrance around the active siteof MB cations and studied in depth the intercalation ofMB into layered �-ZrP in the presence of n-butylamineand the function of the alkylamine in the reaction. Inaddition, our preliminary work [18] showed that newmethylene blue (NMB) shown in Fig. 1, methylenegreen, Meldola blue and Nile blue could also not bedirectly inserted into the gallery of �-ZrP. In 1999,Cooper et al. [23] prepared a soft carbon paste elec-trode containing MB-intercalated �-ZrP and used it asa photoelectrochemical sensor for ascorbic acid. Car-bon paste electrodes containing phenazine, phenoxazineand phenothiazine dyes exchanged with �-ZrP havealso been prepared [24–27]. In the present study, sub-micron particles of NMB-intercalated �-zirconiumphosphate (NMBZrP) were synthesized in the presenceof n-butylamine and deposited onto the surface ofgraphite powder to yield graphite powder-supportedNMBZrP, which was subsequently dispersed intomethyltrimethoxysilane-derived gels to fabricate sur-face-renewable, stable, rigid carbon ceramic electrodescontaining NMB.

2. Experimental

2.1. Materials and solution

Methyltrimethoxysilane (MTMOS, �97%) was pur-chased from ACROS and used without further purifica-tion. Zirconium oxychloride, phosphoric acid,n-butylamine and NMB were purchased from BeijingChemicals Factory. High purity graphite powder (aver-age particle diameter, 8 �m) was obtained from Shang-hai Carbon Plant. The 600-grit emery paper wassupplied by Shanghai Sand Wheel Plant. Other chemi-cals were of analytical grade and were used as received.Ultrapure water obtained from a Millipore Milli-Q

P. Wang et al. / Journal of Electroanalytical Chemistry 519 (2002) 130–136132

Fig. 2. Powder XRD patterns of (a) pure �-ZrP; (b) NMB exchanged�-ZrP and (c) NMBZrP.

2.5. Fabrication of NMBZrP modified CCE

The NMBZrP modified CCEs were prepared by thefollowing procedure. The solution of 1.50 ml methanol,0.5 ml MTMOS and 0.05 ml hydrochloric acid (11 moll−1) was mixed ultrasonically for 2 min, then 2.45 ggraphite powder-supported NMBZrP was added andshaken on a vortex agitator for an additional 3 min.The mixture was added to glass tubes with 3 mm innerdiameter and 8 cm length, and the length of compositematerial in the tubes was controlled to be about 8 mm.In addition, a little extra mixture was needed to beretained on the top of the electrodes, and the mixture inthe tubes was lightly pressed on smooth plastic paperwith a copper stick through the back. After drying atr.t. for 48 h, the electrodes were polished with 600-gritemery paper to remove extra composite material andthen wiped gently with weighing paper. Electric contactwas made by silver paint through the back of theelectrode.

3. Results and discussion

3.1. Preparation and characterization of sub-micronparticles of NMBZrP

Direct intercalation of NMB into layered �-ZrP wasnot successful by the method of Kubota and Gorton[26], so the intercalation was performed according tothe published procedure [22] for MBZrP with somemodifications. The color of the product obtained with-out n-butylamine is pale blue, but that of the productin the presence of n-butylamine is deep blue, suggestingthat the latter contains more NMB than the former.The intercalation of NMB into �-ZrP was proved bypowder XRD, IR and elemental analysis. Fig. 2 showsthe comparative powder XRD patterns of pure �-ZrP,NMB exchanged �-ZrP and NMBZrP. Two new peaksat 21.1 and 10.8 A� suggest that NMB and n-butylamineare co-intercalated into the gallery of �-ZrP and expandthe interlayer distances. However, no new peaks wereobserved for the powder XRD pattern of NMB ex-changed �-ZrP. According to the elemental analysisresults, the molar ratio of NMB to n-butylamine wascalculated to be approximately 3.5:1. IR spectra of pure�-ZrP, NMB, NMB exchanged �-ZrP and NMBZrPare presented in Fig. 3. The main difference betweenNMB and NMBZrP is the absorption bands of 1050and 1073 cm−1, which represent a phosphate vibrationof symmetry group C3y. The two absorption bandsappear at 1050 and 1075 cm−1 for pure �-ZrP. Inaddition, absorption bands at 3510 and 3593 cm−1 forNMBZrP decrease compared with those for �-ZrP.These absorption bands of �-ZrP can be assigned to theasymmetric and symmetric water stretching in �-ZrP.

Fig. 3. FTIR spectra of (a) pure �-ZrP; (b) NMB; (c) NMB ex-changed �-ZrP and (d) MBZrP.

2.4. Preparation of graphite powder-supportedNMBZrP

Fifty milligrams of NMBZrP was first dispersed into500 ml deionized water with the help of sonication atroom temperature (r.t.) for 10 min to yield a stabletransparent blue colloidal solution, then 5 g graphitepowder was added. The mixture was stirred for 2 h.When the stirring was stopped, the supernatant becamecolorless, indicating that NMBZrP was deposited ontographite powder. Graphite powder-supported NMBZrPwas obtained after it was filtered with a Buchner funnel,washed with 50 ml cold water and dried in vacuo at60 °C for about 24 h. The efficient loading of NMBZrPon graphite powder was further confirmed by ESCApeaks at 133.7 eV (P2p), 164.2 eV (S2p), 183.2 eV(Zr3d5/2), 186.5 eV (Zr3d3/2), 399.1 eV (N1s) and 401.5 eV(N1s).

P. Wang et al. / Journal of Electroanalytical Chemistry 519 (2002) 130–136 133

These phenomena indicate that the intercalating pro-cess is accompanied by the partial dehydration of �-ZrP. Framework vibrational absorption bands of NMBare also observed clearly in the spectra of NMBZrP. Inaddition, neither differences between IR spectra of pure�-ZrP and NMB exchanged �-ZrP nor adsorptionbands for NMB in the spectrum of NMB exchanged�-ZrP are observed, suggesting that only a smallamount of NMB can be loaded onto �-ZrP if n-buty-lamine is not used. Furthermore, this also indicates thatthe location of NMB in NMBZrP is not mainly at thesurface of �-ZrP but in the gallery of �-ZrP. The TEMphotograph of NMBZrP particles is shown in Fig. 4,and particle diameters are less than 500 nm.

3.2. Electrode preparation

MTMOS can form a maximum of three siloxanebond entities (Si�O�Si) if the theoretical maximumdegree of condensation during gel formation is

achieved. However, if hydrolysis and condensation donot approach the maximum theoretical value, that is, ifthe degree of crosslinking is less than 100%, the situa-tion is different. Since insufficient water for the MT-MOS hydrolysis step was used to prepareMTOMS-derived sols, we were interested in determin-ing the actual degree of crosslinking in the xerogelcomponent of the carbon ceramic composite to confirmthat additional water for the MTMOS hydrolysis canbe supplied by air humidity. From a solid-state 29Si-NMR study it is possible to identify structurally differ-ent entities, specifically Si�O�Si and Si�OH originatingfrom MTOMS. The conventional Tn notation was used[30,31]. Thus, T represents a tetrahedral silicon atombonded to three oxygen atoms, and the superscript nindicates the connectivity, that is, the number of otherT units attached to the CH3�SiO3 tetrahedron. Theactual degree of crosslinking can be expressed by aver-aging the experimentally observed T species as demon-strated in Eq. (1).

Degree of crosslinking (%)=100(T1+2T2+3T3)/3(1)

Fig. 5 shows the 29Si CP MAS NMR spectrum of thecarbon ceramic composite. It is clear that there is ahigh percentage of T3 species (approximately 65%),about 35% T2 species and essentially no T1 componentpresent. The total degree of crosslinking amounts to88%. Furthermore, the signal at −100 ppm, where apeak due to the SiO4 component is expected to appear,is extremely small. Thus, appreciable hydrolysis of theCH3�Si entity does not occur.

In addition, base (NaOH or NH4OH) or neutral(NH4F) catalysts failed to produce rigid monoliths andyielded only silica-covered graphite powder. If a largerdose of hydrogen chloride was used, then rapid gelationoccurred before the graphite powder-supported NM-BZrP was added. Also, a larger dose of MTMOS-derived gels resulted in poor conductivity of thecomposite due to the percolation conduction mecha-nism of composite material electrodes [32]. All thediffraction peaks of NMBZrP were also observed forthe NMBZrP-methylsilicate composite (not includinggraphite powder) prepared via the sol–gel method be-sides wide peaks of methylsilicate, but no other newpeaks appeared, which revealed that the layered struc-ture of NMBZrP was not changed and no new bulk-material containing Zr, P, Si, C and O formed duringthe sol–gel process. In the process of fabricating theNMBZrP modified CCEs, a little extra mixture wasrequired to be retained on the top of the electrodes inorder to obtain conveniently a complete and uniformsurface when they were first polished. In addition, thecomposite material became fragile and thus it wasdifficult to obtain smooth electrode surfaces if thegelation temperature was higher than 60 °C.

Fig. 4. The TEM photograph of NMBZrP particles. Scale bar: 500nm.

Fig. 5. 29Si CP MAS NMR spectrum of the carbon ceramic com-posite.

P. Wang et al. / Journal of Electroanalytical Chemistry 519 (2002) 130–136134

Fig. 6. Cyclic voltammograms of the NMBZrP-modified CCE in 0.1mol l−1 K2SO4+H2SO4 (pH 0.52) at various scan rates (from innercurve to outer curve: 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 250,300, 350, 400, 450 and 500 mV s−1).

at various scan rates. As shown in Fig. 7, the cathodicpeak currents (Ipc) are linearly proportional to the scanrate (�) at scan rates lower than 120 mV s−1, suggest-ing that peak currents are surface-confined; however, atscan rates higher than 300 mV s−1, the cathodic peakcurrents are proportional to the square root of scanrate, which indicates that the peak current is diffusion-controlled. The transition of the relationship betweenpeak current and scan rate reveals that proton diffusionin the composite is not fast enough for electron transferat scan rates higher than 300 mV s−1. In fact, it is veryfrequent to observe this phenomenon in the voltamme-try of microparticles [34,35]. At a scan rate of 20 mVs−1, the anodic and cathodic peak potential separation(�Ep) is less than 16 mV instead of the value zeroexpected for a reversible surface redox process [36],which might arise due to non-ideal behavior caused bythe effects of charge diffusion and uncompensatedohmic drop. The surface concentration of electroactivespecies, �c, can be calculated to be approximately2.0×10−9 mol cm−2 for n=2 from the slope of theplot of Ipc versus � (��120 mV s−1) by the followingequation [36]:

Ip=n2F2�A�c/4RT (2)

where � is the sweep rate, A is the surface area and theother symbols have their usual meaning. It should bepointed out that the calculated surface concentration isan efficient attribute (per cross section of the electrode)and does not reflect the actual amount of NMB perarea of exposed graphite. In addition, along with theincrease of colloidal NMBZrP solution concentration,the amount of adsorption (NMBZrP) on the surface ofthe graphite powder increased, so the resulting elec-trodes presented cyclic voltammograms with largerpeak currents compared with those of the NMBZrP-modified electrode described in this paper. Also, wehave prepared CCEs containing the same amount asNMBZrP of NMB exchanged �-ZrP. This electrode didnot show any observable Faradaic response because itcontains only a very small amount of NMB (electroac-tive species). However, if a large amount of NMBexchanged �-ZrP was dispersed in the carbon ceramicmatrix in order to obtain an ideal Faradaic currentresponse, the conductivity of the resulting electrodewould drop dramatically because the conductive mech-anism of CCEs is percolation [32] and �-ZrP is not agood electronic conductor [19].

3.4. pH-dependent electrochemical beha�ior of theNMBZrP modified CCE

Additional observations are concerned with thestrong dependence of the voltammetric response ofNMB on solution acidity. In the present work, square-wave voltammetry was adopted to measure the pH-de-

Fig. 7. The dependence of cathodic peak current on scan rate and thesquare root of scan rate.

3.3. Electrochemical beha�ior of the NMBZrP-modifiedCCE

After 4 min of cyclic potential scanning, the NM-BZrP-modified CCE presented an invariable voltam-metric response, suggesting that a stable four-phaseboundary of graphite–NMBZrP–methylsilicate-solu-tion was formed. Fig. 6 shows typical cyclic voltam-mograms of the NMBZrP-modified electrode in 0.1 moll−1 K2SO4+H2SO4 aqueous solution (pH 0.52) atdifferent scan rates. When the potential is scannedbetween +0.5 and −0.1 V, chemically reversible redoxwaves with the formal potential, (Eox+Ered)/2, at +0.143 V, are observed, which can be attributed totwo-electron reduction and oxidation [33] of NMBbecause neither �-ZrP nor carbon ceramic matrix showany electroactivity in the potential range investigated.The electron transfer mechanism may be electron hop-ping. The anodic and cathodic peak potentials shiftsymmetrically, resulting in a constant formal potentialat various scan rates. In addition, the correspondinganodic and cathodic peak currents are almost the same

P. Wang et al. / Journal of Electroanalytical Chemistry 519 (2002) 130–136 135

pendent electrochemical behavior conveniently. Fig. 8presents square-wave voltammograms for the NM-BZrP-modified CCE in 0.1 mol l−1 K2SO4+H2SO4

aqueous solutions with different acidities. It can beclearly seen that along with the increase of pH, thepeak potentials shift to the more negative potentialdirection and the peak currents also decrease gradually.Reduction of NMBZrP in the CCE matrix is accompa-nied by the evolution of protons from solution to thewetting section of the electrode to maintain chargeneutrality. When the pH is varied from 0.52 to 3.95,slower diffusion rate of protons should be the reasonfor the current decrease [37], and the more negativepeak potentials can be elucidated by the Nernst equa-tion [38]. The plot of peak potential versus pH for theNMBZrP-modified CCE presents a good linearity inthe pH range from 0.52 to 3.95. The slope in this pHrange is −86.7 mV pH−1, which is very close to thetheoretical value −87 mV pH−1 expected for the2e−/3H+ redox process at the experimental tempera-ture. As is known, the redox reaction of NMB in strongacidic aqueous solution is a 2e−/2H+ process [33].However, Kubota and Gorton [26] found that theformal potential of carbon paste electrodes containingNMB exchanged �-ZrP was pH-independent due to theacidity of �-ZrP, which was different from our result.This may be caused by the micro-environmental differ-ence between the gallery of �-ZrP and bulk acidicsolutions although direct evidence has not beenobtained.

3.5. Stability and repeatability of surface-renewal

Practical utility of chemically modified electrodes isoften limited by a lack of long-term stability, which canbe attributed mainly to dissolution or leakage of theelectrocatalyst to some extent, especially under hydro-dynamic conditions. No current decrease for an NM-BZrP-modified electrode was observed after 10 h

successive potential cycling in 0.1 mol l−1 K2SO4+H2SO4 aqueous solution (pH 0.52). In addition, only a3.6% current decrease was found when the electrodewas immersed in a stirred acidic solution for 10 days.The high stability of the modified electrode is related tothe chemical and mechanical stability of the silicatematrix, the limited wetting section controlled by methylgroups, especially the strong host–guest interaction[19–21] between NMB and �-ZrP and the stable physi-cal encapsulation of sub-micron particles of NMBZrPin the nanoporous methylsilicate-based matrix [1,2].Also, the possible chemical bonding (Si�O�P andSi�O�Zr) [39–41] between the methylsilicate networkand hydroxyl groups at the surface of NMBZrP parti-cles contributes to the stability. Another main attrac-tion of the NMBZrP-modified CCE is that theelectrode surface can be renewed by simple mechanicalpolishing on emery paper and a fresh surface exposedwhenever needed. As a matter of fact, ten successivepolishings resulted in a relative standard deviation(R.S.D.) of 2.3% for an NMBZrP-modified CCE bymeasuring cathodic peak currents. In addition, 15 NM-BZrP-modified CCEs had almost the same voltammet-ric behavior in 0.1 mol l−1 K2SO4+H2SO4 aqueoussolution (pH 0.52).

4. Conclusions

An efficient and practical method for the construc-tion of surface-renewable, leak-free, electrochemicallyreversible CCEs bulk-modified with NMB has beendeveloped. Although the new concept has been pre-sented in the context of NMBZrP modified CCEs, itcould be extended readily to prepare other CCEs con-taining some interesting electroactive species by meansof inserting electroactive cations or anions into thegallery of layered compounds and depositing sub-mi-cronmeter- or nanometer-sized particles onto the sur-face of graphite powder.

Acknowledgements

The authors thank the Ministry of Science and Tech-nology of China and the Ministry of Science andTechnology of Jilin Province for financial support.Fruitful discussions with Professor Erkang Wang andProfessor Shaojun Dong are also acknowledged.

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