PREPRATION OF CARBON ELECTRODES

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Anal. Chem.

1994,66, 1747-1753

Sol-Gel-Derived Ceramic -Carbon Composite Electrodes:

Introd uction and Scope

of

Applications

Mic ha el Tsionsky, Genia Gun, Victor

Giezer,

and Ovadia Lev

Division of Environmental Sciences, The Fredy and Nadine Herrmann School of Applied Science,

The Hebrew University

of

Jerusalem, Jerusalem,

9

1904, Israel

A new class of composite electrodes made of sol-gel derived

carbon-silica m aterials is introduced. Modified porous com-

posite carbon-silica electrodes can exhibit hydrophobic

or

hydrophilic surface characteristicsand can serveas an indicator

(inert) electrode, as a potentiometric (selective

or

reference)

electrode, and in amperometric sensing and biosensing. The

composite (carbon) ceramic electrodes (CCEs) are rigid,

porous, easily modified chemically and have a renewable

external surface. The electrodes offer higher stability than

carbon pasteelectrodes, and they are more amenable to chemical

modification than monolithic and (organic) composite carbon

electrodes. Experimental examples demonstrating the scope

of electroanalyticalapplications

of

the composite carbon-silica

electrode and its modifications are presented.

Composite electrodes made of carbon powder (or inert

metals) dispersed in insulating organic polym er (e.g., epoxy1

or poly chlorotrifluoroethylene)2) or viscous liquid (e.g.,

paraffin oil or Nujo13) were found useful in th e various fields

of ele ctr ~a na lys is. ~J his paper demonstrates an alternative,

equally versatile, class of ceram ic com posite electrodes (CC Es) .

The electrodes are made of graphite powder dispersed

homogeneously in sol-gel derived cera mic mate rial such as

porous silica or Ormosil (organically modified silica6).

Methods for the preparation of CCEs and examples dem-

onstrating the power and versatility of modified CCEs are

presented.

The evolution of sol-gel te c h n~ lo g y, ~ -~meltless method

for the formation of silica and m etal oxides, and thesub sequent

invention of the sol-gel doping technique,1 provided anal ytica l

chemists with the possibility to tailor supporting inorganic

matrices with the versatility tha t was traditionally attributed

only to organic polymers. 11,12 Indeed , sol-gel derived cera mics

(1) Swofford, H. S.; Carman, R. L., I11 Anal. Chem.

1966,38,

966.

(2 ) Tallma n, D. E.; Chesney, D.

J.;

Anderson,

J.

L. Anal. Chem.

1978,50,1051.

(3) Adams, R. N.; Prater, K. B.

Anal. Chem. 1966,

38. 153.

(4) Petersen, S. L.; Tallman, D. E. Anal. Chem.

1990

62, 459, and references

(5) Wring, S. A.; Hart,

J.

P. Analyst

1992, 117, 1215,

and references therein.

(6) Philipp, G.; Schm idt, H. J.

Non-Cryst. Solids

1984,63,

283.

(7) Sc herer, G.; Brinker,

J . Sol-Gel Science;

Academic Press: San D iego, 1990.

8) Klein, L. C. , Ed. Sol-Gel Technology fo r Thin Films, Fibers, Preforms,

Electronics and Spec ialty Shapes; Noyes: Park Ridge, NJ ,

1990.

(9) Hench, L. L.; West,

J.

K., Eds. Chemical Processingof Advanced Ma terials;

John Wiley and

Sons,

Inc.: New York, 1992.

(10)

First

described

by: Avnir, D.; Levy, D.; Reisfeld, R. J Phys. Chem.

1984,88,

5956.

1 1) Avnir, D.; Braun, S.; Lev. 0.;ttolenghi, M. Sol Gel Optics

11

SPIE, Symp.

Series;

Mackenzey, J. D., Ed. 1992;Vol. 1758.

(12)

Avnir, D.; Levy, D.; Lev,

0 ;

Braun,

S.;

Ottolenghi, M. Organically-doped

Sol-Gel Glasses: O ptics, Photophysics and Chemical Sensing. In

Sol Gel

Optics-Processing and Appl ication s;

Klein, L. C.,

Ed.;

Kluwer Academic:

Dordrecht, the Neth erlands in press.

therein.

0003-2700/94/0366-1747 04.5QI0

1994 American

Chemical

Society

found a plethora of analytical applications in biosensing,13-15

chromatography,16J7 photometric sensing,18-22 and other

application^.^-^ A most useful route for the formation of porous

silica is by using alkoxysilane precursors (e.g., tetramethoxy-

silane or tetraethoxysilane) that hydrolyze (in the presence

of water) and condense to form a colloidal suspension (sol)

tha t after gelation and drying forms the xerogel (Le., dry gel).

A base or acid is used for catalyses and for control of the

specific surface are a and pore size distribution

of

the xerogels.

Most of the applications of sol-gel derived ceramics require

poreless materials which are produced by a final high-

tem per atur e sintering step . Th e sol-gel derived composite-

carbon electrodes that are described here are not sintered.

Sintering lowers material porosity, which is in itself adva nta-

geous, as is the ability to introduce and immobilize heat-

sensitive compounds during the molding step. The surfa ce

properties of the xerogel (such as polarity, ion exchange

capacity, an d concentration of silanol groups) can be modified

by using as precursors compounds containing the silicon-

carbon bond. Th e Si-C bond remains unchanged during the

hydrolyses and polycondensation, and the functional group

(R) remains exposed at the surface of the porous structure

that is denoted by the general formula RySi0,(OH)~y-2x,

where R and O H ar e the surface groups:

(1

)Si(OR),

+

(y)RSi(OR),

+

(4 x y ) H 2 0

-

where

0 5

V/s), the exchanged charg e

was independent of the scan rate. At such high scan rates the

diffusion layer becom es very small and t he solution contributes

only a small amount of reactive species compared with the

adsorbed layer.

Th e amoun t of adsorbed P Q on modified CC E can be used

to estimate the active area of the electrode. The quan tity of

ads orbed PQ w as -40 0 p mol /cm2 . M ~ C r e e r y ~ ~eported a

value of 170 pmol/cm2 for

a

monolayer of adsorbed P Q on

a polished glassy carbon electrod e. Thu s, the ratio of active

Anawic al Chemistty,

Vol. 66 No. 10, May 15, 1994 1751


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Flgure 8. pH dependence of the peak po tential of 9,lO-phenan-

threnequinone modified

CCE

in citric-phosphate buffers (scan rate

100 mV/s).

surfacearea to the geom etricarea (postulating that thespecific

adsorbance of graphite powder is similar to that of glassy

carbon) is approximately a = 2.5 cm2 /cm2 where a= ctive

area/geometric exposed surface).

Quinone Modified pH CCEs. Voltam metric electrodes

often exhibit a faste r response and a lower level of interfe rence s

than potentiometric sensors. Voltammetric polymer-coated

(e.g., N a f i ~ n ~ ~r p ~ l y ( e t h y l e n i m i n e ) ~ ~ )H electrodes and

microelectrode^^^ ^^

have recently been reported. Th e quinone

modified CCEs demonstrate a new class of voltammetric pH

sensors with renewable surface . Figu re 8 shows the dependence

of the potential of the cathodic a nd ano dic peaks of 9,lO-

phenanthrenequ inone modified carbon-silica electrode on the

pH of a buffer solution. An almost Nernestian depend ence

of the anodic peak current on pH is found over a wide pH

ran ge (0-7): dEo/dpH

=

60.2 mV R = 0.9996). At higher

pH , deprotonation of the quinone interferes. Quinhydrone

modified electrode s exhibited a larger linear p H rang e (0-9)

which is only violated near the pKa of hydroquinone (pKaI =

9.85; pKaz

=

11.4 38), Sma ll leaching of the quinones from

the electrode during prolonged continuous cycling does not

affect its accuracy since the p H determination depends on

potential shifts and is not sensitive to the absolute value of the

peak current.

The stability of organically doped CCEs depends

on

the

type of modifier. Open circu it

9,lO-phenanthrenequinone

nd

quinhydrone and ferrocene modified CCEs did not lose any

activity even afte r long duration in aqueous solution. Th e

ferrocene did not loseactivity even afte r exten sivev oltam metr ic

cycling while PQ /C CE and quinhydrone /CCE lost -5-10

activity during 3 h of cycling between -0.2 and 1.2 V (SC E)

(after a 30-min pretreatment of continuous cycling to leach

externally adsorbed dopan ts). Th e stability of the organically

doped electrodes depends

on

he leachability of the modifiers

from the electrodes. Leaching of dopants from sol-gel silica

matrices is well studied and was found to depend on the

interaction of the immobilized species with the supporting

(42) Rubinstein, I

Anal. Chem.

1984, 56, 1135.

(43) Mandler, D.; Kaminski, A.; Willner, 1. Electrochim. Acta

1992,

37, 2765.

(44) Hickman, J. J.; Ofer, D.; Laibinis, P.E.,; Whitesides, M.; Wrighton,S. Science

1991,

252, 688.

Figure 9, Cyclic voltammogram 100 mV/s ) of a com posite carbon-

silica glucose biosensor in 0.1 M phosp hate buffer (pH5.6):

(1)

blank

and (2)

9.8

mM glucose.

matrix , size exclusion (w hich depends on the sol-gel prepar -

ation procedure), and solubility of the compound.18

Hyd rophilic Silver-Silver Chloride CCEs. Modified elec-

trodes are frequently used in potentiometric devices as

reference electrodes or a s ion-selective electrodes. Sol-gel

derived carbon-silica com posite electrodes modified by a

silver-silver chloride couple demonstrate both reference and

selective ion applications. Th e Ag-AgCl couple was chosen

as a demonstrative exam ple due to its popularity as a reference

electrode and its excellent halide selectivity. Therefore,

althoug h more useful (especially in aprotic) applications exist,

the silver-silver chloride modified composite electrode is

demonstrated here.

Silver-silver chloride CCE s were prepared according to

the procedure depicted in the Experimental Section. The

electrode potential (measured relative to standard calomel

electrode) was stable for a t least several days. A pH chan ge

over the range 4-10 did not alter the electrode response. A

logarithmic dependence (range 1-100 mM ; slope 49 mV/Cl-

decade;

R

= 0.997) of the electrode potential on chloride ion

concentration was observed. Th e reference (and halide-

selective) electrode retained its signal even after surface

renewal by polishing with emery paper. This confirms tha t

the bulk of th e porous electrode was modified by th e silver-

silver chloride couple and the modification was not confined

to its outermost surface.

Biosensing. Brau n and co-workers13 dem onstrate d the

possibility of protein immobiliza tion in sol-gel derived silica

matrices and recently this ability was extended to an tib od ie ~. ~

This class of bioceramic materials was applied to produce

silica-based ph0tometric~6.~~nd flow injection analysis

detectors.48 Electroche mical biosensing applications of com-

posite carbon-silica electrodes should be especially attr acti ve

due to the high conductivity of the composite electrodes. Fig ure

(45) Wang,

J.;

Narang,

U.;

Parsad, P. N.; Bright, F. V. Anal. Chem.

1993,

65,

(46)

Shteltzer, S.; Rappoport, S.,

vnir,

D.; Ottolenghi, M ; Braun, S. iorechnol.

(47) Wu,

S.;

Ellerby, L.

M.;

Cohan,

J. S.;

unn, B.; El-Sayed,

M.

A, ; Valentine,

(48)

Tatsu, Y.; Yamashita, K.; amaguchi, M.; Yamamura, S.; amamoto, H.;

2611.

Appl .

Biochem.

1992, 15, 227.

S. ;

Zink, J.

I.

Chem. Mater.

1993,

5 , 115.

Yoshikawa, S.Chem. Lett.

1992, 1619.

1752

Analflical Chemistry, Vol. 66 No. 10 May

15,

1994


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Table 1. Demonstrated Ciaore8 of CompoeHe Ceramic Electrodes (CCEe)

demonstrated active composition of ceramic

class of CCEs configuration surface network

indicator electrode 1. rod external C/[=Si-C HJ or

2 microelec trode C/ [=Si-CeHbl

3. film

organically modif rod external C/ [eS i-C Hs]

inorganic modif rod bulk C/[SiO

biochemical modif coated layer bulk C/[SiO21,

Curren t , uA

' - - I I

40/0

0

6

10 15 20

25

30

35

Glucose conc., mM/ i

Flgure

10.

Calibration curve

of

comp osite carbon-silica glucose

biosensor (scan rate 100 mV/s; pH 5.6;

E =

1.0 V/SCE).

9

depicts the cyclic voltammograms (scan rate 100mV/s) of

the carbon com posite electrode immerse d in a blank solution

(buffered a t pH 5.6 by phosphate buffer) and in a

9.8

mM

glucose solution. Th e increased anodic current, which becomes

apparent at -700 mV ( SC E), is caused by oxidation of the

hydrogen peroxide that was formed during the enzymatic

conversion of glucose to gluconolactone. This was verified by

nitrogen bubbling, which recovered the blank response. Figure

10

demo nstrates a typical glucose calibration curve depicting

the dependehce of the current at

1.0

V (SCE) on glucose

concentration. Th e biosensors described here show relatively

low stability in dry storage. This phenomenon was also

observed in sol-gel derived optical b io ~ e n s o r s .~ ~

CONCLUSIONS

Table

1

summarizes the classes of CCEs that were

demonstrated in this article. Som e of the more important

characteristics of CC Es include the following.

(1)

Electrode

configuration: CC Es enjoy the inherent versatility of the sol-

gel molding technology an d it is possible to cast silica-carbon

matrices in virtually any desired geometrical configuration.

(2) Active surface: CC Es are made of porous mate rial and

thus can operate as flow-through or large surface area

electrodes where the bulk of the electrode is available for

charge transfer. On the other hand, in hydrophobically

modified CC Es only the outermost surfac e s electrochemically

active. However, even when only the external surfa ce is wetted

the electrodes can be bulk m odified, and thus polishing exposes

a new surface area .

(3)

Surface polarity: Th e use of a modified

silica network (O rmosil) provides an ability to tailor electrode

surface attributes such as ion exchange, size exclusion,

polarizability, surface polarity, a nd hydrophobic na ture.

(4)

Type of immobilization: Modification of electrode struc tur e

is

not confined to a selection of the silica precursors.

active

mode of

compound

immobilization

hydrophobic

covalent bonding

hydroquinone or sol-gel doping

Ag/AgCl 1. doping

functional groups

ferrocene

2 im regnation+

rexuction

glucose oxidase doping

demonstrated

application

electrosensing

pH sensing

1. reference

electrode

2 halide-selective

electrode

glucose sensing

Impreg nation by physical or chemical adsorption on prepared

electrodes can be used. Sol-gel doping, Le., incorporation of

the modifiers along with the sol-gel reaction precu rsors,

facilitates the entrapment of heat-sensitive reagents in the

bottleneck str uc tur e tha t is formed du ring sol-gel doping.'*

Adsorption of modifiers on carbon grains prior to their

incorporation in the sta rting sol-gel solution is still ano the r

method th at may beused to protect sensitive (e.g., biochemicals

and enzymes) com pounds from t he aggressive sol-gel solution.

Th e silica-carbon matrices combine some additional

favorable cha rac teris tics which were less emphasized in this

article: high electrical conductivity

>

1 Q-'/cm); physical

rigidity; negligible swelling in aqu eous an d o rga nic solvents;

chemical inertness, which implies low interaction w ith analytes

and slower poisoning by irreversible side reactions; high

photochemical, biodegradational, and thermal stability es-

pecially at low pH; and excellent adhesion to glass supports.

CONCLUDING REMARKS

A new class of carbon-ceramic electrodes is introduced,

and its electroanalytical applications were demonstrated by

amperometric biosensing (glucose sensing) and indicator

electrodes, potentiometric reference and ion-selective elec-

trodes, and voltammetric pH sensors. The electrodes can be

manufactured in virtually any dimension and geometrical

configuration including flat plates, metal-coated electrodes,

monolithic rods and disks, and even microelectrodes. Th e

bulk and external surface of the electrodes can be modified

by covalent bonding, impregnation, or sol-gel doping. Suc h

versatility has traditionally been attributed exclusively to

organic polyme rs, but the evolution of sol-gel technology

changed these (once) accepted notions.

ACKNOWLEDGMENT

The research is supported by a grant from the MOST,

Israel, and Forschungszentrum fur Umw elt und Gesundheit

Gm bH, Neuerberg, Germ any, and the Israel US A Binational

Science Foundation. Th e research is undertaken in col-

laboration with Profs. D. Avnir and M . Ottolenghi of the

Hebrew University. Th e help of

L.

Rabinovits and useful

discussions with

A J.

Bard, I. Turyan,

D.

Mandler, and

S.

Braun are gratefully acknowledged.

Received for review July 20, 1993. Acc epted Decemb er 21,

1993.

Abstract published in Aduance ACS Abstracts, February 15, 1994.

AnalyticalChemistry

Vol.

613,

No. 10,

May 15,

1994

1753

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PREPRATION OF CARBON ELECTRODES