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Fabrication and application of gold microelectrode ensemble based on carbonblack–polyethylene composite electrode

Galina N. Noskova,b Elza A. Zakharova,ab Vladimir I. Chernov,b Anna V. Zaichko,ab Elena E. Elesovab

and Alex S. Kabakaev*a

Received 12th February 2011, Accepted 16th March 2011

DOI: 10.1039/c1ay05074e

A simple and cheap procedure for solid composite electrode fabrication is described, including the

pressure casting of carbon black and polyethylene (CB/PE) concentrate (30 : 70 mass), which is

industrially produced (known as masterbatch) and commercially available. The gold microelectrode

ensemble (Au-MEE) was obtained by electrodeposition of Au on the mentioned composite electrode.

The Au-MEE based on the described CB/PE composite substrate demonstrated better sensitivity,

reproducibility and stability in time than other widely used carbon-based substrates. Analytical

possibility to perform voltammetric determination of the trace As, Hg, Se, Fe, Cr, NO2-ions at CB/PE

based gold electrode is shown.

Introduction

Ensembles of microelectrodes remain an area of intensive

research1 due to their excellent analytical properties, such as fast

mass transfer, low IR drop resistance, low residual currents and

thus high signal/noise ratio. This gives opportunity to use low

conductive media and two-electrode cells in analytical applica-

tions. Methods of microelectrode array fabrication as well as

properties thereof and examples of practical use are described in

several reviews.2–7

Gold is one of the most frequently used materials for micro-

array fabrication8 and biosensors support9 because of its chem-

ical resistance to most reagents, high electron transfer rate in

heterogeneous reactions, catalytic activity (in contrast to the

bulk gold), wider potential range compared to Pt, and ability to

form self-assembled monolayers by organosulfur modification.

Numerous supports for Au-nanoparticles deposition are

known, such as clean glassy carbon,10–13 electrochemically

modified glassy carbon (GC),14 chemically modified GC15 or

carbon nanotubes (CNT),16 indium tin oxide,17 screen-printed

carbon,18 and thin-film of boron-doped diamond.19 One of the

methods for gold ensembles fabrication is a patterned electrode-

position5 of the metal. Conductive particles on the composite

surface in-between of the solid polymer components can be such

a template for gold electrodeposition, and this was particularly

interesting for us to investigate.

Overview of composite electrodes, their classification, princi-

ples and applications is given by Tallman.20 Authors21–25 consider

aTomsk Polytechnic University, 30 Lenina av., Tomsk, Russia. E-mail:ask@tpu.rubTomanalyt laboratory LLC, 240a-14 Frunze, Tomsk, Russia. E-mail:tan@mail.tomsknet.ru; Fax: +7 3822 241795; Tel: +7 3822 253195

1130 | Anal. Methods, 2011, 3, 1130–1135

composites of carbon powder–polyethylene electrode attractive

support because they are inexpensive and easy to fabricate, which

makes them a convenient and effective type of voltammetric

sensors. Recent trends and advances in electroanalysis using

composite solid electrodes were reviewed byNavratil and Barec.26

The authors emphasize that the fabrication of microelectrode

ensembles is not standardized, and practically every paper

proposes its own preparation procedure. This fact complicates

comparison of electrode quality and features.At present, there are

not so many articles on fabrication and voltammetric application

of carbon black–polyethylene composite modified with gold

electrodeposition. Papers we found are concerned with Hg27 and

As28,29 determination. The mentioned papers describe the carbon

soot-based electrode support with considerably bigger conductive

particles. Those papers give no information on the procedure of

electrode production, and the electrode is not characterized.

The present article demonstrates a simple and partially auto-

mated procedure for fabrication of carbon black–polyethylene

composite (CP/PE) electrode made of the commercially available

CB masterbatch and other components. Electrochemical modifi-

cation with gold deposited on its surface is proposed. Electro-

chemical behaviour of such gold deposited electrode is

characterized, revealing its nature as a microelectrode ensemble.

Finally successful applications of such electrodes for voltammetric

determination of As, Hg, Se, Fe, Cr, NO2� ions traces is shown.

Experimental

Reagents and equipment

All reagents were of the highest grade available and were used

without further purification. Double distilled deionised water

was used for all the solutions and subsequent dilutions.

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Russian State Standard solutions (lab attestation reference

solutions, highest accuracy available) of 1 mg mL�1 Fe(III), Cr

(VI), Hg(II), As(III), NO2� were used. The solutions of smaller

concentrations were obtained by diluting the initial standard

solutions with deionized water.

Low density polyethylene (Samsung H082577 04208, Korea)

was used to fabricate the electrode body.

Conductive material for the electrode was made of the indus-

trially produced and commercially available carbon black–

polyethylene from Tomskneftekhim LLC, Tomsk, Russia.

A wide range of similar CB/PE mixtures is used in industry as

a pigment for plastics and rubber, known under the name of

‘masterbatch’ or ‘concentrate’. Carbon nanoparticles are already

perfectly homogenized, thus providing a well-distributed

template of electroactive centres. Commercial masterbatch pack

contains the round-shaped black granules with a size from 1.4 to

5.0 mm. Our masterbatch consists of 70% of heat stabilized high-

density polyethylene (HDPE, hereinafter referred to as ‘PE’) and

30% of carbon black (grade ‘‘N220’’) with the carbon particles

sized 24–33 nm (this corresponds to the standard of the American

Society for Testing and Materials—Standard Classification

System for Carbon Blacks Used in Rubber Products ASTM

D1765).

Voltammetric workstation TA-4 (Tomanalyt LLC, Tomsk,

Russia) connected to a computer was used. It makes it possible to

use different forms of potential sweep at 2–300 mV s�1. Three

electrochemical cells of the device allow for simultaneous

measurements in 3 independent quartz cells, providing built-in

UV irradiation, oxygen removal with nitrogen, ozone, and argon

sparging. Currents ranging from 0.05 nA to 200 mA can be

measured.

Working electrode is described in greater detail further in this

paper. It is the random microelectrode array of Au particles

plated on the surface of the carbon-polyethylene composite

mentioned above. Reference electrode was Ag/AgCl in 1 M KCl.

If the 3-electrode cell is employed, an auxiliary electrode is

platinum or another Ag/AgCl electrode. Linear sweep (LSV) was

used to obtain cyclic voltammograms (CV) and anodic stripping

voltammetry (ASV) curves. The first-order derivative of

dI/dE � E was used for better signal recognition in direct LSV.

In some experiments oxygen was removed from the analyzed

solution using a flow of nitrogen, or a sodium sulfite as the

background solution.

Fig. 1 (a) Exterior view of the composite electrode. 1—body of the electr

surface. (b) SEM image of Au-MEE, on the support of the composition con

This journal is ª The Royal Society of Chemistry 2011

For comparison, authors also used glassy carbon electrode

(GCE) and graphite electrode impregnated with polyethylene

and paraffin (PIGE).

Scanning electron microscopy (SEM) images were obtained

using Philips SEM 515 with micro-analyzer EDAX ECON IV.

Preparation of composite CB/PE electrode

First, the insulator tube is fabricated by pressure casting of low

density polyethylene in press-form at 175 �C and 8.8 MPa.

Dimensions of the finished electrode body are 20 mm (CB/PE

length), 3.9 mm (inner diameter), and 0.55 mm (wall thickness).

Then composite granules are loaded into the vertical type

casting machine (pressure is up to 14 MPa, the volume of the

injection is up to 125 cm3). Next, the composition is melted at

a temperature of 160 �C. The molten composition is kept at

a temperature of 160 �C up until the start of the casting process.

The insulation body of the electrode is fixated in the mold of

the casting machine. Electrical contact, a 1 mm stainless steel

rod, is inserted (see Fig. 1a).

The molten composition is then injected into the plastic tube

(body of the electrode) under the pressure of 6 MPa. The tube is

then cooled down. The CB/PE solid composite electrode is ready.

At present this technique allows producing up to 30 electrodes

per hour.

Finally, the surface of the prepared electrode is cut off with

a chopping knife specially constructed to cut only a sub-milli-

metre layer of electrode rod. The obtained disk-shaped working

surface is used without ground. The prepared electrode is stored

in a shielding cap. Resistance of such electrode is 5 kU or less.

Gold deposition

Working Au-MEE electrodes were based on CB/PE electrode

described above. Gold is electroplated on a freshly-cut surface.

Procedure is as follows:

First, the surface of the carbon black–polyethylene electrode

was renewed by cutting off a thin layer of about 0.3 mm. Then

modification was carried out by way of gold electrodeposition

from solution of 500 or 1000 mg L�1 HAuCl4 in 0.1M HCl. Time

of electrolysis was from 30 to 60 s, potential was 0.0 V vs.

Ag/AgCl, 1 MKCl. Finally, electrodes were cleaned with a water

flow and covered with a shielding cap.

ode (insulator); 2—CB/PE composite; 3—electrical contact; 4—sensing

sisting of 30% carbon black and 70% HDPE.

Anal. Methods, 2011, 3, 1130–1135 | 1131

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Regeneration and activation of the electrode surface are per-

formed under conditions depending on solution composition and

substance to be determined.

Table 1 Range of working potentials for the Au-MEEwith electroactivesurface 0.015� 0.005 cm2 at the background current of 1 mA. Oxygen wasremoved with nitrogen

Background electrolytes, 0.01 M solutions (Ek.Ea), V vs. *Ag/AgCl

HClO4 –0.7.1.3H2SO4 –0.7.1.3HCl –0.7.0.7b

H3Cit –0.7.1.5KNO3 –1.6.1.3Na2SO3 –1.7.0.3a

KNaHPO4 –1.8.1.6Na3Cit –1. 8.1.5KOH –1.6.0.2b

KOH + Na2SO3 –1.4.0.2b

a Background electrolyte oxidation. b Oxidation of Au.

Results and discussion

Characterization of CB/PE-based Au-MEE electrode

Fig. 1b shows an SEM image of the electrode surface after elec-

trolysis from 1000 mg L�1 HAuCl4 for 20 s. As can be seen from

the figure, the electrode surface represents a random ensemble of

microelectrodes, varying in size d ¼ 100–500 nm and distance of

1–3mmbetween them.Thus, goldwas deposited onto the template

of carbon-black particles, and was retained tightly on the surface,

forming Au microelectrodes ensemble (Au-MEE).

Electroactive surface area of the electrode was determined by

coulometry from the charge passed during the reduction of the

gold oxide monolayer, employing the well-known procedure

described in Ref. 30. The registered value was divided by 400 mC

cm�2, giving a surface area of ca. 0.01 cm2. The geometric surface

of the disk-shaped electrode is 0.12 cm2. Thus, the amount of

gold is 10 times less compared to the case of the continuous Au

film.

Having such a large number of independent carbon particles

covered with Au, it becomes clear why the electrode is so

reproducible even after a layer is cut off. Amorphous carbon is

uniformly distributed in the volume of the electrode, so every cut

produces a statistically equivalent surface in terms of energy of

the active centres.

Electrochemical characterization of the Au electrode was

made in 1 mM solution of potassium ferrocyanide + 0.1 M KCl

(Fig. 2). For comparison, the CV curves for graphite electrode

impregnated with polyethylene and paraffin (PIGE) were

obtained. Its peak-shaped form corresponds to the linear diffu-

sion to a plane electrode. The form of CV curves (Fig. 2b) was

close to the form of waves without clear peak, which is typical for

random arrays of microelectrodes.32

For our experimental time scale of W ¼ 0.02–0.10 V s�1, peak

half-width of 0.1 V and D ¼ 10�5 cm2 s�1, the diffusion layer

Fig. 2 CV curves of 1 mM ferrocyanide in 0.1 M KCl, scan rate 20 mV s�1

graphite (Sgeometr ¼ 0.12 cm2). (b) Au-microelectrode ensemble (Seff ¼ 0.013

1132 | Anal. Methods, 2011, 3, 1130–1135

thickness (pDt)0.5 is around d ¼ 10–50 mm.31 Therefore when

R ¼ 50–250 nm is less than layer thickness, the type of diffusion

to a microelectrode is transformed to the radial diffusion.

However, the distance is not sufficient to consider the micro-

electrodes as independent. It leads to overlapping of diffusion

layers, so the CV shape differs from sigmoid. Also the peak

current–scan rate function (0.02–1 V s�1) was compared for plane

electrode (Ipeak f V0.5) and for Au-MEE (Ipeak f V0.3). The

0.3 power of scan rate corresponds to a mixed type of diffusion

and a random array of microelectrodes (case 3 of criterion given

in Ref. 31), which is in agreement with the direct observation of

SEM image.

Unfortunately, further decrease of CB concentration in CB/PE

composite (20%) leads to a fast drop in conductivity. Higher

concentrations of CB (40% and 50%) showed considerably worse

signal/noise ratio and reproducibility.

Table 1 shows the ranges of working potentials for Au-MEE in

certain supporting electrolytes. The range of working potentials

is limited in the cathodic region by the discharge of hydrogen

(or water), and in the anodic region by oxidation of gold or

supporting electrolyte.

for dashed line and 80 mV s�1 for a solid one. (a) Paraffin impregnated

cm2, Sgeometr ¼ 0.12 cm2). Base line is close to zero.

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The Au-MEE electrode is stable in both acidic and basic

media. Slightly acidic solutions are usually convenient for the

analytical practice. In the neutral and alkaline media, pre-

monolayer oxides33 can be formed on the surface. As a result, the

oxidation current (or reduction in backward CV) emerges and

becomes especially noticeable in the regime of the 1st order

derivative.

The special feature of MEE application is the possibility to use

the dilute supporting electrolytes of 0.005–0.01 M. In spite of the

high resistance of working electrodes (up to 5 kOhm), the small

currents of mA and nA scale (at 10�8–10�7 M) do not introduce

any curve shape distortions: i R � 10�3 V.

Table 2 illustrates the stability of gold electrodes with different

supports, which were studied for 10 days by comparing Cmin for

As(III) ASV detection under the same conditions given in the

table heading. The Au–CB/PE electrode had no considerable

decrease in sensitivity for all 10 days of study. The polished

Table 2 Comparison of stability in time for gold plated electrodes with diffeHCl background, Edep ¼ �1.6 V, tdep ¼ 240 s. Time of Au deposition was 30

Support for Au deposition

Cmin, mg L�1

1st day 2nd

Template of CB/PE 0.20 0.2Polished GCE 0.22 0.3Paraffin impregnated graphite (PIGE) 0.23 0.5

Fig. 3 ASV of As, Hg, Se, and 1st derivative of direct LSV for NO2�, Fe, Cr

indicate background, 1st and 2nd concentration respectively. (A) 10 and 20 m

(C) 2 and 4 mg L�1 of Se(IV), tdep ¼ 30 s; (D) 20 and 40 mg L�1 of NO2�; (E) 10

direction for anodic (A–D) and cathodic (E, F) potential sweep. Scan rate (m

This journal is ª The Royal Society of Chemistry 2011

GCE–based gold electrode was stable for 5 days, with Cmin

decreasing to 0.5 mg L�1. Paraffin impregnated graphite-based

support was only stable for 2 days. No signal of As(III) appeared

on both Au-on-GCE and Au-on-PIGE electrodes after 6 days of

use. The Au-MEE continued to give an acceptable calibration

plot even after 30 days of use. The same stability in time is

observed for other elements considered further on.

Gradual decrease in sensitivity of arsenic determination on

GCE- and PIGE-based Au electrodes is caused by an increase in

the residual current, while the CB/PE-based Au electrode was

characterized by practically invariable signal and noise (residual

current).

Such a difference in behavior of tested electrodes can be

explained by the nature of carbonic support and the size of

carbon surface free of Au. For Au-MEE electrode, all the active

centers of CB/PE electrode are covered with Au, with a pure

insulator in between. It leads both to the stability of active

rent supports. Cmin of As ASV detection during 10 working days. 0.01 Ms from 1000 mg L�1 HAuCl4

day 4th day 6th day 10th day

2 0.23 0.24 2.44 0.55 2.4 No signal6 1.8 10.5 No signal

at Au-MEE, measured under conditions given in Table 3. Curves a, b, c

g L�1 of As(III), tdep ¼ 10 s; (B) 0.4 and 0.8 mg L�1 of Hg(II), tdep ¼ 60 s;

and 20 mg L�1 of Fe(III); (F) 20 and 40 mg L�1 of Cr(VI). Arrows indicate

V s�1) is 180 (A), 40 (B), 80 (C) and 20 mV s�1 (D, E, F).

Anal. Methods, 2011, 3, 1130–1135 | 1133

Table 3 Voltammetric determination of various analytes on Au-MEE, developed by the authors and their colleagues in Tomanalyt laboratory. 2-electrode cell (low currents), no oxygen removal

Measured ion Background electrolyte Methoda Edep, V Range of concentrations, mg L�1 LOD (3s), mg L�1

As(III) 0.4 M Na2SO3 ASV –1.6 0.05–100d 0.02d

Hg(II) 0.02 M HNO3 + 0.002 M KCl ASV –0.6 0.08–10d 0.05d

Se(IV) 0.003 M H3Cit ASV –1.5 0.05–100d 0.02d

Fe(III) 0.05 M HCl Cathodic LSVb,c 2–500 0.7Cr(V) 0.03 M HNO3 Cathodic LSVb,c 2–400 0.8NO2

� 0.005 M H2SO4 Anodic LSVb 3–1000 2

a ASV—anodic stripping voltammetry; LSV—linear sweep voltammetry. b The first order derivative was plotted for further calculations.c Electrochemical activation of electrode was included before every sweep. d Range of concentrations is obtained for deposition times varying from300 s to 2 s. LOD was obtained at the highest 300 s deposition time.

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centers (because Au is inert) and to the considerably lower

residual current, because of the absence of electrochemically

active oxygen-containing functional groups.

Electrodes are stable in time, but in case of decrease in activity,

the electrode surface can be easily regenerated by electrochemical

activation, i.e., sequential pulses of alternating voltage. Usually,

the exact voltage is chosen experimentally, based on positive and

negative sides of electrochemical window for a particular back-

ground. Later on, electrochemical activation can be included in

the measurement procedure of the software program of

potentiostat.

Another positive feature of the Au-MEE electrode is the

signal-to-signal repeatability on sequential curves, which is quite

high (1–2%) even at the low currents of several nA.

Further on, we demonstrate the analytical features of

Au-MEE electrodes, exemplifying it by determination of As(III),

Hg(II), Se(IV) by ASV and Fe(III), Cr(VI), NO2-ions by direct LSV.

Fig. 3 shows the signals of identified elements (background, 1st

and 2nd concentration). Table 3 demonstrated conditions of the

experiments, the range of determined concentrations, and the

limit of determination. Evidently, the possible determined

concentrations are much lower than the human consumption

limit. For instance, the As LOD at Au-MEE is comparable to the

best known published examples (Table 1 in Ref. 34). High

sensitivity and possibility of direct voltammetric determination

of NO2�, Fe(III) and Cr(VI) are associated with features of the

gold ensemble: radial diffusion and possible catalytic activity of

Au.

In certain cases, the 1st derivative (Fig. 3, C, D, E) gives extra

benefit of a clear peak-shaped signal instead of a wave shaped

voltammogram, simultaneously eliminating most of the capacity

current. The small peak on background curves can be explained

by formation of premonolayer oxides on the gold surface.33 The

value of those background peaks can be subtracted during

further calculation of concentration.

The developed analytical procedures let us measure: arsenic in

soil, water and food; mercury in soil, water, fish and sea prod-

ucts; and selenium, iron, chromium, and nitrite ions in water.

Reproducibility of signals is usually less than 2%, and accuracy

does not exceed 25% at 10�9–10�8 M concentrations.

Conclusions

This paper proposes a simple and inexpensive method of fabri-

cation of solid composite electrode by compression molding,

based on the CB/PE concentrate. Further, the gold

1134 | Anal. Methods, 2011, 3, 1130–1135

microelectrode ensemble was obtained by electrodeposition of

Au on the mentioned composite electrode substrate. It was

shown that the described electrode was an irregular ensemble of

a large number of gold microelectrodes, which is confirmed by

the form of CV curves and the SEM image. Gold electrodes

deposited on different carbonic supports were compared,

revealing perfect stability of the Au-MEE electrode. The Au-

MEE on CB/PE support combines many useful features such as

high sensitivity and reproducibility, stability and mechanical

durability, simplicity of preparation with mechanized stages, low

cost and commercial availability of high-quality electrode

material. Those features make the CB/PE electrode a good

template for other metal-deposited microelectrode ensembles. To

the best of our knowledge, this paper is the first to demonstrate

a wide range of applications of the Au-deposited CB/PE solid

electrode. It can be used as a very sensitive and stable electrode in

both stripping and direct VA, and that was demonstrated for

determination of As, Hg, Se, Cr, Fe, NO2� ions. At present, the

electrode is used to determine As contents in soil, water and food;

mercury in soil, water, fish and sea products; and selenium and

iron in water.

Acknowledgements

The authors express appreciation to their collaborators in the

SEM Laboratory of Tomsk State University.

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