Sensor development exploiting graphite-epoxy composite as electrode material

A. L. M. Azevedo, R. S. de Oliveira, E. A. Ponzio, F. S. Semaan

Composite materials: mixtures of two or more components with different

properties and distinct boundaries between them.

Homogeneous macroscopic levels,

Heterogeneous at microscopic levels.

Milton, G. W. Theory of composites. Cambridge University Press. 2004. Cambridge. 719p.

Sensors Composites: mixture of at least one phase conductor and

insulator, reaching a different material.

Versatility, moldability, stability, possibility of modification, surface renewal ease, at low cost.

Applicability to diverse environments of analysis and wide range of potential.

Adams R. Carbon Paste Electrodes. Analytical Chemistry, Julho de 1958.

INTRODUCTION: Since the first studies reported by Adams [1], composite sensors have been explored in electroanalysis due

to their advantages compared with metal electrodes. They consist of a dispersion of at least one phase conductor in an

insulating component, generating a new material. By far the most reported for conductive phase is carbon in its many forms

available, with different composites described according to their insulating phases. Such composites were prepared by

mechanical dispersion of suitable amounts of graphite powder (Sigma-Aldrich, USA, 2-20 mM) in epoxy resin (Avipol, Brazil,

2126-3024 Silaex SQ), being then let to cure for at least 24 hrs under pressure and polished with suitable tools.

Work electrode

Surface catalytic

Analytical signals directly dependent not only on the surface area, but also the possibility of adsorption. Brett, A. M. O.; Brett, C. M. A. Electroquímica: princípios, métodos e aplicações. Editora Almedina. Coimbra. 471 p.

Preliminary studies: Proportions of polymer and catalyst. In this study, the epoxy resin (Avipol, Silaex 2126-3024 SQ, Brazil). Technique for the homogenization of the composite. Catalytic surface to be developed. Ideal ratio of composite. Tests to qualify the best composite graphite epoxy.

Universidade Federal Fluminense, Instituto de Química, Niterói RJ, Brasil

e-mail: andreazevedo@id.uff.br, felipesemaan@gmail.com

Advaced studies: The composite set of best features, other tests were administered:

Friability (the condition of being friable) is the ability of a solid substance to be reduced to smaller pieces with little effort. Comparison test between the raw materials and developed new composite. Mass loss was 0.75%. Excluding the white reduction was 0.3% . Hardness at first trials were carried out in an apparatus for tablets. Showed that the composite has a hardness negligible, compared to the source material. Hardness Vickers (HV), more specific hardness test to determine the hardness of materials shown how the composite is soft. Atomic force microscopy (AFM)

0.0 0.5

-50

0

50

25 mV/s

Electrode 1

Electrode 2

Electrode 3

Electrode 4

GCE

Curr

ent (

A)

E / V vs Ag|AgCl

Comparative composite X Glassy Carbon

Figure 5: Comparate among composite and to a glassy carbon commercial electrode with almost same geometrical surface

-0.3 0.0 0.3 0.6 0.9

-100

-50

0

50

100

E / V vs Ag|AgCl

2mV*s-1

5mV*s-1

10mV*s-1

20mV*s-1

50mV*s-1

100mV*s-1

I (A

)

Electroative area

Figure 4: . Cyclic voltammograms obtained with the 65% (graphite, w/w) on 5.0 mmol l-1 hexacyanoferrate in 0.5 mol l−1 KCl at different scan rates from 2 to 100 mV s-1.

Topography – Scan forward Line fit Topography – Scan forward Line fit

Figure 3a.: Topography obtained obtained with the 65% (graphite, w/w). Figure 3b.: 3D projection topography acquired.1

(b) (a)

A suitable stable and low cost material for electroanalytical application is hereby described. Its hardness facilitates polishing, and subsequently recovery of the surface, allowing stable but transient chemical modifications as well as renewal. The methodology of polishing on abrasive surface resulted on low roughness topography. Economically such material showed to be easy to build and cost-effective when compared to glassy carbon commercially available sensors. For analytical purpose such composite is being evaluated as substrate for gold-nanoparticles immobilization in chitosan-modified and cellulose acetate films, with good preliminary results. The electrochemical cell developed can be directly used to determinate many inorganic and/or organic compounds in real samples or even undergoes to chemical modification to more specialized situations.

Acknowledgments

References

Conclusion

[1] R. N. Adams, Anal. Chem. 30 (1958) 1576. [2] K. Kalcher, J.M. Kauffmann, J. Wang, I. Svancara, K. Vytras, C. Neuhold, Z. Yang, Electroanalysis 7 (2009) 598-656. [3] R. Pauliukaite, M. E. Ghica, O. F. Filho, C. M. A. Brett. Anal. Chem. 81 (2009) 5364-5372.

Electroative area

Table 1.: Different compositions were prepared (55-80% w/w) and characterized by thermogravimetry analysis (TGA-DTA) and cyclic voltammetry (CV).

55 60 65 70 75 800.0

0.1

0.2 Anodic

Catodic

GRAPHITE (m m -1)

ELEC

TRO

ATIV

E AR

EA(c

m2 )

Figure 2: Thermal analysis of different compositions

55 60 65 70 75 800.1

0.2

0.3

0.4

Roug

hnes

s (rm

s)

Graphite (%)

Table 2: Calculated roughness (rms) found by AFM results and topography (Tapping mode)

0 20 40 60 80 100 120

(a)

(b)

2 Theta /

(c)

Figure 5: XDR for Graphite powder in (a), Epoxy in (b) and Grahite/Epoxy 65% composite in (c).

6 cm

3,1 mm Internal diameter

External diameter 3,3 mm

(a) (b)

Figure 7a: Body of working electrode Figure 7b: Arrangement of electrodes in electrochemical cell

One disposable electrochemical cell comprising working electrode (composite described), counter electrode of stainless steel, silver conductive epoxy reference (pseudo-reference) electrode was constructed. The electrodes were mounted on a plunger of a 5 mL syringe, being then the syringe filled by insulating polymers for mechanical purposes.