Conductive copolymer-modified carbon fibre microelectrodes: electrode characterisation and electrochemical detection of p-aminophenol

Sensors and Actuators B 97 (2004) 59–66

Conductive copolymer-modified carbon fibre microelectrodes: electrode characterisation and electrochemical detection of p-aminophenol Mamun Jamal, A. Sezai Sarac1 , Edmond Magner∗ Materials and Surface Science Institute, University of Limerick, Limerick, Ireland Received 23 April 2003; received in revised form 17 July 2003; accepted 24 July 2003

Abstract Thin film electro-coated poly[N-vinylcarbazole-co-vinylbenzene sulfonic acid] (p[NVCzVBSA]), poly[carbazole-co-methylthiophene] (p[CzMeTh]) and polycarbazole (p[Cz]) carbon fibre microelectrodes (CFMEs) were characterised by scanning electron microscopy (SEM) and FTIR-ATR spectroscopy. These modified carbon fibre electrodes were found to be effective systems for the determination of para-aminophenol (p-AP). Thin film coated p[NVCzVBSA] was the most suitable modified electrode for the detection of p-AP. © 2003 Elsevier B.V. All rights reserved. Keywords: Carbon fibre microelectrode; Conductive polymers; Cyclic voltammetry; Surface morphology; p-Aminophenol

1. Introduction There is an increasing need for the miniaturisation of chemical sensors due to the continually growing demand for sensors that are capable of operating outside the laboratory environment [1]. One promising approach toward miniaturisation is to couple a micro-fabricated device with appropriate detectors containing a specific receptor, either natural such as an antibody or synthetic, e.g. a molecularly imprinted polymer, for a particular analyte [2]. Polymers are widely used as permselective barriers used to improve the selectivity of sensors and to maintain stability of the microelectrode response [3]. The discovery of polymers with conductive properties opened up the possibility of using such polymers to encapsulate redox proteins and enzymes while allowing direct electrical communication to occur between the redox site and the electrode [4–6]. Carbon fibre electrodes have been used in the detection of biologically important analytes such as dopamine [7,8]. The fibres are stable and readily available from a number of commercial sources. The disposable nature and low cost of these microelectrodes has opened up a wide range of pos-

∗ Corresponding author. Tel.: +353-61-202629; fax: +353-61-213529. E-mail addresses: [email protected] (A.S. Sarac), [email protected] (E. Magner). 1 Co-corresponding author. Permanent address: Department of Chemistry, Istanbul Technical University, Polymer Science and Technology, Maslak, 80626 Istanbul, Turkey. Tel.: +353-61-234174; fax: +353-61-213529.

0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00647-6

sible applications, e.g. in the determination of enzyme substrates [9], neurotransmitters [10], proteins [11] and other redox species [12,13]. As fibres facilitate work in small volumes of solutions while retaining a large electrode surface area, we have recently examined the electro-grafting of a range of copolymers, composed of saturated and unsaturated monomers, onto carbon fibres [14–17]. We also described the preparation of a range of carbazole polymers and established the relationship between the polymerisation parameters and the surface properties of the electrodes used [18]. In this study, a series of conductive polymer-modified carbon fibre electrodes was prepared and characterised by scanning electron microscopy (SEM) and FTIR. The ability of these electrodes to detect para-aminophenol (p-AP) was examined.

2. Experimental 2.1. Materials Polyacrylonitrile (PAN)-based carbon fibres were used for this study (Hexel AS4C) with 12,000 single filaments of diameter 5–6 ␮m per roving. The monomers of N-vinylcarbazole (NVCz), vinylbenzene sulfonic acid (VBSA), 3-methylthiophene (3-MeTh), and carbazole (Cz) were obtained from Merck (synthesis grade) and tetramethylammonium perchlorate (TMAP, Fluka, >99%)) was used as the supporting electrolyte. Para-amino phenol, KH2 PO4 and K2 HPO4 were obtained from Sigma–Aldrich

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(ACS reagents). K3 (FeCN6 ) (ACS reagent) was obtained from Hopkins & Williams. All reagents were used as received. All aqueous solutions were prepared using purified water (18 M
) from an ElgaStat SPECTRUM system.

of 0.5 nA. Average values of the increase in thickness were obtained from SEM images by taking into account the diameter of the ungrafted fibre. The diameters reported for the fibres represent an average of 8–10 measurements on individual fibres.

2.2. Electrode preparation 2.4. FTIR-ATR measurements The area of the fibre electrodes was 0.011 cm2 (fibres were covered to a height of 60 mm in the cell). The electrolyte solution was stirred during the electro-polymerisation process. A galvanometer was used as power supply (Statron, type 3218, VEB Statron Fürstenwalde, Germany) with an adjustable current between 0–4 A and a potential range of 0–75 V. The electrochemical cell used for the preparative electro-copolymerization procedure was a cylindrical shaped glass cell (50 mm diameter, 70 mm height) with an effective volume of about 100 ml. This cell contained a stainless-steel (V2A) cathode. The carbon fibres were placed in a glass tube against a stainless-steel (V2A) plate. Electro-deposition of polymers on the carbon fibre microelectrodes (CFMEs) was performed galvanostatically at 40 ◦ C at a constant current density of 10 A/cm2 , using the carbon fibres as anode. The initial concentrations of monomers used were 0.05 M NVCz, 0.075 M MeCz, 0.05 M VBSA and 0.215 M MeTh, in dimethyl formamide (DMF) containing 0.1 M TMAP. After electrolysis, the carbon fibres were washed thoroughly with water, distilled acetone and THF, and then dried overnight in a vacuum oven at 1 mbar at a temperature of 50 ◦ C. Cyclic voltammetry and chronoamperometry were performed on a CHI 800 electrochemical analyser (CHI Instrument, USA). The three-electrode system used consisted of carbon fibre or polymerised carbon fibre electrodes (typically 35–40 fibres bundled together, inserted into a glass capillary and connected via a copper wire to the potentiostat), Ag/AgCl (BAS), and Pt wire (3 cm in length) as the working, reference and counter electrodes, respectively. For untreated fibres, the actual surface area was determined by cyclic voltammetry using 1 mM ferricyanide. Epoxy was used to seal the capillary tube, allowing ca. 4 mm of the fibres to protrude. The fibres were washed with concentrated nitric acid and then rinsed with copious amounts of water. The buffer used was phosphate (0.1 M, pH 8.0). The stability of the response to p-AP was monitored over a period of 20 days with the current density obtained in a 1 mM solution of p-AP determined daily. The electrodes were stored in buffer when not in use. 2.3. Fibre surface morphology and thickness All electro-grafted fibres were analysed by scanning electron microscopy using an Hitachi S-2700 scanning electron microscope (Nissei Sangyo GmbH, Rathingen, Germany), which was connected to an energy dispersive X-ray micro analyser (EDX) (Kevex type delta V, Foster City, CA, USA). The excitation energy was 10 keV at a beam current

Both polymers and copolymers electro-grafted onto the carbon fibre surface were analysed by FTIR reflectance spectrometry (Perkin-Elmer, Spectrum One, Überlingen, Germany) with an ATR attachment (Universal ATR with a diamond and ZnSe crystal).

3. Results and discussion 3.1. Polymer synthesis and characterisation The structures of each of the monomers used are shown in Fig. 1. Electro-polymerisation of the monomers and their mixtures was performed galvanostatically. Batch quantities of electro-grafted carbon fibres (12,000 fibres), were readily made, allowing for ease of use and characterisation of the fibres. The monomer concentrations and other experimental details of the electro-coating conditions are given in Table 1. Regular growth of polymer was observed in all cases. The rate of polymer deposition rate was faster with the carbazole monomer than with methylthiophene. This difference in rates can be ascribed to the lower oxidation potential of the carbazole monomer, 1.2 V, compared to 1.4 V for methylthiophene [19]. SEM images of films of electro-coated polymer or copolymers onto CFMEs are shown in Fig. 2, and the thickness of the polymer films are listed in Table 1. The values obtained were found to be 340 ± 15, 700 ± 30, 150 ± 10, 6750 ± 200 nm for poly[N-vinylcarbazole-co-vinylbenzene sulfonic acid] (p[NVCzVBSA1]), p[NVCzVBSA2], poly[carbazole-co-methylthiophene] (p[CzMeTh]), and polycarbazole (p[Cz]), respectively. From the film thickness data and AFM images, the smallest (150 nm) and most homogeneous film coating was obtained with CzMeTh. The thickest film was obtained with p[Cz] (6750 nm). This coating was more heterogeneous and showed the presence of grains. Given that the same total charge was passed during the electro-polymerisation procedures and that the p[Cz] film was at least ten times thicker than the other films, it is clear that kinetic factors limit the growth of the films containing substituted carbazoles. The ease of manufacture of large quantities of the modified fibres and the homogeneous nature of the polymeric coatings suggest the possibility of using these electrodes as sensors. Galvanostatically prepared films of homopolymers and copolymers grafted on to CFMEs at a constant current density of ∼10 A cm−2 were analysed by FTIR-ATR spectroscopy. The spectra obtained with CzMeTh-modified

M. Jamal et al. / Sensors and Actuators B 97 (2004) 59–66

61

N

N H

CH2

N-Vinylcarbazole (NVCz)

Carbazole

(Cz)

SO3– Na+

CH3

CH2

S

Vinylbenzenesulfonic acid sodium salt (VBSA)

3-Methylthiohene (MeTh)

Fig. 1. Structures of monomers used.

Table 1 Electro-coating conditions and thickness measurements of polymer-modified fibres Copolymer

I (A cm−2 )

t (min)

Initial [monomer] ratio

Increase in radius (␮m)

NVCzVBSA1 NVCzVBSA2 CzMeTh Cz

10 20 10 10

240 120 30 120

[NVCz]/[VBSA] = 1

0.34 0.70 0.15 6.75

fibres had a peak at 742 cm−1 (Fig. 3), corresponding to C–S in-plane deformation [20]. This peak was absent from spectra of p[Cz]. Similarly, a sharp peak at 745 cm−1 was obtained with NVCz-VBSA1 fibres corresponding to the C–S stretch of the sulfonate group. The presence of bands associated with functional groups of individual monomers indicated that both monomers were incorporated into the copolymer. A peak at 1080 cm−1 in the NVCzVBSA1 film can be assigned to ClO4 − from the supporting electrolyte [21]. Consistent with previous reports [18], the presence of this dopant anion is likely a result of further oxidation of some sites of the polymer surface during the electro-polymerisation process. The surface composition [22] of NVCzVBSA1 and NVCzVBSA2 were determined by XPS to be 76.6% C; 2.9% N; 15.0% O; 2.8% S and 85.8% C; 3.8% N; 9% O; 0.2% S, respectively. These data clearly indicate that inclusion of N groups from the NVCz unit into the polymer had occurred. The ratio of sulphur (originating from vinylbenzene sulfonic acid) to nitrogen present in NVCzVBSA1 was approximately 1.0, but for NVCzVBSA2 was 0.05. This result indicates that inclusion of carbazole unit into the copolymeric structure is more facile, due to its higher reactivity relative to VBSA.

[Cz]/[MeTh] = 0.35

3.2. Morphology The morphologies of the polymers of homopolymer and copolymer depositions from solutions of 0.1 M TMAP/DMF on CFMEs were investigated by scanning electron microscopy (Fig. 2). A globular structure resulting from a three-dimensional nucleation growth mechanism was observed in all cases. The SEM picture of p[Cz] shows small clusters of globules. The morphology of p[CzMeTh], and p[NVCzVBSA] were quite different from p[Cz], i.e. more homogeneous coatings were observed. This structural difference provides additional proof for the incorporation of both monomers into the copolymer. Copolymerization was carried out for two different time periods and current densities (but with the same total charge of 0.01 C) using the monomers NVCz and VBSA. SEM pictures of these films show that the thickness of the resulting copolymer varied with the current density. Increases in radii of 340±15 and 700±30 nm were observed for NVCzVBSA1 (t = 240 min, I = 10 A/cm2 ) and NVCzVBSA2 (t = 120 min, I = 20 A/cm2 ), respectively. Although the charge density was the same for both cases, higher currents over a shorter period produced thicker films of NVCzVBSA2, indicating that kinetic factors play a role in the polymerisation process.

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Fig. 2. SEM images of untreated and polymer-modified carbon fibre microelectrodes.

3.2.1. Electrochemical detection of p-aminophenol The detection of p-aminophenol in the development of immunosensors has been reported [2]. A particular advantage of p-AP is that it is electroactive, with a oxidation potential of 0.3 V [2], while its precursor, p-aminophenyl phosphate is not electroactive at this potential. The electro-

chemical response of p-AP at a range of polymers with different initial ratios of co-monomers were examined. Fig. 4 shows a series of cyclic voltammograms of p-AP obtained at a CFME modified with p[NVCzVBSA] at different scan rates. Quasi-reversible behaviour was obtained (anodic to cathodic peak current ratio of 1.0, Ep of 114 mV at a

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Fig. 3. FTIR-ATR spectrum of p[NVCZVBSA], p[Cz] and p[CzMeTh]-modified carbon fibres.

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M. Jamal et al. / Sensors and Actuators B 97 (2004) 59–66 2.0

(A)

Current density / µA.cm

-2

1.5 1.0 0.5 0.0

-0.4

-0.2

0

0.2

0.4

0.6

0.4

0.6

-0.5 -1.0 -1.5

Potential / V 1.0

Current density / mA.cm

-2

(B) 0.5

0.0 -0.4

-0.2

0

0.2

-0.5

-1.0

Potential / V Fig. 4. Cyclic voltammograms obtained at (A) p[NVCzVBSA] coated carbon fibres and (B) untreated carbon fibres in buffer solution containing 1 mM p-aminophenol at scan rates of 20, 40, 60, 100, 120, 140, 160 mV/s.

scan rate of 100 mVs−1 ). Plots of peak current versus ν1/2 were linear, indicating that the process was operating under diffusion control (Fig. 5). In comparison to the untreated carbon fibre electrodes, the current density obtained at NVCzVBSA1 electrodes was 76% higher. On untreated carbon fibre and glassy carbon electrodes, an additional oxidation peak was observed at 0.33 V. This peak was not observed when the fibres were washed with nitric acid or when

the glassy carbon electrode was cleaned after each scan and is likely due to contamination of the electrode surface. For the other polymers examined, the response obtained with p-AP was less reversible, with higher values of Ep (200 mV [NVCzVBSA-2], 230 mV [CzMeTh] and 237 mV [Cz-12]) and peak current ratios of 1.06 [NVCzVBSA-2], 1.45 [CzMeTh] and 1.5 [Cz]. The current density applied to make the polymer film affected the response to p-AP. The anodic to cathodic peak current ratio was 1.0 for p[NVCzVBSA1] (I = 10 A/cm2 ) and 1.33 for p[NVCzVBSA2] (I = 20 A/cm2 ). Though both electrodes modified with p[NVCzVBSA] showed quasi-reversible behaviour for the oxidation of p-AP, the thin film P[NVCzVBSA1] had higher peak currents with calibration slopes of 0.82 and 0.27 A cm−2 M−1 for p[NVCzVBSA1] and p[NVCzVBSA2], respectively (Fig. 6). p[NVCzVBSA1] contains substantially more sulfonate groups than p[NVCzVBSA2], which appear to influence the response. The presence of the these groups in the polymer film could prove useful for the entrapment of cationic enzymes and proteins. The use of these films to immobilise proteins is currently being investigated. Fig. 6 shows a plot of the current densities obtained from cyclic voltammograms at varying concentrations of p-AP for each of the polymers. A linear relationship was obtained in all cases with the highest response being obtained with p[NVCzVBSA1]. The polymer-modified electrodes all displayed greater sensitivity to p-AP than untreated CFME, with the best response being obtained with p[NVCzVBSA1]. The increased response may be a result of binding of p-AP to the polymer films. A detection limit of 10−6 M (0.1 ␮g/ml) for p-AP in 0.1 M phosphate buffer was obtained. 3.3. Stability of the polymers Although polypyrrole is known to be highly sensitive to over-oxidation, polycarbazole and its copolymers were found to be much more stable to high potentials and high 12

1.5 1 0.5 0 -0.5

0

2

4

6

8

10

12

14

-1

Current density / µA.cm

Current density / mA.cm

-2

-2

2 10 8 6 4 2 0 0

-1.5

(Scan Rate)

1/2

1/2

/ mV .s

-1/2

Fig. 5. Plot of current density vs. square root of scan rate of p[NVCzVBSA1] (䊊) and, untreated () carbon fibre electrodes.

2

4

6

8

10

12

[p-AP] / µM Fig. 6. Plot of current density vs. concentration of p-AP at carbon fibre electrodes, p[NVCzVBSA1] (䊐); p[NVCzVBSA2] (); p[CzMeTh] (䊊); p[Cz] (×); untreated (䉫).

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cil (Project: TUBITAK-COST 523), the Programme for Research in Third Level Institutes and the European Union (QLRT-2000-01670).

120

Relative sensitivity / %

65

100 80 60

References

40 20 0 0

5

10

15

20

Time / Days Fig. 7. Plot of relative sensitivity vs. concentration of p-AP at carbon fibre electrodes, p[NVCzVBSA1] (䊐); p[NVCzVBSA2] (); p[CzMeTh] (䉱); p[Cz] (䊏). Data were normalised to response on day 1.

temperatures [14–17]. At high potentials, oxidation can occur in position 3 or 4 of the thiophene rings, causing degradation of the polymers. The presence of alkyl groups such as methyl groups avoids this type of degradation in thiophene and carbazole, and the polymers were stable up to potentials of 1.4 V in DMF. The polymers were stored under air and at ambient temperatures and were stable for at least 6 months under these conditions. The stability of the response of the electrodes is shown in Fig. 7. The current density decreases over a period of 3–4 days to between 40 and 60% of the original response. After this decrease, the response remained stable for 16 days.

4. Conclusions In summary, poly[N-vinylcarbazole-co-vinylbenzene sulfonic acid], poly[carbazole-co-methylthiophene] and polycarbazole were coated electrochemically on carbon fibre microelectrodes. The monomers used have an effect on the homogeneity of the resultant films, i.e. very homogenous copolymers can be synthesised with p[NVCzVBSA]. The electrochemical behaviour of this homopolymer and copolymers on CFME against p-AP indicated that these electrodes can be used to detect p-AP.

Acknowledgements The authors thank Prof. Dr. J. Springer, Technische Universitat, Berlin, Fachgebiet Makromol. Chemie, for the supply of carbon fibre and permission to perform the SEM micrograph measurements performed by Jörg Nissen (Zentraleinrichtung für Elektronenmikroskopie) and FTIR-ATR measurements performed by Astrid Müller, the financial support of Turkish Scientific and Research Coun-

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Biographies Mamun Jamal obtained an MSc in physical–inorganic chemistry from University of Dhaka (Bangladesh). Currently, he is studying for his PhD

at the University of Limerick. His thesis project is concerned with the development of biosensors for hormone residue analysis. Edmond Magner is a senior lecturer in the Department of Chemical and Environmental Sciences and a member of the Materials and Surface Science Institute at the University of Limerick. After obtaining his BSc (University College, Cork) and PhD (University of Rochester, NY), he was a post-doctoral fellow at Imperial College, London and at the Massachusetts Institute of Technology. Prior to taking up his current post, he worked as a research scientist for MediSense Inc. His research interests lie broadly in the electrochemistry of redox proteins. A. Sezai Sarac is a professor of chemistry at Istanbul Technical University, and a visiting professor in the Materials and Surface Science Institute at the University of Limerick. He received his BSc and MSc From Istanbul Technical University, and his PhD from the University of Missouri Rolla (USA). He was a post-doctoral fellow at the University of Leeds and a visiting professor at the Technical University of Berlin and the University of Regensburg.