**3.2.3 Electrochemical oxidation of ascorbic acid**

A more extended applications of polyluminol films and its derivatives is the electrocatalysis of ascorbic acid (Chen & Lin, 2002; Ashok et al., 2009; Ma et al., 2010; Kumar et al., 2009a; Kumar et al., 2009b). As the potentiodinamic response of films obtained using carbon paste electrodes are different to those synthesised with metallic electrodes; in fact the cyclic voltammogram is more similar to those of copolymers of aniline and luminol, we think that

Electrochemical Preparation and Properties of Novel Conducting Polymers

Derived from 5-Amino-2naphtalensulfonic Acid, Luminol and from Mixtures of Them 111

anodic peak situated in the 0.7 -1.2 Vand 0.9 and 1.2 V for ANS and luminol respectively. The anodic response correspond to the irreversible oxidation of monomers entities, the potential where the oxidation of both monomers is carried out is environ 0.9 V. The effect of the scan rate in the intensity of current was studied, in the Figure 13b can be observed that in the case of ANS the peak heights are linear with the square root of sweep rate in all range studied, this mean that the oxidation of ANS is controlled by diffusion; while in the luminol case the peak heights scale linearly with the square root of sweep rate only in the range between 5 and 100 mVs-1, in consequence for higher sweep rate, the process of oxidation can be controlled by adsorption or a combination of adsorption and diffusion. It is important note that the slopes of both curves are essentially identical, in considering the Randles-Sevcik can be established that the Diffusion coefficient of both monomers should be very similar. The oxidation of both monomers is 0.9 V and in order to both monomes are in the

same regime of mass transfer it is convenient working to sweep rate ≤ 100 mVs-1.

**B)**

(a) (b)

**3.3.2 Electrochemical synthesis and characterization of copolymers** 

anodic peak current with the square root of sweep rate

**-200 0 200 400 600 800 1000 1200 1400 E/mV**

**A)**

**J/**μ**Acm-2**

Fig. 13. (a) Cyclic voltammetric response of: (A) ANS (0.8 mM) and (B) luminol (0.8 mM) on a carbon paste electrode (0.1452 cm2) in H2SO4 1 M. The carbon paste was formed by a mixture 1:1 of luminol and graphite. Potential scan rate 100 mVs-1. (b) Dependence of

**I/** μ**A**

0 2 4 6 8 10 12 14 16

**A)**

**B)**

**V½/ mV½s-½**

The copolymers were synthesised using a H2SO4 (1 M) solution contain luminol (0.8 mM) and ANS (0.8 mM) as working electrode carbon paste electrode was used, 50 potential cycles were carried out between 0 y 0.9 V. Concurrently the same experiment was repeated with a solution contain only a monomer. After their electrosyntesis, copolymers and homopolymers were characterised by cyclic voltametry in 1 M H2SO4 (Figure 14), in orden to avoid the sobreoxidation of films the upper potentials were shifted to lower values. The cyclic voltammogram of copolymer was characterised by present two redox process, attributed in analogy with polyluminol and polyANS to partial and total oxidation of polymer. Comparison of the peak of copolymer with those of luminol and ANS taken under similar conditions indicate that the potential and the current of copolymers peak lies between them. This suggest that the films prepared by this procedure are indeed copolymers and not just mixtures of polyluminol and polyANS. We have tried to modulate the amount of lumiol in the copolymer film. To do this, the electrosynthesis of copolymers

the chemical composition of films in carbon paste correspond to those of a polymer, this supposition is supported by the behaviour of films in neutral solutions. In order to determinate weather the chemical composition of deposits of luminol affect or not the catalytic properties of films, the electoxidation of ascorbic acid (aa) was studied with the films obtained and compared with those of metallic electrodes, the curves obtained are presented in Figure 12, as can be seen, the oxidation peak current augment with increasing aa concentration. The inset of Figure 12 shows that the anode peak current is linearly dependent on the aa concentration in the studied range, to slope of line was obtaining the sensibility that was 43.2 μAmM-1, this value is the double of those obtained using a polyluminol synthetised in a metallic electrode. This result show that the electrocatalytic activite of polyluminol film is enhanced with is obtained in carbon paste electrodes.

Fig. 12. Cyclic voltammetry of polyluminol in PBS containing different concentrations of aa: 0, 0.098, 0.195, 0.279, 0.364, 0.444, 0.522, 0.596 and 0.667 mM. The film was synthesised like those of Figure 8b. The scan rate was 100 mVs-1.

#### **3.3 Electrodeposition of copolymer**

Finally we have tried the electrosynthesis of a novel self-doped polymer formed by ANS and luminol, in fact the electroxidation of both compound can to produce self-doping polymers, However the polyANS is soluble in milieu neuter, this characteristic limit its use in biosensors, while the polyluminol is insoluble but the electrochemical activity is more dependent of pH in comparison with the polyANS in reason of the –SO3H is a strong acid. The combination of both monomers can be produced an insoluble polymer which present electrochemical activity in a wide pH range.

#### **3.3.1 Electrochemical properties of monomers**

In order to determinate the potential which both monomers are oxided, potenciodynamic experiments were carried out (Figure 13a), the voltammograms exhibit a broad irreversible

the chemical composition of films in carbon paste correspond to those of a polymer, this supposition is supported by the behaviour of films in neutral solutions. In order to determinate weather the chemical composition of deposits of luminol affect or not the catalytic properties of films, the electoxidation of ascorbic acid (aa) was studied with the films obtained and compared with those of metallic electrodes, the curves obtained are presented in Figure 12, as can be seen, the oxidation peak current augment with increasing aa concentration. The inset of Figure 12 shows that the anode peak current is linearly dependent on the aa concentration in the studied range, to slope of line was obtaining the sensibility that was 43.2 μAmM-1, this value is the double of those obtained using a polyluminol synthetised in a metallic electrode. This result show that the electrocatalytic

activite of polyluminol film is enhanced with is obtained in carbon paste electrodes.

Fig. 12. Cyclic voltammetry of polyluminol in PBS containing different concentrations of aa: 0, 0.098, 0.195, 0.279, 0.364, 0.444, 0.522, 0.596 and 0.667 mM. The film was synthesised like

Finally we have tried the electrosynthesis of a novel self-doped polymer formed by ANS and luminol, in fact the electroxidation of both compound can to produce self-doping polymers, However the polyANS is soluble in milieu neuter, this characteristic limit its use in biosensors, while the polyluminol is insoluble but the electrochemical activity is more dependent of pH in comparison with the polyANS in reason of the –SO3H is a strong acid. The combination of both monomers can be produced an insoluble polymer which present

In order to determinate the potential which both monomers are oxided, potenciodynamic experiments were carried out (Figure 13a), the voltammograms exhibit a broad irreversible

those of Figure 8b. The scan rate was 100 mVs-1.

electrochemical activity in a wide pH range.

**3.3.1 Electrochemical properties of monomers** 

**3.3 Electrodeposition of copolymer**

anodic peak situated in the 0.7 -1.2 Vand 0.9 and 1.2 V for ANS and luminol respectively. The anodic response correspond to the irreversible oxidation of monomers entities, the potential where the oxidation of both monomers is carried out is environ 0.9 V. The effect of the scan rate in the intensity of current was studied, in the Figure 13b can be observed that in the case of ANS the peak heights are linear with the square root of sweep rate in all range studied, this mean that the oxidation of ANS is controlled by diffusion; while in the luminol case the peak heights scale linearly with the square root of sweep rate only in the range between 5 and 100 mVs-1, in consequence for higher sweep rate, the process of oxidation can be controlled by adsorption or a combination of adsorption and diffusion. It is important note that the slopes of both curves are essentially identical, in considering the Randles-Sevcik can be established that the Diffusion coefficient of both monomers should be very similar. The oxidation of both monomers is 0.9 V and in order to both monomes are in the same regime of mass transfer it is convenient working to sweep rate ≤ 100 mVs-1.

Fig. 13. (a) Cyclic voltammetric response of: (A) ANS (0.8 mM) and (B) luminol (0.8 mM) on a carbon paste electrode (0.1452 cm2) in H2SO4 1 M. The carbon paste was formed by a mixture 1:1 of luminol and graphite. Potential scan rate 100 mVs-1. (b) Dependence of anodic peak current with the square root of sweep rate

#### **3.3.2 Electrochemical synthesis and characterization of copolymers**

The copolymers were synthesised using a H2SO4 (1 M) solution contain luminol (0.8 mM) and ANS (0.8 mM) as working electrode carbon paste electrode was used, 50 potential cycles were carried out between 0 y 0.9 V. Concurrently the same experiment was repeated with a solution contain only a monomer. After their electrosyntesis, copolymers and homopolymers were characterised by cyclic voltametry in 1 M H2SO4 (Figure 14), in orden to avoid the sobreoxidation of films the upper potentials were shifted to lower values. The cyclic voltammogram of copolymer was characterised by present two redox process, attributed in analogy with polyluminol and polyANS to partial and total oxidation of polymer. Comparison of the peak of copolymer with those of luminol and ANS taken under similar conditions indicate that the potential and the current of copolymers peak lies between them. This suggest that the films prepared by this procedure are indeed copolymers and not just mixtures of polyluminol and polyANS. We have tried to modulate the amount of lumiol in the copolymer film. To do this, the electrosynthesis of copolymers

Electrochemical Preparation and Properties of Novel Conducting Polymers

**a)**

> **-40 -30 -20 -10 0 10 20 30 40**

**-60**

**-40**

**-20**

**0**

**J/**μ**Acm-2**

**20**

**40**

**60**

**J/**μ**A cm-2**

**J /**μ**Acm-2**

Derived from 5-Amino-2naphtalensulfonic Acid, Luminol and from Mixtures of Them 113

**a )**

**0 200 400 600 800 1000 E /m V** 

**b)**

**c)**

**0 200 400 600 800 1000 E/mV** 

**c )**

**B)**

**c)**

**C )**

**b )**

**A )**

**b )**

Fig. 15. Voltammograms for (a) polyANS, (b) copolymer and (c) polyluminol films obtained

**a)**

**0 200 400 600 800 1000 E/ m V** 

in H2SO4 (1 M). Potential scan rate 100 mVs-1. The scan rates used during the

electrodeposition were: (A) 5, (B) 25 and (c) 50 mVs-1.

was performed using different ratios of luminol and ANS, in addition the potential scan rate was changed. Concerning the ratio of monomer when the amount of luminol increase in solution the cyclic voltammogram of copolymer present current and potentials mores similar to those of luminol, while that when it is augmented the amount of ANS in comparison to those of luminol in solution, the potentiodynamic response of copolymer is more near to those of polyANS. Thus, it is possible modulate the monomer proportion in the film changing the ratio of monomer in the electrosynthesis solution.

Fig. 14. Voltammograms for (A) polyANS, (B) copolymer and (C) polyluminol films obtained in H2SO4 (1 M). Potential scan rate 100 mVs-1.

On the other hand, as can be been in Figure 14 and Figure 15 the potentiodynamic response is dependent of the scan rate used during the electropolymerization, in the case of homopolymers the current change with this parameter in contrast the potential is the same for all scan rate studied. This indicate that only change the amount of film obtained and the chemical composition is the same, in the copolymer case both current and potential are dependent of potential scan rate Therefore the amount and composition of films synthesized are modified with the scan rate. These facts could be related with the solubility of films, because the products of oxidation of ANS are more soluble that those of luminol, since to lower scan rate the oligomers formed have time for a probable adsorption in the surface of paste carbon, since the polymerization of ANS is enhanced to low scan rate (Figure 15a). The products of luminol are insoluble and are less dependent to the scan rate, since the proportion of luminol and ANS onto the film are dependent of this variable.

The electroactivity of copolymers was analysed in neutral solutions by cyclic voltammetry, as can be showed in Figure 16, the curves obtained show process redox, this indicate that are electrochemical activity to pH neutral. This result is congruent with the behaviour of a self-doped polyaniline. The response was affected by the ratio of monomers in the films, for the films obtained to 5 mVs-1 the behaviour was similar to those of polyANS, while the film synthetised to 100 mVs-1 the cyclic voltammogram was similar to those of polyluminol.

was performed using different ratios of luminol and ANS, in addition the potential scan rate was changed. Concerning the ratio of monomer when the amount of luminol increase in solution the cyclic voltammogram of copolymer present current and potentials mores similar to those of luminol, while that when it is augmented the amount of ANS in comparison to those of luminol in solution, the potentiodynamic response of copolymer is more near to those of polyANS. Thus, it is possible modulate the monomer proportion in the

film changing the ratio of monomer in the electrosynthesis solution.

Fig. 14. Voltammograms for (A) polyANS, (B) copolymer and (C) polyluminol films

proportion of luminol and ANS onto the film are dependent of this variable.

On the other hand, as can be been in Figure 14 and Figure 15 the potentiodynamic response is dependent of the scan rate used during the electropolymerization, in the case of homopolymers the current change with this parameter in contrast the potential is the same for all scan rate studied. This indicate that only change the amount of film obtained and the chemical composition is the same, in the copolymer case both current and potential are dependent of potential scan rate Therefore the amount and composition of films synthesized are modified with the scan rate. These facts could be related with the solubility of films, because the products of oxidation of ANS are more soluble that those of luminol, since to lower scan rate the oligomers formed have time for a probable adsorption in the surface of paste carbon, since the polymerization of ANS is enhanced to low scan rate (Figure 15a). The products of luminol are insoluble and are less dependent to the scan rate, since the

The electroactivity of copolymers was analysed in neutral solutions by cyclic voltammetry, as can be showed in Figure 16, the curves obtained show process redox, this indicate that are electrochemical activity to pH neutral. This result is congruent with the behaviour of a self-doped polyaniline. The response was affected by the ratio of monomers in the films, for the films obtained to 5 mVs-1 the behaviour was similar to those of polyANS, while the film synthetised to 100 mVs-1 the cyclic voltammogram was

obtained in H2SO4 (1 M). Potential scan rate 100 mVs-1.

similar to those of polyluminol.

Fig. 15. Voltammograms for (a) polyANS, (b) copolymer and (c) polyluminol films obtained in H2SO4 (1 M). Potential scan rate 100 mVs-1. The scan rates used during the electrodeposition were: (A) 5, (B) 25 and (c) 50 mVs-1.

Electrochemical Preparation and Properties of Novel Conducting Polymers

**3.3.3 Electrocatalytic oxidation of ascorbic acid** 

suggest that the catalysis is more favourable for this film.

August 1994), pp. 577-580, ISSN 1521-4095

pp. 5294-5303, ISSN 0013-4686

6, (September 1992), pp. 1355-1379, ISSN 0365-6675

**4. Conclusion** 

scan rate.

39.39).

**6. References** 

**5. Acknowledgment** 

Derived from 5-Amino-2naphtalensulfonic Acid, Luminol and from Mixtures of Them 115

Finally the oxidation of aa was analysed in the copolymer and comparated with those of homopolymers, Figure 17 show the ascorbic acid oxidation on the different films, immersed in 1 M H2SO4 solution in the absence and in the presence of different concentrations of ascorbic acid. An increase in intensity of the anodic current peak, as the acid ascorbic concentration was increased is an indication of catalytic oxidation of ascorbic acid mediated for each film. The effect catalytic of polyluminol in the oxidation of aa is well established (Chen & Lin, 2002), however the electrocatalysis in polyANS and copolymer has been not reported. The current obtained for a same concentration of aa is higher in the polyANS this

In conclusion we have found that it is posibble the formation of polymer of ANS, when the electroxidation of monomer is carried out in PANI/Au or carbon paste electrodes. The films obtained are electrochemically active in neutral pH. The charge compensation of this film is carried out principally by ejection of cation, but anion insertion is simultaneuslly presented. On other hand, the electrochemical polymerization of luminol in carbon paste electrodes give a film with characteristic of a seld-doped polymer contraty to the film obtained using metallic electrodes which are dimmers. A polymer was synthetised to luminol and ANS, the cyclic voltammogram obtain show peak intermediate beetween luminol and ANS, the film can catalize the oxidation of aa and is electroachemically active to neutral pH, the proportion of monomers in the film can be modulate by the solution composition and the

This work was supported by the SEP (C06-PIFI-03.18.18) and CONACYT (C01-AINAT-01-

Barbero, C. & Kötz, R. (1994). Electrochemical formation of a selfdoped conductive polymer

Buttry, D.A. & Ward, M.D. (1992). Measurement of interfacial processes at electrode surfaces

Cano-Márquez, A.G.; Torres-Rodríguez, L.M. & Montes-Rojas, A. (2007). Synthesis of fully

in the absence of an supporting electrolyte. The copolymerization of *o*aminobenzensulfonic acid and aniline, *Advanced Materials,* Vol. 6, No. 7-8; (July-

with the electrochemical quartz crystal microbalance. *Chemical Review* Vol. 92, No.

and partially sulfonated polyanilines derived of ortanilic acid: an electrochemical and electromicrogravimetric study, *Electrochimica Acta*, Vol. 52, No. 16, (April 2007),

Fig. 16. Cyclic voltammograms of copolymer synthesised to (A) 5 and (B) 100 mVs-1 in (a) H2SO4 (1 M) and Na2SO4 (1 M). The scan rate was 100 mVs-1.

Fig. 17. Cyclic voltammograms curves for (A) polyANS, (B) polyluminol and (C) copolymer electrode in the prescence of (i) 0.1, (ii) 0.2, (iii) 0.4, (iv) 0.6 and (v) 0.8 mM. Supporting electrolyte H2SO4 1M. The scan rate was 100 mVs-1.
