Abstract
The present work describes the electrochemical behavior of cationic dyes (methylene blue and toluidine blue) as electron mediators adsorbed in the multiwall carbon nanotubes paste in 0.5 KCl at pH 7.0 by using cyclic voltammetry. Based on midpoint potential [E m = (E ap + E cp )/2] and separation of cathodic and anodic peaks (Δ E ), it was not observed interaction of different eletrolytes (LiCl, KCl, BaCl 2 , CaCl 2 , NaCl, NaNO 3 , Na 2 SO 4 and NaClO 4 ), with the cationic dyes by an ion exchange reaction and, as a consequence, absence of leaching of cationic dyes to the solution phase. The kinetics of electron transfer on the surface electrode was not sufficiently fast showing a fairly resistence of carbon nanotube paste modified with the cationic dyes. The midpoint potential and Δ E also were insentive to the pH range (4-8), confirming the protective effect of carbon nanotubes matrix, owing to strong interaction of between the latter and the nitrogen of nitrogen of cationic dyes with carbon nanotube matrix, minimizing the proton interaction under cationic dye. This result is very important for sensor/biosensor preparation, because the eletrooxidation behavior of the analyte will be only affected by its formal potencial shifting. Carbon nanotubes proved to be an efficient solid matrix for the adsorption of mediator electron in comparison to the electrochemical behavior of free cationic dyes in solution phase.
Highlights
Some low molecular weight enzymes such as horseradish peroxidase (HRP), cytocrome c peroxidase and fungal peroxidase, commonly used for building amperometric biosensors, present a kinetic barrier for the direct electron transfer between the active sites of redox enzymes and the electrode surface (KUBOTA et al, 2000)
As it could be observed, no cathodic and anodic peaks were found for the unmodified Multiwall carbon nanotubes (MWCNTs) sensor within the potential range evaluated, whereas a redox couple of the mediator peak having the midpoint potential Em = -113 mV vs saturated calomelane (SCE) (Em=(Eap + Ecp)/2) (MURRAY, 1984), where Eap Ecp are the anodic and cathodic peaks, respectively, was verified for the MWCNT-toluidine blue (TB) sensor
It could be seen, comparing the midpoint potential of -211 mV vs SCE for the free TB in the solution phase (KITANI et al, 1981) at the same pH, that a shift of 98 mV toward more positive values was observed for the dye immobilized onto MWCNT
Summary
Some low molecular weight enzymes such as horseradish peroxidase (HRP), cytocrome c peroxidase and fungal peroxidase, commonly used for building amperometric biosensors, present a kinetic barrier for the direct electron transfer between the active sites of redox enzymes and the electrode surface (KUBOTA et al, 2000). 331-336, July-Sept., 2012 potential, Eo, between 0.08 and -0.25 V (versus SCE), very close to those redox reactions of several enzymes over a large pH range In spite of these features, the use of solution-phase mediators is not preferable for preparing a biosensor, because large amounts of dyes are required and the drawbacks associated with the reference electrode contamination have been noticed (KUBOTA et al, 2000). Overlap of the unhybridized pz orbitals yields a π complex both above and below the plane containing the atoms, which is related to the high electron mobility and high electrical conductivity of graphene (ROTKIN; SUBRAMONEY, 2005) Based on these comments, carbon nanotubes have received significant attention for the preparation of electrochemical sensors (SANTOS et al, 2005) due to their intrinsic properties, such as high surface area and high chemical and electrical properties (TARLEY et al, 2009). The present work describes the electrochemical behavior of methylene blue and toluidine blue as electron mediators adsorbed in the multiwall carbon nanotubes paste
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