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This work evaluated an amperometric biosensor based on multi-wall carbon nanotubes (MWCNT), chemically modified with methylene blue (Met) and horseradish peroxidase (HRP), for detection of phenolic compounds. The dependences of the biosensor response due to the enzyme immobilization procedure, HRP amounts, pH and working potential were investigated. The amperometric response for catechol using the proposed biosensor showed a very wide linear response range (1 to 150 mmol L À1 ), good sensitivity (50 nA cm À2 mmol À1 L), excellent operational stability (after 300 determinations the response remained at 97%) and very good storage stability (lifetime > 3 months). Based on all these characteristics, it is possible to affirm that the material is promising for phenol detection due to its good electrochemical response and enzyme stabilization. The biosensor response for various phenolic compounds was investigated.

Reagentless biosensors sensitive to hydrogen peroxide have been developed and compared. These biosensors are comprised of a carbon paste electrode modified with horseradish peroxidase (HRP) and one phenothiazine (methylene blue), one phenoxazine (meldolas blue) or one phenazine (phenazine methosulfate) dye adsorbed on silica gel modified with niobium oxide (SN). The enzyme was immobilized onto the graphite powder by cross-linking with glutaraldehyde and mixing with one of the electron transfer mediators (dyes) adsorbed on SN. The amperometric response was based on the electrocatalytic properties of the dye to mediate electrons, which were generated in the enzymatic reaction of hydrogen peroxide under catalysis of HRP. The dependence on the biosensor response in terms of pH, buffer, HRP amounts and applied potential was investigated. The best results were found with a biosensor containing methylene blue dye showing an excellent operational stability (around 92% of the activity was maintained after 300 determinations). The proposed biosensor also presented good sensitivity (32.87 nA cm À2 m mol À1 L) allowing hydrogen peroxide quantification at levels down to 0.52 Â 10 À6 mol L À1 an optimum response at pH 6.8 and at a potential of À 50 mV (vs. SCE) and showing a wide linear response range (from 1 to 700 mmol L À1 for hydrogen peroxide).

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