Development of amperometric sensors for uric acid based on chemically modified graphite-epoxy resin and screen-printed electrodes containing cobalt phthalocyanine

Cyclic voltammetry, hydrodynamic voltammetry (HDV) and amperometry were used to investigate the behaviour of uric acid at carbon(graphite)–epoxy resin electrodes modified with the following electrocatalysts: ferrocenes, phthalocyanines, phenoxazines, phenazines and hexacyanoferrate species. As no direct electrocatalytic oxidation of uric acid was apparent, enzyme-coupled mediation was studied using uricase with electrodes modified with ferrocene, cobalt phthalocyanine (CoPC) and Meldola's Blue, in the presence and absence of oxygen. Again, no electrocatalysis was observed so we exploited the specificity of the enzyme uricase, which is known to produce hydrogen peroxide from uric acid. This product has been shown to undergo electrocatalysis at modified electrodes, and preferentially traverses certain membranes, so this rationale was adopted for the ensuing studies. Cyclic voltammetry and amperometry were used to study the electrochemical behaviour of H2O2 in 0.05 mol dm–3phosphate buffer (pH 9.3). Hydrogen peroxide was found to be electroactive at substantially lower potentials (<600 mV) at CoPC-modified electrodes than at unmodified electrodes. The precision of surface–surface reproducibility of epoxy resin electrodes was calculated by cyclic voltammetry to be 7.3%(n= 5). Further studies were performed with and without cellulose acetate (CA) membranes using amperometry in stirred solution. Epoxy resin electrodes were simply dropcoated with CA. The membrane functioned as an effective barrier against a range of potential interferences, but allowed the selective detection of enzymically generated H2O2 from urate. This methodology was then translated to screen-printed electrodes (SPEs). Cyclic voltammetry was first used to establish the precision of electrode manufacture; this was 4.9%(n= 5). The SPEs were then modified by placing each inside a tube, one end of which was covered with a solvent cast (CA) membrance. This electrode–polymer configuration furnished similar permselective properties as the drop-coated devices. The solvent cast membranes were treated with various concentrations of uricase; when the membrane was loaded with 0.48 U of enzyme a measurable H2O2 signal was obtained at the CoPC–SPE for a solution containing 0.18 mmol dm–3 uric acid.