Scheme 1. The proposed mechanism for signal generation in the PED immunosensor.

Fig. 1. The influence of polymer growth time at a growth rate of 0.5 mA cm⁻² on the response of the PED immunosensor for three different counter ions: sulphate (○), nitrate (×) and chloride (). The measurement parameters were 50 μg ml⁻¹ of antigen was injected with a flow rate of 0.5 ml min⁻¹, E₋ as shown and E₊=+0.40 V.

Fig. 2. Pulsed waveform applied at the PED immunosensor.

Fig. 3. Pulsed potential hydrodynamic voltammograms of a PED immunosensor for different electrolytes used during fabrication: 50 μg ml⁻¹ of antigen was injected with a flow rate of 0.5 ml min⁻¹, E₋ as shown and E₊=+0.40 V. Electrolytes: (•) sulphate, (○) nitrate, (▪) chloride, (□) p-toluenesulphonate.

Fig. 4. Cyclic voltammograms of the antibody–polypyrrole modified electrode fabricated in sodium sulphate electrolyte The solid line is before injection of antigen and the dotted line is after injection of antigen. The scan rate was 100 mV s⁻¹ and electrolyte 0.1 M sodium sulphate.

Fig. 5. Typical FIA for 50 μg ml⁻¹ rabbit antigen injections. Carrier solution 0.1 mol l⁻¹ Na₂SO₄; flow rate 0.5 ml min⁻¹; E₋=−0.2 V; E₊=+0.4 V.

Fig. 6. Calibration plot of FIA peak against the logarithm of the antigen concentration. Solid points: the mean and 95% confidence intervals of electrodes prepared with 200 μg ml⁻¹ (nine electrodes). Open circles: the mean of the responses from three electrodes prepared with 50 μg ml⁻¹ antibody. Line is a fit to an equilibrium binding model.

Fig. 7. Response of a PED immunosensor to 50 μg ml⁻¹ antigen as a function of pulse period. Solid points: Time at E₊=+0.4 V for constant 200 ms at E₋=−0.2 V. Open points: Time at E₋=−0.2 V for constant 200 ms at E₊=+0.4 V.

Table 1. The influence of the electrolytes used during the preparation of the antibody–polypyrrole membrane on the amount of antibody incorporated in the polymer and the resultant signal of the PED immunosensor to 50 μg ml⁻¹ antigen

Table 2. The influence of the amount of antibody in solution during preparation of the electrode on the amount of antibody incorporated and on the performance of the final sensor