Fig. 1. Schematic representation of the Acidithiobacillus ferrooxidans biosensor based on multidisk cathode and biomass immobilized in a double filter membrane.

Fig. 2. Amperometric curve obtained with an Acidithiobacillus ferrooxidans sensor after sequential introduction of Fe²⁺ solution. Experimental conditions were: bacterial load 1 μg; pH 1.4; 30 °C; stirring speed 100 rpm.

Fig. 3. Influence of bacterial charge in the biological layer on the Fe²⁺ biosensor response based on Acidithiobacillus ferrooxidans (pH 1.4; 100 rpm; 10⁻⁵ mol L⁻¹ Fe²⁺).

Fig. 4. Pareto Graph: influence of parameters (single, quadratic and interactions) on the refined model describing the amperometric response of an Acidithiobacillus ferrooxidans biosensor for Fe²⁺ determination.

Fig. 5. Fe²⁺ biosensor responses in function of pH and temperature (bacterial charge 1 μg of protein, 1.5 × 10⁻⁴ mol L⁻¹ Fe²⁺). The arrow indicates the optimal experimental conditions for the maximal response of an Acidithiobacillus ferrooxidans sensor for Fe²⁺ determination.

Fig. 6. Amperometric response of the Acidithiobacillus ferrooxidans sensor in function of introduction of S₂O₃²⁻ solution. Experimental conditions: bacterial load 1 μg of protein; pH 4.7; 30 °C; 100 rpm.

Fig. 7. Influence of pH on the amperometric response of the Acidithiobacillus ferrooxidans sensor for the determination of thiosulfate (0.2 mmol L⁻¹, bacterial load 1 μg of protein, stirring speed 100 rpm; 30 °C).

Experimental matrix of the DOE, applied for the modelling and optimisation of the response of an amperometric Acidithiobacillus ferrooxidans sensor used for Fe²⁺ determination (protein quantity 1 μg; Fe²⁺ concentration 1.5 × 10⁻⁴ mol L⁻¹)

The studied response Y corresponds to the current obtained by the biosensor (CV = 2/4%).