Fusarium oxysporum fungal strain was screened and found to be successful for the inter- and extracellular production of platinum nanoparticles. Nanoparticle formation was visually observed, over time, by the colour of the extracellular solution and/or the fungal biomass turning from yellow to dark brown, and their concentration was determined from the amount of residual hexachloroplatinic acid measured from a standard curve at 456 nm. The extracellular nanoparticles were characterized by transmission electron microscopy. Nanoparticles of varying size (10-100 nm) and shape (hexagons, pentagons, circles, squares, rectangles) were produced at both extracellular and intercellular levels by the Fusarium oxysporum. The particles precipitate out of solution and bioaccumulate by nucleation either intercellularly, on the cell wall/membrane, or extracellularly in the surrounding medium. The importance of pH, temperature and hexachloroplatinic acid (H(2)PtCl(6)) concentration in nanoparticle formation was examined through the use of a statistical response surface methodology. Only the extracellular production of nanoparticles proved to be statistically significant, with a concentration yield of 4.85 mg l(-1) estimated by a first-order regression model. From a second-order polynomial regression, the predicted yield of nanoparticles increased to 5.66 mg l(-1) and, after a backward step, regression gave a final model with a yield of 6.59 mg l(-1).

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A mechanism for the bioreduction of H2PtCl6 and PtCl2 into platinum nanoparticles by a hydrogenase enzyme from Fusarium oxysporum is proposed. Octahedral H2PtCl6 is too large to fit into the active region of the enzyme and, under conditions optimum for nanoparticle formation (pH 9, 65 degrees C), undergoes a two-electron reduction to PtCl2 on the molecular surface of the enzyme. This smaller molecule is transported through hydrophobic channels within the enzyme to the active region where, under conditions optimal for hydrogenase activity (pH 7.5, 38 degrees C) it undergoes a second two-electron reduction to Pt(0). H2PtCl6 was unreactive at pH 7.5, 38 degrees C; PtCl2 was unreactive at pH 9, 65 degrees C.

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