Enhanced catalytic stability of acid phosphatase immobilized in the mesospaces of a SiO2-nanoparticles assembly for catalytic hydrolysis of organophosphates

Acid phosphatase (APase) was immobilized in discrete mesospaces of an assembly of silica nanoparticles (SiO2NPA) to be used as a heterogeneous catalyst for the hydrolysis of organophosphates often found as pesticide residues in agricultural products. APase immobilized in SiO2NPA exhibited higher catalytic activity than APase supported on conventional porous supports for the hydrolysis of p-nitrophenyl phosphate (p-NPP) with quantitative yield of p-nitrophenol below pH 5.5 due to the efficient diffusion of substrates in the SiO2NPA. Especially, a heterogeneous catalyst prepared by the co-accumulation method, in which silica nanoparticles (SiO2NPs) dispersion containing APase was simply dried to assemble a composite catalyst (APase/SiO2NPA), exhibited four times faster rate for the hydrolysis of p-NPP than the catalyst prepared by the equilibrium adsorption of APase in pre-assembled SiO2NPA. The catalytic stability of immobilized APase above pH 6.0 was enhanced by surface modification of SiO2NPs with 3-aminopropyltriethoxysilane (APase/SiO2NPA-NH2) due to the strong electrostatic interaction between APase and the protonated amino groups at the pH condition. Such stability enhancement was hardly obtained by cross-linking treatment of SiO2NPA to improve the robustness. These results suggest that electrostatic interaction between APase and SiO2NPs is crucial to enhance catalytic stability in the wide range of pH as well as preparation methods for the stable encapsulation.