Self-assembled lipase nanosphere templated one-pot biogenic synthesis of silica hollow spheres in ionic liquid [Bmim][PF6]

Abstract

The spontaneous self-assembly of hydrophobic enzymatic protein triacylglycerol acylhydrolase (commonly known as lipase and a member of the serine hydrolase family) in hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate [Bmim][PF₆] and in hydrophilic 1-butyl-3-methylimidazolium tetrafluoroborate [Bmim][BF₄] ionic liquids resulted in the formation of lipase enzyme nanocapsules of different morphology. The lipase enzyme capsules were found to retain varying enzyme activity in both cases with both kinds of lipase capsules acting as self-catalyzing functional templates for the hydrolysis of silica precursors into silica. The presence of silica and its interaction with biomolecules was proved by X-ray Photoemission Spectroscopy (XPS). Interestingly, hollow silica spheres were obtained in the case of [Bmim][PF₆] ionic liquid, while solid silica spheres were obtained in the case of [Bmim][BF₄] ionic liquid for the same enzyme. The structural orientation of the enzyme within the capsules, their functional templating to obtain silica particles of varying morphology and finally their combined catalytic activity depend on the initial lipase-ionic liquid interaction. The enzyme activity of all these materials was evaluated against the esterification reaction between oleic acid (fatty acid) and butanol, i.e. biodiesel production. The relative enzyme activity was found to be 93.30% higher in the case of lipase nanocapsules synthesized in [Bmim][PF₆] and its in situ templating action to make hollow silica spheres further enhanced the residual activity. Furthermore time dependent kinetics of esterification by hollow silica spheres has also been shown here. Hollow silica spheres can also be used as a reusable catalyst for up to 6 cycles. This work demonstrates that the choice of ionic liquid is critical in controlling the self-assembly of enzymes as the ionic liquid–enzyme interaction plays a major role in retaining capsule activity and enzyme function.