GNP confinement at the interface of cationic reverse micelles: influence in improving the lipase activity

Abstract

The present work reports thiol-assisted confinement of gold nanoparticles (GNPs) at the interface of reverse micelles with the aim to enhance the interfacial area and thereby the efficiency of surface-active Chromobacterium viscosum lipase. The strong gold capping ability of optimally hydrophobic thiols (1-dodecanethiol and 1,6-hexanedithiol) was aptly utilized to pull GNPs (∼3–5 nm) from the water pool to the oil/water interface of cetyltrimethylammonium bromide (CTAB) reverse micelles. These small sized GNPs were fitted at the microscopic interface of CTAB reverse micelles possibly because of the comparable thickness of the interface (∼1–2 nm) to that of the GNP diameter. Lipase solubilized within this augmented interface enjoys a flexible conformation, which resulted in the improvement of its activity (∼2.5 fold) with respect to only CTAB microemulsion. The activity of lipase within CTAB reverse micelles was thoroughly studied in the presence of mono and dithiols with varying chain length, where a greater improvement in activity was observed with dithiols. Bidentate ligand property of dithiols led to firm localization of higher number of GNPs at the interface which enhanced the total space in vicinity of enzyme at the interfacial domain. Fitting fusion of small sized GNPs within CTAB reverse micellar interface was confirmed by microscopic and spectroscopic studies. Smooth localization of lipase at the enhanced interface was also confirmed from the improvement in its secondary structure (α-helical content) in circular dichroism spectroscopic analysis. Interestingly, large sized GNPs (∼8 and 20 nm) were found to be well fitted at the interface of bigger head group-containing surfactants, cetyltriethylammonium bromide (CTEAB) and cetyltripropylammonium bromide (CTPAB). The hydrolytic efficiency of lipase in 1,6-hexanedithiol included GNP (∼20–25 nm)-doped CTPAB reverse micelles improved by ∼3.4 fold compared to that observed in only CTAB.