Water-in-oil microemulsion doped with gold nanoparticle decorated single walled carbon nanotube: Scaffold for enhancing lipase activity

The present work reports the development of water-in-oil (w/o) microemulsion doped with newly designed nanocomposite comprising of gold nanoparticle (GNP) decorated single walled carbon nanotube (SWNT). This nanocomposite included cationic reverse micelle was used to boost the catalytic activity of a surface-active enzyme, Chromobacterium viscosum lipase (CV lipase). SWNT was non-covalently dispersed using cetyltrimethylammonium bromide (CTAB), cetylalaninetrimethylammonium chloride (CATAC) while GNP was synthesized by reduction of HAuCl4 with reducing/stabilizing agent trisodium citrate. Counterion exchange between cationic SWNT dispersing agent and anionic capping agent of GNP led to the formation of GNP decorated SWNT (SWNT–GNP) nanocomposite. This newly developed SWNT–GNP included CTAB reverse micelle was characterized by several microscopic and spectroscopic techniques. Interfacially located SWNT–GNP included w/o microemulsion (confirmed from biphasic and fluorescence experiment) was used as a proficient host for enhancing the catalytic activity of lipase. Lipase activity within this self-assembled soft nanocomposite improved up to 3.9-fold (second order rate constant, k2 = 1694 ± 16 cm3 g−1 s−1) compared to standard CTAB reverse micelle (k2 = 433 ± 7 cm3 g−1 s−1). In case of cetyltripropyl ammonium bromide (CTPAB) based reverse micelle, the observed lipase activity improved to k2 = 2036 ± 11 cm3 g−1 s−1 in the presence of SWNT–GNP composite. Notably, this catalytic activity of lipase within SWNT–GNP included reverse micelle was till date the highest activity found in any w/o microemulsion. The attainment of flexibility in enzyme conformation at the augmented interface was verified using circular dichroism (CD) spectroscopy.