Immobilized enzyme mediated synthesis of silver nanoparticles using cross-linked enzyme aggregates (CLEAs) of NADH-dependent nitrate reductase

NADH-dependent nitrate reductase CLEAs catalyzed synthesis of silver nanoparticles from silver nitrate has been investigated. NADH-dependent nitrate reductase CLEAs catalyzed reduction of silver ions to silver nanoparticles at pH 7.2 using NADH as electron source and 8-hydroxyquinoline as electron shuttle. Fourier transform infrared spectroscopy (FTIR) and Energy dispersive spectroscopy (EDS) showed that 8-hydroxyquinoline also acts as capping agent of silver nanoparticles. The effects of reaction time, reaction temperature, silver nitrate concentration and amount of CLEAs on silver nanoparticles synthesis were studied with aid of UV-Vis spectrophotometer. Optimum silver nanoparticles synthesis occurred after 24 h reaction of 100 mg CLEAs with 1 mM silver nitrate at pH 7.2 and temperature 35 °C. Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and High resolution transmission electron microscopy (HRTEM) showed the silver nanoparticles were spherical in shape. The particle size calculated using Dynamic light scattering measurements (DLS) was found to be in the range of 5-7 nm. X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns confirmed formation of crystalline silver nanoparticles with face-centred-cubic (fcc) lattice structure. Notably, nitrate reductase CLEAs showed 80% catalytic yield even after 5 cycles of repeated use in silver nanoparticle synthesis at pH 7.2 and temperature 35 °C. Moreover, silver nanoparticles of same morphology and dimensions were formed in each cycle.