In vivo synthesis of semiconductor nanoparticles in Azotobacter vinelandii for light-driven ammonia production

Abstract

Ammonia (NH₃) is an important commodity chemical used as an agricultural fertilizer and hydrogen-storage material. There has recently been much interest in developing an environmentally benign process for NH₃ synthesis. Here, we report enhanced production of ammonia from diazotroph under light irradiation using hybrid composites of inorganic nanoparticles (NPs) and bacteria cells. The primary focus of this study lies in the intracellular biosynthesis of semiconductor NPs within Azotobacter vinelandii, a diazotroph, when bacteria cells are cultured in a medium containing precursor molecules. For example, enzymes in the bacterial cells, such as cystein desulfurase, convert cysteine (Cys) into precursors for cadmium sulfide (CdS) synthesis when supplied with CdCl₂. Photoexcited charge carriers in the biosynthesized NPs are transferred to nitrogen fixation enzymes, e.g., nitrogenase, facilitating the production of ammonium ions. Notably, the intracellular biosynthesis approach minimizes cell toxicity compared to extracellular synthesis due to the diminished generation of reactive oxygen species. The biohybrid system based on the in vivo approach results in five-fold increase of the ammonia production (0.45 mg g_DCW^-1 h^-1) compared to the case of diazotroph cells only (0.09 mg g_DCW^-1 h^-1).