High-Yield Synthesis of Complex Gold Nanostructures in a Fungal System

In this work, gold nanocrystals with various shapes and sizes, including spherical, regular nanoplates, spiral nanoplates, nanowalls, lamellar nanoagglomerates, and spherical nanoagglomerates, were produced at high yield by reacting an aqueous solution of chloroauric acid with the mycelia-free spent medium or the extract of a filamentous fungus, Aspergillus niger . Structural characterizations showed that the plate-like structures were oriented with {111} planes as the basal plane. The results implicated proteins on the fungal cell walls and proteins in the fungal extract as the primary biomolecules involved in the synthesis of gold nanocrystals. Reaction in the spent medium, on the other hand, involved an enzymatic process where small biomolecules and enzymes served as the substrates and catalysts, respectively. The formation of these highly anisotropic structures was kinetically controlled, as shown by the dependence of particle shape and size on environmental variables such as the temperature and pH of the solution.