Size-Controlled Production of Gold Bionanoparticles Using the Extremely Acidophilic Fe(III)-Reducing Bacterium, Acidocella aromatica

Intan Rizki,Naoko Okibe

doi:10.3390/min8030081

Abstract

Recycling of gold-bearing “urban mine” resources, such as waste printed circuit boards (PCBs), is attracting an increasing interest. Some of the gold leaching techniques utilize acidic lixiviants and in order to eventually target such acidic leachates, the utility of the acidophilic Fe(III)-reducing heterotrophic bacterium, Acidocella (Ac.) aromatica PFBC was evaluated for production of Au(0) bionanoparticles (bio-AuNPs). Au(III) ions (as AuCl4−, initially 10 mg/L), were readily adsorbed onto the slightly-positively charged Ac. aromatica cell surface and transported into cytoplasm to successfully form intracellular bio-AuNPs in a simple one-step microbiological reaction. Generally, increasing the initial concentration of formate as e-donor corresponded to faster Au(III) bioreduction and a greater number of Au(0) nucleation sites with less crystal growth within 40–60 h: i.e., use of 1, 5, 10, or 20 mM formate led to production of bio-AuNPs of 48, 24, 13, or 12 nm in mean particle size with 2.3, 17, 62, and 97 particles/cell, respectively. Addition of Cu2+ as an enzymatic inhibitor significantly decreased the number of Au(0) nucleation sites but enhanced crystal growth of individual particles. As a result, the manipulation of the e-donor concentration combined with an enzyme inhibitor enabled the 3-grade size-control of bio-AuNPs (nearly within a normal distribution) at 48, 26 or 13 nm by use of 1 mM formate, 20 mM formate (+Cu2+) or 10 mM formate, respectively, from highly acidic, dilute Au(III) solutions.

Highlights

Industrial demands for precious-group metals (PGMs), together with the growing needs for cost effectiveness and implementation of green technology are increasing rapidly
Higher formate concentrations led to less noticeable induction time and faster Au(III) bioreduction: At 1, 5, 10, and 20 mM formate, Au(III) was completely reduced within 60, 60, 50, and 40 h, respectively (Figure 1b)
Addition of 5 mM Cu2+ simultaneously with 20 mM formate at 1 h did not show a significantly different trend in soluble Au concentrations, when compared to that without Cu2+ (Figure 1b)

Summary

Introduction

Industrial demands for precious-group metals (PGMs), together with the growing needs for cost effectiveness and implementation of green technology are increasing rapidly. As one of the PGMs, Au is recognized as one of the most expensive metals historically in the world [1]. Industrial demands prefer Au mostly in the form of nanoparticles, rather than in bulk metal, owing to a variety of their unique properties. Au(0) nanoparticles (AuNPs) have been applied to many different applications across the field of biology and medicine (e.g., labeling, bio-imaging, drug delivery [2,3]), environment (e.g., pollution control and water purification [4], catalysts for green chemistry [5,6], nanobiosensors in food and agriculture industry [7,8]), and technology (e.g., improving computer memory [9])

Objectives

Methods

Results