Aggregation-regulated bioreduction process of graphene oxide by Shewanella bacteria

Abstract

The bioreduction of graphene oxide (GO) using environmentally functional bacteria such as Shewanella represents a green approach to produce reduced graphene oxide (rGO). This process differs from the chemical reduction that involves instantaneous molecular reactions. In bioreduction, the contact of bacterial cells and GO is considered the rate-limiting step. To reveal how the bacteriaeGO integration regulates rGO production, the comparative experiments of GO and three Shewanella strains were carried out. Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and atomic force microscopy were used to characterize the reduction degree and the aggregation degree. The results showed that a spontaneous aggregation of GO and Shewanella into the condensed entity occurred within 36 h. A positive linear correlation was established, linking three indexes of the aggregation potential, the bacterial reduction ability, and the reduction degree (ID/IG) comprehensively. © 2024 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved.

1. Introduction

As an important member of graphene-family materials, the reduced graphene oxide (rGO) is renowned for its exceptional properties and diverse applications in biological, environmental, catalytic optoelectronic, and storage domains. Typically, rGO is produced by reducing graphene oxide (GO) using chemical reductants such as NaBH4, N2H4, LiAlH4, etc., or through thermal treatment. However, these methods often lead to water pollution and elevated production cost. Seeking an environment-friendly, safe, and economical alternative, the microbial reduction has emerged as a viable solution, offering ease of handling and high yields. Various bacteria, including Shewanella spp., Escherichia coli, Bacillus sphaericus, Geobacter sulfurreducens, Pseudoalteromonas sp., and the mixotroph of activated sludge, have demonstrated their ability to reduce GO to rGO. Particularly, Shewanella, the model bacteria for studying extracellular electron transport (EET), was first used by Salas et al. in the reduction of GO, making a significant step toward green nanomaterials.

Shewanella spp. can reduce GO mildly and effectively at room temperature under both aerobic and anaerobic conditions, and the electrochemical properties of rGO produced by Shewanella spp. have been proven competitive to those prepared by chemical methods. Unlike chemical reduction, where the soluble reducing molecules are directly added in GO, in microbial reduction, electrons generated by Shewanella cells have been recognized as to have the reducing power to drive extracellular reduction reactions via EET. Thus, to ensure sufficient redox binding sites, the contact state of GO and bacteria at the mm scale could be the rate-limiting step in the overall GO bioreduction process. In previous studies, the bacterial reduction experiments were conducted by mixing GO with bacteria and then being cultured in the incubator. Apart from the appearance change of the solution from light color to dark, a spontaneous aggregation between the GO colloids and suspended cells can be observed, leading to a variety of terminologies (e.g., precipitates, complex, composite, hydrogels, aerogel) used to describe these insoluble aggregates containing rGO-semi-finished products and bacteria. Meanwhile, the correlation of GO-reduction degree and the aggregates formation has remained largely at the level of visual observation and empirical judgment. Although the surface and interfacial physiochemical properties of GO and bacteria have been known to be crucial for their attraction, how interaction forces drive GO bioreduction process is still unclear.