Identifying efficient natural bioreductants for the preparation of graphene and graphene-metal nanoparticle hybrids with enhanced catalytic activity from graphite oxide

Abstract

The implementation of green approaches towards the preparation of graphene and graphene-based materials with enhanced functionality from graphite oxide has been relatively little explored. Particularly, the use of bioreductants and the testing of their relative efficacies is an incipient area of research. Here, a pool of 20 environmentally friendly, natural antioxidants have been tested for their ability to reduce graphene oxide. These antioxidants were mostly vitamins, amino acids and organic acids. By establishing a protocol to systematically compare and optimize their performance, several new efficient bioreductants of graphene oxide have been identified, namely, pyridoxine and pyridoxamine (vitamin B₆), riboflavin (vitamin B₂), as well as the amino acids arginine, histidine and tryptophan. These biomolecules were used to prepare reduced graphene oxide–silver nanoparticle hybrids that displayed colloidal stability in water in the absence of additional dispersants. Particularly, hybrids prepared with pyridoxamine exhibited a combination of long-term colloidal stability and exceptionally high catalytic activity among silver nanoparticle-based catalysts in the reduction of p-nitrophenol with NaBH₄. Thus, in addition to expanding substantially the number of green reductants available for graphene oxide reduction, the present results underline the idea that proper selection of bioreductant can be relevant to achieve graphene-based materials with improved performance.

Introduction

Since its first isolation in 2004, graphene has aroused a great deal of interest in the scientific community, and particularly in the materials research realm, given that its exceptional physical properties promise outstanding performance in a large number of potential applications [1], [2], [3]. However, an obvious prerequisite for the practical use of graphene is the availability of suitable methods that allow its mass production and processing. In this regard, one of the most promising and hitherto widely investigated approaches relies on the use of graphite oxide, which typically comprises the exfoliation of this strongly oxygenated derivative of graphite in aqueous or organic medium followed by chemical reduction of the resulting single-layer sheets (graphene oxide sheets) [4], [5], [6]. Hydrazine and some of its derivatives, and later on other reagents commonly used in organic chemistry for the reduction of oxygen-containing groups (e.g., LiAlH₄), were found to be highly effective reductants for graphene oxide, and are still broadly employed today for this purpose [4], [5], [6], [7], [8], [9], [10]. Nevertheless, the high toxicity and environmental hazard of these reagents have recently driven a move towards the exploration of safer alternatives, to better comply with the principles of green chemistry and sustainability. Thus, solvothermal, electrochemical, catalytic [11] and photocatalytic [12], [13], [14], [15], [16], [17] methods as well as the use of environmentally friendly reductants have been proposed during the last two years for the reduction of graphene oxide [18].

Vitamin C was the first green reducing agent that was reported to be effective in the deoxygenation of graphene oxide, affording dispersions of reduced graphene oxide (RGO) sheets in water and some organic solvents whose extent of reduction and colloidal stability matched those of dispersions obtained with hydrazine [19], [20], [21], [22]. Such discovery suggested that other innocuous and abundant biomolecules could also act as efficient reductants in the preparation of RGO. Studies carried out subsequently confirmed indeed that certain saccharides (glucose, fructose and sucrose) and polysaccharides (dextran) [23], [24], [25], polyphenols (e.g., epigallocatechin gallate and related compounds present in tea solutions, or tannic acid) [26], [27], [28] and natural phenolic acids [29], [30], plant extracts [31], a catecholamine polymer [poly(norepinephrine)] [32], the hormone melatonin [33], [34], a tripeptide (l-gluthatione) [35], a protein [bovine serum albumin (BSA)] [36], and bacteria [37], [38] could reduce graphene oxide.

However, the number of biomolecules available at present for the reduction of graphene oxide is still relatively limited, and having a wider range of effective bioreductants to call upon for specific purposes would be highly desirable. There is a large number of natural antioxidants, including amino acids, vitamins and organic acids whose reducing ability towards graphene oxide and potential role in enhancing the functionality of the resulting material is unknown and unexplored. It has been shown in some cases that the use of selected biomolecules lends added value to RGO other than just acting as reductants of the starting graphene oxide sheets. For example, dextran and tea polyphenols have been reported to increase the colloidal stability in water and/or biocompatibility of RGO [24], [26]. However, other potential benefits of using reducing biomolecules in the preparation of graphene-based materials that could be relevant for applications have been, to the best of our knowledge, far less explored [32].

In the present work, (i) we identify a number of small biomolecules (amino acids and vitamins) as efficient bioreductants of grapheme oxide dispersions, and (ii) significantly, we demonstrate that proper selection of specific bioreductants affords graphene-based materials with enhanced performance towards relevant uses. First of all, a set of 20 biomolecules were investigated for their potential use in the reduction of aqueous graphene oxide dispersions. These biomolecules were mainly chosen on the basis of their known reducing and/or antioxidant activity in the biochemical context, although their reducing power towards graphene oxide was unknown. Several of the tested biomolecules were shown to be efficient reductants of graphene oxide. Then, the previously identified bioreductants were used for the preparation of RGO-silver nanoparticle (Ag NP) hybrids. Furthermore, the use of selected biomolecules was shown to afford hybrids that exhibited an attractive combination of long-term colloidal stability in water and exceptionally high catalytic activity, as gauged from the study of a model reaction (reduction of p-nitrophenol to p-aminophenol), making them ideal candidates for practical uses.