Electron transfer study on graphene modified glassy carbon substrate via electrochemical reduction and the application for tris(2,2′-bipyridyl)ruthenium(II) electrochemiluminescence sensor fabrication

Highlights

• Electron transfer of attached graphene via electrochemical reduction was studied.

• The attached graphene was with standing configuration on the electrode.

• Ru(bpy)₃²⁺ was used as the redox probe to evaluate the electron transfer.

• The Electron transfer is much faster than that of tiled graphene modified GCE.

• Stable Ru(bpy)₃²⁺ ECL sensor was fabricated with the standing graphene.

Abstract

In this study, electron transfer behavior of the graphene nanosheets attachment on glassy carbon electrode (GCE) via direct electrochemical reduction of graphene oxide (GO) is investigated for the first time. The graphene modified electrode was achieved by simply dipping the GCE in GO suspension, followed by cyclic voltammetric scanning in the potential window from 0 V to −1.5 V. Tris(2,2′-bipyridyl)ruthenium(II) [Ru(bpy)₃²⁺] was immobilized on the graphene modified electrode and used as the redox probe to evaluate the electron transfer behavior. The electron transfer rate constant (Ks) was calculated to be 61.9±5.8 s⁻¹, which is much faster than that of tiled graphene modified GCE (7.1±0.6 s⁻¹). The enhanced electron transfer property observed with the GCE modified by reductively deposited graphene is probably due to its standing configuration, which is beneficial to the electron transfer comparing with the tiled one. Because the abundant oxygen-containing groups are mainly located at the edges of GO, which should be much easier for the reduction to start from, the reduced GO should tend to stand on the electrode surface as evidenced by scanning electron microscopy analysis. In addition, due to the favored electron transfer and standing configuration, the Ru(bpy)₃²⁺ electrochemiluminescence sensor fabricated with standing graphene modified GCE provided much higher and more stable efficiency than that fabricated with tiled graphene.

Introduction

Ever since its discovery in 2004 [1], graphene has made a profound impact in many areas of science and technology especially for providing new approaches and critical improvements in electrochemistry due to its remarkable physicochemical properties, such as extraordinary electronic properties, electron transport capabilities, strong mechanical strength, excellent thermal and electrical conductivities. Like other nanocarbon family members, graphene has been extensively applied in drug delivery systems [2], [3], [4], [5], catalysis [6] and electroanalytical chemistry (EC) [7], [8], [9], [10], [11], [12], [13]. In the EC applications, attachment of graphene on an electrode can be achieved via either covalent bonding or non-covalent interactions [14], [15], [16]. The former is usually realized via the esterification or amidation reaction through the residual functional groups or by the destruction of the unsaturated double bonds. However, the covalent attachment of graphene usually destroys some of graphene׳s conjugated structure, which might greatly compromise graphene׳s superb properties [14]. Therefore, non-covalent modification methods are preferred when preservation of graphene׳s original properties is expected [11], [17], [18]. Recently, Liu and co-workers have reported the manipulation of the conductivity of graphene papers, at the molecular level, via either covalent bonding or non-covalent π–π stacking interactions using either monofunctional or bifunctional molecules [14]. A multilayered glucose sensor with enhanced detection limit and sensitivity was also successfully fabricated via layer-by-layer immobilization of graphene and pyrene-modified glucose oxidase through π–π stacking interaction [13]. It was believed that the graphene nanosheets could act as good relays for promoting the electron communication between the active center of the enzyme and the electrode, a phenomenon that was also observed by us and others [19], [20]. In our previous work, graphene has also been demonstrated to be able to open pathways for electrical communication through well-passivated monolayers. The non-covalently immobilized graphene on pyrene terminated monolayer could significantly affect the electron transfer between pyrene and the underlying electrode [10].

The preparation of graphene can be very versatile, micro-mechanical stripping [1], matrix-assisted exfoliation [21], chemical vapor deposition [22], [23] and chemical reduction of graphene oxide (GO) [24], [25] are most frequently utilized. Electrochemical reduction of GO is a new and promising green strategy for graphene synthesis and has drawn tremendous attention [26], [27], [28]. For instance, graphene nanosheets can be directly deposited onto a glassy carbon electrode (GCE) through cyclic voltammetric reduction of a GO colloidal solution [29]. A facile approach to the synthesis of high quality graphene nanosheets in large scale through electrochemical reduction of exfoliated GO precursor at cathodic potentials was also reported by Chen et al. [30]. This approach opens up the possibility for assembling graphene biocomposites for electrocatalysis and the construction of EC biosensors. Despite the physical conductivity of graphene and graphene conductive films have been widely studied [31], [32], [33], [34], [35], the study of electron transfer through graphene sheets or between graphene layers has not been reported.

In recent years, ruthenium(II) tris(bipyridine) [Ru(bpy)₃²⁺] has been extensively explored for electrochemiluminescence (ECL) applications due to its superior properties including high sensitivity and stability under moderate conditions in aqueous solution [36]. Considerable attention has been paid to immobilize Ru(bpy)₃²⁺on an electrode surface via sol–gel technique [37], ion-exchanging [38], [39] and self-assembly of the derivatives of Ru(bpy)₃²⁺ [40], [41]. In addition, Ru(bpy)₃²⁺ is consisted of 4d transition state electron and π-orbital-rich groups which allow its immobilization on graphene surface via π-π interaction. π–π stacking interactions usually occur between two relatively large non-polar aromatic rings having overlapping π orbitals. Most importantly, modification via π–π stacking does not disrupt the conjugation of the graphene sheets, which preserves the electronic properties of graphene. Thus, Ru(bpy)₃²⁺ can be used as the redox probe to evaluate the electron transfer behavior of graphene deposited on the electrode. In addition, graphene has been widely explored as electrode modification material to form ion-exchanging composite film for developing solid-state Ru(bpy)₃²⁺ ECL sensors [42], [43]. However, solid-state Ru(bpy)₃²⁺ ECL sensor fabricated with pure electrochemical reduced GO has been scarcely reported and its efficiency compared with other graphene-related ECL sensors has not been studied.

Herein, GO was directly deposited on GCE via electrochemical reduction to afford standing graphene, through which the electron transfer behavior was investigated using Ru(bpy)₃²⁺ immobilized via π–π interaction as redox probe. As a comparison, the electron transfer through the tiled graphene modified electrode was also studied. Meanwhile, the immobilization of Ru(bpy)₃²⁺ on both the standing and the tiled graphene could afford platforms as solid-sate ECL sensors, whose efficiency has also been investigated and compared. The stepwise modified electrodes were also characterized using cyclic voltammetry (CV), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS).