Electrocatalytic Sensing with Reduced Graphene Oxide: Electron

Sep 21, 2016 - These stored electrons can then be transported to oxidized Fc during the anodic scan through the C–C network of RGO. The recycling an...
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Electrocatalytic Sensing with Reduced Graphene Oxide: Electron Shuttling between Redox Couples Anchored on a 2-D Surface Victoria L Bridewell, Christopher J. Karwacki, and Prashant V. Kamat ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00377 • Publication Date (Web): 21 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016

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Electrocatalytic Sensing with Reduced Graphene Oxide: Electron Shuttling between Redox Couples Anchored on a 2-D Surface Victoria L. Bridewell1,2 Christopher J. Karwacki3, Prashant V. Kamat1,2* 1,2

Radiation Laboratory and the Department of Chemistry & Biochemistry University of Notre Dame, Notre Dame, IN 46556, United States and

3

Edgewood Chemical Biological Center, U.S. Army Research, Development and Engineering Command, 5183 Blackhawk Road, APG, MD 21010

1

Notre Dame Radiation Laboratory

2

Department of Chemistry & Biochemistry

3

U.S. Army Research

*Address correspondence to [email protected]

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Abstract The electron storage and shuttling capabilities of reduced graphene oxide (RGO) have been explored by anchoring two redox couples, methyl viologen (MV2+) and ferrocene (Fc). When an RGO modified glassy carbon electrode (RGO/GCE) was subjected to a cathodic scan, a quasireversible reduction of MV2+ was seen indicating a loss of electrons contributed to “charging” of RGO. These stored electrons can then be transported to oxidized Fc during the anodic scan through the C-C network of RGO. The recycling and peak current magnitude of oxidized and reduced forms of Fc during the anodic scan is strongly dependent on the scan rate, concentration and extent of MV2+ reduction, either complete or partial, during the cathodic scan. This electrocatalytic property of RGO film enables the design of sensors and catalysts with the capacity to capture, store and shuttle electrons and corroborate a boost in sensitivity for the electrochemical detection and conversion of low level analytes.

Keywords: graphene oxide, electrocatalysis, electron shuttling, 2-D materials, charge transfer, signal enhancement

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Graphene, a single-layer carbon two-dimensional (2-D) nanosheet with honeycomb lattice structure, has received a great deal of attention in scientific and engineering fields harking back to its emergence since 2004.1–6 This atomically simplistic material exhibits many extraordinary properties such as large surface area, excellent conductivity, and high flexibility thereby revolutionizing the scope and design of composite based materials.7–11 These materials have been extensively utilized to improve the performance of electrodes employed in electrochemical storage,12–16 biological17–19 and environmental electrochemical analysis20–22 Moreover, the coupling of graphene with a catalyst proposes the possibility of fabricating simple, new multicomponent composite materials that exhibit synergistic physicochemical properties towards solar fuel production.23 This same approach can also be applied for enhanced electrochemical detection and catalysis as presented in this study. Simple solution processable graphene oxide (GO) provides a convenient, large scale synthetic method of producing single-layer 2-D materials 23–25 with structural defects. This functionalization can then lead to enhanced sensitivity and selectivity;26,27 however, during chemical exfoliation, the aromatic sp2 network is disrupted leading to a loss in conductivity and π-π interactions with partial restoration achieved via thermal28, chemical29, photocatalytic30 or electrochemical reduction31. In previous studies, we have exploited and demonstrated the electron shuttling capability of photocatalytically reduced graphene oxide (RGO) sheets by anchoring semiconductor and metal nanoparticles.32 In addition, semiconductor-graphene-metal film composites have also been used to detect low level contaminants33 for detection by SERS. The promising field of RGO nanocomposites for sensing and energy applications is dependent upon a sound understanding of the fundamental processes governing these electronic interactions. In the present study, we have investigated RGO composites for control of electron capture, transport and discharge through the 2-D carbon network. By anchoring two redox active species, methyl viologen (MV2+) and ferrocene (Fc), and monitoring the electron transfer between the two via cyclic voltammetry (CV), we have explored the electrocatalytic activity of RGO. Exploring such an electrocatalytic property of RGO can provide an elevated understanding of the underlying electrochemical phenomenon and aid in the design of electrochemical sensors and new hybrid electrode materials with enhanced sensitivity. Results and Discussion Through exploitation of the large surface area and conductive properties of RGO, designing improved low cost, high throughput electrode materials are possible. GO was synthesized following the modified Hummer method34 with additional details and characterization in the supporting information. Previous work has shown that suspended RGO sheets become negatively charged and can then be driven towards a positive electrode when subjected to an externally applied electric field.35 Further exploiting RGO surface chemistry, simplistic adsorption of MV2+ and Fc was accomplished via electrostatic and π-π interactions previously shown.36 In the 3 ACS Paragon Plus Environment

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present work, we modified a glassy carbon electrode (GCE) with an RGO film using electrophoretic deposition methodology (GCE/RGO). The thickness and morphology of the RGO film can be controlled by simple adjustment of the RGO suspension concentration, applied

Figure 1. Cyclic voltammograms of 50 µM MV2+ using (A) pristine GCE and (B) RGO/GCE working electrodes with arrow indicating loss in peak current from 1 to 5 consecutive scans. (C) CV of 25 µM Fc at GCE and (D) RGO/GCE following the addition and pre-step charging of 50 µM MV2+. CV Conditions: 0.1 M TBAP /acetonitrile at scan rate 0.05 V/s vs Ag/AgCl under N2 for 5 consecutive cycles.

voltage or duration of electrophoresis time. Figure 1A-B show the voltammagrams of MV2+ reduction at GCE and GCE/RGO. At bare GCE, reversible reduction of MV2+ is observed at -280 mV vs. Ag/AgCl corresponding to MV+• generation (reaction 1). MV2+ + e  MV+•

(1)

However at the GCE/RGO, quasi-reversible behavior was observed via the loss of the reduction peak intensity. During consecutive voltage scans the reduction peak potential for reaction 1 decreases suggesting that only a fraction of initial MV+• formed during reduction is able to be reoxidized. This behavior suggests that the RGO sheets play a role in the reoxidation of MV+•. Given the electron storage and shuttling properties of RGO, we can expect a fraction of the electrons from MV+• are transferred to RGO and get stored within its network (reaction 2). MV+• + RGO  MV2+ + RGO (e)

(2)

Such electron storage property of nanocarbon materials has been observed in earlier studies.38–40 We have demonstrated that RGO is capable of accepting electrons from photochemically 4 ACS Paragon Plus Environment

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generated MV+• in a solution (Figure S7) via the loss in absorbance at 600 nm with increasing concentration of RGO. Furthermore, the cations present in the CV solution facilitate stabilization of such transferred electrons as demonstrated in the electrochemical reduction of C60 films.41 Figure 1C shows anodic CV scans following the introduction of Fc to the same solution with peaks at 0.60 and 0.53 V vs Ag/AgCl at a GCE (reaction 3). Fc – e  Fc+

(3)

The reversible redox peaks seen for MV2+/MV+• and Fc/Fc+ using GCE electrodes are independent of each other and show no noticeable distortion (Figure 1A and C). As expected, reaction 3 represents normal electrochemical reaction (E) without any additional processes. When the GCE is replaced with GCE/RGO, a different pattern emerges. GCE/RGO system exhibits a larger Fc+ oxidation peak current in the first scan than the one seen during the reverse scan when the scan between cathodic and anodic regions (-0.5  +1.0 V) was extended (Figure 2). The observation of a larger oxidation peak for Fc during the anodic scan after scanning MV2+/MV+• indicates that Fc+ is reduced by stored electrons within the RGO network contributing to the regeneration of Fc (reaction 4). Fc+ + RGO (e)  Fc + RGO

(4)

Reaction 4 thus represents a catalytic reaction (C) that follows the electrochemical processes of reactions (2) and (3). To further analyze the reproducibility and cooperative behavior between MV2+/MV+• and Fc/Fc+ couples at the GCE/RGO electrode, cyclic voltammograms at varying scan rates were recorded (Figure 2). The peak currents increase linearly with scan rate while maintaining asymmetric peak shapes between forward and reverse cycles (Figure S3). The voltammograms follow the same pattern at all scan rates and exhibit reversibility during the voltage scan over the wide potential range. Multiple scans recorded at a fixed scan rate also show reproducibility (Figure S4). These observations Figure 2. Cyclic voltammogram of 250 µM further ascertain that the overall reversibility of the of MV2+ and Fc in 0.1 M TBAP/acetonitrile system is only maintained when MV2+ and Fc are on GCE/RGO at increasing scan rates (5- coupled together through electron shuttling via the C200 mV/s) under N2 vs Ag/AgCl. C network of RGO.42

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The recycling of Fc+ to Fc continues until all the RGO(e) generated by MV2+ reduction are consumed. Such recycling of redox couples to increase the sensitivity of electrochemical detection has been demonstrated by employing two closely spaced electrodes in a microfluidic cell.43 These results suggest that reactions (1) and (3) dictate the reduction of MV2+ and oxidation of Fc+ respectively at a GCE electrode. When the GCE is modified with RGO, all four pathways are observed (reactions 1-4) and are contributing to the overall electrochemical process. The electron storage capability of RGO is thus responsible for the observed electrocatalytic behavior in the oxidation of Fc+ by acting as a mediator to shuttle electrons between the two redox couples (Scheme 1). Blank experiments carried out with GCE show normal reversible CV behavior for all couples (Fig. S5). Similarly, in the presence of O2 both in the same, as well as separate electrolyte solutions, increased Fc oxidation peak currents are still observed with GCE/RGO electrode (Fig. S6).

Scheme 1. Proposed path of methyl viologen initiated electron transfer, storage and shuttling through RGO to ferrocene.

To further probe the extent of adsorption and determine behavior at the RGO interface, rotating disk CV with and without RGO modification were carried out (Figure 3). In a diffusion limited process, the reduced MV2+ at the electrode interface is swept away, thus producing a rotation speed dependent limiting current (Figure 3A).44 This show the electrode response is dictated by diffusion of MV2+ to and away from the GCE electrode. In the case of GCE/RGO, the CV response mimics that of the adsorbed species, independent of rotating speed, because they’ve

Figure 3. Rotating disc CV at (A) GCE with 250 µM of MV2+exhibiting limiting current as a function of rotation speed. (B) Rotating disc CV of 250 µM of MV2+ and Fc0 on GCE/RGO at varying rotation speeds with no limiting currents. CV conditions: acetonitrile/0.1 M TBAP and 0.1 V/s under N2. ACS Paragon Plus Environment

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since become anchored on the RGO film, (Figure 3B). This surface adsorption behavior in turn enables electron shuttling between the two couples. By varying the degree of MV2+ reduction, it is possible to control electron storage in the RGO network in turn dictating the extent of recycling of Fc during oxidation (Figure 4A). By simply integrating both the MV2+ reduction peak and the Fc oxidation peak separately, the net charge consumed between Fc oxidation (QFc0 /Fc+ ) and MV2+reduction (QMV2+/MV+• ) can be quantified (Figure 4B). The linear dependence between these two electrochemical processes confirms interdependence between Fc oxidation and MV2+ reduction in this system. The efficiency of electrocatalyic oxidation of Fc by MV2+ via RGO is 60 % as realized from the slope of the linear plot in Figure 4B. The experiments in Figure 4 not only establish the interdependence of the two processes mediated on an RGO film, but also show the extent (or

Figure 4. Cyclic voltammetry of 500 µM of MV2+ and Fc in 0.1M TBAP/acetonitrile at 0.05 V/s under N2 on GCE/RGO (A) partially reducing MV2+ and (B) the resulting integrated charge (QFc0/Fc+ ) of Fc as a function of integrated MV2+ charge (QMV2+ /MV+• ).

efficiency) with which these processes can be modulated. This interdependence of redox behavior between the two couples can be further exploited to design RGO-based electrochemical sensors with enhanced sensitivity. Since the oxidation of Fc can be modulated by controlling the MV2+ reduction it should be possible to increase the peak current of Fc oxidation by increasing the loading of MV2+. We estimated a Sensitivity Factor (S) as the ratio of the Fc oxidation peak current measured with and without the counterpart, MV2+ (expression 5).

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S = ip (RGO/MV2+ )/ip(RGO)

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(5)

We measured the anodic peak current of Fc oxidation at a constant Fc concentration with increasing MV2+. The dependence of S on the MV2+ concentration added is shown in Figure 5 by an increase in the anodic peak current with increasing MV2+ loading. The loading of MV2+ becomes a limiting factor due to the availability of adsorption sites on RGO as well as MV2+ maintaining an adsorption equilibrium. A maximum S of 7 was obtained in the present case, thus, allowing the detection of Figure 5. Dependence of Fc peak enhancement Fc at concentrations below 3 ppm and (Sensitivity Factor) on the concentration of MV2+. greatly improving its detection by coupling Signal enhancement of 15 µM Fc0 was recorded with the redox process with MV2+ reduction. stepwise increase of MV2+ concentration during cyclic voltammetry experiment (0.1 M TBAP/acetonitrile This proves that the ability of RGO to shuttle electrons between the two redox solution, scan rate at 0.05 V/s, under N2 atmosphere). species is the key towards boosting the sensitivity of electrochemical detection. Although the example selected here for demonstrating enhanced detection sensitivity is a common Fc redox couple, the principle of detection can be expanded to other electroactive species by merely pre-charging RGO. In summary we have explored the electrocatalytic property of reduced graphene oxide in establishing the communication between two redox couples. By adsorbing methyl viologen and ferrocene onto reduced graphene oxide modified glassy carbon electrodes, electron transfer efficiencies of 60% and sub ppm concentration detections of the analyte have been achieved. This cooperative effect between two redox couples on an RGO platform can be further utilized to increase the sensitivity of electrochemical detection of electroactive species with efforts underway to extend this concept towards detection of low levels contaminants in the environment.

Acknowledgments. The research described here was supported by the Army Research Office through the award ARO 64011-CH. VB acknowledges the partial graduate fellowship support received through the University of Wisconsin Materials Research Science and Engineering Center, National Science Foundation Grant DMR-1121288. The Notre Dame Radiation Laboratory is supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences under Award

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Number DE-FC02-04ER15533. This is a document number 5126 from the Notre Dame Radiation Laboratory. Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at: List of materials, detailed reduced graphene oxide synthesis and characterization, RGO/GCE electrode fabrication, experimental conditions for stationary and rotating disc CV, and spectroscopic analysis of electron transfer and storage of MV+● and RGO.

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For TOC use only:

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