Effect of Surface and Bulk Properties of Mesoporous Carbons on the Electrochemical Behavior of GOx-Nanocomposites

ORIGINAL RESEARCH published: 19 February 2019 doi: 10.3389/fchem.2019.00084 Effect of Surface and Bulk Properties of Mesoporous Carbons on the Electrochemical Behavior of GOx-Nanocomposites Tsai Garcia-Perez 1 , Shouzhen Hu 1 , Youngho Wee 2 , Louis Scudiero 3 , Conrad Hoffstater 1 , Jungbae Kim 2* and Su Ha 1* 1 School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, United States, Department of Chemical and Biological Engineering, Korea University, Seoul, South Korea, 3 Department of Chemistry and Materials Science and Engineering Program, Washington State University, Pullman, WA, United States 2 Edited by: Nosang Vincent Myung, University of California, Riverside, United States Reviewed by: Aihua Liu, Qingdao University, China Dunfeng Gao, Fritz-Haber-Institut, Germany Wook Ahn, Soonchunhyang University, South Korea *Correspondence: Jungbae Kim Su Ha Specialty section: This article was submitted to Electrochemistry, a section of the journal Frontiers in Chemistry Received: 12 October 2018 Accepted: 31 January 2019 Published: 19 February 2019 Citation: Garcia-Perez T, Hu S, Wee Y, Scudiero L, Hoffstater C, Kim J and Ha S (2019) Effect of Surface and Bulk Properties of Mesoporous Carbons on the Electrochemical Behavior of GOx-Nanocomposites. Front. Chem. 7:84. doi: 10.3389/fchem.2019.00084 Frontiers in Chemistry | www.frontiersin.org Biofuel cell (BFC) electrodes are typically manufactured by combining enzymes that act as catalysts with conductive carbon nanomaterials in a form of enzyme-nanocomposite. However, a little attention has been paid to effects of the carbon nanomaterials’ structural properties on the electrochemical performances of the enzyme-nanocomposites. This work aims at studying the effects of surface and bulk properties of carbon nanomaterials with different degrees of graphitization on the electrochemical performances of glucose oxidase (GOx)-nanocomposites produced by immobilizing GOx within a network of carbon nanopaticles. Two types of carbon nanomaterials were used: graphitized mesoporous carbon (GMC) and purified mesoporous carbon (PMC). Graphitization index, surface functional groups, hydrophobic properties, and rate of aggregation were measured for as-received and acid-treated GMC and PMC samples by using Raman spectrometry, X-ray photoelectron spectroscopy (XPS), contact angle measurement, and dynamic light scattering (DLS), respectively. In addition to these physical property characterizations, the enzyme loading and electrochemical performances of the GOx-nanocomposites were studied via elemental analysis and cyclic voltammetry tests, respectively. We also fabricated BFCs using our GOx-nanocomposite materials as the enzyme anodes, and tested their performances by obtaining current-voltage (IV) plots. Our findings suggest that the electrochemical performance of GOx-nanocomposite material is determined by the combined effects of graphitization index, electrical conductivity and surface chemistry of carbon nanomaterials. Keywords: graphitized mesoporous carbon, graphitization index, hydrophobic properties, biofuel cells, glucose oxidase, enzymatic nanocomposites INTRODUCTION Self-powered implantable devices such as deep brain neurostimulators, pacemakers, and biosensors for environmental monitoring have enormous potential in medical, agricultural or even military applications (Falk et al., 2012; Katz, 2013). Biofuel cells (BFCs) can be an alternative portable power solution to batteries for powering these devices, due to their capability to continuously convert the chemical energy from organic fuels, such as glucose in fruits or human blood, into electricity 1 February 2019 | Volume 7 | Article 84 Garcia-Perez et al. Electrochemical Behavior of GOx-Nanocomposite (Katz, 2013; MacVittie et al., 2015). Enzymatic BFCs use (1) enzymes to catalyze both oxidation of organic fuels and reduction of oxidizing agents, and (2) conductive materials (such as carbon nanomaterials) to transmit the electrons between the enzymes’ active sites and the electrodes. Thus, the physical properties of both materials—enzymes and nanomaterials—play a key role in the BFC’s electrochemical performances. However, to the best of our knowledge, little attention has been paid to the effects of the carbon nanoparticle’s surface and bulk properties on the overall electrochemical performance of the enzyme electrodes. Graphitized mesoporous carbons (GMC) and purified mesoporous carbons (PMC) are two types of mesoporous carbon materials with similar chemical composition and morphological properties, but different surface and structural properties. This contrast on the properties of GMC and PMC makes them the ideal carbon nanomaterials for investigating the effects of carbon nanomaterials’ properties on the electrochemical performances of BFCs. Graphitized and non-graphitized carbons are structurally different (Franklin, 1951). The graphitization process of the carbon is a method to produce well-organized graphite (Mattia et al., 2006). Non-graphitized carbons exhibit a cross-linked structure where graphitic structures are randomly oriented in a rigid mass. Conversely, graphitized carbons present a compact structure where the graphite layers have a nearly parallel orientation. Graphite layers play a major role in both the surface and bulk properties of these materials. The hydrophobicity and electrical conductivity of the carbon materials, for example, are directly related to the level of graphitization of the carbon materials (Pantea et al., 2003). Hydrophobicity also affects the nanoparticle aggregation process (Nel et al., 2009). It is known that the surface of graphitized materials presents smaller amounts of oxygen functionalities compared to that of non-graphitized materials, which strongly repels water molecules (due to their hydrophobic nature) and decreases electrostatic repulsion among the nanoparticles. Consequently, these nanomaterials can form a compact carbon network with low dispersion in an aqueous medium. These unique properties of nanomaterials have been used to physically entrap large enzyme aggregates within the carbon networks and to form protein-nanocomposite materials (Garcia-Perez et al., 2016). This observation suggests that the performance of this hybrid protein-nanoparticle composite structure highly depends on the graphitization index of the carbon nanomaterial used as the enzyme support. Literature shows that the GMC sample can be used to entrap enzymes to build bioelectrodes (Garcia-Perez et al., 2016; Walcarius, 2017), although no information has been reported in the literature on employing the PMC sample for electrochemical applications. This work aims to study the effects of the bulk and the surface properties of different carbon nanomaterials on the electrochemical performances of glucose oxidase (GOx)nanocomposite bioanode materials under the BFC operation mode. For the present study, a homemad (...truncated)