Journal of Alloys and Compounds 691 (2017) 220e229
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Development of graphite/silicone composites for use as flexible electrode materials ^a a, Grasielli Correa de Oliveira a, Ana Luísa Silva a, Matheus Millen Corre Pedro Pablo Florez-Rodriguez b, Carlos Alberto Rodrigues Costa c, Felipe Silva Semaan a, Eduardo Ariel Ponzio a, * a i, Grupo de Eletroquímica e Eletroanalítica (G2E), Instituto de Química da Universidade Federal Fluminense, Campus Valonguinho, CEP 24020-141, Nitero RJ, Brazil b rio de Química Supramolecular e Nanotecnologia, Instituto de Química da Universidade Federal Fluminense, Campus Valonguinho, CEP 24020Laborato i, RJ, Brazil 141, Nitero c rio de Ci^ Laborato encia de Superfícies (LCS), Brazilian Nanotechnology National Laboratory (LNNano), Centro Nacional de Pesquisa em Energia e Materiais (CNPEM), CEP 13083-970, Campinas, SP, Brazil
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Article history: Received 6 April 2016 Received in revised form 10 August 2016 Accepted 22 August 2016 Available online 24 August 2016
Graphite/silicone composites (SGCE) in different ratios were prepared and characterized for different strategies, such as thermogravimetry (TG), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). A electrochemical characterization of the different electrodes was performed by applying cyclic voltammetry; experiments covered a potential window from 0.2 to 0.8 V (vs. AgjAgCl), using as a probe a K3Fe(CN)6 5 mmol L1 in KCl 0.5 mol L1 solution. Voltammetric profiles for both blank and probe solutions were suitably recorded under a 50 mV s1 scan rate. Results from cyclic voltammetry led to morphological and topographical assessments performed by scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. The only ratio that showed flexible and self-sustaining properties with a reversible voltammetric profile at slow scan rates (5e15 mV s1) was SG1. The use of the SSF/Graphite-Silicone composite electrodes for electroanalytical applications is very promising, since these types of electrodes allow for the monitoring of electroactivity, presenting applicability even with low graphite ratios. © 2016 Elsevier B.V. All rights reserved.
Keywords: Flexible electrode Composite electrodes Graphite Silicone
1. Introduction Electrode composite development began with an early description by Adams [1], in 1958, followed by more defined and categorized studies, especially by Tallman and Petersen, who characterized deeper aspects and gave a more concise definition of this material. Their definition can be simplified as a mixture of at least two phases in which at least one must present conductive properties and the other an insulating material, generating, after mixing and incorporating processes, a conductive material with physical and chemical properties different to those from the
* Corresponding author. Department of Physichal-Chemical, Fluminense Federal ~o Jo~ i, RJ, University, Morro Sa ao Batista s/n, Campus Valonguinho, 24.020-141, Nitero Brazil. E-mail addresses:
[email protected] (A.L. Silva),
[email protected] (E.A. Ponzio). http://dx.doi.org/10.1016/j.jallcom.2016.08.232 0925-8388/© 2016 Elsevier B.V. All rights reserved.
original phases [2,3]. Some particular properties make composite materials an attractive choice for electroanalysis, especially those related to applicabilities at wide pH ranges, broad potential interval of application, low costs, easy preparation and adequate surface renewal, among others [2e4]. Due to their widely exploited and well-established properties, special attention has been given to the use of carbon-derived materials as conductive phases. Among the most popular examples are vitreous carbon, boron doped diamond, graphite, graphene, fullerenes and carbon nanotubes [4]. Considering the insulating phases, focus on polymeric materials is observed, especially regarding resins, like epoxy, polyurethane and silicone, among others [5e9]. Amounts and ratios must be carefully studied and defined in order to reach the best combination possible, leading to the most suitable composition assembling the best chemical, physical and electrical properties for certain purposes [10]. In this context, the present study aimed to assess the use of silicone and
A.L. Silva et al. / Journal of Alloys and Compounds 691 (2017) 220e229
graphite. The choice of using silicone as an insulating phase is related to its numerous applications. Noteworthy among them are moldability (which allows the obtainment of specific formats and shapes), low toxicity, and insolubility in water. In addition, other interesting characteristics, such as high chemical and physical stability and flexibility (since it is a thermoplastic) must be taken into account. These properties have raised many new application possibilities besides those already described in the electroanalysis literature. Some of these new perspectives can be appreciated in the development of portable cells, flexible electrodes, and conductive filaments, among others [11e13]. In 1968, Pungor and Szepesvary [14] presented their first description regarding the preparation and use of silicone rubber/ graphite composite electrodes (SGCE). That study and other papers by Pungor et al. [14e16] gave birth to a series of experiments using this type of electrode in different procedures for the voltammetric determination of various analytes in many kinds of matrices. At this point, it becomes interesting to note that, despite the easy manipulation and well-defined preparation and voltammetric uses of SGCE, a twenty year gap without further voltammetric studies is observed between their description and new applications raised by Ref. [17]. Such lack of studies are not necessarily due to lack of interest, since potentiometric applications can be found in the literature [18e26]. Many results previously reported in the literature present the applicability of SGCE as sensors, such as three different studies by Santos et al. [7,26e28], in which the proposed electrode was applied to the differential pulse voltammetric determination of rutin and propranolol in pharmaceutical formulations, and applied in new strategies using cyclic voltammetry (CV), and square wave voltammetry (SWV) to determine hydrochlorotiazide in commercial samples. In addition to studies focused on electrochemical and electroanalytical approaches, many fields of expertise can clearly take advantage of the development of new flexible electrodes, from electronics, capacitors, batteries, electrochromism, smart textiles, soft robotics and sensors to medical diagnostic tools. Some interesting studies can be found over the past few years, for example in electroencephalography (EEG) [28], soft robotics [29], smart textiles [30], electroenterography (EEnG) and electrohysterography (EHG) [31], lithium-ion batteries [32,33], artificial muscles of dieletric elastomer actuators (DEAs) [34] and health monitoring [35]. For these applications, electrodes need soft substrates to support conductive phases, making the development of flexible electrodes highly promising, since they can contribute to solve the major problems regarding rigid sensors, such as the need for the use of substrates and the fact that they are uncomfortable, many times causing skin irritations that usually evolve to allergies [28,29,32,35]. This paper presents, for the first time, the development of selfsustained flexible composite electrodes based on graphite bonded with silicone, or, in other words, electrodes that do not need substrates, as well as the characterization of the material by scanning electron microscopy (SEM), atomic force microscopy (AFM), thermogravimetry (TG), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), in addition to cyclic voltammetry.
containing 5.0 mmol L1 of potassium ferrycianide (Mallinckrodt®, Ireland) in 0.50 mol L1 KCl (Vetec®, Brazil) were used, prepared immediately prior to use. 2.2. Preparation of the built-in and flexible composite electrodes Composite electrodes were prepared by mixing graphite (Sigma Aldrich®,
