Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem New rotating disk hematite film electrode for riboflavin detection Larissa C. Gribat a , Jerome T. Babauta b , Haluk Beyenal b , Nathalie A. Wall a,⁎ a Department of Chemistry, Washington State University, Pullman, WA 99164-4630, United States b The Gene and Linda Voiland School of Engineering and Bioengineering, Washington State University, Pullman, WA 99164-6515, United States ARTICLE INFO Keywords: Hematite Pec Electrode Electrochemistry RDE Adsorption Flavin Riboflavin Long cycle life Galvanostatic charge—Discharge measurements Cycle stability ABSTRACT The motivation for this work was to develop and test a hematite (α-Fe 2 O 3 ) film electrode for the detection of riboflavin using a less common preparation with rotating disk electrode linear sweep voltammetry. Two types of hematite electrodes were prepared, with a differential method and a one-step method (quicker preparation), and bare glassy carbon electrodes were used for comparison. The differential and one-step hematite electrodes were tested for cyclic voltammetry stability. Colloidal hematite (α-Fe 2 O 3 ) was synthesized from iron nitrate nonahydrate from a two-line ferrihydrite transition and characterized with X-ray powder diffraction and Mössbauer spectroscopy. Both hematite film electrodes were prepared using a three-electrode electrochemical cell and rotating disk voltammetry. Rotating disk hematite electrode stability was demonstrated for up to 1000 cyclic voltammetry scans. A single use of the rotating disk hematite film electrode was shown to demonstrate quantitatively the detection of riboflavin reduction peaks with optimized differential square wave voltammetry. The limits of detection for the glassy carbon and rotating disk hematite film electrodes for riboflavin are 0.0028 ± 0.0009 mM and 0.0084 ± 0.0009 mM, respectively. The detection linear range for the glassy carbon and rotating disk hematite film electrodes is 0.0013–0.1 mM riboflavin. The sensitivities of the one-step and differential rotating disk hematite film electrodes are 0.216 ± 0.001 μA/μM and 0.27 ± 0.02 μA/μM riboflavin, respectively. The sensitivity of the bare glassy carbon electrode is 0.144 ± 0.003 μA/μM riboflavin. 1. Introduction Iron is the fourth most abundant element in the earth's crust, comprising 5.1% of its mass. As such, iron is geologically important; in particular iron oxides are formed upon weathering, affecting soil properties including surface adsorption, soil aggregation, and redox behavior [1]. One of the iron oxides of interest is hematite (α-Fe 2 O 3 ), a crystalline iron mineral that is resistant to dissolution above pH 4 and is also an n-type semiconductor [1–3]. Hematite is involved in environ- mental biological and abiotic electron transfer processes [3–6]. One of the important processes in which hematite is involved is dissimilatory Fe(III) reduction, in which bacteria reduce Fe(III) present in minerals to Fe(II). In anaerobic environments, dissimilatory metal-reducing bacter- ium Shewanella oneidensis MR-1 utilizes cytochromes to transfer elec- trons with redox-active hematite [7–8]. The interaction of hematite with Shewanella oneidensis MR-1 cytochromes OmcA and MtrC and other biomolecular redox interactions with hematite can be studied using electrochemistry [5,9]. Hematite electrodes have been used to model environmental systems, via crystalline hematite or hematite film electrodes [5,9–11]. Hematite electrodes have also been employed as biosensors for analytes such as dopamine, hydroquinone, and NADH, showing micro-to-millimolar detection limits [3,9,12–13]. Hematite electrodes are prepared in a multitude of ways, depending on whether the goal is to produce a metal/metal oxide film or alloy, produce energy from water, or simulate environmental hematite conditions. Photoelectrochemical energy has been produced using hematite electrodes, thanks to hematite's favorable band gap (2.2 eV) and improvement from semiconductor doping and morphology-con- trollable α-Fe 2 O 3 nanostructures in its light to energy conversion [14–19]. These nanostructured hematite electrodes are prepared through a variety of methods including nanoparticle dispersion with sintering, nanowire arrays, spray pyrolysis, atmospheric pressure vapor deposition, and electrodeposition at high temperatures (700–800 °C) [18]. Traditionally, metal oxide electrodes have been prepared for electrocatalytic uses through 1) anodization of metal electrodes, 2) chemical vapor deposition, 3) vacuum evaporation and sputtering or 4) deposition from a colloidal solution [20]. In environmental studies, hematite electrodes are often formed from a colloidal hematite suspen- sion or hematite crystals collected from the environment [9–10]. Hematite films grown from colloidal hematite are advantageous because of their increased surface area-to-volume ratio and reactivity, which bypass some of their intrinsic limitations [21]. A disadvantage is http://dx.doi.org/10.1016/j.jelechem.2017.05.008 Received 17 January 2017; Received in revised form 25 April 2017; Accepted 7 May 2017 ⁎ Corresponding author. E-mail address: nawall@wsu.edu (N.A. Wall). Journal of Electroanalytical Chemistry 798 (2017) 42–50 Available online 08 May 2017 1572-6657/ © 2017 Elsevier B.V. All rights reserved. MARK