Contents lists available at ScienceDirect Biosensors and Bioelectronics journal homepage: www.elsevier.com/locate/bios A symmetric supercapacitor/biofuel cell hybrid device based on enzyme- modified nanoporous gold: An autonomous pulse generator Xinxin Xiao a , Peter Ó Conghaile b , Dónal Leech b , Roland Ludwig c , Edmond Magner a, ⁎ a Department of Chemical and Environmental Sciences, Bernal Institute, University of Limerick, Limerick, Ireland b School of Chemistry & $2 Ryan Institute, National University of Ireland Galway, Galway, Ireland c Department of Food Science and Technology, BOKU-University of Natural Resources and Life Sciences, Muthgasse 18, 1190 Vienna, Austria ARTICLE INFO Keywords: Biofuel cell Supercapacitor Hybrid device Nanoporous gold Osmium redox polymer Pulse generator ABSTRACT The integration of supercapacitors with enzymatic biofuel cells (BFCs) can be used to prepare hybrid devices in order to harvest significantly higher power output. In this study, a supercapacitor/biofuel cell hybrid device was prepared by the immobilisation of redox enzymes with electrodeposited poly(3,4-ethylenedioxythiophene) (PEDOT) and the redox polymer [Os(2,2′-bipyridine) 2 (polyvinylimidazole) 10 Cl] +/2+ (Os(bpy) 2 PVI) on dealloyed nanoporous gold. The thickness of the deposition layer can be easily controlled by tuning the deposition conditions. Once charged by the internal BFC, the device can be discharged as a supercapacitor at a current density of 2 mA cm -2 providing a maximum power density of 608.8 μW cm -2 , an increase of a factor of 468 when compared to the power output from the BFC itself. The hybrid device exhibited good operational stability for 50 charge/discharge cycles and ca. 7 h at a discharge current density of 0.2 mA cm -2 . The device could be used as a pulse generator, mimicking a cardiac pacemaker delivering pulses of 10 μA for 0.5 ms at a frequency of 0.2 Hz. 1. Introduction Enzymatic biofuel cells (BFCs) utilizing oxidoreductases as electro- catalysts can be used to generate electricity from fuels such as sugars or alcohols in combination with dioxygen (Calabrese Barton et al., 2004; Cooney et al., 2008; Leech et al., 2012). BFCs are of interest as power sources for biosensors (Pinyou et al., 2015; Zloczewska et al., 2014), medical implants (e.g. insulin pumps, cardiac pacemakers (MacVittie et al., 2013)), and other devices (Falk et al., 2012; Ó Conghaile et al., 2016). To be able to activate commonly used microelectronic devices (such as commercial pacemakers), appropriate output voltages (mini- mum of 1.4 V) are required (MacVittie et al., 2013). The open circuit voltage (OCV) of glucose and oxygen BFCs is limited by the thermo- dynamic value of 1.179 V (Pankratov et al., 2016), and in practice by the difference between the onset redox potentials of the bioanode and biocathode (Cracknell et al., 2008). The observed OCV can be increased change "The observed OCV can be increased by..." to "The observed OCV can be increased closer to the theoretical value by..." by using direct electron transfer (DET) or by the use of redox mediators with redox potentials closer to those of the enzyme/cofactor (Rasmussen et al., 2015). The OCV can also be increased by using multiple cells connected in series (MacVittie et al., 2013). However, due to the presence of conductive fluids within the body, implantable cell stacks suffer from the problem of short-circuits between individual cells (Andoralov et al., 2013; MacVittie et al., 2013). In such systems, isolation of the cells is essential. Another route is to couple BFCs with external electronic devices to increase the voltage. For example, using a charge pump and a DC-DC converter, a fluidic BFC utilizing PQQ- dependent glucose dehydrogenase and laccase with an intrinsic OCV of 0.47 V was sufficient to power a pacemaker (Southcott et al., 2013). Falk et al. (2014) presented a self-powered wireless lactose biosensing system, consisting of an energy harvesting module including a voltage amplifier and capacitor to build a power source based on a BFC using bilirubin oxidase (BOx) and cellobiose dehydrogenase (CDH). In addition to low voltage outputs, BFCs are also limited by their low current/power densities, which can be improved through efficient substrate diffusion (Murata et al., 2009), enhanced rates of electron transfer between enzymes and electrodes, improving catalytic activity (Suraniti et al., 2013) and loading of enzymes (Flexer et al., 2011), as well as utilizing enzyme cascades for deep and complete oxidation pathways (Kim et al., 2013; Shao et al., 2013; Xu and Minteer, 2012). The introduction of capacitors into the BFC circuit enables the accumulation of charge, resulting in output pulses of higher power. Sode et al. proposed the concept of a “BioCapacitor” with the integra- http://dx.doi.org/10.1016/j.bios.2016.11.012 Received 17 July 2016; Received in revised form 21 October 2016; Accepted 5 November 2016 ⁎ Corresponding author. E-mail address: edmond.magner@ul.ie (E. Magner). Biosensors and Bioelectronics 90 (2017) 96–102 0956-5663/ © 2016 Elsevier B.V. All rights reserved. Available online 09 November 2016 crossmark