www.afm-journal.de FULL PAPER © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.MaterialsViews.com wileyonlinelibrary.com Fei Xiao, Yuanqing Li, Xiaoli Zan, Kin Liao, Rong Xu, and Hongwei Duan* 1. Introduction Flexible biosensors that can accommodate dramatic shape changes are instrumental for the development of portable point- of-care medical products, minimally invasive implantable devices, and compact diagnostic platforms. [1–4] Biosensors based on elec- trochemical readout represent a major class of analytical tools in clinical applications. [5] As the primary functioning components of electrochemical sensors, carbon substrates with immobilized noble metal nanostructures typically serve as working elec- trodes to promote the redox reactions of analytes of interest. [6,7] Recent advances in the production of carbon and metal nano- materials have greatly diversified the possible building blocks for the hybrid electrodes. [8] However, the performance of the hybrid electrodes relies on not only the properties of individual components, such as the electron transport ability of carbon substrates and the catalytic activities of metal nanostructures, but also the effective and scalable structural integration of the two components. Great efforts have been directed to generate stable and high-density loading of nanostructured electrocata- lysts on carbon substrates by taking advantages of the functional groups on the carbon substrates or physically adsorbed organic mediators. [9–12] In addition, the design of flexible electrochemical biosensors creates special needs for freestanding carbon sub- strates with superior mechanical strength and flexibility. Here, we report a triple-component flexible biosensor fabricated by sequen- tially growing MnO 2 nanowire networks and Pt nanoparticles on freestanding reduced graphene oxide (rGOP). Recent discovery of graphene has stimulated con- siderable research activities on the optical, electronic, and mechanical properties of this single-atom-thick 2D carbon material and its hierarchical assemblies. [13–18] The directed assembly of individual graphene nanosheets into macroscopic paper-like materials is of particular interest for the development of flexible electrode because the resultant freestanding graphene paper has shown a unique collection of characteristics, such as mechanical robustness, excellent electrical conductivity, and stability in electrochemical environment, outperforming many other carbon substrates. [19–24] As shown in Figure 1, graphene paper used in this study was fabricated by evaporating the aqueous dispersion of exfoliated graphene oxide (GO) nanosheets in casting molds, followed by chemical reduction. To circumvent the low-loading of metal catalysts resulting from the limited surface area of the 2D rGOP substrate, we have developed template-free electrodeposition of MnO 2 nanowire networks on rGOP to produce a 3D network with large sur- face areas. Metal oxides are actively pursued to support metal particles for heterocatalysis and electrocatalysis. [25–27] Here, the use of MnO 2 nanowire networks gives rise to interconnected porous 3D frameworks with abundant reactive sites and large surface areas for the deposition of nanostructured Pt. Among a large family of noble metals, metallic Pt or Pt oxides are the most catalytically active species used in electrocatalysis. [28,29] A number of wet-chemical and electrochemical approaches have been explored to synthesize Pt nanostructures with controlled sizes and morphologies. We have used ultrasonic-electrodepo- sition to grow Pt nanoparticles on the MnO 2 network coated graphene paper. Ultrasonic-electrodeposition is a modified electrochemical approach in which the use of ultrasonication during electrodeposition process results in the generation of small nanocrystals with good size distribution, providing great versatility in synthesizing nanocatalysts for catalytic or sensing processes. [30–32] Our results also demonstrate that the struc- tural integration of the three building blocks into flexible electrodes successfully overcome the problems associated with Growth of Metal–Metal Oxide Nanostructures on Freestanding Graphene Paper for Flexible Biosensors Flexible biosensors are of considerable current interest for the development of portable point-of-care medical products, minimally invasive implantable devices, and compact diagnostic platforms. A new type of flexible electro- chemical sensor fabricated by depositing high-density Pt nanoparticles on freestanding reduced graphene oxide paper (rGOP) carrying MnO 2 nanowire networks is reported. The triple-component design offers new possibilities to integrate the mechanical and electrical properties of rGOP, the large surface area of MnO 2 networks, and the catalytic activity of well-dispersed and small- sized Pt nanoparticles prepared via ultrasonic-electrodeposition. The sensi- tivity and selectivity that the flexible electrode demonstrates for nonenzymatic detection of H 2 O 2 enables its use for monitoring H 2 O 2 secretion by live cells. The strategy of structurally integrating metal, metal oxide, and graphene paper will provide new insight into the design of flexible electrodes for a wide range of applications in biosensing, bioelectronics, and lab-on-a-chip devices. DOI: 10.1002/adfm.201200191 Dr. F. Xiao, Dr. Y. Li, X. Zan, Prof. K. Liao, Prof. R. Xu, Prof. H. Duan School of Chemical and Biomedical Engineering Nanyang Technological University 70 Nanyang Drive, Singapore 637457 E-mail: hduan@ntu.edu.sg Adv. Funct. Mater. 2012, DOI: 10.1002/adfm.201200191