1558-1748 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSEN.2018.2842081, IEEE Sensors Journal Abstract- Graphene has been entitled as a promising candidate for gas sensing applications, however suffers from low selectivity, as a limit for commercialization. We propose a gas sensor based on reduced graphene oxide (rGO)/Ag nanoparticles (Ag NPs), which can overcome this limitation by benefiting from both localized surface plasmon resonance of Ag NPs, and chemisorption capability of gas molecules on rGO sheets. These simultaneous gas detection mechanisms help to reveal both physical and chemical aspects of gas molecules. We show that gas-induced plasmonic shift is enhanced in rGO/Ag NPs (ΔλLSPR ≈ 5 nm), comparing with Ag NPs (ΔλLSPR ≈ 1.4 nm) versus 250 ppm of nitrogen concentration. Furthermore, electrical measurements prove that gas-induced variation in the conductivity response is higher in rGO/Ag NPs (ΔG/G0 ≈ 3.6%), in comparison with Ag NPs (ΔG/G0 ≈ 1.5%). Finally, we have utilized electro-plasmonic measurement in the proposed structure, leading to about 47% enhancement in the measured sensitivity, when λincident=440 nm is illuminated during nitrogen exposure. The proposed structure allows both electrical sensing and electro-plasmonic sensing by means of a simple measurement configuration, revealing both chemisorption and physisorption finger prints of the test gas. Index Terms: Reduced graphene oxide, Ag nanoparticle, localized surface plasmon, gas sensor I. INTRODUCTION Graphene has been attracted considerable attention owing to its unique optical, electrical, chemical and mechanical properties [1-3]. Additionally, because of its atomic thickness and high surface area, graphene has been applied in different sensing applications such as gas sensors [4-9]. Graphene- based gas sensors are generally based on changes in electronic properties due to adsorption of environmental molecules. However, this mechanism suffers from poor selectivity [10]. In this regard, noble metals have been widely used for realization of hybrid metallic nanostructure/carbon-based sensors recently, in order to achieve highly selective and sensitive gas sensors [11-13]. On the other hand, metallic nanostructures are well established for showing surface plasmon resonance (SPR) [14, 15], which will be localized around the particle [16]. In a metallic particle electrons are confined in three dimensions and light Manuscript received March 14, 2018; revised XXXX XX, 2018. First published XXX XX, 2018; current version published XXXX XX, 2018. S. Khalilifard (khalilifard_87@yahoo.com), S. Darbari (s.darbari@modares.ac.ir) and V. Ahmadi (vahid.ahmadi@modares.ac.ir) are with ECE Faculty, Tarbiat Modares University, Tehran, Iran. P O Box 14115- 111, Tehran 1411713116, Iran. Digital Object Identifier: illumination can cause oscillation of electrons, which induce a large electric field close to the surface of nanoparticle (NP) [17, 18]. Recently, the structures exploiting the properties of localized surface plasmon resonance (LSPR) has attracted considerable interests in different devices, including chemical sensors [19, 27]. LSPR creates sharp spectral absorption that can be used for detection of molecular environment and/or interactions near the nanoparticle surface by spectral shift detection [28-30]. Among metal NPs, noble metals such as silver and gold, have been widely used as LSPR systems forming hybrid gas sensors, owing to their easily accessible and simple synthesis [18]. In this work, reduced graphene oxide (rGO)/Ag nanoparticle heterostructure has been used as a hybrid gas sensor to achieve enhanced sensitivity, and introduce new aspects, which seems promising for improving selectivity at room temperature. Here, we have chosen Ag nanoparticles due to their inherently more intensive and sharper plasmonic spectrum, comparing with Au nanoparticles, which is superior for sensing functionality. More recently, because of very low permeability, graphene has been used as a passivating coating on silver nanoparticles, which prevents surface oxidation of Ag NPs [31, 32]. In the proposed structure we are able to detect both chemisorption-based variation of electrical properties and plasmonic behavior independently, by a simple electrical measurement configuration. This helps to achieve more individual properties of the introduced gas molecules. The proposed sensor, also benefits from simple fabrication and measurement method, which make the proposed structure compatible for integration. II. FABRICATION PROCESS Fig. 1.a displays the fabrication process schematically. A pre- cleaned glass substrate is coated by 100 nm Ni layer (Fig. 1.a), which is patterned to interdigitated electrodes subsequently (Fig. 1.b). The spacing between neighboring electrodes is about 10 μm. The utilized interdigitated electrodes are a common and practical geometric structure for various sensor and transducer applications. Ag NPs are synthesized by solution method [33], in which sodium borohydride (NaBH4) is used as the agent to reduce silver nitrate (AgNO3) and to stabilize the synthesized solution [34]. 7 ml of a 0.001 M solution of AgNO3 is added drop by drop into 30 ml of a 0.002 M solution of NaBH4. Then, the reaction mixture is cooled in Electro-plasmonic gas sensing based on reduced graphene oxide/Ag nanoparticle heterostructure S. Khalili Fard, S. Darbari, Member, IEEE, and V. Ahmadi, Senior Member, IEEE 1