Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay Research Paper Doped nafion-layered double hydroxide nanoparticles as a modified ionic polymer metal composite sheet for a high-responsive humidity sensor Fahimeh Beigi a , Matin Sadat Saneei Mousavi a , Faranak Manteghi a, ⁎ , Mohammadreza Kolahdouz b a Department of Chemistry, Iran University of Science and Technology, Tehran, Iran b School of Electrical and Computer Engineering, University of Tehran, Tehran, Iran ARTICLE INFO Keywords: LDH Composite Humidity sensor Nafion ABSTRACT In this paper, a novel Ionic Polymer Metal Composite (IPMC) has been fabricated by chemical electroless plating integrated with doping Layered Double Hydroxide (LDH) nanoparticles in the nafion polymeric matrix. Platinum, as a noble metal, was used for electrode deposition of the prepared doped nafion and successfully act as a high-quality electrode to transfer electrical signal. This IPMC, in fact, is a smart material suggesting to acts as a high responsive mechanical humidity sensor in biomedical devices since it produced a remarkably higher voltage with higher sensitivity and responsivity in comparison to the previous IPMC humidity sensors in lit- erature. LDH nanoparticles own a layered ionic structure, which makes them highly hydrophilic. This property increases the water uptake of the IPMC. By increasing the penetration of water molecules in the nafion channels, the separation of positive and negative charges becomes easier and leads to higher and faster humidity sensing application. The humidity sensing applications of undoped and LDH-doped IPMC fabricated by facile chemical plating method were measured and compared at different bending angles. It was found that 1% doped nafion sheet had the highest water uptake of 30%, and consequently better humidity sensing compared to the undoped, 0.5%, 1.5%, 2% and 3% doped nafion. This means that doping concentration and modification has an optimized value. 1. Introduction Ionic Polymer Metal Composite (IPMC) is a kind of electro-active polymer (EAP) with remarkable sensing and actuating properties which makes it a suitable soft material to be used in various devices such as biomedical ones due to its high biocompatibility (Zhu et al., 2016). Recently, sensing applications of IPMC have been emerged and re- ported on the displacement, pressure, humidity, force and structural health monitoring (Bonomo et al., 2008; Zangrilli and Weiland, 2011; Matsuura et al., 2014; Smith, 2007; Brufau-Penella et al., 2008; Aureli et al., 2010). This sensor generally consists of an electrically activated polymer layer (usually nafion) sandwiched by two metal electrodes. Inside the polymer, anions, which are covalently bound to the polymer chains, are balanced and made nanochannels by mobile cations. It can bend sharply through electrical conduction and has two major ad- vantages of high curvature and high sensitivity. IPMCs have innate sensing properties in which a force or deformation on an IPMC beam produces a measurable electrical signal. Another amazing feature of this material is that different electrical responses are seen at various ranges of humidity, which is a key for developing a sensitive humidity sensor (Esmaeli et al., 2017). High responsive IPMCs are usually fabricated by facile chemical routes (Pt electrodes) which results in high quality electrodes with no cracks on the surface (Shahinpoor and Kim, 2001; Wang et al., 2016; Chung et al., 2006; Lee et al., 2012; Oguro, 1995; Shahinpoor and Kim, 2004; Kim and Shahinpoor, 2003; Ionic et al., 2016). To increase sensing responsivity, special particles with specific properties can be introduced into the nafion polymer (Narimani et al., 2016). One of the materials that has been introduced into the nafion so far is Layered Double Hydroxide (LDH). LDH are a class of natural and synthetic mixed metal hydroxides, historically described as anion-ex- changing, clay-like materials. LDH are structurally similar to brucite, Mg(OH) 2 , with one notable difference: LDH are mixed-metal hydro- xides and brucite is magnesium hydroxide (Richardson, 2007). To un- derstand the structure of these compounds, it is necessary to start from the structure of brucite where octahedra of Mg 2+ (6-fold coordinated to OH - ) share edges to form infinite sheets. These sheets are stacked on top of each other and are held together by hydrogen bonding. When Mg 2+ ions are substituted by a trivalent ion, having not too different radius (such as Fe 3+ for pyroaurite and Al 3+ for hydrotalcite, respec- tively), positive charges are generated in the hydroxyl sheet. This net https://doi.org/10.1016/j.clay.2018.09.006 Received 12 April 2018; Received in revised form 5 August 2018; Accepted 5 September 2018 ⁎ Corresponding author. E-mail address: f_manteghi@iust.ac.ir (F. Manteghi). Applied Clay Science 166 (2018) 131–136 Available online 21 September 2018 0169-1317/ © 2018 Published by Elsevier B.V. T