Graphene-Coated Porous PDMS As a Gas Sensor for Potential Epidermal Electronics Applications

Abstract

Introduction Epidermal electronics that can intimately integrate with human skin are getting much attention due to ultrathin, soft, and light-weight [1]. These capabilities create interesting opportunities for diverse wearable sensor applications such as sweat monitoring, multiple electrophysiological signal monitoring, subtle motion sensing, ultraviolet sensing, gas monitoring, etc [2]. The authors have been developed graphene-based gas sensors having various forms including electrospun fiber [3], commercially-available yarn [4], stretchable yarn [5], and fabric [6]. In this paper, we present the stretchable porous e-skin based gas sensor comprising of porous polydimethysiloxane (PDMS) as a stretchable substrate and reduced graphene oxide (RGO) as gas sensing materials showed high NO2 sensitivity under 30% strain at room temperature. Method Porous PDMS thin films with the average pore size of 10 um were achieved by breath figure method [7]. The porous PDMS film were functionalized with 3-aminopropyltriethoxysilane (APTES) via chemical vapor treatment, which induced positive charges on the surface of porous PDMS film. Spray coating was performed with a commercial spray coater system (Jinsung, Korea). GO dispersed solution of 1.0 mg/mL was deposited onto the amine functionalized periodic porous PDMS films. The prepared GO deposited porous PDMS films were chemically reduced by immersing them in a reduction agent of hydriodic acid solution at 50 °C for 20 min. To neutralize and remove any residual on the RGO porous PDMS films, the samples were repeatedly rinsed with a sodium bicarbonate solution and deionized water, and then dried in a chemical hood. The stretchable gas sensors were fabricated by deposition silver paste onto the tops of the RGO films to make electrodes by silk screening method. Results and Conclusions The RGO porous PDMS films exhibited distinctive wrinkles and ripples with high density on its surface, which was attributed to the successful coating of RGO sheets on the surface of porous PDMS film. We believe that numerous wrinkles make the RGO porous PDMS films more stable against several deformations. The Raman spectrum of RGO porous PDMS films exhibited typical RGO spectral features, such as the D peak at 1340 cm-1 (defects in graphene) and the G peak at 1580 cm-1 (a stretching mode of C-C bond of the sp2 structure of graphene).We also studied the electrical properties of porous PDMS films coated with RGO sheets using four probe method. A GO coated porous PDMS film yielded an electrical conductivity of about 10-8 S/cm. After chemical reduction, the conductivity of RGO porous PDMS increased by approximately seven orders of magnitude. Our RGO porous PDMS films could maintain stable electrical conductivity values upon about 30% strain deformation. The mechanical stability of the RGO porous PDMS films was evaluated by measuring the electrical conductivity under repeated stretching and releasing. The electrical resistance of the samples remains unchanged even after 5000 cycles. The as-prepared RGO porous films are mechanically robust and can be deformed without device performance degradation, making them very attractive candidates for epidermal electronic devices.Porous PDMS coated with reduced graphene oxide responded to NO2 gas with sensitive and selective manner. When we compare the response (Ra/Rg) of porous PDMS with that of non-porous PDMS, porous PDMS shows 20% stronger response to NO2 compared to non-porous PDMS. The porous nature is considered to contribute to strong response due to increase of reaction possibility with reduced graphene oxide and NO2 gas. We found that the response of gas sensor was increased around 20% upon elongation. We believe that the hidden area of pores in the PDMS matrix can be exposed to the gas molecules upon elongation. Finally, we found that the sensor shows reliability in terms of the response up to 5,000 times elongation-relaxation cycles. In the future, we expect that the thin porous PDMS can be attached to the skin and find the applications in healthcare by sensing gas molecules emitted from skin. Acknowledgement This research was supported the R&D Program (Development of a mobile diet-monitoring technology based on multiple biomarkers by the National Research Foundation of Korea under research projects, NRF-2017M3A9F1033056).