Graphene is a crystalline 2D material and considered the thinnest possible membrane. Because of its high chemical stability, combined with its physical properties, graphene is promising in a variety of applications. Particularly, liquid/graphene interfaces have been exploited for bio-applications on cellular flow sensing, liquid sensing, DNA sequencing, and transparent windows in liquid cells. In such devices, understanding the interaction between water and graphene is crucial for building up novel and smart bio-interfaces. Additionally, the study of reactivity and structure of water at the graphene interface has also generated intriguing questions and controversial results. For instance, several experimental works demonstrate that the charge transfer process that happens between graphene and water molecules is highly dependent on the underlying substrate. Thus, it would be highly desirable to elucidate the above discussion by probing the electrical response of a suspended graphene membrane in contact with water without the presence of any substrate. We also believe that a precise understanding of the electrochemical behavior of water/graphene interface would be fundamental for developing novel and superior electrical, mechanical and optical devices. In this work, we develop a microfluidic platform that integrates suspended graphene membrane windows (with electrical contacts) with buried fluid channels to probe the electrical response of a graphene membrane in contact with water [1,2,3]. The platform design provides a direct probing of the electrical response of the air/graphene/liquid interface without the presence of any underlying substrate.

 I will present a detailed study of the water-induced electromechanical response in suspended graphene atop a microfluidic channel. The graphene membrane resistivity rapidly decreases ~ 25% upon water injection into the channel, defining a sensitive “channel wetting” device – a wetristor. The physical mechanism of the wetristor operation is investigated using two graphene membrane geometries, either uncovered or covered by an inert and rigid lid (h-BN multilayer or PMMA film). The wetristor effect, namely the water-induced resistivity collapse, occurs in uncovered devices only. AFM and Raman spectroscopy indicate substantial morphology changes of graphene membranes in such devices, while covered membranes suffer no changes, upon channel water filling. Our results suggest an electromechanical nature for the wetristor effect, where the resistivity reduction is caused by un-wrinkling of the graphene membrane through channel filling, with an eventual direct doping caused by water being of much smaller magnitude, if any. The wetristor device should find useful sensing applications in general micro- and nano-fluidics and provides novel insights on the interface interactions of 2D materials with liquids.

Acknowledgments: The authors acknowledge the support of FAPEMIG (Rede 2D), CNPQ/MCTI, and INCT de Nanocarbono.

[1] Ferrai et al., Graphene nanoencapsulation action at an air/lipid interface Journal of Materials Science 57, 6223 (2022).

[2] Ferrari et al., Apparent Softening of Wet Graphene Membranes on a Microfluidic Platform ACS Nano 12, 5, 4312 (2018)

[3] Meireles, Leonel et al., Graphene Electromechanical Water Sensor: The Wetristor. Advanced Electronic Materials, 6, 1901167, 2020.