Graphene Electromechanical Water Sensor: The Wetristor

Abstract

between water and graphene is crucial for building up novel and smart biointerfaces. [18,19] Additionally, the study of reactivity and structure of water at the graphene interface has also generated intriguing questions and controversial results. [20,21] 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. [20,22] 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 the present 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. The platform design provides a direct probing of the electrical response of the air/graphene/liquid interface without the presence of any underlying substrate. [23] Our results show a significant change of graphene resistivity (of about 25%) due to the presence of water in the microchannel. The identification of the physical mechanisms behind such strong change in resistivity is not A water-induced electromechanical response in suspended graphene atop a microfluidic channel is reported. The graphene membrane resistivity rapidly decreases to ≈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 (hexagonal boron nitride multilayer or poly(methyl methacrylate) film). The wetristor effect, namely the water-induced resistivity collapse, occurs in uncovered devices only. Atomic force microscopy and Raman spectroscopy indicate substantial morphology changes of graphene membranes in such devices, while covered membranes suffer no changes, upon channel water filling. The results suggest an electromechanical nature for the wetristor effect, where the resistivity reduction is caused by unwrinkling 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 nanofluidics.

A multiscale approach for electronic transport simulation of carbon nanostructures in aqueous solvent

“…Since graphene was discovered, 1 there have been proposals for utilizing it in many different applications due to its unique properties. [2][3][4][5][6] Specifically, high surface area and good electric conductivity make graphene an excellent candidate for the active material in sensors, [7][8][9][10][11] not least for compounds of interest in biology. [12][13][14][15][16] An all-electronic graphene-based sensor operates on the assumption that the chemisorption or physisorption of external agents results in measurable changes of the electronic structure of graphene.…”

“…Since graphene was discovered, 1 there have been proposals for utilizing it in many different applications due to its unique properties. 2–6 Specifically, high surface area and good electric conductivity make graphene an excellent candidate for the active material in sensors, 7–11 not least for compounds of interest in biology. 12–16…”

Observing Electrochemical Reactions on Suspended Graphene: An Operando Kelvin Probe Force Microscopy Approach

“…Recently, reactor‐cells developed with this graphene‐membrane approach were shown to be feasible for characterizing materials in their reaction environment via electron microscopy and X‐ray spectroscopy techniques. [ 3–10 ] However, graphene's high specific surface area makes it particularly susceptible to doping by its operating environment, [ 11 ] which may be influenced by its electromechanical behavior, [ 12,13 ] electrochemical reactions, [ 11 ] functionalization, as well as contamination during preparation of the reactor‐cell, [ 14 ] including exposure to air. [ 15,16 ] Furthermore, the width of the electrical double layer (EDL) formed in liquid environments can have a major influence on doping.…”

“…This complicates analysis and introduces additional ambiguity in developing a model and is thus avoided here. The effects of the interface between suspended graphene and water on the mechanical properties of graphene and on the electrical resistivity of graphene were also investigated recently, [ 12,13 ] but the preparation method involved the use of polymer in both of these studies as well. Another distinction of the work herein is that our suspended graphene regions were only 500 nm in diameter.…”

Nanomechanics of few-layer materials: do individual layers slide upon folding?

“…The resistivity of the graphene membrane rapidly decreases by approx. 25% upon water injection into the channel due to the reduction of the folds in the suspended graphene [ 17 ].…”