Graphene Transistors Are Insensitive to pH Changes in Solution

Abstract

We observe very small gate-voltage shifts in the transfer characteristic of as-prepared graphene field-effect transistors (GFETs) when the pH of the buffer is changed. This observation is in strong contrast to Si-based ion-sensitive FETs. The low gate-shift of a GFET can be further reduced if the graphene surface is covered with a hydrophobic fluorobenzene layer. If a thin Al-oxide layer is applied instead, the opposite happens. This suggests that clean graphene does not sense the chemical potential of protons. A GFET can therefore be used as a reference electrode in an aqueous electrolyte. Our finding sheds light on the large variety of pH-induced gate shifts that have been published for GFETs in the recent literature.

Sensing at the Surface of Graphene Field‐Effect Transistors

“…This effect has been harvested to design the first graphene field-effect transistor (GFET), [4] which has inspired considerable experimental and theoretical work relating to the application of GFETs for high performance label-free chemical and biological sensors. [5][6][7][8][9][10][11][12]37] 2.1. Back-gated GFETs.…”

“…[12] The broad sensing potential of graphene can only be unlocked by the introduction of sensitizer (bio)molecules and structures, e.g. various inorganic groups, [23][24][25][81][82][83][84][85][86][87][88][89][90] organic or organometallic molecules, [37,[91][92][93][94][95][96] DNAs, [97][98][99][100][101] proteins, [102] peptides, [30,31,103,104] nanoparticles, [105,106,107] and 2D heterostructure.…”

“…But this high chemical sensitivity of graphene towards physisorbed gaseous NH 3 is a surprise if considering the fact that graphene is intrinsically chemically inert. [12] Indeed, in more recent studies, inert electrical responses (i.e., no response) for gaseous NH 3 were observed by annealing the exfoliated graphene at high temperature (400 °C) in Ar/H 2 atmosphere to remove possible polymer contaminations and produce atomically clean graphene sheets. [178] This inert sensing behavior of the cleaned graphene sensors is robust even upon the exposure to NH 3 vapor at a concentration of 1000 ppm (Fig.…”

“…[5][6][7] Such defectinduced ionic response can be suppressed by passivating the graphene layer with inert aromatic molecules such as fluorobenzene. [12] As a consequence of its ideal hydrophobic surface with a very small amount (ideally zero) of dangling bonds, such a clean GFET should be inert to the change of electrolyte compositions, and could therefore act as a novel solid-state reference electrode that senses only the electrostatic potential in aqueous electrolytes unless a chemo-adsorption or a physico-adsorption of charged ions is considered. [184] On the contrary, by functionalizing the GFETs with active groups, for example, with proton sensitive phenol or K + sensitive crow ethers conjugated with aromatic molecular anchor groups, a pH response up to ~49 mV/decade or a K + ionic response up to ~40 mV/dec was achieved.…”

“…[3] Since the experimental preparation and observation of the electric field effect in graphene by the Manchester group in 2004, [4] biochemical sensing using graphene electronic devices has been actively pursued. [5][6][7][8][9][10][11][12] The sensing principle roots on a change of the electrical conductance of the graphene channel upon adsorption of a molecule on the sensor surface. [5] The uniqueness of graphene among other solid-state materials is that all carbon atoms are located on the surface, making the graphene surface potentially highly sensitive to any changes of its surrounding environment.…”