Graphene transistors are insensitive to pH changes in solution

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 with liquid-gated graphene field-effect transistors

Liquid-gated graphene field-effect transistors (GFETs) with reliable performance are developed. It is revealed that ideal defect-free graphene should be inert to electrolyte composition changes in solution, whereas a defective one responses to electrolyte composition. This finding sheds light on the large variety of pH or ion-induced gate shifts that have been published for GFETs in the recent literature. As a next step to target graphene-based (bio-) chemical sensing platform, non-covalent functionalization of graphene has to be introduced

Selective Sodium Sensing with Gold-Coated Silicon Nanowire Field-Effect Transistors in a Differential Setup

Ion-sensitive field-effect transistors based on silicon nanowires with high dielectric constant gate oxide layers (e.g., Al2O3 or HfO2) display hydroxyl groups which are known to be sensitive to pH variations but also to other ions present in the electrolyte at high concentration. This intrinsically nonselective sensitivity of the oxide surface greatly complicates the selective sensing of ionic species other than protons. Here, we modify individual nanowires with thin gold films as a novel approach to surface functionalization for the detection of specific analytes. We demonstrate sodium ion (Na+) sensing by a self-assembled monolayer (SAM) of thiol-modified crown ethers in a differential measurement setup. A selective Na+ response of ≈−44 mV per decade in a NaCl solution is achieved and tested in the presence of protons (H+), potassium (K+), and chloride (Cl–) ions, by measuring the difference between a nanowire with a gold surface functionalized by the SAM (active) and a nanowire with a bare gold surface (control). We find that the functional SAM does not affect the unspecific response of gold to pH and background ionic species. This represents a clear advantage of gold compared to oxide surfaces and makes it an ideal candidate for differential measurements

Understanding the Electrolyte Background for Biochemical Sensing with Ion-Sensitive Field-Effect Transistors

Silicon nanowire field-effect transistors have attracted substantial interest for various biochemical sensing applications, yet there remains uncertainty concerning their response to changes in the supporting electrolyte concentration. In this study, we use silicon nanowires coated with highly pH-sensitive hafnium oxide (HfO2) and aluminum oxide (Al2O3) to determine their response to variations in KCI concentration at several constant pH values. We observe a nonlinear sensor response as a function of ionic strength, which Is independent of the pH value. Our results suggest that the signal is caused by the adsorption of anions (Cl-) rather than cations (K+) on both oxide surfaces. By comparing the data to three well-established models, we have found that none of those can explain the present data set. Finally, we propose a new model which gives excellent quantitative agreement with the data