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 silicon nanowire field-effect transistors

All experiments presented in this thesis are focusing on the development of a reliable and stable sensing platform, which allows converting a (bio-) chemical signal into an electrical, processable one. A sensing platform requires a proper design meeting many demands, e.g., stability and differential readout capability with in situ references to prevent misreadings due to non-specific interactions and/or thermal drifts. Nanoscale electronic detection systems, e.g. nanowires, based on an ion-sensitive field-effect transistor (ISFET) implementation (Sensors & Actuators B, 1:1, 2003) do have the potential to meet these boundary conditions. However there is a main difficulty: controlling and understanding the interface between the transducer and the target agents is a crucial factor and needs to be carefully explored to allow reliable detection. In this thesis we developed a fabrication protocol yielding top-down fabricated silicon nanowire field-effect transistors (SiNW FETs) with a high degree of reproducibility which we present in chapter I. The nanowire FETs are operated in the accumulation regime. The electrical characterization of a FET is performed in the linear regime by applying a low source-drain voltage and varying the gate voltage. While changing the back-gate voltage the source-drain current is recorded. With the focus on (bio-) chemical sensing, stable functionalities and operations in electrolyte solutions is mandatory. In chapter II we discuss investigations, which were performed in order to achieve stable working conditions in liquid environments. We show that leakage currents can sufficiently be suppressed with a thin alumina layer covering the sensor. Further we discuss the dual-gate approach. A home-built liquid cell allows the integration of a platinum electrode with which a gate potential can be applied. This electrode acts as liquid gate. The liquid cell combines both types of gating, liquid (top)-gating and back-gating, in order to characterize a device. To target sensing with our platform we focus on ion sensing experiments, which are presented in chapter III. We studied the nanowires as ISFETs and conducted pH-sensing experiments. we show that a nanowire FET can be used as a sensing device. We demonstrate that we can measure pH shifts either by sweeping the back-gate voltage or the liquid potential. In chapter IV we expand the application of our nanowire FETs as ion-sensitive sensors. We study here the sensing capabilities at different salts and concentrations. We demonstrate that using an alumina interface between the FETÕs surface and the liquid results in insensitivity to monovalent as well as divalent ions at a large concentration range up to 10 mM. Above a concentration of 10 mM we observe a sudden transition resulting in a transfer characteristics change. The presented evaluation and the models have to be considered as work in progress. In chapter V we discuss additional investigations, which are of importance for reliable detection of target analytes with our sensing platform. To determine the sensing limit of our nanowires, we analyzed the signal-to-noise ratio in our fabricated nanowire FETs. To target specificity of analytes in solution the surface has to be (i) passivated against pH reactions and (i) functionalized for specific targeting. To address these issues we investigated passivation and functionalization of the surface. Finally we introduce the developed nanowire array platform. The design enables new experiments, e.g., time-resolved correlation measurements as well as differential measurements using multiple functionalization, which is beyond the scope of this thesis. As a last step we discuss first results towards (bio-) chemical sensing using streptavidin and conclude this thesis in chapter VI. The developed sensing platform will allow conducting further experiments to explore the use of nanowire ion-sensitive field-effect transistors as (bio-) chemical sensors

High mobility graphene ion-sensitive field-effect transistors by noncovalent functionalization

Noncovalent functionalization is a well-known nondestructive process for property engineering of carbon nanostructures, including carbon nanotubes and graphene. However, it is not clear to what extend the extraordinary electrical properties of these carbon materials can be preserved during the process. Here, we demonstrated that noncovalent functionalization can indeed delivery graphene field-effect transistors (FET) with fully preserved mobility. In addition, these high-mobility graphene transistors can serve as a promising platform for biochemical sensing applications

Investigation of Protein Detection Parameters Using Nanofunctionalized Organic Field-Effect Transistors

Biodetection using organic field-effect transistors (OFETs) is gaining increasing interest for applications as diverse as food security, environmental monitoring, and medical diagnostics. However, there still lacks a comprehensive, empirical study on the fundamental limits of OFET sensors. In this paper, we present a thorough study of the various parameters affecting biosensing using an OFET decorated with gold nanoparticle (AuNP) binding sites. These parameters include the spacing between receptors, pH of the buffer, and ionic strength of the buffer. To this end, we employed the thrombin protein and its corresponding DNA binding aptamer to form our model detection system. We demonstrate a detection limit of 100 pM for this protein with high selectivity over other proteases <i>in situ</i>. We describe herein a feasible approach for protein detection with OFETs and a thorough investigation of parameters governing biodetection events using OFETs. Our obtained results should provide important guidelines to tailor the sensor’s dynamic range to suit other desired OFET-based biodetection applications

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

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 KCl 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

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