pH sensitivity of interfacial electron transfer at a supported graphene monolayer

Electrochemistry at the Edge of a van der Waals Heterostructure

Artificial van der Waals heterostructures, obtained by stacking two-dimensional (2D) materials, represent a novel platform for investigating physicochemical phenomena and applications. Here, the electrochemistry at the one-dimensional (1D) edge of a graphene sheet, sandwiched between two hexagonal boron nitride (hBN) flakes, is reported. When such an hBN/graphene/hBN heterostructure is immersed in a solution, the basal plane of graphene is encapsulated by hBN, and the graphene edge is exclusively available in the solution. This forms an electrochemical nanoelectrode, enabling the investigation of electron transfer using several redox probes, e.g., ferrocene(di)methanol, hexaammineruthenium, methylene blue, dopamine and ferrocyanide. The low capacitance of the van der Waals edge electrode facilitates cyclic voltammetry at very high scan rates (up to 1000 V s-1), allowing voltammetric detection of redox species down to micromolar concentrations with sub-second time resolution. The nanoband nature of the edge electrode allows operation in water without added electrolyte. Finally, two adjacent edge electrodes are realized in a redox-cycling format. All the above-mentioned phenomena can be investigated at the edge, demonstrating that nanoscale electrochemistry is a new application avenue for van der Waals heterostructures. Such an edge electrode will be useful for studying electron transfer mechanisms and the detection of analyte species in ultralow sample volumes.

Evaluating diverse electrode surface patterns of 3D printed carbon thermoplastic electrochemical sensors

Electrochemical sensing techniques rely on redox reactions taking place at the electrode surface. The configuration of this surface is of the utmost importance in the advancement of electrochemical sensors. The majority of previous electrode manufacturing methods, including 3D printing have produced electrodes with flat surfaces. There is a distinct potential for 3D printing to create intricate and distinctive electrode surface shapes. In the proposed work, 3D printed carbon black polylactic acid electrodes with nine different surface morphologies were made. These were compared to a flat surface electrode. To evaluate the performance of the electrodes, measurements were conducted in three different redox probes (ferrocene methanol, ferricyanide, and dopamine). Our findings highlighted that when electrodes were normalised for the geometric surface area of the electrode, the surface pattern of the electrode surface can impact the observed current and electron transfer kinetics. Electrodes that had a dome and flag pattern on the electrode surface showed the highest oxidation currents and had lower values for the difference between the anodic and cathodic peak current (ΔE). However, designs with rings had lower current values and higher ΔE values. These differences are most likely due to variations in the accessibility of conductive sites on the electrode surface due to the varying surface roughness of different patterned designs. Our findings highlight that when making electrodes using 3D printing, surface patterning of the electrode surface can be used as an effective approach to enhance the performance of the sensor for varying applications.

Graphene-on-gold surface plasmon resonance sensors resilient to high-temperature annealing

Gold films coated with a graphene sheet are being widely used as sensors for the detection of label-free binding interactions using surface plasmon resonance (SPR). During the preparation of such sensors, it is often essential to subject the sensor chips to a high-temperature treatment in order to ensure a clean graphene surface. However, sensor chips used currently, which often use chromium as an adhesion promoter, cannot be subjected to temperatures above 250 °C, because under such conditions, chromium is found to reorganize and diffuse to the surface, where it is easily oxidized, impairing the quality of SPR spectra. Here we present an optimized preparation strategy involving a three-cycle tempering coupled with chromium (oxide) etching, which allows the graphene-coated SPR chips to be annealed up to 500 °C with little deterioration of the surface morphology. In addition, the treatment delivers a surface that shows a clear enhancement in spectral response together with a good refractive index sensitivity. We demonstrate the applicability of our sensors by studying the kinetics of avidin-biotin binding at different pH repeatedly on the same chip. The possibility to anneal can be exploited to recover the original surface after sensing trials, which allowed us to reuse the sensor for at least six cycles of biomolecule adsorption.

Modulation of the kinetics of outer-sphere electron transfer at graphene by a metal substrate

Solid-supported graphene is a typical configuration of electrochemical devices based on single-layer graphene. Therefore, it is necessary to understand the electrochemical features of such heterostructures. In this work, we theoretically investigated the effect of the metal type on the nonadiabatic electron transfer (ET) at the metal-supported graphene using DFT calculations. We considered five metals Au, Ag, Pt, Cu, and Al on which graphene is physically adsorbed. It is shown that all metals catalyze the ET. The electrocatalytic effect increases in the following series Al < Au ≲ Ag ≈ Cu < Pt. The enhanced ET in the presence of the metal substrate is explained by the hybridization of metal and graphene states, due to which the coupling between the reactant in an electrolyte and metal is increased. Metal-dependent electrocatalytic effect is explained both by different densities of states at the Fermi level of the systems and by differences in the behaviour of the tails of hybridized wave functions in the electrolyte region. The shift of the Fermi level with respect to the Dirac point in graphene when charging at the metal/graphene/electrolyte interface does not affect the kinetics due to the small contribution of graphene states to the electron transfer.

A highly durable graphene monolayer electrode under long-term hydrogen evolution cycling

Achieving long term stability of single graphene sheets towards repeated electrochemical hydrogen evolution reaction (HER) cycling has been challenging. Here, we show through appropriate electrode preparation that it is possible to obtain highly durable isolated graphene electrodes, which can survive several hundreds of HER cycles with virtually no damage to the sp²-carbon framework and persistently good electron transfer characteristics.

Faradaic effects in electrochemically gated graphene sensors in the presence of redox active molecules

Field-effect transistors (FETs) based on graphene are promising devices for the direct sensing of a range of analytes in solution. We show here that the presence of redox active molecules in the analyte solution leads to the occurrence of heterogeneous electron transfer with graphene generating a Faradaic current (electron transfer) in a FET configuration resulting in shifts of the Dirac point. Such a shift occurs if the Faradaic current is significantly high, e.g. due to a large graphene area. Furthermore, the redox shift based on the Faradaic current, reminiscent of a doping-like effect, is found to be non-Nernstian and dependent on parameters known from electrode kinetics in potentiodynamic methods, such as the electrode area, the standard potential of the redox probes and the scan rate of the gate voltage modulation. This behavior clearly differentiates this effect from other transduction mechanisms based on electrostatic interactions or molecular charge transfer doping effects, which are usually behind a shift of the Dirac point. These observations suggest that large-area unmodified/pristine graphene in field-effect sensors behaves as a non-polarized electrode in liquid. Strategies for ensuring a polarized interface are discussed.