Quantifying the intrinsic surface charge density and charge-transfer resistance of the graphene-solution interface through bias-free low-level charge measurement

TL;DR

This study introduces a low-noise, bias-free method to quantify the surface charge density and charge-transfer resistance at the graphene-liquid interface, providing insights crucial for bioelectronic applications.

Contribution

It presents a reproducible, minimally perturbative technique for measuring interfacial charge properties of graphene in aqueous solutions, aligning well with theoretical predictions.

Findings

Adsorbed ions have a negative surface charge density of approximately -32.8 mC/m².

Charge transfer resistance is about 6.5 MΩ·cm².

Normalized current noise follows a 1/f characteristic with α≈1.1.

Abstract

Liquid-based bio-applications of graphene require a quantitative understanding of the graphene-liquid interface, with the surface charge density of adsorbed ions, the interfacial charge transfer resistance, and the interfacial charge noise being of particular importance. We quantified these properties through measurements of the zero-bias Faradaic charge-transfer between graphene electrodes and aqueous solutions of varying ionic strength using a reproducible, low-noise, minimally perturbative charge measurement technique. The measurements indicated that adsorbed ions had a negative surface charge density of approximately -32.8 mC m-2 and that the specific charge transfer resistance was 6.5pm0.3 M${\Omega}$ cm2. The normalized current noise power spectral density for all ionic concentrations tested collapsed onto a 1/f characteristic with ${\alpha}$=1.1pm0.2. All the results are in excellent agreement with predictions of the theory for the graphene-solution interface. This minimally-perturbative method for monitoring charge-transfer at the sub-pC scale exhibits low noise and ultra-low power consumption (~ fW), making it well-suited for use in low-level bioelectronics in liquid environments.