pH sensors based on amino-terminated carbon nanomembrane and single-layer graphene van der Waals heterostructures

Abstract

The ability of graphene to transduce an adsorption event of ions into a detectable electrical signal has sparked a lot of interest for its use in sensors. However, a low concentration of the chemically active sites for binding analytes on the graphene surface has significantly prevented its applications so far. Here, we report on implementation of the van der Waals heterostructure based on a monolayer graphene and an ∼1-nm-thick molecular carbon nanomembrane (CNM) in a solution-gated field-effect transistor (FET) for pH sensing. The nondestructive functionalization of a graphene FET with the amino-terminated CNM (NH2-CNM) enables the induction of chemically active groups in the vicinity of the graphene sheet, maintaining its charge carrier transport properties. We applied complementary characterization techniques, including Raman spectroscopy, x-ray photoelectron spectroscopy, and optical and atomic force microscopy as well as field-effect and electrical impedance measurements to characterize the engineered NH2-CNM/graphene devices. We demonstrate their high pH resolution with a minimum detectable pH change of ∼0.01 at pH 2 and ∼0.04 at pH 12, with a response time in the range of seconds, and we apply an electrical double-layer model to rationalize the experimentally observed performance theoretically. The developed device concept enables the engineering of microscale pH sensors for applications in biological and environmental sciences.