High-Throughput In-Operando Terahertz Time-Domain Spectroscopy of Chemical Vapor Deposited Graphene for pH Sensing

Reliable pH sensing in liquid environments underpins applications in energy storage, biosensing, and process monitoring, yet conventional graphene-based electrical devices introduce ohmic contacts, encapsulation layers, and electrochemical artifacts that can obscure intrinsic behavior. These approaches also fail to capture the strong spatial variability of commercial CVD graphene. Here, we present a fully contactless methodology for quantifying pH-dependent conductivity of large-area CVD graphene using through-substrate reflection terahertz time-domain spectroscopy (THz-TDS). A customized liquid cell with a THz transparent window preserves unrestricted access to the graphene–electrolyte interface while enabling centimeter-scale conductivity mapping and in-operando single-point monitoring during sequential pH modulation. By integrating spatially registered Raman mapping with automated quality-metric filtering, we extract statistically robust trends across thousands of pixels rather than isolated device regions. We show that pristine monolayer graphene exhibits the highest baseline conductivity and strongest pH sensitivity, whereas O2-plasma-introduced defects systematically suppress both. This scalable, remote, and inert probing approach establishes THz-TDS as a powerful platform for high-throughput characterization and robust liquid-phase sensing in thin films and 2D systems.