Imaging of CO2 and Dissolved Inorganic Carbon via Electrochemical Acidification–Optode Tandem

Abstract

Dissolved inorganic carbon (DIC) is a key component of theglobal carbon cycle and plays a critical role in ocean acidification andproliferation of phototrophs. Its quantification at a high spatial resolution isessential for understanding various biogeochemical processes. We present ananalytical method for 2D chemical imaging of DIC by combining aconventional CO2 optode with localized electrochemical acidification from apolyaniline (PANI)-coated stainless-steel mesh electrode. Initially, the optoderesponse is governed by local concentrations of free CO2 in the sample,corresponding to the established carbonate equilibrium at the (unmodified)sample pH. Upon applying a mild potential-based polarization to the PANI mesh, protons are released into the sample, shifting thecarbonate equilibrium toward CO2 conversion (>99%), which corresponds to the sample DIC. It is herein demonstrated that theCO2 optode−PANI tandem enables the mapping of free CO2 (before PANI activation) and DIC (after PANI activation) in complexsamples, providing high 2D spatial resolution (approx. 400 μm). The significance of this method was proven by inspecting thecarbonate chemistry of complex environmental systems, including the freshwater plant Vallisneria spiralis and lime-amendedwaterlogged soil. This work is expected to pave the way for new analytical strategies that combine chemical imaging withelectrochemical actuators, aiming to enhance classical sensing approaches via in situ (and reagentless) sample treatment. Such toolsmay provide a better understanding of environmentally relevant pH-dependent analytes related to the carbon, nitrogen, and sulfurcycles.