Probing aqueous interfaces with spin defects

Understanding the physical and chemical properties of aqueous interfaces is important in diverse fields of science, ranging from biology and chemistry to materials science. In spite of crucial progress in surface sensitive spectroscopic techniques over the past decades, the microscopic properties of aqueous interfaces remain difficult to measure. Here, we explore the use of noise spectroscopy to characterize interfacial properties, specifically of quantum sensors hosted in two-dimensional materials in contact with water. We combine molecular dynamics simulations of water interfaced with a model two-dimensional substrate and the calculation of the dynamical properties of a spin defect, representing a quantum sensor, and we investigate the impact of interfacial water and simple ions on the decoherence time of the defect. We show that the Hahn echo coherence time of the quantum sensor is sensitive to motional narrowing and to the hydrogen bonding arrangement and the dynamical properties of water and ions at the interface. We present results as a function of the liquid temperature, strength of the water-surface interaction, and varied monovalent and divalent ions, highlighting the broad applicability of near-surface quantum sensors to gain insight into the properties of aqueous interfaces.