Single-Ion Spectroscopy of h-BN Point Defect Fluorescence in Liquid Environments

Understanding the chemical state of individual ions in solutions is crucial for advancing knowledge of complex chemical systems. However, tracking and analyzing materials at the single-ion level in liquid environments remains a significant challenge. We present a strategy for imaging the single ions in liquid environments, using spectral information provided by point defects in hexagonal boron nitride (h-BN) as the ion sensors. These optically active point defects in h-BN interact with ions, altering their emission properties. This interaction enables the detection and visualization of single ions. Using Li⁺ ions in organic electrolytes as a model system, a spectral shift of over 10 nm was observed upon Li⁺ ion addition, and an over 50 nm red shift with applied electric fields, due to reactions between Li⁺ ion and h-BN point defects. Frequency domain analysis further revealed the rapid dynamics of ion migration and the slow electrochemical reactions. Various ions (H⁺, Li⁺, Na⁺, K⁺, Zn²⁺, Al³⁺) in aqueous solutions were further spectroscopically differentiated, demonstrating that each ion’s distinct electron cloud configuration interacts uniquely with the electron clouds of h-BN defects, producing specific and identifiable spectroscopic signatures. This platform enables the direct visualization of ions and their chemical states in a liquid environment during reactions, offering insights into chemical reactions at the single-ion level. This capability presents potential applications in various fields involving ions in liquids that include battery technology and environmental science.