  Understanding individual ions in solutions is essential for advancing our
knowledge of complex chemical systems. However, tracking and detecting ions at
the single-ion level in liquid environments remains a challenge. We introduce a
strategy for visualization and differentiation of different ions in liquid
environment via point defects in hexagonal boron nitride (h-BN) as the ion
sensor. Ions interacting with the optically active point defects in h-BN alter
emission properties, allowing us to capture these changes and visualize single
ions. Using Li+ in organic electrolytes as a model, we observed a spectral
shift of over 10 nm upon Li+ addition, and an over 50 nm red shift with applied
electric fields due to reactions between Li+ and h-BN point defects. Frequency
domain analysis further revealed the rapid dynamics of ion migration and the
slow electrochemical reactions. We further spectroscopically differentiated
various ions (H+, Li+, Na+, K+, Zn2+, Al3+) in aqueous solution. Each ion, with
its distinct electron cloud configuration, interacts distinctively with the
electron clouds of h-BN defects, resulting in specific and identifiable
spectroscopic signatures. This ion sensing platform enables the direct
visualization and differentiation of individual ions in a liquid environment,
offering insights into chemical reactions at the single-ion level. This
capability presents potential applications in various fields involving ions in
liquids, including but not limited to biology, battery technology, and
environmental science.
