Triboelectric Spectroscopy for Identification of Metal Ion Valence States in Aqueous Solutions

Ion valence state analysis in aqueous solutions is of critical importance as the oxidation state of metal ions governs their chemical reactivity, environmental behavior, and biological effects. Conventional spectroscopic and electrochemical techniques are often limited by low selectivity, poor applicability, complex and lengthy sample preparation, or requiring bulky or expensive instrumentation. Here, we use the functional relationship between triboelectric charge transfer and trajectory position as triboelectric spectroscopy (TES), for the rapid and efficient identification of metal ion valence states in aqueous solution. By analyzing the peaks positions and corresponding magnitudes of charge transfer in TES, we successfully identified 15 different valence states across seven metal elements, achieving a detection accuracy above 90% with detection limits down to the ppb level. Most importantly, this work advances the fundamental understanding of liquid-solid contact electrification by identifying key physicochemical factors (ion valence and nanoparticle surface states) that govern the spatial distribution of charge transfer, helping to address a key unresolved scientific question─the origin of the non-uniform, peak-like charge transfer distribution. The information derived from TES offers a broadly applicable, ultrafast (<0.6 s), non-destructive, low-cost, and portable analytical tool for assessing the oxidation states of metal elements, including both ions and suspended nanoparticles in liquid, demonstrating significant potential for analytical chemistry, real-time environmental monitoring, and biomedical diagnostics.