Quantum Defects in Boron Nitride Let Scientists Watch Single Ions Move in Liquid

For decades, chemists and battery engineers have had to infer what individual ions are doing inside liquids from bulk measurements that average the behavior of trillions of particles at once. Now a team led by researchers at Stanford University and SLAC National Accelerator Laboratory, working with crystal growers at the National Institute for Materials Science in Japan, has demonstrated a way to watch single ions in real time using nothing more exotic than a fluorescent defect in an atomically thin sheet of hexagonal boron nitride. The work, published in Nature Sensors, shows that optically active defects in this two-dimensional material respond to the presence of individual ions with measurable shifts in their emission spectra, opening a window onto processes that have long been hidden at the interface between solids and liquids.

The principle behind the sensor is elegantly simple, even if the engineering behind it is not. Hexagonal boron nitride, a layered crystal often described as a wide-band-gap insulator, hosts point defects that emit light when excited by a laser. These emitters behave much like the nitrogen-vacancy centers in diamond that have become workhorses of quantum sensing, but they live in a material that is only a few atoms thick. When an ion in the surrounding liquid approaches or binds near one of these defects, the local electrostatic environment changes, and the energy levels of the defect shift accordingly. That shift appears as a change in the wavelength of the emitted fluorescence, which the researchers can read out with a spectrally resolved optical imaging system.

To prove the concept, the team used lithium ions in organic electrolytes as their model system, a choice motivated by the central role that lithium-ion dynamics play in rechargeable batteries. When lithium ions were added to the electrolyte surrounding an h-BN flake, individual defects responded with spectral shifts exceeding 10 nanometers, a remarkably large change for a single-ion event. The magnitude of the shift matters enormously for practical sensing: a signal of 10 nanometers is far above the noise floor of a well-calibrated optical system, which means individual ion arrivals can be distinguished unambiguously rather than inferred statistically from ensemble averages.

The researchers then discovered that they could amplify the effect dramatically. By applying an external electric field across the liquid cell, they increased the observed spectral shifts to more than 40 nanometers. This enhancement arises because the field perturbs the local ion distribution near the defect, effectively modulating the electrostatic landscape that the emitter experiences. With such large shifts, the platform becomes capable of real-time monitoring of local ion perturbations, meaning the researchers could follow ions as they arrived, departed, or reorganized around individual defects on the h-BN surface. The temporal resolution of the measurement reaches the millisecond scale, fast enough to capture many of the dynamic processes relevant to electrochemistry.

Perhaps the most striking capability demonstrated in the study is chemical discrimination. Individual defects did not merely register the presence of ions; they distinguished between different ion species. Sodium, magnesium, and aluminum ions each produced characteristic spectral signatures, allowing the researchers to identify which ion was interacting with a given defect. This selectivity emerges from differences in charge, ionic radius, and hydration chemistry among the species, each of which leaves a distinct fingerprint on the defect’s emission spectrum. In experiments spanning lithium, sodium, potassium, and cesium ions in aqueous solutions, the distributions of spectral peak positions differed measurably between ion types, suggesting a general route to ion-specific sensing without the ion-selective membranes that conventional electrochemical sensors require.

The sensitivity of the platform is equally noteworthy. The researchers report detection at concentrations down to the 10 micromolar range, which corresponds to roughly ten billion ions per liter. While that sounds like an enormous number, it represents a dilute regime in which individual binding and unbinding events at the defect surface can be resolved rather than swamped by the ambient population. Combined with the millisecond temporal resolution, this sensitivity places the technique in a regime where it can probe the stochastic, single-molecule dynamics that govern interfacial chemistry, rather than the smooth averages that bulk techniques report.

The experimental apparatus itself reflects careful engineering. The team built microfluidic cells, some equipped with electrodes for electrical testing, that hold the h-BN flakes in contact with electrolytes while an optical system images the fluorescent emission. Wavelength calibration was performed using a calcium fluoride prism and regression models that correlate pixel positions with wavelengths across the field of view, achieving a resolution of roughly 1 to 4 nanometers depending on the spectral region. Drift correction using calibration nanoparticles kept positional uncertainty below one pixel over thousand-frame recordings. Statistical analysis of emitter counts followed Poisson distributions, confirming that the observed signals arose from well-defined, individually addressable defects rather than collective artifacts.

The implications for battery research are immediate. Understanding how lithium ions move, accumulate, and react at electrode-electrolyte interfaces is one of the central unsolved problems in energy storage, and it dictates phenomena from charging speed to degradation and dendrite formation. Existing tools, from cryogenic electron microscopy to synchrotron X-ray methods and Raman spectroscopy, either require freezing the sample, average over large areas, or lack the sensitivity to see single ions. A sensor that sits directly in the liquid environment and reports on individual ion arrivals at a specific surface site offers a fundamentally new vantage point. The same logic extends to environmental chemistry, where detecting trace metal ions in water is a persistent analytical challenge, and to the broader study of interfacial electrochemistry, catalysis, and ion transport through nanoscale channels.

The work also builds on a rapidly maturing body of research into quantum emitters in two-dimensional materials. Since single-photon emission from h-BN was first reported in monolayers, researchers have catalogued defect types, developed plasma-based creation methods, and demonstrated room-temperature spin defects usable as qubits. Recent studies have already used h-BN sensors to detect paramagnetic spins in liquids and to observe water-mediated proton transport at surface defects. The new result extends this quantum-sensing toolkit from spins and protons to the full range of dissolved ions, leveraging the extreme surface sensitivity that only an atomically thin host can provide. Because the defects sit at or near the surface, ions in the adjacent liquid interact with them directly, without the screening and distance penalties that plague buried sensors in bulk crystals.

There are, of course, questions that future work must address. The precise atomic structure of the optically active defects remains an active area of investigation, and theoretical computations performed by the team, in collaboration with the SUNCAT Center for Interface Science and Catalysis at SLAC, point toward defect-mediated interactions that will need further first-principles characterization. Long-term stability of the emitters in aggressive electrochemical environments, calibration across different solvent systems, and the development of arrays of addressable defects for spatially resolved ion mapping are all natural next steps. But the core demonstration stands: a fluorescent defect in a two-dimensional crystal can register the arrival of a single ion in a liquid, tell you what kind of ion it is, and do so in milliseconds. For a field that has spent half a century averaging away the very events that matter most, that is a transformative capability, and one that is likely to spread quickly from battery labs to wherever single ions write the story of chemistry at interfaces.

Subject of Research: Single-ion detection in liquids using fluorescent defects in hexagonal boron nitride

Article Title: Single-ion sensing in liquid using fluorescent hexagonal boron nitride defects

Article References: Wu, Y., Xu, K., Sarker, H. P., Taniguchi, T., Watanabe, K., Abild-Pedersen, F., Majumdar, A., Cui, Y., Tzeng, Y.-K., & Chu, S. (2026). Single-ion sensing in liquid using fluorescent hexagonal boron nitride defects. Nature Sensors. https://doi.org/10.1038/s44460-026-00150-z

Image Credits: AI Generated

DOI: 10.1038/s44460-026-00150-z

Keywords: hexagonal boron nitride, quantum defects, single-ion sensing, fluorescence spectroscopy, lithium ions, two-dimensional materials, battery interfaces, electrochemistry, quantum sensing, spectral shifts, microfluidics, ion discrimination