Liquid-activated quantum emission from pristine hexagonal boron nitride for nanofluidic sensing

Abstract

Liquids confined down to the atomic scale can show radically new properties. However, only indirect and ensemble measurements operate in such extreme confinement, calling for novel optical approaches that enable direct imaging at the molecular level. Here we harness fluorescence originating from single-photon emitters at the surface of hexagonal boron nitride for molecular imaging and sensing in nanometrically confined liquids. The emission originates from the chemisorption of organic solvent molecules onto native surface defects, revealing single-molecule dynamics at the interface through the spatially correlated activation of neighbouring defects. Emitter spectra further offer a direct readout of the local dielectric properties, unveiling increasing dielectric order under nanometre-scale confinement. Liquid-activated native hexagonal boron nitride defects bridge the gap between solid-state nanophotonics and nanofluidics, opening new avenues for nanoscale sensing and optofluidics.

Fig. 1. Liquid-induced fluorescence from pristine hBN crystals.

a, Sketch of the experimental setup. b, Wide-field fluorescence images of an hBN crystal under 3.5 kW cm^–2 and 561 nm laser light illumination with 1 s exposure time. No fluorescence was observed in water, but in ethanol, the entire crystal surface became fluorescent. The images underwent linear contrast enhancement. c, A zoomed-in view of the dashed yellow box in b reveals dense clusters of emission when the laser is turned ON, with 6 ms exposure time. After 10 s of wide-field illumination, the crystal surface reached a stable number of diffraction-limited isolated emitters. d, Localization microscopy-based counting of the emitters as a function of illumination time: after 5 s, a steady state was reached. The dashed line is a fit to an offset exponential relaxation. e, Liquid dependency of the crystal fluorescence, showing strongly activating liquids (type I), mildly activating liquids (type II) and no activation in water (type III). Source data

Fig. 2. Surface of pristine hBN reveals interfacial molecular dynamics.

a, Overlay of a super-resolved image from 5,000 frames showing hopping emitters in isopropanol as the linked trajectories, as well as trapped spots. b, Artist’s view of the correlated activation of neighbouring defects leading to the trajectories. c, Representative intensity traces from the same images, taken from 7 × 7 pixel bins around emitters (dashed yellow box in Fig. 1c) with 6 ms exposure time. The top right trace corresponds to a long defect activation. The top left trace corresponds to a short activation of the same defect, magnified in the bottom panel. d, Distribution of residence times on single defects and for the entire trajectories. The dotted lines are fits to a two-component exponential decay. e, Displacement probability density functions (PDF) of the trajectories after different lag times τ = 6 ms, 24 ms, 66 ms,142 ms. The dashed lines are fits to two-component Gaussians. f, Visualizing the evolution of the two modes of the Gaussian fit in e with increasing lag time. The central region, corresponding to the trapped state, remains of a constant width, whereas the tails, corresponding to hopping, enlarge with time. The solid line is a fit to a standard diffusion curve. Source data

Fig. 3. Spectral properties of surface dipole emitters coupled to both solid and liquid environments.

a, sSMLM splits the fluorescence signal from an emitter into a localization component (left; single-molecule localization microscopy (sSMLM)) and a spectral component (right) on the same camera chip. b, Ensemble spectra of liquid-activated emitters in different type-I solvents, exhibiting a clear ZPL and PSB. c, Visualizing the wavelength shifts of both ZPL (circles) and PSB (squares), which correlate with the dielectric constant of the liquid. The peak positions were obtained by fitting to a sum of Lorentzians. The dashed line indicates the linear solvatochromic range, with a slope of 1 nm per unit. The error bars correspond to standard deviations of fitting parameters from groups of 100 single-molecule spectra. d, Jablonski diagram of processes at play: 561 nm laser excitation induces a dipolar excited state, which can directly emit (orange arrow; ZPL) or with the emission of a phonon (red arrow; PSB). e, Normalized intensity as a function of input-light or output-light polarization angle α relative to the emitter axis. The solid lines correspond to fits to ideal electric dipole emission cos²α + constant. Supplementary Fig. 11 provides more details. f, Sketch of an excited emitter that can interact with the crystal through phonons as well as with the surrounding molecules (yellow ellipses). Source data

Fig. 4. Quantum emission from liquid-activated emitters.

a, Representative PL trace from an isolated emitter in acetonitrile under 0.7 mW confocal excitation. The shaded region corresponds to the 10-s-long trace used for photon statistics. b, Normalized coincidences g⁽²⁾ measured from time-correlated single-photon counting in a Hanbury Brown and Twiss geometry, in hexadecane and acetonitrile. In both liquids, a pronounced antibunching is observed with g⁽²⁾(0) < 0.5 without background correction, proving the single-photon emission. The fluorescent lifetimes, corresponding to the width of the antibunching dip, were found to be 2.73 ± 0.09 ns and 2.20 ± 0.20 ns for acetonitrile and hexadecane, respectively. The spectra are shown in the inset, demonstrating liquid-tunable single-photon emission from 615 to 636 nm. c, Single-photon statistics under pulsed laser excitation, showing a suppression of the central peak due to antibunching. Source data

Fig. 5. Nanoslit-embedded liquid-activated emitters.

a, Sketch of the heterostructure nanoslit device. The red glow indicates an emitter inside the nanoslit. b, Overlay of a super-resolved image of masked ethanol-activated hBN and the atomic force microscopy mapping of the graphene spacers. c, Optical micrograph of the heterostructure. On the purple-coloured part of the image, only graphene spacers on the hBN bottom crystal are present, which leads to masked hBN. The bottom blue region corresponds to the full heterostructure with slits. d, Super-resolved image of acetonitrile-activated emitters embedded in 2.4-nm-high nanoslits, from 30,000 frames with 20 ms exposure time and 1.4 kW cm^–2 illumination. e, Comparison of localization intensity distributions for masked hBN and 2.4 nm nanoslits in acetonitrile, showing no loss of photons but an overall reduction in the number of localizations (Supplementary Fig. 18). f, Illustration of the effect of confinement: the liquid dielectric constant can be changed by confinement (ϵ_conf) and the defect dipole can interact with solvent molecules (yellow ellipses) within a range ℓ_dip, comparable with the confinement size h. g, Representative trajectories in the 2.4-nm-high nanoslits filled with ethanol, overlaid with the super-resolved image. h,i, sSMLM spectra of liquid-activated defects in nanoslits filled with ethanol (h) and acetonitrile (i). The confinement size is tuned from the open geometry to 2.4 nm down to 1.4 nm. The solid lines correspond to two-component Lorentzian fits, and the black dashes indicate the extracted ZPL position. Source data