Peering into nanofluidic mysteries one photon at a time

Nanofluidics, the study of fluids confined within ultra-small spaces, offers insights into the behaviour of liquids on a nanometer scale. However, exploring the movement of individual molecules in such confined environments has been challenging due to the limitations of conventional microscopy techniques. This obstacle prevented real-time sensing and imaging, leaving significant gaps in our knowledge of molecular properties in confinement.

A team led by Professor Radha Boya in the Department of Physics at The University of Manchester makes nanochannels which are only one-atom to few-atom thin using two-dimensional materials as building blocks.

Prof Boya said: "Seeing is believing, but it is not easy to see confinement effects at this scale. We make these extremely thin slit-like channels, and the current study shows an elegant way to visualise them by super-resolution microscopy."

The study's findings are published in the journal Nature Materials.

The partnership with the EPFL team allowed for optical probing of these systems, uncovering hints of liquid ordering induced by confinement.

Thanks to an unexpected property of boron nitride, a graphene-like 2D material which possesses a remarkable ability to emit light when in contact with liquids, researchers at EPFL's Laboratory of Nanoscale Biology (LBEN) have succeeded in directly observing and tracing the paths of individual molecules within nanofluidic structures.

This revelation opens the door to a deeper understanding of the behaviours of ions and molecules in conditions that mimic biological systems.

Professor Aleksandra Radenovic, head of LBEN, explains: "Advancements in fabrication and material science have empowered us to control fluidic and ionic transport on the nanoscale. Yet, our understanding of nanofluidic systems remained limited, as conventional light microscopy couldn't penetrate structures below the diffraction limit. Our research now shines a light on nanofluidics, offering insights into a realm that was largely uncharted until now."

This newfound understanding of molecular properties has exciting applications, including the potential to directly image emerging nanofluidic systems, where liquids exhibit unconventional behaviours under pressure or voltage stimuli.

The research's core lies in the fluorescence originating from single-photon emitters at the hexagonal boron nitride's surface.

Doctoral student Nathan Ronceray, from LBEN, said: "This fluorescence activation came unexpected as neither hexagonal boron nitride (hBN) nor the liquid exhibit visible-range fluorescence on their own. It most likely arises from molecules interacting with surface defects on the hBN crystal, but we are still not certain of the exact mechanism,"

Dr Yi You, a post-doc from The University of Manchester engineered the nanochannels such that the confining liquids mere nanometers from the hBN surface which has some defects.

Surface defects can be missing atoms in the crystalline structure, whose properties differ from the original material, granting them the ability to emit light when they interact with certain molecules.

The researchers further observed that when a defect turns off, one of its neighbours lights up, because the molecule bound to the first site hopped to the second. Step by step, this enables reconstructing entire molecular trajectories.

Using a combination of microscopy techniques, the team monitored colour changes to successfully demonstrate that these light emitters emit photons one at a time, offering pinpoint information about their immediate surroundings within around one nanometer. This breakthrough enables the use of these emitters as nanoscale probes, shedding light on the arrangement of molecules within confined nanometre spaces.

The potential for this discovery is far-reaching. Nathan Ronceray envisions applications beyond passive sensing.

He said: "We have primarily been watching the behaviour of molecules with hBN without actively interacting with, but we think it could be used to visualize nanoscale flows caused by pressure or electric fields.

"This could lead to more dynamic applications in the future for optical imaging and sensing, providing unprecedented insights into the intricate behaviours of molecules within these confined spaces."

The project received funding from the European Research Council, Royal Society University Research Fellowship, Royal Society International Exchanges Award and EPSRC New Horizons grant.