🔬 mesoscale physics

Liquid-Phase Near-Field Infrared Nanoscopy of Ion-Exchange Reactions

This study demonstrates the use of thin SiC membrane-enabled liquid-phase infrared nanoscopy to achieve in situ, chemically specific, and nanometer-scale monitoring of Ca $^{2+}$ /Na $^{+}$ ion exchange and the subsequent formation of calcium terephthalate coordination frameworks under aqueous conditions.

Original authors: Wassie M. Takele, Teferi Sitotaw Yallew, Mason Caron, Yohannes Abate

Published 2026-09-15

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CC BY 4.0

Original authors: Wassie M. Takele, Teferi Sitotaw Yallew, Mason Caron, Yohannes Abate

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Chemistry in water is a world of constant, invisible motion. In the liquid environments that sustain life and drive industrial processes, metal ions and organic molecules are in a perpetual state of meeting, binding, and swapping partners. These interactions, where a metal atom latches onto a specific part of a molecule, dictate how proteins function, how gels expand and contract, and how new materials are built. For scientists, watching these events happen in real time is a major challenge. While microscopes can see the shape of tiny particles, they often struggle to identify what those particles are made of when submerged in water. Conversely, techniques that can identify chemical composition usually lack the sharpness to see the tiny, localized changes where reactions actually begin. Bridging this gap—seeing both the shape and the chemical identity of a reaction as it unfolds in a liquid—has long been a goal for researchers studying everything from battery chemistry to biological signaling.

A team of researchers at the University of Georgia has now demonstrated a way to watch these invisible chemical swaps happen with unprecedented clarity. They focused on a specific reaction where calcium ions replace sodium ions in a solution containing a common organic molecule called disodium terephthalate. When calcium takes the place of sodium, the two molecules bind together to form a new solid structure known as calcium terephthalate. To observe this, the scientists built a specialized viewing window. They placed the liquid reaction inside a tiny cell covered by an incredibly thin, transparent membrane made of silicon carbide, a material strong enough to hold back the liquid but thin enough to let light pass through. This setup allowed them to use a highly sensitive microscope probe, which hovers just above the membrane in the air, to "feel" the chemical vibrations of the molecules trapped underneath.

The researchers used a beam of infrared light, which is sensitive to the way chemical bonds vibrate, to probe the scene. As the calcium ions entered the solution and began to interact with the organic molecules beneath the membrane, the team watched for changes in the way the molecules absorbed this light. They found that the specific vibrations of the chemical groups holding the metal ions shifted noticeably. Before the reaction, the molecules showed a distinct pattern of vibration typical of their sodium-bound state. Once the calcium was introduced, these vibrations changed, signaling that the metal ions had successfully swapped places and that a new coordination environment had formed. The data revealed that the calcium ions were binding to the organic molecules in a specific, stable arrangement, creating a new chemical phase right under the membrane.

Beyond just listening to the chemical vibrations, the team also mapped the physical changes occurring in the liquid. Before the reaction, the area beneath the membrane appeared relatively uniform. After the calcium was added, the microscope revealed the sudden appearance of tiny, distinct particles. These were the new calcium-based structures forming and precipitating out of the solution. By comparing the chemical signals and the physical shapes, the researchers confirmed that the formation of these particles was directly linked to the chemical exchange they had observed. The new structures were not random clumps but well-defined particles with a consistent chemical signature, proving that the reaction had produced a specific, ordered material.

This work demonstrates that it is possible to monitor complex chemical reactions in water with a level of detail previously reserved for dry, solid samples. By using a thin membrane to separate the liquid from the sensitive probe, the researchers avoided the problem of water absorbing the light and drowning out the signal. They showed that they could track the precise moment a metal ion binds to a molecule and watch the resulting material take shape, all while the reaction was happening in its natural, wet environment. This capability opens the door to studying a wide range of processes, from how catalysts work in industrial reactors to how biological molecules assemble in cells, providing a new lens through which to view the dynamic chemistry of the aqueous world.

Technical Summary: Liquid-Phase Near-Field Infrared Nanoscopy of Ion-Exchange Reactions

Problem Statement
In situ monitoring of chemical reactions with both chemical specificity and nanometer-scale spatial resolution under aqueous conditions remains a significant experimental challenge. While aqueous-phase metal–ligand interactions govern critical processes in chemistry, materials science, and biology, direct probing of these coordination environments in solution is limited. Existing techniques face specific drawbacks: Liquid-Phase Transmission Electron Microscopy (LP-TEM) offers high resolution but can perturb liquid-phase chemistry and provides limited direct molecular information on metal–ligand coordination. Atomic Force Microscopy (AFM) provides label-free topographic imaging in liquid but lacks chemical contrast, as it cannot track spectral changes associated with carboxylate-metal ion interactions. Furthermore, standard optical near-field nanoscopy is difficult to apply in liquids due to strong mid-IR water absorption and the risk of liquid contaminating the AFM tip.

Methodology
To address these limitations, the authors developed a home-built, 3D-printed liquid cell for in situ near-field nanoscopy. The device features a 30 nm-thick Silicon Carbide (SiC) membrane that acts as an infrared-transparent window, separating the metallized AFM tip (operating in air) from the liquid sample beneath. This configuration allows the evanescent near-field to couple through the membrane to probe the hydrated sample while preventing tip contamination and minimizing water absorption effects.

The study utilized this platform to monitor the Ca²⁺/Na⁺ ion-exchange reaction between disodium terephthalate (Na₂TP) and a CaCl₂ solution. The experimental approach combined:

Broadband nano-FTIR spectroscopy: To acquire complex near-field spectra (amplitude and phase) and identify vibrational modes.
Single-frequency s-SNOM imaging: To generate spatially resolved near-field amplitude and phase maps at specific excitation frequencies.
Controlled reactant delivery: CaCl₂ solution was injected through a port into the reservoir containing pre-deposited Na₂TP beneath the SiC membrane.

Key Results
The study successfully demonstrated the conversion of Na₂TP to calcium terephthalate (CaTP), a metal–organic framework (MOF)-type coordination phase, beneath the SiC membrane.

Spectral Evidence of Ion Exchange: Nano-FTIR phase spectra acquired before and after CaCl₂ injection revealed distinct changes in the carboxylate stretching modes. The symmetric carboxylate stretching mode ( $\nu_{sy}$ , COO⁻) shifted from 1420 cm⁻¹ to new features at 1405, 1425, and 1450 cm⁻¹. The asymmetric mode ( $\nu_{asy}$ , COO⁻) at 1570 cm⁻¹ remained relatively stable, while the phenyl ring stretching mode ( $\nu_{ph}$ ) near 1515 cm⁻¹ became more pronounced.
Coordination Environment Analysis: The separation between the asymmetric and symmetric carboxylate stretching modes ( $\Delta\nu$ ) was calculated to be approximately 116–123 cm⁻¹. This range suggests a coordination environment with substantial bidentate or bridging character, consistent with Ca²⁺ binding to terephthalate linkers.
Linewidth Changes: The Full Width at Half Maximum (FWHM) of the asymmetric carboxylate band decreased from approximately 50 cm⁻¹ (Na₂TP) to 34 cm⁻¹ (CaTP), indicating a modification of the vibrational response and a more ordered coordination environment upon CaTP formation.
Nanoscale Spatial Resolution: Near-field imaging revealed that the initially extended Na₂TP-rich regions became less pronounced and dispersed after injection. In their place, discrete, localized bright features emerged in the near-field amplitude and phase images, corresponding to the formation of nanoscale CaTP precipitates.
Solid-Phase Validation: Complementary solid-state nano-FTIR measurements on dry CaTP samples confirmed the characteristic vibrational response observed in the liquid-phase experiments, validating the identification of the crystalline CaTP phase.

Significance and Claims
The paper claims to demonstrate that liquid-phase infrared near-field nanoscopy can monitor the chemistry of metal–ligand interactions in situ within hydrated environments. By utilizing a SiC membrane to isolate the probe from the liquid, the authors achieved chemically specific detection of ion-exchange reactions and the subsequent formation of coordination frameworks at the nanoscale.

The authors state that this capability enables the study of catalytic, electrochemical, and biological processes under native aqueous conditions. They emphasize that the symmetric and asymmetric carboxylate stretching modes serve as chemically specific near-field reporters, allowing for the resolution of reactions with nanoscale chemical contrast that were previously inaccessible to standard AFM or LP-TEM. The work establishes a platform for observing metal–ligand coordination and precipitation events with high spatial resolution and molecular specificity in liquid.

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