Visualizing Solution Structure at Solid-Liquid Interfaces using Three-Dimensional Fast Force Mapping

Many interesting phenomena occur within a few nanometers of a solid-liquid interface where classical theories for colloidal interactions break down¹. Solvent molecules and ions organize into unexpected patterns² and diverse processes, such as catalysis³, ion adsorption^4,5, electron transfer^6,7, bio-molecular assembly⁸, particle aggregation⁹, attachment^10,11, and assembly^12,13, can occur. However, few techniques can characterize the solution structure at the interface, particularly with sub-nanometer 3D resolution. In this context, three-dimensional fast force mapping (3D FFM)-a technique based on atomic force microscopy (AFM)-has emerged as a useful tool for determining interfacial solution structure^14,15 and understanding its impact on such phenomena.

In general, AFM techniques employ a cantilever with a nanosized tip to characterize surfaces using two main classes of measurements: topographic imaging that measures the height of a substrate at every xy pixel or force measurements that quantify mechanical properties, colloidal interactions^16,17, or adhesive forces between a functionalized tip and the substrate. Today, the capabilities of this versatile instrument extend far beyond these traditional applications; skilled users operating modern instruments can measure electrical, magnetic, and chemical surface properties by coupling force microscopy to spectroscopy and other methods¹⁸. Perhaps the most fascinating advances have been the ability to image materials and processes in their native solutions, with nanoscale spatial resolution, in real time^19,20,21. This latter capability facilitated the development of 3D FFM, which extends AFM measurements into the third spatial dimension by combining 1D force curves with topographic imaging¹⁴. Specifically, the tip acquires consecutive force curves at each xy coordinate to produce a 3D map of the forces detected by the tip at the solid-liquid interface. The novelty here is that a sufficiently fast and sensitive tip can detect minor force gradients corresponding to the local distribution of molecules to map the interfacial solution structure.

To date, 3D FFM has been developed by only a few research groups, which, in our opinion, is not due to its technical limitations but rather the need for customizing instruments in-house to perform these measurements. However, 3D FFM was recently commercialized and is now accessible to researchers of all relevant disciplines. From a scientific point of view, this technique has a broad and multi-disciplinary appeal. For example, the first 3D FFM experiments were performed on mineral-solution systems^15,22,23,24, where important questions included understanding mechanisms of crystal growth and dissolution, the adsorption of ions and molecules, and the role of hydration layers in particle aggregation and attachment. Successful experiments have identified calcium and magnesium atoms in a dolomite crystal lattice²⁵, visualized solution structure around calcite point defects²⁶, and imaged ion adsorption at mica^27,28 and fluorite^24,29 surfaces.

Beyond visualizing mineral-solution interfaces, 3D FFM can provide insights into fundamental questions in surface and colloidal physics, such as the scaling of short-range colloidal interactions, the structure of electric double layers at a molecular level, and the nature and origins of solvation forces. These measurements have important implications for electrochemistry and battery research, as 3D FFM can map electrode-electrolyte interfaces and probe their response to electric fields³. Other applications in materials science include understanding phenomena that occur at the surfaces of separation membranes, heterogeneous catalysts, and polymer coatings. As this capability develops further, we anticipate that it will also play an important role in imaging biomolecules and delineating the role of interactions, ions, and solvent molecules in their self-assembly.

One of the key aspects for advancing data interpretation in 3D FFM is benchmarking against other experimental and simulation tools that have been previously used to study solid-liquid interfaces. For example, techniques based on X-ray reflectivity or diffraction measure electron density profiles that can be mapped to the distribution of ions and solvent molecules as a function of height from the interface^30,31,32,33. This approach has been successful for a range of mineral-solution systems but remains limited to large atomically smooth surfaces and is often incapable of producing laterally resolved data. Other techniques, such as sum frequency generation spectroscopy, provide evidence of particular aspects of solvent structuring at mineral surfaces, such as the orientation of solvent molecules at the surface, but not direct visualization of the structure^34,35. Moreover, molecular dynamics simulations have advanced significantly and can now routinely probe solvent distribution profiles at crystal surfaces^{4,36,37,38,39}. While each of these techniques has its own challenges and limitations, they form a complementary suite of tools for investigating interfacial solution structure; 3D FFM is poised to contribute significantly to this regard and expand the range of solid-liquid systems that can be studied, as well as the research questions that can be answered.

A pre-requisite for implementing 3D FFM on a particular sample, is the ability to obtain topographic images with the desired spatial resolution. For a detailed experimental protocol on high-resolution AFM imaging, the reader is referred to a recent manuscript by Miller et al.²⁰. For optimal operation of 3D FFM, it is strongly advised to first master the high-resolution imaging technique described therein. Most of the recommendations in that protocol are applicable and necessary for 3D FFM. In the following protocol, we briefly highlight the main steps for high-resolution imaging but focus on specific considerations for 3D FFM.