We thank Dr. Marta Kocun (Asylum Research), Dr. Takeshi Fukuma (Kanazawa), Dr. Ricardo Garcia (CSIC Madrid), Dr. Angelika K&#252;hnle (Bielefeld), Dr. Ralf Bechstein (Bielefeld), Sebastien Seibert (Bielefeld), and Dr. Hiroshi Onishi (Kobe) for useful discussions.
Development of the 3D FFM experimental protocol was supported as part of IDREAM (Interfacial Dynamics in Radioactive Environments and Materials), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science (SC), Office of Basic Energy Sciences (BES). Development of the 3D FFM data analysis code was supported by the Laboratory Directed Research and Development Program (LDRD) at Pacific Northwest National Laboratory (PNNL) through the Linus Pauling Distinguished Postdoctoral Fellowship program to which E.N. is grateful for support. Development of the 3D FFM measurement capability was carried out at PNNL with support from the BES Division of Materials Science and Engineering, Synthesis and Processing Sciences Program. PNNL is a multiprogram national laboratory operated for DOE by Battelle Memorial Institute under contract no. DEAC05-76RL0-1830.

1. Loading and calibrating the AFM tip
<ol>
	<li>Clean the cantilever tip by immersing it in water and isopropanol solvents consecutively for several minutes to remove contaminants and organic adsorbates. Other common methods for cleaning include argon-plasma or ultraviolet-ozone surface treatment. 
	NOTE: Be consistent in the sample and cantilever preparation when comparing different data sets. Changes in the cleaning process might affect the tip properties such as surface chemistry, hydrophilicity, or even shape...

Selecting the AFM tip 
As with any AFM application, the key characteristics of the probe tip are the resonance frequency, cantilever size, tip radius, tip material, and spring constant. Almost all the 3D FFM literature to date has reported the use of stiff, high-frequency tips. The most common examples are silicon-based tips (e.g., AC55TS, PPP-NCH, Tap300-G, etc.) tips that can be utilized in their higher resonance modes14. Other research groups have opted for USC-F5-k30-10 c...

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 adsorption4,5, electron transfer6,7, bio-molecular assembly8, particle aggregation9, attachment10,11, and assembly12,13, 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 structure14,15 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 interactions16,17, 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 time19,20,21. 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 systems15,22,23,24, 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 mica27,28 and fluorite24,29 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 interface30,31,32,33. 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 structure34,35. Moreover, molecular dynamics simulations have advanced significantly and can now routinely probe solvent distribution profiles at crystal surfaces4,36,37,38,39. 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.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>AC55TS AFM tip</td>
			<td>Olympus</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Cypher VRS Atomic Force Microscope</td>
			<td>Asylum Research</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>PPP-NCH AFM tip</td>
			<td>Nanosensors</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Tap300-G AFM tip</td>
			<td>Budget Sensors</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>USC-F5-k30-10 AFM tip</td>
			<td>Nanoworld</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>(Note only one of the AFM tip options is required)</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

visualizing solution structure at solid-liquid interfaces using three-dimensional fast force mapping

Amongst the challenges for a variety of research fields are the visualization of solid-liquid interfaces and understanding how they are affected by the solution conditions such as ion concentrations, pH, ligands, and trace additives, as well as the underlying crystallography and chemistry. In this context, three-dimensional fast force mapping (3D FFM) has emerged as a promising tool for investigating solution structure at interfaces. This capability is based on atomic force microscopy (AFM) and allows the direct visualization of interfacial regions in three spatial dimensions with sub-nanometer resolution. Here we provide a detailed description of the experimental protocol for acquiring 3D FFM data. The main considerations for optimizing the operating parameters depending on the sample and application are discussed. Moreover, the basic methods for data processing and analysis are discussed, including the transformation of the measured instrument observables into tip-sample force maps that can be linked to the local solution structure. Finally, we shed light on some of the outstanding questions related to 3D FFM data interpretation and how this technique can become a central tool in the repertoire of surface science.

Figure 2A presents a schematic of 3D force mapping. Similar to other AFM techniques operating in amplitude modulated mode, an oscillating cantilever is scanned across the surface. In addition to the tip height at each coordinate, instrument observables such as phase shift and amplitude are collected as the tip approaches and retracts from the surface. The result is a 3D dataset of observables-notably the oscillation amplitude, phase shift, and tip deflection-that can be readily converted int...

Read this Scientific Article Protocol about Visualizing Solution Structure at Solid-Liquid Interfaces using Three-Dimensional Fast Force Mapping at JoVE.com

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

Here, we present a protocol for using three-dimensional fast force mapping - an atomic force microscopy technique - for visualizing solution structure at solid-liquid interfaces with the subnanometer resolution by mapping the tip-sample interactions within the interfacial region.

Amongst the challenges for a variety of research fields are the visualization of solid-liquid interfaces and understanding how they are affected by the ...