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

Summary

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.

Abstract

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.

Introduction

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

Protocol

1. Loading and calibrating the AFM tip

1. 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, and hence influence the measured forces⁴⁰.
2. Clean the cantilever holder with water and isopropanol solvents as well....

Results

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.......

Discussion

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 car.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
AC55TS AFM tip	Olympus
Cypher VRS Atomic Force Microscope	Asylum Research
PPP-NCH AFM tip	Nanosensors
Tap300-G AFM tip	Budget Sensors
USC-F5-k30-10 AFM tip	Nanoworld
(Note only one of the AFM tip options is required)