November 2024

Kaolinite is one of the most abundant natural clay minerals within soils at the Earth’s surface and within rock units in the upper crust. *

The interface between aqueous solutions and the facets of kaolinite plays an important role in a wide range of technological applications including tribology, paper production, oil recovery, waste water treatment and medical devices. *

This is made possible by kaolinite’s layered structure, with its two basal surfaces -aluminol and siloxane-exhibiting different properties and reactivity. *

Both macroscopic and nanoscale studies point to a strong dependence of kaolinite’s surface properties on its local hydration structure. No experimental results, however, have systematically and comparatively investigated the hydration landscape of both basal facets to date. *

In the article “Local probing of the nanoscale hydration landscape of kaolinite basal facets in the presence of ions” Clodomiro Cafolla, Tai Bui, Tran Thi Bao Le, Andrea Zen, Weparn J. Tay, Alberto Striolo, Angelos Michaelides, Hugh Christopher Greenwell and Kislon Voïtchovsky combine high-resolution atomic force microscopy (AFM) imaging and force spectroscopy with classical molecular dynamics (MD) simulations to illustrate key differences in the hydration behaviour of the aluminol and siloxane facets of kaolinite particles immersed in water and NaCl solutions. *

This combined approach allows the authors to overcome the limitations of each technique via the advantages of the other. Specifically, AFM images highlight the differences in the first hydration layer of each facet and serve as a basis for force spectroscopy measurements of the full hydration profile at a given location. *

Water densities extracted from MD help interpret the AFM results, both in the absence and in the presence of added Na+ ions. *

Complementary AFM spectroscopy measurements show an excellent agreement between the conservative component and MD’s water density profiles, with discrete hydration layers on both facets and little sensitivity to added ions. *

The dissipative component of the measured AFM tip-sample interactions is more sensitive to the presence of ions, with MD suggesting a link with the local water dynamics and transient instabilities between stable hydration layers. *

These effects are facet-dependant and more pronounced on the aluminol facet where the first water layer is better defined. Increasing the salt concentration allows hydrated ions to form more stable layers, with hints of organised ionic domains. *

The results provide unique insights into both the equilibrium molecular structure and dynamics of the kaolinite facets, potentially informing applications involving interfacial processes. *

The AFM experiments were conducted at 25 ± 0.1 °C using a commercial atomic force microscope equipped with temperature control.
NanoWorld Arrow-UHF silicon AFM probes were used.
The AFM cantilevers were thoroughly washed with pure water (20 times with 100 μl) and then with the solution of interest (40 times with 100 μl).
Experiments were performed at near neutral pH 5.8. This ensured that only the metal ions of interest were present on the AFM cantilever. Thorough cleaning procedures were implemented to avoid any possible sources of contamination. *

During the measurements, the AFM cantilever and the sample were fully immersed in the aqueous ionic solution of interest. The thermal spectrum of the AFM cantilever was used to perform the flexural calibration of the AFM cantilevers. The AFM probes were found to have a flexural spring constant in the range 1.0–4.0 N/m and a resonance frequency of ∼400–900 kHz in water. These values agree with the nominal range and the literature. The AFM cantilever oscillation was photo-thermally driven to ensure greater stability, making sure that the frequency response remained unaffected by any spurious contributions due to the noise produced by mechanical coupling with other experimental components of the system. *

Fig. 2 from Clodomiro Cafolla et al. 2024 “Local probing of the nanoscale hydration landscape of kaolinite basal facets in the presence of ions”:Representative experimental (AFM) and computational (MD) images of both kaolinite facets. For each image, the corresponding atomic arrangement of the facet is superimposed to scale. The green triangle highlights the brightest periodic features appearing in both AFM and MD, showing a good agreement. The MD images represent the density distribution of the first hydration layer over each facet. The insets show the Fast Fourier Transform (FFT) of each image and highlight the first (red), second (orange) and third (cyan) order intensity peaks. The MD results represent a 2D projection for the water oxygen density in the first hydration layer averaged over 3 ns. The scale bar represents 1 nm. The AFM colour scale bar represents a height variation of ∼0.3 nm; the MD colour scale is based on a density range at a fixed height (first hydration layer). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) NanoWorld Arrow-UHF silicon AFM probes were used. — Fig. 2 from Clodomiro Cafolla et al. 2024 “Local probing of the nanoscale hydration landscape of kaolinite basal facets in the presence of ions”:
Representative experimental (AFM) and computational (MD) images of both kaolinite facets. For each image, the corresponding atomic arrangement of the facet is superimposed to scale. The green triangle highlights the brightest periodic features appearing in both AFM and MD, showing a good agreement. The MD images represent the density distribution of the first hydration layer over each facet. The insets show the Fast Fourier Transform (FFT) of each image and highlight the first (red), second (orange) and third (cyan) order intensity peaks. The MD results represent a 2D projection for the water oxygen density in the first hydration layer averaged over 3 ns. The scale bar represents 1 nm. The AFM colour scale bar represents a height variation of ∼0.3 nm; the MD colour scale is based on a density range at a fixed height (first hydration layer). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

*Clodomiro Cafolla, Tai Bui, Tran Thi Bao Le, Andrea Zen, Weparn J. Tay, Alberto Striolo, Angelos Michaelides, Hugh Christopher Greenwell and Kislon Voïtchovsky
Local probing of the nanoscale hydration landscape of kaolinite basal facets in the presence of ions
Materials Today Physics, Volume 46, August 2024, 101504
DOI: https://doi.org/10.1016/j.mtphys.2024.101504

Open Access The article “Local probing of the nanoscale hydration landscape of kaolinite basal facets in the presence of ions” by Clodomiro Cafolla, Tai Bui, Tran Thi Bao Le, Andrea Zen, Weparn J. Tay, Alberto Striolo, Angelos Michaelides, Hugh Christopher Greenwell and Kislon Voïtchovsky is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

Alzheimer’s disease (AD) is the most frequent neurodegenerative disorder in the elderly aged over 65.*

The extracellular accumulation of beta-amyloid (Aβ) aggregates in the brain is considered as the major event worsening the AD symptoms, but its underlying reason has remained unclear.

In the article „Lateral Piezoelectricity of Alzheimer‘s Aβ Aggregates“ by Jinhyeong Jang, Soyun Joo, Jiwon Yeom, Yonghan Jo, Jingshu Zhang, Seungbum Hong and Chan Beum Park the piezoelectric characteristics of Aβ aggregates are revealed.

The vector piezoresponse force microscopy (PFM) analysis results exhibit that Aβ fibrils have spiraling piezoelectric domains along the length and a lateral piezoelectric constant of 44.1 pC N^-1. Also, the continuous sideband Kelvin probe force microscopy (KPFM) images display that the increment of charge-induced surface potential on a single Aβ fibril is allowed to reach above +1700 mV in response to applied forces.

These findings shed light on the peculiar mechano-electrical surface properties of pathological Aβ fibrils that exceed those of normal body components.*

Both KPFM and nanoindentation measurements were performed using a chemically inert conductive diamond AFM tip. (NanoWorld Pointprobe CDT-FMR).

Figure S10 from „Lateral Piezoelectricity of Alzmeiner‘s Aβ Aggregates by Jinhyeong Jang et al.:
Nanoindentation test result of Aβ fibrils. Direct piezoelectric effects are tested by measuring the surface potential change before and after nano-indentation of Aβ fibrils.
(A) Schematic illustration of the nanoindentation test for Aβ fibrils coated on an Au/Cr substrate.
(B) Topgraphy of Aβ fibrils acquired before and after applying nanoindentation. The post-indentation image shows white spots resulting from physical interactions between the AFM tip and the sample, while the approximate structure of the Aβ fibrils remains intact. Height and potential line sections obtained fom the (c) blue and (d) red lines are shown in the topgraphy of Aβ fibrils. The height remains unchanged, while electrical voltage increment and decrement were observed at the blue and red lines, respectively.
(D) During the nanoindentation the Young‘s modulus of Aβ – fibrils was measured to be 3.17 GPa, closely approxmating the literature (Nanoscale 4, 4426-4429 (2012). Both KPFM and nanoindentation measurements were performed using a chemically inert conductive diamond AFM tip (CDT-FMR, NanoWorld, Switzerland). Collected force curves were analyzed using the Hertz model to estimate the elastic moduli.

*Jinhyeong Jang, Soyun Joo, Jiwon Yeom, Yonghan Jo, Jingshu Zhang, Seungbum Hong and Chan Beum Park
Lateral Piezoelectricity of Alzheimer‘s Aβ Aggregates
Advanced Science, Volume 11, Issue 39, October 024, 2406678
DOI: https://doi.org/10.1002/advs.202406678

Open Access The article “Lateral Piezoelectricity of Alzheimer‘s Aβ Aggregates” byJinhyeong Jang, Soyun Joo, Jiwon Yeom, Yonghan Jo, Jingshu Zhang, Seungbum Hong and Chan Beum Park is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.