Determination of the surface hydrophobicity or wettability of nanomaterials and nanoparticles (NPs) is often challenged by the heterogeneous properties of NPs that vary with particle size, shape, surface charge, aggregation states, and surface sorption or coating. This study first summarized inherent limitations of the water contact angle, octanol–water partition coefficient ( K ow ) and surface adsorption of probe molecules in probing nanomaterial hydrophobicity. Then, we demonstrated the principle of a scanning probe method based on atomic force microscopy (AFM) for the local surface hydrophobicity measurement. Specifically, we measured the adhesion forces between functionalized AFM tips and self-assembled monolayers (SAMs) to establish a linear relationship between the adhesion forces and water contact angles based on the continuum thermodynamic approach (CTA). This relationship was used to determine the local surface hydrophobicity of seven different NPs ( i.e. , TiO 2 , ZnO, SiO 2 , CuO, CeO 2 , α-Fe 2 O 3 , and Ag), which agreed well with bulk contact angles of these NPs. Some discrepancies were observed for Fe 2 O 3 , CeO 2 and SiO 2 NPs, probably because of surface hydration and roughness effects. Moreover, the solution pH and ionic strength had negligible effects on the adhesion forces between the AFM tip and MWCNTs or C 60 , indicating that the hydrophobicity of carbonaceous nanomaterials is not influenced by pH or ionic strength (IS). By contrast, natural organic matter (NOM) appreciably decreased the hydrophobicity of MWCNTs and C 60 due to surface coating of hydrophilic NOM. This scanning probe method has been proved to be reliable and robust toward the accurate measurement of the nanoscale hydrophobicity of individual NPs or nanomaterials in liquid environments.
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Unique sequence-dependent properties of trinucleotide repeat monolayers: electrochemical, electrical, and topographic characterization
Trinucleotide repeat (TNR) sequences widely exist in nature and their overgrowth is associated with two dozen neurodegenerative diseases in humans. These sequences have a unique helical flexibility, which affects their biophysical properties. A number of biophysical properties of these sequences have been studied in the past except their surface-tethered monolayers. To address the effect of sequence context and the associated helical flexibility on TNR monolayers, disease-relevant TNRs from three flexibility groups were surface-assembled on gold surfaces. The properties of the TNR films were studied, including charge transfer resistance ( R ct ) by electrochemical impedance spectroscopy (EIS), surface density by chronocoulometry (CC), surface topography by atomic force microscopy (AFM), and electrical conductivity by conducting atomic force microscopy (C-AFM). We found that the TNR film properties are characteristically sequence dependent rather than being dependent on their flexibility rank reported in the literature. The characteristic properties of TNR films studied here may be used for engineering label-free biosensors to detect neurological disorders and build DNA bioelectronics.
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- Award ID(s):
- 1940716
- NSF-PAR ID:
- 10157474
- Date Published:
- Journal Name:
- Journal of Materials Chemistry B
- ISSN:
- 2050-750X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d0tb00507j
