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Designing a functional surface that selectively adsorbs nanoparticles based on their size and shape is essential for developing an advanced adsorption-based, post-synthesis nanoparticle separation device. We demonstrate selective adsorption of larger nanoparticles from solution onto a polyelectrolyte brush by tuning the salt concentration. Specifically, a positively-charged polyelectrolyte brush is created by converting pyridine groups of poly(2-vinylpyridine) to n-methyl pyridinium groups using methyl iodide. The adsorption kinetics and thermodynamics of polyethylene glycol-grafted, negatively charged gold nanoparticles (diameters of 12 and 20 nm) were monitored as a function of salt concentration. In a salt-free solution, the polyelectrolyte brush adsorbs gold nanoparticles of both sizes. As the salinity increases, the areal number density of adsorbed nanoparticles monotonically decreases and becomes negligible at high salinity. Interestingly, there is an intermediate range of salt concentrations (i.e., 15 – 20 mM of NaCl) where the decrease in nanoparticle adsorption is more pronounced for smaller particles, leading to size-selective adsorption of the larger nanoparticles. As a further demonstration of selectivity, the polyelectrolyte brush is immersed in a binary mixture of 12-nm and 20-nm nanoparticles and found to selectively capture larger particles with ~ 90 % selectivity. In addition, the size distribution of as-synthesized gold nanoparticles, with an average diameter of 12 nm, was reduced by selectively removing larger particles by exposing the solution to polyelectrolyte brush surfaces. This study demonstrates the potential of a polyelectrolyte brush separation device to remove larger nanoparticles by controlling electrostatic interactions between polymer brushes and particles
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Relative Strength of Polycation Adsorption on Oxide Surfaces
Polyelectrolyte adsorption to surfaces is widely employed in water treatment and mining. However, little is known of the relative interaction strengths between surfaces and polymer. This fundamental property is assumed to be dominated by electrostatics, i.e. attractive interactions between opposite charges, which are set by the overall ionic strength (“salt concentration”) of the solution, and charge densities of the surface and the polymer. A common, counterintuitive, finding is a range of salt concentration over which the amount of adsorbed polyelectrolyte increases as electrostatic interactions are tempered by the addition of salt. After an adsorption maximum, higher salt concentrations then produce the expected gradual desorption of polyelectrolyte. In this work, the salt response of the adsorption of the same narrow molecular weight distribution polycation, poly(N-methyl-4-vinyl pyridinium), PM4VP, to a variety of surfaces was explored. Oxide powders for adsorption included Al2O3, SiO2, Fe2O3, Fe3O4, TiO2, ZnO and CuO. Planar surfaces included silicon wafer, mica, calcium carbonate and CaF2 single crystals. The PM4VP was radiolabeled with 14C so that sensitive, sub-monolayer amounts could be detected. The position of the peak maximum, or the lack of a peak, in response to added salt was used to rank the electrostatic component of the interaction. The importance of charge regulation, a shift in the surface pKa in response to solution species, was highlighted as a mechanism for adsorption on the “wrong” side of the isoelectric point, and also as a factor contributing to the difficulty of reaching the totally desorbed state even at the highest salt concentrations.
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- Award ID(s):
- 2103703
- PAR ID:
- 10518763
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Langmuir
- ISSN:
- 0743-7463
- 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/10.1021/acs.langmuir.3c03641
