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1. Controlling Surfactant Adsorption on Highly Charged Nanoparticles to Stabilize Bijels

   https://doi.org/10.1021/acs.jpcc.0c01440  

   Boakye-Ansah, S. ( May 2020 , Journal of physical chemistry)

   Bicontinuous particle-stabilized emulsions (bijels) are networks of interpenetrating oil/water channels with applications in catalysis, tissue engineering, and energy storage. Bijels can be generated by arresting solvent transfer induced phase separation (STrIPS) via interfacial jamming of nanoparticles. However, until now, STrIPS bijels have only been formed with silica nanoparticles of low surface charge densities, limiting their potential applications in catalysis and fluid transport. Here, we show how strongly charged silica nanoparticles can stabilize bijels. To this end, we carry out a systematic study employing dynamic light scattering, zeta potential, acid/base titrations, turbidimetry, surface tension, and confocal microscopy. We find that moderating the adsorption of oppositely charged surfactants on the particles is crucial to facilitate particle dispersibility in the bijel casting mixture and bijel stabilization. Our results potentially introduce a general understanding for bijel fabrication with different inorganic nanoparticle materials of variable charge density. 

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2. Microgel and coacervate formation in polyelectrolyte​/multivalent ion mixtures

   Lapitsky, Y. ( August 2017 , Abstracts of Papers, 254th ACS National Meeting & Exposition)

   When polyelectrolytes and oppositely-charged multivalent ions are mixed in aqueous solutions, they can self-assemble into an array of soft materials and complex fluids, ranging from micro- and nanoparticles, to coacervates, to macroscopic gels. Here, we describe the formation and useful/interesting properties of two such materials: (1) submicron particles formed via ionotropic gelation of the cationic polysaccharide chitosan with tripolyphosphate (TPP); and (2) coacervates prepared from mixtures of the synthetic polycation poly(allylamine hydrochloride) (PAH) with either TPP or pyrophosphate (PPi). For chitosan/TPP particles (which are widely explored as potential drug carriers) we show how, by inhibiting chitosan/TPP binding, monovalent salt (NaCl) can be used to: (1) drastically slow down the rapid ionotropic gelation process to facilitate the experimental analysis of how these particles form; (2) enhance the stability of these particles to aggregation; and (3) achieve improved control over particle size. Unlike the gel-like chitosan/TPP ionic networks, which are both soft (with 10^3 - 10^4 Pa storage moduli) and water-rich, mixtures of PAH with TPP and PPi form high-modulus, putty-like coacervates with storage moduli above 10^5 Pa and much lower (26 - 40 wt%) water contents. These moduli and water contents evidently reflect the high ionic crosslink densities enabled by the densely-charged and flexible PAH chains, and strong PAH/PPi and PAH/TPP binding (which also imparts these coacervates with long relaxation times). Besides their bulk properties, we show that the coacervates adhere to diverse substrates (both hydrophilic and hydrophobic) and, when used as wet adhesives, deliver short-term tensile adhesion strengths above 10^5 Pa. Further, the dense crosslinking within PAH/PPi and PAH/TPP coacervates makes them strong barriers to solute diffusion and (regardless of the solute-coacervate binding strength) enables them to release small water-soluble molecules over multiple months. These findings suggest that PAH/PPi and PAH/TPP coacervates can provide a simple route to both underwater adhesion and long-term controlled release. 

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3. Ionically crosslinked cellulose nanocrystals by metal nitrates for the preparation of stable emulsions with tunable interface properties

   https://doi.org/10.1038/s41598-023-48703-3  

   Batta-Mpouma, Joseph ; Kandhola, Gurshagan ; Kim, Jin-Woo ( December 2023 , Scientific Reports)

   Biologically extracted cellulose nanocrystals (CNCs) are rod-like and amphiphilic materials with surface-exposed (hydrophilic sites) and hidden (hydrophobic sites) hydroxyl groups. These physicochemical characteristics make CNCs suitable for use as emulsifying agents to stabilize emulsions. Stable oil-in-water emulsions, using sulfated (i.e., –$${{\text{SO}}}_{3}^{-}$$${\text{SO}}_3^{-}$) CNCs that were ionically crosslinked with alkaline-earth (i.e.,$${{\text{Mg}}}^{2+}$$${\text{Mg}}^{2+}$) or transition-d-block (i.e.,$${{\text{Zn}}}^{2+}$$${\text{Zn}}^{2+}$) metal cations, were developed without the use of any synthetic surfactants or prior functionalization of pure CNCs with hydrophobic molecules. Various emulsion surface properties such as interfacial tension, surface charge, surface chemistry, as well as rheology were characterized. Ionically crosslinked CNCs (iCNCs) adsorbed at the interface of an oil and water and fortified the emulsion droplets (5–30 µm) against coalescence by lowering the interfacial tension from 65 mN/m (i.e., pure CNC mixture with oil) to 25 mN/m (i.e., iCNC mixture with oil) and reducing zeta potential with surface charge values (–30 mV to –10 mV), ideal to maintain droplet layer assembly at the water–oil interface. This study provided an alternative approach to achieve particle-stabilized and surfactant-free emulsions by using divalent metal nitrates to develop “clean” emulsion-based technologies for applications in many industries from agriculture to food to pharmaceuticals.

    

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4. A hybrid lipid membrane coating “shape-locks” silver nanoparticles to prevent surface oxidation and silver ion dissolution

   https://doi.org/10.1039/D0RA01727B  

   Miesen, Thomas J. ; Engstrom, Arek M. ; Frost, Dane C. ; Ajjarapu, Ramya ; Ajjarapu, Rohan ; Lira, Citlali Nieves ; Mackiewicz, Marilyn R. ( April 2020 , RSC Advances)

   null (Ed.)

   The controlled synthesis of stable silver nanoparticles (AgNPs), that do not undergo surface oxidation and Ag + ion dissolution, continues to be a major challenge. Here the synthesis of robust hybrid lipid-coated AgNPs, comprised of l -α-phosphatidylcholine (PC) membranes anchored by a stoichiometric amount of long-chained hydrophobic thiols and sodium oleate (SOA) as hydrophobic binding partners, that do not undergo surface oxidation and Ag + ion dissolution, is described. UV-Visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM), and inductively coupled plasma mass spectrometry (ICP-MS) demonstrate that in the presence of strong oxidants, such as potassium cyanide (KCN), the hybrid lipid-coated AgNPs are stable and do not undergo surface oxidation even in the presence of membrane destabilizing surfactants. UV-Vis studies show that the stability of hybrid lipid-coated AgNPs of various sizes and shapes is dependent on the length of the thiol hydrocarbon chain and can be ranked in the order of increasing stability as follows: propanethiol (PT) < hexanethiol (HT) ≤ decanethiol (DT). UV-Vis and ICP-MS studies show that the hybrid lipid-coated AgNPs do not change in size or shape confirming that the AgNPs do not undergo surface oxidation and Ag + ion dissolution when placed in the presence of strong oxidants, chlorides, thiols, and low pH. Long-term stability studies, over 21 days, show that the hybrid lipid-coated AgNPs do not release Ag + ions and are more stable. Overall, these studies demonstrate hybrid membrane encapsulation of nanomaterials is a viable method for stabilizing AgNPs in a “shape-locked” form that is unable to undergo surface oxidation, Ag + ion release, aging, or shape conversion. More importantly, this design strategy is a simple approach to the synthesis and stabilization of AgNPs for a variety of biomedical and commercial applications where Ag + ion release and toxicity is a concern. With robust and shielded AgNPs, investigators can now evaluate and correlate how the physical features of AgNPs influence toxicity without the confounding factor of Ag + ions present in samples. This design strategy also provides an opportunity where the membrane composition can be tuned to control the release rate of Ag + ions for optimizing antimicrobial activity. 

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5. Tuning surface‐cross‐linking of molecularly imprinted cross‐linked micelles for molecular recognition in water

   https://doi.org/10.1002/jmr.2769  

   Zhang, Shize ; Zhao, Yan ( November 2018 , Journal of Molecular Recognition)

   Molecular recognition in water is an important challenge in supramolecular chemistry. Surface‐core double cross‐linking of template‐containing surfactant micelles by the click reaction and free radical polymerization yields molecularly imprinted nanoparticles (MINPs) with guest‐complementary binding sites. An important property of MINP‐based receptors is the surface‐cross‐linking between the propargyl groups of the surfactants and a diazide cross‐linker. Decreasing the number of carbons in between the two azides enhanced the binding affinity of the MINPs, possibly by keeping the imprinted binding site more open prior to the guest binding. The depth of the binding pocket can be controlled by the distribution of the hydrophilic/hydrophobic groups of the template and was found to influence the binding in addition to electrostatic interactions between oppositely charged MINPs and guests. Cross‐linkers with an alkoxyamine group enabled two‐stage double surface‐cross‐linking that strengthened the binding constants by an order of magnitude, possibly by expanding the binding pocket of the MINP into the polar region. The binding selectivity among very similar isomeric structures also improved.

    

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