Oppositely Charged Nanoparticles Precipitate Not Only at the Point of Overall Electroneutrality

Precipitation of oppositely charged entities is a common phenomenon in nature and laboratories. Precipitation and crystallization of oppositely charged ions are well-studied and understood processes in chemistry. However, much less is known about the precipitation properties of oppositely charged nanoparticles. Recently, it was demonstrated that oppositely charged gold nanoparticles (AuNPs), also called nanoions, decorated with positively or negatively charged thiol groups precipitate only at the point of electroneutrality of the sample (i.e., the charges on the particles are balanced). Here we demonstrate that the precipitation of oppositely AuNPs can occur not only at the point of electroneutrality. The width of the precipitation window depends on the size and concentration of the nanoparticles. This behavior can be explained by the aggregation of partially stabilized clusters reaching the critical size for their sedimentation in the gravitational field.

Bio-Memristor Based on Peptide and Peptide Composite with Gold Nanoparticles

The structure, morphology and electrical properties of thin dipeptide hexamethylenediamide bis (N-monosuccinylglutamlysin) (DPT) layers and a DPT composite with gold nanoparticles deposited on gold and HOPG substrates were studied by probe microscopy and spectroscopy. The chemical formula of DPT is: {HOOC–(CH2)2–CO-L-Glu-L-Lys-NH–(CH2)3}2, and it is a mimetic of nerve growth factor. The results demonstrate that the structure and morphology of DPT thin layers depend significantly on the molecule charge (neutral or anion) and the nature of the substrate–layer interface. It was possible to control the structure and properties of the formed solid layers by changing pH of aqua solution (the charge of the DPT molecule). Bipolar resistive switching was observed in thin DPT layers on graphite and gold surfaces. The crystallization of anions on the surface of gold led to the formation of a ferroelectric unlike graphite. A strong dependence of the morphology of DPT composite layers on the nature of the substrate and the state of its surface is revealed. It indicates the important role of interfacial interactions in the crystallization processes of the DPT layers. The electrical properties of layers also depend on the interaction of DPT with the substrate. An increase in the thickness of the layers significantly affects the morphology and value of the tunneling current. Similar to crystallization of DPT salt on a gold surface, crystallization of DPT composite with gold nanoparticles also leads to the formation of a ferroelectric. The differences found in the structure of DPT composite layers on graphite and gold surfaces can be explained by assuming that the structure of the second and all subsequent layers is completely determined by the structure of the first adsorption layer in DPT-substrate interface. So this layer serves as a template for the growth of all other layers. The results can find practical application in 3D printing technologies. The presence of negative differential conductivity on local tunnel current–voltage characteristics of peptide composites is of great practical importance when used as active elements for amplifying current and power, memory cells in organic electronics. Investigated DPT has rather good memristive characteristics, including good endurance, satisfying ON/OFF current ratio, long retention time and reproducible write-once read-many times (WORM) memory behavior. All this allows us to consider the DPT to be a perspective material of memristor organic electronics. Since it is also a drug, the polymorphism and its dependence on pH can also find application in the pharmaceutical industry.

Existence of a Precipitation Threshold in the Electrostatic Precipitation of Oppositely Charged Nanoparticles

Oppositely charged nanoparticles precipitate rapidly only at the point of electroneutrality, wherein their charges are macroscopically compensated. We investigated the aggregation and precipitation of oppositely charged nanoparticles at concentrations ranging from 10 to 10^-3  mm (based on gold atoms) by using UV/Vis measurements. We employed solutions of equally sized (4.6 nm) gold nanoparticles, which were functionalized and stabilized with either positively or with negatively charged alkanethiols. Results showed that oppositely charged nanoparticles do not precipitate if their concentration is below a certain threshold even if the electroneutrality condition is fulfilled. This finding suggests a universal behavior of chemical systems comprising oppositely charged building blocks such as ions and charged nanoparticles.

Aqueous Assembly of Oxide and Fluoride Nanoparticles into 3D Microassemblies

We demonstrate rapid [∼mm³/(h·L)] organic ligand-free self-assembly of three-dimensional, >50 μm single-domain microassemblies containing up to 10⁷ individual aligned nanoparticles through a scalable aqueous process. Organization and alignment of aqueous solution-dispersed nanoparticles are induced by decreasing their pH-dependent surface charge without organic ligands, which could be temperature-sensitive or infrared light absorbing. This process is exhibited by transforming both dispersed iron oxide hydroxide nanorods and lithium yttrium fluoride nanoparticles into high packing density microassemblies. The approach is generalizable to nanomaterials with pH-dependent surface charge (e.g., oxides, fluorides, and sulfides) for applications requiring long-range alignment of nanostructures as well as high packing density.