Surface-Engineered Cationic Nanocrystals Stable in Biological Buffers and High Ionic Strength Solutions

Abstract

Progress in colloidal synthesis in the last two decades has enabled high-quality semiconductor, plasmonic, and magnetic nanocrystals (NCs). As synthesized, these NCs are usually capped with long-chain apolar ligands. Postsynthetic surface functionalization is required for rendering such NCs colloidally stable in polar media such as water. However, unlike small anionic molecules and polymeric coatings, producing positively charged stable NCs, especially at high ionic strengths, has remained challenging. Here, we present a general approach to achieve aqueously stable cationic NCs using a set of small (<2.5 nm long) positively charged ligands. The applicability of this method is demonstrated for a variety of materials including semiconductor CdSe/CdS core/shell NCs, magnetic Fe@Fe3O4, Fe3O4, and FePt NCs, and three different classes of plasmonic Au NCs including large nanorods. The obtained cationic NCs typically have zeta potential values ranging from +30 to +60 mV and retain colloidal stability for days to months, depending on NC/ligand pair, in several biological buffers at elevated pH and in concentrated salt solutions. This allowed us to demonstrate site-specific staining of cellular structures using fluorescent cationic NCs with several different surface chemistries. Furthermore, colloidal stability of the obtained NCs in the presence of other charged species allowed the assembly of cationic and anionic counterparts driven primarily by electrostatic attraction. With this approach, we prepare highly uniform 3D and 2D binary mixtures of NCs through induced homogeneous aggregation and alternating-charge layer-by-layer deposition, respectively. Such binary mixtures may provide a new route in the engineering of nanocrystalline solids for electronics, thermoelectrics, and photovoltaics.

Self‐Assembly of Proteinaceous Shells around Positively Charged Gold Nanomaterials Enhances Colloidal Stability in High‐Ionic‐Strength Buffers

“…This measurement thus confirms that the entire ensemble of NPs has acquired negative surface charge. The coexistence of cationic and anionic highly charged large species would lead to their fast aggregation, followed either by precipitation of aggregates or, in some cases, formation of organized halo structures, as reported elsewhere . Negative‐stain EM images (Figures and ) confirmed that the resulting structures consisted of a core NP coated with a uniform, ≈4‐nm‐thick proteinaceous shell, as expected for a single layer of AaLS‐13 pentamers.…”

“…We recently reported the preparation of cationic gold nanocrystals derivatized with short (1‐ to 2.5‐nm), positively charged organic ligands . For example, (11‐mercaptoundecyl)‐ N , N , N ‐trimethylammonium bromide (TMABr; Figure B) was found to stabilize 22‐nm AuNPs in water.…”

“…The gradual decrease in absorption of the shelled NPs without accompanying changes in the shape of the absorption band indicates that the NPs slowly settle over days, but retain their integrity. Such behavior is expected for large and heavy nanocrystals because steric and electrostatic repulsion between the NPs is no longer sufficient to outweigh gravity …”

“…In addition to studying spherical gold nanomaterials, we tested whether our shelling procedure could be applied to rod‐shaped nanomaterials. TMABr‐capped AuNRs (13×50 nm) were synthesized as previously reported and incubated with AaLS‐13 pentamer fragments. As for the spherical particles, negative‐stain TEM images show that a single layer (≈4 nm) of the AaLS‐13 protein formed around the AuNRs (Figure B).…”

Production of antimicrobial silver nanoparticles modified by alkanethiol self‐assembled monolayers by direct current atmospheric pressure glow discharge generated in contact with a flowing liquid anode

“…To develop the method of synthesis of size‐defined AgNPs stabilized with (11‐mercaptoundecyl)‐N,N,N‐trimethylammonium chloride (TMA) alkanethiol, a CAPP‐based reaction‐discharge system with direct current atmospheric pressure glow discharge (dc‐APGD) generated in contact with a flowing liquid anode (FLA) was used and optimized in the present contribution. TMA allows to obtain stable nanoparticles, as well as noncytotoxic AgNPs with antimicrobial potential . Previously, dc‐APGD or pulse‐modulated radio‐frequency atmospheric pressure glow discharge (pm‐rf‐APGD), operated between the surface of flowing liquid electrodes and pin‐type W electrodes or a gaseous nozzle jet, was used in our group to synthesize Ag or Ag‐Au core‐shell nanostructures.…”