- [Show abstract] [Hide abstract] ABSTRACT: Nanoparticles covered with ligand shells comprising both positively and negatively charged ligands exhibit Gram-selective antibacterial action controlled by a single experimental parameter, namely the proportion of [+] and [-] ligands tethered onto these particles. Gram selectivity is attributed to the interplay between polyvalent electrostatic and non-covalent interactions that work in unison to disrupt the bacterial cell wall. The [+/-] nanoparticles are effective in low doses, are non-toxic to mammalian cells, and are tolerated well in mice. These results constitute the first example of rational engineering of Gram selectivity at the (macro)molecular level.
- [Show abstract] [Hide abstract] ABSTRACT: A general-purpose procedure based on electrostatic titrations was developed to determine the compositions of mixed self-assembled monolayers (m-SAMs) of oppositely charged ligands on metal nanoparticles (NPs). In this procedure, nanoparticles of unknown m-SAM compositions and charges are titrated with the positively charged NP "standards". These compositions and the differences in the free energies of adsorption of the [+] and [-] are then determined from the points of precipitation at which the overall charge on all NPs is neutralized. The results of the titrations agree well with traditional core-etching/NMR studies. However, the titration-based approach is nondestructive and requires significantly less material than NMR.
- [Show abstract] [Hide abstract] ABSTRACT: Like-charged nanoparticles, NPs, can assemble in water into large, faceted crystals, each made of several million particles. These NPs are functionalized with mixed monolayers comprising ligands terminating in carboxylic acid group ligands as well as positively charged quaternary ammonium ligands. The latter groups give rise to electrostatic interparticle repulsions which partly offset the hydrogen bonding between the carboxylic acids. It is the balance between these two interactions that ultimately enables self-assembly. Depending on the pH, the particles can crystallize, form aggregates, remain unaggregated or even - in mixtures of two particle types - can "choose" whether to crystallize with like-charged or oppositely charged particles.
- [Show abstract] [Hide abstract] ABSTRACT: The mechanism of alternating deposition of oppositely charged gold nanoparticles (AuNPs) was investigated by optical waveguide lightmode spectroscopy (OWLS). OWLS allows monitoring of the kinetics of layer-by-layer (LbL) adsorption of positively and negatively charged nanoparticles in real time without using any labels so that the dynamics of layer formation can be revealed. Positively charged NPs that are already deposited on a negatively charged glass substrate strongly facilitate the adsorption of the negatively charged particles. The morphology of the adsorbed layer was also investigated with atomic force microscopy (AFM). AFM revealed that the interaction between oppositely charged particles results in the formation of NP clusters with sizes varying between 100 and 6000 NPs. The cluster size distribution is found to be an exponentially decaying function, and we propose a simple theory to explain this finding.
- [Show abstract] [Hide abstract] ABSTRACT: In materials design, cracks are often avoided rather than sought. Here, several design principles are implemented that allow cracks to be formed with high reproducibility and control. We show how strains applied to flexible substrates produce nanoscopic cracks mediating size-, shape- and location-selective deposition of nanomaterials.
- [Show abstract] [Hide abstract] ABSTRACT: Gold nanoparticles (NPs) functionalized with 2-fluoro-4-mercaptophenol (FMP) ligands form densely packed NP films at liquid-liquid interfaces, including surfaces of liquid droplets. The process is driven by a gradual lowering of temperature that changes the solution's pH, altering both the energy of interfacial adsorption for NPs traveling from solution to the interface as well as the balance between electrostatic and vdW interactions between these particles. Remarkably, the system shows hysteresis in the sense that the films remain stable when the temperature is increased back to the initial value. The same phenomena apply to gel-air interfaces, enabling patterning of these wet materials with durable NP films.
- [Show abstract] [Hide abstract] ABSTRACT: Interest in assemblies of colloidal particles has long been motivated by their applications in photonics, electronics, sensors and microlenses. Existing assembly schemes can position colloids of one type relatively flexibly into a range of desired structures, but it remains challenging to produce multicomponent lattices, clusters with precisely controlled symmetries and three-dimensional assemblies. A few schemes can efficiently produce complex colloidal structures, but they require system-specific procedures. Here we show that magnetic field microgradients established in a paramagnetic fluid can serve as 'virtual moulds' to act as templates for the assembly of large numbers (∼10(8)) of both non-magnetic and magnetic colloidal particles with micrometre precision and typical yields of 80 to 90 per cent. We illustrate the versatility of this approach by producing single-component and multicomponent colloidal arrays, complex three-dimensional structures and a variety of colloidal molecules from polymeric particles, silica particles and live bacteria and by showing that all of these structures can be made permanent. In addition, although our magnetic moulds currently resemble optical traps in that they are limited to the manipulation of micrometre-sized objects, they are massively parallel and can manipulate non-magnetic and magnetic objects simultaneously in two and three dimensions.
- [Show abstract] [Hide abstract] ABSTRACT: Even minute quantities of electric charge accumulating on polymer surfaces can cause shocks, explosions, and multibillion-dollar losses to electronic circuitry. This paper demonstrates that to remove static electricity, it is not at all necessary to "target" the charges themselves. Instead, the way to discharge a polymer is to remove radicals from its surface. These radicals colocalize with and stabilize the charges; when they are scavenged, the surfaces discharge rapidly. This radical-charge interplay allows for controlling static electricity by doping common polymers with small amounts of radical-scavenging molecules, including the familiar vitamin E. The effectiveness of this approach is demonstrated by rendering common polymers dust-mitigating and also by using them as coatings that prevent the failure of electronic circuitry.
- [Show abstract] [Hide abstract] ABSTRACT: Nanoparticles functionalized with mixed self-assembled monolayers (m-SAMs) comprising positively-and negatively charged thiols are stable in both low and high pH but precipitate sharply at pHprec at which the charges on the particle are balanced. By adjusting the proportion of the (+) and the (-) ligands in the m-SAM, or by changing particle size, the precipitation point can be varied flexibly between pH ~ 4 and pH ~ 7. In addition, changes in the SAMs' com-position and particles' net charge translate into different degrees of cellular uptake. Remarkably, the presence of the (+) thiols allows for the uptake of particles having net nega-tive charge.
- [Show abstract] [Hide abstract] ABSTRACT: Computational algorithms are used to identify sequences of reactions that can be performed in one pot. These predictions are based on over 86 000 chemical criteria by which the putative sequences are evaluated. The "raw" algorithmic output is then validated experimentally by performing multiple two-, three-, and even four-step sequences. These sequences "rewire" synthetic pathways around popular and/or important small molecules.
- [Show abstract] [Hide abstract] ABSTRACT: Although modern chemical databases store a great wealth of structural and reactivity data, this vast “universe” of chemical information has not yet been systematically analyzed. Here, we use computers to derive from the entire body of organic-chemical knowledge the indices that estimate the reactivity and cross influence of functional groups. The major premise of our approach is that in sufficiently large and diverse collections of reactions (as the entire “history” of organic chemistry is), the frequencies with which transformations of certain groups occur, reflect their reactivities. Illustrative examples spanning several classes of reactions demonstrate that our knowledge-based indices capture the well-known reactivity trends. A free-access software is also developed with which other trends can be analyzed for various combinations of functional groups.
- [Show abstract] [Hide abstract] ABSTRACT: Microcrystals of desired sizes are important in a range of processes and materials, including controlled drug release, production of pharmaceutics and food, bio- and photocatalysis, thin-film solar cells and antibacterial fabrics. The growth of microcrystals can be controlled by a variety of agents, such as multivalent ions, charged small molecules, mixed cationic-anionic surfactants, polyelectrolytes and other polymers, micropatterned self-assembled monolayers, proteins and also biological organisms during biomineralization. However, the chief limitation of current approaches is that the growth-modifying agents are typically specific to the crystalizing material. Here, we show that oppositely charged nanoparticles can function as universal surfactants that control the growth and stability of microcrystals of monovalent or multivalent inorganic salts, and of charged organic molecules. We also show that the solubility of the microcrystals can be further tuned by varying the thickness of the nanoparticle surfactant layers and by reinforcing these layers with dithiol crosslinks.
- [Show abstract] [Hide abstract] ABSTRACT: Oppositely charged, nanoionic nanoparticles can act as "universal surfactants" regulating the growth of ionic microcrystals. This phenomenon derives from a subtle interplay between crystal growth and cooperative electrostatic adsorption of the nanoparticles onto crystal faces. In addition to the electrostatic interactions acting in the system, the nature of salts is also important in the sense that for the same Debye screening length, different salts can mediate formation of markedly different assemblies including supraspheres, nanoneedles, or nanoplates. The method can be further extended to coat non-ionic crystals with appropriately functionalized nanoparticles.
- [Show abstract] [Hide abstract] ABSTRACT: This paper reviews techniques currently available for size- and shape-selective purification of nanoscopic objects. The methods discussed range from variants of familiar chromatographic, centrifugation, or filtration techniques, to purification schemes deriving from nanoscale-specific phenomena, including shape-selective reactivity, or propensity to form organized superstructures.Graphical AbstractThis work reviews techniques currently available for size- and shape-selective purification of nanoscopic objects. The methods discussed range from variants of familiar chromatographic, centrifugation, or filtration techniques, to purification schemes deriving from nanoscale-specific phenomena, including shape-selective reactivity, or propensity to form organized superstructures. Figure was taken from Reference .View high quality image (96K)Highlights► Review of the methods of nanoparticle separation and purification. ► Overview of analytical techniques for the size- and shape-selective nanoseparations. ► Purification schemes deriving from nanoscale-specific phenomena.
- [Show abstract] [Hide abstract] ABSTRACT: Dissociation of ionizable ligands immobilized on nanopaticles (NPs) depends on and can be regulated by the curvature of these particles as well as the size and the concentration of counterions. The apparent acid dissociation constant (pK(a)) of the NP-immobilized ligands lies between that of free ligands and ligands self-assembled on a flat surface. This phenomenon is explicitly rationalized by a theoretical model that accounts fully for the molecular details (size, shape, conformation, and charge distribution) of both the NPs and the counterions.
Article: Electrostatics at the nanoscale[Show abstract] [Hide abstract] ABSTRACT: Electrostatic forces are amongst the most versatile interactions to mediate the assembly of nanostructured materials. Depending on experimental conditions, these forces can be long- or short-ranged, can be either attractive or repulsive, and their directionality can be controlled by the shapes of the charged nano-objects. This Review is intended to serve as a primer for experimentalists curious about the fundamentals of nanoscale electrostatics and for theorists wishing to learn about recent experimental advances in the field. Accordingly, the first portion introduces the theoretical models of electrostatic double layers and derives electrostatic interaction potentials applicable to particles of different sizes and/or shapes and under different experimental conditions. This discussion is followed by the review of the key experimental systems in which electrostatic interactions are operative. Examples include electroactive and "switchable" nanoparticles, mixtures of charged nanoparticles, nanoparticle chains, sheets, coatings, crystals, and crystals-within-crystals. Applications of these and other structures in chemical sensing and amplification are also illustrated.
- [Show abstract] [Hide abstract] ABSTRACT: Selective aggregation and precipitation of like-charged nanoparticles (NPs) covered with carboxylate ligands can be induced by different monovalent cations. The ordering of critical concentrations required for NP precipitation is Cs(+) ≫ K(+) > Li(+) > Na(+) > Rb(+) and does not correlate with the size of hydrated cations M(+), nor can it be predicted by the Hofmeister series. On the other hand, different anions have no effect on the precipitation trends. These observations are rationalized by a theoretical model combining the elements of the DLVO theory with molecular-level calculations. The key component of the model is the cation-specific binding of various metal cations to the carboxylate ligands.
Article: Nanoparticle Oscillations and Fronts[Show abstract] [Hide abstract] ABSTRACT: Rhythmic nanoparticles: Chemical oscillations can be coupled to the dynamic self-assembly of nanoparticles. Periodic pH changes translate into protonation and deprotonation of the ligands that stabilize the nanoparticles, thus altering repulsive and attractive interparticle forces. In a continuous stirred-tank reactor, rhythmic aggregation and dispersion is observed; in spatially distributed media, propagation of particle aggregation fronts is seen.