Bartosz A. Grzybowski

Electrostatic self-assembly of binary nanoparticle crystals with a diamond-like lattice

Self-assembly of charged, equally sized metal nanoparticles of two types (gold and silver) leads to the formation of large, sphalerite (diamond-like) crystals, in which each nanoparticle has four oppositely charged neighbors. Formation of these non–close-packed structures is a consequence of electrostatic effects specific to the nanoscale, where the thickness of the screening layer is commensurate with the dimensions of the assembling objects. Because of electrostatic stabilization of larger crystallizing particles by smaller ones, better-quality crystals can be obtained from more polydisperse nanoparticle solutions.

Nanoparticles functionalised with reversible molecular and supramolecular switches

Nanoparticles (NPs) and molecular/supramolecular switches have attracted considerable interest during the past decade on account of their unique properties and prominent roles in the fields of organic chemistry and materials science. Materials derived from the combination of these two components are now emerging in the literature. This critical review evaluates materials which comprise NPs functionalised with well-defined self-assembled monolayers of molecular and supramolecular switches. We draw attention to the fact that immobilisation of switches on NPs does not, in general, hamper their switching ability, although it can impart new properties on the supporting particles. This premise leads us to the discussion of systems in which switching on the surfaces of NPs can be used to modulate reversibly a range of NP properties-optical, fluorescent, electrical, magnetic-as well as the controlled release of small molecules. Finally, we discuss examples in which molecular switches direct reversible self-assembly of NPs (308 references).

The Mosaic of Surface Charge in Contact Electrification

Electrification caused by rubbing two objects creates patches of positive and negative charge on both surfaces. When dielectric materials are brought into contact and then separated, they develop static electricity. For centuries, it has been assumed that such contact charging derives from the spatially homogeneous material properties (along the material’s surface) and that within a given pair of materials, one charges uniformly positively and the other negatively. We demonstrate that this picture of contact charging is incorrect. Whereas each contact-electrified piece develops a net charge of either positive or negative polarity, each surface supports a random “mosaic” of oppositely charged regions of nanoscopic dimensions. These mosaics of surface charge have the same topological characteristics for different types of electrified dielectrics and accommodate significantly more charge per unit area than previously thought.

Self-assembly: from crystals to cells

Self-assembly (SA) is the process in which a systems components—be it molecules, polymers, colloids, or macroscopic particles—organize into ordered and/or functional structures without human intervention. The main challenge in SA research is the ability to “program” the properties of the individual pieces such that they organize into a desired structure. Although a general strategy for doing so is still elusive, heuristic rules can be formulated that guide design of SA under various conditions and thermodynamic constraints. This Review examines SA in both the equilibrium and non-equilibrium/dynamic systems and discusses different SA modalities: energy driven, entropy-driven, templated, and field-directed. Non-equilibrium SA is discussed as a route to reconfigurable (“adaptive”) materials, and its connection to biological systems is emphasized.

Light-controlled self-assembly of reversible and irreversible nanoparticle suprastructures

Nanoparticles (NPs) decorated with ligands combining photoswitchable dipoles and covalent cross-linkers can be assembled by light into organized, three-dimensional suprastructures of various types and sizes. NPs covered with only few photoactive ligands form metastable crystals that can be assembled and disassembled “on demand” by using light of different wavelengths. For higher surface concentrations, self-assembly is irreversible, and the NPs organize into permanently cross-linked structures including robust supracrystals and plastic spherical aggregates.

Swimming bacteria power microscopic gears

Whereas the laws of thermodynamics prohibit extraction of useful work from the Brownian motion of particles in equilibrium, these motions can be “rectified” under nonequilibrium conditions, for example, in the presence of asymmetric geometrical obstacles. Here, we describe a class of systems in which aerobic bacteria Bacillus subtilis moving randomly in a fluid film power submillimeter gears and primitive systems of gears decorated with asymmetric teeth. The directional rotation is observed only in the regime of collective bacterial swimming and the gears’ angular velocities depend on and can be controlled by the amount of oxygen available to the bacteria. The ability to harness and control the power of collective motions appears an important requirement for further development of mechanical systems driven by microorganisms.

Writing Self‐Erasing Images using Metastable Nanoparticle “Inks”

Fans of the “Mission Impossible” movies might recall the selfdestructing messages used to brief the secret agent on the details of his new mission. Even beyond the realm of fictitious espionage, materials that store textual or graphical information for a prescribed period of time are desirable for applications in secure communications. 2] Furthermore, if such materials are rewritable, they can help to limit the use of traditional paper, thereby reducing the costs, both industrial and environmental, associated with paper production and recycling. To date, most research on self-erasing media has relied on the use of photochromic molecules—that is, molecules that isomerize and change color when exposed to light of appropriate wavelength—embedded in or attached to a polymeric or gel matrix. In one widely publicized example, Xerox Corporation recently announced the development of photochromic paper that self-erases in 16 to 24 h. While writing with light can be both rapid and accurate, 7] photochromic “inks” are not necessarily optimal for transforming light-intensity patterns into color variations, because they have relatively low extinction coefficients, are prone to photobleaching, and usually offer only two colors corresponding to the two states of photoisomerizing molecules. Herein, we describe a conceptually different self-erasing material in which both the “writing” and self-erasure of color images are controlled by the dynamic non-equilibrium aggregation of photoresponsive metal (here, gold and silver) nanoparticles (Au and AgNPs “inks”) embedded in thin, flexible organogel films. When exposed to UV light, the trans-azobenzene groups coating the NPs isomerize to cisazobenzene with a large dipole moment. As a result, the NPs aggregate into supraspherical (SS) assemblies, whose apparent color depends on the duration of UV irradiation (Figures 1 and 2). Since the SS are metastable and fall apart spontaneously in the absence of UV irradiation, the two-color and multicolor images written into the films gradually self-erase (Figures 2 and 3). The erasure times can be controlled by the number of dipoles induced on the nanoparticles and can also be accelerated by exposure to visible light or by heating the material. Multiple images can be written into the same film either concurrently or after erasure.

Photoconductance and inverse photoconductance in films of functionalized metal nanoparticles

In traditional photoconductors, the impinging light generates mobile charge carriers in the valence and/or conduction bands, causing the material’s conductivity to increase. Such positive photoconductance is observed in both bulk and nanostructured photoconductors. Here we describe a class of nanoparticle-based materials whose conductivity can either increase or decrease on irradiation with visible light of wavelengths close to the particles’ surface plasmon resonance. The remarkable feature of these plasmonic materials is that the sign of the conductivity change and the nature of the electron transport between the nanoparticles depend on the molecules comprising the self-assembled monolayers (SAMs) stabilizing the nanoparticles. For SAMs made of electrically neutral (polar and non-polar) molecules, conductivity increases on irradiation. If, however, the SAMs contain electrically charged (either negatively or positively) groups, conductivity decreases. The optical and electrical characteristics of these previously undescribed inverse photoconductors can be engineered flexibly by adjusting the material properties of the nanoparticles and of the coating SAMs. In particular, in films comprising mixtures of different nanoparticles or nanoparticles coated with mixed SAMs, the overall photoconductance is a weighted average of the changes induced by the individual components. These and other observations can be rationalized in terms of light-induced creation of mobile charge carriers whose transport through the charged SAMs is inhibited by carrier trapping in transient polaron-like states. The nanoparticle-based photoconductors we describe could have uses in chemical sensors and/or in conjunction with flexible substrates.

Ultrasensitive detection of toxic cations through changes in the tunnelling current across films of striped nanoparticles

Although multiple methods have been developed to detect metal cations, only a few offer sensitivities below 1 pM, and many require complicated procedures and sophisticated equipment. Here, we describe a class of simple solid-state sensors for the ultrasensitive detection of heavy-metal cations (notably, an unprecedented attomolar limit for the detection of CH(3)Hg(+) in both standardized solutions and environmental samples) through changes in the tunnelling current across films of nanoparticles (NPs) protected with striped monolayers of organic ligands. The sensors are also highly selective because of the ligand-shell organization of the NPs. On binding of metal cations, the electronic structure of the molecular bridges between proximal NPs changes, the tunnelling current increases and highly conductive paths ultimately percolate the entire film. The nanoscale heterogeneity of the structure of the film broadens the range of the cation-binding constants, which leads to wide sensitivity ranges (remarkably, over 18 orders of magnitude in CH(3)Hg(+) concentration).

How and Why Nanoparticle’s Curvature Regulates the Apparent pKa of the Coating Ligands

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.