Rapid growth in nanotechnology is increasing the likelihood of engineered nanomaterials coming into contact with humans and the environment. Nanoparticles interacting with proteins, membranes, cells, DNA and organelles establish a series of nanoparticle/biological interfaces that depend on colloidal forces as well as dynamic biophysicochemical interactions. These interactions lead to the formation of protein coronas, particle wrapping, intracellular uptake and biocatalytic processes that could have biocompatible or bioadverse outcomes. For their part, the biomolecules may induce phase transformations, free energy releases, restructuring and dissolution at the nanomaterial surface. Probing these various interfaces allows the development of predictive relationships between structure and activity that are determined by nanomaterial properties such as size, shape, surface chemistry, roughness and surface coatings. This knowledge is important from the perspective of safe use of nanomaterials.

Understanding biophysicochemical interactions at the nano–bio interface

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

http://experimentationlab.berkeley.edu/sites/default/files/AFMImages/butt_AFM.pdf

Force measurements with the atomic force microscope: Technique, interpretation and applications

This feature article highlights work from the authors' laboratories on the synthesis, assembly, reactivity, and optical applications of metallic nanoparticles of nonspherical shape, especially nanorods. The synthesis is a seed-mediated growth procedure, in which metal salts are reduced initially with a strong reducing agent, in water, to produce ∼4 nm seed particles. Subsequent reduction of more metal salt with a weak reducing agent, in the presence of structure-directing additives, leads to the controlled formation of nanorods of specified aspect ratio and can also yield other shapes of nanoparticles (stars, tetrapods, blocks, cubes, etc.). Variations in reaction conditions and crystallographic analysis of gold nanorods have led to insight into the growth mechanism of these materials. Assembly of nanorods can be driven by simple evaporation from solution or by rational design with molecular-scale connectors. Short nanorods appear to be more chemically reactive than long nanorods. Finally, optical applica...

Anisotropic metal nanoparticles: Synthesis, assembly, and optical applications

Surface plasmons (SPs) are coherent oscillations of conduction electrons on a metal surface excited by electromagnetic radiation at a metal -dielectric interface. The growing field of research on such light -metal interactions is known as ‘plasmonics’. 1-3 This branch of research has attracted much attention due to its potential applications in miniaturized optical devices, sensors, and photonic circuits as well as in medical diagnostics and therapeutics. 4-8

Nanostructured plasmonic sensors.

The factors affecting the nucleation and growth of gold nanorods, (Jana et al., Adv. Mater.2001, 13, 1389) have been investigated. It is shown that the size and aspect ratio can be controlled through the use of different sized seed particles. The length of the rods can be tuned from 25–170 nm, while the width remains almost constant at 22–25 nm. The formation of rods requires the presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB). Lower temperature favors rod formation, although this reduces CTAB solubility. The addition of chloride ions or the use of dodecyltrimethylammonium bromide (DTAB) leads to shorter-aspect rods. AuIII and AuI are shown to be quantitatively bound to the CTAB micelles. We propose an electrochemical mechanism for rod formation, whereby the flux of AuI bound to cationic micelles to the seed surface is maximized at points of highest curvature, where the electrical double layer gradient is highest. Initial numerical solutions to the electric potential and field around an ellipsoid in a 1:1 electrolyte are provided, which indicate that the field at the particle tip scales linearly with the aspect ratio. Mean free passage times for ions are found to be shortest at the tips. The results provide a general explanation for the formation of non-equilibrium crystal habits and a mechanism for controlling crystal growth.

Electric-Field-Directed Growth of Gold Nanorods in Aqueous Surfactant Solutions

Mise au point. Rappel des equations fondamentales et des conditions d'etude. Donnees sur le modele de l'ion ponctuel et la theorie de Gouy-Chapman, sur les theories de la double couche pour les electrolytes modeles primitifs, sur la comparaison de la theorie avec des simulations dans le cas des electrolytes symetriques et enfin sur la mise au point d'un modele basique

The statistical mechanics of the electrical double layer

The understanding of static interactions in colloidal suspensions is well established, whereas dynamic interactions more relevant to biological and other suspended soft-matter systems are less well understood. We present the direct force measurement and quantitative theoretical description for dynamic forces for liquid droplets in another immiscible fluid. Analysis of this system demonstrates the strong link between interfacial deformation, static surface forces, and hydrodynamic drainage, which govern dynamic droplet-droplet interactions over the length scale of nanometers and over the time scales of Brownian collisions. The results and analysis have direct bearing on the control and manipulation of suspended droplets in soft-matter systems ranging from the emulsions in shampoo to cellular interactions.

https://science.sciencemag.org/content/sci/313/5784/210.full.pdf

Dynamic Forces Between Two Deformable Oil Droplets in Water

Calculations of Electric Double-Layer Force and Interaction Free Energy between Dissimilar Surfaces

The detailed structure of the double layer is investigated using a model fluid consisting of hard spheres with embedded point charges in a solvent of hard spheres with embedded point dipoles against a hard wall with smeared‐out surface charge. Such a model treats solute and solvent particles on an equal basis, unlike the primitive model of electrolytes. The statistical mechanics is solved using the mean spherical approximation for all interactions. This limits the validity of any results to the regime of low ionic concentrations, where, in this approximation, the model fluid has the correct limiting behavior for bulk thermodynamic quantities. In this regime, simple analytic results for the surface properties are given, which are correct to order (κR). In particular, the surface potential has the classical Stern layer form, with solvent structuring responsible for the inner layer capacitance. This result is the first derivation, as opposed to postulation, of Stern layer behavior. In addition, the polarizat...

Steven L. Carnie

Papers

Electric-Field-Directed Growth of Gold Nanorods in Aqueous Surfactant Solutions

The statistical mechanics of the electrical double layer

Dynamic Forces Between Two Deformable Oil Droplets in Water

Calculations of Electric Double-Layer Force and Interaction Free Energy between Dissimilar Surfaces

The structure of electrolytes at charged surfaces: Ion–dipole mixtures