Impact of concentration and capping ligand length on the organization of metal nanoparticles in Langmuir-Blodgett surface micelles and nanostrands

Plasmonic Properties of Self-Assembled Gold Nanocrescents: Implications for Chemical Sensing

A bottom-up approach, the Langmuir-Blodgett technique, is used for the preparation of composite thin films of gold nanoparticles and polymers: poly(styrene-b-2-vinylpyridine), poly-2-vinylpyridine, and polystyrene. The self-assembly of poly(styrene-b-2-vinylpyridine) at the air-water interface leads to the formation of surface micelles, which serve as a template for the organization of gold nanoparticles into ring assemblies. By using poly-2-vinylpyridine in conjunction with low surface pressure, the distance between nanostructures can be increased, allowing for optical characterization of single nanostructures. Once deposited on a solid substrate, the preorganized gold nanoparticles are subjected to further growth by the reduction of additional gold, leading to a variety of nanostructures which can be divided into two categories: nanocrescents and circular arrays of nanoparticles. The optical properties of individual structures are investigated by optical dark-field spectroscopy and numerical calculations. The plasmonic behavior of the nanostructures is elucidated through the correlation of optical properties with structural features and the identification of dominant plasmon modes. Being based on a self-assembly approach, the reported method allows for the formation of interesting plasmonic materials under ambient conditions, at a relatively large scale, and at low cost. These attributes, in addition to the resonances located in the near-infrared region of the spectrum, make nanocrescents candidates for biological and chemical sensing.

Scaling Laws for Block Copolymer Surface Micelles

Polymers under confinement exhibit different structures and properties from the bulk. While block copolymers (BCPs) create well-defined micelles in solution, two-dimensional (2D) spatial confinement at the air-water interface constrains the chain conformations and deforms the micellar structure, thus forming a surface micelle. The BCP surface micelles open up an opportunity in nanoscience and engineering by serving as an interfacial modifier and structural platform. Nevertheless, a scaling law, a principle governing the micellar structure, is absent. Herein, we report a unified scaling relation to describe the combinational structure of BCP surface micelles in two and three dimensions and further reveal their formation mechanism in line with the suggested scaling relation. We investigated the intrinsic scaling relations in a surface pressure-free environment by introducing a concept of excluded volume-dependent scaling exponent based on the scaling theory of 2D polymers. In addition, an extrinsic scaling relation is derived for the surface pressure-dependent corona scaling.

Controlled Growth of Gold Nanoparticles Preorganized in Langmuir-Blodgett Monolayers

A method is described for the in situ growth of substrate-supported organized gold nanoparticles. Upon exposure to an aqueous solution of a gold salt and a mild reducing agent, the particle size can be significantly increased without any loss of superstructure organization. Furthermore, no secondary nucleation is observed. The surface-supported regrowth procedure can be combined with the Langmuir-Blodgett technique to produce a rich library of plasmonic nanoparticle assemblies. Controlled particle regrowth plays a crucial role in this assembly method because only relatively small metallic nanoparticles can be directly dispersed in polymeric Langmuir-Blodgett films. The versatility of the method is demonstrated through the fabrication of several specific nanoparticle structures, including contiguous plasmonic rings, core-satellite structures, and necklace assemblies. Plasmon extinction spectra are presented for the various nanoparticle superstructures and illustrate the importance of controlling both particle size and assembly architecture in achieving targeted optical properties. The reported approach constitutes a viable bottom-up assembly route for the fabrication of surface-supported nanoparticle superstructures for plasmonic applications.