Clusters and Inverse Emulsions from Nanoparticle Surfactants in
Michael T. Lombardo and Lilo D. Pozzo*
Chemical Engineering Department, University of Washington, Seattle, Washington 98195, United States *S Supporting Information
ABSTRACT: A method is presented for the synthesis of selfassembling nanoparticle surfactants in nonpolar organic solvents. The method relies on the control of long-range steric repulsion imparted by grafted polystyrene and short-range attraction from short-chain thiol molecules with an alcohol or carboxylic functionality. Similar to water-based nanoparticle surfactants, these oil-dispersed materials are found to cluster in dispersion and also to stabilize oil−water interfaces to form water-in-oil emulsions. The clustering process is characterized with dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), UV−vis spectroscopy, and transmission electron microscopy (TEM). Thermogravimetric analysis (TGA) is used to quantify the surface concentration of grafted polymer, which is found to be a parameter of critical importance for the formation of stable clusters. The clustering kinetics and dispersion stability are both affected by the polymer molecular weight, surface concentration, and chemical structure of the thiol molecules that induce particle attraction. Nanometer-sized water-in-oil emulsions are formed by sonication in the presence of nanoparticle surfactants. A large broadening of the optical absorption spectrum in the NIR region is observed because of changes in the collective surface plasmon resonance of the gold particle shell. ■ INTRODUCTION
In a 2002 Science paper by Whitesides and Grzybowski, selfassembly is defined as the assembly of pre-existing components in a reversible and controllable manner through proper design.1
A key aspect of this definition is that steady-state positions are achieved by a balance between attractive and repulsive forces.
There are countless examples of self-assembly ranging from crystals to bacterial colonies.1 Self-assembled nanoparticle structures are one small segment of this wide range. One important structure of self-assembled nanoparticles is clusters, which refers to aggregates of two or more particles that are geometrically packed in three dimensions as opposed to 1D chains2 or 2D arrays.3 Nanoparticle clusters are used in a number of applications such as photothermal therapy,4 bioseparations,5 and imaging.6 One advantage of metal clusters is that they can exhibit different optical properties than their individual building blocks because of the overlap of their plasmon resonance. By placing gold surfaces close together, the surface plasmons create “hot spots” with a large electromagnetic field enhancment.7 This phenomena is utilized in surface-enhanced Raman spectroscopy (SERS) to provide signal enhancement on the order of 106, which allows for the detection of extremely dilute compounds.8 Another advantage is that both orientation-dependent and -independent properties can be achieved.9 Orientation-independent optical properties resulting from tetrahedral configurations of plasmonic particles have been shown to be useful in applications such as metafluids with negative indices of refraction.10 The tetrahedral configuration leads to clusters that interact with light in the same way regardless of orientation with respect to the incident light. This is advantageous for metafluids because the particles are in constant motion and their orientation with the incoming light is not easily controlled. Though much work has been performed to self-assemble particles in water, much less research has been pursued for self-assembly in organic solvents. Moreover, much less has been reported for self-assembly in apolar organic solvents, which is the focus of this work.
There are several techniques used to create nanoparticle clusters (i.e., aggregates of two or more particles), and these are outlined nicely in a recent review by Lu and coworkers.11 Many of these techniques suffer from drawbacks in terms of cost, versatility, scalability, and/or control of cluster size and shape.
The most promising techniques can be classified as bottom-up assemblies that are advantageous over top-down methods (e.g., lithography) because less material is wasted, length scales are determined by the size of the building blocks, and the materials can be processed in bulk quantities. For example, accurate and reproducible particle clusters have been successfully produced through grafting complementary strands of DNA onto gold nanoparticles to induce clustering.12 This technique, however, is limited to aqueous systems because DNA hybridization is limited to polar solvents. It is also rather expensive because of the high costs associated with producing synthetic DNA.
Another technique, evaporation-induced self-assembly, creates
Received: November 19, 2014
Revised: January 12, 2015
Published: January 13, 2015
Article pubs.acs.org/Langmuir © 2015 American Chemical Society 1344 DOI: 10.1021/la504520p
Langmuir 2015, 31, 1344−1352 clusters by forcing the aggregation of particles that are confined within an emulsion droplet upon the evaporation of the dispersed liquid phase. This technique is significantly more scalable than DNA grafting, but it is limited by the size of the initial emulsion. It is therefore difficult to achieve structural control over nanometer-sized clusters.13
Recently, our group developed an inexpensive and scalable technique utilizing nonspecific interactions to form clusters of gold nanoparticles controllably in water.14 This technique renders 12 nm gold particle spheres amphiphilic through the sequential conjugation of long PEG chains and short alkanethiol molecules to the particle surface. By controlling the amount of PEG and the length of the alkanethiol, the cluster size is tunable to between 1 and 9 particles/cluster.15
This work demonstrates an ability to self-assemble particles in water controllably without the use of expensive raw materials, in a scalable fashion and with good control over the final cluster size. Water-based nanoparticle surfactants are also capable of stabilizing oil-in-water emulsions that have been recently demonstrated for use in photoacoustic imaging16 and, potentially, in thrombolytic therapy.17 Moreover, because this simple approach relies on the balance of repulsion from polymers and short-range attraction from alkanethiols, it should also be possible to extend this approach to nonpolar organic solvent systems.