Structures generated from the assembly of metallic nanoparticles (NPs) have garnered significant attention due to their potential technological applications. Now, an international research group has developed strategies which can generate linear assemblies of anisotropic gold nanorods (NRs). Linkage of nanorods with different dimensions led to random and block copolymer structures. The group’s approach was further extended to the co-assembly of random copolymers of gold and palladium NRs.

Eugenia Kumacheva of the University of Toronto, Kun Liu of Jilin University, China, Michael Rubenstein of the University of North Carolina, and their colleagues describe their copolymerization methodologies in the March 3 issue of Angewandte Chemie International Edition (DOI: 10.1002/anie.201309718; p. 2648). Gold NRs with mean diameter of 12 nm and lengths of 99 nm were etched to fine-tune their lengths, and were then functionalized with thiol-terminated polystyrene (PS) molecules. End-to-end association of NRs, which behave as monomers, is initiated when the quality of the solvent for the PS tethers is reduced by adding water to the solution of PS-functionalized NRs in dimethylform-amide (DMF). When the process is repeated with a mixture of short 50-nm in length NRs (SNRs) and 80-nm long NRs (LNRs), co-assembly into linear copolymer structures with limited extent branching is revealed in scanning transmission electron microscopy (STEM) images.

The researchers also used strategies commonly employed in the synthesis of molecular block copolymers to co-assemble block copolymer structures from gold SNRs and LNRs. In the two-prepolymer approach, homopolymer structures from SNRs and LNRs are separately prepared in DMF/water mixture. Upon mixing the solutions of the homopolymers, the chains of SNRs and LNRs copolymerize to give large fractions of diblock and triblock copol-ymers. In the one-prepolymer approach, individual SNRs are introduced right after the assembly of LNR homopolymer in a water/DMF mixture. Analysis of STEM images reveal that the SNR monomers primarily attach to the ends of the LNR blocks or to each other.

Palladium NRs with mean lengths and diameters of 270 nm and 20 nm, respectively, assemble in an end-to-end manner when homopolymerized by adding water to the NR solution in DMF. However, when the co-assembly is initiated from mixtures of individual solutions of gold and palladium NRs in DMF, STEM images reveal copolymer structures where the palladium NRs can be clearly distinguished from the gold NRs due to their rougher surface and lower electron density.

The ability to visualize the assembly of these one-dimensional plasmonic heterostructures at different stages of copolymerization is very useful for mechanistic interpretations, according to the researchers.