Versatile Tetrablock Copolymer Scaffold for Hierarchical Colloidal Nanoparticle Assemblies: Synthesis, Characterization, and Molecular Dynamics Simulation

Abstract

A unique combination of molecular dynamics (MD) simulation and detailed size exclusion chromatography-multiangle light scattering (SEC-MALS) analysis is used to provide important a priori insights into the solution self-assembly of a well-defined and symmetric tetrablock copolymer with two acrylic acid (AA) outer blocks, two polystyrene (PS) inner blocks, and a trithiocarbonate (TTC) central group, prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization. SEC-MALS experiments show that the copolymer forms aggregates in both tetrahydrofuran and N,N-dimethylformamide (DMF), even in the presence of different salts, but not in 1,4-dioxane (dioxane). Combined with MD simulations, these results indicate that the AA units are the main cause of aggregation through intermolecular hydrogen bonding, with additional stabilization by the central TTC. The block copolymer chains self-assemble in dioxane by adding cadmium acetate, originating flowerlike inverse micelles with a cadmium acrylate core and the TTC groups in the outer surface of the PS corona. The micelles were used as nanoreactors in the templated synthesis of a single cadmium selenide (CdSe) quantum dot (QD) in the core of each micelle, whereas the shell TTC groups can be converted into thiol functions for further use of these units in hierarchical nanostructures. Only in dioxane where simulations and SEC-MALS suggest an absence of copolymer aggregates prior to cadmium acetate addition do well-dispersed and highly luminescent CdSe QDs form by templated synthesis. These results provide valuable insights into the self-assembly of RAFT copolymers in different solvent systems as it relates to the preparation of emissive QDs with polymer-spaced thiol functionality for binding to gold nanostructures.

Plasmonic Properties of Colloidal Assemblies

“…[ 65 ] In another example, triblock copolymers made from a bifunctional reversible addition–fragmentation chain transfer (RAFT) agent, in which the RAFT trithiocarbonate group is located in the center of the middle (polystyrene) block, which joins two outer poly(acrylic acid) blocks were used to template the formation of quantum dots. [ 66 ] The as‐prepared quantum dots were functionalized with polystyrene loops, which allowed subsequent attachment of (on average one) gold NPs at a certain distance from the quantum dot. [ 67 ] These heterogeneous dimers showed significant fluorescence enhancement, which can be attributed to the distance‐dependent balance in which near‐field‐promoted increased excitation rates overcompensate de‐excitation pathways, thanks to the polystyrene spacer.…”

Bright and Stable Nanomaterials for Imaging and Sensing

“…Dye-metal nanostructures with increased brightness thus require the use of spacer materials to control the metal-dye distance with precision. This has been attempted using silica [51,131,133,135,136], DNA [137], and polymers [131,[138][139][140]. While DNA and polymer spacers can be too flexible to completely prevent dye-metal contact (leading to emission quenching), this problem can be minimized by using very high-density polymer brushes.…”

Nanoengineering with RAFT polymers: from nanocomposite design to applications

“…One way of circumventing this issue is to use macromolecular ligands, which can be easily prepared via RAFT polymerization. Here, a multiply-binding copolymer design has been proven to be very effective: one type of monomer contains an anchoring group for QD binding, such as thiol, 164 carboxyl, 146,147 imidazole 112,188,189 and 8-hydroxyquinoline 241 moieties. Another part of the polymer adjusts dispersibility and provides additional properties.…”