Nanoparticles reveal Extreme Size-Sorting and Morphologies in Complex Coacervate Superstructures

Abstract

We here provide detailed insight in self-assembled complex coacervate systems exploiting gold nanoparticles for cryoTEM contrast. Nanoparticle-containing dendrimicelles are formed from fifth-generation dendrimer-encapsulated nanoparticles (DENs) and dendrimer-stabilized nanoparticles (DSNs). The complex coacervate structures self-organize in biconcave thin water layers into size-sorted monolayer superstructures. The embedded nanoparticles are a straightforward tool to visualize dendrimicelles and determine the aggregation number and polydispersity. The superstructure shows extreme size-sorting patterns which, contrary to related systems with higher generation dendrimers, consists not only of dendrimicelles but also much bigger complex coacervate nanoassemblies, such as vesicles.

Figure 1

Preparation of dendrimicelles from amine-terminated fifth generation PAMAM dendrimers. Complexation and reduction of Au³⁺ ions inside fifth-generation PAMAM results in DENs as well as DSNs. Addition of a negative-neutral block copolymer, pMAA₆₄pEO₈₈₅ results in formation of dendrimicelles, depending on the charge fraction f, where f = (negative charge from the block copolymers)/(positive charge from the dendrimers). Charge stoichiometric mixing (f = 1) yields well-defined dendrimicelles. Using excess block copolymer to dendrimer (f = 1.5) results in coacervate structures with an increased polydispersity: dendrimicelles as well as much bigger nanostructures, such as complex coacervate vesicles.

Figure 2

PAMAM generation 5-based dendrimicelles made at charge-stoichiometry. (a) cryoTEM micrograph of the dendrimicelle superstructure, with the (40 × 40 nm) inset showing a single dendrimicelle; here, the core clearly reveals smaller gold particles (DENs) as well as ~6 nanometer big particles (DSNs) The red, solid circles in this figure indicate the dendrimicelle core, as identified by the embedded nanoparticles. The green, dotted circle indicates the total dendrimicelle size, as determined from DLS. Scale bar is 100 nm. (b) Heat map plot of dendrimicelle superstructure, with the individual dendrimicelles color-coded according to the core area. (c) The average dendrimicelle core diameter is 26 ± 6 nm. (d) The average dendrimicelle diameter, as determined from measuring dendrimicelle core-core distances is 45 ± 5 nm. (e) Plotting the micelle core area versus the radial distance to the center of the dendrimicelle superstructure shows a slight size-sorting of the dendrimicelles over the superstructure.

Figure 3

Extreme size-sorting of dendrimicelles made under off-stoichiometric conditions. (a) CryoTEM micrograph of the formed dendrimicelle superstructure. (b) Heat map plot of dendrimicelle superstructure, with the individual dendrimicelles color-coded according to the core area as determined from the cryoTEM micrograph in (a), emphasizing the size-sorting present. (c) The average micelle core diameter is 36 ± 18 nm. (d) The average micelle size, as determined from the micelle core-core distances is 48 ± 12 nm. (e) Plotting the micelle core area versus the radial distance to the center of the dendrimicelle superstructure confirms the size-sorting of the dendrimicelles. (f) Schematic illustration of the amplified thin film-templated size sorting. Scale bar is 100 nm.