Size-Sorting and Pattern Formation of Nanoparticle-Loaded Micellar Superstructures in Biconcave Thin Films

Abstract

Biconcave thin water layers represent a template to induce organization of supramolecular structures into ordered monolayers. Here we show how micelles form extensive micrometer-sized pseudo-2D superstructures that reveal size-sorting and geometric pattern formation, as shown by cryo-transmission electron microscopy (cryoTEM). Electron-rich gold particles inside the micelles facilitate direct visualization and determination of size, composition, and ordering of the micellar assemblies over multiple length scales. Some of the patterns observed show intriguing geometric patterns for superstructures, including Fibonacci-like, double-spiral domains that also appear in, for example, sunflower seed head patterns.

Keywords: biconcave template; dendrimers; micelles; self-assembly; size-sorting; superstructures; thin films.

Figure 1

Organization and ordering over multiple length scales. (a) Illustration of a cryoTEM-grid hole consisting of a thin layer of water with patterns of assembled (b) complex coacervate core micelles, consisting of a negative (red)–neutral (green) pMAA₆₄-b-PEO₈₈₅ diblock copolymer and (c) a positively charged (blue) PAMAM core with an encapsulated (d) gold nanoparticle. The dendrimer-encapsulated nanoparticles provide excellent contrast for cryoTEM (scale bars in e–h correspond to the ones in a–d, respectively). DLS reveals (i) micelles with a hydrodynamic diameter of about 50 nm, (j) with a core of about 25 nm as derived from TEM. In (e) and (f), the green circles indicate the dendrimicelle hydrodynamic diameter as determined by DLS, and the red circles indicate the dendrimicelle core size as obtained from the cryoTEM micrographs. The hydrodynamic diameter of the eighth-generation PAMAM is about 14 nm, as determined by DLS (k), and the gold nanoparticles inside the dendrimer are about 2.4 nm (l). (m) Scheme showing the synthesis of nanoparticle-containing dendrimicelles. Complexation and reduction of gold(III) ions in PAMAM dendrimers yield dendrimer-encapsulated nanoparticles. Addition of pMAA₆₄-b-PEO₈₈₅ results in dendrimicelle formation. The blue circle in (g) indicates the dendrimer size as determined by DLS, and the purple circles in (g) and (h) indicate the AuNP.

Figure 2

Ordering of seventh (left), eighth (middle), and ninth (right) generation PAMAM dendrimicelles. (a–c) The darker spots in the cryoTEM images are the 2–3 nm gold nanoparticles residing inside the dendrimers in the micelle core; scale bars correspond to 200 nm. (d–f) Image segmentation results in heat map plots with micelle cores color-coded according to their size. (g–i) Radial distance plots show micelle diameter versus distance to the center of the pattern.

Figure 3

Extreme size sorting of dendrimicelles in biconcave thin films. CryoTEM micrograph (a) of a superstructure obtained from G7-based dendrimicelles obtained at noncharge stoichiometric conditions (f = 1.5). Color- and size-coded heat map highlighting the size sorting (b). Core diameter of the dendrimicelles versus radial distance from the center of the superstructure. Slice from the cryoTEM micrograph (d) and corresponding heat map (e) visualizing the extreme size sorting. Side-view impression of the distribution of differently sized micelles in the thin biconcave water layer in a TEM-grid hole.

Figure 4

Double-spiral sunflower-like patterns in dendrimicelle superstructures. (a) CryoTEM shows large multiple spiral domains; scale bar corresponds to 200 nm. (b) Corresponding color-coded heat maps highlighting geometric features such as intersecting spirals, a feature commonly observed in nature, e.g., (c) sunflower heads, with Fibonacci-related symmetry.