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Back | Show only these items: | Ice template-assisted assembly of spherical PS-b-PAA micelles into novel layer-by-layer hollow spheres

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Ice template-assisted assembly of spherical PS-b-PAA micelles into novel layer-by-layer hollow spheres

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

Novel microscale layer-by-layer hollow spheres are template-assembled by monodispersed spherical PS-b-PAA micelles during the sublimation of the frozen micellar solution.

Type: Object | Advantage: None | Novelty: None | ConceptID: Obj1

The aggregation of nanoscale building blocks, such as clusters,¹ spheres,^2,3 and superlattices,^4–6 has attracted intense interest due to their importance in fundamental research and potential wide-ranging applications.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

To date, a wealthy of methods have been developed to assemble nanoparticles into two- and three-dimensional ordered aggregates, which offers opportunities to explore their novel collective optical, magnetic, and electronic properties.^7–10

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

For examples, Xia et al⁷. produced various uniform aggregates of spherical colloids following an approach combining physical template and capillary forces.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

Jenekhe and Chen¹¹ obtained periodic structures assembled by poly(phenylquinoline)-block-polystyrene via solution evaporation.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

Stucky et al¹². gained micro-sized hollow spheres co-assembled by silica and gold nanoparticles based on cysteine–lysine diblock copolypeptides.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met3

These novel structures have potential applications ranging from electro-optical devices ^13,14 and separation membranes ^15,16 to active biomaterial coating ^17,18 and nano-reactors.^18,19

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

Our group has been studying the micellization of amphiphilic block copolymers and the ordered aggregation of the polymeric micelles.^20,21

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

Recently, we have prepared various morphological aggregates of micelles via different methods.²¹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

One strategy is to control ordered flower-like aggregation of micelles on a adjustable template of water polycrystal.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

The present study is focused on the formation of novel layer-by-layer hollow polymeric spheres by ice template-assisted aggregation of nanoscale micelles.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

To the best of our knowledge, this is the first report of obtaining such novel layer-by-layer hollow aggregates via such a simple and convenient method.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

The PS₃₀-b-PAA₅₁ micelles were firstly prepared as described elsewhere²¹ and characterized by dynamic laser scattering (DLS) and transform electronic microscopy (TEM).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

A drop of the aqueous micelle solution was first placed onto a clean glass slide and quickly frozen in liquid nitrogen.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Then the ice in the frozen micellar sample was sublimated by freeze-drying under vacuum and the resultant samples were observed by scanning electron microscopy (SEM).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

Furthermore, the crystal formation process of pure water was monitored by polarized light microscopy (PLM).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

Fig. 1a shows the hydrodynamic diameter (D_h) distribution f(D_h) of the PS₃₀-b-PAA₅₁ micelles in aqueous solution obtained from DLS.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

Obviously, the diameter distribution of the micelles is monodispersed and the average D_h of the micelles is about 58 nm.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

Fig. 1b displays the TEM images of the micelles and we can clearly see the spherical shape of the micelles with about 40 nm in diameter.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

It must be noted that the hydrodynamic diameter D_h of the micelles measured by DLS is much larger than that observed by TEM.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

This is because the micelles are swollen in water due to the soluble PAA block, while TEM observation shows the dried aggregates.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

Usually, nanoparticles dispersed in water, such as the micelles shown in Fig. 1, aggregate to form random-shaped clusters when the water is removed.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

In order to achieve controlled aggregation of the nanoparticles, physical templates are usually used.⁷

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

Can the solvent be used directly as a template to control the aggregation?

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

To the best of our knowledge, liquid water is not a suitable candidate, but ice may be.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

Compared with other templates that can be added, the ice template can be formed and removed easily by freezing and sublimating.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

Fig. 2 gives the typical PLM image of the thin layer of ice template formed by freezing a drop of water in liquid nitrogen.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

The dark grid-like lines as shown in Fig. 2 divide the ice film into many micro-regions.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

These micro-regions are supposed to shrink gradually to form spherical micro-regions during the slow sublimation of H₂O, which is similar to the sublimation of ice in winter.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

Thus, it is reasonable to think that ice as shown in Fig. 2 can be used as a template to guide the aggregation of the micelles.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

Herein, we aim to use this template to fulfil the controlled aggregation of the micelles shown in Fig. 1.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

Upon first rapidly freezing the micellar aqueous solution in liquid nitrogen and then freeze-drying, the resultant samples were observed by SEM and the SEM images are shown in Fig. 3.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

The resultant sample shown in Fig. 3a looks greatly different from the spheres in Fig. 1b.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

The most remarkable difference is the broken hollow spheres containing a cavity encapsulating a smaller spherical particle and the outer shell is about 100 nm in thickness, which can be seen in the insert-enlarged image in Fig. 3a.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

The fine structure of the shell can be seen in Fig. 3b.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

Obviously, the shell is formed by the packed small particles, which corresponds to the nanoscale spherical PS₃₀-b-PAA₅₁ micelles.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

These results confirm that the microscale layer-by-layer hollow spheres are just resulted from the assembly of the nanoscale PS₃₀-b-PAA₅₁ micelles.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

Thus, it can be concluded that the ice template plays a key role in the ordered assembly of the dispersed PS₃₀-b-PAA₅₁ micelles in water.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

According to what we have obtained above, we propose a schematic illustration of the ice template-assisted assembly of the nanoscale micelles into layer-by-layer hollow spheres in Fig. 4.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

The first step is to freeze the micellar solution instantly (A in Fig. 4), where the micelles are uniformly dispersed in the ice template.