Cluster-mediated assembly enables step-growth copolymerization from binary nanoparticle mixtures with rationally designed architectures

Multicomponent nanoparticle chains structurally analogous to random, block, and alternating copolymers, respectively, have been fabricated by a cluster-mediated self-assembly process.


Synthesis of Fe3O4 NPs: Monodisperse Fe3O4
NPs with a diameter of ~16 nm were synthesized according to a modified literature method. [1] Typically, 36 g of iron oleate (pre-synthesized by reaction of FeCl3· 6H2O and sodium oleate) and 8.6 g of OA were dissolved in 150 g of ODE, followed by degassing at 120 o C for 60 min. The resulting solution was then heated to 320 o C under N2 atmosphere for 1 h. After cooling down to room temperature, the crude solution was purified by centrifugation with the addition of methanol and isopropanol for three times. The sediments were redispersed in 40 mL of hexane for further use.
Synthesis of CoFe2O4 NPs: CoFe2O4 NPs with a diameter of ~6 nm were synthesized by a modified literature method. [2] Generally, 5.6 g of Co(acac)2, 2.0 g of Fe(acac)3, After that, the solution was heated to 295 o C and kept at this temperature for 1 h.

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CoFe2O4 NPs were separated from the growth solution using ethanol followed by centrifugation and were redispersed in 20 mL of hexane.
Synthesis of Au NPs: Monodisperse Au NPs with a diameter of ~8 nm were synthesized by a modified literature method. [3] Typically, 100 mg of HAuCl4· 4H2O, 10 mL of OAm, and 10 mL of hexane were combined in air and magnetically stirred at 15 o C under N2 flow for 10 min. Then, a reducing solution containing 15 mg of TBAB, 1 mL of OAm, and 1 mL of hexane was quickly injected. The reaction mixture was allowed to react at 15 o C for 1 h before 60 mL of ethanol was added to precipitate Au NPs. The precipitated Au NPs upon centrifugation were redispersed in 10 mL of hexane.

Synthesis of Ag NPs:
Colloidal Ag NPs with a diameter of ~16 nm were synthesized by modifying a reported approach. [4] Briefly, 170 mg of AgNO3 and 20 mL of dried OAm were heated at 60 o C under N2. After that, the mixture was quickly heated up to 240 o C and maintained at this temperature for 1 h. Acetone was added to precipitate and purify Ag NPs. The precipitated Ag NPs upon centrifugation were redispersed in 15 mL of hexane.

Synthesis of CsPbBr3 nanocubes:
To synthesize cesium oleate, 0.814 g of Cs2CO3, 40 mL of ODE, and 2.5 mL of OA were mixed and dried under 120 o C for 1 h, and then heated under N2 to 150 o C until all Cs2CO3 was dissolved. CsPbBr3 nanocubes with a diameter of ~15 nm were synthesized according to the protocol reported previously. [5] In a typical synthesis, a mixture of 5 mL of ODE and 0.69 mg of PbBr2 was dried under vacuum at 120 o C for 1 h, followed by the injection of 0.5 mL of dried OA at 120 o C.
After the dissolution of PbBr2, the temperature was raised to 200 o C and the stock solution containing cesium oleate was quickly injected. The reaction mixture was quickly cooled by an ice-water bath. The crude solution was centrifuged, and the sediment was redispersed in 10 mL of hexane for further use.

Synthesis of Ag-Fe3O4 heterodimers:
The synthesis of Ag-Fe3O4 heterodimers was carried out based on a procedure reported previously. [6] Typically, 50 mg of 16 nm Fe3O4 NPs, 50 mg of silver acetate, and 3 mL of OAm were added into 40 mL of toluene.
The mixture was heated to 80 o C under N2 and kept at this temperature for 8 h. After cooling down to room temperature, the stock solution was centrifuged with the addition of ethanol. The precipitated Ag-Fe3O4 dimers upon centrifugation were redispersed in 15 mL of hexane.
Synthesis of PbSO4 clusters: PbSO4 clusters were synthesized by the protocol we reported previously. [7] In brief, lead oleate was synthesized by heating a mixture of 139 Cluster-mediated polymerization of colloidal NPs into homopolymers: Inorganic homopolymers were obtained by a cluster-mediated assembly process, following a modified procedure reported previously. [7] To initiate colloidal polymerization, an appropriate amount of the freshly made PbSO4 cluster solution was added into a hexane solution containing NPs followed by incubation for a certain period of time under ambient conditions. The resulting inorganic polymers were isolated by centrifugation followed by redispersion in hexane for further characterization.

Co-assembly of binary NPs into random copolymers:
The procedure for growing random copolymers was conducted similarly to that used for the synthesis of inorganic homopolymers, except that a mixture containing two types of NPs with similar sizes were co-incubated in the presence of PbSO4 clusters.

Synthesis of inorganic block copolymers by the stepwise copolymerization approaches:
Inorganic block copolymers could be synthesized by the prepolymerization strategy commonly used in molecular copolymerization. In the first "one-prepolymer" approach, block copolymers were synthesized by co-polymerization of NP prepolymers with a second NP monomer with the assistance of PbSO4 clusters. Take Fe3O4-CsPbBr3 block copolymers for example, 16 nm Fe3O4 NPs were first allowed to polymerize to afford prepolymers with desired lengths by incubation for a certain period of time. After that, an appropriate amount of 15 nm CsPbBr3 nanocubes and PbSO4 clusters were added into the solution containing the as-prepared Fe3O4 prepolymers to initiate copolymerization. The resulting copolymer species were isolated by centrifugation and were then re-dispersed in hexane for further characterization. Block copolymers could also be obtained by the "two-prepolymer" approach, in which two kinds of prepolymers with desired lengths were coupled with the assistance of PbSO4 clusters. It should be noted that the yield of block copolymers was much lower than that obtained by the "one-prepolymer" approach.
"One-pot" synthesis of inorganic block copolymers: In this approach, inorganic block copolymers were formed by the direct assembly of binary NPs with distinct sizes, following the procedure used for growing random copolymers. The evolution of block copolymers under this situation was attributed to a depletion-induced phase separation process, where the larger NPs tend to assemble while excluding the smaller ones.
Fabrication of inorganic alternating copolymer-like assemblies: To achieve alternating copolymer-like assemblies, Ag-Fe3O4 heterodimers were used as monomers, which were incubated with the presence of PbSO4 clusters.
Steady-state photoluminescence (PL) spectra were obtained at room temperature on an Edinburgh Instruments FLS920 fluorescence spectrophotometer. The absolute PL quantum yield was obtained using integrating sphere measurements. Fluorescence micrographs were obtained on a Leica DM4000 B LED microscope equipped with a Leica DFC310 FX camera.   nm, respectively. Inset shows a cartoon illustration of the heterodimer. S12 Fig. S5. Illustration of the possible alignment modes of heterodimers during the formation of alternate copolymer-like structures. S13