Constitutionally Selective Dynamic Covalent Nanoparticle Assembly

Abstract

The future of materials chemistry will be defined by our ability to precisely arrange components that have considerably larger dimensions and more complex compositions than conventional molecular or macromolecular building blocks. However, exerting structural and constitutional control in the assembly of nanoscale entities presents a considerable challenge. Dynamic covalent nanoparticles are emerging as an attractive category of reaction-enabled solution-processable nanosized building block through which the rational principles of molecular synthetic chemistry can be extended into the nanoscale. From a mixture of two hydrazone-based dynamic covalent nanoparticles with complementary reactivity, specific molecular instructions trigger selective assembly of intimately mixed heteromaterial (Au-Pd) aggregates or materials highly enriched in either one of the two core materials. In much the same way as complementary reactivity is exploited in synthetic molecular chemistry, chemospecific nanoparticle-bound reactions dictate building block connectivity; meanwhile, kinetic regioselectivity on the nanoscale regulates the detailed composition of the materials produced. Selectivity, and hence aggregate composition, is sensitive to several system parameters. By characterizing the nanoparticle-bound reactions in isolation, kinetic models of the multiscale assembly network can be constructed. Despite ignoring heterogeneous physical processes such as aggregation and precipitation, these simple kinetic models successfully link the underlying molecular events with the nanoscale assembly outcome, guiding rational optimization to maximize selectivity for each of the three assembly pathways. With such predictive construction strategies, we can anticipate that reaction-enabled nanoparticles can become fully incorporated in the lexicon of synthetic chemistry, ultimately establishing a synthetic science that manipulates molecular and nanoscale components with equal proficiency.

Figure 1

Selective assembly of binary or unary aggregates from two complementary dynamic covalent nanoparticle (DCNP) building blocks. Nucleophilic DCNP-1 was constructed using either gold cores (AuNP-1) to give homomaterial binary aggregates or on palladium cores (PdNP-1) to give heteromaterial binary aggregates in combination with electrophilic DCNP-2, which were constructed on gold cores (AuNP-2).

Figure 2

(a–c) Linker-driven assembly of nucleophilic PdNP-1 using dialdehyde linker 3. (d–f) Linker-driven assembly of electrophilic AuNP-2 using dihydrazide linker 4. (b, e) Solvodynamic diameter (⟨d_SD⟩, red symbols and blue symbols) as measured by DLS and absorbance at 520 nm (solid lines) evolution over time after addition of the acid catalyst required for dynamic covalent hydrazone exchange. (c, f) Representative TEM images of PdNP aggregates (c) and AuNP aggregates (f) isolated after assembly of all nanoparticle material. For supplementary TEM images, see Figures S11 and S14, respectively. Conditions (concentrations in terms of molecular species) (a–c): [PdNP-1]₀ = 0.15 mM, [3]₀ = 0.45 mM, [CF₃CO₂H]₀ = 20 mM, 9:1 v/v DMF/H₂O, room temperature; (d–f): [AuNP-2]₀ = 0.15 mM, [4]₀ = 0.45 mM, [CF₃CO₂H]₀ = 20 mM, 9:1 v/v DMF/H₂O, room temperature.

Figure 3

(a) Co-assembly of nucleophilic PdNP-1 and electrophilic AuNP-2 from binary colloidal mixtures. (b,c) Variation in solvodynamic diameter (⟨d_SD⟩, magenta symbols) as measured by DLS and absorbance at 520 nm (line) over time after addition of the acid catalyst required for dynamic covalent hydrazone exchange. (d,f) Representative dark field STEM micrographs and (e,g) EDX maps of heteromaterial aggregates (blue = Au; red = Pd). Inset: Heteromaterial aggregate composition expressed in terms of ratio of nanoparticles (PdNP:AuNP) determined by EDX mapping on three distinct sample regions (for full results see Tables S3 and S4, with accompanying HAADF images in Figures S21 and S22). Assembly conditions (concentrations in terms of molecular species) (b, d, e): [PdNP-1]₀ = 0.105 mM, [AuNP-2]₀ = 0.075 mM (i.e., PdNP:AuNP = 50:50); (c, f, g): [PdNP-1]₀ = 0.075 mM, [AuNP-2]₀ = 0.075 mM (i.e., PdNP:AuNP = 41:59). Both experiments: [CF₃CO₂H]₀ = 20 mM, 9:1 v/v DMF/H₂O, room temperature.

Figure 4

Selective assembly of AuNPs from a binary colloidal mixture of PdNP-1 and AuNP-2 under aqueous (b–c) and anhydrous (d–e) conditions. (b,d) Variation in solvodynamic diameter (⟨d_SD⟩, blue symbols) as measured by DLS and absorbance at 520 nm (line) over time after addition of the acid catalyst required for dynamic covalent hydrazone exchange. (c,e) Representative EDX maps of gold-enriched aggregates (blue = Au; red = Pd). Inset: Heteromaterial aggregate composition expressed in terms of ratio of nanoparticles (PdNP:AuNP) determined by EDX mapping on three distinct sample regions (for full results see Tables S5 and S6, with accompanying HAADF images in Figures S24 and S25). Assembly conditions (concentrations in terms of molecular species): [PdNP-1]₀ = 0.105 mM, [AuNP-2]₀ = 0.075 mM (i.e., PdNP:AuNP = 50:50), [4]₀ = 0.45 mM, [CF₃CO₂H]₀ = 20 mM, room temperature, 9:1 v/v DMF/H₂O (b–c) or anhydrous DMF (d–e).

Figure 5

Selective linker-driven assembly of PdNPs from a binary colloidal mixture of PdNP-1 and AuNP-2, equimolar in nanoparticle-bound hydrazones (b–c) or equimolar in nanoparticles (d–e). (b,d) Variation in solvodynamic diameter (⟨d_SD⟩, red symbols) as measured by DLS and absorbance at 520 nm (line) over time after addition of the acid catalyst required for dynamic covalent hydrazone exchange. (c,e) Representative EDX maps of palladium-enriched aggregates (blue = Au; red = Pd). Inset: Heteromaterial aggregate composition expressed in terms of ratio of nanoparticles (PdNP:AuNP) determined by EDX mapping on three distinct sample regions (for full results see Tables S10 and S11, with accompanying HAADF images in Figures S32 and S33). Assembly conditions (concentrations in terms of molecular species) (b–c): [PdNP-1]₀ = 0.075 mM, [AuNP-2]₀ = 0.075 mM (i.e., PdNP:AuNP = 41:59), [3]₀ = 0.45 mM; (d–e): [PdNP-1]₀ = 0.105 mM, [AuNP-2]₀ = 0.075 mM (i.e., PdNP:AuNP = 50:50), [3]₀ = 0.45 mM; both experiments: [CF₃CO₂H]₀ = 20 mM, room temperature, 9:1 v/v DMF/H₂O.

Figure 6

Summary of simulation results for nucleophilic linker-driven assembly from a binary mixture of nucleophilic PdNP-1 and electrophilic AuNP-2. (a,b) Evolution of gold selectivity parameter S_Au (see Supporting Information for details) over time for input conditions that are equimolar in terms of nanoparticles (a) and equimolar in terms of nanoparticle-bound hydrazones (b) under anhydrous conditions (solid lines) or in the presence of water (dashed lines). For speciation profiles, see Figure S38. (c) Dependence of gold selectivity parameter S_Au on the concentration of nucleophilic linker 4 at different time points under equimolar nanoparticle-bound hydrazone input conditions in the presence of water (light green to dark green: 1 mol equiv, 3 mol equiv, 6 mol equiv, 10 mol equiv, 20 mol equiv of 4). For evolution plot of S_Au against time, see Figure S39b.

Figure 7

Summary of simulation results for electrophilic linker-driven assembly from a binary mixture of nucleophilic PdNP-1 and electrophilic AuNP-2. Evolution of selectivity parameter S_Pd (describing ratio of interparticle linkages favoring incorporation of PdNPs over AuNPs in aggregates, see Supporting Information for details) over time for input conditions that are equimolar in terms of nanoparticle-bound hydrazones (a) or equimolar in terms of nanoparticles (b) over a range of concentrations of electrophilic linker 3 (light green to dark green: [3]₀ = 0.075 mM, 0.15 mM, 0.45 mM, 0.75 mM, 1.50 mM, 2.25 mM, 3.00 mM). (a) [1]₀ = [2]₀ = 0.075 mM; (b) [1]₀ = 0.105 mM, [2]₀ = 0.075 mM.

Figure 8

Summary of heteromaterial assembly experiments. (a) Input conditions expressed in terms of both nanoscale (PdNP:AuNP) and molecular scale ([1]:[2]) stoichiometries. Molar equivalents of linker are given relative to the corresponding complementary nanoparticle-bound hydrazone (i.e., 3 relative to PdNP-1; 4 relative to AuNP-2). (b) Assembly outcomes expressed in terms of mole fraction of each nanoparticle (relative proportions of bars: blue = F_{AuNP(aggregate)}; red = F_{PdNP(aggregate)}) and enrichment factor (total height of bars, E.F. = [F_{Au(aggregate)} – F_Au(input)]/F_Au(input)). For a full summary of numerical results, see Table S2.