Electrostatic co-assembly of nanoparticles with oppositely charged small molecules into static and dynamic superstructures

Abstract

Coulombic interactions can be used to assemble charged nanoparticles into higher-order structures, but the process requires oppositely charged partners that are similarly sized. The ability to mediate the assembly of such charged nanoparticles using structurally simple small molecules would greatly facilitate the fabrication of nanostructured materials and harnessing their applications in catalysis, sensing and photonics. Here we show that small molecules with as few as three electric charges can effectively induce attractive interactions between oppositely charged nanoparticles in water. These interactions can guide the assembly of charged nanoparticles into colloidal crystals of a quality previously only thought to result from their co-crystallization with oppositely charged nanoparticles of a similar size. Transient nanoparticle assemblies can be generated using positively charged nanoparticles and multiply charged anions that are enzymatically hydrolysed into mono- and/or dianions. Our findings demonstrate an approach for the facile fabrication, manipulation and further investigation of static and dynamic nanostructured materials in aqueous environments.

Extended Data Fig 1. Dependence of the titration behavior on nanoparticle size.

Differently sized TMA-functionalized Au NPs (4.8 nm, 8.8 nm, and 13.1 nm) at the same overall concentration of TMA in solution were titrated with the same solution of EDTA^3– (the NPs were prepared analogously to those described in the Methods section). a, Left: Change in the position of Au·TMA’s SPR peak as a function of amount of EDTA^3– added. In all cases, the amount of NP-adsorbed TMA was 20 nmol. The dashed red line denotes the point of electroneutrality (6.7 nmol of triply charged EDTA). Right: Relative dimensions of Au·TMA used in titration experiments. b, Normalized position of Au·TMA’s SPR peak as a function of amount of EDTA^3– added (replotted from a). The normalized data show that the titration profiles are nearly the same irrespective of the NP size, indicating that the interparticle interactions are governed predominantly by electrostatics. The dashed red line denotes the point of electroneutrality (6.7 nmol of triply charged EDTA).

Extended Data Fig 2. Representative SEM images of colloidal crystals co-assembled from TMA-functionalized Au NPs and various multiply charged anions.

The following anions were used: a–c, EDTA^3–; d–j, citrate^3–; k, l, pyrophosphate^4–; m–s, triphosphate^5–; t, u, trimetaphosphate^3–; v, w, hexametaphosphate^6–; x–z, ATP^4–. The size of the NPs was 4.7 nm (panels n–r), 7.4 nm (panels a–f, j–m, and s–z), and 11.4 nm (panels g–i). In all cases, the counterion was Na⁺.

Extended Data Fig 3. Nanoparticle packing on the faces of colloidal crystals.

SEM images of crystals co-assembled from TMA-functionalized 4.7 nm Au NPs and ATP. The magnified images in (b) and (e) show the hexagonal packing of NPs characteristic of the (111) facet of the face-centered cubic (fcc) structure. The magnified image in (g) shows the cubic packing of NPs characteristic of the (100) facet of the fcc structure.

Extended Data Fig 4. SEM images of colloidal crystals co-assembled from negatively charged NPs and an organic trication.

The crystals were prepared using MUS-functionalized 4.7 nm Au NPs and triply charged cations, OMA³⁺, as described in the Methods section.

Extended Data Fig 5. Cryo-STEM images of aggregates of TMA-functionalized Au NPs and P₃O₁₀ ^5– or ATP.

a, Contrast-inverted bright-field cryo-STEM images of Au·TMA/P₃O₁₀ ^5– aggregates. Reconstruction and analysis of the aggregates denoted by circles are shown in Extended Data Fig. 6. b, Contrast-inverted bright-field cryo-STEM image of Au·TMA/ATP aggregates. Reconstruction and analysis of the aggregates denoted by circles are shown in Extended Data Fig. 7. All panels show single images at zero tilt, part of a tilt series spanning the tilt range of 60º.

Extended Data Fig 6. Reconstruction and analysis of Au·TMA/P₃O₁₀ ^5– aggregates.

Labels a–d correspond to the locations indicated with the same labels in Extended Data Fig. 5. Left panel: ‘Atomistic’ models of the aggregates obtained after 3D reconstruction and particle coordinate refinement. Middle panel: Numbers of nearest neighbors in the first coordination shell in a color-coded representation for each NP. Average number of nearest neighbors = 6.4 (±0.8) (measured on ten different aggregates). Right panel: Pair correlation functions; the nearest-neighbor distance, Δ = 8.27 (±0.03) nm.

Extended Data Fig 7. Reconstruction and analysis of Au·TMA/ATP aggregates.

Labels a–d correspond to the locations indicated with the same labels in Extended Data Fig. 5. First panel: Contrast-inverted bright-field cryo-STEM images of individual Au·TMA/ATP aggregates. Second panel: ‘Atomistic’ models of the aggregates obtained after 3D reconstruction and particle coordinate refinement. Third panel: Numbers of nearest neighbors in the first coordination shell in a color-coded representation for each NP. Average number of nearest neighbors = 7.4 (±0.5) (measured on five different aggregates). Fourth panel: Pair correlation functions; the nearest-neighbor distance, Δ = 8.08 (±0.07) nm.

Fig. 1. Electrostatic co-assembly of positively charged nanoparticles and negatively charged small molecules.

a, Typical aggregation behavior of oppositely charged species on a molecular scale (left) and a nanoscale (right). b, Structural formula of (11-mercaptoundecyl)-N,N,N-trimethylammonium (TMA) ligand used for stabilizing gold nanoparticles (NPs) in water (counterion, Br⁻); right: a representative TEM image of TMA-functionalized 7.4 nm Au NPs (Au TMA). c, Examples of multiply charged anions capable of mediating attractive interactions between Au·TMA. d, Representative titration curve for the titration of Au·TMA (here, 11.4 nm; overall 20 nmol of TMA groups) with EDTA trisodium salt. The dashed red line denotes the point of electroneutrality (~6.7 nmol of trianionic EDTA³⁻). e, Representative titration curve for the titration of EDTA (trisodium salt) (6 nmol), with 11.4 nm Au·TMA. The dashed red line denotes the point of electroneutrality (18 nmol TMA on Au NPs).

Fig. 2. Molecular dynamics simulations of electrostatic interactions between positively charged nanoparticles and small anions.

a, Atomistic model of one-half of a TMA-functionalized Au NP (here, interacting with citrate) used in the all-atom simulations. b, Free-energy profile for the interaction between Au·TMA and Cl⁻ (yellow), HPO₄ ²⁻ (blue) or citrate³⁻ (red) (expressed as a function of the distance d between the center of mass of the anion and the center of mass of the closest TMA charged headgroup; errors bars are calculated as the standard error of the mean). c, Snapshots from CG-MD simulations of two TMA-coated Au NPs in the presence of HPO₄ ²⁻ (top) or citrate³⁻ (bottom). d, Distance between the centers of two TMA-functionalized Au NPs in the presence of HPO₄ ²⁻ (blue) or citrate³⁻ (red) as a function of the CG-MD simulation time.

Fig. 3. Dynamics of small anions and the annealing of TMA-functionalized Au NPs.

a, Snapshots from a CG-MD simulation of two Au·TMA NPs in the presence of citrate ions after stable binding of the two NPs (0.9 μs), and at the end of the CG-MD simulation (8.4 μs). Red, green, and gray identify citrates grouped into three different macro-clusters at t = 0.9 μs (the NPs are represented as solid yellow spheres, with TMA ligands omitted for clarity). b, A two-dimensional free-energy surface associated with the configurations of citrate ions as a function of two variables, CONT and DIST (for definitions, see Supporting Information, Section 4.2.2). The areas encircled by dashed-line ovals denote three local energy minima corresponding to citrates interacting with a single NP (gray), located at the interface between the two NPs (red), and an intermediate state (green). The black arrows denote transitions between the three states, with the numbers next to the arrows indicating the relative probability of a given transition. c, Annealing of Au TMA/citrate aggregates at 23 °C over 24 h. d, Annealing of Au·TMA/citrate aggregates at 50 °C over 24 h. e, Representative cryo-STEM bright-field image of an Au·TMA/P₃O₁₀ ⁵⁻ aggregate. The entity inside the turquoise square is a NP₅₅ Mackay cluster. f, Top: Voxel projections of the tomographic reconstruction of the cluster marked in (e). The three projections shown correspond to the 5-fold, 3-fold, and 2-fold symmetry Bottom: Matching projections of a model Mackay cluster. Scale bars in the insets, 50 nm.

Fig. 4. Self-assembly of co-crystals of TMA-functionalized Au NPs and small anions.

a, Schematic illustration of the method for anion-mediated crystallization of Au·TMA (the counterion for multiply charged anions, indicated in green, is Na⁺. Ammonium ions and ammonia are shown in red; carbonate ions and carbon dioxide are shown in blue). b-h, Representative SEM images of colloidal crystals of Au·TMA and various multiply charged anions: P₃O₁₀ ⁵⁻ (b, e, g, h), EDTA³⁻ (f), citrate³⁻ (d), and ATP⁴⁻ (c). The size of Au NPs was 7.4 nm (b, c, e), 11.4 nm (d, f), and 4.7 nm (g, h). For additional images, see Extended Data Fig. 1. i, Optical micrograph of colloidal crystals of 4.7 nm Au·TMA and P_{3O 10} ⁵⁻ j, 1D X-ray diffraction (SAXS) patterns for 4.73 nm Au·TMA/ P3O10⁵⁻ crystals. SAXS data are plots of scattered intensity I(q) (y-axis, arbitrary units) vs. scattering vector q (x-axis, Å^-1). Black traces are experimental data; pink and blue traces are modeled SAXS patterns for perfect lattices (pink: a = 121.3 Å; blue: a = 130.9 Å). k, Proposed models of binding within crystals with a smaller (top; major species) and larger (bottom; minor species) lattice constants. l, 1D SAXS patterns for 4.73 nm Au·TMA/ATP crystals. Black traces are experimental data; pink and blue traces are modeled SAXS patterns for perfect lattices (pink: a = 119.8 Å; blue: a = 130.9 Å).

Fig. 5. Electrostatic co-assembly of negatively charged nanoparticles and positively charged small molecules.

a, Structural formula of 11-mercaptoundecanesulfonate (MUS) used for stabilizing gold NPs in water (counterion, Na⁺); right: a representative TEM image of MUS-functionalized 4.7 nm Au NPs. b, Structural formulas of an organic trication capable of mediating attractive interactions between Au·MUS (octamethyldiethylenetriammonium; OMA³⁺) and a control dication (hexamethylethylenediammonium; HMA²⁺) (counterions, I⁻). c, Solid markers: a representative titration curve for the titration of Au·MUS (4.7 nm; overall 50 nmol of MUS groups) with OMA³⁺. The dashed red line denotes the point of electroneutrality (~16.7 nmol of OMA³⁺). Empty markers: control titration with HMA²⁺. d, Solid markers: a representative titration curve for the titration of OMA³⁺ (20 nmol) with 4.7 nm Au·MUS. The dashed red line denotes the point of electroneutrality (60 nmol MUS on Au NPs). Empty markers: a control titration of HMA²⁺ with the same NPs. e-f, Representative SEM images of colloidal crystals of Au·MUS and OMA³⁺.

Fig. 6. Dissipative self-assembly of gold nanoparticles driven by ATP.

a, Structural formulas of ATP and inorganic triphosphate. b, Schematic representation of the ATP-induced dissipative selfassembly (DSA) of Au·TMA. The assembly/disassembly cycle is coupled to an exergonic reaction of the hydrolysis of ATP into AMP and HPO₄ ²⁻. c, A series of UV/Vis absorption spectra of TMA-functionalized 7.4 nm Au NPs (n _TMA = 19.2 nmol) in the presence of 11.1 units/mL of apyrase before and after injecting a solution of ATP (n _ATP = 54 nmol). d, Changes in the maxima of the dynamic light scattering (DLS) profiles of a solution of the same Au·TMA (with 11.1 units/mL of apyrase) after injecting 40.5 nmol of ATP at t = 0. e, Representative SEM images of 7.4 nm Au·TMA (11.1 units/mL of apyrase) before (t = 0) and after different time intervals following the injection of 54 nmol of ATP. f, Six cycles of DSA of Au·TMA in the presence of 1000 units/mL of alkaline phosphatase (ALPase), followed by monitoring the position of the NPs’ SPR band. For each cycle, 54 nmol of ATP was used. g, Six cycles of DSA of Au·TMA (1000 units/mL of ALPase), followed by monitoring the absorbance at 800 nm. Each cycle was initiated by injecting 54 nmol of ATP. h, Stimulus-dependent disassembly profiles of aggregates of Au·TMA in the presence of ALPase (200 units/mL). Solid triangles denote ATP (12 nmol); empty circles denote inorganic triphosphate (9 nmol). i, Representative cryo-TEM image of 4.7 nm Au TMA/ P₃O₁₀ ⁵⁻ aggregates. j, Representative cryo-TEM image of 4.7 nm Au·TMA/ATP aggregates (the scale bar in the inset corresponds to 20 nm). k, Controlling the lifetimes of dynamic NP aggregates by the amount of ATP injected at t = 0 (in all cases, 200 units/mL of ALPase were used). l, Controlling the lifetimes of dynamic NP aggregates by the amount of a phosphatase enzyme (here, apyrase; in all cases, 54 nmol of ATP were injected at t = 0). In panels b–d and h, the syringe indicates injection of an oligophosphate stimulus and the petrol pump indicates the availability of the stimulus.