Building Reversible Nanoraspberries

Abstract

The adsorption mechanism of small positively charged silica nanoparticles (SiO₂ NPs) onto larger polystyrene latex nanoparticles (PSL NPs) forming hybrid particles was studied. CryoTEM showed the morphology of these supraparticles to be raspberry-like. After surface modification of the SiO₂ NPs, the optimum pH regime to initiate the formation of nanoraspberries was determined. Thereafter, their size evolution was evaluated by dynamic light scattering for different surface charge densities. Reversibility of nanoraspberry formation was shown by cycling the pH of the mixture to make interparticle forces either attractive or repulsive, while their stability was confirmed experimentally. The number of SiO₂ NPs on the PSL NPs as determined with cryoTEM matched the theoretically expected maximum number. Understanding and controlling the relevant parameters, such as size and charge of the individual particles and the Debye length, will pave the way to better control of the formation of nanoraspberries and higher-order assemblies thereof.

Keywords: CryoTEM; Raspberry Nanoparticles; Self-assembly; Silica Nanoparticles; Supraparticles.

Figure 1

(a) Zeta potential measurements of PSL NPs, bare SiO₂ NPs, and surface-modified SiO₂ NPs. (b) Evolution of the hydrodynamic diameter of aggregates in a mixture of 30 nm surface-modified SiO₂ NPs and 100 nm PSL NPs at different pH values over time. Results for pH 9 and pH 12 overlap for the whole range as they do for pH 5 and 6. The lines drawn are a guide to the eye.

Figure 2

CryoTEM images of the mixture of PSL NPs and SiO₂ NPs (a, b) at pH 4 taken 1 month after mixing and (c–h) at pH 2 taken 2 min after mixing. In this case, several images are shown to demonstrate clearly in detail that the PSL NPs are not fully covered with SiO₂ NPs yet (i, j) at pH 2 taken 1 week after mixing, (k–m) at pH 2 taken 1 month after mixing, and (n–p) at pH 12 taken 1 month after mixing.

Figure 3

Cross sections at different heights in a nanoraspberry as obtained from cryoET reconstructions (a–e) show that the PSL nanoparticle is fully covered with SiO₂ NPs. Scale bars = 100 nm. Further, (f) shows the cryoTEM image of a single nanoraspberry, and (g) and (h) show the side and top view of the 3D reconstruction of the same nanoraspberry, respectively.

Figure 4

Dynamic and reversible nature of nanoraspberries as evaluated by cryoTEM characterization made by surface-modified SiO₂ NPs mixed with PSL NPs. (a) Mixture at pH 2. (b) The mixture as shown in (a) brought at pH 10. (c) The mixture shown in (b) brought at pH 2 again. (d) The mixture shown in (c) brought back to pH 10 again. (e) DLS results at various pH corresponding to the cryoTEM images (a) to (d) (see also Figure SI6b). The error bars indicate the standard deviation in the mean, which is typically 10 nm. (f) Zeta potential measurements for PSL NPs, SiO₂ NPs, and mixtures used in the reversibility experiments as a function of pH.

Figure 5

Influence of changing ionic strength I on the Debye length κ^–1 in water at a 25 °C. Ionic strength values corresponding to cycling steps in Figure 4 shown as a, b, c, and d.