In Situ Characterization of Binary Mixed Polymer Brush-Grafted Silica Nanoparticles in Aqueous and Organic Solvents by Cryo-Electron Tomography

Abstract

We present an in situ cryo-electron microscopy (cryoEM) study of mixed poly(acrylic acid) (PAA)/polystyrene (PS) brush-grafted 67 nm silica nanoparticles in organic and aqueous solvents. These organic-inorganic nanoparticles are predicted to be environmentally responsive and adopt distinct brush layer morphologies in different solvent environments. Although the self-assembled morphology of mixed PAA/PS brush-grafted particles has been studied previously in a dried state, no direct visualization of microphase separation was achieved in the solvent environment. CryoEM allows the sample to be imaged in situ, that is, in a frozen solvated state, at the resolution of a transmission electron microscope. Cryo-electron tomograms (cryoET) were generated for mixed PAA/PS brush-grafted nanoparticles in both N,N-dimethylformamide (DMF, a nonselective good solvent) and water (a selective solvent for PAA). Different nanostructures for the mixed brushes were observed in these two solvents. Overall, the brush layer is more compact in water, with a thickness of 18 nm, as compared with an extended layer of 27 nm in DMF. In DMF, mixed PAA/PS brushes are observed to form laterally separated microdomains with a ripple wavelength of 13.8 nm. Because of its lower grafting density than that of PAA, PS domains form more or less cylindrical or truncated cone-shaped domains in the PAA matrix. In water, PAA chains are found to form a more complete shell around the nanoparticle to maximize their interaction with water, whereas PS chains collapse into the core of surface-tethered micelles near the silica core. The cryoET results presented here confirm the predicted environmentally responsive nature of PAA/PS mixed brush-grafted nanoparticles. This experimental approach may be useful for the design of future mixed brush-grafted nanoparticles for nano- and biotechnology applications.

Loading and Delivery Characteristics of Binary Mixed Polymer Brush‐Grafted Silica Nanoparticles

“…The striation arises because only the PAA polymer chains, and not the PS polymer chains, are stained making the PAA polymers appear darker in contrast by cryoEM. In addition, the mixed PAA/PS brush adopts a laterally microphase‐separated structure in DMF with a ripple wavelength of ≈13.8 nm . This ripple pattern is clearly visible as striations within the brush layer in the cryo‐electron image.…”

“…We developed a strategy for loading Nile Red into mixed PAA/PS brush‐grafted nanoparticles which involves sonication of the nanoparticles with excess dye in an organic solvent, transfer to a 50/50 aqueous/organic solvent, and dialysis in aqueous buffer to remove excess dye. We reasoned that since the hydrophobic PS and hydrophilic PAA chains are laterally microphase separated and swollen in DMF, this would be a good solvent for loading hydrophobic dye molecules into the nanoparticles. Gradual transfer of the dye‐loaded nanoparticles into an aqueous buffer (PBS) should result in the dye being trapped within collapsed surface‐tethered micelles of PS within the swollen PAA brush layer.…”

“…The working hypothesis regarding loading of the mixed PAA/PS brush‐grafted nanoparticles with hydrophobic compounds is that loading should be most readily achieved when the nanoparticles are in a good solvent for both polymers. In this environment, the polymeric chains are highly solvated and microphase separated laterally into a ripple structure . In contrast, when the nanoparticles are in an aqueous environment they adopt a different morphology, with the hydrophobic PS chains collapsed in surface‐tethered micelles of PS within the swollen PAA brush layer.…”

“…NP‐1 mixed PAA/PS brus h nanoparticles and NP‐3 PAA brush nanoparticles were prepared for cryo‐electron microscopy (cryoEM) as described previously . Briefly, a 20.0 μL aliquot of the nanoparticles in DMF was stained with 0.5 μL of filtered 1% uranyl acetate aqueous solution, mixed, and allowed to equilibrate on ice for 30 min before preparation of the cryoEM grids.…”

“…Unfavorable interactions between the two types of polymers are minimized by microphase separation, which generates various nanoscale patterns in the polymer layer. A cryo‐electron tomography study has shown that mixed poly(acrylic acid) (PAA)/polystyrene (PS) brush‐grafted 67 nm silica nanoparticles have distinct 3D morphologies in organic and aqueous solvents . In an organic solvent in which both polymers are soluble, the mixed brush layer is highly solvated and microphase separated laterally into a ripple structure.…”