Characterizing the Core-Shell Architecture of Block Copolymer Nanoparticles with Electron Microscopy: A Multi-Technique Approach

Abstract

Electron microscopy has proved to be a major tool to study the structure of self-assembled amphiphilic block copolymer particles. These specimens, like supramolecular biological structures, are problematic for electron microscopy because of their poor capacity to scatter electrons and their susceptibility to radiation damage and dehydration. Sub-50 nm core-shell spherical particles made up of poly(hydroxyethyl acrylate)–b–poly(styrene) are prepared via polymerization-induced self-assembly (PISA). For their morphological characterization, we discuss the advantages, limitations, and artefacts of TEM with or without staining, cryo-TEM, and SEM. A number of technical points are addressed such as precisely shaping of particle boundaries, resolving the particle shell, differentiating particle core and shell, and the effect of sample drying and staining. TEM without staining and cryo-TEM largely evaluate the core diameter. Negative staining TEM is more efficient than positive staining TEM to preserve native structure and to visualize the entire particle volume. However, no technique allows for a satisfactory imaging of both core and shell regions. The presence of long protruding chains is manifested by patched structure in cryo-TEM and a significant edge effect in SEM. This manuscript provides a basis for polymer chemists to develop their own specimen preparations and to tackle the interpretation of challenging systems.

Cellulose Nanocrystals Mimicking Micron-Sized Fibers to Assess the Deposition of Latex Particles on Cotton

“…It is suggested that the difference in chain density between the core and shell chains accounts for the fact that the electronic contrast of the copolymer shell is significantly lower than those of PBA or PEHA cores, resulting in a lower capacity to scatter electrons. 40,49 Electrophoretic mobility measurements led to ζ potential values around +35 mV, confirming that latexes are strongly charged and cationic. It is anticipated that the positive electrostatic charges in the shell will favor a strong attractive interaction with anionic CNCs and cotton fibers.…”

Kinetically stable sub-50 nm fluorescent block copolymer nanoparticles via photomediated RAFT dispersion polymerization for cellular imaging

“…56,57 This result is consistent with our recent findings that negative stain TEM could take into account the thin POEGA shell while this latter is not easily resolved by cryo-TEM. 58 Note the presence of isolated worms in this latter image not visible in the negative-stained TEM image c. Conversion of spheres into worms may be promoted by particle swelling with residual monomer since a non-dried sample is analyzed in cryo-TEM. In the presence of non-reacted monomer, the copolymer chains in the NP have a greater mobility that favors morphology change.…”

A Comprehensive Review on Core‐Shell Polymeric Particles for Enzyme Immobilization

“…In most cases, the justification for the core structure is based on the thermodynamic incompatibility between the polymers or their different solubility in the continuous phase. [34] Synthesis of core-shell materials often does not lead to an ideal core-shell morphology with complete phase separation. The components at the interface will mix in different ways depending on the compatibility of the two polymers involved and the reaction conditions.…”

“…It is a quick and simple way to determine the hydrodynamic radius of colloidal species [50] . Recently, most works involving synthesizing materials with core‐shell structures sought to obtain submicron‐sized particles [34,51,52] …”