Mechanistic Insights into Diblock Copolymer Nanoparticle–Crystal Interactions Revealed via in Situ Atomic Force Microscopy

Abstract

Recently, it has become clear that a range of nanoparticles can be occluded within single crystals to form nanocomposites. Calcite is a much-studied model, but even in this case we have yet to fully understand the details of the nanoscale interactions at the organic-inorganic interface that lead to occlusion. Here, a series of diblock copolymer nanoparticles with well-defined surface chemistries were visualized interacting with a growing calcite surface using in situ atomic force microscopy. These nanoparticles comprise a poly(benzyl methacrylate) (PBzMA) core-forming block and a non-ionic poly(glycerol monomethacrylate) (Ph-PGMA), a carboxylic acid-tipped poly(glycerol monomethacrylate) (HOOC-PGMA), or an anionic poly(methacrylic acid) (PMAA) stabilizer block. Our results reveal three modes of interaction between the nanoparticles and the calcite surface: (i) attachment followed by detachment, (ii) sticking to and "hovering" over the surface, allowing steps to pass beneath the immobilized nanoparticle, and (iii) incorporation of the nanoparticle by the growing crystals. By analyzing the relative contributions of these three types of interactions as a function of nanoparticle surface chemistry, we show that ∼85% of PMAA-PBzMA nanoparticles either "hover" or become incorporated, compared to ∼50% of the HOOC-PGMA-PBzMA nanoparticles. To explain this difference, we propose a two-state binding mechanism for the anionic PMAA-PBzMA nanoparticles. The "hovering" nanoparticles possess highly extended polyelectrolytic stabilizer chains and such chains must adopt a more "collapsed" conformation prior to successful nanoparticle occlusion. This study provides a conceptual framework for understanding how sterically stabilized nanoparticles interact with growing crystals, and suggests design principles for improving occlusion efficiencies.

What Dictates the Spatial Distribution of Nanoparticles within Calcite?

“…Recently, Estroff and coworkers demonstrated three modes of interaction between the nanoparticles and the growing calcite surface via in situ AFM studies: (i) nanoparticle attachment followed by detachment, (ii) nanoparticle, and (iii) incorporation of the nanoparticle by the growing crystals. 62 Which Parameters Dictate Uniform Occlusion? Empirically, it has been shown that anionic surface character is important for driving nanoparticle occlusion within calcite.…”

Model Anionic Block Copolymer Vesicles Provide Important Design Rules for Efficient Nanoparticle Occlusion within Calcite

“…[9][10][11][12][13][14][15] Not only do such synthetic routes provide a straightforward approach to the rational design of new hybrid materials with enhanced physical properties such as hardness, magnetism, color or strong fluorescence, [16][17][18][19][20] but they can also inform our understanding of biomineralization processes. [21][22][23][24] There is no doubt that the nanoparticle surface chemistry dictates their occlusion. [25][26][27] However, the precise design rules required for efficient nanoparticle occlusion within inorganic crystals have not yet been elucidated.…”

How Many Phosphoric Acid Units Are Required to Ensure Uniform Occlusion of Sterically Stabilized Nanoparticles within Calcite?

“…In this case, there is no specific requirement for the nanoparticle surface chemistry because the nanoparticles are merely physically entrapped within an appropriate gel network. However, for active nanoparticle occlusion within growing inorganic crystals, the nanoparticle surface chemistry dictates whether occlusion occurs and also to what extent . In the present study, nanoparticles with low phosphate content are preferentially occluded within the near‐surface of the calcite crystals (see Figure b and Figure S4), presumably because there are insufficient anionic groups to ensure adequate binding to the growing crystal surface.…”

“…Nanoparticle occlusion within inorganic crystals is a complex process that is influenced by many parameters –. Liu and co‐workers reported that nanoparticles can be passively incorporated into calcite using a gel‐trapping method .…”