The controlled self-assembly of nanoparticles (NPs) into complex structures is a significant challenge in materials science. Traditional methods often rely on pre-functionalized NPs with directional bonding characteristics, achieved through patchy particle fabrication or the synthesis of building blocks with specific geometries. However, these approaches are hindered by the difficulties associated with regioselective surface functionalization and shape-specific synthesis at the nanoscale. This research aims to overcome these limitations by developing a novel method for the controlled co-assembly of unvarnished hard nanoparticles and soft block copolymer micelles, leveraging non-covalent interactions to guide the self-assembly process. This approach offers a more versatile and scalable route to the creation of complex colloidal architectures with diverse functionalities compared to previous methods involving intricate surface modification and precise shape control of individual nanoparticles. The ability to program the assembly of simple, unmodified nanoparticles opens avenues for designing advanced materials with tailored properties for various applications.

Prior research has explored the self-assembly of nanoparticles, resulting in diverse colloidal architectures such as low-dimensional clusters, one-dimensional chains, and higher-ordered two- and three-dimensional superstructures. Studies have demonstrated the use of block copolymer micelles with a poly(2-vinylpyridine) (P2VP) corona to adsorb onto silica particles, creating core-satellite structures through hydrogen bonding interactions. This work builds upon these findings by utilizing the hydrogen bonding capabilities of P2VP-based block copolymer micelles to direct the assembly of much smaller silica nanoparticles, allowing for the formation of highly organized multidimensional colloidal structures. The use of unvarnished nanoparticles simplifies the synthesis process and offers a pathway to control assembly solely through the interactions between the soft and hard components.

The study employed block copolymer micelles (PS core and P2VP corona, denoted as A) and silica nanoparticles (denoted as B) of varying sizes (50 nm, 75 nm, 90 nm). The co-assembly process involved mixing the soft and hard NPs in different A:B ratios in ethanol. The resulting structures were characterized using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The valence of the silica nanoparticles varied with the A:B ratio, ultimately reaching a maximum value dependent on the silica nanoparticle size. This self-regulation of valence was attributed to the balance between the adhesive forces of hydrogen bonding and the repulsive forces between the P2VP coronas. The study extended this approach to other hard NPs, including gold nanoparticles (Au NPs, denoted as C) and zeolitic imidazolate framework-8 (ZIF-8) NPs (denoted as D), demonstrating the versatility of the method. The co-assembly process was also investigated on a silicon wafer surface. Micelles were immobilized on a silicon wafer, followed by sequential addition of silica NPs, leading to the formation of colloidal molecules on the surface. A 'grafting-to' method was also utilized to create colloidal brushes on the surface by attaching silica-NPs-capped colloidal oligomers to the immobilized micelles. The entire process relied on noncovalent interactions, primarily hydrogen bonding and coordination interactions, to drive the self-assembly process. Characterization techniques included TEM and SEM throughout the different experiments and steps.

The research demonstrated a self-regulated co-assembly process between soft block copolymer micelles and hard nanoparticles, leading to the formation of well-defined colloidal structures. The valence of the hard nanoparticles (number of micelles attached) was tunable by adjusting the relative size of the nanoparticles and the feeding ratio of the soft and hard components. For example, 50 nm silica nanoparticles exhibited a maximum valence of 2, while 75 nm and 90 nm silica nanoparticles achieved valences of 3 and 4, respectively. The morphology of the resulting structures varied from simple dimers and trimers to complex branched polymers and two-dimensional networks. The formation of Janus-like intermediate structures was proposed as a mechanism for directing the assembly process. The method was successfully extended to other hard nanoparticles, including gold nanoparticles and ZIF-8 nanoparticles, showcasing its versatility. Moreover, the controlled co-assembly was achieved on a surface, both through stepwise sequential addition and a grafting-to method, creating colloidal molecules and brushes. These findings suggest a facile and effective method for building up complex colloidal architectures using simple, unvarnished nanoparticles.

This work provides a significant advancement in the field of nanoparticle self-assembly. The development of a self-regulated co-assembly strategy simplifies the synthesis of complex colloidal structures, eliminating the need for intricate surface functionalization of individual nanoparticles. The ability to tune the valence of hard nanoparticles by simply adjusting the mixing ratio of soft and hard components offers a high level of control over the final architecture. The successful integration of diverse hard nanoparticles (silica, gold, and ZIF-8) highlights the versatility of this approach. Furthermore, demonstrating the feasibility of surface-based assembly expands the potential for applications in materials science and nanotechnology, offering pathways to create patterned surfaces with tailored properties. The observed dynamic valence of the silica nanoparticles, a consequence of the balance between attractive and repulsive forces, indicates a level of self-correction inherent to the system.

This study presents a novel self-regulated co-assembly approach for constructing complex colloidal molecules and polymers using soft block copolymer micelles and hard nanoparticles. The method's simplicity, versatility, and tunability offer a promising platform for creating sophisticated colloidal architectures with a high degree of control. Future research could explore the integration of more diverse nanoparticle types, expanding the range of functionalities achievable through this approach. Further investigation into the precise control of assembly parameters will enhance the predictability and reproducibility of the process. The exploration of the resulting materials' macroscopic properties and their potential applications in diverse fields is crucial for realizing the full potential of this self-assembly method.

While this study demonstrates a significant advance in colloidal self-assembly, certain limitations exist. The current methods are primarily characterized using TEM and SEM, which provide information at the nanoscale. Further characterization techniques may be necessary to fully understand the resulting structures at a larger scale. The current methods focused on co-assembly involving hydrogen bonding and coordination interactions. The generalizability of this method to other types of interactions requires further exploration. The surface-based assembly demonstrated so far is limited to relatively short oligomers. Developing strategies to create longer and more complex structures on surfaces remains a challenge.