Particle-Nanofiber Superstructures

Superstructured particle assemblies are sought as efficient platforms for transferring properties from the nano- to macroscale while combining modularity and versatility with facile and scalable fabrication processes. Such assemblies are achieved by structuring primary particle using various assembly techniques, which enables the control and customization of morphological features across length scales. Ensuring high cohesion within these assemblies is crucial for practical applications, both to mitigate nanotoxicity and bioaccumulation and to prevent the loss of performance resulting from subunit detachment. In this Account, the integration of biobased nanofibers into particle constructs is discussed in terms of their ability to act as universal binders that offer several advantages over case-specific strategies to develop strength in superstructures. Cellulose nanofibers, among others, have a remarkable capacity to confer cohesion to virtually any particulate system, thereby improving strength and toughness and opening several opportunities to manipulate their nano- to macrostructures. At the nano- and microscale, nanofibers can disrupt particle lattices, thereby enhancing access to surface functionalities. At the macroscale, nanofibers enable control over the viscoelastic properties of particle suspensions and govern their consolidation into dried particle-nanofiber constructs. Cellulose nanofibers are the most widely used in supraparticle fabrication, but several other fibrillar nanomaterials from chitin, amyloid, and aramid show promise for a broad range of particle-nanofiber assemblies. This Account presents a detailed review of the ability of nanofibers to enhance cohesion and manipulate supraparticle structures. First, cellulose nanofibers are introduced, and aspects like extraction, surface chemistry, modification, and colloidal properties are discussed, as they play important roles in transferring cohesion from the nanofibrillar network onto the particle-nanofiber assembly. Then, nanofiber-particle interactions in dilute and concentrated regimes are discussed and related to the forces and phenomena that drive the consolidation of these robust constructs. Analyses of mesh size and crowding of associated nanofiber networks are put into perspective and discussed in terms of particle entrapment and particle-nanofiber interactions prior to consolidation. Methods currently employed to fabricate superstructured materials are presented, including casting on superhydrophobic surfaces, templates and hydrophilic substrates, foaming for particle-nanofiber networks at air-liquid interfaces, 3D printing, and spray drying. Finally, practical applications for particle-nanofiber superstructures are introduced and discussed in terms of the gains obtained by using well-defined nanostructured materials and supraparticles.