Mesocrystals are assemblies of smaller crystallites and have attracted attention because of their nonclassical crystallization pathway and emerging collective functionalities. Understanding the mesocrystal crystallization mechanism in chemical routes is essential for precise control of size and microstructure, which influence the function of mesocrystals. However, microstructure evolution from the nucleus stage through various crystallization pathways remains unclear. We propose a unified model on the basis of the observation of two crystallization pathways, with different ferric (oxyhydr)oxide polymorphs appearing as intermediates, producing microstructures of magnetite mesocrystal via different mechanisms. An understanding of the crystallization mechanism enables independent chemical control of the mesocrystal diameter and crystallite size, as manifested by a series of magnetic coercivity measurements. We successfully implement an experimental model system that exhibits a universal crystallite size effect on the magnetic coercivity of mesocrystals. These findings provide a general approach to controlling the microstructure through crystallization pathway selection, thus providing a strategy for controlling magnetic coercivity in magnetite systems.

In the crystallisation of nanomaterials, an assembly-based mechanism termed ‘oriented attachment’ (OA) has recently been recognised as an alternative mechanism of crystal growth that cannot be explained by the classical theory. However, attachment alignment during OA is not currently tuneable because its mechanism is poorly understood. Here, we identify the crystallographic disorder-order transitions in the OA of magnetite (Fe3O4) mesocrystals depending on the types of organic surface ligands on the building blocks, which produce different grain structures. We find that alignment variations induced by different surface ligands are guided by surface energy anisotropy reduction and surface deformation. Further, we determine the effects of alignment-dependent magnetic interactions between building blocks on the global magnetic properties of mesocrystals and their chains. These results revisit the driving force of OA and provide an approach for chemically controlling the crystallographic order in colloidal nanocrystalline materials directly related to grain engineering.

decade, many approaches for synthesizing multifunctional NPs with combined properties have been developed, such as heterogeneous crystal growth, [3][4][5] coassembly of different building blocks, [6,7] and the template-based method involving chemical and/or physical binding. [8,9] In particular, the template-based method is advantageous for utilizing the size-and shape-dependent properties of individual NPs, [8][9][10] new properties resulting from inter-particle coupling, [8,10,11] and independently additional modification of different NPs and the template. [11,12] Among the various multifunctional NPs, magnetic-plasmonic (MP) NPs are regarded as emerging materials that are suitable for a broad range of applications, such as catalysis, [12,13] detection, [14,15] optical devices, [9,16] and therapy. [17][18][19] In particular, MP NPs simultaneously exhibiting magnetic and surface-enhanced Raman scattering (SERS) activities have been researched due to their synergistic properties in biomedical applications. [20][21][22][23][24] In detail, non-destructive and sensitive analysis of the SERS technique and separating ability based on the magnetic properties could provide convenient and significant tools in sensing of target molecules or cells. [20][21][22][23][24][25] However, it is difficult to satisfy the designing criteria for high performances in bio-applications: low remanence for preventing particle aggregation, maximized magnetic Magnetic-plasmonic nanoparticles have received considerable attention for widespread applications. These nanoparticles (NPs) exhibiting surfaceenhanced Raman scattering (SERS) activities are developed due to their potential in bio-sensing applicable in non-destructive and sensitive analysis with target-specific separation. However, it is challenging to synthesize these NPs that simultaneously exhibit low remanence, maximized magnetic content, plasmonic coverage with abundant hotspots, and structural uniformity. Here, a method that involves the conjugation of a magnetic template with gold seeds via chemical binding and seed-mediated growth is proposed, with the objective of obtaining plasmonic nanostructures with abundant hotspots on a magnetic template. To obtain a clean surface for directly functionalizing ligands and enhancing the Raman intensity, an additional growth step of gold (Au) and/or silver (Ag) atoms is proposed after modifying the Raman molecules on the as-prepared magnetic-plasmonic nanoparticles. Importantly, one-sided silver growth occurred in an environment where gold facets are blocked by Raman molecules; otherwise, the gold growth is layer-by-layer. Moreover, simultaneous reduction by gold and silver ions allowed for the formation of a uniform bimetallic layer. The enhancement factor of the nanoparticles with a bimetallic layer is approximately 10 7 . The SERS probes functionalized cyclic peptides are employed for targeted cancer-cell imaging and separation.

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Scheme 1. Schematic of submolecular tuning ligand size and spacing to regulate integrin binding and clustering. Descriptions of the proposed model of independently unraveling the effect of ligand size and spacing on integrin ligation and the clustering of macrophages: "7-18", "13-17", "7-3", and "gold shell". In the "A-B" notation, "A" indicates the liganded GNP size in nm and "B" indicates the liganded GNP edge-to-edge spacing in nm. The "7-3" arrays stimulate the binding of integrin across neighboring liganded GNPs, resulting in robust macrophage adhesion comparable to fully liganded gold shells as a positive control. Relative to the "7-3" ligand arrays, increasing the "ligand spacing only" or "both ligand size and spacing" in the "7-18" and "13-17" liganded sites, respectively, yields binding sites for integrin but suppresses multiple integrin molecules joining the neighboring liganded sites due to the "ligand spacing" playing a dominant role, resulting in poor macrophage adhesion.