Supraparticles: Functionality from Uniform Structural Motifs

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Supraparticles: Functionality from Uniform Structural Motifs Susanne Wintzheimer, Tim Granath, Maximilian Oppmann, Thomas Kister, Thibaut Thai, Tobias Kraus, Nicolas Vogel, and Karl Mandel ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00873 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Supraparticles: Functionality from Uniform Structural Motifs Susanne Wintzheimer,a Tim Granath,b Maximilian Oppmann,a Thomas Kister,c Thibaut Thai,c Tobias Kraus,c,d* Nicolas Vogel,e,* Karl Mandela,b,*

Affiliations: a Fraunhofer Institute for Silicate Research, ISC, Neunerplatz 2, 97082 Würzburg, Germany. b University Würzburg, Chair of Chemical Technology of Materials Synthesis, Röntgenring 11, 97070 Würzburg, Germany. c INM—Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbrücken, Germany d Saarland University, Colloid and Interface Chemistry, Campus D2 2, 66123 Saarbrücken, Germany e Institute of Particle Technology, Friedrich-Alexander Universität Erlangen-Nürnberg (FAU), Haberstrasse 9A, 91058 Erlangen, Germany. * corresponding authors: [email protected] (T.K.), [email protected] (N.V.), [email protected] (K.M.)

Table of Contents (TOC) graphic:

Keywords: supraparticles; colloids; self-assembly; agglomeration; hierarchy; building-blocks; nanostructures; complex particles; functionality Vocabularies: • Supraparticles: Supraparticles combine prefabricated nanoparticles into larger, discrete units with defined structural motifs. • Structural motif: A “structural motif” is a recurring relation between the constituent particles of the supraparticle. Typical examples are the number and distance of nearest neighbors, positions of particles in a crystal structure, or the alignment of anisotropic particles within the supraparticle. In more complex supraparticles, additional motifs include the adjacency or number ratios of different particle types, and their preferential positions, for example in a core-shell structure. • Agglomerates: An agglomerate is a discrete unit of particles that can be dispersed into the original particles without damaging them. By contrast, aggregates contain particles that are rigidly connected, e.g. through a material bridge. • Self-assembly: Self-assembly arranges individual components into organized structures solely through interactions among the components that minimize the free energy. • Emergence: Emergence describes properties caused by a specific order or cooperative effects within a supraparticle that the constituent particles do not have. 1

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Abstract Under the right process conditions, nanoparticles can cluster together to form defined, dispersed structures, which can be termed supraparticles. Controlling the size, shape, and morphology of such entities is a central step in various fields of science and technology, ranging from colloid chemistry and soft matter physics to powder technology and pharmaceutical and food sciences. These diverse scientific communities have been investigating formation processes and structure/property relations of such supraparticles under completely different boundary conditions. On the fundamental side, the field is driven by the desire to gain maximum control of the assembly structures using very defined and tailored colloidal building-blocks, while more applied disciplines focus on optimizing the functional properties from rather ill-defined starting materials. With this review article, we aim to provide a connecting perspective by outlining fundamental principles that govern the formation and functionality of supraparticles. We discuss the formation of supraparticles as a result of colloidal properties interplaying with external process parameters. We then outline how the structure of the supraparticles gives rise to different functional properties. They can be a result of the structure itself (emergent properties), of the colocalization of different, functional building-blocks, or of coupling between individual particles in close proximity. Taken together, we aim to establish structure-property and process-structure relationships that provide unifying guidelines for the rational design of functional supraparticles with optimized properties. Finally, we aspire to connect the different disciplines by providing a categorized overview of the existing, diverging nomenclature of seemingly similar supraparticle structures. 2


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Table of Contents Introduction Discussion General mechanisms of supraparticle formation Synthesis methods Confinement Wet self-assembly (WSA): Emulsions (Non-)surfactant-stabilized emulsions Microfluidic emulsification Dry self-assembly (DSA): Spray-drying Bulk agglomeration Self-limited self-assembly Sonochemically assisted formation Sol-gel processes Specific interactions Function Coupling Surface plasmon resonance and resonant energy transfer Upconversion Photocatalysis Emergence Photonics Porosity Colocalization Nanoparticle formulations Magnetic objects Nomenclature Conclusions

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The past decades brought tremendous progress in making nanoparticles with a wide range of chemical compositions, controlled sizes, and defined morphologies. Superior and useful properties were discovered for nanoparticles in different areas of science and technology, including optics,1–4 magnetics,5,6 electronics,7–12 mechanics,13,14 sensing15,16 or catalysis.17–19 The precise control of composition, size, and morphology that is possible today enables adjustment and ultimately control of the nanoparticle properties. A similar evolution of synthetic capabilities and thus, possibilities already occurred in molecular chemistry some decades before the nano-age. As a step further, supramolecular chemistry takes advantage of readily designed molecular structures and allows for a “chemistry beyond the molecule“. In the words of Jean-Marie Lehn,20 supramolecular chemistry aims at “designing and implementing functional chemical systems based on molecular components held together by noncovalent intermolecular forces”. It is “the buildup (synthesis!) of discrete or extended assemblies of chemical objects.”20 A similar buildup of assemblies is possible with nanoparticles, too. In a highly cited article by Glotzer and Solomon21 nanoparticles are considered as the “atoms” and “molecules” of tomorrow’s materials that may be assembled into useful structures. Such concepts have spurred the search for processes to assemble defined suprastructures using particles interacting via weak, physical interactions similar to supramolecular chemistry. As this review article will show, under the right process conditions, nanoparticles can indeed cluster together to form defined agglomerates.

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Controlling the size, shape and morphology of particle agglomerates is not only a potential next level of nanotechnology. It is already a central step in various fields of science and technology, ranging from soft matter physics to powder technology and pharmaceutical or food sciences. These diverse scientific communities have been investigating formation processes and structure/property relations of units made from smaller building-blocks for very different materials and processes. Fundamental approaches focus on a high level of control of the assembly process using very defined colloidal building-blocks to understand phase diagrams and equilibrium structures, while more applied disciplines are driven by the desire to optimize functional properties from rather ill-defined starting materials. In fact, as a result of the richness and complexity of such systems there are both “worlds”: On the one hand, a focus on precision, i.e., the focus on engineering highly defined complex particles. On the other hand, there are factors considered such as cost and scalability that might play an important role. As a consequence, there might be researchers who are devoted to demonstrate the capability to create highly precise structures - at any cost - while others require inexpensive particles that must be available in large amounts – the price that is paid in the latter case is often a less sophisticated structure. What all these communities have in common is that they share an interest in making mesoscale objects. Chemists have already mastered the precise engineering at the molecular level, i.e., in making well-defined molecules (drugs, catalysts, polymers,…), while engineers excel at the design of structures at the macroscopic level, i.e., in making well-defined buildings, engines and devices. Engineering at the length scales in between these two extrema, by any other approaches than lithography used in the computer chip industry, however, remains a challenge. Assembling 5


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objects at this mesoscale, the length scales from about 100 nm to several tens of micrometers, requires precise building-blocks and a high level of control of the forces interacting between these. Nanoparticles, which are available with well-controlled properties, may provide the ideal toolbox for the engineering at the mesoscale. It seems natural to term a particle that is composed of smaller particulate building-blocks a “supraparticle” in analogy to a “supramolecule”; the latter is defined by the IUPAC as “a system of two or more molecular entities held together and organized by means of intermolecular (noncovalent) binding interactions”.22 In the field of particles, however, there is no coherent nomenclature used, yet. This lack of coherent descriptions seemingly complicates the connection of different scientific communities, as evidenced by the difficulties of finding related literature. A literature search in Scopus and ISI Web of Knowledge yields only around one hundred publications that explicitly use the wording of “supraparticles”. The term has been most often used for either particle systems comprising two or more types of nanoscale building-blocks to form particles with complex nature23–37 or for particles made from a single type of nanoparticle arranged into more complex structures.25,26,30,33,37–57 Many communities that make complex particles from sub-units do not call them „supraparticles“ and may not know of similar activities in other fields - an issue that we attempt to overcome with this review.

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As depicted in Figure 1, the design of supraparticles is flexible. A very large number of different structures can be assembled for a small number of building-blocks by varying fundamental design criteria such as: • Connectivity: density, fractal dimension, order and disorder, or shape • Composition: homo-component versus hetero-/multiple-components • Distribution: different phases within the particles, core-shell, or capsule architecture With this wide variety in supraparticle characteristics comes a wide variety in engineering processes required to make them. Equally diverse are the functionalities, the potential applications of these particles, and the communities that deal with them. This article reviews supraparticle synthesis, structures, and properties. It is not designed as an exhaustive literature collection but rather as a critical discussion with instructive examples.

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Figure 1: Supraparticles formed from colloidally dispersed nanoparticle building-blocks through self-assembly. Structural motifs that recur in the supraparticle and lend it functionality can include connectivity (e.g. the number of nearest neighbors or the fractal dimension), composition (where certain particles are preferably in contact with identical particles, or a different type), and distribution (where the probability of encountering a certain particle, or particle arrangement, depends on its position in the supraparticle) (a). b) Exemplary scanning electron micrographs of a loose connection (left), a solid supraparticle (middle) and a highly ordered and regular structure (right). c) Transmission electron micrographs of supraparticles with difference distributions of small and large gold nanoparticles in supraparticles with a binary crystalline lattice, Janus supraparticles with two sided composed of only one particle type, and core-shell supraparticles where small particles surround a core of large particles. (Reprinted with permission from b) (left) ref 58, Copyright 1985, American Physical Society, b) (middle) ref 59, Copyright 2015, NATIONAL ACADEMY OF SCIENCES, b) (right) ref 60, Copyright 2014, Springer Nature, c) ref 61, Copyright 2016, Royal Society of Chemistry.)

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We restricted ourselves to particles that fit into the following definition: 1. We believe that the term supraparticle implies an entity consisting of defined particles, i.e., that it has been created from colloidally dispersed nanoparticles as the starting material. We excluded all processes where the constituent particles are only transient stages that are not accessible for analysis, modification, or exchange. 2. The resulting supraparticles must again be in a dispersed form, for instance in a colloid, powder, microcomposite, etc. This distinguishes supraparticles from self-assembled nanoparticle films, colloidal crystals, and other nanoparticle arrangements that are infinitely extended in one, two, or three dimensions. 3. A consistent structural motif prevails amongst all particles at least to some approximation and enables to describe the structure of the supraparticle with respect to crystal packing, fractal dimension, connectivity to the nearest neighbor, or a defined average size or shape of the supraparticle. This distinguishes deliberately engineered supraparticles from randomly agglomerated nanoparticles. 4. This structural motif creates additional functionality, i.e., the level of definition of the supraparticular objects must be sufficient to generate function.

A “structural motif” is a recurring relation between the constituent particles of the supraparticle. Typical examples are the number and distance of nearest neighbors, positions of particles in a crystal structure, or the alignment of anisotropic particles within the supraparticle. In more

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complex supraparticles, additional motifs include the adjacency or number ratios of different particle types, and their preferential positions, for example in a core-shell structure. Considering only supraparticles that fulfill these criteria, this review at first briefly introduces general mechanisms and principles of supraparticle formation. This is followed by an overview on methods that have been reported to make supraparticles. We then focus on potential functionalities emerging in supraparticles, in particular if they are special to this class of particles. The final chapter attempts to survey the nomenclature used supraparticles and related structures.

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Discussion

General mechanisms of supraparticle formation The task to create a supraparticle requires bringing together nanoparticles to form a more complex entity. As the supraparticle is formed by a controlled agglomeration (or assembly) of individual nanoparticles, it is clear that the resulting structure directly relates to the balance of forces acting between the particles and the kinetics of the assembly process.62–64 Key to controlling the structure therefore is to understand and manipulate the forces acting on the building-blocks as well as the process conditions. The building-blocks forming the supraparticle in our definition pre-exist as colloidally stable, separated nanoparticles. Consequently, the formation of the supraparticle requires overcoming repulsive forces in between the particles.63,65 In a bulk system, the balance of forces to achieve attractive interactions can be introduced for example via changes in the electrostatic stabilization (i.e., by changing pH or ion concentration), the mixture of mutually attractive colloidal species (i.e., negatively and positively charged colloids), enthalpic interactions, where distinct patches on a particle surface provide attractive interaction to similar patches of a second particle,66–68 or entropic interactions that are typically based on depletion attraction.63,65,66,68 A very high precision of the assembly structure can be achieved by providing defined surface features to which a second particle can anchor following a lock-and-key mechanism.69 The interplay between attractive and repulsive forces governs the density of the supraparticle, i.e., whether its structure is more open or more compact. The stronger the attractive component, the faster the 11


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agglomeration behavior, which, in the extreme case, leads to diffusion-limited agglomeration and loose structures (Figure 1b, left).70,71 A high degree of control over the attractive component can lead to the formation of highly symmetric clusters with defined structures, sometimes referred to as colloidal molecules.65,66,69,72 An alternative approach to enforce the assembly of a defined supraparticle involves external confinement to template the assembly process. If a distinct number of nanoparticulate buildingblocks is forced into a defined volume (most commonly, a liquid droplet), supraparticle formation may largely depend on external process conditions and can thus be manipulated without directly manipulating the interaction potential between the particles. An evaporating droplet will lead to a continuous concentration increase of the constituent particles, until they are forced into direct contact and form a solid supraparticle (Figure 1b, middle).73–76 The resulting morphology is often not governed by the interaction potential of the building-blocks (provided the particles are colloidally stable before emulsification), but by the kinetics of the formation process. Slow evaporation provide time for the individual particles to minimize their free energy and foster the formation of highly ordered and regular structures (Figure 1b, right).60,76,77 Depending on the number of particles within the system and their interactions, different configurations can result, ranging from defined colloidal clusters77 via icosahedral structures60 to spherical colloidal crystals.76 If the assembly process is fast, as for example in spray-drying processes, the formation process is kinetically controlled and will result in less defined supraparticles. Kinetic phenomena can even be exploited to define their morphology: in a mixed dispersion with two particle populations with different sizes, the difference in the time scale of diffusion will drive the smaller particles to the outside of the particle, providing a hierarchically structured supraparticle.78 By 12


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controlling the colloidal stability of different particle populations in confinement, the sequence of agglomeration can be tuned to obtain different supraparticle morphologies.61 Supraparticle formation in confinements is often affected by the tendency of colloidal particles to adsorb to fluid-fluid interfaces.79 Depending on the contact angle of a particle, adsorption can be strong enough to drive the entire population of particles to the interface, forming the hollow particle shells known as colloidosomes.54,80–82 Choosing different particle populations with different contact angles therefore provides further possibilities to tailor the resulting morphology. Even particles that do not segregate to the liquid-liquid interface can be affected by its nature. For example, supraparticles that formed from mixtures of two types of apolar gold nanoparticles had different structures depending on the surfactant used to stabilize the particle-confining oil droplets in water (Figure 1c).61 In summary, the structure formation of supraparticles depends on the characteristics of the individual nanoparticulate building-blocks (determined by the core and surface properties) and the process conditions. In the following, we provide an overview of different supraparticle formation protocols (and mechanisms), discuss their differences, and highlight advantages and limitations of the different methods.

Synthesis methods The methods of supraparticle formation and their underlying mechanisms are diverse; they range from easily scalable processes that do not require specific interaction and provide limited control over supraparticulate structure to the use of particles with well-defined and site-selective interactions based on DNA that are costly but provide exquisite control over connectivity. The 13


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underlying mechanisms of the different formation methods can be roughly divided in those which are dominated by the confinement – and therefore the process conditions; and those strongly affected by the interactions between the particles themselves, yielding unspecific agglomeration or specific arrangement.

Confinement Nanoparticles that are confined in droplets cannot cross the interface that delineates this “soft template”. Velev named the “soft templating” of supraparticles in droplets with liquid-liquid interfaces “wet self-assembly”(WSA), while the droplets in “dry-self-assembly” (DSA) have gasliquid interfaces.83 For both processes, supraparticle formation occurs in three steps and starts with particles that are dispersed in a liquid. This liquid is then dispersed into defined droplets. As the liquid evaporated from the created droplets, particle concentration continuously increases until solid supraparticles form.73–75

Wet self-assembly (WSA): Emulsions The use of emulsion droplets as templates for the formation of solid supraparticles can be classified into three distinct scenarios: i) Supraparticles with a two-dimensional shell, known as colloidosomes, are formed if the constituent colloidal particles are exclusively adsorbed onto the interface of the emulsion droplet.23,54,80 The tendency of colloidal particles to adsorb at liquid interfaces79,84 facilitates the 14


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formation of particle-laden interfaces in emulsion droplets. The adsorption is strongest if the surface of the particles favors neutral wetting, i.e., has a contact angle of approximately 90° at the interface. Strongly adsorbed particles stabilize the emulsion droplets against coalescence and form so-called Pickering emulsions.84,85 ii) Encapsulated systems consisting of a solid core decorated with a particle-based shell result if colloidal particles stabilize a liquid core that is subsequently solidified.54 iii) Solid supraparticles consisting of close-packed particle agglomerates are formed if the inner phase of the emulsion consists of a colloidal dispersion.73 The solvent in the emulsion droplets is subsequently removed until the inside solidifies into a supraparticle. The drying rate often affects supraparticulate structure. A range of established techniques enables the formation of emulsions with droplet sizes ranging from nanometers to millimeters. Droplets with sizes in the upper micrometer to millimeter range are formed by simple shaking of a two-phase system, via droplet shear-off techniques,86 electrospraying,87 or inkjet printing.88 Micrometer droplets form in droplet-based microfluidics,89 membrane emulsification,90 or emulsification using high shear forces, for example using a high shear homogenizer. Emulsions with droplet sizes in the nanometer range are fabricated by miniemulsion techniques.91,92

(Non-)surfactant-stabilized emulsions Velev and Manoharan were the first to prepare densely packed, symmetric clusters from colloidal particles by evaporating oil-in-water emulsions.54,77,93 Velev formed water droplets in 15


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perfluoromethyldecalin that contained negatively charged latex microspheres. The droplets in the fluorinated oil were stored in a chamber with a desiccant until they dried and formed regular supraparticles.94 Manoharan used surfactant-free Pickering emulsions stabilized by charged polystyrene particles that adsorbed at the liquid-liquid interface until evaporation forced them to arrange into clusters. Supraparticles with a size that depended on droplet size and particle concentration resulted with a structure that was not governed by attractive dispersive forces, but by capillary interactions of the oil-water interface with the particles.77,95 The WSA approach can be used for smaller particles, too. Gold nanoparticles with narrow size distributions that were coated with nonpolar alkanethiol shells arranged inside the oil phase of an oil-in-water emulsion into supraparticles that structurally resembled noble gas clusters.96 Such clusters formed only when the emulsion was stabilized by surfactants that prevented the particles from migrating to the liquid-liquid interface. Van Blaaderen used a similar approach to assemble sterically

stabilized

iron

oxide

nanoparticles

into

supraparticles

with

icosahedron,

rhombicosidodecahedron, or face-centered cubic (fcc) structure.60 Their structure depended on the number of dispersed nanoparticles in the droplet, and the authors attributed the arrangement not to attractive interactions but entropy maximization. While the presence or absence of a surfactant leads to fundamentally different assembly routes of supraparticles in emulsions, recent reports suggest that more subtle changes in the surfactant type or concentration can be used to affect supraparticulate structure. Magnetic Fe3O4 and NiFe2O4 nanoparticles stabilized with oleic acid assembled into hexagonally packed supraparticles when the emulsion was stabilized with dodecyltrimethylammonium bromide at a concentration of 20 mg/mL. Increasing the concentration of the surfactant to 200 mg/mL changed the 16


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supraparticles’ structures from hexagonal to hollow, probably by changing the balance between inward water compression and outward diffusion (due to evaporation of the solvent hexane).97 Binary mixtures of alkanethiol-stabilized gold nanoparticles with diameters of 4 and 8 nm assembled into crystalline, Janus, or core-shell supraparticles when the emulsion was stabilized with Triton X-100, X-165 or X-705, respectively.61 The authors attributed this transition to the change of the Laplace pressure caused by different energies of the liquid-liquid interface.

Microfluidic-process-controlled emulsion formation Microfluidic devices offer the highest level of control over emulsification and allow for the fabrication of highly uniform droplets, the precise adjustment of the droplet size, and the design of more complex, multi-compartment liquid architectures.74,75,89,98 This flexibility provides a considerable potential for the design of complex supraparticulate architectures. Figure 2 illustrates typical microfluidic device architectures that enable the fabrication of complex droplets. Elastomer-based devices, fabricated from poly-dimethylsiloxane (PDMS) by soft lithography,99 can be fabricated cheaply and rapidly from re-usable, photolithographicallydesigned masters.100 Single or multiple liquid junctions in serial arrangement within the device (Figure 2a) lead to homogeneous emulsion droplets, core-shell architectures, or droplets containing multiple shells (Figure 2b).101 Glass capillary-based devices with multiple coaxial flows enable to control the flow fields of multiple liquids simultaneously and allow for the fabrication of multi-compartment emulsions (Figure 2c,d).102 The high precision and uniformity

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of the resulting droplet architectures for both types of devices is clearly seen in the images shown in Figure 2c,d.101,102 A disadvantage of conventional droplet-based microfluidics is the low throughput, which typically limits droplet generation to the lower milliliter per hour range.103 The straightforward solution of numbering up the outlets to increase the droplet yield often leads to polydisperse droplets. The droplet pinch-off is driven by the flow of the continuous liquid and is therefore sensitive to even small inhomogeneities in the flow fields.103 The introduction of a triangular nozzle opening into an abrupt change in channel height creates a situation where the pinch-off is solely determined by the geometry of the device, but not the liquid flow (Figure 2f). This device design enables the parallelization of nozzles in a “millipede” design (Figure 2e-h), which is capable of producing droplets at a rate up to 150 mL/h while retaining the high uniformity and control over droplet sizes.103,104

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Figure 2: Microfluidic fabrication of tailored droplet architectures. a,b) PDMS-based devices fabricated by soft lithography enable the design of homogeneous droplets (i), core-shell architectures (ii), and droplets with multiple shells (iii) with high precision and narrow size distributions. c,d) Glass capillary devices enable to control the flow of multiple liquid phases and provide the means to engineer hierarchical emulsions with precise droplet-in-droplet architectures. e) High-throughput microfluidics using the “millipede” device contain a high number of parallel individual nozzles. f) Monodispersity of the droplets is generated via a step-emulsification process where the breakoff of individual droplets is provided by the topography of the device rather than the flow of the continuous phase. gh) Visualization of the high throughput of the device shown macroscopically via a fluorophore-containing inner phase (g) and microscopically (h) to highlight the uniformity of the generated droplets. (Reprinted with permission from a,b) ref 101, Copyright 2009, John Wiley and Sons, c,d) ref 102, Copyright 2012, Royal Society of Chemistry, e) ref 103, Copyright 2016, Royal Society of Chemistry, f-h) ref 104, Copyright 2016, John Wiley and Sons.)

Dry self-assembly (DSA): Spray-drying Liquid droplets in gases template the formation of supraparticles in the absence of a surfactant. The most widely applied method is spray-drying on which focus will therefore be put herein. Spray-drying is a high-throughput method to create aerosol particles. Upon drying, the droplets of the sprayed aerosol force the colloidal building-blocks into complex, densely packed particles, similar to emulsion based processes but with much faster kinetics. During spray-drying, droplets 19


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of nanoparticle sols are generated in a nozzle, and the solvent in these droplets evaporates as the droplets enter a hot zone (typically heated to 100 °C to 200 °C).78,105–107 The atomization (droplet formation) is a determining factor for the final supraparticle size. The droplet size is influenced by the configuration of the atomizer, the liquid flow rate, the density, the viscosity, and the surface tension of the precursor sol. The droplet-to-particle conversion, i.e. the solvent evaporation to form supraparticles, is achieved by heat treatment in a chamber to which the droplets are carried by an air, steam, or gas flow. It should be noted that also methods such as freeze-drying exist which exploit the principle of sublimation to remove the liquid in a particle dispersion.108 Mass and heat transfer during drying can be manipulated via process conditions such as chamber geometry, flow rate of the carrier, pressure, or temperature. The obtained solvent-free supraparticles are collected e.g. with the help of a cyclone, a filter bag, or an electric field precipitator.105,109 Most supraparticles from spray-drying are spherical in nature and resemble raspberries in structure. The term is used sometimes in literature as “raspberry-like particles”, “nanoraspberries”, “nanostructured micro-raspberries”, or simply “raspberry particles”.110–114 The distribution of the precursor particles within the supraparticle is linked to the particle movement inside the droplet due to diffusion and convection: Solvent evaporating from the droplet induces a capillary flow to the gas-liquid interface. Emerging interparticle capillary forces self-assemble the particles into closely packed arrangements within the droplet.105 The resulting supraparticles are typically in a size range from 1 µm to 20 µm.107,115–117 Control of process parameters such as solid concentration, solvent type, evaporation rate, or spray geometry allow for distinct structures of raspberry-type spray-dried particle entities (Figure 3a).105,109,113,114,118 The size of the 20


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nanoparticle precursors dominates the surface roughness and final density or porosity of the obtained supraparticles. The initial particle size distribution also affects the morphology of the final particles as it directly influences the packing density of the particles and the interparticle capillary forces in the droplet.105,109 Non-spherical droplets form for certain viscosities of the precursor sol, drying temperatures, gas flow rates, or present surfactants, for example doughnutshaped droplets that yield doughnut supraparticles (Figure 3d).105,107 Composite supraparticles consisting of (at least) two different nanoparticle components are formed when two components are mixed in the precursor sol and atomized: At first, all particles are homogeneously distributed within the formed droplet but due to thermophoresis, they move to the meniscus region. If all precursor particles move in a similar way, supraparticles with well-mixed components are obtained (Figure 3b,c).113 Precursor particles with different average motion may lead to demixing, where slower particles are first transported to the meniscus region and form a shell around particles that move more slowly.78,105 Encapsulated particle structures are thus obtained (Figure 3e,f) if particles e.g. with significantly differing diameters are spray-dried.105 Much of the published work on supraparticle formation in spray-drying focus on the structure formation itself,118–122 but there are application-driven reports that discuss supraparticle formation, too. In the field of pharmaceutical technology and food processing, spray-drying of building-blocks to form micron-sized nanostructured particles is the state of the art (e.g. instant coffee, milk powder etc.).123–125 Other applications use multicomponent and thus, multifunctional supraparticles that are composed of nanoscale building-blocks with different physical properties. Examples include hydrophobic-hydrophilic core-shell micro-granules for water filtration,126

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nanostructured magnetic photocatalysts for wastewater purification,127 or graphene-encapsulated Fe2O3 nanoparticles for lithium ion battery anodes.128 Spray-drying is as an effective method to force together different nanoparticles via droplet evaporation that are hard to combine with other methods. Particles that are confined in the same droplet at very short time scales (seconds) are forced to approach each other due to the evaporation of the solvent no matter what they are. However, it should be noted that exposure of the spray-dried powder to liquid (solvents) might yield a disintegration of the different components again, depending on whether a (attractive) chemical interaction between the building-blocks took place during spray-drying or not.

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Figure 3: Supraparticle morphologies that can be obtained from spray-drying. a) In the simplest case, uniform, dense particles with a raspberry-like morphology are obtained by careful control of drying-temperature and use of one type of nanoparticle. b,c) A mixture of different sizes or materials of the nanoparticulate building-blocks can yield more complex supraparticles. d) Doughnut-like supraparticles result when the process is performed with high drying temperatures and drying rates. e,f) Eventually, the method allows for encapsulation of solid or nanostructured cores via a nanoparticle-shell by spray-drying the core material together with nanoparticles in dispersion which are designated to form the shell. (Reprinted with permission from a) ref 114, Copyright 2015, American Chemical Society, c) ref 113, Copyright 2017, Elsevier, f) ref 118, Copyright 2003, Royal Society of Chemistry, b,d) own unpublished work, e) ref 129, Copyright 2018, American Chemical Society.)

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Bulk agglomeration Instead of external forces that confine nanoparticles in a restricted space and force them together into a supraparticle, techniques that rely on “internal” forces between the particles to form larger structures are common and may be called bulk agglomeration approaches. Such processes include self-limited self-assembly, sonochemically assisted formation, sol-gel-processes, and specific interactions, primarily introduced via DNA-based surface functionalization.

Self-limited self-assembly Self-limited self-assembly33,44,130–135 can form colloidally stable supraparticles by balancing repulsive and attractive forces. Supraparticle growth is induced when attractive interactions between the nanoparticles are slightly stronger than their repulsive counterparts.130 If the attractive interactions are too dominant, unlimited stochastic agglomeration results in the precipitation of macroscopic agglomerates.44,132,133 In contrast, controlled particle self-assembly stops when the attractive and repulsive interactions reach an equilibrium state (which is, unlike template assisted methods, often thermodynamically controlled) or a metastable secondary minimum.130 Controlled assembly permits tailoring of several fundamental supraparticle properties such as their composition, dimension, and morphology by controlling process conditions such as temperature, ionic strength, solvent, pH, or the presence of ligands/stabilizers/depletants.130 Supraparticle properties can also be tuned by varying the size, shape, or material (organic/inorganic) of the constituent nanoparticles.44,131,136–138 The assembly procedure occasionally alters the shape, size, crystallinity, or chemical composition of the 24


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primary nanoparticles. Ostwald ripening, oriented attachment or oxidation processes can then lead to covalently bound 3D nanostructures (e.g. wires, rings, sheets, hollow spheres or also mesocrystals).132,139–143 The self-assembly of nanoparticles is typically based on van der Waals, electrostatic, entropic (including depletion and steric) interactions, or hydrogen bonding.136,144,145 Self-limited selfassembly can for example take place when the strength of van der Waals forces between particles is sufficiently high to compete with particle interactions with the solvent and finally reach an equilibrium.131,146 In aqueous dispersions, these dipole interactions are often superimposed by electrostatic interparticle forces due to surface charges.138,147,148 A specific case is the generation of induced dipoles by external electromagnetic fields. An inhomogeneous alternating current electric field can induce electric dipoles due to rearrangement of the counter-ion concentration near the particle surfaces and can therefore prompt particle self-assembly.136,149 Ferromagnetic particles bearing magnetic moments can be similarly assembled in an external magnetic field by magnetic dipole-dipole interactions.141,150 Electrostatic interactions between nanoparticles can be used as a tool to regulate their selfassembly.44,137,151–156 These forces are very sensitive to the pH and electrolyte concentration of the colloidal dispersion, which provides possibilities to tailor their magnitude and thus control agglomeration.136 Compared to covalent, electrostatic and van der Waals forces, hydrogen bonding has an intermediate binding strength136,157 and its specificity provides a higher control of the particle assembly and the structure of resulting supraparticles than van der Waals or electrostatics.158–160

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Depletion attraction is due to entropic interactions, steric repulsion due to both entropic and enthalpic forces between nanoparticles. Steric repulsion usually hinders supraparticle formation. Depletion interactions due to small non-adsorbing molecules (such as polymers or smaller particles) dispersed in the solvent can facilitate the formation of supraparticles by both inorganic and organic nanoparticles.69,161,162 Self-limited supraparticle assembly can be divided into one-step and multi-step syntheses.33,163 One-step approaches assemble supraparticles directly after nanoparticle formation within the same synthesis pot and are often based on the so-called limited ligand protection. Capping ligands bind to the formed nanoparticles to restrict their growth and inhibit uncontrolled particle agglomeration through steric interactions. A reduced amount of ligand is used during synthesis such that the obtained nanoparticles are no longer stably dispersed and form supraparticles.33,163 This principle has been applied both in the thermolytic and solvothermal synthesis of supraparticles consisting of In2O3CoO, MnO, ZnO, ZnSe, or PbS163–165 and iron related materials (Figure 4a), such as Fe3O4, ferrite, or α-Fe2O3.134,140,166–169 One-step syntheses may be more convenient and timesaving.163 However, the number of reports on one-step supraparticle syntheses is limited, probably due to the challenge of finding the critical degree of ligand protection. Above this critical ligand concentration, single, colloidally stable nanoparticles are obtained; below it, uncontrolled agglomeration and precipitation occurs.163 Multi-step approaches are more common and have been used to prepare a large variety of particles in the last decade (Figure 4b-d).69,137,141 Modular multi-step approaches form nanoparticles of different sizes,44 materials137 or surface modifications44,155 in a first step and subsequently assemble them into diverse supraparticles with tailored properties. 26


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Figure 4: Examples of supraparticles formed via self-limited self-assembly: a) Low-magnification TEM image of hematite supraparticles (inset: schematic diagram of the supraparticle geometry); b) FESEM micrograph of supraparticles consisting of polystyrol and silica nanoparticles; c) SEM micrograph (inset: TEM image) of ring-like structures formed by Au–Ni core–shell nanorods; d) Optical microscopy images (left column), and schematics (right column) of lock-key polymer supraparticles. Supraparticles of a) were synthesized via a one-step method while b)-d) required multiple step approaches. (Reprinted with permission from a) ref 134, Copyright 2009, Royal Society of Chemistry, b) ref 137, Copyright 2011, Elsevier, c) ref 141, Copyright 2010, Royal Society of Chemistry, d) ref 69, Copyright 2010, Springer Nature.)

Sonochemically assisted formation A solution that is exposed to ultrasound may form small bubbles. The bubbles instantly grow and eventually implode (=cavitation), which creates hotspots that can reach up to 5000 K and 1000 bar at heating and cooling rates of >1010 K/s.170 Such conditions are interesting for supraparticle formation, not least because implosions and sonic waves can accelerate particles in dispersion. The formation of a nanoparticle shell onto a core particle via sonochemistry has often been described in literature.171–174 Typically, the shell precursor undergoes chemical reactions induced 27


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or supported by ultrasound and deposits it on the core material. This chapter focuses on processes where the particles are accessible and not formed in situ, which typically leads to core-satellite supraparticles pioneered by the Gedanken group.175–178 In the process, the surface of a larger core particle is decorated with smaller nanoparticles, surrounding it like satellites. These nanoparticle satellites are typically not densely packed but form a loose shell. Cavitation created by ultrasound accelerates the small particles onto the surface of the larger core particles and induce collisions, which can induce “welding” (i.e. the particle surfaces overcome the repulsion barrier). Silver nanoparticles have thus been bound to larger gold particles for surface enhanced Raman scattering,179 layered double hydroxides (LDHs) have been brought on magnetic composite microparticles for phosphate adsorption in waste water,180 and LDHs on manganese oxide microparticles for metal-air battery catalysts have been reported.181

Sol-gel processes Sol-gel reactions are commonly used for the synthesis of metal oxide nanoparticles,182 and they can be used for the subsequent creation of supraparticles using confinement of constituent particles together with sol-gel precursors.42,111,127,183,184 This requires to control gelation, which can easily lead to uncontrolled bulk structures.185 The embedding of pre-synthesized nanoparticles within a matrix forming a microsphere is among the few supraparticle approaches using the sol-gel method (without the help of confinement); the results have been dubbed “nanostructured microspheres”, “nanocomposites”, or “raisin bread particles”.186–192 One report describes porous microspheres assembled from CeO2 nanocrystals during their sol-gel synthesis in a one-step approach.193 Supraparticles made in presence of sol-gel precursors usually have 28


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sizes of several hundred nm up to a few µm and typically show large polydispersity and irregular morphologies. However, this method is usually easy to handle and therefore offers a high scalability and potential transferability to continuous flow chemistry.

Specific interactions Colloidal interactions are often unspecific: dispersive van der Waals forces and electrostatic interactions cannot be designed to exclusively connect particles A and C out of a mixture of A, B and C, for example. One way to increase specificity is to use geometrically matching, complementary particles, similar to the “lock-and-key” geometries of proteins, but it is yet impossible to systematically generate nanoparticles with such geometries. A recent, alternative approach to achieve specificity is the incorporation of defined chemical interactions between molecules on the surface of the particles. For example, Xiang et al. directed the formation of metal-semiconductor heterodimers by changing the composition of the citric/gallic acid ligand layer.194 Click chemistry has been used to connect Janus particles into linear assemblies.195 Guo et al. created supraparticles by modifying silica and other nanoparticles with polyphenols, assembling them on larger template particles, cross-liking the phenols, and removing the cores.26 By far the most popular route for supraparticle formation with specific chemical interaction is based on DNA molecules on the particle surface. Functionalized nanoparticles can be programmed to self-assemble into 1D, 2D, 3D structures with defined interparticle distance and morphology thanks to the specific conjugation of DNA strands. The surrounding DNA layer endows nanoparticles with electrostatic and steric resistance to colloidal precipitation in high ionic strength environments.196 The negative charges from the 29


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sugar-phosphate backbone need to be shielded for the hybridization of complementary DNA strands. Hence, the self-assembly of DNA-functionalized nanoparticles can be controlled by tuning the ionic strength of the surrounding medium. Temperature is another parameter that affects the self-assembly since high temperatures cause the “melting” (dehybridization) of double stranded DNA. Both of these mechanisms are reversible, making DNA an attractive linker for dynamic self-assembly strategies. The DNA-directed self-assembly of nanoparticles was pioneered by Alivisatos et al. in 1996.197 His approach was to use a single DNA strand as a template on which DNA-functionalized gold nanoparticles self-assembled into dimers or trimers with controlled interparticle distance. Concurrently, Mirkin et al. showed the reversible self-assembly of DNA-functionalized gold nanoparticles using a linker DNA and later modified this method to fabricate discrete coresatellite structures where a 31-nm nanoparticle was surrounded by 8-nm nanoparticles.198 The inherent problem of such assembly is the ‘‘polymerization” of nanoparticles where satellite nanoparticles can bridge with another core nanoparticle creating a network. This issue was limited by controlling the ratio of each building-block and linker DNA during the mixing. Using a similar approach, more complex core-satellite supraparticles were fabricated as delivery platforms (Figure 5a).199 These early examples suffered from nanoparticle ‘‘polymerization” which inevitably leads to the production of undesired assembled byproducts.200 Hence, the attention shifted to the design of asymmetric building-blocks. Early work involved gel electrophoresis to sort a mixture of DNAfunctionalized nanoparticles in order to select building-blocks bearing a specific number of single DNA strands per nanoparticle.201,202 Another approach to anisotropic functionalization of 30


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nanoparticles is to use a solid support as a template to induce geometric restriction, such as magnetic microparticles (Figure 5b),203,204 large silica colloids,23,205 or even a glass substrate.206 The concept is to adsorb nanoparticles on a solid support yielding different surfaces for specific modifications between the area in contact with the support's surface (binding site) and the rest of the nanoparticle surface (exposed area). The latter can be covered with a ‘‘passive” layer of ligands such as PEG molecules206 or glutathione,205 or a preexisting monolayer of DNA strands on the nanoparticles can be modified in an asymmetric manner.178,

179

Janus nanoparticles,

another class of asymmetric building-blocks where half of the surface is functionalized with DNA while the other one is passivated, can be obtained by chemical template without the need of a solid support, too.207 More advanced strategies use electrostatic repulsion and steric hindrance to control the number and positioning of single DNA strands on the surface of nanoparticles.208 These different building-blocks enable a phenomenal diversity of supraparticles ranging from 2D square planar to 3D octahedral morphology. Self-assembled DNA structures can act as a template for DNA-functionalized nanoparticles that anchor on predefined points.209,210 This method has yielded discrete, pyramidal and chiral nanostructures where four different nanoparticles are conjugated to each tip of the DNA pyramid.211 Impressive tubular architectures were achieved by using DNA tile scaffolds forming 3D structures from single spirals to nested spirals (Figure 5c).212 More recently, DNA origami has emerged as a class of scaffold in DNA nanotechnology. DNA origami is made of one long single-stranded genomic DNA that can fold into geometrically defined nanopatterns connected by short staple DNA strands. The folding of one long strand proceeds with limited mistakes increasing the yield of scaffold formation with no need for further 31


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purification steps.213 This powerful self-assembly tool not only allows for precise positioning of nanoparticles on the origami but can be designed into a variety of shapes such as triangles,214–218 tiles,219–223 cages,224 or rings.225,226 Its versatility has been demonstrated with different types of nanoparticles including quantum dots, silver, and gold nanoparticles. Examples of 3D DNA origami include origami cages,224 bundles,227 and octahedra.228 3D structures of self-assembled metal nanoparticles are often described as artificial molecules. The analogy was pushed further with the design of an octahedral DNA frame where nanoparticles can be accurately placed on one of the 6 vertexes (Figure 5d).228 These octahedral origami can be used as building-blocks and further self-assemble into 1D or 2D square arrays with nanoparticles between each origami.

Figure 5: Examples of DNA-based supraparticles: a) TEM images of core-satellite supraparticles self-assembled with different satellite-to-core ratio (r = 2,8,16 and 24). Scale bars, 50 nm. b) TEM micrographs of Janus supraparticles with 11-nm gold nanoparticles assembled selectively on one side of a 50-nm core. The asymmetry of the core nanoparticle was achieved with an encoding step where it is immobilized on a magnetic microbead. c) TEM image of different tube conformations with 5-nm gold nanoparticles. These tubular supraparticles are formed with a DNA tile system where the tiles act as a scaffold. d) Cryo-EM images of octahedron supraparticles with their

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associated 3D design model on different angles. They consist of 7-nm gold nanoparticles self-assembled on the vertex positions on octahedral DNA origami. (Reprinted with permission from a) ref 199, Copyright 2014, Springer Nature, b) ref 204, Copyright 2009, Springer Nature, c) ref 212, Copyright 2009, American Association for the Advancement of Science, d) ref 228, Copyright 2015, Springer Nature.)

Function Supraparticles enable the combination of different nanoscale building-blocks into a single entity. Such supraparticles do not only inherit the functionalities of its constituent building-blocks but may exhibit additional functional properties, thus exceeding the sum of its parts. The principles leading to this additional functionality include coupling, emergence, and colocalization (Figure 6). Coupling changes the physical properties of nanoparticles that interact with their neighbors in close proximity. Emergence describes the appearance of an entirely different physical property due to a specific structural arrangement of the constituent particles. Colocalization simply denotes that nanoparticles are linked to each other as an entity in time and space, forming an object with diverse properties based on the sum of the individual properties of the constituents.

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Figure 6: Functionality in supraparticles. Supraparticles may exhibit additional functional properties beyond their building-blocks. These principles can arise from coupling, emergence, and colocalization. Coupling between the constituent particles in a supraparticle changes the supraparticle properties by energy transfer from one buildingblock to another. Emergence describes properties caused by a defined geometry and order of the building-blocks within a supraparticle. Colocalization changes the functionality by confining different building-blocks within the suprastructure. Free particles move freely by Brownian motion, while the motion of colocalized particles is coherent in space and time.

Several aspects set supraparticles apart from layer-by-layer assemblies of nanoparticles and nanoparticle “gels” that infinitely extend in at least two dimensions. Particles couple strongly only below some finite distance; in a spherical supraparticle, all constituent particles can be within this distance. A spherical supraparticle in a liquid provides a large mass transfer interface. Supraparticles in dispersion are highly mobile and can be easily transferred, concentrated, or diluted as required. All this makes them a convenient “vehicle” to carry joint functionality. In the following, we provide instructive examples of functional properties exhibited by supraparticles using the three categories introduced above. The aim of this review is not to provide a complete overview, and some important examples may have been omitted. In contrast

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we hope to provide an instructive understanding on the three categories that structure this diverse and emerging field by functionalities.

Coupling Close proximity may lead to strong interactions between electrons in nanoparticles and thus, coupling. The connection of separate electronic systems in a modular fashion has been successfully used to create functional supraparticles. For instance, surface plasmon resonances shift, upconversion processes transform energy between different levels, and photocatalysis becomes possible.

Surface plasmon resonance and resonant energy transfer Surface plasmon resonance is due to the coherent excitation of electrons in metal particles and leads to the strong coloration of many noble metal nanocrystals.4,15,16 This color is sensitive to the dielectric environment of the particles, and it changes when the dipoles of multiple plasmonic particles couple. Thus, plasmonic particles packed into a supraparticle will have an optical absorption spectrum that strongly depends on their configuration, and — if the constituent particles are anisotropic — can even exhibit chirality.41 The electromagnetic field in the space between two coupled plasmonic particles is often enhanced, with a factor that depends on the geometry and can become large for sharp edges; supraparticles with nanorods in cellulose acetate exploit this to enhance the Raman signal (“Surface-Enhanced Raman Spectroscopy”, SERS).229 35


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Semiconductor nanocrystals in close proximity can transfer energy, too. The mechanism strongly differs from that in metals and is named after Theodor Förster, who first described it for organic dye molecules. An excited semiconductor particle rapidly (in less than 1 ps) transfers its energy resonantly to a second particle if the spectral overlap is sufficient and the particles are close enough to each other. “Förster resonance energy transfer”, or FRET, is now well understood for “Quantum Dots” (QDs, i.e., small semiconductor nanocrystals); the mechanism has been reviewed by Rogach et al.230 and Chou et al.231 Supraparticles that make use of this coupling contain QDs with different optical absorption but a sufficient spectral overlap. This may simply be a mixture of two sizes of the same particle, for example CdTe QDs with diameters of 2.5 and 3.2 nm.232 In this early example, the mixture of the particles exhibited a simple superposition of the two QD’s optical signatures; adding Ca2+ led to agglomeration and a strong decrease of the (green) emission of the smaller particle. Energy transfer was seen in time-resolved photoluminescence spectra. When semiconductor particles in a supraparticle are very close to each other, higher-order interactions and even tunneling may occur. Recent work has explored the use of conductive ligands (such as pentacene233), J-aggregates,234 and short mercaptoalkanoic ligands of varying size235 to tune distance and coupling. So far, this has only been achieved for coatings and small agglomerates, but it appears feasible to introduce such molecules to QD supraparticles with structural definition and design their optical properties. Combinations of semiconductor and metal nanocrystals often lead to quenching, where the electrons of gold or other metal particles rapidly dissipate the QD’s exciton at a rate that depends 36


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on the distance according to Förster’s d6 relation.236 This principle has been used to precisely measure the distance between QD and metal particle between 5 and 20 nm237 and to create assays based on the formation of QD-gold supraparticles (“complexes”) through hybridization.238 On the other hand, the enhanced electric field close to metal particles can also lead to an enhanced luminescence of QDs. This “Maxwell enhancement”239 is due to the strong adsorption of light by metals, well-known for dye molecules on metal surfaces, and has been reported for QDs on metal surfaces.240 Recent studies on hybrid supraparticles show that the coupling between metal and semiconductor particles leads to a rich set of phenomena. Coupled surface plasmons and excitons, so-called “plexitons”, have intriguing properties that lead to strong and directed optical absorption and emission.241 A film of CdSe/ZnS particles on silver shells around dielectric cores242 exhibit “Rabi splitting” of the incoming energy between plasmonic and excitonic modes. Supraparticles of single gold nanorods and CdTe NCs exhibited chirality only due to the coupling between the surface plasmon and the chiral exciton of the semiconductor.41 A number of very recent reports on combinations between graphene sheets and QD243 or core-shell structures suggest that the design of supraparticles with defined positions of different particles will lead to optical elements with a wide range of tunable properties.

Upconversion Certain QDs can absorb two photons and emit one photon with higher energy. This non-linear optical response is called “upconversion” (UC) and potentially useful for photovoltaics, where it could convert previously “useless” photons into photons that can be converted into electrons;244 bioimaging, where it could convert light that propagates through tissues into light that is easier to 37


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detect;245 and photocatalysis, where it increases the energy of the photons such that they can overcome activation barriers. As with most non-linear effects, the efficiency of UC in most particles is small, and considerable efforts have been made to find high luminescence upconversion materials.246,247 Recently, supraparticles have been used to create upconverting particles or to combine the upconverter with the functional material. A general concept to upconverting supraparticles are the "double quantum dots" that were recently reviewed by Teitelboim et al.248 Quantum dot pairs can be considered as the smallest possible supraparticles after the definition of this review. Their function is based on a defined relative position of the two quantum dots that exchange energy through resonant coupling or overlapping wave functions. Teitelboim and colleagues maintain that the understanding of the photophysical mechanisms in such particles is limited, and the same is true for all upconverting supraparticles that are currently emerging. Upconverting “heterostructures” for dye-sensitized solar cells were prepared using a hydrothermal method from YbF3-Ho and TiO2 in HNO3. The resulting supraparticles were rather ill-defined, with many interfaces between the YbF3-Ho and the TiO2 in fractal agglomerates, and suitable as photoelectrodes for dye-sensitized solar cells. They enhanced the overall photoconversion efficiency by 23% by a combination of multiple effects, but only a part of this improvement was due to upconversion. Perovskite solar cells were enhanced with upconverters based on "raisin bread" supraparticles. Copper sulfide particles (6.5 nm diameter) “raisins” were embedded in silica (using a TEOSbased route, the particles were about 45 nm in diameter), coated with (7 nm thick) shell of erbium 38


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oxide, and embedded in perovskite solar cells to test their efficiency.191 The supraparticles enhanced upconversion by a factor of 1000, which increased the solar cell’s power conversion efficiency by about 10%.

Photocatalysis In photocatalysis, light needs to be absorbed and transferred to promote a chemical reaction. Combinations of different particles in supraparticles can improve the efficiency of this process, for example by combining one particle that efficiently absorbs light (perhaps upconverting it) with a second particle that provides catalytic specificity. They are an interesting addition to the well-established hybrids of inorganic nanoparticles and organic dyes;249 inorganic nanoparticles are less prone to degradation and their structure and position in the hybrid (i.e., the supraparticle) is easier to analyze than dye-sensitized materials. Upconversion supraparticles for photocatalysis were made by coating NaYF4:Yb,Tm platelets with a carbon film to enable the deposition of CdS nanoparticles on the surface. The resulting NaYF4:Yb,Tm@C@CdS supraparticles rapidly degraded model pollutants (Rhodamine B and methylene blue) under visible light, while the components without the CdS did not have any effect because they do not absorb in that spectral region. In addition, the carbon “connector” layer strongly adsorbed the dye molecules, thus aiding the decomposition reaction.250 Plasmonic particles that act as “nano-antennae” can produce hot carriers and heat. Both can support (or enable) reactions in a catalytic particle in close proximity. For example, Swearer et al. recently decorated plasmonic aluminum nanocrystals (with diameters on the order of 100 nm) 39


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with smaller catalytic Pd nanoparticles (on the order of 10 nm).251 They suggest that hot carriers from the Al induced hydrogen desorption from Pd and thus, increase the selectivity of photohydrogenation reactions such as the reduction of acetylene of ethylene. The enhanced absorption of the supraparticle was the main factor leading to an increased reactivity. A very recent report on “transient” supraparticles demonstrates the switching of photocatalytic activity by forming either active or inactive supraparticles with different structures.252 Ammonium-modified

titania nanoparticles

were

mixed with

aldehyde-modified

gold

nanoparticles in water. Changes in pH and the addition of cucurbit[6]uril led to the reversible formation of supraparticles where either individual titania particles were surrounded by gold particles, or titania particles formed larger agglomerates that excluded the gold particles. Only the first type was catalytically active and decolorized methylene blue under UV radiation. Figure 7 depicts a selection of supraparticle systems where coupling among the nanoparticulate building-block determines the ultimate functionality.

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Figure 7: Coupling between particles in supraparticles. a) Multiple quantum dots (QDs) around a single AuNP (configuration 1) and multiple AuNPs around a single QD (configuration 2) that exhibit energy transfer upon optical irradiation have been realized using AuNPs with diameters of 6, 13, and 30 nm (electron micrographs). b) A bioconjugate of QD-peptide-AuNP provides controlled quenching as a function of the spacer length. (Reprinted with permission from a) ref 238, Copyright 2015, American Chemical Society, b) ref 237, Copyright 2007, American Chemical Society.)

Emergence Emerging properties arise from a specific structure of the building-blocks within a supraparticle rather than from the material of the individual building-blocks itself.

Photonics Structural coloration is a typical emergent property of supraparticles. The observed optical effects arise predominantly from the controlled arrangement of monodispersed colloidal particles within the supraparticle and mimic the fascinating optical properties of the natural world.253 The colloidal particles used as building-blocks can be conveniently synthesized with diameters in the same size range as the wavelength of visible light. The wavelength-scale nature of the buildingblocks enables strong interactions with visible light, which are macroscopically observed as color. Figure 8a graphically illustrates the different optical effects that can be observed in supraparticles. Details on the origin of the effects and the tailoring of supraparticles to maximize these effects can be found elsewhere.76,253,254 Supraparticles with ordered colloidal arrangements display structural color arising from Bragg reflection at individual colloidal lattice planes (Figure 8a,b). Depending on the size of the colloidal building-blocks, the wavelength of constructive interference can be shifted throughout the visible spectrum (Figure 8b).59 Furthermore, the macroscopic color in these particulate materials can be tailored by additive 41


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mixing, enabling the design of arbitrary color.255 In disordered supraparticles, coloration results from coherent scattering of the individual building-blocks, which allows for the design of angleindependent color effects (Figure 8c).256 With increasing size of the colloidal building-blocks, the observable optical effect changes to polychromatic rings as a result of grating diffraction (Figure 8d).83 Grating diffraction from ordered layers of colloidal particles at the surface of the supraparticle are responsible for the effect.59,83 The addition of absorbing elements can reduce the amount of light scattered at defects within the supraparticle and therefore enhance the contrast of the structural color.253 Figure 8d exemplarily shows a supraparticle exhibiting grating diffraction coloration while the red hue of the entire particle results from the incorporation of gold nanoparticles.83 Macroscopically, the color can be observed both in dispersion (Figure 8e)257 and on solid surface, enabling the design of structural color-pigments based paints.258,259 More complex optical properties can be implemented by controlling phase separation within individual particles.260,261 An example of this strategy, providing means to create Janus-type particle with multiple color properties, is illustrated in Figure 8g-j.260 A triphase microfluidic device is used to create droplets with two compartments, in which one is composed of a colloidal dispersion while the other one contains a photocurable monomer and magnetic nanoparticles. Upon solidification, the colloidal particles assemble into a hemispherical photonic crystal, while the other face remains uncolored (Figure 8g). The resulting Janus particles can thus be used to create dynamic, magnetically switchable color substrates (Figure 8j). While the arrangement of building-blocks within a supraparticle is the primary origin of structural color, it is important to realize that optical effects can be further manipulated on consecutive levels of hierarchy. The arrangement of supraparticles themselves can manipulate the 42


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macroscopic coloration. Figure 13f exemplarily shows how anisotropic particles can be temporarily aligned on a surface via magnetic forces, forming a defined line pattern on the substrate. These arrangement produces grating diffraction effects which render the surface colorful in the presence of a magnetic field.262 Similarly, such anisotropic particles can be oriented in a bulk dispersion to create vivid optical effects when rotated by magnet.263

Figure 8: Structural color as an emergent optical property of supraparticles. a-d) Graphic illustration (a) and examples (b-d) of optical properties arising from defined structural arrangements within a supraparticle. b) Ordered colloidal layers within the ball give rise to Bragg reflection colors. Depending on the size of the constituent colloidal particles, the interference conditions can be tailored throughout the visible range. c) Disordered colloidal arrangements can cause isotropic structural coloration by coherent scattering. d) Ordered, two-dimensional arrangements at the particle surface support grating diffraction, visible as polychromatic rings on the balls. The addition of absorbing materials (gold nanoparticles in the example), reduces incoherent scattering thus increases the optical effect. Supraparticle exhibit structural color both in dispersion (e) and when used as a pigment on a solid surface (f). g-j) Example of more complex supraparticles, exhibiting a Janus structure with a non-colored (I) and a colored (II) side which can be magnetically agitated to switch on and off the observable color. (Reprinted with permission from a,b) ref 59, Copyright 2015, PNAS, c) ref 256, Copyright 2014, John Wiley and Sons, d) ref 83, Copyright 2008, John Wiley and Sons, e) ref 257, Copyright 2006, American Chemical Society, f) ref 258, Copyright 2013, John Wiley and Sons, g-j) ref 260, Copyright 2012, John Wiley and Sons.)

Porosity

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Porosity naturally emerges in supraparticles from the formation of interstitial sites. The coassembly of polymeric and inorganic particles enables the design of more complex porous structures, for example from Ludox silica nanoparticles with a defined number of polymer colloidal particles (Figure 9a).264 Larger structures using similar particle mixtures resulted in inverse opal supraparticles after combustion of the polymer particles. These supraparticles feature a nanoscale, interconnected porosity with high order (Figure 9b).257 Hierarchical porous structures were made by assembling micron-scale polymer particles with silica colloidal particles. The polymer particles left microscopic pores after combustion, while interconnected, nanoscale porosity arose from the interstitial sites of the silica colloid matrix (Figure 9c).265 Porosity in colloidosomes81 is determined by the size of interstitial sites of the colloidal monolayer shell. Controlling the arrangement of the particle monolayer therefore enables a control of the porosity.266

Figure 9: Control of porosity in supraparticles. a) Discrete, nanoscale pores result from the controlled co-assembly of a defined number of polymer colloids with silica nanoparticles. b) Large numbers of polymer colloidal particles with silica nanoparticles give rise to inverse-opal supraparticles with interconnected, nanoscale and ordered porosity.

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c) Co-assembly of micron-scale polymer particles with silica colloids creates hierarchically porous structures, featuring large, micron-scale pores and a nanoscale porosity arising from the interstitial sites of the silica colloids. (Reprinted with permission from a) ref 267, Copyright 2007, American Chemical Society, b) ref 257, Copyright 2006, American Chemical Society, c) ref 265, Copyright 2018, American Chemical Society.)

Colocalization A third aspect where the supraparticulate nature of objects determines the specific functional properties is the colocalization of the constituent building-blocks. The characteristics of colocalization are rather trivial: the components that are connected in a supraparticle move together and form a distinguishable common entity that can be removed, concentrated, and observed individually. Emissions from the individual particles are superimposed, and even if the individual emissions are entirely unchanged (because there is neither coupling nor emergence), their concentration in a single position can provide additional benefits. In the following, we summarize how this colocalization is exploited to create what we call “supraparticles as nanoparticle formulations”. These allow for a better handling of nanoparticles, their collective transfer to coatings (as ballistic objects), their unification to form reservoirs or to combine optical building-blocks. Besides these supraparticles as nanoparticle formulations, magnetic supraparticles are another very important field where colocalization yields functionality. Figure 10 depicts sketches of typical supraparticle systems as nanoparticle formulations that represent the above fields. Magnetic objects are not depicted in this figure as Figure 11 depicts a more detailed sketch on such systems.

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Nanoparticle formulations Ceramics with grain sizes below 100 nm are called nanocrystalline ceramics. Their fracture toughness and sintering behavior are improved over larger grains, and optical transparent ceramics can be made for optical filters and sensors.268 It is difficult, however, to fully disperse the nanocrystals, eliminate aggregates and agglomerates, and to compact them without cracks or large pores.268 Ideally, one would like to work with individual nanoparticles, which is practically impossible. Thus, it is desired to colocalize individual nanoparticles in larger, very well-defined entities which form a nanocrystalline powder – ideally with pre-programmed redispersibility.269 In fact, the ceramics community was probably first to suggest combining individual nanoparticles into slightly larger particles (e.g. on the submicrometer scale) “in which the properties generated by the nanoscale material are preserved.”106 This simplifies the handling of any powders containing nanoparticles by making it easy and safe.270 Colocalization reduces dust formation270 and increases the uniformity in morphology and constituency.269 Pecharromán and Esteban-Cubillo270 proposed to create “micro/nano composites”, by colocalizing nanoparticles on or within micron sized particle hosts to form a supraparticulate structure. Edelson and Glaeser had already described a form of supraparticles for sinter processes with titania 30 years ago.271 They prepared uniform titania particles with a diameter of about 0.35 µm from primary particles of 6 nm diameter in a sol-gel approach and reported improved sintering properties. Another example is from Slamovich and Lange, who prepared supraparticles in 1990 to obtain zirconia ceramics by colocalizing ZrO2 and Y2O3 particles in a supraparticle.272 Okuyama et al. suggested spray-drying as a flexible method to

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colocalize nanoparticles from dispersion in supraparticles, yielding powders that are easy to handle.106 Recently, this idea was expanded by equipping nanoparticles with well-defined “spring” functionalities. This modification with silanes and polymers on the individual nanoparticles enabled to control the redispersion of the supraparticles to individual nanoparticles or aggregated substructures both in liquids and polymer composites.112,113 The “spring” functionality provided by the silanes/polymers is based on a combination of spacing effect among the nanoparticles and a “like-solves-like” effect when the supraparticles are added to a medium in which they are subsequently easily disintegrated via shear-inducing energy input (Figure 10a). The “Rocatec™ process” and similar coating processes employ structures that are not called supraparticles by the manufacturers but fulfill its definitions. In such processes, small particles, typically made of silica but sometimes also of metals or other ceramics, are brought onto a larger core particle often made of corundum to form a type of core-satellite supraparticles or core-shell supraparticles. These are then used for a tribochemical coating process, typically in a sandblasting setup. The particles are accelerated by air pressure to a high speed and impact on a target substrate as ballistic objects. Colocalization ensures that all small particles are carried with the larger particle that carries the main impulse. Upon impact, a tribochemical reaction occurs, eventually leaving the small particles connected/implanted with the target surface.129,273–276 Supraparticle functionality of colocalization is only of temporary interest as the functionality is mainly intended to act as transport vehicle before the impact of the particles on the target surface. Yet, the concept of colocalization of smaller particles in a larger supraparticle entity is important as individual nanoparticles could not be accelerated in a comparable manner as it is the case 47


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when they are colocalized. The principle of the process is depicted in Figure 10b along with electron micrographs of such a ballistic particle system.129 Voids and pores between the smaller packed particles in a supraparticle can be exploited as reservoir, for example in an inhalable spray-dried drug vehicle277 or as a general carrier for drugs.278 A sophisticated reservoir system with gradual drug release, which unfolds its functionality due to the nanoparticulate architecture of the reservoir supraparticle, was reported by Hassan et al.279 Korin et al. designed a reservoir supraparticle whose nanoparticle backbone architecture was shear sensitive, resulting in drug release at regions of high shear stress,280 for example in vascular narrowing for preventing thrombosis.280 Mandel et al. recently used a shearsensitive supraparticle for a coating with a refreshable anti-bacterial surface functionality (Figure 10c).114 Nanoparticles with optical properties (such as luminescence) combined into larger particulate entities do not necessarily yield different properties due to coupling or emergence; functionality can also be obtained from simple colocalization. A representative example is the recent work by Montanarella et al.281 who report the combination of three types of Cd(Se,ZnS) core/(Cd,Zn)S shell nanocrystals with emission in the red, green, and blue in one supraparticle (Figure 10d). The colocalized combination of colors yielded a white light emitting particle dispersion when excited with a UV laser. A change in the combination of the building-blocks within the supraparticle enables to tune the emission to any desired color (Figure 10d). Although a certain degree of Förster-type energy transfer was observed, this coupling of the nanoparticulate building-blocks was not the dominant property in the system. Instead, the emission properties were mainly governed by (simple) colocalization. 48


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Particles composed of several optical building-blocks are furthermore employed as barcode particles and find application for instance as anti-counterfeit tags, as markers, or as tracers. Many options exist to create a particle-based barcode. and we refer to recent review articles in this field.282,283 The basic working principle is the superposition of characteristic luminescence peaks from a set of nanoparticles that are combined in distinct ratios within one supraparticle. Peak positions and relative intensities generate a distinct code. Coupling or emergence are usually undesired as they would alter the code in a non-controllable way. Different types of nanoparticulate building-blocks have been reported for optical barcode particles, including QDs,284–286 carbon-based dots,287 and lanthanide ion containing particles.288,289 Most of them were embedded, for example by swelling a polymer host-bead in organic solvents followed by a diffusion-driven incorporation of the nanoparticles,290 by enclosing the nanoparticles within an emulsion and subsequent evaporation of the solvent in the droplets,288 or via a sol-gel reaction to “polymerize” silica (as the designated host matrix for the nanoparticles) in emulsion droplets.183,184,192

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Figure 10: Examples for supraparticles as nanoparticle formulations, i.e., functional supraparticles obtained from colocalization of nanoparticles. a) Handleable nano-objects, i.e. easy to handle microparticles (called nanostructured micro-raspberries due to their raspberry-like appearance) composed of modified nanoparticles which can be redispersed upon mechanical input (i: principle, ii and iii: SEM images of a raspberry particle at different magnifications). b) Ballistic objects: Colocalized nanoparticles on a carrier particle can be accelerated and targeted on a substrate surface (i: principle, ii and iii: SEM images of such a ballistic particle at different magnifications). c) Reservoir objects composed of nanoparticles forming a supraparticle to host active agents such as anti-bacterial agents (i: principle, ii SEM images of a coating containing reservoir particles; insets different magnifications and view angles of the system). d) Objects with combined optical building-blocks, namely QDots to yield different colors in one supraparticle (i: different colors of supraparticles in dispersion ii scanning transmission electron microscopy image of two supraparticles (scale bar 200 nm). The inset shows a magnification of one of the particles (scale bar 20 nm). iii-iv: 2D high angle annular dark field scanning transmission electron microscopy images (scale bars 50 nm) of a supraparticle for which electron tomography has been performed (iii), for a 3D visualization of the 3D tomographic reconstruction (iv). v: Slice through the center of the 3D reconstruction of the supraparticle (scale bar 50 nm). Individual QDots can be clearly recognized vi: Fast Fourier transform of the slice of panel d (scale bar 0.1 nm−1). (Reprinted with permission from a) ref 112, Copyright 2017, Elsevier, b) ref 129, Copyright 2018, American Chemical Society, c) ref 114, Copyright 2015, American Chemical Society, d) ref 281, Copyright 2017, American Chemical Society.)

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Magnetic objects The most common use of colocalization is found in magnetic particles. Magnetic nanoparticles have been thoroughly reviewed6 and extensively used to cause motion in a magnetic field. Welldispersed magnetic nanoparticles cannot be magnetically moved or separated from a fluid with typically employed magnetic field gradients, instead these particles move together with the liquid and form ferrofluids.291 If, however, such nanoparticles are clustered into particles in a size range between 100 nm to 10 µm, magnetic steering becomes possible with realistic field gradients. The magnetic nanoparticles are typically assembled by incorporation in a polymer or silica matrix.292 The obtained particles are typically nanocomposites but have some characteristics of supraparticles and are typically employed as carrier particles (Figure 11).

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Figure 11: Colocalization of magnetic nanoparticles can yield carrier, switch, imaging or structural functionality provided by the obtained supraparticles.

An interesting property in addition to their motion in field gradients is a certain switchability of such magnetic particles via an external magnetic field that can either lead to inductive heating of the particles, a triggered reaction in the system, or a magnetic rotation of a particle system.169,262,263,292–296 In addition, the magnetic components can be used for imaging and eventually, a certain arrangement of magnetic building-blocks as such might yield structural functional features (which will be described later in this chapter). The following sections summarize the different (carrier, switching, imaging and structural) functionalities of magnetic supraparticles and a selection of these systems is depicted in Figure 12. Biomedical applications have recently been thoroughly reviewed25 and are not covered here in any detail.

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Carrier functionality of magnetic supraparticles The application of magnetic supraparticles (also sometimes referred to as nanocomposite microparticles)190,292,297,298 in the purification of “bio-fluids” such as cell lysates is probably the best example for commercially successful complex magnetic particles. Such commercial magnetic beads are equipped with specific biomolecules that selectively bind biological target objects. Once linked to the target, the magnetic beads are magnetically separated from the solution together with their load, i.e. they act as selective catchers or carriers.299 In the last decade, these carrier particles were also combined with other nanoscale building-blocks that allowed for an optical identification of the beads for tracking, tracing, or sorting, among other tasks.297,300,301 When combined with catalysts, magnetic carrier particles can be dispersed in a fluid to catalyze a reaction and subsequently be recovered magnetically.302 Photocatalysts have been colocalized with magnetic particles to obtain magnetically recoverable photocatalytically active particles, too (Figure 12a depicts a TEM image of a CoFe2O4 nanoparticles decorated 3D urchin-like TiO2 photocatalyst particle).169,303 Magnetic carrier particles have been extensively employed in the field of water purification. The main interest in this field is the combination of magnetic particles with adsorber moieties to obtain a magnetically recoverable adsorber. One approach is to directly combine magnetic nanoparticles with chemically active nanoadsorbers. For example, a system was reported where magnetic nanoparticles and graphene were directly connected.304 Another approach is to equip magnetic nanocomposite supraparticles (Figure 12b depicts the basic particle which is an iron oxide nanoparticle in silica matrix structure) with (nanostructured) adsorbers on their surface to form a complex core-shell supraparticle system . This was for instance done to 53


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selectively adsorb phosphate in waste water which could be subsequently recovered via magnetically harvesting the adsorber particles.180,305 Recently, a chemically reactive luminescent adsorber at the surface on a magnetic carrier particle (Figure 12c depicts the resulting composite particle) was employed as magneto-optical detector by magnetically harvesting the particles from a fluid containing a target species and reading out the concentrated particles with a fiber optical analyzer.293 Other magnetic marker or tracer particles can be added to fluids or materials and later be recovered magnetically to be identified through a code that is inherent to the particle for counterfeit protection or substance tracing and labeling for recycling purposes. Examples reported include magnetic carrier particles with Raman active optical codes,306 DNA codes,307 or upconverting nanocrystals294 (the latter was used for cell sorting). The use of magnetic carrier particles as magnetically steerable vehicles for drugs was recently reviewed, including multifunctional carriers with switchable release systems and labels.308

Switching functionality of magnetic supraparticles Magnetic supraparticles and composites can be “switched” through external fields through several approaches covered in excellent reviews.309–311 An AC magnetic field can inductively heat the magnetic nanoscale building-blocks within the supraparticle, for example to release drugs or treat tumors via hyperthermia.312–314 Thomas et al. reported on a system where magnetic nanoparticles acted as “plugs” for a porous particle system that could be magnetically removed, thus releasing the drugs (Figure 12d).295 54


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Magnetic switching can also occur through the rearrangement of magnetic objects by rotation or displacement, which is distinct to a magnetic movement as described in the chapter on carrier particles. Chong et al. made chain-like magnetic supraparticles that rotated in a magnetic field and acted as nanostirrers.315 Mandel et al. recently reported on a system composed of rod-like supraparticles additionally modified with a thin nanostructure of fluorescent MOFs (Figure 12e).263 It was possible to switch between isotropic and anisotropic optical properties based on magnetic rotation of the particles (Figure 12e).263 Change of optical properties from switching the arrangement of magnetic supraparticles is of course a very prominent topic in the field of magnetic photonic crystals, a field which was inspired by the work of Yin et al. in 2007.165 The group created chains of magnetic supraparticles which could be arranged in dispersion via a magnetic field to a periodic grating, yielding interference colors when illuminated with white light. Since then, several approaches of dispersed particular structures that can be arranged in a magnetic field to yield switchable photonic crystal properties have been proposed (Figure 12f).316

Imaging functionality of magnetic supraparticles Magnetic nanoparticles are promising candidates for imaging (magnetic resonance imaging, magnetic particle imaging) in biomedical applications. Multimodal imaging requires particles that carry additional function, for example an optically active component, and supraparticles have been used for such combinations.36,317–322 Detailed review articles describe the combination of magnetic building-blocks with luminescent (fluorescent) or plasmonic particles for imaging.301,323

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Theranostic supraparticles combine imaging capabilities with active drug release or hyperthermia.324 Generally, current systems mainly provide colocalization but no desired coupling: undesired quenching of fluorescence by magnetic nanoparticles is an unwanted form of interaction.320,325

Structural functionality of magnetic supraparticles There are a few reports in literature on the change of magnetic properties that are caused by a specific arrangement of the magnetic nanoparticles in a supraparticle. Abramson et al. found that when arranging magnetic nanoparticles to microellipsoidal supraparticles, the magnetic moments of the nanoparticles were all orientated and caused alignment of the supraparticles in fluids.326 An anisotropic chain-like supraparticle arrangement of magnetic nanocubes was reported to yield superior inductive heating properties.327 Granath et al. reported on supraparticles composed of patchy superparamagnetic iron oxide nanoparticles which evolved into hollow balloons that were strongly magnetic and very lightweight (Figure 12g depicts the principle and the resulting appearance of the balloons).296 Very recently, a system of nanostructured micro-raspberry supraparticles was reported that had controllable burst properties as a redispersible system of magnetic nanoparticles.303

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Figure 12: a-c) Magnetic supraparticles with a carrier functionality: a) as photocatalyst-carrier; b) to remove and recover resources; c) to act as magneto-luminescent substance detector; (d), a switching functionality by remote heating via introduction of an oscillating magnetic field for release of loaded drugs in a complex particle entity; a switching & structural functionality yielding magneto-optical effects such as change between isotropy and anisotropy (e) and colors (f; i-iii: structure of the magnetic chain-like supraparticles in a magnetic field (scale bars: 20 µm (i), 2 µm (ii) and 200 nm (iii)); iv-vi: principle of color generation: vii: photograph of colors obtained from magnetic particle structures.) and a structural functionality yielding super-lightweight magnetic microballoons (g). (Reprinted with permission from a) ref 169, Copyright 2016, Royal Society of Chemistry, b) ref 293, Copyright 2012, American Chemical Society, c) ref 293, Copyright 2016, American Chemical Society, d) ref 295, Copyright 2010, American Chemical Society, e) ref 263, Copyright 2017, American Chemical Society, f) ref 262, Copyright 2012, American Chemical Society, g) ref 296, Copyright 2016, American Chemical Society.)

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Nomenclature The term supraparticle was, to the best of our knowledge, first used in 1971 to describe and classify structures of particle aggregates in clay domains and sub-domains.328 It was also used in a biochemical context by Bachellerie et al. to depict the arrangement of extranucleolar elements isolated from mammalian cell nuclei.329,330 Lawless et al. were the first to use the term in colloidal science to describe the arrangement of TiO2 with CdS particles bridged by bifunctional mercaptocarboxylic acids.331 The first formal definition of the term “supraparticle” was given by the Velev group, who used it to describe ordered or multicomponent clusters assembled from different types of colloidal particles.28,48,52,54,73,83,93,94,332 Around the same time, a review from Antonietti and Göltner, entitled “Superstructures of Functional Colloids: Chemistry on the Nanometer Scale” appeared, in which the authors discussed the connection between supramolecular chemistry and chemical nanotechnology. The authors realized that both fields follow similar principles and differ only in scale.333 According to the proposed concept, superstructures consist of prefabricated colloidal subunits which are selfassembled into more complex agglomerates. This means that the agglomeration goes beyond a merely statistical arrangement and ideally follows a particular hierarchy. Since these superstructures in the definition are composed of colloids, the definition coincides with that of Velev, except for the restriction to particles. Following this, supraparticles are a more specific form of superstructures – particles made from (hierarchical) self-assembled particles.

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A few years later, Edwards et al. summarized the processes involved in the controlled assembly of nanoparticulate building-blocks into superstructures under the heading “supraparticle chemistry”, and again highlighted the similarities to supramolecular chemistry.334 Therefore, the terms “superstructure”, “suprastructure”,74,146 and “supraparticle” often mean the same. We suggest to emphasize the analogy with the field of supramolecular chemistry by using the prefix “supra” and to clearly indicate that the structure is a particle by writing “supraparticle”. Terms such as colloidal molecules,46 supramolecular colloidal materials130 or supracolloid335 have been used to describe objects that fulfill above definitions. But not all do: the term “colloidal molecule” has been associated with at least two other definitions. It dates back to the early years of colloidal respectively macromolecular chemistry and has been used for large molecules that behave as colloids.336–341 A recent definition by Blaaderen342,343 uses “colloidal molecules” to denote a very specific form of particle agglomerates in which the number and position of subunits are precisely defined.21,65,72,335,344,345 Other terms currently in use do not specify a degree of order as required for supraparticles, such as “colloidal/nanoparticle aggregate”,25,39,53,74,346 “colloidal/nanoparticle cluster”39,65,67,96,334 or “assembled colloids/nanoparticles”.39,81,97 One may add adjectives such as “hierarchical”, “defined”, or “structured” or simply use the term “supraparticle”, as suggested above. The building-blocks of hierarchical structures are frequently referred to as “primary particles” that form “secondary particles”.33,35,39,347,348 Consequently, higher leveled structures would have to be named “tertiary” or “quaternary” particles, which in fact is not uncommon.348,349 Some authors consider the number of building-blocks at different levels, for example, by using the 59


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terms “higher-order clusters”, “supraparticles”, or “supraballs” for objects comprising N

Supraparticles: Functionality from Uniform Structural Motifs

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