PERSPECTIVE www.advmat.de Multifunctional, Hybrid Materials Design via Spray-Drying: Much more than Just Drying Susanne Wintzheimer, Leoni Luthardt, Kiet Le Anh Cao, Inhar Imaz, Daniel Maspoch, Takashi Ogi, Andreas Bück, Damien P. Debecker, Marco Faustini, and Karl Mandel* 1. Introduction Spray-drying is a popular and well-known “drying tool” for engineers. This perspective highlights that, beyond this application, spray-drying is a very If a strawberry has a wonderful taste and a kiwi has one as well, how interesting and powerful tool for materials chemists to enable the design of would a fruit hybrid of both taste multifunctional and hybrid materials. Upon spray-drying, the confined space This thought translated to the nano of a liquid droplet is narrowed down, and its ingredients are forced together and the molecular world would mean: If upon “falling dry.” As detailed in this article, this enables the following nanoparticles and molecules alone pos- material formation strategies either individually or even in combination: sess fantastic properties, what would nanoparticles and/or molecules can be assembled; precipitation reactions as properties look like after a merge Up to today, scientists have mastered well as chemical syntheses can be performed; and templated materials can be designing nanoparticles in all kinds of designed. Beyond this, fragile moieties can be processed, or “precursor compositions, sizes, and structures and materials” be prepared. Post-treatment of spray-dried objects eventually they have established myriads of highly enables the next level in the design of complex materials. Using spray-drying functional molecules. Now, we have all to design (particulate) materials comes with many advantages—but also with these moieties as building blocks at hand to develop more complex entities. The many challenges—all of which are outlined here. It is believed that upcoming century will be the century multifunctional, hybrid materials, made via spray-drying, enable very unique of the next hierarchical level, where we property combinations that are particularly highly promising in myriad will look at what objects and properties applications—of which catalysis, diagnostics, purification, storage, and can be obtained upon joining nano and information are highlighted. molecular building blocks. To achieve this, many different strategies are at hand, for S. Wintzheimer, L. Luthardt, K. Mandel I. Imaz, D. Maspoch Inorganic Chemistry Catalan Institute of Nanoscience and Nanotechnology (ICN2) Department of Chemistry and Pharmacy CSIC Friedrich-Alexander-Universität Erlangen-Nürnberg and Barcelona Institute of Science and Technology Egerlandstraße 1, 91058 Erlangen, Germany Campus UAB, Bellaterra, Barcelona 08193, Spain E-mail: karl.mandel@fau.de I. Imaz, D. Maspoch S. Wintzheimer, K. Mandel Departament de Química Fraunhofer-Institute for Silicate Research ISC Facultat de Ciències Neunerplatz 2, 97082 Würzburg, Germany Universitat Autònoma de Barcelona K. L. A. Cao, T. Ogi Bellaterra 08193, Spain Chemical Engineering Program D. Maspoch Department of Advanced Science and Engineering ICREA Graduate School of Advanced Science and Engineering Pg. Lluís Companys 23, Barcelona 08010, Spain Hiroshima University A. Bück 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan Institute of Particle Technology Department of Chemical and Biological Engineering Friedrich-Alexander-Universität Erlangen-Nürnberg Cauerstraße 4, 91058 Erlangen, Germany The ORCID identification number(s) for the author(s) of this article D. P. Debecker can be found under https://doi.org/10.1002/adma.202306648 Université catholique de Louvain (UCLouvain) Institute of Condensed Matter and Nanosciences (IMCN) © 2023 The Authors. Advanced Materials published by Wiley-VCH Place Louis Pasteur, 1, 348, Louvain-la-Neuve Belgium GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. DOI: 10.1002/adma.202306648 Adv. Mater. 2023, 35, 2306648 2306648 (1 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de instance, self-limited self-assembly,[1] or assembly through spe- is therefore to draw the attention of our fellow scientists to this cific molecular, e.g., DNA-based, protein-based, interactions.[2] technique. Many approaches require highly controlled and tailored chem- From a fundamental science perspective, it is highly interest- istry and are often only successful in specific cases. ing to study the properties that result from the cooperative inter- An alternative approach is to neglect to pay attention to chem- play of the (sometimes very different) building blocks that were ical “preferences” or “affinities” and rather use “brute force” to joined by force and that would not have come together in any join moieties. Such a brute force method is to put together the other way. But there is also a great application perspective: the target moieties in a confined space and narrow down this space powder of micrometer-sized entities can be easily handled and more and more until the moieties have no other option than to treated like standard additives and thus, upon its employment, join as a new composite entity. A concrete way of doing this is via equip materials with novel properties. Pushing it to the extremes, dispersing nano- or molecular building blocks in a dispersion liq- inside of such an entity an own ecosystem can be designed, e.g., uid, then forming a droplet and ultimately evaporating the liquid. to be a little microreactor on its own, thus acting as a highly in- Techniques to realize such an approach include emulsion-based teractive object. assembly (including microfluidic process-controlled emulsions, Herein, we do not aim at reviewing spray-drying and its princi- or surfactant-based as well as surfactant-free emulsions) or the ples and achievements as such, as informative reviews on this do droplet evaporation on a surface.[3] exist already.[4,12–14] Rather, we aim at giving a perspective on the Among these methods, spray-drying stands out as it is the future potential of obtaining multifunctional, hybrid and to date most “chemistry-independent” and versatile approach, as it is unthinkable materials upon employing spray-drying as a tool to simple, fast, amenable to continuous processing, as well as scal- assemble a plethora of building blocks by force. able and yields reproducible results.[4–7] In principle, any nano or molecular building blocks in dispersion are fed into a noz- zle, which creates small droplets. These droplets are then dried 2. Spray-Drying as a Highly Versatile Tool for in a hot chamber, resulting in typically micrometer-sized entities Material Chemists composed of the aforementioned building blocks, which are sub- sequently joined. This process is universally applicable as long as 2.1. Material Formation via Spray-Drying the chemical moieties are firstly dispersible/soluble in a liquid and secondly, the liquid can be evaporated before the moieties Upon spray-drying, the confined space of a liquid droplet is take off to the gas phase. These criteria are rather simple and narrowed down and the liquid dispersion medium is dried can be fulfilled for most moieties and thus, in principle, make (Figure 1). Although seemingly simple, this process can be ex- the process chemistry-independent. Moreover, as it is simple, it ploited for a variety of material formation strategies: a) nanopar- makes it also easily scalable and obviously, the process is fast, ticles and/or molecules can be assembled; b) precipitation re- making it superior to many other assembly methods regarding actions can be performed; c) chemical syntheses/reactions are the listed aspects. As the building blocks during spray-drying are possible upon the drying event; d) templated materials can be by the process colocalized as an entity in time and space, this fact designed; and of course, e) any combinations of (a) to (d) at alone, or secondary effects such as coupling among the build- the same time can be achieved. Beyond this, f) fragile moieties ing blocks or the so-called “emergence,” yield new properties that can be processed, or g) “precursor materials” can be prepared. could not be achieved with the individual building blocks alone.[8] h) Post-treatment of spray-dried material eventually enables the The ability to join building blocks with hardly having to pay at- next level of material design. tention to finely designing chemical affinities and the possibility Whenever nanoparticles are involved in the spray-drying pro- to perform “chemistry in a droplet” upon the spray-drying event cess, as defined earlier,[8] we will denote the formed entities as render this technique very powerful. In the field of pharmaceu- “supraparticles” throughout this Perspective article. ticals/drug design[9] as well as foods,[10] spray-drying has been employed as a versatile tool for a long time already. Also, for ba- sic chemical drying or impregnation operations, spray-drying is 2.1.1. Nanoparticle and/or Molecule Assembly well-established and performed in the industry.[11] However, we believe that the enormous potential to design novel, multifunc- The self-assembly of building blocks, such as nanoparti- tional materials via spray-drying was unjustifiably neglected too cles and molecules within a single droplet upon solvent much in the past by materials chemists. Our intention herein evaporation[8,15] can produce a variety of compositions (e.g., single-component,[16] multi-component,[17,18] inorganic-organic hybrid,[19] metal oxide hybrid[20,21] ) and structures (e.g., solid,[22] hollow,[23] core–shell[23] ) with unique properties and potential ap- M. Faustini plications. Recently, many types of unique building blocks such Sorbonne Université as nanofibers,[24] nanorods,[25] and metal–organic frameworks Collège de France (MOFs)[26,27] have become available and offer potential advan- CNRS tages. However, maintaining the distinct properties of the build- Laboratoire Chimie de la Matière Condensée de Paris (LCMCP) Paris F-75005, France ing blocks during self-assembly is highly important. To address M. Faustini this, researchers should aim to hybridize the material proper- Institut Universitaire de France (IUF) ties of building blocks with the features of the aggregate parti- Paris 75231, France cles, leading to the development of new materials with unique Adv. Mater. 2023, 35, 2306648 2306648 (2 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 1. Upon spray-drying the confined space of a liquid droplet is narrowed down and the liquid dispersion medium is dried. Using this principle, for instance, a) nanoparticles and/or molecules can be assembled, b,c) precipitations (b), or chemical reactions (c) can be performed, d) templated systems can be created, and e) any combination of (a–d) is possible. Beyond, f) fragile moieties can be processed or g) precursor materials can be synthesized. h) Post-treatment of spray-dried material eventually enables the next level of material design. physical and chemical properties. Despite the potential of this spheres (nanoscale drug particles to enhance bio-availability);[29] approach, there are still several challenges that need to be ad- bimetallic nanoparticles for use in reduction and removal of dressed. One of the main challenges is achieving precise control chlorinated ethene and ethylene;[30] and molecular explosives of the particle structure, particularly when the assembly involves (hexanitrohexaazaisowurtzitane).[31] By precipitation, the phase two or more different types of particles or molecules with differ- transition of the dissolved substance into a dispersed solid phase ent properties. To overcome this challenge, adjusting several fac- is achieved. The properties of the precipitated material depend on tors such as process parameters (e.g., drying gas temperature, gas the thermodynamics and kinetics of the precipitation reactions. flow rate, nozzle configuration), physicochemical properties of In brief, precipitation reactions are triggered by a local or nanoparticles (e.g., size, shape, density, surface potential, chem- global excess in thermodynamic state variables, most commonly ical composition, concentration, and mixing ratio in the case of the solubility of the solid in the liquid phase. The correspond- multiple components), droplet properties (e.g., droplet size, sur- ing solids concentration is a saturation concentration (cs ) de- face tension, viscosity, boiling point, polarity), the drying rate of termined by thermodynamics; the local excess is typically ex- the solvent, and nanoparticle diffusion rate within droplets is pressed in terms of the supersaturation S = (c − cs ). The kinet- essential. ics of nucleation and further growth of the nuclei, e.g., into crys- tals, often follow a power law as a function of S, e.g., Sg with a material-specific parameter g. For control of nucleation, e.g., to 2.1.2. Precipitation Reactions control the size and shape of the nuclei, it is of paramount impor- tance to control the supersaturation or the underlying saturation One of the main virtues of spray-drying is that almost any concentration. solid-containing liquid can be processed. If at least one of Supersaturation can be achieved along three routes, all of them the solids is initially dissolved in the feed, then solid forma- manipulating the saturation concentration: 1) Controlled cool- tion by precipitation (also: nucleation) together with subse- ing: Solubility decreases with decreasing temperature. By con- quent growth and agglomeration of the primary solids build- trolled cooling of the feed during processing, the saturation tem- ing blocks can be utilized for product design. Examples with perature (concentration) is reached, and nucleation starts. In nanoscale precipitates are the production of drug-loaded micro- spray-drying, it is only of limited use as the feed (and droplet) Adv. Mater. 2023, 35, 2306648 2306648 (3 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de temperature is fixed by the adiabatic saturation temperature of fates, etc.[31,32] The typical reaction consists of two steps: 1) hy- the gas used in the spray-dryer. Only if the feed temperature is droxylation of metal precursors to form reactive hydroxy groups; higher than the gas saturation temperature, control on supersat- and 2) polycondensation reactions (oxolation, olation) through uration can be achieved. 2) Reduction of solvent volume: In spray- the formation of branched oligomers, inorganic polymers, and drying this is achieved by evaporation of the liquid. As solids vol- crystalline networks, typically after thermal annealing.[32] The na- ume remains constant, solids concentration increases to and be- ture of the inorganic network depends on the precursors, pH, yond the saturation concentration and precipitation is initiated. water content, and aging. Indeed, the condensation degree of As the liquid volume decreases further, a steady production rate sol–gel-derived oxides evolves with time, the aging of the pre- of nuclei (with constant properties) can be maintained. The speed cursor solution is a key parameter affecting the properties and of evaporation can be manipulated by the gas-side conditions, al- the nature of the final materials obtained by spray-drying. Spray- lowing for a wide range of nucleation rates and nuclei proper- drying sol–gel systems requires a delicate control of the reactivity: ties. 3) Reaction-induced precipitation: The basis for this reaction one must design the process in order to minimize the condensa- type is that the reactants have a higher solubility than the reaction tion rate in the precursor solution before spraying and to trig- product. ger the condensation during drying. As mentioned above, long New opportunities for material and product design can be reaction times can represent a limitation. For instance, this is- achieved, e.g., by (ad-)mixing at least two reactants (e.g., by a sue can be overcome by using chemical additives in solution, droplet–droplet collision where one droplet is rich in component inducing rapid modification of the pH during the evaporation, A and the other in component B). The reaction product is gen- leading to condensation and oxide formation. Having these gen- erated and then precipitates according to its level of supersatura- eral considerations in mind, typical examples of spray-dried ox- tion. The use of multiple immiscible fluid phases in one droplet ides spheres comprise conventional oxides, such as SiO2 , TiO2, with preferential precipitation of the components in one specific or ZrO2 , and even multimetallic oxides, such as cobaltites or fluid phase presents a new opportunity over spatial arrangement perovskites with exotic properties for applications in thermo- of the components and the properties of the precipitates in the electricity, magnetoresistance and piezoelectricity.[33] Going be- spray-dried particle. Thereby, complex structured patterns, like yond oxides, another way to obtain ceramic spheres, such as Turing patterns[28] or as generated by classical spinodal decom- carbides, nitrides or boron-nitrides, involves the utilization of position are imaginable, also in parallel with control of the do- polymer-derived ceramics (PDCs).[34] Preceramic polymers, such main sizes of the different components in the droplet and parti- as polysilazane or polyvinylsilane, can be processed as polymers cle, respectively. Spatial patterning in general has been shown for into micronic or nanometric spheres by spray-drying.[35] The molecular multiphase systems, e.g., acetone–water–malto dex- cured preceramic polymer can then be converted into ceramic trin solutions,[29] and is routinely applied in food engineering to material upon heating to a high temperature in an inert atmo- protect nutrients from the environment. sphere, typically above 1000°C. During this pyrolysis step, the polymer undergoes a series of chemical reactions, such as de- composition, condensation, and crystallization, leading to the for- 2.1.3. Chemical Syntheses/Reactions mation of ceramic materials.[34] Going beyond traditional applications, among the emerging Spray-drying can become a very useful tool for the synthesis of functionalities of spray-dried inorganic spheres, one of the most new molecules and materials, as it is a fast, one-step, and re- exciting directions is related to photonic metamaterials. Sub- producible process to mix reactants under well-controlled tem- micrometric sol–gel-based polycrystalline TiO2 spheres obtained peratures; can be performed under good manufacturing prac- by spray-drying have been exploited as all-dielectric Mie res- tices; can be easily scaled up; and the final product is a dry solid. onators for a plethora of applications including vanishing back- With all these advantages, spray-drying has begun to be used for scattering, enhanced directivity of a light emitter, beam steering, the efficient synthesis of not only a plethora of inorganic mate- and large Purcell factors.[36] As shown in Figure 2a, those spher- rials (e.g., oxides, ceramics, etc.), but also of molecular materi- ical nanoresonators possess a structural color that depends on als in which coordination, supramolecular and organic chemistry their size and their dielectric properties that can be tuned taking are involved. For all these syntheses, however, the inherent fast advantage of the chemical versatility of spray-drying for fabricat- drying-kinetic nature of spray-drying can limit its use to perform ing complex metal oxides and including other optical functional- reactions that require long reaction times. In these cases, imple- ities. From a practical point of view, the spray-drying method cou- mentation of these chemical reactions in spray-drying should re- pled with inorganic polymerization will allow for a direct trans- quire innovative solutions, such as the pre-treatment of the re- formation of the precursors’ solution in ready-to-use metamate- actant mixture before its spray-drying using a continuous-flow rials to be sprayed on a surface or adapted for the production of reactor placed at the entrance of the spray-dryer. meta-pigments or meta-paints. A clear example of a chemistry that can be performed in spray- Besides solgel reactions, the scope of chemistries accessi- drying is the sol–gel process, which enables the transformation ble by spray-drying also includes coordination chemistries. In of a molecular precursor into a metal oxo-macromolecular net- 2013, Maspoch et al. first reported the use of spray-drying as work or oxide nanoparticles based on inorganic polymerization a rapid, continuous method to synthesize MOFs,[37] which are reactions. This chemistry is thus ideally suited to prepare ox- organic-inorganic hybrid, crystalline, porous materials assem- ide spheres when coupled with spray-drying.[30] Typical precur- bled from metal ions or clusters, and organic linker molecules. sors are metallo-organic compounds such as alkoxides, chelated MOFs have shown promise for applications such as gas stor- alkoxides, or metallic salts such as metal chlorides, nitrates, sul- age, catalysis, drug delivery, thermal energy storage, sensing, and Adv. Mater. 2023, 35, 2306648 2306648 (4 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 2. Schematic illustration of the synthetic versatility of spray-drying. a) Synthesis and SEM images of sol–gel derived TiO2 microsphere resonators (scale bars: 500 nm) (a); b) spherical beads of an imine-linked COF (Scale bar = 2 μm); c) MOF HKUST-1 (scale bar = 2 μm); and d) sol–gel derived porous metallic iridium microspheres (Scale bar = 200 nm). a inset) Adapted with permission.[36] Copyright 2018, John Wiley and Sons. d inset) Adapted with permission.[48] Copyright 2020, Royal Society of Chemistry. decontamination, among many others.[38] The spray-drying syn- sis of covalent organic frameworks (COFs), a class of porous thesis of MOFs (Figure 2b) relies on the formation and further crystalline materials analogous to MOFs, which are created via drying of micrometric droplets containing the desired metal- covalent linkage of organic monomers (Figure 2c).[46] A possi- lic and organic precursors. Rapid evaporation of the solvent in- ble explanation for the amenability of these condensations to duces the formation of nanoscale MOF crystals at the air–liquid spray-drying synthesis is that water, which is generated as a side- interface. These crystals subsequently accumulate and merge product during the formation of the condensation product, is into compact or hollow spherical MOF superstructures or beads spontaneously removed upon entry of microdroplets into the hot upon drying.[37] Today, a wide range of MOFs (e.g., MIL fam- chamber. This drives the equilibrium in the forward direction, ac- ily, ZIF family, etc.), coordination compounds with function- cording to Le Chatelier’s principle, thus providing high rates of alities such as thermochromism or spin crossover, and even conversion.[47] hydrogen-bonded porous frameworks can be synthesized using spray-drying at the gram- and kilogram-scales.[39] A critical as- pect in spray-drying synthesis of MOFs is control over reaction 2.1.4. Templated Materials Design kinetics. Indeed, conditions that promote rapid reaction and pre- cipitation upon mixing of the reagents can lead to the formation Template-assisted spray-drying has emerged as the predominant of amorphous phases or unwanted species before spray-drying approach for the synthesis of nanostructured materials.[13,49] The even begins. This problem can be resolved through the use of a incorporation of templates provides insightful guidance for the T-junction connector or a three-fluid nozzle,[40] attachments that assembly of new types of nanostructured materials with novel shorten the contact time between the precursor solutions.[41,42] frameworks and compositions, leading to their extended func- Contrarily, spray-drying syntheses of MOFs containing high- tions and prospects. However, there are still challenges to be nuclearity clusters (e.g., zirconium-oxo-hydroxo clusters) that re- addressed for the practical applications of nanostructured parti- quire longer reaction times may result in low yields and/or af- cles using the template-assisted spray-drying method. Figure 3 ford materials with poor sorption capabilities, due to fast drying- illustrates important issues that need to be considered in the fu- kinetics. This problem can be overcome by adding a continuous- ture to overcome these challenges and further advance the field. flow reactor at the entrance of the spray-dryer, to induce nucle- Firstly, the selection of an appropriate template is an essential ation before spray-drying.[43] factor in the synthesis of nanostructured materials. One of the Spray-drying has also been proven utile in covalent chemistry. key considerations in template selection is its cost-effectiveness For example, it has been used to improve the stability or modify and the ability to recycle for multiple synthesis cycles, which the mechanical properties of polymers, by chemical crosslinking may be feasible for large-scale production, and reduce the over- of polymer or biopolymer chains using transformations such as all cost as well as environmental impact of the process. The Diels–Alder or click chemistry reactions.[44] In other, more re- typical templates including SiO2 ,[50] CaCO3 ,[51] polymer (e.g., cent examples, Schiff-base condensations have been employed PMMA, PSL),[52,53] amphiphilic triblock copolymer (e.g., F127, for spray-drying modifications of MOFs[45] or for the synthe- P123),[54] and alkali salt (e.g., NaCl, KCl)[55] have been used for the Adv. Mater. 2023, 35, 2306648 2306648 (5 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 3. Challenges and perspective for the practical applications of nanostructured particles using template-assisted spray-drying method. fabrication of nanostructured materials. However, the removal 2.1.5. Combinations of (a)–(d) (Sections 2.1.1–2.1.4) of these organic/inorganic templates may require harsh chem- ical treatments (e.g., organic solvent, strong acid, or base) or pro- Besides pursuing one of the aforementioned four different ma- longed exposure to high temperatures, which may destroy the terial formation strategies via spray-drying, a combination of two structure of the material. Secondly, controlling the properties of or more of them is also possible and opens up the design of even used templates plays a significant role in determining the struc- more diverse and complex hybrid materials. One approach in this ture, composition, and morphology of the final materials. For context is the combination of chemical reactions and templating example, the use of surfactants can help controlling the size of during spray-drying. In a typical process, molecular metallic pre- the template, while surface modification techniques can be em- cursors and templates co-assemble by evaporation-induced self- ployed to tailor the surface charge and chemical functionality of assembly mechanism to form organic/inorganic nanocompos- the template. In addition, the selection of an appropriate solvent ites. The molecular precursor typically leads to an inorganic con- system can help ensuring the dispersity of the template and pro- tinuous network as discussed above (Section 2.1.3). Instead, the moting efficient assembly of the nanostructured material. The organic agents form micelles or other structures, which act as a positioning of the template particles within the microsphere is sacrificial mold for the final porous material (Section 2.1.4). One influenced by several critical factors, including: (i) the mixing timely development in the field consists in going beyond porous ratio between the nanoparticles, serving as building blocks, and oxides and ceramics. Starting from molecular precursors, spray- the template particles, (ii) the size ratio between these nanoparti- drying synthesis can be extended to fabricate porous metallic ma- cles and template particles, and (iii) the surface potential of both, terials, including noble metals, with important implications for the nanoparticles, and template particles. Furthermore, the posi- catalysis for instance. This can be done by using bifunctional tioning of template particles is affected by the difference between polymeric templates, in which polymeric colloids can be used as the solvent evaporation rate of the droplet and the diffusion rate structuring agents and reducing agents to form a metallic porous of colloids inside the droplet.[56] These factors collectively deter- network starting from metallic salts by carbothermal reductions mine how templates are distributed within the microspheres, im- at a mild temperature (see Figure 2d, Section 2.1.3).[48] Another pacting the final structure and properties of the resulting nanos- combined spray-drying approach is the MOF or COF synthesis tructured materials. Thirdly, after removing templates, the inner (Section 2.1.3) merged with the sole assembly of molecular or pore networks of generated products can be further functional- nanoparticle building blocks (Section 2.1.1). Such composites of- ized (e.g., metal loading, heteroatom-doping), which significantly fer many advantages over other material classes: for instance, enhances the application scope of the resulting materials. In ad- they can prevent guest materials from aggregating or degrading dition, the systematic evaluation of the correlation between struc- in harsh environments; control and sustain the release of guest tures and properties is crucial for understanding the underlying molecules; and provide superior adsorption or selectivity relative mechanisms of the synthesis process and optimizing the perfor- to analogous non-composite materials.[57] In this context, MOF- mance of the materials. Finally, using a simulation approach is based and COF-based composites incorporating various mate- an effective and promising tool for predicting optimal conditions rials, including inorganic nanoparticles (e.g., Pd, Pt, Au, CeO2 , for obtaining the desired target structure in the template-assisted and hybrid core–shell Au/CeO2 nanoparticles), polymers (e.g., spray-drying method. polystyrene), organic molecules (such as dyes) and salts, have Adv. Mater. 2023, 35, 2306648 2306648 (6 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de been synthesized.[41,58,59] In another approach, precipitation reac- was slightly lowered after spray, but an enormous gain in stabil- tions during spray-drying (Section 2.1.2) can be combined with ity was achieved. Hybrid catalysis, as an emerging field where different procedures. For example, one way of coupling precipita- chemo and enzymatic catalysts are teaming up,[67] entails the tion and chemical synthesis is co-spray-drying calcium hydroxide preparation of chemo-enzymatic hybrid materials in which the and ammonium fluoride precursors to initiate a co-precipitation activity of both partners must be preserved. In the case of very reaction that yields nanoscaled CaF2 powder.[60] When composite fragile enzymes, these must be stabilized (e.g., via complexation precursor solutions, consisting of various 3,5-dinitrobenzoates in a polyelectrolyte) forming so-called enzyme-polyelectrolyte (Fe, Co(Cu)), are united by ultrasonic spray-drying, nanoparticles complexes,[68] to preserve their activity upon processing via spray- precipitate in the process and self-assemble into core–shell mi- drying.[69] crostructures, depending on their respective size, therefore suc- cessfully enabling the combination of precipitation and nanopar- ticle assembly (chapter a).[61] Further, templated reactions can 2.1.7. Formation of “Precursor Materials” be linked with precipitation. An example is the preparation of porous SBA-15-type SiO2 microparticles (Section 2.1.4).[16] Here, The particles obtained via spray-drying can be exploited as precur- TEOS as a precursor for SiO2 is spray-dried with polymeric tem- sors to being further converted into well-defined objects which plates (F127 and Eudragit RS). While the former precipitates dur- are otherwise difficult to obtain. For example, tailored zeolite ing spray-drying, the latter is removed in a subsequent calcina- nanocrystals have been formed upon hydrothermal treatment tion step to form porous structures. of amorphous metallo-silicate microspheres formed by reactive spray-drying.[70] The method takes advantage of the spray process to shape the precursor materials in the form of spherical particles 2.1.6. Processing of Fragile Moieties which feature a high concentration and a high dispersion of the heteroatom (e.g., Ti when the TS-1 zeolite is targeted). The poros- Spray-drying processing is primed to solve some of the chal- ity is then filled with a structure-directing agent solution and the lenges we face when handling fragile moieties that are needed paste is treated in hydrothermal conditions. As opposed to con- in hybrid materials, mostly owing to the very short processing ventional hydrothermal methods, very low amounts of structure- time. A first example is the preparation of functionalized oxides, directing agents and solvent (water) are needed, and the crys- such as hybrid silica or metallo-silicates bearing organic moi- tallization can be completed at lower autoclave temperatures. eties. It is well-known that organosilanes do not react at the same In another example, the potential of spray-drying was demon- rate as the usual alkoxide precursors (e.g., tetraethyl orthosilicate strated as an alternative synthesis process for the production of (TEOS) in silica-based sol–gel chemistry),[62] which complicates perovskite absorbers in powder form. Various Pb-containing and the one-step preparation of hybrid materials, often leading to in- Pb-free perovskites were produced, owing to phase stability and homogeneous materials. Even when successful, for example us- high shelf-life. Subsequently, whenever desired, the produced ab- ing reactivity modifiers, the texture of the material can be strongly sorber powders could be used as an “instant powder” starting ma- affected by the introduction of organosilanes in the synthesis terial for a coating solution and the deposition of absorber layers medium. The rapidity of the spray process allows a facile addition in solar cell manufacturing.[71] of organosilanes to incorporate the needed organic functionality Another approach is the exploitation of spray-drying for dis- at the surface of the silica. This has been exploited to moderate persing small metal nanoparticles on the surface of a relatively the hydrophilicity of the materials, as exploited in heterogeneous “innocent” support (e.g., silica). Such deposition is often a chal- catalysis.[63] Interestingly, in this case, the texture can still be con- lenge, due to the poor interactions between the support and the trolled by exploiting templating strategies; this allows exploiting metal, and due to the tendency of the latter to sinter. Taking a the presence of the organic functionalities without compromis- sidestep, aerosol-assisted sol–gel allows trapping the metal pre- ing the advantageous textural properties of the mesoporous ma- cursor (e.g., Pd,[72] Au,[73] Ag,[74] Cu salts[75] ) into the matrix of terials that are accessible via spray.[64] a (porous) silica material. Even if the metal shows low solubil- Biological materials can also be processed by spray. In the food ity in silica, its homogeneous dispersion is forced by the rapid industry, the common example is the milk that is dried by spray- polycondensation reactions occurring during aerosol processing. drying to give the powder we use to prepare baby formula. Now, Then, thermal treatment can be used to trigger the migration when it comes to functional materials, the idea is to maintain of the metal toward the surface. This phenomenon is referred the biological activity of the biological moieties that are forced to to as “exsolution” (in particular when starting from crystalline come together with other components. In one example, spray- lattices and applying reducing thermal treatments,[76] but also drying was used to fabricate highly concentrated, self-assembled by extension for all cases where a highly dispersed element is type I collagen microparticles enabling the encapsulation of ther- forced towards the surface under thermal effect, see Figure 4). apeutic biomolecules by preserving the native properties of frag- Playing with the severity of the heating step it is possible to fine- ile collagen after drying.[65] Spray-drying is thus ideally suited for tune the extent of the migration towards the surface and the fabricating ready-to-use collagen microparticles easily injectable subsequent sintering to obtain supported nanoparticles of differ- for tissue regeneration and 3D bioprinted scaffolds. ent sizes. Interestingly, the formed nanoparticles—which arise For example, enhancing the storage and thermal stability of from the bulk of the material—tend to exhibit stronger interac- active enzymes is of utmost importance, and microencapsu- tions with their supporting material, and therefore higher stabil- lation with appropriate support (e.g., alginate)[66] using spray- ity. Recently, this was successfully exploited to design Cu-SiO2 drying has been demonstrated. In this case, enzymatic activity catalysts[77] and also with other carriers, such as ceria, alumina, Adv. Mater. 2023, 35, 2306648 2306648 (7 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 4. Schematic representation of the preparation of metal-based heterogeneous catalysts by the “spray + exsolution” process. A precursor solution containing solvent(s), precursor(s) of the oxidic support (e.g., alkoxides), a sacrificial porogen, and a metal precursor (e.g., chloroauric acid, copper nitrate, palladium chloride, etc.) is aerosolized and dried. In the recovered dried particles, the metal precursor is trapped and homogeneously dispersed into the oxide matrix. Upon thermal treatment, the metal species—which feature low solubility in the oxide—are forced to migrate towards the surface of the particles or pores, where they form small metal or metal oxide nanoparticles. Depending on the severity and duration of the thermal treatment, these nanoparticles can further sinter to form larger nanoparticles. This process may result in materials that are very different (in terms of NP size, size distribution, shape, degree of interaction with the support, spatial distribution, etc.) from those obtained by the more classical deposition (impregnation, deposition, grafting) of metal precursors onto a preformed support. This emerging strategy was shown and is expected to lead to new metal-based catalysts with properties and performance that cannot be reached otherwise. titania, zirconia.[78,79] An important note is that some metal pre- rial limitations (see 2.2.) when using fragile substances or per- cursors may react very quickly during the spray processing, under mit surface modification that would negatively interfere with the the effect of the sudden temperature and concentration increase. material formation during spray-drying. They consequently need to be stabilized, e.g., with functional More advanced processing approaches are the creation of alkoxides. In the case of chloroauric acid that readily decomposes a next level of hierarchy based on spray-dried materials as during aerosol processing and forms relatively large nanoparti- building blocks. In this context, one could imagine the fab- cles embedded inside the support matrix instead of being ho- rication of particle-based materials of all “dimensions.” For mogeneously dispersed, the substitution of a small fraction of example, “0-dimensional” multi-hierarchical millimeter-scaled TEOS by (3-mercaptopropyl)trimethoxysilane (MPTMS) success- particles[17,86] have recently been presented. As “2-dimensional” fully stabilized the gold precursor and led to small (≈3 nm) Au objects first coatings based on spray-dried particles have already nanoparticles after calcination.[73] been developed[87–89] and “3-dimensional” objects can be envis- aged via powder-based additive manufacturing[90] or could in future be exploited via 3D printing techniques (see Figure 5). 2.1.8. Post-Treatment of Spray-Dried Material The creation of “1-dimensional” objects could eventually also be achieved by transferring nanoparticle assembly concepts[91] to Based on the extensive experience gained in the formation the microscale, such as a linear particle arrangement with the of a wide variety of materials and structures by spray-drying help of external fields. and on the way to their application, the further processing of spray-dried materials has currently started to attract attention and will continue to do so in the future. The probably most straight-forward post-synthetic treatment of spray-dried materi- 2.2. Spray-Drying: Technical Advantages and Limitations als is their chemical surface functionalization through either co- valent coupling reactions (e.g., click chemistry or DDC/EDC cou- Spray-drying comes with many advantages—but there are also plings) with target molecules such as enzymes, chromophores, limitations. Both sides of the medal are outlined in the following. or peptides, via reactions with organosilanes (e.g., aminopropyl- Furthermore, this Perspective will focus on material-related for- triethoxysilane),[80] or via electrostatic interactions (e.g., car- mulation opportunities and challenges that are independent of boxylic or phosphonic acids on metal oxide surfaces) with ligands equipment size. Scale transition of spray-drying, for constant for- such as surfactants.[81] In this context, the post-synthetic load- mulation properties, will introduce additional operational chal- ing of spray-dried particles with molecules including drugs, en- lenges on the engineering side, e.g., flow field design, avoidance zymes, dyes, fertilizers, or pesticides with or without attachment of excessive droplet–droplet or droplet–particle collisions, that onto them should also be mentioned as it enables their appli- merit an own perspective. cation as storage materials.[82,83] Furthermore, advanced physi- On the plus side, as obvious from the previous section, stands cal and chemical surface layer deposition methods could also be out the possibility to enable the design of diverse materials, hy- exploited for post-synthetic surface modifications. Using atomic brids, and exotic combinations. A few examples are macro and layer deposition, porous spray-dried iron oxide particles,[84] as mesoporous high-entropy-alloy (HEA) particles (Figure 6a),[53] well as amorphous drug-loaded particles,[85] have recently been silica-enzyme particles (Figure 6b),[81] silicon–carbon–graphene successfully surface-modified with various metal oxide (Al2 O3 , combinations (Figure 6c),[79] iron oxide-polymer (e.g., polylac- ZnO, and TiO2 ) coatings. In general, post-synthetic surface tic acid) composites (Figure 6d),[6] lipid–silica structures con- functionalization may circumvent spray-drying-inherent mate- taining living cells (Figure 6e),[80] zeolite–enzyme–polyelectrolyte Adv. Mater. 2023, 35, 2306648 2306648 (8 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 5. a) Particles consisting of supraparticles, b) i.e., supra-supraparticles, and powder-based additive manufacturing using supraparticles. a1,b1) Their fabrication processes are visualized in schemes and a2,b2,b3) SEM studies show the obtained material. b1,b2) Adapted with permission.[90] Copyright 2020, John Wiley and Sons. a1,a2) Adapted with permission.[17] Copyright 2023, John Wiley and Sons. hybrids (Figure 6f),[60] LiMnO2 –carbon black materials,[82] or iron Tuning the drying of the solvent can thus be used as a tool to oxide–MOF hybrids.[37] induce a gradient of properties from the surface to the core to Moreover, spray-drying is in essence a process based on fast core–shell, graded, or hollow particles of complex structures.[97] evaporation. Controlling kinetics is of paramount importance in For instance, spray-drying was used to fabricate porous spheres the final chemical composition and the structuration process. with a core–shell structure by using a mixed surfactant approach From a chemical point of view, the fast drying induces quenching where the shell and the core exhibit different ordered mesoporos- of the system into metastable states. As a result, whatever non- ity and pore sizes.[98] Gradients of precursors concentration can volatile species are in the initial solution, they will be found in the be used to induce controlled phase separation of several com- final material with a degree of dispersion that will depend on the pounds (such as salt crystals, nanoparticles, or biomolecules) at chemical affinities and the kinetics of the evaporation-related pro- the core of spheres.[99] Conversely, evaporation-driven gradients cess. Playing on kinetics has two main advantages. The first im- of polarity can promote the migration of hydrophobic entities portant advantage is related to the fact that fast drying promotes (nanoparticles) at the outer surface of the spheres.[100] the kinetic control of the structuration into local minima through Furthermore, spray-drying is unrivaled in its tunability for con- thermal quenching ideally suited to prepare materials with inti- tinuously transforming a solid-containing liquid into a powder mate mixing of non-soluble precursors to obtain mixed oxides, consisting of structured particles. The main handles for tun- alloys, or composites. One representative example concerns the ing are gas-side properties (e.g., type of gas, temperature, initial fabrication of high entropy alloys, multi-principal element alloys, moisture loading, flow configuration (with or against spray di- composed of five or more homogeneously mixed metals in nearly rection), flow pattern (laminar, turbulent)), feed properties (tem- equimolar ratios.[53,95] The second advantage of fast kinetics con- perature (heated, superheated, subcooled), solids concentration, cerns the structuration and the possibility to achieve controlled use of surfactants or other chemical enhancers), spray proper- matter phase separation. Indeed the evaporation of the droplets ties (e.g., initial droplet size distribution and velocity distribution, taking place at the liquid/air interface leads to thermal and chem- droplet cone (hollow or full), or droplet and particle re-circulation ical gradients (solutes concentrations, surface tension, and po- in the apparatus (e.g., by internal classification of particles and larity) from this interface towards the heart of the volume.[96] refeeding of too small particles; forced agglomeration of droplets Adv. Mater. 2023, 35, 2306648 2306648 (9 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 6. Exotic combinations of diverse materials via spray drying. a) Porous high-entropy-alloy particles; b) silica-enzyme (hexose oxidase) particles: c) silicon–carbon–graphene combinations; d) iron oxide–polymer composites; e) lipid-silica structures containing living cells; and f) zeolite–enzyme– polyelectrolyte hybrids. a) Adapted with permission.[53] Copyright 2022, American Chemical Society. b) Adapted with permission.[92] Copyright 2010, Royal Society of Chemistry. c) Adapted with permission.[93] Copyright 2017, Elsevier. d) Reproduced under the terms of the Creative Commons CC-BY license.[5] Copyright 2022, The Authors. Published by Wiley-VCH. e) Reproduced under the terms of the Creative Commons CC-BY license.[94] Copyright 2015, The Authors. Published by American Chemical Society. f) Reproduced with permission.[69] Copyright 2021, Royal Society of Chemistry. by intersecting spray (also for initiation of reaction-induced pre- • Exploration of search space for optimized product design: Find- cipitation)). Each of these opportunities can be detailed and opti- ing the right set of operation and material parameters for mized for a given product; it is up to the expertise and experience a desired product property is a multidimensional prob- of the spray-dryer operator to achieve the best possible product lem. Within the set of admissible solutions (parameter val- with the least amount of investment (material, energy). ues), some may fulfill additional constraints, e.g., maxi- With more and more operational and product characterization mized sustainability of the process and product, or min- data available nowadays, e.g., by extended process monitoring imum production effort, as demonstrated by Przybyl and and offline characterization (e.g., electron microscopy, X-ray to- Kozela (e.g., deep and convolutional neural networks),[102] mography), spray-drying can profit from the consequent use of Ming et al. (particle swarm optimization-enhanced arti- AI methods like other disciplines, e.g., catalysis research or parti- ficial neural network),[103] or Fiedler et al. (efficient de- cle technology at large.[101] The main perspective uses of AI meth- sign of experiments using a multi-step machine learning ods in spray-drying are: approach).[104] Adv. Mater. 2023, 35, 2306648 2306648 (10 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de • Elucidation of fundamental relationships between operational, ma- tallinity, etc.) of the final product. To obtain a homogeneous terial parameters and product properties: Utilizing spray-drying solid product, this solution/dispersion must be kinetically sta- data across different disciplines, e.g., material science, phar- ble for the entire duration of the atomization process, which maceuticals, or food and feed, AI methods may find fun- can range from minutes to hours. Indeed, unstable solutions or damental relationships between the parameters and product suspensions can cause various problems. For instance, they can properties, independent of the field of application. For this, block the spray-dryer nozzle, leading to inconsistent droplet for- a general open-access database, collecting and curating avail- mation and particle-size distribution; and they can provoke the able experimental evidence across the applications, needs to formation of unwanted byproducts such as agglomerates, non- be established by the spray-drying community (academia and homogeneous samples, and unstable particles, ultimately afford- industry). ing the final product in low yields. Additionally, the synthesis • Data-driven modeling for process monitoring and control: AI of some materials is time-dependent, and the evolution of the methods must be consequently used to develop fast, robust, reaction or the aging time during the processing of the precur- and computationally efficient surrogate models for spray- sor solution can dramatically affect subsequent atomization.[30] drying. This will then allow large-scale optimization of spray- Such is the case for many sol–gel-derived syntheses of metal- drying processes (e.g., considering whole process chains) as oxo polymers, as both the size and degree of condensation are well as allow for improved process monitoring and control of time-dependent, and the aging time can drastically influence the spray-dryer operation. Monitoring and control will for instance texture of the resulting spray-dried polymeric products. Similar not only enable improved product quality but also reduce off- problems have been observed in the spray-drying synthesis of spec product quantities, and operational failures, thereby con- high-nuclearity beads of MOFs. To avoid clogging of the nozzle or tributing to sustainable process operation. low crystallization of the MOF beads, the activation time of this process should be precisely controlled before atomization. This However, in spray-drying, there are also challenges: Spray- is achieved through a continuous-flow reactor, where the primary drying as a thermal drying process relies on the phase transition nucleus of the MOF is grown.[43] Accordingly, researchers have of the solvent from liquid to vapor. The phase transition is typ- dedicated great effort to optimizing the stability of the feed solu- ically highly energy-intensive, e.g., water has a specific evapora- tion/dispersion. Strategies include optimizing the formulation of tion enthalpy of about 2250 kJ per kg , ammonia of 1369 kJ kg-1 the suspension, adding appropriate stabilizers, or, in some cases, and ethanol of 846 kJ kg-1 . Typically, these theoretic minima are keeping the dispersion in the feed vessel under constant agitation not met (with excess in the range of 50 and 150%) which renders to facilitate the mixing of ingredients and prevent any sedimen- spray-drying one of the costliest and by energy demand environ- tation or separation, or even continuously preparing the suspen- mentally very challenging drying processes. Another challenge sion just before atomization.[97,106] is the typically low temperature of the exhaust (i.e., moist) gas Mechanical stability is very often a desired property, either which is hard to integrate into other process units. To achieve for intermediate processing or as a final product quality indi- a more sustainable and environmentally friendlier process, typi- cator in specific applications such as dental restoration. In the cally: 1) high gas (outlet) temperatures or 2) high solids contents latter, stable spray-dried nanoparticle clusters made up of mul- in the initial droplets are sought. In case (1), the low moisture tiple nanofillers are employed in high-performance dental resin loading and high temperature allows for easier integration in composites.[107] Mechanical stability can in principle be achieved other process steps, but results in low efficiency in the use of the in various ways, for example, by designed slurry properties (e.g., drying capacity of the gas; in case (2), earlier crust formation may size and shape distribution of primary particles, surface charge set in, leading to either hollow particles or fractured particles, if distribution, tailoring particle–particle interaction[108] and par- the remaining liquid is trapped and the generated vapor fractures ticle microstructure[109] ), by the use of additives (e.g., clays in the formed shell. Huge gains in energy efficiency can be obtained porcelain manufacturing[110] to improve bonding, plasticity, and by: 3) switching to different solvents with lower specific evapora- deformability), or by chemical reactions during drying that im- tion enthalpy (if permissible), 4) consequent recovery of the latent prove bonding of suspended primary particles. In general, while heat from the outlet gas. Especially the last option is very promis- smaller primary particles yield a higher deformation resistance in ing, if instead of a carrier gas the superheated vapor is used for the structures formed by spray-drying, fracture stress and strain spray-drying. In that case, the liquid can be removed easily from of the resulting supraparticles depend directly on their diame- the outlet gas by condensation. The use of superheated steam ter. If highly mechanically stable primary particles (e.g., silica) has been considered, e.g., in the food and dairy industry.[105] A are used, the spray-drying process leads to less stable supraparti- fundamental understanding of the high temperatures on drying cles as less energy is needed to break the weaker inter-particle kinetics and achievable particle properties is still lacking. Also, a bonds than to deform the primary particles.[111] Another opera- balance between the competing interests of product quality, pro- tional dimension to improve mechanical stability is by selection cess efficiency, and environmental impact, still needs to be found. of drying conditions, i.e., to avoid too fast evaporation rates that Another challenge in spray-drying is that precursor solu- either yield hollow particles with thin solid layer or avoid exces- tions/dispersions can be complex mixtures comprising a myr- sive pressure generation during drying leading to deformed or iad of ingredients, including nanoparticles, sol–gel inorganic or cracked particles.[108,112] hybrid polymers, oligomers, organic molecules, metal salts, and Regarding the processing of fragile materials, e.g., enzymes, surfactants. The stability of the precursor solution/dispersion be- via spray-drying, the shear stress acting on the droplet, the pro- fore atomization is crucial, as it strongly influences the physi- cess temperature (despite its variability over a wide range) as well cal and chemical properties (e.g., particle size, morphology, crys- as sudden liquid-to-solid interface changes during this process Adv. Mater. 2023, 35, 2306648 2306648 (11 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 7. Example of tailored heterogeneous catalysts: a) mesoporous Au–SiO2 catalyst featuring small (3–4 nm) gold nanoparticles, b) hollow zeolite microspheres in which enzymes can be loaded and trapped to form a chemo-enzymatic catalyst, c) macroporous IrO2 electrocatalysts for proton exchange membrane water electrolyzers. a) Adapted with permission.[73] Copyright 2022, American Chemical Society. Reproduced with permission.[83] Copyright 2019, Royal Society of Chemistry. c) Adapted with permission.[125] Copyright 2018, John Wiley and Sons. represent essential challenges. This is why they require new ap- surface and the utilization of the (often expensive) active ele- proaches or modifications to classical spray-drying. One option, ment. This is classically obtained by impregnation or grafting particularly suited for temperature-sensitive products, is spray- methods. A forthcoming approach is to use reactive aerosol pro- freeze-drying. There, the spray is propelled in a very cold envi- cessing to trap and disperse active metal species in the matrix ronment, e.g., liquid nitrogen, producing solid particles. The sol- of another (porous) oxide. From that point on, depending on vent is then removed by sublimation. First applications have been the type of catalyst that is targeted, two phenomena can be ex- presented in the field of biotics,[113] while the requirements and ploited: mobility or miscibility. First, (noble) metals may have opportunities for the generation of nanostructured particles have a low miscibility in the surrounding oxide, and a simple cal- not been explored for this technique. cination leads to their “exsolution” from the bulk of the mate- rials towards its surface (see Figure 4).[72,79] Once on the sur- face, further migration and sintering can occur, depending on 2.3. Applications of Spray-Dried Structures the severity and duration of the thermal treatment. The proto- cols can be finely tuned to promote mobility and adjust metal In the following, we outline the fields of applications where nanoparticle size (Figure 7a). Interestingly, the fact that the metal we foresee the greatest potential and dynamic development for nanoparticles emerge from the bulk of the carrier—instead of spray-dried materials. We do not attempt to review the current being deposited on top of it—was found to enhance the stabil- state of the art (which for some of the discussed application fields ity of metal nanoparticles during catalytic reactions, for exam- has been reviewed recently[12] ) but rather aim at giving exemplary ple in the dehydrogenation of ethanol to acetaldehyde.[77] This insights and draw a vision of what could become possible in the “metal exsolution” strategy will probably offer new opportunities fields of: I) catalysis, II) diagnostics, III) purification, IV) storage for the preparation of metal-based catalysts, and possibly bimetal- and V) information. lic or multimetallic catalysts. Indeed, synergy between various metals is a common phenomenon in heterogeneous catalysis, when gains are obtained in terms of intrinsic activity, selectiv- 2.3.1. Catalysis ity, or stability when blending two (or more) metals.[116] Classical two-step methods which involve the impregnation or deposition Heterogeneous catalysis is at the heart of a large majority of of the metals on the surface of a preformed support offer lim- chemical processes, and our ability to engineer these functional ited control over nanoparticle formation, (de)alloying, and metal- materials at the nanoscale is considered as key for a transition to- support interactions. Here, starting from a situation where all wards more sustainable horizons in industrial chemistry.[114] Ad- metal species are first dispersed in the support matrix, and then vanced photo-, electro-, and thermal catalysts are exploited in cru- forced to migrate towards the surface to form supported particles cial processes such as hydrogen production, CO2 reduction, and will expectedly provide new opportunities to tune these formu- pollutant degradation. Considering the objective to shift from a lations. Second, it is possible to leverage rapid aerosol-assisted petro-based society toward a bio-based one, it has become evi- sol–gel polycondensation reactions to stabilize homogeneously dent that a new generation of catalysts is needed.[115] While petro- dispersed metal oxide species. In fact, many active sites in het- chemicals are rather apolar and must mainly undergo function- erogeneous catalysts consist of highly dispersed (isolated) metal alization reactions, bio-based chemicals are generally polar and oxide species, while more condensed species (oligomeric, poly- must undergo de-functionalization before entering subsequent meric, or crystalline oxide species) may be catalytically inactive. chemical processes and products. Aerosol-assisted solgel methods have been shown to allow stabi- Spray-drying techniques come in handy to tackle some of the lizing formulations that feature a high degree of dispersion. The existing challenges in heterogeneous catalyst preparation. Het- high proportion of isolated metal oxide species (e.g., Ti sites in se- erogeneous catalysis is a phenomenon that takes place at the lective epoxidation,[117] Mo sites in olefin metathesis,[118] Ga sites surface of a solid that exposes catalytically active sites. Hence, in acetalization,[119] etc.) allowed reaching high activity. An im- in most cases, the idea in catalyst preparation is to disperse an pending approach exploits simultaneously both strategies to pre- active component onto a porous carrier, to maximize the active pare bifunctional catalysts. Recently, a mesoporous Au-Sn-SiO2 Adv. Mater. 2023, 35, 2306648 2306648 (12 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de catalyst was prepared in one step by an aerosol-assisted sol–gel satile and efficient approach to preparing electrocatalysts for ap- synthesis.[120] Upon calcination, isolated Sn oxide species were plications in fuel cells, electrolysis, batteries, and other electro- stabilized in the silica matrix, and small gold nanoparticles were chemical technologies based on catalytic layers. Ideally, the elec- formed at its surface. The catalyst was able to catalyze the oxida- trocatalysts must meet several requirements: high electrical con- tion of glycerol (on Au NP) and the rearrangement of the formed ductivity (percolation of the electron conducting network), high dihydroxyacetone with methanol (on Sn species), to yield the tar- electrocatalytic activity, high porosity, and accessibility for liq- geted methyl lactate. Importantly, the spatial proximity between uid and gas transport. The ideal catalytic material must be fabri- the two catalytic active species of this bifunctional catalyst fa- cated via green, and cost-effective processes, compatible with cur- vored a faster substrate channeling, thereby hampering the for- rent industrial deposition methods, based on catalytic inks.[126] mation of side products and favoring high yield for the final prod- Spray-drying enables the fabrication of particles with control uct. It should be noted that previous attempts to prepare such over the size, morphology, porosity, and high surface area, which bifunctional catalysts using classical preparation methods were can enhance electrocatalytic performance by providing more sur- unsuccessful.[121] face area for electrochemical reactions to occur and by favor- Hybrid materials are also important in heterogeneous catal- ing mass transfer. In addition, spray-drying facilitates the homo- ysis. Tuning the hydrophilicity of the catalyst surface can serve geneous mixing of the precursor materials, resulting in a high to boost catalytic performance, by controlling the adsorption be- and uniform distribution of active sites on the surface of the havior of reactants, intermediates, products, or poisons.[122] Us- electrocatalyst. Most of the current examples of spray-dried elec- ing functionalized (methylated or fluorinated) alkoxides, one-pot trocatalysts concern carbon-based spheres decorated or doped aerosol-assisted sol–gel was shown to provide easy access to vari- with metal catalysts for oxygen evolution reactions (OERs),[127] ous surface-functionalized oxide formulations; this was exploited methanol electro-oxidation[128] or the hydrogen evolution reac- to enhance hydrophobicity and thereby boost reaction rates in or- tion (HER).[129] In a highly promising example, bifunctional elec- ganic reactions that generate water as one of the products.[63] In a trocatalysts composed of doped mesoporous carbon-based micro- similar way, the method is expected to provide a platform for the spheres were made by spray-drying from cost-effective and bio- synthesis of catalysts with tailored molecular active sites. For ex- logical precursors from eggs.[130] For the challenging OER in pro- ample, thiol function can be oxidized to sulfonate, and exploited ton exchange membrane water electrolyzers, only expensive and in acid-catalyzed reactions. Aminated functions can be used to rare Ir-based materials combine high activity and stability.[126] In “click” molecular catalysts. this context, recently several spray-dried porous Ir-based catalysts Finally, the tremendous opportunities offered by enzymes have been fabricated showing outstanding activity and stability, should not be overlooked when it comes to the development as shown in Figure 6c.[125,131] While still in its infancy, spray- of more sustainable chemical processes. Yet, many useful cat- drying represents an emergent tool to design such multifunc- alytic functions are missing in the biocatalytic toolbox, and, to tional electrocatalytic materials. Future perspectives could con- harness the power of enzymes, it is essential to envisage their sist in integrating additional structural features to manage and immobilization,[123] and their use in conjunction with chemo- control bubble formations or additional functions such as self- catalysts and in particular with heterogeneous catalysts.[67] In healing or light or magnetic-responsive materials to boost the this particular field of “hybrid catalysis”, advanced particle de- electrocatalytic activity. sign is essential to bring together components that are es- sentially very different. Processing enzymes directly in spray- drying is challenging because these fragile proteins are prone 2.3.2. Diagnostics (Ex Vivo) to rapid deactivation. Thus, stabilization strategies have to be developed. In this perspective, it was shown that enzymes While we refrain from giving a perspective on in vivo diagnos- can be protected when complexed with polyelectrolytes (e.g., tics (as this lies outside the authors’ expertise and is rarely ad- poly(allylamine hydrochloride)[69] ), or with biopolymers (e.g., dressed with spray-dried materials[132] ), ex vivo diagnostics dis- chitosan[66,124] ), and this allows preparing solid hybrid biocata- plays a broad application field and includes from our point of view lysts. Other stabilization methods, for example using reverse mi- sensing (detection/recording/indication) of chemical substances celles, or crosslinked enzyme aggregates, could become handy (gases, solvents, biomolecules, ions, …) and physical conditions in this perspective. In another approach, spray-drying can be ex- (temperature, humidity, mechanical forces, …). Diagnostics for ploited to prepare hollow microstructures, of which the central these purposes are usually based on a molecular recognition or cavity is exploited to load and trap a controlled amount of en- a physical/chemical change resulting in optical or electrical sig- zyme (Figure 7b). The concept was demonstrated for the com- nal changes of the respective molecule or (nano)material. Spray- bination of glucose oxidase and TS-1 zeolite nanocrystals (for drying of such building blocks, e.g., together with a support ma- which hybridization by adsorption or grafting yielded mediocre trix or templates, offers their size increase for facile handling and results).[83] In a “Lego-like” fashion, starting from any kind of the creation of highly porous hybrid materials. Diverse spray- nano-objects (e.g., clays, zeolites, nanoparticles) and any kind of dried materials have already been proven to serve this purpose;[12] enzymes, we foresee that such an approach will allow the prepa- e.g., as temperature,[5,27,133] gas,[88,89,134] dopamine[135] or me- ration of complex hybrid catalysts featuring various chemo- and chanical sensors,[136,137] to name but a few. As diverse sensing enzymatic active species, of particular interest in cascade chemi- building blocks can be flexibly combined via the spray-drying pro- cal reactions. cess, the creation of materials capable of sensing multiple trig- Spray-drying also has a great potential to serve the current ef- gers at once is conceivable in the future. These may significantly forts towards a sustainable energy transition as it offers a ver- contribute to the development of highly miniaturized sensor Adv. Mater. 2023, 35, 2306648 2306648 (13 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de inants. These composites can exhibit synergic or collective ad- sorption properties that can target the simultaneous removal of multiple contaminants, or that can improve their removal perfor- mance for a specific contaminant. Following this strategy, Liu et al. used spray-drying to produce starch-chitosan hydrogel micro- spheres that can simultaneously adsorb organic dyes (methylene blue and Eosin Y) and several heavy metal ions (Cd2+ , Cu2+ , and Ni2+ ).[139] More recently, the excellent adsorption capacities of porous MOFs (in particular, UiO-66 and UiO-66-(SH)2 ) and inor- ganic nanoparticles (in particular, cerium-oxide NPs) were com- bined to form spherical, microscale bead-like composites that can efficiently and simultaneously remove multiple heavy met- als from water, including As(III and V), Cd(II), Cr(III and VI), Cu(II), Pb(II), and Hg(II) (Figure 8c).[58,140] 2.3.4. Storage Supraparticles have unique properties and characteristics that make them ideal for storage applications. In recent years, ad- vanced rechargeable batteries and supercapacitors with high en- ergy/power density and long cycle life are intensively researched to meet the needs of various energy storage systems.[141] De- spite significant progress in developing new materials in this field (e.g., carbon-based materials, transition metal oxides, al- loys, and metal chalcogenides), several challenges should be ad- Figure 8. a) Schematic illustration of water purification using spray- dressed. One of the main challenges is achieving high electro- drying-derived composite materials, such as b,c) silica@Fe3 O4 (b) and chemical performance. This requires a deep understanding of UiO-66@CeO2 (c). b) Adapted with permission.[138] Copyright 2018, John the underlying physics and chemistry of the superparticles, as Wiley and Sons. c) Adapted with permission.[58] Copyright 2020, American Chemical Society. well as the ability to control their size, shape, composition, and surface chemistry. Another challenge is the limited stability of the supraparticles, particularly in the presence of moisture and high arrays but will also come along with new research challenges re- temperature. This can result in a decrease in their performance garding materials cross-sensitivities, spectral cross-talk, or cross- and efficiency over time. There are several solutions that can ad- leaching of sensitive agents. Future developments in, e.g., bio- dress the current situation. One of the most promising solutions logical sensing molecules will extend the library of available sen- is the use of advanced materials and surface coatings that can sitive building blocks—given their sufficient stability for their enhance the stability, performance, and durability of the supra- spray-drying processing—and may boost the application of spray- particles. For example, the use of carbon-based materials (e.g., dried sensor materials in the current but also in new application graphene, carbon nanotubes) can improve the electrical conduc- fields. tivity and stability of the supraparticles. Similarly, the use of sur- face coatings (e.g., polymers, silica, metal oxides) can protect the supraparticles from moisture and high temperature and enhance 2.3.3. Purification their surface chemistry for better interaction with the electrolyte. Supraparticle-based storage can be not only considered in Water is a finite and non-substitutable resource, which is essen- terms of energy but also in substance storage, such as in the field tial for our life, industry, and agriculture. This is why the man- of agriculture pesticides[142] and fertilizers[143] or in the field of agement of water resources and especially, water treatment and biomedicine drugs.[144] In these fields, the stability of supraparti- purification, are of imminent importance. Water purification is cles (as carriers) during storage and handling is important.[145] the process of removing undesirable chemicals, biological con- However, supraparticles may agglomerate or degrade over time taminants, suspended solids, and gases from water. In this pro- (on purpose or accidentally), which can affect their properties cess, spray-drying can be valuable as a versatile method to design, and performance. On the other hand, the optimization of the produce, shape, and scale up new materials that can efficiently drug-carrier ratio and spray-drying conditions to achieve optimal adsorb and remove contaminants from water (Figure 8a). For substance loading and release behavior is essential. To address instance, simply the superior structural properties (e.g., defined these challenges, for drug delivery applications, the use of natu- porosity) that can be obtained from spray-drying nanoscale build- ral biopolymers (e.g., chitosan, alginate) can improve the stabil- ing blocks are exploitable for water purification, such as the ul- ity and mucoadhesive properties of the supraparticles, which can trafast dye removal from wastewater (Figure 8b).[138] Also, we en- enhance their efficacy. Another solution is the addition of sur- vision the use of spray-drying to create superior adsorbent com- factants or co-solvents to the spray solution, which can enhance posites by mixing two or more materials able to capture contam- the stability of the drug and carrier material, leading to more Adv. Mater. 2023, 35, 2306648 2306648 (14 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de efficient drug loading and release. In the case of fertilizer or pes- environmental stimuli. Taken together, such a supraparticle is ca- ticide delivery systems, the focus also is on natural biopolymers pable of reporting its identity and its history (in terms of experi- (e.g., chitosan or cellulose) that are available at low cost, do not enced environmental triggers).[148] Such particles are envisioned release any toxic substances to the environment, and are fully to be easily applicable as additives to employ arbitrary materials degradable after completion of the delivery process. The use of with intelligence, i.e., to enable perceptual matter.[148,151] This is novel carrier materials (e.g., MOFs, mesoporous silica) can pro- of great interest as it would deliver the bridge to connect the ma- vide high surface area and tunable pore size, which can improve terials with the modern, digital world (Figure 9) and enable track- the loading and release of active ingredients in the supraparticles. ing and tracing of objects, predictive maintenance, recycling, and The significance of encapsulation becomes evident through many more applications.[152] the application of spray-drying in various fields, particularly in To create CSPs, nano and also molecular building blocks the domain of material science and engineering.[146] One area with characteristic physical properties need to be joined. These where encapsulation plays a critical role is in the development building blocks might be very different, for instance, to enable and design of inorganic–organic healing agents for self-healing an optical or magnetic ID, nano and molecular moieties such concrete, offering transformative perspectives for enhancing the as organic dyes, lanthanides, MOFs, quantum dots, nanosilica, durability and resilience of civil infrastructure.[147] The spray- nano-polymer-particles, nano-iron-oxides, and many more are drying enables the creation of micrometer-sized particles that can employed.[6,7,21,137,148] Often, the ID is based on ratiometric and encapsulate healing agents effectively. This encapsulation shields spectral pattern principles that can be designed via a flexible the healing agents from direct contact with the harsh environ- choice of building block combinations and ratios.[6,148,153] This ment (concrete), ensuring their protection and controlled release flexibility in choice is key and can only be guaranteed if the when cracks form. By incorporating inorganic-organic healing process of joining the building blocks comes without too many agents (e.g., minerals and polymers) into the encapsulation pro- restrictions, i.e., this is another example where spray-drying is cess, synergistic benefits can be achieved, where the inorganic superior compared to many other techniques. Furthermore, in components provide mechanical reinforcement, and the organic order to incorporate these intelligent particles into materials, components offer flexibility and self-healing capability. The versa- the processing of CSPs should be possible in a similar man- tility of spray-drying allows for precise control over particle size, ner to standard additives. The fact that spray-drying produces morphology, and composition, resulting in improved compati- a micrometer-sized, free-flowing powder further enhances the bility and durability of the encapsulated agents within the con- benefits of using CSPs. crete matrix. The perspectives of spray-drying in this domain are Designing functional CSPs for all application aspects as men- promising and forward-looking, as ongoing research explores in- tioned above will be a long journey and will require a lot of ef- novative approaches to optimize the encapsulation process, tai- fort to precisely control building block arrangement, structure, lor the release kinetics of healing agents, and enhance the over- and thereby interaction and ultimately cooperative interplay in all performance of self-healing concrete. The utilization of spray- the whole supraparticle to yield the desired interactive function- drying for encapsulation not only underscores the significance alities. To succeed in this endeavor, the precise arrangement of of encapsulation itself, but also opens new avenues for advanc- building blocks upon spray-drying will be crucial and will de- ing the field of self-healing materials, particularly in the area of mand further research—but will be rewarded with a direct ap- concrete technology. Furthermore, the integration of responsive plication perspective as spray-drying ensures that a scalable way polymers, smart materials, and sensor networks can elevate the of manufacturing is right at hand. capabilities of self-healing concrete, creating a dynamic infras- tructure that adapts and repairs in real-time. 3. Conclusion In addition, there are also challenges specific to the spray- drying method that must be considered. One major challenge is In our view, spray-drying is a technique whose potential has controlling the size, shape, and morphology of the supraparti- been underestimated by most (material) chemists today. The sim- cles produced using spray-drying, as well as achieving uniform ple tool has undeservedly been neglected as part of a materials dispersion of the starting materials and/or templates. To address chemist’s laboratory. Clearly, one has to be open to leaving the these challenges, the development of new types of templates or possibly more familiar world of wet chemistry and dealing with solvents to achieve more precise control over particle properties is a transition of wet chemistry to the gas phase to ultimately end highly desirable. By addressing these challenges and perspectives up with a solid phase. related to the spray-drying method, researchers can continue to Also, one has to accept to leave the regime of thermodynamic improve the synthesis of supraparticles using this approach and control and enter the world of fast kinetics—where control is enable further advancements in the fields of storage applications. more difficult to obtain and where deep insights, e.g., regarding formation aspects, are hard to gain. Understanding fully the pro- cesses and mechanisms that occur when doing chemistry in a 2.3.5. Information carriers and providers spray-dryer will still be a long journey and will require the close collaboration of chemists with engineers, theoreticians, model- Recently, the concept of communicating supraparticles (CSPs) ing experts, and physicists. Additionally, it is of utmost impor- was introduced.[148–150] CSPs are supraparticles that are equipped tance to further address the challenges of spray-drying such as with nano building blocks that deliver a unique identification the continuous development of new technological advances in (ID) pattern for each particle. Moreover, the supraparticles are spray-dryers to cover the synthesis of more molecules and ma- equipped with building blocks that are specifically vulnerable to terials using different chemistries. In this way, this could enable Adv. Mater. 2023, 35, 2306648 2306648 (15 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 9. The concept of cuommunicating supraparticles (CSP) to bridge the analog and the digital world. Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[150] Copyright 2023, The Authors, published by Wiley-VCH. the use of spray-drying in fields where it is still underused, for CRC1411 (Deutsche Forschungsgemeinschaft (DFG, German Research example, organic chemistry. Furthermore, it would become pos- Foundation)- Project-ID 416229255 - SFB 1411). sible to use spray-drying in more “extreme” conditions such as Open access funding enabled and organized by Projekt DEAL. spray-freeze-drying, spray pyrolysis, or even in coupled processes with other stimulations such as radiations, light, magnetic fields, or reactive gases. Conflict of Interest But progress in this field is considered as highly rewarding, as The authors declare no conflict of interest. spray-drying enables fast, easy, and simple creation of very new multifunctional, hybrid material types and classes, considering that any combinations of nanoparticles or molecules and in situ Keywords chemistry (“chemistry in a droplet”) is in principle possible— often independently of the original chemistry or classical chem- chemistry in a droplet, functional materials, hybrid materials, nanoparticle assembly, spray-drying, structure formation, supraparticles ical reaction-constraints. In particular, we expect many high- performance materials in diverse application fields in the near Received: July 7, 2023 future that will attract the attention of materials chemists more Revised: August 30, 2023 and more towards spray-drying. Published online: October 15, 2023 Acknowledgements S.W. and K.M. gratefully acknowledge funding by the NanoMatFutur [1] a) E. Piccinini, D. Pallarola, F. Battaglini, O. Azzaroni, Mol. Syst. Des. grant 03XP0149. K.M. and A.B. acknowledge funding by the project Eng. 2016, 1, 155; b) Y. Xia, T. D. Nguyen, M. Yang, B. Lee, A. Santos, Adv. Mater. 2023, 35, 2306648 2306648 (16 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. 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Pigliacelli, P. Sánchez Moreno, Reichstein, P. Groppe, A. Wolf, B. Fett, H. Zhou, R. Pujales-Paradela, M. B. Violatto, L. Talamini, I. Tirotta, R. Piccirillo, M. Zucchetti, F. Miller, S. Müssig, S. Wenderoth, K. Mandel, Adv. Funct. Mater. L. Morosi, R. Frapolli, G. Candiani, P. Bigini, P. Metrangolo, F. 2021, 31, 2011089. Baldelli Bombelli, ACS Nano 2017, 11, 9413; c) D. Li, L. Sun, [153] a) S. Müssig, J. Reichstein, F. Miller, K. Mandel, Small 2022, 18, Y. Zhang, M. Yu, J. Guo, C. Wang, Mater. Chem. Front. 2017, 1, e2107511; b) F. Miller, S. Wintzheimer, J. Prieschl, V. Strauss, K. 521. Mandel, Adv. Opt. Mater. 2021, 9, 2001972. Susanne Wintzheimer has been a junior principal investigator in inorganic chemistry at the Friedrich- Alexander-Universität Erlangen-Nürnberg (FAU), Germany since 2020. Since 2021, she has addition- ally been the scientific head of the Particle Technology Group at the Fraunhofer Institute for Silicate Research ISC in Würzburg, Germany. She holds a Ph.D. in chemistry (graduated: 2017) from the Uni- versity of Bordeaux, France. Her research not only focuses on the development of supraparticle-based materials such as coatings or millimeter-scaled beads but also on the combination of catalytically active and supportive materials within supraparticles for heterogeneous catalysis. To date, she has published over 35 articles. Leoni Luthardt completed her Master of Science in chemistry at the Friedrich-Alexander-University Erlangen-Nürnberg (FAU) in 2022. During her Master’s thesis, she worked on the design and charac- terization of luminescent temperature indicator supraparticles with a tunable two-step color change. She started her Ph.D. in February 2023 at FAU in the field of inductively heatable nano- and supraparti- cles and their versatile applications. Adv. Mater. 2023, 35, 2306648 2306648 (20 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Kiet Le Anh Cao received his B.Sc. from the University of Science, Vietnam National University, Ho Chi Minh City in 2014 and his M.Sc. from Hanyang University, Korea in 2018. He obtained his Ph.D. degree in chemical engineering from Hiroshima University in 2021. He is currently working as a Japan Society for the Promotion of Science (JSPS) Postdoctoral Fellow in the Chemical Engineering Program, Grad- uate School of Advanced Science and Engineering, Hiroshima University, Japan. His research focuses on the rational design and development of innovative nanostructured materials for applications in energy storage and carbon capture/conversion. Inhar Imaz received his Ph.D. in materials science from the Université Bordeaux I in 2005, where he studied the formation of heterometallic metal–organic architectures from tetrahedral building blocks. He joined the CIN-2 (ICN-CSIC) center in 2005 as a postdoctoral fellow. At present, he is a senior re- searcher at the Catalan Institute of Nanoscience and Nanotechnology (ICN2). His research interests are focused on controlling the supramolecular assembly of molecules, metal ions, and nanoscale building blocks at the macro and nanoscale for the design of novel metal–organic frameworks and functional supramolecular architectures with interesting properties and applications in gas storage, and catalysis. Daniel Maspoch is an ICREA research professor at the Institut Català de Nanociència i Nanotecnolo- gia (ICN2). He received his B.S. degree at the Universitat de Girona and his Ph.D. degree at the Uni- versitat Autònoma de Barcelona & Institut de Ciència de Materials de Barcelona. He worked as a post- doctoral fellow at Northwestern University. His research interests include reticular materials (MOFs, COFs, and MOPs) and delivery systems. Takashi Ogi is a professor in the Chemical Engineering Program, Graduate School of Advanced Sci- ence and Engineering, Hiroshima University. He received his Ph.D. (engineering) (2008) in chemical engineering at Hiroshima University. His research concerns nanostructured particles and their appli- cations as well as aerosol science and technology research field. He has over 150 scientific SCI papers, 40 review papers, 20 book chapters, and more than 20 patents. He is currently on the editorial board of Advanced Powder Technology and Journal of Nanoparticle Research. Adv. Mater. 2023, 35, 2306648 2306648 (21 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202306648 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Andreas Bück is a professor for Particle Technology at Friedrich-Alexander Universität Erlangen- Nürnberg, Germany, since 2017. He holds a Ph.D. in chemical engineering (graduated: 2012) from Otto von Guericke University Magdeburg, Germany. His research interests focus on particle formu- lation processes involving drying, e.g., spray-drying, coating, and agglomeration, combining experi- mental and numerical methods towards tailored processes and products. To date, he has published more than 100 peer-reviewed papers. Damien P. Debecker is a professor at the Faculty of Bio-Science Engineering of UCLouvain (Belgium), and is leading a research group at the Institute of Condensed Matter and Nanosciences since 2012. His research focuses on developing innovative preparation routes to heterogeneous catalysts and biocatalysts, targeting sustainable catalytic processes: upgrading of bio-based chemicals, greener organic syntheses, CO2 capture and hydrogenation, flow chemistry, etc. He has published about 140 papers. Marco Faustini is a professor at Sorbonne Université in Paris and member of Institut Universitaire de France. He was trained as a materials engineer in Italy and obtained his Ph.D. in physics and chemistry of materials at Université Pierre et Marie Curie (France). His expertise lies in the fabrication of porous materials for environment- and energy-related applications by soft-chemistry and self-assembly. He co-authored more than 70 articles and 11 patents, has been awarded an ERC Starting Grant and has received several awards such as the CNRS Bronze Medal and D. Ulrich award from the International Sol-Gel Society. Karl Mandel has been a professor for inorganic chemistry at the Friedrich-Alexander University Erlangen-Nürnberg, Germany since 2020, and head of the Particle Technology Group at the Fraun- hofer Institute for Silicate Research ISC in Würzburg, Germany since 2014. He holds a Ph.D. in chem- istry (graduated: 2013) from the Julius-Maximilians University Würzburg, Germany. His research in- terest is in creating supraparticles—mostly via spray-drying— to find new and unexpected properties and employing them as smart objects to contribute to sustainability. To date, he has published about 100 papers. Adv. Mater. 2023, 35, 2306648 2306648 (22 of 22) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH