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ERROR: type should be string, got "https://doi.org/10.1007/s00396-020-04629-0\nColloid and Polymer Science (2020) 298:921–935 https://doi.org/10.1007/s00396-020-04629-0\nColloid and Polymer Science (2020) 298:921–935 INVITED ARTICLE Abstract Inspired by the path followed by Matthias Ballauff over the past 20 years, the development of thermosensitive core-shell microgel\nstructures is reviewed. Different chemical structures, from hard nanoparticle cores to double stimuli-responsive microgels have\nbeen devised and successfully implemented by many different groups. Some of the rich variety of these systems is presented, as\nwell as some recent progress in structural analysis of such microstructures by small-angle scattering of neutrons or X-rays,\nincluding modelling approaches. In the last part, again following early work by the group of Matthias Ballauff, applications with\nparticular emphasis on incorporation of catalytic nanoparticles inside core-shell structures—stabilising the nanoparticles and\ngranting external control over activity—will be discussed, as well as core-shell microgels at interfaces. Keywords Microgel . Small-angle scattering . SANS . Catalysis . Responsive interfaces Introduction emulsions [11–14], nanoreactors, or smart carriers for catalyt-\nically active nanoparticles [15, 16]. Since their first synthesis in 1986 by Pelton and Chibante [1],\nso-called smart microgels are subject of a steadily increasing\nnumber of studies, and in the period from 2018 to 2019, only\non the topic of N-isopropylacrylamide (NIPAM), which is still\nthe most studied responsive polymer, about 1600 publications\ncan be found in the web of science. A lot of these publications\ndeal with smart microgels. Microgels are fascinating materials\nbecause they are able to respond to changes of external pa-\nrameters like temperature, pH, ionic strength, or external fields\nby changing their state of swelling [2–4]. This makes them\ninteresting for applications in sensors [5], optics, and colloidal\ncrystals [6–8]. Moreover, they are also potentially useful in\ndrug delivery [9], as smart surface coatings, actuators in bio-\nmedical devices [10], stabilisers in switchable Pickering Besides NIPAM, other acrylamide monomers can be used to\nmake microgels which also exhibit a so-called volume phase\ntransition temperature (VPTT). The most prominent ones are\nN-vinylcaprolactam (VCL), N-isopropylmethacrylamide\n(NIPMAM), and N,n-propylacrylamide (NNPAM) [17, 18]. However, also some other monomers appear in the literature\nfrom time to time. The main difference between the respective\nmicrogels is their different VPTT. More details about copoly-\nmer microgels can be found in a review by one of us [19]. Copolymerisation of these monomers can be used to systemat-\nically vary the transition temperature. Moreover,\ncopolymerisation with acrylic or other organic acids can be\nused to tune and enhance the properties of these colloidal gels\n[20–22]. In this area, Matthias Ballauff’s group started to con-\ntribute at the end of the 1990s of the last century [23]. At about\nthe same time, Matthias’ group also started to work on core-\nshell microgels [24]. Increasingly complex microgel architec-\ntures like core-shell structures are of growing relevance, since\nthey allow to create improved properties and additional func-\ntions like, e.g. response to several stimuli, or a particular swell-\ning behaviour. Starting from pioneering works by M. Ballauff\net al. [25–27], this approach gave rise to increased interest in\nmaking nanoparticle-microgel (NP-MG) hybrids for different\napplications in optics [28, 29] and catalysis [27]. Recent advances in stimuli-responsive core-shell microgel particles:\nsynthesis, characterisation, and applications Julian Oberdisse1 & Thomas Hellweg2 Received: 15 January 2020 /Revised: 25 February 2020 /Accepted: 25 February 2020\n# The Author(s) 2020\n/ Published online: 26 March 2020 * Thomas Hellweg\nthomas.hellweg@uni-bielefeld.de 1\nLaboratoire Charles Coulomb (L2C), University of Montpellier,\nCNRS, 34095 Montpellier, France 2\nDepartment of Physical and Biophysical Chemistry, Bielefeld\nUniversity, Universitätsstr. 25, 33615 Bielefeld, Germany Introduction At the\nend, we will conclude and give perspectives for possible fu-\nture developments. While M. Ballauff’s group continued exploiting the particu-\nlar PS core/microgel shell system as tunable carrier for catalytic\napplications as discussed in “Applications of core-shell\nmicrogels in catalysis”, many other nanoparticle core/\nthermosensitive shell systems have been created since then. The common approach is usually to introduce a surface modi-\nfication of the NPs, typically by hydrophobising silane cou-\npling agents like methacryloxypropyl-trimethoxysilane (MPS)\noffering the possibility to start the polymerisation of NIPAM or\nsimilar monomers from the surface [39]. This technique is also\nsuitable for silica or silica-coated metallic nanoparticles [39]. For example, Nun et al. studied the hydrodynamic radius of a\nsilica core/p(NIPAM) shell particle in a recent article [40], with\na focus on the cross-linking density. Their surprising finding is\nthat the size evolution of the NP hybrid with temperature may\nbe non-monotonic, presumably due to anisotropic heterogene-\nities in cross-linking. A similar silane-grafting technology has\nbeen used to formulate temperature-sensitive carbon core/\nmicrogel shell particles [41]. When direct polymer-grafting\nfrom the particles was not feasible, in some cases, silica layers\nwere formed first, followed by silane surface modification and\npolymerisation. This has been the case, e.g. for magnetic nano-\nparticles. Haematite [42] or maghemite [43] particles have been\ncovered by silica, the surface of which was then used to grow\nthe cross-linked p(NIPAM) layer. Due to the anisotropy of these\nparticles, the alignment of such systems has been studied in\nexternal fields [42]. Moreover, the aspect ratio of the particles\nis then found to be temperature-dependent, due to the isotropic\nswelling of the p(NIPAM) layer. The same pathway of grafting\non a silica layer was followed with non-NIPAM polymers and\nother magnetic particles, like, e.g. manganite [44]. Back in M. Ballauff’s group, a yolk-type encapsulation of gold NPs was\ngenerated by first growing a silica layer on gold cores, then\npolymerising the cross-linked p(NIPAM) layer, before finally\netching the silica layer. This technique thus led to individual\ngold cores freely “swimming” inside a microgel shell, without\nbeing covalently connected to it. The consequence is great col-\nloidal stability of the latter, with tuning of optical and catalytic\nproperties by the microgel swelling [45, 46]. A thorough scat-\ntering study of amine surface-modified gold cores with a\np(NIPAM) shell was proposed by Dulle et al. [28]. Introduction At about the\nsame time at the beginning of the century, doubly This manuscript is dedicated to Matthias Ballauff on the occasion of his\ntransition to the state of a very active and busy senior-professor in 2019. * Thomas Hellweg\nthomas.hellweg@uni-bielefeld.de * Thomas Hellweg\nthomas.hellweg@uni-bielefeld.de 922 Colloid Polym Sci (2020) 298:921–935 thermosensitive core-shell microgels (MG-MG) have also been\ndeveloped [30, 31]. Both NP-MG and MG-MG particles will\nbe reviewed in this article, with a particular emphasis on their\nstructural properties as seen by small-angle scattering. dependent thickness of the grafted p(NIPAM) layer both di-\nrectly in the TEM pictures, and its effect via the resulting\nrepulsion between PS cores [35]. In an earlier article, the scat-\ntering model describing such core-shell structures, based on a\nhomogeneous core surrounded by a homogeneous shell, with\nadditional terms for network fluctuations and heterogeneities,\nhas been used to compare the thermal swelling properties of\nsuch core-shell particles to the ones of macrogels [24]. This\nmodel system continuously attracted interest of scientists in\nthe field, including variations of the chemical protocol deter-\nmining the type of core-shell morphology [36–38]. A recent review has reported on the evolution of the last\n3 years, in the field of microgel synthesis, topology, biologi-\ncal applications, drug delivery, microgel packing, and theo-\nretical approaches [4]. Another review focusing specifically\noncore-shellmicrogelsdatesbackto2013[32],whileSuzuki\nand colleagues have proposed a focus review of their many\ncontributions, including heterogeneous structures like Janus\nmicrogels and microgel nanocomposites, which may have a\ncore-shell structure and will be further discussed below [33]. Here, we adopt a different point of view, starting with the\npioneeringcontributionsatthebeginningofthecentury,more\norlessfollowingtheguidanceofM.Ballauff’swork.Wethen\npropose a special focus on recent progress in understanding\npolymer-based core-shell structures stemming from scatter-\ning methods. The article is thus organised as follows. In “The\nroute to stimuli-responsive multi-domain structures”, we\nsummarise different approaches to system developments of\ncore-shell and multi-domain structures, with various organic\nor inorganic nanoparticles, up to pure polymer core-shell\nmicrogels. The section “Internal architecture of core-shell\nmicrogels” is devoted to novel insights into the structure\nandpropertiesofcore-shellmicrogels,withthedifferentscat-\ntering models developed to describe isotropic but non homo-\ngeneous structures. This is followed by “Applications of\ncore-shell microgels in catalysis” dealing with potential ap-\nplications, in particular catalysis with embedded nanoparti-\ncles, of controllable activity via the swelling state of the\nmicrogel. “Core-shell microgels at surfaces and interfaces”\ndeals briefly with core-shell microgels at interfaces. Introduction By using\na combination of approaches, these authors characterised both The route to stimuli-responsive multi-domain\nstructures With relevance to the discussion in “Internal architecture of\ncore-shell microgels”, it is noteworthy that they have used a\ndeconvolution technique for spherical symmetries of the corre-\nlation function determined by indirect Fourier transform. y\nThe technique of removing the silica at high NaOH con-\ncentrations was also used to produce hollow microgel particles\n[47], which have more recently triggered quite some interest\nin their physical properties, in terms of individual structure,\nand compression in dense assemblies. Dubbert et al. have\nreported on a detailed small-angle neutron scattering\n(SANS) modelling study of the hollow cavity [48]. The void\nis found to be smaller than the original sacrificial silica core,\nand it is big for highly cross-linked, stiff shells (but then less\nthermosensitive), whereas it becomes rather small (albeit more\nthermosensitive) in the opposite case. The authors conclude\nthat a compromise has to be sought between both properties. In further studies, they added a second outer layer, and thus\nreach multilayer particles, either core-shell-shell, or hollow-\nshell-shell (after etching) [49, 50]. Silica-p(NIPAM)-\np(NIPMAM) and void-p(NIPAM)-p(NIPMAM) are thus dou-\nble thermoresponsive. In the presence of the core particle,\nswelling is restricted by the grafting via MPS onto the silica. After etching of the core, both shells can adapt more freely to\ntemperature changes, and a clear two-step variation is found. These authors underline the interest of the shell-shell geome-\ntry: while the outer shell governs the interaction with other\nparticles and stability, the state of the inner one may influence\nthe material encapsulated in the centre of the microgel. The\nstudy of the compression of hollow microgel particles embed-\nded in dense suspensions of homogeneous microgels has been\nproposed by Scotti et al. [51]. Using SANS and contrast\nmatching of the regular microgel particles, they managed to\nspecifically observe the response of the hollow ones to pres-\nsure by the neighbouring regular particles of same elastic\nproperties. The intriguing linear swelling curves motivated structural\nstudies in our groups [59–61]. In the framework of the PhD\nthesis of Marian Cors, the detailed temperature-dependent\nstructure of the core and the shell polymers has been studied\nby SANS using isotopic labelling. The corresponding model\nand the main results will be outlined in “Internal architecture\nof core-shell microgels”. The route to stimuli-responsive multi-domain\nstructures One of the early pioneers in this field was Matthias Ballauff. In absence of photoinitiators, the seeded precipitation poly-\nmerisation of NIPAM on a non-thermosensitive nanoparticle\n(NP) surface allowed growing and cross-linking the NIPAM\nchains, with however imperfections in the grafting of the\nmicrogel on the NP [24, 34]. He and his colleagues then trans-\nferred a well-established method for the photochemical syn-\nthesis of spherical polyelectrolyte brushes to the synthesis of\ncore-shell microgels predominantly with a polystyrene (PS)\ncore and a responsive and cross-linked poly(NIPAM) shell\n[26]. This method guaranteed grafting from the core particle. The resulting dense PS core is highly visible in electron mi-\ncroscopy, and the authors could see the temperature- 923 Colloid Polym Sci (2020) 298:921–935 peculiarities of such a system was pointed out early by Lyon\net al. [54]: the shell and the core exert forces on each other, and\nunequal volume phase transition temperatures imply the trans-\nmission of stresses at the interface, and thus impact of the\nswelling of the core on the shell and vice versa. By aiming\nat a higher difference in swelling temperature combining\np(NIPMAM) with p(NNPAM), Zeiser et al. opened the road\nto an unexpected phenomenon, linear swelling [55]. Indeed,\nthe swelling curves measured through the hydrodynamic ra-\ndius show a peculiar linear domain, where the radius does not\nchange abruptly, but continuously, over a large temperature\nrange located between the two transition temperatures, of the\ncore and of the shell. As shown in Fig. 1a, this behaviour is\nrobust: normalising the hydrodynamic radius in suspension or\nthe ellipsometric thickness when adsorbed to a surface repro-\nduces the same linear swelling at intermediate temperatures\n(see also “Core-shell microgels at surfaces and interfaces”). One may note that this is only the case when the transition\ntemperature of the shell is below the one of the core—in the\ninverse case, a two-step transition is found [30]. Incidentally,\nthis remains true in other solvents, as shown by Lee and Bae\nfor a p(MMA-HEMA) in 1-propanol [56]. For comparison,\nwe have also reproduced in Fig. 1b the more classical two-step\nbehaviour observed in a system where the VPTTof the shell is\nabove the one of the core. the density profile and the internal network structure as a func-\ntion of different shelling steps increasing the shell thickness. The route to stimuli-responsive multi-domain\nstructures Besides being a force of proposition of general core-shell\nstructures, Matthias’ group also pioneered the use of NP core/\nMG shell particles as carriers for nanoparticles, in particular of\npalladium, platinum, silver, and gold [25, 62–65]. His group\naccomplished to show that core-shell particles are able not\nonly to stabilise the catalytically active particles but also to\nallow to control their catalytic activity—this will be reviewed\nin “Applications of core-shell microgels in catalysis” below. However, it is worth noting in the present section on synthesis\nthat a variety of systems has been designed where nanoparti-\ncles are generated within the pre-existing core-shell microgel\ncarrier system serving also as a nanoreactor and template. The\nmicrogel is thus created beforehand, and NPs are embedded—\non the contrary to the above discussion the NPs do not form\nthe core, and depending on the chemistry, their exact\nlocalisation may be tuned. With respect to NP-MG particles, core-shell microgels\nconsisting of both a microgel core and microgel shell [52]\nintroduce a new functionality to core-shell systems: now the\ncore also undergoes a volume phase transition, and by con-\nveying different thermal properties to the core and the shell,\ne.g. by introducing acrylic acid monomers, tunable swelling\nproperties have been achieved [53]. The step to combinations\nof acrylamides in the core and the shell was quickly mastered\nby Berndt and Richtering, who studied the swelling curves of\np(NIPAM) and p(NIPMAM) with LCSTs of 34 and 44 °C,\nused either as cores or as shells [30]. Of course, the difference\nin transition temperatures triggers new effects, the presence\nand magnitude of which however depends on the relative\nthickness of the core and shell, as well as on their strength,\ni.e. the degree of cross-linking. Berndt et al. have presented a\nSANS study of these effects in 2006 [31]. One of the Early work by Suzuki [66, 67] reported on gold NPs being\ngenerated in situ close to the core surface and eventually\nforming a gold layer of AuNPs using modified p(NIPAM-\nglycidyl methacrylate) copolymer microgels. These microgels\ncomprise epoxy groups in the core which can be subsequently Colloid Polym Sci (2020) 298:921–935 924 presence of hydrophobic domains of SDS, the particles lost\nthermosensitivity. When performing the polymerisation of\nstyrene in the swollen state (under appropriate pH conditions),\nthe PS particles are found to form a non-compact surface layer. Internal architecture of core-shell microgels The tuning of the localisation of in-microgel generated\nnanoparticles has been promoted by Suzuki’s group in the past\nyears to design new types of thermoresponsive particles, with\nthermoresponsive microgels of poly(N-isopropylacrylamide)\ncross-linked with N,N′-methylene-bis-acrylamide as cores or\nseeds for the polymerisation which are surrounded by a hard\npolystyrene shell [72–74]. Following a seeded emulsion pro-\ntocol in the presence of p(NIPAM) microgel particles, the PS\nparticles where found to form raspberry-like structures under\nhigh temperature, surfactant-free polymerisation, with howev-\ner some remaining thermosensitivity, presumably due to in-\ncomplete coverage by PS [72]. An example of TEM pictures\nillustrating such a MG particle core-shell structure is shown in\nFig. 2, for different mole percentages of styrene in the feed,\nand the SDS concentration given in the sample codes. In In a first approximation, most microgel particles are globular,\nand are conveniently described as spheres. Photon correlation\nspectroscopy (PCS) gives the hydrodynamic radius of these\nspheres, whereas small-angle scattering measures the radius\nof gyration, or the corresponding equivalent hard sphere radius. Comparison of both immediately shows that the microgel par-\nticles are in general not homogeneous hard spheres, but possess\na temperature-dependent gradient profile, with, e.g. dangling\nchains at the surface. In scattering, this is immediately visible\nin the high-q range, where deviations from a Porod q−4-law are\neasily measured. Indeed, a progressive radial decrease of den-\nsity may be caused by different monomer and cross-linker re-\nactivities: microgels are thus spatially inhomogeneous and pos-\nsess a fuzzy surface [75], unless cross-linker is added Fig. 1 Swelling curves of core-shell microgel particles. (a) p(NIPMAM) core\nwith p(NNPAM) shell in H2O. Comparison between particles in bulk (PCS)\nand on surfaces (ellipsometry), normalised to the high-temperature average of\neither the hydrodynamic radius or the ellipsometric thickness, for core cross-\nlinker contents of 10%mol. (b) p(NIPAM) core with p(NIPMAM) shell in\nD2O. Panel (a) reprinted with permission from reference [57]. Copyright\n2017 American Chemical Society. Panel (b) reprinted with permission from\nreference [58]. Copyright 2005 American Chemical Society linker contents of 10%mol. (b) p(NIPAM) core with p(NIPMAM) shell in\nD2O. Panel (a) reprinted with permission from reference [57]. Copyright\n2017 American Chemical Society. Panel (b) reprinted with permission from\nreference [58]. Copyright 2005 American Chemical Society Fig. 1 Swelling curves of core-shell microgel particles. (a) p(NIPMAM) core\nwith p(NNPAM) shell in H2O. The route to stimuli-responsive multi-domain\nstructures Under pH-induced deswelling, this layer does not become\nmore compact, but NPs seem to diffuse inside the microgel\n[73]. Finally, in a very recent contribution on the same subject,\nthe Suzuki group presented seeded polymerisation of styrene\nin presence of microgels below the volume phase transition\ntemperature [74]. While raspberry-like structures are again\nfound, the authors point out issues of colloidal stability of\nthe microgels which are related to electrostatic repulsion,\nwhich in turn also governs the location of the polystyrene\nseeds. Last but not least, noteworthy cryo-electron tomogra-\nphy images (and movies) allow understanding of the three-\ndimensional arrangement of the polystyrene particles inside\nthe microgels. modified by reaction with 2-aminoethane thiol to introduce\nSH and also NH2 groups [66, 67]. Brändel et al. optimised\ncatalytic response of silver NPs also located around the core in\ncore-shell microgels with linear thermoresponse, the structure\nof which will be further discussed below [68]. Suzuki and\nKawaguchi also achieved high loadings of in situ synthesised\nmagnetic particles in the spherical microgel core using again\ncopolymerisation of glycidyl methacrylate to introduce\nimmobilising functions [69], whereas Xu et al. provided an\nexample of a cylindrical core-shell structure loaded with mag-\nnetite particles stabilised by the shell. They thus gained an-\nisotropy on the mesoscale, which can moreover be oriented\nwith low magnetic fields [70]. By localising beta-diketone groups capable of complexing\nand reducing aureate ions in the core of core-shell microgel\nparticles, Thies et al. managed to grow individual gold NPs in\neach core [71]. Moreover, they used these particles as a seed for\nfurther growth, with anisotropic rod-like shapes at high ionic\nconcentrations, and avoiding the formation of secondary gold\nnuclei. Internal architecture of core-shell microgels Comparison between particles in bulk (PCS)\nand on surfaces (ellipsometry), normalised to the high-temperature average of\neither the hydrodynamic radius or the ellipsometric thickness, for core cross- linker contents of 10%mol. (b) p(NIPAM) core with p(NIPMAM) shell in\nD2O. Panel (a) reprinted with permission from reference [57]. Copyright\n2017 American Chemical Society. Panel (b) reprinted with permission from\nreference [58]. Copyright 2005 American Chemical Society Colloid Polym Sci (2020) 298:921–935 925 Fig. 2 Electron micrographs. a, b\nNIPAM-styrene composites\nNS200 (0.5), c, d NS200 (6.5),\nand dried on solid substrates at\n25 °C. The first number\nrepresents the styrene\nconcentration in mmol in the feed,\nthe second one the SDS\nconcentration. (a, c) FE-SEM\nimages. Scale bars are 1 μm. (b,\nd) TEM images, scale bars\nrepresent 300 nm. Reprinted with\npermission from reference [72]. Copyright 2014 American\nChemical Society continuously to reach a more even distribution [76]. In the first\npart of this article, “multi-domain” thermosensitive microgel\nstructures have been reviewed following the original pathway\nof Matthias Ballauff. Some of them are well described by a\ncore-shell structure, or, more general, a non-monotonous den-\nsity profile. As described in “The route to stimuli-responsive\nmulti-domain structures”, a particular class of core-shell\nmicrogels was obtained by the combination of largely different\nVPTTs, where the core swells first when decreasing the tem-\nperature [55]. The outstanding property of such core-shell to-\npologies [32] is the linear evolution of their radius as a function\nof temperature between the two polymer phase transition tem-\nperatures as shown in Fig. 1a [55, 57, 77]. This linear response\nmay be favourable in particular for sensors and actuators, but it\nremains unclear how this property is related to the structure of\nthe shell. A possible explanation of the limited core swelling\nbetween the two VPTTs has been proposed and has been called\nthe “corset effect” [55]. This concept claims that the shell em-\nbedding the core is not swollen at the same temperature, thus\nrestricting the volume phase transition of the core. It is unclear,\nhowever, how well this structural picture of a well-defined shell\nrestricting the core is appropriate. It is the aim of the present\nsection to review the corresponding small-angle scattering and\nmodelling approach. suited, and it is particularly useful for particles suspended in\nsolvent at various temperatures. Internal architecture of core-shell microgels This technique is capable of\nsimultaneously measuring both average particle sizes and poly-\ndispersity, as well as the local structure of macromolecules. SANS and isotopic substitution may reveal the distribution of\nmonomers within particles, as for example in core-shell\nmicrogels having one monomer deuterated, which is the ultimate\ngoal of our approach extracting radial density profiles based on a\nform-free model. In this section, we will discuss modelling\napproaches to small-angle scattering from such systems. Contrast matching through the solvent and employing isotopic\nsubstitution of either shell or core monomers enables studying\nthe monomers selectively in SANS, and thereby gives\naccess to independent measurements of their monomer\ndensity profiles. One should note that deuteration causes small\ndifferences in swelling, as published by us for p(NIPMAM) [60],\nbut its impact remains negligible far from the VPTT. We add that\nalthough small-angle scattering is not the only technique to ob-\nserve microgels [78–80], it is indeed very powerful due to the\npossibilities of deuteration allowing differentiating core and shell\npolymer, in bulk suspensions, without fluorescence labelling. Core-shell models for small-angle scattering The common point of the various fuzzy sphere models\nproposed in the literature discussed above is that predefined\nparameters fix and limit the possibilities for unexpected struc-\ntures. For example, a given number of shells may be used as a\nstarting point, with possible variation of thickness and posi-\ntion of the interfaces. In some cases, unforeseen fit results may\nmotivate an extension of the model, e.g. by adding a shell. In\nsome occasions, the choice of a given model has been guided\nby a first form-free modelling [85]. In others, the form-free\nmethod proved to be essential to discover an unusual density\ndistribution—microgels less dense in the centre—with\nultralowly cross-linked particles [86]. “Form-free” modelling\nis obtained by the construction of any general profile based on\nthe densities in (thin) concentric shells. The intensity can thus\nbe calculated easily by analytical Fourier transformation. Such\nmodels have also been applied to copolymer micelles [87]. In\nVirtanen et al. and also in our approach outlined below, the\nmethod has further been extended to include particle polydis-\npersity [86]. In the past, such approaches have been applied successfully\nto describe the structure of core-shell microgel particles inves-\ntigated by small-angle neutron scattering. The resulting scat-\ntering curves were analysed with different core-shell models\nbased on the above-mentioned concepts [30, 31, 58, 82]. In\nthese experiments, the absence of isotopic substitution did not\nallow differentiation of the core and shell monomers. Such\nmeasurements and analyses showed the presence of both\nmonomers, and appeared to endorse the intuition that the shell\nis located just outside the core—we will see below that de-\npending on the system, this is not necessarily the complete\npicture. The strength of these core-shell models is that they\nare given by analytical formulas of the radial density profiles. It is thus straightforward to Fourier transform them and com-\npare the result with the experimental intensities, and proceed\nwith fitting, needing to fix only a small number of parameters:\nthe location/radii and densities of the core and the shell, and\npossibly interfacial fuzziness [31, 82, 83]. As already\ndiscussed, by adding additional layers, core-shell-shell parti-\ncles have been synthesised. After silica core etching, hollow-\nshell-shell structures have been observed [49, 50]. Core-shell models for small-angle scattering The scattering of isotropic particles is usually described as a\nsum of terms, the first of which is linked to the density profile, The synthesis of complex architectures of microgels thus trig-\ngers the necessity of structural characterisation. SANS is well Colloid Polym Sci (2020) 298:921–935 926 while the others account for chain scattering, as well as for\nheterogeneities and dynamic fluctuations [81]. Usually, a\nLorentzian term is used to describe the internal polymer mesh,\nwith a mesh size extracted from fitting the correlation length,\nbut other polymer scattering terms like Debye functions or\ngeneralized Gaussian coils may be applied empirically [59]. Single compartment microgels (“core-only”) can be modelled\nin a first approximation by sphere scattering—one of the char-\nacteristics of which is the steep high-q Porod law, which can\nbe masked by internal and chain contributions, or by fuzziness\nof the interface, which also smears oscillations, just like poly-\ndispersity. For more complicated geometries, the density\nprofile may be defined piecewise, and constant but\ndistinct densities in the core and in the shell are an obvious\nstarting point to describe core-shell particles. Again, introduc-\ning fuzziness allows a more realistic description of the inter-\nface, and most differences between such fixed-form models\nactually stem from the type of decay describing the interfaces. Often, the decays are described by Gaussians or piecewise\npolynomials, like parabolic decays. By combining piecewise\nparabolic functions, different geometries can be constructed,\nand namely an empty shell can be described easily [31, 82,\n83]. For completeness, let us mention another model based on\nFlory-Rehner theory recently developed by Boon and\nSchurtenberger. In p(NIPAM) microgels, this model gave ac-\ncess to the non-homogeneity of the spatial distribution of\ncross-linker within microgel particles [84]. unravel the structure of hollow microgels and their precursors,\nsilica-microgel core-shell particles. By varying the isotopic\ncomposition of the solvent between light and heavy water,\nthe contributions of the silica core and of the protonated poly-\nmer layer to scattering could be varied via their respective\ncontrast. In particular, Dubbert et al. have succeeded in fitting\na single model discriminating the inorganic silica core from\nthe shell under all contrast situations [48]. Finally, by compar-\ning this result to the one of hollow microgels, the evolution of\nthe shell with the etching of the core could be followed in\ndetail [48]. Core-shell models for small-angle scattering Light scat-\ntering has been discussed qualitatively—these authors conjec-\nture that a generalisation of core-shell models with different\nshells might have been able to quantitatively account for the\nobserved intensity features, in particular for the shift in first\nminima [49]. SANS and detailed modelling has been used to Form-free density profiles of spherically symmetric\nmicrostructures In recent contributions, we have implemented a reverse Monte\nCarlo (RMC) optimisation of general, form-free density pro-\nfiles Φ(r) [59]. Such arbitrary profiles allow the description of\nSANS intensities without a priori parametrisation. The mono-\nmer density profiles evolve stochastically, driven by a χ-square\nminimisation. The aim is to reach agreement between the ex-\nperimental and theoretical scattering functions. The resulting\ndensities have been demonstrated to be robust with respect to\nstarting conditions or other simulation parameters. In a first\narticle, we have confronted the density profiles of the most\nsimple “core-only” microgels to the models of the literature,\nand a performance equivalent to the well-established\n(parabolic) fuzzy sphere model was evidenced. In this ap-\nproach, it was supposed that the microgels are polydisperse as\nmeasured by AFM, and spherically symmetric. The contribu-\ntion of polymer chains is visible in the mid- to high-q range, and\nis successfully described by the generalised coil model [88]. Our aim of understanding the outstanding linear swelling\nproperties of core-shell microgels motivated an in-depth form- 927 Colloid Polym Sci (2020) 298:921–935 free modelling of their internal microstructure. This could be\nachieved with contrast-variation SANS combined with the\ndetermination of the core and shell density profiles. Due to\nthe unexpected geometries, it was important to be able to\nanalyse the data without any a priori assumptions on the shape\nof the density profiles. Our approach combined several mea-\nsurements: at first, a pure p(NIPMAM) core (“core-only”),\nonto which either deuterated or hydrogenated p(NNPAM)\nshells have been grown. It is stressed that all cores are the\nsame in our studies [57, 59, 61, 89], i.e. the second synthesis\nstep of the shell has been performed with pre-existing cores,\nenabling thus direct comparisons. free modelling of their internal microstructure. This could be\nachieved with contrast-variation SANS combined with the\ndetermination of the core and shell density profiles. Due to\nthe unexpected geometries, it was important to be able to\nanalyse the data without any a priori assumptions on the shape\nof the density profiles. Our approach combined several mea-\nsurements: at first, a pure p(NIPMAM) core (“core-only”),\nonto which either deuterated or hydrogenated p(NNPAM)\nshells have been grown. It is stressed that all cores are the\nsame in our studies [57, 59, 61, 89], i.e. the second synthesis\nstep of the shell has been performed with pre-existing cores,\nenabling thus direct comparisons. Form-free density profiles of spherically symmetric\nmicrostructures and experimental intensity is measured by χi\n2. In the inset, the\ninitial guess and the final fit are compared to the experimental\nintensity. In order to find a physically acceptable solution to\nthis ill-posed problem, a compromise based on the L-shaped\n“stability plot” with smoothing term χr\n2 (see [86]) was sought\nby systematically varying the relative importance of fit quality\nand profile smoothness. These functions are also shown in\nFig. 3b, χT\n2 representing the virtual annealing temperature\nallowing progressive freezing of a solution. The result of our\nsimulation is a smooth volume fraction profile of monomers\nleading by Fourier transformation to an intensity prediction\ncompatible with the experimental intensity. We have first investigated the density profiles of the “core-\nonly” system synthesised using NIPMAM with a LCST of\nabout 44 °C [59], and which has been used as the core of\nthe deuterated core-shell particles later [61, 89]. We have first\nperformed PCS and direct imaging providing details on poly-\ndispersity and shape which are necessary to set parameters for\nthe SANS analysis. An example of this analysis showing the\npolymer chain and density profile parts of the scattered inten-\nsity is shown in Fig. 4a. The resulting profiles describe the\n“core-only” microgels as a function of cross-linking and tem-\nperature (shown in Fig. 4b), for subsequent comparison with\ncores within added shells. This representation of the particle\nmorphology gives direct access both to the internal density\n(which is directly linked to the amount of water) in the centre,\nand to the details of the particle interface, which is shown to\nbecome smoother with decreasing temperature. In our studies, the monomer density profile Φ(r) is defined\non Np = 30 homogeneous shells of fixed thickness set to ΔR =\n2 nm as building units. The profile Φ(r) is then transformed\ninto the scattering length density ρ(r), and ultimately into the\nscattered intensity I(q). Through the low-angle intensity and\nparticle concentration, the total number of monomers in a\nmicrogel particle is known. In some cases, interactions be-\ntween particles generate a very weak structure factor\nappearing in the low-q domain, in particular below the\nVPTT where microgels are swollen. For this reason, usually\nvery low volume fractions are measured; moreover, it has\nbeen checked that the results are virtually identical when cut-\nting off the low-q oscillations [61]. Form-free density profiles of spherically symmetric\nmicrostructures synthesized the shell with\np(NIPAM) and its VPTT of ca. 33 °C [60, 90, 91, 93, 94];\nthus, the linear region is larger in our case, extending from ca. 20 to 45 °C. These microgels are similar to the ones investi-\ngated by Zeiser [55], but they are smaller and possess H-D\ncontrast. The smaller size with respect to Zeiser et al. allows\nmeasuring the entire form factor in a standard small-angle\nscattering experiment, and thus gives access to a more detailed\nanalysis of the measured intensities as described below. It\nbecomes thus thinkable that the underlying nature of the corset\neffect may be understood on a structural basis. For the H core/\nD shell microgels, measurements in 12 v%/88 v% H2O/D2O\nare of particular interest. They have been designed to contrast-\nmatch the deuterated shell polymers and investigate the core\nstructure in presence of the invisible shell chains. Recent data\nfrom IR-spectroscopy indicated interpenetration of the core with the shell polymers [77], further motivating a detailed\nlook into the spatial extension of both monomers. It is again\nunderlined that the “core-only” microgel particles investigated\nin our previous paper are identical to the ones studied here, i.e. the shell synthesis has been performed with the same samples\nas in [59], and otherwise identical conditions as in the article\nby Zeiser et al. [55]. We have measured a series with various\ncore cross-linker contents (CCC), for several temperatures\nranging from 15, 30, 35, 40, and finally 55 °C, encompassing\nthe VPTT of core and shell polymers. As illustrated in Fig. 5a, the unexpected result of our anal-\nysis is that the cores embedded in core-shell microgels occupy\na larger space than the original “core-only” microgels [61]. Due to conservation of monomer numbers—this is exactly\nthe advantage of using the same cores—the cores possess\nthe same mass and thus the same low-q intensity in absence\nof interaction. They are thus characterised by a lower volume\nfraction, which contradicts the prediction of core compression\nby the (hypothetical) corset effect. This unexpected experi-\nmental result could only be rationalised by swelling of the\ncore by the shell monomers. Fig. 4 a Fitted chain and profile contributions to scattered intensity of a\nmicrogel (CC = 10 mol%) at 55 °C. b Density profiles of the H-\np(NIPMAM) core with a CCC of 10 mol% in D2O. Adapted with\npermission from reference [59]. Form-free density profiles of spherically symmetric\nmicrostructures The physical model and the elementary RMC steps of the\nalgorithm are illustrated in Fig. 3a. Several different initial\nconditions in terms of distributions of monomers across the\nshells have been produced in order to verify the robustness of\nour approach. The progression of the algorithm is shown in\nFig. 3b. Monte Carlo steps are performed by random moves of\ngroups of Nmove monomers from any shell to another, respect-\ning maximum packing of 100%. This quantity is seen to be\ndecreased to one with simulation time, progressively fine-\ntuning the profile. The deviation between the theoretical curve In a following paper, we have reported on the synthesis and\nscattering analysis of particles made of a p(NIPMAM) core\nand a p(NNPAM) shell [61]. These experiments have been set\nup to verify the spatial structure of the core embedded in a\nshell. We have based these measurements on the NIPMAM\ncore monomer used by Berndt et al. (VPTT ≈45 °C) [18, 60,\n83, 90–92], but have taken NNPAM monomer (VPTT ≈ Fig. 3 a Shell model of average microgel particle indicating moves of\ngroups of monomers between shells. b Simulation parameters as a\nfunction of the number of MC steps (χi\n2, χT\n2, Nmove, the number\nmonomers moved in one step, as indicated in the legend. The lowest\nvalue is (1). Once χ2 falls below 1 the profiles and fits are averaged\nover the next 100,000 steps. In the inset, the initial guess for the\nintensity (in red) and the final simulation result (in green) are compared\nto the experimental intensity (black). Reprinted with permission from\nreference [59]. Copyright 2018 American Chemical Society Fig. 3 a Shell model of average microgel particle indicating moves of\ngroups of monomers between shells. b Simulation parameters as a\nfunction of the number of MC steps (χi\n2, χT\n2, Nmove, the number\nmonomers moved in one step, as indicated in the legend. The lowest\nvalue is (1). Once χ2 falls below 1 the profiles and fits are averaged over the next 100,000 steps. In the inset, the initial guess for the\nintensity (in red) and the final simulation result (in green) are compared\nto the experimental intensity (black). Reprinted with permission from\nreference [59]. Copyright 2018 American Chemical Society 928 Colloid Polym Sci (2020) 298:921–935 23 °C) [60] for the shell synthesis, thereby generating the\nlinear swelling. Berndt et al. Form-free density profiles of spherically symmetric\nmicrostructures Copyright 2018 American Chemical\nSociety In the same paper, we have estimated the shell profiles\nwithout having the corresponding measurements with core-\nmatching [61]. This was achieved by comparing the H-H with\nthe H-D monomer density profiles, which by subtraction pro-\nvide the D-profile. Based on the assumption that H-H and D-D\nsyntheses gave rigorously identical particle size distributions,\nthe location of the monomers of the shell and thus their radial\ndensity profile could be estimated indirectly. In a third article,\nwe have then directly measured the radial density profile of\nthe shell monomers by contrast matching the hydrogenated\ncore monomers in 77 v%/23 v% H2O/D2O solvent mixtures. The resulting SANS curves have been analysed by applying\nour form-free multi-shell RMC procedure to the data, and the\nresult is shown in Fig. 5b [89]. From the comparison of Fig. 5a and b, which have been\nobtained on the same microgels for different solvent mixtures\n(different scattering contrast), it results that the core mono-\nmers are diluted in the centre and therefore extend further\ntowards the outside. This is caused by the presence of the\n(so-called) shell monomers, which are seen in Fig. 5b to reach\ndeeply into the core. The shell monomers form both a small\nexternal shell and an internal insoluble skeleton with a density\ngradient which restricts the swelling of the core once its VPTT\nis reached. For comparison, a homogeneous blend of core and\nshell monomers within the microgel particle would generate a\nsingle swelling step, whereas regions of varying monomer\nratio would respond at different temperatures. It would be\ninteresting to conceive a mechanical model describing the\nswelling and shrinking of such heterogeneous microgel parti-\ncles. Indeed, one may conjecture that it is this gradient struc-\nture which causes the linear swelling between the VPTTs of\nthe monomers involved. Moreover, it can be guessed that the Fig. 4 a Fitted chain and profile contributions to scattered intensity of a\nmicrogel (CC = 10 mol%) at 55 °C. b Density profiles of the H-\np(NIPMAM) core with a CCC of 10 mol% in D2O. Adapted with\npermission from reference [59]. Copyright 2018 American Chemical\nSociety Fig. 4 a Fitted chain and profile contributions to scattered intensity of a\nmicrogel (CC = 10 mol%) at 55 °C. b Density profiles of the H-\np(NIPMAM) core with a CCC of 10 mol% in D2O. Applications of core-shell microgels\nin catalysis One of these systems, shown in Fig. 6, exhibits excellent\nre-usability and even after the fifth catalytic cycle there is\nnearly no loss in activity observable. In between the catalytic\nreactions, the hybrids are separated from the reaction mixture\nusing centrifugation steps. The use of core-shell microgels in catalysis was pioneered by\nthe Ballauff group [15, 16, 25, 27, 62, 95, 96] and still repre-\nsents an active area of research which is close to potential\napplications. Especially in catalysis based on metal\nnanocrystals, core-shell microgels are very useful, since they\nstabilise the nanoparticles and prevent aggregation. This al-\nlows for an easy separation of the hybrid particles from the\nreaction mixture which enhances largely the re-usability of the\ncatalysts and leads to a straightforward work-up of the reac-\ntion mixture. This topic was addressed by Suzuki and co-\nworkers [97]. Shuli Dong and co-workers [98] have prepared another\ncore-shell system which can be used for catalysis. The hard\ncore of this system consists of silica-coated Au nanorods\nwhich are used as seeds in the microgel synthesis. As in some\nof our old works, MPS is used to enhance the incorporation of\nthe inorganic core into the shell [39, 99]. In addition to\nNIPAM, the authors use 1-vinlyimidazole as comonomer. This enhances the binding of spherical gold NPs which the\nauthors generate from HAuCl4 by reduction using NaBH4 in\npresence of the core-shell microgels. These particles present a\nhierarchically organised functional structure and the nanorod\ncore provides the possibility to shrink the particles by irradia-\ntion in the near-infrared (see also related works by\nKumacheva et al. [100, 101]). Hence, light can be used to\nexternally control the degree of shrinking of the shell of these\nparticles and hence, to control the activity of the spherical Au\nNPs embedded in the shell. Shuli Dong et al. also use the\nreduction of p-nitrophenol to measure the catalytic activity. Moreover, as also already shown by Matthias Ballauff and\nco-workers, the responsive hydrogel part of such hybrid ma-\nterials can be used to produce non-Arrhenius behaviour which\nis a sign for the control of the catalytic activity [25, 27, 63, 95]. Meanwhile, several authors tried to improve the carrier parti-\ncles, and the recent publications in this context will be pre-\nsented in the following. As already mentioned, Suzuki et al. Form-free density profiles of spherically symmetric\nmicrostructures Adapted with\npermission from reference [59]. Copyright 2018 American Chemical\nSociety Colloid Polym Sci (2020) 298:921–935 929 Fig. 5 Results of SANS analysis (CCC = 10%mol). a Density profiles of\nthe core monomers of the H-p(NIPMAM) core D7-p(NNPAM) shell\nsystem with an index-matched shell (H2O/D2O = 12 v%/88v%). The\nfigure was reproduced from [61]. b Density profile of the D7-\np(NNPAM) shell monomers of same system with contrast matched core\n(H2O/D2O = 77 v%/23v%). Figure 5 b was reproduced from [89] with\npermission from the The Royal Society of Chemistry p(NNPAM) shell monomers of same system with contrast matched core\n(H2O/D2O = 77 v%/23v%). Figure 5 b was reproduced from [89] with\npermission from the The Royal Society of Chemistry p(NNPAM) shell monomers of same system with contrast matched core\n(H2O/D2O = 77 v%/23v%). Figure 5 b was reproduced from [89] with\npermission from the The Royal Society of Chemistry Fig. 5 Results of SANS analysis (CCC = 10%mol). a Density profiles of\nthe core monomers of the H-p(NIPMAM) core D7-p(NNPAM) shell\nsystem with an index-matched shell (H2O/D2O = 12 v%/88v%). The\nfigure was reproduced from [61]. b Density profile of the D7- p(NNPAM) shell monomers of same system with contrast matched core\n(H2O/D2O = 77 v%/23v%). Figure 5 b was reproduced from [89] with\npermission from the The Royal Society of Chemistry linear swelling and shrinking property is intimately related to\nthe exact shape of the density gradient. five different hybrid systems with Au NPs were made. Also,\nSuzuki et al. use the reduction of p-nitrophenol as model re-\naction to follow the activity of the catalyst. Hence, the results\nare nicely comparable to the works by Ballauff et al. Applications of core-shell microgels\nin catalysis have prepared core-\nshell poly (NIPAM-co-glycidylmethacrylate) microgels with a\nrigid core mainly consisting of the glycidyl methacrylate\n(GMA) comomer [97]. The core-shell structure is obtained\nin a simple one-pot precipitation polymerisation due the faster\npolymerisation of GMA. In some cases, in a different reaction\nstep, the authors added 2-amino-ethanol to the reaction mix-\nture leading to a localisation of positive charges at the inter-\nface between GAM core and p(NIPAM) shell. This is after-\nwards helpful to generate catalytically active nanoparticles\n(NPs) which are localised at this internal interface. In total, Another article which also uses the same model reaction\nwas published in 2019 by Tzounis and co-workers [102]. In\nthis work, the authors describe the formation of a hierarchi-\ncally ordered core-shell microgel with a core made of silver\nand gold covered by a cross-linked p(NIPAM-co-allylamine)\nshell. In this thermoresponsive shell, silver nanoparticles are\ngenerated by reduction and prevented from bleeding by the\nallylamine. Hence, the particles have the following structure:\nAuAg@p(NIPAM-co-allylamine)@Ag. The distribution of 930 Colloid Polym Sci (2020) 298:921–935 Fig. 7 High-resolution TEM (a) and STEM (b) image of the particles\nprepared by Yang et al. The STEM-EDS image nicely reveals the\ndistribution of the silver nanoparticles in the hybrid core-shell\nmicrogels. Reproduced with permission by Springer Nature from\nRef. [96] the Ag particles inside the polymer shell is described by the\nterm “satellites” by the authors. The AuAg cores are obtained by first making a\nAu@p(NIPAM-co-allylamine) system and subsequently over-\ngrowing the gold cores with silver. In the reduction of p-nitro-\nphenol, this new structure seems to have superior turnover\ncompared to other microgel nanoparticle hybrids and very\nhigh-rate constants are achieved. Another way to achieve stronger persistence of the nano-\nparticles in the microgel network is the use of carboxylic acid\ngroups. Based on this idea, Yang et al. have prepared core-\nshell microgels with carboxylic acid groups in the shell [103]. The microgel particles have thus a p(styrene-co-NIPAM) @\np(NIPAM-co-methacrylic acid) structure and are made in a\ntwo-step synthesis. The shell has a remarkably high nominal\nmethacrylic acid content of 30 mol% with respect to NIPAM. Following the synthesis, the core-shell microgels were dried\nand redispersed in ethanol. In this medium, the authors re-\nduced [Ag(NH3)2]+ to generate the Ag NPs. The obtained\nhybrid particles were characterised by TEM and Fig. 7a shows\na typical image. In Fig. 7b, a STEM-EDS image is shown. Applications of core-shell microgels\nin catalysis Here, the silver particles appear as red dots. The silver parti-\ncles are mainly formed in the shell indicating that the\nmethacrylic acid has an influence on the Ag localisation. Also, Yang et al. have used the reduction of p-nitrophenol to\nstudy the catalytic activity of the new core-shell hybrid\nmicrogels, and the observed behaviour is similar compared\nto the findings presented in Matthias Ballauff’s pioneering\nworks on this topic. However, the swelling curves of these\nparticles seem to exhibit a two-step transition which is differ-\nent compared to the PS@p(NIPAM) particles used by\nMatthias. Fig. 7 High-resolution TEM (a) and STEM (b) image of the particles\nprepared by Yang et al. The STEM-EDS image nicely reveals the\ndistribution of the silver nanoparticles in the hybrid core-shell\nmicrogels. Reproduced with permission by Springer Nature from\nRef. [96] were used: p(NIPAM-co-AAc)@p(NNPAM), p(NIPMAM-\nc o - A A c ) @ p ( N N PA M ) , a n d p ( N N PA M - c o -\nAAc)@p(NNPAM). In all cases, the core contained acrylic\nacid as comonomer to improve the binding of the metal\nNPs. In this context, the swelling behaviour of the carriers\nand the stability of silver NPs inside the polymer network\nwas investigated by photon correlation spectroscopy, trans-\nmission electron microscopy, and by following the surface\nplasmon resonance of the NPs. Depending on the cross- were used: p(NIPAM-co-AAc)@p(NNPAM), p(NIPMAM-\nc o - A A c ) @ p ( N N PA M ) , a n d p ( N N PA M - c o -\nAAc)@p(NNPAM). In all cases, the core contained acrylic\nacid as comonomer to improve the binding of the metal\nNPs. In this context, the swelling behaviour of the carriers\nand the stability of silver NPs inside the polymer network\nwas investigated by photon correlation spectroscopy, trans-\nmission electron microscopy, and by following the surface\nplasmon resonance of the NPs. Depending on the cross- Also, one of us has recently started to work on catalytically\nactive core-shell microgels [68] using carboxylic acid groups\nto achieve better fixation of the metal NPs as well. In this\nwork, acrylamide-based, thermoresponsive core-shell\nmicrogels with a linear phase transition region are exploited\nas improved carriers for catalytically active silver nanoparti-\ncles. The particles are slightly different compared to those\ndescribed in [57]. The following general microgel structures Fig. Applications of core-shell microgels\nin catalysis 6 Image of the successful system from the work of Suzuki et al. This core-shell system shows excellent re-usability as catalyst. Reproduced from\nRef. [97] (Open Access Publication in ACS Omega) Fig. 6 Image of the successful system from the work of Suzuki et al. This core-shell system shows excellent re-usability as catalyst. Reproduced from\nRef. [97] (Open Access Publication in ACS Omega) 931 Colloid Polym Sci (2020) 298:921–935 linker content of the microgel core, very good stability of the\nNPs inside the microgel network was observed, with nearly no\nbleeding or aggregation of the NPs over several weeks for\ncore cross-linker contents of 5 and 10 mol%. The architecture\nof the hybrid particles in the swollen state was investigated\nwith cryogenic transmission electron microscopy. The parti-\ncles exhibit a core-shell structure, with the silver NPs located\nmainly at the interface between the core and shell. Such a\nhybrid architecture was not yet used before and leads to im-\nproved stability of the Ag nanocrystals inside the microgels. Also, in this work, reduction of p-nitrophenol was used as\nmodel reaction. By studying the reaction at different temper-\natures, a good switchability of the catalytic activity was re-\nvealed as shown in Fig. 8. in catalysis. In addition to these topics, core-shell microgels\ndeposited on surfaces, e.g. as actuators or at interfaces to make\nsmart emulsions are of growing relevance. This is of course\nrelated to the growing interest in adsorbed microgels and smart\nPickering emulsions in general [105]. In this context, recent\nwork was published by Yi Gong and co-workers [106]. These\nauthors studied the interfacial rheology of core-shell microgels\nat the decane-water interface. The particles were p(NIPAM-co-\nAAc)@poly-2,2,2-trifluoroethyl methacrylate (PTFMA) core-\nshell systems and the authors measured the interfacial pressure\nin a Langmuir trough. The microgel monolayers at the oil-water\ninterface were exposed to compression expansion cycles. The\nwork shows that the elastic behaviour of the monolayer is dom-\ninated by the properties of the shell. Also, Vasudevan and co-workers have studied core-shell\nmicrogels in this case with silica cores of variable diameter\nand p(NIPAM) shells of variable thickness with respect to\ntheir behaviour at the oil-water interface [107]. First, the au-\nthors presented a theoretical model for the wetting behaviour\nof this type of inorganic-organic core-shell microgel as a func-\ntion of the specific geometrical and compositional features of\nthe different particles. Applications of core-shell microgels\nin catalysis The model is in line with experimental\nfindings showing that the occupied area per core-shell\nmicrogel is strongly dependent on both varied parameters,\nnamely the core size and shell thickness. Below a critical shell\nthickness, the cores are found to be located in a position where\nthey just touch the oil-water interface. Beyond a critical shell\nthickness, the core detaches from the interface. Compared to\nhard sphere particles at the oil-water interface such hard-soft\nhybrids exhibit a subtle interplay between position at the in-\nterface, interfacial dynamics, and might have interesting ap-\nplications in interfacial rheology. This is in line with the ob-\nservations by Yi Gong et al. [106]. Similar to previous works, the obtained Arrhenius plots\nshow that the swelling of the core and shell can influence\nthe catalytic activity of the silver nanoparticles. As mentioned\nbefore, the cross-linker content of the core seems to be a very\nimportant parameter for the switchability of the catalytic ac-\ntivity. A higher cross-linker content of the core appears to be\nconnected to a stronger influence of the carrier swelling/\nshrinking degree on the catalytic activity of the silver NPs. Besides the often used reduction of p-nitrophenol, also\nCongo red was used in a recent example of silver NPs in PS\ncore/MG shell particles, where the NPs can be located only in\nthe shell. Dyes are a major problem in regions with intense\ntextile production due to release in the environment. Especially, Congo red is problematic since it is harmful for\nmammals. Hence, it would be desirable to have an efficient\nprocedure to decompose this dye. The work by Naseem et al. reveals an enhanced reduction efficiency of Congo red by this\nparticular core-shell microgel dispersion type [104]. Besides the behaviour of core-shell microgels at liquid-\nliquid interfaces, their behaviour at solid surfaces has also\nbeen studied. This topic is of great relevance when applica-\ntions as sensors or actuators are envisaged. In a work on par-\nticles made of a p(NIPMAM) core and a p(NNPAM) shell, we\nhave shown that these core-shell microgels still exhibit a linear Core-shell microgels at surfaces\nand interfaces In the previous sections, we have already discussed structure\nand properties of core-shell microgels and also their application PNN10@PNN (right). Especially the last system shows a drastic change\nof the reaction rate. Reproduced from reference [68], American Chemical\nSociety ACS Omega Fig. 8 Arrhenius plots for the different synthesised microgels showing\nthe enormous influence of the swelling/shrinking state of the core-shell\nparticles on the activity of the Ag NPs. The figure presents the hybrid\nsystems based on PNI10@PNN (left), PMAM10@PNN (middle), and\nPNN10@PNN (right). Especially the last system shows a drastic change\nof the reaction rate. Reproduced from reference [68], American Chemical\nSociety ACS Omega PNN10@PNN (right). Especially the last system shows a drastic change\nof the reaction rate. Reproduced from reference [68], American Chemical\nSociety ACS Omega Fig. 8 Arrhenius plots for the different synthesised microgels showing\nthe enormous influence of the swelling/shrinking state of the core-shell\nparticles on the activity of the Ag NPs. The figure presents the hybrid\nsystems based on PNI10@PNN (left), PMAM10@PNN (middle), and 932 Colloid Polym Sci (2020) 298:921–935 swelling behaviour even if confined to a solid interface (Si-\nwafer) [57]. In this work, we have used ellipsometry to mea-\nsure the film thickness as a function of temperature while the\nwafers were immersed in water. Such surface coatings might\nbe suitable actuating units in, e.g. etalons [108] giving rise to a\nlinear piezo-like response. Related to this, it is of course inter-\nesting to study adsorbed core-shell microgels with respect to\ntheir mechanical properties and changes. According to\nSchulte el al., this can be done by nano-indentation experi-\nments [109]. For sensing applications, adsorbed core-shell\nmicrogels can be used as templates for making composite\nfilms for surface enhanced Raman scattering (SERS). This\nwas shown by Sen Yan and co-workers using poly(styrene-\nco-N-isopropylacrylamide)@polyacrylic acid microgels\nwhich were deposited in an ordered array and subsequently\nsputtered with an Au film [110]. Such a process yields very\nnice highly ordered surface coatings. This is shown in Fig. 9. scattering techniques and in particular small-angle scattering\nwith simulation-aided structural analysis coincides with our\nown research interests over the past years. Core-shell microgels at surfaces\nand interfaces Other topics which\nmay be looked at as related, like protein or enzyme adsorption\nonto microgels also studied by Matthias Ballauff [16, 111,\n112] have not been included in this review for lack of space,\nalthough other pioneering studies have been recently pub-\nlished by other groups active in this field [50, 113]. Due to our inclination towards scattering techniques, a trait\nof character we share with Matthias, we have mostly restricted\nourselves to core-shell particles of spherical symmetry. Naturally, nanorod structures as studied by Dong et al. [114], the groups of Dietsch and Schurtenberger [42, 43],\nand the ones of Ballauff and Müller [70] can also be\napprehended in detail by scattering due to the high degree of\nsymmetry. Computer simulations have certainly not been\ntreated here at the level they deserve, as they now allow to\nfollow the formation of complex topologies in detail\n[115–118]. Finally, self-assembly of microgels, as well as their\ninterfacial activity and their possible application as switchable\ncell culture substrates for vertebrate cells have also been omit-\nted [119]. Fig. 9 The upper row (images a,\nb) shows SEM micrographs of a\nPSN@PAA monolayered\nfilm. Images c, d show a\nPSN@PAA/Au composite film.\nThe insets show the macroscopic\noptical appearance of the prepared\nfilms. The obtained hybrid film\nexhibits excellent SERS\nenhancement factors and are\nshown to be re-usable several\ntimes for sensing without any loss\nof performance. Reproduced with\npermission from reference [110].\nCopyright 2020 Elsevier Conclusion and perspectives On the occasion of this special issue in honour of the contri-\nbutions of Matthias Ballauff and his group to the field of\ncolloid and polymer science, we have organised this review\non core-shell thermosensitive microgel particles taking contri-\nbutions by Matthias and co-workers as starting point and guid-\nance. We have deliberately focused our article on either soft\ncopolymer microgel core-shell structures, or microgel shells\ngrown onto hard nanoparticle cores, as recent progress using If one looks at recent progress in the field of microgels, one\ncan only be surprised by the liveliness of the subject, several\ndecades after their discovery, and at least 20 years of in-depth\nstudies. Current directions of research seem to develop to-\nwards more complex topologies, like multi-shell or hollow\nstructures with multi-responsiveness. Obviously, the main\ndriving force comes from chemical synthesis, which can Fig. 9 The upper row (images a,\nb) shows SEM micrographs of a\nPSN@PAA monolayered\nfilm. Images c, d show a\nPSN@PAA/Au composite film. The insets show the macroscopic\noptical appearance of the prepared\nfilms. The obtained hybrid film\nexhibits excellent SERS\nenhancement factors and are\nshown to be re-usable several\ntimes for sensing without any loss\nof performance. Reproduced with\npermission from reference [110]. Copyright 2020 Elsevier Colloid Polym Sci (2020) 298:921–935 933 7. Gasser U, Fernandez-Nieves A (2010). Phys Rev E 81(5). https://\ndoi.org/10.1103/physreve.81.052401 explore the infinite possibilities of organic structures. This will\nallow for fine-tuning of the interactions between microgel\nhosts and guest molecules, by introducing specific functions\nin well-defined positions inside the microgels. Scattering stud-\nies may then allow (or not, depending on contrast situations)\ndesigning and understanding the function of such structures. If\nhosting molecules or simply solvent has an impact on the\nintrinsic swelling properties of microgels, then one may also\nuse them as sensor capable of detecting local concentration\nchanges. 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Langmuir 20:2123 Compliance with ethical standards 22. Hoare T, Pelton R (2006). Langmuir 22:7342 23. Kim JH, Ballauff M (1999). Colloid Polym Sci 277:1210 Conflict of interest\nThe authors declare that they have no conflicts of\ninterest. Conflict of interest\nThe authors declare that they have no conflicts of\ninterest. 24. Dingenouts N, Nordhausen C, Ballauff M (1998). Macromolecules 31:8912 25. Lu Y, Mei Y, Ballauff M, Drechsler M (2006). J Phys Chem B 110:\n3930 Open Access This article is licensed under a Creative Commons\nAttribution 4.0 International License, which permits use, sharing, adap-\ntation, distribution and reproduction in any medium or format, as long as\nyou give appropriate credit to the original author(s) and the source, pro-\nvide a link to the Creative Commons licence, and indicate if changes were\nmade. 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Nowadays, he studies\nthe structure and dynamics of var-\nious soft matter systems in bulk,\nin solution, and sometimes at in-\nterfaces. His primary research interest is the investigation of nanostruc-\ntures using scattering experiments, in polymer nanocomposites and, more\nrecently, microgels, combined with modelling approaches often based on\ncomputer simulations. 101. Publisher’s note Springer Nature remains neutral with regard to jurisdic-\ntional claims in published maps and institutional affiliations. (\n)\ny\nC8CP05911J Colloids Surf A Physicochem Eng Asp 452:46. https://doi. org/10.1016/j.colsurfa.2014.03.090 115. Moreno AJ, Verso FL (2018). Soft Matter 14(34):7083. https://doi. org/10.1039/c8sm01407h 116. Rudyak VY, Gavrilov AA, Kozhunova EY, Chertovich AV\n(2018). Soft Matter 14(15):2777. https://doi.org/10.1039/\nc8sm00170g 117. Gelissen APH, Scotti A, Turnhoff SK, Janssen C, Radulescu A,\nPich A, Rudov AA, Potemkin II, Richtering W (2018). 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