Nanogels and microgels are soft, deformable, and penetrable objects with an internal gel-like structure that is swollen by the dispersing solvent. Their softness and the potential to respond to external stimuli like temperature, pressure, pH, ionic strength, and different analytes make them interesting as soft model systems in fundamental research as well as for a broad range of applications, in particular in the field of biological applications. Recent tremendous developments in their synthesis open access to systems with complex architectures and compositions allowing for tailoring microgels with specific properties. At the same time state-of-the-art theoretical and simulation approaches offer deeper understanding of the behavior and structure of nano- and microgels under external influences and confinement at interfaces or at high volume fractions. Developments in the experimental analysis of nano- and microgels have become particularly important for structural investigations covering a broad range of length scales relevant to the internal structure, the overall size and shape, and interparticle interactions in concentrated samples. Here we provide an overview of the state-of-the-art, recent developments as well as emerging trends in the field of nano- and microgels. The following aspects build the focus of our discussion: tailoring (multi)­functionality through synthesis; the role in biological and biomedical applications; the structure and properties as a model system, e.g., for densely packed arrangements in bulk and at interfaces; as well as the theory and computer simulation.

Gold nanoparticles are encapsulatedwithin thermoresponsive pNIPAMmigrogels through an easy two‐stepprotocol. The core/shell structure ofthe composite is characterized by TEM,AFM, PCS, and UV‐vis spectroscopy. The restricted environment and thehigh porosity of the microgel shell arestudied through the overgrowth of thegold core.

Gold nanoparticles and nearby fluorophores interact via electromagnetic coupling upon light excitation. We determine the distance and wavelength dependence of this coupling theoretically and experimentally via steady-state and time-resolved fluorescence spectroscopy. For the first time, the fluorescence quenching of four different dye molecules, which absorb light at different wavelengths across the visible spectrum and into the near-infrared, is studied using a rigid silica shell as a spacer. A comprehensive experimental determination of the distance dependence from complete quenching to no coupling is carried out by a systematic variation of the silica shell thickness. Electrodynamic theory predicts the observed quenching quantitatively in terms of energy transfer from the molecular emitter to the gold nanoparticle. The plasmonic field enhancement in the vicinity of the 13 nm gold nanoparticles is calculated as a function of distance and excitation wavelength and is included in all calculations. Relative radiative and energy transfer rates are determined experimentally and are in good agreement with calculated rates. We demonstrate and quantify the severe effect of dye-dye interactions on the fluorescence properties of dyes attached to the surface of a silica nanoparticle in control experiments. This allows us to determine the experimental conditions, under which dye-dye interactions do not affect the experimental results.

Nanoparticles and in particular gold nanorods have interesting optical properties arising from two well-differentiated plasmon modes. The frequency of such modes can be altered by their chemical environment and coupling with neighboring rods. This study investigates new composite materials made of gold nanorods adsorbed on thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) microgels. It is shown that the thermally induced collapse of the polymer network inside the particles leads to a red shift of the longitudinal plasmon band of the gold rods, which is found to be fully reversible.

We present a general mechanism for the solution self-assembly of crystalline-core micelles (CCMs) from triblock copolymers bearing a semicrystalline polyethylene (PE) middle block. This approach enables the production of nanoparticles with tunable dimensions and surface structures. Depending on the quality of the solvent used for PE, either spherical or worm-like CCMs can be generated in an easy and highly selective fashion from the same triblock copolymers via crystallization-induced self-assembly upon cooling. If the triblock copolymer stays molecularly dissolved at temperatures above the crystallization temperature of the PE block, worm-like CCMs with high aspect ratios are formed by a nucleation and growth process. Their length can be conveniently controlled by varying the applied crystallization temperature. If exclusively spherical micelles with an amorphous PE core are present before crystallization, confined crystallization within the cores of the preformed micelles takes place and spherical CCMs are formed. For polystyrene-block-polyethylene-block-poly(methyl methacrylate) triblock terpolymers a patch-like microphase separation of the corona is obtained for both spherical and worm-like CCMs due to the incompatibility of the PS and PMMA blocks. The structure of the patch-like corona depends on the selectivity of the employed solvent for the PS and PMMA corona blocks, whereby nonselective solvents produce a more homogeneous patch size and distribution. Annealing of the semicrystalline PE cores results in an increasingly uniform crystallite size distribution and thus core thickness of the worm-like CCMs.

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Soft hydrogel particles show a rich structural and mechanical behaviour compared to hard particles, both in bulk and when confined in two dimensions at a fluid interface. Moreover, encapsulation into hydrogel shells makes it possible to transfer the tunability of soft steric interactions to hard nanoparticle cores, which bear interest for applications, e.g. in terms of optical, magnetic and reinforcement properties. In this work, we investigate the microstructures formed by hard core-soft shell particles at liquid-liquid interfaces upon compression. We produced model particles with the same silica core and systematically varied the shell-to-core ratio by synthesising shells with three different thicknesses. These particles were spread at an oil-water interface in a Langmuir-Blodgett trough and continuously transferred onto a solid support during compression. The transferred microstructures were analysed by atomic force and scanning electron microscopy. Quantitative image analysis provided information on the particle packing density, the inter-particle distance, and the degree of order of the monolayers. We discovered several essential differences compared to purely soft hydrogel particles, which shed light on the role played by the hard cores in the assembly and compression of these composite monolayers.

The volume phase transition of colloidal microgels made of N-isopropylacrylamide (NIPAM) is well-studied and it is known that the transition temperature can be influenced by copolymerization. A series of poly( N-isopropylacrylamide- co-allylacetic acid) copolymers with different contents of allylacetic acid (AAA) was synthesized by means of a simple radical polymerization approach. The thermoresponsive behavior of these particles was studied using dynamic light scattering (DLS). Further characterization was done by employing transmission electron microscopy (TEM) and zeta potential measurements. TEM observations reveal the approximately spherical shape and low polydispersity of the copolymer particles. In addition, the measured zeta potentials provide information about the relative surface charge. Since these copolymers are much more sensitive to external stimuli such as pH and ionic strength than their pure PNIPAM counterparts, the volume phase transition was investigated at two different pH values and various salt concentrations. At pH 10 for the copolymer microgels with the highest AAA content, a significant shift of the volume phase transition temperature toward higher values is found. For higher AAA content, a change in pH from 8 to 10 can induce a change in radius of up to 100 nm making the particles interesting as pH controlled actuators.