Colloidal nanoparticles (NPs) have become versatile building blocks in a wide variety of fields. Here, we discuss the state-of-the-art, current hot topics, and future directions based on the following aspects: narrow size-distribution NPs can exhibit protein-like properties; monodispersity of NPs is not always required; assembled NPs can exhibit collective behavior; NPs can be assembled one by one; there is more to be connected with NPs; NPs can be designed to be smart; surface-modified NPs can directly reach the cytosols of living cells.

Highly uniform, core-shell microgels consisting of single gold nanoparticle cores and cross-linked poly-N-isopropylacrylamide (PNIPAM) shells were prepared by a novel, versatile protocol. The synthetic pathway allows control over the polymer shell thickness and its swelling behavior. The core-shell structure was investigated by electron microscopy and atomic force microscopy, whereas the swelling behavior of the shell was studied by means of dynamic light scattering and UV-vis spectroscopy. Furthermore, the latter method was used to investigate the optical properties of the hybrid particles. By modeling the scattering contribution from the PNIPAM shells, the absorption spectra of the gold nanoparticle cores could be recovered. This allows the particle concentration to be determined, and this in turn permits the calculation of the molar mass of the hybrid particles as well as the refractive index of the shells.

2D arrays of Au-PNIPAM core-shell nanocrystals were fabricated using convective deposition and spin-coating. The particle density and ordering were studied by AFM. Annealing at 700 °C removes the polymer shell, while retaining a monolayer of well-separated gold nanoparticles. The surface plasmon modes of the colloid monolayers could be measured by spectroscopic ellipsometry.

Coating metal nanocrystals with responsive polymers provides a model case of smart, functional materials, where the optical properties can be modulated by external stimuli. However the optical response is highly sensitive to the polymer shell morphology, thickness and dielectric contrast. In this paper we study the nature of cross-linked, thermoresponsive polymer shells for the first time using four different scattering approaches to elucidate the density profile of the shells. Each scattering method provides unique information about the temperature-induced changes of shell thickness in terms of hydrodynamic radius and radius of gyration, the pair-distance distribution functions of the shells as well as the dynamic network fluctuations. Only a combination of these different scattering techniques allows to develop a morphological model of the core-shell particles. We further demonstrate control of the cross-linker distribution in core-shell synthesis by semi-batch precipitation copolymerization. Conducting the polymerization in three steps, we show for the first time that the polymer shell thickness can be successively increased without affecting the shell morphology and response behavior.

Hydrophobic forces play a key role in the processes of collapse and reswelling of thermoresponsive polymers. However, little is known about the dynamics of these processes. Here, thermoresponsive poly(N-isopropylacrylamide)-encapsulated gold nanoparticles (Au-PNIPAM) are heated via nanosecond laser flash photolysis. Photothermal heating via excitation of the localized surface plasmon resonance of the Au nanoparticle cores results in rapid PNIPAM shell collapse within the 10 ns pulse width of the laser. Remarkably, reswelling of the polymer shell takes place in less than 100 ns. A clear pump fluence threshold for the collapse of the PNIPAM shell is demonstrated, below which collapse is not observed. Reswelling takes longer at higher laser intensities.