Functionalization of Planet-Satellite Nanostructures Revealed by Nanoscopic Localization of Distinct Macromolecular Species

Turning on hotspots: supracolloidal SERS probes made brilliant by an external activation mechanism

We achieved external activation of local hot-spot sites in supracolloidal assembly structures. The concept was demonstrated by boosting surface-enhanced Raman scattering (SERS) efficiency by one order of magnitude through a heating-induced process. Our approach involves assembling gold nanoparticles with distinct dimensions, i.e. 16 and 80 nm, into well-defined planet-satellite-type arrangement structures using thermoresponsive (poly(N-isopropylacrylamide)) star polymer linkers. Insights into the assembly process were obtained by calculations within the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory framework. We observe one order of magnitude increase in SERS enhancement by a heating-induced volume-phase transition. This magnification aligns with simulations run using the finite-difference time-domain (FDTD) method. The implications of this adaptive supracolloidal concept are twofold: Firstly, our approach bypasses limitations of existing systems that are associated with the limited accessibility of electromagnetic hot-spot sites in strongly coupled, static assemblies of plasmonic nanoparticles, by providing the capability of dynamic hot-spot re-configuration. Second, these externally activated probes offer promising opportunities for the development of messenger materials and associated sensing strategies.

Materials with Tunable Optical Properties for Wearable Epidermal Sensing in Health Monitoring

Advances in wearable epidermal sensors have revolutionized the way that physiological signals are captured and measured for health monitoring. One major challenge is to convert physiological signals to easily readable signals in a convenient way. One possibility for wearable epidermal sensors is based on visible readouts. There are a range of materials whose optical properties can be tuned by parameters such as temperature, pH, light, and electric fields. Herein, this review covers and highlights a set of materials with tunable optical properties and their integration into wearable epidermal sensors for health monitoring. Specifically, the recent progress, fabrication, and applications of these materials for wearable epidermal sensors are summarized and discussed. Finally, the challenges and perspectives for the next generation wearable devices are proposed.

Gold nanoparticle ring arrays from core-satellite nanostructures made to order by hydrogen bond interactions

Polyethylene glycol-grafted gold nanoparticles are attached to silica nanoparticle cores via hydrogen bonding in a controlled fashion, forming well-defined core-satellite structures in colloidal solution. For separating these complex structures effectively from the parental nanoparticles, a straightforward and easy protocol using glass beads has been developed. The attached gold nanoparticles show unique surface mobility on the silica core surface, which allows for nanoparticle rearrangement into a 2D ring pattern surrounding the silica nanoparticle template when the core-satellite structures are cast to a planar surface. When etching away the silica core under conditions in which the polymer shell fixes the satellites to the substrate, highly ordered ring-shaped patterns of gold nanoparticles are formed. By variation of the size of the parental particles - 13 to 28 nm for gold nanoparticles and 39 to 62 nm for silica nanoparticles - a great library of different ring-structures regarding size and particle number is accessible with relative ease. The proposed protocol is low-cost and can easily be scaled up. It moreover demonstrates the power of hydrogen bonds in polymers as a dynamic anchoring tool for creating nanoclusters with rearrangement ability. We believe that this concept constitutes a powerful strategy for the development of new and innovative nanostructures.

Controlled Arrangement of Gold Nanoparticles on Planar Surfaces via Constrained Dewetting of Surface-Grafted RAFT Polymer

Linear and four-arm star polystyrene samples prepared by RAFT polymerization were grafted to gold surfaces directly via their thiocarbonylthio-end groups. Nanoscale polymer patterns were subsequently formed via constrained dewetting. The patterned polymer films then served as a template for the precise arrangement of gold nanoparticles in a monolayer with a well-defined and regular structure. Using star polymers as a linker between the planar gold surface and the particles, the structural stability of the arranged particles can be further enhanced. The surface-bound nanocomposite films made of polymer and nanoparticles can also reversibly switch their nanostructures by simple wetting or dewetting treatment.

Recent Progress in Macromolecule-Anchored Hybrid Gold Nanomaterials for Biomedical Applications

Gold nanoparticles (AuNPs), with elegant thermal, optical, or chemical properties due to quantum size effects, may serve as unique species for therapeutic or diagnostic applications. It is worth mentioning that their small size also results in high surface activity, leading to significantly impaired stability, which greatly hinders their biomedical utilizations. To overcome this problem, various types of macromolecular materials are utilized to anchor AuNPs so as to achieve advanced synergistic effect by dispersing, protecting, and stabilizing the AuNPs in polymeric-Au hybrid self-assemblies. In this review, the most recent development of polymer-AuNP hybrid systems, including AuNPs@polymeric nanoparticles, AuNPs@polymeric micelle, AuNPs@polymeric film, and AuNPs@polymeric hydrogel are discussed with respect to their different synthetic strategies. These sophisticated materials with diverse functions, oriented toward biomedical applications, are further summarized into several active domains in the areas of drug delivery, gene delivery, photothermal therapy, antibacterials, bioimaging, etc. Finally, the possible approaches for future design of multifunctional polymer-AuNP hybrids by combining hybrid chemistry with biological interface science are proposed.

Thermoreversible Surface Polymer Patches: A Cryogenic Transmission Electron Microscopy Investigation

Hybrid core-shell type nanoparticles from gold nanoparticle cores and poly( N-isopropylacrylamide) shells were investigated with regard to their structural plasticity. Reversible addition-fragmentation chain transfer polymerization was used to synthesize well-defined polymers that can be readily anchored onto the gold nanoparticle surface. The polymer shell morphologies were directly visualized in their native solution state at high resolution by cryogenic transmission electron microscopy, and the microscopic results were further corroborated by dynamic light scattering. Different environmental conditions and brush architectures are covered by our experiments, which leads to distinct thermally induced responses. These responses include constrained dewetting of the nanoparticle surface at temperatures above the lower critical solution temperature of poly( N-isopropylacrylamide), leading to surface polymer patches. This effect provides a novel approach toward breaking the symmetry of nanoparticle interactions, and we show first evidence for its impact on the formation of colloidal superstructures.

Phase separation in mixed polymer brushes on nanoparticle surfaces enables the generation of anisotropic nanoarchitectures

The preparation of nanoparticles and their targeted connection with other functional units is one key challenge in developing nanoscale devices. Herein, we report an experimental strategy toward the development of anisotropic nanoparticle architectures. Our approach is based on phase separation of binary mixed polymer brushes on gold nanoparticle surfaces leading to Janus-type structures, as revealed by scanning transmission electron microscopy and electron energy-loss spectroscopy and, additionally, corroborated by computer simulation. We show that such structures can be used for the site-selective functionalization with additional nanosized entities.

Uniform Distance Scaling Behavior of Planet-Satellite Nanostructures Made by Star Polymers

Planet-satellite nanostructures from RAFT star polymers and larger (planet) as well as smaller (satellite) gold nanoparticles are analyzed in experiments and computer simulations regarding the influence of arm number of star polymers. A uniform scaling behavior of planet-satellite distances as a function of arm length was found both in the dried state (via transmission electron microscopy) after casting the nanostructures on surfaces and in the colloidally dispersed state (via simulations and small-angle X-ray scattering) when 2-, 3-, and 6-arm star polymers were employed. This indicates that the planet-satellite distances are mainly determined by the arm length of star polymers. The observed discrepancy between TEM and simulated distances can be attributed to the difference of polymer configurations in dried and dispersed state. Our results also show that these distances are controlled by the density of star polymers end groups, and the number of grabbed satellite particles is determined by the magnitude of the corresponding density. These findings demonstrate the feasibility to precisely control the planet-satellite structures at the nanoscale.

Gold-Planet-Silver-Satellite Nanostructures Using RAFT Star Polymer

The hierarchical self-assembly of distinct nanoelements into precisely ordered nanostructures requires efficient and flexible fabrication strategies. Herein, we report the precise fabrication of bimetallic gold-planet-silver-satellite nanoparticle-arrangements employing RAFT star polymers as particle linker connecting gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) with judiciously modified surface activity. The strengths of this approach include the adjustability of interparticle distances by tailoring the star polymer molar mass. The prepared nanoassemblies have well-defined structures in which a planet AuNP (∼13 nm) is encompassed by several satellite AgNPs (∼8 nm), thus incorporating the properties of both AuNPs and AgNPs, as confirmed by transmission electron microscopy and UV-vis spectra. Our results highlight the general applicability of RAFT star polymers as a nanosynthesis platform for synthesizing noble metal nanocomposites.