Spherical Gold-Nanoparticle Assemblies with Tunable Interparticle Distances Mediated by Multifunctional RAFT Polymers

Encapsulation of gold nanoclusters: stabilization and more

Gold nanoparticles with only a few atoms, known as gold nanoclusters (AuNCs), have dimensions below 2 nm and feature singular properties such as size dependent luminescence. AuNCs are also highly photostable and have catalytic activity, low toxicity and good biocompatibility. With these properties, they are extremely promising candidates for application in bioimaging, sensing and catalysis. However, when stabilized only with small capping ligands, their use is hindered by lack of colloidal stability. Encapsulation of the AuNCs can contribute to provide a more robust protection and even to improve their properties. Here, we review the encapsulation of AuNCs in polymers, silica and metal organic frameworks (MOFs) for applications in bioimaging, sensing and catalysis.

Near-Infrared-Triggered Photothermal Aggregation of Polymer-Grafted Gold Nanorods in a Simulated Blood Fluid

Gold nanorods were decorated with thermoresponsive copolymers of tailored architecture and constructed from N-isopropyl acrylamide and acrylamide. The copolymers were prepared via reversible addition-fragmentation chain transfer polymerization (RAFT) and immobilized on the gold nanorod surface taking advantage of the aurophilicity of its inherently formed trithiocarbonate groups. The topology as well as the average molecular weight of the copolymers was altered using either a monofunctional or 3-arm star RAFT agent. Two-dimensional arrays of the self-assembled core-shell nanostructures were fabricated by drop-casting showing tunable interparticle spacings. In a simulated blood fluid, the lower critical solution temperature of the nanohybrids could be modified over a significant temperature range around body temperature by adjusting the copolymer composition, the architecture, and/or the size of the polymer. The intrinsic photothermal properties of the gold nanorods were utilized to trigger particle aggregation by irradiation at 808 nm in the optical window of human tissues. In effect, a new nanohybrid system with remotely controllable aggregation via an external NIR-light stimulus for nanomedical applications was developed.

Nanoengineering with RAFT polymers: from nanocomposite design to applications

Reversible addition–fragmentation chain-transfer (RAFT) polymerization is a powerful tool for the precise formation of macromolecular building blocks that can be used for the construction of well-defined nanocomposites.

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.

Precise Self-Assembly and Controlled Catalysis of Thermoresponsive Core-Satellite Multicomponent Hybrid Nanoparticles

The construction of multicomponent hybrid nanomaterials with well-controlled architecture, especially bearing an ordered homogeneity and distribution of the subunits with tunable functions, is a key challenge in chemistry and material science. Herein, we reported a versatile and novel strategy to fabricate core-satellite multicomponent nanostructures with tunable interparticle distances and catalysis properties by the combination of surface-initiated reversible addition-fragmentation chain transfer (SI-RAFT) polymerization and self-assembly. The arrangement and interparticle distance of gold satellites could be precisely tuned by the SI-RAFT polymerization process and the feeding ratio of gold nanoparticles (AuNPs) and the core nanoparticle. It is worth to note that multilayered core-satellite nanostructures have been fabricated by a high-feeding ratio of AuNPs and magnetite NP (MNP)@SiO₂-PNIPAm. Notably, the core-satellite MNP@SiO₂-PNIPAm-Au nanoparticles exhibited excellent thermoresponsive behaviors with the change of temperature. Furthermore, the catalytic efficiency of MNP@SiO₂-PNIPAm-Au nanoparticles via the reduction of 4-nitrophenol to 4-aminophenol can be well modulated by the nanoparticle size, temperature, and polymer feed ratio. This strategy for precise construction of core-satellite nanostructures would open a new pathway to construct multicomponent functional nanostructures.

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.

Polyethylene-Grafted Gold and Silver Nanoparticles Using Catalyzed Chain Growth (CCG)

We report an efficient synthesis route for the formation of gold/silver-core⁻PE-shell nanohybrids in a simple self-assembly approach using PE with strong aurophilicity and argentophilicity, via thiol- and trithiocarbonate terminated moieties. This united the unique properties of polyethylene (PE) with gold and silver nanoparticles, using the well-defined end-group design of PE. These nanocomposites showed a similar solubility as PE, as confirmed by dynamic light scattering, and could be fully incorporated into a polyethylene matrix with different particle contents, as visualized by transmission electron microscopy. Using UV/vis-spectroscopy, we observed reversible, thermoresponsive aggregation/deaggregation properties in the nanohybrids, validating the strong and effective anchoring of PE on gold/silver surfaces.

Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection

Colloidal gold nanoparticles (Au NPs) have been used in several biological applications, which include the exploitation of size- and shape-dependent Localized Surface Plasmon Resonance (LSPR) in biosensing devices. In order to obtain functional and stable Au NPs in a physiological medium, surface modification and functionalization are crucial steps in these endeavors. Reversible addition-fragmentation chain transfer (RAFT) polymerization meets this need offering the possibility of control over the composition and architecture of polymeric shells coating Au NPs. Furthermore, playing with a careful choice of monomers, RAFT polymerization allows the possibility to design a polymer shell with the desired functional groups aiming at Au based nanocomposites suitable for biorecognition and biotargeting. This review provides important aspects concerning the synthesis and optical properties of Au NPs as well as concepts of RAFT polymerization. Understanding these concepts is crucial to appreciate the chemical strategies available towards RAFT-polymer coated Au core-shell nanostructures, which are here reviewed. Finally, examples of applications in opto-biodetection devices are provided and the potential of responsive "smart" nanomaterials based on such structures can be applied to other biological applications.

Metal Free Reversible-Deactivation Radical Polymerizations: Advances, Challenges, and Opportunities

A considerable amount of the worldwide industrial production of synthetic polymers is currently based on radical polymerization methods. The steadily increasing demand on high performance plastics and tailored polymers which serve specialized applications is driven by the development of new techniques to enable control of polymerization reactions on a molecular level. Contrary to conventional radical polymerization, reversible-deactivation radical polymerization (RDRP) techniques provide the possibility to prepare polymers with well-defined structures and functionalities. The review provides a comprehensive summary over the development of the three most important RDRP methods, which are nitroxide mediated radical polymerization, atom transfer radical polymerization and reversible addition fragmentation chain transfer polymerization. The focus thereby is set on the newest developments in transition metal free systems, which allow using these techniques for biological or biomedical applications. After each section selected examples from materials synthesis and application to biomedical materials are summarized.

Structure and Dynamics of a N-Methylfulleropyrrolidine-Mediated Gold Nanocomposite: A Spectroscopic Ruler

A mechanistic understanding of the structure and dynamics of a chemically tunable N-methylfulleropyrrolidine (8-NMFP)-assisted gold nanocomposite and its aggregation via a controllable interparticle interaction is reported as a function of the molar ratio and pH of the medium. Electronic structure calculations adopting density functional theory methods implied electrostatic interactions to play a dominant role between 8-NMFP and citrate-capped gold nanoparticles. MM⁺ molecular mechanics force field computations revealed intermolecular gold-gold interactions, contributing toward the formation of spherical composite aggregates. Corroborating these, optical absorption spectra showed the usual surface plasmon band along with a higher-wavelength feature at ∼600-650 nm, indicative of the aggregated nanocomposite. pH-controlled reversible tuning of the plasmonic features in the composite was evident in a pH interval ∼5-6.8, revealing prevalent interparticle electrostatic interactions. In addition, photoluminescence (PL) and time-correlated single-photon counting studies revealed a strong nanocomposite interaction with a pure fluorescent dye, Rhodamine B, indicating excitation energy transfer from the dye to the composite. The dye upon interaction with the nanocomposite showed a significant quenching of its PL intensity and shortening of lifetime. Energy coupling between the metal nanoparticle composite and the emitting molecular dipole resulted in a long-range surface energy transfer (SET) from the donor dye to the surface plasmon modes of the nanoparticle following a donor-acceptor distance dependence of 1/r⁴. This molecular beacon with correlation between the nanoscale structure and the nonradiative nanometal SET can be used as a spectroscopic/molecular ruler in probing advanced functional materials.

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.

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

The development of a straightforward method is reported to form hybrid polymer/gold planet-satellite nanostructures (PlSNs) with functional polymer. Polyacrylate type polymer with benzyl chloride in its backbone as a macromolecular tracer is synthesized to study its localization within PlSNs by analyzing the elemental distribution of chlorine. The functionalized nanohybrid structures are analyzed by scanning transmission electron microscopy, electron energy loss spectroscopy, and spectrum imaging. The results show that the RAFT (reversible addition-fragmentation chain transfer) polymers' sulfur containing end groups are colocalized at the gold cores, both within nanohybrids of simple core-shell morphology and within higher order PlSNs, providing microscopic evidence for the affinity of the RAFT group toward gold surfaces.

The Structure of Gold-Nanoparticle Networks Cross-Linked by Di- and Multifunctional RAFT Oligomers

Gold nanoparticle (AuNP) network structures featuring particles from the two-phase Brust-Schiffrin synthesis and linear RAFT oligomers of styrene with two and multiple trithiocarbonate (TTC) groups along their backbone have been investigated in detail. Insights into the internal structures of these particle networks could be obtained from small-angle X-ray scattering experiments, showing that primary AuNPs are cross-linked by the employed molecular linker. The extent of AuNP network formation was investigated by means of dynamic light scattering and UV/visible extinction spectroscopy, showing an abrupt attenuation of network formation after a critical degree of polymerization of the cross-linker is exceeded. Analysis of transmission electron micrographs indicated a three-dimensional shape of the particle superstructures, which is evenly filled with the primary AuNPs. From the results obtained in this study, guidelines for the fabrication of nanoparticle networks from the self-assembly with macromolecular cross-linkers are suggested.