Colloidally Stable and Surfactant-Free Protein-Coated Gold Nanorods in Biological Media

In this work, we investigate the ligand exchange of cetyltrimethylammonium bromide (CTAB) with bovine serum albumin for gold nanorods. We demonstrate by surface-enhanced Raman scattering measurements that CTAB, which is used as a shape-directing agent in the particle synthesis, is completely removed from solution and particle surface. Thus, the protein-coated nanorods are suitable for bioapplications, where cationic surfactants must be avoided. At the same time, the colloidal stability of the system is significantly increased, as evidenced by spectroscopic investigation of the particle longitudinal surface plasmon resonance, which is sensitive to aggregation. Particles are stable at very high concentrations (<i>c</i>_Au 20 mg/mL) in biological media such as phosphate buffer saline or Dulbecco’s Modified Eagle’s Medium and over a large pH range (2–12). Particles can even be freeze-dried (lyophilized) and redispersed. The protocol was applied to gold nanoparticles with a large range of aspect ratios and sizes with main absorption frequencies covering the visible and the near-IR spectral range from 600 to 1100 nm. Thus, these colloidally stable and surfactant-free protein-coated nanoparticles are of great interest for various plasmonic and biomedical applications

Colloidally Stable and Surfactant-Free Protein-Coated Gold Nanorods in Biological Media

In this work, we investigate the ligand exchange of cetyltrimethylammonium bromide (CTAB) with bovine serum albumin for gold nanorods. We demonstrate by surface-enhanced Raman scattering measurements that CTAB, which is used as a shape-directing agent in the particle synthesis, is completely removed from solution and particle surface. Thus, the protein-coated nanorods are suitable for bioapplications, where cationic surfactants must be avoided. At the same time, the colloidal stability of the system is significantly increased, as evidenced by spectroscopic investigation of the particle longitudinal surface plasmon resonance, which is sensitive to aggregation. Particles are stable at very high concentrations (<i>c</i>_Au 20 mg/mL) in biological media such as phosphate buffer saline or Dulbecco’s Modified Eagle’s Medium and over a large pH range (2–12). Particles can even be freeze-dried (lyophilized) and redispersed. The protocol was applied to gold nanoparticles with a large range of aspect ratios and sizes with main absorption frequencies covering the visible and the near-IR spectral range from 600 to 1100 nm. Thus, these colloidally stable and surfactant-free protein-coated nanoparticles are of great interest for various plasmonic and biomedical applications

Photochemical Synthesis of Polymeric Fiber Coatings and Their Embedding in Matrix Material: Morphology and Nanomechanical Properties at the Fiber–Matrix Interface

In this contribution, we present a three-step pathway to produce a novel fiber coating, study its embedding in epoxy resin and characterize its nanomechanical properties at the interface between fiber and matrix. Inorganic surfaces were sulfhydrylated for subsequent use in thiol-initiated ene photopolymerization. The influence of water on the sulfhydrylation process was studied to find conditions allowing monomolecular deposition. Surface morphology as well as SH-content were evaluated by UV/vis spectroscopy, atomic force microscopy and spectroscopic ellipsometry. Brush-like polymer layers (PS and PMMA) were introduced by UV-light initiated surface polymerization of vinyl monomers. Polymer growth and morphology were studied. After embedding, the nanomechanics of the interfacial region of the fibers was studied. AFM force spectroscopy allowed the mapping of the stiffness distribution at the cross-section of the composite with high spatial resolution. Elastic moduli were determined by Hertzian contact mechanics. The individual phases of the composite material (fiber, interphase, and matrix) can be clearly distinguished based on their mechanical response. The synthesis, morphology, and mechanical properties of an interphase based on a polymeric graft-film swollen with matrix material are shown, and perspectives of these novel coatings for improved matrix–fiber compatibility and interfacial adhesion are discussed

Injectable Shear-Thinning Fluorescent Hydrogel Formed by Cellulose Nanocrystals and Graphene Quantum Dots

In the search for new building blocks of nanofibrillar hydrogels, cellulose nanocrystals (CNCs) have attracted great interest because of their sustainability, biocompatibility, ease of surface functionalization, and mechanical strength. Making these hydrogels fluorescent extends the range of their applications in tissue engineering, bioimaging, and biosensing. We report the preparation and properties of a multifunctional hydrogel formed by CNCs and graphene quantum dots (GQDs). We show that although CNCs and GQDs are both negatively charged, hydrogen bonding and hydrophobic interactions overcome the electrostatic repulsion between these nanoparticles and yield a physically cross-linked hydrogel with tunable mechanical properties. Owing to their shear-thinning behavior, the CNC-GQD hydrogels were used as an injectable material in 3D printing. The hydrogels were fluorescent and had an anisotropic nanofibrillar structure. The combination of these advantageous properties makes this hybrid hydrogel a promising material and fosters the development of new manufacturing methods such as 3D printing

The Role of Substrate Wettability in Nanoparticle Transfer from Wrinkled Elastomers: Fundamentals and Application toward Hierarchical Patterning

We report on the role of surface wettability during the printing transfer of nanoparticles from wrinkled surfaces onto flat substrates. As we demonstrate, this parameter dominates the transfer process. This effect can further be utilized to transfer colloidal particles in a structured fashion, if the substrates are patterned in wettability. The resulting colloidal arrangements are highly regular over macroscopic surface areas and display distinct pattern features in both the micrometer and nanoscale regime. We study the obtained structures and discuss the potential of this approach for creating hierarchical particle assemblies of high complexity. Our findings not only contribute to a better understanding of technologically relevant colloidal assembly processes, but also open new avenues for the realization of novel materials consisting of nanoparticles. In this regard, the presented structuring method is especially interesting for the design of optically functional surface coatings

Silver-Overgrowth-Induced Changes in Intrinsic Optical Properties of Gold Nanorods: From Noninvasive Monitoring of Growth Kinetics to Tailoring Internal Mirror Charges

We investigate the effect of surfactant-mediated, asymmetric silver overgrowth of gold nanorods on their intrinsic optical properties. From concentration-dependent experiments, we established a close correlation of the extinction in the UV/vis/NIR frequency range and the morphological transition from gold nanorods to Au@Ag cuboids. Based on this correlation, a generic methodology for <i>in situ</i> monitoring of the evolution of the cuboid morphology was developed and applied in time-dependent experiments. We find that growth rates are sensitive to the substitution of the surfactant headgroup by comparison of benzylhexadecyldimethylammonium chloride (BDAC) with hexadecyltrimethyl­ammonium chloride (CTAC). The time-dependent overgrowth in BDAC proceeds about 1 order of magnitude slower than in CTAC, which allows for higher control during silver overgrowth. Furthermore, silver overgrowth results in a qualitatively novel optical feature: Upon excitation inside the overlap region of the interband transition of gold and intraband of silver, the gold core acts as a retarding element. The much higher damping of the gold core compared to the silver shell in Au@Ag cuboids induces mirror charges at the core/shell interface as shown by electromagnetic simulations. Full control over the kinetic growth process consequently allows for precise tailoring of the resonance wavelengths of both modes. Tailored and asymmetric silver-overgrown gold nanorods are of particular interest for large-scale fabrication of nanoparticles with intrinsic metamaterial properties. These building blocks could furthermore find application in optical sensor technology, light harvesting, and information technology

Shape-Specific Patterning of Polymer-Functionalized Nanoparticles

Chemically and topographically patterned nanoparticles (NPs) with dimensions on the order of tens of nanometers have a diverse range of applications and are a valuable system for fundamental research. Recently, thermodynamically controlled segregation of a smooth layer of polymer ligands into pinned micelles (patches) offered an approach to nanopatterning of polymer-functionalized NPs. Control of the patch number, size, and spatial distribution on the surface of spherical NPs has been achieved, however, the role of NP shape remained elusive. In the present work, we report the role of NP shape, namely, the effect of the local surface curvature, on polymer segregation into surface patches. For polymer-functionalized metal nanocubes, we show experimentally and theoretically that the patches form preferentially on the high-curvature regions such as vertices and edges. An <i>in situ</i> transformation of the nanocubes into nanospheres leads to the change in the number and distribution of patches; a process that is dominated by the balance between the surface energy and the stretching energy of the polymer ligands. The experimental and theoretical results presented in this work are applicable to surface patterning of polymer-capped NPs with different shapes, thus enabling the exploration of patch-directed self-assembly, as colloidal surfactants, and as templates for the synthesis of hybrid nanomaterials

Controlled Living Nanowire Growth: Precise Control over the Morphology and Optical Properties of AgAuAg Bimetallic Nanowires

Inspired by the concept of living polymerization reaction, we are able to produce silver–gold–silver nanowires with a precise control over their total length and plasmonic properties by establishing a constant silver deposition rate on the tips of penta-twinned gold nanorods used as seed cores. Consequently, the length of the wires increases linearly in time. Starting with ∼210 nm × 32 nm gold cores, we produce nanowire lengths up to several microns in a highly controlled manner, with a small self-limited increase in thickness of ∼4 nm, corresponding to aspect ratios above 100, whereas the low polydispersity of the product allows us to detect up to nine distinguishable plasmonic resonances in a single colloidal solution. We analyze the spatial distribution and the nature of the plasmons by electron energy loss spectroscopy and obtain excellent agreement between measurements and electromagnetic simulations, clearly demonstrating that the presence of the gold core plays a marginal role, except for relatively short wires or high-energy modes