Light Scattering versus Plasmon Effects: Optical Transitions in Molecular Oxygen near a Metal Nanoparticle

The localized surface plasmon of a metal nanoparticle can influence the optical properties of a molecule in the plasmon field. In a previous study of molecular oxygen adjacent to nanodisks on a flat substrate, we showed that a plasmon field can increase the probability of the O₂(a¹Δ_g) → O₂(X³Σ_g^–) radiative transition at 1275 nm. For the present study, we set out to ascertain if metal nanoparticles suspended in a liquid solvent could likewise induce measurable plasmonic effects on optical transitions in oxygen. Metal nanoparticles were prepared with the intent of selectively perturbing the 765 nm O₂(X³Σ_g^–) → O₂(b¹Σ_g⁺) absorption transition. Because O₂(b¹Σ_g⁺) efficiently decays to O₂(a¹Δ_g), we used the spectrally distinct O₂(a¹Δ_g) → O₂(X³Σ_g^–) phosphorescent transition at 1275 nm to probe the potential plasmon effects at 765 nm. Although we indeed observed nanoparticle-mediated effects on the O₂(X³Σ_g^–) → O₂(b¹Σ_g⁺) transition, our present data are readily explained in terms of a nanoparticle-dependent change in the path length of light propagation through the sample. We modeled the latter using features of radiative transfer theory. As such, we cannot claim to observe a plasmonic effect on oxygen from these nanoparticles suspended in solution. Instead, our results point to the general importance of considering the effects of light scattering, certainly for experiments on suspended metal nanoparticles. Indeed, the extent to which light scattering can influence such optical experiments leads us to infer that many claims of a plasmonic effect could be misassigned

Shape-Templated Growth of Au@Cu Nanoparticles

We report the formation of copper nanoparticles with various morphologies and low polydispersity, using Au nanoparticles as templates. This seeded growth strategy is based on the reduction of Cu²⁺ with hydrazine in water at low temperature. Additionally, the use of poly­(acrylic acid) as capping agent allows synthesis under aerobic conditions. The dimensions of the resulting Au@Cu nanoparticles can be readily tuned through either the dimensions of the Au cores or the Cu/Au molar ratio. Although Au and Cu show a significant lattice mismatch, epitaxial growth of Cu onto single crystal Au nanorods was confirmed through high-resolution electron microscopy and electron diffraction analysis. The effects of core morphology on the optical properties of the core–shell nanoparticles were analyzed by vis-NIR spectroscopy and were found to agree with simulations based on the boundary element method. This work contributes to understand the strong effect of interband transitions on the optical response of Au@Cu and to confirm the importance of tuning the localized surface plasmon resonance away from the interband transitions

Static and Dynamic Plasmon-Enhanced Light Scattering from Dispersions of Polymer-Grafted Silver Nanoprisms in the Bulk and Near Solid Surfaces

Polarized (VV) and depolarized (VH) static (SLS) and dynamic light scattering (DLS) experiments were conducted in dispersions of sterically stabilized silver nanoprisms in three different solvents where strong plasmon-enhanced scattering was observed. In the dilute regime, hydrodynamic sizes obtained from VV and VH were in good agreement with TEM data. VV correlation functions revealed two relaxation modes, reflecting the translational and rotational diffusions unambiguously. Increasing the concentration, the bimodal nature of the correlation functions was retained, and it appeared that the VH correlation function was more strongly influenced. Evanescent-wave DLS was shown to probe rotational and translational diffusion in the vicinity of a hard wall. It is suggested that DLS methodologies can be successfully applied to this type of metallic nanoparticles for characterization and exploration of their dynamics

Effect of the Cross-Linking Density on the Thermoresponsive Behavior of Hollow PNIPAM Microgels

We report on the fabrication of thermally responsive hollow pNIPAM particles through the oxidation of the metal core in an Au@pNIPAM system. The selective oxidation of the Au core is achieved by addition of AuCl₄^– to an aqueous dispersion of Au@pNIPAM particles in the presence of cetyltrimethylammonium bromide (CTAB). We fabricate hollow pNIPAM particles with three cross-linking densities (<i>N,N</i>′-methylenebis­(acrylamide), BA, at 5%, 10%, and 17.5%). The study of the effect of the amount of BA within the microgel network was performed by dynamic light scattering (DLS), transmission electron microscopy (TEM), and atomic force microscopy (AFM), showing its key role in determining the final hollow structure and thermal response. While the thermal responsiveness is largely achieved at low cross-linking densities, the hollow structure only remains at larger cross-linking densities. This was further confirmed by cryo-TEM analysis of hollow pNIPAM particles below and above the volume phase transition temperature (VPTT). Thus, it clearly shows (i) the shrinking of particle size with the temperature at low cross-linking density and (ii) the dependence of particle size on the amount of cross-linker for the final hollow pNIPAM structure. Observed differences in the hollow pNIPAM structure are attributed to different elastic contributions (Π_elas), showing higher elasticity for microgels synthesized at lower amount of BA

Nickel Nanoparticle-Doped Paper as a Bioactive Scaffold for Targeted and Robust Immobilization of Functional Proteins

Cellulose-based materials are widely used in analytical chemistry as platforms for chromatographic and immunodiagnostic techniques. Due to its countless advantages (<i>e.g.</i>, mechanical properties, three-dimensional structure, large surface to volume area, biocompatibility and biodegradability, and high industrial availability), paper has been rediscovered as a valuable substrate for sensors. Polymeric materials such as cellulosic paper present high protein capture ability, resulting in a large increase of detection signal and improved assay sensitivity. However, cellulose is a rather nonreactive material for direct chemical coupling. Aiming at developing an efficient method for controlled conjugation of cellulose-based materials with proteins, we devised and fabricated a hybrid scaffold based on the adsorption and <i>in situ</i> self-assembly of surface-oxidized Ni nanoparticles on filter paper, which serve as “docking sites” for the selective immobilization of proteins containing polyhistidine tags (His-tag). We demonstrate that the interaction between the nickel substrate and the His-tagged protein G is remarkably resilient toward chemicals at concentrations that quickly disrupt standard Ni-NTA and Ni-IDA complexes, so that this system can be used for applications in which a robust attachment is desired. The bioconjugation with His-tagged protein G allowed the binding of anti-<i>Salmonella</i> antibodies that mediated the immuno-capture of live and motile <i>Salmonella</i> bacteria. The versatility and biocompatibility of the nickel substrate were further demonstrated by enzymatic reactions

Silver Ions Direct Twin-Plane Formation during the Overgrowth of Single-Crystal Gold Nanoparticles

It is commonly agreed that the crystalline structure of seeds dictates the crystallinity of final nanoparticles in a seeded-growth process. Although the formation of monocrystalline particles does require the use of single-crystal seeds, twin planes may stem from either single- or polycrystalline seeds. However, experimental control over twin-plane formation remains difficult to achieve synthetically. Here, we show that a careful interplay between kinetics and selective surface passivation offers a unique handle over the emergence of twin planes (in decahedra and triangles) during the growth over single-crystalline gold nanoparticles of quasi-spherical shape. Twinning can be suppressed under conditions of slow kinetics in the presence of silver ions, yielding single-crystalline particles with high-index facets

Gold Nanooctahedra with Tunable Size and Microfluidic-Induced 3D Assembly for Highly Uniform SERS-Active Supercrystals

Shape-controlled synthesis of uniform noble metal nanoparticles (NPs) is crucial for the development of future plasmonic devices. The use of nanocrystals with well-defined morphologies and crystallinity as seed particles is expected to provide excellent shape control and monodispersity. We report the aqueous-based seed-mediated growth of monodisperse gold octahedra with wide range of sizes (50–150 nm in side length) by reducing different amounts of HAuCl₄ on preformed single crystalline gold nanorods using butenoic acid as reducing agent. Butenoic acid plays a key role as a mild reducing agent as well as favoring the thermodynamic control of the reaction. The uniformity of the as-prepared Au octahedra combined with the use of a microfluidic technique based on microevaporation will allow the self-assembly of octahedra into uniform 3D supercrystals. Additionally, these plasmonic substrates exhibit high and uniform SERS signals over extended areas with intensities increasing with the Au nanoparticle size

Seedless Synthesis of Single Crystalline Au Nanoparticles with Unusual Shapes and Tunable LSPR in the near-IR

The plasmonic properties of metal nanoparticles have acquired great importance because of their potential applications in very diverse fields. Metal nanoparticles with localized surface plasmon resonances (LSPR) in the near-infrared (NIR, 750–1300 nm) are of particular interest because tissues, blood, and water display low absorption in this spectral range, thus facilitating biomedical applications. Cetyltrimethylammonium chloride (CTAC) was used to induce the seedless formation of highly anisotropic, twisted single crystalline Au nanoparticles in a single step. The LSPR of the obtained particles can be tuned from 600 nm up to 1400 nm by simply changing the reaction temperature or the reagents concentrations. The tunability of the LSPR is closely associated with significant changes in the final particle morphology, which was studied by advanced electron microscopy techniques (3D Tomography and HAADF-STEM). Kinetic experiments were carried out to establish the growth mechanism, suggesting that slow kinetics together with the complexation of the gold salt precursor to CTAC are key factors favoring the formation of these anisotropic particles

Seedless Synthesis of Single Crystalline Au Nanoparticles with Unusual Shapes and Tunable LSPR in the near-IR

The plasmonic properties of metal nanoparticles have acquired great importance because of their potential applications in very diverse fields. Metal nanoparticles with localized surface plasmon resonances (LSPR) in the near-infrared (NIR, 750–1300 nm) are of particular interest because tissues, blood, and water display low absorption in this spectral range, thus facilitating biomedical applications. Cetyltrimethylammonium chloride (CTAC) was used to induce the seedless formation of highly anisotropic, twisted single crystalline Au nanoparticles in a single step. The LSPR of the obtained particles can be tuned from 600 nm up to 1400 nm by simply changing the reaction temperature or the reagents concentrations. The tunability of the LSPR is closely associated with significant changes in the final particle morphology, which was studied by advanced electron microscopy techniques (3D Tomography and HAADF-STEM). Kinetic experiments were carried out to establish the growth mechanism, suggesting that slow kinetics together with the complexation of the gold salt precursor to CTAC are key factors favoring the formation of these anisotropic particles