Munish Chanana

Strongly Coupled Plasmonic Modes on Macroscopic Areas via Template-Assisted Colloidal Self-Assembly

We present ensembles of surface-ordered nanoparticle arrangements, which are formed by template-assisted self-assembly of monodisperse, protein-coated gold nanoparticles in wrinkle templates. Centimeter-squared areas of highly regular, linear assemblies with tunable line width are fabricated and their extinction cross sections can be characterized by conventional UV/vis/NIR spectroscopy. Modeling based on electrodynamic simulations shows a clear signature of strong plasmonic coupling with an interparticle spacing of 1–2 nm. We find evidence for well-defined plasmonic modes of quasi-infinite chains, such as resonance splitting and multiple radiant modes. Beyond elementary simulations on the individual chain level, we introduce an advanced model, which considers the chain length distribution as well as disorder. The step toward macroscopic sample areas not only opens perspectives for a range of applications in sensing, plasmonic light harvesting, surface enhanced spectroscopy, and information technology but also eases the investigation of hybridization and metamaterial effects fundamentally.

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 (cAu 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.

Magnetic Porous Sugar-Functionalized PEG Microgels for Efficient Isolation and Removal of Bacteria from Solution

Here, we present a new microparticle system for the selective detection and magnetic removal of bacteria from contaminated solutions. The novelty of this system lies in the combination of a biocompatible scaffold reducing unspecific interactions with high capacity for bacteria binding. We apply highly porous poly(ethylene glycol) (PEG) microparticles and functionalize them, introducing both sugar ligands for specific bacteria targeting and cationic moieties for electrostatic loading of superparamagnetic iron oxide nanoparticles. The resulting magnetic, porous, sugar-functionalized (MaPoS) PEG microgels are able to selectively bind and discriminate between different strains of bacteria Escherichia coli . Furthermore, they allow for a highly efficient removal of bacteria from solution as their increased surface area can bind three times more bacteria than nonporous particles. All in all, MaPoS particles represent a novel generation of magnetic beads introducing for the first time a porous, biocompatible and easy to functionalize scaffold and show great potential for various biotechnological applications.

Protein-Assisted Assembly of Modular 3D Plasmonic Raspberry-like Core/Satellite Nanoclusters: Correlation of Structure and Optical Properties

We present a bottom-up assembly route for a large-scale organization of plasmonic nanoparticles (NPs) into three-dimensional (3D) modular assemblies with core/satellite structure. The protein-assisted assembly of small spherical gold or silver NPs with a hydrophilic protein shell (as satellites) onto larger metal NPs (as cores) offers high modularity in sizes and composition at high satellite coverage (close to the jamming limit). The resulting dispersions of metal/metal nanoclusters exhibit high colloidal stability and therefore allow for high concentrations and a precise characterization of the nanocluster architecture in dispersion by small-angle X-ray scattering (SAXS). Strong near-field coupling between the building blocks results in distinct regimes of dominant satellite-to-satellite and core-to-satellite coupling. High robustness against satellite disorder was proved by UV/vis diffuse reflectance (integrating sphere) measurements. Generalized multiparticle Mie theory (GMMT) simulations were employed to describe the electromagnetic coupling within the nanoclusters. The close correlation of structure and optical property allows for the rational design of core/satellite nanoclusters with tailored plasmonics and well-defined near-field enhancement, with perspectives for applications such as surface-enhanced spectroscopies.

Formation Mechanism for Stable Hybrid Clusters of Proteins and Nanoparticles.

Citrate-stabilized gold nanoparticles (AuNP) agglomerate in the presence of hemoglobin (Hb) at acidic pH. The extent of agglomeration strongly depends on the concentration ratio [Hb]/[AuNP]. Negligible agglomeration occurs at very low and very high [Hb]/[AuNP]. Full agglomeration and precipitation occur at [Hb]/[AuNP] corresponding to an Hb monolayer on the AuNP. Ratios above and below this value lead to the formation of an unexpected phase: stable, microscopic AuNP-Hb agglomerates. We investigated the kinetics of agglomeration with dynamic light scattering and the adsorption kinetics of Hb on planar gold with surface-acoustic wave-phase measurements. Comparing agglomeration and adsorption kinetics leads to an explanation of the complex behavior of this nanoparticle-protein mixture. Agglomeration is initiated either when Hb bridges AuNP or when the electrostatic repulsion between AuNP is neutralized by Hb. It is terminated when Hb has been depleted or when Hb forms multilayers on the agglomerates that stabilize microscopic clusters indefinitely.

Biocompatible Magnetite Nanoparticles Trapped at the Air/Water Interface

In the last decade, iron oxide nanoparticles (NPs)—known as super-paramagnetic iron oxide nanoparticles (SPION)—have received significant attention due to their applicability in a variety of domains. In the biomedical field, iron oxide nanoparticles are very promising as drug-delivery systems, magneticresonance-imaging contrast enhancers (clinical diagnosis), inflammation-responsive or anti-cancer agents, or for labeling and cell separation. In addition to their potential medical applications, magnetic nanoparticles are of great interest in materials science, for the development of magnetic data-recording media and nanocomposite permanent magnets, as well as in catalysis and colloid chemistry, where ferrofluids represent a topic of high interest. This study is focused on Fe3O4 NPs grafted with the thermosensitive and biocompatible copolymer MEO2MA/OEGMA, [10] whose interfacial behavior is yet unknown. Many papers have been dedicated to the formation, organization, and stability of Langmuir films of iron oxide (Fe2O3 or Fe3O4) NPs and their Langmuir–Blodgett transfer. The system studied herein is completely different from the iron oxide NPs already studied at the air/water interface due to the unique properties of the polymer used. A monodisperse population of uncharged Fe3O4 cores with a diameter of 6.4 nm, which are grafted with catechol-terminated copolymers of 2-(2-methoxyethoxy) ethyl methacrylate (MEO2 MA) and oligo(ethylene glycol) methacrylate (OEGMA), has been investigated (Figure 1). Due to the presence of oligo(ethylene glycol) side groups on the surfaces, the Fe3O4@MEO2MA90-co-OEGMA10 NPs (90 and 10 represent the molar fractions of MEO2MA and OEGMA, respectively) can be dispersed in water, exhibiting a high colloidal stability against salt and, at the same time, they can be well dispersed in organic solvents such as chloroform, ethanol or toluene. This specific copolymer in water (no salt) is hydrophilic and becomes hydrophobic at temperatures above 40 8C. The experimental details are presented in the Supporting Information. Since the polymer and the polymer-dressed NPs behave exactly in the same way, showing that the polymer dictates the surface activity, we will describe here mainly the results obtained with the polymer-dressed NPs because this system can be investigated with a larger number of methods giving complementary information. The surface activity of the Fe3O4@MEO2MA90-co-OEGMA10 NPs is based on the amphiphilic character of the copolymer shell. This (oligo ethylene glycol) methyl ether methacrylate polymer has a graft structure (Figure 1) composed of an apolar carbon–carbon backbone which leads to a competitive hydrophobic effect and multiple oligo(ethylene glycol) side chains of which ether oxygen atoms form stabilizing hydrogen bonds with water. Moreover, the ethylene oxide motif can adopt a configuration with the oxygen atoms on one side of the molecule and with the two methylene groups on the other, thus giving the molecule both a hydrophilic and a hydrophobic surface. The hydrophobicity can be tuned by changing the molar fraction of the two monomers. The polymer can be dispersed both in water and in chloroform; therefore, the corresponding films have been prepared either by adsorption from aqueous bulk solution or by spreading from a chloroform solution at the interface. Using, for example, a NP bulk concentration of 1.5 10 3 mg mL , a constant surface pressure value of approximately 23 mN m 1 is reached after 20 h (Figure 2 A). Compression isotherms of Langmuir layers formed by spreading certain amounts of NPs are shown in Figure 2 B. By compression, the surface pressure increases continuously to 25 mN m 1 (critical pressure pc of the polymer as well as the NP film). During further compression, a plateau region appears at which the surface pressure increases only slightly up to a maximum value of 27 mN m . It is very important to highlight that no hysteresis of the compression/ expansion isotherms is observed when the interfacial film is compressed to surface pressures below the critical pressure of the Langmuir layer (Figure 2 C), suggesting that no loss of material from the interface occurs. Based on this observation, we can calculate the interfacial concentration of NPs corresponding to the critical pressure. The critical concentration amounts to (7.7 0.6) 10 4 mg cm . This value is in good agreement with the NP interfacial concentration of 8.2 10 4 mg cm , which can be calFigure 1. Schematic representation of the Fe3O4 @ MEO2MA90-co-OEGMA10 NPs

Bidirectional Nanoparticle Crossing of Oil–Water Interfaces Induced by Different Stimuli: Insight into Phase Transfer

Swap transactions: Bidirectional spontaneous transfer of gold nanoparticles coated with stimuli-responsive polymer brushes across oil-water interfaces has been implemented (see picture). The water-to-oil transfer of the gold nanoparticles is dictated by the ionic strength in water, while the nanoparticle oil-to-water transfer occurs only when the environmental temperature is reduced below 5 °C.

Polymer brush controlled bioinspired calcium phosphate mineralization and bone cell growth.

Polymer brushes on thiol-modified gold surfaces were synthesized by using terminal thiol groups for the surface-initiated free radical polymerization of methacrylic acid and dimethylaminoethyl methacrylate, respectively. Atomic force microscopy shows that the resulting poly(methacrylic acid) (PMAA) and poly(dimethylaminoethyl methacrylate) (PDMAEMA) brushes are homogeneous. Contact angle measurements show that the brushes are pH-responsive and can reversibly be protonated and deprotonated. Mineralization of the brushes with calcium phosphate at different pH yields homogeneously mineralized surfaces, and preosteoblastic cells proliferate on both the nonmineralized and mineralized surfaces. The number of living cells on the mineralized hybrid surfaces is ca. 3 times (PDMAEMA) and 10 times (PMAA) higher than on the corresponding nonmineralized brushes.

A novel class of potential prion drugs: preliminary in vitro and in vivo data for multilayer coated gold nanoparticles

Gold nanoparticles coated with oppositely charged polyelectrolytes, such as polyallylamine hydrochloride and polystyrenesulfonate, were examined for potential inhibition of prion protein aggregation and prion (PrPSc) conversion and replication. Different coatings, finishing with a positive or negative layer, were tested, and different numbers of layers were investigated for their ability to interact and reduce the accumulation of PrPSc in scrapie prion infected ScGT1 and ScN2a cells. The particles efficiently hampered the accumulation of PrPSc in ScN2a cells and showed curing effects on ScGT1 cells with a nanoparticle concentration in the picomolar range. Finally, incubation periods of prion-infected mice treated with nanomolar concentrations of gold nanoparticles were significantly longer compared to untreated controls.

Langmuir and Gibbs Magnetite NP Layers at the Air/Water Interface

The interfacial properties of Fe(3)O(4)@MEO(2)MA(90)-co-OEGMA(10) NPs, recently developed and described as promising nanotools for biomedical applications, have been investigated at the air/water interface. These Fe(3)O(4) NPs, capped with catechol-terminated random copolymer brushes of 2-(2-methoxyethoxy) ethyl methacrylate (MEO(2)MA) and oligo(ethylene glycol) methacrylate (OEGMA), with molar fractions of 90% and 10%, respectively, proved to be surface active. Surface tension measurements of aqueous dispersions of the NPs showed that the adsorption of the NPs at the air/water interface is time- and concentration-dependent. These NPs do not behave as classical amphiphiles. Once adsorbed at the air/water interface, they do not exchange with NPs in bulk, but they are trapped at the interface. This means that all NPs from the bulk adsorb to the interface until reaching maximum coverage of the interface, which corresponds to values between 6 × 10(-4) and 8 × 10(-4) mg/cm(2) and a critical equilibrium surface tension of ∼47 mN/m. Moreover, Langmuir layers of Fe(3)O(4)@MEO(2)MA(90)-co-OEGMA(10) NPs have been investigated by measuring surface pressure-area compression-expansion isotherms and in situ X-ray fluorescence spectra. The compression-expansion isotherms showed a plateau region above a critical surface pressure of ∼25 mN/m and a pronounced hysteresis. By using a special one-barrier Langmuir trough equipped with two surface pressure microbalances, we have shown that the NPs are squeezed out from the interface into the aqueous subphase, and they readsorb on the other side of the barrier. The results have been supported by TEM as well as AFM experiments of transferred Langmuir-Schaefer films on solid supports. This study shows the ability of Fe(3)O(4)@MEO(2)MA(90)-co-OEGMA(10) NPs to transfer from hydrophilic media (an aqueous solution) to the hydrophobic/hydrophilic interface (air/water interface) and back to the hydrophilic media. This behavior is very promising, opening studies of their ability to cross biological membranes.