Magnetic-plasmonic nanoparticles for the life sciences: calculated optical properties of hybrid structures.

Magnetic-plasmonic nanoparticles, combining magnetic and plasmonic components, are promising structures for use in life sciences. Optical properties of core-shell magnetite-gold nanostructures, such as the wavelength of the plasmon resonance, the extinction cross-section, and the ratio of scattering to absorption at the plasmon wavelength are critical parameters in the search for the most suitable particles for envisioned applications. Using Mie theory and the discrete dipole approximation (DDA), optical spectra as a function of composition, size, and shape of core-shell nanospheres and nanorods were calculated. Calculations were done using simulated aqueous media, used throughout the life sciences. Our results indicate that in the advantageous near-infrared region (NIR), although magnetic-plasmonic nanospheres produced by available chemical methods lack the desirable tunability of optical characteristics, magnetic-plasmonic nanorods can achieve the desired optical properties at chemically attainable dimensions. The presented results can aid in the selection of suitable magnetic-plasmonic structures for applications in life sciences. FROM THE CLINICAL EDITOR: In this basic science study, magnetic-plasmonic nanoparticles are studied for future applications in life sciences. Optical properties of core-shell magnetite-gold nanostructures, such as the wavelength of the plasmon resonance, the extinction cross-section, and the ratio of scattering to absorption at the plasmon wavelength are critical parameters in the search for the most suitable particles for proposed future applications.

Resolving enantiomers using the optical angular momentum of twisted light.

Circular dichroism and optical rotation are crucial for the characterization of chiral molecules and are of importance to the study of pharmaceutical drugs, proteins, DNA, and many others. These techniques are based on the different interactions of enantiomers with circularly polarized components of plane wave light that carries spin angular momentum (SAM). For light carrying orbital angular momentum (OAM), for example, twisted or helical light, the consensus is that it cannot engage with the chirality of a molecular system as previous studies failed to demonstrate an interaction between optical OAM and chiral molecules. Using unique nanoparticle aggregates, we prove that optical OAM can engage with materials' chirality and discriminate between enantiomers. Further, theoretical results show that compared to circular dichroism, mainly based on magnetic dipole contributions, the OAM analog helical dichroism (HD) is critically dependent on fundamentally different chiral electric quadrupole contributions. Our work opens new venues to study chirality and can find application in sensing and chiral spectroscopy.

Tunability of Size and Magnetic Moment of Iron Oxide Nanoparticles Synthesized by Forced Hydrolysis.

To utilize iron oxide nanoparticles in biomedical applications, a sufficient magnetic moment is crucial. Since this magnetic moment is directly proportional to the size of the superparamagnetic nanoparticles, synthesis methods of superparamagnetic iron oxide nanoparticles with tunable size are desirable. However, most existing protocols are plagued by several drawbacks. Presented here is a one-pot synthesis method resulting in monodisperse superparamagnetic iron oxide nanoparticles with a controllable size and magnetic moment using cost-effective reagents. The obtained nanoparticles were thoroughly characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) measurements. Furthermore, the influence of the size on the magnetic moment of the nanoparticles is analyzed by superconducting quantum interference device (SQUID) magnetometry. To emphasize the potential use in biomedical applications, magnetic heating experiments were performed.

Direct Fabrication of Monodisperse Silica Nanorings from Hollow Spheres - A Template for Core-Shell Nanorings.

We report a new type of nanosphere colloidal lithography to directly fabricate monodisperse silica (SiO2) nanorings by means of reactive ion etching of hollow SiO2 spheres. Detailed TEM, SEM, and AFM structural analysis is complemented by a model describing the geometrical transition from hollow sphere to ring during the etching process. The resulting silica nanorings can be readily redispersed in solution and subsequently serve as universal templates for the synthesis of ring-shaped core-shell nanostructures. As an example we used silica nanorings (with diameter of ∼200 nm) to create a novel plasmonic nanoparticle topology, a silica-Au core-shell nanoring, by self-assembly of Au nanoparticles (<20 nm) on the ring's surface. Spectroscopic measurements and finite difference time domain simulations reveal high quality factor multipolar and antibonding surface plasmon resonances in the near-infrared. By loading different types of nanoparticles on the silica core, hybrid and multifunctional composite nanoring structures could be realized for applications such as MRI contrast enhancement, catalysis, drug delivery, plasmonic and magnetic hyperthermia, photoacoustic imaging, and biochemical sensing.

Broadband nonreciprocal quadrupolarization-induced asymmetric transmission (Q-AT) in plasmonic nanoparticle aggregates.

Non-reciprocal asymmetric transmission, i.e., the dependence of optical transmittance on the direction of light propagation in the material, can be used in optical isolators or photonic circuits. Broadband asymmetric transmission is observed in near-field coupled aggregates of small plasmonic nanoparticles, even for unpolarized light. Non-reciprocity is demonstrated and, using a phenomenological model, induced electric quadrupole moments are identified as the root cause of the effect.

Selective uptake of rare earths from aqueous solutions by EDTA-functionalized magnetic and nonmagnetic nanoparticles.

Magnetic (Fe3O4) and nonmagnetic (SiO2 and TiO2) nanoparticles were decorated on their surface with N-[(3-trimethoxysilyl)propyl]ethylenediamine triacetic acid (TMS-EDTA). The aim was to investigate the influence of the substrate on the behavior of these immobilized metal coordinating groups. The nanoparticles functionalized with TMS-EDTA were used for the adsorption and separation of trivalent rare-earth ions from aqueous solutions. The general adsorption capacity of the nanoparticles was very high (100 to 400 mg/g) due to their large surface area. The heavy rare-earth ions are known to have a higher affinity for the coordinating groups than the light rare-earth ions but an additional difference in selectivity was observed between the different nanoparticles. The separation of pairs of rare-earth ions was found to be dependent on the substrate, namely the density of EDTA groups on the surface. The observation that sterical hindrance (or crowding) of immobilized ligands influences the selectivity could provide a new tool for the fine-tuning of the coordination ability of traditional chelating ligands.

Ala-7, His-10 and Arg-12 are crucial amino acids for activity of a synthetically engineered µ-conotoxin.

Cone snail toxins or conotoxins are often small cysteine-rich peptides which have shown to be highly selective ligands for a wide range of ion channels such as voltage-gated sodium channels (Na(V)s). Na(V)s participate in a wide range of electrophysiological processes. Consequently, their malfunction has been associated with numerous diseases. The development of subtype-selective modulators of Na(V)s remains highly important in the treatment of such disorders. In order to expand our knowledge in the search for novel therapeutics to treat Na(V)-related diseases, we explored the field of peptide engineering. In the current study, the impact of well considered point mutations into a bioactive peptide that was found to be a very potent and selective inhibitor of Na(V)s (i.e. Midi R2) was examined. We designed two peptides, named Midi R2[A7G] and Midi R2[H10A, R12A] which have mutations at position 7, and both 10 and 12, respectively. Electrophysiological recordings indicated that an Ala to Gly mutation at position 7 increased IC50-values from the nanomolar range to the micromolar range. For Midi R2[H10A, R12A] at a concentration of 10 µM, activity is even reduced to 0-10% for all of the tested Na(V)-channels. Circular dichroism measurements proved that overall structural conformations did not change. These findings suggest that the minimal space between the second and the third intercysteine loop of Midi R2 is the sequence RRWARDHSR and that His at position 10 and Arg at position 12 are crucial amino acids for the potency and specificity of Midi R2. In this way, new insights into the structure-activity relationships of µ-conotoxins were found.

Synthesis and Characterization of Holmium-Doped Iron Oxide Nanoparticles.

Rare earth atoms exhibit several interesting properties, for example, large magnetic moments and luminescence. Introducing these atoms into a different matrix can lead to a material that shows multiple interesting effects. Holmium atoms were incorporated into an iron oxide nanoparticle and the concentration of the dopant atom was changed in order to determine its influence on the host crystal. Its magnetic and magneto-optical properties were investigated by vibrating sample magnetometry and Faraday rotation measurements. The luminescent characteristics of the material, in solution and incorporated in a polymer thin film, were probed by fluorescence experiments.

Improved functionalization of oleic acid-coated iron oxide nanoparticles for biomedical applications.

Superparamagnetic iron oxide nanoparticles can provide multiple benefits for biomedical applications in aqueous environments such as magnetic separation or magnetic resonance imaging. To increase the colloidal stability and allow subsequent reactions, the introduction of hydrophilic functional groups onto the particles' surface is essential. During this process, the original coating is exchanged by preferably covalently bonded ligands such as trialkoxysilanes. The duration of the silane exchange reaction, which commonly takes more than 24 h, is an important drawback for this approach. In this paper, we present a novel method, which introduces ultrasonication as an energy source to dramatically accelerate this process, resulting in high-quality water-dispersible nanoparticles around 10 nm in size. To prove the generic character, different functional groups were introduced on the surface including polyethylene glycol chains, carboxylic acid, amine, and thiol groups. Their colloidal stability in various aqueous buffer solutions as well as human plasma and serum was investigated to allow implementation in biomedical and sensing applications. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11051-012-1100-5) contains supplementary material, which is available to authorized users.