Bertrand Vileno

Magnetic Properties of Gold Nanoparticles: A Room-Temperature Quantum Effect

Gold nanoparticles elicit a huge research activity in view of their applications in diagnostics,[1, 2] therapy,[3] drug or gene delivery,[ 4] sensing[5, 6, 7] and imaging.[8] Gold nanoparticles also display interesting catalytic[9, 10] and optical[11, 12, 13, 14] properties. This Communication focuses on the least understood and so far unused property of gold: its becoming magnetic when prepared in the form of nanoparticles. All these desirable properties, bound together in one nanometric piece of matter, possibly self-organized thanks to its ligands, make functionalized gold nanoparticles a treasurable entity for nanosciences. The ex nihilo magnetic properties of functionalized gold (and other diamagnetic metals, such a Ag or Cu) nanoparticles, that is, their ferromagnetic-like behavior, are well documented, though still poorly understood.[15] This unexpected property opens new perspectives in materials science, in particular for the design of metamaterials. One may also envisage applications in information storage and processing: nanometric magnetic particles with no obvious temperature limitation and possibly self-organizing are currently much sought-after by the computer industry and developing a room-temperature magnetic semiconductor is paramount for the realization of spintronics technologies. Herein, we wish to present the results of our own investigations into the magnetic properties of functionalized gold nanoparticles. We have made attempts at understanding this magnetic behavior using both traditional techniques (e.g. superconducting quantum interference device, SQUID, magnetometry) and other methods less common in this field, such as zerofield 197Au NMR (nuclear magnetic resonance) and SANS (smallangle neutron scattering). We also directly probed the local magnetic field at the surface of gold nanoparticles using paramagnetic TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] radicals and ESR (electron spin resonance) spectrometry. Surprisingly, none of these experiments provided a clearer picture in fine. These “negative” results led us to pondering whether or not the explanation could be elsewhere. Our hypothesis is that the magnetism of gold (and possibly other metals) could very well originate in self-sustained persistent currents. We shall demonstrate hereafter that this hypothesis is indeed very plausible and would actually reconcile all of the experimental data reported to date.Striking results are often obtained when SQUID magnetometry is performed on functionalized Au nanoparticles, such as dodecanethiol-coated ones. Rather than being diamagnetic, as expected, the nanoparticles can be found to be para- or ferromagnetic at room temperature and above. When hysteresis is observed, the magnetization curve looks like that of a soft ferromagnet and exhibits a remnant magnetization MR and a coercive field HC, though both are rather weak. These parameters have been observed to have values that vary by orders of magnitude from sample to sample[15] (see Figure ESI-1 of the Supporting Information). Very often, the magnetization does not saturate. Diamagnetic samples are more diamagnetic than the bulk metal. Also, the magnetic observables show little dependence on temperature between 2 and 400 K. The measurements reported so far have been performed by totally independent groups, on systems that were synthesized using known chemical procedures. Figure 1 compares the magnetization of bulk gold with that of two diamagnetic samples of gold nanoparticles. It can be seen that nanoparticles have a much larger absolute diamagnetic susceptibility than massive gold. Figure 2 compares two samples of gold nanoparticles, exhibiting a paramagnetic behavior and a ferromagnetic-like one. There is a weak but clear hysteresis, and the magnetization does not really saturate even at high field values.

A fully enzymatic method for site-directed spin labeling of long RNA

Site-directed spin labeling is emerging as an essential tool to investigate the structural and dynamical features of RNA. We propose here an enzymatic method, which allows the insertion of a paramagnetic center at a specific position in an RNA molecule. The technique is based on a segmental approach using a ligation protocol with T4 RNA ligase 2. One transcribed acceptor RNA is ligated to a donor RNA in which a thio-modified nucleotide is introduced at its 5′-end by in vitro transcription with T7 RNA polymerase. The paramagnetic thiol-specific reagent is subsequently attached to the RNA ligation product. This novel strategy is demonstrated by introducing a paramagnetic probe into the 55 nucleotides long RNA corresponding to K-turn and Specifier Loop domains from the Bacillus subtilis tyrS T-Box leader RNA. The efficiency of the coupling reaction and the quality of the resulting spin-labeled RNA were assessed by Mass Spectrometry, Electron Paramagnetic Resonance (EPR) and Nuclear Magnetic Resonance (NMR). This method enables various combinations of isotopic segmental labeling and spin labeling schemes, a strategy that will be of particular interest to investigate the structural and dynamical properties of large RNA complexes by NMR and EPR spectroscopies.

Pertinent parameters in photo-generation of electrons: Comparative study of anatase-based nano-TiO2 suspensions

In the field of solar fuel cells, the development of efficient photo-converting semiconductors remains a major challenge. A rational analysis of experimental photocatalytic results obtained with material in colloïdal suspensions is needed to access fundamental knowledge required to improve the design and properties of new materials. In this study, a simple system electron donor/nano-TiO2 is considered and examined via spin scavenging electron paramagnetic resonance as well as a panel of analytical techniques (composition, optical spectroscopy and dynamic light scattering) for selected type of nano-TiO2. Independent variables (pH, electron donor concentration and TiO2 amount) have been varied and interdependent variables (aggregate size, aggregate surface vs. volume and acid/base groups distribution) are discussed. This work shows that reliable understanding involves thoughtful combination of interdependent parameters, whereas the specific surface area seems not a pertinent parameter. The conclusion emphasizes the difficulty to identify the key features of the mechanisms governing photocatalytic properties in nano-TiO2.

Experimental and theoretical study of the n-doped successive polyanions of oligocruciform molecular wires: up to five units of charge.

The electronic structure of polyanions of sterically encumbered triisopropylsilyl-substituted linear and cyclic oligo(phenyleneethynylene)s (Monomer, Trimer, Pentamer, and Triangle) is investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and UV/Vis-near-infrared (NIR) spectroscopies, cyclic voltammetry, and theoretical calculations (DFT). Increasing anion orders are generated sequentially in vacuo at room temperature by chemical reaction with potassium metal up to the pentaanion. The relevance of these compounds acting as electron reservoirs is thus demonstrated. Even-order anions are EPR silent, whereas the odd species exhibit different signatures, which are identified after comparison of the measured hyperfine couplings by ENDOR spectroscopy with those predicted by DFT calculations. With increasing size of the oligomers the electron spin density is first distributed over the backbone carbon atoms for the monoanions, and then further localized at the outer phenyl rings for the trianion species. Examination of the UV/Vis-NIR spectra indicates that the monoanions (T(.-) , P(.-) ) exhibit two transitions in the Vis-NIR region, whereas a strong absorption in the IR region is solely observed for higher reduced states. Electronic transitions of the neutral monoanions and trianions are redshifted with increasing oligomer size, whereas for a given oligomer a blueshift is observed upon increasing the charge, which suggests a localization of the spin density.

Cu(II) Binding to the Peptide Ala-His-His, a Chimera of the Canonical Cu(II)-Binding Motifs Xxx-His and Xxx-Zzz-His

Peptides and proteins with the N-terminal motifs NH2-Xxx-His and NH2-Xxx-Zzz-His form well-established Cu(II) complexes. The canonical peptides are Gly-His-Lys and Asp-Ala-His-Lys (from the wound healing factor and human serum albumin, respectively). Cu(II) is bound to NH2-Xxx-His via three nitrogens from the peptide and an external ligand in the equatorial plane (called 3N form here). In contrast, Cu(II) is bound to NH2-Xxx-Zzz-His via four nitrogens from the peptide in the equatorial plane (called 4N form here). These two motifs are not mutually exclusive, as the peptides with the sequence NH2-Xxx-His-His contain both of them. However, this chimera has never been fully explored. In this work, we use a multispectroscopic approach to analyze the Cu(II) binding to the chimeric peptide Ala-His-His (AHH). AHH is capable of forming the 3N- and 4N-type complexes in a pH dependent manner. The 3N form predominates at pH ∼ 4-6.5 and the 4N form at ∼ pH 6.5-10. NMR experiments showed that at pH 8.5, where Cu(II) is almost exclusively bound in the 4N form, the Cu(II)-exchange between AHH or the amidated AHH-NH2 is fast, in comparison to the nonchimeric 4N form (AAH). Together, the results show that the chimeric AHH can access both Cu(II) coordination types, that minor changes in the second (or further) coordination sphere can impact considerably the equilibrium between the forms, and that Cu kinetic exchange is fast even when Cu-AHH is mainly in the 4N form.

Measurement of white wines resistance against oxidation by Electron Paramagnetic Resonance spectroscopy

Free radical theory of aging hypothesizes that oxygen-derived radicals are responsible for the storage-related flavor instability in wine. In an optimal situation, a balanced-distribution exists between oxidants and antioxidants among wines intrinsic/extrinsic metabolites. Based on the kinetic study of POBN-1-hydroxyethyl spin adduct (POBN-1-HER) formation in wines initiated via the Fenton reaction, a novel tool based on EPR spin trapping methodology was developed to quantify wines resistance against oxidation. Antioxidant capacities of wines were evaluated according to POBN-1-HER maximum signal intensity (Imax POBN-1-HER) and rate formation (rPOBN-1-HER) kinetic parameters. Low Imax POBN-1-HER and rPOBN-1-HER values suggest that endogenous antioxidants in wine are able to quench a substantial amount of radicals capable to take part in deleterious oxidative reactions. This will be very valuable in understanding aging potential and will provide an avenue to better control the process by knowing how it might be possible to change wines resistance against oxidation.

Disentangling Magnetic Hardening and Molecular Spin Chain Contributions to Exchange Bias in Ferromagnet/Molecule Bilayers

We performed ferromagnetic resonance and magnetometry experiments to clarify the relationship between two reported magnetic exchange effects arising from interfacial spin-polarized charge transfer in ferromagnetic metal (FM)/molecule bilayers: the magnetic hardening effect and spinterface-stabilized molecular spin chains. To disentangle these effects, we tuned the metal phthalocyanine molecule central sites magnetic moment to enhance or suppress the formation of spin chains in the molecular film. We find that both effects are distinct, and additive. In the process, we extend the list of FM/molecule candidate pairs that are known to generate magnetic exchange effects, experimentally confirm the predicted increase in anisotropy upon molecular adsorption, and show that spin chains within the molecular film can enhance magnetic exchange. Our results confirm, as an echo to progress regarding inorganic spintronic tunnelling, that spintronic tunnelling across structurally ordered organic barriers has been reached through previous magnetotransport experiments.

Development of an electron paramagnetic resonance methodology for studying the photo-generation of reactive species in semiconductor nano-particle assembled films

ABSTRACT An experimental approach involving electron paramagnetic resonance is proposed for studying photo-generated reactive species in semiconductor nano-particle-based films deposited on the internal wall of glass capillaries. This methodology is applied here to nano-TiO2 and allows a semi-quantitative analysis of the kinetic evolutions of radical production using a spin scavenger probe.

Potential of EPR spin-trapping to investigate in situ free radicals generation from skin allergens in reconstructed human epidermis: cumene hydroperoxide as proof of concept

Abstract The first step in the development of skin sensitisation to a chemical, and in the elicitation of further allergic contact dermatitis (ACD), is the binding of the allergen to skin proteins after penetrating into the epidermis. The so-formed antigenic adduct is then recognised by the immune system as foreign to the body. Sensitising organic hydroperoxides derived from autoxidation of natural terpenes are believed to form antigens through radical-mediated mechanisms, although this has not yet been established. So far, in vitro investigations on reactive radical intermediates derived from these skin sensitisers have been conducted in solution, yet with experimental conditions being far away from real-life sensitisation. Herein, we report for the first time, the potential use of EPR spin-trapping to study the in situ generation of free radicals derived from cumene hydroperoxide CumOOH in a 3D reconstructed human epidermis (RHE) model, thus much closer to what may happen in vivo. Among the undesirable effects associated with dermal exposure to CumOOH, it is described to cause allergic and irritant dermatitis, being reported as a significant sensitiser. We considered exploiting the usage of spin-trap DEPMPO as an extensive view of all sort of radicals derived from CumOOH were observed all at once in solution. We showed that in the EpiskinTM RHE model, both by incubating in the assay medium and by topical application, carbon radicals are mainly formed by redox reactions suggesting the key role of CumOOH-derived carbon radicals in the antigen formation process.