Paola Franchi

Water-Soluble Gold Nanoparticles Protected by Fluorinated Amphiphilic Thiolates

The preparation and the properties of gold nanoparticles (Au NPs) protected by perfluorinated amphiphiles are described. The thiols were devised to form a perfluorinated region close to the gold surface and to have a hydrophilic portion in contact with the bulk solvent to impart solubility in water. The monolayer protected clusters were prepared, in an homogeneous phase using sodium thiolates because of the low nucleophilicity of the alpha-perfluorinated thiols, and fully characterized with (1)H, (19)F NMR spectrometry, IR and UV-vis absorption spectroscopies, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). Au NPs with core diameters ranging from 1.6 to 2.9 nm, depending on the reaction conditions, were obtained. Water-soluble NPs (MPC-F8-PEGs) were obtained with the thiol HS-F8-PEG ending with a short poly(ethylene glycol) unit (PEG-OMe 550), whereas thiols with shorter PEG chains give rise to NPs insoluble in water. MPC-F8-PEGs undergo an exchange reaction with amphiphilic alkyl thiols. ESR investigations, using a hydrophobic radical probe, indicate that the MPC-F8-PEG monolayer shows a greater hydrophobicity compared to the analogous hydrogenated monolayer.

Unraveling Unidirectional Threading of α-Cyclodextrin in a [2]Rotaxane through Spin Labeling Approach

We present here the results of a CW-ESR investigation of a double spin labeled α-cyclodextrin-based [2]rotaxane that is characterized by the presence of nitroxide labels both at the wheel and at the dumbbell. This was accomplished by synthesizing a spin labeled α-CD (the wheel) that was mechanically blocked on a thread containing a nitroxide unit by a Cu(I) catalyzed azide-alkyne cycloaddition (CuAAC). Both ESI-MS analysis and NMR spectroscopy were used to provide evidence of the threading processes. Because of the unsymmetrical structure of both the wheel and the axle, two different geometrical isomers could be predicted on the basis of the orientation of the CD along the thread. By combining molecular dynamic calculations and information extracted from the CW-ESR spectra, we were able to determine the geometrical nature of the isomer that was isolated as the only species. The ESR spectra showed J-coupling between the two mechanically connected nitroxide units and were analyzed by a model assuming three main molecular states. The intramolecular noncovalent nature of spin exchange was confirmed by reversibly switching the magnetic interaction on-off by changing the pH of the solution in the presence of a competing macrocyclic host.

Nitroxide Radicals as Hydrogen Bonding Acceptors. An Infrared and EPR Study.

The equilibrium constants for the formation of hydrogen-bonded complexes with phenol, benzyl alcohol and diphenyl amine of a persistent nitroxide radical (TEMPO) have been measured for the first time by IR spectroscopy in solution. By making use of the data obtained with IR measurements it was possible to quantitatively determine the effect of hydrogen bonding on the nitrogen hyperfine-splitting constant. On this basis it is shown that EPR spectroscopy can be used as an alternative to IR spectroscopy for the determination of the thermodynamic parameters of hydrogen bond formation with nitroxides by following the dependence of the experimental EPR nitrogen hyperfine splitting of these radicals upon changing the nature of the solvent. The experimental data obtained from both IR and EPR spectroscopy indicate that nitroxides are versatile hydrogen bond acceptors giving hydrogen bonds of strength similar to that of ethers or esters. The corresponding Abrahams beta 2H values have been determined as 0.46.

Increasing the Persistency of Stable Free-Radicals: Synthesis and Characterization of a Nitroxide Based [1]Rotaxane

A persistent paramagnetic [1]rotaxane based on beta-cyclodextrin showing an increased persistency under reductive conditions has been prepared and characterized for the first time.

Self-Organization of Mixtures of Fluorocarbon and Hydrocarbon Amphiphilic Thiolates on the Surface of Gold Nanoparticles

Self-assembled monolayers composed of a mixture of thiolate molecules, featuring hydrocarbon or perfluorocarbon chains (H- and F-chains) terminating with a short poly(oxoethylene) (PEG) moiety, are the most extreme example of surfactant immiscibility on gold nanoparticles reported so far. The phase segregation between H-chains and F-chains and the consequent, peculiar folding of PEG chains are responsible for the increased affinity of a selected radical probe for the fluorinated region, which increases as the size of the fluorinated domains decrease, independently of the shape of such domains. This feature has been revealed by ESR measurements and an in silico innovative multiscale molecular simulations approach in explicit water. Our results reveal an underlying mechanism of a transmission of the organization of the monolayer from the inner region close to the gold surface toward the external hydrophilic PEG region. Moreover, this study definitively proves that a mixed monolayer is a complex system with properties markedly different from those characterizing the parent homoligand monolayers.

Gold nanoparticles protected by fluorinated ligands for 19F MRI

Gold nanoparticles coated with fluorinated ligands (F-MPCs) present features suitable for (19)F MRI as observed from phantom experiments. Cellular uptake, by HeLa cells, and toxicity of fluorescent dye-decorated F-MPCs are presented together with their ability to bind hydrophobic molecules allowing for a potential combination of targeting, delivery and imaging features.

Nitroxide biradicals as thread units in paramagnetic cucurbituril-based rotaxanes

The first example of paramagnetic rotaxane containing cucurbit[6]urils has been reported and characterized both by ESR and NMR spectroscopy.

Recyclable Catalyst Reservoir: Oxidation of Alcohols Mediated by Noncovalently Supported Bis(imidazolium)‐Tagged 2,2,6,6‐Tetramethylpiperidine 1‐Oxyl

Bis(imidazolium)‐tagged 2,2,6,6‐tetramethylpiperidine 1‐oxyl (TEMPO) catalysts were adsorbed on different supports such as silica gel, silica gel modified with highly cross‐linked polymeric imidazolium networks, and highly cross‐linked polymeric imidazolium networks entrapping magnetic particles. These systems provided a convenient tool for the oxidation of both primary and secondary alcohols working as recyclable reservoirs for the bis(imidazolium)‐tagged TEMPO catalysts. By using EPR spectroscopy it was demonstrated that the catalyst was released as the corresponding oxoammonium salt in the solution during the recycling step, thus promoting the oxidative process in a homogeneous fashion. After solvent removal, the catalyst was readsorbed on the support allowing an easy recovery and recycle of the catalytic material up to 13 consecutive cycles with no loss in activity. The bis(imidazolium)‐tagged TEMPO catalyst could be used in only 1 mol % both for the oxidation of benzylic and aliphatic alcohols. The catalytic material was highly recyclable if used on silica or imidazolium‐modified silica gel in 10 mol % loading. Loading could be scaled down to 1 mol % and the catalyst proved to be recyclable up to 8 cycles only with imidazolium‐modified silica gel. Such a “catalyst‐sponge‐like” system permits to combine the benefits of homogeneous and heterogeneous catalysis.

SOMO–HOMO Conversion in Distonic Radical Anions: An Experimental Test in Solution by EPR Radical Equilibration Technique

Bond dissociation energies (BDEs) of O-H bond in the hydroxylamines deriving from neutral and deprotonated forms of 4-carboxy-TEMPO have been measured in solution (acetonitrile and dimethylsulfoxide) by using the EPR radical equilibration technique. The experimental results confirm that stabilizing interaction between a remote negative charge and stable radicals, occurring in gas phase, is completely lost in polar solvents as predicted by theoretical computations.

End-to-end Distance determination in a cucurbit[6]uril-based rotaxane by PELDOR spectroscopy.

Distance determination: The use of pulsed electron-electron double resonance (PELDOR/DEER) spectroscopy to determine the distances among the end units of a paramagnetic cucurbit[6]uril-based rotaxane is demonstrated.