Poly(lipoic acid)-Based Nanoparticles as Self-Organized, Biocompatible, and Corona-Free Nanovectors.

Herein we present an innovative approach to produce biocompatible, degradable, and stealth polymeric nanoparticles based on poly(lipoic acid), stabilized by a PEG-ended surfactant. Taking advantage of the well-known thiol-induced polymerization of lipoic acid, a universal and nontoxic nanovector consisted of a solid cross-linked polymeric matrix of lipoic acid monomers was prepared and loaded with active species with a one-step protocol. The biological studies demonstrated a high stability in biological media, the virtual absence of "protein" corona in biological fluids, the absence of acute toxicity in vitro and in vivo, complete clearance from the organism, and a relevant preference for short-term accumulation in the heart. All these features make these nanoparticles candidates as a promising tool for nanomedicine.

Chromatographic NMR spectroscopy: the effect of hollow silica microspheres on magnetic field inhomogeneities and resonance lineshapes.

Micrometric hollow silica spheres can effectively reduce magnetic field inhomogeneities when employed as a stationary phase in the context of NMR chromatography. We here provide a description of the NMR line broadening phenomenon for physically representative collections of hollow spheres with different geometries and filling factors. Our results highlight how, within the explored conditions, a proper modelling of the line broadening phenomenon should consider the enhanced relaxation of the spins during their diffusion across the spherical shells, and possibly other slow motional effects.

Nanoparticle-Assisted NMR Spectroscopy: Enhanced Detection of Analytes by Water-Mediated Saturation Transfer.

Nanoparticle-assisted "NMR chemosensing" is an experimental protocol that exploits the selective recognition abilities of nanoparticle receptors to detect and identify small molecules in complex mixtures by nuclear Overhauser effect magnetization transfer. Although the intrinsic sensitivity of the first reported protocols was modest, we have now found that water spins in long-lived association at the nanoparticle monolayer constitute an alternative source of magnetization that can deliver a remarkable boost of sensitivity, especially when combined with saturation transfer experiments. The approach is general and can be applied to analyte-nanoreceptor systems of different compositions. In this work, we provide an account of the new method and we propose a generalized procedure based on a joint water-nanoparticle saturation to further upgrade the sensitivity, which ultimately endows selective analyte detection down to the micromolar range on standard instrumentation.

Molecular-Dynamics-Simulation-Directed Rational Design of Nanoreceptors with Targeted Affinity.

Here, we demonstrate the possibility of rationally designing nanoparticle receptors with targeted affinity and selectivity for specific small molecules. We used atomistic molecular-dynamics (MD) simulations to gradually mutate and optimize the chemical structure of the molecules forming the coating monolayer of gold nanoparticles (1.7 nm gold-core size). The MD-directed design resulted in nanoreceptors with a 10-fold improvement in affinity for the target analyte (salicylate) and a 100-fold decrease of the detection limit by NMR-chemosensing from the millimolar to the micromolar range. We could define the exact binding mode, which features prolonged contacts and deep penetration of the guest into the monolayer, as well as a distinct shape of the effective binding pockets characterized by exposed interacting points.

Ion pairing in 1-butyl-3-methylpyridinium halide ionic liquids studied using NMR and DFT calculations.

We present the 1H, 13C and 15N NMR chemical shifts of bulk ionic liquids based on 1-butyl-3-methylimidazolium (the cation also known as 1-butyl-3-picolinium) halides (Cl-, Br- and I-) and tribromide (Br3-) salts. A characterization in solution of the analogous ICl2- and I3- salts is also reported. A series of DFT calculations has been run to predict the features of the NMR spectra of the pure ILs based on a few selected supramolecular ionic aggregates. To test the effect of temperature, and vibrational and conformational motions, only for the chloride salt, we also run first-principles molecular dynamics simulations of the ion pair in the gas phase, using the ADMP scheme (Atom Centered Density Matrix Propagation molecular dynamics model). The aim of our investigation is to test whether a simple DFT based approach of ion-pairing in ionic liquids is capable of providing reliable results and under which conditions the protocol is robust. We obtained a very good agreement between the calculated and experimental spectra for the three halides, where the bulk structure of the ILs is dominated by H-bond interactions between the X- anion (X = Cl, Br and I) and the ortho protons of the pyridinium ring (a structural arrangement not too different from the solid-state structure of pyridinium halides). In contrast, when the H-bond is weak, as in the Br3- case, a number of supramolecular arrangements exist in solution and the simple DFT calculations of a few selected cases cannot exhaustively explore the complete energy landscape. Moreover, the dynamic effects due to thermal motion, evaluated by ADMP MD simulations of the chloride salt, appear to be not very significant.

NMR Quantification of Carbohydrates in Complex Mixtures. A Challenge on Honey.

The knowledge of carbohydrate composition is greatly important to determine the properties of natural matrices such as foodstuff and food ingredients. However, because of the structural similarity and the multiple isomeric forms of carbohydrates in solution, their analysis is often a complex task. Here we propose an NMR analytical procedure based on highly selective chemical shift filters followed by TOCSY, which allows us to acquire specific background-free signals for each sugar. The method was tested on raw honey samples dissolved in water with no other pretreatment. In total, 22 sugars typically found in honey were quantified: 4 monosaccharides (glucose, fructose, mannose, rhamnose), 11 disaccharides (sucrose, trehalose, turanose, maltose, maltulose, palatinose, melibiose and melezitose, isomaltose, gentiobiose nigerose, and kojibiose), and 7 trisaccharides (raffinose, isomaltotriose, erlose, melezitose, maltotriose, panose, and 1-kestose). Satisfactory results in terms of limit of quantification (0.03-0.4 g/100g honey), precision (% RSD: 0.99-4.03), trueness (bias % 0.4-4.2), and recovery (97-104%) were obtained. An accurate control of the instrumental temperature and of the sample pH endows an optimal chemical shift reproducibility, making the procedure amenable to automation and suitable to routine analysis. While validated on honey, which is one of the most complex natural matrices in terms of saccharides composition, this innovative approach can be easily transferred to other natural matrices.

Nanoparticle-Assisted Affinity NMR Spectroscopy: High Sensitivity Detection and Identification of Organic Molecules.

A simple and effective method for high-sensitivity NMR detection of selected compounds is reported. The method combines 1D NMR diffusion filter experiments and small monolayer-protected nanoparticles as high-affinity receptors. Once bound to the nanoparticles, the diffusion coefficient of the analyte decreases in such way that spectral editing based on diffusion filters can separate its signals from those of other mixture components. Using nanoparticles functionalized with Zn2+ -triazacyclonane complexes, detection and identification of phosphorylated organic molecules can be achieved. Diphenyl phosphate can be detected at 25 micromolar concentration with good selectivity. The selectivity toward organic carboxylates is enhanced at pD=3.75. In these conditions, commercial tablets containing betamethasone phosphate and a large excess of benzoate could be successfully analyzed.

Chromatographic NMR Spectroscopy with Hollow Silica Spheres.

The use of micrometric hollow silica spheres is described as a strategy to reduce magnetic field inhomogeneities in the context of NMR chromatography. When employed as a stationary phase, hollow silica microspheres allow the use of common solution-state NMR instruments to measure the diffusion coefficient perturbation induced by the interaction of the analytes with the silica surface.

Turning Supramolecular Receptors into Chemosensors by Nanoparticle-Assisted "NMR Chemosensing".

By exploiting a magnetization transfer between monolayer-protected nanoparticles and interacting analytes, the NMR chemosensing protocol provides a general approach to convert supramolecular receptors into chemosensors via their conjugation with nanoparticles. In this context, the nanoparticles provide the supramolecular receptor not only with the "bulkiness" necessary for the NMR chemosensing approach but also with a different selectivity as compared to the parent receptor. We here demonstrate that gold nanoparticles of 1.8 nm core coated with a monolayer of 18-crown-6 ether derivatives can detect and identify protonated primary amines in methanol and in water, and even discriminate between two biogenic diamines that are selectively detected over monoamines and α-amino acids.

Nanoparticle-assisted NMR detection of organic anions: from chemosensing to chromatography.

Monolayer-protected nanoparticles provide a straightforward access to self-organized receptors that selectively bind different substrates in water. Molecules featuring different kinds of noncovalent interactions (namely, hydrophobic, ion pairing, and metal-ligand coordination) can be grafted on the nanoparticle surface to provide tailored binding sites for virtually any class of substrate. Not only the selectivity but also the strength of these interactions can be modulated. Such recognition ability can be exploited with new sensing protocols, based on NMR magnetization transfer and diffusion-ordered spectroscopy (DOSY), to detect and identify organic molecules in complex mixtures.

Characterization of Paramagnetic Reactive Intermediates: Predicting the NMR Spectra of Iron(IV)-Oxo Complexes by DFT.

The relative energies of spin states of several iron(IV)-oxo complexes and related species have been calculated with DFT methods by employing the B3LYP* functional. We show that such calculations can predict the correct ground spin state of Fe(IV) complexes and can then be used to determine the (1) H NMR spectra of all spin states; the spectral features are remarkably different, hence calculated paramagnetic (1) H NMR spectra can be used to support the structure elucidation of numerous paramagnetic complexes. Applications to a number of stable and reactive iron(IV)-oxo species are described.

Viologen-based ionic liquid crystals: induction of a smectic A phase by dimerisation.

The stability of thermotropic ionic liquid crystals is essentially due to micro-phase segregation between the ionic heads and the long alkyl chains. Here we show, using newly synthesized viologen dimers, that the structure of the central core is another key parameter to play with in order to tune the mesomorphic behaviour.

"NMR chemosensing" using monolayer-protected nanoparticles as receptors.

A new sensing protocol based on NMR magnetization transfer sequences and the molecular recognition abilities of nanoparticles allows the detection and identification of organic molecules in complex mixtures.

Dynamic covalent capture of hydrazides by a phosphonate-target immobilized on resin.

A protocol is described that permits the self-selection of hydrazides from a small library by a phosphonate-target immobilized on resin. Hydrazides are captured by a neighbouring aldehyde group through reversible hydrazone bond formation. Stabilizing intramolecular interactions between the phosphonate-target and functional groups of the hydrazides drive the selection process. The phosphonate-target is introduced onto commercially available Tentagel resin through straightforward synthetic steps. The functionalized resin could be conveniently characterized by HR-MAS NMR spectroscopy using a recently developed transverse relaxation filter that eliminates the strong phase defects commonly observed with CPMG sequences. In addition, a protocol was developed to quantitatively remove the captured hydrazides from resin in order to analyse their composition by LC/MS. Kinetic experiments were used to study hydrazone formation and exchange on resin yielding similar results to those obtained previously in solution. Competition experiments showed that the system reaches thermodynamic equilibrium if multiple hydrazides are added to the resin. Finally, competition experiments showed that the immobilized phosphonate-target indeed amplifies the capture of those hydrazides able to develop stabilizing interactions with the target. Importantly, the obtained amplification profile was nearly identical to the ones obtained previously in solution studies. Notably, the observed amplification factors for the self-selected hydrazides were higher, which was attributed to steric effects imposed by the resin.

Synthesis of Ion-Exchange Catalysts by Introduction of Fluorinated Ponytails into Novel Mesoporous Polymers.

A novel synthetic procedure for the functionalisation of styrenic cross-linked polymers with perfluorinated acyl chains has been reported. The effective significant grafting of the fluorinated moieties is supported by {1H}-13C and {19F}-13C NMR characterisations. This kind of polymer appears promising as catalytic support for a variety of reactions requiring a highly lipophilic catalyst. Indeed, the improved lipophilicity of the materials resulted in enhanced catalytic properties of the corresponding sulfonic materials in the reaction of esterification of a solution in a vegetable oil of stearic acid with methanol.

Molecular Mechanisms Underlying Detection Sensitivity in Nanoparticle-Assisted NMR Chemosensing.

Nanoparticle-assisted nuclear magnetic resonance (NMR) chemosensing exploits monolayer-protected nanoparticles as supramolecular hosts to detect small molecules in complex mixtures via nuclear Overhauser effect experiments with detection limits down to the micromolar range. Still, the structure-sensitivity relationships at the basis of such detection limits are little understood. In this work, we integrate NMR spectroscopy and atomistic molecular dynamics simulations to examine the covariates that affect the sensitivity of different NMR chemosensing experiments [saturation transfer difference (STD), water STD, and high-power water-mediated STD]. Our results show that the intensity of the observed signals correlates with the number and duration of the spin-spin interactions between the analytes and the nanoparticles and/or between the analytes and the nanoparticles' solvation molecules. In turn, these parameters depend on the location and dynamics of each analyte inside the monolayer. This insight will eventually facilitate the tailoring of experimental and computational setups to the analyte's chemistry, making NMR chemosensing an even more effective technique in practical use.

Lanthanide-based NMR: a tool to investigate component distribution in mixed-monolayer-protected nanoparticles.

Gd(3+) ions, once bound to the monolayer of organic molecules coating the surface of gold nanoparticles, produce a paramagnetic relaxation enhancement (PRE) that broadens and eventually cancels the signals of the nuclear spins located nearby (within 1.6 nm distance). In the case of nanoparticles coated with mixed monolayers, the signals arising from the different coating molecules experience different PRE, depending on their distance from the binding site. As a consequence, observation of the signal broadening patterns provides direct information on the monolayer organization.

Thermal behaviour and electrochemical properties of bis(trifluoromethanesulfonyl)amide and dodecatungstosilicate viologen dimers.

We report the synthesis and characterization of dimeric viologen salts (1',1''-(alkane-1,n-diyl)bis(1-ethyl-4,4'-bipyridinium) with n = 4-10) with bis(trifluoromethanesulfonyl)amide (bistriflimide, Tf(2)N(-)) as a counteranion. For n = 4, 5 and 6, and for the nonylviologen cation (1,1'-dinonyl-4,4'-bipyridinium) we also prepared salts with the totally inorganic dodecatungstosilicate anion, SiW(12)O(40)(4-), featuring a poly-charged surface and nanosized dimensions. The materials have been characterized by means of calorimetric techniques, X-ray diffraction and solid state NMR and a comparison is made with analogous monomeric viologen salts exhibiting smectic mesophases. A strong odd-even effect is observed in the melting points and in the thermal behaviour of the bistriflimide dimeric systems, similar to what was reported for dipolar calamitic liquid crystal dimers, although the studied viologen dimers are not mesomorphic. By increasing the size of the counteranion we have observed a destabilization of the crystal phases and of the mesophases in favour of a glassy amorphous state. Implications on the design of novel ionic liquid crystals are discussed. The electrochemical behaviour in solution has been investigated by cyclic voltammetry measurements: interestingly, the odd-even effect is clearly visible also in the redox potentials. The spin-pairing of the viologen radical cations formed at each end of the dimer is responsible for the observed redox trend. Insights on the structure of the spin-paired dimer have been obtained by DFT calculations.

Uniform water-mediated saturation transfer: A sensitivity-improved alternative to WaterLOGSY.

In the study of small molecule ligands and candidate macromolecular targets, water spins in long-lived association with macromolecules (proteins or nanoparticles) constitute a remarkable source of magnetization that can be exploited to reveal ligand-target binding. In this work we show how the selective saturation of water spins complemented with adiabatic off-resonance spin-locks can remove the NOE contribution of bulk water in the final difference spectrum, leading to uniformly enhanced signals that reveal weak ligand-target interactions.

Selective NMR detection of N-methylated amines using cavitand-decorated silica nanoparticles as receptors.

We report a strategy for the realization of NMR chemosensors based on the spontaneous self-assembly of lower rim pyridinium-functionalized tetraphopshonate cavitands on commercial silica nanoparticles. These nanohybrids enable the selective detection of physiologically relevant N-methylated amines, with a limit of detection of 31 µM, via STD-based NMR experiments, achieving for the first time fine structural selectivity in nanoparticle-assisted NMR chemosensing.