Viologen-templated arrays of cucurbit[7]uril-modified iron-oxide nanoparticles.

Magnetic and fluorescent assemblies of iron-oxide nanoparticles (NPs) were constructed by threading a viologen-based ditopic ligand, DPV(2+), into the cavity of cucurbituril (CB[7]) macrocycles adsorbed on the surface of the NPs. Evidence for the formation of 1:2 inclusion complexes that involve DPV(2+) and two CB[7] macrocycles was first obtained in solution by (1)H NMR and emission spectroscopy. DPV(2+) was found to induce self-assembly of nanoparticle arrays (DPV(2+)⊂CB[7]NPs) by bridging CB[7] molecules on different NPs. The resulting viologen-crosslinked iron-oxide nanoparticles exhibited increased saturation magnetization and emission properties. This facile supramolecular approach to NP self-assembly provides a platform for the synthesis of smart and innovative materials that can achieve a high degree of functionality and complexity and that are needed for a wide range of applications.

Radical-cation dimerization overwhelms inclusion in [N]pseudorotaxanes.

Suppression of the dimerization of the viologen radical cation by cucurbit[7]uril (CB7) in water is a well-known phenomenon. Herein, two counter-examples are presented. Two viologen-containing thread molecules were designed, synthesized, and thoroughly characterized by (1)H DOSY NMR spectrometry, UV/Vis absorption spectrophotometry, square-wave voltammetry, and chronocoulometry: BV(4+), which contains two viologen subunits, and HV(12+), which contains six. In both threads, the viologen subunits are covalently bonded to a hexavalent phosphazene core. The corresponding [3]- and [7]pseudorotaxanes that form on complexation with CB7, that is, BV(4+)⊂(CB7)2 and HV(12+)⊂(CB7)6, were also analyzed. The properties of two monomeric control threads, namely, methyl viologen (MV(2+)) and benzyl methyl viologen (BMV(2+)), as well as their [2]pseudorotaxane complexes with CB7 (MV(2+)⊂CB7 and BMV(2+)⊂CB7) were also investigated. As expected, the control pseudorotaxanes remained intact after one-electron reduction of their viologen-recognition stations. In contrast, analogous reduction of BV(4+)⊂(CB7)2 and HV(12+)⊂(CB7)6 led to host-guest decomplexation and release of the free threads BV(2(·+)) and HV(6(·+)), respectively. (1)H DOSY NMR spectrometric and chronocoulometric measurements showed that BV(2(·+)) and HV(6(·+)) have larger diffusion coefficients than the corresponding [3]- and [7]pseudorotaxanes, and UV/Vis absorption studies provided evidence for intramolecular radical-cation dimerization. These results demonstrate that radical-cation dimerization, a relatively weak interaction, can be used as a driving force in novel molecular switches.

Photophysical evaluation of a new functional terbium complex in FRET-based time-resolved homogenous fluoroassays.

A new functional luminescent lanthanide complex (LLC) has been synthesized with terbium as a central lanthanide ion and biotin as a functional moiety. Unlike in typical lanthanide complexes assembled via carboxylic moieties, in the presented complex, four phosphate groups are chelating the central lanthanide ion. This special chemical assembly enhances the complex stability in phosphate buffers conventionally used in biochemistry. The complex synthesis strategy and photophysical properties are described as well as the performance in time-resolved Förster Resonance Energy Transfer (FRET) assays. In those assays, this biotin-LLC transferred energy either to acceptor organic dyes (Cy5 or AF680) labelled on streptavidin or to quantum dots (QD655 or QD705) surface-functionalised with streptavidins. The permanent spatial donor-acceptor proximity is assured through strong and stable biotin-streptavidin binding. The energy transfer is evidenced from the quenching observed in donor emission and from a decrease in donor luminescence decay, both associated with simultaneous increase in acceptor intensity and in the decay time. The dye-based assays are realised in TRIS and in PBS, whereas QD-based systems are studied in borate buffer. The delayed emission analysis allows for quantifying the recognition process and for auto-fluorescence-free detection, which is particularly relevant for application in bioanalysis. In accordance with Förster theory, Förster-radii (R0) were found to be around 60 Å for organic dyes and around 105 Å for QDs. The FRET efficiency (η) reached 80% and 25% for dye and QD acceptors, respectively. Physical donor-acceptor distances (r) have been determined in the range 45-60 Å for organic dye acceptors, while for acceptor QDs between 120 Å and 145 Å. This newly synthesised biotin-LLC extends the class of highly sensitive analytical tools to be applied in the bioanalytical methods such as time-resolved fluoroimmunoassays (TR-FIA), luminescent imaging and biosensing.

Supramolecular luminescent lanthanide dimers for fluoride sequestering and sensing.

Lanthanide complexes (Ln=Eu, Tb, and Yb) that are based on a C2 -symmetric cyclen scaffold were prepared and characterized. The addition of fluoride anions to aqueous solutions of the complexes resulted in the formation of dinuclear supramolecular compounds in which the anion is confined into the cavity that is formed by the two complexes. The supramolecular assembly process was monitored by UV/Vis absorption, luminescence, and NMR spectroscopy and high-resolution mass spectrometry. The X-ray crystal structure of the europium dimer revealed that the architecture of the scaffold is stabilized by synergistic effects of the EuFEu bridging motive, π stacking interactions, and a four-component hydrogen-bonding network, which control the assembly of the two [EuL] entities around the fluoride ion. The strong association in water allowed for the luminescence sensing of fluoride down to a detection limit of 24 nM.

Towards High-Performance Photo-Fenton Degradation of Organic Pollutants with Magnetite-Silver Composites: Synthesis, Catalytic Reactions and In Situ Insights.

In this study, Fe3O4/Ag magnetite-silver (MSx) nanocomposites were investigated as catalysts for advanced oxidation processes by coupling the plasmonic effect of silver nanoparticles and the ferromagnetism of iron oxide species. A surfactant-free co-precipitation synthesis method yielded pure Fe3O4 magnetite and four types of MSx nanocomposites. Their characterisation included structural, compositional, morphological and optical analyses, revealing Fe3O4 magnetite and Ag silver phases with particle sizes ranging from 15 to 40 nm, increasing with the silver content. The heterostructures with silver reduced magnetite particle aggregation, as confirmed by dynamic light scattering. The UV-Vis spectra showed that the Fe:Ag ratio strongly influenced the absorbance, with a strong absorption band around 400 nm due to the silver phase. The oxidation kinetics of organic pollutants, monitored by in situ luminescence measurements using rhodamine B as a model system, demonstrated the higher performance of the developed catalysts with increasing Ag content. The specific surface area measurements highlighted the importance of active sites in the synergistic catalytic activity of Fe3O4/Ag nanocomposites in the photo-Fenton reaction. Finally, the straightforward fabrication of diverse Fe3O4/Ag heterostructures combining magnetism and plasmonic effects opens up promising possibilities for heterogeneous catalysis and environmental remediation.

Stable and highly fluorescent europium(III) chelates for time-resolved immunoassays.

Derivatives of 4-[2-(4-isothiocyanatophenyl)ethynyl]-2,6,-bis{[N,N-bis(carboxymethyl)-amino]methyl}pyridine europium(III) (1) bearing one (6) or two (7) additional iminodiacetate coordinating arms have been synthesized. 6 and 7 were significantly more stable than 1 as evidenced by competition experiments with ethylenediaminetetraacetic acid (EDTA) and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). While the luminescence quantum yield of 1 remained modest, the other two complexes displayed substantial luminescence efficiency. The introduction of a supplementary iminodiacetate arm in 6 brought important improvements to both the stability and the luminescence properties of the Eu complex. In contrast, although 7 is more luminescent than 1, the introduction of a second iminodiacetate coordinating arm brings no further benefit on the photophysical properties. The most promising results were obtained with the nine-dentate chelate 6 and its Eu complex, which was conjugated to biotin and applied within the frame of a bioaffinity immunoassay of human C-reactive protein.

Activated phosphonated trifunctional chelates for highly sensitive lanthanide-based FRET immunoassays applied to total prostate specific antigen detection.

The first example of an activated phosphonated trifunctional chelate (TFC) is presented, which combines a non-macrocyclic coordination site for lanthanide coordination based on two aminobis-methylphosphonate coordinating arms, a central bispyrazolylpyridyl antenna and an N-hydroxysuccinimide ester in para position of the central pyridine as an activated function for the labeling of biomaterial. The synthesis of the TFC is presented together with photo-physical studies of the related Tb and Eu complexes. Excited state lifetime measurements in H2O and D2O confirmed an excellent shielding of the cation from water molecules with a hydration number of zero. The Tb complex provides a high photoluminescence (PL) quantum yield of 24% in aqueous solutions (0.01 M Tris-HCl, pH 7.4) and a very long luminescence lifetime of 2.6 ms. The activated ligand was conjugated to different biological compounds such as streptavidin, and a monoclonal antibody against total prostate specific antigen (TPSA). In combination with AlexaFluor647 (AF647) and crosslinked allophycocyanin (XL665) antibody (ABs) conjugates, homogeneous time-resolved Fluorescence Resonance Energy Transfer (FRET) immunoassays of TPSA were performed in serum samples. The Tb donor-dye acceptor FRET pairs provided large Förster distances of 5.3 nm (AF647) and 7.1 nm (XL665). A detailed time-resolved FRET analysis of Tb donor and dye acceptor PL decays revealed average donor-acceptor distances of 4.2 nm (AF647) and 6.3 nm (XL665) within the sandwich immunocomplex and FRET efficiencies of 0.79 and 0.68, respectively. Very low detection limits of 1.4 ng mL(-1) (43 pM) and 2.4 ng mL(-1) (74 pM) TPSA were determined using a KRYPTOR fluorescence immunoanalyzer. These results demonstrate the applicability of our novel Tb-bioconjugates for highly sensitive clinical diagnostics.

Viologen-cucurbituril host/guest chemistry - redox control of dimerization versus inclusion.

Two calix[4]arene systems, C234+ and C244+ - where 2 corresponds to the number of viologen units and 3-4 corresponds to the number of carbon atoms connecting the viologen units to the macrocyclic core - have been synthesized and led to the formation of [3]pseudorotaxanes when combined with either CB[7] or CB[8]. The [3]pseudorotaxanes spontaneously dissociate upon reduction of the bipyridinium units as the result of intramolecular dimerization of the two face-to-face viologen radical cations. CB[7] and CB[8]-based [2]pseudorotaxanes containing monomeric viologen guest model compounds, MC32+ and MC4+, do not undergo decomplexation and dimerization following electrochemical reduction of their bipyridinium units.