Aldrik H. Velders

A Coordination Cage with an Adaptable Cavity Size

A hexanuclear coordination cage can increase the size of its cavity from nearly zero to more than 500 Å(3), which allows the encapsulation of two coronene molecules.

Chiral Salan Aluminium Ethyl Complexes and Their Application in Lactide Polymerization

Synthetic routes to aluminium ethyl complexes supported by chiral tetradentate phenoxyamine (salan-type) ligands [Al(OC(6)H(2)(R-6-R-4)CH(2))(2){CH(3)N(C(6)H(10))NCH(3)}-C(2)H(5)] (4, 7: R=H; 5, 8: R=Cl; 6, 9: R=CH(3)) are reported. Enantiomerically pure salan ligands 1-3 with (R,R) configurations at their cyclohexane rings afforded the complexes 4, 5, and 6 as mixtures of two diastereoisomers (a and b). Each diastereoisomer a was, as determined by X-ray analysis, monomeric with a five-coordinated aluminium central core in the solid state, adopting a cis-(O,O) and cis-(Me,Me) ligand geometry. From the results of variable-temperature (VT) (1)H NMR in the temperature range of 220-335 K, (1)H-(1)H NOESY at 220 K, and diffusion-ordered spectroscopy (DOSY), it is concluded that each diastereoisomer b is also monomeric with a five-coordinated aluminium central core. The geometry is intermediate between square pyramidal with a cis-(O,O), trans-(Me,Me) ligand disposition and trigonal bipyramidal with a trans-(O,O) and trans-(Me,Me) disposition. A slow exchange between these two geometries at 220 K was indicated by (1)H-(1)H NOESY NMR. In the presence of propan-2-ol as an initiator, enantiomerically pure (R,R) complexes 4-6 and their racemic mixtures 7-9 were efficient catalysts in the ring-opening polymerization of lactide (LA). Polylactide materials ranging from isotactically biased (P(m) up to 0.66) to medium heterotactic (P(r) up to 0.73) were obtained from rac-lactide, and syndiotactically biased polylactide (P(r) up to 0.70) from meso-lactide. Kinetic studies revealed that the polymerization of (S,S)-LA in the presence of 4/propan-2-ol had a much higher polymerization rate than (R,R)-LA polymerization (k(SS)/k(RR)=10.1).

Reversible Phase Transfer of (CdSe/ZnS) Quantum Dots between Organic and Aqueous Solutions

Ttrioctylphosphine oxide (TOPO) stabilized CdSe/ZnS quantum dots (QD) were modified with 6-ferrocenyl-1-hexanethiol (FcHT) or 11-ferrocenyl-1-undecanethiol (FcUT) via ligand exchange. The presence of ferrocenyl thiol ligands on the surface of the QDs was proven by diffusion ordered NMR spectroscopy. Upon replacement of the initial TOPO ligand with ferrocene derivatives the emission of the QDs decreased. Phase transfer of ferrocene-modified QDs from organic solvents into water was achieved by complexation reactions with beta-cyclodextrin (beta-CD). The QDs coated with ferrocene thiols are soluble in nonpolar solvents and are transferred into the aqueous phase upon formation of host-guest complexes between the ferrocene units and the cavity of beta-CD. The reversibility of the phase transfer was probed by the addition of naphthalene and adamantane derivatives to the aqueous phase containing QD-[Fc-CD] adduct.

Strategies for patterning biomolecules with dip-pen nanolithography.

Dip-pen nanolithography (DPN) is an atomic force microscopy (AFM)-based lithography technique, which has the ability to fabricate patterns with a feature size down to approximately 15 nm using both top-down and bottom-up approaches. DPN utilizes the water meniscus formed between an AFM tip and a substrate to transfer ink molecules onto surfaces. A major application of this technique is the fabrication of micro- and nano-arrays of patterned biomolecules. To achieve this goal, a variety of chemical approaches has been used. This review concisely describes the development of DPN in the past decade and presents the related chemical strategies that have been reported to fabricate biomolecular patterns with DPN at micrometer and nanometer scale, classified into direct- and indirect DPN methodologies, discussing tip-functionalization strategies as well.

Multimodal Tumor-Targeting Peptides Functionalized with Both a Radio- and a Fluorescent Label

The use of monolabeled tumor-targeting peptides for molecular imaging is widespread. However, it is often desirable to use the same compound for different clinical applications, e.g., combined pre- and intraoperative tumor detection. On the basis of their detection sensitivity, the combination of radioactivity and fluorescence is probably the most valuable in multimodal molecular imaging. In this review, we compare multimodal peptide derivatives and discuss the influence of the diagnostic labels on receptor affinity and biodistribution. On the basis of the described constructs, we propose improvements for the design of future multimodal tumor-targeting peptide derivatives.

NMR Characterization of Fourth-Generation PAMAM Dendrimers in the Presence and Absence of Palladium Dendrimer-Encapsulated Nanoparticles

High-resolution solution NMR spectroscopy has been used to characterize the structure of Pd dendrimer-encapsulated nanoparticles (DENs), consisting of approximately 55-atom nanoparticles encapsulated within fourth-generation, hydroxyl-terminated poly(amidoamine) PAMAM dendrimers (G4-OH). Detailed analysis of 1D and 2D NMR spectra of dendrimers with (G4-OH(Pd(55))) and without (G4-OH) nanoparticles unambiguously demonstrate that single nanoparticles are encapsulated within individual dendrimers. This conclusion is based on the following results. First, the NMR data show that signals arising from the innermost methylenes of G4-OH(Pd(55)) are more highly influenced by the presence of the Pd nanoparticles than are the terminal functional groups. This means that DENs are encapsulated within dendrimers rather than being adsorbed to their surface, as would be the case for aggregates consisting of multiple dendrimers and nanoparticles. Second, extraction of DENs from within their dendrimer hosts results in an increase in the NMR intensity associated with the interior methylenes, which corroborates the previous point. Third, NMR pulse-field gradient spin-echo experiments demonstrate that G4-OH and G4-OH(Pd(55)) have identical hydrodynamic radii, and this finding excludes the presence of dendrimer/nanoparticle aggregates.

Small-Volume Nuclear Magnetic Resonance Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is one of the most information-rich analytical techniques available. However, it is also inherently insensitive, and this drawback precludes the application of NMR spectroscopy to mass- and volume-limited samples. We review a particular approach to increase the sensitivity of NMR experiments, namely the use of miniaturized coils. When the size of the coil is reduced, the sample volume can be brought down to the nanoliter range. We compare the main coil geometries (solenoidal, planar, and microslot/stripline) and discuss their applications to the analysis of mass-limited samples. We also provide an overview of the hyphenation of microcoil NMR spectroscopy to separation techniques and of the integration with lab-on-a-chip devices and microreactors.

The Formation of Large-Area Conducting Graphene-Like Platelets

The treatment of a suspension of graphite oxide (GO) with sodium azide leads to a material that, after reduction, features amino groups at the top and bottom of the sheets. These groups react through microcontact printing with an isothiocyanate monolayer on a silicon oxide substrate to form covalent bonds that strongly attach to the particles on the surface. With ultrasonication it is possible to obtain exfoliation of the sheets that are not covalently bound to the surface leaving single-layer platelets attached to the substrate. The azido derivative can be also used to functionalize the graphene oxide with long alkylic chains through a click chemistry approach. This functionalization results in the exfoliation of this material in dimethylformamide. The novel materials were fully characterized by different techniques including IR spectroscopy, thermogravimetric analysis (TGA), scanning and transmission electron microscopy (SEM and TEM), X-Ray photoelectron spectroscopy (XPS), and solid state NMR spectroscopy. The material with amino groups, after the reduction step, is conductive with a resistivity only approximately seven times larger than that of unprocessed graphite. This implies that after reduction of the GO, the conjugated sp2 network is largely restored. We consider this to be an important step towards a chemical approach for forming conducting large-area platelet films of single-layer graphene.

Ratiometric Fluorescent Detection of an Anthrax Biomarker at Molecular Printboards

Anthrax is an acute disease, concurrently a potential biological warfare agent caused by Bacillus Anthracis. The accurate, rapid, sensitive, and selective detection of Bacillus spores plays a vital role in order to prevent a biological attack or outbreak of disease. Bacterial spores contain a main core cell which is enclosed by protective layers. As a major component of these protective layers, bacterial spores contain up to 1 M dipicolinic acid (DPA), accounting for 5−15 % of the dry mass of the bacterial spore. Hence, DPA is a convenient biomarker for these spores. In recent years a number of biological and chemical detection methods for Bacillus Anthracis spores have been investigated. Biological methods are based on polymerase chain reactions and immunoassays. Important chemical methods employ vibrational spectroscopy (FT-IR, Raman and SERS) and photoluminescence. Among them, lanthanide (Ln3+)-based luminescent detection of DPA has been most promising owing to the unique photophysical properties of Ln3+-DPA chelates, including their bright luminescence upon sensitization by DPA, the long luminescence lifetimes compared to free Ln3+, and the concomitantly high luminescence enhancement ratio upon coordination of DPA to the Ln3+ center. Besides the use of DPA itself as a sensitizer, ratiometric fluorescent detection of anthrax spores can be achieved through the displacement of a different sensitizer by DPA.

Simple 3D Printed Scaffold-Removal Method for the Fabrication of Intricate Microfluidic Devices

An easy and cheap fabrication method for intricate polydimethylsiloxane microfluidic devices is presented. The acrylonitrile butadiene styrene scaffold‐removal method uses cheap, off‐the‐shelf materials and equipment for the fabrication of intricate microfluidic devices. The versatility of the method is proven by the fabrication of 3D multilayer, ship‐in‐a‐bottle, selective heating, sensing, and NMR microfluidic devices. The methodology is coined ESCARGOT: Embedded SCAffold RemovinG Open Technology.