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Weekly UpdatesMultiple electrically wired surface enzyme logic gates
A significant step in the development of biocomputing has been made with the fabrication of the first electrically wired enzyme system assembled on an electrode surface capable of performing various logic operations depending on the applied potential, permitting electrical interfacing between biomolecular computing systems and 'normal' electronics (M Pita, E Katz; J. Am. Chem. Soc., 2008, 130, 36) (Scheme 1). Glucose oxidase (GOx) (surface concentration =2 x [10.sup.-12] mol [cm.sup.2]) reconstituted on an FAD-cofactor-modified electrode surface provides an electrically wired biocatalytic interface on which an Au wire electrode (ca. 0.2[cm.sup.2] area) modified by a pyrroloquinoline quinone (pQQ) monolayer covalently binds a cystamine monolayer self-assembled on the electrode surface. Carboxylic groups of the PQQ monolayer covalently bind to 3-aminophenylboronic acid. The phenylboronic acid groups are themselves coupled with the vicinal hydroxyl groups of the FAD to immobilise the cofactor. Once reconstituted, the Apo-GOx on the FAD-functionalised surface gives the electrically wired enzyme, which communicates with the conductive support through the electron transfer mediated by the co-immobilised PQQ monolayer. The lysine residues of the reconstituted COx allow microperoxidase-11 (MP-11; conc = 5 x [10.sup.-10] mol [cm.sup.-2]) to be covalently immobilised at the top of the GOx layer. The resulting GOx/MP-11-modified electrode--which catalyses three different reactions: oxidation of glucose and reduction or oxidation of [H.sub.2][O.sub.2]--performs various Boolean logic operations (OR, XOR, AND-OR) upon addition of glucose and/or [H.sub.2][O.sub.2] and application of different potentials. Addition of glucose (c. 5mM) and [H.sub.2][O.sub.2] (c. 5mM) are the input signals '1', while the absence of the respective substrate is input '0'. The output signals are the currents generated by the enzyme-modified electrode upon application of constant potentials 0.45, 0.18, and -0.10 V in the absence or presence of glucose and [H.sub.2][O.sub.2], corresponding to the combinations of the input signals '0,0'; '0,1'; '1,0', and '1,1'. The current range from 0.4 to 1.4 [mu]A is considered to be an undefined output signal. For example, at the applied potential of 0.45V, both biocatalytic units, GOx and MP-11, are activated for the oxidation of glucose and [H.sub.2][O.sub.2], respectively. However, the system demonstrates only a low background current (output signal '0') in the absence of both substrates (input signals '0,0'). In the presence of any substrate, glucose or [H.sub.2][O.sub.2] (input signals '0,1' or '1,0') or both of them (input signals '1,1'), the modified electrode generates the anodic currents corresponding to the oxidation of glucose or [H.sub.2][O.sub.2] or both of them (output signal '1'). Thus, the system performs OR logic operation, corresponding to the Boolean logic addition: A + B. At an applied potential of 0.18V, the electrode can be made to perform XOR (Exclusive-OR) operation, whilst AND-OR logic gates performing the following AB + A Boolean logic operations occur at -01.10V.
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MOFs for high-density gas adsorption
Reaction of Ag(I) with 3,5-bis(trifluoromethyl)-1,2,4-triazolate gives rise to a neutral, hydrogen-free, extended 3D nanotubular fluorous metal-organic framework (FMOF1), consisting of tetranuclear dusters [[Ag.sub.4][Tz.sub.6]] connected by three-coordinate Ag(I) centres (C Yang, X Wang, M A Omary; J. Am. Chem. Soc., 2007, 129, 15454) (Scheme 2). The fluortylined channels and cavities of the framework show hysteretic adsorption of hydrogen with a volumetric capacity of 41 kg/[m.sup.3] at 77K and 64 bar. The frame. work also exhibits very high adsorptions for oxygen and nitrogen with volumetric uptake of ca 550 and 400kg/[m.sup.3] at 77K even at very low pressures (<[10.sup.-2] bar).
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Sell-cleaning and controlled release smart films
Smart nanocomposite films that possess self cleaning and/or controlled release capabilities can be obtained by infiltrating temperature-responsive poly(N-isopropylacrylamide) gel (PNIPAAm) into vertically-aligned multi-walled carbon nanotube (VA-MWNT) arrays (W Chen, L Qu, D Chang et al, Chem. Commun., 2008, 163) (Scheme 3). It is the temperature-induced reversible rod-coil conformational transition of PNIPAAm in aqueous solutions that enables the infiltrated polymer chains to expand out or collapse within the nanotube gaps for self-cleaning, for applications such as antifouling substrates, artificial Gecko feet, and controlled release actions, such as functional membranes and sensors. With so many stimuli (temperature, solvent, pH, photo, ionic, electrical) responsive polymers already developed, this concept represents a versatile means of developing various functional polymer and aligned carbon nanotube-based multifunctional smart nanocomposite materials and devices with switchable surface characteristics.
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Selective N[O.sub.x] optical sensing
Redox-active covalently bound monolayers of osmium polypyridyl complexes on glass substrates undergo selective electron transfer with parts per million (ppm) levels of N[O.sub.2] (1-10 ppm) and N[O.sub.x] (800-2550 ppm) (A Gulino, T Gupta, P G Mineo, M E van der Boom, Chem. Commun., 2007, 4878) (Scheme 4). The accompanying decrease of the metal to ligand charge transfer within the optical range 350-800nm due to the formation of osmium(III) polypyridyl complexes, provides this sensing device with an optical readout, which can be reset with water saturated stream of nitrogen.
Water soluble magnetite nanoparticles
Reacting iron(III) acetylacetonate (Fe[(acac).sub.3]) in the polyol medium/Methylene glycol (TREG) at elevated temperature without any surfactants produces new class of magnetite nanoparticles (J Wan, WCai, X Meng, Chem. Commun., 2007, 5004). In this reaction polyol TREG plays a triple role as high-boiling solvent, reducing agent, and stabiliser to efficiently control the particle growth and prevent interparticle aggregation. The nanoparticles are uniform in size (8-1.1nm), highly crystalline, and superparamagnetic (80 emu [g.sup.-1]) at room temperature. The unique hydrophilic surface structures of the particles lead to the particles being stable not only in aqueous solution at neutral pH but also in physiological buffer. In vitro experiments indicate that these magnetite nanoparticles have an excellent MRI enhancement effect, unusual cancer cellular affinity, thus good biocompatibility. These novel magnetite nanoparficles should have great potential as high-performance magnetic resonance imaging (MRI) contrast agents.
Magnetic transition metal room temperature ionic liquids
In related studies, reaction of trihexyl(tetradecyl)phosph onium, 1-decyl-3- methylimidazolium (PR4), or 1-butyl-3-methylimidazolium (C4mim) halides with the corresponding metal halides, such as Fe[Cl.sub.4], and Mn[Br.sub.4], or metathesis with alkali salts of metal-based anions, generates transition metal based room temperature ionic liquids which show paramagnetic behaviour (R E Del Sesto, T M McCleskey, A K Burrell et al, Chem. Commun., 2008, 447). Although the ionic liquids are simply paramagnetic, they respond strongly to an applied magnetic field. For example, droplets of the [C10mim][Fe[Cl.sub.4]] and [PR4][Fe[Cl.sub.4]] salts, which are initially immiscible in water, can be easily manipulated with the application of an external strong magnetic field. [C10mim][Fe[Cl.sub.4]] will completely dissolve in the aqueous phase after several hours, whereas [PR4][Fe[Cl.sub.4]] droplets remain intact almost indefinitely (over several months). The potential thus exists for using phosphonium-based room temperature ionic liquids for magnetic transport through aqueous systems as well as for the fabrication of magnetic/electrochromic switching devices.
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Biodegradable microcapsules
Degradable microcapsules suitable for drug delivery, comprised of dextran modified with alkyne and azide groups can be rapidly generated through Cu(I)-catalysed 'click' reactions between azides and alkynes (Huisgen 1,3-dipolar cycloaddition) yielding triazole crosslinks--Scheme 5 (B G De Geest, W Van Camp, F E Du Prez et al, Chemical Commuications, 2008,190). The microcapsules can encapsulate macromolecular compounds and release them in a tailored, controlled fashion determined by the dextran propargyl carbonate (dex-CMC) and dextran azidopropyl carbonate (dex-N3) ratio, under physiological conditions. The mild, non-harmful reaction conditions for 'click' chemistry are especially attractive for the encapsulation of labile biological drugs.
Microporous polymers
High surface area (BET surface area up to 842[m.sup.2] [g.sup.2]) porous poly(phenylene butadiynylene) networks are produced by the palladium-catalysed homocoupling of 1,3,5-triethynylbenzene and 1,4-diethynylbenzene (J-X Jiang, F Su, H Niu et al, Chem. Commun., 2008, 486). These polymers which exhibit good chemical and thermal stability, are composed solely of carbon-carbon and carbon-hydrogen bonds. Since they are highly conjugated, there is a wealth of opportunity for producing porous materials with specific functionalities; for example, by attaching metals to the alkyne and/or alkene bonds in the network to facilitate catalysis, or introducing high binding energy sites, such as metals or transition metal complexes, for hydrogen storage.
Hydrogen sensing nanoparticles
The conductivity of solid-state films of alkylamine (eg octylamine and dodecylamine)-coated Pd, PdAg, and PdAu monolayer-protected clusters increases irreversibly upon initial exposure to 100% [H.sub.2] to varying degrees and with different reaction kinetics then exhibits stable, reversible changes in the presence of hydrogen concentrations (F J Ibanez, F P Zamborini; J. Am. Chem. Soc., 2008, 130, 622) (Scheme 6). The magnitude of the reversible conductivity changes depend on the alkyl chain length and alloy composition. Films of TOABr-coated Pd and PdAg nanoparticles, for instance, show stable, reversible increases in conductivity in the presence of [H.sub.2] concentrations from 9.6 down to 0.11%. This work provides a simple approach towards preparing films of chemically synthesised Pd-containing nanoparticles with controlled reactivity to hydrogen for sensing and catalysis applications.
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