Linear self-assembly and grafting of gold nanorods into arrayed micrometer-long nanowires on a silicon wafer via a combined top-down/bottom-up approach.

Macroscopically long wire-like arrangements of gold nanoparticles were obtained by controlled evaporation and partial coalescence of an aqueous colloidal solution of capped CTAB-Au nanorods onto a functionalised 3-mercaptopropyl trimethoxysilane (MPTMS) silicon substrate, using a removable, silicon wafer with a hydrophobic surface that serves as a "handrail" for the initial nanorods' linear self-assembly. The wire-like structures display a quasi-continuous pattern by thermal annealing of the gold nanorods when the solvent (i.e. water) is evaporated at temperatures rising from 20°C to 140°C. Formation of both single and self-replicating parallel 1D-superstructures consisting of two or even three wires is observed and explained under such conditions.

Plasmonic enhancement of dye sensitized solar cells via a tailored size-distribution of chemically functionalized gold nanoparticles.

A substantial and stable increase of the current density Jsc of ruthenium (Ru) dye sensitized solar cells (DSC) of up to 16.18% and of the power efficiency of up to 25.5% is demonstrated in this article via plasmonic enhancement. The key aspect of this work is the use of a tailored bimodal size distribution of functionalized gold nanoparticles (AuNPs) that have been chemically immobilized onto the mesoporous titanium dioxide (TiO2) layer via short, stable dithiodibutyric acid linkers. The size distribution of the AuNPs is a result of theoretical calculations that aimed at the perfection of the absorption characteristics of the complete solar cell system over a wide range of wavelengths. The functionalization of the AuNPs serves to bind them at a close but defined distance to TiO2-particles and additionally to chemically protect them against potential corrosion by the electrolyte. Simulations of near field (enhanced absorption) and far field (scattering) contributions have been used to tailor a complex AuNPs bimodal size distribution that had subsequently demonstrated experimentally a close to optimum improvement of the absorbance over a wide wavelength range (500-675 nm) and therefore an impressive DSC efficiency enhancement. Finally, the modified DSCs are exhibiting pronounced longevity and stable performance as confirmed via long time measurements. In summary, the presented systems show increased performance compared to non plasmonic enhanced cells with otherwise identical composition, and are demonstrating a previously unpublished longevity for iodide electrolyte/AuNPs combinations.

Cell uptake and cytotoxicity of a novel cyclometalated iridium(III) complex and its octaarginine peptide conjugate.

The synthesis and characterisation of iridium(III) bis(2-(2,4-difluorophenyl)pyridinato-N, C2')-2(4-carboxylphenyl)imidazo[4,5-f][1,10]phenanthroline perchlorate, [Ir(dfpp)(2)(picCOOH)](+) and its octaarginine conjugate [Ir(dfpp)(2)(picCONH-Arg(8))](9+) are reported. Both complex and conjugate exhibit intense and long-lived luminescence, which is O(2) and pH sensitive. Conjugation to the polyarginine peptide renders the complex very water soluble. The uptake of the parent iridium(III) complex and conjugate are compared in two mammalian cell lines; SP2 myeloma and Chinese hamster ovary (CHO). Both complexes internalise into the cytoplasm, however dye uptake rate and distribution vary with peptide conjugation and with cell identity. Whereas transmembrane transport is thought to have been facilitated by the dimethyl sulfoxide (DMSO) used as co-solvent (0.05% v/v) for the parent complex, the octaarginine, the dye-conjugate (iridium-R(8)) is membrane permeable in water only. Both complexes exhibit high cytotoxicity, evident through blebbing and vacuole formation within living cells, indicative of apoptosis, within 30min of exposure to the probe. The IC(50) recorded for the cells in the dark was independent, in the case of the parent complex, of the identity of the cell, with IC(50) of 84.8µM and 88µM respectively for SP2 and CHO cells. The IC(50) approximately doubled for the polyarginine conjugate and displayed a significant dependence on cell type with IC(50) of 35µM and 54.1µM respectively for SP2 and CHO cells. These IC(50) values were recorded in the dark. However under irradiation cell death is considerably faster. Evidence from imaging suggests that the conjugate penetrates the nucleus whereas the parent does not, indicating that nuclear penetration may play a role in cytotoxicity.

Electron transfer and switching in rigid [2]rotaxanes adsorbed on TiO2 nanoparticles.

Bistable [2]rotaxanes have been attached through a bulky tripodal linker to the surface of titanium dioxide nanoparticles and studied by cyclic voltammetry and spectroelectrochemical methods. The axle component in the [2]rotaxane contains two viologen sites, V(1) and V(2), interconnected by a rigid terphenylene bridge. In their parent dication states, V(1)(2+) and V(2)(2+) can both accommodate a crown ether ring, C, but are not equivalent in terms of their affinity towards C and have different electrochemical reduction potentials. The geometry and size of the tripodal linker help to maintain a perpendicular [2]rotaxane orientation at the surface and to avoid unwanted side-to-side interactions. When the rigid [2]rotaxane or its corresponding axle are adsorbed on a TiO(2) nanoparticle, viologen V(2)(2+) is reduced at significantly more negative potentials (-0.3 V) than in flexible analogues that contain aliphatic bridges between V(1) and V(2). These overpotentials are analysed in terms of electron-transfer rates and a donor-bridge-acceptor (D-B-A) formalism, in which D is the doubly reduced viologen, V(1)(0), adjacent to the TiO(2) surface (TiO(2)-V(1)(0)), B is the terphenylene bridge and A is viologen V(2)(2+). We have also found that, in contrast with earlier findings in solution, no molecular shuttling occurs in rigid [2]rotaxane adsorbed at the surface. The observations were explained by the relative position of the viologen stations within the electrical double layer, screening of V(2)(2+) by the counterions and high capacity of the medium, which reduces the mobility of the crown ether. The results are useful in transposing of solution-based molecular switches to the interface or in the design and understanding of the properties of systems comprising electroactive and/or interlocked molecules adsorbed at the nanostructured TiO(2) surface.

High sensitivity carbon nanotube based electrochemiluminescence sensor array.

Ink jet printed carbon nanotube forest arrays capable of detecting picomolar concentrations of immunoglobulin G (IgG) using electrochemiluminescence (ECL) are described. Patterned arrays of vertically aligned single walled carbon nanotube (SWCNT) forests were printed on indium tin oxide (ITO) electrodes. Capture anti-IgG antibodies were then coupled through peptide bond formation to acidic functional groups on the vertical nanotubes. IgG immunoassays were performed using silica nano particles (Si NP) functionalized with the ECL luminophore [Ru(bpy)(2)PICH(2)](2+)], and IgG labelled G1.5 acid terminated PAMAM dendrimers. PAMAM is poly(amido amine), bpy is 2,2'-bipyridyl and PICH(2) is (2-(4-carboxyphenyl)imidazo[4,5-f][1,10]phenanthroline). The carboxyl terminal of [Ru(bpy)(2)PICH(2)](2+) (fluorescence lifetime ≈ 682±5 ns) dye was covalently coupled to amine groups on the 800 nm diameter silica spheres in order to produce significant ECL enhancement in the presence of sodium oxalate as co-reactant in PBS at pH 7.2). Significantly, this SWCNT-based sensor array shows a wide linear dynamic range for IgG coated spheres (10(6) to 10(12) spheres) corresponding to IgG concentrations between 20 pM and 300 nM. A detection limit of 1.1±0.1 pM IgG is obtained under optimal conditions.

Introducing negative charges into bis-p-phenylene crown ethers: a study of bipyridinium-based [2]pseudorotaxanes and [2]rotaxanes.

This paper describes novel host-guest systems comprising viologen cations (guests) and the derivatives of bis-para-phenylene-34-crown-10 (hosts) with anionic groups COO(-) or SO(3)(-). The structure of the resulting charge-compensated host-guest complexes, their association constants and their electrochemical behaviour have been studied. In the solid state, the viologen cations thread the negatively charged crown ethers forming electroneutral zwitterion-like [2]pseudorotaxane salts; in solution this threaded geometry is preserved. The association constants of [2]pseudorotaxane salts incorporating the 1,1'-diethylviologen moiety in solution are significantly higher than those of previously reported analogues. The extrapolated association free energies in non-aqueous media exceed -40 kJ mol(-1) at 25 degrees C. This significant increase of the interaction free energy makes these compounds stable even in aqueous solutions. The association constants of [2]pseudorotaxane salts incorporating sterically more hindered 1,1'-diethyl-3,3'-dimethylviologen moieties are significantly lower. Structurally related [2]rotaxane salts, in which the oppositely charged ionic components are mechanically interlocked, have been prepared in good yields. It has been shown that [2]rotaxane salts incorporating anti-isomers of bisfunctionalised crown ethers are cycloenantiomeric. In both [2]pseudorotaxane and [2]rotaxane salts, the electrostatic interactions between the viologen moieties and the negatively charged crown ethers lead to very significant negative shifts of viologen reduction potentials up to 450 mV. The findings of the present study are valuable for the design of nanoscale molecular electronic devices.

Quantitative conformational study of redox-active [2]rotaxanes, part 1: Methodology and application to a model [2]rotaxane.

This paper reports a novel methodology for the conformational analysis of [2]rotaxanes. It combines NMR spectroscopic (COSY, NOESY and the recently reported paramagnetic line-broadening and suppression technique) and electrochemical techniques to enable a quantitative analysis of the co-conformations of interlocked molecules and the conformations of their components. This methodology was used to study a model [2]rotaxane in solution. This [2]rotaxane consists of an axle that incorporates an electron-poor, doubly positively charged viologen that threads an electron-rich crown ether. It has been shown that the axle of the [2]rotaxane in its dicationic state adopts a folded conformation in solution and the crown ether is localised at the viologen moiety. Following a one-electron reduction of viologen, the paramagnetic radical cation of the [2]rotaxane retains its folded conformation in solution. The data also demonstrate that in the radical cation the crown ether remains localised at the viologen, despite its reduced affinity for the singly reduced viologen. The combined quantitative NMR spectroscopic and electrochemical characterisation of the electromechanical function of the model [2]rotaxane in solution provides an important reference point for the study of switching in structurally related bistable [2]rotaxanes, which is the subject of the second part of this work.

Quantitative conformational study of redox-active [2]rotaxanes, part 2: Switching in flexible and rigid bistable [2]rotaxanes.

Translational movement of the macrocycle in two structurally similar bistable [2]rotaxanes, which is induced by a four-step electrochemical process in solution, has been investigated by using a methodology developed in the preceding article (Chem. Eur. J. 2008, 14, 1107-1116). Both [2]rotaxanes contain a crown ether that can be accommodated by either of two interconnected viologen recognition sites. These sites are substantially different in terms of their affinity towards the crown ether and they possess considerably different electrochemical reduction potentials. The two [2]rotaxanes differ in the length and the rigidity of a bridge that links these sites. A combination of molecular mechanics modelling and NOE spectroscopy data provides information about the conformations of both [2]rotaxanes in the parent oxidation state when the crown ether exclusively populates the strong recognition site. To determine the population of the recognition sites at subsequent stages of reduction, a paramagnetic NMR technique and cyclic voltammetry were used. The key finding is that the flexibility of the connecting bridge element between the recognition sites interferes with shuttling of the crown ether in [2]rotaxanes. It can be demonstrated that the more flexible trimethylene bridge is folded, thus limiting the propensity of the crown ether to shuttle. Consequently, the crown ether populates the original site even in the second reduced state of the flexible [2]rotaxane. On the contrary, in the [2]rotaxane in which two viologen sites are connected by a larger and more rigid p-terphenylene bridge, the predominant location of the crown ether at the weak recognition site is achieved after just one single electron reduction.

A tripodal [2]rotaxane on the surface of gold.

Tripodal [2]rotaxane, 3, and the structurally related axle, 2, incorporating a viologen moiety, a crown ether, and three thiol anchoring groups have been synthesized. Analogous monopodal derivatives, 1, have also been prepared. Self-assembled monolayers of the above tripodal and monopodal systems on gold have been studied by cyclic voltammetry. It has been shown that a thiol anchoring group is required to attach the monopodal viologen 1 to the surface of gold and that the maximum surface coverage of 1 corresponds to 2.7 x 10(-10) mol.cm(-2). The adsorbed monopodal viologen 1 does not thread bis-p-phenylene-34-crown-10 ether, 6. However, the tripodal axle 2 adsorbed on the surface of gold threads the crown ether 6 to form a hetero [2]rotaxane. In the case of the tripodal axle 2, the surface coverage is 7 x 10(-11) mol.cm(-2), while for the tripodal [2]rotaxane 3 the surface coverage reaches 1.1 x 10(-10) mol.cm(-2).