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ABSTRACT: 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.
ChemPhysChem 02/2012; 13(3):797-810. · 3.41 Impact Factor
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ABSTRACT: 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.
Chemistry 02/2008; 14(4):1095-106. · 5.93 Impact Factor
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ABSTRACT: 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.
Chemistry 02/2008; 14(4):1117-28. · 5.93 Impact Factor
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ABSTRACT: 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.
Chemistry 02/2008; 14(4):1107-16. · 5.93 Impact Factor
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ABSTRACT: 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).
Langmuir 12/2007; 23(24):12147-53. · 4.19 Impact Factor
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ABSTRACT: Here, we report on a facile method for the preparation of an aqueous dispersion of 3.6 nm gold nanoparticles electrostatically stabilized by a weakly physisorbed ligand, namely, 4-(dimethylamino)pyridine (DMAP). The nature and extent of the interaction of this ligand with the surface of a gold nanoparticle has been examined. We also report on the thermally promoted ripening of these nanoparticles under mild conditions to yield a dispersion of 11.3 nm gold nanoparticles. The role of the weakly physisorbed DMAP ligand in facilitating thermally promoted ripening under mild conditions has also been examined.
Langmuir 10/2007; 23(20):10262-71. · 4.19 Impact Factor
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ABSTRACT: We report here on the application of Surface Element Integration (SEI) to evaluate the potential energy of interactions between
alkane-modified multiwalled carbon nanotubes (MWCNTs) and tetraoctylammonium bromide (TOAB) stabilised gold nanoparticles.
The interacting objects are treated as cylinders and spheres, respectively, with corresponding alkyl chains extending perpendicularly
from their surfaces. In such case the widely used Derjaguin approximation is invalid. Thus SEI was used to calculate the van
der Waals, osmotic and elastic interactions. The results show that it is possible to control the self-assembly process of
the gold nanoparticles at the surface of modified MWCNT in terms of size- and type-selectivity.
Journal of Computer-Aided Materials Design 03/2007; 14(1):151-165. · 1.30 Impact Factor
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ABSTRACT: Several linear and branched DNA structures from 80-200 nm with a biotine molecule in the middle have been prepared. These structures have been decorated by addition of positively charged gold nanoparticles carrying 4-(dimethylamino)pyridine ligands. Streptavidin binds to the central biotine molecule introducing a 20 nm gap in the structure in which a biotinylated nanoparticle can be introduced. The simplest structure (80 nm, linear) is formed by 4 oligonucleotides. By changing some of these components changes on length, shape, and recognition system easily can be introduced.
Nucleosides Nucleotides & Nucleic Acids 02/2007; 26(10-12):1605-9. · 0.90 Impact Factor
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ABSTRACT: This paper reports the findings of a detailed study of the self-assembly of gold nanoparticles at the surface of carbon nanotubes (CNTs). The study included the development of a predictive model for the interactions (charge transfer, van der Waals, osmotic, elastic, nonelastic, and covalent) between tetraoctylammonium bromide-stabilized (TOAB) gold nanoparticles and alkyl- and alkylthiol-modified multiwalled carbon nanotubes (MWCNTs). It also included the measurement of the coverage of gold nanoparticles at the surface of the above MWCNTs as a function of increasing alkyl chain length. One key finding is that it is possible to predict with a high degree of accuracy using the above model the measured coverage of gold nanoparticles adsorbed, either noncovalently or covalently, at the surface of a MWCNT. Another key finding is that, as predicted, under well-defined conditions the measured coverage of nanoparticles is very sensitive to the nature of the modified CNT surface and the contiguous environment, providing valuable insights that will underpin the rational design of functional nanoscale devices assembled from nanoparticle and CNT building blocks.
The Journal of Physical Chemistry B 10/2005; 109(34):16310-25. · 3.70 Impact Factor
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ABSTRACT: A new NMR technique, namely paramagnetic suppression spectroscopy (PASSY), has been used to study the oxidation-state dependent structural conformation and supramolecular function of redox-active rotaxanes and catenanes in solution and at the surfaces of nanoparticles. Specifically, this technique has been used to study the structural conformation and supramolecular function in solution of a redox-active [2]rotaxane and the corresponding radical cation formed by a one-electron reduction. The findings of this and related studies provide important insights into the design of nanoscale devices based on rotaxanes and catenanes.
Journal of the American Chemical Society 07/2005; 127(22):8067-76. · 9.91 Impact Factor
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ABSTRACT: Reported is the use of DNA to template the assembly of nanowires and protein-functionalized nanogap electrodes. Specifically, the use of DNA to template the assembly of gold nanowires between conventionally patterned gold contacts on a silicon wafer substrate. Also the use of DNA to template the assembly of protein-functionalized gold nanogap electrodes on a silicon wafer substrate. Of particular significance is the finding that suitably modified gold nanoparticles recognize and bind selectively the protein-functionalized nanogap between the above electrodes and are localized there. This and related work forms part of a broader effort directed toward the development of the alternative bottom-up fabrication technologies that will be needed to extend Moore's Law beyond 2012.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
06/2005;
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ABSTRACT: Here we report on the DNA-templated self-assembly of conducting gold nanowires between gold electrodes lithographically patterned on a silicon oxide substrate. An aqueous dispersion of 4-(dimethylamino)pyridine-stabilized gold nanoparticles was prepared. These nanoparticles recognize and bind selectively double-stranded calf thymus DNA aligned between the gold electrodes to form a linear nanoparticle array. Continuous polycrystalline gold nanowires are obtained by electroless deposition that enlarges and enjoins the individual gold nanoparticles. The above nanowires were structurally characterized using a range of electron and scanning probe microscopies and electrically characterized at room temperature using a standard probe setup. The results of these characterizations show these wires to be 20 nm high and 40 nm wide, to be continuous between interdigitated gold electrodes with an interelectrode spacing of 0.2 or 1.0 μm, and to possess a resistivity of 2 × 10-4 Ωm. These DNA-templated nanowires, the smallest reported to date, exhibit resistivities consistent with reported findings and current theory. The use of DNA as a template for the self-assembly of conducting gold nanowires represents a potentially important approach to the fabrication of nanoscale interconnects.
03/2005;
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ABSTRACT: The use of DNA to template the assembly of nanoscale wires and protein-functionalized nanogap contacts is described: Specifically, the use of DNA to template the assembly of gold nanowires between conventionally patterned gold contacts on a silicon wafer substrate. Also described is the use of DNA to template the assembly of protein-functionalized nanogap gold contacts on a silicon wafer substrate. Of particular significance is the finding that suitably modified gold nanoparticles recognize and bind selectively the protein-functionalized nanogap and are localized there.
The Analyst 01/2005; 129(12):1171-5. · 4.23 Impact Factor
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ABSTRACT: Reported is the preparation and characterization of phosphonic acid-modified and alkoxy silane-modified multiwalled carbon nanotubes (MWCNTs). Also reported is the use of these modified MWCNTs to template the assembly of titanium dioxide and silica nanoparticles, respectively. Some potential applications of these findings are considered.
08/2004;
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ABSTRACT: This paper demonstrates a novel facile method for fabrication of patterned arrays of gold nanoparticles on Si/SiO2 by combining electron beam lithography and self-assembly techniques. Our strategy is to use direct-write electron beam patterning to convert nitro functionality in self-assembled monolayers of 3-(4-nitrophenoxy)-propyltrimethoxysilane to amino functionality, forming chemically well-defined surface architectures on the 100 nm scale. These nanopatterns are employed to guide the assembly of citrate-passivated gold nanoparticles according to their different affinities for amino and nitro groups. This kind of nanoparticle assembly offers an attractive new option for nanoparticle patterning a silicon surface, as relevant, for example, to biosensors, electronics, and optical devices.
Langmuir 05/2004; 20(9):3766-8. · 4.19 Impact Factor
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ABSTRACT: A cation-modified multiwalled carbon nanotube is used to template the noncovalent self-assembly in solution of a gold nanowire from crown-modified gold nanoparticles. The driving force for self-assembly is formation of the surface-confined pseudorotaxane that results from the electron-poor cation threading the electron-rich crown.
04/2004;
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ABSTRACT: A silicon wafer was modified by chemisorption of a monolayer of a cation precursor and exposed to blue light through a mask. In the regions exposed to blue light, the cation precursor was converted to cation. These cations were recognized and bound selectively by nanoparticles modified by adsorption of crown. As a consequence, these crown-modified nanoparticles were adsorbed at only the desired regions and pattern transfer was affected.
03/2004;
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ABSTRACT: This paper describes the self-assembly of a heterosupramolecular system consisting of a tripodal [2]rotaxane adsorbed at the surface of a titanium dioxide nanoparticle. The tripodal [2]rotaxane consists of a dumbbell-shaped molecule, incorporating two electron-poor viologens, threading an electron-rich crown ether. The [2]rotaxane also incorporates a bulky tripodal linker group at one end and a bulky stopper group at the other end. The [2]rotaxane is adsorbed, via the tripodal linker group, at the surface of a titanium dioxide nanoparticle. The structure and function of the resulting hetero[2]rotaxane have been studied in detail by (1)H NMR spectroscopy and cyclic voltammetry. A key finding is that it is possible to electronically address and switch the above hetero[2]rotaxane.
Journal of the American Chemical Society 01/2004; 125(50):15490-8. · 9.91 Impact Factor
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ABSTRACT: This paper reviews recent experimental approaches to the development of surface nanostructures from nanoparticles. The formation of nanowires by electron beam writing in films of gold nanoparticles passivated with a specially designed class of ligand molecules (dialkyl sulfides) is presented, together with illustrations of practical nanostructures. Potential applications of this methodology are discussed. Another alternative to the controlled fabrication of arrays of nanoparticles, based on nanocrystals which contain molecular recognition elements in the ligand shell, is also surveyed. These particles aggregate in the presence of specifically designed molecular dications which act as a molecular binder. Finally, recent work on the formation of nanoscale surface architectures using x-ray patterning of self-assembled monolayers is introduced. Current and potential future applications of these surface nanostructures are discussed.
Journal of Physics Condensed Matter 10/2003; 15(42):S3047. · 2.55 Impact Factor
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ChemPhysChem 09/2003; 4(8):884-9. · 3.41 Impact Factor
