Efficient self-assembly of DNA-functionalized fluorophores and gold nanoparticles with DNA functionalized silicon surfaces: the effect of oligomer spacers.

Although strategies for the immobilization of DNA oligonucleotides onto surfaces for bioanalytical and top-down bio-inspired nanobiofabrication approaches are well developed, the effect of introducing spacer molecules between the surface and the DNA oligonucleotide for the hybridization of nanoparticle-DNA conjugates has not been previously assessed in a quantitative manner. The hybridization efficiency of DNA oligonucleotides end-labelled with gold nanoparticles (1.4 or 10 nm diameter) with DNA sequences conjugated to silicon surfaces via hexaethylene glycol phosphate diester oligomer spacers (0, 1, 2, 6 oligomers) was found to be independent of spacer length. To quantify both the density of DNA strands attached to the surfaces and hybridization with the surface-attached DNA, new methodologies have been developed. Firstly, a simple approach based on fluorescence has been developed for determination of the immobilization density of DNA oligonucleotides. Secondly, an approach using mass spectrometry has been created to establish (i) the mean number of DNA oligonucleotides attached to the gold nanoparticles and (ii) the hybridization density of nanoparticle-oligonucleotide conjugates with the silicon surface-attached complementary sequence. These methods and results will be useful for application with nanosensors, the self-assembly of nanoelectronic devices and the attachment of nanoparticles to biomolecules for single-molecule biophysical studies.

Kinetics of electroless deposition: the copper-dimethylamine borane system.

A kinetic study of the electroless deposition of copper on gold, using dimethylamine borane (DMAB) as a reducing agent, has been carried out. The copper deposition rate in the electroless bath was determined to be 50 nm min(-1), through electrochemical stripping of the copper deposits as well as from direct measurements of the film thickness using atomic force microscopy (AFM). Comparison with a galvanic cell setup, where the two half-reactions were physically separated, yielded a lower deposition rate of 30 nm min(-1). An important kinetic effect of the surface on the oxidation of the reducing agent, and thus on the overall process, was therefore revealed. The efficiency of the process was measured over time, revealing the contribution of side reactions in the cathodic half-cell, particularly during the initial stages of the electroless process.

Interference lithographic nanopatterning of plant and bacterial light-harvesting complexes on gold substrates.

We describe a facile approach for nanopatterning of photosynthetic light-harvesting complexes over macroscopic areas, and use optical spectroscopy to demonstrate retention of native properties by both site-specifically and non-specifically attached photosynthetic membrane proteins. A Lloyd's mirror dual-beam interferometer was used to expose self-assembled monolayers of amine-terminated alkylthiolates on gold to laser irradiation. Following exposure, photo-oxidized adsorbates were replaced by oligo(ethylene glycol) terminated thiols, and the remaining intact amine-functionalized regions were used for attachment of the major light-harvesting chlorophyll-protein complex from plants, LHCII. These amine patterns could be derivatized with nitrilotriacetic acid (NTA), so that polyhistidine-tagged bacteriochlorophyll-protein complexes from phototrophic bacteria could be attached with a defined surface orientation. By varying parameters such as the angle between the interfering beams and the laser irradiation dose, it was possible to vary the period and widths of NTA and amine-functionalized lines on the surfaces; periods varied from 1200 to 240 nm and linewidths as small as 60 nm (λ/4) were achieved. This level of control over the surface chemistry was reflected in the surface topology of the protein nanostructures imaged by atomic force microscopy; fluorescence imaging and spectral measurements demonstrated that the surface-attached proteins had retained their native functionality.

Photocatalytic nanolithography of self-assembled monolayers and proteins.

Self-assembled monolayers of alkylthiolates on gold and alkylsilanes on silicon dioxide have been patterned photocatalytically on sub-100 nm length-scales using both apertured near-field and apertureless methods. Apertured lithography was carried out by means of an argon ion laser (364 nm) coupled to cantilever-type near-field probes with a thin film of titania deposited over the aperture. Apertureless lithography was carried out with a helium-cadmium laser (325 nm) to excite titanium-coated, contact-mode atomic force microscope (AFM) probes. This latter approach is readily implementable on any commercial AFM system. Photodegradation occurred in both cases through the localized photocatalytic degradation of the monolayer. For alkanethiols, degradation of one thiol exposed the bare substrate, enabling refunctionalization of the bare gold by a second, contrasting thiol. For alkylsilanes, degradation of the adsorbate molecule provided a facile means for protein patterning. Lines were written in a protein-resistant film formed by the adsorption of oligo(ethylene glycol)-functionalized trichlorosilanes on glass, leading to the formation of sub-100 nm adhesive, aldehyde-functionalized regions. These were derivatized with aminobutylnitrilotriacetic acid, and complexed with Ni(2+), enabling the binding of histidine-labeled green fluorescent protein, which yielded bright fluorescence from 70-nm-wide lines that could be imaged clearly in a confocal microscope.

Micrometer and nanometer scale photopatterning of proteins on glass surfaces by photo-degradation of films formed from oligo(ethylene glycol) terminated silanes.

Exposure of films formed by the adsorption of oligo(ethylene glycol) (OEG) functionalized trichlorosilanes on glass to UV light from a frequency-doubled argon ion laser (244 nm) causes photodegradation of the OEG chain. Although the rate of degradation is substantially slower than for monolayers of OEG terminated thiolates on gold, it is nevertheless possible to form micrometer-scale patterns by elective adsorption of streptavidin to exposed regions. A low density of aldehyde functional groups is produced, and this enables derivatization with nitrilotriacetic acid via an amine linker. Complexation with nickel enables the site-specific immobilization of histidine-tagged yellow and green fluorescent proteins. Nanometer-scale patterns may be fabricated using a Lloyd's mirror interferometer, with a sample and mirror set at right angles to each other. At low exposures, partial degradation of the OEG chains does not remove the protein-resistance of the surface, even though friction force microscopy reveals the formation of patterns. At an exposure of ca. 18 J cm(-2), the modified regions became adhesive to proteins in a narrow region ca. 30 nm (λ/8) wide. As the exposure is increased further the lines quickly broaden to ca. 90 nm. Adjustment of the angle between the sample and mirror enables the fabrication of lines of His-tagged green fluorescent protein at a period of 340 nm that could be resolved using a confocal microscope.

Self-assembly of semifluorinated n-alkanethiols on {111}-oriented Au investigated with scanning tunneling microscopy experiment and theory.

The adsorption of semifluorinated alkanethiols on Au/mica was studied by scanning tunneling microscopy (STM). The adlayer structure produced is based on a p(2 x 2) structure though lines of molecules displayed extensive kinks and bends. In addition, a considerable variation in the contrast of molecular features is found. Molecular modeling calculations confirm that, for the fluorinated thiols, inequivalently adsorbed molecules within a p(2 x 2) registry are present, an aspect that endows the local structure of the adlayer with a higher flexibility in comparison to nonfluorinated thiols, where one adsorption site is strongly favored in a (radical 3 x radical 3) R30 degrees structure. Simulated STM imaging on the optimized systems successfully recovered the effects on the molecular feature contrast induced by the flexibility of the fluorinated thiol adlayer.

Evaporation and deposition of alkyl-capped silicon nanocrystals in ultrahigh vacuum.

Nanocrystals are under active investigation because of their interesting size-dependent properties and potential applications. Silicon nanocrystals have been studied for possible uses in optoelectronics, and may be relevant to the understanding of natural processes such as lightning strikes. Gas-phase methods can be used to prepare nanocrystals, and mass spectrometric techniques have been used to analyse Au and CdSe clusters. However, it is difficult to study nanocrystals by such methods unless they are synthesized in the gas phase. In particular, pre-prepared nanocrystals are generally difficult to sublime without decomposition. Here we report the observation that films of alkyl-capped silicon nanocrystals evaporate upon heating in ultrahigh vacuum at 200 degrees C, and the vapour of intact nanocrystals can be collected on a variety of solid substrates. This effect may be useful for the controlled preparation of new quantum-confined silicon structures and could facilitate their mass spectroscopic study and size-selection.