Petri Pulkkinen

Inkjet-printed gold electrodes on paper: characterization and functionalization.

Gold nanoparticles were synthesized and inkjet-printed on a paper substrate and IR-sintered to produce conductive electrodes. The electrodes were further functionalised by using self-assembled octadecanethiol monolayers (SAMs). The effect of sintering, print quality, and SAM formation were examined by topographical, chemical and electrical methods. With optimised printing parameters, a volume resistivity of ~1.6 × 10(-7) Ω m was attained by a single print layer.

Poly(ethylene imine) and tetraethylenepentamine as protecting agents for metallic copper nanoparticles.

The aim of this research was to explore the use of amine-containing polymeric and low-molar-mass organic protecting agents in the preparation of copper nanoparticles. Particles were synthesized using poly(ethylene imine) (PEI) or tetraethylenepentamine (TEPA) as protecting agents. The resulting particles were studied with UV-vis spectrometry, thermogravimetry, scanning electron microscopy, and transmission electron microscopy, wide-angle X-ray scattering with heating, X-ray photoelectron spectroscopy, and Auger electron spectroscopy. The average crystal sizes for the particles were at room temperature 8.5 and 19.4 nm for PEI and TEPA, respectively, and some surface oxidation was observed. The particles were sintered on paper, and the resistance and resistivity were measured. For Cu/PEI samples, the protecting agent was removed upon sintering at relatively low temperatures (between 150 and 200 degrees C). At this temperature range, particles exhibited a rapid increase in the crystal size. Sintered particles exhibited high conductivity, indicating that these kinds of materials might find use in paper-based printing.

Synthesis and characterization of copper sulfide nanocrystallites with low sintering temperatures

To study nano-inks with relatively low sintering temperatures for fabrication of functional electronics on paper by inkjet printing technology, we have successfully prepared copper sulfide nanocrystallites protected by self-assembled monolayers. Systematic characterization was performed on as-prepared nanoparticles by FTIR, NMR, thermogravimetric analysis (TGA), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and wide-angle X-ray scattering (WAXS) with heating. The copper sulfide nanocrystallites with crystal sizes <1.5 nm show a hexagonal Cu2S phase at low temperatures but undergo significant consolidation/crystallization from 100 to 240 °C, accompanying a transformation from the hexagonal Cu2S phase to the cubic Cu1.8S phase when heated up to ca. 150 °C. The protective ligand burnout during heating is closely associated with the nanocrystallite consolidation. Further, the copper sulfide nanoparticles were deposited on paper and sintered at 240 °C in air. The sintered particles are composed of large crystals of cubic Cu1.8S with no serious degradation due to oxidation. The resistivity of the sintered particles was of the order of 1 × 10−5 (Ω m).

Simple and Efficient Separation of Atomically Precise Noble Metal Clusters

There is an urgent need for accessible purification and separation strategies of atomically precise metal clusters in order to promote the study of their fundamental properties. Although the separation of mixtures of atomically precise gold clusters Au25L18, where L are thiolates, has been demonstrated by advanced separation techniques, we present here the first separation of metal clusters by thin-layer chromatography (TLC), which is simple yet surprisingly efficient. This method was successfully applied to a binary mixture of Au25L18 with different ligands, as well as to a binary mixture of different cluster cores, Au25 and Au144, protected with the same ligand. Importantly, TLC even enabled the challenging separation of a multicomponent mixture of mixed-monolayer-protected Au25 clusters with closely similar chemical ligand compositions. We anticipate that the realization of such simple yet efficient separation technique will progress the detailed investigation of cluster properties.

Mixed-Monolayer-Protected Au25 Clusters with Bulky Calix[4]arene Functionalities

Although various complex, bulky ligands have been used to functionalize plasmonic gold nanoparticles, introducing them to small, atomically precise gold clusters is not trivial. Here, we demonstrate a simple one-pot procedure to synthesize fluorescent magic number Au25 clusters carrying controlled amounts of bulky calix[4]arene functionalities. These clusters are obtained from a synthesis feed containing binary mixtures of tetrathiolated calix[4]arene and 1-butanethiol. By systematic variation of the molar ratio of ligands, clusters carrying one to eight calixarene moieties were obtained. Structural characterization reveals unexpected binding of the calix[4]arenes to the Au25 cluster surface with two or four thiolates per moiety.

Control of the morphology of lipid layers by substrate surface chemistry.

In this study, surface coatings were used to control the morphology of the deposited lipid layers during vesicle spreading, i.e., to control if liposomes self-assemble on a surface into a supported lipid bilayer or a supported vesicular layer. The influence of the properties of the surface coating on formation of the deposited lipid layer was studied with quartz crystal microbalance and two-wavelength multiparametric surface plasmon resonance techniques. Control of lipid self-assembly on the surface was achieved by two different types of soft substrate materials, i.e., dextran and thiolated polyethylene glycol, functionalized with hydrophobic linkers for capturing the lipid layer. The low-molecular-weight dextran-based surface promoted formation of supported lipid bilayers, while the thiolated polyethylene glycol-based surface promoted supported vesicular layer formation. A silicon dioxide surface was used as a reference surface in both measurement techniques. In addition to promoting supported lipid bilayer formation of known lipid mixtures, the dextran surface also promoted supported lipid bilayer formation of vesicles containing the cell membrane extract of human hepatoblastoma cells. The new dextran-based surface was also capable of protecting the supported lipid bilayer against dehydration when exposed to a constant flow of air. The well-established quartz crystal microbalance technique was effective in determining the morphology of the formed lipid layer, while the two-wavelength surface plasmon resonance analysis enabled further complementary characterization of the adsorbed supported lipid bilayers and supported vesicular layers.

Complexation of calix[4]arene protected gold nanoparticles with pyridinium and bipyridinium compounds

Calix[4]arene and calix[4]arene/alkanethiol protected gold nanoparticles with narrow size distributions were synthesised and characterized with NMR, thermogravimetric analysis (TGA) and a transmission electron microscope (TEM). NMR and light scattering (LS) were used to study the complexation of the nanoparticles with a pyridinium modified polyethylene oxide (Pyr-PEO2k-Pyr) complexant and a small 16 carbon pyridinium compound (Pyr-C16). Results give clear evidence of complexation induced aggregation of the nanoparticles as pyridinium proton signals shift upon changing the host : guest ratio and LS shows a change from a narrow size distribution into a broad one. The addition of alkanethiols with longer dimensions than that of the calixarene derivative and the type of the complexant can be used to tune the complexation. The studies also provide evidence of induced fit complexation into calix[4]arene cavities and solution phase interdigitation (secondary monolayer formation) when the nanoparticles are complexed with Pyr-C16.

Interparticle distance in monolayers controlled by soft spacers

This paper addresses the use of graft polymer layers as spacers to control interparticle distance in 2-dimensional monolayers. Gold nanoparticles grafted with thermosensitive PNIPAM-based polymers with a large range of molecular mass and different degrees of hydrophobicity have been studied. The hydrophobicity of the polymer is adjusted by incorporation of a comonomer n-propylamine. The resulting copolymer, PNIPAM-co-NPAM, exhibits lower collapse transition temperature and increased cooperativity in the collapse process with n ≈ 150 compared to n ≈ 100 for PNIPAM, n being the number of monomers per collapse domain. Langmuir isotherms of these polymers under moderate compression follow closely a π ≈ c3 behavior with corresponding critical exponent ν = 3/4 as predicted for 2-dimensional polymer conformation in good solvent. Nanoparticles grafted with these polymers form stable Langmuir monolayers where the graft polymer chains adopt a 2-D stretched conformation that tethers the nanoparticles to the interface. The nanoparticle cores are thus isolated by the polymer shells resulting in nanoparticle areas that increase with polymer chain length. Correspondingly, the interparticle distance is found to vary with chain length as Dp ≈ N0.8. For the Au-PNIPAM-NPAM, a moderate increase in temperature to near-θ conditions decreases the nanoparticle area by about 30% through lateral collapse of the polymer layer. This thermally induced molecular collapse in a 2-D monolayer is an unusual and novel observation that may be attributed to cooperative effects of the collapse transition of the new copolymer PNIPAM-co-NPAM.

Gold Nanoparticles: Calixarene Complexation in a Mixed Calixarene/Alkanethiol Monolayer

This paper describes a detailed study on the complexation of pyridinium derivatives with calixarenes bound to gold nanoparticles (AuNPs). The studied calixarene derivatives are mixed with alkanethiols to form mixed monolayers on AuNP surfaces. The key findings are: (i) even a small amount (less than 11 mol%) of calixarenes can retain their complexation abilities among a majority of alkanethiols in a mixed monolayer, showing that it is possible to dilute the active calixarene (and possibly other receptors) in gold surfaces, (ii) the chain length of the alkanethiol compared with the calixarene spacer length can be used to fine tune the complexation ability of the calixarene, and there exist calixarene–alkanethiol mixed monolayer compositions in which the particles become unstable due to mismatching ligand spacer lengths, (iii) calixarenes with very short spacers bound to the gold surface can experience an enhancement in the delocalized π-electron density available for cation complexation, likely due to the proximity of the gold-bound sulphur to the calixarene cavity.