Nadejda Krasteva

Gold-nanoparticle/organic linker films: self-assembly, electronic and structural characterisation, composition and vapour sensitivity

Gold-nanoparticle/organic films were prepared via layer-by-layer self-assembly using dodecylamine-stabilised Au-nanoparticles and poly(propyleneimine) (PPI) dendrimers of generation one to five (G1-G5) or hexadecanedithiol (HDT) as linker compounds. TEM and FE-SEM images revealed that the bulk of the films consisted of nanoparticles with diameters of about 4 nm. XPS was used to study the chemical composition of the films. The C 1s and N 1s signals of an AuPPI-G4 film were interpreted qualitatively according to the dendrimer structure. The absence of the nitrogen signal in case of an AuHDT film indicated that the dodecylamine ligands were quantitatively exchanged during film assembly. About 76% of the sulfur atoms were bound to the nanoparticles. the remainder being present as free thiol (S H) groups. All films displayed linear current voltage characteristics and Arrhenius-type activation of charge transport. The conductivities of the AuPPI films decreased exponentially over approximately two orders of magnitude (6.8 x 10(-2) to 1.0 x 10(-3) ohms(-1) cm(-1)) with a five-fold increase of the dendrimer generation number. Dosing the films with solvent vapours caused their resistances to increase. Using different solvent vapours demonstrated that the sensitivity of this response was determined by the solubility properties of the linker compounds. Microgravimetric measurements showed that absorption of analyte was consistent with a Langmuir adsorption model. These measurements also revealed a linear correlation between the electrical response (deltaR/Rini) and the concentration of absorbed analyte. The absorption of d4-methanol from a saturated vapour atmosphere was studied by neutron reflectometry with an AuPPI-G4 film. This measurement indicated condensation of methanol on top of the film and a uniform distribution of the analyte across the film thickness.

Gold-nanoparticle/dithiol films as chemical sensors and first steps toward their integration on chip

Networked films comprised of Au-nanoparticles and organic dithiols were deposited onto silicon or glass substrates via repetitive self-assembly from solution. The substrates were equipped with interdigitated electrode structures for electrically addressing the films. Dodecylamine stabilized Au-nanoparticles with a core diameter of 4 (+/-0.8) nm were used for film assembly. The metal cores were networked through 1,n-alkylenedithiols with chain lengths varying from 6 to 20 methylene units. With increasing number of methylene units, the conductivity of the films decreased by several orders of magnitude without following a monoexponential decay. When dosing the films with organic vapors (toluene, 1-propanol, 4-methyl-2-pentanone) their resistance increased reversibly. The amplitudes of this response increased strongly with increasing length of the linker molecules. In contrast, the sensitivity to water vapor was marginal for all alkylenedithiol-linked films. However, insertion of polar amide groups into the backbone of the linker decreased the sensitivity to toluene and significantly enhanced the sensitivity to water. The deposition of sensor films on chip from organic solvents could be directed by using a patterned CaO mask. After film deposition, the lift-off of nanoparticles from protected parts of the substrate was achieved by dissolving the mask in aqueous solution at ambient temperature.