Single-component monolayers of dendrimers and
two-component monolayers consisting of dendrimers
and n-alkanethiols immobilized on Au substrates are
described. Single-component monolayers are prepared
by exposing an Au substrate to ethanolic solutions of amine- or
hydroxy-terminated polyamidoamine (PAMAM)
dendrimers. The resulting monolayers are highly stable and nearly
close-packed for dendrimer generations
ranging from 4 to 8 (G4−G8). Electrochemical ac-impedance
measurements indicate that the dendrimer surface
is very porous toward the electroactive redox couple
Fe(CN)6
3-/4-.
Ferrocene-terminated dendrimer monolayers
have also been investigated. Exposure of higher-generation
dendrimer monolayers to ethanolic solutions of
hexadecanethiol (C16SH) results in a dramatic compression of the
dendrimers, and causes them to reorient on
the surface from an oblate to prolate configuration. The
dendrimers originally present on the surface do not
desorb as a consequence of this configurational change. Comparison
of the extent of adsorption of C16SH in
different media (vapor-phase N2, hexane, and ethanol) shows
that solvation of the dendrimers is the primary
driving force for the structural change. Finally, the reactivity
and stability of the dendrimer monolayers is
investigated by on-surface functionalization of the dendrimer monolayer
with 4-(trifluoromethyl)benzoyl chloride.
The physical and chemical properties of the single- and
two-component monolayers are evaluated by using
reflection infrared spectroscopy, ellipsometry, contact-angle
measurements, ac-impedance spectroscopy, cyclic
voltammetry, and surface acoustic wave (SAW)-based analyte-dosing
experiments.
Au colloids in the 2-3-nm size regime were prepared by in situ reduction of HAuCl(4) in the presence of poly(amidoamine) dendrimers. The dendrimers encapsulate the colloids, imparting stability to the aqueous colloidal solutions. The nanocomposite materials can be isolated by precipitation. The dendrimer generation used in the synthesis controls the size of the resultant colloids: lower-generation dendrimers give rise to larger colloids. The materials were characterized by infrared and UV-vis spectroscopy and transmission electron microscopy.
Passivated gold nanoparticles were synthesized through a microwave-assisted process in a two-phase system, in the presence of 1-dodecanethiol. An average particle size of 1.8 nm of the gold nanoparticles obtained and 0.35 S.D. was determined through HRTEM and STEM analysis. It was observed that these nanoparticles spontaneously self-assemble into self-supported superstructures of 1 μm in diameter avg and 400 nm thickness, yielding an off-white powder which can be handled as a simple powder. XRD analysis indicates that n-alkanethiol molecules used as a passivating compound, besides protecting against crystal growth, interact to form cubic ordered arrays between the nanoparticles. This interaction leads to the superstructure formation, with an average distance between nanoparticles in the array, of 3.56 nm. Theoretical calculations and molecular dynamics simulations were performed to analyze the resulting structure.
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