This manuscript describes the stepwise, ligand-directed assembly, characterization, and prospective applications of three-dimensional Au and Ag nanoparticle, multlilayered films. Films were prepared by successive treatments of a Au nanoparticle monolayer with a bifunctional cross-linker and colloidal Au or Ag solutions. Changes in film electrical and optical properties are reported for a series of bifunctional cross-linkers of varying molecular lengths. Interestingly, these films exhibit Beer's law behavior despite the presence of strong interparticle optical coupling. Multilayer films with greater than six exposures to 2-mercaptoethylamine and Au colloid were highly conductive and resembled bulk Au in appearance. In contrast, films of similar particle coverage generated using a longer cross-linker (1,6-hexanedithiol) exhibited higher transmission in the near-infrared region and exhibited a reduced conductivity. Measurement of the multilayer morphology with atomic force microscopy , electrostatic force microscopy, and field emission scanning electron microscopy revealed a porous, discontinuous morphology composed of large, continuous regions of aggregated nanoparticles. This, in turn, results in a surface roughness contribution to surface plasmon scattering and surface-enhanced Raman scattering observed for Au, Au/Ag, and Ag colloid multilayers. Particulate multilayer films made using horseradish peroxidase as a cross-linker remained enzymatically active, even beneath three layers of colloidal Au. Multilayers could also be prepared on surfaces patterned by microcontact printing. These data show how Au colloid multilayers grown in solution are a viable alternative to evaporated metal films for a number of applications.

Mixed monolayers of (ferrocenylcarboxy)-alkanethiol/n-alkanethiol have been investigated electrochemically in 2:1 (v:v) chloroethane:butyronitrile solvent in the temperature range of 120K to 150K. Cyclic voltammetry of these monolayers shows large oxidation-reduction peak potential separations indicative of electron transfer rate control.The voltammetric waveshapes are also broadened; this and curved log[i] vs. time transients observed in potential step experiments are interpreted as a dispersion in the reaction rates of the ferrocene sites. This paper considers origins and three models for such kinetic dispersion: (i) Using simulations, the observed kinetic dispersion effects can be successfully represented by a Gaussian distribution among the formal potentials E 0 ' of the surface redox sites. While only an apparent kinetic dispersion (having a thermodynamic origin), we show by simulations that its presence affects potential step log[k APP ,] vs. tj plots, depressing the apparent reorganizational barrier energies (X) and elevating the apparent rate constants (k°), consistent with previous experimental observations. Similarly, cyclic voltammetric simulations with a Gaussian E 0 ' distribution give excellent fits to experimental 2 voltammograms with mid-point average rates (that with voltammograms can be simulated to fit both the experimental waveshape and AE PEAK ) that are roughly 6-fold smaller than the average rate (determined from a fit to the experimental AE PEAK assuming a homogeneous population). The temperature and chain length dependence of simulations are also consistent with experimental observations and indicate that the dispersion has little effect on accurate determination of X (from an activation analysis) or ß (from a plot of log(k°) vs. chain length), (ii) A Gaussian distribution of reorganizational energies, which is a real kinetic dispersion, has consequences on the appearance and the analysis of data quantitatively equivalent to those of a distribution of formal potentials, (iii) A kinetic dispersion model based on a Gaussian distribution of tunneling distances (or equivalently electronic coupling parameter) from the electrode surface is also evaluated. This model predicts curved potential step log[i] vs. time plots, and in analysis of log[k A p Pl) ] vs. rj plots, undistorted results for X but alteration of the apparent k°.

The preparation, characterization, and electrochemical properties of two types of conductive Au films are described. Both films are made entirely by wet chemical procedures. In the first, successive treatment of a Au colloid monolayer/glass substrate with (i) 2-mercaptoethylamine and (ii) colloidal Au in solution leads to systematic buildup of a Au colloid multilayer. After seven to eight layers of Au nanoparticles have been deposited, the multilayers become conductive. Cyclic voltammograms of several different redox couples show that the peak-to-peak separation decreases as the number of layers increases. In the second type of film, a solution of hydroxylamine and Au3+ are used to selectively enlarge the size of a preimmobilized colloidal Au monolayer. Once the particles coalesce, the resulting film can be used to generate voltammograms with narrow peak separations. The ability to form conductive Au films using entirely wet-chemical steps may be valuable for fabrication of electrodes with complex shapes.