Poly(amidoamine) (PAMAM) dendrimers can be used as a polymeric template/stabilizer/ reservoir to prepare stable gold-dendrimer nanocomposites by reducing PAMAM-tetrachloroaurate polysalts using hydrazine. In the gold-dendrimer nanocomposite, the presence of elemental gold is indicated by its characteristic plasmon absorption peak at 529 nm in aqueous solution and is visualized by transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDXS). Electrostatic layer-by-layer assembly of the gold-dendrimer nanocomposite using poly(sodium 4-styrenesulfonate) (PSS) as the oppositely charged polyelectrolyte leading to nanoscale uniform multilayers of gold-dendrimer nanoclusters is reported. UV-vis absorption spectra from the consecutive multilayers indicated that each bilayer growth is regular, even though a 20 nm absorption bathochromic shift takes place in the film. TEM of PSS/gold-dendrimer nanocomposite film demonstrates that gold nanoparticles (5-20 nm) appear as aggregates within a gold-dendrimer nanocomposite monolayer, an observation borne out by crystalline gold electron diffraction patterns obtained from PSS/gold-dendrimer nanocomposite. Atomic force microscopy (AFM) of PSS/gold-dendrimer nanocomposite film indicates that the nanoclusters are arrayed with high uniformity at the nanometer scale.

Peroxidase-catalyzed template-guided polymerization of aniline has been carried out in aqueous micellar solutions of surfactants, such as sodium dodecylbenzenesulfonate, (SDBS), hexadecyltrimethylammonium bromide (CTAB), and polyoxyethylene(10) isooctylphenyl ether (Triton X-100). The properties of these enzymatically synthesized polyanilines strongly depend on the structure of the surfactants used in the formation of micelles as shown by the UV-vis-near-IR absorption spectroscopy. The micelles formed by strong acid surfactants such as SDBS are suitable templates for the enzymatic synthesis of conducting polyaniline. In solution, the micelles may serve as nanoreactors for the aniline monomer prior to the reaction and provide the necessary low-pH local environment for the growth of conducting polyaniline. The formation of low-pH local environment is further confirmed by the pH change of the bulk solution after the loading of aniline onto the micelles. FTIR, NMR, and UV-vis-near-IR absorption spectroscopy as well as cyclic voltammetry are used in the characterization of the synthesized polyaniline. The enzymatically synthesized SDBS micellar polyanilines are electrically active and soluble in organic solvents such as DMSO and DMF in emeraldine base form.

This work describes the development of thin film optical sensors for pH, metal ions (ferric and mercury), and 2,4-dinitrotoluene detection. To fabricate the pH sensor, a fluorescent molecule, 1-hydroxypyren-3,6,8-trisulfonate, was assembled with a polycation by an electrostatic layer-by-layer assembly technique. The fluorescent indicator molecule exhibits distinct and well-defined emission peaks for protonated and deprotonated forms. The relative peak positions and intensity of fluorescence of the protonated and deprotonated forms change in response to pH variations. For metal ion (ferric and mercury) and 2,4dinitrotoluene sensing, the indicator molecules were covalently incorporated into poly(acrylic acid) and subsequently assembled with a polycation employing electrostatic layer-by-layer assembly. The sensor is based on the fluorescence quenching of indicator molecules by electron transfer from indicator to electrondeficient analytes such as ferric ions, mercury, and 2,4-dinitrotoluene. Fluorescence intensities decreased with increasing concentration of analytes. Quenching behavior follows Stern-Volmer bimolecular quenching kinetics. Linear increase in absorbance, film thickness, and emission intensity was observed as a function of number of bilayers deposited in all these films.

We report here a detailed experimental study, establishing the surface-initiated mechanism for the photoinduced relief grating formation on azobenzene functionalized polymer films. The grating formation process is found to be strongly dependent on the nature of the surface (20−200 Å), while the birefringent grating appears to be a purely bulk process. Restriction of the surface of the azobenzene polymer film through the deposition of ultrathin top layers of transparent polyelectrolytes by electrostatic self-assembly dramatically decreases the relief grating formation. Thicker polyelectrolyte multilayer film (250 Å) on the surface resulted in the formation of purely birefringent grating, without any surface features.

Holographic surface relief gratings (SRGs) were fabricated on composite films assembled by electrostatic layer-by-layer (ELBL) deposition of a polyelectrolyte, poly(dimethyl diallylammonium chloride) (PDAC), and an azo dye, Congo Red (CR). Surface modulation and first-order diffraction efficiency of the SRG were found to increase with the thickness of the PDAC/CR films. Polarized absorption spectra indicated an oriented growth of CR on the PDAC film. Analysis of the film thickness, FTIR, and FT-Raman results confirmed that the electrostatic attraction between CR and PDAC, as well as the π−π interaction between CR chromophores resulting in the formation of J aggregates, lead to formation of PDAC/CR composite films. Photochemical changes of the PDAC/CR films after irradiation were investigated by UV−vis absorption, FTIR, and FT-Raman spectroscopy. The results indicate that in addition to trans ⇔ cis photoisomerization of CR in the composite film, an irreversible photochemical degradation of CR also simultaneously occurs. Recording SRG on PDAC/CR films by s- and p-polarized beams show different behavior compared to spin-coated films of polymers containing functionalized azo chromophores. Our results indicate that the volume collapse due to the photodegradation of CR in the polymeric matrix, as well as gradient force-induced migration due to trans ⇔ cis isomerization cycling of CR contribute to the formation of SRG on the composite films. This approach provides a methodology to fabricate SRGs for optical information storage applications by using the facile ELBL technique to assemble commercially available azo dyes and polyelectrolytes.