Thiol adsorption on metal oxide nanoparticles.

The adsorption of 2-naphthalenethiol (2-NPT) and methanethiol (MT) on 13 different metal oxide nanoparticles, of approximately 30 nm average primary particle size, has been investigated. In the case of 2-NPT, which is fluorescent, a screening method to assess adsorption was developed that consists of mixing the nanoparticles with a dilute ethanolic solution of 2-NPT and performing several cycles of centrifuging and rinsing with ethanol. Fluorescence measurements on the re-dispersed particle suspensions were then used to diagnose whether or not adsorption had occurred. Complementary experiments were performed by mounting powder samples of each of the metal oxide nanoparticles onto sample stubs and performing X-ray photoelectron spectroscopy (XPS) before and after in situ dosing with MT. In both cases, adsorption was observed only on ZnO, TiO2, and In2O3. Adsorption did not occur on Al2O3, CeO2, Fe2O3, Gd2O3, Ho2O3, NiO, SiOx, WO3, Y2O3, and ZrO2. Density functional theory (DFT) calculations were performed using small metal oxide clusters, assuming that dissociative adsorption occurs by replacement of a hydroxyl group attached to a metal site and the formation of water. The theoretical and experimental results generally agree, suggesting that this is indeed the adsorption mechanism for most of the nanoparticles. The agreement also suggests that the size and geometry of the nanoclusters play a minor role and that the relative strengths of the metal-sulfur and metal-hydroxyl bonds dictate thiol adsorption. This work has important implications related to the functionalization of metal oxide nanoparticles and surfaces.

Thiol Adsorption on and Reduction of Copper Oxide Particles and Surfaces.

The adsorption of 1-dodecanethiol at room temperature and at 75 °C on submicron cuprous and cupric oxide particles suspended in ethanol has been investigated by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy. Thiol adsorption occurs in all cases via Cu-S bond formation, with partial dissolution of CuO at 75 °C and formation of a copper-thiolate complex replacement layer. Regardless of temperature, the surface of the CuO particles is essentially completely reduced to either Cu2O or metallic copper, as evidenced by loss of the characteristic Cu(2+) XPS features of dried powder samples. Companion ultrahigh-vacuum studies have been performed by dosing clean, oxygen-dosed, and ozone-treated single crystal Cu(111) with methanethiol (MT) gas at room temperature. In the latter case, the surface corresponds to CuO/Cu(111). XPS confirms MT adsorption in all cases, with an S 2p peak binding energy of 162.9 ± 0.1 eV, consistent with methanethiolate adsorption. Heating of MT-covered Cu(111) and oxygen-dosed Cu(111) leads to decomposition/desorption of the MT by 100 °C and formation of copper sulfide with an S 2p binding energy of 161.8 eV. Dosing CuO/Cu(111) with 50-200 L of MT leads to only partial reduction/removal of the CuO surface layers prior to methanethiolate adsorption. This is confirmed by ultraviolet photoelectron spectroscopy (UPS), which measures the occupied states near the Fermi level. For both the colloidal CuO and single crystal CuO/Cu(111) studies, the reduction of the Cu(2+) surface is believed to occur by formation and desorption of the corresponding dithiol prior to thiolate adsorption.

Photocatalytic Activity and Fluorescence of Gold/Zinc Oxide Nanoparticles Formed by Dithiol Linking.

Monolayer-protected gold nanoparticles (AuNPs) with average diameters of 2-4 nm have been covalently attached to zinc oxide nanorods using dithiol ligands. Electron microscopy and Raman spectroscopy show that ozone treatment or annealing at 300 or 450 °C increases the average diameter of the AuNPs to 6, 8, and 14 (±1) nm, respectively, and decomposes the organic layers to various degrees. These treatments locate the AuNPs closer to the nanorods. Heating and subsequent ozone exposure changes the color of the as-prepared nanocomposite powder from blue to purple due to oxidation of the outer layer of the AuNPs, and heating to 300 °C changes it to pink due to oxygen desorption. ZnO nanorods have a bimodal photoluminescence spectrum that consists of an ultraviolet excitonic peak and a visible, surface defect-related peak. Ozone treatment and annealing of the nanocomposite decreases the intensities of both peaks due to quenching by the AuNPs, but the visible peak is affected less. The photocatalytic efficiency of the nanocomposites toward oxidative degradation of rhodamine B has been measured and follows the order 300 °C > 450 °C > ozone treated ≈ as-prepared ≈ bare ZnO. The greater efficiency of the annealed samples likely arises from decreased electron-hole pair recombination rates.

Directed adhesion and patterning by ultraviolet irradiation of TiO2(110).

Forces of adhesion between a hydroxylated silicon oxide tip and a TiO(2)(110) surface, before and after irradiation of the surface with 254 nm light, were measured using atomic force microscopy. The work of adhesion before and after irradiation was 32 and 166 mJ/m(2), respectively, but a difference was observed only if ultraviolet light exposure was used in the presence of oxygen. The change in adhesion correlated strongly with decreasing water contact angle, which changed from ca. 70 to 0° because of irradiation. The contrast in adhesion between irradiated and nonirradiated regions of the surface makes possible a simple method of patterning molecules with micrometer, and potentially nanoscale, resolution. As an example, fluorescein was selectively adsorbed onto hydrophilic regions of the surface by spin coating an ethanolic fluorescein solution onto TiO(2)(110) that had been irradiated through a photomask.

Template-directed patterning of polymers and biomaterials.

A novel method of patterning surfaces with synthetic or biological polymers is demonstrated. It consists of using microcontact printing to pattern a gold surface with an adsorbate that imparts hydrophilicity; the remainder of the surface is covered with one that imparts hydrophobicity. 16-Mercaptohexadecanoic acid (MHDA) and 1H,1H,2H,2H-perfluorodecanethiol, respectively, have been used as the hydrophilic and hydrophobic adsorbates. This functionalized gold surface then serves as a template for patterning hydrophilic polymers and biomaterials, which are either spin-coated or drop-cast onto the surface. Using this methodology, it is shown by atomic force microscopy, scanning electron microscopy (SEM), and fluorescence microscopy that micron-scale patterns of a poly(ethylene)-block-poly(ethylene oxide) copolymer, poly-L-tryptophan, and bovine collagen can be fabricated, with these mimicking the MHDA patterns. For the block copolymer, it is found by atomic force microscopy that the heights of the polymer patterns decrease as their widths decrease. This is believed to be due to the inherent instability of tall, narrow polymer structures and the tendency of the polymer to minimize its exposed surface area. For poly-L-tryptophan, two different molecular weights of this polyamino acid have been studied, and different morphologies within the patterned regions are observed. While oligomeric poly-L-tryptophan (1,000-5,000 g/mol) gives smooth MHDA-covered patterns, the higher molecular weight (15,000-50,000 g/mol) yields fibrous ones.

A fiber optic, ultraviolet light-emitting diode-based, two wavelength fluorometer for monitoring reactive adsorption.

Construction and use of an ultraviolet light-emitting diode-based fluorometer for measuring photoluminescence (PL) from powder samples with a fiber optic probe is described. Fluorescence at two wavelengths is detected by miniature photomultiplier tubes, each equipped with a different band pass filter, whose outputs are analyzed by a microprocessor. Photoluminescent metal oxides and hydroxides, and other semiconducting nanoparticles, often undergo changes in their emission spectra upon exposure to reactive gases, and the ratio of the PL intensities at two wavelengths is diagnostic of adsorption. Use of this instrument for reactive gas sensing and gas filtration applications is illustrated by measuring changes in the PL ratio for zirconium hydroxide and zinc oxide particles upon exposure to air containing low concentrations of sulfur dioxide.

A compact Raman converter for UV-VIS spectrometers.

A small form factor, easily constructed converter that adapts fiber coupled UV/VIS CCD detector-based spectrometers into a right angle scattering Raman spectrometer is described. Its design philosophy and design are discussed. An example measurement, the depolarization ratio of carbon tetrachloride, a classic Raman test compound, is presented. The unique instrument features a blue-violet (405 nm wavelength) diode laser that takes advantage of the inverse fourth power wavelength dependence of Raman scattering. The converter also features Glan-Thompson polarizing prisms that enable measurement of depolarization ratios. The spectrometer is also capable of measuring a standard Raman spectrum. A fiber optic link offers flexibility when adapting the converter to any spectrometer system that accepts a fiber optic input. The performance of the instrument is critically discussed in the context of an example measurement.

Photometer for monitoring the thickness of inkjet printed films for organic electronic and sensor applications.

Inkjet printed organic thin films are being used for a variety of electronic and sensor applications with advantages that include ease of fabrication and reproducibility. Construction and use of a low-cost photometer based on a light-emitting diode (LED) light source and a photodiode detector are described. The photometer attaches to the exit of the printer with the transparent substrate onto which the film is printed passing between the LED and photodiode. By measuring the output voltage of the detector, the transmittance and absorbance of the inkjet printed film can be calculated in real-time. Since absorbance is linearly proportional to thickness in the Beer-Lambert regime, the thickness of the film may be monitored and controlled by varying the number of passes through the printer. Use of the photometer is demonstrated for inkjet printed films of monolayer-protected colloidal gold nanoparticles that function as chemical vapor sensors. The photometer may find applications in both research and quality control related to the manufacture of organic electronic devices and sensors and enables "feedback-controlled" inkjet printing.

Anomalous vapor sensor response of a fluorinated alkylthiol-protected gold nanoparticle film.

Monolayer-protected gold nanoparticle films generally swell and increase their electrical resistance when exposed to organic vapors. Films of gold nanoparticles protected by 1H,1H,2H,2H-perfluorodecanethiol (PFDT) exhibit an anomalous response in which the resistance decreases for all vapors investigated. Electron microscopy illustrates that the PFDT-functionalized gold nanoparticles are hexagonally ordered with an interparticle separation of 3 nm. Quartz crystal microbalance measurements confirm substantial mass uptake, but the relatively large interparticle separation and insulating properties of the gold particles lead to a porous film whose electrical resistance is strongly influenced by changes in the relative permittivity and reversible, vapor-induced changes in film morphology.

Peptide surface modification of P(HEMA-co-MMA)-b-PIB-b-P(HEMA-co-MMA) block copolymers.

Peptide surface modification of poly[(methyl methacrylate-co-hydroxyethyl methacrylate)-b-isobutylene-b-(methyl methacrylate-co-hydroxyethyl methacrylate)] P(MMA-co-HEMA)-b-PIB-b-P(MMA-co-HEMA) triblock copolymers with different HEMA/MMA ratios has been accomplished using an efficient synthetic procedure. The triblock copolymers were reacted with 4-fluorobenzenesulfonyl chloride (fosyl chloride) in pyridine to obtain the activated polymers [poly{(methyl methacrylate-co-fosyloxyethyl methacrylate)-b-isobutylene-b-(methyl methacrylate-co-fosyloxyethyl methacrylate)}] P(MMA-co-FEMA)-b-PIB-b-P(MMA-co-FEMA), with an activating efficiency of 80-90%. The resulting polymers were soluble in chloroform, and their solutions were used to coat thin uniform films with a predetermined thickness on smooth steel surfaces. The presence of reactive activating groups on the film surface was confirmed by X-ray photoelectron spectroscopy (XPS), dye labeling, and confocal laser scanning microscopic studies. Activation of the triblock copolymer films was also achieved under heterogeneous conditions in polar (acetonitrile) and nonpolar (hexanes) media. The extent of activation was controlled by varying the dipping time and polarity of the medium. Peptide attachment was accomplished by immersing the coated steel strips into aqueous buffer solution of Gly-Gly or GYIGSR. XPS and solubility studies revealed successful attachment of peptides to the polymer surface. Virtually all remaining activating groups were successfully replaced in the subsequent step by a treatment with Tris(hydroxymethyl)amino methane in a buffered methanol/water mixture.

Encapsulation of zinc oxide nanorods and nanoparticles.

A simple method for encapsulating zinc oxide nanoparticles within an organic matrix is described that consists of dispersing them in an ethanolic solution, adding an organothiol, and stirring while heating. Electron microscopy, photoemission, Raman spectroscopy, and thermal gravimetric analyses demonstrate that partial dissolution of the oxide occurs, accompanied by encapsulation within a matrix consisting of a 1:2 zinc/thiol complex. Using this methodology, it is possible to surround ZnO within diverse matrices, including fluorescent ones. The process is demonstrated for 1-dodecanethiol (DDT) and fluorescent 2-naphthalenethiol (NPT). For DDT, ZnO nanorods become surrounded by a layer of the zinc-thiol complex that is greater than 100 A thick. In the case of NPT, significantly greater dissolution of the ZnO occurs, with the encapsulated rods taking on a spherical geometry, as evidenced by electron microscopy.

Novel photoluminescence properties of surface-modified nanocrystalline zinc oxide: toward a reactive scaffold.

Nanocrystalline zinc oxide has been functionalized with 11-triethoxysilylundecal to introduce chemical reactivity for subsequent molecular attachment while preserving its unique photoluminescence (PL) properties. Silane bonding is confirmed using FTIR and photoelectron spectroscopy, with a total Si concentration of 3 to 4 atomic %. PL measurements demonstrate a 2-fold enhancement of its UV peak and maintenance of its visible peak compared to control samples. Enhancement of the UV peak is likely due to a decrease in the surface-dependent nonradiative recombination process. These results lay the framework for sensor fabrication, with the aldehyde groups available to bind analytes that would alter the PL spectrum.

Ultraviolet photoelectron spectroscopy study of the thermochromic phase transition in urethane-substituted polydiacetylenes.

Threshold solid-state ionization energies determined from ultraviolet photoelectron spectra are reported for the thermochromic polydiacetylenes (PDAs) from the bis-ethyl- and bis-n-propyl urethanes of 5,7-dodecadiyn-1,12-diol (ETCD and PUDO, respectively) and the nonthermochromic 1,6-bis-p-toluenesulfonate of 2,4-hexadiyne-1,6-diol (PTS) at temperatures above and below the thermochromic phase transition. PDA-PTS has an ionization energy of 5.66 eV which does not change significantly as the temperature is raised above 140 degrees C. At 25 degrees C, PDA-ETCD and PDA-PUDO have threshold ionization energies of 5.65 and 5.51 eV, respectively. The ionization energies of these PDAs increase by approximately 0.34 eV as temperature is raised above 140 degrees C and returns to the lower values as temperature is reduced to 25 degrees C. The magnitude of the increase in ionization energy on heating to temperatures above the thermochromic transition is very close to the shift in energy of the electronic spectrum over the same temperature range. These observations suggest that the structural changes that take place in the course of the thermochromic transition are primarily associated with the valence band and are consistent with partial relief of mechanical strains.

Template-directed adsorption of block copolymers on alkanethiol-patterned gold surfaces.

Functionalized alkanethiols have been self-assembled on gold to modify the wetting properties of the surface and promote or hinder the adsorption of block copolymers containing both hydrophobic and hydrophilic blocks. X-ray photoelectron spectroscopy (XPS) studies of spin-coated polyethylene-block-poly(ethylene oxide) (PE-b-PEO) copolymers on 16-mercaptohexadecanoic acid (MHDA)-, octadecanethiol (ODT)-, and 1H,1H,2H,2H-perfluorodecanethiol (PFDT)-covered surfaces have been performed. In the case of an 80 wt % PEO block copolymer, spin-coating on a gold surface precovered with MHDA results in a polymer film thick enough to completely attenuate Au 4f photoelectrons; spin-coating on the more hydrophobic ODT and PFDT monolayers leads to significantly thinner polymer films and incomplete attenuation of the gold photoelectrons. The opposite results are observed when a 20 wt % PEO block copolymer is used. Angle-resolved XPS studies of the 80 wt % PEO block copolymer spin-coated onto an MHDA-covered surface indicate that the PE blocks of the polymer segregate to the near-surface region, oriented away from the hydrophilic carboxylic acid tails of the monolayers; the surface concentration of PE is further enhanced by annealing at 90 degrees C. Microcontact printing and dip-pen nanolithography have been used to pattern gold surfaces with MHDA, and the surfaces have been backfilled with ODT or PFDT, such that the unpatterned regions of the surface are covered with hydrophobic monolayers. In the case of backfilling with PFDT, spin-coating the 80 wt % PEO copolymer onto these patterned surfaces and subsequent annealing results in the block copolymer preferentially adsorbing on the MHDA-covered regions and forming well-defined patterns that mimic the MHDA pattern, as determined by scanning electron microscopy and atomic force microscopy. Significantly worse patterning, characterized by micron-sized polymer droplets, results when the surface is backfilled with ODT instead of PFDT. Using PFDT and MHDA, polymer features having widths as small as 500 nm have been formed. These studies demonstrate a novel method to pattern block copolymers with nanoscale resolution.

Conjugated polymers in an arene sandwich.

A series of poly(p-arylene butadiynylene)s containing zero, one, and two co-facial pi-pi interactions per repeat unit were synthesized and characterized. A surprisingly selective and high-yielding Diels-Alder cycloaddition of anthracene and nonsymmetric, sterically hindered anhydrides proved essential to generating the cofacial arene-containing monomers. Single-crystal X-ray structures display nearly parallel cofacial arenes that are within the van der Waals contact distances. The precursor molecules with cofacial arenes undergo reversible one- and two-electron oxidations to the radical cation and dication in CH2Cl2. The anhydrides were converted to N-alkyl imides to increase the solubility. High-molecular weight poly(p-arylene butadiynylene)s were prepared via Pd/Cu(I)/benzoquinone oxidative coupling of the diacetylene monomers. The resulting polymers are highly emissive in solution and thin films. The ionization potentials were measured using ultraviolet photoelectron spectroscopy with thin films. Last, fluorescence measurements of polymer thin films during continuous irradiation indicate that the most hindered polymer is more resistant to photobleaching.

High ionization potential conjugated polymers.

We report the synthesis of a series of poly(p-phenylene ethynylene)s (PPEs) with high ionization potentials and associated high excited-state electron affinities. Their photophysical properties were investigated using steady-state and time-resolved fluorescence techniques. The ionization potentials of the polymer thin films were determined using ultraviolet photoelectron spectroscopy (UPS), and those with the highest ionization potentials displayed high sensitivity for the detection of electron-donating aromatic compounds. The effects of sterics, chemical structure, and electronic properties on the polymers' sensory responses were investigated by fluorescence quenching experiments in both solution and solid thin films. In addition, we report that in some cases the excited-state charge-transfer complexes (exciplexes) of the PPEs with analytes were observed. These latter effects provide promising opportunities for the formation of sensitive and selective chemical sensors.

Peroxidase-catalyzed in situ polymerization of surface orientated caffeic acid.

Nanoscale surface patterning and polymerization of caffeic acid on 4-aminothiophenol-functionalized gold surfaces has been demonstrated with dip pen nanolithography (DPN). The diphenolic moiety of caffeic acid can be polymerized by biocatalysis with laccase or horseradish peroxidase. In the present study, the DPN patterned features were polymerized in situ through the use of the peroxidase. Using samples prepared by DPN, microcontact printing, and adsorption on macroscopic substrates, the products were characterized by electrostatic force microscopy (EFM), MALDI-TOF, X-ray photoelectron spectroscopy (XPS), UV-vis, and FT-IR. The in situ surface polymerization resulted in the formation of a quinone structure, while the phenyl ester formed in bulk polymerization reactions was not detected. A different coupling site was observed when comparing the polymers obtained from solution (bulk) vs the surface DPN reactions. The structural differences were attributed to surface-induced pre-organization and orientation of the monomers prior to the enzymatic polymerization step. The results of this study expand the application of DPN technology to surface modification and surface chemistry reactions wherein stereo-regularity and regioselectivity can be exploited.