Alkyl and Aromatic Amines as Digestive Ripening/Size Focusing Agents for Gold Nanoparticles.

Both long chain alkyl thiols and alkyl amines behave as size focusing agents for gold nanoparticles, a process that is under thermodynamic control. However, amines do not oxidize surface gold atoms while thiols do oxidize surface gold to gold(I) with evolution of hydrogen gas. Therefore, alkyl amines participate in digestive ripening by a different mechanism. The efficiency of alkyl amines for this process is described and compared, and ultimate gold particle size differences are discussed. Reported herein is a detailed investigation of alkyl chain lengths for alkyl amines, aromatic amines (aniline), and unusually reactive amines (2-phenylethyl amine). Also, two methods of preparation of the crude gold nanoparticles were employed: gold ion reduction/inverse micelle vs. metal vaporization (Solvated Metal Atom Dispersion-SMAD).

Hydrogen generation from water/methanol under visible light using aerogel prepared strontium titanate (SrTiO3) nanomaterials doped with ruthenium and rhodium metals.

Nanostructured strontium titanate visible-light-driven photocatalysts containing rhodium and ruthenium were synthesized by a modified aerogel synthesis using ruthenium chloride and rhodium nitrate as dopant precursors, and titanium isopropoxide and strontium metal as the metal sources. The well-defined crystalline SrTiO(3) structure was confirmed by means of x-ray diffraction. After calcination at 500 °C, diffuse reflectance spectroscopy shows an increase in light absorption at 370 nm due to the presence of Rh(3 + ); however an increase of the calcination temperature to 600 °C led to a decrease in intensity, probably due to a loss of surface area. An increase in the rhodium doping level also led to an increase in absorption at 370 nm; however, the higher amounts of dopant lowered the photocatalytic activity. The modified aerogel synthesis allows greatly enhanced H(2) production performance from an aqueous methanol solution under visible light irradiation compared with lower surface area conventional materials. We believe that this enhanced activity is due to the higher surface areas while high quality nanocrystalline materials are still obtained. Furthermore, the surface properties of these nanocrystalline aerogel materials are different, as exhibited by the higher activities in alkaline solutions, while conventional materials (obtained via high temperature solid-state synthesis methods) only exhibit reasonable hydrogen production in acidic solutions. Moreover, an aerogel synthesis approach gives the possibility of thin-film formation and ease of incorporation into practical solar devices.

Loss of hydrogen upon exposure of thiol to gold clusters at low temperature.

Gold-acetone-dodecanethiol and gold-acetone-phenylethanethiol colloids were prepared by the solvated metal atom dispersion method. Hydrogen evolution occurred at fairly low temperature, when the melting acetone-gold solvate encountered the thiol molecules, due to S-H bond scission. The gas inside the reactor was analyzed by gas chromatography, and the moles of H(2) was determined by calibration curves obtained from a series of known concentration samples. The gold-to-thiolate ratio was calculated using thermal gravimetric analysis. The average particle diameter was also calculated using the gold-to-thiolate ratio.

Direct synthesis of aqueous quantum dots through 4,4'-bipyridine-based twin ligand strategy.

We report a new class of derivatized 4,4'-bipyridinium ligands for use in synthesizing highly fluorescent, extremely stable, water-soluble CdSe and CdTe quantum dots (QDs) for bioconjugation. We employed an evaporation-condensation technique, also known as solvated metal atom dispersion (SMAD), followed by a digestive ripening procedure. This method has been used to synthesize both metal nanoparticles and semiconductors in the gram scale with several stabilizing ligands in various solvents. The SMAD technique comprised evaporation condensation and stabilization of CdSe or CdTe in tetrahydrofuran. The as-prepared product was then digestively ripened in both water and dimethyl formamide, leading to narrowing of the particle size distributions. The ligands were synthesized by nucleophilic substitution (S(N)2) reactions using 4,4'-bipyridine as a nucleophile. Confocal microscopy images revealed the orange color of the nanocrystalline QDs with diameters of ~5 nm. The size has been confirmed by using transmission electron microscopy. As a part of our strategy, 85% of the 4,4'-bipyridinium salt was synthesized as propionic acid derivative and used to both stabilize the QDs in water and label basic amino acids and different biomarkers utilizing the carboxylic acid functional group. Fifteen percent of the 4,4'-bipyridinium salt was synthesized as N-propyl maleimide and used as a second ligand to label any protein containing the amino acid cysteine by means of a 1,4-Michael addition.

Solubility of gold nanoparticles as a function of ligand shell and alkane solvent.

The solubility of ca. 5.0 nm gold nanoparticles was studied systematically as a function of ligand shell and solvent. The ligands were octane-, decane-, dodecane- and hexadecanethiols; the solvents were the n-alkanes from hexane to hexadecane and toluene. Supernatant concentrations in equilibrium with precipitated superclusters of nanoparticles were measured at room temperature (23 °C) with UV-Vis spectrophotometry. The solubility of nanoparticles ligated with decane- and dodecanethiol was greatest in n-decane and n-dodecane, respectively. In contrast, the solubility of nanoparticles ligated with octane- and hexadecanethiol showed decreasing solubility with increasing solvent chain length. In addition the solubility of the octanethiol ligated system showed a nonmonotonic solvent carbon number functionality with even numbered solvents being better solvents than neighboring odd numbered solvents.

Transformation of indium nanoparticles to ß-indium sulfide: digestive ripening and visible light-induced photocatalytic properties.

We report the transformation of polydispersed dodecanethiol stabilized indium nanoparticles, obtained from bulk indium shot by evaporation/condensation solvated metal atom dispersion (SMAD) technique, into highly monodispersed partially alkyl thiolate-capped ß-indiumsulfide (In(2)S(3)) by a postpreparative digestive ripening in high boiling point t-butyltoluene (190 °C) solvent. Upon digestive ripening, the as-prepared polydispersed black indium nanoparticles showed a characteristic color transition from black to cream, pale yellow, yellow, and finally to brown, indicating the transformation of the indium metal nanoparticles into intermediates composed of indium thiolates, sulfides, and polysulfides, and finally into the product In(2)S(3) nanoparticles whose surfaces are partially capped with thiolates. The transformed product (In(2)S(3)) was characterized with UV-vis, XRD, EDX, SEM, XPS, and TEM. From XRD and TEM measurements, the average size of the transformed In(2)S(3) nanoparticles is 5 nm. The optical absorbance of the as-prepared sample showed absorption peaks around 538 and 613 nm; upon digestive ripening these two peaks disappeared and stabilized at 375 nm, providing evidence of strong quantum confinement of excitons. The visible light-induced photocatalytic activity test with the In(2)S(3) nanoparticles showed that 95% of Rhodamine B (RhB) dye degraded after 100 min of irradiation with visible light.

Size Focusing of Nanoparticles by Thermodynamic Control through Ligand Interactions. Molecular Clusters Compared with Nanoparticles of Metals.

Ligand-capped metal entities come in two sizes, (1) molecular clusters of 10-200 metal atoms and (2) nanoparticles of 2000-10000 metal atoms. In numerous cases, certain "magic sizes" have been found to be most accessible and stable, clusters of 25, 38, 55, and 102 atoms and nanoparticles of 3500-5000 atoms or 4-5 nm. The most familiar and studied system is that of gold (metal) and thiol (ligand). Herein, the methods of synthesis of these gold clusters versus gold nanoparticles are carefully compared. In the cluster case, an important intermediate is the (Au(+)SR(-))n polymer, which is not the case in the synthesis of nanoparticles either from metal (vapor) atoms or metal ions. Also, it is shown that thiol can act as both a reductant (Au(3+) → Au(+)) and as an oxidant (Au(0) → Au(+)). The thermodynamic forces responsible for the favored formation of certain size clusters and nanoparticles are discussed.

Synthesis of indium nanoparticles: digestive ripening under mild conditions.

Here we report the synthesis of monodispersed indium nanoparticles by evaporation/condensation of indium shot using the solvated metal atom dispersion (SMAD) technique, followed by digestive ripening in low boiling point (BP 38 °C) methylene chloride and in a high boiling point (BP 110 °C) toluene solvent. The as-prepared SMAD indium nanoparticles are polydispersed with particle size ranging from 25 to 50 nm, but upon digestive ripening (heating of colloidal material at the boiling point of solvent in presence of excess surface active ligands) in methylene chloride, a remarkable reduction of particle size was achieved. In higher boiling solvent (toluene), where the indium nanoparticles at reflux temperature are probably melted, it does not allow the best result, and less monodispersity is achieved. We employed different surface active ligands (amine, phosphine, and mixed ligands) to passivate these indium nanoparticles. The temporal evolution of the surface plasmon of indium nanoparticles was monitored by in situ UV-vis spectroscopy, and particles were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The merits of this synthesis procedure are the use of bulk indium as starting material, tuning the particle size in low boiling point solvent, particle size adjustment with the choice of ligand, and a possible scale up.

Virucidal properties of metal oxide nanoparticles and their halogen adducts.

Selected metal oxide nanoparticles are capable of strongly adsorbing large amounts of halogens (Cl(2), Br, I(2)) and mixed halogens. These solid adducts are relatively stable thermally, and they can be stored for long periods. However, in the open environment, they are potent biocides. Herein are described studies with a number of bacteriophage MS2, phiX174, and PRD-1 (virus examples). PRD-1 is generally more resistant to chemical disinfection, but in this paper it is shown to be very susceptible to selected interhalogen and iodine adducts of CeO(2), Al(2)O(3), and TiO(2) nanoparticles. Overall, the halogen adducts of TiO(2) and Al(2)O(3) were most effective. The mechanism of disinfection by these nanoparticles is not completely clear, but could include abrasive properties, as well as oxidative powers. A hypothesis that nanoparticles damage virons or stick to them and prevent binding to the host cell is a consideration that needs to be explored. Herein are reported comparative biocidal activities of a series of adducts and electron microscope images of before and after treatment.

Biocidal properties of metal oxide nanoparticles and their halogen adducts.

Nanosized metal oxide halogen adducts possess high surface reactivities due to their unique surface morphologies. These adducts have been used as reactive materials against vegetative cells, such as Escherichia coli as well as bacterial endospores, including Bacillus subtilis and Bacillus anthracis (Delta Sterne strain). Here we report high biocidal activities against gram-positive bacteria, gram-negative bacteria, and endospores. The procedure consists of a membrane method. Transmission electron micrographs are used to compare nanoparticle-treated and untreated cells and spores. It is proposed that the abrasive character of the particles, the oxidative power of the halogens/interhalogens, and the electrostatic attraction between the metal oxides and the biological material are responsible for high biocidal activities. While some activity was demonstrated, bacterial endospores were more resistant to nanoparticle treatment than the vegetative bacteria.

A multifunctional biocide/sporocide and photocatalyst based on titanium dioxide (TiO2) codoped with silver, carbon, and sulfur.

Composite nanostructured samples of Ag (0.5-20%)/(C, S)-TiO(2) were synthesized and characterized by EDX, XRD, FT-IR, UV-vis, BET, XPS, and zeta potential measurements. Photocatalytic and biocidal tests revealed that the amount of the codoped silver (Ag(+)) in (C, S)-TiO(2) played a crucial, distinctive role in the photodegradation of gas-phase acetaldehyde as well as in the inactivation of Escherichia coli cells and Bacillus subtilis spores. Very interestingly, Ag/(C, S)-TiO(2) nanoparticles (crystallite size <10 nm) have shown very strong antimicrobial properties without light activation against both E. coli (log kill >8) and B. subtilis spores (log kill >5) for 30 min exposures, compared with P25-TiO(2). Thus, for the first time, we have demonstrated that titanium dioxide (an environmentally friendly photocatalyst) codoped with silver, carbon, and sulfur can serve as a multifunctional generic biocide as well as a visible light activated photocatalyst.

Novel dye-sensitized solar cell architecture using TiO2-coated vertically aligned carbon nanofiber arrays.

A novel dye-sensitized solar cell (DSSC) architecture based on vertically aligned carbon nanofibers coated with a thin nanoneedle-textured anatase TiO2 film is demonstrated. An encouraging overall conversion efficiency of approximately 1.09% and a rather high open-circuit voltage of approximately 0.64 V have been achieved. The efficient charge separation at the TiO2-CNF junction and the large outer TiO2 surface of this core-shell architecture provide new methods to tune the materials and interfaces in solar cells.

Gold nanoparticle superlattices.

Ordered metal nanoparticle assemblies--superlattices--have captivated and stirred the imagination of scientists and engineers alike and promise great prospect for future technologies. This potential though will greatly be determined by the understanding and control that can be exerted on the assembling processes. This tutorial review presents a brief account of the factors that govern the formation of superlattices and then presents several examples of gold nanoparticle superlattices that are distinguished by the size of participating particles, chain length/functional group of the capping agent, the substrates on which they form etc.

Biocidal activity of nanocrystalline silver powders and particles.

While the activity of the SMAD powders is lower than that of pure silver nitrate, it has the ability to kill bacteria very effectively and over long periods of time.

Synthesis, characterization, and visible light activity of new nanoparticle photocatalysts based on silver, carbon, and sulfur-doped TiO2.

New nanoparticle photocatalysts based on silver, carbon, and sulfur-doped TiO2 having only the homogeneous anatase crystalline phase and high surface area were successfully synthesized by a modified sol-gel route. The catalysts were characterized by EDX, XRD, BET, UV-vis, IR, and Raman spectroscopy. The effects of the experimental parameters on the visible light reactivity of the catalysts were evaluated for the photodegradation of gaseous acetaldehyde as a model indoor pollutant. The activity results show that the silver(I) ion, Ag(+), doping significantly promotes the visible light reactivities of carbon and sulfur-doped TiO2 catalysts without any phase transformation from anatase to rutile. Moreover, Ag/(C, S)-TiO2 photocatalysts degrade acetaldehyde 10 times faster in visible light and 3 times faster in UV light illuminations than the accredited photocatalyst P25-TiO2. The commendable visible photoactivities of Ag/(C, S)-TiO2 new nanoparticle photocatalysts are predominantly attributable to an improvement in anatase crystallinity, high surface area, low band gap and nature of precursor materials used.

Synthesis and characterization of a new Tiara Pd(II) thiolate complex, [Pd(SC12H25)2]6, and its solution-phase thermolysis to prepare nearly monodisperse palladium sulfide nanoparticles.

A new tiara Pd(II) thiolate complex, [Pd(SC12H25)2]6, has been synthesized and fully characterized by X-ray single crystal analysis, elemental analysis, MALDI, 1H NMR, powder XRD, IR, Raman, and UV/vis. It was found that, in each complex cluster, the six palladium atoms form a nearly planar hexagonal ring and the adjacent palladium atoms are bridged by sulfur atoms from both sides. Then the complex was further used as a single-source precursor to prepare nearly monodisperse palladium sulfide (PdS) nanoparticles through the high-temperature-induced decomposition in diphenyl ether. The obtained nanoparticles are 2.87 +/- 0.51 nm in diameter and protected by a layer of thiolate species on the surface.

Gram-scale synthesis of aqueous gold colloids stabilized by various ligands.

We have developed a method for the large-scale synthesis of gold nanoparticles (Au NPs) in an aqueous medium stabilized by various water-soluble ligands. Significantly, the narrow size-distribution of the particles is achieved without employing size-selective procedures. The versatility of the procedure is demonstrated for the preparation of three colloidal systems stabilized by different ligands. Transmission electron microscopy (TEM), zeta-potential measurements and UV-vis spectroscopy are used to characterize the three colloidal systems.

First principles density functional study of the adsorption and dissociation of carbonyl compounds on magnesium oxide nanosurfaces.

The adsorption and dissociation of three carbonyl compounds, formaldehyde, acetaldehyde, and acetone, on the magnesium oxide nanosurface, consisting of four stacked (MgO)3 hexagons, is investigated by first principles density functional theory (DFT). In the case of formaldehyde, strongly chemisorbed species, with carboxylate-like structures, are initially formed. These may subsequently undergo heterolytic cleavage of an aldehyde C-H bond to form formate ions involving a surface oxide ion and a hydride ion adsorbed over the magnesium dication [(MgH+)(HCOO-)]. For acetaldehyde, besides this reaction leading to the formation of acetate, the methyl hydrogen of the adsorbed species also tends to attach itself to a surface oxide ion, yielding surface hydroxyl ions and adsorbed [CH2=C(H)OMg]+. These results are in accord with our previous experimental and theoretical results. In particular, the shift of the aldehyde C-H vibration band to higher frequency and the appearance of OH bands in the infrared spectrum are clearly accounted for. For acetone, the mechanism is found to be similar, i.e., a methyl hydrogen shift to yield surface enolate. Again, this is in agreement with experimental studies.

Low-temperature metallic alloying of copper and silver nanoparticles with gold nanoparticles through digestive ripening.

We describe a remarkable and simple alloying procedure in which noble metal intermetallic nanoparticles are produced in gram quantities via digestive ripening. This process involves mixing of separately prepared colloids of pure Au and pure Ag or Cu particles and then heating in the presence of an alkanethiol under reflux. The result after 1 h is alloy nanoparticles. Particles synthesized according to this procedure were characterized by UV-vis spectroscopy, EDX analysis, and high-resolution electron microscopy, the results of which confirm the formation of alloy particles. The particles of 5.6+/-0.5 nm diameter for Au/Ag and 4.8+/-1.0 nm diameter for Cu/Au undergo facile self-assembly to form 3-D superlattice ordering. It appears that during this digestive ripening process, the organic ligands display an extraordinary chemistry in which atom transfer between atomically pure copper, silver, and gold metal nanoparticles yields monodisperse alloy nanoparticles.

Amphiphilic templating of magnesium hydroxide.

The synthesis of lamellar mesostructured Mg(OH)2 was achieved through a surfactant templating route. Amphiphilic compounds with different anionic headgroups (phosphate, sulfate, sulfonate, and carboxylate) were used as surfactants. Control of d spacing was achieved through the use of different alkyl carboxylate amphiphiles. It is proposed that the interaction between the highly reactive oxygen atoms of the anionic surfactants and the highly electrophilic Mg atom leads to the formation of high charge density at the interface between the surfactant molecules and the inorganic precursor. This interaction is very strong and the existence of strong bonds between the headgroup molecules of the surfactant and the Mg atom locks the structure in a preferred orientation, i.e., lamellar mesostructure. The strong interaction thus precludes any phase transformation, and only the lamellar phase of Mg(OH)2 is obtained. Calcination of the surfactant by heating in oxygen flow leads to the collapse of the lamellar mesophase and results in the formation of nonporous MgO.