A. M. Burov

Study of polyol synthesis reaction parameters controlling high yield of silver nanocubes

The influence of the parameters and conditions of sodium sulfide-induced reaction of polyol synthesis of silver nanoparticles on the yield of cube-shaped particles and the optical properties of colloids is studied. The protocol proposed by Skrabalak et al. for the synthesis of nanocubes in small volumes (Nature Protocols, 2007, vol. 2, p. 2182) is taken as an initial variant for optimization. The effects of the reagent concentrations, degree of ethylene glycol oxygenation, the presence of impurities, reaction time, and temperature are studied. Suspensions containing nanoparticles with different shapes and sizes, including polydisperse particles of irregular shapes, silver nanocubes with a yield of 0 to 97%, nanoprisms, and nanorods, can be produced by varying the synthesis parameters. The key parameters controlling the yield of nanocubes are the degree of ethylene glycol oxygenation and the presence of trace amounts of ions of other metals (not silver). It is established that variations in the reaction time make it possible to vary the sizes of nanocubes in the range of 30–60 nm. Suspensions with high contents of cube-shaped particles are shown to exhibit three maxima in the plasmon extinction resonance spectrum at wavelengths of 350, 390, and, depending on the particle size, 435–470 nm.

Enzymatic formation of gold nanoparticles by submerged culture of the basidiomycete Lentinus edodes

We report for the first time that the medicinal basidiomycete Lentinus edodes can reduce Au(III) from chloroauric acid (HAuCl4) to elemental Au [Au(0)], forming nanoparticles. Several methods, including transmission electron microscopy, electron energy loss spectroscopy, X-ray fluorescence, and dynamic light scattering, were used to show that when the fungus was grown submerged, colloidal gold accumulated on the surface of and inside the mycelial hyphae as electron-dense particles mostly spherical in shape, with sizes ranging from 5 to 50nm. Homogeneous proteins (the fungal enzymes laccase, tyrosinase, and Mn-peroxidase) were found for the first time to be involved in the reduction of Au(III) to Au(0) from HAuCl4. A possible mechanism forming Au nanoparticles is discussed.

Biosynthesis of gold nanoparticles by Azospirillum brasilense

Plant-associated nitrogen-fixing soil bacteria Azospirillum brasilense were shown to reduce the gold of chloroauric acid to elemental gold, resulting in formation of gold nanoparticles. Extracellular phenoloxidizing enzymes (laccases and Mn peroxidases) were shown to participate in reduction of Au+3 (HAuCl4) to Au0. Transmission electron microscopy revealed accumulation of colloidal gold nanoparticles of diverse shape in the culture liquid of A. brasilense strains Sp245 and Sp7. The size of the electron-dense nanospheres was 5 to 50 nm, and the size of nanoprisms varied from 5 to 300 nm. The tentative mechanism responsible for formation of gold nanoparticles is discussed.

Tuning of plasmon resonance of gold nanorods by controlled etching

Production of gold nanorods with preset geometric characteristics is of fundamental importance for many fields of application, because even slight variations in their absolute sizes or axial ratio may noticeably change the positions and intensities of the maxima in the resonance absorption and scattering of light. In this work, a method has been proposed for tuning the plasmon resonance of gold nanorods in a wide wavelength range via their controlled etching along the major axis with a complex of Au3+ ions and cetyltrimethylammonium bromide. The synthesized gold nanorods possess longitudinal plasmon resonance at a wavelength of 920 nm, an average thickness of 16 nm, and an axial ratio of about 5. A chloroauric acid solution added to a colloid of the nanorods etches their ends and decreases the axial ratio thereof, while their thickness remains unchanged, thereby shifting the longitudinal plasmon resonance toward shorter wavelengths. The axial ratio of resulting nanoparticles and the position of plasmon resonance are entirely determined by the ratio between gold atoms in the initial nanorods and gold ions in the solution. This makes it possible to vary the axial ratio from 5 to 1 and finely tune plasmon resonance in a wavelength range from 920 to 520 nm. It has also been shown that the proposed method can be used for controlled etching of gold nanospheres.

Changes in biofilm formation in the nonflagellated flhB1 mutant of Azospirillum brasilense Sp245

Bacteria Azospirillum brasilense Sp245 with mixed flagellation are able to form biofilms on various surfaces. A nonflagellated mutant of this strain with inactivated chromosomal copy of the flhB gene (flhB1) was shown to exhibit specific traits at the later stages of biofilm formation on a hydrophilic (glass) surface. Mature biofilms of the flhB1::Omegon-Km mutant Sp245.1063 were considerably thinner than those of the parent strain Sp245. The biofilms of the mutant were more susceptible to the forces of hydrodynamic shear. A. brasilense Sp245 cells in biofilms were not found to possess lateral flagella. Cells with polar flagella were, however, revealed by atomic force microscopy of mature native biofilms of strain Sp245. Preservation of a polar flagellum (probably nonmotile) on the cells of A. brasilense Sp245 may enhance the biofilm stability.

Biological synthesis of gold nanoparticles by the xylotrophic basidiomycete Lentinula edodes

This is the first study to demonstrate that the medicinal basidiomycete Lentinula edodes can reduce gold (III) ions from hydrogen tetrachloaurate (chloroauric acid) H[AuCl4] to the elementary state with the formation of spherical nanoparticles (nanospheres). When a culture was grown under submerged conditions in the presence of chloroauric acid, the appearance of an intense purple-red color of L. edodes filamentous hyphae was recorded, which indicates that gold ions were reduced to gold nanoparticles. Using transmission electron microscopy and X-ray fluorescence, we observed accumulation of colloidal gold by the fungal mycelium in the form of electron-dense nanospheres of 5 to 50 nm in diameter on the surface and inside fungal cells.

Immunoelectron microscopy investigation of the cell surface of Azospirillum brasilense strains

The genus-specific surface protein antigens of Azospirillum brasilense strains were visualized immunochemically. The procedure used for cell sample preparation was optimized to ensure that the surface protein structures were detected on cells in situ. Gold and gold-silver nanoparticles were conjugated to antibodies raised against the flagellin of A. brasilense type strain Sp7, against the lipopolysaccharide of A. brasilense Sp245, and against the genus-specific protein determinants of A. brasilense Sp7. Electron microscopic analysis using nanoparticle-labeled antibodies revealed antigenic determinants of the polar flagellum on the A. brasilense Sp245 cell surface, which in these bacteria are normally screened from the surroundings by a lipopolysaccharide sheath. Pili-like structures were detected on the Sp245 wild-type strain and on its Fla– Swa– Omegon-Km mutant SK048, which are presumably involved in microcolonial spreading in these bacteria.

Serological Relationships of Azospirilla Revealed by Their Motility Patterns in the Presence of Antibodies to Lipopolysaccharides

Motility of the serologically different Azospirillum brasilense strains Sp245 (serogroup I) and Sp7 (serogroup II) was studied in the presence of antibodies to their lipopolysaccharides (LPS). A procedure was proposed in order to determine the motility patterns indicating the specificity of the interaction between the anti-LPS antibodies and bacteria. Analysis of the effect of such antibodies on motility of 25 strains (A. brasilense, A. lipoferum, A. irakense, and Azospirillum sp.) revealed bacteria exhibiting antigenic cross reactions with A. brasilense Sp7 or Sp245. The effect of anti-LPS antibodies on motility of azospirilla was in agreement with the results of immune agglutination analysis of bacterial cells and of immunodiffusion analysis of the LPS preparations. According to our results, strains Azospirillum sp. SR81 and A. brasilense SR14 should be included into serogroups I and II, respectively.

Green synthesis of nanoparticles with extracellular and intracellular extracts of basidiomycetes

Au, Ag, Se, and Si nanoparticles were synthesized from aqueous solutions of HAuCl4, AgNO3, Na2SeO3, and Na2SiO3 with extra- and intracellular extracts from the xylotrophic basidiomycetes Pleurotus ostreatus, Lentinus edodes, Ganoderma lucidum, and Grifola frondosa. The shape, size, and aggregation properties of the nanoparticles depended both on the fungal species and on the extract type. The bioreduction of the metal-containing compounds and the formation rate of Au and Ag nanoparticles depended directly on the phenol oxidase activity of the fungal extracts used. The biofabrication of Se and Si nanoparticles did not depend on phenol oxidase activity. When we used mycelial extracts from different fungal morphological structures, we succeeded in obtaining nanoparticles of differing shapes and sizes. The cytotoxicity of the noble metal nanoparticles, which are widely used in biomedicine, was evaluated on the HeLa and Vero cell lines. The cytotoxicity of the Au nanoparticles was negligible in a broad concentration range (1–100 µg/mL), whereas the Ag nanoparticles were nontoxic only when used between 1 and 10 µg/mL.

A New Type of SERS Tags: Au@Ag Core/Shell Nanorods with Embedded Aromatic Molecules

SERS tags are a new class of nanoprobes consisting of metal nanoparticles with adsorbed Raman active molecules. The brightness of a single tag depends on the Raman cross section of the reporter molecules and on the enhancing properties of plasmonic particles. Here, nine types of composite AuNR(core, 81 × 25 nm)@R@Ag(shell, 14 nm) nanorods (NRs) with thiolated aromatic molecules (R, nine different molecules) embedded between metal layers were synthesized. The Raman efficiency of reporters was evaluated by measuring normal Raman spectra in ethanol. The surface-enhanced Raman scattering (SERS) was evaluated for molecules adsorbed on Au nanorods before and after Ag shell growth. The coverage of the functionalized Au nanorods with an Ag shell results in an increase in SERS enhancement factor by two orders of magnitude, from 2.5 × 104 for the initial Au NRs to 8.5 × 106 for composite particles. Finite-difference time-domain (FDTD) simulations showed that the ultrahigh enhancement of Raman scattering inside composite nanorods can be explained by the enhancement of the EM field at the boundary of metal layers. Thus, for fabricated composite particles, we have a specific variant of EM SERS enhancement at the boundary of Au/Ag metals, which is different from the local field enhancement near plasmonic particles.