Sujoy K. Das

GOLD NANOPARTICLES: MICROBIAL SYNTHESIS AND APPLICATION IN WATER HYGIENE MANAGEMENT

A green chemical method to synthesize nanogold-bioconjugate and its eco-friendly promising role to purify contaminated waters has been described. Gold nanoparticles of 10 nm average diameter are produced on the surface of Rhizopus oryzae , a fungal strain, by in situ reduction of chloroauric acid (HAuCl(4)). The nanogold-bioconjugate (NGBC) showed strong adsorption capacity toward different organophosphorous pesticides. The EDXA study confirms adsorption of pesticides on the conjugate material surface. Morphological changes of the NGBC material after adsorption of organophosphorous pesticides were detected by atomic force micrographs. NGBC shows high antimicrobial activity against several Gram-negative and Gram-positive pathogenic bacteria as well as the yeasts Saccharomyces cerevisiae and Candida albicans . The treatment of microbial cells with NGBC caused rupture of cell membrane as revealed in scanning electron and fluorescence micrographs. These unique characteristics of NGBC have been successfully utilized to obtain potable water free from pathogens and pesticides in a single operation.

Synthesis, characterization and catalytic activity of gold nanoparticles biosynthesized with Rhizopus oryzae protein extract

A simple one-pot green chemical method for the biosynthesis of gold nanoparticles (AuNPs) by reducing chloroauric acid (HAuCl4) with protein extract of Rhizopus oryzae to produce novel gold nano-bio-conjugates (AuNBC) is described. AuNBCs, having sizes ranging from 5 to 65 nm, were synthesized by altering the HAuCl4–protein extract ratio. The conjugates were characterized by spectroscopic, electron microscopic, light scattering and electrophoretic mobility measurements. It was found that the aqueous AuNBC suspensions exhibited excellent stability over a wide range of ionic strength, pH and temperature. The effect of pH and ionic strength indicated that stabilization is due to electrostatic repulsion arising from the negative charge of the conjugate proteins. The AuNBCs were stable at temperatures lower than the denaturation temperature of the fungal proteins. The catalytic activity of the as-synthesized AuNBCs was quantified by analysing the reduction of p-nitrophenol by borohydride. The conjugates exhibited interesting size and shape dependent catalytic activity, which was stronger than that observed for AuNPs prepared by conventional chemical methods. The catalytic activity was found to be sensitive to both the surface-area-to-volume ratio and the thickness of the protein coating on the NP.

Biomineralization Mechanism of Gold by Zygomycete Fungi Rhizopous oryzae

In recent years, there has been significant progress in the biological synthesis of nanomaterials. However, the molecular mechanism of gold biomineralization in microorganisms of industrial relevance remains largely unexplored. Here we describe the biosynthesis mechanism of gold nanoparticles (AuNPs) in the fungus Rhizopus oryzae . Reduction of AuCl(4)(-) [Au(III)] to nanoparticulate Au(0) (AuNPs) occurs in both the cell wall and cytoplasmic region of R. oryzae . The average size of the as-synthesized AuNPs is ~15 nm. The biomineralization occurs through adsorption, initial reduction to Au(I), followed by complexation [Au(I) complexes], and final reduction to Au(0). Subtoxic concentrations (up to 130 μM) of AuCl(4)(-) in the growth medium increase growth of R. oryzae and induce two stress response proteins while simultaneously down-regulating two other proteins. The induction increases mycelial growth, protein yield, and AuNP biosynthesis. At higher Au(III) concentrations (>130 μM), both mycelial and protein yield decrease and damages to the cellular ultrastructure are observed, likely due to the toxic effect of Au(III). Protein profile analysis also confirms the gold toxicity on R. oryzae at high concentrations. Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis shows that two proteins of 45 and 42 kDa participate in gold reduction, while an 80 kDa protein serves as a capping agent in AuNP biosynthesis.

Microbial Synthesis of Multishaped Gold Nanostructures

The development of methodologies for the synthesis of nanoparticles of well-defined size and shape is a challenging one and constitutes an important area of research in nanotechnology. This Full Paper describes the controlled synthesis of multishaped gold nanoparticles at room temperature utilizing a simple, green chemical method by the interaction of chloroauric acid (HAuCl4 x 3H20) and cell-free extract of the fungal strain Rhizopus oryzae. The cell-free extract functions as a reducing, shape-directing, as well as stabilizing, agent. Different shapes of gold nanocrystals, for example, triangular, hexagonal, pentagonal, spherical, spheroidal, urchinlike, two-dimensional nanowires, and nanorods, are generated by manipulating key growth parameters, such as gold ion concentration, solution pH, and reaction time. The synthesized nanostructures are characterized by UV/Vis and Fourier-transform infrared spectroscopy, transmission electron microscopy, and energy dispersive X-ray analysis studies. Electron diffraction patterns reveal the crystalline nature of the nanoparticles and a probable mechanism is proposed for the formation of the different structural entities.

Interaction of Chromium with Resistant Strain Aspergillus versicolor: Investigation with Atomic Force Microscopy and Other Physical Studies

The interaction of chromium and a chromate resistant Aspergillus versicolor strain has been studied by atomic force (AFM) and transmission electron (TEM) microscopies. The nanomechanical properties such as cell wall rigidity and elasticity were measured by force spectroscopy and found to be 0.61 +/- 0.08 N/m, and 20.5 +/- 2.1 MPa, respectively. On chromium binding, ultrastuctural changes of the cell wall along with the formation of layered structures on the cell wall were observed. TEM and AFM micrographs demonstrate the accumulation of chromium on the cell wall, which were rough and irregular compared with the smooth pristine mycelia. The surface roughness, cell wall rigidity and elasticity increased to 35.5 +/- 3.5 nm, 0.88 +/- 0.05 N/m, and 62.5 +/- 3.5 MPa, respectively, from the corresponding values of 5.2 +/- 0.68 nm, 0.61 +/- 0.02 N/m, and 20.5 +/- 2.1 MPa for the pristine mycelia. X-ray photoelectron spectroscopy and Fourier transform infrared studies suggest that bound chromium was reduced to its trivalent state by the cell wall components. The reduced chromium species on the cell surface further electrostatically bind chromate ions forming layered structure on the cell wall.

Adsorption behavior of mercury on functionalized aspergillus versicolor mycelia: atomic force microscopic study.

The adsorption characteristics of mercury on Aspergillus versicolor mycelia have been studied under varied environments. The mycelia are functionalized by carbon disulfide (CS(2)) treatment under alkaline conditions to examine the enhance uptake capacity and explore its potentiality in pollution control management. The functionalized A. versicolor mycelia have been characterized by scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDXA), attenuated total reflection infrared (ATR-IR), and atomic force microscopy (AFM) probing. SEM and AFM images exhibit the formation of nanoparticles on the mycelial surface. ATR-IR profile confirms the functionalization of the mycelia following chemical treatment. ATR-IR and EDXA results demonstrate the binding of the sulfur groups of the functionalized mycelia to the mercury and consequent formation metal sulfide. AFM study reveals that the mycelial surface is covered by a layer of densely packed domain like structures. Sectional analysis yields significant increase in average roughness (R(rms)) value (20.5 +/- 1.82 nm) compared to that of the pristine mycelia (4.56 +/- 0.82 nm). Surface rigidity (0.88 +/- 0.06 N/m) and elasticity (92.6 +/- 10.2 MPa) obtained from a force distance curve using finite element modeling are found to increase significantly with respect to the corresponding values of (0.65 +/- 0.05 N/m and 32.8 +/- 4.5 MPa) of the nonfunctionalized mycelia. The maximum mercury adsorption capacity of the functionalized mycelia is observed to be 256.5 mg/g in comparison to 80.71 mg/g for the pristine mycelia.

Bioinspired Metal Nanoparticle: Synthesis, Properties and Application

Nanomaterials with a characteristic dimension in the range of 1-100 nanometers (nm) are at the leading edge of nanoscience and nanotechnology. In recent years nanomaterials, and specifically metal nanoparticles, have received particular interest in diverse field ranging from material science to biotechnology (Guo et al. 2005, Daniel and Astruc 2004, Huang et al. 2007). Although widespread interest in nanomaterials is recent, the concept was introduced over 40 years ago. Nanomaterials have actually been produced and used by humans for hundreds of years: for example, the beautiful ruby red colour of some glass is due to gold nanoparticles (AuNP) trapped in the glass matrix. In the decorative glaze known as luster, found on some medieval pottery, the special optical properties of the glaze arose from metallic spherical nanoparticles which were dispersed in the glaze in a random fashion. Michael Faraday in 1857 on his pioneering work “Experimental relations of gold (and other metals) to light” (Faraday, 1857) explain the properties of this glaze. Now with advances of science and technology, the morphology of this material, which contains metallic nanoparticles, has been understood. Because of extremely small size and high surface volume ratio of nanoparticles, the physicochemical properties of nanoparticles-containing materials are quite different to those of the bulk materials (El-Sayed 2001). Thus, nanomaterials have potential applications in electronics and photonics, catalysis, information storage, chemical sensing and imaging, environmental remediation, drug delivery and biological labelling (Guo et al. 2005, Daniel and Astruc 2004, Huang et al. 2007). It is well known that the optical, electronic, and catalytic properties of metal nanoparticles are greatly influenced by their size, shape, and crystal structure. For example, silver (Ag) and gold (Au) nanocrystals of different shapes possess unique optical scattering responses (Daniel and Astruc 2004, Roduner 2006). Whereas highly symmetric spherical particles exhibit a single scattering peak, anisotropic shapes such as rods, triangular prisms, and cubes exhibit multiple scattering peaks in the visible wavelengths due to highly localized charge polarizations at corners and edges (Mie 1908). Thus, synthesis of metal nanoparticles with defined morphology gained much interest. A variety of strategies have been developed for the synthesis of metal nanoparticles (MNPs) and nanomaterials. Optimizing the nanomaterial synthesis has now become a prolific area of investigation. In the first part of this chapter, we will review the synthetic methods for MNPs production, with particular regard to biosynthesis in viable organisms and protein extracts. In the second part, we will discuss the most recent finding on the biosynthetic mechanism, the properties of the nanobioconjugates, and recent applications of MNPs.

Structural and nanomechanical properties of Termitomyces clypeatus cell wall and its interaction with chromium(VI).

Alterations of cell surface properties accompanying the complex life cycle of Termitomyces clypeatus have been monitored using atomic force microscopy (AFM). A new hyphae/mycelium is developed on cell division, and the cell wall of the mycelium undergoes a process of internal reorganization (or maturation) followed by morphological and chemical alterations. The changes of the surface ultrastructures during the growth process are correlated to the corresponding changes in relative viscoelasticity and rigidity of the cell wall by employing force spectroscopy. The cell wall rigidity and elasticity are found to be 0.34+/-0.02 N/m and 27.5+/-2.1 MPa, respectively, at the early logarithmic phase, on maturation increase to reach 0.81+/-0.08 N/m and 92.5+/-12 MPa, respectively, at the stationary phase, and thereafter decrease to 0.62+/-0.06 N/m and 61.6+/-6.6 MPa at the death phase. The alterations of the ultrastructural and nanomechanical properties of the cell surface as functions of growth phases affect the interaction involving chromium and T. clypeatus.

Nanoengineering of a Supramolecular Gel by Copolymer Incorporation: Enhancement of Gelation Rate, Mechanical Property, Fluorescence, and Conductivity

In the quest to engineer the nanofibrillar morphology of folic acid (F) gel, poly(4-vinylpyridine-co-styrene) (PVPS) is judiciously integrated as a polymeric additive because of its potential to form H-bonding and π-stacking with F. The hybrid gels are designated as F-PVPSx gels, where x denotes the amount of PVPS (mg) added in 2 mL of F gel (0.3%, w/v). The assistance of PVPS in the gelation of F is manifested from the drop in critical gelation concentration and increased fiber diameter and branching of F-PVPSx gels compared to that of F gel. PVPS induces a magnificent improvement of mechanical properties: a 500 times increase of storage modulus and ∼62 times increase of yield stress in the F-PVPS5 gel compared to the F gel. The complex modulus also increases with increasing PVPS concentration with a maximum in F-PVPS5 gel. Creep recovery experiments suggest PVPS induced elasticity in the otherwise viscous F gel. The fluorescence intensity of F-PVPSx gels at first increases with increasing PVPS concentration showing maxima at F-PVPS5 gel and then slowly decreases. Gelation is monitored by time-dependent fluorescence spectroscopy, and it is observed that F and F-PVPSx gels exhibit perfectly opposite trend; the former shows a sigmoidal decrease in fluorescence intensity during gelation, but the latter shows a sigmoidal increase. The gelation rate constants calculated from Avrami treatment on the time-dependent fluorescence data manifest that PVPS effectively enhances the gelation rate showing a maximum for F-PVPS5 gel. The hybrid gel exhibit 5 orders increase of dc conductivity than that of F-gel showing semiconducting nature in the current-voltage plot. The Nyquist plot in impedance spectra of F-PVPS5 xerogel exhibit a depressed semicircle with a spike at lower frequency region, and the equivalent circuit represents a complex combination of resistance-capacitance circuits attributed to the hybrid morphology of the gel fibers.

Triarylamine-Cored Dendritic Molecular Gel for Efficient Colorometric, Fluorometric, and Impedometeric Detection of Picric Acid

Detection of nitroaromatics at ultralow concentration is a major security concern in defense, forensics, and environmental science. To this end, a new triarylamine-cored dendritic gelator (OGR) was synthesized, which produced thermoreversible, thixotropic, and fluorescent gels in n-octanol. On gelation, both π-π* transitions and the emission peak of the gelator show redshifts with a 4.5-fold increase of fluorescence intensity in the gel state indicating J-aggregation. The nitrogen lone-pair electrons of OGR make it a donor, and electron transfer occurs to acceptor nitroaromatics causing fluorescence quenching, which is further promoted due to its acidity. The Stern-Volmer rate constants measured for different nitroaromatics showed that it senses picric acid (PA) best. The contact-mode technique with OGR-treated paper strips can allow naked-eye detection of PA under UV light down to 10-11  m concentration within 30 s. Reusability of the gel is achieved by treating OGR@PAx with NaOH solution. Impedance spectroscopic results indicated a decrease of both charge-transport resistance and Warburg impedance on successive addition of PA. The limits of detection of PA determined from fluorescence and impedance measurements match well. Thus, the OGR gel is a reusable, low-cost, specific sensor for PA by naked-eye colorimetric, fluorescence, and impedance techniques.