[Show abstract][Hide abstract] ABSTRACT: A composite of 1,4-dihexyloxy-poly(p-phenylenevinylene) (DHPPV) and gold nanoparticles (AuNPs) was prepared by an efficient and simple in situ method. The formation of DHPPV-AuNP composites was confirmed through high-resolution transmission electron microscopy (TEM); UV/vis absorption spectra showed a large blue shift in the polymer absorption peak, which was greater than 60 nm in solution, accompanied by a remarkable change in the intensity, whereas the photoluminescence (PL) spectra in the solution showed only a marginal decrease in intensity in the presence of AuNPs. Solid-state UV/vis spectra of DHPPV-AuNP also showed a decrease in the intensity of the shoulder peak in the region of 400-450 nm. Interesting features were observed in the solid-state PL spectra, where the efficient energy transfer from AuNP to DHPPV results in the complete disappearance of the 585 nm peak with a dominant peak appearing at 635 nm. TEM analysis confirmed that AuNPs were embedded in the DHPPV matrix systematically, thus presenting a simple tool to assemble hybrid nanowires comprising π-conjugated organic/polymeric systems and inorganic nanoparticles with likely applications in nanosized optoelectronic devices. The optical properties of DHPPV-AuNP could be further tuned by treating the composite with octadecane thiol or sodium sulfide, resulting in a further blue shift of 65 nm.
The Journal of Physical Chemistry B 10/2010; 114(46):14821-6. · 3.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, we report the spontaneous formation of fibrous structures consisting of assemblies of Au–Ag core-shell nanoparticles
(NPs) from a solution consisting of Au–Ag core-shell NPs and l-ascorbic acid (AA). AA acted both as the reducing agent for the generation of NPs and also as the mediator for the formation
of fibers. The process of fiber formation involved three steps—reduction of HAuCl4 to Au NPs by AA, subsequent formation of Au–Ag core-shell NPs after addition of AgNO3, and spontaneous formation of fibers from the mixtures in water. It took typically about 30days to form complete fibers
that are of lengths of several hundred micrometers to millimeters, although nanofibers started forming from the first day
of solution preparation. The width of each of these fibers was typically about 1–4µm with length of each segment of fiber
bundle, on the order of 40µm. Formation of fibers was also observed in absence of AgNO3. These fibers consisted of Au NPs and polymer of AA degradation products and were not electrically conducting. Also, low
concentrations of AgNO3 produced fibers with low electrical conductivity. However, it was observed that increase in the amount of AgNO3 leads to the formation of fibers that were electrically conducting with conductivity values in the range of metallic conductivity.
Spectroscopic and electron microscopic investigations were carried out to establish the formation of fibers. The details of
fiber formation mechanism under different conditions and electrical conductivities of the fibers are discussed in the article.
[Show abstract][Hide abstract] ABSTRACT: Herein we report on the use of chitosan–Ag NP composite as an efficient catalyst in selective C–C coupling of phenolic compounds in the presence of molecular iodine. This is the first report on the C–C coupling with Ag NPs as the catalyst. The reactions were carried out with phenol and a few of its derivatives as well as naphthols. The results indicate that the reaction proceeded via the formation of p-iodophenol followed by o–p coupling leading to the products. The reactions were complete in 3h with high yields. When the para-position of phenol was blocked by a substituent then iodination at the ortho-position could only be observed and the formation of coupled products did not occur. In the case of naphthols o–o coupling could only be observed. Based on the results a mechanism is proposed for the coupling reaction occurring at the surface of Ag NP.
Journal of Molecular Catalysis A Chemical 01/2009; 304:153-158. · 3.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Herein we report the generation of Au nanoparticles (NPs) by sparingly soluble acetanilide in water. We also report the formation of linear chain-like superstructures of self-assembled Au NPs, in the presence of excess acetanilide. This was achieved in two different ways. In the first method, acetanilide was added, with increasing concentration, into aqueous HAuCl(4) to produce Au NPs as well as for the formation of assembly, which varied according to the concentration of acetanilide. The other route involved formation of spherical Au NPs at the lowest concentration of acetanilide, which was followed by the formation of assembly of various lengths upon further addition of variable amount of acetanilide. The assemblies were stable in aqueous solution for days with characteristic UV-vis absorption spectra consisting of two peaks. While the wavelength of the first peak remained the same, the position of the second peak changed to longer wavelength with increasing acetanilide concentration. Interestingly, the linear chain-like arrays could be broken into individual particles by first dilution of the solution concentration followed by treatment with ultrasonic waves. The individual Au NPs again formed linear chain-like arrays upon addition of excess acetanilide.
Journal of Colloid and Interface Science 09/2008; 324(1-2):230-5. · 3.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on the formation of linear assemblies of citrate-stabilized spherical gold nanoparticles (GNPs) in the presence of acetanlide in aqueous medium. The length of the assembly was dependent on the concentration of acetanilide. At higher concentrations branching of the assembly occurred instead of an increase in the length of the assembly. Interestingly, the catalytic properties of the assemblies were dependent on the concentration of the acetanilide. This means the effective surface areas of the metal nanoparticles in the assembly were much dependent on the length of the assembly, which could be controlled by the concentration of acetanilide. The results indicate a linear dependence of the effective surface area on the concentration of acetanilide. Also, the observations suggest that the presence of NaBH 4 in the reaction medium led to the fusion of GNPs into fibrous form thus reducing the effective surface area for catalysis.
The Journal of Physical Chemistry C 07/2008; 112(30). · 4.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, we report on the catalytic activity of a new metal nanoparticle-polymer composite consisting of Ag nanoparticles (NPs) and environmentally friendly ('green') chitosan. The polymer (chitosan) not only acted as the reducing agent for the metal ions, but also stabilized the product NPs by anchoring them. The majority of the particles produced in this way had sizes less than 5 nm. The catalytic activity of the composite was investigated photometrically by monitoring the reduction of 4-nitrophenol (4NP) in the presence of excess NaBH(4) in water, under both heterogeneous and micro-heterogeneous conditions. The reaction was first order with respect to the concentration of 4NP. We also observed that the apparent rate constant, k(app), for the reaction was linearly dependent on the amount of Ag NPs present in the composite. Moreover, the turn-over frequency (TOF) of the catalyst was found to be (1.5 ± 0.3) × 10(-3) s(-1), when the reaction was carried out under heterogeneous conditions. The Ag NPs in the composite retained their catalytic activities even after using them for ten cycles. Our observations also suggest that the catalytic efficiency under micro-heterogeneous conditions is much higher than under heterogeneous conditions. Thus the composite we have represents an ideal case of an environmentally friendly and stable catalyst, which works under heterogeneous as well as micro-heterogeneous conditions with the advantage of nanoscopic particles as the catalyst.
[Show abstract][Hide abstract] ABSTRACT: Escherichia coli expressing recombinant green fluorescent protein was used to test the bactericidal efficacy of a newly synthesized chitosan–Ag-nanoparticle composite. The composite was found to have significantly higher antimicrobial activity than its components at their respective concentrations. The one-pot synthesis method led to the formation of small Ag nanoparticles attached to the polymer which can be dispersed in media of pH ≤ 6.3. The presence of a small percentage (2.15%, w/w) of metal nanoparticles in the composite was enough to significantly enhance inactivation of E. coli as compared with unaltered chitosan. Fluorescence spectroscopy indicated that bacterial growth stopped immediately after exposure of E. coli to the composite, with release of cellular green fluorescent protein into the medium at a faster rate than with chitosan. Fluorescence confocal laser scanning and scanning electron microscopy showed attachment of the bacteria to the composite and their subsequent fragmentation. Native protein gel electrophoresis experiments indicated no effect of the composite on bacterial proteins.
International Journal of Food Microbiology 01/2008; · 3.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report the results of investigation of the interaction of silver with presynthesized ZnS nanoparticles (NPs) that was stabilized
by cetyl trimethyl ammonium bromide (CTAB). The photoluminescence properties of ZnS NPs were followed in the presence of Ag+ ions, Ag NPs and by the synthesis of Ag@ZnS core-shell nanoparticles. We observed that CTAB stabilized ZnS NPs emitted broadly
in the region from 350–450 nm, when excited by 309 nm light. In the presence of Ag+ ions the emission peak intensity up to 400 nm was reduced, while two new and stronger peaks at 430 nm and 550 nm appeared.
Similar results were obtained when Ag NPs solution was added to ZnS solution. However, when Ag@ZnS NPs were synthesized, the
emission in the 350–450 nm region was much weaker in comparison to that at 540 nm, which itself appeared at a wavelength shorter
than that of Ag+ ion added ZnS NPs. The observations have been explained by the presence of interstitial sulfur and Zn2+, especially near the surface of the nanocrystals and their interaction with various forms of silver. In addition, our observations
suggest that Ag+ ions diffuse into the lattice of the preformed ZnS NPs just like the formation of Ag+ doped ZnS NPs and thus changes the emission characteristics. We also have pursued similar experiments with addition of Mn2+ ions to ZnS and observed similar results of emission characteristics of Mn2+ doped ZnS NPs. We expect that results would stimulate further research interests in the development of fluoremetric metal
ion sensors based on interaction with quantum dots.
Bulletin of Materials Science 01/2008; 31(3):533-539. · 0.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this article, we report the formation of assembly of Au nanoparticles (NPs) of different sizes in the presence of p-aminoacetanilide. Citrate stabilized spherical Au NPs assembled into a linear array, the extent of which depended on the
concentration of p-aminoacetanilide in the medium. Higher concentrations led not only to the formation of longer assemblies but also branched
ones. In addition, substantial fusion of NPs was observed at higher concentrations. UV-Vis spectra showed the appearance of
a second peak at higher wavelength — the position of which shifted to the red with increasing concentration of p-aminoacetanilide. Interestingly, the second peak could not only be influenced by the concentration of p-aminoacetanilide but also by choosing different sizes of the spherical NPs at the initial stages. For example, when the particles
were larger the shift could be observed at higher wavelengths than those starting with smaller particles. The present method
allows organization of NPs into linear arrays based on the molecular properties of the ‘assembler’ i.e. p-aminoacetanilide. Also, the optical properties of the assembled NPs could be tuned with the choice of suitable sizes of the
Journal of Chemical Sciences 01/2008; 120(6):547-555. · 1.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper we report the results on the use of L-ascorbic acid (AA) in assembling metal nanoparticles (NPs) into three-dimensional fibrous structures. The degradation product of AA leads to the formation of fibrous structures, which has been used as a template for deposition of metal NPs such as Au, Pt, and Ag. We also report that AA can be used as the reducing agent in generating Au NPs. The spontaneous fiber formation and formation of Au NPs by AA have been coupled to generate fibers made up of composite of Au NPs and the polymer from the degradation products of AA. These fibers appear in the form of a fiber bundle with branched structures having overall dimensions on the order of several millimeters. They have typical widths of 1-4 microm with length of each segment of fiber bundle on the order of 40 microm. The composite fiber bundle has been found to be electrically conducting with surface resistivity on the order of 2.16x10(3) Omegacm. UV-vis spectroscopy, X-ray diffraction, transmission and scanning electron microscopic measurements were used to establish the formation of fibrous structures in the medium.
Journal of Colloid and Interface Science 08/2007; 311(1):303-10. · 3.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Herein we report a new method of synthesis of Au nanoparticles (NPs) of multi-branched and spherical shapes. HAuCl4 when reduced by Fe2+ ions in aqueous solution, in the presence of sodium dodecyl sulfate (SDS), produced multi-branched Au NPs including a shape that could be termed as a "nanofan." On the other hand spherical particles were produced in the presence of Na2S and SDS or starch (instead of SDS). UV-vis, X-ray diffraction, transmission electron microscopic (TEM) studies revealed the difference in optical properties and shapes under various conditions. The essential conclusion of this report is that stabilizing agent and other reagents do have special role in controlling the shape of the NPs produced.
Journal of Nanoscience and Nanotechnology 07/2007; 7(6):1730-5. · 1.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report the experimental results on temperature-dependent studies of interactions between a novel biocompatible thermosensitive
polymer hydrogel and different stabilizing agent capped gold nanoparticles (Au NPs) with particle size ranging from 5 to 20nm.
Stabilizing agents such as thioglycolic acid, tryptophan, and phenylalanine have been used as capping agents for Au NPs. The
poly-N-isopropyl acrylamide-co-acrylic acid (pNIPAm-AAc) with 3.0±0.7μm in size was synthesized by radical polymerization of
a selected mixture of N-isopropyl acrylamide (NIPAm), methylene-bis-acrylamide and acrylic acid (AAc). The capped Au NPs were mixed with a solution
of pNIPAm-AAc hydrogel. The temperature-dependent properties of the mixture were studied by UV–vis spectroscopy, dynamic light
scattering based particle size analysis, and transmission electron microscopy (TEM). The observations indicated change in
the lower critical solution temperature (LCST) depending on the nature of the stabilizer, with hydrophobic ones lowering the
value while hydrophilic stabilizers increasing the same. Also, the optical absorption due to Au NPs, when stabilized with
hydrophobic groups, reduced significantly at above LCST along with significant blue shift of wavelength maximum.
Journal of Nanoparticle Research 12(4):1331-1348. · 2.18 Impact Factor