Continuous Flow Reactor for the Controlled Synthesis and Inline Photocatalysis of Antibacterial Ag2 S Nanoparticles.

The present study is focused on the integration of microreactors to synthesize visible light active nanophotocatalysts for inline photocatalytic degradation of organic dye and antibacterial activity. A wire-assisted and a rapid laser micromachining technique has been employed for the fabrication of polydimethylsiloxane (PDMS) and poly(methyl methacrylate) (PMMA)-based microreactors, respectively. By varying the design and chemical reagents involved, different sizes of visible light active Ag2 S nanoparticles were prepared via a continuous microfluidics approach using fabricated microreactors. When polyvinylpyrrolidone (PVP) was utilized as the capping agent during the reaction, smaller particles of the size of ~ 15 nm were observed. The photocatalytic performance of these nanoparticles has been evaluated inline by employing the single-inlet planar microreactor as a function of flow rate and channel length. The photocatalyst durability test and a comparative photocatalytic efficiency study between the microreactor and the conventional beaker reactor have also been carried out. Under visible light, these nanoparticles exhibit a remarkable enhancement of ~ 94.5% in the inline microreactor-based photocatalytic degradation of methylene blue dye. The slower the flow rate and longer the channel length, gradual enhancement in the performance has been observed. Also, these nanoparticles express an antibacterial effect with very high efficacy even at very low (2 mg mL-1 ) concentration toward the inhibition of Escherichia Coli.

Modal engineering of Surface Plasmons in apertured Au Nanoprisms.

Crystalline gold nanoprisms of sub-micrometric size sustain high order plasmon modes in the visible and near infrared range that open a new realm for plasmon modal design, integrated coplanar devices and logic gates. In this article, we explore the tailoring of the surface plasmon local density of states (SP-LDOS) by embedding a single defect, namely a small hole, carved in the platelet by focused ion beam (FIB). The change in the SP-LDOS of the hybrid structure is monitored by two-photon luminescence (TPL) microscopy. The dependency of the two-dimensional optical field intensity maps on the linear polarization of the tightly focused femtosecond laser beam reveals the conditions for which the hole defect significantly affects the initial modes. A detailed numerical analysis of the spectral characteristics of the SP-LDOS based on the Green dyadic method clearly indicates that the hole size and location can be exploited to tune or remove selected SP modes.

Tailoring and imaging the plasmonic local density of states in crystalline nanoprisms.

Surface plasmon (SP) technologies exploit the spectral and spatial properties of collective electronic oscillations in noble metals placed in an incident optical field. Yet the SP local density of states (LDOS), which rule the energy transducing phenomena between the SP and the electromagnetic field, is much less exploited. Here, we use two-photon luminescence (TPL) microscopy to reveal the SP-LDOS in thin single-crystalline triangular gold nanoprisms produced by a quantitative one-pot synthesis at room temperature. Variations of the polarization and the wavelength of the incident light redistribute the TPL intensity into two-dimensional plasmonic resonator patterns that are faithfully reproduced by theoretical simulations. We demonstrate that experimental TPL maps can be considered as the convolution of the SP-LDOS with the diffraction-limited Gaussian light beam. Finally, the SP modal distribution is tuned by the spatial coupling of nanoprisms, thus allowing a new modal design of plasmonic information processing devices.

Imaging symmetry-selected corner plasmon modes in penta-twinned crystalline Ag nanowires.

Using dual-plane leakage radiation microscopy, we investigate plasmon propagation in individual penta-twinned crystalline silver nanowires. By measuring the wavevector content of the light emitted in the substrate, we unambiguously determine the effective index and the losses of the mode propagating in these structures. The experimental results, in particular, the unexpectedly low effective index, reveal the direct influence of the nanowire crystallinity and pentagonal structure on the observed plasmon modes. By analogy with molecular orbitals of similar symmetry, the plasmon modes are also determined numerically in good agreement with the observed values. We further investigate the effect of wire geometry (length, diameter) on the effective index and propagation loss. Our results show that, beyond dissipation concerns, the morphological and structural control obtained in crystalline colloidal plasmonic nanostructures can be exploited to finely tune their optical properties.

Self-assembled monolayers induced inter-conversion of crystal structure by vertical to lateral growth of aluminium doped zinc oxide thin films.

In this communication, we demonstrate the inter-conversion of crystal structure of aluminium doped zinc oxide (AZO) thin films from highly (002) plane oriented vertical growth to (103) plane oriented lateral growth by adjusting the polarity of the self-assembled monolayers (SAMs) on glass substrates at room temperature.

Improved charge separation properties of organic hetero-junction solar cells by self-assembled monolayers anchored Ag nanoparticles.

We investigate the effect of self-assembled monolayers and localized surface plasmons of silver nano-particles on an organic solar cell consisting of zinc phthalocyanine as an active layer. The device was fabricated by covalent attachment of silver nanoparticles on n-type silicon substrates using self-assembled monolayer of 4-mercaptophenol. Power conversion efficiency is increased up to 8 times as compared to a reference device with merely 0.13% photo-conversion efficiency containing no self-assembled monolayers and silver nano-particles. We believe that improved conductivity at the interface due to the aromatic self-assembled monolayer and the increased local electric field experienced by the active layer in presence of silver nano-particles act in synergy towards the higher population of excitons and dissipation of charge.

Discriminate crystallinities of tin doped indium oxide films on self-assembled monolayers modified glass substrates.

Tin doped indium oxide (ITO) films have generated tremendous research interest and received widespread applications in optoelectronic devices due to a good combination of desired optical and electrical properties. Their electrical properties vary depending on the crystallinity of the film. A good quality ITO film should have low resistivity, which can be achieved with highly crystalline films deposited at very high temperature. Thus, film quality is sensitive to the deposition conditions. Generally, low-temperature deposition of ITO results in poor quality films due to amorphous growth. In this study, we have demonstrated that crystallinity of the ITO films can be improved even at room temperature (RT) using self-assembled monolayers (SAMs) modified glass substrates. The present study demonstrates that SAM with -SH terminal group is necessary for the high-quality ITO growth, while SAMs with other terminal groups (-NH(2) and -CH(3)) generate ITO films with moderate crystallinity. Various properties of such films were investigated using X-ray diffraction, X-ray photoelectron depth profile, four-point probe, and Hall measurements. It is confirmed from such measurements that ITO film deposited on -SH terminated SAM substrate has excellent crystallinity, conductivity, and optical transmission.

Recent advances in fabrication of anisotropic metallic nanostructures.

The current review describes a critical survey of various methods for the shape- and size-selective synthesis of anisotropic metallic nanostructures, especially, of gold and silver. Although substantial progress in the size and shape control of metallic nanostructures has been made for years, we focus a concise review of most recent advances for the fabrication of anisotropic metallic nanostructures. We begin with the novel synthesis protocols of various anisotropic nanostructures with specific classification, and discuss briefly their remarkable size- and shape-dependant properties. The growth mechanism is also introduced independently for such structures illustrating the importance of several key variables like ligand to metal ion ratios, ligand chain lengths, nature of stabilizers or capping agents, solvents, temperature, and additive ions. Finally, applications of these nanostructures have been discussed.

Electron transfer to sulfides and disulfides: intrinsic barriers and relationship between heterogeneous and homogeneous electron-transfer kinetics.

The electron-acceptor properties of series of related sulfides and disulfides were investigated in N,N-dimethylformamide with homogeneous (redox catalysis) and/or heterogeneous (cyclic voltammetry and convolution analysis) electrochemical techniques. The electron-transfer rate constants were determined as a function of the reaction free energy and the corresponding intrinsic barriers were determined. The dependence of relevant thermodynamic and kinetic parameters on substituents was assessed. The kinetic data were also analyzed in relation to corresponding data pertaining to reduction of diaryl disulfides. All investigated reductions take place by stepwise dissociative electron transfer (DET) which causes cleavage of the C(alkyl)--S or S--S bond. A generalized picture of how the intrinsic electron-transfer barrier depends on molecular features, ring substituents, and the presence of spacers between the frangible bond and aromatic groups was established. The reduction mechanism was found to undergo a progressive (and now predictable) transition between common stepwise DET and DET proceeding through formation of loose radical anions. The intrinsic barriers were compared with available results for ET to several classes of dissociative- and nondissociative-type acceptors, and this led to verification that the heterogeneous and the homogeneous data correlate as predicted by the Hush theory.

Role of polyfunctional organic molecules in the synthesis and assembly of metal nanoparticles.

The role of polyfunctional organic molecules in the synthesis of differently shaped metallic nanostructures and their assembly is investigated. These molecules could be used as spacer ligands and also for surface passivation of nanoparticles, especially with the objective of controlling their electronic and optical properties depending on their length scales. We investigate the role of several such molecules, such as 4-aminothiophenol, tridecylamine, Bismarck brown R and Y, mordant brown, fat brown, chrysoidin (basic orange), and 3-aminobenzoic acid in the synthesis and assembly of various nanoparticles of gold and silver. For example, the use of 4-ATP helps in the formation of rod shaped micelles in aqueous acetonitrile as confirmed by transmission electron microscopy (TEM) suggesting their role as soft templates. In addition, 4-ATP has also been used for the formation of heteroassembly of spherical nanoparticles of gold and silver at controlled pH. Significantly, triangular and hexagonal gold nanoplates are formed at room temperature by similar polyfunctional dye molecule, Bismarck brown R (BBR), while other analogous dye molecules give only arbitrary shaped gold nanoparticles. Further confirmation of their role in shape determination comes from linear amine molecules such as tridecylamine, which give only spherical nanoparticles both for silver and gold. In essence, our study confirms the role of various such organic molecules in shape controlled synthesis of nanoparticles. We also report optical and electrochemical properties of few of these nanostructures as a function of their shape.

Wolff rearrangement of alpha-diazoketones using in situ generated silver nanoclusters as electron mediators.

[reaction: see text] We report Wolff rearrangement of alpha-diazoketones by in situ generated silver nanoclusters (Ag(n)(), 2-4 nm) from silver(I) oxide (Ag(2)O) involving a nonclassical electron-transfer process. Our results show that Ag(n)()(+)/Ag(n)()(0) redox couple allows the initial removal of an electron from alpha-diazoketone and its back-donation after chemical reaction(s). Controlled potential coulometry (CPC) of various alpha-diazoketones results in the realization of Wolff-rearranged carboxylic acids in excellent yields.

Organic dye molecules as reducing agent for the synthesis of electroactive gold nanoplates.

Highly crystalline, hexagonal and triangular nanoplates of gold are synthesized in high yield by a new wet chemical method using multifunctional molecules, Bismarck brown R (BBR) and Bismarck brown Y (BBY). This method involves a simple approach by keeping a mixture of aqueous HAuCl4 solution and BBR/BBY solution in presence of poly(vinyl pyrrolidone) for 24 h. These nanostructures show unprecedented electrochemical properties exhibiting surface confinement effect. The UV-visible (UV-vis) spectrum shows certain distinct features with absorptions at 300, 400, and 650 nm extending up to the near infrared region. Selected area electron diffraction patterns of these nanoparticles show highly oriented (111) crystal facets. X-ray diffraction analysis also confirms the predominant orientation in the (111) crystal planes with lattice constant approximately 4.07 angstroms of face-centered-cubic (fcc) gold. X-ray photoelectron (XP) and Fourier transform infrared (FTIR) spectroscopic analysis shows the presence of a fraction of reducing molecules as surface passivating agent either in the unreacted molecular state or as a mixture of reacted and unreacted product, which probably undergoes charge transfer with gold nanocrystals giving absorption at approximately 300 nm.

Electron transfer behavior of monolayer protected nanoclusters and nanowires of silver and gold.

Understanding the electron transfer behavior of nanometer sized, both metallic and semiconducting particles and wires is important due to the fundamental interest in size and shape dependent electronic properties and also because of its applications in nano-electronic devices like single electron transistors and molecular switches. Monolayer protected nanoclusters enable one simple and elegant method of synthesis of these types of metallic and semiconducting materials using interfacial chemistry as has been successfully used in several applications ranging from catalysis to molecular electronics. The success of this type of nanostructured materials is due in part to the well known protecting/stabilizing action of the ligands (also known as surface passivating/capping agents), which facilitate the synthesis and processing of these hydrophobic colloids in solution form. The present article discusses the electron transfer behavior of silver nanowires and nanoparticles with varied sizes. In particular, we have investigated the electrochemical properties of silver nanowires (diameter 70 nm, length several micrometers) and compared with the behavior of similar relatively larger sized nanoparticles (size 40 nm). A critical analysis of the redox behavior of silver nanowires and nanoparticles is presented in aqueous medium under various electrolytic conditions along with a comparison of analogous properties of smaller sized (2-7 nm) silver and gold nanoclusters.

Controlled interlinking of Au and Ag nanoclusters using 4-aminothiophenol as molecular interconnects.

This work describes the formation of interlinked gold and silver nanoclusters at controlled pH using 4-aminothiophenol (ATP) as a molecular interconnect. UV-visible spectra give on intercrystal plasmon resonance band in the region 550-580 nm. The crystalline heteroassembly formation is also evident from the transmission electron microscopic (TEM) images, whereas X-ray photoelectron spectroscopic (XPS) analysis of the aggregates shows the presence of charged -N species, indicating electrostatic interaction of -N with Ag nanoclusters. Furthermore, electrochemical studies of these heteroassembled systems suggest that silver nanoclusters are not fully passivated by the monolayers of ATP and are accessible for redox reactions.