Vinay K. Gupta

Photocatalytic degradation of methyl orange using polymer–titania microcomposites

Photodegradation of an organic dye was studied experimentally using novel polymer-titania microcomposites. These microcomposites were prepared from titanium dioxide (TiO(2)) nanoparticles embedded within cross-linked, thermally-responsive microgels of poly(N-isopropylacrylamide) and contained interpenetrating linear chains of poly(acrylic acid) that functionalize the nanoparticles of TiO(2). Because these microcomposites settle more than a hundred times faster than freely suspended TiO(2) nanoparticles, they are extremely useful for simple gravity separation of the photocatalyst in applications that employ titania nanoparticles. Methyl orange (MO) was used as a model contaminant to investigate the degradation kinetics using the microcomposites in aqueous suspensions. Kinetics of the photodegradation were evaluated by monitoring the changes in methyl orange concentration using UV-Vis spectroscopy. The photocatalytic behavior of functional microcomposites containing 65 wt% titania was studied and the influence of the solution pH as well as the total titania concentration in solution was explored. The results indicated that pH of the solution changes the surface interactions between the poly(acrylic acid), titania, and methyl orange and this interplay determined the overall degradation kinetics of the chemical contaminants. Nearly identical reaction rate constants were observed in acidic solutions for the microcomposites when compared to freely suspended titania. The latter showed higher rate constants than the microcomposites at a neutral pH. Release of the titania from the microcomposites was observed under basic conditions. Complete degradation of the microcomposites was observed after prolonged (7-13 h) UV irradiation. However, the microcomposites were easily regenerated by addition of microgels and no loss of photocatalytic activity was observed.

Photo-conversion of CO2 using titanium dioxide: enhancements by plasmonic and co-catalytic nanoparticles

Converting carbon dioxide (CO2) to hydrocarbons that can be used as fuels is beneficial from both environmental and economic points of view. In this study, nanoparticles are designed to enhance the photoreduction of CO2 on a titanium dioxide (TiO2) catalyst. An increase in catalytic activity is reported when silver (Ag), platinum (Pt) or bimetallic Ag-Pt and core-shell Ag@silica (SiO2) nanoparticles are used with the TiO2 semiconductor catalyst. Nanoparticles with different elemental composition or geometrical structure facilitate successive photo-excitation steps-generation, transport, storage and interfacial transfer of electrons and holes. Results show that while the addition of either type of nanoparticles augments product formation rates, bimetallic co-catalysts improve product selectivity. When both bimetallic co-catalysts and Ag@SiO2 nanoparticles are used in combination, product yields are enhanced more than seven fold in comparison to native TiO2 and high selectivity for methane (CH4) is observed. When the bimetallic Ag-Pt co-catalysts are tuned, a selectivity of CH4 of approximately 80%, as compared to 20% with only TiO2, can be achieved.

Reversible end-to-end assembly of gold nanorods using a disulfide-modified polypeptide

Directing the self-assembly of colloidal particles into nanostructures is of great interest in nanotechnology. Here, reversible end-to-end assembly of gold nanorods (GNR) is induced by pH-dependent changes in the secondary conformation of a disulfide-modified poly(L-glutamic acid) (SSPLGA). The disulfide anchoring group drives chemisorption of the polyacid onto the end of the gold nanorods in an ethanolic solution. A layer of poly(vinyl pyrrolidone) is adsorbed on the positively charged, surfactant-stabilized GNR to screen the surfactant bilayer charge and provide stability for dispersion of the GNR in ethanol. For comparison, irreversible end-to-end assembly using a bidentate ligand, namely 1,6-hexanedithiol, is also performed. Characterization of the modified GNR and its end-to-end linking behavior using SSPLGA and hexanedithiol is performed using dynamic light scattering (DLS), UV-vis absorption spectroscopy and transmission electron microscopy (TEM). Experimental results show that, in a colloidal solution of GNR-SSPLGA at a pH∼3.5, where the PLGA is in an α-helical conformation, the modified GNR self-assemble into one-dimensional nanostructures. The linking behavior can be reversed by increasing the pH (>8.5) to drive the conformation of the polypeptide to a random coil and this reversal with pH occurs rapidly within minutes. Cycling the pH multiple times between low and high pH values can be used to drive the formation of the nanostructures of the GNR and disperse them in solution.

CONTROLLING MOLECULAR ORDER IN HAIRY-ROD LANGMUIR-BLODGETT-FILMS - A POLARIZATION-MODULATION MICROSCOPY STUDY

The interplay of molecular weight, layer thickness, and thermal annealing in controlling molecular order in ultrathin Langmuir-Blodgett films is characterized with the use of polarization-modulation laser-scanning microscopy. The degree and direction of molecular alignment can be imaged rapidly and sensitively through the magnitude and orientation of linear dichroism in Langmuir-Blodgett films of rodlike poly(phthalocyaninatosiloxane) (PcPS). Images are presented for films as thin as two molecular layers (∼44 angstroms). Molecular alignment along the transfer direction is much stronger for films of PcPS with ∼25 repeat units (∼10 nanometers long) than for those with ∼50 repeat units (∼20 nanometers long). Enhancement of alignment by thermal annealing is also much greater for PcPS-25 than PcPS-50. Intimate interaction with the substrate suppresses improvement in alignment by annealing, evident by an anomalously small increase in anisotropic absorption of the first two layers.

Bisphosphonate-modified gold nanoparticles: a useful vehicle to study the treatment of osteonecrosis of the femoral head

Legg-Calvé-Perthes disease (LCPD) is a juvenile form of osteonecrosis of the femoral head that presents in children aged 2-14 years. To date, there is no effective medical therapy for treating LCPD largely due to an inability to modulate the repair process, including the predominance of bone resorption. This investigation aims to evaluate the feasibility of using gold nanoparticles (GNPs) that are surface modified with a bisphosphonate compound for the treatment of osteonecrosis at the cellular level. Studies have found osteoclast-mediated resorption to be a process that contributes significantly to the pathogenesis of femoral head deformities arising from Perthes disease. Our in vitro model was designed to elucidate the effect of alendronate-(a bisphosphonate) modified GNPs, on osteoclastogenesis and osteoclast function. RAW 264.7 macrophage cells were cultured with recombinant mouse receptor activator of NF-κB ligand (RANKL), which stimulates osteoclastogenesis, and were then treated with alendronate-modified GNPs for 24, 48, and 72 h. Cell proliferation, osteoclast function, and osteoclast morphology were evaluated by trypan blue dye exclusion assay, tartrate-resistant acid phosphatase (TRAP) staining, and transmission electron microscopy (TEM) imaging. Comparative studies were performed with GNPs that were only stabilized with citrate ions and with alendronate alone. Neither osteoclastogenesis nor osteoclast function were adversely affected by the presence of the citrate-GNP. Alendronate-modified GNPs had an enhanced effect on inducing osteoclast apoptosis and impairing osteoclast function when compared to unbound alendronate populations.

Polarization modulation laser scanning microscopy: A powerful tool to image molecular orientation and order

To image the orientational order in a broad class of biological and manufactured materials, a new microscope has been developed that integrates laser scanning microscopy with polarization modulation polarimetry. Polarimetry allows quantitative characterization of the molecular orientation and the degree of order through characterization of optical anisotropy. Combined with laser scanning microscopy, it is used here to image the anisotropy with high spatial resolution, sensitivity, and speed. The design of the microscope is presented; and the vast improvement in sensitivity achieved using PM-LSM over conventional polarization microscopy is illustrated by imaging the linear dichroism of ultrathin Langmuir–Blodgett polymer films. PM-LSM allows imaging of the magnitude and orientation of linear dichroism in films as thin as three molecular layers (~66 A) at high resolution by rastering a diffraction limited spot of laser light across the sample. The rate of image acquisition is over 2000 pixels/s, two to three orders of magnitude faster than the previous methods of imaging optical anisotropy.

Concentration‐dependent effects of alendronate and pamidronate functionalized gold nanoparticles on osteoclast and osteoblast viability

Severe osteoporotic diseases, such as Pagets disease, Osteogenesis Imperfecta, and Legg Calve Perthes disease, lack treatments that address the pathobiology of the diseases, as well as, long-term and prospective studies. Bisphosphonates, which are known to dramatically hinder the viability of osteoclast cells, along with gold nanoparticles (GNP) are a potential theranostic for osteoporotic diseases. We evaluated GNP functionalized with two different bisphosphonates, namely, alendronate and pamidronate. RANKL differentiated murine pre-osteoclasts (Raw 264.7) and murine osteoblasts (7F2) were treated with varying concentrations ranging from 0.1-5 µM of free and GNP bound bisphosphonates. GNPs with an average size of ∼15 nm were functionalized with alendronate and pamidronate through surface modification by self-assembly. MTT viability assay results show no changes in viability of the osteoclasts when treated with free bisphosphonates in the range of 1-5 µM, but significant decrease on treatment with functionalized GNP at concentrations above the range of 0.1-1 µM depending on the bisphosphonate. Osteoblast cell viability is maintained at all but the highest concentrations used. Qualitative and quantitative characterization by Western Blot for RANKL expression in the osteoblast cell line shows that expression is largely maintained. These results provide a basis for methods that use bisphosphonate functionalized GNP in the treatment of osteoporotic bone diseases.

Chemical Mechanical Polishing: Role of Polymeric Additives and Composite Particles in Slurries

Publisher Summary This chapter highlights the role of polymeric additives in the slurry and the use of novel composite abrasive particles incorporating polymers as promising alternatives to traditional inorganic oxide nanoparticles. Polymeric additives are of fundamental importance to the planarization process as they can control the aggregation, dispersion, surface hardness, and stability of the inorganic oxide nanoparticles. Abrasive composites range from simple inorganic particles surface decorated with organic moieties to compressible polymeric microspheres that incorporate inorganic oxide nanoparticles. Typically, slurries used in mechanical polishing/planarization (CMP) consist of a liquid solvent and a solid dispersant. The liquid phase is generally deionized water with additives like oxidizers, complexing agents inhibitors, and surfactants. The solid phase consists of abrasives, which are typically metal oxides such as alumina (Al2O 3 ), silica (SiO 2 ), and ceria (CeO 2 ). While silica and alumina particles are commonly used for polishing copper and tungsten, silica and ceria are used to polish SiO 2 , poly-Si, and silicon nitride. The stability of the abrasive particles is an important criterion for CMP as particle sedimentation or aggregation often leads to the formation of scratches on the wafer surface after polishing. The CMP process draws upon a broad base of knowledge from the fields of science and engineering, which range across colloidal science, surface and interfacial phenomena, polymer physics and chemistry, tribology, fluid mechanics and dynamics, and surface metrology.