Ramasis Goswami

Cellular Uptake and Fate of PEGylated Gold Nanoparticles Is Dependent on Both Cell-Penetration Peptides and Particle Size

Numerous studies have examined how the cellular delivery of gold nanoparticles (AuNPs) is influenced by different physical and chemical characteristics; however, the complex relationship between AuNP size, uptake efficiency and intracellular localization remains only partially understood. Here we examine the cellular uptake of a series of AuNPs ranging in diameter from 2.4 to 89 nm that are synthesized and made soluble with poly(ethylene glycol)-functionalized dithiolane ligands terminating in either carboxyl or methoxy groups and covalently conjugated to cell penetrating peptides. Following synthesis, extensive physical characterization of the AuNPs was performed with UV-vis absorption, gel electrophoresis, zeta potential, dynamic light scattering, and high resolution transmission electron microscopy. Uptake efficiency and intracellular localization of the AuNP-peptide conjugates in a model COS-1 cell line were probed with a combination of silver staining, fluorescent counterstaining, and dual mode fluorescence coupled to nonfluorescent scattering. Our findings show that AuNP cellular uptake is directly dependent on the surface display of the cell-penetrating peptide and that the ultimate intracellular destination is further determined by AuNP diameter. The smallest 2.4 nm AuNPs were found to localize in the nucleus, while intermediate 5.5 and 8.2 nm particles were partially delivered into the cytoplasm, showing a primarily perinuclear fate along with a portion of the nanoparticles appearing to remain at the membrane. The 16 nm and larger AuNPs did not enter the cells and were located at the cellular periphery. A preliminary assessment of cytotoxicity demonstrated minimal effects on cellular viability following peptide-mediated uptake.

One-Phase Synthesis of Water-Soluble Gold Nanoparticles with Control over Size and Surface Functionalities

We report a simple and efficient synthetic method to prepare gold nanoparticles (AuNPs) in aqueous phase using HAuCl(4) and poly(ethylene glycol) (PEG) ligands appended with bidentate anchoring groups. Our approach provides narrow size distribution nanocrystals over the size range between 1.5 and 18 nm; this range is much wider than those achieved using other small molecules and polymer ligands. The NP size was simply controlled by varying the molar ratio of Au-to-PEG ligand precursors. Further passivation of the as-prepared AuNPs permitted in situ functionalization of the NP surface with the desired functional groups. The prepared AuNPs exhibit remarkable stability in the presence of high salt concentrations, over a wide range of pHs (2-13), and a strong resistance to competition from dithiothreitol (DTT). These results are a clear manifestation of the advantages offered by our synthetic approach to prepare biocompatible AuNPs, where modular, multifunctional ligands presenting strong anchoring groups and hydrophilic PEG chains are used.

Determination of crystallite size in a magnetic nanocomposite using extended x-ray absorption fine structure

An extended x-ray absorption fine structure was collected for a soft magnetic material comprising very fine nanoscale crystallites of nickel within coarser iron matrix grains. Using a simple spherical model and the spectra of bulk standards, the nickel crystallite size was estimated. Comparison with transmission electron microscopy images confirms that this technique yields a size weighted toward smaller crystallites, whereas Scherrer analysis yields sizes weighted toward larger crystallites. The iron crystallite size was also estimated by this technique in order to ascertain the effect of a nonspherical morphology. This technique shows promise for in situ analyses of materials containing nanoscale crystallites and as a complement to Scherrer analyses.

A survey of sensitization in 5xxx series aluminum alloys

The 5xxx series (Al-Mg-based) aluminum alloys suffer from intergranular corrosion and intergranular stress corrosion cracking when the alloy has become “sensitized.” Sensitization refers to insidious precipitation of β phase (Mg2Al3), which is problematic when present at grain boundaries. The β phase is electrochemically active and may preferentially dissolve. This paper reviews the relevant works that have documented the degree of sensitization for various 5xxx series alloys, providing a holistic overview of the issue, along with attention to the bulk composition, heat treatment, and microstructure.

Microwave-induced transformation of rice husks to SiC

Samples of rice husks were transformed to β (3C)-SiC by microwave processing in controlled conditions of temperature and vacuum. This simple and fast way of producing powdered samples of silicon carbide is technologically important if this material is to be used for electronics, sensors, biotechnology, and other applications. Using x-ray diffraction it was found that the microwave processed sample at 1900 °C consists of β (3C)-SiC phase. Raman scattering measurements confirmed the formation of β (3C)-SiC phase. Transmission electron microscopy revealed the presence of stacking faults along the [111] direction. The presence of 6H/4H stacking faults in 3C phase is explained in terms of their total energies. The presence of these stacking faults with a ∼1 eV band offset between the host 3C and hexagonal stacking faults implies that these stacking faults provide a conduction barrier, and the interfaces between the stacking faults and host lattice act as a heterojunction that may provide potential utility for ...

Energy Transfer Sensitization of Luminescent Gold Nanoclusters: More than Just the Classical Förster Mechanism

Luminescent gold nanocrystals (AuNCs) are a recently-developed material with potential optic, electronic and biological applications. They also demonstrate energy transfer (ET) acceptor/sensitization properties which have been ascribed to Förster resonance energy transfer (FRET) and, to a lesser extent, nanosurface energy transfer (NSET). Here, we investigate AuNC acceptor interactions with three structurally/functionally-distinct donor classes including organic dyes, metal chelates and semiconductor quantum dots (QDs). Donor quenching was observed for every donor-acceptor pair although AuNC sensitization was only observed from metal-chelates and QDs. FRET theory dramatically underestimated the observed energy transfer while NSET-based damping models provided better fits but could not reproduce the experimental data. We consider additional factors including AuNC magnetic dipoles, density of excited-states, dephasing time, and enhanced intersystem crossing that can also influence ET. Cumulatively, data suggests that AuNC sensitization is not by classical FRET or NSET and we provide a simplified distance-independent ET model to fit such experimental data.

Formation of Nanodimensional 3C-SiC Structuresfrom Rice Husks

We have demonstrated that large quantities of β-SiC nanostructures can be obtained from rice husk agricultural waste by using controlled conditions in a thermogravimetric setup. This simple and inexpensive method of producing these structures on a large scale is critical for applications in nanoelectronics, nanosensors, and biotechnology. The temperature and atmosphere are two critical elements in forming either α-cristobalite (SiO2) or β-SiC. Using different characterization methods (x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy), we have shown that pyrolysis of rice husks in argon atmosphere at 1375°C results in simultaneous formation of carbon nanotubes, β-SiC nanowires/nanorods, and β-SiC powder.

Substitution of silicon within the rhombohedral boron carbide (B4C) crystal lattice through high-energy ball-milling

Boron carbide (B4C) is a ceramic with a structure composed of B12 or B11C icosahedra bonded to each other and to three (C and/or B)-atom chains. Despite its excellent hardness, B4C fails catastrophically under shock loading, but substituting other elements into lattice sites may change and possibly improve its mechanical properties. Density functional theory calculations of elemental inclusions in the most abundant polytypes of boron carbide, B12-CCC, B12-CBC, and B11Cp-CBC, predict that the preferential substitution site for metallic elements (Be, Mg and Al) is the chain center atom and that for non-metallic elements (N, P and S) it is generally the chain end atom of the three-atom chain in B4Cs rhombohedral crystal lattice. However, Si, a semi-metal, seems to prefer the chain center in B12-CCC and icosahedral polar sites in both B12-CBC and B11Cp-CBC. As a first step to testing the feasibility of elemental substitutions experimentally, Si atoms were incorporated into B4C at low temperatures (∼200–400 °C) by high-energy ball-milling. High-resolution transmission electron microscopy showed that the Si atoms were uniformly dispersed in the product, and the magnitude of the lattice expansion and Rietveld analysis of the X-ray diffraction data were analyzed to determine the likely sites of Si substitution in B4C. Further corroborative evidence was obtained from electron spin resonance spectroscopy, magic-angle spinning nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy characterizations of the samples. Thus, a simple, top-down approach to manipulating the chemistry of B4C is presented with potential for generating materials with tailored properties for a broad range of applications.

Selective switching of GaN polarity on Ga-polar GaN using atomic layer deposited Al2O3

Patterned layers of Al2O3 prepared by atomic layer deposition (ALD) are used to produce structures of alternating polarity (c-plane orientation) of GaN on Ga-polar GaN epilayers. Annealing prior to epitaxial growth allows the amorphous ALD layers to attain sufficient crystallinity, which enables N-polar GaN growth over the annealed ALD Al2O3 and Ga-polar growth over the bare substrate. Transmission electron microscopy and electron channeling contrast imaging were carried out to characterize the structural nature of the material and confirm crystallinity. ALD Al2O3 may be suitable for fabricating novel variable-polarity devices, particularly where growth on native, Ga-polar GaN substrates is beneficial.

Probing the Quenching of Quantum Dot Photoluminescence by Peptide-Labeled Ruthenium(II) Complexes.

Charge transfer processes with semiconductor quantum dots (QDs) have generated much interest for potential utility in energy conversion. Such configurations are generally nonbiological; however, recent studies have shown that a redox-active ruthenium(II)–phenanthroline complex (Ru2+-phen) is particularly efficient at quenching the photoluminescence (PL) of QDs, and this mechanism demonstrates good potential for application as a generalized biosensing detection modality since it is aqueous compatible. Multiple possibilities for charge transfer and/or energy transfer mechanisms exist within this type of assembly, and there is currently a limited understanding of the underlying photophysical processes in such biocomposite systems where nanomaterials are directly interfaced with biomolecules such as proteins. Here, we utilize redox reactions, steady-state absorption, PL spectroscopy, time-resolved PL spectroscopy, and femtosecond transient absorption spectroscopy (FSTA) to investigate PL quenching in biological assemblies of CdSe/ZnS QDs formed with peptide-linked Ru2+-phen. The results reveal that QD quenching requires the Ru2+ oxidation state and is not consistent with Förster resonance energy transfer, strongly supporting a charge transfer mechanism. Further, two colors of CdSe/ZnS core/shell QDs with similar macroscopic optical properties were found to have very different rates of charge transfer quenching, by Ru2+-phen with the key difference between them appearing to be the thickness of their ZnS outer shell. The effect of shell thickness was found to be larger than the effect of increasing distance between the QD and Ru2+-phen when using peptides of increasing persistence length. FSTA and time-resolved upconversion PL results further show that exciton quenching is a rather slow process consistent with other QD conjugate materials that undergo hole transfer. An improved understanding of the QD–Ru2+-phen system can allow for the design of more sophisticated charge-transfer-based biosensors using QD platforms.