Somdev Tyagi

During the last few years increased interest has been shown in investigating the amorphous magnetic alloys of transition metals with metalloids prepared by techniques other than the most widely used splat cooling technique. For example, amorphous alloys have been obtained by the spark erosion /1, 2/, gas atomization with water quenching /3/, and the capillary quenching techniques /4/. Powder samples of amorphous Fe^Si^B^g produced by the spark erosion method have recently been investigated by Berkowitz et al. /2/ using X-ray diffraction, Moss-bauer, and low and high-field magnetization measurements. The powder samples consisted of mainly spherical particles in the ranges 0.5 to 5 jam, 10 to 20 jam, and 20 to 30 jam. Compared with the Curie point, Tc = 717K for the ribbon samples obtained by the splat quenching technique, the particles show a gradual decrease in the T^ values with decreasing particle fraction size. The 0.5 to 5 jam range has T^ = 628 K as determined by the high-field magnetization data. This lowering of TQ has been interpreted as due to the decreased chemical short-range ordering (CSRO) caused by the higher quenching rates achieved by 7 6 the spark erosion technique (10 versus 10° K/s for the splat quenching method).

Spin-momentum locking in the surface mode of topological insulators (TI) leads to the surface accumulation of spin-polarized electrons caused by bias current flows through TI samples. Here, we demonstrate that scanning tunneling microscopy can be used to sense this surface spin-polarized electron accumulation. We present experimental results of this sensing for Sn-doped Bi$_2$Se$_3$ samples by employing Fe-coated W tips as well as non-magnetic W tips. We observe a linear increase in the spin-accumulation as a function of bias current through TI samples.

One of the most widely used methods for surface-enhanced Raman scattering (SERS) employs silver or gold nanoparticles either in colloidal suspension or in the dry-drop form. In such substrates the SERS amplification factors depend critically on the interparticle distances. Here, we report that microwave absorption as a function of temperature in dry-drop substrates can be used as a probe to demarcate temperature regions for thermal annealing to produce SERS substrates with very high amplification factors. Copyright (c) 2011 John Wiley & Sons, Ltd.

In order to analyze a sample using SERS, the analyte has to be brought in intimate contact with the substrate. This can be problematic when, let's say, the molecules of interest in trace amounts are located in large volumes. For example a biotoxin aerosol in a large room or a trace amount of bio-hazardous substances mixed in large volumes of water or other liquids. In principle it is possible to filter out the molecules of interest and then deposit them on the SERS substrate for further analyses. In practice this is very cumbersome and therefore is rarely used. Here we discuss flexible and porous SERS substrates that have been fabricated by depositing silver nano-particle inks on woven or spun fabrics made of glass fiber or cellulose followed by thermal annealing at 170-200 degrees C for 10-15 minutes. Use of microwave absorption at about 10 GHz in the polymer-nanoparticle matrix to monitor the sintering process and to optimize the SERS amplification is also discussed. By varying the annealing time, different levels of nanoparticle clustering and the consequent SERS amplification can be achieved. Sampling of large volumes using the SERS filter substrates to detect airborne molecules is also discussed.

Metallic nanoparticle inks - colloidal suspensions of silver or gold nanoparticles in water or other organic solvents - can be sintered at relatively low temperatures (70 - 200 degrees C). With appropriate thermal treatment the sintering can be controlled to fabricate nanoparticle substrates with a distribution of clusters sizes and interparticle distances. Such substrates exhibit relatively high (10(8) - 10(9)) surface enhanced Raman scattering (SERS) amplification factors (AFs). The high AFs in such substrates arise from several mechanisms. The 'dimers' - two nanoparticles separated by a nanometer-size gap - are known to produce amplification of the local electric field orders of magnitude larger than at the surface of an isolated single nanoparticle due to surface plasmon resonance. Furthermore, the lack of translational symmetry in the clusters leads to localizations of electromagnetic excitations to very small regions that can create SERS hot spots. Here we report that microwave absorption (similar to 10 GHz) as a function of thermal annealing in dry-drop substrates can be used to monitor the sintering process in metallic nanoparticle inks. The predominant contribution to microwave absorption comes from electrically resistive weak links that are formed between nanoparticles as a result of the thermal treatment. Just before the creation of these weak links, such nanoparticle pairs are also the ones that make a major contribution to the SERS AFs. This leads to a correlation between the observed microwave absorption and the SERS signal intensities. We also present a simple model that describes the microwave absorption as a function of the isothermal annealing treatment.

Hyaluronic acid (HA) is a high molecular weight glycosaminoglycan found in the extracellular matrix and joints in the body. Elevated levels in the serum are associated with liver disease so detection at concentrations in the microgram per liter range is useful for monitoring cirrhosis progression. Conventional methods can measure HA in this range but they take several days and require multiple preparation steps. Surface-enhanced Raman scattering (SERS) could be an alternative as it yields specific signatures stemming from molecular vibrations and has been shown to provide large enhancement factors. However, due to its intrinsic negative charge HA does not readily adsorb on metallic surfaces. To overcome this, a functionalized SERS substrate was developed to immobilize HA. A cysteamine self-assembling monolayer in the trans conformation allows the HA carboxyl group to attach to the ligand's amine group. The SERS signal can be used to monitor the concentration of cysteamine trans conformers in order to optimize HA attachment. In addition, SERS analysis shows an increase in HA Raman band intensity when immobilized to the substrate as compared to free HA on the substrate. Correlations between HA concentration and Raman band intensity are also discussed.

Surface enhanced Raman scattering (SERS) is now a well-established technique to greatly amplify the normally weak Raman scattering signals. The amplification is achieved by using SERS substrates - specially structured metallic substrates with nano-scale morphological features. One of the most widely used methods for SERS amplification employs nanoparticles of silver or gold either in colloidal suspension or in dry-drop form. In such substrates SERS amplification factors (AF) exceeding 10(12) have been reported. The reproducibility of the colloid-based substrates, however, is a problem. The lack of reproducibility can be caused by a variety of factors that can change the interparticle distances. In this paper we show that thermal annealing of SERS substrates fabricated using commercially available nano-particle inks can be used to create thermally stable substrates with high reproducibility. It appears that thermal annealing destroys the unstable hot-spots with very high AF's but still leaves the sample with high AF sites yielding spatially averaged substrate AF's exceeding 10(8).

Changes of optical properties of wound tissue in hairless rats were quantified by diffuse photon density wave methodology at near-infrared frequencies. The diffusion equation for semi-infinite media was used to calculate the absorption and scattering coefficients based on measurements of phase and amplitude with a frequency domain device. There was an increase in the absorption and scattering coefficients and a decrease in blood saturation of the wounds compared with the nonwounded sites. The changes correlated with the healing stage of the wound. The data obtained were supported by immunohistochemical analysis of wound tissue. These results verified now by two independent animal studies could suggest a noninvasive method to detect the progress of wound healing.

Objective and Design: Myeloperoxidase (MPO) and proinflammatory cytokines play an important role in the development of inflammation. These markers are generally measured using tedious ELISA procedures. In this study, a novel technique utilizing antibody conjugated quantum dot nanoparticles was developed to detect myeloperoxidase, IL-1 α and TNF-α in vivo in the dextran sodium sulfate (DSS) model of experimental colitis. Materials and Methods: Colitis was induced in animals (n=8 animals/ group) by feeding 4% DSS solution ad libitum for seven to eight days. Quantum Dots exhibiting fluorescence at various wavelengths were conjugated to MPO, IL-1 α and TNF-α polyclonal antibodies and tested in vivo at various stages of colitis. Tissue sections obtained were imaged with confocal microscope. The image intensity obtained from the tissue specimen was correlated with clinical activity measured as Disease Activity Index (DAI). Results: Myeloperoxidase, IL-1α and TNF-α were visualized with quantum dots on various days of disease. The intensity of quantum dots increased with increase in inflammation. The increase in intensity showed an excellent correlation with the DAI based on the clinical parameters. Conclusion: The study demonstrated that multiple biomarkers can be detected simultaneously and their quantitative expression correlated well with clinical disease severity. This novel technology should facilitate design of a novel optical platform for imaging various biomarkers of inflammation, early detection of acute and chronic disease markers and inflammation-mediated cancer markers. This detection may also facilitate determination of therapeutic success.

Raman spectroscopy is now a well-established analytical tool for obtaining rapid and compound specific information for chemical analysis. However, Raman scattering - inelastic scattering of photons - cross sections are typically of the order of 10(-30) cm(2) per molecule and thus Raman signals are usually weak. In Surface Enhanced Raman Scattering (SERS) the signals can be greatly amplified by using specially structured metallic (usually Ag, Au, and Cu) substrates. SERS substrates can be fabricated by a variety of methods. Here, we report a method for fabricating SERS substrates from commercially available silver nanoparticle based printing inks. For dilute inks (similar to 1-2% Ag by weight) the method involves the airbrushing of inks on heated (similar to 100 degrees C) quartz or polymer substrates followed by heating at 170 degrees C for about 20 minutes. The heating treatment removes the polymer coating used to prevent aggregation of Ag particles in the colloidal suspension and allows partial sintering of particles. More concentrated inks (similar to 20 - 30% Ag by weight) can be applied to various substrates at room temperature followed by the thermal treatment. SERS spectra of Rhodamine 6G, and beta-carotene molecules are reported. SERS amplification factors of more than 10(6) can be easily obtained reproducibly.