Somdev Tyagi

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).

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.

Chronic wounds represent one of the most serious complications of diabetes. Lack of quantitative assessment of healing progress makes diabetic wound management a clinical challenge. We constructed an optical device based on near infrared diffuse optical spectroscopy and monitored the change in wound optical properties during heating. A single source, four detector frequency domain instrument with multiple wavelengths was employed in a streptozotocin induced diabetic rat animal model. Optical properties including absorption and reduced scattering coefficients were measured. Our results show that there is significant difference in the absorption and reduced scattering coefficient of the wounds between diabetic and controls rats, and such difference persists throughout the healing period. Our technique would be highly useful in monitoring and quantifying the wound healing process.

We report results of our recent efforts to develop nano-tools to study proteins and their interactions in complex environments that exist on the cell membrane and inside the cells. Due to the spatial constraints imposed on the mobility of cell constituents, it is reasonable to expect that the nature and dynamics of the biomolecular interactions in a living cell would be substantially different from those routinely observed in dilute solutions. Nanotechnology has begun to provide tools with which to monitor processes that occur in membranes and intracellular regions. Nano-optics is a rich source of such emerging tools. Tapered optical fibers coated with metallic films can effectively confine excitation light to sub-wavelength linear dimensions and cubic nanometer excitation volumes. This leads not only to a resolution that exceeds the diffraction-limited values, but also to the elimination of the background signal. Thus, highly localized and specific regions of cellular function can be investigated. By immobilizing silver colloidal nanoparticles on such tapered fibers we have also fabricated surface enhanced Raman scattering (SERS) probes. Nanoprobes have been found to enable detection of fluorescent antibody molecules immobilized on a functionalized glass surface and polychromic quantum dots in picomolar solutions. In addition, we have successfully inserted nanoprobes with dimensions of 30-80 nm into both adherent insect and mammalian cells with maintenance of their viability. We summarize our development of optical nanoprobes with the motivation to detect cell-surface and intracellular proteins of the interleukin-5 system in native cellular environments, through quantum dot fluorescence and SERS.

Recently, two of us have developed the technology for preparing homogeneous thin films of the spinel CdCr 2 Se 4 with varying amounts of In impurity. Ferromagnetic resonance (FMR) and spin wave resonance (SWR) measurements at 10 GHz at temperatures (T) varying between 4 and 300 K were used to elucidate the magnetic state -- ferromagnet (FM) re-entrant (REE) or spin glass (SG), of the material as a function of In content. However, it is realized that a thorough understanding is not possible without making resonance measurements at several frequencies. For instance, it is impossible to delineate various contributions to the peak-to-peak linewidth (Γ PP ) without knowing its frequency dependence. The present report is based on magnetic resonance data on thin films of CdCr 2-x In x Se 4 at several frequencies between 3 and 36 GHz and at 77 < T < 200 K. We will also report on the remarkable sensitivity of the resonance parameters to exposure, of the sample, to modest (-1 mW) amounts of light from a tungsten lamp over a period of a few seconds.