Low Raman Cross Section Research Articles

Self-assembled vertically aligned gold nanorod superlattices for ultra-high sensitive detection of molecules

We show that self-assembled vertically aligned gold nanorod (VA-GNRs) superlattices can serve as probes or substrates for ultra-high sensitive detection of various molecules. d-glucose and 2,4,6-trinitrotoluene (TNT) have been chosen as model systems due to their very low Raman cross-sections (5.6 × 10−30 cm2·molecule−1·sr−1 for d-glucose and 4.9 × 10−31 cm2·molecule−1·sr−1 for TNT) to show that the VA-GNR superlattice assembly offers as low as yoctomole sensitivity. Our experiment on mixed samples of bovine serum albumin (BSA) and d-glucose solutions demonstrate sensitivity for the latter, and the possible extension to real samples. Self-assembled superlattices of VA-GNRs were achieved on a silicon wafer by depositing a drop of solvent containing the GNRs and subsequent solvent evaporation in ambient conditions. An additional advantage of the VA-GNR monolayers is their extremely high reproducible morphology accompanied by ultrahigh sensitivity which will be useful in many fields where a very small amount of analyte is available. Moreover the assembly can be reused a number of times after removing the already present molecules. The method of obtaining VA-GNRs is simple, inexpensive and reproducible. With the help of simulations of monolayers and multilayers it has been shown that superlattices can achieve better sensitivity than monolayer assembly of VA-GNRs.

Sensitive Raman spectroscopy of lipids based on drop deposition using DCDR and SERS

Raman spectroscopy is widely used for study of lipids and membrane models. A severe limitation of this technique lies in the low Raman cross section requiring high sample concentrations. We report sensitive detection of synthetic 1,2‐dimyristoyl‐3‐trimethylammonium‐propane (DMTAP) lipid employing two Raman techniques with improved sensitivity: drop coating deposition Raman (DCDR) and surface‐enhanced Raman scattering (SERS) spectroscopies. DCDR provided well‐reproducible DMTAP spectra without considerable loss of its solution properties if measured from the ‘coffee ring’ pattern of a drop dried on a SpectRIMTM plate. DMTAP was detected at ~10 μM initial solution concentration, which is about three orders of magnitude lower than that for conventional Raman spectroscopy. Moreover, SERS spectra from dried ring of Ag hydrosol/DMTAP system were obtained down to ~0.3 μM DMTAP concentration, which means that sensitivity of SERS is about five orders of magnitude higher than that of conventional Raman spectroscopy. In contrast to the DCDR technique, good SERS spectra of DMTAP were obtained only from some spots of the ring containing big nanoparticle aggregates, and the structural properties of DMTAP were significantly perturbed by adsorption on the Ag nanoparticles. Copyright © 2013 John Wiley & Sons, Ltd.

Characterization of SAMs Derived from Octadecyloxyphenylethanethiols by Sum Frequency Generation

Sum frequency generation (SFG) vibrational spectroscopy analyses were performed on self-assembled monolayers (SAMs) formed via the adsorption of 2-(4-(octadecyloxy)phenylethanethiol (R1ArMT), 2-(3,5-bis(octadecyloxy)phenylethanethiol (R2ArMT), and 2-(3,4,5-tris(octadecyloxy)phenyl-ethanethiol (R3ArMT) on gold. SFG spectra showed that the monolayers formed were closely packed and well ordered. It was determined, using orientation analysis, that as additional octadecyloxy chains were attached to the aromatic ring the apparent methyl group tilt increased and conformational order decreased. Aromatic C–H modes exhibited in the SFG spectra suggest the aromatic rings have a C2 axis along the surface normal. Model phenylethanethiols with different symmetries along the benzene ring were synthesized and used to identify vibrational modes and the surface orientation of the alkoxyphenylethanethiols. SAMs derived from R1ArMT exhibit aromatic C–H stretching modes due to the C2v symmetry at the ring. In contrast, SAMs derived from R2ArMT and R3ArMT exhibit no aromatic C–H stretching modes due to their low Raman cross section.

Pure rotational Coherent anti‐Stokes Raman spectroscopy of water vapor and its relevance for combustion diagnostics

In this work, we report for the first time the rotational coherent anti‐Stokes Raman spectroscopy (CARS) spectrum of water vapor, recorded through experiments at 400 K and 670 K for Raman shifts up to 600 cm−1. Using the standard dual‐broadband pure rotational CARS approach, the resonant structure was overwhelmed by a strong non‐resonant background. By employing a polarization technique, this background could be suppressed and the resonant structure discerned. The spectral line structure matched spectra from spontaneous rotational Raman scattering found in the literature. The CARS signal intensity at the highest peak from water vapor at 400 K was more than five orders of magnitude weaker than for N2 because of a low Raman cross section and a larger number of transitions. Due to the weak resonant signal, water vapor is an unsuitable candidate for CARS thermometry and relative concentration measurements in combustion. Still, rotational CARS spectra from product gases in combustion will be affected by the presence of water, which is briefly discussed. Copyright © 2013 John Wiley & Sons, Ltd.

Improving the sensitivity of Raman signal of ZnO thin films deposited on silicon substrate

Raman spectroscopy results of DC reactive sputtering of ZnO thin films deposited on silicon substrates are reported. Monocrystalline silicon wafers are frequently used as substrates for thin film deposition, and when working on thin films materials of rather low Raman cross-sections such as ZnO, it is difficult and even sometimes hopeless to extract the Raman information relevant to the film from the silicon signal.In this paper, we show that using a 457nm wavelength of the laser excitation combined with the effect of silicon Raman selection rules through convenient orientation of the silicon substrate, the 520cm−1 Si signal can be sufficiently attenuated to give access to the ZnO Raman information, whatever the important difference of intensity between them.

Bottom-up optimization of SERS hot-spots

The bottom-up optimization of signal-to-noise ratios for SERS tags employing SAMSA fluorescein as the reporter was achieved using a bifunctional linker with low Raman cross section. By holding the nanoparticles at subnanometer separation, the linker allowed us to optimize the hot-spot thereby significantly reducing the fluorescence background.

SURFACE-ENHANCED RAMAN SCATTERING: METAL NANOSTRUCTURES COATED WITH LANGMUIR-BLODGETT FILMS

This review deals with surface-enhancved Raman scattering (SERS) employing Langmuir-Blodgett (LB) films, which serve as model systems for developing theoretical and experimental studies to elucidate the SERS effect. In addition, LB films have be used as integral parts of molecular architectures for SERS-active substrates. On the other hand, SERS and surface-enhaced resonant Raman scattering (SERRS) have allowed various properties of LB films to be investigated, especially those associated with molecular-level interactions. In the paper, emphasis is placed on single molecule detection (SMD), where the target molecule is diluted on an LB matrix of spectral silent material (low Raman cross section). The perspectives and challenges for combining SERS and LB films are also discussed.

Simple Strategy To Improve Surface-Enhanced Raman Scattering Based on Electrochemically Prepared Roughened Silver Substrates

We develop an easy and effective pathway to improve surface-enhanced Raman scattering (SERS) effects of probe molecules of Rhodamine 6G (R6G) adsorbed on electrochemically prepared roughened Ag substrates. In general SERS studies, SERS-active metal substrates are first prepared. Then probe molecules are adsorbed on them to evaluate the relative SERS effects. In this study, we employ electrochemical oxidation-reduction cycle (ORC) treatments in 0.1 M KCl solutions containing probe molecules of 2 x 10(-5) M R6G to prepare R6G-adsorbed SERS-active Ag substrates for one step. Encouragingly, based on this strategy, the SERS intensity of adsorbed R6G can be increased by 1 order of magnitude, as compared with that of R6G adsorbed on a roughened Ag substrate beforehand, which was generally shown in the literature. Moreover, this improved SERS effect based on this strategy is also effective for 2 x 10(-9) M probe molecules, which is at a level of single molecule detection based on Ag colloids. It is also effective for probe molecules of ClO(4)(-) with low Raman cross section and for other electrochemically prepared SERS-active substrates of Au. Further analyses indicate that the increase in SERS activity in this new method is most likely due to the incorporation of more chloride ions into the substrate.

Comparison of SERS Performances of Co and Ni Ultrathin Films over Silver to Electrochemically Activated Co and Ni Electrodes

In this work, the surface-enhanced Raman scattering (SERS) spectra of pyridine (py) on thin films of Co and Ni electrodeposited on an Ag electrode activated by oxidation−reduction cycles (ORC) are presented. The SERS spectra from the thin films were compared to those of py on activated bare transition metal electrodes. It was verified that the SERS spectra of py on 3 monolayers (ML)-thick films of Ni and Co presented only bands assignable to the py adsorbed on transition metal surfaces. It was also observed that even for 50 ML-thick transition metal films, the py SERS intensity was ca. 40% of the intensity from the 3 ML-thick films. The relative intensities of the SERS bands depended on the thickness of the films, and for films thicker than 7 ML for Co and 9 ML for Ni they were very similar to those of the bare transition metal electrodes. The transition metal thin films over Ag activated electrodes presented SERS intensities 3 orders of magnitude higher than the ones from bare transition metal electrodes. These films are more suitable to study the adsorption of low Raman cross-section molecules than are ORC-activated transition metal electrodes.

Pulsed remote Raman system for daytime measurements of mineral spectra

A remote Raman system has been developed utilizing a 532 nm pulsed laser and gated intensified charged couple device (ICCD) detector in the oblique geometry. When the system is set for 50 m sample distance it is capable of measuring Raman spectra of minerals located at distances in the range of 10–65 m from the telescope. Both daytime and nighttime operations are feasible and the spectra of minerals can be measured in a short period of time, of the order of a few seconds. In oblique geometry, measured sampling depth is more than 30 m, during which the system maintains very high performance without any adjustments. Much longer sampling depth (0.1–120 m) has been observed when the system is configured in the coaxial geometry. Clear advantages of using a gated detection mode over the continuous (CW) mode of operation in reducing the background signal and eliminating long-lived fluorescence signals from the Raman spectra are presented. The performance of the pulsed Raman system is demonstrated by measuring spectra of Raman standards including benzene (C 6H 6) and naphthalene (C 10H 8), a low Raman cross section silicate mineral muscovite (KAl 2(Si 3Al)O 10(OH) 2), and a medium Raman cross section mineral calcite (CaCO 3).

Raman Scattering from Vibrational Modes in Layered CdxHg1–xI2 Solid Solution

AbstractRaman spectra at 15 and 300 K of CdxHg1–xI2 solid solutions for 0 < x < 0.44 are investigated. One mode behaviour is observed for the low frequency phonons and the A1g mode, while a two mode behaviour is exhibited by the higher frequency vibrations. The expected mass, force, and temperature induced splitting of the B1g2 and Eg1 degenerated modes is not observed. This fact proposes another different experimental evidence of the low Raman cross section of the B1g2 rigid layer mode. For the first time the degeneracy of the Eg2 mode with the A1g is lifted and experimentally found.

Use of an unintensified charge-coupled device detector for low-light-level Raman spectroscopy

Spontaneous Raman scattering from adsorbates on surfaces is a powerful vibrational spectroscopy technique that is due to the very large frequency range and high resolution currently possible with visible lasers and spectrometers. Because of the extremely low Raman cross sections of nonresonant molecules, one must use either some sort of signal-enhancement technique or long integrations using multichannel detection in order to have monolayer sensitivity. We have used a backthinned, unintensified, liquid-nitrogen-cooled charge-coupled device (CCD) detector to obtain high-quality multichannel unenhanced Raman scattering spectra from organic monolayers on surfaces. This system has roughly a factor-of-10 greater sensitivity for very low-light-level visible excitation Raman spectra than intensified multichannel detectors currently on the market. A description of our detection system is given along with a comparison with other commercially available multichannel detectors suitable for low-light-level visible Raman spectroscopy. We discuss the advantages and disadvantages of using CCD’s, the special problems that one encounters in their use as Raman detectors, and our methods for overcoming these problems. Prospects for the future use of CCD’s for low-light-level Raman spectroscopy will also be discussed.

Measurement of the molecule–silver separation dependence of surface enhanced Raman scattering in multilayered structures

We report details and results of experiments in which surface enhanced Raman scattering from molecules is studied as a function of their spacing from a rough silver enhancing layer. Two types of experiments were performed. In the first, an organic monolayer chemisorbed on aluminum oxide is covered with a smooth low Raman cross section polymer spacer layer and rough silver is placed on top of the spacer. The Raman scattering from the monolayer is studied as a function of the spacer layer thickness. As a result of our diffusion and tunneling experiments, we conclude that direct contact of the molecules with scattering from the molecules. We observe an enhancement of roughly 106 for no polymer spacer thickness, and a fall-off of the enhancement by a factor of 10 with each subsequent 35–50 Å spacer thickness. We also observe a similar drop off of the broadband background under the Raman scattering. In the second type of experiment, the Raman scattering from a high Raman cross section polymer is studied as a function of its thickness in similar multilayered structures. The results of these variable thickness experiments are consistent with those of the spacer experiments. We conclude that in our samples, the major cause of surface enhanced Raman scattering is electromagnetic in nature.