Probing the effect of molecular orientation on the intensity of chemical enhancement using graphene-enhanced Raman spectroscopy

A rational approach to investigate the effect of molecular orientation on the intensity of chemical enhancement using graphene-enhanced Raman spectroscopy (GERS) is developed. A planar molecule, copper phthalocyanine (CuPc), is used as probe molecule. Annealing allows the CuPc molecule in a Langmuir-Blodgett film to change orientation from upstanding to lying down. The UV-visible absorption spectra prove the change of the molecular orientation, as well as the variation of the interaction between the CuPc molecule and graphene. The Raman spectra of the molecule in the two different orientations are compared and analyzed. The results show that chemical enhancement is highly sensitive to the molecular orientation. The different molecular orientations induce different magnitudes of the interaction between the molecule and graphene. The stronger the interaction, the more the Raman signal is enhanced. Furthermore, the sensitivity of GERS to molecular orientation is promising to determine the orientation of the molecule on graphene. Based on this molecular orientation sensitive Raman enhancement, quantitative calculation of the magnitude of the chemical enhancement is implemented for a series of Pc derivatives. It shows that the magnitude of the chemical enhancement can be used to evaluate the degree of interaction between the molecules and graphene. Moreover, an understanding of the chemical enhancement in GERS is promoted and the effect of molecular orientation on the intensity of chemical enhancement is explained.     

Study on stress evolution in SiC particles during crack propagation in cast hybrid metal matrix composites using Raman spectroscopy

The evolution of stress in the SiC particles during crack propagation under monotonic loading in a cast hybrid MMC was investigated by micro Raman spectroscopy. The experiment was carried out in situ in the Raman spectroscopy. Experimental results showed that cracks due to monotonic loading propagated by the debonding of the particle/matrix interface and particle fracture. Secondary cracks those formed in front of the main crack tip coalesced with the main crack in subsequent loading and final failure occurred. A high decrease in stress (several hundreds in MPa) was observed with the interfacial debonding at the interface and with the particle fracture on the particle. Moreover, the critical tensile stresses for particle–matrix interface debonding and particle fracture developed in hybrid MMC were also estimated during the crack propagation.     

Discrimination of bacteria using surface-enhanced Raman spectroscopy

Raman spectroscopy has recently been shown to be a potentially powerful whole-organism fingerprinting technique and is attracting interest within microbial systematics for the rapid identification of bacteria and fungi. However, while the Raman effect is so weak that only approximately 1 in 10(8) incident photons are Raman scattered (so that collection times are in the order of minutes), it can be greatly enhanced (by some 10(3)-10(6)-fold) if the molecules are attached to, or microscopically close to, a suitably roughened surface, a technique known as surface-enhanced Raman scattering (SERS). In this study, SERS, employing an aggregated silver colloid substrate, was used to analyze a collection of clinical bacterial isolates associated with urinary tract infections. While each spectrum took 10 s to collect, to acquire reproducible data, 50 spectra were collected making the spectral acquisition times per bacterium approximately 8 min. The multivariate statistical techniques of discriminant function analysis (DFA) and hierarchical cluster analysis (HCA) were applied in order to group these organisms based on their spectral fingerprints. The resultant ordination plots and dendrograms showed correct groupings for these organisms, including discrimination to strain level for a sample group of Escherichia coli, which was validated by projection of test spectra into DFA and HCA space. We believe this to be the first report showing bacterial discrimination using SERS.     

Encapsulation of indomethacin using coaxial ultrasonic atomization followed by solvent evaporation

We have encapsulated indomethacin into poly (lactide-co-glycolide) (PLGA) using coaxial ultrasonic atomization technique. The specific aims of this study were to evaluate the effect of drug loading and a change in relative concentration of polymer in the inner and outer layers of coflowing spray liquids on the physicochemical characteristics of the particles. Indomethacin, a non steroidal anti-inflammatory drug, was selected as a model compound. The micro/nanocapsules prepared using a drug free PLGA solution as an outer layer showed higher encapsulation efficiency. Thermal analysis of the formulations indicated that indomethacin was dissolved within the PLGA matrix. The formulations prepared with 25mg indomethacin showed relatively smaller particle size compared with the formulations prepared with 50 mg indomethacin. The particles, in general, showed bi- and tri-modal distribution. Irrespective of the compositions of the liquids 1 and 2, all the particles were smooth and spherical. A cross-section view of the particles revealed the presence of three different internal morphologies. These formulations were a mixture of hollow or solid spheres, and single or multiple spheres encapsulated into a larger sphere. To the best of our knowledge, this is the first study revealing the cross-sectional view of particles prepared with coaxial ultrasonic atomization technique.     

Molecular weight dependent structural regimes during the electrospinning of PVA

The cumulative effects of molecular weight and concentration on the structural transitions in the electrospun polymer have been studied. Experiments have been conducted with water as the solvent for molecular weights of polyvinylalcohol (PVA) ranging from 9500 g/mol to 155,000 g/mol. The structural regimes for beads, beaded fibers, complete fibers and flat ribbons have been mapped. The development of a stable fiber structure generally corresponds to the onset of significant molecular entanglements.     

Processes and states during polymer film formation by simultaneous crosslinking and solvent evaporation

Coating film formation with simultaneous crosslinking and solvent evaporation, accompanied by passage of the polymer film through glass transition region, is a complex process by which temporary or permanent anisotropic and gradient network structures can be formed. Evaporation and crosslinking are processes that are interdependent. The changes in structure (growth of branched molecules and network evolution) are a function of reaction kinetics, which gets diffusion controlled when the system passes through the glass transition region. Structural changes are determined by branching, gelation, and network build-up and depend on the architecture of network precursors. Thermodynamic interactions of polymer with solvents affect the solvent activity which determines the vapor pressure of the solvent over the film and thus the evaporation rate. The glass transition temperature increases as a result of both the decreasing solvent content and conversion of functional groups into bonds. By interplay of these two factors more or less solvent can be locked in by vitrification. The roles and intensity of these basic processes and interrelations are discussed. Some older results are reviewed and new experimental evidence is added. The interrelations are illustrated by time dependences of solvent evaporation and conversion of functional groups for solvent-based high-solids polyurethane systems composed of a hydroxyfunctional star oligomer and triisocyanate and by the role of the ratio of evaporation to crosslinking rates. Evidence was obtained of gradient formation in which appearance of a glassy surface layer is an important event in the history of film formation that determines solvent retention and other film characteristics.     

Detection of alkaline phosphatase using surface-enhanced Raman spectroscopy

A new approach was developed to detect the activity of alkaline phosphatase (ALP) enzyme at ultralow concentrations using a surface-enhanced Raman scattering (SERS) technique. The approach is based on the use of gold nanoparticles as a SERS material whereas 5-bromo-4-chloro-3-indolyl phosphate (BCIP) is used as a substrate of ALP. The enzymatic hydrolysis of BCIP led to the formation of indigo dye derivatives, which were found to be highly SERS active. For the first time, we were able to detect ALP at a concentration of approximately 4 x 10(-15) M or at single-molecule levels when ALP was incubated with BCIP for 1 h in the Tris-HCl buffer. The same technique also was successfully employed to detect surface-immobilized avidin, and a detection limit of 10 ng/mL was achieved. This new technique allows the detection of both free and labeled ALP as a Raman probe in enzyme immunoassays, immunoblotting, and DNA hybridization assays at ultralow concentrations.     

Flow injection analysis and real-time detection of RNA bases by surface-enhanced Raman spectroscopy.

Surface-enhanced Raman scattering (SERS) spectroscopy has been successfully interfaced with a flow injection analysis system to detect RNA bases in real time. Four of the major base components of RNA, uracil, cytosine, adenine, and guanine, were introduced into the flow injection system and were mixed with a Ag sol prior to SERS measurements. Several experimental parameters including pH, temperature, flow rate, and tubing materials were examined, and their impact on the SERS spectra is presented here. The feasibility of interfacing flow injection based SERS detection methods with liquid or high-performance liquid chromatography for the detection of individual components in a complex mixture is also assessed.     

Lactate and sequential lactate-glucose sensing using surface-enhanced Raman spectroscopy

Lactate production under anaerobic conditions is indicative of human performance levels, fatigue, and hydration. Elevated lactate levels result from several medical conditions including congestive heart failure, hypoxia, and diabetic ketoacidosis. Real-time detection of lactate can therefore be useful for monitoring these medical conditions, posttrauma situations, and in evaluating the physical condition of a person engaged in strenuous activity. This paper represents a proof-of-concept demonstration of a lactate sensor based on surface-enhanced Raman spectroscopy (SERS). Furthermore, it points the direction toward a multianalyte sensing platform. A mixed decanethiol/mercaptohexanol partition layer is used herein to demonstrate SERS lactate sensing. The reversibility of the sensor surface is characterized by exposing it alternately to aqueous lactate solutions and buffer without lactate. The partitioning and departitioning time constants were both found to be approximately 30 s. In addition, physiological lactate levels (i.e., 6-240 mg/dL) were quantified in phosphate-buffered saline medium using multivariate analysis with a root-mean-square error of prediction of 39.6 mg/dL. Finally, reversibility was tested for sequential glucose and lactate exposures. Complete partitioning and departitioning of both analytes was demonstrated.     

Measurement of polystyrene nanospheres using excimer laser fragmentation fluorescence spectroscopy

Monodisperse polystyrene nanospheres with a mean diameter of 102 nm are photofragmented with 193 nm light in N2 at laser fluences from 1 to 20 J/cm2. Carbon atom fluorescence at 248 nm from the disintegration of the particles is used as a signature of the polystyrene. The normalized fluorescence signals are self-similar with an exponential decay lifetime of approximately 10 ns. At fluences above 17 J/cm2, optical breakdown occurs and a strong continuum emission is generated that lasts significantly longer. A non-dimensional parameter, the photon-to-atom ratio (PAR), is used to interpret the laser-particle interaction energetics. Carbon fluorescence from polystyrene particles is compared with that from soot, and a similarity between the two particles is observed when normalized with PAR. Carbon emission from bulk polystyrene was also measured. Similar emission signals were observed, but the breakdown threshold of the surface is significantly lower at 0.2 J/cm2.     

Review Polarised Raman spectroscopy for the study of molecular orientation distributions in polymers

Polarised Raman spectroscopy is a vibrational spectroscopic technique that is used widely for the chemical and physical analyses of materials since it is both non-destructive and suitable for remote analysis. In particularly over the last 40 years, the technique has been developed and applied for the study of molecular orientation distributions in polymers. Compared to other analytical techniques, polarised Raman spectroscopy has the following advantages, (1) quantitative and precise measurement of molecular orientation distributions, and (2) study of these distributions in both the crystalline and amorphous phases. Knowledge obtained from the technique is of both academic and industrial interest to study relationships between microstructure and macroscopic physical properties in polymers. In this paper, polarised Raman spectroscopy is reviewed with regard to the study of molecular orientation distributions in polymeric materials. The basis of polarised Raman scattering is first described, and this is followed by the procedure for obtaining spectra. It is shown how Raman scattering intensities for different polarised scattering geometries can be interpreted to give parameters and functions representing quantitative measures of the degree of molecular orientation. Factors affecting the evaluation of these parameters are also summarised. Finally, the usefulness of the technique is demonstrated by practical applications including a study of molecular orientation distributions in poly(ethylene terephthalate) (PET) fibres.     

Characterization of microorganisms using UV resonance Raman spectroscopy and chemometrics

The past decade has seen an increased interest in the application of several physicochemical analytical techniques for the rapid detection and identification of microorganisms. We report the development of UV resonance Raman (UVRR) spectroscopy for the reproducible acquisition of information rich Raman fingerprints from endospore-forming bacteria belonging to the genera Bacillus and Brevibacillus. UVRR was conducted at 244 nm, and spectra were collected in typically 60 s. Cluster analyses of these spectra showed that UVRR spectroscopy could be used to discriminate between these microorganisms to species level, and the clustering pattern from this phenotypic classification was highly congruent with phylogenetic trees constructed from 16S rDNA sequence analysis. Therefore, we conclude that UVRR spectroscopy when coupled with chemometrics constitutes a powerful approach to the characterization and speciation of microorganisms.     

Discrimination of bacteria and bacteriophages by Raman spectroscopy and surface-enhanced Raman spectroscopy

Detection of pathogenic organisms in the environment presents several challenges due to the high cost and long times typically required for identification and quantification. Polymerase chain reaction (PCR) based methods are often hindered by the presence of polymerase inhibiting compounds and so direct methods of quantification that do not require enrichment or amplification are being sought. This work presents an analysis of pathogen detection using Raman spectroscopy to identify and quantify microorganisms without drying. Confocal Raman measurements of the bacterium Escherichia coli and of two bacteriophages, MS2 and PRD1, were analyzed for characteristic peaks and to estimate detection limits using traditional Raman and surface-enhanced Raman spectroscopy (SERS). MS2, PRD1, and E. coli produced differentiable Raman spectra with approximate detection limits for PRD1 and E. coli of 10(9) pfu/mL and 10(6) cells/mL, respectively. These high detection concentration limits are partly due to the small sampling volume of the confocal system but translate to quantification of as little as 100 bacteriophages to generate a reliable spectral signal. SERS increased signal intensity 10(3) fold and presented peaks that were visible using 2-second acquisitions; however, peak locations and intensities were variable, as typical with SERS. These results demonstrate that Raman spectroscopy and SERS have potential as a pathogen monitoring platform.     

Noninvasive detection of concealed liquid explosives using Raman spectroscopy

We present a Raman spectroscopic method for the noninvasive detection of liquid explosives within bottles, and other packaging, of substantially higher sensitivity and wider applicability than that currently available via conventional Raman spectroscopy. The approach uses a modification of the spatially offset Raman spectroscopy (SORS) concept, which permits the interrogation of a wide range of containers, including transparent, colored, and diffusely scattering plastic and glass beverage, medicine, and cosmetic bottles, with no change in experimental geometry. The enhanced sensitivity is achieved by the technique's inherent ability to effectively suppress fluorescence and Raman contributions originating from the wall of the container. The application is demonstrated on the noninvasive detection of hydrogen peroxide solution, a critical component of a number of liquid explosives. In contrast to conventional Raman spectroscopy, the modified SORS concept enables the detection of concealed hydrogen peroxide solution in all the studied cases.     

Molecular recognition in monolayers and species detection by surface-enhanced resonance Raman spectroscopy

The current emphasis in efforts to produce systems capable of highly specific molecular recognition has produced a wide variety of compounds such as crown ethers, cryptands, cyclodextrins and other inclusion systems. A more desirable approach, and one obviating laborious organic synthesis, would be based upon a mechanism more like that seen in the in vivo antigen-antibody reaction. Sites having the capability for specific molecular recognition based on a predetermined template molecule would allow realization of systems of the desired specificity. The technique of cosorption of a silane and a surface-active molecule onto a glass surface has been comprehensively described by Sagiv and by Maoz and Sagiv and has indicated the feasibility of this approach, e.g. with surface-active dyes. In the present study, adsorbed monolayers were produced with sites based on chosen template molecules, using the Sagiv method, and the systems then reconstituted with the original template molecule as well with molecules of closely similar structure (i.e. porphyrins or chlorophylls). A high degree of recognition was evidenced, as shown by the use of surface-enhanced resonance Raman spectroscopy as the detection tool. It was also shown that chemically dissimilar species can be reconstituted into sites formed by other species, provided that the molecular shapes are compatible. The ease of resorption into performed sites is strongly dependent on the presence of amphiphilic character in the molecule re-entering a site.     

Monitoring the glycosylation status of proteins using Raman spectroscopy

Protein-based biopharmaceuticals are becoming increasingly widely used as therapeutic agents, and the characterization of these biopharmaceuticals poses a significant analytical challenge. In particular, monitoring posttranslational modifications (PTMs), such as glycosylation, is an important aspect of this characterization because these glycans can strongly affect the stability, immunogenicity, and pharmacokinetics of these biotherapeutic drugs. Raman spectroscopy is a powerful tool, with many emerging applications in the bioprocessing arena. Although the technique has a relatively rich history in protein science, only recently has Raman spectroscopy been investigated for assessing posttranslational modifications, including phosphorylation, acetylation, trimethylation, and ubiquitination. In this investigation, we develop for the first time Raman spectroscopy combined with multivariate data analyses, including principal components analysis and partial least-squares regression, for the determination of the glycosylation status of proteins and quantifying the relative concentrations of the native ribonuclease (RNase) A protein and RNase B glycoprotein within mixtures.     

Defect characterization in graphene and carbon nanotubes using Raman spectroscopy

This review discusses advances that have been made in the study of defect-induced double-resonance processes in nanographite, graphene and carbon nanotubes, mostly coming from combining Raman spectroscopic experiments with microscopy studies and from the development of new theoretical models. The disorder-induced peak frequencies and intensities are discussed, with particular emphasis given to how the disorder-induced features evolve with increasing amounts of disorder. We address here two systems, ion-bombarded graphene and nanographite, where disorder is represented by point defects and boundaries, respectively. Raman spectroscopy is used to study the 'atomic structure' of the defect, making it possible, for example, to distinguish between zigzag and armchair edges, based on selection rules of phonon scattering. Finally, a different concept is discussed, involving the effect that defects have on the lineshape of Raman-allowed peaks, owing to local electron and phonon energy renormalization. Such effects can be observed by near-field optical measurements on the G' feature for doped single-walled carbon nanotubes.