Near-infrared excited Raman optical activity

Measurements of near-infrared scattered circular polarization Raman optical activity (SCP-ROA) are presented using laser excitation at 780 nm for samples of S-(-)-alpha-pinene and L-alanyl-L-alanine. These are the first measurements of ROA outside the blue-to-green visible region between 488 and 532 nm. Comparison of Raman and ROA intensities measured with excitation at 532 and 780 nm demonstrate that the expected frequency to the fourth-power dependence for Raman scattering and the corresponding fifth-power dependence for ROA are observed. It can be concluded that, to within this frequency dependence, the same level of efficiency of Raman and ROA measurements using commercial instrumentation with 532 nm excitation is maintained with the change to near-infrared excitation at 780 nm.     

Control of enhanced Raman scattering using a DNA-based assembly process of dye-coded nanoparticles

Enhanced Raman scattering from metal surfaces has been investigated for over 30 years. Silver surfaces are known to produce a large effect, and this can be maximized by producing a roughened surface, which can be achieved by the aggregation of silver nanoparticles. However, an approach to control this aggregation, in particular through the interaction of biological molecules such as DNA, has not been reported. Here we show the selective turning on of the surface enhanced resonance Raman scattering effect on dye-coded, DNA-functionalized, silver nanoparticles through a target-dependent, sequence-specific DNA hybridization assembly that exploits the electromagnetic enhancement mechanism for the scattering. Dye-coded nanoparticles that do not undergo hybridization experience no enhancement and hence do not give surface enhanced resonance Raman scattering. This is due to the massive difference in enhancement from nanoparticle assemblies compared with individual nanoparticles. The electromagnetic enhancement is the dominant effect and, coupled with an understanding of the surface chemistry, allows surface enhanced resonance Raman scattering nanosensors to be designed based on a natural biological recognition process.     

Novel method for preparing controllable and stable silver particle films for surface-enhanced Raman scattering spectroscopy

A new surface-enhanced Raman scattering (SERS) active substrate has been developed based on our previous study. Small silver nanoparticles on a quartz slide can be enlarged by using a mixture of commercially available reagents called Silver Enhancer and Initiator. The optical properties and characteristics of the new substrate have been investigated by ultraviolet-visible (UV-Vis) spectroscopy and atomic force microscopy (AFM). The results indicate that the small silver nanoparticles grow and some silver aggregates emerge on the quartz slide after the slide is immersed into the Silver Enhancer and Initiator Mixture (SEIM). The average diameter of the silver nanoparticles on the substrate becomes approximately double after the immersion into SEIM for 20 s. 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (BPENB) was used as a Raman probe to evaluate the enhancement ability of the new silver substrate. It has been found that the SERS intensity can be increased about 10 times by using the substrate treated by SEIM compared with that without being treated by SEIM. Interestingly enough, the SERS enhancement increases with time. This may be due to the reorganization of silver nanoparticles on the quartz surface. The new substrate can remain active for more than 90 days. The adsorption mode of BPENB on the new substrate and the dependence of the BPENB configurations on the concentration of BPENB in methanol solution have also been investigated by SERS or UV-Vis spectroscopy. The SERS spectra of a self-assembled monolayer (SAM) BPENB film adsorbed on a silver substrate treated by SEIM show that BPENB molecules are chemically absorbed through the Ag-N bond. Consequently, a nearly perpendicular orientation of BPENB on the silver surface is proposed. The SERS spectra of BPENB SAMs on the new substrates fabricated from methanol solutions with different concentrations are compared. The concentration dependence of the SERS spectra reveals that the BPENB molecules are adsorbed on the silver film as monomers when the film is prepared from the solution with a lower concentration (<4 x 10(-6) M) and as aggregates when it is prepared from the solution with a higher concentration (>1 x 10(-5) M).     

New strategy for ready application of surface-enhanced resonance Raman scattering/surface-enhanced Raman scattering to chemical analysis of organic films on dielectric substrates

Dropping of appropriately concentrated AgNO3 and NaBH4 solutions, as well as laser-ablated Ag sols, onto organic molecules results in the formation of aggregated Ag nanoparticles that can induce surface-enhanced Raman scattering (SERS) for the molecules. The addition of flocculating agents such as alkali halides can further increase the Raman signals. We demonstrate in this work that Raman spectra can be obtained even for 0.01 monolayers of R6G on Si simply by spreading silver nanoparticles and/or fabricating Ag nanoparticles and nanoaggregates at the gaps and vacant sites of R6G molecules. The application prospect of the present methodology is extremely high, not only because of its simplicity but also because of the fact that the observation of vibrational spectra is one of the most incisive methods for understanding the chemical and physical phenomena on a variety of surfaces.     

Surface-enhanced Raman scattering on single-wall carbon nanotubes

Exploiting the effect of surface-enhanced Raman scattering (SERS), the Raman signal of single-wall carbon nanotubes (SWNTs) can be enhanced by up to 14 orders of magnitude when the tubes are in contact with silver or gold nanostructures and Raman scattering takes place predominantly in the enhanced local optical fields of the nanostructures. Such a level of enhancement offers exciting opportunities for ultrasensitive Raman studies on SWNTs and allows resonant and non-resonant Raman experiments to be done on single SWNTs at relatively high signal levels. Since the optical fields are highly localized within so-called "hot spots" on fractal silver colloidal clusters, lateral confinement of the Raman scattering can be as small as 5 nm, allowing spectroscopic selection of a single nanotube from a larger population. Moreover, since SWNTs are very stable "artificial molecules" with a high aspect ratio and a strong electron-phonon coupling, they are unique "test molecules" for investigating the SERS effect itself and for probing the "electromagnetic field contribution" and "charge transfer contribution" to the effect. SERS is also a powerful tool for monitoring the "chemical" interaction between the nanotube and the metal nanostructure.     

Chitosan-coated anisotropic silver nanoparticles as a SERS substrate for single-molecule detection

Surface-enhanced Raman spectroscopy (SERS) is a technique that has become widely used for identifying and providing structural information about molecular species in low concentration. There is an ongoing interest in finding optimum particle size, shape and spatial distribution for optimizing the SERS substrates and pushing the sensitivity toward the single-molecule detection limit. This work reports the design of a novel, biocompatible SERS substrate based on small clusters of anisotropic silver nanoparticles embedded in a film of chitosan biopolymer. The SERS efficiency of the biocompatible film is assessed by employing Raman imaging and spectroscopy of adenine, a significant biological molecule. By combining atomic force microscopy with SERS imaging we find that the chitosan matrix enables the formation of small clusters of silver nanoparticles, with junctions and gaps that greatly enhance the Raman intensities of the adsorbed molecules. The study demonstrates that chitosan-coated anisotropic silver nanoparticle clusters are sensitive enough to be implemented as effective plasmonic substrates for SERS detection of nonresonant analytes at the single-molecule level.     

Structure and vibrational motion of insulin from Raman optical activity spectra

The Raman optical activity (ROA) spectroscopic technique has been applied in the past to many biologically relevant systems including peptides, proteins, sugars, and even viruses. However, theoretical interpretation of the spectra relies on lengthy quantum-chemical computations, which are difficult to extend to larger molecules. In the present study, ROA and Raman spectra of insulin under a range of various conditions were measured and interpreted with the aid of the Cartesian-coordinate tensor transfer (CCT) method. The CCT methodology yielded spectra of insulin monomer and dimer of nearly ab initio quality, while at the same time reproducing the experimental data very well. The link between the spectra and the protein structure could thus be studied in detail. Spectral contributions from the peptide backbone and the amino acid side chains were calculated. Likewise, specific intensity features originating from the α-helical, coil, β-sheet, and 3(10)-helical parts of the protein could be deciphered. The assignment of the Raman and ROA bands to intrinsic molecular coordinates as based on the harmonic force field calculation revealed their origin and degree of locality. Alternatively, the relation of the structural flexibility of insulin to the inhomogeneous broadening of spectral bands was studied by a combination of CCT and molecular dynamics (MD). The present study confirms the sensitivity of the ROA technique to some subtle static and dynamic changes in molecular geometry, and many previous ad hoc or semiempirical spectral-structure assignments could be verified. On the other hand, a limitation in longer-range tertiary structure sensitivity was revealed. Unlike for smaller molecules with approximately equal contributions of the electric dipole (α), quadrupole (A), and magnetic dipole (G') polarizabilities, only the electric dipolar polarization (α) interactions seem to dominate in the protein ROA signal. The simulations concern the largest molecule for which such spectra were interpreted by a priori procedures and significantly enhance protein folding studies undertaken by this technique.     

Fabrication of Flexible Metal‐Nanoparticle Films Using Graphene Oxide Sheets as Substrates

Graphene‐based sheets that possess a unique nanostructure and a variety of fascinating properties are appealing as promising nanoscale building blocks of new composites. Herein, graphene oxide sheets are used as the nanoscale substrates for the formation of silver‐nanoparticle films. These silver‐nanoparticle films assembled on graphene oxide sheets are flexible and can form stable suspensions in aqueous solutions. They can also be easily processed, forming macroscopic films with high reflectivity. Raman signals of graphene oxide in such hybrid films are increased by the attached silver nanoparticles, displaying surface‐enhanced Raman scattering activity. The degree of enhancement can be adjusted by varying the quantity of silver nanoparticles on the graphene oxide sheets.     

Tuning luminescence intensity of RHO6G dye using silver nanoparticles

The photoluminescence (PL) from rhodamine (RHO6G) dye dispersed in ethanol has been studied in the presence of different amounts of citrate stabilized silver nanoparticles of size, ∼10 nm. Enhancement as well as quenching of luminescence intensity has been observed and it was found that luminescence intensity can be tuned by adding various amounts of silver nanoparticles to the RHO6G dye dispersion. The luminescence spectra of dye consist of two peaks at 440 nm and 550 nm. Peak at 440 nm shows an enhancement in intensity at all the concentrations of added silver nanoparticles with the maximum intensity for dye with 0.25 ml silver nanoparticles (82% enhancement in the luminescence intensity). PL intensity of intense peak at 550 nm of dye molecules was found to be quenched in presence of silver nanoparticles and maximum quenching was found to be 41% for the dye with 1 ml silver nanoparticles. However, for lowest concentration of silver nanoparticles viz. (0.01 ml), enhancement in intensity was observed (13% enhancement than the dye molecules). The quenching as well as enhancement in the intensity can be understood by considering the possibility of three different phenomena. It has been reported earlier that when metal nanoparticles are in close proximity to the fluorophores, quenching of luminescence occurs, whereas when metal nanoparticles are located at certain distance, enhancement in luminescence is observed. This effect has been explained by coupling of surface plasmon resonance from metal nanoparticles with fluorophore, resulting in the increase of excitation and emission rate of the fluorophore in the localized electromagnetic field. The quenching and enhancement of luminescence intensity of the dye molecules can also be explained as the transfer of electrons from dye to the silver nanoparticles and to an extent it can be attributed to the aggregation of dye molecules upon addition of silver nanoparticles.     

Growth of Ag, Au, Cu, and Pt nanostructures on surfaces by micropatterned laser-image formations

Silver, gold, copper and platinum nanoparticles (NPs) were grown on surfaces in the form of patterns by the exposure of laser radiation onto droplets of metal ion solutions and the aid of a reducing agent. The generation of patterns from metallic NPs was achieved by combining induced growth of NPs and nanostructures by laser incidence directly on surfaces and optical image formation techniques for transferring the patterns. Near-UV (363.8 nm) and visible (532 nm) laser wavelengths were used for the laser-induced growth of NPs into microstructures on glass, quartz, stainless steel, silicon, and gold-on-silicon substrates. The sizes of the patterns formed were on the micrometer scale and the sizes of the transferred patterns were on the millimeter scale. The patterns formed were generated by optical transference of image and interference of laser beams. Ag and Au substrates were highly active in surface enhanced Raman spectroscopy (SERS). The enhanced Raman activity was measured for SERS probe molecules: 9H-purin-6-amine (adenine) and 1,2-bis (4-pyridyl)-ethane analytes on Ag and Au substrates, respectively. The enhancement factors obtained were 1.8×10(5) and 6.2×10(6), respectively.     

Controlled Deposition of Silver Nanoparticles in Mesoporous Single‐ or Multilayer Thin Films: From Tuned Pore Filling to Selective Spatial Location of Nanometric Objects

Silver nanoparticle assemblies are embedded within mesoporous oxide thin films by an in situ mild reduction leading to nanoparticle–mesoporous oxide thin‐film composites (NP@MOTF). A quantitative method based on X‐ray reflectivity is developed and validated with energy dispersive spectroscopy in order to assess pore filling. The use of dilute formaldehyde solutions leads to control over the formation of silver nanoparticles within mesoporous titania films. Inclusion of silver nanoparticles in mesoporous silica requires more drastic conditions. This difference in reactivity can be exploited to selectively synthesize nanoparticles in a predetermined layer of a multilayered mesoporous stack leading to complex 1D‐ordered multilayers with precise spatial location of nanometric objects. The metal oxide nanocomposites synthesized have potential applications in catalysis, optical devices, surface‐enhanced Raman scattering, and metal enhancement fluorescence.     

The importance of protonation in the investigation of protein phosphorylation using Raman spectroscopy and Raman optical activity

The effect of protonation on amino acid monomers and protein phosphorylation was studied by means of a combination of Raman scattering and Raman optical activity (ROA). In the past, identifying spectral variations in phosphorylated proteins arising from either the phosphate stretch or amide vibrational modes has proven to be challenging mainly due to the loss of amide and P═O band intensity in the presence of phosphate. By contrast, we have developed a novel strategy based on the careful monitoring of the sample pH and thereby modified the protonation state, such that these difficulties can be overcome and phosphate-derived vibrations are readily visualized with both Raman and ROA. Variations in pH-dependent spectral sets of phosphorylated amino acid monomers serine and threonine demonstrated that the protonation state could be determined by the intensity of the monobasic (-OPO(3)H(-)) phosphate stretch band occurring at ~1080 cm(-1) versus the dibasic (-OPO(3)(2-)) band measured at ~980 cm(-1) in both Raman and ROA. Furthermore, by adjustment of the pH of aqueous samples of the phosphoprotein α-casein and comparing this result with dephosphorylated α-casein, spectral variations in phosphate stretch bands and amide bands could be easily determined. Consequently, structural variations due to both protonation and dephosphorylation could be distinguished, demonstrating the potential of Raman and ROA for future investigations of phosphoprotein structure and interactions.     

Role of polyfunctional organic molecules in the synthesis and assembly of metal nanoparticles

The role of polyfunctional organic molecules in the synthesis of differently shaped metallic nanostructures and their assembly is investigated. These molecules could be used as spacer ligands and also for surface passivation of nanoparticles, especially with the objective of controlling their electronic and optical properties depending on their length scales. We investigate the role of several such molecules, such as 4-aminothiophenol, tridecylamine, Bismarck brown R and Y, mordant brown, fat brown, chrysoidin (basic orange), and 3-aminobenzoic acid in the synthesis and assembly of various nanoparticles of gold and silver. For example, the use of 4-ATP helps in the formation of rod shaped micelles in aqueous acetonitrile as confirmed by transmission electron microscopy (TEM) suggesting their role as soft templates. In addition, 4-ATP has also been used for the formation of heteroassembly of spherical nanoparticles of gold and silver at controlled pH. Significantly, triangular and hexagonal gold nanoplates are formed at room temperature by similar polyfunctional dye molecule, Bismarck brown R (BBR), while other analogous dye molecules give only arbitrary shaped gold nanoparticles. Further confirmation of their role in shape determination comes from linear amine molecules such as tridecylamine, which give only spherical nanoparticles both for silver and gold. In essence, our study confirms the role of various such organic molecules in shape controlled synthesis of nanoparticles. We also report optical and electrochemical properties of few of these nanostructures as a function of their shape.     

Synthesis of Starch-Stabilized Silver Nanoparticles and Their Antimicrobial Activity

In this study, silver nanoparticles were prepared using silver nitrate as the metal precursor, starch as protecting agent, and sodium borohydride (NaBH₄) as a reducing agent by the chemical reduction method. The formation of the silver nanoparticles was monitored using ultraviolet-visible absorption spectroscopy, cyclic voltammetry, and particle size analyzer and characterized by transmission electron microscopy (TEM) and x-ray diffraction (XRD). Synthesis of nanoparticles were carried out by varying different parameters, such as reaction temperature, concentration of reducing agent, concentration of silver ion in feed solution, type and concentration of the stabilizing agent, and stirrer speed expressed in terms of particle size and size distribution. Dispersion destabilization of colloidal nanoparticles was detected by Turbiscan. It was observed that size of the starch stabilized silver nanoparticles were lower than 10 nm. The microbial activity of synthesized silver nanoparticles was examined by modified Kirby-Bauer disk diffusion method. Silver nanoparticles were tested for their antibacterial activity against Gram negative bacteria such as Escherichia coli, Pseudomonas aeruginosa, and Gram positive bacteria such as Staphylococcus aureus and Staphylococcus epidermidis. High bacterial activity was observed at very low concentrations of silver (below 1.39 μg/ml). The antifungal activity of silver nanoparticles has been assayed against Candida albicans.     

Green synthesis of silver nanoparticles using Glaucium corniculatum (L.) Curtis extract and evaluation of its antibacterial activity

The metal nanoparticles, due to interesting features such as electrical, optical, chemical and magnetic properties, have been investigated repeatedly. Also, the mentioned nanoparticles have specific uses in terms of their antibacterial activity. The biosynthesis method is more appropriate than the chemical method for producing the nanoparticles because it does not need any special facilities; it is also economically affordable. In the current study, the silver nanoparticles (AgNPs) were obtained by using a very simple and low‐cost method via Glaucium corniculatum (L.) Curtis plant extract. The characteristics of the AgNPs were investigated using techniques including: X‐ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy. The SEM and TEM images showed that the nanoparticles had a spherical shape, and the mean diameter of them was 53.7 and 45 nm, respectively. The results of the disc diffusion test used for measuring the anti‐bacterial activity of the synthesised nanoparticles indicated that the formed nanoparticles possessed a suitable anti‐bacterial activity.Inspec keywords: silver, nanoparticles, antibacterial activity, nanomedicine, nanofabrication, X‐ray diffraction, transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectraOther keywords: green synthesis, silver nanoparticles, Glaucium corniculatum Curtis extract, antibacterial activity, metal nanoparticles, biosynthesis method, X‐ray diffraction, transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, SEM, TEM, spherical shape, disc diffusion test, Ag     

Phosphorylation detection and characterization in ribonucleotides using Raman and Raman optical activity (ROA) spectroscopies

Raman and Raman optical activity (ROA) spectra are presented for adenosine and seven of its derivative ribonucleotides. Both of these spectroscopic techniques are shown to be sensitive to the site and degree of phosphorylation, with a considerable number of marker bands being identified for these ribonucleotides. ROA spectra are shown to provide the most sensitive diagnostic tool for phosphorylation characterization and quantification.     

Single-molecule spectroscopic study of enhanced intrinsic phycoerythrin fluorescence on silver nanostructured surfaces

In this paper, we report on steady-state and time-resolved single-molecule fluorescence measurements performed on a phycobiliprotein, R-phycoerythrin (RPE), assembled on silver nanostructures. Single-molecule measurements clearly show that RPE molecules display a 10-fold increase in fluorescence intensity, with a 7-fold decrease in lifetime when they are assembled on silver nanostructured surfaces, as compared to control glass slides. The emission spectrum of individual RPE molecules also displays a significant fluorescence enhancement on silver nanostructures as compared to glass. From intensity and lifetime histograms, it is clear that the intensities as well as lifetimes of individual RPE molecules on silver nanostructures are more heterogeneously distributed than that on glass. This single-molecule study provides further insight on the heterogeneity in the fluorescence intensity and lifetimes of the RPE molecules on both glass and SiFs surfaces, which is otherwise not possible to observe using ensemble measurements. Finite-difference time-domain calculations have been performed to study the enhanced near-fields induced around silver nanoparticles by a radiating excited-state fluorophore, and the effect of such enhanced fields on the fluorescence enhancement observed is discussed.     

Gold/palladium and silver/palladium colloids as novel metallic substrates for surface-enhanced Raman scattering

New surface-enhanced Raman scattering (SERS) substrates, composed of gold or silver colloidal nanoparticles doped with palladium, were prepared. These novel colloids are stable and maintain a satisfactory SERS efficiency, even after long aging. The interest in doping the coinage metal nanoparticles with palladium is due to the well-known catalytic activity of this metal. Transmission electron microscopy (TEM) and ultraviolet-visible absorption spectroscopy were used to characterize the shape and size of the metal particles. It was found that these bimetallic colloidal nanoparticles have a core-shell structure, with gold or silver coated with palladium clusters.     

Plasma reduction of silver compounds for fabrication of surface-enhanced Raman scattering substrates

Several silver compounds were reduced by low-pressure air plasma to produce porous nanostructured surfaces as surface-enhanced Raman scattering (SERS) substrates. This method is advantageous because substrates are easy to prepare and the silver metal surface is inherently clean without spectroscopic background. Silver compounds were melted into 1-2 mm slugs on quartz slides and plasma treated for different lengths of time. Silver chloride was found to be the best compound to make reproducible and stable SERS substrates. SERS activity of the substrates was tested using L-tryptophan, 4-mercaptobenzoic acid, and adenine.