Surface-enhanced Raman Scattering Study Research Articles

Optimization of SERS-active substrates for near-field Raman spectroscopy

As a first step towards near-field Raman, we chose to study surface enhanced Raman scattering (SERS)-active substrates to cope with the weakness of Raman scattering (small cross-section and low concentration). We concentrated our work on localized surface plasmon (LSP) since they turned out to play a great part in SERS and we put forward the relation between LSP resonance and Raman enhancement. Roughness of our samples is controlled either by annealing process or electron-beam lithography (EBL); this latter technique proved to best suit to our study. Substrates are characterized by extinction spectroscopy which determines the LSP resonance and then Raman spectrum of a probe molecule, trans-1,2-bis(4-pyridyl)ethylene (BPE) is recorded. We show that maximum of enhancement is obtained when the LSP resonance is red-shifted (50 nm) compared to the excitation laser line (632.8 nm).

Surface-Enhanced Raman Scattering Studies on Aggregated Gold Nanorods

Surface-enhanced Raman scattering (SERS) of adsorbed molecules on gold nanorods (NRs) with dimensions of 10 nm × 27 nm was studied on silica surface with low to high surface coverage of NRs. The study was carried out to investigate both the dependence of the SERS intensity on the number of NRs and the NRs spacing on the silica surface. SERS of adsorbed molecules such as 2-aminothiophenol (2-ATP) and the capping molecules (hexadecyltrimethylammonium bromide) was studied on these surfaces using a near-IR laser excitation source (1064 nm). To produce silica surfaces covered with NRs, two approaches were used. In the first approach, monodispersed NRs gradually deposited from solution to silica surface and their number was increased by increasing the deposition time. In the second one, the NRs were first aggregated in solution and then deposited on the surface. Although using the first approach it was possible to prepare surfaces with high NR surface coverage, SERS intensity was found to be stronger for adsorb...

Adsorption of 4-Biphenylisocyanide on Gold and Silver Nanoparticle Surfaces: Surface-Enhanced Raman Scattering Study

The adsorption behavior of 4-biphenylisocyanide (BPNC) has been studied by means of surface-enhanced Raman scattering (SERS). BPNC has been found to adsorb on silver and gold via the carbon lone pair electrons assuming a standing geometry with respect to the surfaces. The presence of the ν(CH) band in the SERS spectra indicated a vertical orientation of the biphenyl ring of BPNC on Ag and Au. Neither a substantial red shift nor a significant band broadening of the ring breathing modes implied that a direct ring π orbital interaction with metal substrates should be quite low. On the basis of the electromagnetic surface selection rule, we attempted to explain the orientation of the adsorbate on Ag and Au surface by determining the relative enhancement factor of each vibrational band. The band analysis indicated that BPNC should have a slightly more vertical orientation on silver than on gold. The frequency of the ν(NC) band on gold increased by ∼10 cm-1 than that on silver surface. Such a higher blue shift of the ν(NC) stretching vibration in the gold sol SER spectrum may be due to a higher electronegativity of gold.

Adsorption of Polyethyleneimine on Silver Nanoparticles and Its Interaction with a Plasmid DNA: A Surface-Enhanced Raman Scattering Study

Raman spectroscopy is applied in this work to study the adsorption of poly(ethyleneimine) (PEI) on Ag nanoparticles obtained by reduction with citrate, as well as to the study of the interaction between PEI and a plasmid. The surface-enhanced Raman spectroscopy (SERS) affords important information about the interaction and orientation of the polymer on the particles. In particular we have found that this polymer interacts with the surface through their amino groups in an interaction which also involves a change in the protonation state of amino groups as well as an increase of the chain order. This interaction implies a charge-transfer effect as deduced from the strong resonant effect in Raman spectra obtained at different excitation wavelengths. The complex formed by PEI and a plasmid, obtained by encoding the HBV (hepatitis B virus) genome inside the EcoRI restriction site of pGEM vector, was also studied by SERS. The interaction between both polymers leads to a conformational change affecting both macromolecules that can be detected by Raman at different excitation wavelengths. PEI undergoes a change to a more disordered structure as well as an increase of the number of protonated amino groups. The plasmid undergoes a structural change from A-DNA structure to B-DNA, along with a change in the superhelicity resulting in a more lineal structure when the plasmid interacts with PEI.

Surface-Enhanced Raman Scattering Study of 4-Biphenylcarboxylic Acid and a Liquid Crystalline Oligomer on Different Metal Surfaces

Surface-enhanced Raman spectra of 4-biphenylcarboxylic acid (BPCA) and ethyl 4‘-[4‘-oxy-4-biphenylcarbonyloxy]-4-biphenylcarboxylate with a poly(propylene oxide) coil of 12 (hereafter named P-12-4) adsorbed on two different metal substrates, Ag and Au colloid monolayers, were examined, to determine the relationship between adsorption strength and geometry. A significant downshift of νCOO- by 60 cm-1 and lack of a C−H aromatic stretching vibration band indicated that BPCA lies flat on the Au surface, while a smaller shift of 40 cm-1 to a lower wavenumber and weak appearance of the C−H aromatic stretching mode implied adsorption through mainly oxygen lone pair electrons of carboxylate with the benzene ring tilted on the Ag surface. These orientations were confirmed by the desorption behavior of a liquid crystalline (LC) oligomer, P-12-4, which contains the BPCA moiety, on the Au and Ag colloid monolayers with increasing temperature.

Self-Assembled Monolayers of Organoselenium Compounds on Gold: Surface-Enhanced Raman Scattering Study

To understand the binding nature of organoselenium compounds on gold, we have examined the adsorption behavior of several representative organoselenium compounds, i.e., benzeneselenol (BSe), diphenyl diselenide, dibenzyl diselenide, dioctyl diselenide, and benzyl phenyl selenide (BPSe) on the Au surface by virtue of surface-enhanced Raman spectroscopy (SERS). BSe chemisorbs on gold as selenolate with a tilted orientation. Upon adsorption, the Se–Se bonds of diselenides are cleaved to form selenolates, analogous to the formation of thiolate monolayers from disulfides. BPSe adsorbs on gold without any C–Se bond scission. The benzyl moiety of BPSe assumes a rather vertical stance while the phenyl moiety is more tilted to the gold surface.

Adsorption of 1,4-Benzenedithiol on Gold and Silver Surfaces: Surface-Enhanced Raman Scattering Study

The adsorption behavior of 1,4-benzenedithiol (1,4-BDT) on colloidal gold and silver surfaces has been investigated by means of surface-enhanced Raman scattering (SERS). 1,4-BDT chemisorbed dissociatively on both gold and silver surfaces but as mono- and dithiolate, respectively. Regardless of the bulk concentration of 1,4-BDT, only a monolayer was assembled on the silver surface with a flat orientation by forming two Ag–S bonds. On the gold surface, the monothiolate species,1,4-BDT−1, appeared to assume a rather flat orientation at a very low surface coverage, but as the surface coverage was increased, the adsorbate took a perpendicular orientation. Furthermore, when the bulk concentration of 1,4-BDT was close to that required for a full-monolayer coverage limit, a band assignable to the S–S stretching vibration appeared at ∼536 cm−1 in the gold sol SERS spectra. A separate ellipsometry measurement performed with vacuum-evaporated gold substrates revealed that up to tetralayers could be assembled on gold in 1 mM n-hexane solution of 1,4-BDT while at best a bilayer formed in either methanol or ethanol solution. The different adsorbate structure of 1,4-BDT on gold and silver was overall quite comparable to that of p-xylene-α,α′-dithiol.

Surface-Enhanced Raman Scattering Study of 4-Biphenylcarboxylic Acid

The interfacial structures of biphenyl (BP), benzoic acid (BA), and 4-biphenylcarboxylic acid (BPCA) on a Au colloid monolayer surface were investigated using surface-enhanced Raman scattering (SERS). The absence of the C−H stretching vibrational mode in the SERS spectrum of BP indicated that the two benzene rings of BP were coplanar on the Au colloid surface, while their twisted structure was suggested in the bulk state. The downward frequency shift of the carboxylate stretching vibrational mode of BA implied the direct chemical adsorption of the carboxylate group onto the Au surface with a flat orientation. In BPCA, an absence of the C−H stretching vibrational mode and the red shift of the carboxylate stretching vibrational mode were observed. This fact strongly indicated that both the two benzene rings and the carboxylate group were adsorbed on the Au surface with flat orientation via their π-systems.

Surface-Enhanced Raman Scattering Study on Passivating Films of Ag Electrodes in Lithium Batteries

Li/Ag and Li/Al electrochemical cells composed of smooth Ag or Al electrode sheets, respectively, and matched with nonaqueous electrolytes were assembled and discharged. Discharged Ag electrodes were investigated using micro-Raman spectromety. Raman bands attributed to the components of the passivating film on the surface of the discharged Ag electrode, such as Li2CO3 and other species, were observed. No characteristic bands from discharged Al electrodes were present in the Raman spectra. This indicates that the Raman signals for the passivating layer are surface-enhanced by discharged Ag electrodes. The formation of a nanometer-scale rough surface caused by a Li−Ag alloy reaction during the discharging process is an important factor in the enhancement. It was also shown that the passivating films formed in various electrolytes could be distinguished by SERS.

Vibrational behavior of transition metal cupferronato complexes. Raman and SERS studies on nickel(II) cupferronato complexes

Raman and surface enhanced Raman scattering (SERS) spectra of the cupferron [PhN 2O 2]NH 4 and new NiL 2A n complexes, with L=PhN 2O 2 −, n=1, A= o-phenanthroline, o,o′-bipyridine and n=2, A=H 2O, o-NH 2Py, o-C 6H 4(NH 2) 2 have been recorded and discussed. Vibrational analysis of the neutral ligand and newly prepared nickel(II) cupferronato complexes suggests electron delocalization through the coordinated N-nitroso- N-hydroxylaminato (ONNO) unit and the bidentate coordination of the anionic ligand to the metal center through the oxygen atoms. A Raman band of the new nickel(II) complexes was found at about 1300 cm −1. The NiL 2A n , complexes with n=1, A= o-phenanthroline, o,o′-bipyridine and n=2, A=H 2O were found to adsorb on different Ag SERS substrates. The best SERS spectra were recorded on the nanometer Ag colloid particles. The SERS bands associated to the chemical interaction between the ONNO unit and Ag colloidal particles were observed. o-Phenanthroline and o,o′-bipyridine ring vibrational modes were assigned in the SERS spectra of the corresponding complexes. SERS spectra of the title compounds bring additional arguments for the bidentate coordination of the ONNO group.

Adsorption Characteristics of p-Xylene-α,α‘-dithiol on Gold and Silver Surfaces: Surface-Enhanced Raman Scattering and Ellipsometry Study

The adsorption behavior of p-xylene-α,α‘-dithiol (p-XDT) on colloidal gold and silver surfaces has been investigated by means of surface-enhanced Raman scattering (SERS). p-XDT chemisorbed dissociatively on both the gold and silver surfaces but as mono- and dithiolate, respectively. Regardless of the bulk concentration of p-XDT, only a monolayer was assembled on the silver surface with a flat orientation by forming two Ag−S bonds. On the gold surface, the monothiolate species, p-XDT1-, appeared to assume a rather flat orientation at a very low surface coverage, but as the surface coverage was increased, the adsorbate took a perpendicular orientation. Furthermore, when the bulk concentration of p-XDT was close to that required for a full-monolayer coverage limit, a band assignable to the S−S stretching vibration appeared at ∼509 cm-1 in the gold sol SER spectra. A separate ellipsometry measurement performed with vacuum-evaporated gold substrates revealed that up to trilayers could be assembled on gold in 1...

Near-Infrared Surface-Enhanced Raman Scattering Study of Ultrathin Films of Azobenzene-Containing Long-Chain Fatty Acids on a Silver Surface Prepared by Silver Mirror and Nitric Acid Etched Silver Foil Methods

Near-infrared (NIR) Fourier transform (FT) surface-enhanced Raman scattering (SERS) spectra were measured for Langmuir−Blodgett (LB) and dipping films of azobenzene-containing long-chain fatty acids (nAmH, n = 8, 12, m = 3, 5) on silver substrates prepared by silver mirror and nitric acid etched silver foil methods. FT-IR spectra were also obtained for some of the films on silver mirror, silver foil, and CaF2 plate. The dipping films on the silver mirror always show much greater SERS effect than those on the silver foils, revealing that the silver mirror method is more powerful than the silver foil method in preparing the substrates suitable for the SERS measurements. The fatty acids assume the COO- form in the dipping films on the silver mirrors, while they mainly take the COOH form in the LB films on the silver mirror and CaF2 plate. Probably, silver reacts with the COOH groups during the dipping of the silver mirrors into the chloroform solutions of the azobenzene fatty acids. Bands at 1603 and 853 cm-1 due to the phenyl ring modes are enhanced markedly in the NIR-SERS spectra of the dipping films of 8A3H and 12A3H on the silver mirrors. Based upon the SERS “surface selection rule”, it may be concluded that the phenyl rings are nearly perpendicular to the silver mirror surfaces. Such enhancement is much weaker for the dipping film of 8A5H. Therefore, it seems that the length of the spacer (m) of the fatty acid is an important factor for the great SERS enhancement.

Surface Enhanced Raman Scattering Studies of 1,4-Diaminoanthraquinone Dye in Ag Colloid

Surface enhanced Raman scattering studies are carried out for 1,4-diaminoanthraquinone dye [1,4-DAAQ] in Ag colloid in order to understand the enhancement effect and the nature of interaction between the dye molecule and Ag surface. Detailed analysis of the enhanced vibrational bands in the SERS spectrum shows that the 1,4-DAAQ molecule is not adsorbed through its coordinating sites, because of intramolecular hydrogen bond formation. The 1,4-DAAQ molecule is seen having 'flat on' orientation. Laser induced fluorescence measurements in different solvents and in different pH confirm the intramolecular hydrogen bond formation.

Photosensitive Oligomer Formation and Laser-induced Polymerization of Aniline on a Gold Electrode Surface: A Surface-Enhanced Raman Scattering Study

Abstract The process of laser-induced electropolymerization of aniline was studied by in situ surface-enhanced Raman scattering (SERS) spectroscopy and cyclic voltammetry (CV). Formation of p-coupled aniline oligomers was confirmed at an electrode potential below the oxidation potential of aniline, and the oligomers were found to be the source compound for photo-induced electropolymerization.

Surface-Enhanced Raman Scattering (SERS) Study of Mercaptoethanol Monolayer Assemblies on Silver Citrate Hydrosol. Preparation and Characterization of Modified Hydrosol as a SERS-Active Substrate

The self-assembling technique was employed for modification of a citrate-reduced silver hydrosol with mercaptoethanol (ME). The results of investigation of the structure and properties of both the self-assembled monolayers (SAMs) of ME and the modified hydrosol as a substrate for surface-enhanced Raman scattering (SERS) spectroscopy are presented. ME chemisorbs on the hydrosol by rupture of the S−H bond with concomitant bonding of the sulfur headgroup to Ag. Chemisorption is accompanied by gauche−trans isomerization of ME. The isomerization occurs cooperatively at submonolayer coverage due to interactions of neighboring adsorbed molecules rather than due to the chemisorption itself. The chemisorption of ME takes place on a less than 5 min time scale and leads to formation of a SAM with definite density and gauche−trans equilibrium, the parameters of which depend on the bulk concentration of ME but not on the time of incubation. It was revealed that densely packed and highly ordered (97% of trans-isomers) SAM is formed at an ME concentration higher than 250 μM. ME induces concentration-dependent aggregation of the hydrosol and makes it SERS active. After 30 min of incubation with 250 μM ME the modified hydrosol has the highest Raman enhancement only slightly altered during the next 40−50 min. Preactivation of the hydrosol surface with ClO4-, NO3-, Cl-, Br-, and I- anions does not improve SAM formation or Raman enhancement. Using β-carotene as a probe molecule it was well demonstrated that the structured surface of the modified hydrosol provides favorable conditions for increasing the number of adsorption sites and increasing the adsorption constant for an analyte. It also protects an analyte from direct contact with the metal surface, thereby preventing distortion of the electronic system of adsorbed molecule. These advantages make the modified hydrosol a useful tool for SERS probing of chromophores with intense electronic transitions in the visible region.

Observation of the Surface-Enhanced Raman Scattering Spectrum of Uranyl Ion

We wish to report the surface-enhanced Raman scattering (SERS) spectrum of uranyl ion, as a uranyl nitrate complex. The uranyl-ion group has a very characteristic linear structure. The oxygen-uranium bond in this group is known to be only slightly weaker than that of the carbon-oxygen in CO2. The vibrational frequency of the symmetric stretch for the uranylion is Raman active and is located around 800 Δcm-1 , varying with compounds, There has been a previous Raman and SERS study of uranyl superphthalocyanine (UO2SPc). That study dealt primarily with ligand interactions but did note a unique interaction of the UO2SPc molecule and silver. A weak SERS enhancement of less than 102 was noted; the weak enhancement was believed to be caused by damping effects of the superpthalocyanine molecule. To our knowledge, our work is the first to show a SERS enhancement for an inorganic species, UO2(NO3)2, on silver.

Surface-Enhanced Fourier Transform Raman Scattering Study on the Adsorption Structure of an RNA Triple Helix at a Silver Electrode

In this research, we studied the adsorption structure of an RNA triplex, poly[rU]poly[rA]poly[rU], at a silver electrode by surface-enhanced Fourier transform Raman scattering (FT-SERS) spectroscopy and compared it to those of the corresponding single-stranded poly[rA] and duplex poly[rA] · poly[rU]. Some interesting phenomena have been observed. At the ex situ electrochemically roughened silver electrode, the SERS behavior of the duplex RNA is close to that of the single-stranded poly[rA], thereby indicating that the duplex RNA adsorbed at the electrode might be partly destabilized. However, on the highly positively charged surfaces, the SERS spectra revealed that the triplex was predominantly adsorbed at the electrode via the phosphate-moiety-directed mechanism, and thus the helical structure of the triplex molecules was well preserved; furthermore, since the electrode potential was set to approach the potential of zero charge (pzc) of the silver metal, in the spectral region between 1800 and 600 cm−1 the triplex gives rise to SERS spectra similar to those of the corresponding duplex, and also yields only two enhanced signals at 734 and 1382 cm−1, due to the ring-breathing and ring-vibration modes of the adsorbed adenine residues, respectively, along with the disappearance of some bands originating from the corresponding rU residues and phosphate groups. In fact, a dramatic transition of the SERS spectra of the triplex at the electrode was found to occur between −0.2 and −0.4 V. On the basis of the structural characteristics of triple-helical nucleic acids, we concluded that in this case there are two types of competitive adsorbed species—the triplex itself and the unpaired adenine residues in the incomplete region of the triplex RNA—which might be responsible for the unique potential-dependent transition.

Surface-Enhanced Raman Scattering (SERS) Study of the Interaction between Hematoporphyrin and Guanine

The interaction between hematoporphyrin and guanine has been investigated by the Raman spectroscopic method. Irradiation of guanine solution mixed with hematoporphyrin was observed through the change in the Raman spectra. It was observed that the band at 1534 cm−1 shifted and the bands at 1274 cm−1 and 1190 cm−1 disappeared in the presence of hematoporphyrin. A prolonged irradiation time of five hours caused the SERS spectra of guanine to disappear. These results indicate that the guanine was modified in the photosensitized reactions in the presence of hematoporphyrin. We used SERS to study the photodynamic modification of nucleic acid base by hematoporphyrin. We found that illumination of guanine and adenine with the 514.5-nm argon-ion laser line in the presence of hematoporphyrin resulted in the selected degradation of guanine.

Surface-Enhanced Raman Scattering (SERS) Study on the Air Oxidation of Copper and its Reactions with Azoles

Surface-Enhanced Raman Scattering (SERS) on HNO3-roughened copper foils has been used to follow the air oxidation of copper at room temperature and to study the surface reactions with azole compounds. It has been found that azoles react with metallic copper as well as copper oxides. SERS and IR studies show that the reaction of azoles with metallic copper is faster than with cuprous and cupric oxides.

Surface Enhanced Raman Scattering Study of Flavin Adenine Dinucleotide on Silver Electrode

Surface Enhanced Raman Scattering Study of Flavin Adenine Dinucleotide on Silver Electrode