Quantitative analysis of methyl green using surface-enhanced resonance Raman scattering

Surface-enhanced resonance Raman scattering (SERRS) spectra of aqueous solutions of the triphenylmethane dye methyl green have been obtained for the first time by use of citrate-reduced silver colloids and a laser excitation wavelength of 632.8 nm. Given the highly fluorescent nature of the analyte, which precluded collection of normal Raman spectra of the dye in solution and powdered state, it was highly encouraging that SERRS spectra showed no fluorescence due to quenching by the silver sol. The pH conditions for SERRS were optimised over the pH range 0.5–10 and the biggest enhancement for SERRS of this charged dye was found to be at pH 2.02, thus this condition was used for quantitative analysis. SERRS was found to be highly sensitive and enabled quantitative determination of methyl green over the range 10⁻⁹ to 10⁻⁷ mol dm⁻³. Good fits to correlation coefficients were obtained over this range using the areas under the vibrational bands at 1615 and 737 cm⁻¹. Finally, a limit of detection of 83 ppb was calculated, demonstrating the sensitivity of the technique.     

Substrates for the at-line coupling of capillary electrophoresis and surface-enhanced Raman spectroscopy

In this preliminary study, we evaluated four different types of substrate for the at-line coupling of capillary electrophoresis and surface-enhanced (resonance) Raman spectroscopy, CE-SER(R)S, with emphasis on spectral repeatability. We tested Sub1: etched silver foil, Sub2: a vapour-deposited silver film, Sub3: a silver oxalate-precoated silica TLC plate and Sub4: a silica TLC plate on which colloid and poly(l-lysine) were manually added to the analyte spots, used earlier in at-line CE-SE(R)RS. All substrates were first tested by manual spotting using trans-1,2-bis(4-pyridyl)ethylene (BPE) as a model compound for SERS and crystal violet (CV) as a model compound for SE(R)RS. The spectral features of the SE(R)RS spectra of BPE and CV showed a most satisfactory repeatability on all substrates. As expected, the signal intensities varied considerably between runs; this implies that quantification in at-line CE-SE(R)RS should rather be done by means of an on-line absorbance detector. In addition, the suitability of Sub1, Sub2 and Sub4 as deposition substrates after CE was explored using two cationic dyes: CV and basic fuchsin (BF). Good-quality SERRS spectra could be recorded on all three substrates. Although Sub1 and Sub2 have a poor water-sorptivity, they were found to be good substrates for at-line CE-SERRS. They do not require any post-deposition addition of silver colloid and could therefore become attractive alternatives for Sub4.     

IR, Raman and SERS spectra of 2-phenoxymethylbenzothiazole

The FT-IR and FT-Raman spectra of 2-phenoxymethylbenzothiazole were recorded and analyzed. The surface enhanced Raman scattering (SERS) spectrum was recorded in a silver colloid. The vibrational wavenumbers of the compound have been computed using the Hartree–Fock/6-31G* basis and compared with the experimental values. The appearance of the Ag–O stretching mode at 237 cm⁻¹ in the SERS spectrum along with theoretically calculated atomic charge density, leads us to suggest that the molecule is adsorbed through the oxygen atom with the molecular plane tilted on the colloidal silver surface. The direction of charge transfer contribution to SERS has been discussed from the frontier orbital theory.     

Surface-enhanced Raman spectra of rhodamine 19 octadecylamide

Surface-enhanced Raman scattering (SERS) of the cationic and the neutral form of rhodamine 19 octadecylamide (R19OA) has been studied in the silver citrate colloid using NIR excitation at 1064 nm. Cationic molecules readily adsorb onto negatively charged silver nanoparticles through a positively charged xanthene part of the molecules resulting in surface enhancement of Raman scattering. Due to a lack of the positive charge in molecular structure, SERS spectrum of neutral molecules is not observed. Nevertheless, a broad band appearing at 1240 cm^?1 in the spectrum of the cationic form indicates conversion of the cationic into the neutral species occurring close to the silver surface. The observed band most likely arises from a vibration of the ring formed in the molecular structure after conversion, but before complete desorption of the neutral molecules from the metal surface. Upon addition of HCl and NaOH in the silver sol, equilibrium is shifted toward the cationic and the neutral form of R19OA, respectively, followed by corresponding changes in the Raman spectrum. In addition, FT-SERS spectra of two rhodamine dyes, rhodamine 19 (R19) and rhodamine 6G (R6G), that are structurally related to R19OA, have been studied under the same experimental conditions for comparison.     

An application of the DFT-based scaled quantum mechanical force field method to a weakly bonded system: N2O4

The adequacy of the density functional derived scaled quantum mechanical force field method of Rauhut and Pulay has been tested for a weakly bound dimer, N₂O₄. The Becke3–Lee–Yang–Parr/6-31G^* harmonic force field has been scaled with the transferable scale factors of Rauhut and Pulay (developed originally for `common' organic compounds without any reference to the N–O bond) resulting in 20.7 and 38 cm⁻¹ for the mean and the maximal individual deviations, respectively, as compared with experimental spectra. Consequently, the force field method of Rauhut and Pulay may give reliable results for non-common compounds as well.     

Semi-quantitative trace analysis of nuclear fast red by surface enhanced resonance Raman scattering

The dye nuclear fast red has been detected and determined semi-quantitatively by means of surface enhanced resonance Raman scattering (SERRS) and surface enhanced Raman scattering (SERS), using laser exciting wavelengths of 514.5 and 632.8 nm, respectively, by employing a citrate-reduced silver colloid. A good linear correlation is observed for the dependence of the intensities of the SERRS bands at 989 cm⁻¹ (R=0.9897) and 1278 cm⁻¹ (R=0.9872) on dye concentration over the range 10⁻⁹ to 10⁻⁷ M, when using an exciting wavelength of 514.5 nm. At dye concentrations above 10⁻⁷ M, the concentration dependence of the SERRS signals is non-linear. This is almost certainly due to the coverage of the colloidal silver particles being in excess of a full monolayer of the dye. A linear correlation is also observed for the dependence of the intensities of the SERS bands at 989 cm⁻¹ (R=0.9739) and 1278 cm⁻¹ (R=0.9838) on the dye concentration over the range 10⁻⁸ to 10⁻⁶ M when using an exciting wavelength of 632.8 nm. Strong fluorescence prevented collection of resonance Raman scattering (RRS) spectra from powdered samples or aqueous solutions of the dye using an exciting wavelength of 514.5 nm, but weak bands were observed in the spectra obtained from both powdered and aqueous samples of the dye using an exciting wavelength of 632.8 nm. A study of the pH dependence of SERRS/SERS and UV–VIS absorption spectra revealed the presence of different ionisation states of the dye. The limits of detection for nuclear fast red by SERRS (514.5 nm), SERS (632.8 nm) and visible spectroscopy (535 nm) are 9, 89 and 1000 ng ml⁻¹, respectively.     

Direct evidence of electron flow via the heme c group for the direct electron transfer reaction of fructose dehydrogenase using a silver nanoparticle-modified electrode

The direct electron transfer reaction of fructose dehydrogenase (FDH) from Gluconobacter sp. on alkanethiol-modified silver nanoparticles (AgNPs) was examined using cyclic voltammetry and surface-enhanced resonance Raman scattering (SERRS). Using cyclic voltammetry, catalytic oxidation currents (based on the direct electron transfer reaction of FDH) were observed from a potential of approximately −100 mV (vs. Ag/AgCl, 3 M NaCl) in the presence of d-fructose, without a mediator. A comparison of the SERRS spectra and the resonance Raman spectra of FDH in solution indicated that the heme c site retained its six-coordinated low-spin heme after immobilization. Moreover, SERRS also demonstrated that the heme c of the adsorbed FDH was the electron transfer site within the enzyme.     

A surface enhanced Raman study of the adsorption of tris(2,2-bipyridine)ruthenium(II) and substituted analogues on chemically-deposited silver films

The surface enhanced resonance Raman spectroscopy (SERRS) of a series of tris(2,2′-bipyridine)ruthenium(II) complexes on chemically produced silver films is reported. The SERR spectra of [Ru(bipy)₃]²⁺, several tris complexes of Ru(II) containing substituted 2,2′-bipyridine (4,4′-dimethyl-,4,4′diphenyl-, 4,4′-diamino- and 4,4′-diethylcarboxylate-2,2′-bipyridine) ligands and the neutral cis-bis complexes [Ru(bipy)₂(NCS)₂] and [Ru(bipy)₂Cl₂] show very high band intensities. The large enhancement arises from the combination of the inherent resonance Raman effect and the surface plasmon resonance (due to the rough nature of the silver film). The molecules are not chemisorbed on the silver surface and hence the enhancement occurs solely via the electromagnetic mechanism. Ale SERR spectra are virtually free of the fluorescence which dominates the corresponding RR spectra thus illustrating the use of SERRS in the vibrational spectroscopy of strongly luminescing species. The SERRS spectra of the substituted 2,2′-bipyridine complexes are discussed.     

Preparation of L-Cys–Au colloid self-assembled nanoarray electrode based on the microporous aluminium anodic oxide film and its application to the measurement of dopamine

A novel method for fabricating a nanoarray electrode combining the template technique with the self-assembled approach was developed. The glassy carbon electrode was modified with the Au nanoarray using micropores of aluminum anodic film as template. Then, the Au nanoarray electrode was self-assembled with L-cysteine (L-Cys) and gold colloid, respectively. In order to evaluate the electrochemical characteristics of L-Cys–Au colloid self-assembled nanoarray electrode, was chosen as molecule probe and cyclic voltammetry was used. In addition, the functional nanoarray electrode was applied to measuring dopamine (DA). The resulting L-Cys–Au colloid self-assembled nanoarray electrode demonstrated that the linear calibration range extended over three orders of magnitude of DA concentrations (1.0 × 10⁻⁹–1.0 × 10⁻⁶ mol/L) and the detection limit was 5.0 × 10⁻¹⁰ mol/L.     

Surface enhanced raman and resonance raman spectroscopy in a non-aqueous electrochemical environment: Tris(2,2′-bipyridine)ruthenium(II) adsorbed on silver from acetonitrile

This letter reports the first observation of both surface enhanced Raman scattering (SERS) and surface enhanced resonance Raman scattering (SERRS) from the transition metal complex tris(2,2′-bipyridine)ruthenium (II), Ru(bpy)₃²⁺, adsorbed on a silver electrode from acetonitrile (ACN). The assignment of these spectra as valid examples of SERS and SERRS in a non-aqueous environment is based on the following criteria: (1) in situ demonstration of monolayer surface coverage of Ru(bpy)₃²⁺ using double potential step chronocoulometry (DPSCC); (2) the Raman signals are most intense after surface roughening by anodization; (3) the Raman spectra are potential dependent in the non-faradaic potential region; (4) the measured enhancement factors are greater ilian 10⁶; (5) the surface spectra are frequency shifted relative to their bulk counterpart; and (6) several other molecules also exhibit non-aqueous SERS and SERRS behavior. These results are highly significant in that generality of surface enhanced Raman spectroscopy has been extended into the rich domain of nonaqueous electrochemistry.     

Immunoassay utilizing biochemistry reaction product via surface-enhanced Raman scattering in near field

In recent years, surface-enhanced Raman scattering (SERS) has been a topic of keen interest with regard to the detection of trace biological materials[1―8]. A wide range of SERS studies aimed at investigating structure, topology, and composition of biome…     

SERRS study of [Ru(CN)<Subscript>5</Subscript>(pyS)]<Superscript>4−</Superscript> SAM and cytochrome c: A suggestion toward the heterogeneous molecular recognition

Surface-Enhanced Raman Scattering (SERS) spectra of [Ru(CN)₅(pyS)]⁴⁻ (RupyS) complex self-assembled monolayer (SAM) were obtained on gold and silver surfaces at 632.8 and 413.1 nm excitation radiations, respectively. The bands assigned to the heme iron of the cytochrome c (cyt c) metalloprotein group were observed by using the RupyS SAM on silver at 413.1 nm. The Surface-Enhanced Resonance Raman Scattering (SERRS) spectra of the RupyS SAM on silver in the cyt c solution obtained at −0.2 and +0.2 V present bands at 1,365 and 1,374 cm⁻¹ characteristic of the heme group, indicating the reduced and oxidized states of this protein, respectively. The bands observed at 1,464, 1,504, and 1,638 cm⁻¹ are used to confirm the redox state of cyt c. The presence of the oxidized and reduced bands in function of different applied potential is an evidence that the protein is interacting with the modifier. This paper is dedicated to Prof. Francisco Nart, in memoriam.     

Electron paramagnetic resonance of carbon adsorbents from phenol-formaldehyde oligomers and their ability to adsorb noble metals

In order to establish the interrelation between the electronic characteristics of carbons that are manifested in the EPR spectra and their ability to adsorb complex gold and silver ions, the EPR spectra of carbon adsorbents from phenol-formaldehyde resins of various nature, unactivated and activated under various conditions, and the adsorption of [Au(CN)₂]^– and [Ag(CN)₂]^– from cyanide solutions on these same adsorbents were investigated. A parallel relation was observed between the amount of paramagnetic centers on carbon and its adsorption capacity and selectivity in relation to these anions.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 6, pp. 742–745, November–December, 1990.     

Charge distribution and 13C NMR spectra of cyanine dyes with bridging groupings in the chromophore

The electronic structure of cationoid and anionoid cyanine dyes was calculated by the CNDO/2 method. The calculation results were compared with the data of the ¹³C NMR spectra. It was shown that the anionoid dyes of the malonocyanine class have all the inherent properties of a polymethine state, i.e., alternation of charges and leveling of bonds, and these are also characteristic of cationic streptopolymethines. According to the calculation data, the introduction of bridging groupings causes considerable changes in the electronic structure of the cyanine dyes, as confirmed by the results of the study of their ¹³C NMR spectra.Translated from Teoreticheskaya i Ékperimental'naya Khimiya. Vol. 23, No. 6, pp. 687–692, November–December, 1987.     

Contribution of singlet oxygen to the accelerated decolorizing of dye mixtures

Binary mixtures of dispersed dyes, which during irradiation with polychromatic light give an effect of accelerated decolorization, were studied. Quantum yields of the formation of singlet oxygen by individual dyes were estimated during irradiation of solutions in ethyl acetate with light of 435 and 546 nm as well as quantum yields of the reaction of dyes with singlet oxygen. It was found that the contribution of the reaction of dyes with singlet oxygen to the effects of accelerated decolorization were vanishingly small. The quantum yield of the photodestruction of dyes in a mixture was 10^–3-10^–5 and the quantum yield of reaction with singlet oxygen was less than 10^–6. It was concluded that accelerated decolorization of the systems studied is not determined by reaction with singlet oxygen.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2014–2017, September, 1991.     

Voltammetric Determination of Rubeanic Acid

Procedures for the voltammetric determination of rubeanic acid (RA) at a mercury-film electrode were proposed. They are based on the oxidation of RA at –0.70 to –0.80 V or on the reduction of mercury sulfide, the product of RA oxidation, at –0.82 to –0.85 V (versus a saturated silver–silver chloride electrode) in a 1 M NaOH solution. The oxidation and reduction currents are linear functions of RA concentration in the ranges from 9 × 10^–6 to 3 × 10^–4 M and from 5 × 10^–6 to 3 × 10^–4 M, respectively.     

Identification and characterization of active and inactive species for surface-enhanced resonance Raman scattering

The surface-enhanced resonance Raman scattering (SERRS) activity of a statistically significant number of silver nanoparticles has been studied using a correlated SERRS mapping and transmission electron microscopy (TEM) method. TEM allowed the nature of each entity to be directly identified, and the SERRS activity was obtained from the corresponding SERRS map. Particles in various states of aggregation were analyzed to establish relative activities. It was established that SERRS activity is dependent on the specific batch of colloid tested. By averaging different colloid batches, it was shown that increasing SERRS activity is observed with increasing numbers of particles in the aggregates. By reducing the surface coverage of the particles to the extent that single moieties could be examined optically, the ratio of the relative activities of single particles, dimers, trimers, and larger aggregates was estimated. High-resolution TEM images of a number of active and inactive particles are reported. However, no clear correlation between microstructure and SERRS activity was observed.     

Silver-particle-based surface-enhanced resonance Raman scattering spectroscopy for biomolecular sensing and recognition

We demonstrate in this work that 2-μm-sized Ag (μAg) powders can be used as a core material for constructing biomolecular sensing/recognition units operating via surface-enhanced resonance Raman scattering (SERRS). This is possible because μAg powders are very efficient substrates for both the diffuse reflectance IR and the surface-enhanced Raman scattering–SERRS spectroscopic characterization of molecular adsorbates prepared in a similar manner on silver surfaces. Besides, the agglomeration of μAg particles in a buffer solution can be prevented by the layer-by-layer deposition of cationic and anionic polyelectrolytes such as poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). In this particular study, we used rhodamine B isothiocyanate (RhBITC) as a SERRS marker molecule, and μAg powders adsorbed consecutively with RhBITC and PAH–PAA bilayers were finally derivatized with biotinylated poly(l-lysine). On the basis of the nature of the SERRS peaks of RhBITC, those μAg powders were confirmed to selectively recognize streptavidin molecules down to concentrations of 10⁻¹⁰ g mL⁻¹. Since a number of different molecules can be used as SERS–SERRS marker molecules, the present method proves to be an invaluable tool for multiplex biomolecular sensing/recognition via SERS and SERRS.     

Langmuir Aggregation of Bordeaux R on a Cationic Surfactant and Its Application to Sensitive Quantitative Determination of Copper

The microphase adsorption-spectral correction (MPASC) technique has been applied to the study of the interaction between cetyl trimethylammonium bromide (CTMAB) and the dye Bordeaux R (BR) at pH 9.6. The aggregation of BR on the CTMAB surface obeys the Langmuir adsorption isotherm. The aggregation of BR on CTMAB accelerates the complexation between Cu(II) and BR. Results at 25°C show that the adsorption constant of the CTMAB–BR 1:1 aggregate is 6.80 × 10⁴. In the presence of CMTAB, the Cu–Br complex with a mole ratio of 2:1 has a cumulative stability constant of 1.08 × 10¹¹. The cooperative adsorption and complexation have been applied successfully in a sensitive determination of trace amounts of copper.     

Vibrational spectra of certain triphenylmethane dyes and their structure in solution

A combined analysis of two types of vibrational spectra (Raman and infrared) has proven the lactone structure of colorless solid preparations of phenolphthalein and thymolphthalein. It has been shown that phenolsulfophthalein, its alkyl derivatives, and bromcresol purple exist in the solid state primarily in the form of an intensely colored zwitterion; bromphenol blue and bromthymol blue exist primarily in the form of a colorless sultone. For the ionized forms of triphenylmethane dyes (cationic and anionic chromophore systems) vibrations with a frequency near 1360 cm^–1 are characteristic in the IR spectra and also in the Raman spectra.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 6, pp. 688–698, November–December, 1990.