Surface-enhanced Raman Scattering Spectral Features Research Articles

An approach for in-situ detection of gold colloid aggregates amyloid formations within the hippocampus of the Cohen’s Alzheimer’s disease rat model by surface enhanced raman scattering methods

BackgroundAmyloid beta (Aβ) peptides, such as Aβ1–40 or Aβ1–42 are regarded as hallmark neuropathological biomarkers associated with Alzheimer’s disease (AD). The formation of an aggregates by Aβ1–40 or Aβ1–42-coated gold nano-particles are hypothesized to contain conformation of Aβ oligomers, which could exist only at an initial stage of fibrillogenesis. New MethodThe attempt of in-situ detection of externally initiated gold colloid (ca. 80 nm diameter) aggregates in the middle section of the hippocampus of the Long Evans Cohen’s Alzheimer’s disease rat model was conducted through the Surface Enhanced Raman Scattering (SERS) method. ResultsThe SERS spectral features contained modes associated with β-sheet interactions and a significant number of modes that were previously reported in SERS shifts for Alzheimer diseased rodent and human brain tissues; thereby, strongly implying a containment of amyloid fibrils. The spectral patterns were further examined and compared with those collected from in-vitro gold colloid aggregates which were formed from Aβ1–40 - or Aβ1–42 -coated 80 nm gold colloid under pH ∼4, pH ∼7, and pH ∼10, and the best matched datasets were found with that of the aggregates of Aβ1–42 -coated 80 nm gold colloid at ∼pH 4.0. The morphology and physical size of this specific gold colloid aggregate was clearly different from those found in-vitro. Comparison with Existing Method(s)The amyloid fibril with a β-sheet conformation identified in previously reported in AD mouse/human brain tissues was involved in a formation of the gold colloid aggregates. However, to our surprise, best explanation for the observed SERS spectral features was possible with those in vitro Aβ1–42 -coated 80 nm gold colloid under pH ∼4. ConclusionsA formation of gold colloid aggregates was confirmed in the AD rat hippocampal brain section with unique physical morphology compared to those observed in in-vitro Aβ1–42 or Aβ1–40 mediated gold colloid aggregates. It was concluded that a β-sheet conformation identified in previously reported in AD mouse/human brain tissues was in volved in a formation of the gold colloid aggregates.

2-thiazoline-2-thiol functionalized gold nanoparticles for detection of heavy metals, Hg(II) and Pb(II) and probing their competitive surface reactivity: A colorimetric, surface enhanced Raman scattering (SERS) and x-ray photoelectron spectroscopic (XPS) study

A novel 2-thiazoline-2-thiol functionalized gold (Au-TT) nanosensor was developed for the selective and sensitive detection of toxic heavy metal ions, Hg(II) and Pb(II) by colorimetry, and for studying its competitive surface reactivity using SERS and XPS. The detection mechanism and surface reactivity is based on competitive binding affinity of thiocarbonyl S or thiazoline ring N/S atom with the metal ions and the nanoparticles (NPs). Due to diff ;erences in the binding affinities of the metal ions towards the active binding sites of TT, addition of different metal ions resulted in variation of colour and SERS spectral features. Of the various metal ions studied, viz. Cu(II), Cd(II), Co(II), Zn(II), Ni(II), Hg(II), Pb(II), Ca(II), Fe(II), Mn(II) and Sn(II), only Hg(II) and Pb(II) showed distinctive colorimetric and SERS spectral response that was quantified using XPS. This study, thus, realizes a selective and sensitive visual based nanosensor for Hg(II) and Pb(II) with limit of detection (LOD) of ∼ 0.1 ppm. SERS revealed the formation of Hg(TT)2 and Pb(TT)2 complexes on the Au NPs surface with the former remaining bonded to the NPs, resulting in enhanced Raman intensity for the 1026 cm−1 band, while the latter gets desorbed from the surface leading to reduced SERS intensity.

Interfacial structures of 1-methyladenine, 3-methyladenine, 7-methyladenine, and 9-methyladenine on gold nanoparticles by Raman spectroscopy

Interfacial structures of 1-methyladenine (1 MA), 3-methyladenine (3 MA), 7-methyladenine (7 MA), and 9-methyladenine (9 MA) on gold nanoparticles (AuNPs) were investigated by means of surface-enhanced Raman scattering (SERS). Different positions of the methyl group in the purine ring of adenine were found to result in not only dissimilarity among Raman spectral features but also surface binding schemes. Density functional theory (DFT) calculations predicted that the amino-9H tautomeric form would be most stable among the conformers of 1 MA, 3 MA, 7 MA, and 9 MA including amino and imino tautomeric conformers. SERS spectral features were analyzed with appropriate vibrational assignments based on DFT calculations. After considering the enhancement factors, the Raman spectra of 1 MA, 3 MA, 7 MA, and 9 MA on AuNPs were dissimilar, suggesting the methyl group at the purine ring of methyladenine may be significantly affecting the binding on AuNPs. Our Raman study indicates the position of the methyl group in methyladenine may play a significant role in coordinating metal surfaces.

SERS investigations on orientation of 2-bromo-3-methyl-1,4-dimethoxy-9,10-anthraquinone on silver nanoparticles

Silver nanoparticles (Ag NPs) were prepared by solution combustion method with urea as fuel. Silver nanoparticles were characterized by UV–visible spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Surface-enhanced Raman scattering (SERS) of 2-bromo-3-methyl-1,4-dimethoxy-9,10-anthraquinone (BMDMAQ) adsorbed on silver nanoparticles was investigated. The orientation of BMDMAQ on silver nanoparticles was inferred from nRs and SERS spectral features. Density functional theory (DFT) calculation was also performed to study the theoretical performance. The observed spectral features such as the high intensity of C–H out-of-plane bending mode and ring C–C stretching mode revealed that BMDMAQ adsorbed on silver surface through ‘stand-on’ orientation. Anthraquinone (AQ) derivatives have wide biomedical application which includes laxatives, antimalarials and antineoplastics used in the treatment of cancer. This present study would help to identify the interaction of drug molecules with DNA.

Orientation of 1,4-dimethoxy-3–bromomethylanthracence-9,10-dione on silver nanoparticles: SERS studies

Silver nanoparticles (Ag NPs) are prepared by solution combustion method with glycine as fuel. Silver nanoparticles are characterized by UV–visible spectroscopy, X-ray Diffraction (XRD) and High Resolution Transmission Electron Microscopy (HRTEM). The optical adsorption spectrum shows that the surface plasmon resonance peak of silver is observed at 380nm. The prepared silver nanoparticles exhibit cubic crystalline structure with grain size of 59nm. HRTEM image shows that the silver nanoparticles have strain and fivefold symmetry is formed by twinning in the crystal structure. Surface-enhanced Raman scattering (SERS) of 1,4-dimethoxy-3-bromomethyl-9,10anthraquinone dione (DMBMAD) adsorbed on silver nanoparticles has been investigated. The orientation of DMBMAD on silver nanoparticles has been inferred from nRs and SERS spectral features. The observed spectral feature corroborated that DMBMAD would absorb on silver surface with ‘stand-on’ orientation through the high intensity of CH out-of-plane bending mode, CBr stretching, ring stretching and CO. In present case we used the DMBMAD and Ag NPs as component molecules for energy gap analysis, whereas DMBMAD serve as donor and Ag NPs as acceptor. The calculated HOMO and LUMO energy gap shows that charge transfer occur within molecule and Ag NPs.

Raman spectral characteristics of 4-aminobenzenethiol adsorbed on ZnO nanorod arrays

Interest in the surface-enhanced Raman scattering (SERS) of 4-aminobenzenethiol (4-ABT) has surged recently. The SERS spectral features are highly dependent on the measurement conditions; a notable example is the appearance of b2-type bands that are not evident in the normal Raman (NR) spectrum. In an effort to discover new information and make any necessary corrections, we measured the Raman spectrum of 4-ABT adsorbed on a semiconducting material that would only enable the chemical enhancement mechanism; accordingly, the Raman spectrum of 4-ABT adsorbed on ZnO nanorods grown on an indium tin oxide substrate was measured for the first time. In the NR spectrum of the zinc salt of 4-ABT, which was taken as a reference, only the a1-type bands of 4-ABT were identified. However, in the surface Raman spectrum of 4-ABT on ZnO, the b2-type bands were also clearly evident, suggesting that the b2-type bands arose owing to its adsorption onto ZnO. The b2-type bands were also observed for 4-ABT analogs adsorbed on ZnO; this confirms that the b2-type bands were not a result of a surface-catalyzed photoreaction. Based on electric-potential and excitation-wavelength dependence studies, the a1- and b2-type bands were attributed to a charge-transfer (CT) transition from the surface defect levels of ZnO to the unoccupied La and Lb states (in Platt's notation) of 4-ABT, respectively; the bands gained intensity via the Herzberg-Teller coupling terms. The enhancement factor associated with the CT transition was estimated to be around 22, suggesting that it is, at best, a moderately effective process.

Surface-Enhanced Raman Scattering of 4-Aminobenzenethiol on Ag and Au: pH Dependence of b2-Type Bands

The surface-enhanced Raman scattering (SERS) of 4-aminobenzenehtiol (4-ABT) has seen a surge of interest recently, since its SERS spectral features are dependent not only on the kinds of SERS substrates but also on the measurement conditions. The most unusual SERS feature is the appearance of b2-type bands in the region 1100–1500 cm–1, in contrast to their absence in the normal Raman spectrum, but their origin is not yet clarified. However, propositions have been made suggesting that their appearance is associated with either a charge transfer phenomenon or a surface-induced photoreaction product such as 4,4′-dimercaptoazobenzene (4,4′-DMAB). In this work, we found that the b2-type bands of 4-ABT are strongly affected also by the solution pH. Regardless of the excitation wavelength and kind of SERS substrates, the b2-type bands appeared very weakly or negligibly at acidic pHs, while they were observed very distinctly at basic pHs. For the case of 4,4′-DMAB, any such pH dependence was not observed at all i...

Characterization of self-assembled phenyl and benzyl isothiocyanate thin films on Au surfaces

Structures of self-assembled monolayers (SAMs) prepared from benzyl isothiocyanate (BZIT) and phenyl isothiocyanate (PHIT) were comparatively investigated by means of surface enhanced Raman scattering (SERS) on gold nanoparticle surfaces. Both the molecules are assumed to have upright geometries binding via the sulfur atom, from the SERS spectral features. The broad and split bands at ∼2100 cm -1 could be ascribed to the combination band v 9a + v 12 of the phenyl ring vibrations through a Fermi-resonance interaction with the v 3 (NCS) band in the neat-liquid state. Such broad bands became quite symmetric upon adsorption on Au. UV-vis absorbance spectroscopy and cyclic voltammetry (CV) were used to compare the kinetics of the formation of SAMs on gold. Our results suggest that BZIT should have a relatively faster self-assembly than PHIT, as indicated from signal decrease in the CV graph, which suggests a more vertical stance for BZIT in line with Raman measurements.

Surface-Enhanced Raman Scattering of Benzyl Phenyl Sulfide in Silver Sol: Excitation-Wavelength-Dependent Surface-Induced Photoreaction

Ag sol prepared by the citrate-reduction method is a promising surface-enhanced Raman scattering (SERS) substrate that can be activated not only by 514.5 nm radiation but also by 568 nm and 632.8 nm radiation. Using these facts, the SERS of benzyl phenyl sulfide (BPS) in silver sol is revisited to assess the excitation-wavelength dependence of the C−S bond scission of BPS on Ag to produce surface-bound benzenethiolate. The SERS spectral features clearly indicate that the C−S bond cleavage of BPS on Ag occurs far more favorably under green light at 514.5 nm than under red light at 632.8 nm. The laser power-dependence study suggests further that the photodissociation of BPS proceeds on Ag via an initial single-photon absorption process rather than a multiphoton absorption one.

Adsorption Characteristics of Anthraquinone-2-carboxylic Acid on Gold

The adsorption of anthraquinone-2-carboxylic acid (AQ-2-COOH) on a gold surface has been investigated by reflection−absorption infrared (RAIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), surface-enhanced Raman scattering (SERS), quartz crystal microbalance (QCM), and ab initio quantum mechanical calculation. The RAIR spectral data clearly showed that the molecule was chemisorbed on gold as carboxylate, after deprotonation, with its two oxygen atoms bound symmetrically to the gold substrate; the molecular plane was determined to be tilted by 30° from the surface normal. The XPS and SERS spectral features were also consonant with the RAIR data. On the other hand, in CV, symmetrical redox peaks arising from a surface-confined species were reversibly identified at −647.5 mV, and the surface coverage determined from CV, that is, 1.8 × 10-10 mol/cm2, was consistent with that estimated from QCM. The adsorption rate of AQ-2-COOH on gold was, however, much slower than that on sil...