Raman scattering of L‐tryptophan enhanced by surface plasmon of silver nanoparticles: vibrational assignment and structural determination

Vibrational bands of L ‐tryptophan which was adsorbed on Ag nanoparticles (∼10 nm in diameter) have been investigated in the spectral range of 200–1700 cm⁻¹ using surface‐enhanced Raman scattering (SERS) spectroscopy. Compared with the normal Raman scattering (NRS) of L ‐tryptophan in either 0.5 M aqueous solution (NRS‐AS) or solid powder (NRS‐SP), the intensified signals by SERS have made the SERS investigation at a lower molecular concentration (5 × 10⁻⁴ M ) possible. Ab initio calculations at the B3LYP/6‐311G level have been carried out to predict the optimal structure and vibrational wavenumbers for the zwitterionic form of L ‐tryptophan. Facilitated with the theoretical prediction, the observed vibrational modes of L ‐tryptophan in the NRS‐AS, NRS‐SP, and SERS spectra have been analyzed. In the spectroscopic observations, there are no significant changes for the vibrational bands of the indole ring in either NRS‐AS, NRS‐SP, or SERS. In contrast, spectral intensities involving the vibrations of carboxylate and amino groups are weak in NRS‐AS and NRS‐SP, but strong in SERS. The intensity enhancement in the SERS spectrum can reach 10³–10⁴‐fold magnification. On the basis of spectroscopic analysis, the carboxylate and amino groups of L ‐tryptophan are determined to be the preferential terminal groups to attach onto the surfaces of Ag nanoparticles in the SERS measurement. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of an aliphatic spacer group on the adsorption mechanism on the colloidal silver surface of L‐proline phosphonodipeptides

A comparative study of molecular structures of five L ‐proline (L ‐Pro) phosphonodipeptides: L ‐Pro‐NH‐C(Me,Me)‐PO₃H₂ (P1), L ‐Pro‐NH‐C(Me,iPr)‐PO₃H₂ (P2), L ‐Pro‐L ‐NH‐CH(iBu)‐PO₃H₂ (P3), L ‐Pro‐L ‐NH‐CH(PA)‐PO₃H₂ (P4) and L ‐Pro‐L ‐NH‐CH(BA)‐PO₃H₂ (P5) has been carried out using Raman and absorption infrared techniques of molecular spectroscopy. The interpretation of the obtained spectra has been supported by density functional theory calculations (DFT) at the B3LYP; 6–31 + + G** level using Gaussian 2003 software. The surface‐enhanced Raman scattering (SERS) on Ag‐sol in aqueous solutions of these phosphonopeptides has also been investigated. The surface geometry of these molecules on a silver colloidal surface has been determined by observing the position and relative intensity changes of the Pro ring, amide, phosphonate and so‐called spacer (−R) groups vibrations of the enhanced bands in their SERS spectra. Results show that P4 and P5 adsorb onto the silver as anionic molecules mainly via the amide bond (∼1630, ∼1533, ∼1248, ∼800 and ∼565 cm⁻¹), Pro ring (∼956, ∼907 and ∼876 cm⁻¹) and carboxylate group (∼1395 and ∼909 cm⁻¹). Coadsorption of the imine nitrogen atom and PO group with the silver surface, possibly by formation of a weaker interaction with the metal, is also suggested by the enhancement of the bands at 1158 and 1248 cm⁻¹. P1, P2 and P3 show two orientations of their main chain on the silver surface resulting from different interactions of the  C CH₃,  NH and  CONH fragments with this surface. Bonding to the Ag surface occurs mainly through the imino atom (1166 cm⁻¹) for P2, while for P1 and P3 it occurs via the methyl group(s) (1194–1208 cm⁻¹). The amide group functionality (CONH) is practically not involved in the adsorption process for P1 and P2, whereas the C_s P bonds do assist in the adsorption. Copyright © 2008 John Wiley & Sons, Ltd.     

Food additives characterization by infrared,Raman, and surface‐enhanced Raman spectroscopies

Fourier‐transform infrared (FT‐IR), Raman (RS), and surface‐enhanced Raman scattering (SERS) spectra of β‐hydroxy‐β‐methylobutanoic acid (HMB), L ‐carnitine, and N‐methylglycocyamine (creatine) have been measured. The SERS spectra have been taken from species adsorbed on a colloidal silver surface. The respective FT‐IR and RS band assignments (solid‐state samples) based on the literature data have been proposed. The strongest absorptions in the FT‐IR spectrum of creatine are observed at 1398, 1615, and 1699 cm⁻¹, which are due to ν_s(COOH) + ν(CN) + δ(CN), ρ_s(NH₂), and ν(C O) modes, respectively, whereas those of L ‐carnitine (at 1396/1586 cm⁻¹ and 1480 cm⁻¹) and HMB (at 1405/1555/1585 cm⁻¹ and 1437–1473 cm⁻¹) are associated with carboxyl and methyl/methylene group vibrations, respectively. On the other hand, the strongest bands in the RS spectrum of HMB observed at 748/1442/1462 cm⁻¹ and 1408 cm⁻¹ are due to methyl/methylene deformations and carboxyl group vibrations, respectively. The strongest Raman band of creatine at 831 cm⁻¹ (ρ_w(R NH₂)) is accompanied by two weaker bands at 1054 and 1397 cm⁻¹ due to ν(CN) + ν(R NH₂) and ν_s(COOH) + ν(CN) + δ(CN) modes, respectively. In the case of L ‐carnitine, its RS spectrum is dominated by bands at 772 and 1461 cm⁻¹ assigned to ρ_r(CH₂) and δ(CH₃), respectively. The analysis of the SERS spectra shows that HMB interacts with the silver surface mainly through the  COO⁻, hydroxyl, and  CH₂ groups, whereas L ‐carnitine binds to the surface via  COO⁻ and  N⁺(CH₃)₃ which is rarely enhanced at pH = 8.3. On the other hand, it seems that creatine binds weakly to the silver surface mainly by  NH₂, and C O from the  COO⁻ group. Copyright © 2006 John Wiley & Sons, Ltd.     

Characterization of sodium alginate and its block fractions by surface‐enhanced Raman spectroscopy

The surface‐enhanced Raman scattering (SERS) of sodium alginates and their hetero‐ and homopolymeric fractions obtained from four seaweeds of the Chilean coast was studied. Alginic acid is a copolymer of β‐D ‐mannuronic acid (M) and α‐L guluronic acid (G), linked 1 → 4, forming two homopolymeric fractions (MM and GG) and a heteropolymeric fraction (MG). The SERS spectra were registered on silver colloid with the 632.8 nm line of a He Ne laser. The SERS spectra of sodium alginate and the polyguluronate fraction present various carboxylate bands which are probably due to the coexistence of different molecular conformations. SERS allows to differentiate the hetero‐ and homopolymeric fractions of alginic acid by characteristic bands. In the fingerprint region, all the poly‐D ‐mannuronate samples present a band around 946 cm⁻¹ assigned to C O stretching, and C C H and C O H deformation vibrations, a band at 863 cm⁻¹ assigned to deformation vibration of β‐C₁ H group, and one at 799–788 cm⁻¹ due to the contributions of various vibration modes. Poly‐L ‐guluronate spectra show three characteristic bands, at 928–913 cm⁻¹ assigned to symmetric stretching vibration of C O C group, at 890–889 cm⁻¹ due to C C H, skeletal C C, and C O vibrations, and at 797 cm⁻¹ assigned to α C₁ H deformation vibration. The heteropolymeric fractions present two characteristic bands in the region with the more important one being an intense band at 730 cm⁻¹ due to ring breathing vibration mode. Copyright © 2009 John Wiley & Sons, Ltd.     

Nanostructured thin films as surface‐enhanced Raman scattering substrates

Nanoporous thin films with silver nanoparticles were synthesized with a bottom–up approach, and its potential as effective surface‐enhanced Raman scattering (SERS) substrates was demonstrated. The use of mesoporous titania films as substrates allowed to control the growth of nanoparticles on the film surface. Atomic force microscopy measurements, Ultraviolet‐visible and X‐ray diffraction analysis confirmed the photoreduction of Ag⁺ to Ag⁰ with the formation of nanoparticles with crystallite dimensions of 32 to 36 nm. The new substrates allowed the detection of two analytes (rhodamine B isothiocyanate and cytochrome c), present in solutions at very low concentrations, highlighting their potential in SERS sensing. Reproducibility, homogeneity, enhancement factor of the substrate, consistency of results and detection limits were also assessed. Copyright © 2012 John Wiley & Sons, Ltd.     

Comparison of SERS effectiveness of copper substrates prepared by different methods: what are the values of enhancement factors?

Surface‐enhanced Raman scattering (SERS) spectroscopy is an analytical method for the detection of low amounts of analytes adsorbed on an appropriate coinage metal (Au, Ag, Cu) surface. Generally, the values of the enhancement factor are the highest on silver, lower on gold and relatively very low on copper. In this study, we have focused on the estimation of the enhancement factors of copper surface/substrates formed by different preparation procedures. The SERS activity of large electrochemically prepared substrates and colloidal systems is compared. The surface morphology of the large substrates was studied using scanning electron microscopy and atomic force microscopy. The size distribution of colloidal nanoparticles was monitored by dynamic light scattering. The values of enhancement factor are in both cases more than 10⁵ for the FT‐SERS spectra, demonstrating the fundamental role of nanostructured copper as a substrate material at the excitation wavelength (1064 nm) used. Copyright © 2011 John Wiley & Sons, Ltd.     

Rapid atto‐molar level detection of surface‐enhanced Raman spectroscopy technique based on glycidyl methacrylate–ethylene dimethacrylate (GMA–EDMA) porous material

A novel miniature device for rapid ultra‐sensitive surface‐enhanced Raman scattering (SERS) detection was developed in the present study. The device was made of a syringe, a piece of filter, and a Teflon tube. Therefore, it was with advantages of simplicity, miniaturization, and easy operability. The tube was filled in advance with the glycidyl methacrylate‐ethylene dimethacrylate powder porous material which has been proved to increase the sensitivity of normal SERS dramatically, then the mixture solution containing the analyte, silver colloid, and NaCl solution passed through the porous material by the action of the syringe. SERS signals were collected from the surface of the material. Rhodamine 6G (R6G), p‐aminothiophenol (PATP), and thiabendazole (TBZ) were employed as the probe molecules in the present work. R6G at microlitre‐scale can be detected at an extremely low concentration of 10^–18 mol/l, and the relative standard deviation of spot to spot is 14.16% at the intensity of the band at 609 cm⁻¹. The concentrations of PATP and TBZ that can be detected with the method are 10⁻¹¹ mol/l and 1.3 × 10⁻⁶ mol/l, respectively. This method not only has achieved the ultra‐sensitive detection of dye and pesticide but also realized the simple, rapid, and small sample quantity requirement detection, and it is of great potential use for lots of analytes. Copyright © 2013 John Wiley & Sons, Ltd.     

Self‐assembled silver nanoparticle monolayer on glassy carbon: an approach to SERS substrate

In this paper, the fabrication of an active surface‐enhanced Raman scattering (SERS) substrate by self‐assembled silver nanoparticles on a monolayer of 4‐aminophenyl‐group‐modified glassy carbon (GC) is reported. Silver nanoparticles are attached to the substrate through the electrostatic force between the negatively charged silver nanoparticles and the positively charged 4‐aminophenyl groups on GC. The active SERS substrate has been characterized by means of tapping‐mode atomic force microscopy (AFM), indicating that large quantities of silver nanoparticles are uniformly coated on the substrate. Rhodamine 6G (R6G) and p‐aminothiophenol (p‐ATP) are used as the probe molecules for SERS, resulting in high sensitivity to the SERS response, with the detection limit reaching as low as 10⁻⁹ M . This approach is easily controlled and reproducible, and more importantly, can extend the range of usable substrates to carbon‐based materials for SERS with high sensitivity. Copyright © 2007 John Wiley & Sons, Ltd.     

Vibrational characterization of L‐valine phosphonate dipeptides: FT‐IR,FT‐RS,and SERS spectroscopy studies and DFT calculations

Four L ‐valine (L ‐Val) phosphonate dipeptides that are potent inhibitors of zinc metalloproteases, namely, L ‐Val‐C(Me)₂‐PO₃H₂ (V1), L ‐Val‐CH(iP)‐PO₃H₂ (V2), L ‐Val‐CH(iB)‐PO₃H₂ (V3), and L ‐Val‐C(Me)(iP)‐PO₃H₂ (V4), are studied by Fourier‐transform infrared (FT‐IR) spectroscopy, Fourier‐transform Raman spectroscopy (FT‐RS), and surface‐enhanced Raman scattering (SERS). The band assignment (wavenumbers and intensities) is made based on (B3LYP/6‐311 + + G**) calculations. Comparison of theoretical FT‐IR and FT‐RS spectra with those of SERS allows to obtain information on the orientation of these dipeptides as well as specific‐competitive interactions of their functionalities with the silver substrate. More specifically, V1 and V4 appear to interact with the silver substrate mainly via a  C_sgCH₃ moiety localized at the  N_amideC_sg(CH₃)P molecular fragment. In addition, the  POH and isopropyl units of V4 assist in the adsorption process of this molecule. In contrast, the  C_αNH₂ and  PO₃H⁻ groups of V2 and V3 interact with the silver nanoparticles, whereas their isopropyl and isobutyl fragments seem to be repelled by the silver substrate (except for the  CH₂  of V3), similar to the  C_β(CH₃)₂ fragment of L ‐Val for all L ‐Val phosphonate dipeptides investigated in this work. The adsorption mechanism of these molecules onto the colloidal silver surface is also affected by amide bond behavior. Copyright © 2010 John Wiley & Sons, Ltd.     

SERS studies of DNA nucleobases using new silver poly(methyl methacrylate) nanocomposites as analytical platforms

The implementation of polymer‐based composites provides a plausible alternative to develop efficient, handy and scalable substrates for surface‐enhanced Raman spectroscopy (SERS) aiming the widespread use of this technique for chemical analysis and molecular sensing. In this research, new poly(methylmethacrylate) based nanocomposites for SERS were prepared by in situ miniemulsion polymerization in the presence of organically capped metal silver nanoparticles. The ensuing composites have been investigated as analytical platforms for SERS detection of DNA constituents for variable analytical conditions. Finally, we show that in special cases, selective detection of DNA bases by SERS can be possible by varying the pH of the solution under analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

IR/Raman spectroscopy and DFT calculations of cyclic di‐amino acid peptides. Part III: comparison of solid state and solution structures of cyclo(L‐Ser‐L‐Ser)

B3‐LYP/cc‐pVDZ calculations of the gas‐phase structure and vibrational spectra of the isolated molecule cyclo(L ‐Ser‐L ‐Ser), a cyclic di‐amino acid peptide (CDAP), were carried out by assuming C₂ symmetry. It is predicted that the minimum‐energy structure is a boat conformation for the diketopiperazine (DKP) ring with both L ‐seryl side chains being folded slightly above the ring. An additional structure of higher energy (15.16 kJ mol⁻¹) has been calculated for a DKP ring with a planar geometry, although in this case two fundamental vibrations have been calculated with imaginary wavenumbers. The reported X‐ray crystallographic structure of cyclo(L ‐Ser‐L ‐Ser), shows that the DKP ring displays a near‐planar conformation, with both the two L ‐seryl side chains being folded above the ring. It is hypothesized that the crystal packing forces constrain the DKP ring in a planar conformation and it is probable that the lower energy boat conformation may prevail in the aqueous environment. Raman scattering and Fourier‐transform infrared (FT‐IR) spectra of solid state and aqueous solution samples of cyclo(L ‐Ser‐L ‐Ser) are reported and discussed. Vibrational band assignments have been made on the basis of comparisons with the calculated vibrational spectra and band wavenumber shifts upon deuteration of labile protons. The experimental Raman and IR results for solid‐state samples show characteristic amide I vibrations which are split (Raman: 1661 and 1687 cm⁻¹, IR: 1666 and 1680 cm⁻¹), possibly due to interactions between molecules in a crystallographic unit cell. The cis amide I band is differentiated by its deuterium shift of ∼30 cm⁻¹, which is larger than that previously reported for trans amide I deuterium shifts. A cis amide II mode has been assigned to a Raman band located at 1520 cm⁻¹. The occurrence of this cis amide II mode at a wavenumber above 1500 cm⁻¹ concurs with results of previously examined CDAP molecules with low molecular weight substituents on the C^α atoms, and is also indicative of a relatively unstrained DKP ring. Copyright © 2009 John Wiley & Sons, Ltd.     

On‐site detection of phosgene agents by surface‐enhanced Raman spectroscopy coupled with a chemical transformation approach

Phosgene and its analogs are greatly harmful to the public health, environmental safety and homeland security as widely used industrial substances with extremely high toxicity. In order to rapidly evaluate the emergency risk caused by these chemicals, a new highly sensitive method based on surface‐enhanced Raman spectroscopy (SERS) technique for measurement of phosgene agents was developed for the first time. Coupled with a chemical transformation approach, the highly toxic phosgene was conveniently converted to a SERS‐sensitive probe, i.e. iodine (I₂), with low toxicity or non‐toxicity. The characteristic SERS peak in 459 cm⁻¹ was used for quantitation and was presumed as a formation of triiodide anion (I₃⁻), which was induced in an iodide (I⁻)‐aggregation Au NPs system. The total measurement can be completed in ~20 min with the limits of detection of ~60 µg/l (phosgene) and ~30 µg/l (diphosgene), respectively, on a portable Raman spectrometer. This work is the first report of SERS measurement on phosgene and diphosgene in a quantitative level. This method is expected to meet the requirements of on‐site detection of phosgene agents, promote emergency responses and raise more opportunities for the portable SERS applications. Copyright © 2015 John Wiley & Sons, Ltd.     

Comparison of adsorption mechanism on colloidal silver surface of alafosfalin and its analogs

This study reports the Raman (FT‐RS) and absorption infrared (FT‐IR) spectra, based on calculated wavenumbers and normal modes of vibrations, of the following compounds: L ‐Ala‐L ‐NH‐CH(Me)‐PO₃H₂ (alafosfalin, A1), L ‐Ala‐D ‐NH‐CH(Me)‐PO₃H₂ (A2), L ‐Ala‐L ‐NH‐CH(Et)‐PO₃H₂ (A3), D ,L ‐Ala‐D,L ‐NH‐CH(Et)‐PO₃H₂ (A4), L ‐Ala‐D ‐NH‐CH(iPr)‐PO₃H₂ (A5), L ‐Ala‐D,L ‐NH‐CH(iPr)‐PO₃H₂ (A6), L ‐Ala‐D,L ‐NH‐CH(tBu)‐PO₃H₂ (A7), L ‐Ala‐D,L ‐NH‐CH(iBu)‐PO₃H₂ (A8), L ‐Ala‐D,L ‐NH‐CH(cBu)‐PO₃H₂ (A9), L ‐Ala‐D,L ‐NH‐CH(nPA)‐PO₃H₂ (A10), β‐Ala‐D ‐NH‐CH(Me)‐PO₃H₂ (A11), and D,L ‐Ala‐NH‐C(Me,Me)‐PO₃H₂ (A12). The equilibrium geometries and vibrational wavenumbers are calculated using density functional theory (DFT) at the B3LYP; 6–31 + + G** level of theory using Gaussian'03, GaussSum 0.8, and GAR2PED software. We briefly compare and analyze the experimental and calculated vibrational wavenumbers in the range of 3600–400 cm⁻¹. In addition, Raman wavenumbers are compared to those from surface‐enhanced Raman scattering (SERS) for the phosphonodipeptides of alanine (Ala) adsorbed on a colloidal silver surface. The geometry of these molecules etched on the silver surface is deduce from the observed changes in both the intensity and breadth of Raman bands in the spectra of the bound vs free species. For example, A7, A8, A1, A3, and A4 appear to adsorb onto the colloidal silver particles mainly through the phosphonate terminus, and for A3 and A4, through the  C‐NH₂ and  CONH fragments. The most dominant SERS bands of A5, A6, A9, A10, and A11 are due to the amide bond vibrations, as well as to the vibrations of the  C‐NH₂ group (A9 and A10) and the C C group (A6 and A11). The differences recorded for the A5, A6, A9, A10, and A11 and those of A2 and A12 are due to interactions between the amine and methyl groups with the silver surface, and they reflect vibrational characteristic of these groups. Copyright © 2008 John Wiley & Sons, Ltd.     

Effects of solid substrate on the SERS‐based immunoassay: a comparative study

A comparative study of the solid substrates used in surface‐enhanced Raman scattering (SERS) based immunoassay is made in this paper. Five different substrates were prepared and divided into two groups with and without SERS activity. They are (1) a poly‐L ‐lysine slide, (2) a glutaraldehyde (GA)‐aminosilane slide, (3) a substrate assembled with silver nanoparticles, (4) a substrate assembled with silver nanoparticles and functionalized with GA–aminosilane and (5) a substrate assembled with gold nanoparticles, of which the first two are substrates are without SERS activity and the latter three are with SERS activity because of the existence of the metallic nanoparticles. The SERS experimental results show that the immunoassay performed on an SERS‐active substrate is more effective than that employing the inactive substrate. Among the inactive substrates, the GA–aminosilane slide with a better ability for antibody immobilization leads to a more sensitive immunoassay than the poly‐L ‐lysine slide. Moreover, for SERS‐based immunoassay, the substrate with assembled silver nanoparticles has an advantage of higher SERS enhancement capacity over the substrate assembled with gold nanoparticles. This work indicates that SERS‐active substrates play important and positive roles in sensitive SERS‐based immunoassay. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of amino acid modifications on the molecular structure of adsorbed and nonadsorbed bombesin 6–14 fragments on an electrochemically roughened silver surface

This work used infrared absorption and Raman spectroscopy to determine the structure of seven modified fragments (residues 6–14 of the polypeptide chain) of bombesin (BN^6–14). The peptides studied are cyclo[D ‐Phe⁶, His⁷, Leu¹⁴]BN^6–14, [D ‐Phe⁶, Leu‐NHEt¹³, des‐Met¹⁴]BN^6–14, [D ‐Phe⁶, Leu¹³‐®‐p‐chloro‐Phe¹⁴]BN^6–14, [D ‐Phe⁶, β‐Ala¹¹, Phe¹³, Nle¹⁴]BN^6–14, [D ‐Tyr⁶, β‐Ala¹¹, Phe¹³, Nle¹⁴]BN^6–14, [D ‐Tyr⁶, β‐Phe¹¹, Phe¹³, Nle¹⁴OH]BN^6–14 and [D ‐Cys⁶, Asn⁷, D ‐Ala¹¹, Cys¹⁴]BN^6–14. These peptides are potent bombesin agonists useful in the treatment of tumors. Surface‐enhanced Raman scattering (SERS) spectroscopy was also used to examine the behavior of these molecules on an electrochemically roughened silver surface. The SERS spectra reveal that substituting native amino acids in these molecules with synthetic ones changes their adsorption state slightly on an electrochemically roughened surface of silver. The peptides [D ‐Tyr⁶, β‐Ala¹¹, Phe¹³, Nle¹⁴]BN^6–14 and [D ‐Tyr⁶, β‐Phe¹¹, Phe¹³, Nle¹⁴OH]BN^6–14 tend to adsorb strongly on this surface via C fragment (∼1400 cm⁻¹). The observed medium enhancement of the Trp⁸ residue and amide bond Raman signals indicate further interactions between these fragments and the surface. [D ‐Phe⁶, Leu‐NHEt¹³, des‐Met¹⁴]BN^6–14 and [D ‐Cys⁶, Asn⁷, D ‐Ala¹¹, Cys¹⁴]BN^6–14 are shown to be coordinated to the silver through  CONH , CO, and the indole ring. The strongest SERS bands (∼1506, ∼1275, ∼1149, and ∼1007 cm⁻¹) of [D ‐Phe⁶, Leu¹³‐®‐p‐chloro‐Phe¹⁴]BN^6–14 and [D ‐Phe⁶, β‐Ala¹¹, Phe¹³, Nle¹⁴]BN^6–14 suggest that these two peptides bind to the silver via Trp⁸ and  CONH . In the case of cyclo[D ‐Phe⁶, His⁷, Leu¹⁴]BN^6–14, the formation of a peptide/Ag complex is confirmed by the strong SERS bands involving Trp⁸ and  CONH vibrations, which are accompanied by a SERS signal due to the CO vibrations. For these analogs, the relative potency for inhibition of binding of ¹²⁵I‐[Tyr⁴]BN to rat pancreas acini cells was correlated with the behavior of the amide bond on the silver surface, while the contribution of the structural components to the ability to interact with the rGRP‐R was correlated with the SERS patterns. Copyright © 2008 John Wiley & Sons, Ltd.     

New SERS feature of β‐carotene: consequences for quantitative SERS analysis

While recording SERS spectra of pure β‐carotene at sub‐micromole concentrations for reference purpose, we discovered an unusual spectral response never reported before. In pre‐resonance conditions with the 532‐nm line, SERS of β‐carotene with AgNPs exhibits among the strong υ(CC) mode at 1512 cm⁻¹ unshifted from normal Raman spectrum, additional strong bands at 1649, 1575 and 1387 cm⁻¹ as well as other medium bands not observed in the Raman spectrum of the crystalline powder. Such behavior is explained in terms of selection rules relaxation upon cyclohexene terminal rings of the β‐carotene interaction with the NP surface. AFM images of the SERS system suggested dimers and trimers clustering of the nanoparticles with adsorbed β‐carotene. In light of the new SERS feature the consequences in correct interpretation of the SERS imaging from complex biosystems containing carotenoids are discussed. Relative intensity ratio of the β‐carotene band at 1512 cm⁻¹ and water against concentration allowed a reliable SERS calibration curve for 50 to 500 nmol l⁻¹ concentration range and provided quantitative SERS assessment of the carotenoid content in the sea urchin (Paracentrotus lividus) gonads extracts. Copyright © 2015 John Wiley & Sons, Ltd.     

Fabrication of periodical Ag–Au compound nanostructure films with controllable Ag nanoparticle aggregate patterns: a study on surface‐enhanced Raman scattering

The Ag–Au compound nanostructure films with controllable patterns of Ag nanoparticle (NP) aggregates were fabricated. A strategy of two‐step synthesis was employed toward the target products. Firstly, the precursor Au NP (17 nm) films were synthesized as templates. Secondly, the Ag NPs (45 nm) were deposited on the precursor films. Three types of Ag NP aggregates were obtained including discrete Ag NPs (discrete type), necklace‐like Ag NP aggregates (necklace type), and huddle‐like Ag NP aggregates (huddle type). The surface‐enhanced Raman scattering (SERS) property was studied on these nanostructures by using the probing molecule of rhodamine 6G under the excitation laser of 514.5 nm. Interestingly, the different types of samples showed different enhancement abilities. A statistical method was employed to assess the enhancement. The relative enhancement factor for each Ag NP was estimated quantitatively under the ratio of 1 : 25 : 18 for the discrete‐type, necklace‐type, and huddle‐type samples at the given concentration of 10⁻⁸ mol/l. This research shows that the enhancement ability of each Ag NP is dependent on the aggregate morphology. Moreover, the different enhancement abilities displayed different limit detection concentrations up to 10⁻⁸, 10⁻¹¹, and 10⁻⁹ mol/l, separately. The understanding of the relationship between the defined nanostructures and the SERS enhancement is very meaningful for the design of new SERS substrates with better performance. Copyright © 2015 John Wiley & Sons, Ltd.     

Vibrational spectra and crystal structure of the di‐amino acid peptide cyclo(L‐Met‐L‐Met): comparison of experimental data and DFT calculations

Experimental Raman and FT‐IR spectra of solid‐state non‐deuterated and N‐deuterated samples of cyclo(L ‐Met‐L ‐Met) are reported and discussed. The Raman and FT‐IR results show characteristic amide I vibrations (Raman: 1649 cm⁻¹, infrared: 1675 cm⁻¹) for molecules exhibiting a cis amide conformation. A Raman band, assigned to the cis amide II vibrational mode, is observed at ∼1493 cm⁻¹ but no IR band is observed in this region. Cyclo(L ‐Met‐L ‐Met) crystallises in the triclinic space group P1 with one molecule per unit cell. The overall shape of the diketopiperazine (DKP) ring displays a (slightly distorted) boat conformation. The crystal packing employs two strong hydrogen bonds, which traverse the entire crystal via translational repeats. B3‐LYP/cc‐pVDZ calculations of the structure of the molecule predict a boat conformation for the DKP ring, in agreement with the experimentally determined X‐ray structure. Copyright © 2009 John Wiley & Sons, Ltd.     

Ion beam engineered nano silver silicon substrates for surface enhanced Raman spectroscopy

We demonstrate a method for engineering substrates for surface enhanced Raman spectroscopy (SERS) by Ag⁻ ion implantation in Si. The implantation dose and beam current density are chosen such that the Ag concentration in Si exceeds the solid solubility limit, causes aggregation of Ag and nucleates Ag nano particles. The embedded nano particles are then partially exposed by a wet etch process. Our measurements show that the so fabricated nano‐composite substrates are very effective as stable and reproducible SERS substrates. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis of indium–silver bimetallic nanocomposites for surface-enhanced Raman scattering

We report on the synthesis of indium–silver bimetallic nanocomposites by chemical reduction method under atmospheric condition and their activity for surface-enhanced Raman scattering (SERS). It is found that the indium–silver bimetallic nanocomposites have better SERS activity with larger enhancement factors (EF) than pure silver nanoparticles with similar size. The SERS EF can reach 10⁷ for 4-mercaptobenzoic acid and 10⁹ for crystal violet and rhodamine 6G adsorbed on the nanocomposites and the detection limits can be at least down to 10^?7 and 10^?10 M, respectively. The results demonstrate that the indium–silver bimetallic nanocomposites are promising as SERS substrate for a myriad of chemical and biological sensing applications.