Adsorption of S-S containing proteins on a colloidal silver surface studied by surface-enhanced Raman spectroscopy

We present a Raman and surface-enhanced Raman scattering (SERS) study of the following proteins containing S-S group(s): alpha chymotrypsin (alpha-CHT), insulin, lysozyme, oxytocin (OXT), Streptomyces subtilisin inhibitor (SSI), and trypsin inhibitor (STI). The SERS study is performed in order to understand the adsorption mechanism of the above-mentioned proteins on a colloidal silver surface. The SERS spectra presented here show bands associated mainly with aromatic amino acid vibrations. In addition, two distinct vibrations of the -C-S-S-C- fragment are observed in the Raman and SERS spectra, i.e., nu(SS) and nu(CS). The enhancement of the nu(SS) vibration in the SERS spectra yields evidence that the intact disulfide bridge(s) is (are) located near the silver surface. This finding is supported by the presence of the nu(CS) mode(s). The presence of nus(COO-) and nu(C-COO-) in the SERS spectra in the 1384-1399 cm(-1) and 909-939 cm(-1) regions, respectively, indicate that the negatively charged COO- groups (aspartic and glutamic acids) assist in the binding on the positively charged silver surface. The Raman amide I and III bands observed in the 1621-1633 and 1261-1289 cm(-1) ranges, respectively, indicate that the alpha-helical conformation is favored for binding to the surface over the random coil or beta-sheet conformations. In addition, the presence of the imino group of Trp and/or His indicates that these amino acid residues may also bind to the silver sol.     

Preparation of silver nanoparticles by photo-reduction for surface-enhanced Raman scattering

A substrate for surface-enhanced Raman scattering (SERS) has been developed. Based on the surface-catalyzed reduction of Ag⁺ by citrate on the silver nanoparticles surface under light irradiation, small silver seeds on a quartz slide can be enlarged. The optical properties and characteristics of the silver films have been investigated by ultraviolet-visible spectroscopy, scan electron microscope and atomic force microscopy (AFM). The results indicate that the particle size and shape are different at different reduction time. At the first 3 h, some triangular and hexagonal nanoparticles formed; with the reduction proceeding, the shape of the silver particles became irregular and the size became larger. The silver films obtained are very suitable as SERS active substrate. The relationship between SERS intensity and the reduction time has been investigated for 1,4-bis[2-(4-pyridyl)ethenyl]-benzene molecule adsorbed on the silver film. The SERS intensity reached a maximum at 8 h reduction. The AFM measurements indicate that roughness features with an average size of 100 nm are present on the surface, which yielded the strongest SERS signal. Pyridine was used as a probe molecule to investigate the enhancement factor (EF) of the silver films. According to the formalism of Tian and co-workers, the EF of the silver films is estimated to be 3.4 × 10⁵. The silver film that can remain active for more than 50 days would seem to be suitable for various analytical applications.     

Reproducible surface-enhanced Raman scattering spectra of bacteria on aggregated silver nanoparticles

Surface-enhanced Raman scattering (SERS) is proven to be a powerful technique for rapid identification and discrimination of microorganisms. However, due to the heterogeneous nature of the samples, the acquisition of reproducible spectra hinders the further development of the technique. In this study, we demonstrate the influence of the experimental conditions on SERS spectra. Then, we report a simple sample preparation method coupled with a light microscope attached to a Raman spectrometer to find a proper spot on the sample to acquire reproducible SERS spectra. This method utilizes the excited surface plasmons of the aggregated silver nanoparticles to visualize the spots on the sample. The samples are prepared using the concentrated silver colloidal solutions. The collection time for one spectrum is 10 s and each spectrum is a very good representative of the other spectra acquired from the same sample. The nature of the surface charge of the silver nanoparticles influences the spectral features by determining the strength of the interactions between nanoparticles and bacteria and the aggregation properties of the nanoparticles. Although increasing the colloid concentration in the sample resulted in reproducible spectra from arbitrary points on the sample, a great variation from sample to sample prepared with the different colloidal solution concentrations is observed.     

Effect of oxidation on surface-enhanced Raman scattering activity of silver nanoparticles: a quantitative correlation

We quantitatively studied, using X-ray photoelectron spectroscopy (XPS), oxidation of substrate-immobilized silver nanoparticles (Ag NPs) in a wide range of conditions, including exposure to ambient air and controlled ozone environment under UV irradiation, and we correlated the degree of silver oxidation with surface-enhanced Raman scattering (SERS) enhancement factors (EFs). The SERS activity of pristine and oxidized Ag NPs was assessed by use of trans-1,2-bis(4-pyridyl)ethylene (BPE) and sodium thiocynate as model analytes at the excitation wavelength of 532 nm. Our study showed that the exposure of Ag NPs to parts per million (ppm) level concentrations of ozone led to the formation of Ag(2)O and orders of magnitude reduction in SERS EFs. Such an adverse effect was also notable upon exposure of Ag NPs under ambient conditions where ozone existed at parts per billion (ppb) level. The correlated XPS and SERS studies suggested that formation of just a submonolayer of Ag(2)O was sufficient to decrease markedly the SERS EF of Ag NPs. In addition, studies of changes in plasmon absorption bands pointed to the chemical enhancement as a major reason for deterioration of SERS signals when substrates were pre-exposed to ambient air, and to a combination of changes in chemical and electromagnetic enhancements in the case of substrate pre-exposure to elevated ozone concentrations. Finally, we also found UV irradiation and ozone had a synergistic effect on silver oxidation and thus a detrimental effect on SERS enhancement of Ag NPs and that such oxidation effects were analyte-dependent, as a result of inherent differences in chemical enhancements and molecular binding affinities for various analytes.     

Active control of silver nanoparticles spacing using dielectrophoresis for surface-enhanced Raman scattering

We demonstrate an active microfluidic platform that integrates dielectrophoresis for the control of silver nanoparticles spacing, as they flow in a liquid channel. By careful control of the nanoparticles spacing, we can effectively increase the surface-enhanced Raman scattering (SERS) signal intensity based on augmenting the number of SERS-active hot-spots, while avoiding irreversible aggregation of the particles. The system is benchmarked using dipicolinate (2,6-pyridinedicarboxylic acid) (DPA), which is a biomarker of Bacillus anthracis. The validity of the results is discussed using several complementing characterization scenarios.     

Adsorption characteristics of 4,4′-bipyridine molecules on gold nanosphere films studied by surface-enhanced Raman scattering

We have measured and compared the surface-enhanced Raman scattering (SERS) spectra of 4,4′-bipyridine (bpy) molecules deposited (adsorbed) on the films of gold nanospheres (NSs, mean diameter 35 ± 5 nm), as prepared by the salting-out method using NaClO₄ and NaOH. The bpy-deposited (adsorbed) films were prepared by casting an aliquot of the bpy-ethanol solutions in different concentrations (less and above the monolayer converge on the gold surface), or on immersion of the NS films into the concentrated bpy solution. The enhancement of the SERS spectra of bpy was larger for the NS film prepared from NaClO₄ than that from NaOH. But, the SERS intensity was independent of the concentration of the bpy solution. The changes in the intensities and the shapes of the SERS spectra were distinctly observed on immersion of the bpy-deposited (adsorbed) NS films into pure ethanol, and these spectral changes were prominent for the NS film prepared from NaClO₄. This must be attributed to considerably coalesced structures of the NS film prepared from NaClO₄. From the comparison of the intensity changes before and after immersion into ethanol, it is suggested that the vertical orientation of the bpy molecules adsorbed on the NS films changes into the flat orientation with respect to the surface of the gold nanoparticles at the solid-liquid interface, on immersion of the film into pure ethanol.     

Type I collagen-templated assembly of silver nanoparticles and their application in surface-enhanced Raman scattering

Silver nanoparticles (Ag NPs) are one of the active substrates that are employed extensively in surface-enhanced Raman scattering (SERS), and aggregations of Ag NPs play an important role in enhancing the Raman signals. In this paper, we fabricated two kinds of SERS-active substrates utilizing the electrostatic adsorption and superior assembly properties of type I collagen. These were collagen-Ag NP aggregation films and nanoporous Ag films. Two probe molecules, 4-aminothiophenol (4-ATP) and methylene blue (MB), were studied on these substrates. These substrates showed reproducible SERS intensities with relative standard deviations (RSDs) of 8-10% and 11-14%, respectively, while the RSDs of the traditional thick Ag films were 12-28%. Also, the intensities for the 4-ATP spectrum on the collagen-templated nanoporous Ag film were approximately one order higher than those on the DNA-templated Ag?film.     

Long-range structuring of nanoparticles by mimicry of a cholesteric liquid crystal

Patterning nano-objects is an exciting interdisciplinary research area in current materials science, arising from new optical and optoelectronic properties and the need to miniaturize electronic components. Many techniques have been developed for assembling nanoparticles into two- and three-dimensional arrays. Most studies involving liquid crystals as templates have dealt with colloidal particles and nematic and smectic phases. Here, we demonstrate the long-range ordering of nanoparticle assemblies that adopt the helical configuration of the cholesteric liquid crystalline phase. Because we used glass-forming cholesterics, the nanostructures could be examined by transmission electron microscopy. The platinum nanoparticles form periodic ribbons that mimic the well-known 'fingerprint' cholesteric texture. Surprisingly, the nanoparticles do not decorate the original cholesteric texture but create a novel helical structure with a larger helical pitch. By varying the molar fraction of cholesterol-containing mesogen in the liquid crystal host, we show that the distance between the ribbons is directly correlated to the pitch. Therefore this inherent lengthscale becomes a simple control parameter to tune the structuring of nanoparticles. These results demonstrate how such an assembly process could be modulated, providing a versatile route to new materials systems.     

Polarization-dependent surface-enhanced Raman scattering from a silver-nanoparticle-decorated single silver nanowire

Single silver nanowires produced by DC electrodeposition in porous anodic alumina templates were surface-functionalized with the bifunctional molecule 4-aminobenzenthiol (ABT) then exposed to aqueous silver nanoparticles resulting in a silver nanoparticle-decorated silver nanowire. The polarization dependent surface-enhanced Raman (SERS) signal from this system showed significant intensity anisotropy when measured at a midsection of the nanowire, where the largest SERS intensities were observed when the incident light was polarized perpendicular to the nanowire's long axis but was almost isotropic near the tip of the nanowire. The observed effects were accounted for in terms of the electromagnetic fields concentrated in the collection of hot spots created through the ABT-linker-driven nanoparticle-nanowire self-assembly process.     

3D nano-arrays of silver nanoparticles and graphene quantum dots with excellent surface-enhanced Raman scattering

Active surface-enhanced Raman scattering (SERS) substrates, 3D nano-arrays of Ag nanoparticles (NPs) and graphene quantum dots (GQDs), were prepared using a photochemical approach and an electrophoresis deposition technique with the formation mechanism addressed. The GQDs (ca. 6?nm average) fit into the inter-particle gaps between Ag NPs, as verified by their scanning electron microscopy and high-resolution transmission electron microscopy. This deliberately designed 3D assembly of Ag NPs and GQDs could promote the synergistic effects of both components to further enhance the SERS performances according to both electromagnetic mechanism and chemical mechanism. Preliminary experiments show that the 3D substrates exhibited strong SERS signals comparing with bare Si substrates. This work provides a promising way for 3D SERS substrates.     

Degradation of curcumin dye in aqueous solution and on ag nanoparticles studied by ultraviolet-visible absorption and surface-enhanced Raman spectroscopy

The chemical degradation of curcumin (CU) in aqueous solution and on silver nanoparticles was studied by means of ultraviolet (UV)-visible absorption and surface-enhanced Raman (SERS) spectroscopy at different pH levels and upon light irradiation. CU undergoes a chemical degradation in aqueous solution mainly when the pH is increased. The CU degradation is catalytically enhanced in the presence of Ag nanoparticles. In general, CU degradation implies two steps: (1) the breakdown of the interring chain connecting the two CU aromatic side rings, producing smaller phenolic compounds rich in carboxylate groups, and (2) polymerization of the resulting phenolic products, giving rise to phenolic polymeric products. The degradation-polymerization mechanism can be modulated depending on the experimental conditions. The chemical photoproducts resulting from the visible light irradiation are similar to the polycatechol products obtained when catechol is adsorbed on Ag nanoparticles.     

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).     

Ultraviolet-visible and surface-enhanced Raman scattering spectroscopy studies on self-assembled metalloporphyrin films on organic monolayer-modified ultra-thin silver substrates

A self-assembled monolayer (SAM) film of 5,10,15,20-tetra-(para-chlorophenyl)-porphyrin terbium (or lutetium) hydroxy compound (TbOH/LuOH) fabricated on a silver substrate premodified with a SAM of 4-mercaptopyridine (PySH) was studied by ultraviolet-visible (UV-Vis) spectroscopy and surface-enhanced Raman scattering (SERS) spectroscopy. PySH can modify the substrate and deliver its pyridyl group pointing out from the silver surface. Thus, we can investigate the effects of the central metals of the metalloporphyrins in the formation of the composite films. For the TbOH-PySH composite film, a new absorption band arising from TbOH appears at 425 nm, and a band at 512 nm due to the PySH-modified silver substrate shifts to a longer wavelength (538 nm). The results suggest that TbOH is successfully assembled on the top of PySH-modified silver film and that there is an interaction between TbOH and PySH in the film. The frequency shifts and relative intensity changes of bands due to PySH in the SERS spectra imply the coordination of the pyridyl group on Tb in the SAM. As for the LuOH-PySH composite film, its SERS spectrum shows bands arising from both the LuOH and PySH moieties, indicating that LuOH is assembled on the PySH-modified silver film. Furthermore, a band at 1221 cm(-1) due to the in-plane C-H bending mode of PySH disappears, implying that the pyridyl moiety of PySH becomes more parallel to the silver surface upon the formation of the LuOH-PySH composite film. Additionally, an absorption band at 515 nm shifts to a longer wavelength (541 nm) and becomes broad upon the formation of the composite film, suggesting an interaction between LuOH and PySH in the film. By comparing the spectral changes between the two self-assembled composite films, we find that the central metal is crucial in the formation of the composite films. The structure and orientation of the composite films depend on the central metal of the metalloporphyrin compounds.     

Spectroscopic tags using dye-embedded nanoparticles and surface-enhanced Raman scattering

Surface-enhanced Raman scattering is capable of providing rich vibrational information at the level of single molecules and single nanoparticles, but the practical applications of this enormous enhancement effect are still a challenge. Here we report a new class of dye-embedded core-shell nanoparticles that are highly efficient for surface Raman enhancement and could be used as spectroscopic tags for multiplexed detection and spectroscopy. The core-shell particles contain a metallic core for optical enhancement, a reporter molecule for spectroscopic signature, and an encapsulating silica shell for protection and conjugation. A surprising finding is that organic molecules with an isothiocyanate (-N=C=S) group or multiple sulfur atoms are compatible with silica encapsulation. In comparison with fluorescent dyes and quantum dots, enhanced Raman probes contain a built-in mechanism for signal amplification and provide rich spectroscopic information under ambient experimental conditions.     

Ultraviolet-visible and surface-enhanced Raman scattering spectroscopy studies on self-assembled films of ruthenium phthalocyanine on organic monolayer-modified silver substrates

A self-assembled film of ruthenium phthalocyanine (RuPc) fabricated on a silver substrate pre-modified with a monolayer of 4-mercaptopyridine (PySH) or 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (BPENB) was studied by ultraviolet-visible (UV-Vis) and surface enhanced Raman scattering (SERS) spectroscopy. PySH or BPENB was used as a ligand to link RuPc since they not only modify the silver substrate, but also deliver a pyridyl group pointing out from the silver surface. Therefore, we can explore the relationship between the structure and orientation of metallophthalocyanine and the substrate modified by the two kinds of organic-monolayers with the different conjugates and molecular lengths. UV-Vis bands due to the organic-monolayer (PySH or BPENB) modified silver films shift to longer wavelengths and a new band arising from the metallophthlocyanine appears, suggesting the binding of RuPc to PySH/BPENB, as well as the interaction between the marcocycle of RuPc and the ring of PySH/BPENB. Vibrational bands arising from both the RuPc and PySH/BPENB moieties appear clearly in the SERS spectra of the RuPc-PySH/BPENB composite films, indicating that RuPc is successfully assembled on the top of PySH/BPENB film. The shifts and relative intensity changes of bands due to PySH or BPENB in the SERS spectra imply the binding of the metallophthalocyanine to the pyridyl group in the composite films. Furthermore, the comparison of the SERS spectra revealed that the orientations of PySH and BPENB in the two kinds of composite films are different; the BPENB moiety in the RuPc-BPENB composite film is more perpendicular to the silver surface compared with the PySH moiety in the RuPc-PySH composite film.     

New surface-enhanced Raman spectroscopy substrates via self-assembly of silver nanoparticles for perchlorate detection in water

Perchlorate (ClO4-) has recently emerged as a widespread contaminant in drinking water and groundwater supplies in the United States, and a need exists for rapid detection and monitoring of this contaminant. In this study, surface-enhanced Raman spectroscopy (SERS) was studied as a means of ClO4- detection, and new sol-gel-based SERS substrates were developed by self-assembly of silver colloidal nanoparticles with various functionalized silane reagents. These substrate materials were tailored to allow detection of ClO4- in water with improved sorptivity, stability, and sensitivity and with the ability to detect ClO4- at concentrations as low as 10(-6) M (or 100 microg/L) with good reproducibility. Similar techniques were used to fabricate capillary SERS flow cells by assembling functionalized silver nanoparticles capable of attracting ClO4- to the SERS surface or the internal wall of glass capillaries. These capillary flow cells could be readily configured to allow for in situ, nondestructive detection of ClO4- via fiber optics.     

Base effects on fabrication of silver nanoparticles embedded silica nanocomposite for surface-enhanced Raman scattering (SERS)

In this paper, we studied on the effect of organic bases in the case of ethylene glycol based fabrication of silver nanoparticles embedded silica nanocomposite (Ag SNC) without heating. Considering their chemical structures, butylamine (BA), ethanolamine (EA), triethanolamine (TEA), tributylamine (TBA), octylamine (OA) and Jeffamine 500 (JA) were used as an organic base. In addition, the effect of the concentrations of AgNO3 and organic bases on the formation of Ag SNC was also examined. In conformity with the characteristics of Ag SNC, SERS signal intensity of benzenethiol on Ag SNC was measured. As a result, the SERS signal intensity of Ag SNCs was strongly dependent on the reaction conditions. Furthermore, when reacted under the best reaction condition with concentrations of AgNO3 and OA, 3 mM and 5 mM, respectively, a large-scale production of Ag SNC was possible under the mild conditions.     

Mechanism of cellular uptake of graphene oxide studied by surface-enhanced Raman spectroscopy

The last few years have witnessed rapid development of biological and medical applications of graphene oxide (GO), such as drug/gene delivery, biosensing, and bioimaging. However, little is known about the cellular uptake mechanism and pathway of GO. In this work, surface-enhanced Raman scattering (SERS) spectroscopy is employed to investigate the cellular internalization of GO loaded with Au nanoparticles (NPs) by Ca Ski cells. The presence of Au NPs on the surface of GO enables detection of enhanced intrinsic Raman signals of GO inside the cell. The SERS results reveal that GO is distributed inhomogeneously inside the cell. Furthermore, internalization of Au-GO into Ca Ski cells is mainly via clathrin-mediated endocytosis, and is an energy-dependent process.     

Semi-quantitative analysis of gentian violet by surface-enhanced Raman spectroscopy using silver colloids

The viability of the application of surface-enhanced Raman spectroscopy (SERS) to the semi-quantitative analysis of the triphenylmethane dye gentian violet was examined by using activated borohydride-reduced silver colloids. Raman and SERS spectra of aqueous solutions of gentian violet at different pH values were acquired for the first time and equally intense SERS signals were obtained at both acidic and alkaline pH values. Two maxima intensities observed in the pH profile revealed the presence of different ionization states of the dye. The pH conditions for SERS were optimized over the pH range 1 to 12 and the biggest enhancement for SERS of this charged dye was found to be at pH 2.0; thus, this condition was used for semi-quantitative analysis. A good linear correlation was observed for the dependence of the signal intensities of the SERS bands at 1620 cm(-1) (R = 0.999) and 1370 cm(-1) (R = 0.952) on dye concentration over the range 10(-6) to 10(-4) mol/L, using laser excitation at 514.5 nm. At concentrations of dye above 10(-2) mol/L, the concentration dependence of the SERS signals is nonlinear. This is explained as due to the precipitation of metallic silver as well as due to saturation caused by complete coverage of the SERS substrate. A series of intensities of the band at 1620 cm(-1) measured from dye molecules proved that the single-molecule limit of gentian violet is attained at the concentration of 10(-9) mol/L.     

Diagnosis of breast cancer by analysis of sialic acid concentrations in human saliva by surface-enhanced Raman spectroscopy of silver nanoparticles

Breast cancer is the most common type of malignant tumor among women and their second leading cause of cancer-related deaths.The most common method for screening and diagnosis is mammography.Nonetheless,two main problems have been identified.First,the dose of radiation received during the test prevents the method from the use on women who are ＜ 40 years old.Second,there can be mammogram failure owing to the lack of tumor contrast with the fibrous tissue.Therefore,there is a need for screening methods that will help to identify high-risk cases.We developed a biological marker test that can help to identify them.Increased levels of sialic acid (SA) in saliva are known to correlated with breast cancer.In this study,we evaluated the feasibility of Raman spectroscopy as a method for quantification of SA in saliva,using citrate-reduced silver nanoparticles (cit-Ag-NPs) as a surface-enhanced Raman spectroscopy (SERS) substrate.Quantification of SA was accomplished by measuring its intensity in saliva and comparing it with a calibration curve of SA standards.The mean SA concentration in saliva was found to be significantly higher among 100 breast cancer patients (18.3 ± 9.4 mg·dL-1;mean ± SD) than among 106 healthy controls (3.5 ± 1.0 mg·dL-1).The SERS test showed sensitivity of 94％ and specificity 98％ for detection of patients with breast cancer,assuming that SA concentration ＞ 7 mg·dL-1 is a cutoff for positive test results.Our findings prove the usefulness of this SERS technique as a simple,convenient,and highly sensitive method of quantitative analysis of SA in saliva.The simplicity of this nanotechnological test may help to substantially reduce the mortality among patients with breast cancer by providing women with a simple,noninvasive screening test that can be applied regardless of age or density of breast tissue.