Raman,infrared, SERS and theoretical study of 3‐(1‐phenylpropan‐2‐ylamino) propanenitrile,fenproporex

Infrared, Raman and surface‐enhanced Raman scattering (SERS) spectra of 3‐(1‐phenylpropan‐2‐ylamino)propanenitrile (fenproporex) have been recorded. Density functional theory (DFT) with the B3LYP functional was used for optimizations of ground state geometries and simulation of Raman and SERS vibrational spectra of this molecule. Bands of the vibrational spectra were assigned in detail. The comparison of SERS spectra obtained by using colloidal silver and gold nanoparticles with the corresponding Raman spectrum reveals enhancement and shifts in bands, suggesting a possible partial charge‐transfer mechanism in the SERS effect. Information about the orientation of fenproporex on the nanometer‐sized metal structures is also obtained. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering of 4‐mercaptopyridine on sub‐monolayers of α‐Fe2O3 nanocrystals (sphere,spindle, cube)

We report surface‐enhanced Raman scattering (SERS) spectra from 4‐mercaptopyridine (4‐Mpy) adsorbed on sub‐monolayers of α‐Fe₂O₃ nanocrystals (sphere, spindle, cube). The maximum enhancement factor has been estimated to be about 10⁴ compared to that of 4‐Mpy in solution. A possible mechanism has been proposed that the charge transfer between the α‐Fe₂O₃ nanocrystals and the 4‐Mpy molecules is most likely responsible for the observed enhancement of Raman intensity of adsorbed 4‐Mpy molecules as surface plasmon resonances have not occurred. Copyright © 2009 John Wiley & Sons, Ltd.     

Multiscale electromagnetic SERS enhancement on self‐assembled micropatterned gold nanoparticle films

In this work, we demonstrate a cascaded, multiplicative electromagnetic enhancement effect in surface‐enhanced Raman scattering (SERS) on periodically micropatterned films made of colloidal gold nanoparticles, prepared by a self‐assembly approach, without implying lithography procedures. The multiplicative enhancement effect is obtained by combining surface plasmon near‐field enhancement due to nanoscale features with far‐field photonic coupling by periodic microscale features. The effect is observed for both internal Raman reporters (molecules attached to the Au colloids before their assembly) and external Raman probes (molecules adsorbed on the samples after film assembly). The ability of the patterned films for far‐field light coupling is supported by reflectivity spectra, which present minima/maxima in the visible spectral range. Finite‐difference time‐domain computer simulations of the electric field distribution also support this interpretation. The fabricated dual‐scale SERS substrates exhibit a good spot‐to‐spot reproducibility and time stability, as proved by the SERS response over a time scale longer than 1 month. The experimental demonstration of this cascaded electromagnetic enhancement effect contributes to a better understanding of SERS and can affect future design of SERS substrates. Moreover, such dual‐scale colloidal films prepared by convective self‐assembly can be of general interest for the broader field of nanoparticle‐based devices. Copyright © 2014 John Wiley & Sons, Ltd.     

New insights on surface‐enhanced Raman scattering based on controlled aggregation and spectroscopic studies,DFT calculations and symmetry analysis for 3,6‐bi‐2‐pyridyl‐1,2,4,5‐tetrazine adsorbed onto citrate‐stabilized gold nanoparticles

A systematic study on the surface‐enhanced Raman scattering (SERS) for 3,6‐bi‐2‐pyridyl‐1,2,4,5‐tetrazine (bptz) adsorbed onto citrate‐modified gold nanoparticles (cit‐AuNps) was carried out based on electronic and vibrational spectroscopy and density functional methods. The citrate/bptz exchange was carefully controlled by the stepwise addition of bptz to the cit‐AuNps, inducing flocculation and leading to the rise of a characteristic plasmon coupling band in the visible region. Such stepwise procedure led to a uniform decrease of the citrate SERS signals and to the rise of characteristic peaks of bptz, consistent with surface binding via the N heterocyclic atoms. In contrast, single addition of a large amount of bptz promoted complete aggregation of the nanoparticles, leading to a strong enhancement of the SERS signals. In this case, from the distinct Raman profiles involved, the formation of a new SERS environment became apparent, conjugating the influence of the local hot spots and charge‐transfer (CT) effects. The most strongly enhanced vibrations belong to a₁ and b₂ representations, and were interpreted in terms of the electromagnetic and the CT mechanisms: the latter involving significant contribution of vibronic coupling in the system. Copyright © 2010 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering and DFT calculations studies of 3,3′‐diethylthiatri‐ carbocyanine iodide

As an infrared Raman probe, the molecule 3,3′‐diethylthiatricarbocyanine iodide (DTTC) has received much attention in the past decades due to its potential applications in Raman imaging, single‐cell detection, cancer diagnosis, and surface‐enhanced Raman scattering (SERS). In this work, ordinary Raman, SERS, and theoretical Raman spectra were investigated to estimate the DTTC suspension. More specifically, the original gold nanospheres (60 nm diameter) and gold nanorods were encoded with DTTC and stabilized with a layer of thiol–polyethylene glycol as Raman reporter; SERS data were also obtained from the samples. Hartree–Fock theory and density functional theory (DFT) calculation were applied to calculate the optimized Raman spectra of DTTC in water on the B3LYP/6‐31G level. Subsequently, the obtained experimental spectra from DTTC were carefully compared with the theoretically calculated spectra, and good agreement was obtained between the theoretical and experimental results.The bands between 500 and 3100 cm⁻¹ in the ordinary Raman and SERS spectra were assigned as well. This work will facilitate the development of ultrasensitive SERS probes for advanced biomedical imaging applications. Copyright © 2011 John Wiley & Sons, Ltd.     

Spectral characterization and intracellular detection of Surface‐Enhanced Raman Scattering (SERS)‐encoded plasmonic gold nanostars

Plasmonic gold nanostars offer a new platform for surface‐enhanced Raman scattering (SERS). However, due to the presence of organic surfactant on the nanoparticles, SERS characterization and application of nanostar ensembles in solution have been challenging. Here, we applied our newly developed surfactant‐free nanostars for SERS characterization and application. The SERS enhancement factors (EF) of silver spheres, gold spheres and nanostars of similar sizes and concentration were compared. Under 785 nm excitation, nanostars and silver spheres have similar EF, and both are much stronger than gold spheres. Having plasmon matching the incident energy and multiple ‘hot spots’ on the branches bring forth strong SERS response without the need to aggregate. Intracellular detection of silica‐coated SERS‐encoded nanostars was also demonstrated in breast cancer cells. The non‐aggregated field enhancement makes the gold nanostar ensemble a promising agent for SERS bioapplications. Copyright © 2012 John Wiley & Sons, Ltd.     

Direct observation of surface‐enhanced Raman scattering in ZnO nanocrystals

We have been able to observe the surface‐enhanced Raman scattering (SERS) from 4‐mercaptopyridine (4‐Mpy) molecules adsorbed on ZnO nanocrystals, which display 10³ enhancement factors (EFs). An excitation wavelength‐dependent behavior is clearly observed. Another molecule BVPP is also observed to have surface‐enhanced Raman signals. The chemical enhancement is most likely responsible for the observed enhancement, since plasmon resonances are ruled out. The research is important not only for a better understanding of the SERS mechanism, but also for extension of the application of Raman spectroscopy to a variety of adsorption problems on a semiconductor surface. Copyright © 2009 John Wiley & Sons, Ltd.     

Visible light response of silver 4‐aminobenzenethiolate and silver 4‐dimethylaminobenzenethiolate probed by Raman scattering spectroscopy

Silver thiolate is a layered compound with a Raman spectrum that is known to change with time, becoming the same as the surface‐enhanced Raman scattering (SERS) spectrum of the parent thiol molecule adsorbed on Ag nanoparticles. On this basis, the Raman scattering characteristics of silver 4‐aminobenzenethiolate (Ag‐4ABT) compounds were investigated to determine whether certain peaks that are identifiable in the SERS spectrum of 4‐aminobenzenethiol (4‐ABT) but absent in its normal Raman spectrum were also apparent in the Ag salt spectrum. For comparative purposes, the Raman scattering characteristics of silver 4‐dimethylaminobenzenethiolate (Ag‐4MABT) were also examined. Raman spectra acquired while spinning the sample were typified by only a₁‐type vibrational bands of Ag‐4ABT and Ag‐4MABT, whereas in the static condition, several non‐a₁‐type bands were identified. The spectral patterns acquired in the static condition were similar to the intrinsic SERS spectra of 4‐ABT or 4‐dimethylaminobenzenethiol (4‐MABT) adsorbed on pure Ag nanoparticles. Notably, the CH₃ group vibrational bands were observable for Ag‐4MABT irrespective of the sample rotation. In addition, no decrease in intensity during irradiation with a visible laser was observed for any of the bands, suggesting that no chemical conversion actually took place in either 4‐ABT or 4‐MABT. The preponderance of evidence led to the conclusion that the non‐a₁‐type bands observable in the SERS spectra must be associated with the chemical enhancement mechanism acting on the Ag nanoparticles. The chemical enhancement effect was more profound at 514.5 nm than at 632.8 nm, and was more favorable for 4‐ABT than 4‐MABT at both wavelengths. Copyright © 2012 John Wiley & Sons, Ltd.     

Rapid detection of 2,2′,4,4′‐tetrabromodiphenyl ether (BDE‐47) using a portable Au‐colloid SERS sensor

In situ rapid detection and identification of polybrominated diphenyl ethers, a group of well‐known persistent organic pollutants, present a great challenge. To develop a portable and sensitive surface‐enhanced Raman scattering (SERS) sensor for rapid 2,2′,4,4′‐tetrabromodiphenyl ether (BDE‐47) detection, we adopted the most commonly used Au nanoparticles, which are effective in the analysis of hydrophobic BDE‐47 with a simple optimization in citrate content and sampling technique. Qualitative and quantitative determination of BDE‐47 was achieved using a portable Raman spectrometer. The SERS response exhibited a linear dependence on the BDE‐47 concentration up to 1000 nM with a detection limit of 75 nM. The density function theory‐calculated Raman spectra agreed well with the experimental observations, and the results justified the existence of electromagnetic enhancement and charge transfer mechanism. This in situ SERS platform allows easy and reliable detection of hydrophobic molecules such as BDE‐47 in complex matrices. Copyright © 2014 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering studies of carbon nanotubes using Ag‐core Au‐shell nanoparticles

Surface‐enhanced Raman scattering from carbon nanotube bundles adsorbed with plasmon‐tunable Ag‐core Au‐shell nanoparticles (Ag@Au nps) was carried out for the first time. By utilizing nanoparticles whose plasmon resonance peak (541, 642 nm) closely matches the commonly used Raman excitation sources (532, 632.81 nm), we can observe a large enhancement in the Raman signatures of carbon nanotubes. We obtain greater enhancement in the Raman signal for the above case when compared to nanotubes adsorbed with conventional Ag, Au or other ‘off resonant’ Ag@Au nps. The power‐dependent SERS experiment on single‐walled nanotubes (SWNTs) with resonant Ag@Au nps reveals a linear behavior between the G‐band intensity and the photon flux density, which is in agreement with the vibrational pumping model of SERS. The observed enhancement by resonance matching is pronounced for carbon nanotubes and may lead to insights into understanding nanotube–nanoparticle interaction. Copyright © 2009 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman spectroscopy of 4‐tert‐butylpyridine on a silver electrode

We report the observation of large surface‐enhanced Raman scattering (SERS) (10⁶) for 4‐tert‐butylpyridine molecules adsorbed on a silver electrode surface in an electrochemical cell with electrode potential set at − 0.5 V. A decrease in electrode potential to − 0.3 V was accompanied by a decrease in relative intensities of the vibrational modes. However, there were no changes in vibrational wavenumbers. Comparison of both normal solution Raman and SERS spectra shows very large enhancement of the intensities of a₁, a₂, and b₂ modes at laser excitation of 488 nm. Enhancement of the non‐totally symmetric modes indicates the presence of charge transfer as a contributor to the enhancement. Copyright © 2011 John Wiley & Sons, Ltd.     

Design of plasmonic bowtie nanoring array with high sensitivity and reproducibility for surface‐enhanced Raman scattering spectroscopy

A metallic bowtie nanoring array is designed to gain high sensitive and reproducible substrate for surface‐enhanced Raman scattering (SERS) spectroscopy. The localized surface plasmon resonance (LSPR), the electric field enhancement factors (EFs) and the electric field distribution of the bowtie and bowtie nanoring array are numerically investigated by means of the finite‐difference time domain (FDTD) method. After the optimization of the particle size and the array period, the maximum electromagnetic field EF approaches 153, and the corresponding SERS electromagnetic enhancement factor (EMEF) reaches 5.4 × 10⁸. This highly sensitive and reproducible substrate can be a good candidate for SERS applications. Copyright © 2011 John Wiley & Sons, Ltd.     

Effect of surface morphology on surface‐enhanced Raman scattering of 4‐aminobenzenethiol adsorbed on gold substrates

The substrate‐dependent surface‐enhanced Raman scattering (SERS) of 4‐aminobenzenethiol (4‐ABT) adsorbed on Au surfaces has been investigated. 4‐ABT is one of the very unique adsorbate molecules whose SERS spectral patterns are known to be noticeably dependent on the relative contribution of chemical enhancement mechanism vs electromagnetic enhancement mechanism. The SERS spectral patterns of 4‐ABT adsorbed on gold substrates with various surface morphology have thus been analyzed in terms of the symmetry types of the vibrational modes. Almost invisibly weak b₂ type vibrational bands were observed in the SERS spectra of the 4‐ABT adsorbed on Au colloidal sol nanoparticles or commercially available Au micro‐powders because of the weak contribution of the chemical enhancement. However, greatly enhanced b₂ vibrational bands were observed in the spectra of the 4‐ABT molecules adsorbed on the synthesized Au(Zn) sponge or the electrochemically roughened Au(ORC) foil caused by the strong contribution of the chemical enhancement mechanism. Copyright © 2008 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering study of Ag@PPy nanoparticles

Surface‐enhanced Raman scattering (SERS) spectra of Ag@polypyrrole (PPy) nanoparticles with both 488 and 1064 nm excitation were investigated. Experimental results as well as theoretical analysis demonstrated that electromagnetic (EM) enhancement and charge transfer (CT) both rebounded to the SERS effect of Ag@PPy nanoparticles. When near‐IR excitation (1064 nm) was used for the SERS measurements, the contribution from CT was amplified relative to that from EM because the energy of the near‐IR excitation is far from the surface plasmon resonance of the nanosized Ag particles. The increased doping level of PPy, leading to optimal energy matching between the Fermi levels of the Ag nanoparticles and the energy levels in PPy molecules, could obviouslyenhance the SERS signal. These results suggested that the SERStechnique wasan effective tool for investigating the doping effect and interface interaction in metal‐conductingpolymer composite nanoparticles. In particular, the SERS technique with near‐IR excitation could give more information regarding the contribution of the charge‐transfer mechanism to the spectral enhancement of this kind of system. Copyright © 2010 John Wiley & Sons, Ltd.     

Fabrication and characterization of SERS‐active silver clusters on glassy carbon

In this article, a novel technique for the fabrication of surface enhanced Raman scattering (SERS) active silver clusters on glassy carbon (GC) has been proposed. It was found that silver clusters could be formed on a layer of positively charged poly(diallyldimethylammonium) (PDDA) anchored to a carbon surface by 4‐aminobenzoic acid when a drop containing silver nanoparticles was deposited on it. The characteristics of the obtained silver clusters have been investigated by atomic force microscopy (AFM), SERS and an SERS‐based Raman mapping technique in the form of line scanning. The AFM image shows that the silver clusters consist of several silver nanoparticles and the size of the clusters is in the range 80–100 nm. The SERS spectra of different concentrations of rhodamine 6G (R6G) on the silver clusters were obtained and compared with those from a silver colloid. The apparent enhancement factor (AEF) was estimated to be as large as 3.1 × 10⁴ relative to silver colloid, which might have resulted from the presence of ‘hot‐spots’ at the silver clusters, providing a highly localized electromagnetic field for the large enhancement of the SERS spectra of R6G. The minimum electromagnetic enhancement factor (EEF) is estimated to be 5.4 × 10⁷ by comparison with the SERS spectra of R6G on the silver clusters and on the bare GC surface. SERS‐based Raman mapping technique in the form of line scanning further illustrates the good SERS activity and reproducibility on the silver clusters. Finally, 4‐mercaptopyridine (4‐Mpy) was chosen as an analyte and the lowest detected concentration was investigated by the SERS‐active silver clusters. A concentration of 1.6 × 10⁻¹⁰ M 4‐Mpy could be detected with the SERS‐active silver clusters, showing the great potential of the technique in practical applications of microanalysis with high sensitivity. Copyright © 2006 John Wiley & Sons, Ltd.     

Application of surface‐enhanced Raman scattering in cell analysis

In surface‐enhanced Raman scattering (SERS), the scattered intensity is drastically increased due to a resonant interaction with surface plasmons of coin metals. SERS is a nondestructive spectroscopic method applied also to biomedical samples. It inherits the advantages of normal Raman spectroscopy and at the same time overcomes the inherent low sensitivity problem. These properties endow SERS with exciting opportunities to be a successful analytical tool for cell analysis. SERS can be used to detect only molecules located on or close to the metallic nanostructures which can support surface plasmon resonances for the enhancement of the Raman signals. Therefore, these metallic nanostructures play a key role in the application of SERS in cell analysis. By incorporating the SERS substrates into the biosamples, molecular structural probing and cellular imaging become possible. In the past decade, analysts worldwide have developed many schemes to study the chemical changes and component distribution in cells by using SERS. In this paper, the application of SERS in cell analysis is reviewed. Copyright © 2011 John Wiley & Sons, Ltd.     

Identification of two Ag‐2,2′:6′,2″‐terpyridine surface species on Ag nanoparticle surfaces by excitation wavelength dependence of SERS spectra and factor analysis: evidence for chemical mechanism contribution to SERS of Ag(0)–tpy

Measurement and interpretation of the excitation wavelength dependence of surface‐enhanced Raman scattering (SERS) spectra of molecules chemisorbed on plasmonic, e.g. Ag nanoparticle (NP) surfaces, are of principal importance for revealing the charge transfer (CT) mechanism contribution to the overall SERS enhancement. SERS spectra, their excitation wavelength dependence in the 445–780‐nm range and factor analysis (FA) were used for the identification of two Ag‐2,2′:6′,2″‐terpyridine (tpy) surface species, denoted Ag⁺–tpy and Ag(0)–tpy, on Ag NPs in systems with unmodified and/or purposefully modified Ag NPs originating from hydroxylamine hydrochloride‐reduced hydrosols. Ag⁺–tpy is a spectral analogue of [Ag(tpy)]⁺ complex cation, and its SERS shows virtually no excitation wavelength dependence. By contrast, SERS of Ag(0)–tpy surface complex generated upon chloride‐induced compact aggregate formation and/or in strongly reducing ambient shows a pronounced excitation wavelength dependence attributed to a CT resonance (the chemical mechanism) contribution to the overall SERS enhancement. Both the resonance (λ_exc = 532 nm) and off‐resonance (λ_exc = 780 nm) pure‐component spectra of Ag(0)–tpy obtained by FA are largely similar to surface‐enhanced resonance Raman scattering (λ_exc = 532 nm in resonance with singlet metal to ligand CT (¹ MLCT) transition) and SERS (λ_exc = 780 nm) spectra of [Fe(tpy)₂]²⁺ complex dication. Copyright © 2014 John Wiley & Sons, Ltd.     

SERS study on surface chain geometry of atactic poly (methyl methacrylate) film and nanosphere

The surface chain geometry of atactic poly (methyl methacrylate) (a‐PMMA) film and nanosphere (NS) was revealed by surface‐enhanced Raman scattering (SERS) spectra. The Ag nanoparticles and nanoplates were prepared by electrochemical deposition and chemical synthesis for SERS substrates. The experimental results suggested that the molecular chain axis of a‐PMMA film adopted a trans‐conformation on bonding to Ag surface ascribed to the short‐range chemical (CHEM) effect according to the SERS selection rules. However, for the well‐coated monolayer of a‐PMMA NSs, the α‐CH₃ in polymer chains stood vertically to the Ag surface due to the giant local electromagnetic effect, then the chain conformation presented in the interface between a‐PMMA NSs and Ag metal was adopted the opposite orientation compared with a‐PMMA film. The Raman enhancement of the Ag nanoparticles was more prominent than that of the Ag nanoplates due to the free energies of face‐centered cubic crystal faces in nanoparticles, but the single crystals with (111) plane of Ag nanoplates could improve the stability of SERS signals when the annealed temperature was above T_g of a‐PMMA NSs. The present work can provide some useful information of surface chain geometry and conformation of NSs for designing various materials with well‐defined structure via a‐PMMA NSs template. Copyright © 2013 John Wiley & Sons, Ltd.     

Substituent effect on structure and surface activity of N‐methylpyridinium salts studied by FT‐IR,FT‐RS,SERS and DFT calculations

The substituent effect on structure and surface activity of mono‐ and disubstituted N‐methylpyridinium salts was investigated by means of Raman, infrared and surface‐enhanced Raman spectroscopy (SERS). The significant differences observed in Raman and infrared spectra have been correlated with marker bands assigned to in‐plane and out‐of‐plane vibrations, respectively. This vibrational analysis, complemented by quantum chemical calculations (B3LYP/6‐311++G(d,p)) was a basis for investigation of the surface activity of the studied compounds. Significant differences in their SERS spectra related to the enhancement mechanism and adsorbate orientation have been observed and analyzed. Copyright © 2012 John Wiley & Sons, Ltd.     

Detection of protein molecules by surface‐enhanced Raman spectroscopy‐based immunoassay using 2–5 nm gold nanoparticle lables

Here we report the synthesis of 2–5 nm size gold nanoparticle labels for surface‐enhanced Raman Spectroscopy (SERS) based immunoassay to detect protein molecules. The Au nanoparticles were conjugated with fluorescein isothiocyanate (FITC) and goat anti‐h‐IgG (immunoglobin) and the resultant particles were used for the detection of h‐IgG. Commercially available nitrocellulose strip and silver enhancement method were used for SERS‐based immunoassays. The FITC acts as a Raman probe, and vibrational fingerprint of this molecule was used for the detection of h‐IgG in concentration ranging from 1 to 100 ng/µl. Our Raman probe is robust and small in size and has high water solubility with minimum steric effect during antigen–antibody binding. Copyright © 2007 John Wiley & Sons, Ltd.