Direct visual evidence for the chemical mechanism of surface‐enhanced resonance Raman scattering via charge transfer

We describe the chemical and electromagnetic enhancements of surface‐enhanced resonance Raman scattering (SERRS) for the pyridine molecule absorbed on silver clusters, in which different incident wavelength regions are dominated by different enhancement mechanisms. Through visualization we theoretically investigate the charge transfer (CT) between the molecule and the metal cluster, and the charge redistribution (CR) within the metal on the electronic intracluster collective oscillation excitation (EICOE). The CT between the metal and the molecule in the molecule–metal complex is considered as an evidence for chemical enhancement to SERRS. CR within the metal on EICOE is considered as an evidence for the electromagnetic enhancement by collective plasmons. For the incident wavelength from 300 to 1000 nm, the visualized method of charge difference density can classify the different wavelength regions for chemical and electromagnetic enhancement, which are consistent with the formal fragmented experimental studies. Copyright © 2008 John Wiley & Sons, Ltd.     

Direct visual evidence for chemical mechanisms of SERRS via charge transfer in Au20–pyrazine–Au20 junction

The essence of the chemical mechanism for surface‐enhanced resonance Raman scattering (SERRS) is the charge transfer (CT) between the metal and the molecule at the resonant electronic transition, which results in the mode‐selective enhancement in the SERRS spectrum. The site‐orientated CT can directly interpret the mode‐selective chemical enhancement in SERRS. However, it is a great challenge to intutively visualize the orientation and site of the CT. In this paper, for the pyrazine–Au₂ complex, a three‐dimensional (3D) cubic representation is built to provide direct visual evidence for chemical mechanisms of SERRS via CT from the Au₂ cluster to pyrazine at the resonant electronic transition. The relationship between the mode‐selective enhancements in SERRS and the site‐orientated CT was clearly revealed. The intracluster excitation (analog of plasmon excitation in large naonoparticles) was also visualized by the 3D cubic presentation, which provided the direct evidence of local electromagnetic field enhancement of SERRS. To study the quantum size effect and the coupling effect of the nanoparticles, the photoexcitation mechanisms of the Au₂₀–pyrazine complex and the Au₂₀–pyrazine–Au₂₀ junction were also investigated. The tunneling charge transfer from one Au₂₀ cluster to another Au₂₀ cluster outside the pyrazine in Au₂₀–pyrazine–Au₂₀ junction was also revealed visually. The calculated normalized extinction spectra of Au nanoparticles using the generalized Mie theory reveal that the resonance peak is red‐shifted due to the coupling between particles. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical study on SERRS of rhodamine 6G adsorbed on Ag2 cluster: chemical mechanism via intermolecular or intramolecular charge transfer

The problem of the chemical enhancement of rhodamine 6G (R6G) adsorbed on silver cluster has been theoretically investigated by charge difference densities (CDDs) to show the direct charge transfer (CT) evidence. For surface‐enhanced resonance Raman scattering (SERRS) of R6G excited at 514.5 nm, the enhancements of v(151) and v(154) result from weak intermolecular (from Ag to R6G) CT and the strong intramolecular CT [similar to that of resonance Raman scattering (RRS) of R6G], respectively. The possibility of the SERRS of R6G contributed from pure intermolecular CT is also discussed, when the incident light is close to the new metal–R6G CT excited state at 1571.4 nm. Meanwhile compared with the absorption process the fluorescence yield of R6G is investigated by transition densities and CCDs. Copyright © 2008 John Wiley & Sons, Ltd.     

Nanostructures and nanostructured substrates for surface—enhanced Raman scattering (SERS)

We review the performance of various nanoscaled structures needed to support the propagation of the surface plasmons responsible for surface‐enhanced Raman scattering (SERS), and assess the potential for the optimisation of the compromise between enhancement and reproducibility that they provide, and hence their utility for relevant applications. We divide these nanostructures into those comprising structured arrays of discrete nanoparticles in two or three dimensions, and those comprising structured or regularly patterned surfaces in two or three dimensions. The most promising in terms of this compromise are those that involve the tethering of functionalised metal nanoparticles to surfaces. They are not only reproducible, but the functionalisation provides a measure of selectivity to relevant target analytes that the majority of SERS applications require. Copyright © 2008 John Wiley & Sons, Ltd.     

Charge‐transfer contributions in surface‐enhanced Raman scattering from Ag,Ag2S and Ag2Se substrates

The degree of charge‐transfer in Ag–4‐mercaptopyridine (Mpy) and Ag₂S–4‐Mpy systems is investigated by use of surface‐enhanced Raman spectroscopy (SERS). Ag₂S and Ag₂Se nanoparticles are prepared on the basis of the former formation of Ag nanoparticles to make the SERS analytical objects comparable. We utilize the intensity of the non‐totally symmetric modes (either b₁ or b₂) as compared with the totally symmetric a₁ modes to measure the degree of charge‐transfer. We find ~25% of charge‐transfer contribution for Ag–4‐Mpy, whereas 81 ~ 93% for Ag₂S–4‐Mpy. It means that the charge‐transfer resonance contribution dominates the overall enhancement in SERS of Ag₂S–4‐Mpy. Energy level diagram is applied to discuss the likely charge‐transfer transition between Ag, Ag₂S, Ag₂Se and 4‐Mpy. This article may point out the link among the three main resonance sources and could enable some insights into the electronic pathways available to the metal‐molecule and semiconductor‐molecule systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Theoretical investigation on surface‐enhanced Raman evidence for conformation transition of dimercaptoazobenzene adsorbed on gold nanoclusters

Probing and controlling the configurations and mechanical motions of the azobenzenes adsorbed on the metal substrates are preliminary for their applicability in various functional devices. In this work, we presented a detailed investigation of Raman properties of the dimercaptoazobenzene (DMAB) bound to gold nanoclusters using density functional calculations. It is demonstrated that the spectral features of the trans conformation of DMAB are quite different from the cis conformation, and the Raman intensities of the trans‐DMAB are much larger. Magnitude of chemical enhancement for the adsorbed trans‐DMAB is found to be close to or less than that for the adsorbed cis‐DMAB for the molecule–cluster complexes. This change trend can be, to a large extent, governed by the energy difference between the highest occupied energy level of the molecule and the lowest unoccupied energy level of the gold. Moreover, it is further demonstrated that differences in Raman intensities of the two conformations can be amplified for the cluster–molecular–cluster junctions, and thus chemical enhancement is much larger for the trans conformation than the cis conformation, possibly facilitating the experimental identification of the trans/cis DMAB. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterization of indigo carmine with surface‐enhanced resonance Raman spectroscopy (SERRS) using silver colloids and island films,and theoretical calculations

The application of resonance Raman (RR) and surface‐enhanced resonance Raman (SERR) spectroscopies to the qualitative and semiquantitative analysis of the artificial dye indigo carmine has been examined using sodium‐citrate‐reduced silver colloid and island films with various roughnesses. Additional, the Raman spectrum of the solid state and density functional theory (DFT) calculations helped to a better understanding of the fully optimized geometry and of the vibrational wavenumbers of the dye. A strong chemical interaction of indigo carmine with the silver colloidal particles was observed mainly at very low concentration of 0.03 × 10⁻⁹ M and with silver film surfaces at a concentration of 10⁻⁴ M . The indigo carmine orientation possibilities while going to different metallic substrates are discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

Photodissociation effects on pulsed laser deposited silver oxide thin films: surface‐enhanced resonance Raman scattering

Temporal Raman scattering measurements with 488, 532 and 632 nm excitation wavelengths and normal Raman studies by varying the power (from 30 W/cm² to 2 MW/cm²) at 488 nm were performed on silver oxide thin films prepared by pulsed‐laser deposition. Initially, silver oxide Raman spectra were observed with all three excitation wavelengths. With further increase in time and power, silver oxide photodissociated into silver nanostructures. High‐intensity spectral lines were observed at 1336 ± 25 and 1596 ± 10 cm⁻¹ with 488 nm excitation. No spectral features were observed with 633 nm excitation. Surface‐enhanced resonance Raman scattering theory is used to explain the complex behavior in the intensity of the 1336/1596 cm⁻¹ lines with varying power of 488 nm excitation. Copyright © 2011 John Wiley & Sons, Ltd.     

Halide‐ion‐assisted increase of surface‐enhanced hyper‐Raman scattering: a clear observation of the chemical effect

We show that the increase of surface‐enhanced hyper‐Raman scattering (SEHRS) intensity of organic dye molecules adsorbed on single silver (Ag) colloid aggregate in the presence of halide ions is a direct evidence of the chemical effect in the enhancement mechanism. Time‐dependent SEHRS measurements before and after adding halide ions enabled us to distinctly observe the chemical effect. The presence of the halide ions results to a more stable chemical interaction between metal and dye molecule, making it more resistant against photodegradation effects. This study can contribute in elucidating the chemical effect mechanism and aid in the development of SEHRS as a useful spectroscopic tool. Copyright © 2008 John Wiley & Sons, Ltd.     

Incorporation of Ag nanoparticles into membrane mimetic systems composed by phospholipid layer‐by‐layer (LbL) films to achieve surface‐enhanced Raman scattering as a tool in drug interaction studies

The synergistic effect produced by nanoparticles when incorporated into different systems used as analytical tools represents a growing research field nowadays. On the other hand, the study of interactions involving pharmacological drugs and biological membranes using phospholipids as mimetic systems is a research field already well established. Here, we combine both the anionic phospholipid dipalmitoyl phosphatidyl glycerol (DPPG) and negative Ag nanoparticles (AgNP) to form layer‐by‐layer (LbL) multilayered films using the cationic polymer poly(allylamine hydrochloride) (PAH) as the supporting polyelectrolyte, which were further investigated in the presence of a phenothiazine compound (methylene blue—MB). The molecular architecture of the LbL films in terms of controlled growth, morphology with micro and nanometer spatial resolutions, and dispersion of both AgNP and MB within the DPPG matrix was determined combining spectroscopy [ultraviolet–visible (UV–Vis) absorption and micro‐Raman spectroscopy] and microscopy [scanning electron microscopy (SEM) and atomic force microscopy (AFM)]. The results showed that the LbL films can be grown in a controlled way at nanometer thickness scale with the surface morphology susceptible to the presence of both AgNP and MB. The surface‐enhanced phenomenon was applied to investigate the LbL films taking the advantage of the strong surface‐enhanced resonance Raman scattering (SERRS) signal presented by the MB molecules. Besides, as MB is a pharmacological drug of interest, its molecular arrangements when dispersed in LbL films containing DPPG, which is the biological membrane mimetic system here, were investigated. In this case, the AgNP played a key role in achieving the MB SERRS signal. Copyright © 2009 John Wiley & Sons, Ltd.     

Direct evidence for conversion of p‐aminothiophenol to p,p'‐dimercaptoazobenzene by in situ reduplicative surface‐enhanced Raman scattering measurements

There has been a surge of interest in the surface‐enhanced Raman scattering (SERS) of p‐aminothiophenol (PATP), since its SERS spectra are dependent on the measurement conditions. However, there is a dispute over the origin of the so‐called b₂ modes in SERS spectrum of PATP recently. Some researchers propose that these bands come from selective chemical enhancement, while others conclude that these bands are due to the a_g modes of p,p'‐dimercaptoazobenzene (DMAB) produced from PATP by surface photoreaction. To solve this problem, we have studied the SERS spectra of PATP on Au and Ag nanoparticles by in situ measurement under various conditions. The results proved that the b₂ modes are not due to PATP but due to the a_g modes of DMAB. The key of the method is to ensure the SERS spectra taken from the same point in reduplicative measurements. The result showed that the stable SERS spectrum of PATP was essentially from DMAB. The reversibility of the PATP SERS spectra in previous studies is due to the variety of the measurement points, which is in nature of different PATP conversions to DMAB under different conditions. Copyright © 2013 John Wiley & Sons, Ltd.     

pH‐sensing nanostar probe using surface‐enhanced Raman scattering (SERS): theoretical and experimental studies

Local pH environment has been considered to be a potential biomarker for tumor diagnosis because solid tumors contain highly acidic environments. A pH‐sensing nanoprobe based on surface‐enhanced Raman scattering (SERS) using nanostars under near‐infrared excitation has been developed for potential biomedical applications. To theoretically investigate the effect of protonation state on SERS spectra of p‐mercaptobenzoic acid (pMBA), we used the density functional theory (DFT) with the B3LYP functional to calculate Raman vibrational spectra of pMBA‐Au/Ag complex in both protonated and deprotonated states. Vibrational spectral bands were assigned with DFT calculation and used to investigate SERS spectral changes observed from experiment when varying pH value between five and nine. The SERS peak position of pMBA at ~1580 cm⁻¹ was identified to be a novel pH‐sensing index, which has small but noticeable downshift with pH increase. This phenomenon is confirmed and well‐explained with theoretical simulation. The study demonstrates that SERS is a sensitive tool to monitor minor structural changes due to local pH environment, and DFT calculations can be used to investigate Raman spectra changes associated with minor differences in molecular structure. Copyright © 2013 John Wiley & Sons, Ltd.     

Using surface‐enhanced Raman scattering (SERS) and fluorescence spectroscopy for screening yeast extracts,a complex component of cell culture media

Yeastolate or yeast extract, which are hydrolysates produced by autolysis of yeast, are often employed as a raw material in the media used for industrial mammalian cell culture. The source and quality of yeastolate can significantly affect cell growth and production; however, analysis of these complex biologically derived materials is not straightforward. The best current method, liquid chromatography–mass spectrometry (LC‐MS), is time‐consuming and requires extensive expertise. This study describes the use of surface‐enhanced Raman scattering (SERS) and fluorescence excitation–emission matrix (EEM) spectroscopy coupled with robust principal component analysis (ROBPCA) for the rapid and facile characterization and discrimination of yeast extracts in aqueous solution. SERS using silver colloids generates time‐dependent signals, where adenine is the strongest contributor, and the spectra are stable and reproducible (< ~3%) at 180 min after mixing. Combining this spectral behavior with chemometric methods enables SERS to be used in discriminating between different yeastolate sources, for assessing lot‐to‐lot variability, and, potentially, to monitor storage‐induced compositional changes. Fluorescence EEM combined with multiway ROBPCA also provides a rapid and inexpensive method for the discrimination of yeastolate, yielding results in terms of sample discrimination very similar to that obtained with SERS. However, the EEM data does not provide the same level of chemical information that is provided by the SERS. Thus, the combination of these two methodologies has the potential to be extremely useful in biopharmaceutical manufacturing, as well as for the rapid characterization and screening of biogenic hydrolysates from animal or plant sources. Copyright © 2011 John Wiley & Sons, Ltd.     

Historical organic dyes: a surface‐enhanced Raman scattering (SERS) spectral database on Ag Lee–Meisel colloids aggregated by NaClO4

In the present study, several natural organic dyes used in antiquity, especially in textile dyeing, were analysed by surface‐enhanced Raman scattering (SERS) spectroscopy, in order to build a wide database that could integrate the data previously published in the literature. In particular, we reported for the first time the SERS spectra of 11 dyes: dragon's blood, sandalwood, annatto, safflower yellow and red, old fustic, gamboge, catechu, kamala, aloe and sap green. Silver colloids (Ag colloids) prepared according to the Lee–Meisel procedure, i.e. by reduction of a silver nitrate (AgNO₃) aqueous solution with trisodium citrate dihydrate, were used as substrate. As its efficiency had been tested in a previous work, sodium perchlorate (NaClO₄) 1.8 M was again employed as aggregating agent, giving the best results when added to the silver nanoparticles after the analyte. Copyright © 2011 John Wiley & Sons, Ltd.     

Validation of isotope dilution surface‐enhanced Raman scattering (IDSERS) as a higher order reference method for clinical measurands employing international comparison schemes

Isotope dilution surface‐enhanced Raman scattering (IDSERS) was established as a higher order reference measurement method for clinical measurands in international comparison schemes and in direct comparison with established mass spectrometric methods. IDSERS represents the most accurate and precise quantitative Raman technique and is capable of providing SI‐traceable amount‐of‐substance measurements. IDSERS has now been validated by participation in two international ring trials organized within the framework of the Metre Convention on the determination of biomarkers in human serum. The concentration of creatinine has been measured in two human serum samples supplied by the study organizer. The results obtained by IDSERS agree excellently with the data reported by participating national metrology institutes using mass spectrometry, and the associated uncertainties are within the claimed limits of equivalence. The performance and applicability of IDSERS were thereby convincingly demonstrated. This has led to the approval of the method as a higher order reference measurement procedure, which is now listed in a publicly accessible database maintained by the International Bureau of Weights and Measures (BIPM). Copyright © 2013 John Wiley & Sons, Ltd.     

Application of calixarene to high active surface‐enhanced Raman scattering (SERS) substrates suitable for in situ detection of polycyclic aromatic hydrocarbons (PAHs) in seawater

The characteristics of the sol–gel matrix embedding Ag nanoparticles functionalized with 25,27‐dimercaptoacetic acid‐26,28‐dihydroxy‐4‐tert‐butylcalix[4]arene (DMCX) suitable for the in situ detection of polycyclic aromatic hydrocarbons (PAHs) in seawater is presented. The DMCX‐functionalized silver nanoparticles were produced by the thermal reduction method in xerogel film. The silver colloid blocks were formed in the sol–gel matrix, with a diameter ranging from 50 to 120 nm. DMCX forming the monolayer on the silver nanoparticle surface contributes to the surface‐enhanced Raman scattering (SERS) activity due to the aggregation of silver nanoparticles and the preconcentration of PAH molecules within the zone of electromagnetic enhancement. When selected, PAH molecules e.g. pyrene and naphthalene were adsorbed onto the SERS substrate; Raman band positions of PAH were slightly shifted. A calibration procedure reveals that this type of SERS substrate has a limit of detection of 3 × 10⁻¹⁰ mol/l for pyrene and 13 × 10⁻⁹ mol/l for naphthalene in artificial seawater. The Raman signal response on a pyrene concentration change in artificial seawater was evaluated using a 671‐nm Raman setup with a flow‐through cell. This type of SERS substrate will be suitable for the in situ trace detection of pollutant chemicals in seawater. Copyright © 2012 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman spectroscopy (SERS) on silver colloids for the identification of ancient textile dyes. Part II: pomegranate and sumac

The effectiveness of surface‐enhanced Raman spectroscopy (SERS) spectrocsopy on Ag colloids has been successfully demonstrated for the identification of a yellow dye in two ancient wool threads found in the Royal Tumulus of In Aghelachem, Libyan Sahara, belonging to the Garamantian period (2nd–3rd century A.D.). High‐performance liquid chromatography (HPLC) highlighted the presence of ellagic acid in the extracts from the threads, excluding other chromophores. This result, together with the abundance of malic acid detected by gas chromatography‐mass spectrometry (GC‐MS), suggested the possible use of pomegranate rind or sumac berries as source of the yellow dye, both plants being documented in the Fezzan area during the Garamantian period. HPLC analyses and SERS spectra acquired on the extracts of the ancient threads were therefore compared with those obtained from pomegranate and sumac extracts of the corresponding fruits and reference dyed wool samples, allowing us to identify the yellow dye as deriving from pomegranate (Punica granatum L.). SERS spectra of ellagic acid and dyes extracted from pomegranate rind and sumac berries are reported here for the first time. A methodological improvement is also presented, based on the use of NaClO₄ as aggregating agent, that leads to a significant increase of the signal‐to‐noise ratio in the SERS spectra. Copyright © 2010 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.