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.     

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.     

Nanogaps in 2D Ag‐nanocap arrays for surface‐enhanced Raman scattering

The surface‐enhanced Raman scattering substrate of Ag–Ag nanocap arrays are prepared by depositing Ag film onto two‐dimensional (2D) polystyrene colloidal nanosphere templates. When the original colloidal arrays are used as the substrate for Ag deposition, surface‐enhanced Raman scattering (SERS) enhancements show the strong size‐dependence behaviours. When O₂‐plasma etched 2D polystyrene templates are used as the substrate for Ag deposition to form nanogaps, the gap sizes between adjacent Ag nanocaps from 5 to 20 nm generate even greater SERS enhancements. When SiO₂ coverage is deposited to isolate the Ag nanocaps from the neighbours, the SERS signals are enhanced more. The significant SERS effects are due to the coupling between Ag nanocaps controlled by the distance, which enhances the local electric‐field intensity. Copyright © 2013 John Wiley & Sons, Ltd.     

Adsorption of neurotensin‐family peptides on SERS‐active Ag substrates

Kinetensin (KN) and its amino acids 1–8 fragment ([des‐Leu⁹]KN), neuromedin N (NMN), and xenopsin (XP) and its two analogs (XP‐1 and XP‐2) belong to the neurotensin family of peptides and are known to stimulate the growth of human tumors. In this work, we report Fourier transform‐Raman and surface‐enhanced Raman scattering (SERS) studies of these peptides and discuss their structures, orientation, and mode of adsorption onto a highly specific, electrochemically roughened SERS‐active Ag electrode that is characterized by the formation of a 50–150 nm Ag island on its surface. We show that the investigated peptides bind preferentially to this surface by substantial electronic overlap between the metal surface and the π‐orbitals of the benzene rings of the Phe, Tyr, and Trp residues, which forces them to take parallel or almost parallel orientations with respect to the surface. In addition, the –CH₂–, –CNH₂, and –COO^‐ molecular fragments are involved in interactions with (binding to or in close proximity with) the Ag surface. The SERS data show that the adsorption modes in each of these cases are very similar. In addition, we show that the specific differences in the amino acid sequences do not significantly affect the orientation of the investigated peptides on the Ag substrate. This result implies that the N‐termini of the neurotensin‐family peptides do not influence the mode for adsorption onto the Ag substrates. Copyright © 2012 John Wiley & Sons, Ltd.     

B2 bradykinin receptor antagonists: adsorption mechanism on electrochemically roughened Ag substrate

In this paper, the surface‐enhanced Raman scattering (SERS) spectra of the potent B₂ bradykinin receptor antagonists, [D‐Arg⁰,Hyp³,Thi^5,8,L‐Pip⁷]BK, Aaa[D‐Arg⁰,Hyp³,Thi^5,8,L‐Pip⁷]BK, [D‐Arg⁰,Hyp³,Thi⁵,D‐Phe⁷,L‐Pip⁸]BK, and Aaa[D‐Arg⁰,Hyp³,Thi⁵,D‐Phe⁷,L‐Pip⁸]BK, were measured when immobilized onto a highly specific electrochemically roughened SERS‐active Ag substrate characterized by the formation of a 50 – 150 nm Ag islands on its surface. The observed SERS bands corresponding to different vibrational modes of the molecule, attached to or near Ag, and the variations in these bands resulting from competitive interactions of the functional groups of the peptides with the SERS‐active Ag surfaces and reorientation occurring over time of adsorption were analyzed in this study. Copyright © 2012 John Wiley & Sons, Ltd.     

Single‐molecule surface‐enhanced Raman scattering of fullerene C60

We achieved single‐molecule surface‐enhanced Raman scattering (SM‐SERS) spectra from ultralow concentrations (10⁻¹⁵ M) of fullerene C₆₀ on uniformly assembled Au nanoparticles. It was found that resonant excitation at 785 nm is a powerful tool to probe SM‐SERS in this system. The appearance of additional bands and splitting of some vibrational modes were observed because of the symmetry reduction of the adsorbed molecule and a relaxation in the surface selection rules. Time‐evolved spectral fluctuation and ‘hot spot’ dependence in the SM‐SERS spectra were demonstrated to result from the single‐molecule Raman behavior of the spherical C₆₀ on Au nanoparticles. Copyright © 2010 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.     

Potential‐induced Raman behavior of individual (R)‐di‐2‐naphthylprolinol molecules on a Ag‐modified Ag electrode

The applicability of surface‐enhanced Raman spectroscopy is demonstrated to probe the adsorption behavior of individual molecules on a Ag electrode. High‐quality SERS spectra of (R)‐di‐2‐naphthylprolinol (DNP) were obtained from ultradilute solutions (10⁻¹² M ) on the Ag‐nanoparticle‐modified Ag electrode, which is attributed to the high electromagnetic (EM) effect of the SERS‐active system as well as to the strong adsorption and interaction of DNP molecules with Ag. The stable SERS spectra present remarkable potential dependence, which gives evidence for the behavior of individual DNP molecules on the Ag surface. Based on statistical analysis for the probability of DNP molecules located in ‘hot spots’, we propose an SERS mechanism for individual molecules in the electrode system, in combination with the hot‐spot model and orientation of the probe molecules. Copyright © 2010 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.     

CW measurements of resonance Raman profiles,line‐widths,and cross‐sections of fluorescent dyes: application to Nile Blue A in water and ethanol

The aim of this work is to illustrate the power of recently developed methods for measuring resonance Raman scattering (RRS) spectra of efficient fluorophores (using a standard continuous wave excitation and a charge‐coupled device (CCD)‐based Raman spectrometer), by applying them to a detailed study of a specific fluorophore: Nile Blue A. A combination of methods are used to measure the RRS properties of Nile Blue A in water (quantum yield (QY) of 4%) and ethanol (QY of 22%) at excitation wavelengths between 514 and 647 nm, thus covering both pre‐resonance and RRS conditions. Standard Raman measurements are used in situations where the fluorescence background is small enough to clearly observe the Raman peaks, while the recently introduced polarization‐difference RRS and continuously shifted Raman scattering are used closer to (or at) resonance. We show that these relatively straightforward methods allow us to determine the Raman cross‐sections of the most intense Raman peaks and provide an accurate measurement of their line‐width; even for broadenings as low as ∼ 4 cm ^{− 1}. Moreover, the obtained Raman excitation profiles agree well with those derived from the optical absorption by a simple optical transform model. This study demonstrates the possibility of routine RRS measurements using standard Raman spectrometers, as opposed to more complicated time‐resolved techniques. Copyright © 2013 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.     

Surface‐enhanced Raman scattering characteristics of CuO : Mn/Ag heterojunction probed by methyl orange: effect of Mn2+ doping

Interest in the synthesis of hybrid substrates for surface‐enhanced Raman scattering (SERS) has surged recently. Hereof, in the present work, a hybrid SERS substrate CuO : Mn/Ag heterojunction has been synthesised. To accomplish this, the nanostructred Ag island film and CuO : Mn nanoparticles are synthesised by vacuum thermal evaporation method and sol–gel method respectively, and thereafter, a heterojunction between the CuO : Mn and Ag is fabricated by adsorption of CuO : Mn (10^‐3 m in ethanol) on Ag island film. Further, the SERS sensitivity of CuO : Mn/Ag heterojunctions has been studied by probing methyl orange. We observed that with Mn‐doping in the lattice of CuO, the SERS signal is enhanced considerably because of ferromagnetic ordering in CuO : Mn. DFT/B3LYP/6‐311 G(d, p) method is used to calculate the energy of HOMO and LUMO level of methyl orange. Copyright © 2016 John Wiley & Sons, Ltd.     

Experimental and theoretical studies of Raman spectroscopy on 4‐mercaptopyridine aqueous solution and 4‐mercaptopyridine/Ag complex system

Raman scattering and surface‐enhanced Raman scattering (SERS) have been used to study the behavior of 4‐mercaptopyridine (4‐Mpy) dissolved in water and adsorbed on silver mirrors. In order to gain the actual structure and the theoretical modes of the 4‐Mpy dissolved in water and adsorbed on the surface of silver mirror, ab initio calculation at the Hartree–Fock (HF) level and density functional theory (DFT) at Beck's three‐parameter Lee‐Yang‐Parr (B3LYP) level were performed to calculate the vibrational modes and wavenumbers. 4‐Mpy/2H₂O and 4‐Mpy/Ag complex systems were optimized, and then the corresponding Raman spectra were calculated and analyzed. Compared with the experimental results, the calculated results of 4‐Mpy and 4‐Mpy/2H₂O complex systems obtained from DFT method were more accurate. Among the results calculated with HF method, the one with three Ag atoms was economical, which took less computer time but gave equivalent results to those with more noumber of Ag atoms. Copyright © 2007 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering‐based approach for DNA detection at low concentrations via polyvinyl alcohol‐protected silver grasslike patterns

A simple method is demonstrated to detect DNA at low concentrations on the basis of surface‐enhanced Raman scattering (SERS) via polyvinyl alcohol‐protected silver grasslike patterns (PVA‐Ag GPs) grown on the surface of the common Al substrate. By the SERS measurements of sodium citrate and thymine, the PVA‐Ag GPs are shown to be an excellent SERS substrate with good activity, stability and reproducibility. With the use of the tested molecule of thymine, the enhancement factor of the PVA‐Ag GPs is up to ~1.4 × 10⁸. The PVA‐Ag GPs are also shown to be an excellent SERS substrate with good biocompatibility for DNA detection, and the detection limit is down to ~10⁻⁵ mg/g. Meanwhile, the assignations of the Raman bands and the adsorption behaviors of the DNA molecules are also analyzed. In this work, the geometry optimization and the wavenumber analysis of adenine–Ag and guanine–Ag complexes for the ground states are performed using density functional theory, B3LYP functional and the LanL2DZ basis set. The transition energies and the oscillator strengths of adenine–Ag and guanine–Ag for the lowest six singlet excited states were calculated by using the time‐dependent density functional theory method with the same functional and basis set. The results show that the charge transfer in the adenine–Ag and guanine–Ag complexes should be the chemical factor for the SERS of the DNA molecules. Lastly, this method may be employed in large‐scale preparation of substrates that have been widely applied in the Raman analysis of DNA because the fabrication process is simple and inexpensive. Copyright © 2011 John Wiley & Sons, Ltd.     

Experimental parameters for the SERS of nitrate ion for label‐free semi‐quantitative detection of proteins and mechanism for proteins to form SERS hot sites: a SERS study

We have explored the effects of the experimental parameters on the surface‐enhanced Raman scattering (SERS) intensities of NO₃⁻ and proteins observed by a heat‐induced SERS method developed by our group. The results have shown that a strong SERS signal can be obtained at pH 4.0, using an Ag colloid prepared with the reduction time of 15 min (the average size of Ag nanoparticle is 56.5 nm) dilution prepared Ag colloid by a factor of 2 by use of a 5 mM citrate buffer, using 6 mM NaNO₃ and drying the sample at 100 °C, respectively. Based on the results, two possible mechanisms for proteins to form SERS hot sites during the sample preparations are proposed. A semi‐quantitative SERS detection of ribonuclease B has been investigated. Also, NaNO₂, Mg (NO₃)₂, MgSO₄ and Na₂SO₄ have been found to be suitable for the heat‐induced SERS method. Importantly, samples prepared by the heat‐induced SERS method are so stable that these samples can be used as a standard and transferred to different laboratories for direct comparison. Namely, it can overcome uncontrollable aggregation of Ag colloids in a solution sample. All these advantages and the simplicity of experimental setup have demonstrated that the heat‐induced SERS method using NaNO₃ as an electrolyte is very promising for label‐free routine and quantitative detection of proteins. Copyright © 2011 John Wiley & Sons, Ltd.     

SERS microRaman spectral probing of adsorbate‐containing,liquid‐overlayed nanosponge Ag aggregates assembled from fractal aggregates

Adsorbate‐containing, nanosponge Ag aggregates overlayed by a thin (~1.5 mm) liquid layer are reported as a new type of sample for Surface‐enhanced Raman scattering (SERS) microRaman spectral measurements and adsorbate (analyte) detection. Macroscopic Ag aggregates (of about 1.5 × 1.0 × 0.025 mm size) with the nanosponge internal morphology (revealed by Scanning electron microscopy (SEM)) were prepared by 3D assembling of fused fractal aggregates (D = 1.84 ± 0.04) formed in Ag nanoparticle hydrosol/HCl/adsorbate systems with 2,2’‐bipyridine (bpy) and/or a cationic free‐base tetrakis(2‐methyl‐4‐pyridiniumyl) porphine (H₂TMPyP) as the testing adsorbates. For SERS microRaman measurements, the macroscopic aggregate was overlayed by a thin (~1.5 mm) layer of the residual liquid. Preparation procedure, nanoscale imaging, and SERS spectral probing including the determination of the detection limits of the adsorbates revealed the following advantages of the adsorbate‐containing, liquid‐overlayed 3D nanosponge aggregate as a sample for SERS microRaman spectral measurements: (1) localization of adsorbate (analyte) into hot spots and, simultaneously, prevention of the analyte decomposition during the spectral measurement (carried out without an immersion objective), (2) fast and simple sample preparation, and (3) minimization of sample volume and an efficient concentration of hot spots into the focus of the laser beam. The advantages of the nanosponge Ag aggregates are further demonstrated by the 40 fmol limit of detection of bpy as Ag(0)‐bpy surface complex, as well as by preservation of the native structure of the cationic free‐base porphyrin H₂TMPyP. Copyright © 2015 John Wiley & Sons, Ltd.     

Fabrication of surface‐enhanced Raman scattering‐active ZnO/Ag composite microspheres

We show in this paper how zinc oxide (ZnO)/silver (Ag) composite microspheres can be prepared by the reduction of Ag(NH₃)₂⁺ with the reducing agent formaldehyde in aqueous solution on the surface of ZnO microspheres. During the preparation, Sn²⁺ was absorbed on the surface of ZnO microspheres for sensitization and activation, and then Ag(NH₃)₂⁺ was reduced to Ag nanoparticles by the reducing agent to obtain ZnO/Ag composite microspheres. SEM and TEM images revealed silver nanoparticles with a diameter ranging from tens to 100 nm. X‐Ray photoelectron spectra (XPS), X‐ray diffraction (XRD) patterns and UV‐vis spectra were used to characterize the structure of the ZnO/Ag composite microspheres. The origin of the surface‐enhanced Raman scattering properties was traced to the surface of the ZnO/Ag composite microspheres. The enhancement factor was estimated in detail, and the enhancement mechanism for the SERS effect was also investigated. Copyright © 2007 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.     

Surface‐enhanced Raman scattering of 4‐aminobenzenethiol on silver: confirmation of the origin of b2‐type bands

In order to resolve the dispute on the origin of the b₂‐type bands in the surface‐enhanced Raman scattering (SERS) of 4‐aminobenzenethiol (4‐ABT), we have measured its SERS spectra under a variety of conditions, including variable temperature and rotation, electrochemistry, and pH, as well as in the presence of a reducing agent. For comparison, the SERS spectra of 4‐nitrobenzenethiol (4‐NBT) and methyl orange (MO), a prototype azo compound, were also measured. First, we found that 4‐ABT on Ag is not subjected to photoreaction, although 4‐NBT is highly photoreactive on a silver surface. In the electrochemical environment, b₂‐type bands of 4‐ABT lost their intensity at very negative potentials, but the intensity recovered immediately upon raising the potential. In addition, b₂‐type bands were observed under rotation even after lowering the potential. The disappearance and reappearance of the b₂‐type bands could also be observed by bringing the sample of 4‐ABT on Ag into contact consecutively with a borohydride solution and water. This is because the surface potential of Ag is lowered by contact with a borohydride solution. Besides, we found that not only the normal Raman but also the SERS spectral features of 4‐ABT are hardly affected by pH variation, while the spectral features of MO are greatly affected, especially in the region of the NN stretching vibration, suggesting that the possibility of a photoconversion of 4‐ABT to an azo compound is low. Altogether, the b₂‐type bands were attributed to 4‐ABT, appearing in conjunction with the chemical enhancement mechanism in SERS. Copyright © 2011 John Wiley & Sons, Ltd.     

Survival of molecular information under surfaced‐enhanced resonance Raman (SERRS) conditions

In the present paper, we discuss the molecular information that can be derived from surface‐enhanced resonance Raman Scattering (SERRS) experiments performed with different excitation wavenumbers, which are close to resonance with an excited electronic state of the molecule [surface‐enhanced Raman dispersion spectroscopy (SERADIS)]. We specifically consider the situation, where a molecule is physisorbed to a site characterized by a local electric field with a direction independent of the direction of the external, exciting field. The molecular information available in this experimental situation is compared with the information available in a corresponding Raman dispersion spectroscopy (RADIS) experiment performed on a free molecule or a molecule physisorbed to a site, where the local field is isotropic. The consequences for resonance Raman scattering (RRS) and RADIS, when the molecule is adsorbed in the highly anisotropic hot spot (HS), are discussed; here it is shown that only the molecular information originating from the symmetric part of the scattering tensor can survive in SERRS and in SERADIS. Besides, it is shown that the depolarization ratio can no longer be used to discriminate between totally and non‐totally symmetric modes in the polarized surface‐enhanced Raman scattering (SERS) spectra. These results have implications for the resonance Raman spectra, but even more important for the application of the resonance Raman effect in the investigation of excited vibronic molecular states, in general, and in the investigation of electronic states in larger bio‐molecules, such as the various metallo‐porphyrins. Copyright © 2009 John Wiley & Sons, Ltd.