Surface Enhanced Raman Intensities Research Articles

Molecular Tunnel Junction-Controlled High-Order Charge Transfer Plasmon and Fano Resonances.

Quantum tunneling plays an important role in coupled plasmonic nanocavities with ultrasmall gap distances. It can lead to intriguing applications such as plasmon mode excitation, hot carrier generation, and construction of ultracompact electro-optic devices. Molecular junctions bridging plasmonic nanocavities can provide a tunneling channel at moderate gap distances and therefore allow for the facile fabrication of quantum plasmonic devices. Herein we report on the large-scale bottom-up fabrication of molecular junction-bridged plasmonic nanocavities formed from Au nanoplate-Au nanosphere heterodimers. When the molecular junction turns from insulating to conductive, a distinct spectral change is observed, together with the emergence of a high-order charge transfer plasmon mode. The evolution of the electron tunneling-induced plasmon mode also greatly affects the Fano resonance feature in the scattering spectrum of the individual heterodimers. The molecular conductance at optical frequencies is estimated. The molecular junction-assisted electron tunneling is further verified by the reduced surface-enhanced Raman intensities of the molecules in the plasmonic nanocavity. We believe that our results provide an interesting system that can boost the investigation on the use of molecular junctions to modulate quantum plasmon resonances and construct molecular plasmonic devices.

Photo-induced surface-enhanced Raman spectroscopy from a diphenylalanine peptide nanotube-metal nanoparticle template

UV irradiation of aligned diphenylalanine peptide nanotubes (FF-PNTs) decorated with plasmonic silver nanoparticles (Ag NPs) enables photo-induced surface-enhanced Raman spectroscopy. UV-induced charge transfer facilitates a chemical enhancement that provides up to a 10-fold increase in surface-enhanced Raman intensity and allows the detection of a wide range of small molecules and low Raman cross-section molecules at concentrations as low as 10–13 M. The aligned FF-PNT/Ag NP template further prevents photodegradation of the molecules under investigation. Our results demonstrate that FF-PNTs can be used as an alternative material to semiconductors such as titanium dioxide for photo-induced surface-enhanced Raman spectroscopy applications.

Reproducibility in surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is an intense ongoing hot topic because it is an attractive tool for sensing or detecting molecules in trace amounts. Despite its high specificity and sensitivity, the SERS technique has not been established as a routine analytic method most likely due to the low reproducibility of the SERS signal. This review considers the influence factors to produce the poor reproducibility during the SERS measurement. This review starts with the discussion of calculation of surface-enhanced Raman intensity in order to explain the reason why it is so difficult to achieve a high reproducibility of SERS measurement from the origin of enhancement mechanism. Then we focus on the fabrication of SERS substrates generally including two types: ➀ single particles and ➁ arrays on substrate that are directly used to detect molecules or other components. In addition, we discuss the molecule factors and optical system for the reproducibility for sample-to-sample or spot-to-spot on a substrate. In the final part of this review, some effects resulting in the irreproducibility of Raman bands’ position from recent literatures are discussed.

Surface enhanced Raman scattering of pyridazine molecule:absorption configuration and enhancementmechanism via bond polarizabilities

The temporal molecular bond polarizabilities of pyridazine molecule absorbed on Ag electrode were explored from its surface enhanced Raman intensities, showing that the two nitrogen atoms are the absorption sites and the enhancement is dominated by the charge transfer mechanism. The behaviors of the charges on the skeletal bonds were demonstrated under various applied voltages, which shows the conjugation effect. This algorithm is universal for the surface systems as long as their Raman spectra are available.

ChemInform Abstract: Charge Shift of Adsorbed Pyridazine Molecules to the Silver Electrode Surface at Various Applied Voltages as Interpreted from the Surface Enhanced Raman Intensities.

Abstract An algorithm which enables one to calculate the bond polarizability derivatives from the surface enhanced Raman (SER) intensities has been employed to study the adsorption of pyridazine molecules on the silver electrode. It is inferred from the bond polarizability derivatives that as the applied voltage varies from −0.4 V to −0.6 V the electron charge in the bonding orbital n 1 + n 2 , formed from the lone pair orbitals n 1 and n 2 of the two adjacent nitrogen atoms, shifts towards the silver electrode surface, whereas from −0.6 V to −0.8 V it is the charge in the antibonding orbital n 1 − n 2 that shifts towards the electrode surface. The corresponding SER frequencies of those modes involving the stretch motion of the NN bond initially decrease and then increase. At the same time a charge shift occurs in the benzene-like ring which obeys the bond conjugate rule.

Raman intensity interpretation of pyridine liquid and its adsorption on the Ag electrode via bond polarizabilities

The temporal bond polarizabilities of pyridine adsorbed on the Ag electrode under various applied voltages are obtained from their surface enhanced Raman intensities. In so doing, the delicate bond behaviors of pyridine molecule in the surface enhanced Raman process are well demonstrated, including the effects by the charge transfer and electromagnetic mechanisms. Furthermore, the adsorption effect is well reflected by the bond polarizabilities after relaxation as contrasted with the calculated bond electronic densities in the ground state. The work of pyridine liquid is also shown because its comparison with that under adsorption deepens our understanding of the Raman process. Though the method is semi-classical and simple as contrasted with those based on the quantum chemistry, it indeed offers us a very clear physical picture. This work demonstrates that this approach is quite universal for the Raman active systems even under adsorption as far as their Raman profiles are well measured.

A temporal study of the surface enhanced Raman bond polarizabilities of ethylene thiourea on the Ag electrode: The electromagnetic and charge transfer mechanisms

Based on the surface enhanced Raman intensities of ethylene thiourea on the silver electrode, the temporal bond polarizabilities are elucidated to offer important information of the surface enhancement mechanisms. The results demonstrated that both the electromagnetic and charge transfer mechanisms are responsible for the large enhancement. The relaxation rates of the various bond polarizabilities are displayed. All obey a single exponentially decaying function, independent of potentials and mechanisms. The characteristic time for the relaxation due to charge transfer mechanism is larger than that via the electromagnetic mechanism due to that its relaxation requires longer time for the charges to re-distribute.

Ultrasonic Trapping of Microparticles in Suspension and Reaction Monitoring Using Raman Microspectroscopy

An ultrasonic standing wave around 2 MHz has been used for trapping and concentration of suspended micrometer-size particles in a flow cell, whereas Raman microspectroscopy was used as a nondestructive technique to provide molecular information about the trapped particles. With this approach, detection and discrimination of different polymer microparticles based on their characteristic Raman spectra was performed. Dextran, poly(vinyl alcohol), and melamine resin-based beads, with and without functionalization, were used for this purpose. Furthermore, taking advantage of the flow-through characteristics of the cell and the versatility of the employed flow system, full control over the media surrounding the trapped particles was achieved. This allowed us to perform chemical reactions on the trapped particles and to monitor spectral changes in real time. Here retention of cation-exchanger beads loaded with silver ions and subsequent reduction of the silver ions was demonstrated. In this way, surface-enhanced Raman (SER) active beads were prepared and retained in the focus of the Raman microscope by means of the ultrasonic field. Injection of analytes in the flow system thus allowed recording of their SER spectra. Using 9-aminoacridine, a linear dependence of the found SER signal in the range from 1 to 10 microM has been achieved. The repeatability in the recorded SER intensities was on the order of 4-5%. This included bead retention, surface-enhanced Raman layer synthesis, and analyte detection.

Surface-enhanced Raman, normal Raman and infrared spectra of chlorobenzoic acids

IR and Raman spectra of p-, m-, and o-chlorobenzoic acids (CBAs) have been recorded in solid and in different solutions. Vibrational assignments have been proposed based essentially on group frequency, band shape, intensity, and some internal consistency of CBAs. Examinations of the carboxylic acid and the carboxylate vibrations show that chlorobenzoic acids (CBA's) exist as hydrogen-bonded dimers in solid and monomeric acids in organic solvents such as diethyl ether, chloroform, and ethanol, while existing as carboxylate ions in sodium hydroxide solutions. Surface-enhanced Raman (SER) spectra of CBAs adsorbed on silver-coated capillary tube and silver-coated glass plates have also been collected. Experimental results indicate that the detected adsorption species exist as carboxylates on silver surfaces. Explanation of the dependency of the SER intensity of p-chlorobenzoic acid on acidity has been suggested on the basis of the chemical mechanism. From the experimental Raman data, the degree of enhancement is estimated to be 1.0 × 10 5 for the p-chlorobenzoic acid adsorbed on the substrates. The adsorption behaviours of these acids on silver surfaces are discussed.

Effect of molecular oxygen on the surface-enhanced Raman intensity of adsorbed molecules on silver, copper and gold electrodes

The effect of molecular oxygen (O 2) on the surface-enhanced Raman intensities of pyridine (py) adsorbed on silver, copper and gold electrodes has been investigated. A fast decrease in the SERS intensity with the time of electrode exposure to an electrolyte solution saturated with O 2 has been observed for py adsorbed on silver and copper electrodes. In the case of copper in solutions containing benzotriazole, the decrease in the SERS intensities was not so fast as for py. A less pronounced decrease in the SERS intensity with time of O 2 exposure was observed for py adsorbed on gold electrodes. Part of the SERS intensity of py adsorbed on silver and copper is irreversibly lost after oxygen exposure at open circuit, while the SERS intensity of py adsorbed on gold is totally recovered. The results suggest that O 2 reacts with the SERS-active sites, oxidizing them and precluding the short-range (charge-transfer) mechanism of enhancement from being effective. This oxidation of the SERS-active sites is irreversible for silver and copper, but reversible for gold electrodes.

Relation between surface enhanced Raman intensity and surface concentration for 2-amino 5-nitro pyridine adsorbed on roughened polycrystalline silver electrodes

A quantitative correlation between surface enhanced Raman (SER) intensity and surface coverage was obtained for 2-amino 5-nitro-pyridine (ANP) adsorption on roughened polycrystalline silver from aqueous 0.09 M KClO 4 + 0.02 M NaOH by recording in the same cell and on the same electrode surface the SER spectrum of ANP at −0.4 V vs. the molar calomel electrode and, immediately after, its surface concentration Γ ANP. The latter concentration was determined by stepping the applied potential from −0.4 V, where ANP is electroinactive, to −1.2 V where the adsorbed ANP molecules are electroreduced instantaneously, and by measuring the corresponding charge using the double-potential-step chronocoulometric method. In this way the effect of any irreproducibilities in the roughening procedure on the SER intensity vs. Γ ANP relationship was minimized. The surface roughening was made quite reproducible and stable by carrying out the oxidation-reduction cycle (ORC) in neutral 0.09 M KClO 4 in the absence of the adsorbate; this ORC ensures the reversibility of ANP adsorption, which satisfies the Frumkin isotherm with a standard Gibbs energy of adsorption of −43 kJ mol −1. The stability of the surface roughness was checked by measuring the photoemission current at −1.4 V on the freshly roughened electrode in the absence of scavengers. The SER intensity of ANP increases proportionally to Γ ANP up to maximum coverage. The SER spectra of ANP on colloidal silver and on silver electrodes in 10 −3 M HClO 4 were also recorded and an assignment of the bands of all these spectra was provided.

The bond polarizability derivatives of adsorbed methylviologen on the rough silver electrode and their implications as interpreted from surface enhanced Raman intensities

The bond polarizability derivatives of adsorbed methylviologen (MV) on a rough silver electrode under various applied voltages were elucidated from the surface enhanced Raman intensities using an algorithm proposed by us previously [B. Tian, G. Wu and G. Liu, J. Chem. Phys., 87 (1987) 7300]. The results reveal much fruitful information concerning the properties of adsorbed MV 2+ and MV + under the chemical mechanism of surface enhanced Raman scattering as well as the adsorption configurations of these species.

Time development of surface enhanced Raman intensities of the adsorbed pyrazine molecules on the silver electrode as the applied voltage varies

AbstractIn this work, the time development of surface enhanced Raman intensities of the vibrational modes involving the ring skeleton and the C—H motions of the pyrazine molecule as the applied voltage on the silver electrode is shifted between −0.2 V and −0.5 V (vs. saturated calomel electrode) is reported. The observation shows that there are two processes of enhancement involved during this shift of applied voltage. One is vibrational mode dependent while the other is not. The detailed behavior of these two processes of enhancement is discussed along with the retarded response of the Helmholtz double layer structure to the change of the applied voltage on the electrode.

Charge shift of adsorbed pyridazine molecules to the silver electrode surface at various applied voltages as interpreted from the surface enhanced Raman intensities

An algorithm which enables one to calculate the bond polarizability derivatives from the surface enhanced Raman (SER) intensities has been employed to study the adsorption of pyridazine molecules on the silver electrode. It is inferred from the bond polarizability derivatives that as the applied voltage varies from −0.4 V to −0.6 V the electron charge in the bonding orbital n 1 + n 2, formed from the lone pair orbitals n 1 and n 2 of the two adjacent nitrogen atoms, shifts towards the silver electrode surface, whereas from −0.6 V to −0.8 V it is the charge in the antibonding orbital n 1 − n 2 that shifts towards the electrode surface. The corresponding SER frequencies of those modes involving the stretch motion of the NN bond initially decrease and then increase. At the same time a charge shift occurs in the benzene-like ring which obeys the bond conjugate rule.

Diffusion of ethylene and xenon in thin pyrazine layers

The rate of diffusion of C{sub 2}H{sub 4} and Xe through thin pyrazine films condensed on coldly deposited silver films was studied by following the time evolution of the surface-enhanced Raman intensity of the diffusant, as a measure of its accumulation at the silver-pyrazine interface. The form of the intensity versus time curve was reproduced well by solving the diffusion equation with the appropriate boundary conditions and including, in addition, contributions from depolarization effects that become significant at high diffusant surface concentration. The temperature dependence of the diffusion coefficient of C{sub 2}H{sub 4} through the pyrazine film was found to be of the form D = 6 {times} 10{sup {minus}11} exp(-0.7 kcal/RT) cm{sup 2}/s, indicating that the pyrazine films deposited on cold surfaces (40-60 K) have an amorphous structure. In addition to diffusion data. SERS spectra were measured at various silver film deposition temperatures for the mixed C{sub 2}H{sub 4}-pyrazine layer adsorbed on the silver surface after C{sub 2}H{sub 4} diffusion through the pyrazine film occurred. The spectra showed evidence for two different types of SERS-active sites on these rough Ag films, with the relative numbers of each type of site depending on the surface preparation temperature of the Agmore » film. Since the diffusion coefficient was apparently independent of the Ag surface preparation temperature, these studies also indicate that neither of these SERS-active sites are located at the bottom of long narrow pores as postulated by the pore model of Albano et al. Comparison between diffusion coefficients determined for C{sub 2}H{sub 4} and Xe shows that the diffusion coefficient follows a M{sup {minus}{1/2}} mass dependence as predicted by simple kinetic theory treatments of diffusion.« less

Temperature dependent surface enhanced Raman spectroscopy of piperidine in AgBr sol

The temperature dependent surface enhanced Raman (SER) spectra of piperidine in AgBr sols are presented with emphasis on the study of the intensity variation as the temperature increases. Most of the SER intensities decrease as the temperature increases. This is interpreted as due to the increase of the adsorption distance from the sol particle surface. It is also inferred from the various slopes of the decrease in intensity of various modes that there is variation of the SER effect in the dimension of a single bond, i.e. in 1.5 Å. Moreover, from the temperature dependent SER spectra, it is concluded that due to thermal agitation, both axial and equatorial piperidine molecules are adsorbed on the sol particle surface as the temperature increases. The energy difference between these two forms are calculated to be around 20 kcal/mol which is believed to be larger than that in solution due to the adsorption effect. Besides, two peaks at 1390 and 1231 cm −1 show an anomalous positive thermal effect which could be due to the very complicated SERS mechanism. Finally, it is discussed that the temperature dependent SERS study possesses potentiality in revealing the chemical structure near the sol particle and noble metal electrode surfaces.

The molecular polarizability derivatives and their implications as interpreted from the surface enhanced Raman intensities: A case study of piperidine

The distinct variation of the surface enhanced Raman spectral pattern of piperidine under different applied voltages is analyzed with the Wolkenstein’s approximation to reveal its very molecular implication. The calculated effective polarizability derivatives show that there are both chemical and electromagnetic mechanisms in the surface enhanced Raman scattering. The chemical mechanism results in: (1) The farther a bond is away from the metal surface, the larger its polarizability derivative. (2) The more negative the applied voltage (vs standard calomel electrode), the larger the polarizability derivatives are and the weaker the adsorption. (3) There is sharing of the electron charge between the ring skeleton of the piperidine molecule and the Ag electrode. The electromagnetic mechanism implies equatorial C–H bonds possess larger polarizability derivatives than those of the corresponding axial C–H bonds. The analysis confirms the head-on adsorption of the piperidine molecule on the Ag electrode through the nitrogen atom as interpreted independently from other spectroscopic analyses. The behavior of the polarizability derivatives as a function of applied voltages at the Ag electrode reveals certain structural information of the molecules adsorbed on the Ag electrode.

Normal, resonance enhanced, and surface enhanced raman spectra of ruthenium-pyridyl complexes

Normal Raman (NR) spectra of solid and solution phase 2,2′-bipyridine and solid phase Ru(bpy)(PVP)(ClO 4) 2 are shown. Resonance Raman (RR) spectra of solid and solution phase Ru(bpy)(pyr) 4 (ClO 4) 2 and Ru(pyr) 6 (BF 4) 2 are presented. Surface-enhanced Raman (SER) spectra of 2,2′-bipyridine, Ru(bpy)(pyr) 4(ClO 4) 2, (Ru(pyr) 6(BF 4) 2, and Ru(bpy)(PVP)(ClO 4) 2 at silver electrodes are presented. Above, pyr=pyridine, bpy-2,2′-bipyridine, and PVP=poly(4-vinylpyridine). The effect of elctrode potential on the SER spectra of these compounds is shown by spectra acquired at applied potentials from 0.0 to −1.0 V, and plots of SER intensity vs. electrode potential. Excitation profiles for selected bands of each compound are presented. Comparison of SER spectra of Ru(bpy)(pyr) 4(ClO 4) 2 and polymeric Ru(bpy)(PVP)(ClO 4) 2 suggests no potential effect intrinsic to the SER mechanism. Plots of SER intensity vs. electrode potential show the SER effect is dependent on the individual surface-active species. SER excitation spectra for these compounds show that colored ruthenium complexes exhibit a maximum with respect to excitation wavelength. The SER maxima are shifted toward longer wavelengths in comparison with the known UV-vis solution absorbance maxima.

Irreversible loss of adatoms on Ag electrodes during potential cycling determined from surface enhanced Raman intensities

Irreversible loss of adatoms on Ag electrodes during potential cycling determined from surface enhanced Raman intensities

Irreversible loss of adatoms on Ag electrodes during potential cycling determined from surface enhanced raman intensities

Surface enhanced Raman scattering of adsorbates (Ag 0-Cl −, H 2O and pyridine) on Ag electrodes in 1M KC1 and 1M KCl+0.05M pyridine electrolytes was monitored continuously with an optical multichannel analyzer system as the electrode potential was cycled over various ranges within nonfaradaic regions of the oxidation-reduction cycle. A systematic investigation was performed of the potential dependence of SERS of pyridine in 1M KX + 0.05M pyridine electrolytes, where X = F, Cl, Br and I. Since the surface coverage of the adsorbates is reversible with potential cycling within a potential range, it was possible to determine potential dependences of the irreversible loss in the SERS enhancement factor which occurs as the electrode potential is ramped toward the potential of zero charge (PZC). The results provide strong support for the role of adatoms on the electrode surface in the overall enhancement mechanism. There is evidence that the strongly bound adsorbates immobilize the adatoms at positive potentials but allow the adatoms to migrate and become lost at surface defects as the potential approaches the PZC where the adsorbates are less tightly bound.