Characterization of surface water on Au core Pt-group metal shell nanoparticles coated electrodes by surface-enhanced Raman spectroscopy

We utilized the strategy of 'borrowing SERS activity', by chemically coating several atomic layers of a Pt-group metal on highly SERS-active Au nanoparticles, to obtain the first SERS (also Raman) spectra of surface water on Pt and Pd metals, and propose conceptual models for water adsorbed on Pt and Pd metal surfaces.     

Rh和Pt超薄层电极上的CO吸附行为(英文)

利用沉积在粗糙金电极上的过渡金属超薄层电极技术 ,我们获得了氢及一氧化碳在Rh和Pt表面上吸附的拉曼信号 ,并对两者之间的相互作用进行了分析 ..我们还进行了二氧化碳在这两种金属表面的还原行为的初步研究 ,以及对不同方式获得的一氧化碳吸附拉曼信号的特点进行了分析 .     

Surface-enhanced Raman scattering characteristics of 4-nitrobenzenethiol adsorbed on palladium and silver thin films

Palladium is an important catalytic metal, and it is desirable to develop a surface-enhanced Raman scattering (SERS) technique to investigate the reagent and product species adsorbed on its surface. Unfortunately, Pt-group metals, e.g., Pt and Pd, have been commonly considered as non- or weak-SERS-active substrates. In this work, Ag and Pd thin films were deposited very efficiently and evenly onto the surface of glass substrates by using only corresponding metal nitrate salts (AgNO₃ and Pd(NO₃)₂) with butylamine in ethanolic solutions. In this process, pure ethanol was used for Ag deposition, while an ethanol–water (8:2) mixture was used for Pd deposition. The as-prepared Ag and Pd films exhibited SERS activity over a large area. The surface-induced photoconversion capabilities of these Ag and Pd films were then tested on 4-nitrobenzenethiol by means of SERS. It was found that at least under visible laser irradiation, the surface-catalyzed photoreaction occurs more readily on a Ag film than on a Pd film for the conversion of 4-nitrobenzenethiol to 4-aminobenzenethiol, even though Pd is known to be an important transition metal with high catalytic activity.     

In situ dynamic measurements of the enhanced SERS signal using an optoelectrofluidic SERS platform

A novel active surface-enhanced Raman scattering (SERS) platform for dynamic on-demand generation of SERS active sites based on optoelectrofluidics is presented in this paper. When a laser source is projected into a sample solution containing metal nanoparticles in an optoelectrofluidic device and an alternating current (ac) electric field is applied, the metal nanoparticles are spontaneously concentrated and assembled within the laser spot, form SERS-active sites, and enhance the Raman signal significantly, allowing dynamic and more sensitive SERS detection. In this simple platform, in which a glass slide-like optoelectrofluidic device is integrated into a conventional SERS detection system, both dynamic concentration of metal nanoparticles and in situ detection of SERS signal are simultaneously possible with only a single laser source. This optoelectrofluidic SERS spectroscopy allows on-demand generation of 'hot spots' at specific regions of interest, and highly sensitive, reliable, and stable SERS measurements of the target molecules in a tiny volume (～500 nL) of liquid sample without any fluidic components and complicated systems.     

Periodic trends in electrode-chemisorbate bonding: benzonitrile on platinum-group and other noble metals as probed by surface-enhanced Raman spectroscopy combined with density functional theory.

Detailed intramolecular vibrational spectra obtained by means of surface-enhanced Raman scattering (SERS) for benzonitrile adsorbed on seven electrode surfaces-four Pt-group metals (platinum, palladium, rhodium, and iridium) and the Group IB metals (copper, silver, and gold)-are reported with the aim of exploring the metal-dependent nature of surface-chemisorbate interactions. The Pt-group surfaces were prepared as ultrathin electrodeposited films on gold, enabling the SERS activity inherent to the substrate to be imparted to the overlayer material. Benzonitrile was selected as a "model" organic adsorbate since it displays a rich array of coupled aromatic ring as well as substituent modes which collectively can provide insight into the various molecular perturbations induced by surface coordination via the nitrile substituent. The experimental spectra are compared with ab initio calculations of vibrational frequencies, bond geometries, and charge distributions obtained by means of Density Functional Theory (DFT), which yields valuable insight into the underlying structural reasons for the sensitivity of the experimental coordination-induced frequency shifts to the nature of the intramolecular mode and the metal surface. The DFT results also form an invaluable aid in making SER spectral assignments, along with providing detailed information on the coupled atomic displacements involved in each vibrational mode. Benzonitrile surface coordination was modeled in the DFT calculations by binding the nitrile group to metal atoms and small metal clusters. While the majority of the aromatic-ring SER frequencies are altered only slightly (approximately < 5 cm(-1)) upon surface coordination, several modes (especially nu(1), nu(6a)) are blue-shifted substantially (by up to 50 cm(-1)). These shifts were identified by DFT as arising from mode coupling to the nitrile substituent, especially involving the C-CN bond that is compressed upon nitrile coordination, associated with metal-adsorbate back-donation. The small (<5 cm(-1)) red-shifts seen for ring vibrations not involving coupled substituent motion apparently arise from increased antibonding aromatic electron density. The metal-dependent frequency shifts seen for these coupled aromatic vibrations as well as for the more localized C-N nitrile stretching mode are consistent with increased back-donation anticipated in the sequence d(10) < d(9) < d(8) within a given Periodic row. Overall, the findings provide a benchmark illustration of the virtues of DFT in interpreting complex vibrational spectra for larger polyatomic adsorbates.     

Self-assembled metal colloid films: two approaches for preparing new SERS active substrates

In this paper, we propose two new approaches for preparing active substrates for surface-enhanced Raman scattering (SERS). In the first approach (method 1), one transfers AgI nanoparticles capped by negatively charged mercaptoacetic acid from a AgI colloid solution onto a quartz slide and then deoxidizes AgI to Ag nanoparticles on the substrate. The second approach (method 2) deoxidizes AgI to Ag nanoparticles in a colloid solution and then transfers the Ag nanoparticles capped by negatively charged mercaptoacetic acid onto a quartz slide. By transfer of the AgI/Ag nanoparticles from the colloid solutions to the solid substrates, the problem of instability of the colloid solutions can largely be overcome. The films thus prepared by both approaches retain the merits of metal colloid solutions while they discharge their shortcomings. Accordingly, the obtained Ag particle films are very suitable as SERS active substrates. SERS active substrates with different coverages can be formed in a layer-by-layer electrostatic assembly by exposing positively charged surfaces to the colloid solutions containing oppositely charged AgI/Ag nanoparticles. The SERS active substrates fabricated by the two novel methods have been characterized by means of atomic force microscopy (AFM) and ultraviolet-visible (UV-vis) spectroscopy. The results of AFM and UV-vis spectroscopy show that the Ag nanoparticles grow with the increase in the number of coverage and that most of them remain isolated even at high coverages. Consequently, the surface optical properties are dominated by the absorption due to the isolated Ag nanoparticles. The relationship between SERS intensity and surface morphology of the new active substrates has been investigated for Rhodamine 6G (R6G) adsorbed on them. It has been found that the SERS enhancement depends on the size and aggregation of the Ag particles on the substrates. Especially, we can obtain a stronger SERS signal from the substrate prepared by method 1, implying that for the metal nanoparticles capped with stabilizer molecules such as mercaptoacetic acid, the in situ deoxidization in the film is of great use in preparing SERS active substrates. Furthermore, we have found that the addition of Cl- into the AgI colloid solution changes the surface morphology of the SERS active substrates and favors stronger SERS enhancement.     

金属镍掺杂改进纳米TiO2的表面增强拉曼散射性能

通过溶胶-水热法合成纯的和不同量Ni离子掺杂的TiO₂纳米粒子, 将其作为表面增强拉曼散射(SERS)活性基底, 研究了金属Ni掺杂对于纳米TiO₂ SERS性能的改进. 结果表明, 适量的Ni掺杂能够在纳米TiO₂的能隙中靠近导带底的位置形成丰富的掺杂能级, 促进TiO₂-to-molecule的电荷转移过程, 进而提高纳米TiO₂基底对吸附分子的SERS增强能力, 显著改进纳米TiO₂的SERS性能.     

Electrochemical and surfaced-enhanced Raman spectroscopic investigation of CO and SCN- adsorbed on Au(core)-Pt(shell) nanoparticles supported on GC electrodes

Core-shell Au-Pt nanoparticles were synthesized by using a seed growth method and characterized by transmission electron microscopy, X-ray diffraction, and UV-vis spectroscopy. Au(core)-Pt(shell)/GC electrodes were prepared by drop-coating the nanoparticles on clean glassy carbon (GC) surfaces, and their electrochemical behavior in 0.5 M H2SO4 revealed that coating of the Au core by the Pt shell is complete. The electrooxidation of carbon monoxide and methanol on the Au(core)-Pt(shell)/GC was also examined, and the results are similar to those obtained on a bulk Pt electrode. High quality surface-enhanced Raman scattering (SERS) spectra of both adsorbed CO and thiocyanate were observed on the Au(core)-Pt(shell)/GC electrodes. The potential-dependent SERS features resemble those obtained on electrochemically roughened bulk Pt or Pt thin films deposited on roughened Au electrodes. For thiocyanate, the C-N stretching frequency increases with the applied potential, yielding two distinctly different dnu(CN)/dE. From -0.8 to -0.2 V, the dnu(CN)/dE is ca. 50 cm(-1)/V, whereas it is 90 cm(-1)/V above 0 V. The bandwidth along with the band intensity increases sharply above 0 V. At the low-frequency region, Pt-NCS stretching mode at 350 cm(-1) was observed at the potentials from -0.8 to 0 V, whereas the Pt-SCN mode at 280 cm(-1) was largely absent until around 0 V and became dominant at more positive potentials. These potential-dependent spectral transitions were attributed to the adsorption orientation switch from N-bound dominant at the negative potential region to S-bound at more positive potentials. The origin of the SERS activity of the particles is briefly discussed. The study demonstrates a new method of obtaining high quality SERS on Pt-group transition metals, with the possibility of tuning SERS activity by varying the core size and the shell thickness.     

Functionalization of Ag nanoparticles with the bis-acridinium lucigenin as a chemical assembler in the detection of persistent organic pollutants by surface-enhanced Raman scattering

Organochlorine pesticide endosulfan has been detected for the first time by using surface-enhanced Raman scattering (SERS) at trace concentrations. The bis-acridinium dication lucigenine was successfully used as a molecular assembler in the functionalization of metal nanoparticles to facilitate the approach of the pesticide to the metal surface. From the SERS spectra valuable information about the interaction mechanism between the pesticide and lucigenin can be deduced. In fact, endosulfan undergoes an isomerization upon adsorption onto the metal, while the viologen undergoes a rotation of the acridinium planes to better accommodate the pesticide molecule. An interaction between the N atom of the central acridinium ring and the pesticide Cl-CC-Cl fragment is verified through a charge-transfer complex. The present study affords important information which can be applied to the design of chemical sensor systems of persistent organic pollutants based on the optical detection on functionalized metal nanoparticle.     

Effect of Graphene Characteristics on Morphology and Performance of Composite Noble Metal-Reduced Graphene Oxide SERS Substrate

Graphene/noble metal substrates for surface enhanced RAMAN scattering (SERS) possess synergistically improved performance, due to the strong chemical enhancement mechanism accounted to graphene and the electromagnetic mechanism raised from the metal nanoparticles. However, only the effect of noble metal nanoparticles characteristics on the SERS performance was studied so far. In attempts to bring a light to the effect of quality of graphene, in this work, two different graphene oxides were selected, slightly oxidized GOS (20%) with low aspect ratio (1000) and highly oxidized (50%) GOG with high aspect ratio (14,000). GO and precursors for noble metal nanoparticles (NP) simultaneous were reduced, resulting in rGO decorated with AgNPs and AuNPs. The graphene characteristics affected the size, shape, and packing of nanoparticles. The oxygen functionalities actuated as nucleation sites for AgNPs, thus GOG was decorated with higher number and smaller size AgNPs than GOS. Oppositely, AuNPs preferred bare graphene surface, thus GOS was covered with smaller size, densely packed nanoparticles, resulting in the best SERS performance. Fluorescein in concentration of 10⁻⁷ M was detected with enhancement factor of 82 × 10⁴. This work demonstrates that selection of graphene is additional tool toward powerful SERS substrates.     

Raman spectroscopy on transition metals

Surface-enhanced Raman spectroscopy (SERS) has developed into one of the most important tools in analytical and surface sciences since its discovery in the mid-1970s. Recent work on the SERS of transition metals concluded that transition metals, other than Cu, Ag, and Au, can also generate surface enhancement as high as 4 orders of magnitude. The present article gives an overview of recent progresses in the field of Raman spectroscopy on transition metals, including experimental, theory, and applications. Experimental considerations of how to optimize the experimental conditions and calculate the surface enhancement factor are discussed first, followed by a very brief introduction of preparation of SERS-active transition metal substrates, including massive transition metal surfaces, aluminum-supported transition metal electrodes, and pure transition metal nanoparticle assembled electrodes. The advantages of using SERS in investigating surface bonding and reaction are illustrated for the adsorption and reaction of benzene on Pt and Rh electrodes. The electromagnetic enhancement, mainly lightning-rod effect, plays an essential role in the SERS of transition metals, and that the charge-transfer effect is also operative in some specific metal–molecule systems. An outlook for the field of Raman spectroscopy of transition metals is given in the last section, including the preparation of well-ordered or well-defined nanostructures, and core-shell nanoparticles for investigating species with extremely weak SERS signals, as well as some new emerging techniques, including tip-enhanced Raman spectroscopy and an in situ measuring technique. Figure Electric-field enhancement of a SERS-active Rh surface decorated with small nanohemispheres     

Nanoparticle-mirror sandwich substrates for surface-enhanced Raman scattering

Sandwich surface-enhanced Raman scattering (SERS) substrates (3S) utilizing coupling between continuous metal films and plasmonic particles were fabricated using silver mirrors, electrochemically roughened films, and various sizes of silver nanoparticles. The effect of excitation wavelength and nanoparticle size on SERS spectra of poly(vinylpyridine), selected as a model compound, was studied to determine the optimum conditions for the strongest SERS signal. The Raman enhancement resulted from the plasmon coupling of silver nanoparticles to the underlying continuous film as well as the lateral plasmon coupling between the silver nanoparticles. The formation of the charge transfer complex was also observed. The 3S configuration was used to obtain SERS spectra of dipicolinic acid (DPA), a chemical signature for Bacillus anthracis.     

Silica-void-gold nanoparticles: temporally stable surface-enhanced Raman scattering substrates

Reproducible detection of a target molecule is demonstrated using temporally stable solution-phase silica-void-gold nanoparticles and surface-enhanced Raman scattering (SERS). These composite nanostructures are homogeneous (diameter = 45 +/- 4 nm) and entrap single 13 nm gold nanoparticle cores inside porous silica membranes which prevent electromagnetic coupling and aggregation between adjacent nanoparticles. The optical properties of the gold nanoparticle cores and structural changes of the composite nanostructures are characterized using extinction spectroscopy and transmission electron microscopy, respectively, and both techniques are used to monitor the formation of the silica membrane. The resulting nanostructures exhibit temporally stable optical properties in the presence of salt and 2-naphthalenethiol. Similar SERS spectral features are observed when 2-naphthalenethiol is incubated with both bare and membrane-encapsulated gold nanoparticles. Disappearance of the S-H Raman vibrational band centered at 2566 cm(-1) with the composite nanoparticles indicates that the target molecule is binding directly to the metal surface. Furthermore, these nanostructures exhibit reproducible SERS signals for at least a 2 h period. This first demonstration of utilizing solution-phase silica-void-gold nanoparticles as reproducible SERS substrates will allow for future fundamental studies in understanding the mechanisms of SERS using solution-phase nanostructures as well as for applications that involve the direct and reproducible detection of biological and environmental molecules.     

Surface-enhanced Raman scattering investigation of the adsorption of 2-mercaptobenzoxazole on smooth copper surfaces doped with silver colloidal nanoparticles

The adsorption of 2-mercaptobenzoxazole on copper has been investigated by means of surface-enhanced Raman scattering (SERS) by doping smooth copper surfaces with silver colloidal nanoparticles. The metal surfaces have been characterized by means of atomic force microscopy measurements. The compound adsorbs on the Cu/Ag surfaces in its ionized thiolic form, adopting a tilted orientation with respect to the metal surface. The anion is chemisorbed through the sulfur and nitrogen atoms on the smooth copper surface, and the silver colloidal nanoparticles only enhance the Raman signal due to the electromagnetic mechanism. SERS data have been interpreted with the help of DFT calculations on models of the ligand bound to copper adclusters.     

Recent advancements in optical DNA biosensors: exploiting the plasmonic effects of metal nanoparticles

The emerging field of plasmonics, the study of electromagnetic responses of metal nanostructures, has revealed many novel signal enhancing phenomena. As applied to the development of label-free optical DNA biosensors, it is now well established that plasmon-based surface enhanced spectroscopies on nanostructured metal surfaces or metal nanoparticles can markedly improve the sensitivity of optical biosensors, with some showing great promise for single molecule detection. In this review, we first summarize the basic concepts of plasmonics in metal nanostructures, as well as the characteristic optical phenomena to which plasmons give rise. We will then describe recent advances in optical DNA biosensing systems enabled by metal nanoparticle-derived plasmonic effects, including the use of surface enhanced Raman scattering (SERS), colorimetric methods, "scanometric" processes, and metal-enhanced fluorescence (MEF).     

Development of Novel Substrates for Surface-enhanced Raman and Surface-enhanced Hyper Raman Spectroscopy

Since its discovery two decades ago, surface-enhanced Raman scattering (SERS) has been explored extensively as a useful technique in the study of molecular behaviors at interfaces and in chemical and biochemical analysis. At solid-liquid interface, SERS has been practiced mainly in aqueous solution on either aggregated metal colloids or roughened metal electrodes. However, both aggregated metal colloids and roughened electrodes have their own problems as SERS substrates. One of the intriguing questions in exploring SERS application in chemistry is that can SERS-activity be gained and regulated from the dispersed metal nanoparticles immobilized on a SERS-inactive smooth electrode surface. The very essence of this question is to explore the effect on SERS-activity when the main features of two conventional SERS-surfaces, namely metal colloids and electrode, are combined. Same question can also be asked for the nonlinear three-photon surface-enhanced hyper Raman scattering (SEHRS).     

Functionalization of Ag nanoparticles with dithiocarbamate calix[4]arene as an effective supramolecular host for the surface-enhanced Raman scattering detection of polycyclic aromatic hydrocarbons

We report the use of 25,27-diethyl-dithiocarbamic-26,28-dihydroxy-p-tert-butylcalix[4]arene in the functionalization of Ag nanoparticles for pyrene detection by surface-enhanced Raman scattering (SERS). SERS spectra provided information about the calixarene orientation on the metal surface and the interaction mechanism with pyrene. Thus, in this work, we have combined a powerful spectroscopy technique such as SERS, the electronic plasmon-based properties of nanostructured metals, the molecular size-selective recognition of calixarene, and the strong chelating properties of the dithiocarbamate group toward the metal surface in the detection of polycyclic aromatic hydrocarbons.     

Intracellular SERS hybrid probes using BSA–reporter conjugates

Surface-enhanced Raman scattering (SERS) hybrid probes are characterized by the typical spectrum of a reporter molecule. In addition, they deliver information from their biological environment. Here, we report SERS hybrid probes generated by conjugating different reporter molecules to bovine serum albumin (BSA) and using gold nanoparticles as plasmonic core. Advantages of the BSA-conjugate hybrid nanoprobes over other SERS nanoprobes are a high biocompatibility, stabilization of the gold nanoparticles in the biological environment, stable reporter signals, and easy preparation. The coupling efficiencies of the BSA–reporter conjugates were determined by MALDI-TOF-MS. The conjugates’ characteristic SERS spectra differ from the spectra of unbound reporter molecules. This is a consequence of the covalent coupling, which leads to altered SERS enhancement and changes in the chemical structures of the reporter and of BSA. The application of the BSA–reporter conjugate hybrid probes in 3T3 cells, including duplex imaging, is demonstrated. Hierarchical cluster analysis and principal components analysis were applied for multivariate imaging using the SERS signatures of the incorporated SERS hybrid nanoprobes along with the spectral information from biomolecules in endosomal structures of cells. The results suggest more successful applications of the SERS hybrid probes in cellular imaging and other unordered high-density bioanalytical sensing.

Towards single-microorganism detection using surface-enhanced Raman spectroscopy

The identification and discrimination of microorganisms is important not only for clinical reasons but also for pharmaceutical clean room production and food-processing technology. Vibrational spectroscopy such as IR, Raman, and surface-enhanced Raman scattering (SERS) can provide a rapid ‘fingerprint’ on the chemical structure of molecules and is used to obtain a ‘fingerprint’ from microorganisms as well. Because of the requirement that a single bacterium cell and noble metal nanoparticles must be in close contact and the lack of a significant physical support to hold nanoparticles around the single bacterium cell, the acquisition of SERS spectra for a single bacterium using colloidal nanoparticles could be a challenging task. The feasibility of SERS for identification down to a single bacterium is investigated. A Gram-negative bacterium, Escherichia coli, is chosen as a model for the investigation. Because the adsorption of silver nanoparticles onto the bacterial cell is an exclusive way for locating nanoparticles close to the bacterium cell, the absorption characteristics of silver nanoparticles with different surface charges are investigated. It is demonstrated that the citrate-reduced colloidal silver solution generates more reproducible SERS spectra. It is found that E. coli cells aggregate upon mixing with silver colloidal solution, and this may provide an additional benefit in locating the bacterial cell under a light microscope. It is also found that a laser wavelength in the UV region could be a better choice for the study due to the shallow penetration depth. It is finally shown that it is possible to obtain SERS spectra from a single cell down to a few bacterial cells, depending on the aggregation properties of bacterial cells for identification and discrimination.