SERS chemical enhancement of 2,4,5-trichlorophenoxyacetic acid absorbed on Au20, its Au-Ag and Au-Cu bimetallic clusters: a DFT study (original) (raw)
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Role of Surface Metal Clusters in SERS Spectra of Ligands Adsorbed on Ag Colloidal Nanoparticles
Journal of Physical Chemistry C, 2008
The role of surface metal clusters in the surface enhanced Raman scattering (SERS) enhancement mechanism was investigated by coupling Raman measurements and DFT (density functional theory) calculations for pyrazole adsorbed on Ag colloidal nanoparticles. The adsorption process of the ligand can be suitably described by a model system constituted of pyrazolide anions chemically interacting with (Ag 4 ) +2 surface clusters since the DFT approach allows a complete depiction of the SERS profile to be obtained, including both band frequencies and relative intensities.
The Journal of Physical Chemistry C, 2011
Adsorption of 1,10-phenanthroline (PHEN) on the Au core ÀAg shell (Au@Ag) bimetallic nanocolloid surface has been investigated. Bimetallic nanocolloids have been synthesized using β-cyclodextrin (β-CD) in alkaline solution. Nanocolloids of different Au core : Ag shell ratio were synthesized keeping the same Au core size. Again, for the preparation, the total metal concentrations were always kept constant. The structure and composition of the bimetallic particles were characterized by UVÀvis and high-resolution transmission electron microscopy. Detailed normal Raman (NRS) and surface-enhanced Raman spectra (SERS) of PHEN in aqueous solution have been studied. The relative contributions of the electromagnetic (EM) and charge-transfer (CT) mechanism to the overall enhancement of the SERS bands of the PHEN molecule have been estimated. PHEN molecules are adsorbed on the Ag shell surface through both the nitrogen atoms with the molecular plane almost perpendicular to the surface, which has been confirmed from AgÀN stretching vibration. The three-dimensional finite difference time domain (3D-FDTD) method has been applied to simulate the local electric field on the spherical Au@Ag nanocolloid for various core/shell ratios. Comparative spectral information revealed the highest SERS effect from the 1:4 Au core ÀAg shell bimetallic nanocolloid of 29 nm size. This observation has also been supported by theory. Thus, experiment and theory keep abreast the attachment of PHEN molecules on a new substrate from SERS studies.
The Journal of Physical Chemistry A, 2010
A detailed theoretical investigation has been carried out at the density functional level of theories to investigate the nature of Raman intensities of the -PdO stretching mode of a model nerve agent DFP (diisopropylfluorophosphate) when bound to different gold (Au 8 , Au 20 ) and oxide-supported gold (] 2+ , M ) Ca, Mg) clusters. All of these clusters and the DFP-bound clusters are fully optimized, and the computed energetics shows that DFP attaches itself weakly to these clusters. The normal Raman spectra calculations on these clusters show that there is substantial enhancement of the -PdO stretching mode of DFP compared to the isolated species. This enhancement has been found to be due to the polarization of the -PdO bond of DFP when bound to the clusters. Significant enhancement in intensity has been observed in the case of Au n · · · DFP (n ) 8, 20), M 16 O 16 · · · Au 8 · · · DFP, and [M 16 O 15 · · · Au 8 ] 2+ · · · DFP (M ) Ca, Mg) clusters. The resonance Raman calculations on the Au n · · · DFP (n ) 8, 20) reveals that this enhancement could be made quite large and selective, which is a feature that is unique to the nerve agents and could be used as a property for detecting them.
Chemical Physics Letters, 2009
Surface-enhanced Raman scattering (SERS) of pyridine adsorbed on ultrathin nanocrystalline Au and Ag films generated at the liquid-liquid interface has been investigated. The shifts and intensification of bands formed with these films comprising metal nanoparticles are comparable to those found with other types of Au and Ag substrates. SERS of rhodamine 6G adsorbed on Ag films has also been studied. The results demonstrate that nanocrystalline metal films prepared by the simple method involving the organic-aqueous interface can be used effectively for SERS investigations.
SERS Comparison from Au, Ag, and Au-Ag Alloys: Insights by the First Principles
2018
Surface enhanced Raman scattering (SERS) is presented via a case study of 2,2’-bipyridyl(22BPY) molecules adsorbed on pure Au and Ag as well as on Au-Ag alloy nanodiscs. ExperimentalSERS spectra from Au and Ag nanodics show similar peaks, but those from Au-Ag alloyreveal new spectral features. The physical enhancement factors due to surface nano-texture wereconsidered by numerical simulations of light intensity distribution for the nano-textured Au, Ag,and Au-Ag alloy surfaces, but found to cause only minor differences. For the chemical insights ofenhancement, the density functional theory (DFT) calculations were performed using Au20, Ag20,and Au10Ag10 clusters of a pyramidal structure for SERS modeling. Binding of 22BPY to the clusterswas simulated by considering possible arrangements of vertex and planar physical as well aschemical adsorption models. A qualitative match with experimental SERS results for the pure Auand Ag was obtained. DFT models showing spectral shifts of 22BPY S...
Qualitative analysis of some alkanethiols on Au nanoparticles during SERS
RSC Advances, 2016
The surface enhanced Raman spectroscopy enhancement factors (SERS EFs) for different AuNPsurfactant systems is measured and the observed trend is theoretically and qualitatively investigated. Four thiolated coumarin derivatives (HS-(CH 2) 11-NHCO-coumarin, HS-(CH 2) 11-NHCO-indole, HS-(CH 2) 11triphenylimidazole and HS-(CH 2) 11-hydroquinone respectively) together with a stabilizing surfactant, HS-(CH 2) 11-PEG-COOH, were adsorbed in ratios of 1% and 50% onto different sized Au nanoparticles. Similar simulation experiments were done to determine the influence of the nanoparticle-surfactant binding energy (E b) on SERS EFs. Other parameters that were qualitatively investigated were the dependency of the Raman enhancement on (a) the size of Au nanoparticles, (b) the relative surface-to-molecule orientation, (c) the molecular orbitals (HOMOs and LUMOs), (d) the molecule electro-and nucleophilic centres and cross-sectional areas as well as (e) the molecular electrostatic potentials. It was observed that each of these parameters contribute to the observed trend and the final Raman signal enhancement.
Effect of the structure and charge of Au10 clusters on adsorption of hydrocarbons
Russian Chemical Bulletin, 2010
The interaction of gold clusters Au 10 of different structural and charge states with various hydrocarbons was studied by the PBE density functional method. Saturated hydrocarbons interact weakly with the neutral cluster Au 10 , for charged Au 10 + the alkane-cluster bond energies increase threefold. Unsaturated hydrocarbons interact with cluster surface more strongly than saturated hydrocarbons, while coordination to the benzene ring is possible for aromatic compounds PhC 2 H, PhC 2 H 3 , and PhC 2 H 5. The low coordinative gold atoms located on the peaks and edges of the cluster are the active adsorption site of the cluster. The appearance of a positive charge on the cluster leads to a greater increase in the hydrocarbon-gold cluster bond energy than the transition from the planar 2D structure to the three dimensional (3D) structure of the neutral cluster.
The Journal of Chemical Physics, 2008
p-aminothiophenol (PATP) is an important molecule for surface-enhanced Raman spectroscopy (SERS). It can strongly interact with metallic SERS substrates and produce very strong SERS signals. It is a molecule that has often been used for mechanistic studies of the SERS mechanism as the photon-driven charge transfer (CT) mechanism is believed to be present for this molecule. Recently, a hot debate over the SERS behavior of PATP was triggered by our finding that PATP can be oxidatively transformed into 4,4 0 -dimercaptoazobenzene (DMAB), which gives a SERS spectra of so-called ''b 2 modes''. In this perspective, we will give a general overview of the SERS mechanism and the current status of SERS studies on PATP. We will then demonstrate with our experimental and theoretical evidence that it is DMAB which contributes to the characteristic SERS behavior in the SERS spectra of PATP and analyze some important experimental phenomena in the framework of the surface reaction instead of the contribution ''b 2 modes''. We will then point out the existing challenges of the present system. A clear understanding of the reaction mechanism for nitrobenzene or aromatic benzene will be important to not only understand the SERS mechanism but to also provide an economic way of producing azo dyes with a very high selectivity and conversion rate.
Chinese Journal of Analytical Chemistry, 2011
Surface-enhanced Raman scattering (SERS) was used for the detection of 2,6-pyridinedicarboxylic acid (DPA), a biomarker for bacterial spores. By immobilizing gold nanoparticles of diameter 60 nm on a polished Au electrode using PVP as an adhesive layer, the steady and highly sensitive SERS substrates were fabricated. By using strong acid and alkali to adjust the pH value of DPA molecules, the molecular adsorption configuration of DPA via the SERS characteristics under various pH values were studied and the changes of SERS intensity were compared when HCl, H 2 SO 4 and HNO 3 were separately added to the DPA solution to get a pH value of 1.3. The changes of the adsorption form were also analyzed when NaCl, MgSO 4 and NaNO 3 were separately added to the DPA solution. The results showed that the anion of acids could play a role as a bridge when DPA molecules were protonated by the addition of acids, which could greatly enhance the SERS signal; however, when the pH value is greater than the second dissociation constant of DPA, because of steric effect and the disappearance of the bridge, the SERS signal decreases gradually. Compared with NO 3 and SO 4 2-, Clhas a much stronger binding force with protonated N on DPA; consequently, the addition of HCl can get the maximum SERS signal enhancement.
Onset of Intense Surface Enhanced Raman Scattering and Aggregation in the Au@Ag System
Journal of Spectroscopy, 2015
Gold core/silver shell (Au@Ag) nanoparticles of ~37 ± 5 nm diameter generate intense SERS (λEX=785 nm) responses in solution when they interact with the SERS labels rhodamine 6G (R6G), 4-mercaptopyridine (MPY), and 4-mercaptobenzoic acid (MBA). Herein the relationship between SERS intensity, aggregation, and adsorption phenomenon isobserved by titrating Au@Ag with the above labels. As the labels adsorb to the Au@Ag, they drive aggregation as evidenced by the creation of NIR extinction peaks, and the magnitude of this NIR extinction (measured at 830 nm) correlates very closely to magnitude of the intense SERS signals. The label MBA is an exception since it does not trigger aggregation nor does it result in intense SERS; rather intense SERS is recovered only after MBA coated Au@Ag is aggregated with KCl. An “inner filter” model is introduced and applied to compensate for solution extinction when the exciting laser radiation is significantly attenuated. This model permits a summary of ...