Surface-enhanced resonance Raman scattering and background light emission coupled with plasmon of single Ag nanoaggregates

We investigated the optical properties of isolated single aggregates of Ag nanoparticles (Ag nanoaggregates) on which rhodamine 6G molecules were adsorbed to reveal experimentally a correlation among plasmon resonance Rayleigh scattering, surface-enhanced resonance Raman scattering (SERRS), and its background light emission. From the lack of excitation-laser energy dependence of background emission maxima we concluded that the background emission is luminescence, not Raman scattering. The polarization dependence of both SERRS and background emission was the same as that of the lowest-energy plasmon resonance maxima, which is associated with a longitudinal plasmon. From the common polarization dependence, we identified that the lowest-energy plasmon is coupled with both SERRS and background emission. In addition, we revealed that the lowest-energy plasmon with a higher quality factor (Q factor) yields larger SERRS and background emission intensity. Also, we identified that the Q factor dependence of the SERRS intensity was similar to that of the background emission intensity. This similarity directly supported us to demonstrate an enhancement of both SERRS and background emission by coupling with a common plasmon radiative mode.     

Matrix infrared spectroscopic and theoretical of the difluoroamino metal fluoride molecules: F2NMF (M = Cu, Ag, Au)

The difluoroamino coinage metal fluoride molecules F(2)NMF (M = Cu, Ag, Au) have been made via spontaneous reactions of coinage metals and NF(3) in solid argon and neon matrixes during sample annealing without formation of the M(NF(3)) complexes. Comparisons between the matrix infrared spectra and the density functional frequency calculations provide strong support for identification of the F(2)NMF molecules, which are found to have doublet ground states with C(2v) or near C(2v) geometries. The F(2)NCuF molecule can isomerize to the less stable FNCuF(2) isomer upon UV-visible irradiation, while no similar reactions were observed for the silver and gold species. The M-N bonds in the F(2)NMF molecules are stronger than those in the FNMF(2) isomers with the Ag-N bond being longest and weakest in both cases.     

Matrix Infrared Spectroscopy and Quantum‐Chemical Calculations for the Coinage‐Metal Fluorides: Comparisons of Ar?AuF,Ne?AuF,and Molecules MF2 and MF3

The reactions of laser‐ablated Au, Ag, and Cu atoms with F₂ in excess argon and neon gave new absorptions in the M? F stretching region of their IR spectra, which were assigned to metal‐fluoride species. For gold, a Ng? AuF bond was identified in mixed neon/argon samples. However, this bonding was much weaker with AgF and CuF. Molecules MF₂ and MF₃ (M=Au, Ag, Cu) were identified from the isotopic distribution of the Cu and Ag atoms, comparison of the frequencies for three metal fluorides, and theoretical frequency calculations. The AuF₅ molecule was characterized by its strongest stretching mode and theoretical frequency calculations. Additional evidence was observed for the formation of the Au₂F₆ molecule.     

Quantitative evaluation of blinking in surface enhanced resonance Raman scattering and fluorescence by electromagnetic mechanism

We analyze blinking in surface enhanced resonance Raman scattering (SERRS) and surface enhanced fluorescence (SEF) of rhodamine 6G molecules as intensity and spectral instability by electromagnetic (EM) mechanism. We find that irradiation of intense NIR laser pulses induces blinking in SERRS and SEF. Thanks to the finding, we systematically analyze SERRS and SEF from stable to unstable using single Ag nanoparticle (NP) dimers. The analysis reveals two physical insights into blinking as follows. (1) The intensity instability is inversely proportional to the enhancement factors of decay rate of molecules. The estimation using the proportionality suggests that separation of the molecules from Ag NP surfaces is several angstroms. (2) The spectral instability is induced by blueshifts in EM enhancement factors, which have spectral shapes similar to the plasmon resonance. This analysis provides us with a quantitative picture for intensity and spectral instability in SERRS and SEF within the framework of EM mechanism.     

Gaseous HgH2, CdH2, and ZnH2

Gaseous HgH2, CdH2, and ZnH2 molecules were synthesized by the direct gas-phase reaction of excited mercury, cadmium, and zinc atoms with molecular hydrogen. The molecules were identified by their high-resolution infrared emission spectra, and the metal-hydrogen bond lengths were determined from the rotational analysis of the antisymmetric stretching fundamental bands.     

黄曲霉素B1在银团簇表面吸附的表面增强拉曼光谱

采用密度泛函理论(DFT)的B3LYP方法和6-311g(d, p)(C, H, O)/LanL2DZ(Ag)基组, 优化得到黄曲霉素分子AFB₁与Ag小团簇形成的复合物AFB₁-Ag_n (n=2, 4, 6)的稳定结构, 并计算了三种复合物的表面增强拉曼光谱(SERS)和预共振拉曼光谱(SERRS), 与实验结果相一致. 计算结果显示: 三种复合物表面增强拉曼光谱中C＝O伸缩振动模的增强因子约为10²-10³, 是由于极化率改变引起的静化学增强. 根据含时密度泛函理论(TDDFT)方法计算得到的吸收光谱, 分别选择407.5、446.2和411.2 nm作为入射光, 计算三种复合物的共振拉曼光谱, 发现在SERRS光谱中, Ag―O伸缩振动的增强因子达到10⁴量级, 主要是由电荷转移产生的共振增强引起的.     

Spectral variations in background light emission of surface-enhanced resonance hyper Raman scattering coupled with plasma resonance of individual silver nanoaggregates

We demonstrate the origin of spectral variations in background light emission of surface enhanced resonance hyper Raman scattering (SERHRS) from single Ag nanoaggregates. Ag nanoaggregate-by-nanoaggregate variations in background light emission spectra are related to plasma (plasmon) resonance spectra. Temporal variations in background light emission spectra with temporal blueshifts in plasma resonance spectra are also observed under continuous laser excitation. Both types of the variations in background light emission are reproduced by multiplying background light emission spectra measured from a Ag microaggregate by Lorentz function spectra derived from plasma resonance spectra. The reproduction reveals that second electromagnetic (EM) enhancement by plasma resonance is the origin of the variations. Additionally, spectral variations in background light emission of SERHRS are similar to that of surface enhanced resonance Raman scattering (SERRS). The similarity indicates that both types of background light emission commonly obtain second EM enhancement from identical plasma resonance.     

Infrared Spectroscopic Investigations of J-aggregates of Protonated Tetraphenylporphine

Protonated tetraphenylporphine(H2TPP) J-aggregates were prepared by aggregation on the liquid-air interface,Using FTIR spectroscopy,the authors observed the infrared absorption spectra of H2TPP and its J-aggregates.The IR spectra of H2TPP J-aggregates show significant changes compared with that of H2TPP monomer.Intensity changes(e.g.,strong enhancement of the in-plane vibronic mode and weakening of the out-of-plane vibronic mode of phenyi and porphyrin skeletal) were interpreted on the basis of stacking effects.Observation of the same type of bands collapse into single band was explained by the increase in the symmetry of H2TPP molecules.And the new bands at 1635 and 3407 cm-1 indicate the aggregates containing a large amount of bound water.     

C[bond]H- - -O hydrogen bonds in beta-sheetlike networks: combined X-ray crystallography and high-pressure infrared study

Close interactions of the C(alpha)[bond]H- - -O type have been analyzed via X-ray crystallography and high-pressure infrared spectroscopy. The results demonstrate that the C(alpha)[bond]H- - -O interactions can offer an additional stability to the beta-sheet formation. X-ray structural data suggest that while 1-acetamido-3-(2-pyrimidinyl)-imidazolium bromide exhibits a bilayer stacking, the PF(6)(-) salt reveals a beta-sheetlike pattern. The appearance of the free-NH infrared absorption indicates that the conventional N[bond]H- - -O or N[bond]H- - -N hydrogen bonds do not fully dominate the packing for the PF(6)(-) salt. The high-pressure infrared study suggests that the C(alpha)[bond]H- - -O hydrogen bonds are the important determinants for the stability of the PF(6)(-) salt. This study also verifies that the imidazolium C[bond]H stretching frequency shifts to a longer wavelength upon the formation of the C[bond]H- - -O hydrogen bonds.     

Surface-enhanced resonance Raman scattering and density functional calculations of hemicyanine adsorbed on colloidal silver surface

Resonance Raman (RR) and surface-enhanced resonance Raman scattering (SERRS) of 4'-(N,N'-dimethylaminostyryl)-4-propylpyridinium bromide (hemicyanine, HC dye) in acetonitrile solution and on a colloidal silver surface have been investigated. The structure of the dye in the ground (S0) and excited (S1) electronic states was optimized using density functional calculations along with the B3LYP and the configuration interaction with the singlet excitation (CIS) methods, respectively, using the 6-31G basis set. The vibrational frequencies of the molecule were computed at the optimized geometry and compared with the observed Raman bands. A complete normal-mode analysis has been carried out because it is essential for the accurate assignment of the vibrational spectra. From the observed enhancement along various in-plane and out-of-plane vibrations in the SERRS spectrum and from theoretical calculations, it has been inferred that the interaction with the silver surface occurs via the nitrogen lone pair of the pyridyl or the dimethylamino group of the molecule with a tilted orientation. The observed red-shifts in the SERRS spectrum along various vibrations indicate strong interaction (chemisorption) of the HC dye with the silver surface. This is also supported by the presence of a Ag-N stretching vibration at 241 cm(-1). The effect of the dye concentration on the orientation of the molecule is also discussed.     

NOx分子在[Ag]-AlMOR分子筛中的吸附

采用Dmol3程序中密度泛函理论(DFT)的广义梯度方法GGA/BLYP和DND基组研究了银离子交换的丝光沸石([Ag]-AlMOR)结构及其对NOx分子吸附性能的影响, 获得吸附复合物的平衡几何结构参数、吸附能以及红外振动频率等数据. 结果表明, NOx分子与丝光沸石间的主要作用力为NOx分子中的氮(或氧)原子上的孤对电子和Ag+间的静电作用力. 吸附能数据表明, NOx分子以η1-N模式吸附在[Ag]-AlMOR分子筛中的结构更稳定; 在η1-N模式中, NOx分子吸附作用强度的次序为NO>NO2>N2O. 红外振动频率结果表明, 吸附态NOx分子中N—O和N—N键伸缩振动频率的位移趋势与N—O和N—N 键变化规律基本相一致. 另外, 对[Ag]-AlMOR分子筛的抗硫、抗水及抗氧化性能也进行了研究和分析.     

New molecular complexes of fullerenes C60 and C70 with tetraphenylporphyrins [M(tpp)], in which M=H2, Mn, Co, Cu, Zn, and FeCl

New molecular complexes of fullerenes C60 and C70 with tetraphenylporphyrins [M(tpp)] in which M-H2, MnII, CoII, CuII, ZnII and Fe(III)Cl, have been synthesised. Crystal structures of two C60 complexes with H2TPP, which differ only in the number of benzene solvated molecules, and C60 and C70 complexes with [Cu(tpp)] have been studied. The fullerene molecules form a honeycomb motif in H2TPP.2C60. 3C6H6, puckered graphite-like layers in H2TPP.2C60.4C6H6, zigzag chains in [Cu(tpp)].C70.1.5C7H8.0.5C2HCl3 and columns in [Cu(tpp)]2.C60. H2TPP has van der Waals contacts with C60 through nitrogen atoms and phenyl groups. Copper atoms of the [Cu(tpp)] molecules are weakly coordinated with C70, but form no shortened contacts with C60. The formation of molecular complexes with fullerenes affects the ESR spectra of [M(tpp)] (M = Mn, Co and Cu). [Mn(tpp)] in the complex with C70 lowers its spin state from S = 5/2 to S = 1/2, whereas [Co(tpp)] and [Cu(tpp)] change the constants of hyperfine interaction. ESR, IR, UV-visible and X-ray photoelectron spectroscopic data show no noticeable charge transfer from the porphyrinate to the fullerene molecules.     

Probing of porphyrin surface chemistry in systems with laser-ablated Ag nanoparticle hydrosol: role of thiosulfate anions

The influence of sodium thiosulfate (THS) concentration in Ag colloid/THS/H(2)TMPyP and Ag colloid/H2TMPyP/THS systems (H2TMPyP = 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin) was investigated by a combination of surface-enhanced resonance Raman scattering (SERRS) spectroscopy, surface plasmon extinction (SPE) measurements, and transmission electron microscopy (TEM). THS was found to have a strong impact on Ag nanoparticle surface structure and aggregation state and on interaction with H2TMPyP probe molecules, as evidenced by variations of the SERRS spectrum. In the Ag colloid/THS/H2TMPyP system, when laser-ablated Ag colloid was THS pretreated prior to the porphyrin addition, a critical threshold THS concentration (4 x 10(-5) M) was discovered. At concentrations below the threshold, THS mainly reduces the number of Ag+ adsorption sites. This leads to increased Ag nanoparticle aggregation prior to the porphyrin addition and significant weakening of the overall SERRS signal. Dominant contributions in the SERRS spectrum correspond to free base H2TMPyP and Ag+ containing the AgTMPyP form. At concentrations above the threshold, THS mediates also the formation and stabilization of new adsorption sites, probably Ag(0) sites. This induces a turn in the aggregation state of the pretreated Ag-c/THS system, an increase of the overall SERRS signal, and the appearance of a new spectral form of Ag metalated porphyrin.     

Zinc and cadmium dihydroxide molecules: matrix infrared spectra and theoretical calculations

Laser-ablated zinc and cadmium atoms were mixed uniformly with H2 and O2 in excess argon or neon and with O2 in pure hydrogen or deuterium during deposition at 8 or 4 K. UV irradiation excites metal atoms to insert into O2 producing OMO molecules (M = Zn, Cd), which react further with H2 to give the metal hydroxides M(OH)2 and HMOH. The M(OH)2 molecules were identified through O-H and M-O stretching modes with appropriate HD, D2, (16,18)O2, and (18)O2 isotopic shifts. The HMOH molecules were characterized by O-H, M-H, and M-O stretching modes and an M-O-H bending mode, which were particularly strong in pure H2/D2. Analogous Zn and Cd atom reactions with H2O2 in excess argon produced the same M(OH)2 absorptions. Density functional theory and MP2 calculations reproduce the IR spectra of these molecules. The bonding of Group 12 metal dihydroxides and comparison to Group 2 dihydroxides are discussed. Although the Group 12 dihydroxide O-H stretching frequencies are lower, calculated charges show that the Group 2 dihydroxide molecules are more ionic.     

Infrared spectroscopy of hydrogen-bonded 2-fluoropyridine-water clusters in supersonic jets

The hydrogen-bonded clusters of 2-fluoropyridine with water were studied experimentally in a supersonic free jet and analyzed with molecular orbital calculations. The IR spectra of 2-fluoropyridine-(H2O)(n) (n = 1 to 3) clusters were observed with a fluorescence detected infrared depletion (FDIR) technique in the OH and CH stretching vibrational regions. The frequencies of OH stretching vibrations show that water molecules bond to the nitrogen atom of 2-fluoropyridine in the clusters. The hydrogen-bond formation between aromatic CH and O was evidenced in the 1:2 and 1:3 clusters from the experimental and calculated results. The overtone vibrations of the OH bending mode in hydrogen-bonded water molecules appear in the IR spectra, and these frequencies become higher with the increase of the number of water molecules in the clusters. The band structure of the IR spectra in the CH stretching region changes depending on the number of coordinating water molecules.     

Ag(111)表面Ag配位结构的分等级组装

表面辅助的金属有机纳米结构因其结构稳定性和潜在应用受到广泛关注。在金属有机纳米结构中,金属原子来源于外部沉积的金属或金属表面原子。外部沉积的金属原子种类多样,取决于目标纳米结构。然而,金属表面原子受限于表面科学常用的金、银和铜单晶金属表面。金属有机纳米结构大多包括Au配位或是Cu配位结构,而只有少量的用表面Ag原子构成。分子金属相互作用的进一步研究有助于预期纳米结构的精确控制形成。至于构建基元,有机分子通过M―C、M―N和M―O键与表面金属原子配位。末端炔反应或者乌尔曼耦合能够实现C―M―C节点的形成。Cu和Au原子能够与含有末端氰基或吡啶基官能团的分子配位形成N―M―N键。另外,表面Ag增原子能够通过Ag―N配位键与酞菁分子配位。然而,M―O配位键的相关研究较少。因此,我们计划使用末端羟基分子与Ag增原子配位形成金属有机配位纳米结构去研究O―Ag节点。我们通过扫描隧道显微镜利用4, 4’-二羟基-1, 1’: 3’, 1’’-三联苯分子(4, 4’-dihydroxy-1, 1’: 3’, 1’’-terphenyl,H3PH)和Ag增原子成功构筑了一系列二维有序纳米结构。在室温下,蒸镀的H3PH分子自组装形成由环氢键连接的密堆积结构。当退火温度提升到330 K,一种新的纳米结构出现了,该结构由O―Ag配位键和氢键共同作用形成。进一步地提升退火温度至420 K,蜂巢结构和共存的二重配位链出现,这两种结构中仅由O―Ag―O键构成。为分析金属分子反应路径和O―Ag―O键的能量势垒,我们对该体系进行密度泛函理论计算。计算结果显示,O―Ag键形成的能量势垒是1.41 eV,小于O―Ag―O节点1.85 eV的能量势垒。这也解释了分等级金属-有机纳米结构形成的原因。我们的实验结果提供了一种利用有机小分子和金属增原子来设计和构筑分等级二维纳米结构的有效方法。     

Variations in steady-state and time-resolved background luminescence from surface-enhanced resonance Raman scattering-active single Ag nanoaggregates

We observed a background luminescence emission that was associated with surface-enhanced resonance Raman scattering (SERRS) of rhodamine 6G (R6G) molecules adsorbed on single Ag nanoaggregates and investigated the origin of the background luminescence. Thanks to the observation of single nanoaggregates, we clearly identified nanoaggregate-by-nanoaggregate variations in the steady-state and time-resolved background luminescence spectra of each nanoaggregate. From the variations in the steady-state spectra, two kinds of key properties were revealed. First, the background luminescence spectra were divided into four components: one fluorescence band corresponding to the monomers of R6G and three Lorentzian bands whose maxima were red-shifted from the fluorescence maximum of the monomer by several tens of nanometers. On the basis of the red-shifted luminescence maxima, and experimental and theoretical studies of background luminescence, we attributed the three background luminescences to fluorescence from aggregates (dimer and two kinds of higher-order aggregates) of R6G molecules on an Ag surface. Second, a positive correlation was observed between wavelengths of background luminescence maxima and wavelengths of plasmon resonance maxima. This positive correlation invoked the idea that the dipoles of both the background luminescence and the plasmon radiation are coupled with each other. From the key observations in the steady-state background luminescence spectra, we propose that two factors contribute to the variations in the steady-state background luminescence spectra; one is the aggregation (monomer, dimer, and two kinds of higher-order aggregates) of R6G molecules on an Ag surface, and the other is plasmon resonance maxima of single Ag nanoaggregates. Considering these two factors, we propose that the variations in the time-resolved background luminescence spectra are associated with deaggregation of R6G molecules (higher- to lower-order aggregates) and temporal shifts in the plasmon resonance maxima of single Ag nanoaggregates.     

Silver-particle-based surface-enhanced resonance Raman scattering spectroscopy for biomolecular sensing and recognition

We demonstrate in this work that 2-μm-sized Ag (μAg) powders can be used as a core material for constructing biomolecular sensing/recognition units operating via surface-enhanced resonance Raman scattering (SERRS). This is possible because μAg powders are very efficient substrates for both the diffuse reflectance IR and the surface-enhanced Raman scattering–SERRS spectroscopic characterization of molecular adsorbates prepared in a similar manner on silver surfaces. Besides, the agglomeration of μAg particles in a buffer solution can be prevented by the layer-by-layer deposition of cationic and anionic polyelectrolytes such as poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). In this particular study, we used rhodamine B isothiocyanate (RhBITC) as a SERRS marker molecule, and μAg powders adsorbed consecutively with RhBITC and PAH–PAA bilayers were finally derivatized with biotinylated poly(l-lysine). On the basis of the nature of the SERRS peaks of RhBITC, those μAg powders were confirmed to selectively recognize streptavidin molecules down to concentrations of 10⁻¹⁰ g mL⁻¹. Since a number of different molecules can be used as SERS–SERRS marker molecules, the present method proves to be an invaluable tool for multiplex biomolecular sensing/recognition via SERS and SERRS.     

Single molecular detection of a perylene dye dispersed in a Langmuir-Blodgett fatty acid monolayer using surface-enhanced resonance Raman scattering

The Langmuir-Blodgett (LB) monolayer technique was used to fabricate single molecule LB monolayer containing bis(phenethylimido)perylene (PhPTCD), a red dye dispersed in arachidic acid (AA) with an average doping of 1 molecule per microm2. The monolayer was transferred onto Ag island films to obtain spatially resolved surface-enhanced resonance Raman scattering (SERRS) spectra. The mixed LB monolayers were fabricated with a concentration, on average, of 1, 6, 19 and 118 PhPTCD molecules per microm2 in AA. The AA provides a two-dimensional host matrix whose background signal does not interfere with the detection of the probe molecule's SERRS signal. The properties of the single molecule detection were investigated using micro-Raman with a 514.5-nm laser line. The Ag island surfaces coated with the LB monolayer were mapped with spatial steps of 3 microm and global chemical imaging of the most intense SERRS band in the spectrum was also recorded. The SERRS and surface-enhanced fluorescence (SEF) of the neat and single molecule LB monolayer were recorded in a temperature range from liquid nitrogen to + 200 degrees C. Neat PhPTCD LB monolayer spectra served as reference for the identification of characteristic signatures of the single molecule behavior. The spatial resolution of Raman-microscopy experiments, the multiplicative effect of resonance Raman and SERRS, and the high sensitivity of the new dispersive Raman instruments, allow SERRS to be part of the family of single molecular spectroscopies.     

Synthesis,stability, and spectroscopic and electronic properties of three benzocyclohepta[alpha]azulenylium ions

Novel pi-systems, three benzocyclohepta[alpha]azulenylium ions, 7a-c, are synthesized, and their stability and properties have been characterized in terms of the position of the benzo-annulation and compared with those of the parent cyclohepta[alpha]azulenylium ion 4. Benzocyclohepta[6,7-alpha]azulenylium ion (7a) (p K R+ = 7.3, E red = -0.567 V vs Ag/Ag+) and benzocyclohepta[6,5-alpha]azulenylium ion (7b) (p K R+ = 5.1, E red = -0.482 V vs Ag/Ag+), which are annulated with benzene on the position having a high bond order of 4, are not appreciably destabilized compared with cyclohepta[alpha]azulenylium ion (4) (p K R+ = 7.3, E red = -0.458 V vs Ag/Ag+). On the other hand, benzocyclohepta[7,6-alpha]azulenylium ion (7c) (p K R+ = 1.6, Ered = -0.197 V vs Ag/Ag+) is considerably destabilized, probably due to enhanced contribution of the quinoid structure of the benzene ring, which is annulated on the position having a low bond order of the cyclohepta[alpha]azulenylium ion moiety. Furthermore, the cations 7a and 7b are more stable than 12,13-dihydrobenzocyclohepta[7,6-alpha]azulenylium ion (25) (p K R+ = 4.8, E red = -0.513 V vs Ag/Ag+), which is a dihydrogenated compound of 7a, while 7c is less stable than 25. These features are reflected in the considerable red shift of the longest absorption maximum of the electronic spectrum of 7c, as compared with those of 4, 7a, and 7b, and in the chemical shifts of the protons and their coupling constants of the (1)H NMR spectra. Furthermore, the (1)H NMR spectra and electronic spectra of 5H-benzocyclohepta[6,7-alpha]azulen-5-one (8a) and 7H-benzocyclohepta[6,5-alpha]azulen-7-one (8b) in acidic media have also been studied to clarify the spectral characteristics similar to those of 7a and 7b.