Layered core−shell bimetallic silver−gold colloids in the size range of 10−16 nm have been prepared by the seed-growth method. Silver nuclei were covered by gold shells of various thicknesses without any stabilization agent. Interfacial (Ag)Au colloid−2,2‘-bipyridine films were prepared from these bimetallic colloids and used for the purpose of analysis of transmission electron microscopy (TEM) images and electron diffraction. Both observed and calculated TEM images were used to characterize the prepared nanoparticles. On the basis of the analysis of TEM images, the calculated TEM image contrast, and results obtained by electron diffraction, energy-dispersive X-ray analysis, and other experiments, the core−shell structure of the prepared (Ag)Au nanoparticles was revealed. Particles were found to consist of a silver core and a gold shell enriched with silver.

Core−Shell (Ag)Au Bimetallic Nanoparticles: Analysis of Transmission Electron Microscopy Images

Layered core-shell bimetallic silver-gold nanoparticles were prepared by overdeposition of Au over Ag seeds by the seed-growth method using tetrachloroauric acid, with hydroxylamine hydrochloride as the reductant. The effects of pH, reduction rate, and seeding conditions on the morphology and surface plasmon extinction of the bimetallic nanoparticles were investigated. Nanoparticles prepared by a rapid reduction in the neutral ambient and assembled into two-dimensional nanoparticulate films by adsorption of 2,2'-bipyridine were characterized by energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, surface-enhanced Raman scattering spectroscopy, and transmission electron microscopy. The results are consistent with Ag core and Ag/Au-alloyed shell composition of the nanoparticles. Evidence of the presence of Ag on the surface of the nanoparticles, of enrichment of the Ag/Au alloy shell by Ag toward or at the nanoparticle surface, and of modification of the nanoparticle surface by adsorbed chlorides is also provided. Reduction of the size of the Ag seeds, alloying of Ag and Au in the shell of the nanoparticles, and modification of their surfaces by adsorbed chlorides are tentatively attributed to positive charging of the nanoparticles during the electrocatalytic overdeposition of Au over Ag seeds.

Bimetallic (Ag)Au nanoparticles prepared by the seed growth method: two-dimensional assembling, characterization by energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, and surface enhanced raman spectroscopy, and proposed mechanism of growth.

A sandwich structure consisting of Ag nanoparticles (NPs), p-aminothiophenol (p-ATP) self-assembled monolayers (SAMs), and Ag NPs was fabricated on glass and characterized by surface enhanced Raman scattering (SERS). The SERS spectrum of a p-ATP SAM in such sandwich structure shows that the electromagnetic enhancement is greater than that on Ag NPs assembled on glass. The obtained enhancement factors (EF) on solely one sandwich structure were as large as 6.0 +/- 0.62 x 10(4) and 1.2 +/- 0.62 x 10(7) for the 7a and 3b(b(2)) vibration modes, respectively. The large enhancement effect of p-ATP SAMs is likely a result of plasmon coupling between the two layers of Ag NP (localized surface plasmon) resonance, creating a large localized electromagnetic field at their interface, where p-ATP resides. Moreover, the fact that large EF values (approximately 1.9 +/- 0.7 x 10(4) and 9.4 +/- 0.7 x 10(6) for the 7a- and b(2)-type vibration modes, respectively) were also obtained on a single sandwich structure of Au NPsp-ATP SAMsAg NPs in the visible demonstrates that the electromagnetic coupling does not exist only between Ag NPs but also between Au and Ag NPs. The lower EF values on Au-to-Ag NPs compared to those on Ag-to-Ag NPs demonstrate that the Au-to-Ag coupling must be less effective than the Ag-to-Ag coupling for the induction of SERS in the visible.

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Surface enhanced Raman scattering of p-aminothiophenol self-assembled monolayers in sandwich structure fabricated on glass.

Pyrazinamide (pza) and 2-mercaptopyridine (2-MPy) are shown to form Ag colloid−adsorbate films at the interface between an aqueous Ag colloid and a solution of an adsorbate in dichloromethane. By contrast, under the same conditions, 4-mercaptopyridine (4-MPy) forms Ag organosol aggregates by the interface transfer of adsorbate-covered Ag colloidal particles from the aqueous phase to the organic phase. This result confirms that formation of Ag colloid−adsorbate films is an adsorbate specific process. Transmission electron and optical microscopy and surface plasmon adsorption measurements are used to characterize the morphology of the deposited Ag colloid−adsorbate structures while SERS spectra of the incorporated adsorbates probe their internal structure. Furthermore, we demonstrate that, for a particular adsorbate, the process of Ag colloid−adsorbate film formation, the morphology of the deposited films, and the SERS spectra are substantially influenced by the Ag content of the parent colloid (Ag−pza and ...

Surface-Enhanced Raman Scattering of Mercaptopyridines and Pyrazinamide Incorporated in Silver Colloid−Adsorbate Films

Surface-enhanced Raman scattering (SERS) is currently undergoing rapid development as an ultra-sensitive analytical technique in the biological field. Label-free and extrinsic SERS labeling are two independent approaches that are commonly used to detect biomolecules directly and indirectly. The label-free detection method is a direct, convenient method for the detection of biomolecules and is more reliable than extrinsic SERS labeling, but its major limitations are poor selectivity in some complex mixtures and poor sensitivity at relatively low sample concentrations. However, label-free, highly sensitive, selective detection of biomolecules based on SERS is possible. With the development of novel SERS-active substrates, Raman instruments, and methods for sample pretreatment, the label-free protocol is becoming increasingly powerful and promising. This review highlights recent developments in label-free SERS-based techniques for bimolecular detection, including direct biomolecule identification, biomolecule–ligand recognition via spectral differences among molecular bridges, and indirect phenolic molecule detection based on azo couplings.

Label-free detection in biological applications of surface-enhanced Raman scattering

The SERS spectra of ethinyl-labeled estradiol (17αa-ethinyl-1,3,5[10}estratriene-3,17β-diol) have been investigated for both glass-deposited Ag-ethinylestradiol films originating from the corresponding metal liquid like films (MELLFs) and a Ag colloid/ethinylestradiol+dimethyl sulfoxide (solvent) SERSactive system. The SERS spectra of the glass-deposited, interfacial Ag-17α-ethinylestradiol films show the presence of three forms of coordinated ethinylestradiol. Bonding of the ethinyl group of ethinylestradiol to Ag via the σ-π type of bond is characterized by the band at 1971 cm -1 , acetylinic coordination by the band at 1805 cm -1 , and coordination via the ethinyl group rehybridized into sp 2 (C=C) by the band at 1558 cm -1

Surface-Enhanced Raman Scattering Spectroscopic Study of 17.alpha.-Ethinylestradiol on Silver Colloid and in Glass-Deposited Ag-17.alpha.-Ethinylestradiol Film

Surface enhanced Raman scattering and surface plasmon absorption of glass-deposited silver-2,2'-bipyridine films formed from interfacial silver metal liquid-like films

Pressure-tuning infrared and solution Raman spectroscopic studies of 17β-estradiol and several A-ring and 17α-ethynylestradiol derivatives

The study of colored organometallic complexes by dispersive Raman spectroscopy has been limited due to fluorescence or photodecomposition caused by the visible laser used as the excitation source. As a solution to this problem, FT-Raman spectroscopy with a near-infrared laser source has been useful in lowering fluorescence or photolysis in these samples. To investigate the utility of this technique, we have obtained and assigned the FT-Raman spectra of a series of arene chromium tricarbonyl complexes and of cyclopentadienyl manganese tricarbonyl. Some bands previously unobserved by dispersive Raman spectroscopy were seen, including a band assigned to a 13CO satellite in the spectrum of methylbenzoate chromium tricarbonyl. In addition, FT-Raman data for bovine serum albumin (BSA) and Protein-A are presented. Keywords: FT-Raman spectroscopy, metal carbonyl, proteins, organometallics, near infrared.

Application of FT-Raman spectroscopy to the study of selected organometallic complexes and proteins

The complete Raman spectrum of crystalline octacarbonyldicobalt(0), Co2(CO)8, has been successfully obtained for the first time by using a Fourier transform instrument together with a spinning sample cell to avoid decomposition of Co2(CO)8 upon exposure to the near-IR laser radiation used to excite the spectrum. Previously, only limited regions of the Raman spectrum could be obtained with visible laser excitation because of extensive sample burning. The new FT-Raman results, combined with the related IR data from the literature, provide a better description of the vibrational properties of this important binary metal carbonyl complex.

Steven M. Barnett

Papers

Surface-Enhanced Raman Scattering Spectroscopic Study of 17.alpha.-Ethinylestradiol on Silver Colloid and in Glass-Deposited Ag-17.alpha.-Ethinylestradiol Film

Surface enhanced Raman scattering and surface plasmon absorption of glass-deposited silver-2,2'-bipyridine films formed from interfacial silver metal liquid-like films

Pressure-tuning infrared and solution Raman spectroscopic studies of 17β-estradiol and several A-ring and 17α-ethynylestradiol derivatives

Application of FT-Raman spectroscopy to the study of selected organometallic complexes and proteins

Spinning-cell FT-Raman spectroscopy : application to crystalline octacarbonyldicobalt(0), Co2(CO)8