Determining the catalytic activity and the reaction kinetics are key issues when new catalysts are developed, characterized, and introduced. Catalysis at the nanoscale employing nanoparticles has great potential because of their new catalytic properties, high surface-to-volume ratios, and high surface reactivity. [1] In principle, reactions at the surface of metal structures can be studied using molecular surfacespecific spectroscopic techniques. [2] The most versatile of these is surface-enhanced Raman scattering (SERS), which has been frequently applied in investigations of different types of reactions at electrochemical interfaces in situ [3] to address, for example, the formation of reaction intermediates, [4] the dependence of electroorganic reactions on electrode potential, [5] and electron transfer in protein systems. [6] Herein we demonstrate that SERS can be used to study directly the kinetics of a catalytic reaction in situ. Our approach is novel by allowing the structural characterization of the reactant and product surface species in the reaction as well as investigating rate constants in the same experiment. This was possible by using separate gold and platinum nanoparticles that were simultaneously attached to the same glass surface. Our method is independent of the optical absorption properties of the reaction products and/or the catalysts.In order to investigate a metal-catalyzed reaction with SERS or other plasmon-supported approaches, [7] bifunctional metal structures are needed that have plasmonic properties and also act as a catalyst. [8] A number of catalytically active composite nanostructures have been reported to enhance the Raman signals of dye molecules, [9] and SERS has been used to monitor the structural evolution of bimetallic catalytically active Au-Pt nanoparticles. [10] However, direct observations of a catalytic process by SERS have been rare as they require bifunctional nanomaterials. [11] Our approach is different from those previously reported based on composite nanostructures with plasmonic (Au) and catalytic (Pt, [11a] or Pd) [11b] properties, as we have used separate gold and platinum nanoparticles that are simultaneously immobilized on a glass surface. Scanning tunneling microscopy (STM) data indicate that owing to the proximity of the platinum and gold nanoparticles, the molecules can interact with the platinum nanoparticles whilst they reside in the local optical fields provided by the localized surface plasmons of the gold nanoparticles. The versatility, stability, and general applicability of the immobilized gold nanoparticles for studying catalytic reactions are demonstrated by the quantification of the reaction products and the determination of the kinetics with different catalysts. The results reported therefore have implications both for basic catalysis research and analytical applications.Gold nanoparticles 40 nm in diameter and platinum nanoparticles less than 2 nm in diameter were prepared by reported procedures. [12] Mixtures of these gold and platinum nano...

Monodisperse, citrate-stabilized gold nanoparticles of sizes ranging from 15 to 40 nm were synthesized and characterized by small angle X-ray scattering and UV-vis experiments. Identical surface properties of nanoparticles of different sizes to avoid variation in the chemical surface-enhanced Raman scattering (SERS) enhancement, as well as selection of experimental conditions so that no aggregation took place, enabled the investigation of enhancement of individual nanospheres. Enhancement factors (EFs) for SERS were determined using the dye crystal violet (CV). EFs for individual gold nanospheres ranged from 10 2 to 10 3 , in agreement with theoretical predictions. An increase of the EFs of individual spheres with size can be correlated to changes in the extinction spectra of nanoparticle solutions. This confirms that the increase in enhancement with increasing size results from an increase in electromagnetic enhancement. Beyond this dependence of EFs of isolated gold spheres on their size, EFs were shown to vary with analyte concentration as a result of analyte-induced aggregation. This has implications for the application of nanoparticle solutions as SERS substrates in quantitative analytical tasks.

Gold and silver nanoparticles can be immobilized on glass slides using aminosilane linkers. Here, we demonstrate that particle monolayer surfaces can also be generated by simultaneous immobilization of both gold and silver nanoparticles with the same organosilane linker. These new surfaces display surface-enhanced Raman scattering (SERS) enhancement typical for gold or silver monolayers, depending on the ratio of the two types of nanoparticles and, at the same time, have the capability to probe complex analytes composed from various molecules which adsorb at only one of the metals. The reported results from scanning electron microscopy, scanning force microscopy, and UV/vis absorbance for surfaces containing one or two types of nanoparticles indicate that an enhancement level above 104 is related to nanoaggregates that form in the 2D plane. High and stable enhancement factors over a wide range of analyte concentrations along with high homogeneity of the enhancement at the microscopic scale make the plasmonic nanoparticle mix-and-match surfaces ideal substrates for use in microscopic SERS sensing.