Solution-Based Assembly of Metal Surfaces by Combinatorial Methods

Abstract

Generation of combinatorial libraries has become an essential tool in synthetic organic chemistry, 1 but only recently have these methods been used for rapid screening of materials properties of inorganic substances. 2,3 The initial report described the use of vapor deposition and masking techniques to vary the composition of multi-metal superconducting materials; 2 more recently, a similar mask-driven approach to spatially-selective vapor deposition led to discovery of materials exhibiting colossal magnetoresistance. 3 Compared to traditional routes to property optimization in mixed-metal systems, e.g. preparation of separate samples with varying metal ratios, 4 combinatorial methods potentially offer significant advantages in speed and cost. We recently described a new approach to fabrication of macroscopic Au surfaces that is based on self-assembly of nanosized Au particles from solution. 5,6 Reductive deposition of Ag onto these substrates leads to Ag-clad colloidal Au arrays 7 that exhibit an increased enhancement factor (EF) for surface enhanced Raman scattering (SERS). 8 We describe herein a new, solution-based combinatorial strategy for synthesis of surfaces exhibiting nanometer-scale variation in mixed-metal composition and architecture. With this method, continuous or stepped gradients in the size and number density of surface features can be generated simultaneously over different regions of a single substrate. To demonstrate this approach, we have optimized the SERS response of a glass slide upon which the coverage of colloidal Au and the extent of Ag coating were varied. Over a 2 cm × 2 cm sample area, changes in EF spanning almost three orders of magnitude were realized, and the changes in nanostructure responsible for these effects were elucidated using atomic force microscopy (AFM). Our results show significant changes in SERS EFs over small alterations in surface morphology, demonstrating the power of solution-based combinatorial approaches for synthesis of mixed-metal materials.The general combinatorial assembly approach is described in Scheme 1. Prior work has shown that the number of colloidal Au particles bound to functionalized surfaces can be predicted from immersion times in colloidal solution using initial (time) 1/2 kinetics. 9 Thus, when a (3-mercaptopropyl)trimethoxysilane (MPTMS)-coated glass slide attached to a motorized translation stage is immersed at a fixed rate into a 17 nM aqueous solution of 12-nm diameter colloidal Au, a gradient in particle coverage is generated in the direction of immersion. After sample rotation by 90°, fixed-rate immersion into a Ag + -containing solution designed to specifically deposit Ag on Au 7b leads to a gradient in particle size over the new immersion direction. The resulting surface exhibits a continuous variation in nanometer scale morphologysas defined by particle coverage and particle sizesthat results from known, repeatable immersion conditions. 10 In our apparatus, a stepper motor with a 2°step yields a 3 µm translation. Over a 3 cm × 3 cm r...

Three‐layer substrates for surface‐enhanced Raman scattering: preparation and preliminary evaluation

“…The density of the Au submonolayer and the amount of electrolessly deposited Ag needed for optimum SERS enhancement have been previously determined through a combinatorial experiment. 75 However, an unidentified component of the commercial Ag plating solution, which can be displaced by strongly adsorbing molecules, results in a significant fluorescent background when examining weakly binding analytes. 75 -77 Vapor deposition of a thin Ag film over the two-layer architecture eliminated this added fluorescence and maintained the basic morphology of the optimized twolayer substrate.…”

“…More specifically, 12 nm diameter colloidal Au was self-assembled on a glass support to control the nanometer-scale roughness. The surface-bound Au nanoparticles were then coated with chemically reduced Ag in accordance with the combinatorial experiment 75 and overcoated by thermal evaporation of a discontinuous Ag film (Scheme 1). The well-defined surface morphology of the Au submonolayer and the optical properties of the Ag layers enabled this approach to exploit the advantages of both thin metal films and colloidal particles while circumventing many of the drawbacks.…”

Deposition and Characterization of Platinum and Palladium Nanoparticles on Highly Oriented Pyrolytic Graphite

“…[Tomita &Tamai,1974;Baker et al, 1982] By immersing the graphite electrode into a platinum plating solution were obtained Pt Nps in a narrow particle size distribution also for mean crystallite diameters smaller than 4 nm.…”

“…[Garcia-Gutierrez et al, 2008] In this chapter, a colloidal method of synthesis has been proposed to obtain metallic Nps; the polyol method has been reported to produce small Nps as the final product. [Alvarez et al, 1997;Henglein et al, 2000;Grabar, 1995;Baker, 1996] The gas-phase method of platinum and palladium salt particles disposed on a graphite surface by H 2 was used in this work. This method has the potential to yield nanocrystals that are disposed in direct contact with a substrate surface.…”

Fuel cells for transportation program: FY1997 national laboratory annual report

“…Figure 1 shows the concentration of CO as a function of time ( 6 ) . The horizontal line at 0.96% CO is the concentration of CO in the inlet test gas.…”