Interfacial chemical effect evidenced on SERS spectra of polyaniline thin films deposited on rough metallic supports

Abstract

Surface-enhanced Raman scattering (SERS) has proved to be an effective technique for studying the structural properties of conducting polymer thin films. The enhancement process has a twofold origin, electromagnetic and chemical. The electromagnetic enhancement, which is the dominant mechanism in SERS generation, consists in the excitation of localized and delocalized surface plasmons (SPs) in the metallic support of the thin film. The Raman emission of the adsorbed molecules on the metal surface (the most efficient being Ag, Au and Cu) is due to the intense evanescent electromagnetic field located at the interface between the metal and the surrounding medium. The second enhancing mechanism for SERS is of chemical origin, involving the formation of new chemical bonds between the molecules and the metal surface, with the polarizability thus becoming considerably higher than that of the free molecules. This mechanism is as a rule accompanied by a metal-molecule or molecule-metal charge transfer, which partly accounts for the success of the SERS studies on conducting polymers. Unfortunately, these studies have revealed that the chemical effects at the polymer-metal interface varied substantially depending on the various types of polymer and metallic support. In this context, polyaniline containing two different entities (a reduced and an oxidized state) in its repeating units exhibits specific alterations in its SERS spectra depending on the type of metallic support. This paper presents new results concerning the structure of emeraldine-base and emeraldine-salt polyaniline thin films deposited on rough Ag and Au supports. The effect on the SERS spectra of the polymer-metal and polymer-ambient interface chemical reactions is also described. The presence of an interface compound depending on the oxidizing properties of the metallic support has a strong influence on the SERS spectra, no matter how the PAN films were deposited on the support (whether by solvent evaporation or by an electrochemical process, i.e. cyclic voltammetry). When an emeraldine base is doped with HSO 4 − ions, it turns into an emeraldine salt, which exhibits a disordered state in its macromolecular chain, leading to a modified profile of the ∼1162 cm −1 Raman line which is associated with the C-H bond of the quinoid ring. The Lorentzian profile is altered by the addition of a Gaussian profile component. For a rough Ag support, the SERS spectra show that the transformation of emeraldine base films into emeraldine salt films is a reversible process.

Sensitivity of Optical Fiber Sensor Based on Surface Plasmon Resonance: Modeling and Experiments

“…The values of these two parameters were previously determined from AFM images of gold surface: q=0.51, d=5.75 nm [15,32]. By a statistical analysis of the height distribution, d is close to three-times the root mean square roughness (also, namely, Rq) for a Gaussian height distribution.…”

Improved surface‐enhanced Raman scattering of polypyrrole electrodeposited on roughened substrates composed of Au–Ag bimetallic nanoparticles

“…3,4 Occurring on roughened metal substrates, SERS in principle provides a powerful means of obtaining vibrational information on adsorbate-surface interactions in view of its unique sensitivity and excellent wavenumber resolution. 5,6 Many techniques were used to obtain roughened metal substrates, including plasma treatment, 7 sputtering coating, 8 mechanically polishing by using abrasives, 9 vacuum evaporation deposition 10 and electrochemical and laser deposition. 11 However, a more controllable and reproducible surface roughness can be generated through the control of the electrochemical oxidation-reduction cycle (ORC) procedures.…”

Preparation, Optimization, and Characterization of SERS Sensor Substrates Based on Two-Dimensional Structures of Gold Colloid

“…The prepared substrate was then dried for about 30 min at about 100°C and then rinsed again three times with methanol. When all methanol was evaporated, in order to find an optimal preparation procedure, the silanized substrate was immersed at different temperatures (i.e., 30°C, 40°C, 50°C, and 60°C) for different times (5,15,30, and 60 min) in a colloidal solution of gold nanoparticles, which had been highly concentrated before use by centrifugation for 10 min (7,800 rpm). Then, the substrate was rinsed with distilled water.…”

“…The interaction of light with gold nanoparticles is dominated by strong optical resonances due to the excitation of localized surface plasmons [2,3]. One of the main consequences of plasmon excitation is the high electromagnetic field created at the surface of the nanoparticle, which is useful in enhancing the sensitivity of Raman spectroscopy in (bio) molecular detection [4][5][6][7]. Self-assembled gold particles on solid substrates are versatile tools for surface-enhanced Raman scattering (SERS) since the intensity of the electromagnetic field is strongly dependent on the nanoparticle size, shape, interparticle distance, and interparticle separation [8].…”