Hybrid Gold–Conductive Metal Oxide Films for Attenuated Total Reflectance Surface Enhanced Infrared Absorption Spectroscopy

Abstract

The use of conductive metal oxide (CMO) films as supporting layers for attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) is treated theoretically and experimentally. The greater mid-infrared transparency of thin layers of indium zinc oxide (IZO), as compared to metals, is verified through IR reflectivity measurements and the Drude model. IZO layers sputtered on silicon micromachined internal reflection elements (Si μIREs) are found to have a thin surface layer with slightly different plasma frequency and electronic scattering time compared to the bulk material. The complex permittivity and refractive index of the IZO can be extracted using the Drude model. This allows application of the Bruggeman effective medium theory to calculate the ATR-SEIRAS response of a layer of gold prolate spheroids supported on an IZO film. Calculated ATR-SEIRAS spectra for a 1 nm thick organic film, modeled as a Lorentz oscillator, predict an order of magnitude improvement in absorbance strength using the IZO film as a base layer compared to a conventional, gold covered internal reflection element. These predictions are qualitatively verified by the electrodeposition of gold nanoparticles on an IZO modified Si internal reflection element and the study of the potential controlled adsorption of a pyridine derivative. The IZO/Au layers are found to be very mechanically stable and can withstand large potential perturbations. This is demonstrated through the in situ study of the repeated reductive desorption of a self-assembled monolayer of 4-mercaptobenzoic acid.

Optimization of a Commercial Variable Angle Accessory for Entry Level Users of Electrochemical Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy (ATR-SEIRAS)

“…22,24–27 The reduced IR transparency of Si due to interference from Si–O vibrations and phonon modes can be overcome through the use of micromachined ATR wafers. 15,28–30 In practice, silicon hemispheres and hemicylinders dominate the electrochemical ATR-SEIRAS literature. Hemispheres and hemicylinders are potentially advantageous in home-built IR spectroelectrochemical optics as they can be used to collimate the incident IR radiation.…”

“…13 Variable-angle accessories can readily be adapted for electrochemical ATR-SEIRAS with the installation of a suitable internal reflection element (IRE) and either a homebuilt 14 or commercially available spectroelectrochemical cell. 15 Despite the facile nature of these commercial accessories, novice users are still faced with the daunting task of determining optimal ATR-SEIRAS configurations. This includes the selection of IRE material and geometry, the decision whether to include a wire-grid polarizer, the pros and cons of using antireflective (AR) coatings, and the selection of the optimal angle of incidence (AOI).…”

Probing Heterogeneity in Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy (ATR-SEIRAS) Response with Synchrotron Infrared Microspectroscopy

“…The SEIRAS-active Au@ITO layers were prepared following a previously reported procedure 18 with minor modifications. A 25 nm thick indium tin oxide film (ITO) was deposited onto a microgrooved Si internal reflection element (IRE) (Si m-groove IRE, IRUBIS GmbH) using a home-built RF magnetron sputtering unit.…”

“…We have recently advocated the use of thin films of conductive metal oxide (CMO) as the supporting layer in electrochemical ATR-SEIRAS experiments. 18 Metal island films can be directly electrodeposited on these surfaces and the resulting metal-island modified surfaces have been shown to give larger enhancement factors than traditional metal films grown directly on infrared prisms. These metal@CMO films are often quite heterogeneous in appearance, and the electrodeposited nanoparticles are unequally distributed on the conductive metal oxide surface.…”

Infrared Spectroscopy with a Fiber-Coupled Quantum Cascade Laser for Attenuated Total Reflection Measurements Towards Biomedical Applications

“…A fiber probe with an ATR prism necessitates a smaller crystal, down to single-reflection, with concomitant signal reduction. This signal reduction can be counteracted with the use of e.g., surface-enhanced infrared absorption spectroscopy (SEIRAS) [43]. SEIRAS can be achieved by coating the ATR prism with metal nanoparticles, but these substrates often have difficulties with stability and reproducibility.…”