Surface enhanced IR absorption, SEIRA

Comparison of AIREs with the Phenomenon of Surface-Enhanced IR Absorption (SEIRA)... 

Due to the surface sensitivity surface enhanced fluorescence has become particularly popular in the characterisation of thin molecular films, such as Langmuir-Blodgett films and self-assembled biomembranes. Two surface enhanced spectroscopic techniques (surface enhanced IR absorption, SEIRA, and surface enhanced fluorescence, SEF) were recently applied to the study of biomembrane systems by the group of Reiner Salzer [323]. With SEIRA, specific fingerprints of biomolecules could be obtained with a tenfold IR intensity enhancement With SEF signal enhancement factors greater than 100 were obtained. The enhancement factor was very dependent on the properties and structure of the metal clusters used. With the two techniques biomembranes formed from vesicles with embedded nicotinic acetylchoHne receptors were spectroscopically characterized. 

Enhancement of absorption bands in the IR spectra of ultrathin films in the presence of discontinnons (islandlike) nnder- and ovemanolayers of Ag and An was discovered by Hartstein et al. [356] in the early 1980s. Although these researchers believed that they observed an increase in the vCH band intensities for p-nitro-benzoic acid (p-NBA), benzoic acid, and 4-pyridine-COOH films, it was recently shown [350] that the spectra reported are in actual fact due to fully saturated hydrocarbons (possibly vacuum pump oil). In any case, this discovery has stimulated various research activities and led to the development of surface-enhanced IR absorption (SEIRA) spectroscopy. To date, the SEIRA phenomenon has been exploited in chemical [357] and biochemical IR sensors (see [357-360] and literature therein), in studying electrode-electrolyte interfaces [171, 361-365], and in LB films and SAMs [364, 366-370]. Other metals that demonstrate this effect are In [371] and Cu, Pd, Sn, and Pt [372-375]. The metal films can be prepared by conventional metal deposition procedures such as condensation of small amounts of metal vapor on the substrate, spin coating of a colloidal solution, electrochemical [388], or reactive deposition [299] (see also Section 4.10.2). 

LC-IR using surface-enhanced IR absorption spectroscopy (SEIRAS) was recently designed in order to develop a highly sensitive and rapid analysis method for polymer additives [506]. The method, which consists of spraying the LC eluents on to a metal film of Ag on a BaF2 substrate, allows an enhancement factor of about 90. 

Furthermore, it is possible to increase the sensitivity of the IR technique, and thus the probability of detecting transient surface species characterized by (very) low absorption coefficients (such as the intermediate species present during the first steps of the polymerization reaction on the Phillips catalyst) by exploiting the surface-enhanced infrared absorption (SEIRA) effect. It is known that molecules adsorbed on metal island films or particles exhibit 10-1000 times more intense infrared absorption than would be expected from conventional measurements without the metal (253-256). The possibility of performing SEIRA spectroscopy should therefore be expected to provide an opportunity to better investigate the nature of the intermediate species, not only for ethene polymerization on Cr(II)/ Si02, but for other reactions on solid catalysts. 

Since the enhancement of IR absorption is one of the important characteristics in abnormal IR effects, it may be interesting to compare the AIREs with the phenomenon of surface-enhanced infrared absorption (SEIRA) reported in the literature. The SEIRA was discovered by Hartstein et al. [96] in the early 1980s and describes the phenomenon of the enhancement of IR absorption for some specific... 

A new in situ Fourier transform IR (Fl lR) method, surface-enhanced IR absorption spectroscopy (SEIRAS Osawa 1997), was employed that could observe the surface of the platinum electrode on a silicon prism while electrochemically polarizing it in... 

A related effect has been described for IR spectroscopy - Surface Enhanced Infrared Absorption spectroscopy (SEIRA). However, as the enhancement factors are significantly lower than for SERS and both the required metal particle size and the activation distance between the target molecule and the particle are more than one order of magnitude smaller, no practically applicable SEIRA sensors have been demonstrated up to now. 

Itmumerable mechanistic studies of alcohol oxidation on Pt-based electrocatalysts in acidic media have been published over the last few years. Methanol, " ethanol ° and ethylene glycol have been the most studied substrates and their oxidation paths on Pt or Pt alloys have been substantiated using a variety of in situ, extra situ and operando techniques as well as quantum mechanical calculations. The experimental techniques include reflection IR spectroscopy (IR), surface enhanced IR asbsorption spectroscopy (SEIRAS), " attemrated total reflection-IR absorption spectroscopy (ATR-IRAS), differential electrochemical mass spectroscopy (DEMS), single potential alteration IR spectroscopy... 

This model explains why SEIRA is observed in both s- and p- polarized IRRAS [384] and ATR [391, 405] spectra and in normal-incidence transmission spectra [377] and why the enhancement is not uniformly spread over each metal island but occurs mainly on the lateral faces of the metal islands [378, 384, 385]. The quasi-static interpretation of the SEIRA also defines the material parameters necessary for excitation and observation of SPR (1) The resonance frequency determined from the general Mie condition must be as low as possible and (2) Ime((Ures) must be as small as possible. The maximum enhancement effect should be observed for the absorption bands near the Mie (resonance) frequency of the particle. As mentioned in Section 3.9.1, the resonance frequencies of metal particles lie in the visual or near-IR range. However, they can be shifted into the mid-IR range by (1) increasing the aspect ratio of the ellipsoids, (2) adding the support to an immersion medium, (3) coating the particles by a dielectric shell [24, 406], or (4) varying the optical properties of the support [24, 349, 350, 384]. As emphasized by Metiu [299], the surface enhancement effect is not restricted to metals but can also be observed for such semiconductors as SiC and InSb. 

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