Improvement of the Limiting Thickness of the Measurable Surface Layer on Polymer Films Using Fourier Transform Infrared Attenuated Total Reflection Spectroscopy with the Use of Silver Island Films

Abstract

Improvement in the detection limit of the surface layer on polymer films has been demonstrated with the use of Ag films under Kretschmann's attenuated total reflection (ATR) geometry. A remarkable absorption enhancement for 2.5-nm-thick polydimethylsiloxane (PDMS) or for 4.0-nm-thick polycarbonate (PC) coated on polyethyleneterephthalate (PET) films was observed. The magnitude of the enhancement factor is 10 for PDMS and 2 for PC compared with results for the conventional ATR method. In the case of PDMS, the limiting thickness of the measurable surface layer is about 0.5 nm with the use of the spectral subtraction technique. The correlation between the difference in the enhancement factor and the enhancement mechanism is also discussed.

Electroless Reduction of Silver Chloride Precipitates for the Preparation of Highly Sensitive Substrates for Surface-Enhanced Infrared Absorption (SEIRA) Measurements

“…Enhancement of the Prepared Substrate. In the literature, 10,12–143233 the observed peak intensities of pNBA at 1345 cm –1 ranged from 5 to 100 mAU. For instance, Nishikawa et al 13 observed band intensities of 50 mAU for the deposition of 600 ng/cm 2 of pNBA on AgNPs@Ge substrates prepared by the PVD method.…”

“…12 In general, the particles produced by PVD have a prolate ellipsoidal shape, and the height of the film must typically be in the range 4–10 nm to have a significant SEIRA effect. 13–16 Particles produced by electroless reduction are generally round in shape, with those of diameter around 100 nm giving the most intense SEIRA signals. Although the morphologies of the particles differ significantly, the reported enhancement factors are comparable and range from one to two orders of magnitude depending the metals used 2 and their morphologies.…”

Interpretation of IR Spectra of Ultrathin Films

“…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) Im ε(ω res ) must be as small as possible.…”

“…For the other metals the effect is weaker (e.g., a factor of 10 was achieved for the Au needle substrates [376]). The SEIRA was measured using transmission [377][378][379][380][381][382][383], IRRAS [359,360,384], ATR [265, 350, 357, 358, 361-364, 376, 385], and DRIFTS [370,386] for analytes with polar as well as apolar functional groups [376] and various basic molecular structures (e.g., aromatics [377,387], aliphatics [376,380,385], inorganics [380,388], polymers [385], pesticides [357,376], and biomaterials [358,359,389]). However, the observed enhancement was reported to be not equivalent for all absorption bands, being significantly greater for polar groups having large dipole moment gradients [390] and molecules that bind to metals [387].…”

“…It has been invoked to explain the high dependence of SEIRA on the molecular structure of the adsorbed molecules [374,383,390] and the band shape anomaly, especially for p-polarization [397]. On the other hand, it was shown [385,391,398,399] that chemical attachment to an islandlike metal film is not required for surface enhancement. Furthermore, an increase in the ATR spectra of polymer films was observed in contact with smooth Ni nanolayers evaporated onto a KRS-5 prism [372,373], which implies predominance of the electromagnetic mechanism.…”