A surface-enhanced Infrared reflection spectroscopy has been developed for the characterization of thin organic films on low reflective substrates. The substrates we studied were BaF2, Ge, microscope slide glass, and poly(acrylonltrlle-butadlenestyrene) resin. The sensitivity of the reflection Infrared spectroscopy at these surfaces Is remarkably Improved by evaporating very thin silver films on the sample surfaces. The same results were obtained also by casting thin organic films on metal-coated substrates. The Improvement of the spectral sensitivity Is due to the enhancement of the Incident Infrared field at the metal surfaces. We show that this technique Is promising as a surface analytical tool for semiconductors and Insulators. The optimum optical condHIons to obtain the spectra are discussed experimentally and theoretically.

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Qualitative and quantitative analysis of nanogram amounts of substances by transmission infrared spectroscopy with the use of silver island films was carried out. The effect of silver thickness, the comparison of several plates (Ge, BaF2, and ZnSe), the dynamic range of intensity enhancement vs. sample amount, and the enhancement stability after sample deposition were investigated extensively to determine the optimum conditions. The limits of detection (LOD) of several substances were also described. LOD values ranging from 5 ng/cm2 for polydimethylsiloxane to 10 ng/cm2 for diethylphthalate were obtained, and these values show a remarkable improvement in LOD in comparison with results from normal transmission spectroscopy. A quantitative study was also undertaken. A good linear relationship with respect to the amount of triphenyl phosphate, ranging from 25 to 250 ng/cm2, was obtained. The data were reproducible with a coefficient of variation of 15%, and this variation was comparable to that obtained with several surface-enhanced Raman scattering methods.

The intensity enhancement of infrared spectra with Ag island films in surface electromagnetic wave spectroscopy (SEWS), which is commonly associated with the ATR method, was observed with the use of an ordinary transmission configuration. We refer to this as “transmission SEWS” or “T-SEWS.” With 10 nm Ag, the magnitude of enhancement factor was about 200 for the NO2 symmetric stretching band at 1350 cm−1 with p-nitrobenzoic acid. In order to estimate its effectiveness as a qualitative analytical method, T-SEWS spectra were compared with normal transmission spectra with the use of several samples. The results showed that T-SEWS is useful in detecting and identifying nanogram amounts of substances which do not have reactive functional groups—e.g., triphenyl phosphate (TPP) or polydimethylsiloxane (PDMS), which do not have -COOH, -SO3H, -C=S, etc. A usable spectrum was observed from as little as 25 ng/cm2 of TPP with the use of a standard triglycine sulfate detector without a beam condenser. Combined with microspectroscopy, T-SEWS shows promise for picogram order analysis and chromatographic applications.

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.

Surface-enhanced infrared spectroscopy was applied to the trace analysis of secretions on human skin. The secretions were transferred onto silver-coated barium fluoride substrates by pressing the substrates on the skin at several parts of the human body (wrist, palm, cheek, abdomen and sole). The infrared spectra of the substrates were measured by the transmission mode.Small amounts of lactate, triglycerides, fatty acids and fatty acid esters were identified from the secretions.For detecting lactate, the sole is the best sampling point on the human body. Lactate is a product of anaerobic metabolism in the glycolytic pathway.1 Since lactate plays an important role in energy metabolism, the determination of lactate in biological samples has been the subject of intensive study.2 Recently, Karube et al. showed a remarkable increase of lactate in human secretions after under going hard exercises.3 They also suggested that there exists an intimate correlation between fatigue and lactate production. The degree of fatigue may thus be estimated from the amounts of lactate found in sweat.So far, lactate has been analyzed using electrochemical biosensors.4,5 In conventional sensors, the concentration changes of hydrogen peroxide or oxygen caused by the enzymatic oxidation of lactate is usually detected. However, this method is relatively complicated and suffers from several types of interference due to reducing substances, such as ascorbic acid. In the present work, we applied a simple and sensitive infrared spectroscopy technique to the trace analysis of lactate on human skin.Infrared spectroscopy is useful for the characterization of organic materials. Though the development of Fourier-transform infrared (FT-IR) spectrometers has led to a marked improvement in the sensitivity of this method, the sensitivity is still insufficient for the trace analysis of lactate on human skin. In this study, surface-enhanced infrared absorption (SEIRA) spectroscopy6-9 was used to provide a further improvement in the sensitivity of FT-IR. SEIRA is a phenomenon in which the infrared absorption intensity of molecules adsorbed on or near to very thin metal films of silver, gold, copper, or indium is remarkably ExperimentalBarium fluoride (BaF2) plates (20X20X4 mm) were used as testing substrates for the SEIRA measurements. After the substrates were cleaned in ethanol and acetone, silver films were deposited from a tungsten basket in a JEOL JEE 4C vacuum evaporator at a pressure lower than 5X10-5 Torr. Although the largest absorption enhancement was observed on silver films of 8 -10 nm in thickness (estimated with a Tencor profilometer), the band shape was slightly distorted. To avoid this, the silver thickness was restricted to 6 nm. The metal films were not continuous, but consisted of metal particles of about 30 nm in average diameter.The silver-coated substrates were pressed against human skin at the wrist, palm, cheek, abdomen, or sole at a pressure of 25 g/ cm2 for 20 s; the secretions on the skins were transferred to the substrate. Th...

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Identification of subnanometre surface layers on polymer films, which are only analysable by x-ray photoelectron spectroscopy (XPS) or secondary ion mass spectrometry (SIMS), have been demonstrated by the use of transmission surfaceenhanced infrared absorption (SEIRA) spectroscopy with the use of silver island lilms: 1.0 nm thick polydimethylsiloxane (PDMS) or 0.5 nm thick polymethylphenylsiloxane (PMPHS) surface layers, which are semisolid materials, on polyethyleneterephthalate (PET) films were successfully identifiable with the use of transmission SEIRA by measuring the PDMS or PMPHS that was transferred onto the silver-deposited BaF, substrates under pressure. This approach completely eliminates strong spectral interference from PET films by measuring the transferred surface layer and greatly improves the signal-to-noise ratio of the surface layer absorp tion with the use of silver island films. The detection limit of PDMS surface layers on PET films is -0.2 run. The present method gives a considerable amount of information about surfaces, such as chemical composition and chemical structure. This is a highly promising method for analysing subnanometre surface layers which consist of liquid or semisolid materials, such as oil, lubricant, plasticizer and surface-active agent on polymer materials.