Fresenius J Anal Chem (1998) 362 : 15–20
© Springer-Verlag 1998
O R I G I N A L PA P E R
H. D. Wanzenböck · B. Mizaikoff · N. Weissenbacher · R. Kellner (†)
Surface enhanced infrared absorption spectroscopy (SEIRA) using external reflection on low-cost substrates
Received: 20 January 1998 / Revised: 3 June 1998 / Accepted: 5 June 1998
Abstract A novel approach for spectroscopic trace analysis is introduced by combining surface enhanced infrared absorption (SEIRA) spectroscopy with external reflection techniques on disposable inexpensive substrates. SEIRAactive surfaces produced by electrochemical deposition of silver on smooth metal surfaces and glass improve the sensitivity of IR reflection measurements significantly since the infrared absorption of organic substances such as p-nitrobenzoic acid is considerably increased in the vicinity of rough noble metal surfaces. The enhancement properties of thus prepared substrates are characterized and compared using IR-spectroscopy. These low-cost substrates used in single- and multiple external reflection arrangements, respectively, yield a significant increase of the detection level compared to conventional reflection absorption infrared spectroscopy (RAIRS) up to one order of magnitude. Hence, a notable step towards a wide-spread application of SEIRA in routine IR reflection analysis is presented.
Introduction Presently, mass spectroscopy (MS), nuclear magnetic resonance (NMR) and vibrational spectroscopic techniques such as infrared and Raman spectroscopy are the predominantly applied molecule-specific methods of characterizing organic and inorganic species. Among the main advantages of vibrational spectroscopy are the comparably easy quantification and the fact that samples can be analyzed under a variety of conditions. During the recent decades infrared spectroscopy has been established as a routine molecule-specific technique
Dedicated to the memory of Professor Dr. Robert Kellner H. D. Wanzenböck · B. Mizaikoff (Y) · N. Weissenbacher · R. Kellner Vienna University of Technology, Institute of Analytical Chemistry, Getreidemarkt 9/151, A-1060 Vienna, Austria email:
[email protected]
for qualitative and quantitative purposes. However, conventional transmission spectroscopy cannot provide a sufficient sensitivity level for trace concentrations since most organic species are relatively weak absorbers in the midinfrared region. A significantly higher sensitivity can be achieved using an external reflection setup with the analyte deposited on the surface of an appropriate IR-reflective substrate. This technique is referred to as reflection absorption infrared spectroscopy (RAIRS) and has successfully been used for investigating thin films and interfacial phenomena [1]. The application of RAIRS benefits from simple sample preparation and high sensitivity enabling the detection of ultra-thin analyte layers on a smooth surface, usually an IR-reflective metal mirror [2, 3]. The interpretation of the resulting IR spectra is comparable to transmission spectra considering the surface selection rules resulting in different spectral intensities related to the angle of incidence. For a layer thickness in the range of several µm (> λ of the incident light) the spectral intensity corresponds to the doubled intensity of the absorption of the analyte layer. In contrast, for thin layers (thickness < λ⁄4 of the incident light) the spectral response decays with the decreasing field amplitude when approaching the reflection plane due to the node formation of the electromagnetic field at the metal surface [4]. This behavior is dependent on the angle of incidence and on the increasing strength of the electromagnetic field resulting in a 4 times stronger absorption intensity near grazing incidence [5]. By modifying the metal surface with a rough silver film significant enhancement of the spectral intensities is obtained due to the occurrence of surface enhanced infrared absorption (SEIRA). However, the roughness of the silver film is below the wavelength of the incident light; hence, no loss due to diffuse reflectance has to be contemplated. The surface enhancement in RAIRS can be regarded as an established physical effect although most SEIRA studies are performed using an attenuated total reflection (ATR) setup with internal reflection elements [6–9]. Knoll et al.
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[10] observed a shift of the dispersive curves of surface plasmons of rough metal surfaces with underlayers of monomolecular organic films [11–13]. Kamata et al. [14] performed spectroscopic studies of stearic acid monolayers on an ultra-thin silver film without obtaining a strong spectral enhancement [15]. The first significant spectral enhancement in a specular external reflection configuration was reported by Nishikawa et al. [16] using low reflective surfaces such as germanium (Ge), barium fluoride (BaF2) or glass. For incident radiation near the Brewster angle an enhancement factor of about 50 was observed applying a metal-underlayer and metal-overlayer configuration. However, with nonmetallic substrates anomalous negative absorptions occur as a result of the complex optical substrate properties [17, 18]. Johnson and Aroca [19] discovered an exponential decrease of the absorbance with increasing distance from the metal surface. The surface enhancement for external reflection is reported to be most efficient within a 5 nm distance from the metal surface. Up to now, all studies on SEIRA-RAIRS report on the preparation of the noble metal films by physical vapor deposition (PVD), an expensive high vacuum method requiring precise and extensive control of all deposition pa-
rameters to ensure the reproducibility of SEIRA-active surfaces. Hence, the applicability of this method for routine substrate preparation is restricted. In this work the electrochemical production of SEIRAactive substrates for external reflection measurements is suggested. The advantages of this approach are obvious: (i) affordable equipment as no vacuum system is required, (ii) fast fabrication of SEIRA-substrates since the electrodeposition of metal layers requires less then 60 seconds, (iii) pretreatment of substrates is kept at a minimum and can be performed under ambient conditions, (iv) electrochemical deposition has the potential of in-situ generation and recycling of SEIRA-active metal surfaces, (v) the electrolyte solution provides an economical source for the required noble metals, (vi) low-cost substrates can be prepared since massive noble metal substrates are not necessary. With low-priced carrier materials such as flat metal surfaces, metal coated glass surfaces and metal tubes in addition to a low cost process for the noble metal deposition it is possible to manufacture large amounts of recyclable substrates efficiently and cheaply. Hence, non-resuable SEIRA-substrates for single application may be used.
Experimental Planar configuration
Fig. 1 Schematic of the electrodeposition of a silver film. An additional roughening of the silver surface could be accomplished by repeated silver deposition with the procedure described above and by etching the surface with a freshly prepared mixture of NH3 : H2O2 = 1 : 1 Fig. 2 AFM image (left) of the sputtered continuous gold underlayer on a glass substrate and (right) of the subsequently electrodeposited silver layer with a thickness of approx. 40 nm
Flat substrates were produced from glass slides used in conventional microscopy (76 × 26 mm). Since glass represents an insulating carrier material, continuous conductive metal films were applied to ensure conductivity. While in other investigations typically chromium was used for this purpose, for these studies only a single layer of chemically inert gold with a thickness of 0.1 µm was deposited using a sputtering process. These gold covered glass slides were subsequently used as carrier for the electrodeposited silver layer. Furthermore, these gold surfaces provided the reference surface for the SEIRA measurements. The galvanization of the surface with silver was performed in a cyanidic silver bath with the composition 2.5 g AgNO3, 3.5 g KCN and 4.0 g Na2CO3 in 100 mL dist. H2O as shown in Fig. 1. The conductive base substrates were rinsed with water and then processed directly without further preconditioning. Electrodeposition was performed at a potential of 3.0 V and a current of 30 mA. After 5 s of metal deposition the substrate was removed from the galvanic bath and the metal surface was rinsed
17 Fig. 3 Setup for multiple external reflection experiments showing (a) a planar configuration and (b) a tubular configuration. The analyte layer is deposited as thin film on top of the silver film
with water and methanol. These process parameters stand for the deposition of a silver layer of roughly 20 nm thickness. Occasionally, repeated deposition of metal or etching with a mixture of NH3 : H2O2 = 1 : 1 was used for further increasing the roughness of the metal surface. For multiple external reflection two reflecting surfaces, the SEIRA-active silvercoated substrate and a gold mirror, were positioned parallel in a distance of 2 mm. The infrared beam was reflected 25 times between those planar surfaces at an angle of incidence of 45°. The surface topology was investigated using atomic force microscopy (AFM). Figure 2 depicts exemplary the sputtered gold underlayer and the subsequently electrodeposited silver layer of approx. 40 nm thickness. Tubular configuration Copper tubes with a circular cross section were used as carrier material for the silver layers. The substrates were rinsed with methanol and distilled water prior to the metal layer deposition. A continuous silver film was deposited on the copper tube by cementation from a 10 mM aqueous AgNO3 solution. Due to the difference in the electrochemical potentials of silver and copper, an electrochemical open-circuit deposition of the dissolved noble metal ion (silver) on the less noble metal surface (copper) takes place without applying an external potential. The tube was used as external multireflection element (“light pipe”) guiding the infrared beam as depicted in Fig. 3. Depending on the diameter and length of the tube, the number of external reflections can be varied. In order to increase the surface enhancement effect, the surface could be roughened additionally by etching as described above. Spectrometer setup The analyte was applied to the metal surface as organic overlayer. Analytes – exemplary in this study p-nitrobenzoic acid (PNBA) – were prepared in an organic solvent such as methanol in a concentration of 1 mM. The spectrum of PNBA is given in Fig. 4.
Fig. 4 Spectrum of PNBA recorded on a germanium ATR crystal
A defined volume of this solution was deposited on the surface of the reflection element. After evaporation of the volatile solvent a homogeneous thin organic film remained on the metal substrate. The total amount of analyte deposited was in the µg to ng range and could exactly be calculated from the concentration and deposition volume of the solution. The substrates could be used for repeated measurements by rinsing the surface with a solvent in order to remove the analyte entirely. The infrared spectra were recorded on a Bruker IFS 66 and IFS 88 FTIR spectrometer using MCT-detectors. Measurements were performed with unpolarized radiation. For RAIRS-measurements in the single external reflection mode an optical bench allowing adjusting the angle of incidence between 15° and 85° was used as shown in Fig. 5. At larger angles a significant ellipsoidal distortion of the sampled spot occurred enlarging the probed area. Hence, the sample was deposited in a confined area of approximately 12 mm2 in order to avoid this effect. The multireflection setup for flat substrates involves two reflecting metal surfaces of 50 mm length placed parallel with 2 mm
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Fig. 5 External reflection unit with variable angle of incidence
distance. The light was guided between those reflecting planes interacting with the analyte layer on one of the two surfaces. The angle of incidence was 45°. For the tubular configuration the IR beam was coupled in and out of the multireflection element using spherical mirrors since it has to be considered that besides meridional rays also skew rays propagate inside this hollow waveguide. The number of external reflections in the tube depends on the angle of incidence, the diameter and the length of the metal tube. Infrared spectra were recorded with 4 cm–1 resolution by coadding 1024 scans. All spectra of analytes shown in this work represent subtractive spectra in order to eliminate the spectral contributions from the metal substrate.
Fig. 6 External reflection at 82° incidence angle showing the spectra of a thin film of 8.35 µg PNBA (5 µL of 0.01 M solution) deposited on (a) reference gold layer produced by sputtering, (b) electrochemically deposited silver layer galvanized from a cyanidic silver bath
Results The spectral enhancement of the silver layers was investigated by comparing representative absorption band intensities of an exemplary analyte on the eletrodeposited silver surface with respect to the band intensity on a reflecting reference substrate. The metal carrier substrate – smooth bare gold surface and copper tubing, respectively – was used as reference. Single external reflection For sample preparation an amount of 8.35 µg PNBA was deposited on the metal surface as a thin film covering an area of roughly 4 cm2. Due to the properties of the previously described optical bench the size of the radiation sampling spot interrogating the surface varies with the angle of incidence. Hence, for the evaluation of the surface enhancement effect only measurements with the same incidence angle can be compared. Using external reflection with an incidence angle of 82° enabled to obtain a spectrum of PNBA with the reference gold mirror as well as with the electrodeposited silver layer. With the analyte deposited on the galvanized silver layers the absorption intensites of PNBA were enhanced by more than one order of magnitude as illustrated in Fig. 6. The strongest bands were observed at 1700 cm–1 and 1543 cm–1 and are typically attributed to vibrations of a free carboxyl group. Hence, it has to be assumed that the carboxyl group does not interact with the metal layer in
Fig. 7 Variation of incidence angle in an external reflection setup. The surface enhanced spectrum of 8.35 µg PNBA (5 µL of 0.01 M solution) on a silver layer galvanized from a cyanidic bath (Ag(CN)2bath; 3 s; 2 V; < 5 mA) is shown at incidence angles of (a) 20°, (b) 30°, (c) 40°, (d) 50°, (e) 60°, (f) 70°, (g) 75°, (h) 78°, (i) 80°, (j) 82°, (k) 85°
the present case. This behavior suggests the presence of a protective layer due to oxidation on the silver surface. For geometrical and optical reasons the intensity of IR spectra is dependent on the incidence angle of the radiation [5, 18]. The variation of the spectral characteristics
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with the angle of incidence of unpolarized light is shown in Fig. 7 for the surface enhanced spectra on the same substrate with a galvanized silver layer. It could be shown that all spectra on silver show a higher absorption compared to the reference due to the surface enhancement effect of the electrodeposited metal film. The intensities of the spectra vary by a factor of 7.5 depending on the angle of incidence. It has to be noted that the absorption bands at 1345 cm–1, 1425 cm–1, 1543 cm–1, 1600 cm–1 and 1698 cm–1 are enhanced to the same extent. For obtaining the highest total absorption of the analyte on a silver galvanized surface an incidence angle of 82° has to be selected. Multiple external reflection Planar configuration. Using the setup described previously the infrared beam (angle of incidence 45°) was reflected 12 times by the electrodeposited silver film coated with a thin film of an organic analyte. In Fig. 8 the spectra of 8.35 µg PNBA deposited on various metal layers are shown. Using the blank gold mirror as reference surface, the spectrum of PNBA can hardly be recognized. With an electrodeposited silver layer the absorption intensities of PNBA could be enhanced significantly by more than one order of magnitude due to the surface enhancement effect attributed to the rough silver layer. The spectrum with absorption bands at 1698 cm–1 and 1307 cm–1 suggests the existence of the free carboxylic acid. Since the typical formation of carboxylic acid salts on bare silver does not occur, the existence of a protective layer on the silver surface can be assumed again. After etching with NH3 : H2O2 the silver layer exposed comparably higher spectral enhancement, indicating increased roughness of the metal surface. However, this activation of the silver film resulted also in spectral changes. The absorption band at 1700 cm–1 decreased in intensity while new bands emerged around 1587 cm–1 and 1382 cm–1. These spectral features suggest the formation of silver salts of the carboxylic acid. It can be assumed that the etching process has activated the silver surface, revealing a highly reactive metal substrate. Hence, despite the higher spectral enhancement these spectra become difficult to interpret. Similar results could be obtained with silver layers produced by chemical reduction using NaBH4. Tubular configuration. Multiple external reflection on surface enhancing substrates could also be achieved with a rotation-symmetrical setup. Within this configuration the ray path and the angle of incidence is not exactly defined due to the multitude of different meridional and skew rays similar to a fiber optic hollow waveguide. However, surface enhanced spectra could be obtained using a copper tube with an inner diameter of 8 mm. The spectrum of 8.35 µg PNBA covering the bare copper surface shows comparably well resolved absorption bands. After depositing a rough silver film on the etched
Fig. 8 Planar multireflection measurements (45°, 12 reflections on analyte covered area) showing spectra of 5 µL 0.01 M PNBA deposited on a glass slide with a metal layer of (a) sputtered gold, (b) sputtered gold with a 20 nm silver overlayer deposited in a single galvanization step, (c) sputtered gold with a 40 nm silver overlayer deposited in a double galvanization process, (d) a silver film etched for 10 s with a 1 : 1 mixture of H2O2 and NH3
Fig. 9 Measurements in a tubular multireflection setup (approx. 7 reflections) showing spectra of 10 µL 0.01 M PNBA deposited on the inner surface of (a) copper tube, (b) silver layer cemented on a copper tube, (c) silver layer galvanized on a copper tube
copper surface, a significant enhancement of the bands occurred as demonstrated in Fig. 9. Using a cementation process for depositing the silver layer on the etched copper surface the absorption intensity of the 1345 cm–1 NO2-band could be increased by a factor of 3.5. With regard to the weak 1698 cm–1 absorption band the spectrum suggests the deprotonization of the PNBA carboxylic acid group by the formation of a metal salt on the silver surface. For a silver layer deposited by electrogalvanization from a cyanidic bath a notable enhancement of the same magnitude was observed. However, the electrodeposited metal surface revealed no interaction with the carboxylic group of PNBA as the C = O vibration of the carboxylic acid at 1698 cm–1 is present.
Conclusion A novel type of low-cost substrates for surface enhanced reflection absorption infrared spectroscopy (SE-RAIRS)
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has been developed. By electrodeposition of thin silver layers on a smooth metal surface spectral enhancement up to one order of magnitude could be obtained for the exemplary analyte p-nitrobenzoic acid (PNBA). Further increase of sensitivity could be achieved by using a multireflection setup enabling several external reflections on the SEIRA substrate. With this cost-effective approach routine FTIR analysis can benefit from the increased sensitivity of the surface enhanced infrared absorption spectroscopy for the first time. Hence, trace amounts of organic substances may be analyzed on disposable one-way SEIRA substrates. Using a tubular configuration as demonstrated with a copper tube, the first step towards a SE-RAIRS/FTIR analysis method for flow systems was realized. Due to the fact that surface enhancement could also be obtained with cylindrical surfaces, new impulses for the optimization of fiber optic sensors emerge since a hollow fiber approach utilizing the increased sensitivity of SEIRA may become feasible. However, the possible reaction of analytes with the activated metal surface suggests that further efforts towards ultra-thin protective layers for the SEIRA substrate surface are required in order to ensure facile interpretation of spectral data. Acknowledgement The Austrian Science Fund (FWF) is acknowledged for financial support of this work under project 10386–CHE.
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