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Multiple internal reflection in surface enhanced infrared absorption spectroscopy (SEIRA) and its significance for various analyte groups
Journal of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular
Structure 410-41
I (1997) 535-538
Multiple internal reflection in surface enhanced infrared absorption spectroscopy (SEIRA) and its significance for various analyte groups Heinz D. WanzenbCick*, Boris Mizaikoff, Norbert Weissenbacher, Institute for Analytical Chemistry,
Vienna University
Robert Kellner
of Technology, Getreidemarkt 9/ISI. A-1060, Vienna, Austria
Received 26 August 1996; accepted 6 September
1996
Abstract The presence of rough metal surfaces is known to increase the infrared absorption of selected analytes. It could be shown that this surface enhancement effect applies to a wide range of chemical substrates with different molecular structure, although the observed enhancement is not equivalent for all absorption bands. For carboxyl groups, interactions with the metal surface were observed suggesting the formation of an adsorption complex. Furthermore, the potential of surface enhanced infrared absorption (SEIRA) as quantitative method is demonstrated. 0 1997 Elsevier Science B.V. Keywords: SEIRA; Surface enhanced infrared; Metal island films; Attenuated
1. Introduction Infrared spectroscopy is a widely used molecular specific analytical technique but experiences considerable restrictions in the field of trace analysis. Time consuming preconcentration steps or long optical paths demanding a larger sample volume have to be used. Surface enhanced IR absorption spectroscopy offers an alternative to increase the analyte signal by introducing a rough metal surface. An organic substance present on or near a rough metal layer results in a drastic change of the absorption properties of infrared light. Recently, the same substrate was successfully used for surface enhanced infrared spectroscopy and surface enhance Raman scattering demonstrating that the physical phenomenon is corresponding [ 11. * Corresponding 0022-2860/97/$17.00
author. 0 1997 Elsevier
PII SOO22-2860(96)09666-4
Science
total reflection; FITR
The effect is contributed to electromagnetic mechanisms [2] and the excitation of electronic surface states resulting in advantageous dielectric properties [3] of the surface. The effective mechanism is not yet completely understood. During this work SEIRA was applied to multiple attenuated total reflection spectroscopy using conventional ATR-crystals analogous to Hartstein et al. [4]. The organic analyte was supplied as overlayer. Up till now surface enhanced absorption has also been reported for IR-transmission spectroscopy [5], external reflection spectroscopy [6] and single attenuated total reflection [7]. For SEIRA shifts of absorption bands were observed [8] which are in general attributed to chemisorption. Characterizing the metal island films with atomic force microscopy (AFM), it was shown that the size of metal clusters correlates with the achieved enhancement [9]. The question arising is whether or not this principle can be applied to a
B.V. All rights reserved.
536
H.D. Wanzenbiick et al./Journal
of Molecular
Siructure
410-411
(1997) 535-538
variety of chemical substances regardless of their chemical structure, since this effect could provide a significant improvement especially in the field of chemical sensors [ IO- 131.
2. Experimental The solvents (methanol, acetone, hexane) and analytes used were of analytical grade (p.a.). The crystals for ATR-measurements had trapezoidal shape (50 x 20 x 2 mrnI45”) and were made of polycrystalline zinc selenide or germanium. Prior to metal deposition the surface was polished with 0.3 pm A1203 and rinsed with methanol and hexane. Sputtering of silver layers was performed under ambient conditions at 1 x lo4 mbar base pressure. During deposition a target current of 120 mA and 4.0 x lO-3 mbar argon pressure was maintained. Control of the layer thickness was achieved by repeated rotation of the sample over an aperture allowing a deposition rate of 0.58 + 0.07 nm per rotation. The obtained surfaces were investigated by atomic force microscopy (AFM). The deposition of gold was performed by physical vapor deposition (PVD) at a deposition rate of 0.06 nm s-’ and a base pressure < 8 x 1O-7 mbar. The deposited mass was monitored by a quartz micro-balance. By variation of the deposition angle also substrates with needle-shaped surface structures could be obtained.
-1
1600
-7-r--,
1400
1500
1300
wavenumber
1100
9ocl
700
[cm-i]
Fig. 2.5 plO.001 M hexachloroethylene acid in methanol deposited on the surface of Ge-ATR-crystal (a) without metal and (b) with a 7.0 nm Ag layer deposited by sputtering.
3. Results and discussion Several measurements were performed on GeATR-crystals with silver !ayers of a nominal thickness varying in the range from 1.2 to 7.0 nm. For increasing enhancement with growing metal layer thickness the maximum achievable enhancement could be observed for a silver sputter-layer with a nominal thickness of 7.0 nm. With this rough metal island films, an increase of absorption up to a factor of 50 was achieved. A surface enhancement effect is demonstrated for analytes with polar as well as apolar functional groups and various basic molecular structures (aromatics, aliphatics) as illustrated
r,
7,-r-r-
1600
1700
1200
1000
600
wavenumber [cm-l] Fig. 1. 5 ~1 0.001 M p-nitrobenzoic acid (835 ng) in methanol deposited on the surface of Ge-ATR-crystal (a) without metal and (b) with a 7.0 nm Ag layer deposited by sputtering.
Fig. 3. Calibration curve of PNBA: A solution of p-nitrobenzoic acid in methanol is deposited on a ZnSe-ATR-crystal with a 4.7 nm Ag sputter-layer. The calibration covers a range from 1700 ng (10 x 10e9 mol) down to 17 ng (100 x IO-” mol).
H.D.Wanzenbiick
et al./Joumal
of
Molecular
Structure
410-41
I (1997)
535-538
531
absorbance
1800 1600 1400
1200
1000 800
1800 1600 1400 1200 1000 800 600
600
wavanumber[cm-I] Fig. 4.5 ).d 0.001 M terbutryne (1.2 pg) crystal with gold needle films. Analyte gold deposited orthogonal to the surface under a grazing angle of 18” (b) crystal
wavenumber[cm-1]
deposited on a ZnSe-ATRon (a) crystal with 1.2 nm and 4.8 nm gold deposited without any metal.
for p-nitrobenzoic acid and hexachloroethylene in Figs. 1 and 2. A linear correlation between the concentration and signal intensity was obtained (see Fig. 3) for p-nitrobenzoic acid with an analyte surface coverage below 1700 ng corresponding to 10 nmol analyte. For gold films, in general the achieved signal enhancement was below the values obtained for silver-layers. Still, a significant enhancement above a factor of 10 was achieved for the gold needle substrates. In Figs. 4 and 5, the application of SEIRA to practically relevant analytes is depicted.
Fig. 6. ZnSe-ATR-crystal with gold needle films prepared by deposition of I.2 nm gold in an orthogonal angle to the surface followed by the deposition of 4.8 nm gold under a grazing angle of 18”. Bottom: 5 ~lO.001 M p-endosulfane (2.0 pg) deposited on a ZnSe-ATR-crystal (a) without any metal (b) crystal with a gold needle film top: 5 pl 0.001 M p-metazachlorine (1.4 pg) deposited on a ZnSe-ATR-crystal (c) without any metal (d) with a gold needle film.
Two sured ATR mass
pesticides (terbutryne, endosulfane) were meaby conventional ATR and surface enhanced in 5 nmol analyte amounts (corresponding to a of 1.2 and 2.0 pg, respectively). Despite the
1800 1600 1400 1200 1000 800 600 wavenumber[cm-1] I
1 m’,i-~~T-rrr-,1,r,‘l
1800
1600
1400
1200
1000 800 600
wavenumbar[cm-11 Fig. 5. 5 pl 0.001 M p-endosulfane (2.0 pg) deposited on a ZnSeATR-crystal with a gold needle film. Analyte on a crystal (a) without any metal (b) crystal with 1.2 nm gold deposited orthogonal to the surface and 4.8 nm gold deposited under a grazing angle of 18”.
Fig. 7. Mixture of 2 pesticides on ZnSe-ATR-crystal with gold needle films prepared by I .2 nm gold deposition orthogonal to the surface and 4.8 nm gold deposition under a grazing angle of 18” (a) measured spectrum of I:1 mixture of endosulfane (2.0 pg) and metazachlorine (1.4 pg) on ZnSe with gold needle film; (b) spectrum of the same I:1 mixture calculated by spectra addition from spectra of single compounds (see Fig. 6) and (c) the difference spectrum of (a) and (b) showing the distinction of the real mixture due to cross interactions.
538
H.D. Wanzenbiick et al./Joumal of Molecular Structure 4/O-411 (1997) 535-538
different molecular structures, enhanced IR spectra could be obtained. Furthermore, it could be shown that a surface enhancement effect is also achieved analyzing a mixture of substances. In Fig. 6, the enhancement of endosulfane (2.0 pg) and metazachlorine (1.41 pg) by the same metal layer is shown for 5 nmol pure compound. As depicted in Fig. 7, an enhanced IR spectrum could be obtained for a 1: 1 mixture of both compounds. This simultaneous measurement of 5 nmol endosulfane and 5 nmol metazachlorine was simulated by spectral superposition of absorption bands of the unperturbed pure compounds from Fig. 6. As illustrated in Fig. 7 by the difference spectrum (c) no significant difference between the calculated spectrum (b) and the measured spectrum (a) occurred. This indicates that no relevant interaction or perturbation occurs in the mixture of the analytes. For a low coverage competitive adsorption or displacement of one species from surface sites was not observed. This gives rise to the expectation that the analysis of complex real life samples will not be obstructed by cross interactions.
4. Conclusion In the present work, a surface enhancement effect could be measured for aromatics, heteroaromatics and aliphatics with numerous different substituents. An increased absorption was observed for several functional groups such as amino-groups, chlorides, carboxylic acids, nitro-groups and sulfoxy-groups. The result obtained for this multitude of molecular structures gives rise to the expectation that the detection limit for a large variety of analytes can be significantly lowered by using SEIRA. At low concentration levels
the method sis. For the of complex considered
could also be used for quantitative analyidentification of substances the possibility formation with the metal layer has to be when interpreting the spectra.
Acknowledgements This work was supported by the Fonds zur Forderung wissenschaftlicher Forschung under project FWF 10386 CHE. The authors are grateful to W. Theiss, T. Luyven and M. Arntzen (1. Physic. Institute, RVVTH Aachen, Germany) and A. Kiick (Institute f. solid state electronics, Vienna University of Technology) for the deposition of metal layers.
References [I] [2] [3] [4]
E. Johnson, R. Aroca, J. Phys. Chem. 99 (1995) 9325. M. Osawa, K. Ataka, Surf. Sci. Lett. 262 (1992) Lll8. W. Theiss, update communication. A. Hartstein, J. Kirtley, J. Tsang, Phys. Rev. Lett. 45(3) 201. [5] Y. Nishikawa, K. Fujiwara, T. Shima, Appl. Spec. 44(4) (1990) 691. [6] Y. Nishikawa, K. Fujiwam, K. Ataka, M. Osawa, Anal. Chem. 65 ( 1993) 556. [7] A. Hatta, Y. Suzuki, W. Suetaka, Appl. Phys. A 35 (1984) 135. [8] K.P. Ishida, P.R. Griffiths, Anal. Chem. 66 (1994) 522. [9] H.D. Wanzenbock, B. Edl-Mizaikoff, G. Fdedbacher, M. Grasserbauer, R. Kellner, M. Amtzen, T. Luyven, W. Theiss, P. Grosse, Mikrochim. Acta (1996) in press. [ 101 R. Kellner, R. Gobel, R. Gotz, B. Lendl, B. Edl-Mizaikoff, M. Tacke, A. Katzir, SPIE 2508 (1995) 212. [I 11 M. Jakusch, B. Edl-Mizaikoff, R. Kellner, A. Katzir, Sensors Actuators (1995) submitted for publication. [ 121 B. Edl-Mizaikoff, R. G&z, R. Kellner, SPIE 2508 (1995) 253. [I31 B. Edl-Mizaikoff, R. Gobel, R. Krska, K. Taga, R.Kellner, M. Tacke, A. Katzir, Sensors Actuators 29 (1-3) (1995)58.
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular
Structure 410-41
I (1997) 535-538
Multiple internal reflection in surface enhanced infrared absorption spectroscopy (SEIRA) and its significance for various analyte groups Heinz D. WanzenbCick*, Boris Mizaikoff, Norbert Weissenbacher, Institute for Analytical Chemistry,
Vienna University
Robert Kellner
of Technology, Getreidemarkt 9/ISI. A-1060, Vienna, Austria
Received 26 August 1996; accepted 6 September
1996
Abstract The presence of rough metal surfaces is known to increase the infrared absorption of selected analytes. It could be shown that this surface enhancement effect applies to a wide range of chemical substrates with different molecular structure, although the observed enhancement is not equivalent for all absorption bands. For carboxyl groups, interactions with the metal surface were observed suggesting the formation of an adsorption complex. Furthermore, the potential of surface enhanced infrared absorption (SEIRA) as quantitative method is demonstrated. 0 1997 Elsevier Science B.V. Keywords: SEIRA; Surface enhanced infrared; Metal island films; Attenuated
1. Introduction Infrared spectroscopy is a widely used molecular specific analytical technique but experiences considerable restrictions in the field of trace analysis. Time consuming preconcentration steps or long optical paths demanding a larger sample volume have to be used. Surface enhanced IR absorption spectroscopy offers an alternative to increase the analyte signal by introducing a rough metal surface. An organic substance present on or near a rough metal layer results in a drastic change of the absorption properties of infrared light. Recently, the same substrate was successfully used for surface enhanced infrared spectroscopy and surface enhance Raman scattering demonstrating that the physical phenomenon is corresponding [ 11. * Corresponding 0022-2860/97/$17.00
author. 0 1997 Elsevier
PII SOO22-2860(96)09666-4
Science
total reflection; FITR
The effect is contributed to electromagnetic mechanisms [2] and the excitation of electronic surface states resulting in advantageous dielectric properties [3] of the surface. The effective mechanism is not yet completely understood. During this work SEIRA was applied to multiple attenuated total reflection spectroscopy using conventional ATR-crystals analogous to Hartstein et al. [4]. The organic analyte was supplied as overlayer. Up till now surface enhanced absorption has also been reported for IR-transmission spectroscopy [5], external reflection spectroscopy [6] and single attenuated total reflection [7]. For SEIRA shifts of absorption bands were observed [8] which are in general attributed to chemisorption. Characterizing the metal island films with atomic force microscopy (AFM), it was shown that the size of metal clusters correlates with the achieved enhancement [9]. The question arising is whether or not this principle can be applied to a
B.V. All rights reserved.
536
H.D. Wanzenbiick et al./Journal
of Molecular
Siructure
410-411
(1997) 535-538
variety of chemical substances regardless of their chemical structure, since this effect could provide a significant improvement especially in the field of chemical sensors [ IO- 131.
2. Experimental The solvents (methanol, acetone, hexane) and analytes used were of analytical grade (p.a.). The crystals for ATR-measurements had trapezoidal shape (50 x 20 x 2 mrnI45”) and were made of polycrystalline zinc selenide or germanium. Prior to metal deposition the surface was polished with 0.3 pm A1203 and rinsed with methanol and hexane. Sputtering of silver layers was performed under ambient conditions at 1 x lo4 mbar base pressure. During deposition a target current of 120 mA and 4.0 x lO-3 mbar argon pressure was maintained. Control of the layer thickness was achieved by repeated rotation of the sample over an aperture allowing a deposition rate of 0.58 + 0.07 nm per rotation. The obtained surfaces were investigated by atomic force microscopy (AFM). The deposition of gold was performed by physical vapor deposition (PVD) at a deposition rate of 0.06 nm s-’ and a base pressure < 8 x 1O-7 mbar. The deposited mass was monitored by a quartz micro-balance. By variation of the deposition angle also substrates with needle-shaped surface structures could be obtained.
-1
1600
-7-r--,
1400
1500
1300
wavenumber
1100
9ocl
700
[cm-i]
Fig. 2.5 plO.001 M hexachloroethylene acid in methanol deposited on the surface of Ge-ATR-crystal (a) without metal and (b) with a 7.0 nm Ag layer deposited by sputtering.
3. Results and discussion Several measurements were performed on GeATR-crystals with silver !ayers of a nominal thickness varying in the range from 1.2 to 7.0 nm. For increasing enhancement with growing metal layer thickness the maximum achievable enhancement could be observed for a silver sputter-layer with a nominal thickness of 7.0 nm. With this rough metal island films, an increase of absorption up to a factor of 50 was achieved. A surface enhancement effect is demonstrated for analytes with polar as well as apolar functional groups and various basic molecular structures (aromatics, aliphatics) as illustrated
r,
7,-r-r-
1600
1700
1200
1000
600
wavenumber [cm-l] Fig. 1. 5 ~1 0.001 M p-nitrobenzoic acid (835 ng) in methanol deposited on the surface of Ge-ATR-crystal (a) without metal and (b) with a 7.0 nm Ag layer deposited by sputtering.
Fig. 3. Calibration curve of PNBA: A solution of p-nitrobenzoic acid in methanol is deposited on a ZnSe-ATR-crystal with a 4.7 nm Ag sputter-layer. The calibration covers a range from 1700 ng (10 x 10e9 mol) down to 17 ng (100 x IO-” mol).
H.D.Wanzenbiick
et al./Joumal
of
Molecular
Structure
410-41
I (1997)
535-538
531
absorbance
1800 1600 1400
1200
1000 800
1800 1600 1400 1200 1000 800 600
600
wavanumber[cm-I] Fig. 4.5 ).d 0.001 M terbutryne (1.2 pg) crystal with gold needle films. Analyte gold deposited orthogonal to the surface under a grazing angle of 18” (b) crystal
wavenumber[cm-1]
deposited on a ZnSe-ATRon (a) crystal with 1.2 nm and 4.8 nm gold deposited without any metal.
for p-nitrobenzoic acid and hexachloroethylene in Figs. 1 and 2. A linear correlation between the concentration and signal intensity was obtained (see Fig. 3) for p-nitrobenzoic acid with an analyte surface coverage below 1700 ng corresponding to 10 nmol analyte. For gold films, in general the achieved signal enhancement was below the values obtained for silver-layers. Still, a significant enhancement above a factor of 10 was achieved for the gold needle substrates. In Figs. 4 and 5, the application of SEIRA to practically relevant analytes is depicted.
Fig. 6. ZnSe-ATR-crystal with gold needle films prepared by deposition of I.2 nm gold in an orthogonal angle to the surface followed by the deposition of 4.8 nm gold under a grazing angle of 18”. Bottom: 5 ~lO.001 M p-endosulfane (2.0 pg) deposited on a ZnSe-ATR-crystal (a) without any metal (b) crystal with a gold needle film top: 5 pl 0.001 M p-metazachlorine (1.4 pg) deposited on a ZnSe-ATR-crystal (c) without any metal (d) with a gold needle film.
Two sured ATR mass
pesticides (terbutryne, endosulfane) were meaby conventional ATR and surface enhanced in 5 nmol analyte amounts (corresponding to a of 1.2 and 2.0 pg, respectively). Despite the
1800 1600 1400 1200 1000 800 600 wavenumber[cm-1] I
1 m’,i-~~T-rrr-,1,r,‘l
1800
1600
1400
1200
1000 800 600
wavenumbar[cm-11 Fig. 5. 5 pl 0.001 M p-endosulfane (2.0 pg) deposited on a ZnSeATR-crystal with a gold needle film. Analyte on a crystal (a) without any metal (b) crystal with 1.2 nm gold deposited orthogonal to the surface and 4.8 nm gold deposited under a grazing angle of 18”.
Fig. 7. Mixture of 2 pesticides on ZnSe-ATR-crystal with gold needle films prepared by I .2 nm gold deposition orthogonal to the surface and 4.8 nm gold deposition under a grazing angle of 18” (a) measured spectrum of I:1 mixture of endosulfane (2.0 pg) and metazachlorine (1.4 pg) on ZnSe with gold needle film; (b) spectrum of the same I:1 mixture calculated by spectra addition from spectra of single compounds (see Fig. 6) and (c) the difference spectrum of (a) and (b) showing the distinction of the real mixture due to cross interactions.
538
H.D. Wanzenbiick et al./Joumal of Molecular Structure 4/O-411 (1997) 535-538
different molecular structures, enhanced IR spectra could be obtained. Furthermore, it could be shown that a surface enhancement effect is also achieved analyzing a mixture of substances. In Fig. 6, the enhancement of endosulfane (2.0 pg) and metazachlorine (1.41 pg) by the same metal layer is shown for 5 nmol pure compound. As depicted in Fig. 7, an enhanced IR spectrum could be obtained for a 1: 1 mixture of both compounds. This simultaneous measurement of 5 nmol endosulfane and 5 nmol metazachlorine was simulated by spectral superposition of absorption bands of the unperturbed pure compounds from Fig. 6. As illustrated in Fig. 7 by the difference spectrum (c) no significant difference between the calculated spectrum (b) and the measured spectrum (a) occurred. This indicates that no relevant interaction or perturbation occurs in the mixture of the analytes. For a low coverage competitive adsorption or displacement of one species from surface sites was not observed. This gives rise to the expectation that the analysis of complex real life samples will not be obstructed by cross interactions.
4. Conclusion In the present work, a surface enhancement effect could be measured for aromatics, heteroaromatics and aliphatics with numerous different substituents. An increased absorption was observed for several functional groups such as amino-groups, chlorides, carboxylic acids, nitro-groups and sulfoxy-groups. The result obtained for this multitude of molecular structures gives rise to the expectation that the detection limit for a large variety of analytes can be significantly lowered by using SEIRA. At low concentration levels
the method sis. For the of complex considered
could also be used for quantitative analyidentification of substances the possibility formation with the metal layer has to be when interpreting the spectra.
Acknowledgements This work was supported by the Fonds zur Forderung wissenschaftlicher Forschung under project FWF 10386 CHE. The authors are grateful to W. Theiss, T. Luyven and M. Arntzen (1. Physic. Institute, RVVTH Aachen, Germany) and A. Kiick (Institute f. solid state electronics, Vienna University of Technology) for the deposition of metal layers.
References [I] [2] [3] [4]
E. Johnson, R. Aroca, J. Phys. Chem. 99 (1995) 9325. M. Osawa, K. Ataka, Surf. Sci. Lett. 262 (1992) Lll8. W. Theiss, update communication. A. Hartstein, J. Kirtley, J. Tsang, Phys. Rev. Lett. 45(3) 201. [5] Y. Nishikawa, K. Fujiwara, T. Shima, Appl. Spec. 44(4) (1990) 691. [6] Y. Nishikawa, K. Fujiwam, K. Ataka, M. Osawa, Anal. Chem. 65 ( 1993) 556. [7] A. Hatta, Y. Suzuki, W. Suetaka, Appl. Phys. A 35 (1984) 135. [8] K.P. Ishida, P.R. Griffiths, Anal. Chem. 66 (1994) 522. [9] H.D. Wanzenbock, B. Edl-Mizaikoff, G. Fdedbacher, M. Grasserbauer, R. Kellner, M. Amtzen, T. Luyven, W. Theiss, P. Grosse, Mikrochim. Acta (1996) in press. [ 101 R. Kellner, R. Gobel, R. Gotz, B. Lendl, B. Edl-Mizaikoff, M. Tacke, A. Katzir, SPIE 2508 (1995) 212. [I 11 M. Jakusch, B. Edl-Mizaikoff, R. Kellner, A. Katzir, Sensors Actuators (1995) submitted for publication. [ 121 B. Edl-Mizaikoff, R. G&z, R. Kellner, SPIE 2508 (1995) 253. [I31 B. Edl-Mizaikoff, R. Gobel, R. Krska, K. Taga, R.Kellner, M. Tacke, A. Katzir, Sensors Actuators 29 (1-3) (1995)58.