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Fibre-optic SERS sensors with conically etched tips
Journal of Molecular Structure 563±564 (2001) 163±166
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Fibre-optic SERS sensors with conically etched tips C. Viets, W. Hill* Institut fuÈr Spektrochemie und Angewandte Spektroskopie (ISAS), P.O. Box 101352, 44013 Dortmund, Germany Received 31 August 2000; accepted 17 October 2000
Abstract Conical ®bre-optic SERS sensors are produced by etching with hydro¯uoric acid covered with an organic overlayer. These newly presented sensor tips show large SERS intensities and can easily be recycled. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Fibre-optic sensors; SERS; Raman spectroscopy
1. Introduction
2. Experimental
Fibre-optic sensors based on surface enhanced Raman scattering (SERS) can be used for the sensitive detection of submonomolecular layers remote from the spectrometer or at dif®cultly accessible locations [1±3]. In the so-called optode design, a single ®bre carries both, the laser radiation and the Raman scattered radiation which is back-scattered from the SERS-active surface directly attached to the ®bre tip. Such SERSactive ®bre tips can be prepared by vacuum deposition of metal ®lms over nanoparticles [3], slow evaporation of metal island-®lms [3], immobilisation of colloidal particles [4] and evaporation of metals on pre-roughened ®bre tips [3,5]. So far, the latter have been the only kind of recyclable ®bre tips.
2.1. Instrumentation
* Corresponding author. Tel.: 149-231-1392-217; fax: 149-2311392-120. E-mail address: [email protected] (W. Hill).
Raman and SERS measurements were carried out using a 0.5 m triple monochromator with subtractive dispersion of the two ®rst stages (DILOR XY) and a nitrogen-cooled CCD detector (Wright Instruments). The exciting laser light had a wavelength of 702 nm, was produced by a cw Ti:sapphire laser (Coherent 890, pumped by an Ar 1 laser (Spectra-Physics 2000)) and ®ltered by a monochromator (Spectrolab, Labspec III) to remove low-intensity sidebands and the broadband spontaneous emission. A confocal laser microscope (Olympus BX40 with objective Olympus, MPlan 10 £ /0.25) was used to focus laser light with a power of 1.5 mW on the uncoated end of the ®bre sensors and thus to couple it into the ®bre core. This way, the SERS-active tips were illuminated through the ®bres and the scattered light was detected through the microscope after passing the same ®bre. Scanning electron microscope (SEM) images were obtained by using a device by JEOL (type JSM6400) with an acceleration voltage of 25 kV.
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(00)00876-0
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C. Viets, W. Hill / Journal of Molecular Structure 563±564 (2001) 163±166
Fig. 1. Structure and internal re¯ections for a conical, SERS-active ®bre tip.
2.2. Fibre sensor preparation Hard polymer-clad silica ®bres (SpecTran, HCPM0200T-06; ù 200 mm, N.A. 0.37) were cleaved into ®bre sections of about 8 cm. After the removal of the buffer, the tips of the ®bre sections were soaked in concentrated nitric acid for 3 h, so that 3 cm of the hard polymer-cladding could be removed mechanically. Employing a technique similar to the preparation of tips for the optical near-®eld microscopy [6],
the unclad ®bre tips were etched in 40% aqueous hydro¯uoric acid that was overcoated with an organic liquid. The used organic liquids were m-xylene with an etching time of 1 h and iso-octane with an etching time of 2 h. After etching, the ®bres were rinsed with deionised water. For reference, the endface of a ®bre section with cladding was grinded under an inclination angle of 608 with 30-mm grains polishing paper, rinsed with water and ultrasonically cleaned in acetone [5]. The ®bre tips were then evaporated with 75 nm silver using a conventional evaporation chamber (type BA710V, Balzers). The silver-coated ®bre tips were immersed in a 10 mM solution of thiophenol (.99%, Aldrich) in ethanol for 10 min and subsequently rinsed with ethanol. The reference chemical thiophenol forms chemically bonded, stable monolayers on silver surfaces [7] and exhibits a comparatively large Raman cross section. 3. Results and discussion Both etching methods gave conical ®bre tips (Fig. 1)
350
300
Intensity (arb. units)
a 250
200
150
b
100
50
c
0 1150
1100
1050
1000
950
900
850
800
-1
Raman Shift (cm ) Fig. 2. SERS spectra of thiophenol recorded through single-®bre SERS-sensors prepared by: (a) m-xylene/hydro¯uoric acid etching, (b) isooctane/hydro¯uoric acid etching, and (c) 30 mm grain size grinding with 608 inclination.
C. Viets, W. Hill / Journal of Molecular Structure 563±564 (2001) 163±166
(a)
(b)
165
(c)
Fig. 3. SEM images of silver coated ®bre tips etched with hydro¯uoric acid and overcoatings of iso-octane (a) or m-xylene (b and c), respectively. (c) The very tip of the ®bre shown in (b) with an additional 50 fold enlargement, and the tip surface is hereby observed under an angle of 458.
that showed smooth cone ¯anks but also astonishingly high SERS intensities (Fig. 2). It may be assumed that the smooth-looking cone surfaces exhibited atomic roughness features that could not be resolved in the SEM pictures (Fig. 3), but enhanced the electromagnetic ®elds at the surface. The ®bre tip etched with iso-octane/hydro¯uoric acid showed similar thiophenol SERS intensities as grinded, 608 angled ®bre tips (Fig. 2) which exhibited superior SERS enhancement in comparison with other recyclable tips [8]. The SERS intensities from the tip prepared with a mxylene overcoating were even three times higher (Fig. 2). All three types of tips can be recycled by dissolving and re-evaporating the silver. Different contributions may enhance the SERS intensities from conically etched tips: enlargement of the SERS-active surface, multiple internal re¯ections of the laser, or angle effects such as the excitation of delocalised plasmons [5]. The silver surface at a conically etched tip is clearly larger than that of a 608 inclined ®bre tip. However, it does not posses roughness features in the nm-scale which are wellknown to produce high SERS enhancement. Multiple re¯ections of the laser in the tip (Fig. 1) may lead to a summation of weakly enhanced Raman intensities from the smooth cone surface. Smaller cone angles enable larger numbers of internal re¯ections and therefore higher SERS enhancements. Fig. 3 shows that the tips have different cone angles: iso-octane preparation gave a cone of 258 and m-xylene prepara-
tion resulted in a 108 cone. According to geometrical calculations, laser light guided by the ®bre is approximately 7±8 times re¯ected in a 258 tip and 15±17 times in a 108 tip. This difference in the number of re¯ections accounts for about two times larger SERS intensities at 108 tips than at 258 tips. The measurement of three times enlarged SERS intensities for the 108 tip hints to further contributions to the SERS enhancement. For instance, the excitation of delocalised plasmons at a certain angle in total re¯ection geometry may be involved [9]. The magnitude of this contribution to the SERS enhancement could not be determined at the conical tips, but it is known to result in additional enhancement factors up to six at angled ®bre tips [5,8]. Further contributions to the SERS enhancement may arise from SERS enhancing structures at the cone tip. Fig. 3 shows that the structure of the very tip differs for the two etching methods: both endfaces have a diameter of approximately 15 mm, but the one prepared with iso-octane is more or less a ¯at plane while the one prepared with m-xylene is tilted and shows a shallow cavity. 4. Conclusions Hydro¯uoric acid etching offers a new possibility to produce recyclable ®bre-optic SERS sensors with considerable SERS enhancements. The enhancement
166
C. Viets, W. Hill / Journal of Molecular Structure 563±564 (2001) 163±166
is governed by the multiple laser light re¯ections in the conical ®bre tip and thus by the cone angle of the tips. This cone angle and thus the enhancement can be varied by using different organic overlayers in the etching process. Acknowledgements This investigations were supported by the Bundesministerium fuÈr Bildung und Forschung (BMBF), Germany (grant 13N6886). Special thanks are due to U. Barth who helped with the ®bre tip etching and to M. Kahl who recorded the SEM images.
References [1] J.M. Bello, V.A. Narayanan, D.L. Stokes, T. Vo-Dinh, Anal. Chem. 62 (1990) 2437. [2] K.I. Mullen, K.T. Carron, Anal. Chem. 63 (1991) 2196. [3] C. Viets, W. Hill, Sens. Actuators B 51 (1998) 92. [4] E. Polwart, R.L. Keir, C.M. Davidson, W.E. Smith, D.A. Sadler, Appl. Spectrosc. 54 (2000) 522. [5] C. Viets, W. Hill, J. Raman Spectrosc. 31 (2000) 625. [6] P. Hoffmann, B. Dutroit, R.-P. SalatheÂ, Ultramicroscopy 61 (1995) 165. [7] C.J. Sandroff, D.R.J. Herschbach, J. Phys. Chem. 86 (1982) 3277. [8] C. Viets, W. Hill, J. Mol. Struct., in press. [9] A.D. Boardman, in: A.D. Boardman (Ed.), Electromagnetic Surface Modes, Wiley, NewYork, 1982, p. 33.
www.elsevier.nl/locate/molstruc
Fibre-optic SERS sensors with conically etched tips C. Viets, W. Hill* Institut fuÈr Spektrochemie und Angewandte Spektroskopie (ISAS), P.O. Box 101352, 44013 Dortmund, Germany Received 31 August 2000; accepted 17 October 2000
Abstract Conical ®bre-optic SERS sensors are produced by etching with hydro¯uoric acid covered with an organic overlayer. These newly presented sensor tips show large SERS intensities and can easily be recycled. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Fibre-optic sensors; SERS; Raman spectroscopy
1. Introduction
2. Experimental
Fibre-optic sensors based on surface enhanced Raman scattering (SERS) can be used for the sensitive detection of submonomolecular layers remote from the spectrometer or at dif®cultly accessible locations [1±3]. In the so-called optode design, a single ®bre carries both, the laser radiation and the Raman scattered radiation which is back-scattered from the SERS-active surface directly attached to the ®bre tip. Such SERSactive ®bre tips can be prepared by vacuum deposition of metal ®lms over nanoparticles [3], slow evaporation of metal island-®lms [3], immobilisation of colloidal particles [4] and evaporation of metals on pre-roughened ®bre tips [3,5]. So far, the latter have been the only kind of recyclable ®bre tips.
2.1. Instrumentation
* Corresponding author. Tel.: 149-231-1392-217; fax: 149-2311392-120. E-mail address: [email protected] (W. Hill).
Raman and SERS measurements were carried out using a 0.5 m triple monochromator with subtractive dispersion of the two ®rst stages (DILOR XY) and a nitrogen-cooled CCD detector (Wright Instruments). The exciting laser light had a wavelength of 702 nm, was produced by a cw Ti:sapphire laser (Coherent 890, pumped by an Ar 1 laser (Spectra-Physics 2000)) and ®ltered by a monochromator (Spectrolab, Labspec III) to remove low-intensity sidebands and the broadband spontaneous emission. A confocal laser microscope (Olympus BX40 with objective Olympus, MPlan 10 £ /0.25) was used to focus laser light with a power of 1.5 mW on the uncoated end of the ®bre sensors and thus to couple it into the ®bre core. This way, the SERS-active tips were illuminated through the ®bres and the scattered light was detected through the microscope after passing the same ®bre. Scanning electron microscope (SEM) images were obtained by using a device by JEOL (type JSM6400) with an acceleration voltage of 25 kV.
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(00)00876-0
164
C. Viets, W. Hill / Journal of Molecular Structure 563±564 (2001) 163±166
Fig. 1. Structure and internal re¯ections for a conical, SERS-active ®bre tip.
2.2. Fibre sensor preparation Hard polymer-clad silica ®bres (SpecTran, HCPM0200T-06; ù 200 mm, N.A. 0.37) were cleaved into ®bre sections of about 8 cm. After the removal of the buffer, the tips of the ®bre sections were soaked in concentrated nitric acid for 3 h, so that 3 cm of the hard polymer-cladding could be removed mechanically. Employing a technique similar to the preparation of tips for the optical near-®eld microscopy [6],
the unclad ®bre tips were etched in 40% aqueous hydro¯uoric acid that was overcoated with an organic liquid. The used organic liquids were m-xylene with an etching time of 1 h and iso-octane with an etching time of 2 h. After etching, the ®bres were rinsed with deionised water. For reference, the endface of a ®bre section with cladding was grinded under an inclination angle of 608 with 30-mm grains polishing paper, rinsed with water and ultrasonically cleaned in acetone [5]. The ®bre tips were then evaporated with 75 nm silver using a conventional evaporation chamber (type BA710V, Balzers). The silver-coated ®bre tips were immersed in a 10 mM solution of thiophenol (.99%, Aldrich) in ethanol for 10 min and subsequently rinsed with ethanol. The reference chemical thiophenol forms chemically bonded, stable monolayers on silver surfaces [7] and exhibits a comparatively large Raman cross section. 3. Results and discussion Both etching methods gave conical ®bre tips (Fig. 1)
350
300
Intensity (arb. units)
a 250
200
150
b
100
50
c
0 1150
1100
1050
1000
950
900
850
800
-1
Raman Shift (cm ) Fig. 2. SERS spectra of thiophenol recorded through single-®bre SERS-sensors prepared by: (a) m-xylene/hydro¯uoric acid etching, (b) isooctane/hydro¯uoric acid etching, and (c) 30 mm grain size grinding with 608 inclination.
C. Viets, W. Hill / Journal of Molecular Structure 563±564 (2001) 163±166
(a)
(b)
165
(c)
Fig. 3. SEM images of silver coated ®bre tips etched with hydro¯uoric acid and overcoatings of iso-octane (a) or m-xylene (b and c), respectively. (c) The very tip of the ®bre shown in (b) with an additional 50 fold enlargement, and the tip surface is hereby observed under an angle of 458.
that showed smooth cone ¯anks but also astonishingly high SERS intensities (Fig. 2). It may be assumed that the smooth-looking cone surfaces exhibited atomic roughness features that could not be resolved in the SEM pictures (Fig. 3), but enhanced the electromagnetic ®elds at the surface. The ®bre tip etched with iso-octane/hydro¯uoric acid showed similar thiophenol SERS intensities as grinded, 608 angled ®bre tips (Fig. 2) which exhibited superior SERS enhancement in comparison with other recyclable tips [8]. The SERS intensities from the tip prepared with a mxylene overcoating were even three times higher (Fig. 2). All three types of tips can be recycled by dissolving and re-evaporating the silver. Different contributions may enhance the SERS intensities from conically etched tips: enlargement of the SERS-active surface, multiple internal re¯ections of the laser, or angle effects such as the excitation of delocalised plasmons [5]. The silver surface at a conically etched tip is clearly larger than that of a 608 inclined ®bre tip. However, it does not posses roughness features in the nm-scale which are wellknown to produce high SERS enhancement. Multiple re¯ections of the laser in the tip (Fig. 1) may lead to a summation of weakly enhanced Raman intensities from the smooth cone surface. Smaller cone angles enable larger numbers of internal re¯ections and therefore higher SERS enhancements. Fig. 3 shows that the tips have different cone angles: iso-octane preparation gave a cone of 258 and m-xylene prepara-
tion resulted in a 108 cone. According to geometrical calculations, laser light guided by the ®bre is approximately 7±8 times re¯ected in a 258 tip and 15±17 times in a 108 tip. This difference in the number of re¯ections accounts for about two times larger SERS intensities at 108 tips than at 258 tips. The measurement of three times enlarged SERS intensities for the 108 tip hints to further contributions to the SERS enhancement. For instance, the excitation of delocalised plasmons at a certain angle in total re¯ection geometry may be involved [9]. The magnitude of this contribution to the SERS enhancement could not be determined at the conical tips, but it is known to result in additional enhancement factors up to six at angled ®bre tips [5,8]. Further contributions to the SERS enhancement may arise from SERS enhancing structures at the cone tip. Fig. 3 shows that the structure of the very tip differs for the two etching methods: both endfaces have a diameter of approximately 15 mm, but the one prepared with iso-octane is more or less a ¯at plane while the one prepared with m-xylene is tilted and shows a shallow cavity. 4. Conclusions Hydro¯uoric acid etching offers a new possibility to produce recyclable ®bre-optic SERS sensors with considerable SERS enhancements. The enhancement
166
C. Viets, W. Hill / Journal of Molecular Structure 563±564 (2001) 163±166
is governed by the multiple laser light re¯ections in the conical ®bre tip and thus by the cone angle of the tips. This cone angle and thus the enhancement can be varied by using different organic overlayers in the etching process. Acknowledgements This investigations were supported by the Bundesministerium fuÈr Bildung und Forschung (BMBF), Germany (grant 13N6886). Special thanks are due to U. Barth who helped with the ®bre tip etching and to M. Kahl who recorded the SEM images.
References [1] J.M. Bello, V.A. Narayanan, D.L. Stokes, T. Vo-Dinh, Anal. Chem. 62 (1990) 2437. [2] K.I. Mullen, K.T. Carron, Anal. Chem. 63 (1991) 2196. [3] C. Viets, W. Hill, Sens. Actuators B 51 (1998) 92. [4] E. Polwart, R.L. Keir, C.M. Davidson, W.E. Smith, D.A. Sadler, Appl. Spectrosc. 54 (2000) 522. [5] C. Viets, W. Hill, J. Raman Spectrosc. 31 (2000) 625. [6] P. Hoffmann, B. Dutroit, R.-P. SalatheÂ, Ultramicroscopy 61 (1995) 165. [7] C.J. Sandroff, D.R.J. Herschbach, J. Phys. Chem. 86 (1982) 3277. [8] C. Viets, W. Hill, J. Mol. Struct., in press. [9] A.D. Boardman, in: A.D. Boardman (Ed.), Electromagnetic Surface Modes, Wiley, NewYork, 1982, p. 33.