Low background SERS measurement using unfiltered optical fibre probes

Jiaying Ma¹ and Ying-Sing Li^2*
1Department of Chemistry
Purdue University
West Lafayette, IN 47907

^2*Chemistry Department
The University of Memphis
Memphis, TN 38152

* Author to whom correspondence should be addressed. Email: yingli [at] cc.memphis.edu

43 Abstract

44 Introduction

Utilising optical fibres in Raman spectroscopic sampling enables the technique to be used for remote sensing, in situ monitoring and process control, eliminating elaborated optical alignment [ref. 1-8]. Using optical fibres in Raman signal detection can also add new features to the instrument design. For instance, the use of collection fibre bundles can increase collection efficiency and extend the depth of field. The use of the light scrambling effect of that inevitably occurs along the fibres may improve the accuracy in polarization ratio measurements; whilst the application of imaging compression fibre bundles [ref. 9] can enable us to record Raman images without any scanning. However, laser light transmission through optical fibres generates strong Raman scattering and fluorescence emission within the fibres, inducing strong background interference to Raman measurements, especially in the low frequency region (below 1500 cm^-1). Optical filters have been incorporated to efficiently reduce this background interference [ref. 10-12]. However, using optical filters also has some limitations, such as the loss of optical power. For interference band pass filters with FWHM of 10 nm, typical laser transmission is 50 to 75%. If one wishes to record spectra at low shifts the filters have a narrower bandpass and have a typical transmission of only 30%. For some applications where a small size of the probe is required, the size of the filters can be a limitation. In this report, a new method of reducing the background interference is described based on the spatial distribution of the background. The background light generated inside the optical fibres (mostly, Raman scattering from the core materials) propagates efficiently through the optical fibres only if it satisfies the total internal reflection condition of the fibres, otherwise, high loss will prevent the light from reaching the terminal ends of the fibres. As a consequence, the majority of the background light will be restricted within the light cone defined by the numerical aperture (NA) of the fibres. If the sample surface is optically smooth and hence of only poor scattering efficiency eg samples deposited on SERS substrates and thin film specimens, the spatially dependence of the background will be preserved. Therefore, it is possible in these cases to have the collection fibre arranged so that the interference light from the excitation fibre will not be collected, and the background can be efficiently reduced.

Two probe configurations have been designed to take the advantage of this spatial distribution. In the first design, normal multimode fibres are used for both the excitation and collection fibres, and the filaments are spatially arranged to collect the minimum background interference. In the second probe design, the terminal ends of both the excitation and the collection fibres are tapered to a certain angle. This alters the angle of the core of light leaving the fibres in such a way that light from the excitation fibre does not satisfy total internal reflection within the collection fibre and hence, results in high attenuation. Probe designs and experimental results will be presented in this report; potential applications and limitations of the designs will also be addressed.

45 Experimental

The first probe design. Figure 1 shows the configuration of the probe. The laser light passing through the tunable filter was coupled to the excitation fibre via a 10 X microscope objective lens. At the other end of the excitation fibre, a graded refractive index (GRIN) lens (0.29 P, Melles Griot.) was used to focus the laser beam into a spot upon the sample coated on a glass plate. The collection fibre was set as close as possible to the sample spot but without any contact to the substrate. This arrangement allowed the collection optics on the spectrometer to receive the signals from the collection fibre and to focus them onto the entrance slit of the double monochromator. Silica-clad-silica-core optical fibre of a 100μm, -core-diameter (General Fibre Optics Co., N.A.=0.22) was used as the excitation fibre and a silica-clad-silica-core fibre with a core-diameter of 600μm (General Fibre Optics Co., N.A.=0.22) was used as the collection fibre.

Fig. 1.The first probe configuration based on spatial optimization.

The second probe design. Silica-clad-silica-core fibres of 600μm core diameter (3M, NA = 0.39) were used for both excitation and collection. Fig. 2 shows the general layout of the probe. Laser light after passing through a premonochromator (Spex 1405) was coupled into the excitation fibre via a 10 X microscopic objective lens. The angled ends of both the excitation and collection fibres were set parallel to each other (see the enlarged part of Fig. 2). The surface-enhanced Raman scattering (SERS) active substrate was positioned between the fibre ends and kept as close as possible to the fibre ends but without contact. Raman signals from the collection fibre were received by an elliptical mirror and transmitted to the spectrometer for subsequent spectral analysis. The objective lens and the excitation fibre were positioned using a Multimode Fibre Coupler (Newport Co.). The other end of the excitation fibre and both ends of the collection fibre were positioned using fibre optic manipulators (Newport Co.).

Fig. 2 The second fibre probe configuration with tapered fibres.

Chemicals and Substrates. Benzoic acid (BA), p-nitrophenol (P.NITRO PHENOL) and methyl red (MR) were obtained from Aldrich and were used as received. 95% ethanol was used as solvent for preparing the sample solutions. In each measurement, one drop (0.04 ml) of the sample solution was applied to the glass or alternative substrate plate. The procedures for preparing the SERS active substrate were those reported earlier[ref. 13].

46 Results and Discussion

The first probe design.

In Figure 3 (A, B, and C) we show the SER spectra of benzoic acid collected with the collection fibre set at 0°, 25° and 35°, respectively. The spectra showed that the spectral intensity of the acid did not vary much with the collection angle whereas the background intensity varied dramatically with the collection angle. This was especially so when the collection angle was 35° or larger, i.e. when the collection fibre was located outside the cone of the scattering laser beam whence the extent of the background interference appeared to be much reduced. The results demonstrated that the SERS signal was evenly distributed while the background was highly spatially dependent. In Figure 4 we see the SER spectrum of p.nitro phenol (1.0 x 10^-3 M) collected at 35° along with that recorded without using any optic fibre probe. The quality of the spectrum is certainly comparable to that obtained without the fibres, indicating that the fibre background interference can be significantly reduced by the use of an appropriate fibre probe configuration. We have also collected the surface-enhanced resonance Raman (SERR) spectra of 1.0 x 10^-6 M methyl red with the probe set at a range of angles. In a similar way to the results obtained for the SER spectra, the background is highly angular dependent however the SERR signal is more evenly distributed.

Fig. 3 SER spectra of benzoic acid (1.0×10^-3 M) collected using the first probe with a collection angle of A) 0°; B) 25°; C) 35°.

Fig. 4 SER spectrum of p.nitro phenol (1.0×10^-3 M):
A) collected with the first probe set at a collection angle of 35°;
B) collected without using any optic fibre probe.

47 The second probe design.

When the cross section of the optical fibre is not normal to the fibre axis, light emitted from the fibre will be altered to one side of the fibre as shown in Fig. 2. The emission angles θ₊ and θ_–) of the light are related to the refractive index of the core material (n₂), the refractive index of the cladding (n₁), and the tilt angle (α) of the cross section surface from the normal position to the fibre axis, by the following equations:

From equation (1), θ_-will decrease with increasing α. This means that the emission from the fibre is tilted more to one side than the other with increasing tilt angle α. On the other hand, the total light collection efficiency will decrease as the tilted angle increases as shown by equation (3). Thus, we have to find an optimized balance of these two factors. In the special case chossing sin α = NA/n, α becomes the orthogonal angle of the critical angle of total reflection within the fibre. From equation (1), one obtains sinθ_– = 0, which means that the light should propogate normal to the end surface of the fibre on one side of the fibre end section as shown in Fig. 2. The light emitted from the excitation fibre is thus supposed to contain both the excitation laser light and the background light. If the ends of both excitation and collection fibres are modified in the appropriate way and the two end surfaces are set to be parallel to one other (as shown in Fig. 2), light reaching the collection fibre from the excitation fibre will not satisfy the total reflection condition within the collection fibre and therefore will undergo massive attenuation throughout the length of the collection fibre. As a result, low background interference will be expected. Furthermore, the excitation area and the collection area will have the maximum overlap, and the best collection efficiency for Raman scattering will be obtained.

48 The NA of the collection fibre is determined by the f-number of the collection optic of the spectrometer (in this case the elliptical mirror) and has a value of 0.39. The core material of the fibres used in the present study is pure silica which has a refractive index n₂ = 1.46, and from the formula sin α = NA / n₂  α is calculated to be about 15°. This combined with equation (1) gives θ₊ = 41°. If the end of the fibre is normal to the fibre axis, the total divergent angle would be 46°. Thus, the efficiency of these fibres will be close to 90% of that ‘normal’ fibres.

Fig. 5 shows the SER spectra of p.nitrophenol collected at three different angles between the fibre axis and with the end surface angle set at α=15°. Bands marked with “S” are attributed to the silica of the fibre. 180¦ between the two fibres corresponds to a linear 0° configuration of collection. This arrangement would allow much of the light out of the excitation fibre to pass into the collection fibre and cause much of the laser light and the background from the excitation fibre to enter collection fibre. Thus, the background is extremely intense in this case as shown in Fig 5A. As the angle between the two optic fibres is decreased, less light from the excitation fibre enters the collection fibre resulting in lower background interference (see Fig. 5B. When the angle reaches 150°, the end surfaces of the two fibres are parallel. This fibre configuration should give the maximum collection efficiency. Fig.5C shows that the intensity ratio of the SER signal of p.nitro phenol and the silica Raman band reaches a maximum while the background is lower than those in previous cases. If the angle between the two fibres is further reduced, overlap between the excitation and the collection cones drops very quickly whence reduced Raman signals as well as lower backgrounds result. The background appearing in all the SER spectra can be attributed to the diffusion of the substrate and the residue of background. In this study, the background reaching the collection fibre from the excitation fibre would not completely lost because the collection fibre used in the experiment is short.

Fig. 5.SER spectra of p.nitro phenol (1.0×10-3 M) collected with the second probe at different angles between the excitation and collection fibres: A) 180°; B) 160°; C) 150°.

We have also collected a normal Raman spectrum of sodium sulfate film with the second probe. The result demonstrates that the probe design can also be used successfully for Raman measurements on thin solid films.

49 Conclusion

In this work, simple optical fibre probes of high collection efficiency and low background interference have been successfully designed and tested exploiting the difference in the spatial distribution of the silica fibre background and the Raman scattering from the samples. The technique provides a useful alternative to optical filter based fibre technologies. The probes we have devised should be useful in detecting Raman spectra of sample films, solid powders, or small amounts of solution samples. They seem to be especially useful for SERS measurements. The operations required to fabricate the probes are simple.

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Received 13th January 1998, received in revised format 2nd March 1998, accepted 3rd March 1998.

REF: Int. J. Vib. Spect., [www.irdg.org/ijvs] 2, 1, 42-49 (1998)