Development of a Fiber-Optics Microspatially Offset Raman

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Development of a fibre optics microSORS sensor for probing layered materials Peter Vandenabeele, Claudia Conti, Anastasia Rousaki, Luc Moens, Marco Realini, and Pavel Matousek Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b01978 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

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Analytical Chemistry

Development of a fibre optics micro-SORS sensor for probing layered materials

Peter Vandenabeele*1, Claudia Conti2, Anastasia Rousaki3, Luc Moens3, Marco Realini2 , Pavel Matousek4

1. Ghent University, Department of Archaeology, Sint-Pietersnieuwstraat 35, B-9000 Ghent (Belgium). [email protected] 2. Consiglio Nazionale delle Ricerche, Istituto per la Conservazione e la Valorizzazione dei Beni Culturali (ICVBC), Via Cozzi 53, 20125, Milano, Italy. 3. Ghent University, Department of Analytical Chemistry, Krijgslaan 281 (S-12), B9000 Ghent (Belgium). 4. Central Laser Facility, Research Complex at Harwell, STFC Rutherford Appleton Laboratory, Harwell Oxford, OX11 0QX, United Kingdom. * Author for correspondence Content: Graphical abstract Abstract Key-words Introduction Experimental Results and discussion a. Assembly of the fibre optics micro-SORS sensor b. Testing and application of the fibre optics micro-SORS sensor c. Critical evaluation of the fibre optics micro-SORS sensor Conclusions Acknowledgements References Figure captions

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Graphical abstract


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Abstract

Micro-spatially-offset Raman spectroscopy (micro-SORS) has been proposed as a valuable approach to sample molecular information from layers that are covered by a turbid (non-transparent) layer. However, when large magnifications are involved the approach is not straightforward, as spatial constraints exist to position the laser beam and the objective lens with the external beam delivery or, with internal beam delivery, the maximum spatial offset achievable is restricted. To overcome these limitations, we propose here a prototype of a new micro-SORS sensor, which uses bare glass fibres to transfer the laser radiation to the sample, and to collect the Raman signal from a spatially offset zone to the Raman spectrometer. The concept also renders itself amenable to remote delivery and to the miniaturisation of the probe head which could be beneficial for special applications, e.g. where access to sample areas is restricted. The basic applicability of this approach was demonstrated by studying several layered structure systems. Apart from proving the feasibility of the technique, also practical aspects of the use of the prototype sensor are discussed.

Key-words: Raman spectroscopy, micro-SORS, Spatially Offset Raman Spectroscopy (SORS), Fibre optics



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Introduction

Raman spectroscopy is a well appreciated spectroscopic technique, that has shown to be a helpful diagnostic tool in many research fields, including among others geology1,2, microbiology3,4, medicine5,6, cultural heritage research7,8, forensics9,10 and polymer sciences11,12. The technique has, indeed, many favourable characteristics, such as its non-destructive nature, the possibility for in situ analysis13,14 and the relatively easy interpretation of the spectra by comparing the spectra against a reference database. Molecular spectra of inorganic as well as of organic materials can be identified and when using a microscope system, a small spectral footprint (down to ca. 1 µm) can be achieved.

Raman spectroscopy is often combined with complimentary techniques, such as X-ray fluorescence15,16: thus the molecular information can be combined with the elemental composition. However, when interpreting the results from multiple techniques, one should be aware of the different spatial resolution of the methods: different lateral as well as axial resolution should be taken into account when interpreting the results. For example, X-rays can penetrate relatively deeply into many materials, and if layered structures are present, it is often not straightforward to distinguish between the different layers by using standard laboratory spectrometers. Opposite to this Raman spectroscopy is often limited to the top layers if they are not transparent for the laser. However, a possible approach to obtain molecular information from deeper layers, covered by a turbid layer, is spatially offset Raman


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spectroscopy (SORS)17. This approach was for instance successfully demonstrated for analysing liquids in sealed non-transparent containers18, bone tissue inside a body19, or cancer research20.

In contrast to standard Raman spectroscopy in reflective (backscattering) mode (Fig. 1-a), in SORS experiments a certain zone of the turbid object is irradiated with a laser, while the Raman signal is collected from a slightly different, spatially offset, zone. As a consequence, the information that is obtained, originates from deeper layers, under a turbid (opaque) top layer. If small details or thin layers have to be studied, one has to rely on micro-SORS variant21,22. Two main micro-SORS approaches have been proposed: (i) defocusing micro-SORS and (ii) full microSORS. During defocusing micro-SORS the object is positioned out of focus over a defocusing distance ∆z, to enlarge the areas of laser illumination and Raman signal collection (Fig. 1-b). The advantage of this approach is its relatively simple setup: the spectra can be recorded by using a commercial Raman microscope. By using full micro-SORS (Fig. 1-c,d), the laser is focussed in a spot at a distance ∆x from the Raman collection zone on sample surface. The latter approach has more practical constraints, but it yields better contrast improvement between the different layers and larger penetration depths. Recently, a prototype of a full micro-SORS probehead has been developed and tested for art analysis23.

Despite the relatively straightforward nature of full micro-SORS there are some practical issues with its implementation, especially when high magnifications microscope objectives with a short working distance are required. As a consequence,



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even when using long working distance objectives, with externally delivered laser beam, there is insufficient space to focus the laser beam on a spot nearby the Raman collection spot (Fig. 1-d’). Alternatively, with internally delivered laser beam (Fig. 1-c), the spatial offset can also be set by internal imaging optics of microscope although a range of spatial offsets deployable in this arrangement is restricted by the field of view of the microscopes24,25.

To overcome these problems, we propose here a novel approach, which allows us to use a small laser spot and sample the Raman signal on relative short distances ∆x from the laser and with no restrictions on setting larger offsets. In this setup optical fibres are brought in contact with the surface, to transfer the laser radiation to the object and to collect the Raman signal from the object and transfer it to the spectrometer.

Experimental

Micro-SORS Raman spectra were obtained by using a Kaiser System Hololab 5000R modular Raman microspectrometer. A 785 nm diode laser (Toptica Photonics AG) was used as excitation source, and the excitation fibre (length = 0.5 m) was directly coupled to the laser (no laser line filter used before coupling). The laser power on the sample was reduced to ca. 20 mW. The collection fibre (length = 0.5 m) was coupled directly to the filter and spectrometer part of the Kaiser system, omitting the standard Raman microscope and probehead. Detection was performed by a back illuminated deep depletion Pelletier cooled (−70 °C) CCD detector (Andor). The


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Raman signal was collected in the spectral interval of 150 cm−1 to 3500 cm−1. Spectral acquisition time was varied, depending on the sample and spectral resolution was ca. 4 cm-1. For comparison, reference spectra were recorded on the same spectrometer, but by using the Leica microscope attachment, equipped with a 10x objective. Spectra were recorded using the Kaiser Holograms software, exported in the Galactic SPC format and processed in Matlab. All reported Raman spectra are not baseline corrected, nor corrected for blank or dark signal.

For these experiments, two 62.5 µm diameter multimode low-OH glassfibres (Fisfiber, Oriskany, NY, USA) were used and stripped to the bare fibre. These fibres were implemented in the experimental set up described below, and connected with an FC/PC coupler to the laser and spectrometer respectively. In the experimental set up, the fibres were mounted on Thorlabs micrometer stage system to allow for controlled movement of the fibres with respect to each other.

For these proof-of-concept experiments, we selected a series of samples which allowed evaluating the performance of fibre optics micro-SORS with different compounds and thickness of the top layer (ranging from 50 µm to 500 µm). A mockup sample was created, consisting of well-known, homogeneous materials that are decent Raman scatterers. For this reason, a sandwich was made, consisting of Teflon (thickness: 600 µm, on top of an aluminium plate), covered with one or two layers of non-transparent white PVC tape (thickness: ca. 150 µm). Both materials are commonly available in the local do-it-yourself store. Teflon is commonly used for plumbing, PVC tape as an electrical isolator. As second example an inorganic



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powder in a polystyrene container (thickness: ca. 500 µm) was selected. For this experiment, K2CO3 (99.99% pure, Sigma Aldrich) was used. The third sample consisted of an organic liquid (nitrobenzene, p.a., Sigma Aldrich) in a polystyrene vessel (thickness: ca. 500 µm). The fourth sample has been selected in order to demonstrate the system’s capability on detecting layers with a thickness even below 100 µm. It is an artificially prepared two-layer system simulating the application of a spray paint coating on stone: PY151 acrylic spray colorant was spread with a thickness of 50 µm on a marble stone (1 cm thick), consisting of calcite (CaCO3).

Results and discussion

a. Assembly of the fibre optics micro-SORS sensor

For these experiments, a prototype of a fibre optics micro-SORS sensor was designed (Fig. 2). To do so, a 62.5 µm (low OH) multimode glass fibre was cut in two with Kevlar scissors. The plastic sleeve (mantle as well as filling fibres) was removed over a distance of ca. 5 cm using a stripping tool. As we wanted to mount the fibres in a parallel way next to each other, and as it was necessary to control the distance between the fibres accurately and maintain their positioning stability, one of the fibres was taped on the edge of a translation stage, while the other one was mounted on the edge of a small breadboard that was level with the translation stage. Thus, both fibres were parallel with an adjustable inter-fibre distance. As bare fibres are very brittle and flexible the fibres were mounted on two needles with a diameter of 0.6 mm. Thus it was possible to make the set-up more resistant to mechanical damage


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and minimise the bending of the fibres. The fibres were connected to the laser and the spectrometer by using a TC/PC connector. The excitation fibre was stationary and the collection fibre was mounted on the travelling stage.

b. Testing and application of the fibre optics micro-SORS sensor

The prototype of the fibre optics micro-SORS sensor was tested by analysing a mock-up sample that consisted of white PVC tape mounted on a teflon carrier. Reference Raman spectra of the pure (bulk) materials were recorded by Raman microscopy (Fig. 3-a,c) and compared with fibre optics Raman spectra of these bulk materials (∆x = 0 µm, Fig. 3-b,d). It can be seen that the Raman spectra that were obtained with the prototype were highly similar, although in the case of teflon a different background signal is observed due to strong Raman signature of silica fibre acquired through the use of unfiltered fibres. The spectral background as recorded with the fibre optics Raman sensor was influenced by the contact of both fibres with the sample as it varied the amount of scattered radiation from the input into the collection fibre. In this context, a spectrum was recorded (Fig. 3-e) with the fibre optics sensor, without placing a sample. As such, a blank spectrum was recorded, revealing that the fibre signal can indeed interfere with the obtained Raman signal. In the current measurements, the influence was minimal, but in future applications, in order to improve the sensitivity of the approach, one can think on different strategies to reduce this influence, either by introducing micro-filters at the tips of the fibres and/or by mathematically correcting for these signals.



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Based on the spectra of the bulk samples (Fig. 3-a-d), the two most prominent Raman bands were selected for the micro-SORS experiments. It was checked that these bands did not overlap too much with Raman features from the other sources. The Raman bands positioned at 635 and 730 cm-1 were selected for PVC and Teflon, respectively. We constructed two test samples, where a single and a double layer of PVC tape was placed on top of the Teflon. The fibre optics micro-SORS sensor was used to record spectra with increasing distances between the excitation and collection fibre, ranging from 0 to 1500 µm, in stepsizes of 125 µm. The ratio of the net Raman intensities (linear baseline correction of the Raman band base, to reduce the influence of fluorescence) of the Teflon/PVC Raman bands was plotted as a function of the distance between the fibres (Fig. 4). The resulting graphs clearly show that the measured intensity ratio increases with increasing distance. This way, we demonstrate that the fibre optics micro-SORS sensor is able to achieve in a straightforward way information from a deeper layer, which is present under a turbid top layer. Moreover, when comparing both graphs, the influence of the thickness of the top layer can clearly be seen, as with thin layers the signal tends to increase at shorter inter-fibre distances. This might open possibilities to also estimate nondestructively layer thicknesses of turbid layers. The evolution of the intensity ratio as a function of the inter-fibre distance for the thin layer seems to be more subject to statistical variation.

In the second sample K2CO3 was analysed through the walls of a polystyrene (PS) container. This set-up was selected, as an example of the analysis of an inorganic salt, in powdery form. Spectra were recorded with increasing distances


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between the excitation and collection fibre (∆x = 200 µm steps were used) (Fig. 5). From these experiments it can be observed that, with increasing inter-fibre distance (∆x) the intensity of the aromatic ring breathing band of PS, located at 1001 cm-1, decreases, while the intensity of the ν(CO32-) stretching vibration (1060 cm-1) of the salt decreases less. This is even more evident, when plotting the band intensity ratios (Fig. 5-b), showing that also in this case it is possible to obtain information of deeper layers, overcoming the turbid barrier of 500 µm.

The feasibility of using fibre optics micro-SORS was also tested in the analysis of a liquid inside a turbid container. In this case, nitrobenzene (C6H5NO2) was measured through a polystyrene (PS) beaker. In Fig. 6-a the recorded spectra are presented. As the spectra are scaled towards the PS Raman band (1001 cm-1), it is clearly seen how the intensity of the 1342 cm-1 band of nitrobenzene increases with increasing distance between the fibres of the sensor. Thus, it is demonstrated that, with increasing inter-fibre distance, increasingly deeper areas are probed. When plotting the Raman band intensity ratios (Fig. 6-b), a similar effect is observed. A 3step pattern is observed when plotting the intensity ratios, which might suggest some chemical interaction between the nitrobenzene and the polystyrene container.

Finally, in order to evaluate the feasibility of fibre optics micro-SORS for analysing thin layers, a sample consisting of an acrylic coating on a calcite rock was studied. The homogeneous thin coating (50 µm) on top of this sample is clearly less thick than the turbid layers in previous samples (150, 300, 500 µm). Raman spectra were recorded with increasing distances between the fibres, between 0 and 140 µm



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(∆x = 20 µm steps were used) (Fig. 7-a). When normalising the spectra to the 1245 cm-1 Raman band of the coating, it is observed that the relative intensity of the ν(CO32-) stretching vibration (1086 cm-1) of calcite increases with increasing distance between the fibres – i.e. an increasingly larger part of the deeper layer is sampled. The same effect is observed when plotting the ratio as a function of the distance between the fibres (Fig. 7-b), demonstrating the micro-SORS capabilities of the fibre optics sensor.

c. Critical evaluation of the fibre optics micro-SORS sensor

In the previous paragraph we demonstrated the effectiveness of the prototype fibre optics sensor to perform micro-SORS experiments. In the current stage of development, we can also make a few remarks on the practical use of this concept.

Firstly, in the current set-up, the fibres that were used both have a diameter of 62.5 µm. As during the experiments both fibres are brought in contact with the sample, this fibre diameter can be considered as the equivalent of the laser spot size in conventional full micro-SORS experiments. However, where in conventional experiments the working distance of high magnification lenses is a limiting factor to the applicability, in principle it is possible to use glass fibres with smaller diameters. It is to be estimated that decreasing the fibre diameter will on the one hand increase the axial resolution that can be obtained with the micro-SORS experiments, but on the other hand, the signal intensity of the approach might be reduced. To achieve sufficient sensitivity, measuring times or laser power should be increased, but care


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has to be taken not to burn the sample, especially as diminishing the excitation fibre diameter inherently results in higher power densities.

Another issue that requires attention is related to the fact that, in contrast with full micro-SORS there is a physical contact between the sensor and the sample, as the fibres are touching it during the experiment. However, as the bare fibres are very thin and tend to bend easily, we estimate there is little chance that this touching physically alters the sample surface, even if brittle materials are involved. Positioning the sensor and assuring proper contact with the sample surface has to be assured and influences the repeatability of the experiments. Especially, as bare fibres are flexible, it is not always easy to maintain the proper distance between the fibres. Moreover, these tiny fibres are very fragile and care has to be taken not to break or damage them. As the fibres are in contact with the sample surface, there is a risk of contaminating the fibre surfaces.

An additional downside is the inability to set the exact zero spatial offset, i.e. the full overlap between illumination and Raman collection zones on sample the surface representing a conventional Raman measurement and typically used in scaled subtraction to retrieve pure signatures of sublayers. Instead approximation of this situation with fibres in contact with each other has to be used.

Finally, as was shown in figure 3-e, the fibres generate a signal in the final spectrum. In a Raman spectroscopy fibre optics probe head, this effect could be reduced by including appropriate micro-filters at the fibre tips; eg. laser line filter on



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the laser fibre tip and Raman edge filter on the collection fibre tip. When the laser light from the excitation fibre enters the probe head, it passes through a bandpass filter, to eliminate the signal that is generated in the excitation fibre. Likewise, before the light enters the collection fibre, a band blocking filter is used to eliminate the laser line. Any laser scattered light entering the collection fibre would generate interfering Raman and fluorescence induced by the collection fibre itself. In the fibre optics micro-SORS sensor, this interference could also be reduced by working with shorter fibres. Moreover, if the effect is too large, appropriate spectral corrections for the dark signal, can be implemented in the procedures.

On positive side the simple setup renders itself amenable to miniaturisation of the probe head and remote sensing that could be highly advantageous in situations where access to particular regions of sample is restricted.

Conclusions

In this work, we propose a prototype of an optical fibre-based sensor for full micro-SORS experiments. The advantage of this approach is that relative small laser spot sizes can be achieved in a relatively straightforward way, and that the excitation and collection zones can be very close to each other. Thus, it is possible to retrieve the molecular composition of the sublayer/content concealed by a turbid top layer/ container with thickness range from 50 µm to 500 µm by using full micro-SORS. The effectiveness of the approach demonstrated by analysing a sandwich structure of polymer layers and the practical aspects of using this approach are discussed.


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Acknowledgements

The authors are grateful to Ghent University for financially supporting their research through the concerted research actions programme (GOA-BOF).

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Figure captions

Figure 1

Comparison of (a.) the conventional Raman microscopy configuration (in reflection mode) with (b.) defocusing micro-SORS, (c.) internal beam delivery full micro-SORS and (d.) external beam delivery full micro-SORS. When the working distance is small, there are practical constraints to perform external beam delivery full micro-SORS experiments (d’.).

Figure 2

Experimental set-up of the fibre optics micro-SORS sensor. a.: schematic drawing of the different components; b.: picture of the sensor prototype that was used in these experiments.

Figure 3

a. micro-Raman spectrum of PVC tape; b. fibre optics Raman spectrum of PVC tape; c. micro-Raman spectrum of teflon; d. fibre optics Raman spectrum of teflon; e. blank fibre optics Raman spectrum. The Raman bands that are used for the fibre optics micro-SORS experiments are marked.

Figure 4

Fibre optics micro-SORS experiment: PVC tape on Teflon. Net Raman band intensity ratios are plotted as a function of the distance between the fibres. a: double layer of tape (300 µm); b: single layer of tape (150 µm). For each datapoint, a fibre optics Raman spectrum was recorded with 3 accumulations of 60s (785 nm, ca. 20mW at the sample).



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Figure 5

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Fibre optics micro-SORS experiment: K2CO3 Powder in a PS container. a. Detail of selected Raman spectra recorded with specific distances between the fibres (scaled to maximal intensity in this range and offset for clear visibility). The Raman bands that are used for the fibre optics microSORS experiments are marked; b. plot of the Raman band intensity ratios as a function of the distance between the fibres; For each datapoint a fibre optics Raman spectrum was recorded with 3 accumulations of 20s (785 nm, ca. 20mW at the sample).

Figure 6

Fibre optics micro-SORS experiment: Liquid in a polystyrene container. a. Raman spectra of nitrobenzene in a polystyrene container as recorded with specific distances between the fibres (scaled to maximal intensity in this range and offset for clear visibility). Raman bands used for the plot were marked; b. Plot of the net Raman band intensity ratios as a function of the distance between the fibres.

For each datapoint a fibre optics

Raman spectrum was recorded with 3 accumulations of 60s (785 nm, ca. 20mW at the sample).

Figure 7

Fibre-optics micro-SORS experiment: Coating on CaCO3 rock. a. Raman spectra as recorded with specific distances between the fibres. The Raman bands that are used for the fibre-optics SORS experiments are marked. For each datapoint a fibre-optics Raman spectrum was recorded with 9 accumulations of 20s (785 nm, ca. 20mW at the sample). The spectra are intensity-scaled to the 1245 cm-1 Raman band of the coating.


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The inset shows the relative intensity of the CaCO3-band at 1086 cm-1, as a function of the inter-fibre distance; b. Plot of the net Raman band intensity ratios as a function of the distance between the fibres.



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Comparison of (a.) the conventional Raman microscopy configuration (in reflection mode) with (b.) defocusing micro-SORS, (c.) internal beam delivery full micro-SORS and (d.) external beam delivery full micro-SORS. When the working distance is small, there are practical constraints to perform external beam delivery full micro-SORS experiments (d’.). 1016x571mm (96 x 96 DPI)

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Experimental set-up of the fibre optics micro-SORS sensor. a.: schematic drawing of the different components; b.: picture of the sensor prototype that was used in these experiments. 1016x571mm (96 x 96 DPI)



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a. micro-Raman spectrum of PVC tape; b. fibre optics Raman spectrum of PVC tape; c. micro-Raman spectrum of teflon; d. fibre optics Raman spectrum of teflon; e. blank fibre optics Raman spectrum. The Raman bands that are used for the fibre optics micro-SORS experiments are marked. 1016x571mm (96 x 96 DPI)


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Fibre optics micro-SORS experiment: PVC tape on Teflon. Net Raman band intensity ratios are plotted as a function of the distance between the fibres. a: double layer of tape (300 µm); b: single layer of tape (150 µm). For each datapoint, a fibre optics Raman spectrum was recorded with 3 accumulations of 60s (785 nm, ca. 20mW at the sample). 1016x571mm (96 x 96 DPI)



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Fibre optics micro-SORS experiment: K2CO3 Powder in a PS container. a. Detail of selected Raman spectra recorded with specific distances between the fibres (scaled to maximal intensity in this range and offset for clear visibility). The Raman bands that are used for the fibre optics micro-SORS experiments are marked; b. plot of the Raman band intensity ratios as a function of the distance between the fibres; For each datapoint a fibre optics Raman spectrum was recorded with 3 accumulations of 20s (785 nm, ca. 20mW at the sample). 1016x571mm (96 x 96 DPI)


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Fibre optics micro-SORS experiment: Liquid in a polystyrene container. a. Raman spectra of nitrobenzene in a polystyrene container as recorded with specific distances between the fibres (scaled to maximal intensity in this range and offset for clear visibility). Raman bands used for the plot were marked; b. Plot of the net Raman band intensity ratios as a function of the distance between the fibres. For each datapoint a fibre optics Raman spectrum was recorded with 3 accumulations of 60s (785 nm, ca. 20mW at the sample). 1016x571mm (96 x 96 DPI)



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Fibre-optics micro-SORS experiment: Coating on CaCO3 rock. a. Raman spectra as recorded with specific distances between the fibres. The Raman bands that are used for the fibre-optics SORS experiments are marked. For each datapoint a fibre-optics Raman spectrum was recorded with 9 accumulations of 20s (785 nm, ca. 20mW at the sample). The spectra are intensity-scaled to the 1245 cm 1 Raman band of the coating. The inset shows the relative intensity of the CaCO3-band at 1086 cm 1, as a function of the interfibre distance; b. Plot of the net Raman band intensity ratios as a function of the distance between the fibres. 1016x571mm (96 x 96 DPI)


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Graphical abstract 1016x571mm (96 x 96 DPI)


Development of a Fiber-Optics Microspatially Offset Raman

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