Suspended-core optical fibres for organic dye absorption spectroscopy

Photonic sensor of liquids based on suspended-core fibres Barbara Wajnchold*a, Michał Grabkaa, Szymon Pustelnya, Wojciech Gawlika, Paweł Mergob

aMarian Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Cracow, Poland;

bFaculty of Chemistry, Marie Curie-Skłodowska University, pl. M. Curie-Skłodowskiej 3, 20-031 Lublin, Poland

ABSTRACT

The small size of the core (about 1 μm) of the suspended-core optical fibres gives rise to evanescent wave in the surrounding channels. That process allows efficient coupling between light and liquid introduced into the channels and application of the fibre for analytic purposes. In the presented work, the channels of a suspended-core fibre were filled with water and aqueous solutions of oxazine 725 perchlorate and their absorption spectra were measured. While the spectra of the water-filled fibre were consistent with the Lambert-Beer law, the absorptivity of the fibre filled with oxazine 725 perchlorate solution demonstrated an increased sensitivity caused by aggregation of the oxazine molecules, independent on the fibre length.

Keywords: suspended-core fibre, chemical sensing

1. INTRODUCTION A photonic-crystal fibre (PCF) is a special microstructure optical waveguide made by stacking capillaries and rods. Such fibres have unique optical properties important for various applications. Particularly, the fibres with suspended core offer a possibility to use evanescent waves for coupling light to matter in a controlled way [1, 2], to observe surface plasmon effects [3], perform absorption and/or fluorescence measurements [4], and produce such nonlinear effects as supercontinuum generation [5]. The suspended-core fibres possess large channels which can be filled with gas or liquid. This offers a possibility of exerting extra control over the light propagation which is desired from a point of spectroscopy and sensing applications.

Many groups presented various approaches to filling microstructured fibres with foreign media. Some of them achieved a lateral access to the channels by using a laser [6], an ion beam [7], a fusion splicer [8] or by drilling [9]. All these methods, however, are challenging and require special equipment. The simplest method of filling the fibres is to dip a fibre tip in a medium and use either capillary forces for liquid filling or diffusion for gas filling. Despite its simplicity, this method allows also a selective fibre filling [10].

The suspended-core PCFs were used for measuring numerous physical quantities. Important examples include studies of absorption spectra of acetylene [11] and aqueous nickel (II) chloride solution [12]. Also fluorescence of organic dyes [13], semiconductor quantum dots [14], and fluorophore labelled antibodies [15] were investigated with the suspended-core fibres.

*[email protected], www.if.uj.edu.pl/pl/ZF/

Microstructured and Specialty Optical Fibres, edited by Kyriacos Kalli, Alexis Mendez, Proc. of SPIE Vol. 8426, 84261M · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.924061

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The present work aims at studying the possibility of using suspended-core fibres for detection of micromolar amounts of a given chemical compound in solutions. The channels of a suspended-core fibre were filled with water and the aqueous solution of oxazine 725 perchlorate [3,7-bis(diethylamino)phenoxazin-5-ium]. The absorption spectra of oxazine solutions were analyzed versus various parameters.

The choice of the oxazine as a test compound was dictated by its interesting molecular electronic structure allowing the charge transfer effect. Oxazine 725 perchlorate is a very popular organic dye. It was used in dye lasers, in particular in the distributed feedback laser [16], as an electron acceptor in single carbon nanotube electron storage [17], as a light harvesting complex in simulations of photochemical reactions [18] and for optical investigation of molecular sieves [19] and biochemical processes [20].

2. PROPERTIES OF SUSPENDED-CORE FIBRES In our work the suspended-core fibres with the solid core and three channels aligned along the fibre axis were applied. Figure 1 presents three of the different structures fabricated in the Laboratory of Optical Fibre Technology at the Maria Curie-Skłodowska University in Lublin, Poland. The structures differ in core diameters and hole sizes.

Figure 1. Scanning-electron-microscope (SEM) images of the suspended-core fibres with core diameters of 1.3 μm, 1.2 μm,

0.8 μm and hole sizes of 14.7 μm x 10.9 μm, 34.9 μm x 22.5 μm, 13.0 μm x 8.9 μm respectively (from left to right).

In suspended-core fibres, light is guided due to different refractive indices of the core and the cladding based on the modified total internal reflection effect. It is important to note that the light is not present only in the solid core but also exists in the surrounding channels as an evanescent wave. Our former theoretical studies [1, 2] reveal that the amplitude of an evanescent field is related to the core diameter, the light wavelength of light, and the refractive indices of the fibre core and the medium introduced into the channels. The fraction of the evanescent wave increases with decreasing core diameter and ration of the glass and medium refractive indices, and with increasing wavelength of the light.

The three suspended-core structures filled with air and water were modelled using the finite element method (FEM) in COMSOL Multiphysics. The modelling yielded transverse mode profiles which were compared with the beam profiles measured with a camera beam profiler. A good agreement was obtained between the modelled and measured profiles (Fig. 2).

Next, the percentage of the light in the holes was calculated as a function of the light wavelength. The percentage is defined as the ratio Φ of the light intensity in the holes to the total intensity (in the core and holes). The results depicted in Fig. 3 demonstrate that the percentage of light in the holes filled with water varies from 1.5% up to 32% for different fibre structures and different wavelengths. That high percentage indicates that the coupling of the core-guided light to the liquid in the holes is possible.



Figure 2. SEM image of the suspended-core fibre (a), the FEM model of one of modes (b), and the camera beam profiler

picture (c) [1]. Arrows show the electrical field direction and colours depict the intensity of the mode, the highest intensity is red, the lowest intensity is blue.

Figure 3. Percentage of the power of light in the holes according to FEM model for three different core diameters (1.34 μm,

1.22 μm, 0.82 μm) and holes filled with water.

3. EVANESCENT-WAVE SENSING USING SUSPENDED-CORE FIBRES 3.1 Fibre filling and absorption measurements.

As mentioned above, the large dimensions of the channels in the suspended-core fibres allow liquid filling by a simple dipping method. Typically, the pieces of fibres used in the experiment were about 20 cm long. That length is sufficient to suppress the cladding modes by, for example bending a fibre, yet short enough to fill the whole fibre by means of capillary forces only. All fibres were inspected under a microscope after every filling and checked if fibres were fully filled by a given liquid (Fig. 4).

Figure 4. Microscope side view of the fibre partly filled with a water solution of Oxazine 725 Perchlorate (magnification

100x).

(c)(b) (a)



Our approach to the fibre chemical sensing is based on easily applicable and inexpensive setup (Fig. 5). It consists of a halogen lamp with a collimator, two 60x microscope objectives for coupling light into the suspended-core fibre and a fibre spectrometer.

The absorption measurements were performed in the same sequence for all samples. At the beginning, the empty fibre was fixed in the collimators and the setup was aligned and the transmission spectrum I0(λ) was recorded. Next, one tip of the fibre was dipped in a given solution for about 15 minutes. Such filled fibre was fixed in the coupler holders, the setup was realigned and the transmission spectrum I(λ) of the filled fibre was recorded. The two spectra were converted into normalized absorption coefficient A according to the Lambert-Beer law:

( )( ) ( ) ( ),)()(

ln1/1 0 λλαλλ

λ Φ=⎟⎟⎠

⎞⎜⎜⎝

⎛= mc

II

LcmA (1)

where L is the fibre length, α the absorption coefficient, and cm the molar concentration of the measured medium. Since the light beam was much wider than the fibre core, its manipulation was easy and the results were reproducible.

According to the mechanism of an evanescent-wave sensing in suspended-core fibres, described in Sec. 2, it is possible to predict that in the fibres the normalized absorbance would be lowered by factor Φ relative to the standard cuvette spectra. This reduction is due to the fact that only a part of light propagating in the fibre interacts with the medium filling the channels.

Figure 5. Setup used for measuring the absorption of the liquid-filled suspended-core fibres.

3.2 Suspended-core fibres filled with water.

To reach the best performance, the suspended-core fibre with the smallest available core, 0.8 μm, was chosen (see Fig. 3). The fibre was filled with water by the above described dipping method and the absorption spectrum was measured. The recorded spectrum was compared with the water spectrum in a standard reference cuvette. The graph in Fig. 6 shows the trend discussed in Sec. 3.1, i.e. the reduction of the amplitude of the normalized spectrum of the water in the fibre relative to that in a cuvette. In that case the advantage of performing absorption measurements with the fibre



rather than with a cuvette is just the elongation of the light-water interaction path. While generally cuvettes are few centimetres long, the fibres could be much longer which significantly increases the sensitivity. Furthermore, they allow measurements with microliter volumes of liquids.

Figure 6. Normalized absorption spectra of water in a 1 cm cuvette and in a 20 cm long suspended-core fibre.

3.3 Suspended-core fibres filled with the oxazine 725 perchlorate solution.

As the second step of our study of sensors based on suspended-core fibres, we investigated a possibility of increasing the sensor sensitivity by kind of functionalization of its internal capillary surfaces. In this work water solutions of oxazine 725 perchlorate [3,7-bis(diethylamino)phenoxazin-5-ium perchlorate], (Fig. 7) was used. That organic compound has an ionic structure so it dissociates in water into ions. Thus, after filling the fibre, the fluorophore protons accumulate near the negatively charged capillary walls. This effect increases the absorption and improves the detection sensitivity. The scheme of the cross section of the fibre and the idea of an accumulation process of oxazine ions is depicted in Fig. 8.

Figure 7. Chemical formula of oxazine 725 perchlorate and the sample absorption spectrum of the oxazine 725 perchlorate

aqueous solution in the cuvette.



Figure 8. Scheme of an accumulation of oxazine 725 perchlorate ions onto the walls of the channels of the suspended-core

fibres

We recorded absorption spectra of water solutions of oxazine 725 perchlorate introduced into fibres at different concentrations ranging from 0,05 μM to 0,6 μM. A sample spectrum of a 0.2 μM solution is depicted in Fig. 9. The spectrum shows an intense band at 662 nm which is neighboured by a shoulder at 612 nm. It is well visible that the spectrum in the fibre has significantly higher amplitude than the one in the cuvette. The other properties of those spectra are collected and compared in Table 1. It could be noticed that in the table the absorbance in a cuvette, in contrast to the absorption signal in the 23 cm long fibre, is normalized to the optical path length. However, in the case of a fibre filled with oxazine the absorption originates from a short (less than 1 cm) end-section of the fibre (as described below), therefore we decided that we could compare both quantities.

Figure 9. Absorption spectra of oxazine 725 perchlorate aqueous solution at 0.2 μM concentration in a 1-cm cuvette and in

a 23 cm long suspended-core fibre.



Table 1. Comparison of the spectral data for solutions in the cuvette and in the suspended-core fibre.

The first band position Absorbance The second band

position Absorbance

Suspended-core fibre 662 nm 7,308 a.u. 612 nm 5,70 a.u.

Cuvette 654 nm 0,250 1/cm 612 nm 0,140 1/cm

In addition to the observation of the increased oxazine absorption, our further studies revealed that the absorption took place just at the very beginning of the dipped fibre end. This hypothesis was supported by cutting off a 1 cm long piece of the dipped end of the fibre. In the case only the water signal was recorded in the fibre. The location of the oxazine ions only very close to the dipped fibre end confirms very efficient surface interaction between ions and the glass.

Figure 10. Absorption spectra of oxazine 725 perchlorate aqueous solution at 0,2-μM concentration in the suspended-core

fibre before and after cutting off the fibre end.

4. CONCLUSIONS In the presented work we investigated the perspectives of designing a photonic sensor based on the evanescent-wave sensing in the suspended-core fibres. The model of such fibres was developed and the mode analysis was performed using the finite element method. These fibres were filled with water and used for measurements of water absorption spectra. The measurements confirmed the well-known reduction of absorptivity caused by the reduced penetration of the fibre channels by an evanescent wave. We also attempted to increase the sensitivity of the suspended-core fibre sensor by functionalization of the capillary glass surface. The series of measurements of absorption spectra of water solutions of oxazine 725 perchlorate inside the suspended-core fibre was made and revealed that fluorophore ions aggregate in a short distance close to the dipped end of the fibre. In comparison to standard cuvettes, increased sensitivity of absorption detection is observed. That effect is independent of a fibre length.



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Suspended-core optical fibres for organic dye absorption spectroscopy

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