Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 168 (2016) 810 – 813
30th Eurosensors Conference, EUROSENSORS 2016
Optimization of an evanescent field sensor based on D-shaped plastic optical fiber for chemical and biochemical sensing F. Sequeiraa,b*, L. Bilroa,c, A. Rudnitskayab,d, M. Pesaventoe, L. Zenif, N. Cennamof a. Instituto de Telecomunicações, Aveiro, Portugal b. CESAM, University of Aveiro, Aveiro, Portugal c. I3N/FSCOSD, Department of Physics, University of Aveiro, Aveiro, Portugal d. Department of Chemistry, University of Aveiro, Aveiro, Portugal e. Department of Chemistry, University of Pavia, Pavia, Italy f. Department of Industrial and Information Engineering, Second University of Naples, Aversa, Italy
Abstract The experimental investigations of the performance of a D-shaped Plastic Optical Fiber (POF) sensor are presented. The phenomenon behind this optical fiber sensor based platform is the variation of the transmitted light at the output of the POF with the external refractive index. The resolution of the optical sensing platform is strongly dependent on the length of the sensing region, decreasing from 10-1 to 10-3 RIU with length varying from 1 cm to 5 cm. The sensor production procedure is very easy, fast and low-cost. The obtained resolution allows further development for chemical and biochemical sensing by chemical receptor, e.g. molecularly imprinted polymer. A good resolution was obtained using sensing area length of 5 cm. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Plastic optical fibers; optical sensors; refractive index sensors; remote sensing; chemical and biochemical sensing
1. Introduction Numerous studies related to refractive index (RI) plastic optical fiber (POF) sensors can be found in the literature. Bilro et al. reported theoretical modelling of D-shaped POFs at different macrobending conditions and external RI, which was validated by experimental results [1]. Feng et al. reported that the best performance of a POF based RI sensor with a tapered structure was achieved at 633 nm, for RI ranging from 1.33 to 1.41 [2]. An optimization of * Corresponding author. Tel.: +351 234 377 900; fax: +351 234 377 900. E-mail address:
[email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.279
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depth and curvature radius of a D-shaped POF sensor, aiming to increase linearity range and sensitivity to RI, was reported by Feng et al. with the best results obtained for a depth of 500 µm and a curvature radius of 5 cm [3]. Recently, a new LSPR based U-bent POF sensor platform has been presented by Gowri and Sai, with an attained resolution of 8.5x10-5 RIU [4]. Cennamo et al. have reported several chemical sensors based on plasmonic phenomena in a D-shaped POF [5-7], with length of sensing region of 1 cm and typical resolution of about 10-4 RIU. The novelty of this work is the optimization of the sensing region length in a D-shaped POF sensor platform that would allow an easy and low cost implementation, features that will push further development for chemical and biochemical sensing applications. This simple approach would replace or be complementary to the plasmonic sensing phenomena in optical fiber chemical sensors and biosensors [8,9]. 2. Materials and methods 2.1. D-shaped POF sensors POFs with 1 mm of diameter from Asahi Kasei, DB-1000, with polymethyl methacrylate (PMMA) core of 980 µm and fluorinated polymer cladding of 20 μm, were used. Three fibers were cut to about 21 cm, by a POF cutter and embedded in grooves on planar supports with different lengths (1, 3 and 5 cm. The polishing process was carried out using a 5 µm polishing paper, by strokes with an “8-shaped” pattern, in order to remove the cladding and part of the core. After this step, the polishing process was completed by 3 µm and 1 µm polishing papers. Sensors with a sensing region length of 1, 3 and 5 cm were manufactured (see Fig. 1a). Response of the fibers was studied by placing water-glycerin solutions with different refractive indices on the sensing area. Refractive index of the solutions of glycerin in water was measured by an Abbe refractometer (Model RMI, from Exacta and Optech Labcenter). The solutions were prepared using Milli-Q water and glycerin was purchased from Carlo Erba Reagenti. 2.2. Experimental setup The intensity-based detection scheme allowed the measurement of the transmitted light that passed through the fiber with the D-shaped region. The experimental setup (Fig. 1b) comprised a stabilized power supply, LED (wavelength centered at 650 nm), optical coupler (50:50), two photodetectors and a Picoscope. Output data, time, reference and sensor signals outputs, in mV (Vreference and Vsensor, respectively), were logged into a PC by means of Picoscope’s software. The self-referenced transmitted signal (k) was used to correct source fluctuations and variations due to external conditions, as shown in equation 1.
k
Vsensor Vreference
(1)
(a)
(b)
Fig. 1. (a) D-shaped POF sensors, with 1 cm, 3 cm and 5 cm; (b) Outline of optical sensing setup.
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All the D-shaped POF sensors were tested using solutions with increasing refractive index varying from 1.332 to 1.385. Each test was started by measuring sensor signal in water, which was further used as a reference for normalization: I
k s olution
(2)
k water
2.3. D-shaped sensor parameters After placing the test solution on the D-shaped sensor, signals were recorded for 5 minutes, and the average of the normalized signal (I) with respective standard deviation (δI) was calculated with MATLAB software. Between measurements sensor surface was washed repeatedly with the next test solutions in order to clean the surface and eliminate any residues of the previous solution. The obtained normalized smooth signal (I) is a function of the refractive index of the tested solutions (ns). If the external refractive index is altered by Δns, there is a change in the obtained transmitted signal of ΔI. The sensitivity (S) of the D-shaped sensors is defined by:
S
'I 'ns
(3)
The resolution (Δn) is defined as the minimum change in refractive index that can be detected and defined as:
'n
1 GI max >RIU @ S
(4)
where δImax is the maximum value of standard deviation. 3. Results and discussion Figure 2 shows the normalized smooth signal (I) versus the refractive index (ns), for three different lengths of the sensing region (1, 3 and 5 cm). The sensitivity and resolution of the sensors, as defined in equations 3 and 4, are strongly dependent on the sensing length. The parameters for the three sensors tested are shown in Table 1.
Fig. 2. Response of the D-shaped POF sensors to the refractive index.
F. Sequeira et al. / Procedia Engineering 168 (2016) 810 – 813
The highest sensitivity was obtained for the sensor platform with D-shaped length of 5 cm. Although sensitivity of this sensor is still low, the resolution of about 10-3 RIU allows further developments of these D-shaped sensors for chemical and biochemical applications. Table 1. Length, sensitivity and resolution of the D-shaped sensors. Sensing length (cm)
Sensitivity ( au.RIU-1 )
Resolution (RIU)
1
0.1
1 x 10-1
3
1.44
1 x 10-2
5
3.828
5 x 10-3
4. Conclusions The developed D-shaped sensor is easy to produce, by a very fast and low-cost procedure. In this platform the length of the sensing region is a very important parameter for the optimization of sensitivity and resolution with external refractive index variation. The highest sensitivity (and lowest resolution around 10-3 RIU) was obtained for the sensing length of 5 cm. These results enable the application of this optical platform for chemical and biochemical evanescent field sensing. Acknowledgements This work is funded by FCT/MEC through national funds and when applicable co-funded by FEDER – PT2020 partnership agreement under the projects UID/EEA/50008/2013 (project sWAT) and hiPOF (PTDC/EEITEL/7134/2014), PhD fellowship (Filipa Sequeira: SFRH/BD/88899/2012) and investigator grant (Lúcia Bilro: IF/01664/2014; project INITIATE). Alisa Rudnitskaya wishes to acknowledge financial support from CESAM (UID/AMB/50017), FCT/MEC through national funds and the co-funding by the FEDER, within the PT2020 Partnership Agreement and Compete 2020 and through fellowship SFRH/BPD/104265/2014. References [1] [2] [3] [4] [5] [6] [7]
[8] [9]
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