Highly Sensitive Surface Plasmon Resonance-based

Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 168 (2016) 1249 – 1252

30th Eurosensors Conference, EUROSENSORS 2016

Highly sensitive surface plasmon resonance-based optical fiber multi-parameter sensor J.S. Velázquez-González1,2, D. Monzón-Hernández1,*, F. Martínez-Piñón2, and I. Hernández-Romano3 1 Centro de Investigaciones en Óptica A.C., Lomas del Bosque 115 C.P. 37150, León, Guanajuato. México. Instituto Politécnico Nacional, Centro de Investigación e Innovación Tecnológica (CIITEC), Cerrada Cecati S/N. Col. Santa Catarina, C.P. 02250, Azcapotzalco, Ciudad de México, México. 3 CONACyT-Electronic Department, Sede Palo Blanco, University of Guanajuato, Carr. Salamanca-Valle de Santiago Km 3.5+1.8, C.P. 36885, Salamanca, Guanajuato, México. 2

Abstract We proposed and demonstrated a compact, simple-to-construct and highly sensitive optical fiber sensor capable of simultaneously measuring refractive index (RI) and temperature (T) based on surface plasmon resonance (SPR) effect. The device structure consists of a single mode fiber (SMF) spliced between two multimode fibers where whole SMF was coated with chromium and gold. The thicknesses of the layers of chromium and the gold were 5 nm and 30 nm, respectively. By covering half of the sensing region of the device with a polymer with high thermo-optic coefficient (TOC), it is feasible to detect RI and T changes with sensitives of 2664.540 nm/RIU and -2.852 nm/ºC, respectively. © Published by Elsevier Ltd. This © 2016 2016The TheAuthors. Authors. Published by Elsevier Ltd.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. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Optical fiber multi-parameter sensor, Surface plasmon resonance (SPR), Polydimethylsiloxane (PDMS), Refractive index and temperature simultaneous measurement, Gold deposition.

* Corresponding author. Tel.: 52 477-441-4200; fax: 52 477-441-4209. 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.2016.11.437

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J.S. Velázquez-González et al. / Procedia Engineering 168 (2016) 1249 – 1252

1. Introduction In the last five years, fiber optic sensors capable of monitoring two variables, at the same time, have attracted a lot of attention. Some of these sensors have been implemented by adding an extra sensing part to the original singleparameter sensor making the device size bigger which is a disadvantage when they need to be placed in a small chip. For this purpose, Bragg gratings [1], Fabry-Perot interferometers [2] and tapered optical fibers [3] have been used. On the other hand, SPR sensors have shown high resolution and sensitivity for monitoring RI [4], T [5], molecules [6], and viruses [6]. Some of these devices have been implemented by using etched fiber, tapered fiber, D-polish fiber, Hetero-core fiber, Photonic Crystal Fiber (PCF), and Gratings [7-11]. Here, we report a compact, simple-to-fabricate and highly sensitive fiber optic sensor for simultaneous measurement of refractive index and temperature based on SPR. The sensing part consists of a SMF segment spliced between two MMF (MMF-SMF-MMF). The mismatch between the cores of the SMF-MMF generates cladding modes in the SMF segment where the evanescent field interacts with the surrounding media. Taking advantages of this effect, SPR can be excited by placing a gold thin film in the SMF section. Covering half of the length of the gold-coated SMF with PDMS polymer, the RI and T of a solution could be detected simultaneously. It is worth to notice, that by covering bare gold-coated SMF section with a solution, two SPRs were exited and used for detection. One resonance is related to the RI changes (different solution around bare gold-coated SMF section) and the second with the T variation (fluctuation of the RI of the polymer due to TOC). The RI and the T sensitivities of this device were 2664.540 nm/RIU and -2.852 nm/ºC, respectively.

2. Fabrication process and experimental results The structure of the fiber device that was used to excite the SPR is shown in Fig. 1. It was made of a conventional segment of cleaved SMF (core diameter of 9 µm) spliced between two MMF (core diameter of 62.5 µm), where the sensing region (SMF) had a length of 5 mm. By the evaporation method, a film of chromium and subsequently another of gold were deposited on the SMF section with thickness of 5nm and 30 nm, respectively. Half of the length of the SMF was embedded in PDMS layer, by heating the sensor during 6 hours at 80 °C using a Peltier hotplate the polymer was cure. This device was place in transmission experimental setup, see Fig. 2. Light from a white light source was couple to the MMF and travelled through the sensor, by monitoring the transmission spectrum of the device with an optical spectrum analyser (OSA) the RI and the T of solutions were determined.

Fig. 1. Schematic diagram of the optical fiber structure used in this work.

In the first experiment, the whole sensor was dipped in different solution (water/glycerol); the RI of the solutions were 1.346, 1.365, and 1.388 and the temperature was 20 °C. In the transmission spectra two dips appeared, one was related with the RI ሺߣ௕௔௥௘ ሻ and other was related with the T ሺߣ௉஽ெௌ ሻ of the solution, see Fig. 3 a). As the sensor was immersed in different solution, the position of the ߣ௕௔௥௘ was changing, as shown in Fig. 3 a). The sensitivity of the bare gold-coated SMF section to RI changes were 2464.549, see Fig. 3 b). It should be noticed that the position of the ߣ௉஽ெௌ did not move as the sensor was put in different solution, that means that such dip position did not depend on the RI at all. The second experiment consisted in heating the sensor in steps of 5 °C from 20 to 60 °C when it was set in a solution whose RI was 1.357. By changing the temperature of the solution and the polymer, both dips positions

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were affected as shown in Fig. 4 a). The temperature sensitivity of the first and second valleys at this RI, were -0.280 and -2.852 nm/ºC, respectively; see Fig. 3 b). The RI and T changes as a function of SPR dips shift can be mathematically expressed by Eq. (1).

Fig. 2. Experimental setup for simultaneous measurement of refractive index and temperature.

Fig. 3. Experimental: a) Transmission spectra of the dual-channel SPR fiber sensor at three different solutions at a temperature of 20 °C, b) Characterization curve of the dual-channel SPR fiber sensor response to refractive index changes.

Fig. 4. Experimental: a) Transmission spectra of the dual-channel SPR fiber sensor immersed in a solution of RI of 1.357 at four temperatures 25, 35, 45, 55°C, and b) Characterization curve of the dual-channel SPR fiber sensor response to temperature changes.

ଵ ȟܶ Ͳ ቂ ቃ ൌ ሺଶǤ଼ହଶሻሺଶସ଺ସǤହସ଴ሻ ቂ ȟ݊ ʹǤͺͷʹ

െʹͶ͸ͶǤͷͶͲ ȟߣ௕௔௥௘ ቃ൤ ൨ ȟߣ௉஽ெௌ െͲǤʹͺͲ

(1)

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4. Conclusions A compact, simple-to-construct and highly sensitive optical fiber surface plasmon resonance (SPR) multiparameter sensor for simultaneous measurement of RI and T of a solution is proposed and demonstrated. The fiber structure used in this work is a MMF-SMF-MMF with a SMF section of 5 mm, which is coated with a gold thin layer of 30 nm, and half of this region was covered with a polymer with high TOC (PDMS). When the sensor is surrounding by air, a single resonance dip appears around 900 nm (due to PDMS section) and then, when the sensor is immersed in a solution a second resonance dip appears (bare section), this dip depends on the RI of the solution. Changes in the RI only affect the dip of the bare section ሺߣ௕௔௥௘ ሻ, while T variation of the whole device affects both dips (ߣ௕௔௥௘ and ߣ௉஽ெௌ ). The sensor detection can be improved by increasing the separation between the ߣ௕௔௥௘ and ߣ௉஽ெௌ ; to accomplish that task, the SMF section could be coated with a proper metal or dielectric layer [12], so, the superior limit of the dynamic range would increase (in this work this limit was a RI of 1.388). Polymer can be also be doped with materials sensitive to one specific specie chemical or biological agent to determine its concentration as well as the RI of the solution. We strongly believe that this sensor could be easily integrated in a microfluidic chip for biological applications or in a sensing networks. Acknowledgements All the authors are grateful to the Consejo Nacional de Ciencia y Tecnología (CONACyT) of Mexico for financial support. Likewise, Dirección de Investigación, the mechanical and optical workshop from Centro de Investigaciones en Óptica A.C. for their support and assistance in the fabrication of some samples. Besides I. Hernández-Romano acknowledges the funding through the project: “Cátedras CONACyT 2015” and, J.S. Velázquez-González acknowledges to IPN for the BEIFI scholarship and CONACyT for their support through a Ph.D. scholarship.

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