Photochemical Properties of Porphyrin Films

Photochemical Properties of Porphyrin Films Covering Surfaces of Tapered Optical Fibers A. Veselov1, C. Thür2, V. Chukharev1, M. Guina2, H. Lemmetyinen1, N. Tkachenko1 1

Tampere University of Technology, Department of Chemistry and Bioengineering, Tampere, 33720, Finland; phone: +358 40 739 0760; fax: +358 3 3115 2108 2 Tampere University of Technology, Optoelectronics Research Centre, Tampere, 33720, Finland; phone: +358 3 3115 3400; fax: +358 3 3115 3400 e-mail: [email protected] ABSTRACT We report the fabrication and characterization of tapered fibers covered with porphyrin monolayer films prepared by Langmuir-Blodgett (LB) deposition method. The studied molecule was 10 mol-% 5,10,15,20tetrakis(pentafluorophenyl)porphyrin (PFP) entrapped in an octadecylamine (ODA) matrix. PFP molecules, deposited on plane glass surfaces, have relatively long fluorescence lifetime (~ 4 ns) together with high fluorescence efficiency. Therefore, such photoactive materials, example of which are PFP molecules, hold much promise for the development of chemical sensors and efficient light harvesting devices.

Keywords: tapered fiber, chemical sensor, evanescent waves, photochemistry, Langmuir-Blodgett method

INTRODUCTION For light propagation in an optical fiber, the power fraction in the evanescent field is significantly expanded when the fiber is tapered to a diameter of a few microns. Therefore, the use of fiber tapers as substrates for organic films enables efficient interaction between the evanescent field and organic molecules thus giving more advantages over traditional spectroscopy methods. 5,10,15,20-Tetrakis(pentafluorophenyl)porphyrin (PFP) is one of the attractive candidate as a sensory molecule due to its high fluorescence efficiency [1, 2] and long fluorescence lifetime, up to few nanoseconds even at concentrations of 10 mol-% and higher [3]. In this research, Langmuir-Blodgett (LB) method has been used as the deposition technique of PFP molecules; entrapping matrix molecules used were octadecylamine (ODA). One of the objective of this study was to investigate organization of the PFP films on curved surfaces of optical fibers and PFP interaction with the evanescent electromagnetic field at the fiber surface, which is essential for the development of chemically assisted fiber-optic sensors [4, 5]. Another aim of this work was to study the geometry of the fiber tapers and the deposition of the films along tapered fiber. As it is widely known, for sensor applications the cladding layer has to be made significantly thinner since the light does not penetrate deep into it. The common approaches to make the cladding layer thinner are to use chemical etching or to apply fiber tapering procedure that is described in work [6]. When the optical fiber is tapered, the core/cladding interface is redefined in such a way that the light propagation inside the core of the waist of the taper spreads to the cladding; this fact enhances the interaction of the evanescent field with the outer medium so that small changes of the outer medium can modulate the transmission properties of the taper. The typical sensor application of tapered fiber is to cover the coned part with sensing molecules and monitor the change in the layer absorption or emission by collecting the signal light delivered through the fiber core [7]. The 20th International Conference on Optical Fibre Sensors, edited by Julian Jones, Brian Culshaw, Wolfgang Ecke, José Miguel López-Higuera, Reinhardt Willsch, Proc. of SPIE Vol. 7503, 75031K © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.835098 Proc. of SPIE Vol. 7503 75031K-1

efficiency of such a sensor depends on the evanescent field which is the optical field extending outside the fiber interface as described elsewhere [8]. When the fiber has been sufficiently tapered, i.e. to tens of micrometers and below, the evanescent field extends to the fiber surface and interacts with the molecules deposited on it. This interaction can be observed as gradual attenuation of the light propagating through the tapered waveguide. The PFP layer deposited on a fiber prepared for sensor applications can be used as an indicator of the evanescent field, thus enabling optimization of the fiber taper profile. This, however, requires knowledge of the PFP fluorescence properties when deposited on the fiber surface. To the best of our knowledge, there are no reports so far on the optical properties of PFP layers deposited on tapered fibers.

RESULTS AND DISCUSSION Absorption and emission spectra of PFP films covering tapered optical fibers, having tips diameter 10, 15 and 20 μm, conical and adiabatic parts of 20 mm and 15 mm, respectively, were measured using the setup shown in Fig. 1 [9]. As one can see from Fig. 2 [9], absorbance of PFP films on tapered optical fiber is at least 80 times higher than that for PFP films covering glass plate. The results of absorption measurements are brought together in Table 1. Emission spectra of tapered fibers and reference glass plates covered with PFP films are shown in Fig. 3, where fluorescence spectra of fiber with tips diameter of 20 μm was chosen as typical example. It is clear from Fig. 3 that PFP entrapped in ODA matrix shows different aggregation properties depending on whether it has been deposited on a glass plane or on an optical waveguide with a curved surface.

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Variations of peak intensity ratios in emission spectra obtained at different points along tapered fiber, covered with PFP film, were observed. A higher ratio value (peak at 650 nm to peak at 705 nm) corresponds to a uniform part of the fiber, while a lower ratio value corresponds to a conical region of the fiber where waveguide diameter induces significantly along its axis. Table 1. Absorbance of different tapered optical fibers (tapers) and reference glass sample covered with 10% PFP in ODA matrix monolayer

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0.588

506

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426

0.018

504

0.003

537

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578

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425

0.397

504

0.193

538

0.077

580

0.078

644

0.029

Taper d = 15 µm

426

1.224

504

0.729

537

0.316

578

0.389

646

0.146

Taper d = 10 µm

424

1.549

505

1.264

538

0.758

578

0.833

644

0.398

chloroform

Proc. of SPIE Vol. 7503 75031K-2

Emission spectra studies of tapered fibers, regarding geometry optimization of conical part, covered with PFP films were carried out using the same scheme that is shown in Fig. 1 and marked as ‘Emission measurements’. Distribution of emission intensity along fiber optical axis, taken at PFP fluorescence maximum at 650 nm, clearly shows interaction between PFP molecules and light propagating in tapered fiber (Fig. 4). The position of distribution maximum is qualitatively predicted with the help of ray optics [8].

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Using fluorescence lifetime microscopy (FLM), it has been (.9 also found out that unique property of PFP molecules, namely 0,000 0,0 quite long fluorescence lifetime, vanishes when the PFP film 350 400 450 500 550 600 650 700 is deposited on curved surface of tapered fiber, in this case the wavelength, nm lifetime is only few tenths of nanosecond. To put it more exactly, PFP molecules entrapped in LB films aggregate in the Fig 2. Absorption spectra of tapered fibers with different tips diameters and glass plate [9] same way as most other porphyrins.

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The motivation for this study was utilization of a porphyrin monolayer to probe the evanescent field at the surface of a tapered fiber. The presented results show that the aim can be accomplished with the help of a PFP film deposited as a probe layer using the LB method. Besides, fluorescence intensity can be used to probe the intensity of the evanescent wave at the tapered fiber surface, though the measured fluorescence intensity was lower than expected due to the higher quenching efficiency for fluorescence on a surface with radius of curvature approaching tens of micrometers. Another possible application of the studied structures is related to fiber optical chemical sensors. In this case at least three parameters, absorbance, fluorescence intensity and fluorescence lifetime, can be affected by the analyte in question, which provides fascinating opportunities for more detailed studies.

CONCLUSIONS The obtained results show that a PFP monolayer can be deposited on a tapered fiber using the LB technique. Optimal tapered fiber diameter, regarding interaction of light with PFP molecules, was found to be around 2 mm, with its conical part being about 20 mm.

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XX)M A unique property of PFP LB films is the rather long 600 625 650 675 700 725 750 775 800 lifetime of the excited state of PFP entrapped in ODA, wavelength, nm although the studied porphyrins are highly aggregated in such films, as evidenced by the emission spectrum differing Fig 3. Emission spectra of PFP film covering surface of strongly from that of monomers in solution. However, this tapered fiber (left y-axis) in comparison with that of unique property vanishes when the film is deposited on a reference glass plate (right y-axis) [9] curved surface, such as tapered fiber. At a curvature radius of ten micrometers the PFP aggregates behave similarly to most porphyrins, exhibiting emission spectra that consist of two peaks. Moreover, in the case of PFP films on a tapered waveguide surface, the fluorescence lifetime is shortened to less than a nanosecond.


Results of emission distribution along tapered fiber can be qualitatively described basing on ray optics [8]. With such a structure the evanescent field at the fiber surface can be probed. Furthermore, the absorbance of the films is significantly higher in such a structure compared to reference glass plates that is the ultimate challenge to use the deposited films in sensor applications.

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400 The above-mentioned LB method produces not stable films 2 20 regarding solubility in such solvents as chloroform, o iIiIiIiIiIiIjj/I dichloromethane or similar ones. Covalently attached molecular 0 024681012 202224262830323436 films of porphyrin can be obtained using self-assembling Tapered fiber Z, mm monolayer (SAM) technique described elsewhere [10]. This Fig 4. Distribution of emission intensity taken along method produces extremely stable molecular layers and is utterly optical axis of tapered fiber important for producing sensors that can be immersed into different types of solvents. Experiments, that utilize SAM method for porphyrin deposition on tapered surfaces of multimode fibers, are carried out nowadays. Moreover, next step will be utilization of the SAM technique for molecular deposition of different types of porphyrins on the surfaces of hollow core fibers (HCF). 0

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Acknowledgment This study was funded by the Academy of Finland within the framework of the “ActiveFiber” project.

REFERENCES [1] S.-K. Lee, I. Okura, “Photostable Optical Oxygen Sensing Material: Platinum Tetrakis(pentafluorophenyl)porphyrin Immobilized in Polystyrene”, Analytical Communications, 34, 185-188, 1997. [2] D.-G. Zhu, D.-F. Cui, M.C. Petty, ”Gas sensing using Langmuir-Blodgett films of a ruthenium porphyrin”, Sensors and Actuators B, 12, 111-114, 1993. [3] A.V. Efimov, M. Anikin, N.V. Tkachenko, A.F. Mironov, H. Lemmetyinen, “Irregular behavior of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin in Langmuir–Blodgett films”, Chem. Phys. Lett., 289, 572-578, 1998. [4] S. Guo, S. Albin, “Transmission property and evanescent wave absorption of cladded multimode fiber tapers”, Optics Express 11, 215-223, 2003. [5] A.G. Mignani, R. Falciai, L. Ciaccheri, “Evanescent Wave Absorption Spectroscopy by Means of Bi-tapered Multimode Optical Fibers”, Applied Spectroscopy, 52, 546-551, 1998. [6] M. Rusu, S. Kivistö, C. Gawith, O. Okhotnikov, “Red-green-blue (RGB) light generator using tapered fiber pumped with a frequency-doubled Yb-fiber laser”, Optics Express, 13, 8547-8554, 2005. [7] C. McDonagh, C.S. Burke, B.D. MacCraith, “Optical Chemical Sensors”, Chem. Rev., 108, 400-422, 2008. [8] A.W. Snyder, J.D. Love, Optical Waveguide Theory, Kluwer Academic Publishers, 2000. [9] A. Veselov, C. Thür, V. Chukharev, M. Guina, H. Lemmetyinen, N. Tkachenko, “Photochemical properties of porphyrin films covering curved surfaces of optical fibers”, Chem. Phys. Lett., 471, 290-294, 2009. [10] V. Chukharev, T. Vuorinen, A. Efimov, N. V. Tkachenko, M. Kimura, S. Fukuzumi, H. Imahori, H. Lemmetyinen, “Photoinduced Electron Transfer in Self-Assembled Monolayers of Porphyrin−Fullerene Dyads on ITO”, Langmuir, 21, 6385-6391, 2005.
