Merging porphyrins and structured optical fibres: future technology for chemical sensors Cicero Martelli1,2, John Canning1, Maxwell J. Crossley3, Danial Stocks3, Maxine Sintic3 Interdisciplinary Photonics Laboratories, School of Chemistry, University of Sydney, 206 National Innovation Centre, Eveleigh 1430, Sydney, NSW, Australia 2 School of Electrical and Information Engineering, The University of Sydney, NSW 2006, Australia 3 School of Chemistry, The University of Sydney, NSW 2006, Australia Author email address:
[email protected]
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Abstract: Spectroscopic characterisation of water soluble porphyrins using a structured optical fibre are presented and discussed. Porphyrin thin-films were also fabricated inside the holes of structured fibres. The thin-films self-assemble inside the fibres leading to energy coupling between the molecules. These are the first steps towards future chemically tailored optical fibre sensors for molecular detection.
Keywords: porphyrin, microstructured fiber, thin-films, self-assembly, spectroscopy, biological sensing, chemical sensing
INTRODUCTION Cellular metabolism in animals and photosynthesis in plants are the two most important biological processes that make life possible the way we know it. They are both related to consumption or production of oxygen to generate energy. In the cellular metabolism, oxygen is responsible to remove electrons used in the production of highly energetic molecules, such as adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADP). Carbohydrates (source of energy for all animals) are produced during photosynthesis and oxygen is generated as a by-product. Porphyrins are the common link between these two processes [1]. In cellular metabolism, it is an iron porphyrin called heme and in photosynthesis, the chlorophyll. This alone explains the importance of the porphyrins and justifies why they are so intensively studied. Consequently, much attention has been given in the development of synthetic porphyrin compounds that mimic nature’s own functions [2]. A striking feature about porphyrins, both natural and man made, is the ability to selectively tailor their chemistry with functional end groups, potentially making porphyrins an ideal chemical framework in which to develop future species-specific sensors. Similarly to what heme does in nature, for instance. Given the vast literature describing research about porphyrin thin films on planar substrates, we propose the development of high quality porphyrin thin-film waveguide sensors, both integrated and fibre, for the detection of species such as oxygen, nitrogen etc. Oxygen, by way of example, is responsible for non-radiative decay of excited electrons and its concentration can be estimated by measuring either luminescence intensity or lifetime of the organic compounds. These properties can be enhanced by the inherent sensitivity associated with long interaction lengths provided by fibre waveguides, particularly those taking yet another advantage offered by so-called structured optical fibres such as photonic crystal and Fresnel fibres [3,4]. In order to demonstrate the first step towards this technology, we present the first work on integrating porphyrin thin films within structured optical fibres. Spectroscopic studies of a water soluble porphyrin compound, with other applications involving in-fibre biosensing [5], and the fabrication of selforganised thin-films inside the holes of structured fibres (for optoelectronic sensors and device fabrication) is presented. Solid core fibres have important advantages over hollow core diffractive fibres for both sensing and device applications: 1) broader spectral band propagation that is not so sensitive to the refractive index of the sample, and 2) lower tolerance limits in fibre fabrication. They also allow more advanced resonant based sensing techniques using fibre components such as Bragg gratings to be fabricated. The transmission band of hollow core band-gap fibres is very sensitive to the hole material refractive index, which limits and complicates spectroscopic analysis. Every sample concentration has a shift in the transmission band associated with a change in refractive index and this needs to be considered. This also affects propagation loss and in some cases the shifted fibre transmission window does not coincide with the spectral range of interest [6]. Hence, we focus our studies on solid core air-silica structured fibres as waveguide devices.
19th International Conference on Optical Fibre Sensors, edited by David Sampson, Stephen Collins, Kyunghwan Oh, Ryozo Yamauchi, Proc. of SPIE Vol. 7004, 700406, (2008) 0277-786X/08/$18 doi: 10.1117/12.785933
Proc. of SPIE Vol. 7004 700406-1 2008 SPIE Digital Library -- Subscriber Archive Copy
MATERIALS AND METHODS Two different systems are investigated: 1) spectroscopic characterisation of the water soluble compound, 5,10,15,20Tetrakis(4-sulfonatophenyl)porphyrinatomanganese(III) acetate (TPPSMn-OAc), using the evanescent part of the propagating filed of a 6-ring air-silica structured fibre, and 2) the fabrication and analysis of self-assembled porphyrin thin-films inside the micro-channels of a 3-ring fibre samples, the molecule has four alkane chains bonded to meso carbons of the porphyrin skeleton and two chlorine atoms linked to the opposite directions of the central metal (Tin IV) – it is specifically designed to form stable films on silica surfaces. Different fibres were used only to demonstrate the flexibility in fabricating custom tailored designs. The fibre cross-sections as well as a schematic representation of the molecules are presented in Figure 1. SO3H
=' N N-[ _6' 3 LLy)
1
SO3H
b)
a)
Fig. 1. a) Water soluble porphyrin molecule and the 6-ring air-silica structured fibre used in the spectroscopic measurements and b) Tin(IV) porphyrin molecule used to produce the thin-films on the 3-ring structured fibre (inset).
1. Organic molecules The water soluble compound is the 5,10,15,20-Tetrakis(4-sulfonatophenyl)porphyrinatomanganese(III) acetate (TPPSMn-OAc). Reference bulk measurements were performed using a 1 cm long quartz cuvette and a commercially available spectrophotometer. Three different solutions of TPPSMn-OAc and distilled water ( 1) 1.75x10-4 M, 2) 8.77x10-5 M, 3) 4.38x10-5 M) are calibrated. The first two presented saturation at the B-band. The two B and Q absorption bands (~460 and ~560 nm respectively), signature of all porphyrin compounds, arise from π-π* transitions from the four frontier orbitals: two π orbitals (a1u and a2u) and a degenerate pair of π* orbitals (egx and egy). Due to configurational interaction the two absorption bands arise from the transitions a1u→eg and a2u→eg. Both bands are influenced by the external environment and are generally shifted to shorter wavelengths along the spectrochemical series for d-transition metals [7]. Monitoring these bands can also provide information about the surrounding environment, such as Ph and dielectric constant of the solvent. The dichloro[5,10,15,20-tetra(heptyl)porphyrinato]tin(IV) molecules are designed to directly bond to the silica surface reactive sites (e.g. silanol groups) through covalent bond between the central metal and the oxygen atom of the surface. Bonding to the surface results in displacement of the Cl and H to form HCl as a byproduct (SiOH + XCl2 → S-OXCl + HCl, where X represents the porphyrin structure). This leads to superior film adhesion properties generating robust and stable films. The uniformity as well as the surface coverage of the film can also be controlled by engineering the surface reactive sites and roughness. The alkane chains provide close packing and alignment of the molecules on the surface leading, consequently, to the self-assembly of an organised thin-film [8]. The seminal building block for organic circuits. Substances interacting with the film will change its impedance, for example. 2. Structured optical fibres The air-silica structured fibres have an average roughness on the internal hole walls
