Layer-by-Layer assembly of a water–insoluble platinum complex for optical fiber oxygen sensors
Introduction
The development of oxygen sensors has focused the working lines of many researchers due to the important applications that require them. Some of the most relevant ones belong to the fields of biomedicine, food industry, air quality or safety at work [1]. Available electronic sensors for this aim are based on electric potentials, so that a reference electrode is needed [2]; furthermore, an electric signal is required, which makes them not recommended for hazardous environments with a high inflammability risk (such as the chemical industry) [2]. This background has motivated the development of optical devices as an alternative to overcome these drawbacks. In this manner, sensing materials whose optical properties vary in the presence of oxygen have been used to implement oxygen sensors. The general working principle is based on the measurement of these parameters, such as the color, which is known as colorimetry [3]: the color is registered by a Charge-Couple Device (CCD) camera and it can be parameterized, for instance in Commission Internationale d’Eclairage L*a*b (CIElab) space, so that the oxygen concentration can be determined [4].
A relevant type of sensing materials used to develop optical oxygen sensors shows a luminescence emission that decreases as the target gas concentration increases: this effect is known as quenching, and it is reversible in most cases [1]. The straightforward implementation consists of monitoring this emission in time to determine the oxygen concentration [5], [6]. Although there are other kinds of measurements based on the emission lifetime which are more robust, a more complex instrumentation is needed [7], [8]. This type of compounds is required to show a strong luminescence emission, a significant Stokes shift, as well as photo and temporal stability. Metallo-porphyrins meet these requirements [9]: the porphyrin backbone is present in several important biomolecules such as hemoglobin or chlorophyll, both of them related with gas interaction [10]. Different materials are available: each one is characterized by the chains attached to the backbone, although the optical properties are mainly determined by the metallic atom core. The target gas gets coordinated with it, producing the transduction. Their complex chemical structure make them non soluble in water. The material used in this work is platinum tetrakis pentrafluorophenyporphine (PtTFPP), which has been successfully used to develop oxygen sensors thanks to its excellent features such as long lifetime emission, photo stability or significant Stokes shift (around 250 nm) just to mention the most relevant ones [11].
Although colorimetry with CCD camera has been already used with metallo-porphyrins [12], the transduction could be also controlled by an optical fiber. In fact, the exciting light can be coupled into the fiber to illuminate the sensing material, as well as the emission signal is collected into it and guided back toward the receptor. The system can be simplified and made more robust if the compound is directly attached to the fiber, specifically onto a cleaved ended pigtail [13]. This intrinsic architecture allows taking advantage of the features that optical fiber offers [14]: low dimensions, light weight, multiplexation of different sensors, and electromagnetic immunity Moreover, optical fiber is a passive element, so that no electrical feeding is required, which is very important in oxygen applications related to dangerous environments. The challenge when implementing this type of devices is the deposition of the sensing film onto the fiber due to its reduced dimensions and passive nature (it is made of silica). Some methods used so far are Langmuir–Blodgett [15] or dip coating [16], [17], but the resulting sensors have shown a low reproducibility when the sensing material has to be deposited at the end of the fiber [18]. In this context, Layer-by-Layer (LbL) method allows films to be deposited (and even designed) at nanometric scale. This procedure is basically based on the electrostatic assembly of molecules that show opposite electrical charge densities, which is not affected either by the dimensions or geometry of the substrate. One of the standard requirements for the LbL deposition is that the material to be deposited has to be soluble in water, which initially, makes the LbL technique incompatible with many of oxygen sensing materials, for instance, PtTFPP.
In order to get advantage of the synergy between PtTFPP, optical fiber and LbL method, this work presents an oxygen sensor prepared with these three elements. The initial incompatibility between the sensing material and LbL can be overcome using a PtTFPP emulsion, which would allow the deposition of PtTFPP by LbL. To the best of our knowledge, this is the first time that PtTFPP is deposited by means of the Layer-by-Layer technique.
Section snippets
Sensing material
The reagent employed in this work shows a luminescence emission that is affected by the presence of oxygen. The product belongs to the category of porphyrins, which are molecules with a metallic transition atom in its core that gets coordinated with the target gas, in this case, molecular oxygen. Typically, Ruthenium complexes have been used to detect this gas, as well as Palladium and Platinum materials. The material used has a Pt core, and its absorption spectrum shows a significant peak at
Experimental set up
The sensor was implemented using PCS fiber: on one hand, the cladding was easy to remove just applying a flame and cleaning thereafter the segment with ethanol; on the other hand, the core dimensions of the fiber are higher compared to standards fibers, which increases the interface between the substrate and the sensing film. In order to enhance the sensitivity, 2 cm of the cladding were removed [39]. The sensor was connected to a bifurcated fiber, so that one of the branches was connected to a
Sensing layer characterization
The processes described in Section 2.2 yielded into a thin layer deposited onto the fiber. Several tests were performed to check whether the sensing material had been properly attached onto the substrate, and so, validate the construction method. In this manner, a similar sensing film was deposited onto a glass slide (same kind of substrate) to firstly make an Energy-Dispersive X-ray spectroscopy (EDX) analysis and check the presence of the elements that constitute the sensing material. The
Conclusions
LbL method can be used to deposit sensing materials even if they are not soluble in water. This is the case of PtTFPP, which offers excellent properties for oxygen sensing. An optical fiber sensor has been prepared based on the reversible quenching effect that this gas produces over PtFPP. The luminescence emission is decreased when the oxygen concentration is increased, following the Stern–Volmer relationship with a R2 = 0.9991. In addition, the baseline was recovered for all the experiments;
Acknowledgements
Tancial support from the Spanish Ministerio de Educación y Ciencia through project TEC2010-17805. Special thanks to CEMITEC for the utilization of the SEM and EDX as well as Nadetech Inc. for the fabrication and tune-up of the robot used for the deposition of the nanocoatings. Indications provided by Santiago Medina from the University of Granada are also acknowledged.
Cesar Elosua received his MS degree in electrical and electronic engineering from the Public University of Navarra (UPNA, Pamplona, Spain) in 2004. In the same year, he obtained a scholarship from the Science and Technology Spanish Ministry and he joined the optical fiber sensor group at the Department of Electrical and Electronic Engineering of the UPNA. During 2008, he was a visiting Ph.D. student at the University of Limerick and at the City University of London, working on Artificial Neural
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Cesar Elosua received his MS degree in electrical and electronic engineering from the Public University of Navarra (UPNA, Pamplona, Spain) in 2004. In the same year, he obtained a scholarship from the Science and Technology Spanish Ministry and he joined the optical fiber sensor group at the Department of Electrical and Electronic Engineering of the UPNA. During 2008, he was a visiting Ph.D. student at the University of Limerick and at the City University of London, working on Artificial Neural Networks algorithms. He became a lecturer of this department in 2009, receiving his PhD degree in the next year. His research interests include optical fiber sensors and networks, organometallic chemistry and data mining techniques.
Nerea de Acha obtained her MS degree in Electric and Electronic Engineering in 2014 from the Public University of Navarre (UPNA, Pamplona, Spain). Her research interests include optical fiber sensors based on nanostructured coatings.
M. Hernaez obtained his MS degree in Electrical and Electronic Engineering and his PhD from the Public University of Navarra (UPNA, Pamplona, Spain) in 2007 and 2011 respectively, while he was working as Research Associate. In 2010, he was a visiting scientist at the Photonics Research Group in Aston University (Birmingham, UK). Since 2013 he has been holding a teaching assistant position at UPNA. His research interests include optical fiber sensors based on nanostructured coatings and nanostructured materials.
Ignacio R. Matias is a Professor in the Electrical and Electronic Engineering Department at the Public University of Navarra, Spain. He received his MS (1992) and Ph.D. (1996) in electrical and electronic engineering from the Polytechnic University of Madrid. He has coauthored more than 200 book chapters, journal and conference papers related to optical fiber sensors and electronic applications.
Francisco J. Arregui is a Full Professor at the Public University of Navarre, Pamplona, Spain. He was part of the team that fabricated the first optical fiber sensor by means of the Layer-by-Layer assembly method at Virginia Tech, Blacksburg, VA, USA, in 1998. He is the author of around 300 scientific journal and conference publications, most of them related to optical fiber sensors based on nanostructured coatings. Prof. Arregui has been an Associate Editor of the journals “IEEE Sensors Journal”, “International Journal on Smart Sensing and Intelligent Systems” and “Journal of Sensors”. In fact, Journal of Sensors (Hindawi) was founded in 2007 by Prof. Arregui who served as the Editor-in-Chief from 2007 to 2011. He is also the Editor of the books “Sensors Based on Nanostructured Materials” (Springer) and “Optochemical Nanosensors” (Taylor & Francis).