Elsevier

Sensors and Actuators B: Chemical

Volume 203, November 2014, Pages 935-940
Sensors and Actuators B: Chemical

Discrete O2 sensors produced by a spotting method on polyolefin fabric substrates

https://doi.org/10.1016/j.snb.2014.06.103Get rights and content

Abstract

New solid-state phosphorescent oxygen sensors produced by solvent spotting on hydrophilic polyolefin fabrics are described. The method comprises of a simple spotting of dye solution, followed by a short post-treatment of water washing and baking (<4 h), resulting in sensors with uniform distribution of dye molecules in a discrete spot. The resulting sensors exhibited high brightness, optimal lifetime signals (27–31 μs at 21 kPa and 49–53 μs at 0 kPa O2), quasi-linear Stern–Volmer plots and temperature dependence. Compared to the commercial O2 sensors (PS coating on the micro porous support), the new polyolefin-based sensors performed well, showing good wettability and fast response time in liquid (3 min for wetlaid and 7 min for spunbond PP vs. 32 min for the reference). The homogeneity of the sensors was confirmed by microscopic analysis using PLIM (phosphorescence lifetime imaging). Both sensors exhibit cross-sensitivity to humidity which is investigated in this paper through the use of confocal microscopy.

Introduction

Sensing of molecular oxygen (O2) by phosphorescence/fluorescence quenching method is used increasingly in biological research [1], clinical and medical applications, process control in the chemical industry [2], in food [3] and pharmaceutical packaging, mainly because of its stable, disposable, robust, easy-to-use characteristics and not being prone to electrical interferences [4]. Such sensor systems hold many advantages over traditional O2 measurement techniques like Clarke-type electrodes [5]. In addition, they have a reversible response and can measure O2 non-invasively and in a contact-less manner [6].

Solid-state sensors usually consist of an O2-sensitive indicator dye encapsulated within an O2-permeable polymer matrix [4], [7]. The properties of the dye and encapsulation matrix (for instance its O2 solubility, permeability, mechanical attributes, compatibility with the dye, fabrication process and samples used) determine the final characteristics and operational performance of the sensor, particularly its sensitivity to O2 and measurement range, response time, cross-sensitivity (e.g. to temperature, humidity, etc.), the magnitude of luminescent signals (intensity and lifetime) and their changes in response to O2 [4]. Platinum [8] and palladium [9] porphyrins are common indicator dyes used in O2 sensors [5], particularly those excitable in the red (580–650 nm) and emitting in the near infra-red spectral region (700–800 nm).

Hydrophobic polymers with high and moderate oxygen permeability have been used as encapsulation matrices, for instance, polystyrene, polydimethylsiloxane and fluorinated polymers [4]. Conventional sensors based on thin-film polymeric coatings require an additional support material which improves their mechanical properties and aids handling and optical measurements [10]. These sensors are usually produced from organic solvent cocktail by drying [11], or by polymerisation or curing of liquid precursors [12]. Other dye incorporation methods include adsorption [13], covalent immobilisation [14], solvent crazing [15], and polymer swelling methods [7]. Micro porous materials including polymeric membranes and fabric which have appropriate mechanical and light-scattering properties, and permeability to O2 can be used in sensor fabrication as dye encapsulation matrix or sensor support material [16]. Still, many of the aforementioned fabrication techniques are not very suited for large-scale applications such as packaging, in which the sensor should exhibit high robustness, reproducibility between batches and low cost (less than 1c per cm2) [4]. For pharmaceutical and food packaging applications the sensor should be mass produced from minimal number of ingredients [17], easily incorporated into the packaging, provide an adequate shelf-life [6] and low hazard.

Polyolefins such as polypropylene (PP) and polyethylene (PE) are commodity polymers which represent over half the total polymers produced in the world. The mechanical and gas-permeability properties of PP and PE are suitable for oxygen sensing [18]. Some viable PE and PP-based oxygen sensors have been created by solvent-crazing [15] and hot-melt extrusion [19] methods. However there are obstacles regarding insolubility of these polymers in common organic solvents and limited processability. More recently, non-woven polyolefin materials have been developed for a range of industrial applications including textiles, membranes, filtration systems [20] and charge separators in Li-ion batteries [21]. These materials are cost-effective, have suitable chemical and thermal stability, gas permeability, uniformity and variable thicknesses (normally 50–200 microns) [20], [22]. They are also micro-porous, light-scattering, have a large surface area [21], [22], [23], [24] and can be surface-modified (e.g. by grafting of treatments) to make them more hydrophilic and wettable [25], [26]. In our previous study [16], we described solid-state O2 sensors produced from such materials by polymer swelling batch method. However, such sensors are hard to produce in a continuous process and individually rather than in large sheets. The basic method also uses an excess of toxic solvent, increasing the costs and potential hazard. As the sensors produced are larger than that required for the application (5–10 mm spots are usually optimal), they require further post-processing.

Here, we describe an alternative spotting micro-method of sensor incorporation in polyolefin fabric membranes. The advantages are that it can produce discrete sensor spots with controlled location and size, in a continuous and scalable manner using standard liquid dispensing or printing equipment. It also reduces consumption of the solvent and valuable sensor materials and lowers production cost. Studied materials consisted of PP monofibres bound together by either the wetlaid or the spunbond method into flat flexible sheets or tape.

Section snippets

Materials

The 700/30 K non-woven wetlaid polypropylene (PP) (thickness  160 μm, pore size  18 μm) and 700/70 non-woven spunbond polypropylene (thickness  155 μm, pore size  17 μm), both grafted with acrylic chains, were from Freudenberg Nonwovens (UK). Platinum (II)-benzoporphyrin dye (PtBP) was from Luxcel Biosciences (Ireland). The ethyl acetate (EtAc, ≥99%), tetrahydrofuran (THF), toluene and butanone (all of HPLC grade), Kolliphor® P188 (M.W.  1800 Da, polyethylene glycol/(polyethylene glycol + polypropylene

Optimisation of sensor fabrication and initial testing

Polyolefins have a high chemical resistance and are compatible with most solvents (i.e. they do not dissolve). The large surface area of the non-woven polyolefin substrates potentially allows the incorporation of dye by solvent-mediated methods [7]. In this case, impregnation efficiency is determined by the solvent and conditions used; polymer swelling, diffusion of the solvent and dye into polymer, evaporation rate and drying time. Previously, grafted and non-grafted PP non-wovens were

Conclusions

Discrete solid-state oxygen sensors based on polyolefin fabrics and PtBP phosphorescent dye were fabricated by solvent spotting. This method, which can be easily up scaled with currently available manufacturing equipment, provides simple and controllable fabrication of discrete oxygen sensors which are reproducible, robust and have good distribution of the lifetime signal throughout the sensor. The method is also advantageous, as it exhibits low polymer and solvent consumption, uses a low

Caroline A. Kelly received the BSc. Degree in Forensic and Environmental Chemistry from Dublin Institute of Technology in 2012. She is currently a Ph.D. student under Prof. Dmitri B. Papkovsky in the Biophysics and Bioanalysis Research Lab since 2013. Her research is focused on the design and fabrication of solid-state oxygen sensors with particular emphasis on food packaging applications.

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    Caroline A. Kelly received the BSc. Degree in Forensic and Environmental Chemistry from Dublin Institute of Technology in 2012. She is currently a Ph.D. student under Prof. Dmitri B. Papkovsky in the Biophysics and Bioanalysis Research Lab since 2013. Her research is focused on the design and fabrication of solid-state oxygen sensors with particular emphasis on food packaging applications.

    Dr. Claudio Toncelli received his Msc Degree in Industrial Chemistry from the University of Pisa in 2007. He obtained his PhD in Product Technology in 2011. After one year in a biomedical polymer company, he joined as post-doctoral researcher at the Biochemistry Department of University College Cork in 2013. He is currently a post-doctoral researcher in the Department of Oceanography in the Hellenic Centre for Marine research. His expertise includes the correlation between fabrication methods and performances in optical oxygen sensing for their use in large-scale applications with particular focus on morphological and mechanical properties of the matrix.

    Dr. Joe Kerry graduated from University College Galway in 1986. He received his PhD in Microbiology at University College Galway in 1995. Afterwards, Dr. Kerry joined University College Cork where he is currently Senior Lecturer and Head of the food packaging group in the School of Food and Nutritional Sciences. His expertise includes use and manipulation of modified atmosphere packaging systems for use with foods, use of extrusion technology for the manufacture of food products/packaging materials, applications and sensor/new SMART packaging technology developments within the area of food packaging.

    Prof. Dmitri B. Papkovsky graduated from the Chemistry Department of Moscow State University in 1982. He received his PhD in 1986 from the Institute of Biochemistry, Russian Academy of Science, Moscow. In 1997, Prof. Papkovsky joined Biochemistry Department of University College Cork, where he is currently Associate Professor of Biochemistry and Head of the Biophysics and Bioanalysis Research Lab. His present research interests include quenched-luminescence oxygen sensing and its applications, time-resolved and phase-resolved fluorescence spectroscopy, phosphorescentporphyrin probes, fluorescence based (bio)analytical techniques.

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    These authors contributed equally to this work.

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