A novel colorimetric oxygen sensor: dye redox chemistry in a thin polymer film
Introduction
There is much current work on the development of thin film optical sensors for the detection of oxygen in both the gas phase and fluid solution [1], [2], [3]. In such devices, the sensor film gives a response in terms of some optical property as a function of oxygen concentration. Changes in both luminescence and absorbance have been used as sensor response, and the relative advantages of each have been discussed in terms of specific applications by others [4], [5], [6], [7], [8], [9], [10]. A particularly useful characteristic of absorbance based devices is the fact that these can be used qualitatively and even semi-quantitatively by sight alone, without the need for expensive spectroscopic equipment, a feature of some significance in the development of sensors for food packaging [5], [11], [12], [13].
Absorbance based sensors can be divided into two main groups, those using biological [6], [14], [15] or synthetic oxygen binders [4], [7], [8], [9], [16], [17], [18] and those using redox chemistry [5], [10], [12], [13], [19], [20], [21], [22]. In both cases, the indicator has a different colour or intensity of colour in the presence of oxygen.
Sensors based on oxygen complexation are inherently reversible, but those using biological materials can suffer from stability problems and often need the addition of a competitive binder to remove oxygen before they can be used again [6], [14], [15], while those using synthetic complexing agents can show poor sensitivity [8] and a small response range [4]. In principle, redox based sensors could be reversible, however, most involve the consumption of both oxygen and a chemical component in the sensor formulation [10], [19]. Even so, it is possible to use redox chemistry to design a sensor with reasonable longevity, good stability over sensor lifetime, and high sensitivity [11], [13], [20]. Typical formulations incorporate a dye that can exist in both leuco and dye forms, the exchange between which makes up the sensor response; together with a reductant such as a reducing sugar, which under the conditions of sensor formulation is capable of reducing the dye to the leuco form [5], [12], [20]. In the presence of oxygen, a kinetic quasi-equilibrium is set up in which the rate of oxidation of the leuco form to dye by oxygen is equal to reduction of dye to the leuco form by reductant. In principle, the mode of sensors response is simple and can be represented as shown below.where RA is reducing agent, RAoxid the oxidised reducing agent, Dye (ox) the coloured form of the dye, and Leuco Dye (red) is the colourless form of the dye.
However, the kinetics of the reactions are complex, pH dependent, and involve acid/base catalysis [22], [23], [24], [25]. True thermodynamic equilibrium is never reached while the sensor is operating, but over time both oxygen and reductant are consumed, until oxygen and sensor components reach true thermodynamic equilibrium, and the sensor ceases to function. Sensor response time is determined by the rate at which the quasi-equilibrium is reached, and both the degree of quasi-reversibility and sensor longevity are determined by the reservoir of reducing components held in the sensor.
While a variety of support substrates have been used in the preparation of these types of sensors [12], [20], there have been few reports of sensors in which the sensors components are held in the thin polymer films which are popular for luminescence based oxygen sensors[1], [2], [3], [26] and absorbance based CO2 sensors [26], [27], [28]. Polymeric films offer advantages with respect to ease of fabrication, fast response, polymer/plasticiser controlled response tuneability and the ability to withstand humidity. In this paper, results from studies of a sensor using 2,6-dichloroindophenol (2,6-DCIP) as the indicator dye with fructose and base as the reducing system, all held in an ethyl cellulose polymer film, are presented.
Section snippets
Materials
Ethyl cellulose (EC), fructose and 1 M tetraoctylammonium hydroxide (TBAOH) in methanol were purchased from Aldrich, the sodium salt of 2,6-DCIP was purchased from Sigma. Castor oil (glyceryl ricinoleate) was obtained from Lancaster, and Santicizer™ 141 (2-ethyl hexyl diphenyl phosphate) and Santicizer™ 278 (7-(2,6,6,8-tetramethyl-4-oxa-3-oxo-nonyl)benzyl phthalate) were obtained from Monsanto.
Reduction experiments
Stock solutions (I) of fructose in base were prepared by dissolving 0.135 g of fructose in a total of
Dye
Although methylene blue has been used as the redox active dye in sensors of this type [5], [12], [20], there are two undesirable features of the equilibrium chemistry of this dye. Firstly, at moderate to high concentrations the absorption spectrum does not follow Beer’s law [23], [29] and secondly, it is unstable in alkaline solution [23], [30]. 2,6-DCIP is an attractive alternative since the redox chemistry is similar to that of methylene blue [22], [25], the absorption spectrum obeys Beer’s
Conclusion
A simple, reversible, colorimetric sensor for oxygen, based on the redox chemistry of 2,6-DCIP in the presence of fructose and base, in a thin ethyl cellulose polymer film, has been demonstrated. The sensor is colourless in the absence of oxygen, but gives a strong blue colouration in the presence of oxygen at ca. 30 Torr and above. At oxygen pressures in the range of 0–50 Torr, the absorbance of the film shows reasonable linearity with the partial pressure of oxygen. Sensor response is rapid,
Acknowledgments
I would like to thank P. Douglas for assistance in preparing this manuscript, and EPSRC for financial support.
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