Sensors and Actuators B 144 (2010) 49–55 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Monitoring time and temperature by methylene blue containing polyacrylate film Yulia Galagan a,∗ , Sheng-Hao Hsu b , Wei-Fang Su a,b a Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan b Institute of Polymer Science and Engineering, National Taiwan University, Taipei, Taiwan article info Article history: Received 14 July 2008 Received in revised form 21 September 2009 Accepted 5 October 2009 Available online 6 November 2009 Keywords: Methylene blue Polymer matrix Kinetics Color transition Diffusion coefficient Activation energy abstract The redox characteristic of methylene blue in polyacrylate film has been investigated for applications in monitoring time and temperature. This monitoring capability is based on a color change from colorless to blue under exposure to air. The oxidation process is controlled by the air diffusion which is affected by the chemical structure of the polyacrylate. The activation energy of diffusion, which was calculated by Arrhenius equation, goes up with increasing the rigid structure of the polyacrylate. The results correlate well with the glass transition temperature of different polyacrylates. The unique feature of the methy- lene blue to change color under oxidation, can be used for the fabrication of low cost visual sensors for monitoring time and temperature. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Visual time monitoring that is based on color transition has found many applications in different fields of science and technol- ogy. There are a number of time–temperature integrators used in food and pharmaceutical industry which help to determine safe storage period and freshness of the products [1–6]. Time indicating devices are also widely used in manufacturing passes and permits [7,8]. Many time indicators are based on dye diffusion [6,9–11] or on reaction of a reduced dye with oxygen [5,7,12,13]. Methylene blue (MB) is one of the most common redox dyes. The easy reduction of MB to leuco methylene blue (LMB) and its re-oxidation (Fig. 1) have extensive use throughout industry. For example, the commer- cial colorimetric oxygen indicator is based on this reaction [14–20]. The working principle of the oxygen indicator is based on the fast oxidation of colorless LMB in the presence of oxygen to blue MB. The reduction of MB can occur as a result of an electrochromic reaction using various reducing agents. One system, The Ageless Eye ® oxy- gen indicator [19,20] (Mitsubishi Gas Company) uses MB, glucose and alkali for the reaction. Upon anaerobic condition MB is reduced by glucose to leuco form, which can be readily re-oxidized by O 2 with color change. The reduction of MB can also occur as a result of electron and proton transfer from a sacrificial electron donor (SED) ∗ Corresponding author. Present address: Holst Centre/TNO, High Tech Campus 31, Eindhoven, The Netherlands. Tel.: +31 40 277 4300; fax: +31 40 274 6400. E-mail addresses: yulia.galagan@tno.nl (Y. Galagan), suwf@ntu.edu.tw (W.-F. Su). under ultraviolet (UV) irradiation (photochromism) [21,22]. Mills and coworkers [23] reported a novel UV-activated oxygen indicator based on nanoparticles of titania for electron transfer and reduction of MB by triethanolamine using UV light (315–380 nm). This oxygen indicator is successfully used in food and pharmaceutical packag- ing. Finally, the redox reaction of MB has found wide applications in optical storage media with limited access [24]. In this technol- ogy CD and DVD disks with optical transparent LMB coating are produced. The transparent coating allows access to the data when LMB reacts with oxygen and turns into colored MB, now limiting data access. The reversible reaction of reduction and re-oxidation of MB is also widely used in data recording industry and holography [25,26]. The opportunity of this and other similar dyes in the appli- cations of sensors and data storage is very promising. Polymers are ideal carriers for presenting the dye molecule in a convenient and useful form. Therefore, the more comprehensive knowledge about the mechanism and kinetics of the above mentioned reac- tions in polymer media can open new ways of their usages. In most cases, attention has been paid to investigate the reduction kinetics of MB to leuco dye [21,27,28]. The oxidation of LMB blue is very fast under sufficient amounts of oxygen. However, if LMB is situated in polymer media, the oxidation speed will be slowed down in air due to a diffusion process. Therefore, the investigation of diffusion-kinetics of LMB oxidation is important for future devel- opment and inventions. In particular, oxidation within an extended period can be used for “time monitoring”. The principle of oxida- tive color transition can be exploited for the development of visual 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.10.011