Critical toxic gases have been used in the several industrial plants. So workers were exposed to many kinds of toxic gases without proper safety guard in the field. Low concentration of toxic gases injures the eyes, nose, and respiratory organs and cause allergic reaction [1]. These gas leaks emerged as a social issue, because Exposure of toxic gases was fatal for the long term. The World Health Organization(WHO) has set a safe exposure stand respectively [2]. Colorimetric detection of gases has been used for years. They are based on the change in color of pH indicators incorporated in a porous matrix [3]. There were many researches of sensor detecting toxic gases used by pH indicators [4], but there were many disadvantages during the measurement. Expensive analytical instruments are needed, and it takes a long time to obtain the results [5]. In this study we found that it is better and easier to use simple pH indicators to detect toxic gases. We made convenient pH indicator-based films for detecting 4 types of toxic gases easily. Using polymeric binder, the pH indicators were embeded in a polymeric film. To improve sensitivity and reactivity, we added other additive for enhancement. So we could be observed by naked eyes without aid of analytical devices, and decrease the response time. As a result, we made films which reacted with toxic gases and which were faster than commercialized ones.
To make pH indicator-based film sensors, we adopted a method of dip coating. Main substrate was non-woven fabric wiper(WW-2109), polymeric binder was a sodium alginate and cross linking agent was calcium chloride. To improve reactivity, benzoquinone was a additive as electron transfer agent. At first, we made a solution which added sodium alginate(1wt%), benzoquinone, and pH indicator(0.1wt%). Prepared films were impregnated again in the above solution. Then they were impregnated in a calcium chloride solution(10wt%) again. After finish to make sensor, toxic gases(20, 10ppm) was exposed constantly by mass flow controller(MFC). To observe degree of the color change, Spectrophotometer was used and the graph of ΔE*a*b* was drew every 5 seconds.
Sodium alginate can use to crosslink with calcium chloride by ionic linkage. There was a good resistance to washing pH indicator in wet environment. From adding benzoquinone, the film`s color changed to the other color in the 10, 20ppm concentration of gases, and it reacted completely with in a minute. When colorimetric sensors contacted toxic gases exposed by MFC, they changed color from blue to yellow or yellow to blue, because of movement of proton. Benzoquione help a movement of proton. The change started after 10 seconds, when films contacted with toxic gases. Humidity is a important requirement. If humidity was high, its reactivity was more faster.
After exposure of toxic gases, we used a spectrophotometer to confirm the degree of color change. Spectrophotometer digitize a image from using L*a*b* color coordinate. When we compared with before and after, we could know degree of the color change.
Without crosslinking reaction, dye will get out of the wiper, but using the a crosslinking of calcium alginate, we improve water repellency of films in wet environment. By coating pH indicators and benzoquinone on films, it is possible to make films with more faster reactivity with in a minute. In the field, we can determine a gas leak by naked eye within a minute, and prevent unexpected accidents with this films.
This work was supported by Fundamental Technology R&D Program for Society of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (Grant NO. 2013M3C8A3075845).
References:
[1] Bret F. Bessac and Sven-Eric Jordt, Sensory Detection and Responses to Toxic Gases, PROCEEDINGS OF THE AMERICAN THORACIC SOCIETY. VOL 7, Apr. 2010.
[2] Liang Feng and Kenneth S.Suslick, A Simple Highly Sensitive Colorimetric Detection Method for Gaseous Formaldehyde, American Chemical Society, VOL 132, Dec, 2009
[3] J.Courbat and D.Briand, Evaluation of pH indicator-based colorimetric films for ammonia detection using optical waveguides, Sensors and Actuators B: Chemical, VOL 143,Sep, 2009
[4] Martina O`Toole and Dermot Diamond, Inkjet printed LED based pH chemical sensor for gas sensing, Analytica Chimica Acta, VOL 652,Jul, 2009
[5] Yoshio Suzuki and Koji Suzuki, Portable Sick House Syndrome Gas Monitoring System Based on Novel Colorimetric Reagents for the Highly Selective and Sensitive Detection of Formaldehyde, Environmental Science & Technology, VOL 37, Dec, 2003