Colorimetric CO2 Indicators

Abstract

Carbon dioxide, CO₂, is an essential part of life, in that through green plant photosynthesis it is used to generate food and fuel and is generated in both aerobic and anaerobic respiration. Industrially, it is used in fire extinguishers, supercritical fluid extractions, and food packaging. Environmentally, it is in the atmosphere, hydrosphere, and biosphere and is responsible for global warming and the acidification of the oceans. The monitoring of CO₂ in the gas phase is usually carried out using FTIR spectroscopy, whereas the measurement of dissolved CO₂ usually involves an electrochemical device. Excitingly, the most recent forms of CO₂ indicators appear to offer significant advantages over current methods, such as simplicity, low cost, and portability. This Account highlights the work of the Mills group on transforming CO₂ colorimetric indicator technology from the usual water-based (i.e., "wet") indicator form to dry CO₂-sensitive inks, pigments, plastics, and adhesives. Initially, the basic theory associated with colorimetric CO₂ indicators is described, and the simple relationship between indicator absorbance and the partial pressure of CO₂, P_CO2, established. The early work on CO₂-sensitive inks is then described, where such inks comprise a hydrophilic pH-sensitive dye anion, coupled with a lipophilic quaternary ammonium cation, dissolved in a nonaqueous solution of a polymer which, when cast, forms a dry ink film that gives a reversible color response when exposed to CO₂ both in the gas phase and dissolved in solution. The ability to tune the sensitivity of a CO₂ ink film to the desired application through the judicious choice of the pH indicator dye and base concentration is described. The dependence of the sensitivity of a CO₂ ink film on temperature is used to create a temperature indicator, and the ability to tune the ink, to respond to high levels of CO₂, is used to create a fizziness indicator for carbonated drinks. Very sensitive CO₂ inks are used to make a vacuum and a general air-pressure indicator. The more recent development in CO₂ indicator technology is described in which CO₂ inks are used to coat silica particles to make a range of different CO₂-sensitive pigments, which, when incorporated into a plastic, through extrusion, produce a range of novel CO₂-sensitive plastic films that have many notable advantages over their ink film counterparts. Examples are then given of such plastic films being used for dissolved CO₂ measurements in salt water, for food packaging, and as an early wound-infection indicator. Finally, the recent incorporation of a CO₂-sensitive pigment into a pressure sensitive adhesive to make an after opening freshness tape is described briefly. Although most commercial CO₂ indicators are assessed by eye and so are limited to qualitative analysis, this work shows that colorimetric CO₂ indicators can be used for quantitative analysis through absorbance measurements. Nowadays, such measurements can be readily made using just a digital camera and color analysis software via digital camera colorimetry, DCC, which is likely to have a significant impact on the widespread use of the CO₂ indicators described herein, their commercial viability, and their potential areas of application.