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, PCO2, 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.