Colorimetric CO2 Indicators

Andrew Mills

doi:10.1021/accountsmr.2c00226

Colorimetric CO2 Indicators

2023-06-19

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DOI：10.1021/accountsmr.2c00226

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Conspectus Carbon dioxide, CO2, is an essential part of life, inthat through green plant photosynthesis it is used to generate foodand fuel and is generated in both aerobic and anaerobic respiration.Industrially, it is used in fire extinguishers, supercritical fluidextractions, and food packaging. Environmentally, it is in the atmosphere,hydrosphere, and biosphere and is responsible for global warming andthe acidification of the oceans. The monitoring of CO2 inthe gas phase is usually carried out using FTIR spectroscopy, whereasthe measurement of dissolved CO2 usually involves an electrochemicaldevice. Excitingly, the most recent forms of CO2 indicatorsappear to offer significant advantages over current methods, suchas simplicity, low cost, and portability. This Account highlightsthe work of the Mills group on transformingCO(2) colorimetric indicator technology from the usual water-based(i.e., wet) indicator form to dry CO2-sensitiveinks, pigments, plastics, and adhesives. Initially, the basic theoryassociated with colorimetric CO2 indicators is described,and the simple relationship between indicator absorbance and the partialpressure 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, coupledwith a lipophilic quaternary ammonium cation, dissolved in a nonaqueoussolution of a polymer which, when cast, forms a dry ink film thatgives a reversible color response when exposed to CO2 bothin the gas phase and dissolved in solution. The ability to tune thesensitivity of a CO2 ink film to the desired applicationthrough the judicious choice of the pH indicator dye and base concentrationis 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 ageneral 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 differentCO(2)-sensitive pigments, which, when incorporated into aplastic, through extrusion, produce a range of novel CO2-sensitive plastic films that have many notable advantages over theirink film counterparts. Examples are then given of such plastic filmsbeing 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 intoa pressure sensitive adhesive to make an after opening freshness tapeis described briefly. Although most commercial CO2 indicators are assessedby eye and so are limited to qualitative analysis, this work showsthat colorimetric CO2 indicators can be used for quantitativeanalysis through absorbance measurements. Nowadays, such measurementscan be readily made using just a digital camera and color analysissoftware via digital camera colorimetry, DCC, which is likely to havea significant impact on the widespread use of the CO2 indicatorsdescribed herein, their commercial viability, and their potentialareas of application.

Keywords:

CARBON-DIOXIDE

PLASTIC FILM

OPTICAL SENSORS

FRESHNESS

LABEL

Colorimetric CO2 Indicators

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