A Europium-Based Optical Sensor for the Detection of Carbon Dioxide and Its Application for a Fermentation Reaction

A new europium-based complex, K[Eu(hfa)4] with hfa = hexafluoroacetylacetonate is synthesized and its structure confirmed via X-ray crystallography. The structure unravels an anionic octa-coordinate complex, K[Eu(hfa)4], as opposed to the neutral hexacoordinate complex Eu(hfa)3 routinely/ubiquitously presumed to be the case in the literature. The complex displayed pH-dependent, “on–off” emission changes in solution and exhibited a pKa of 6.13 ± 0.06 in ethylene glycol. In solution, the sensor complex exhibited drastic variation in emission intensity corresponding to changes in the concentration of CO2 gas purged. Based on multiple purge cycles of N2 and CO2, the emission intensity changes can be correlated to the concentration of CO2 in the solution. The sensor’s ability to quantify the CO2 presence is based on emission variations of the 5D0 → 7F2 line in the Eu(III) complex at 618 nm. The sensor exhibits a linear response to CO2 concentrations in the range of 0–25% (0–8.50 mM or 0–189.95 mmHg). Based on calibration data, the limit of detection (LOD) is determined to be 0.57% (0.19 mM or 4.33 mmHg) in solution. The I100/I0 ratio is determined to be 80.29 ± 3.79. The percent change in intensity from purging N2 to 100% CO2 is 7911.16%. Over the course of seven cycles of purging different concentrations of CO2, there is essentially no deviation in the emission intensity of the sensor in solution, indicating stability and reversibility. In addition to the analytical characterization of the sensor, the mechanism of CO2 sensing is investigated using cyclic voltammetry, IR, and Raman spectroscopy. These data indicate the reduction of europium(III) to europium(II) in an alkaline medium and suggest changes in the hfa ligand chemistry (association/dissociation and protonation) due to CO2 purging. The potential use of the sensor complex for real-life applications is herein evaluated via a well-known fermentation reaction. The CO2 generated during yeast’s anaerobic respiration in sucrose media is quantified using the sensor complex and a calibrated, commercial CO2 probe; both exhibit similar CO2 concentration values, validating the calibration curve and the viability of the complex as a bona fide sensor. Based on the data collected, a highly stable, brightly red-emissive Eu(III) complex with the ability to differentiate concentrations of CO2 in solution is hereby developed and characterized with benefits for various CO2 sensing applications.