Luminescence lifetime-based carbon dioxide optical sensor for clinical applications

The development of both an optical planar and capillary based carbon dioxide sensor, which final aim is pCO2 monitoring in adipose tissue of critically ill patients, is reported. The sensor is based on the measuring principle of phase fluorometry using a dual luminophore referencing scheme (DLR) to convert the CO2 dependent intensity signal into phase domain. The CO2 sensors have been prepared by incorporating two appropriate luminophores and a phase transfer agent in a same hydrophobic polymer as matrix. The short-lifetime luminophore used as pH indicator is 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS). The second inert luminophore is the long-lifetime dye Ruthenium(II) tris(4,7-diphenyl-1,10-phenanthroline) (Ru(dpp)32+), which has been made insensitive to oxygen by immobilising in a suitable oxygen impermeable polymer. As phase transfer agent, tetraoctylammonium hydroxide (TOA-OH) has been chosen. Both sensor types have been characterised with respect to optimise sensitivity and mechanical stability. For this purpose, several polymers, such as ethylcellulose, eudragit RL100 (EG), copolymer eudragit/poly(ethylene glycol) (PEG) and silicone have been examined as appropriate matrix for incorporation of two indicators. The largest phase shift up to 13° and 15° has been observed in the case of silicone and copolymer EG/PEG, respectively, and they have been in detail examined in terms of sensitivity and stability. The presented sensors enable the measurement of pCO2 in the range from 0 to 150 mmHg, with a resolution of 0.5 mmHg and an accuracy of ±1 mmHg absolute or less than 7% of the read-out value. All measurements have been carried out only in aqueous solutions before clinical measurements.