Development of an opto-chemical carbon dioxide sensor for aquaculture and oceanography applications

In this paper an optical carbon dioxide (CO₂) lifetime-based sensor designed for the continuous monitoring of low CO₂ levels in aqueous solution in fields of fish farming and in oceanography is described. The presented optical measurement unit consists of a CO₂ sensitive layer mounted onto the end of sensing probe and an optoelectronic interrogation unit in order to convert the measurement signal into detectable signal. The measurement technique is the phase modulation fluorometry using Resonance Energy Transfer (RET) as sensing scheme. RET measurements were made by incorporation of ruthenium-(II)-tis-4,7-diphenyl-1,10-phennathroline (Rudpp)₃²- together with pH indicator ion-paired with tetraoctylammonium (TOA-OH) as the cunterion in a highly gas-permeable matrix. As a protective coating a high gas permeable Teflon with added graphite particles was used to eliminate interferences by ionic species. The performance of the whole sensor system is presented with respect to spectral characteristics, dynamic range (0 - 60 mg/l of dissolved CO₂), required sensor system specifications, temperature behavior and long term stability. First field tests performed in real fish farming conditions are presented as well.