Microsecond lifetime-based optical carbon dioxide sensor using luminescence resonance energy transfer

A lifetime-based optical sensor for the measurement of dissolved and gaseous carbon dioxide has been developed The basic principle is radiationless energy transfer from ruthenium(II)-4,4'-diphenyl-2,2'-bipyridyl as luminescent donor to thymol blue (a common pH indicator) as acceptor, both embedded in a hydrophobic matrix. In the presence of carbon dioxide thymol blue is protonated and changes its color from blue to yellow resulting in a decrease in the rate of energy transfer and consequently an increase in decay time. The decay time as a pCO(2)-dependent parameter was measured in the frequency domain with a blue light emitting diode as a light source modulated at 75 kHz. The present sensor displays a phase shift up to 16 degrees in the range 0-30 hPa of pCO(2) corresponding to a change in decay time from 0.38 to 1.05 mu s. The detection limit was found to be 0.5 mu M (22 ppb) of dissolved carbon dioxide and the response times are of the order of 15 s when going from 0 to 30 hPa pCO(2). The temperature behavior of the sensor has been studied in detail and a linear relationship Arrhenius plot has been found. The effect of molecular oxygen as a potential quencher of the luminescence was investigated in detail. The carbon dioxide sensor is stable over weeks, and is robust. (C) 1999 Published by Elsevier Science B.V. All rights reserved.