Researchers have proposed decay-time-based schemes to sense oxygen, pH and CO2, although only oxygen optodes have found practical applications. A research group from the Institute of Analytical Chemistry at the University of Regensburg in Germany has proposed schemes for the design of a pH optode with lifetimes in the microsecond range. After excitation with a blue light-emitting diode, the optodes prove highly luminescent. Modulation frequencies of 75 to 200 kHz allow lifetime measurements with the same solid-state instruments used for optical oxygen sensing. This is an advantage over lifetime sensors with decay times in the nanosecond range and others that need UV light sources, according to the researchers.