Effect of buffers and pH in antenna sensitized Eu(III) luminescence

The photophysics of a europium(III) complex of 1,4,7,10-tetraazacycododecane-1,4,7-triacetic acid-10-(2-methylene)-1-azathioxanthone is investigated in three buffer systems and at three pH values. The buffers—phosphate buffered saline (PBS), HEPES, and universal buffer—had no effect on the europium luminescence, but a lower overall emission intensity is determined in HEPES. It is found that this is due to quenching of the 1-azathioxanthone first excited singlet state by HEPES. The effect of pH on the photophysics of the complex is found to be minimal, and protonation of the pyridine nitrogen was found to be irrelevant. Even so, pH is shown to change the intensity ratio between 1-azathioxanthone fluorescence and europium luminescence. It is concluded that the full photophysics of a potential molecular probe should be investigated to achieve the best possible results in any application. For instance moving from HEPES to PBS improves the signal of the investigated europium(III) complex significantly.