fluorescence based metal ion detection in aqueous samples is highly sensitive and selective
i need scientific reasons for these high selectivity and sensitivity ?
Hi Jehangir,
concerning the sensitivity of fluorescence detection methods:
"Why is fluorescence rather than absorption used for high-sensitivity detection? Fluorescence is more sensitive because of the different ways of measuring absorbance and fluorescence. Light absorbance is measured as the difference in intensity between light passing through the reference and the sample. In fluorescence the intensity is measured directly, without comparison with a reference beam. Consider a 10−10 M solution of a substance with a molar extinction coefficient of 105 M−1 cm−1. The absorbance will be 10−5 per cm, which is equivalent to a percentage transmission of 99.9977%. Even with exceptional optics and electronics, it will be very difficult to detect the small percentage of absorbed light, 0.0023%. Even if the electronics allow measurement of such a low optical density, the cuvettes will show some variability in transmission and surface reflection, which will probably exceed the intensity difference due to an absorbance of 10−5. In contrast, fluorescence detection at 10−10 M is readily accomplished with most fluorometers. This advantage is due to measurement of the fluorescence relative to a dark background, as compared to the bright reference beam in an absorbance meas-urement. It is relatively easy to detect low levels of light, and the electronic impulses due to single photons are measurable with most photomultiplier tubes." (in: Fluorescence Sensing, Principles of Fluorescence Spectroscopy pp 623–673)
About selectivity... yes, you can detect only fluorescent molecules or fluorescence quenchers. But mostly selectivity is determined by your ligands, how selective they bind the metal ions if another ions are present. And the more stable are these complexes - the better sensitivity you have with the same method of detection.
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