Adam,

If you are forming ground state complexes between quencher and fluorophore, you will have static quenching. Then plotting F0/F versus concentration will generate a curve with positive deflection from the expected line. However plotting t0/t will yield a straight line that only provides information on the dynamic quenching.

Here t0 is the lifetime in absence of quencher, t is the lifetime, F0 is the fluorescence intensity in absence of quencher, F is the fluorescence intensity.

This assumes that quenching occurs when fluorophore and quencher are in van der Waals contact.

Reference: "Principles of Fluorescence Spectroscopy" by Lakowicz

Hope that is helpful.

Best

John

Having the slope increase (=an upward curvature) means some static quenching taking place (= binding of the quencher) or something that is called "quenching sphere of action" meaning that a quencher is capable of quenching even without binding (described for Tryptophan and Acrylamide). I guess that this becomes true when the concentration of the quencher is so high that diffusion doesn´t work so well anymore.

I also aggree with John´s explanation. The Lakowicz book really is the reference for these kinds of questions.

Cheers

NAture of quenching decides the slope. Please read the book "principles of fluorescence spectroscopy" by Joseph R. Lakowicz to get answers to your question.