I made a Pb sensor using carbon quantum dots (CQDs). I add buffer, CQDs, and a given metal to a quartz cuvette of the recommended type. Before adding a metal, the cuvette will emit a yellow-green fluorescence that you can confirm visually by using a 390 nm LED flashlight (see attached, phone camera distorts the color). On the left, you see the response with Pb. On the right, you see the response with another metal (e.g. Cd) under illumination with the UV flashlight. The cloudiness of the Pb sample is due to excess reagent that I used to increase the fluorescence for the purpose of this question.

Now, when I run these samples in a fluorimeter (Horiba Fluorolog3), the fluorimeter cannot tell the difference between the two samples. Both samples are excited at 390 nm. Emission is collected from 405-600 nm. Both samples have the same emission profile with the expected peak at 550 nm. The peak is at 100,000 counts before and after adding metal even though I can visually confirm quenching of the green fluorescence. I ran the necessary controls with water, buffer, and metals which show no fluorescence. Can anyone think of a reason why the fluorimeter does not show any fluorescence quenching while my eyes easily can?