Dear Yousra Sbaoui thank you for posting this very interesting technical question which is certainly of interest to other RG members as well. First of all, it is well known that quercetin is highly soluble in diethyl ether and methanol, and soluble in ethanol. In this context, please have a look at the following potentially useful links about the solubility of quercetin:

COMPOUND SUMMARY Quercetin

and

On the Solubility of Quercetin

https://core.ac.uk/download/pdf/205809123.pdf

(see attached pdf files)

Also please note that the solubility of quercetin has been the subject of various previous discussion on RG, e.g.

Which solvent is suitable for quercetin solubility?

https://www.researchgate.net/post/which_solvent_is_suitable_for_quercetin_solubility

(23 answers)

and

Is Quercetin soluble in (THF) tetrahydrofuran?

https://www.researchgate.net/post/Is_Quercetin_soluble_in_THF_tetrahydrofuran

(3 answers)

Getting back to your original question I assume that your sample of quercetin was not very pure and perhaps partially oxidized. As you know, quercetion is a typical polyphenol and as such it is a good anti-oxidant. On the other hand, being an anti-oxidant means that the compound is easily oxidized itself. For more information about the oxidation of quercetin please have a look at the following useful article:

Identification of the Products of Oxidation of Quercetin by Air Oxygen at Ambient Temperature

Article Identification of the Products of Oxidation of Quercetin by ...

This paper is freely available as public full text on RG. It seems quite likley that your sample of quercetin contains dark, insoluble oxidation products, so that a purification step is needed prior to you experiments.

A good procedure for making very pure quercetin crystals has been reported in the following RG reference:

The crystal and molecular structure of quercetin: A biologically active and naturally occurring flavonoid

Article The crystal and molecular structure of quercetin: A biologic...

I suggest a very simple purification method: Just dissolve your impure quercetion in diethyl ether or methanol and filter the solution through a folded paper filter to remove the dark insoluble particles. Then let the filtrate evaporate in the fumehood over night and collect the pure quercetin for further experiments.

Good luck with your work and best wishes, Frank Edelman