To determine the concentration of CQDs, you need to get weight yield of your dots synthesized from your natural precursor. For this you need the measure the weight of your dots after solidification and add your respective solvent.
Suppose you are taking 5 gm of precursor and after your synthesis process you need to weight your sample. if you are getting it in liquid then solidify it to get the weight yield suppose 100% efficiency you will get 5 gm. After then adding your solvent suppose 50ml.
Conc will be : 5gm/50ml= 1gm/ml= 1mg/L
then calculate molarity.
Another concept is of Quantum Yield: which require UV/Vis and fluorescence spectra and quinine sulphate as reference material.
take a UV/Vis and fluorescence spectra of your CQDs and then in the same concentration take the fluorescence spectra of quinine sulphate. Then taking the normalized spectra for both to calculate Qyield.