Have you checked the volume of your reaction? I guess that you succeeded in the past because you worked at higher dilution. At high concentration (i.e. small reaction volume) the EDC activated CD acts as a cross-linker. Another point to check is the order and speed of addition (I would add the CDs on PEI and dropwise)

Good luck.

Thanks Javier!

I tried to use QD/PEI also without EDC. I added QD to PEI. The strange thing is the with ratio of 50 the solution is not clear and aggragates

and with ratio of 500 it's clear.

Do you have any suggestion what QD concentration to work with?

Thanks

This last result is somehow expected: both PEI and QD-COOH are polyfunctional molecules that interact between them (base plus acid yields salt pals water). When the ratio QD/PEI is very high, QDs are not shared by several PEI molecules and there is no cross-linking. The point here is to calculate the amine/carboxylic group ratio. Some time ago we estimated the number of -NH2 groups by molecule as 20.7, 120.5 and 367.5 for PEI 2K, 10K and 25 K respectively. Please, recall that PEI is a very hygroscopic material that interacts almost with any surface (take a look at DOI: 10.1002/cctc.201700776) and this two facts may alter your stoichiometry.

Good luck.