I fell dialysis method easy and relabel method. 

Were u have to dialysis your polymer/nanoparticulate solution with know volume of dialysate for a period of time. Amount of drug in dialyzes fluid can be determined using UV or HPLC.

U can refer the folowing article for detailed methodology:

International Journal of Biological Macromolecules 51 (2012) 1109– 1115.

You can also try centrifugal method. Where you have to centrifuge your  sample solution at high rpm for a period of time. 

Note: %EE may not give you accurate results. Its always suggested to carryout drug content studies. 

Crushed PLGA spheres are to be kept over night in a little amount of solvent in which drug is soluble. Next day the mixture is filtered and diluted with solvent, and absorption is measured. With the help of standard curve amount encapsulated can be calculated.  Otherwise, as Naga mentioned, dialysis is best method.

Thanks Jagadevappa, unfortunately my drug is insoluble in most solvents (i.e. DCM) so im unable to obtain a standard curve in that solvent, only for water.  

Dear Ali,

Its not only solvent but the surfactants and polymer we are using in nanoparticle preparation also contribute largely during UV-Visible spectroscopy. If possible, directly go for HPLC instead of UV spectroscopy to over come the excipient interference issue.  For better suggestion please share which drug you are using.

Thanks for your feedback @Jayrajsinh, i'm using doxycyline hyclate.