oxic nature of both CaP and mPEG-PE, there is great value in developing NP~mPEG-PE/CaP~ as an siRNA delivery system.

RESULTS {#s2}
=======

Synthesis and characterization of mPEG-PE {#s2_1}
-----------------------------------------

The synthetic route to prepare mPEG-PE is illustrated in Figure [1A](#F1){ref-type="fig"}. The amine groups of PE was reacted with the carboxyl groups of mPEG-COOH to form mPEG-PE \[[@R35], [@R36]\]. The reaction was performed in TCM and the EDC/NHS was used as a coupling agent to reduce reaction times. The residual PE and EDC/NHS was removed with anhydrous diethyl ether. The structure of mPEG-PE was confirmed by FT-IR spectra and ^1^H NMR spectra. The ^1^H NMR spectra of mPEG-PE is shown in Figure [1D](#F1){ref-type="fig"}. The proton peak of PE (−CH~3~ at δ 0.9, −CH~2~− at δ 1.3, −CH = CH− at δ 5.34) (Figure [1C](#F1){ref-type="fig"}) were observed in the spectra of mPEG-PE. The existence of methoxy group (CH~3~O− at δ 3.4) and the PEG segment (−CH~2~CH~2~O− at δ 3.7) (Figure [1B](#F1){ref-type="fig"}) peaks from mPEG-COOH demonstrated successful conjugation.

![Synthetic route for mPEG-PE (**A**). ^1^H NMR spectra of mPEG-COOH (**B**), PE (**C**) and mPEG-PE (**D**).](oncotarget-07-2855-g001){#F1}

FT-IR analysis demonstrated that the C = O groups appeared at 1738 cm^−1^ in the IR of PE and mPEG-PE (Figure [2](#F2){ref-type="fig"}). The characteristic peaks of CO-NH and C-O-C were present at approximately 1570 cm^−1^ and 1110 cm^−1^, respectively. These results further indicated the successful synthesis of mPEG-PE.

![Infrared spectroscopy of PE (**A**), mPEG-PE (**B**), CaP (**C**) and mPEG-PE/CaP (**D**).](oncotarget-07-2855-g002){#F2}

Preparation and characterization of NP~mPEG-PE/CaP~ {#s2_2}
---------------------------------------------------

The mPEG-PE micelles were generated using a thin film hydration method with the amphiphilic block copolymer. The anionic micelles were formed through the self-assembly of a thin film of the mPEG-PE in an aque