• 2019-07
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  • br In the case of the ACN a


    In the case of the ACN a detailed report on its synthesis by SFEP was not published before, so a complete characterization by FT-IR, 1H-NMR, and DSC can be found in the Supplementary material (Figures S4, S5 and S6, respectively) all confirming the obtention of the poly(2MBA) core PEGMA-shell nanogels. ACNs synthesis presented a similar beha-vior with respect to the composition drift of CCNs, they were enriched also in PEG but not as much as the CCNs (Table 1). Small particle sizes of ACNs could be easily modulated by modification of the feed monomer composition: the higher the PEGMA content in the feed, the smaller the particle size; probably due to the stronger stabilization force of the PEGMA on nucleated MK2206 [42]. Moreover, negative surface charges (ζ potential, Table 1) unimodal, and narrow distributions (PDI < 0.03) (Supplementary material, Figure S2) were obtained for both nanocarriers (ACN1 and ACN2).
    Fig. 2a and b illustrate the Dh and ζ potential for both CCN and ACN nanocarriers at different values of pH from 3 to 10. CCN1 and CCN2 at pH 3 were highly swollen due to the protonation of the amine groups, exhibiting large values of Dh and high positive surface charge. As the pH was increased, the ζ potential became neutral and the particle size was reduced, finding an isoelectric point at pH values of 8.1 and 7.4 for CCN1 and CCN2, respectively. A more basic medium leads to small 
    collapsed nanoparticles. An opposite behavior was seen in the anionic systems, in which the highly swollen state was found at pH 10 since the core network is deprotonated and highly anionic, giving rise to an in-crease in the Dh. The isoelectric point shall be found below pH 3, therefore the surface charge is always negative throughout the studied pH range. The rate of change of ζ potential with respect to pH (mV/pH unit) for both systems (CCN and ACN) show similar values, -6.8 and -8.1 for CCN and ACN, respectively, which could be an indication that similar processes of deprotonation were carried out during the experi-ments.
    For comparison purposes, the synthesis of nanogels by SFEP of DEAEMA or 2MBA without PEGMA (nanocores) was attempted.
    As expected the sizes were larger, the distribution of sizes broader, and the ζ−potential showed higher values (positive or negative) than those shown for ACN/CCN in Table 1 (see Figure S2 in Supplementary material). Hence, the PEG-shell produces a decrease in the surface charge, which is a good characteristic for the nanocarriers. According to He et al., [21] nanoparticles bearing more positive or negative surface charge they are more attractive to phagocytic cells, due to electrostatic interactions. For this reason, a neutral nanocarrier is preferable for transportation throughout the bloodstream at pH 7.4 in order to avoid the immune system.
    The obtained nanogels show a spherical morphology by AFM in sizes of 80 and 100 nm for ACN1 and CCN1, respectively (Figure S7 in Supplementary material). As is well known, the AFM measurements differ from DLS due to the collapsed state of the nanogels in the former technique [36].
    Additionally, the temperature sensitivity of the CCN was tested,
    Fig. 2. Physicochemical characteristics of cationic nanogels (CCN1 and CCN2) and anionic nanogels (ACN1 and ACN2): a) Dh as function of pH, and b) ζ potential as function of pH. Loading and release of CDDP in both nanogels: c) Release profiles for ACN1 as function of time, and d) Release profiles for CCN1 as function of time.
    since it was reported that a volume phase transition temperature (VPTT) was found for PDEAEMA nanogels at certain pH-values [43]. Figures S8 and S9 in the Supplementary material show that a VPTT was indeed found; a change in pH from 7.4 to 6.8 resulted in a shift of the VPPT to higher temperatures, which is in agreement with previous reports [43].
    Finally, CCNs and ACNs were synthesized using a fluorescent crosslinker, and the resulting nanogels exhibited green fluorescence, these nanocarriers are useful for cell-internalization studies. Their mean physical and chemical characteristics are shown in Table 1. Results show that these nanogels are slightly larger than the ones synthesized using EGDMA crosslinker and they show a more positive ζ-potential value. More insights into the characteristics of these nanogels are matter of future studies.
    3.2. Loading and release rate of CDDP
    CCN and ACN systems were loaded with CDDP, following the pre-viously described procedure. The magnitude of the surface charge was slightly attenuated for both systems (CCN and ACN) when the CDDP was added (Figure S2 in Supplementary material). The results of the loading of CDDP into anionic and cationic nanogels were similar, around 30 wt% of CDDP in each nanogel, resulting also in good DLE values of up to 89% (Figure S10 in Supplementary material), similar values of DLE were reported in the literature [44]. A possible interac-tion is expected between the nanogel core and the CDDP through metal coordination with carboxylic acids or amine groups, leading to a good loading of the drug in the nanocarriers (Fig. 3) [25].