d to C-O bond stretching. The absorption at about 1644 cm^−1^ is due to residual bound water in the samples. Compared with the SNPs curve, two new absorption bands at 1727 and 1572 cm^−1^ appeared in OSA-SNPs curves \[[@pone.0150043.ref016]\]. The peaks at 1727 cm^−1^ were the stretching vibration of C = O \[[@pone.0150043.ref028]\]. This indicated the formation of ester carbonyl groups in the OSA-SNPs \[[@pone.0150043.ref029]--[@pone.0150043.ref030]\]. The new peak at 1572 cm^−1^ was due to the asymmetric stretching vibration of carboxylate RCOO− \[[@pone.0150043.ref030]\]. As presented in [Table 1](#pone.0150043.t001){ref-type="table"}, the DS of was 0.024 and 0.036, respectively. The intensities of OSA-SNP with DS of 0.036 absorption bands at 1727 and 1572 cm^−1^ were higher than those of 0.024 ([Fig 3](#pone.0150043.g003){ref-type="fig"}), which showed that the intensities of two peaks (1727 and 1572 cm^−1^) increased with the DS. The result was consistent with previous reports \[[@pone.0150043.ref028]--[@pone.0150043.ref029], [@pone.0150043.ref031]\].

![FTIR of taro starch nanoparticles (A), octenyl succinic anhydride modified taro starch nanoparticles with degree of substitution of 0.024 (B) and 0.036 (C).](pone.0150043.g003){#pone.0150043.g003}

3.4 X-ray diffraction pattern analysis {#sec018}
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The X-ray diffraction patterns of SNPs and OSA-SNPs are shown in [Fig 4](#pone.0150043.g004){ref-type="fig"} and the crystallinity is shown in [Table 1](#pone.0150043.t001){ref-type="table"}. SNPs exhibited diffraction peaks at Bragg angles (2*θ*) of 5.71°, 14.51°, 17.09°, 19.52°, 22.40°, and 24.01°, representing the B-type crystalline structure. This result was in agreement with the report of Dai et al. \[[@pone.0150043.ref003]\]. In their study, SNPs prepared from taro starch after gelatinization, debranching, and retrogradation also showed a B-type X-ray patterns. The X-ray diffraction patterns of SNPs did not change after OSA modification. The OSA-SNPs