br Fig FT IR spectra of GNR DHHC GNR DHHC
Fig. 3. FT-IR spectra of GNR, DHHC-GNR, DHHC, DOX-DHHC-GNRAH and AHA (A). UV–vis-NIR spectra (B), size distributions (C) and zeta potentials (D) of GNR, DHHC-GNR, DOX-DHHC-GNR and DOX-DHHC-GNRAH. HRTEM images of GNR (E) and DOX-DHHC-GNRAH (F).
formation of DHHC-GNRAH.
In order to maximize therapeutic eﬀect and minimize toxic side eﬀects, photothermal therapy is usually performed in the first NIR biological window (700–950 nm) (Mackey, Ali, Austin, Near, & Elsayed, 2014). Some studies have demonstrated that GNR with a LSPR wave-length of around 800 nm is desirable for PTT (Maestro et al., 2014). In the present study, GNR was synthesized by seed-mediated growth method and its UV–vis-NIR spectrum is shown in Fig. 3B. It was found that the LSPR peak of our GNR was centered at 802 nm, confirming that
it is qualified for NIR-mediated PTT. Because of the excellent adhesion ability of catechol groups (Heo et al., 2012), DHHC was conjugated onto the GNR surface via Au-catechol bonds. It has been reported that the Au-catechol bonds are more stable than Au-S bonds in intracellular microenvironment (Lee, Lee, Kim, & Park, 2010). The LSPR peak of DHHC-GNR showed a slight red-shift (∼8 nm) to 810 nm as compared to that of GNR, which was attributed to the slight aggregation of DHHC-GNR (Wang, Xu et al., 2014). Neither DOX loading nor AHA decoration changed the peak position.
The hydrodynamic sizes of GNRs before and after modification were evaluated by DLS. As indicated in Fig. 3C, all GNRs showed a bimodal size distribution. This phenomenon is consistent with those reported in the literature (Liu, Pierre-Pierre, & Huo, 2012; Xu et al., 2017). In fact, the small peaks are ascribed to the rotational diﬀusion of GNRs rather than actual particle sizes, while the big peaks represent real hydro-dynamic sizes. Therefore, the small size peaks were considered to be meaningless and the big size peaks were further investigated (Liu et al., 2012). The average sizes of GNR, DHHC-GNR, DOX-DHHC-GNR and DOX-DHHC-GNRAH were 24.5, 45.1, 65.8 and 94.0 nm, respectively. The gradual increase in average size was attributed to the progressive formation of Vorinostat (SAHA, MK0683) layers (Song, Zhou, & Duan, 2012).
The zeta potentials of above-mentioned nanoparticles were further investigated, and the results are shown in Fig. 3D. Due to the presence of CTAB molecules, the zeta potential of the GNR was + 38.4 mV (2010b, Li, Chen, Wu, Wang, & Peng, 2010). After conjugation with DHHC, the zeta potential decreased to + 25.3 mV because of the re-placement of CTAB by DHHC. The zeta potential after DOX loading further increased to + 27.8 mV due to its cationic nature. In contrast, the AHA decoration significantly decreased the zeta potential to -27.6 mV, which further verified the successful decoration of AHA onto DOX-loaded DHHC-GNR conjugate.
The morphologies of GNR and DOX-DHHC-GNRAH were observed by HRTEM. As shown in Fig. 3E, the average length and aspect ratio of CTAB-capped GNRs were about 50 nm and 3.9, respectively, which is qualified for NIR-triggered photothermal therapy (Mackey et al., 2014). The decoration of GNRs with polymers (Fig. 3F) did not aﬀect the size and morphology. It is also observed that the polymer layer on the GNR surface was not obvious. This could be attributed to the low contrast between polymer and carbon membrane, and the same phenomenon occurred in previously reported studies (Choi et al., 2010; Tao et al., 2014; Zhu et al., 2017).
3.2. Stability of DOX-DHHC-GNRAH
Good stability of nano therapeutic agents under physiological con-dition is of great significance for their accumulation in tumor sites (Akiyama, Mori, Katayama, & Niidome, 2009). The stability of our GNR and DOX-DHHC-GNRAH in PBS (pH = 7.4) and water was investigated. As shown in Fig. 4A, the two kinds of nanoparticles showed similar stability in water but significantly diﬀerent behaviors in PBS. DOX-DHHC-GNRAH exhibited excellent stability in both PBS and water, while GNR suspension in PBS rapidly turned dark-purple and precipitation occurred. The UV–vis-NIR absorption spectra in Fig. 4B showed that the absorption spectrum of DOX-DHHC-GNRAH in PBS was nearly the same as that in water. However, the LSPR peak of GNR in PBS became ex-ceedingly weak and even flat, suggesting the formation of GNR ag-gregates, which is in accordance with reported result (Huang, Barua, Kay, & Rege, 2009). Furthermore, the absorption spectra of DOX-DHHC-GNRAH dispersed in water and PBS for diﬀerent time periods were monitored by UV–vis spectroscopy. As shown in Fig. 4C and D, the LSPR peaks did not change obviously over the 5-day period, indicating the excellent stability of DOX-DHHC-GNRAH.