br integrated into nano Se via electrostatic
integrated into nano-Se via electrostatic interaction (Fig. S6C). Besides, the hydrodynamic size of MCDION-Se exhibited no significant increase for 24 h, meanwhile the solution of MCDION-Se presented the typical "Tyndall Eﬀect" under illuminating with an external light source for standing time (Fig. S6E). These results suggested that MCDION-Se possessed excellent stability and could be well applied in vivo. Fur-thermore, MCDION-Se still showed excellent response ability to an external magneter (Fig. S6F), implying that MCDION-Se maintained good queneching ability for T1 imaging. MCDION-Se presented the ef-fective release of Mn2+ and good pH-responsive T1 relaxation rate (Figs. S7A and B), indicating that nano-Se coated onto MCDION-1 cannot aﬀect the pH-responsive contrast eﬀect.
Confocal laser scanning microscopy (CLSM) observed that HeLa Angiotensin I treated with FITC-labeled MCDION-Se presented typical green fluorescence (Fig. S8A), and the fluorescence intensity gradually in-creased with the concentration of MCDION-Se increasing, suggesting that MCDION-Se could be eﬀectively internalized by HeLa cells. The corresponding fluorescence quantification analysis was similar to the CLSM images and further confirmed that the cellular uptake behavior of MCDION-Se was concentration-dep endent (Fig. S8B). The eﬀective uptake ability of HeLa cells for MCDION-Se encouraged us to explore its application for cell MR imaging. Next, MR images of HeLa cells gra-dually brightened with the concentration of MCDION-Se increasing, implying that more particles entered into cells and that Mn2+ ions were released in cells (Fig. 5A). Besides, compared to free Mn2+ ions, the image of MCDION-Se was brighter, which might be attributed to more Mn2+ ions being released intracellularly from MCDION-Se. In the in vivo MRI experiment, the T1-weight MR images of tumors
in mice injected with free Mn2+ ions gradually brightened, reached a maximum at 30 min postinjection, and then began to weaken (Fig. 5B). The T1-weighted images of tumors treated with MCDION-1 and MCDION-Se were similar to those of tumors treated with free Mn2+ ions, but the brightest images appeared at 45 min postinjection, im-plying that the blood circulation times of MCDION-1 and MCDION-Se were longer than that of free Mn2+ ions. Additionally, the images of tumors of mice treated with MCDION-1 and MCDION-Se were brighter than those of mice treated with free Mn2+ ions, which could be at-tributed to the eﬀective contrast ability of the MCDIONs in response to the weakly acidic microenvironment of tumors. In addition, the corre-sponding MRI signal-to-noise ratio change ( SNR) in the tumor region was recorded as shown in Fig. 5C. The maximum SNR was 30.35% for free Mn2+ ions, 58.05% for MCDION-1, and 54.31% for MCDION-Se, suggesting that pH-responsive MCDION-1 and MCDION-Se had a better contrast eﬀect for tumors than Mn2+ ions. Moreover, the coating of nano-Se cannot significantly decrease the contrast ability for MCDION; therefore, MCDION-Se might be a good pH-responsive theranostic agent.
To assess the theranostic eﬀect of MCDION-Se, it was first necessary to investigate its Fenton-like reaction eﬃciency using 3,3′,5,5′-tetra-methylbenzidine (TMB) to detect ·OH . It can be seen in Fig. 4D that the color of the H2O2 solution treated with MCDION-Se gradually turns from colorless to blue, and the corresponding absorption intensity sig-nificantly strengthened with decreasing pH (Fig. 4F), indicating that the Fenton-like reaction eﬃciency of MCDION-Se was pH-dependent. The color and absorption intensity of the H2O2 solution also significantly increased as the concentration of MCDION-Se increased (Fig. 4E,G), suggesting that the generation of ·OH was dose-dependent. In addition,