The exploration of magnetic resonance imaging (MRI) agents possessing excellent performances and high biosafety is of great importance for both fundamental science research and biomedical applications. In this study, we present that monodisperse Fe2O3 supraparticles (SPs) can act as T 1-weighted MRI agents, which not only possess a distinct off–on MRI switch in the tumor microenvironment but also are readily excreted from living bodies due to its quasi-amorphous structure and hierarchical topology design. First, the Fe2O3 SPs have a surface-to-volume ratio obviously smaller than that of their building blocks by means of self-assembly processes, which, on the one hand, causes a rather low r 1 relaxivity (0.19 mM–1 s–1) and, on the other hand, can effectively prevent their aggregation after intravenous injection. Second, the Fe2O3 SPs have a dramatic disassembly/degradation-induced active T 1-weighted signal readout (more than 6 times the r 1 value enhancement and about 20 times the r 2/r 1 ratio decrease) in the tumor microenvironment, resulting in a high signal-to-noise ratio for imaging performances. Therefore, they possess excellent in vivo imaging capacity, even with a tumor size as small as 5 mm3. Third, the disassembled/decomposed behaviors of the Fe2O3 SPs facilitate their timely clearance/excretion from living bodies. In particular, they exhibit distinct renal clearance behavior without any kidney damage with the right dosage. Fourth, the favorable biodegradability of the as-prepared Fe2O3 SPs can further relieve the concerns about the unclear biological effects, particularly on nanomaterials, in general.

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For example, soft-template fabrication is one of representative methods for various inorganic SPs, [4c,5] which however possess two typical problems of two-step fabrication systems. First of all, they need lots of organic solvents and surfactants, which inevitably results in environmental concerns. Then, monodispersity of the resultant products is often not ideal, causing some difficulty in their property studies and applications. [1b,6] To address these issues, we previously developed a self-limited strategy for onestep in situ fabricating various (CdS, CdSe, ZnS, PbS, Au@CdSe core@shell, etc.) SPs with narrow size distribution. [7] Because the SPs are self-assembled based on the intrinsic forces among the building blocks, any surfactants and organic solvents are completely needless. Correspondingly, the fabrication processes are carried out in 100% water solutions. Then, to be appropriate for biomedical applications, a few modified fabrication systems have been further explored for the corresponding SPs. [3c,d] Especially, we recently presented a urea modulated system for in situ fabricating citrate modified Fe 2 O 3 SPs. The products are quasi-amorphous, 15 nm in diameter, and have the capacities for active T 1-weighted magnetic resonance imaging (MRI) in vivo. [8] As a new-type of SPs, it is significant to well understand their fabrication approach, engineer their surface chemistry, as well as explore their new application potentials. In this study, we systematically study the eco-friendly fabrication system of the hierarchical Fe 2 O 3 SPs. First, by means of urea as pH modulator, monodisperse Fe 2 O 3 SPs with 7 to 90 nm in diameters can be one-step in situ self-assembled in 100% water solution. Then, in addition to citrate, it is found that the SPs can be fabricated by various carboxyl stabilizers, indicating the versatility of the proposed fabrication system. Third, by means of gallic acid (GA) modification, the corresponding products (denoted as Fe 2 O 3 @GA SPs) can be endowed nearinfrared light-absorbing properties and subsequent photothermal therapy potentials. Fourth, the Fe 2 O 3 @GA SPs exhibit size-dependent photothermal performances and cellular uptake. In terms of 8 nm sized Fe 2 O 3 @GA SPs, their photothermal conversion efficiency and cellular uptake are 66.63% and 8.4 pg Fe/cell, which are well higher than that of 25 and 47 nm sized products. In this study, the eco-friendly fabrication system of the hierarchical monodisperse Fe 2 O 3 supraparticles (SPs) is systematically studied. First, by means of urea as pH modulator, monodisperse Fe 2 O 3 SPs with 7 to 90 nm in diameters can be one-step in situ self-assembled in 100% water solution. Then, the SPs can be reliably fabricated by various carboxyl stabilizers, indicating the versatility of the proposed fabrication system. Third, by means of gallic acid (GA) modification, the corresponding products (denoted as Fe 2 O 3 @GA SPs) can be endowed near-infrared light-absorbing properties and photothermal therapy potentials. Fourth, the Fe 2 O 3 @GA ...

By magnetic separation and subsequent plasmon enhanced fluorescence, an assay platform with a signal output from completely "zero" background to fluorescence amplification is achieved, using quantum dots as reporters. So, it well breaks through the conventional "turn-on" strategy in both lower and upper limits. The sensitivity for hyaluronidase sensing is enhanced 10(4)-10(6) times as compared with previous fluorescence methods.