Controllable Synthesis of Manganese Oxide Nanostructures from 0-D

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Controllable Synthesis of Manganese Oxide Nanostructures from 0-D to 3-D and Mechanistic Investigation of Internal Relation between Structure and T1 Relaxivity Zhenghuan Zhao, Jianfeng Bao, Chen Fu, Ming Lei, and Jingliang Cheng Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04100 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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Chemistry of Materials

Controllable Synthesis of Manganese Oxide Nanostructures from 0-D to 3-D and Mechanistic Investigation of Internal Relation between Structure and T1 Relaxivity Zhenghuan Zhao,1* Jianfeng Bao,2 Chen Fu,1 Ming Lei,1 and Jingliang Cheng3 1

2

College of Pharmaceutical Sciences, Southwest University, Chongqing 400715, China.

College of Medical Technology and Engineering, Henan University of Science and Technology, Luoyang 471000, China.

3

Department of Radiology, First Affiliated Hospital, Zhengzhou University, Zhengzhou 450052, China.

*email: [email protected]

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ABSTRACT Since manganese oxide nanomaterials attract wide attention in biomedical and energetic field, understand the inner-relationship between their properties and structures is fundamental and urgently needed. However, controllable synthesize metal oxide nanomaterials with diverse morphologies is still a persistent challenge. Herein, various anisotropic manganese oxide nanostructures from zerodimensional (0-D) to three-dimensional (3-D) were successfully fabricated through thermal decomposition. We observed that chloride ions can assist the formation of 0-D nanooctaherals, nanocubes, and nanooctapods due to its binding capacity to the manganese ions on the nanocrytal surface. Interestingly, the procedural heating up process can affect the decomposition rate of the manganese-oleate, which drive a substantial reduction in the surface free energy by sharing a common crystallographic orientation and lead to the formation of 1-D and 3-D nanostructures by oriented attachment growth. On the basis of systematic analyses, surface-to-volume ratio, surface manganese ion density, and geometrical confinement determined by specific morphology are found to be the key parameters to achieve high-performance T1 relaxivity. Moreover, the screened out manganese oxide nanocubes with high r1 value exhibit good contrast ability in T1-weighted MRI imaging in vitro and in vivo, showing a great potential to lesion detection in T1 contrast imaging. This study build a link between controllable synthesis of manganese oxide nanomaterials and its property, thus, provide rational design clue to develop high-performance magnetic oxide nanomaterials, especially in biomedical and energy fields.


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INTRODUCTION Due to the wide application in biomedical application, catalysis, energy and data storage devices, and chemical analysis, developing new magnetic oxide nanostructure with unique feature and high performance is of great importance for both fundamental science and technical application.1-7 Among various magnetic oxide nanomaterials (e.g. iron oxide, manganese oxide, and gadolinium oxide), manganese oxide (MO) nanomaterials with unique physical properties have recently got extensive attention,8, 9 especially as contrast agent for shortening of the spin-lattice (or longitudinal) relaxation time, T1.10, 11 Compared to the traditional superparamagnetic iron oxide nanoparticles (e.g. magnetite), utilizing MO nanomaterials as contrast agent can avoid the misled diagnosis cause by the confusion between lesion and signals from bleeding, calcification, or metal deposits.12 Unfortunately, suffering from its relative low T1 relaxivity, high dosage MO nanoparticles is acquired to assess lesion detection. This disadvantage increases the potential risk of advertise effect and limits the application of MO nanoparticles in clinical diagnosis.13 Based on the Solomon, Bloembergen, and Morgan (SBM) theory, T1 contrast ability is highly depend on the chemical exchange efficiency between magnetic ions and proton.14, 15 It has been recently noticed that controlling morphology of magnetic oxide nanocrystals or forming three-dimensional (3-D) magnetic oxide nanoclusters have been shown theoretically and experimentally to improve T1 or T2 relaxivity.16-18 Therefore, systematic synthesis of series of MO nanoparticles with various structures to figure out the inner relationship between the structure of MO nanoparticles and it T1 contrast property is urgent and can accelerate the development of high-performance manganese oxide based T1 contrast agent. In generally, the precise control of the morphology of zero-dimensional (0-D) nanostructures are based on tuning the different growth rate in specific direction in crystal growth process. The most common way to fabricate the anisotropic nanomaterials with various morphologies is through introduction of a selective capping agent.19-22 The presence of a capping agent can change the order ACS Paragon Plus Environment


of free energies for different crystallographic planes, and thus their relative growth rates and form the anisotropic nanoparticles. Whereas, synthesis of complicated nanostructure in high dimensional is controlled by the aggregation-based crystal growth. By slowing down the growth of the nuclei, the nearby nanoseeds may share a common crystallographic to elimination high energy surface and form the complicated nanostructures by oriented attachment and secondary growth.23-27 In the past decades, some process on synthesis of monodisperse metal nanocrystructures (e.g. Au, Pt, Pd, and their alloys) with a variety of anisotropic morphologies has been successful synthesized, including rod, spheroid, cube, octahedron, octahedron, and octapods.28-32 Unfortunately, it is still huge challenge to precise control the morphology of MO nanomaterials from 0-D to 3-D due to their diverse crystal packing structure, strong metal-oxygen covalent bonding, and variable oxidation states.33, 34 Over the past years, thermal decomposition of metal (e,g, Fe, Mn, and Gd) precursor in high-boiling-point solvents have been developed to produce magnetic oxide nanoparticles with high crystallinity, narrow size distribution (σ≤5 %), and large scale.35-40 Recent progress showed that anisotropic magnetic oxide nanoparticles could form by controlling the reaction temperature, selecting the proper surfactant, and introducing proper selective capping agents.16, 41-43 However, synthesis of various MO nanostructures from 0-D to 3-D and explore their formation mechanism is rare. In this study, we report a morphology controllable synthesis of MO nanostructures by thermal decomposition of Mn-oleate complex through procedural heat up process and chloride assistant growth. We observed that the heating rate and chloride ion can effectively affect the morphology of the final product. Various 0-D MO nanostructures with the morphologies of cubes, octapods, and octahedral were successfully synthesized. Additionally, we obtained not only 1-D rod-shaped MO nanostructures, but also self-assemblied nanocluster and nanowhisker MO nanostructures in 3-D by oriented attachment. More importantly, this work provided adequate sources to explore the internal ACS Paragon Plus Environment

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relation between the morphology and T1 relaxation time shortening effects in MRI. The obtained MO nanostructures with high surface-to-volume ratio, surface manganese ion density, and geometrical confinement can exhibit strong T1 contrast capacity. Additionally, further cell and animal study shows that the screened out MO nanocubes with highest r1 values are suitable as high-performance T1 contrast agents for in vivo MRI. These results may provide clues to develop high-performance nanomaterials, especially metal oxide nanomaterials, based on the morphology-capacity relationship in biomedical and energy application.

EXPERIMENTAL SECTIONS Experimental Sections Reagent. Oleic acid (tech 90%), manganese(II) chloride tetrahydrate (tech 90%), phenyl ether (99%), and 1-octadecene (90%) were purchased from Alfa Aesar. NaCl (AR), sodium hydroxide, hexane, tetrahydrofuran, dopamine hydrochloride, sodium oleate, isopropanol, and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All chemicals were used as received without further purification. Characterizations. Transmission electron microscopy (TEM) images were captured on a JEM-2100 microscope at an accelerating voltage of 200 kV. The X-ray diﬀraction (XRD) patterns were obtained X-ray powder diffraction (PANalytical X'Pert³ Powder). The element analysis of Mn was determined by inductively coupled plasma mass spectroscopy (ICP-MS). M-H curves were obtained by on a Quantum Design MPMS-XL-7 system. The MRI testing and T1 relaxation time measurements were tested at a 0.5 T NMR120-Analyst NMR Analyzing&Imaging system (Niumag Corporation, Shanghai, China). In vivo MR images measurements were performed on a 7 T MRI scanner (Varian 7 T micro MRI System).



Synthesis of 0-D truncated MO nanooctahedrons (MOOHs). In a typical experiment, 0.6 g Mn-oleate (0.97 mmol), 155 µL oleic acid, and 20 mg sodium chloride were dissolved in 12 mL 1-octadecene at room temperature. The mixture was degassed in vacuum for 30 min and backfilled with argon to remove any low volatile impurities and oxygen at room temperature. And then, we heated the reaction solution to 320 °C rapidly and maintained this mixture at that temperature for 1 h. The resultant solution was then cooled to room temperature and mixed with 30 mL isopropanol to precipitate the nanoparticles. The nanoparticles were separated by centrifugation and washed 3 times with ethanol. After washing, the nanoparticles were dissolved in hexane for long term storage at 4 °C. Synthesis of 0-D MO nanocubes (MOCs). In a typical experiment, 0.6 g Mn-oleate (0.97 mmol), 155 µL oleic acid, and 20 mg sodium chloride were dissolved in 12 mL 1-octadecene at room temperature. The mixture was degassed in vacuum for 30 min and backfilled with argon to remove any low volatile impurities and oxygen at room temperature. And then, we heated the reaction solution to 200 °C with a constant heating rate of 3.3 °C min-1, and kept at that temperature for 20 minutes. After cooling this mixture to room temperature, we heated it to 320 °C rapidly and maintained this mixture at that temperature for 40 min. The resultant solution was then cooled to room temperature and mixed with 30 mL isopropanol to precipitate the nanoparticles. The nanoparticles were separated by centrifugation and washed 3 times with ethanol. After washing, the nanoparticles were dissolved in hexane for long term storage at 4 °C. Synthesis of 0-D MO nanooctapods (MOOPs). The procedure of synthesis of MOOPs is similar to that of MOCs. After heating to 320 °C, the solution was kept at that temperature for 150 min. The nanoparticles were separated by centrifugation and washed 3 times with ethanol. After washing, the nanoparticles were dissolved in hexane for long term storage at 4 °C.


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Synthesis of 1-D MO nanorods (MORs). In a typical experiment, 0.8 g Mn-oleate (1.30 mmol) and 206 µL oleic acid were dissolved in 12 mL 1-octadecene at room temperature. The mixture was degassed in vacuum for 30 min and backfilled with argon to remove any low volatile impurities and oxygen at room temperature. And then, we heated the reaction solution to 200 °C with a constant heating rate of 3.3 °C min-1, and kept at that temperature for 20 minutes. After cooling this mixture to room temperature, we heated it to 320 °C rapidly and maintained this mixture at that temperature for 1 h. The resultant solution was then cooled to room temperature and mixed with 30 mL isopropanol to precipitate the nanoparticles. The nanoparticles were separated by centrifugation and washed 3 times with ethanol. After washing, the nanoparticles were dissolved in hexane for long term storage at 4 °C. Formation of 2-D manganese oxide superlattice. The formation of 2-D superlattice manganese oxide superlattice was carried out as follows. Typically, 0.1 mL manganese oxide nanospheres dispersed in hexane was gently deposited on the copper grid. The copper grid was placed in the fume hood to slowly evaporate hexane overnight. After the hexane was removed completely under vacuum, 2-D manganese oxide superlattice was obtained. Synthesis of 3-D branched MO nano-assembled structure (BMOAs). In a typical experiment, 0.6 g Mn-oleate (0.97 mmol), 155 µL oleic acid, and 18 mg sodium chloride were dissolved in 10 mL phenyl ether at room temperature. The mixture was degassed in vacuum for 30 min and backfilled with argon to remove any low volatile impurities and oxygen at room temperature. And then, we heated the reaction solution to 210 °C with a constant heating rate of 2 °C min-1, and kept at that temperature for 20 min. After cooling this mixture to room temperature, we heated it to 260 °C rapidly and maintained this mixture at that temperature for 1 h. The resultant solution was then cooled to room temperature and mixed with 30 mL isopropanol to precipitate the nanoparticles. The



nanoparticles were separated by centrifugation and washed 3 times with ethanol. After washing, the nanoparticles were dissolved in hexane for long term storage at 4 °C. Synthesis of 3-D spherical MO nano-assembled structure (SMOAs). In a typical experiment, 0.6 g Mn-oleate (0.97 mmol), 155 µL oleic acid, and 10 mg sodium chloride were dissolved in 10 mL phenyl ether at room temperature. The mixture was degassed in vacuum for 30 min and backfilled with argon to remove any low volatile impurities and oxygen at room temperature. And then, we heated the reaction solution to 210 °C with a constant heating rate of 2 °C min-1, and kept at that temperature for 20 min. After cooling this mixture to room temperature, we heated it to 260 °C rapidly and maintained this mixture at that temperature for 1 h. The resultant solution was then cooled to room temperature and mixed with 30 mL isopropanol to precipitate the nanoparticles. The nanoparticles were separated by centrifugation and washed 3 times with ethanol. After washing, the nanoparticles were dissolved in hexane for long term storage at 4 °C. Synthesis of 3-D MO nanowhiskers (MOWs). In a typical experiment, 0.6 g Mn-oleate (0.97 mmol) and 155 µL oleic acid were dissolved in 10 mL phenyl ether at room temperature. The mixture was degassed in vacuum for 30 min and backfilled with argon to remove any low volatile impurities and oxygen at room temperature. And then, we heated the reaction solution to 210 °C with a constant heating rate of 2 °C min-1, and kept at that temperature for 20 min. After cooling this mixture to room temperature, we heated it to 260 °C rapidly and maintained this mixture at that temperature for 40 min. The resultant solution was then cooled to room temperature and mixed with 30 mL isopropanol to precipitate the nanoparticles. The nanoparticles were separated by centrifugation and washed 3 times with ethanol. After washing, the nanoparticles were dissolved in hexane for long term storage at 4 °C. In vitro MRI imaging. We incubated 1×107 HepG2 cells with MOCs at 37 °C for 0.25, 1.5, and 3 h in the culture media with the pH value of 7.4. The cells were harvested and washed with PBS ACS Paragon Plus Environment

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buffer three times to remove the free MOCs. Then we concentrated the cells at the button of EP tube by centrifugation and performed T1-weighted MRI imaging on a 0.5 T NMI20-Analyst NMR system. The samples were scanned using a multi-echo T1-weighted fast spin echo imaging sequence (TR/TE=100/12 ms, 256 matrices, thickness =1mm, NS=16). In vivo liver MR imaging. Animal experiments were executed according to the protocol approved by Institutional Animal Care and Use Committee of Southwest University. We chose BALB/c mouse (6 weeks old, 18–22 g) as model. After intravenously injected MOCs or MOSs-9 into the mice (2 mg Mn/kg of mouse body weight each), the coronal and transverse plane MR images were scanned using a sequence (TR/TE = 400/10 ms, 256× 256 matrices, thickness = 1mm, FOV = 50× 50) on a Varian 7 T microMRI scanner. The MR images were obtained at pre-injection and 1 h post-injection (n = 3/group). To qualify the signal enhancement, we calculated the signal-to-noise ratio (SNR) by the equation: SNRliver = SIliver / SDnoise, Where SI represents signal intensity and SD represents standard deviation.

RESULTS AND DISCUSSION Synthesis and Characterization of MO nanostructure from 0-D to 2-D. We synthesized MO nanostructures by thermal decomposition of Mn-oleate in 1-octadecene (ODE) solvent. Due to the isotropic growth of crystal facet is dominated, MO nanospheres (MOSs) was formed without any other treatment. The diameter of MOSs can be tuned by extending the synthesis in a reproducible way (Supporting Information, Figure S1). Interestingly, solvent evaporation from colloidal dispersions results in the MOSs align quite regularly to form a highly ordered 2-D superlattice structure, which could be ascribed to the extremely narrow size distribution (Supporting Information, Figure S2).44 To obtain the anisotropic MO nanostructures, we introduce chloride ions to bind to the surface manganese ion to reduce the order of free energies of crystallographic planes. Additionally, a ACS Paragon Plus Environment


procedural heat up process was utilized to control the decomposition rate of Mn-oleate, which can effectively affect the formation rate of nucleus, saturation level of growth materials, and the reaction equilibrium. By only introducing trace amount of chloride ions into this system, we obtained uniform truncated MO nanooctahedrons (MOOHs). The transmission electron microscopy (TEM) images show that the as-prepared products are uniform truncated octahedron morphology with high yield (> 95%). The edge lengths of these MOOHs are appropriate 22.44 ± 1.43 nm (Figure 1a and Supporting Information, Figure S3). To better visualize the three-dimensional structure of this nanostructure, we analyzed it along [110] and [111] zone axis by high-resolution TEM (HRTEM). We noticed that the interplanar spacing distances are 2.58 and 1.58 Å, which could be assigned to the (111) and (220) plane of manganese oxide, respectively (Figure 1b,c). These results suggest that the truncated MOOHs are mainly exposed by eight {111} facets, which are polar facet composed of Mn cations and exhibit much higher energy density than the nonpolar facet.45 Additionally, we synthesized MO nanostructures with procedural heating process without adding chloride ions. Interestingly, elongated rod-shaped and cross-shaped MO nanostructures were generated. TEM images indicate that the aspect ratio of these MO nanorods (MORs) are approximate 14 (with the length of 284.22 ± 11.91 nm and width of 19.61 ± 1.47 nm), which is located in the typical range of nanorods (Figure 1d,e and Supporting Information, Figure S3).46, 47 Also, the widths of the branches of cross-shaped are same, suggesting that they grow out from a central core. After carefully surveying these unique nanoparticles, we found that the edges of these nanorods are not smooth. The HRTEM image reveal the uniform lattice fringes with the length of 2.24 Å, that is (200) plane of manganese oxide, in independent area (Figure 1f). This unusual phenomenon may imply the anisotropic growth of MO nanostructure proceeded by the oriented attachment mechanism. It is interesting that when we introduced chloride and procedural heating process simultaneous in the system, we obtained the MO nanostructure with cubic morphology. TEM images show that all ACS Paragon Plus Environment

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as-prepared MO nanocubes (MOCs) are nearly cubic morphologies with the side length of 9.33 ± 0.68 nm in high yield (Figure 1g and Supporting Information, Figure S3). Along the [100] zone axis, HRTEM images reveal that all MOCs possess the same interplanar distance of about 1.58 Å, which could be assigned to the (220) plane of manganese oxide (Figure 1h). These results suggest that all obtained MOCs are composed by six {100} basal facets. Fascinatingly, we made MO nanooctapods (MOOPs) by only extending the reaction time to form MOCs. The TEM images show that MOOPs with the average length between two nearby armed points is 25.87 ± 1.79 nm (Figure 1i and Supporting Information, Figure S3). We observed specific lattice fringe, that is (220) plane of manganese oxide with the value of 1.58 Å, across this nanostructure (Figure 1j). The formation of MOOPs is probably due to the epitaxial growth process on the MOCs, which is clearly revealed by survey the as-prepared product in the reaction system at a certain time interval (Supporting Information, Figure S4). X-ray powder diffraction (XRD) pattern was employed to identify the crystallographic structure of all samples. MOOHs, MORs, and MOOPs nanostructures exhibit a typical MnO diffractogram pattern (JCPDS no. 01-072-1533), whereas MOCs nanoparticles show mixed MnO (JCPDS no. 01-072-1533) and Mn3O4 (JCPDS no. 01-075-1560) diffractogram patterns (Figure 2a). These different crystallographic patterns could be ascribed to the partial oxidation from Mn2+ to Mn3+ in air, which has been proved by the previous research.48, 49 Since the size of MOCs is significantly smaller than that of MOOHs, MORs, and MOOPs, MOCs exhibit much higher surface-to-volume ratio. This feature increases the oxidation probability of MOCs and leads to its mixture phase. Consistent with XRD analysis, the X-ray photoelectron spectroscopy (XPS) surveys in Mn 2p3/2 spectra show the partial oxidation of MOCs as well. The Mn 2p3/2 peaks of MOOHs, MORs, and MOOPs are measured at a binding energy of 640.58, 640.36 and 640.41 eV, which are agree with the value of MnO nanocrytals.50 Compared to MOOHs, MORs, and MOOPs, the peaks of MOCs slightly shift ACS Paragon Plus Environment


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from typical MnO pattern to Mn3O4 pattern with the value of 641.03 eV (Figure 2b). These results indicate that the MOOHs, MORs, and MOOPs are typical MnO phase, while MOCs are mixture phase of MnO and Mn3O4. Formation mechanism of manganese oxide nanostructure from 0-d to 2-D. To understand the formation mechanism, careful analysis of samples taken from the reaction solution of each MO nanostructures at the certain time interval was conducted. The TEM analysis clearly indicates that the formations of different morphologies nanostructures are highly dependent on the reaction condition. Nucleation represents the very first stage of any crystallization process, while it is hardly to be observed in TEM analysis.51 Thus, we analyzed the dynamic growth process of MO nanostructure after the nuclei formed by TEM and evaluated dynamic thermal decomposition process of Mn-oleate by fourier transform infrared spectroscopy (FT-IR). We observed the irregular crystal nuclei with the sizes of ~3-6 nm in all samples after the aging time of 10 min in TEM images (Figure 3). Due to the higher nucleation temperature than other samples, the sizes of crystal nuclei in MOOH reaction solution are much larger than others. The successful formation of these crystal nuclei proves that 200 °C is enough high for the nucleation process. For the growth processes of MOCs, small quasi-spherical nanocrystals are generated after the nucleation process. With continuous consumption of the monomer at 320 °C, the nanostructure with cubic morphology formed. Besides, the size and uniformity of cubic nanostructures are all improved follow the Ostwald ripening process along with the elapse of reaction time (Figure 3b-d). Additionally, FT-IR analyses show a gradually increase of the peak at 1711cm-1, corresponding to the carboxyl group of oleic acid, along with the extension of reaction time (Supporting Information, Figure S5 ).37,

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Coupled the successful formation of nuclei and growth of nanocrystals, these results clearly show the decomposition of Mn-oleate during the reaction. Surprisingly, dynamic growth processes of MOOPs are similar to that of MOCs. Along with extension of reaction time of MOCs, the epitaxial ACS Paragon Plus Environment

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growth process happened and formed the MOOPs (Figure e-h). For the growth of MOOHs, we noticed the formation of octahedron and tetrahedron nanostructure after reaction for 35 min (Figure 3k). On the basis of that the surface of octahedral and tetrahedron MO nanostructure are enclosed by {111} plane, this result may reveal that the chloride ions is prefer to bind to manganese ions on {111} plane with high metal ion package density and surface energy at high temperature. Interestingly, the TEM images of product took from the reaction solution of MORs clearly showed self-assemble process along specific direction, especially the product took at aging time of 5 min at 320 °C (Figure 3m-p). These results indicate that the anisotropic growth of MORs may proceed by the oriented attachment mechanism. On the basis of TEM analysis results, a formation mechanism of 0-D and 1-D anisotropic MO nanostructures was proposed (Supporting Information, Figure S6). The morphology of a crystal is dependent on the specific surface energies associated with the facets of crystal. Based on the well know Wulff facets theory, a crystal has to be bounded by facets giving a minimum total surface energy when it is stable.53, 54 Additionally, the morphology of a crystal can also be considered in terms of growth kinetics, by which the fast growth rate on one facet causes the disappearance of the facet and vice versa.55 The final morphology of a crystal could be controlled by introducing appropriate organic or inorganic agent to change the free energies to alter their growth rates and direction. The typical structure of MO nanocrystals is based on a cubic rock-salt structure with three low-energy facets, {100}, {110}, and {111}. The surface energies are following the order of {111}>{110}>{100} with the surface-energy ratio of 1, 1.41, and 1.73, respectively.56-58 Only using oleate as the surfactant, various crystal planes are equally capped and stabilized. Thus, the isotropic growth of crystal facets is dominated, resulting in the formation of MOSs.59, 60 When the procedural heating up and Cl- ions are simultaneously introduced, anisotropic growth happened and formed MOCs. XPS survey in Cl 2p spectrum show the trace amount of Cl- ions on the surface of MOCs ACS Paragon Plus Environment


(Supporting Information, Figure S7). Since the exposed surface of MOCs is {100} facet, this result suggests that the Cl- ions can bind to the manganese ions on {100} plane.61 Thus, a possible Cl- ions assisted MOCs formation mechanism is proposed. The relative low temperature caused by the procedural heating up process lead to the poly-facets nucleus expose large amount of {100} plane with the lowest surface energy on its surface. The existence Cl- ions can bind to the manganese ions on the {100} plane, resulting in the preservation of {100} plane and formation of MOCs in the further heating up process. Along the increasing of the reaction time, the amount of chloride ion is sufficient to preserve {100} plane and results in the epitaxial growth on the MOCs to form the MOOPs.43 Without adding chloride ions, the temperature drop caused by the procedural heating may slow down the growth of the nuclei.42 When two nanoparticles approach each other closely enough, they are mutually attracted by van der Waals forces. Along with the rapid rise of reaction temperature again, the strength to eliminate high energy surface of the pairs will drive a substantial reduction in the surface free energy by sharing a common crystallographic orientation and form the MORs by oriented attachment growth.62 Without applying procedural heating up process, the decomposition rate of the Mn-oleate and nucleation improved due to the high reaction temperature (320 °C), resulting the increase of proportion of {111} facet on the poly-facet surface. Since the {111} plane exhibit the highest metal package density and lowest attachment energy among the basic planes, the absorption of chloride ions on manganese oxide {111} facet is more favorable than others in thermodynamics. The XPS analysis on MOOHs in Cl 2p shows a typical peak of Cl element (Supporting Information, Figure S7). On the basis of that the exposed surface of MOOHs is {111} plane, this results indicate that the introduced Cl- ions can bind to the manganese ions on the {111} facet at high nucleation temperature and assist the formation of MOOHs.61 Synthesis and Characterization of 3-D manganese oxide nanostructure. To obtain the 3-D MO nanostructures, we synthesized MO nanostructure with procedural heating process in phenyl ether in ACS Paragon Plus Environment

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either the presence or absence of chloride. It worthwhile to note that the boiling temperature of phenyl ether is lower than that of ODE, which may slow down the growth of nuclei and promote their assembly. We firstly demonstrated that a uniform 3-D branched MO nano-assembled (BMOAs) structure can be obtained by adding chloride. The edge lengths of these nano-assembled structures are approximate 120.47 ± 7.98 nm (Figure 4a and Supporting Information, Figure S8). After carefully surveying this unique 3-D nanostructure, we found that the morphology of this 3-D nanostructure is similar with the morphology of nanooctapods assembled nanoclusters (Figure 4d).43 To better visualize the structure, we analyzed this 3-D nanostructures by HRTEM. The HRTEM images reveal the uniform lattice fringes with the length of 1.58 Å, that is (220) plane of manganese oxide (Figure 4g). More importantly, the HRTEM images clearly showed a sharp edge, which is same to the octapod nanocrystal and prove the assembled units may be the octapod MO nanoseeds. We have further investigated the growth process of this 3-D nanostructure by analyzed the samples taken from the reaction solution. The TEM images obviously showed the process that pairs of octapod nanoseeds attached to the each other to reduce the surface free energy and form the unique 3-D nanostructure. Additionally, we observed several irregular nanostructures caused by the secondary growth of octapod nanoseeds, which can be ascribed to the high monomer concentration (Figure 5a-c). Interestingly, under the similar procedure with reduced chloride amount, an assembled 3-D MO nanostructure with nearly spherical morphology formed (Figure 4b). Further TEM and HRTEM analysis showed that the edge length of this 3-D spherical MO nano-assembled structure (SMOAs) is about 100.52 ± 6.23 nm with the interplanar spacing distances of 1.58 Å, that is (220) plane of manganese oxide (Figure 4e,h and Supporting Information, Figure S8). In addition, the HRTEM images reveal that the SMOAs are composed of many small nanoseeds with poly facet, which is different to BMOAs and cause the differences on morphologies. Surprisingly, without adding chloride, we obtained 3-D MO nanowhiskers (MOWs). TEM analysis indicates that MOWs ACS Paragon Plus Environment


is highly uniform and composed with many curved nanowhiskers (Figure 4c). The aspect ratio of each nanowhisker are approximate 20 (with the length of 103.50 ± 7.14 nm and width of 5.11 ± 0.21 nm), which is between the values of nanorod and nanowire (Supporting Information, Figure S8). After carefully surveying these unique nanostructures, we found that most nanowhiskers have two branches even if some display 3, 4, or 5 branches (Figure 4f). Based on the TEM images of the intermediate products taken from the reaction solution, the formation of this assembled nanowhisker may be caused by the oriented attachment (Figure 5d-f). It should be noted that all the nanowhiskers are single-crystalline as evidenced by the HRTEM analysis. The HRTEM images show the uniform lattice fringes with the length of 2.24 Å, that is (200) plane of manganese oxide (Figure 4i). We then employed XRD pattern to identify the crystallographic structure of all 3-D MO nanostructures (Figure 6a). All samples exhibit a typical MnO diffractogram pattern (JCPDS no. 01-072-1533). Consistent with XRD analysis, the XPS surveys in Mn 2p3/2 spectra show that the valence of manganese ions in the BMOAs, SMOAs, and MOWs are +2. The Mn 2p3/2 peaks of BMOAs, SMOAs, and MOWs are measured at a binding energy of 640.38, 640.48, and 640.38 eV, which are agree with the value of MnO nanocrytals (Figure 6b-d). These results indicate that the BMOAs, SMOAs, and MOWs are typical MnO phase. Magnetic property of manganese oxide nanostructures. Magnetic properties of all 0-D, 1-D, and 3-D MO nanostructures were investigated at room temperature (300 K) by the superconducting quantum interference device (SQUID) magnetometer. Due to the super-exchange interactions between neighboring Mn2+ cations, bulk MnO nanoparticles is classical antiferromagnet with Neel temperature (TN) of 100 K.63, 64 However, the field-independent magnetization (M-H) curves show that the magnetic moments of all MO nanostructures reveal the linear growth trend as the increase of applied magnetic field (Figure 7a,b). Moreover, all samples exhibit nearly zero values for both coercivity and remanence at 300 K, indicating that all MO nanostructures exhibit paramagnetic ACS Paragon Plus Environment

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behavior at room temperature (Figure 7c,d). This phenomenon could be ascribed to the presence of uncompensated spins on the particle surface.37, 65 For MOOHs, MOCs, and MOOPs, the unsaturated magnetic moments reach to 1.61, 1.92, and 1.78 emu/g under magnetic field of 5 T at 300 K, respectively. These differences on the magnetization values could be ascribed to the different noncompensated surface spin. Theoretically, the MO nanstructures with higher surface-to-volume ratio on the surface exhibit larger amount of noncompensated surface spin and higher magnetization values.37 The surface-to-volume ratios of MOOHs, MOCs, and MOOPs are 0.34, 0.67, and 0.41 nm-1, respectively. Thus, MOCs exhibit the highest magnetization values among the 0-D nanostructures. Similarity with 0-D MO nanostructures, the SMOAs, BMOAs, and MOWs in 3-D exhibit the increased unsaturated magnetic moments with the values of 2.08, 2.19, and 2.43 emu/g, respectively, due to the increased noncompensated surface spins. Additionally, temperature dependence of the magnetization of all sample were measured with zero-field-cooling (ZFC) and field-cooling (FC) procedures in an applied magnetic field of 100 Oe in the temperature range 5-300 K. The ZFC/FC analyses clearly show that all MO nanostructures show obvious phase transition from ferromagnetic to paramagnetic phases at the relative low temperature. Due to the increase of noncompensated surface spin, the blocking temperature (TB) values of MOOHs, MOOPs, and MOCs increase with the values of 15.99, 20.09, and 21.98 K, respectively (Figure 7e). Interestingly, the MORs, BMOAs, SMOAs, and MOWs growth by oriented attachment exhibit much higher TB than 0-D MO nanostructures with the values of 65.03, 40.02, 34.01, and 50.02 K, respectively (Figure 7f). These phenomena could be ascribed to their multi-core structures caused by the oriented attachment growth. Although all MO nanostructures show different TB, their TB values are all below 300 K, indicating the paramagnetic behaviors at room temperature and enabling these MO nanostructures for biomedical application. Shortening effect of MO nanostructures on the proton spin-lattice relaxation. Recently, ACS Paragon Plus Environment


biocompatible MO nanoparticles were proposed as a new T1 MRI contrast agent. However, the inner-relationship between the crystal structures and T1 contrast abilities is unclear. Due to the as-prepared MO nanostructures are hydrophobic and unsuitable for surveying their T1 contrast abilities, we transferred all nanostructures to aqueous media by small molecular, dopamine, to form thin-layer coating on the surface. This thin-layer coating can not only endow the good water solubility but also provide proton abundant environment to the nanostructure, which are critically important to produce MRI contrast signal.10, 63 The transferred nanostructures are stable in water without precipitation for at least 1 month. TEM analyses further confirm that the transferred nanostructures are well dispersed in water without any clustering or aggregation (Supporting Information, Figure S9). We then focused on the T1 relaxation shortening capacities of all obtained MO nanostructures by a 0.5 T MRI scanner. Interestingly, the MO nanostructures with different crystal structures exhibit different T1 relaxivities. The r1 values of MOOHs, MORs, MOCs, MOOPs, BMOAs, SMOAs, and MOWs are 1.56, 0.48, 11.50, 1.89, 0.46, 0.42, 2.26 mM-1s-1, respectively (Figure 8a and Support information, Figure S10 and Table S1). Since the surface coating ligand is the same, the different r1 values could be attributed to their different crystal structures. To simplify the investigation on the inner-relationship between the T1 shorten effect and crystal structure of MO nanostructures, we divided all nanostructures into two part based on their complexities. One is 0-D and 1-D nanostructure, the other is the 3-D nanostructures. Interestingly, we found that the r1 values of 0-D and 1-D MO nanostructures are highly dependent on their surface-to-volume ratio. The surface-to-volume ratio of MOCs, MOOPs, MOOHs, and MORs were calculated to be 0.67, 0.41, 0.34, and 0.17, nm-1 (Figure 8b and Supporting Information, Table S1). Along with the decrease of surface-to-volume ratio, the T1 relaxation of MOCs, MOOPs, MOOHs, and MORs exhibited the obvious reduction with the values of 11.50, 1.89, 1.56, and 0.48 mM-1s-1. These results could be ascribed to the fact that the surface-to-volume ratio can mainly determine the surface exposed ACS Paragon Plus Environment

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paramagnetic ions number. Theoretically, the T1 relaxation enhancement is mainly related to the inner sphere regime that protons achieve chemical exchange with the surface paramagnetic ions directly. Thus, the higher surface-to-volume ratio, the more paramagnetic ions expose on surface and the higher T1 relaxation enhancement capacity exhibit.66,

67

Additionally, we noticed that MO

nanostructures with metal rich surface may possess high density of accessible metal ions, fast exchange of water, and multicenter coordination for water molecules, which can further improve its high T1 relaxivity (Figure 8c).68 To verify this inference, MOCs with {100} facet exposed and MOOHs with {111} facet exposed were chosen as representative example. Accordingly, MOSs with mean diameter of 9 and 18 nm (denoted as MOSs-9 and MOSs-18) were used for comparison because of the similarity surface-to-volume ratio (that is, MOCs with MOSs-9 and MOOHs with MOSs-18). Since the {100} facet of MO nanostructure is a metal-rich surface, the manganese ions density on the surface of MOCs exposed {100} facet is much higher than that on the MOSs-9 with the mixed exposed facet. MOCs manifest exceptionally stronger T1 contrast effects than MOSs-9. The r1 value of MOCs is 11.50 mM-1s-1, which is approximate 2.64 times higher than that of MOSs-9 (4.46 mM-1s-1). Similarly, the MOOHs exposed (111) facet show slightly higher r1 value than MOSs-18 with the value of 1.28 mM-1s-1 (Supporting information, Figure S11 and Table S2). These results indicate that MO nanostructures with high surface manganese ion intensity can further improve their T1 relaxivities. It is worthwhile to note that the increment of r1 values caused by MOOHs is lower compared to the r1 value improvement caused by MOCs. These results could be attributed to its relatively low surface-to-volume ratio, which result in the proportion of manganese ions on surface of MOOHs is significantly lower than that of MOCs and limit the improvement. For the 3-D MO nanostructures, the surface-to-volume ratio decrease with the increase of sizes. Besides, the secondary growth blocks the way of proton to enter into the inside spaces of BMOAs and SMOAs, which further reduce their T1 relaxivities. The r1 values of BMOAs and SMOAs are ACS Paragon Plus Environment


remarkably lower than that of 0-D MO nanostructures with the values of 0.46 and 0.42 mM-1s-1. It is worthwhile to notice that the MOWs reveal the comparable T1 relaxivity with the value 2.26 mM-1s-1 to 0-D MO nanostructures due to the geometrical confinement. In theory, T1 relaxivity is highly dependent on certain properties of T1 contrast agent, such as molecular tumbling time (τD). Ideally, long τD is benefit to the chemical exchange between water and magnetic ions and achieve strong T1 relaxation shortening effect. Compared to the BMOAs and SMOAs, the stretched nanowhiskers allow the proton to enter into the inside space of MOWs. More importantly, the specific structure of nanowhiskers can produce confined space to restrict diffusion of water proton in it, which can increase the τD in the inner sphere and improves its T1 contrast enhancement.69, 70 Concurrently, r2/r1 ratio is an important reference to decide whether a given contrast agent is suit to T1 MRI contrast imaging. To investigate the T1 contrast imaging ability of different MO nanostructures, we evaluated the r2/r1 ratio of all samples. The r2/r1 ratio of MOOHs, MORs, MOCs, MOOPs, BMOAs, SMOAs, and MOWs are 2.81, 5.21, 1.71, 3.05, 6.24, 6.79, and 4.01, respectively (Figure 8a and Supporting Information, Figure S12 and Table S3). On the basis of that the low r2/r1 ratio (< 5) leads to T1-dominated contrast effect, these results indicated that MOOHs, MOCs, MOOPs, and MOWs can serve as T1 contrast agent in T1-weighted imaging. However, the relatively high r2/r1 ratios suggest that MORs, BMOAs, and SMOAs are not good candidate to achieve T1-weighted contrast imaging.71,

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T1-weighted phantom images analyses show that T1 signal

intensity changes of MORs, BMOAs, and SMOAs along with the concentration increase are negligible due to their low r1 values and relative high r2/r1 ratio. However, MOOHs, MOCs, MOOPs, and MOWs exhibit gradual enhancement of signal intensity with the increase of concentration, indicating that these samples have the potential to generate MRI contrast enhancement on T1-weighted sequences (Figure 8d). Notably, the MOCs with highest r1 value and lowest r2/r1 ratio exhibited significantly higher T1 signal than others at same concentrations, suggesting that MOCs ACS Paragon Plus Environment

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may serve as highly sensitive T1 contrast agents for biomedical application in vitro and in vivo. In vitro and in vivo MRI imaging. Based on its high T1 relaxivity, we further chose the MOCs as representative samples to do T1-weighted MRI imaging in vitro and in vivo. Prior to do the MRI imaging, we investigate the cytotoxicity of MOCs by MTT assay. MOCs showed negligible cytotoxicity after 24 h of incubation, even at concentration up to 50 µg/mL of Mn, suggesting that MOCs has acceptable biocompatibility and minimal side effects (Supporting Information, Figure S13). To test the MRI contrast ability in vitro, we incubated the HepG2 cells with MOCs with the concentration of 0.4 mM for different times and conducted the T1-weighted MRI imaging on a 0.5 T MRI scanner. T1-weighted MR images show that the cell treated by PBS for 3 h exhibit negligible T1 signal, indicating that cell could not show T1 signal without the assistant of contrast agent in T1-weighted imaging (Figure 8e). Interestingly, the T1 signal in T1-weighted MR images of MOCs treated cells was negligible during the first 0.25 h but then gradually brightened with the elapse of incubation time. These time-dependent T1 signal changes reflected the uptake process of MOCs by HepG2 cells. The total amounts of Mn ions in the cells before and after treatment were also measured by inductively coupled plasma mass spectrometry (ICP-MS). It appears that the cells treated by PBS for 3 h contained 186.43 ng Mn ions, likely due to Mn being a natural cellular constituent and acting as a cofactor for enzymes.73, 74 Remarkably, the amount of Mn ions in cells dramatically increased to 202.62, 393.09, and 870.37 ng after incubation with MOCs for 0.25, 1.5, and 3 h, further confirm the change of T1 signal is caused by the uptake of MOCs by cells (Figure 8f). To further evaluate the ability of MOCs for in vivo imaging, we performed the T1-weighted MRI contrast imaging using BALB/c mice as model in vivo by a 7 T MRI scanner. Because of the high enrichment of nanomaterials in the liver, we focused on the liver as the targeted region. After intravenous injection of MOCs or MOSs-9 at a dose of 2 mg Mn/kg of mouse body weight, we indeed observed significant signal attenuation in the liver region at 1 h post-injection (p.i.) at both ACS Paragon Plus Environment


coronal and transverse planes (Figure 8g). Since the MOSs-9 show lower r1 value than MOCs, the T1 signal enhancement in liver region of mice treated by MOSs-9 are much weaker than that of the mice treated by MOCs (Supporting Information, Figure S14). To quantify the contrast, we identified the liver as the region of interest (ROI) and calculated the signal-to-noise ratio (SNR) and SNRpost/SNRpre values. It appears that the MR signal changes (∆SNR) in the liver region is as high as 68.58 and 78.04 % at coronal and transverse plane for the mice treated by MOCs, respectively, which are significantly higher than that of mice treated by MOSs-9 (Figure 8h and Supporting Information, Figure S14 and Table S4). These results demonstrated that MOCs have the ability to show strong MR contrast enhancement in T1 imaging in vivo, which may be a potential candidate to assist early and accurate lesion detection in T1-weighted MRI contrast imaging, particular in liver.

CONCLUSION In summary, we report a strategy to prepare MO nanostructures from 0-D to 3-D by thermal decomposition. We successfully fabricated anisotropic MO nanooctahedral, nanorods, nanocubes, nanooctapods, nanooctapod and nanopolyfacet assembled nanoclusters, and nanowhiskers with different physical properties. The key factor to the formation of these anisotropic MO nanostructures are introducing chloride and applying procedural heat up process. The chloride can bind to the manganese ions exposed on the MO nanocrystal and result in the anisotropic growth. Whereas, the procedural heat up process can affect the decomposition rate of the manganese-oleate and lead to the oriented attachment growth to form self-assembled 3-D nanostructures. More importantly, T1 relaxivity analysis indicates that the obtained MO nanostructures show morphology-dependent T1 relaxation time. There are three essential factors to affect their T1 contrast abilities, those are surface-to-volume ratio, surface manganese ion density, and geometrical confinement caused by specific morphology, which may light up road to rational design of new generation ACS Paragon Plus Environment

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high-performance T1 contrast agent. We demonstrated that MOCs with the highest T1 contrast ability among all samples shows significant T1 signal in cellular and animal level T1-weighted MRI imaging, thus, showing a great potential to lesion detection. Besides, these MO nanostructures with controllable morphology may provide adequate sources and rational design clue to develop new candidate to energy materials.

ASSOCIATED CONTENT Supporting Information The Support Informatuon is available free of charge on the ACS Publications website at Detailed methods, TEM analyses, growth process of MO nanostructure, MRI relaxivity, and MTT assay (PDF)

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (81601607 and ACS Paragon Plus Environment


81601470), Research Funds for the Central Universities (XDJK2016C182), and Open Research Fund of State Key Laboratory of Molecular Vaccinology and Molecular Diagnosticsh (2016KF02).


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Figure 1. Controlled synthesis of 0-D and 1-D anisotropic MO nanostructures. TEM images of monodispersed (a) MOOHs, (d, e) MORs, (g) MOCs, and (i) MOOPs, respectively. Insets show geometric model. The side lengths of MOOHs, MOCs, and MOOPs are 22.44, 9.33, and 25.87 nm. The size of MORs is 284.22 nm in length and 19.61 nm in width. HRTEM images of (b, c) MOOHs, (f) MORs, (h) MOCs, and (j) MOOPs, respectively. Insets show geometric model. These results suggest that MOOHs and MOCs are composed by {111} and {100} basal facet. The MORs reveal uniform lattice fringes in independent area, which may be ascribed to the oriented attachment growth.


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Figure 2. Crystal structure characterization of 0-D and 1-D MO nanostructures. (a) XRD pattern images of MOOHs, MORs, MOCs, and MOOPs, respectively. The XRD patterns can be assigned to MnO phase (circular, JCPDS No. 01-072-1533) and Mn3O4 phase (triangle, JCPDS no. 01-075-1560). (b) XPS analyses of MOOHs, MORs, MOCs, and MOOPs, respectively. Compared to MOOHs, MORs, and MOOPs, the peak of MOCs slightly shift from typical MnO to Mn3O4 phase. These results indicate that MOOHs, MORs, and MOOPs are typical MnO phase, while MOCs are mixture phase of MnO and Mn3O4.



Figure 3. Dynamic growth process of MO nanostructures. TEM images of MO nanostructures withdrawn from MOCs reaction solution after aging of (a) 10 min and (b) 15 min at 200 °C and (c) 10 min and (d) 40 min at 320 °C. TEM images of MO nanostructures withdrawn from MOOPs reaction solution after aging of (e) 10 min at 200 °C and (f) 120 min, (g) 150 min, and (h) 180 min at 320 °C. TEM images of MO nanostructures withdrawn from MOOHs reaction solution after aging of (i) 10 min, (j) 15 min, (k) 35 min, and (l) 60 min at 320 °C. TEM images of MO nanostructures withdrawn from MORs reaction solution after aging of (m) 10 min at 200 °C and (n) 5 min, (o) 25 min and (p) 60 min at 320 °C. These morphologies evolution indicate that the formation MOCs, MOOPs, and MOOHs are caused by the anisotropic growth and MORs is caused by oriented attachment growth.


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Figure 4. Controlled synthesis of 3-D MO nanostructures. TEM images of monodispersed (a) BMOAs, (b) SMOAs, and (c) MOWs, respectively. The higher magnification TEM images of (d) BMOAs, (e) SMOAs, and (f) MOWs, respectively. These results clearly show that the BMOAs, SMOAs, and MOWs are composed by many MO nanooctapod, nanopolyfacet, and curved nanowhiskers. HRTEM images of (g) BMOAs, (h) SMOAs, and (i) MOWs, respectively. The interplanar distance of 1.58 Å and 2.24 Å are assigned to the (220) and (200) plane of the MnO crystal.



Figure 5. Analyses the assembled process of BMOAs and MOWs. TEM images of (a-c) BMOAs and (d-f) MOWs withdrawn from reaction solution after aging different time during the growth process. These results clearly show that the formation of BMOAs and MOWs are based on the self-assembly crystal growth and secondary growth.


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Figure 6. Crystal structure characterization of 3-D MO nanostructures. (a) XRD pattern images of BMOAs, SMOAs, and MOWs, respectively. The XRD patterns can be assigned to MnO phase (JCPDS No. 01-072-1533), indicating their MnO phases. XPS analyses of (b) BMOAs, (c) SMOAs, and (d) MOWs, respectively. Consistent with the XRD analyses, BMOAs, SMOAs, and MOWs show that the valence of manganese ions in the BMOAs, SMOAs, and MOWs are +2.



Figure 7. Analyses the magnetic properties of different MO nanostructures. The smooth M-H curves of (a) 0-D and 1-D and (b) 3-D MO nanostructures measured at 300 K using a superconducting quantum interference device magnetometer. The M-H curves of (c) 0-D and 1-D and (d) 3-D MO nanostructures in low-magnetic field areas, showing nearly zero values for both coercivity and remanence and indicating that all MO nanostructures exhibit paramagnetic behavior at room temperature. The ZFC/FC curves of (e) 0-D and 1-D and (f) 3-D MO nanostructures measured under an applied magnetic field at 50 Oe, further confirming their paramagnetic behavior at room temperature.


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Figure 8. T1 MRI relaxivities of MO nanostructures and MRI contrast imaging in vitro and in vivo. (a) The comparisons of r1 values and r2/r1 ratios of MOOHs, MORs, MOCs, MOOPs, BMOAs, SMOAs, and MOWs, respectively. Since the surface coating ligand is the same, these results indicate that the T1 relaxivities of MO nanostructures are highly dependent on their crystal structures. (b) The comparisons of surface-to-volume ratio and T1 relaxivity and (c) metal package model of the MnO {111}, {110}, and {100} facets. These results indicate that the T1 relaxivities are affect by the surface-to-volume ratio and magnetic ions density on the surface. (d) T1 MRI phantom studies of all MO nanostructures at different Mn concentration (mM) in 1% agarose at 0.5 T. (e) T1-weighted MR images of HepG2 cells incubated with MOCs for different time durations. (f) Total amount of Mn ions in HepG2 cells incubated with MOCs for different times. The concentration of Mn ions was tested by ICP-MS (**p < 0.01, n = 3/group). (g) In vivo MR images of BALB/c mice at 0 and 1 h after intravenous injection of MOCs (2 mg Mn kg-1) at coronal (upper) and transverse (lower) plane. These results indicate that MOCs possess significant higher T1 signal in liver region. (h) Quantification of relative liver contrast collected at different time after administration of MOCs (**p < 0.01, n = 3/group).



ToC figure


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Controllable Synthesis of Manganese Oxide Nanostructures from 0-D

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