Controlled Self-assembly Of Ultrasmall Fe₃O₄ Nanoparticles And Its Application In Magnetic Resonance Imaging Of Tumors

Magnetic resonance imaging(MRI),as a non-invasive tomography technique,has the advantages of high spatial resolution and unaffected by the depth of tissue penetration.However,due to the low sensitivity of MRI,contrast agents(CAs)were required to increase the signal-to-noise ratio during MRI,thereby improving the imaging performance.Common CAs can be divided into T1 and T2 types.Ti-type CAs,represented by gadolinium and manganese,brighten the imaging site(easier to distinguish),but tend to exhibit higher nephrotoxicity.T2-type CAs represented by magnetic iron oxides(IONPs)can darken the imaging site while maintaining high biocompatibility.However,the imaging time is too long,and the image edge is blurred and difficult to distinguish.In order to overcome the shortcomings of common CAs,researchers have developed ultrasmall superparamagnetic iron oxide nanoparticles(USPIONs)below 4 nm as potential ideal CAs.USPIONs not only exhibit the excellent biocompatibility of IONPs,but also exhibit a breakthrough in demonstrating good T1 MRI performance.In addition,this T1 MRI performance effect will be correspondingly transformed into T2/T1 MRI performance with the assembly/dissociation of USPIONs.Taking advantage of this feature,numerous T2-T1/T1-T2 assembled/dissociated USPIONs biological probes have been developed for the diagnosis of different diseases.Throughout the design of these probes in assembly,there are mainly two types:one is to load USPIONs through a carrier,the other is to self-assemble USPIONs through surface modification.However,due to the uncontrollable loading effect of the former,it is often difficult to complete the conversion of T2-T1 MRI signals.The size of the latter is often uncontrollable during self-assembly,and it cannot be further functionalized after assembly.In view of the above background,this thesis is dedicated to realizing the controllable self-assembly of USPIONs and improving their application effect in MRI detection.The following four research works were carried out:(1)Synthesis of pH-responsive USPIONs aggregates based on microemulsion-confined biomineralization and its application in tumor MRI detection:USPIONs were clustered within microemulsion droplets by the microemulsion method.Then,calcium ions and carbonate ions were introduced to complete the biomineralization of the surface of the microemulsion droplets.The calcium carbonate formed on the surface solidifies the clustered USPIONs inside,thus realizing the assembly of USPIONs without carrier and surface modification.This assembly method was convenient and fast,the size of the assembled PEG-USPIONs@CaCO3 nanoprobe was about 200 nm,and the conversion T1-T2 MRI signal before and after loading was realized.The modification of calcium carbonate endows the nanoprobes with the ability of pH response:at normal tissue sites(pH≥7),nanoprobes were assembled and exhibit T2 MRI signals;at tumor sites or lower pH conditions(pH<7),the calcium carbonate layer was dissolved,releasing the inner USPIONs,which restores the T1 MRI signal.Through the conversion of the T2-T1 MRI signal,the probe was finally successfully applied to the MRI detection of mouse tumors models.(2)Synthesis of pH/H2O2 dual-responsive USPIONs aggregates based on microemulsion confinement and its application in tumor MRI detection:Dopamine and 2-aminophenylboronic acid were modified on the surface of two batches of USPIONs by ligand exchange,respectively.These two molecules could react with each other to form boron ester bonds.Subsequently,the two USPIONs were allowed to react and assemble in microemulsion droplets by microemulsion method.In this process,by adjusting the oil-water ratio and the amount of emulsifier in the microemulsion method,the purpose of adjusting the size of the assembly was achieved.Then,tannic acid(TA)that can be complexed with iron was introduced to increase the hydrophilicity of the assembly and further stabilize the assembly to achieve the purpose of post-assembly modification.It is found that the size of the prepared USPIONs@TA nanoprobes could be precisely regulated within 100-400 nm,and the transition of T1-T2 MRI signals before and after assembly could be realized.The complexation of surface TA to iron would be weakened in acidic environment,thus releasing the internal assembly,which is the first response.The boron-ester bonds between the assembled bodies are cut off by H2O2 and dispersed into free USPIONs to realize T1 MRI signal recovery,which is the second response.In tumor detection,it was further demonstrated that the pH/H2O2 dual-responsive nanoprobe exhibite a more stable MRI signal than the single-responsive probe.(3)Study on the Anionic Control of Super-Homogeneous Assembly of USPIONs in Metal-Organic Frameworks(MOF):Based on the regulation of anions on the growth rate of MOFs,the effects and laws of different metal ions and different anions on the homogeneous self-assembly of USPIONs in MOFs were systematically studied,and a nanocomposite with ultra-uniformly dispersed USPIONs in MOFs(USPIONs(?)MOF)was prepared.Compared with the traditional USPIONs@MOF nanocomposites,the prepared USPIONs(?)MOF exhibit a completely new morphology,and the whole material was composed of a large number of uniformly dispersed USPIONs,which completes the homogeneous assembly of MOFs without surface active sites.Through a series of characterization,the assembly mechanism of this homogeneous composite nanomaterial was proposed,and the assembly mechanism was verified,which further proved the universality of this homogeneous assembly.(4)Synthesis of tumor hypoxia-responsive USPIONs(?)MOF Super-homogeneous aggregates and their application in tumor MRI detection:In the above USPIONs(?)MOF homogeneous aggregates,the ligands used contain azo bonds.Based on this,by verifying the cleavage of azo bond in anoxic environment,the changes of MRI performance of homogeneous aggregates before assembly,after assembly and after cleavage were systematically studied.It was found that compared with ordinary composite materials,homogeneous aggregates exhibit higher MRI signal intensity,more stable structure and less leakage,which were more suitable for nano-probes.By controlling the amount of USPIONs during the assembly process,the size of the homogeneous aggregates could be further controlled to obtain USPIONs assemblies with different properties.Finally,a group of materials with the best size and performance were selected,and their good T2-T1 MRI detection performance was verified in mouse tumor models.