Dynamic Nuclear Polarization Polarizing Agents Carrier-Study On The Preparation And Properties Of Thermoresponsive Microgels

In modern times,the nuclear magnetic resonance(NMR)theory has become increasingly perfect.Nuclear magnetic resonance imaging(MRI)developed on this basis has also made great achievements in the medical field,but scientists are still pursuing higher nuclear magnetic sensitivity,such as the use of dynamic nuclear polarization(DNP),which is based on the Overhauser effect,shifts the polarization of electrons to the nucleus,greatly increasing the polarizability of the nucleus,thereby increasing the NMR signal.However,due to the limitations of the frequency of NMR itself,the signal enhancement of NMR by DNP will be affected and it will not achieve the desired effect.In recent years,using the hydrogel's environmental-sensitive properties,it has been applied to DNP technology,which can not only increase the polarization multiple of DNP,but also obtain a higher-resolution imaging spectrum in MRI experiments.Therefore,it has been widely studied by researcher's attention.The hydrogels currently used for DNP technology are mainly concentrated on N-isopropylacrylamide(NIPAM)thermoresponsive hydrogels.However,the hydrogels currently studied are prepared by solution polymerization and belong to macro hydrogels.The macro hydrogels obtained in the DNP technology have a lower polarization enhancement factor,so it is greatly significance to develop non-toxic and enhanced smart hydrogel material for DNP-polarized.Based on a large number of literature investigations,this paper reviewed the synthesis and properties of hydrogels and the research progress of DNP technology.Using NIPAM and N-acryloyloxysuccinyl(NASI)imines as raw materials,4-amino-TEMPO Radical is a labeling molecule and thermoresponsive spin-labeled poly(N-isopropylacrylamide-N-acryloyloxysuccinimide)(P(NIPAM-NASI))was prepared by soap-free emulsion polymerization.The microgel was characterized by NMR and EPR techniques.The size of the microgel was measured by a nanoparticle size analyzer.The thermosensitivity and polarization enhancement mechanism of the spin-labeled microgels were investigated by DNP spectrometer(0.06T DNP and 0.35T DNP),which provided a valuable theoretical basis for the preparation of microgels with high polarization enhancement factors.The main research results are as follows:(1)High-purity target products were obtained by experiments;the prepared microgels were all submicron-sized and had an average particle size of 300 nm to 600 nm with a wide particle size distribution.(2)At the same temperature,the polarization enhancement factor of the microgel increases with the increase of the microwave power;when the radical concentration of the microgel is too low,the polarization enhancement factor is small;when the radical concentration is too high,the concentration of free electrons increases,the polarization enhancement factor slightly decreases.(3)1H enhancement of 0.35T DNP at the same power is consistent with 1H enhancement of 0.06T DNP,and is much higher than that reported in the literature;the polarization enhancement factor of microgel is less affected by the space;the polarization of 1H on free radicals in the gel was significantly enhanced after DNP polarization,and the SNR and contrast of the MRI images were significantly increased,indicating that the prepared microgels have potential applications in the enhancement of nuclear magnetic imaging.(4)The polarization enhancement factor of microgels does not change significantly with lower temperature.When the temperature reaches the volume phase transition temperature(VPTT)of the microgels,the polarization enhancement factor of the microgels decreases sharply with increasing temperature.(5)The phase transition temperature range and temperature sensitivity range of the prepared microgels were characterized by measuring the polarization enhancement times and T2 relaxation time curves at variable temperature and varying power.Among them,the VPTT range of Y,A-1 and A-2 was 30?C-34?,and the sensitive range was about 4?.The VPTT range of B-1 and B-2 was 30?-32?,and the sensitive range was about 3?;The VPTT range of C-l and C-2 was 28?-32?,and the sensitive range was about 40C.The comparison of the two methods shows that the use of the enhancement factor to characterize the thermoresponsive properties of the microgel is easier and more convenient than using the T2 relaxation time.