Abstract: Superparamagnetic Fe 3 O4 nanoparticles with polyacylic acid coating and high hydrophilicity were synthesized by simple coprecipitation method .The influence of Fe2 +:Fe3+( mole ratio) on the nanoparticle size was investigated . The crystal phase of Fe 3 O4 was confirmed by high resolution transmission electron microscopy and X-ray diffraction analysis .Infrared spectrum and thermogravimetric analysis indicate that poly acrylic acid is successfully coated onto the surface of the nanoparticles .The size of particles and their distribution is obtained from TEM .Average crystal size is infered from X-ray diffraction analysis.The average particles size from TEM and XRD are 7.2 nm, 6.7 nm, 8.3 nm and 9. 1 nm, 8.2 nm, 9.1 nm, respectively, corresponding to Fe2+:Fe3+ =1:1.5, 1:1.75 and 1:2. Dynamic laser scatter analysis reveals that it has some larger particles . The nanoparticles show superparamagnetic properties under room temperature .Relaxation analysis indicates that the sample 1:1. 5 is more suitable for T1 contrast agents and 1:1.75 and 1:2 are for T2 ones.It provides a new way for the optimization of the MR contrast agents .

Key words: Superparamagnetic Fe 3 O4 nanoparticles with polyacylic acid coating and high hydrophilicity were synthesized by simple coprecipitation method .The influence of Fe2 +:Fe3+( mole ratio) on the nanoparticle size was investigated . The crystal phase of Fe 3 O4 was confirmed by high resolution transmission electron microscopy and X-ray diffraction analysis .Infrared spectrum and thermogravimetric analysis indicate that poly acrylic acid is successfully coated onto the surface of the nanoparticles .The size of particles and their distribution is obtained from TEM .Average crystal size is infered from X-ray diffraction analysis.The average particles size from TEM and XRD are 7.2 nm, 6.7 nm, 8.3 nm and 9. 1 nm, 8.2 nm, 9.1 nm, respectively, corresponding to Fe2+:Fe3+ =1:1.5, 1:1.75 and 1:2. Dynamic laser scatter analysis reveals that it has some larger particles . The nanoparticles show superparamagnetic properties under room temperature .Relaxation analysis indicates that the sample 1:1. 5 is more suitable for T1 contrast agents and 1:1.75 and 1:2 are for T2 ones.It provides a new way for the optimization of the MR contrast agents .