title: Coating thickness of magnetic iron oxide nanoparticles affects R2 relaxivity.
abstract: OBJECTIVE
  To evaluate the effect of coating thickness on the relaxivity of iron oxide nanoparticles.
  
  
  METHODS
  Monocrystalline superparamagnetic iron oxide nanoparticles (MIONs), coated with a polyethylene glycol (PEG)-modified, phospholipid micelle coating, with different PEG molecular weights, were prepared. The particle diameters were measured with dynamic light scattering (DLS) and electron microscopy (EM). The R1 and R2 of MIONs were measured using a bench-top nuclear magnetic resonance (NMR) relaxometer. pH was varied for some measurements. Monte Carlo simulations of proton movement in a field with nanometer-sized magnetic inhomogeneities were performed.
  
  
  RESULTS
  Increasing the molecular weight of the PEG portion of the micelle coating increased overall particle diameter. As coating thickness increases, the R2 decreases and the R1 increases. Changing pH has no effect on relaxivity. The Monte Carlo simulations suggest that the effect of coating size on R2 relaxivity is determined by two competing factors: the physical exclusion of protons from the magnetic field and the residence time for protons within the coating zone.
  
  
  CONCLUSIONS
  Coating thickness can significantly impact the R2, and the R2/R1 ratio, of a MION contrast agent. An understanding of the relationship between coating properties and changes in relaxivity is critical for designing magnetic nanoparticle probes for molecular imaging applications using MRI.
authors.affiliation: Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia 30332, USA.
authors.forename: Leslie E W
authors.initials: LE
authors.lastname: LaConte
authors.forename: Nitin
authors.initials: N
authors.lastname: Nitin
authors.forename: Omar
authors.initials: O
authors.lastname: Zurkiya
authors.forename: Daniela
authors.initials: D
authors.lastname: Caruntu
authors.forename: Charles J
authors.initials: CJ
authors.lastname: O'Connor
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Hu
authors.forename: Gang
authors.initials: G
authors.lastname: Bao
mesh_terms: D003198:Computer Simulation; D003287:Contrast Media / Q000737:chemistry*; D005290:Ferric Compounds / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008279:Magnetic Resonance Imaging; D008823:Micelles; D009010:Monte Carlo Method; D053758:Nanoparticles / Q000737:chemistry*; D010743:Phospholipids / Q000737:chemistry; D011092:Polyethylene Glycols / Q000737:chemistry; D013499:Surface Properties
chemical_list: D003287:Contrast Media; D005290:Ferric Compounds; D008823:Micelles; D010743:Phospholipids; D011092:Polyethylene Glycols