Water-based Synthesis of PAA-Coated Iron Oxide Nanoparticles

For the water-based, step-wise synthesis of polyacrylic acid-coated iron oxide nanoparticles (PAA-IONPs), three solutions were prepared; an iron salt solution [0.62 g of FeCl₃. 6H₂O and 0.32 g of FeCl₂. 4H₂O in dilute HCl solution (100 μL of 12 N HCl in 2.0 mL H₂O)]; an alkaline solution [1.8 mL of 30 % NH₄OH solution in 15 mL of N₂ purged DI water]; and a stabilizing agent solution [820 mg of polyacrylic acid in 5 mL of DI water]. To synthesize the PAA-IONP, the iron salt solution was added to the alkaline solution under vigorous stirring. The resulting dark suspension of iron oxide nanoparticles was stirred for approximately 30 seconds before addition of the stabilizing agent solution and stirred for 1 h. The resulting suspension of PAA-IONPs was then centrifuged at 4000 rpm for 30 minutes and the supernatant was washed three times with DI water to get rid of free polyacrylic acid and other unreacted reagents using an amicon 8200 cell (Millipore ultra-filtration membrane YM – 30 k). Finally, the PAA-IONP suspension was purified using magnetic column, washed with phosphate buffer saline (pH = 7.4) and concentrated using the amicon 8200 cell system. The iron concentration and magnetic relaxation of the PAA-IONPs was determined as previously reported [Josephson et. al. Bioconjugate Chem. 1999, 10, 186–191]. The successful coating of the IONPs with PAA was confirmed by the presence of a negative zeta-potential (ζ = −48 mV) and the characteristic acid carbonyl bands on the FT-IR spectroscopic analysis of the nanoparticles (Supporting Information 1 and 5).